摘要:

physicochemical topics, thereby encouraging scientists of different disciplines to contribute their varied viewpoints to a coiiinion theme. A recent Discussion is :- The Royal Soci- of Chemistry- No.75 lntraamolecwlar Kinetics No. 75 in the series, this publication is the result of a general discussion held at the University of Warwick in April 1983. Contents: The Spiers Meniorlal Lecture; Vibrational Redistribution within Excited Electronic States of Polyatomic Molecules Inrraniolecular R e h u t i o n o f 1. vcited States lsomerization of Intcrnal~ncrgy-selected Ions Kinetics of Ion-Molecule Collision Coinple\es in the Gas Phase, E\periinent and Theory lntrainolccular Decay 01' Soinc Open-shell Pulya t o niic Ca lions On tlic Theory u i Iiitrdniolccul~r I n e r g y Transfer Pulsed Laser Preparation and Ouaniuin Superposition Statc Evolution in ReguLtr and Irregular Systems A Ouantuiii-iiicclianical Internal-collision Model for State-sclcctcd Uniinolccular Decoiiiposilio n The Correspondence Principle and Intramolecular Dynamics lntrainoleculdr Dcphasiiig.t'icusecond Evolution of Wavepacket States in a Molecule with Int erinediate-casc level Struct urc Energy Conversion in van der Waals C'u~~iplc\c\ ol s-Tetrarine and Argon Tim-dependent Processes in Polyatuinic Molecules During and After Intense Intrarcd Irradiation Energy Distributions in tlic (.N(X'L+) bragnient froiii tlie Infrared Multiplepholun Dissociation ol' CI. ICN. A Coinparison between 1:xperiiiiental Results and the Predictions ot Statistical Theories of ChFO + Product Energy Partitioning in the Decoiii- position of State-selectively Excited HOON and IIOOD Low-power Inl-rarcd Laser I'hoiolysis o f Tetramethy ldioxetan Uniinolecular Reactions lnduccd by Vibrational Overtone Excitation Uniiiiolecular Decomposition of t-Butylhydro- peroxide by Direct Excitation of the 6-0 0-11 Stretching Overtone I'icosecond-jet Spectroscopy and Photoclieinistry.Energy Redistribution and its Iiiipact'on Coherence, Isoincrization, Ihssociatiun and Solvalioii knergy Redistribution in Large Molecules. Duect St ud y o f In1 rainolucular Rehxa lion in the Gas Phase with Picosecond Gating Rotation-dependent Intrainolecuhr I'r~)cessc.sofSO:(A'A.) in a Superwnic Jct Role of Rotation-Vibration Interaction in Vibrational Keh\ation. Energy Kcdistribution in k,xcitcd Singlet I~'ornialdc1iyde Sub-lhppler.Spectroscopy of Benrcnc in the "('liaiinel-lliree" Region Intraiiiulccular 1:lectronic Kclau~tion and I'liotois~)iiieruati[)n Processes in tlie lsuhted Azabenrene Molecules Pyridinc, Pyrazinc and I'yriiiiidinc Softcover 434pp 0 85186 658 1 Price f25.00 ($48.00) Rest of the World f26.00 RSC Members f 16.25 Faraday Discussions of the Chemical Society 7< lnrruniolei u h r Kincrit I Faraday Symposia are usually held annually and are confined to more specialiscd topics than Discussions, with particular reference to recent rapidly developing lines of rescuch. A recent Symposium is :- No.l?The Hydrophobic Interadion No. 17 in the series, this publication is the result of a symposium on The Hydrophobic Interaction held at the Uiiiversity of Reading in December 1982.Contents: Hydrophobic Interdctionr a llistaric.11 Per spect ivr llydrupliobic Ilydration Geometric Kelaution in Water. Its Role in Precise Vapour-pressure Measureiiients of the SolubilkdtiorI of Benzene by Aqueous Sodiuiii Octylsulphate Solutions Nuclear Magnetic Resonance R e b u t i o n Investigation of Tetrahydrofuran and Methyl Iodide Clathrdtes Infrared and Nuckar Magnetic Kcwnance Studies Pertaining to the (age Model t o r Solutions oS Acetone in Water Irothernial Transport Properties in Solutions o f S y mmet r ica I Tet ra-alk y hmnioniuiii Bromides Thermodynamics of Cavity I'oriiiaiion in Water. A Molecular Dynamics Study Molecular Librations and Solvent Oricnt- ational Correlations in Hydrophobic Phenomena Monte Carlo Computer Siniulation Study of the Hydrophobic Effect.Potential ot Mean Force for ECfir)gaq at 25 and SOv C Hydroplicibic Moments and Protein Structure Application 01' the Kirkwood-Buff Theory to the t'roblcin 01 Hydrophobic Interactions Ihentangleinent of Ilydrophubic and IFlcctrosi~tic Contributions t o the I.ilni Pressures O i Ionic Surfactants llydrophobir. Intcracliuns in Dilute Su lut io ns u t 1'0 1 y (vin y I a Ico lio I) ('onioriii;tiionaI and 1:unc.i ional I'ropertics of tiaeiiwglobin in Water+Alcohol Mixtures. Dependence o f Bull. Electrostatic and tlydrupliohic I n t c r x t i o n s upon ptl and KCI concentrations Softcover 24Opp 0 85186 668 9 Price f36.50 ($70.00) Rest of the World f38.50 RSC Members f 23.75 ORDERING RSC Members should send their orders to: The Royal Society of Chemistry.The Membership Officer. 30 Russell Square, Non-RSC Members The Royal Society of Chemistry, Distribution Centre, Blackhorse Road, L London WC1 B 5DT. Letchworth, Herts SO6 IHN, England. Faradaj Symposia of the Chemical Society hGi 17 I hc HI drophohr' Inrcrm rron 1 9 X ? (viii)physicochemical topics, thereby encouraging scientists of different disciplines to contribute their varied viewpoints to a coiiinion theme. A recent Discussion is :- The Royal Soci- of Chemistry- No.75 lntraamolecwlar Kinetics No. 75 in the series, this publication is the result of a general discussion held at the University of Warwick in April 1983. Contents: The Spiers Meniorlal Lecture; Vibrational Redistribution within Excited Electronic States of Polyatomic Molecules Inrraniolecular R e h u t i o n o f 1.vcited States lsomerization of Intcrnal~ncrgy-selected Ions Kinetics of Ion-Molecule Collision Coinple\es in the Gas Phase, E\periinent and Theory lntrainolccular Decay 01' Soinc Open-shell Pulya t o niic Ca lions On tlic Theory u i Iiitrdniolccul~r I n e r g y Transfer Pulsed Laser Preparation and Ouaniuin Superposition Statc Evolution in ReguLtr and Irregular Systems A Ouantuiii-iiicclianical Internal-collision Model for State-sclcctcd Uniinolccular Decoiiiposilio n The Correspondence Principle and Intramolecular Dynamics lntrainoleculdr Dcphasiiig. t'icusecond Evolution of Wavepacket States in a Molecule with Int erinediate-casc level Struct urc Energy Conversion in van der Waals C'u~~iplc\c\ ol s-Tetrarine and Argon Tim-dependent Processes in Polyatuinic Molecules During and After Intense Intrarcd Irradiation Energy Distributions in tlic (.N(X'L+) bragnient froiii tlie Infrared Multiplepholun Dissociation ol' CI.ICN. A Coinparison between 1:xperiiiiental Results and the Predictions ot Statistical Theories of ChFO + Product Energy Partitioning in the Decoiii- position of State-selectively Excited HOON and IIOOD Low-power Inl-rarcd Laser I'hoiolysis o f Tetramethy ldioxetan Uniinolecular Reactions lnduccd by Vibrational Overtone Excitation Uniiiiolecular Decomposition of t-Butylhydro- peroxide by Direct Excitation of the 6-0 0-11 Stretching Overtone I'icosecond-jet Spectroscopy and Photoclieinistry. Energy Redistribution and its Iiiipact'on Coherence, Isoincrization, Ihssociatiun and Solvalioii knergy Redistribution in Large Molecules.Duect St ud y o f In1 rainolucular Rehxa lion in the Gas Phase with Picosecond Gating Rotation-dependent Intrainolecuhr I'r~)cessc.sofSO:(A'A.) in a Superwnic Jct Role of Rotation-Vibration Interaction in Vibrational Keh\ation. Energy Kcdistribution in k,xcitcd Singlet I~'ornialdc1iyde Sub-lhppler. Spectroscopy of Benrcnc in the "('liaiinel-lliree" Region Intraiiiulccular 1:lectronic Kclau~tion and I'liotois~)iiieruati[)n Processes in tlie lsuhted Azabenrene Molecules Pyridinc, Pyrazinc and I'yriiiiidinc Softcover 434pp 0 85186 658 1 Price f25.00 ($48.00) Rest of the World f26.00 RSC Members f 16.25 Faraday Discussions of the Chemical Society 7< lnrruniolei u h r Kincrit I Faraday Symposia are usually held annually and are confined to more specialiscd topics than Discussions, with particular reference to recent rapidly developing lines of rescuch.A recent Symposium is :- No.l?The Hydrophobic Interadion No. 17 in the series, this publication is the result of a symposium on The Hydrophobic Interaction held at the Uiiiversity of Reading in December 1982. Contents: Hydrophobic Interdctionr a llistaric.11 Per spect ivr llydrupliobic Ilydration Geometric Kelaution in Water. Its Role in Precise Vapour-pressure Measureiiients of the SolubilkdtiorI of Benzene by Aqueous Sodiuiii Octylsulphate Solutions Nuclear Magnetic Resonance R e b u t i o n Investigation of Tetrahydrofuran and Methyl Iodide Clathrdtes Infrared and Nuckar Magnetic Kcwnance Studies Pertaining to the (age Model t o r Solutions oS Acetone in Water Irothernial Transport Properties in Solutions o f S y mmet r ica I Tet ra-alk y hmnioniuiii Bromides Thermodynamics of Cavity I'oriiiaiion in Water.A Molecular Dynamics Study Molecular Librations and Solvent Oricnt- ational Correlations in Hydrophobic Phenomena Monte Carlo Computer Siniulation Study of the Hydrophobic Effect. Potential ot Mean Force for ECfir)gaq at 25 and SOv C Hydroplicibic Moments and Protein Structure Application 01' the Kirkwood-Buff Theory to the t'roblcin 01 Hydrophobic Interactions Ihentangleinent of Ilydrophubic and IFlcctrosi~tic Contributions t o the I.ilni Pressures O i Ionic Surfactants llydrophobir. Intcracliuns in Dilute Su lut io ns u t 1'0 1 y (vin y I a Ico lio I) ('onioriii;tiionaI and 1:unc.i ional I'ropertics of tiaeiiwglobin in Water+Alcohol Mixtures. Dependence o f Bull. Electrostatic and tlydrupliohic I n t c r x t i o n s upon ptl and KCI concentrations Softcover 24Opp 0 85186 668 9 Price f36.50 ($70.00) Rest of the World f38.50 RSC Members f 23.75 ORDERING RSC Members should send their orders to: The Royal Society of Chemistry. The Membership Officer. 30 Russell Square, Non-RSC Members The Royal Society of Chemistry, Distribution Centre, Blackhorse Road, L London WC1 B 5DT. Letchworth, Herts SO6 IHN, England. Faradaj Symposia of the Chemical Society hGi 17 I hc HI drophohr' Inrcrm rron 1 9 X ? (viii)

ISSN:0300-9599    

DOI:10.1039/F198480FX041

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

AUTHOR INDEX xxxv Sabbatini, L., 1029 Sacco, A., 2669 Sanders, J. V., 571 Sangster, D. F., 291 1 Sarka, K., 521 Sasahira, A., 473 Sasse, W. H. F., 571 Satchell, P. W., 2395 Sato, K., 841 Sato, Y., 341 Savino, V., 759 Sayers, C. M., 1217 Schiller, R. L., 1257 Schmidt, J., 1 Schmidt, P. P., 2017 Schneider, H., 3275, 3285 Schulz, R. A., 489, 1323 Scott, J. M. W., 739, 1651, 2287, Scott, S. K., 3409 Segall, R. L., 2609 Sehested, K., 2929, 2969 Seidl, V., 1367 Sem, P., 297 Serratosa, J. M., 2225 Seyama, H., 237 Seyedmonir, S. R., 87, 2269 Shanahan, M. E. R., 37 Sheppard, A., 2999 Sherwood, P. M. A., 135, 2099, Shindo, Y., 879, 2199 Shiotani, H., 2145 Shizuka, H., 383, 341 Siekhaus, W. J., 61 Sircar, S., 1101, 2489 Smart, R. St C., 2957, 2609 Smith, I. M., 3021 Smith, R., 3233 Snow, R.L., 3463 Solar, S., 2929 Solar, W., 2929 Solymosi, F., 1841 Soma, M., 237 Soupart, J-B., 3209 Sourisseau, C., 3257 Spink, J. A., 3469 Spoto, G., 1875, 1891 Spotswood, T. M., 3147 Staricco, E. H., 2631 Stassinopoulou, K., 3095 Stedman, D. H., 285 Stout, D. R., 3481 Strohbusch, F., 1757 Strumolo, D., 1479 Struve, P., 813, 2167 Styring, M. G., 3051 Subramanian, R., 2405 2881, 3359 2549, 2867 Sundar, H. G. K., 3491 Sutcliffe, L. H., 669, 3021 Sutton, H. C., 2301 Sutton, L. E., 635 Suzuki, H., 803 Suzuki, T., 1925, 3157 Symons, M. C. R., 423, 1005, Szamosi, J., 1645 Szczepaniak, W., 2935 Takagi, Y., 1925 Takahashi, K., 803 Takahashi, N., 629 Takanaka, J., 941 Takao, S., 993 Takasaki, S., 803 Takegami, H., 1221 Tam, S-C., 2255 Tamamushi, R., 2751 Tamaru, K., 29, 1567, 1595 Tamilarasan, R., 2405 Tanabe, S., 803 Tanaka, K., 2563,2981 Tanaka, T., 119 Taniewska-Osinska, S., 1409 Tascon, J.M. D., 1089 Teo, H. H., 981, 1787 Tetenyi, P., 3037 Thomas, J. K., 1163 Thompson, L., 1673 Thomson, M., 1867 Thomson, S. J., 1689 Tiddy, G. J. T., 789, 3339 Tittarelli, P., 2209 Tominaga, T., 941 Tomkinson, J., 225 Tonelli, C., 1605 Toprakcioglu, C., 13,413 Tran, T., 1867 Trasatti, S., 913 Tripathi, A. D., 1517 Tronc, E., 2619 Troncoso, G., 2127 Truscott, T. G., 2293 Tsurusaki, T., 879 Tuck, J. J., 309 Turner, P. S., 2609 Tusk, M., 1757 Tvarbikova, Z., 2639 Tyrrell, H. J. V., 1279 Ueki, Y.. 341 Ueno, A., 803 Unno, H., 1059 Valencia, E., 2127 van de Ven, T. G. M., 2677 van Ommen, J. G., 2479 van Truong, N., 3275, 3285 Vargas, I., 1947 2767, 2803, 21 1, 1999 Vedrine, J.C., 1017 Veith, J., 2313 Velasco, J. R., 3429 Vesala, A., 2439 Vickerman, J. C., 1903 Vincent, B., 2599 Vinek, H., 1239 Vink, H., 507, 1297 Waghorne, W. E., 1267 Wagley, D. P., 47 Walker, R. W., 435, 3187, 3195, Wallington, T. J., 2737 Wang, G-W., 1039 Watkins, P. E., 2323 Watkiss, P. J., 1279 Watt, R. A. C., 489 Webb, G., 1689 Webster, B. C., 255, 267 Weiner, E. R., 1491 Wells, C. F., 2155. 2445 Wells, J. D., 1233 Whang, B. C. Y., 291 1 Whittle, E., 2323 Wichterlova, B., 2639 Wiesner, S., 3021 Wilhelmy, D. M., 563 Williams E. H., 3147 Williams, P. A., 403 Williams, R. J. P., 2255 Wokaun, A., 1305 Wolff, T., 2969 Wood, S. W., 3419 Woolf, L. A., 549, 1287 Wright, C. J., 1217 Wu, D. C., 1795 Wiirflinger, A., 3221 Wyn-Jones, E., 1915 Yamabe, M., 1059 Yamamoto, S., 941 Yamashita, H., 1435 Yamauchi, H., 2033 Yamazaki, A., 3245 Yariv, S., 1705 Yasumori, I., 841 Yeates, S.G., 1787 Yide, X., 969, 3103 Ylikoski, J., 2439 Yokokawa, T., 473 Yoneda, N., 879 Yonezawa, T., 1435 Yoshida, S., 119, 1435 Zambonin, P. G., 1029 Zanderighi, L., 1605 Zecchina. A., 2209, 2723, 1875, Zipelli, C., 1777 Zundel, G., 553 348 1, 2827 1891AUTHOR INDEX xxxv Sabbatini, L., 1029 Sacco, A., 2669 Sanders, J. V., 571 Sangster, D. F., 291 1 Sarka, K., 521 Sasahira, A., 473 Sasse, W. H. F., 571 Satchell, P. W., 2395 Sato, K., 841 Sato, Y., 341 Savino, V., 759 Sayers, C. M., 1217 Schiller, R. L., 1257 Schmidt, J., 1 Schmidt, P. P., 2017 Schneider, H., 3275, 3285 Schulz, R. A., 489, 1323 Scott, J. M. W., 739, 1651, 2287, Scott, S.K., 3409 Segall, R. L., 2609 Sehested, K., 2929, 2969 Seidl, V., 1367 Sem, P., 297 Serratosa, J. M., 2225 Seyama, H., 237 Seyedmonir, S. R., 87, 2269 Shanahan, M. E. R., 37 Sheppard, A., 2999 Sherwood, P. M. A., 135, 2099, Shindo, Y., 879, 2199 Shiotani, H., 2145 Shizuka, H., 383, 341 Siekhaus, W. J., 61 Sircar, S., 1101, 2489 Smart, R. St C., 2957, 2609 Smith, I. M., 3021 Smith, R., 3233 Snow, R. L., 3463 Solar, S., 2929 Solar, W., 2929 Solymosi, F., 1841 Soma, M., 237 Soupart, J-B., 3209 Sourisseau, C., 3257 Spink, J. A., 3469 Spoto, G., 1875, 1891 Spotswood, T. M., 3147 Staricco, E. H., 2631 Stassinopoulou, K., 3095 Stedman, D. H., 285 Stout, D. R., 3481 Strohbusch, F., 1757 Strumolo, D., 1479 Struve, P., 813, 2167 Styring, M. G., 3051 Subramanian, R., 2405 2881, 3359 2549, 2867 Sundar, H.G. K., 3491 Sutcliffe, L. H., 669, 3021 Sutton, H. C., 2301 Sutton, L. E., 635 Suzuki, H., 803 Suzuki, T., 1925, 3157 Symons, M. C. R., 423, 1005, Szamosi, J., 1645 Szczepaniak, W., 2935 Takagi, Y., 1925 Takahashi, K., 803 Takahashi, N., 629 Takanaka, J., 941 Takao, S., 993 Takasaki, S., 803 Takegami, H., 1221 Tam, S-C., 2255 Tamamushi, R., 2751 Tamaru, K., 29, 1567, 1595 Tamilarasan, R., 2405 Tanabe, S., 803 Tanaka, K., 2563,2981 Tanaka, T., 119 Taniewska-Osinska, S., 1409 Tascon, J. M. D., 1089 Teo, H. H., 981, 1787 Tetenyi, P., 3037 Thomas, J. K., 1163 Thompson, L., 1673 Thomson, M., 1867 Thomson, S. J., 1689 Tiddy, G. J. T., 789, 3339 Tittarelli, P., 2209 Tominaga, T., 941 Tomkinson, J., 225 Tonelli, C., 1605 Toprakcioglu, C., 13,413 Tran, T., 1867 Trasatti, S., 913 Tripathi, A.D., 1517 Tronc, E., 2619 Troncoso, G., 2127 Truscott, T. G., 2293 Tsurusaki, T., 879 Tuck, J. J., 309 Turner, P. S., 2609 Tusk, M., 1757 Tvarbikova, Z., 2639 Tyrrell, H. J. V., 1279 Ueki, Y.. 341 Ueno, A., 803 Unno, H., 1059 Valencia, E., 2127 van de Ven, T. G. M., 2677 van Ommen, J. G., 2479 van Truong, N., 3275, 3285 Vargas, I., 1947 2767, 2803, 21 1, 1999 Vedrine, J. C., 1017 Veith, J., 2313 Velasco, J. R., 3429 Vesala, A., 2439 Vickerman, J. C., 1903 Vincent, B., 2599 Vinek, H., 1239 Vink, H., 507, 1297 Waghorne, W. E., 1267 Wagley, D. P., 47 Walker, R. W., 435, 3187, 3195, Wallington, T. J., 2737 Wang, G-W., 1039 Watkins, P. E., 2323 Watkiss, P. J., 1279 Watt, R. A. C., 489 Webb, G., 1689 Webster, B. C., 255, 267 Weiner, E. R., 1491 Wells, C. F., 2155. 2445 Wells, J. D., 1233 Whang, B. C. Y., 291 1 Whittle, E., 2323 Wichterlova, B., 2639 Wiesner, S., 3021 Wilhelmy, D. M., 563 Williams E. H., 3147 Williams, P. A., 403 Williams, R. J. P., 2255 Wokaun, A., 1305 Wolff, T., 2969 Wood, S. W., 3419 Woolf, L. A., 549, 1287 Wright, C. J., 1217 Wu, D. C., 1795 Wiirflinger, A., 3221 Wyn-Jones, E., 1915 Yamabe, M., 1059 Yamamoto, S., 941 Yamashita, H., 1435 Yamauchi, H., 2033 Yamazaki, A., 3245 Yariv, S., 1705 Yasumori, I., 841 Yeates, S. G., 1787 Yide, X., 969, 3103 Ylikoski, J., 2439 Yokokawa, T., 473 Yoneda, N., 879 Yonezawa, T., 1435 Yoshida, S., 119, 1435 Zambonin, P. G., 1029 Zanderighi, L., 1605 Zecchina. A., 2209, 2723, 1875, Zipelli, C., 1777 Zundel, G., 553 348 1, 2827 1891

ISSN:0300-9599    

DOI:10.1039/F198480BX043

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

JOURNAL OF THE CHEMICAL SOCIETY F A R A D A Y T R A N S A C T I O N S , P A R T S I A N D I 1 The Journal of the Chemical Society is published in six sections, of which five are termed Transactions; these are distinguished by their subject matter, as follows: Dalton Transactions (Inorganic Chemistry). All aspects of the chemistry of inorganic and organometallic compounds; including bioinorganic chemistry and solid-state inorganic chemistry; of their structures, properties, and reactions, including kinetics and mechanisms; new or improved experimental techniques and syntheses. Faraday Transactions I (Physical Chemistry). Radiation chemistry, gas-phase kinetics, electrochemistry (other than preparative), surface and interfacial chemistry, heterogeneous catalysis, physical properties of polymers and their solutions, and kinetics of polymerization, etc.Faraday Transactions II (Chemical Physics). Theoretical chemistry, especially valence and quantum theory, statistical mechanics, intermolecular forces, relaxation phenomena, spectroscopic studies (including i.r., e.s.r., n.m.r., and kinetic spec- troscopy, etc.) leading to assignments of quantum states, and fundamental theory. Studies of impurities in solid systems. Perkin Transactions I (Organic Chemistry). All aspects of synthetic and natural product organic, organometallic and bio-organic chemistry, including aliphatic, alicyclic, and aromatic systems (carbocyclic and heterocyclic). Perkin Transactions ZI (Physical Organic Chemistry). Kinetic and mechanistic studies of organic, organometallic and bio-organic reactions.The description and application of physicochemical, spectroscopic, and theoretical procedures to organic chemistry, including structure-activity relationships. Physical aspects of bio-organic chemistry and of organic compounds, including polymers and biopolymers. Authors are requested to indicate, at the time they submit a typescript, the journal for which it is intended. Should this seem unsuitable, the Editor will inform the author. The sixth section of the Journal of the Chemical Society is Chemical Communications, which is intended as a forum for preliminary accounts of original and significant work, in any area of chemistry that is likely to prove of wide general appeal or exceptional specialist interest. Such preliminary reports should be followed up eventually by full papers in other journals (e.g.the five Transactions) providing detailed accounts of the work. NOTES I t has always been the policy of the Faraday Transactions that brevity should not be a factor influencing acceptability for publication. In addition however to full papers both sections carry at the end of each issue a section headed ‘Notes’, which are short self-contained accounts of experimental observations, results, or theory that will not require enlargement into ‘full’ papers. The -Notes section is not used for preliminary communications. The layout of a Note is the same as that of a paper. Short summaries are required. The procedure for submission, administration, refereeing, editing and publication of Notes is the same as for full papers.However, Notes are published more quickly than papers since their brevity facilitates processing at all stages. The Editors endeavour to meet authors’ wishes as to whether anarticle is a full paper or a Note, but since there is no sharp dividing line between the one and the other, either in terms of length or character of content, the right is retained to transfer overlong Notes to the full papers section. As a guide a Note should not exceed I500 words or word-equivalents.NOMENCLATURE AND SYMBOLISM Units and Symbols. The Symbols Committee of The Royal Society, of which The Royal Society of Chemistry is a participating member, has produced a set of recommendations in a pamphlet ‘Quantities, Units, and Symbols’ (1975) (copies of this pamphlet and further details can be obtained from the Manager, Journals, The Royal Society of Chemistry, Burlington House, London W IV OBN).These recommendations are applied by The Royal Society of Chemistry in all its publications. Their basis is the ‘Systeme International d’Unites’ (SI). A more detailed treatment of units and symbols with specific application to chemistry is given in the IUPAC Manual of Symbols and Terminology for Physicochemical Quantities and Units (Pergamon, Oxford, 1979). Nomenclature. For many years the Society has actively encouraged the use of standard IUPAC nomenclature and symbolism in its publications as an aid to the accurate and unambiguous communication of chemical information between authors and readers. In order to encourage authors to use IUPAC nomenclature rules when drafting papers, attention is drawn to the following publications in which both the rules themselves and guidance on their use are given: Nomenclature of Organic Chemistry, Sections A , B, C, D, E, F, and H (Pergamon, Oxford, 1979 edn).Nomenclature of Inorganic Chemistry (Butterworths, London, 197 1, now published by Pergamon). Biochemical Nomenclature and Related Documents (The Biochemical Society, London, 1978). A complete listing of all IUPAC nomenclature publications appears in the January issues of J. Chem. SOC., Faraday Transactions. It is recommended that where there are no IUPAC rules for the naming of particular compounds or authors find difficulty in applying the existing rules, they should seek the advice of the Society’s editorial staff. FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY Marlow Medal and Prize Applications are invited for the award of the Marlow Medal for 1985 and prize of €1 00.The award will be open to any member of the Faraday Division of the Royal Society of Chemistry who, by the age of 32, had made in the judgement of the Council of the Faraday Division, the most meritorious contribution to physical chemistry or chemical physics. The award will be rnade on the basis of publications (not necessarily in the Transactions) on any subject normally published in J. Chem. SOC., Faraday Transactions I and / I , that carry a date of receipt for publication not later than the candidate’s 32nd birthday. Candidates should be members and under 34 on 1 st January 1985, the closing date for applications, which may be made either by the candidate himself or on his behalf by another member of the Society.Copies of the rules of the award and application forms may be obtained from: M rs Y. A. Fish, The Royal Society of Chemistry, Burlington House. London W1 V OBN (ii)THE FARADAY DIVISION OF THE ROYAL SOCtETY OF CHEMISTRY SYMPOSIUM NO. 19 Molecular Electronic Structure Calculations- Methods and Applications University of Cambridge, 12-1 3 December 1984 Molecular electronic structure calculations have now developed into a powerful predictive tool and are necessary in several different fields to aid the understanding and interpretation of experimental observations. The meeting will review the current state of this rapidly developing discipline and will bring together experts on some of the most advanced methods and their applications. The meeting will provide an opportunity for discussion and comparison of the various techniques currently in use.It will therefore not only be a valuable forum for discussion among research workers in the field, but should also show the non-specialist what theoretical calculations can be expected to achieve now and in the near future. The final programme and application form may be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 79 (in conjunction with the Polymer Physics Group) Polymer Liquid Crystals University of Cambridge, 1-3 April 1985 The object of the meeting will be to discuss all aspects of the developing subject of polymeric liquid crystals. The hope is to bring together scientists from the fields of conventional polymer science and monomeric liquid crystals who are active in this field. The discussion is aimed at understanding the following facets: (a) The chemical characteristics that give rise to polymer liquid crystalline behaviour.(b) The nature of the high local anisotropy of these systems and their structural organisation at the molecular, micron and macroscopic levels. (c) The physical properties and their industrial exploitation, with particular reference to the influence of external force fields such as flow, electric and magnetic fields. (d) The inter-relations of polymer liquid crystals with small-molecule mesophases, conventional flexible polymers and biopolymers which exhibit liquid-crystalline behaviour.The conference will include a Poster Session for which contributions are invited. Abstracts (300 words) should be sent as soon as possible (and in any event no later than 31 January 1985) to the Chairman of the Organising Committee: Professor 6. R. Jennings, Electro-Optics Group. Department of Physics, Brunel University, Uxbridge UB8 3PH The preliminary programme may be obtained from : Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1 V OBNTHE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 80 Physical Interactions and Energy Exchange at the Gas-Solid Interface McMaster University, Hamilton, Ontario, Canada, 23-25 July 1985 Organising Committee : Professor J.A. Morrison (Chairman) Dr M. L. Klein Professor G. Scoles Professor W. A. Steele Professor F. S. Stone Dr R. K. Thomas The discussion will be concerned with certain aspects of current research on the gas-solid interface: elastic, inelastic and dissipative scattering of atoms and molecules from crystal surfaces, and the structure and dynamics of physisorbed species, including overlayers. Emphasis will be placed on the themes of physical interactions and energy exchange rather than on molecular-beam technology or the phenomenology of phase transitions on overlayers. The interplay between theory and experiment will be stressed as they relate to the nature of atom and molecule surface interaction potentials, including many-body effects.The preliminary programme may be obtained from: Professor J. A. Morrison, Institute for Materials Research, McMaster University, Hamilton, Ontario, Canada L8S 4M1 or: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN, U.K. THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM NO. 20 Phase Transitions in Adsorbed Layers University of Oxford, 17-1 8 December 1985 Organising Committee : Professor J. S. Rowlinson (Chairman) Dr E. Dickinson Dr R. Evans Mrs Y. A. Fish Dr N. Parsonage Dr D. A. Young The aim of the meeting is to discuss phase transitions at gas/liquid, liquid/liquid and solid/fluid interfaces, and in other systems of constrained geometry or dimensionality less than three.Emphasis will be placed on molecularly simple systems, whereby liquid crystal interfaces and chemisorption phenomena are excluded. Further information may be obtained from: Professor J. S. Rowlinson, Physical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 302.THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 81 Lipid Vesicles and Membranes Loughborough University of Technology, 15-1 7 April 1986 Organising Committee: Professor D. A. Haydon (Chairman) Professor D. Chapman Mrs Y. A. Fish Dr M. J. Jaycock Dr I. G. Lyle Professor R. H. Ottewill Dr A. L. Smith Dr D. A. Young The aim of the meeting is to discuss the physical chemistry of lipid membranes and their interactions, in particular theoretical and spectroscopic studies, polymerised membranes, thermodynamics of bilayers and Iiposomes, mechanical properties, encapsulation and interaction forces between bilayers leading to fusion but excluding preparation and characterisation methodology.Contributions for consideration by the Organising Committee are invited and abstracts of about 300 words should be sent by 1 May 1985 to: Professor D. A. Haydon, Physiological Laboratory, Downing Street, Cambridge CB2 3EG Full papers for publication in the Discussion Volume will be required by December 1985. ~~ FARADAY DIVISION INFORMAL AND GROUP MEETINGS Division - Full-day Endo wed Lecture Symposium Phase Equilibrium and Interfacial Structure To be held at Imperial College, London on 10 December 1984 Further information from Mrs Y.A. Fish, The Royal Society of Chemistry, Burlington House, London W1 V OBN High Resolution Spectroscopy Group CARS, Diode Laser and Microwave Spectroscopy To be held at the University of Reading on 16-1 8 December 1984 Further information from Dr J. M. Hollas, Department of Chemistry, University of Reading, Whiteknights, Reading RG6 2AD Colloid and Interface Science Group with the Colloid and Surface Chemistry Group of the SCI Colloidal Aspects of Cohesive Sediments To be held a t the SCI Headquarters, London on 18 December 1984 Further information from Dr J. W. Goodwin, School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS - - ~~ ~~ Neutron Scattering Group Neutrons in Magnetism To be held at the University of Southampton on 20 December 1984 Further information from Dr R.B. Rainford, Department of Chemistry, University of Southampton, Southampton SO9 5NHPolymer Physics Group New Year Soiree To be held in Cambridge on 2 January 1985 Further information from Dr A. Windle, Department of Metallurgy and Materials Science, Pembroke Street, Cambridge CB2 302 Gas Kinetics Group Multiphoton Processes in Chemical Kinetics To be held at the University of Leicester on 4 January 1985 Further information from: Dr I. W. M. Smith, Department of Physical Chemistry, University Chemical Laboratory, Lensfield Road, Cambridge CB2 1 EP Division - Full-day Endowed Lecture Symposium The 4th State of Matter: A Chemical Challenge for the Future To be held at the Scientific Societies Lecture Theatre, London on 10 January 1985 Further information from Mrs Y.A. Fish, The Royal Society of Chemistry, Burlington House, London W1 V OBN Division - Half-da y Endowed Lecture Symposium (jointly with Perkin Division) The Physical Organic Chemistry of Hydrogens To be held at University College London on 26 February 1985 Further information from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1 V OBN Theoretical Chemistry Group Group Meeting To be held at King's College, London on 6 March 1985 Further information from Dr G. Doggett, Department of Chemistry, University of York, York YO1 5DD Division Annual Congress: Solid State Chemistry To be held at the University of St Andrews on 25-28 March 1985 Further information from Professor P.A. H. Wyatt, Department of Chemistry, University of St Andrews, The Purdie Building, St Andrews KY16 9ST Polymer Physics Group 6th Churchill Conference To be held at Churchill College, Cambridge on 1-4 April 1985 Further information from Professor I. M. Ward, Department of Physics, University of Leeds, Leeds LS2 9JT Statistical Mechanics and Thermodynamics Group Liquids-Dynamic and Static Properties To be held at the University of Bristol on 10-1 1 April 1985 Further information from Dr E. Dickinson, Procter Department of Food Science, University of Leeds, Leeds LS2 9JT Neutron Scattering Group Small-angle Neutron Scattering from Organised Systems To be held at Imperial College, London on 17-1 8 April 1985 Further information from Dr R.W. Richards, Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow G1 1 XL Gas Kinetics Group with SERC Summer School in Gas Kinetics To be held at the University of Cambridge on 24 June to 3 July 1985 Further information from Dr I. W. M. Smith, Department of Chemistry, University Chemical Laboratory, Lensfield Road, Cambridge CB2 1 EP Industrial Physical Chemistry Group with the Food Chemistry Group Water Activity: A Credible Measure of Technological Performance and Physiological Viability To be held at Girton College, Cambridge on 1-3 July 1985 Further information from Professor F. Franks, Department of Botany, Downing Street, Cambridge CB2 3EAPolymer Physics Group Biennial Conference To be held at the University of Reading on 11-1 3 September 1985 Further information from Professor Bassett, J.J. Thompson Physical Chemistry Laboratory, University of Reading, Whiteknights, Reading RG6 2AF Industrial Physical Chemistry Group A Molecular Approach to Lubrication and Wear To be held at Girton College, Cambridge on 23-25 September 1985 Further information from Mr M. P. Dare-Edwards, Shell Research Ltd, Thornton Research Centre, Chester CHI 3SH Neutron Scattering Group jointly with the Materials Testing Group of the Institute of Physics Industrial Uses of Particle Beams To be held at the Institute of Physics, London on 26 September 1985 further information from Dr J. G. Booth, Department of Chemistry, University of Salford, Salford M5 4 W - - Division Annual Congress: Structure and Reactivity of Gas-Phase Ions To be held at the University of Warwick on 8-11 April 1986 Further information from Professor K.R. Jennings, Department of Molecular Sciences, University of Warwick, Coventry CV4 7AL 30TH INTERNATIONAL CONGRESS OF PURE AND APPLIED CHEMISTRY Advances in Physical and Theoretical Chemistry Manchester, S 1 3 September 1985 The Faraday Division is mounting the following symposia as part of the 30th IUPAC Congress: A. B. C. D. Reaction Dynamics in the Gas Phase and in Solution This symposium will examine the ways in which modern techniques allow detailed study of the dynamical motion of molecules which are undergoing chemical reaction or energy exchange. Micellar Systems The symposium will discuss various aspects of micellization, including size and shape factors, micellization in biological systems, chemical reactions in micellar systems, micelle structure and solubilization.Emphasis will also be given to modern techniques of examining micellar systems, including small-angle neutron scattering, neutron spin echo, photocorrelation spectroscopy, NM R and use of fluorescent probes. Surface Science of Solids The symposium will centre on recent advances in the study of kinetics and dynamics at surfaces and of phase transitions in adsorbate layers on single crystal surfaces. Both experimental and theoretical aspects will be reviewed with an emphasis on metal single crystal surfaces. New Electrochemical Sensors (in collaboration with the Electroanalytical Group of the Analytical Division) The symposium will cover such topics as the fundamentals of the subject, new gas sensors based on membrane electrodes and on ceramic oxides, the development of new ion- selective electrodes and the synthesis of new guest-host carriers, the development of CHEMFETS and other integrated devices together with the theory of the operation of such devices, and finally the development of biosensors including for instance enzyme electrodes, direct electron transfer to biological molecules and new potentiometric techniques for protein analysis.The second circular, giving details of all the symposia of the Congress and listing invited speakers may be obtained from: Dr J. F. Gibson, 30th IUPAC Congress, Royal Society of Chemistry, Burlington House, London W1V OBN (vii)-Faraday Faruday Discussions syn8posic8~ I Faraday Discussions take p h c e twice a year and are designed to cover t h e broad a F c t s of physicocheinical topics, thereby encouraging scientists of different disciphnes t o contribute their varied viewpoints to a common thcnic.A recent Discussion is :- @= The Royal Society No.75 lntrarnolecular Kinetics No.75 in the series, this publication is the result of a general discussion held at the University of Warwick in April 1983. Contents: Thc Spiers Memorial Lecture; Vibrational Redistribution within Cxciled tlectronic States ot Polyatuniic Molcculcs Intrmiolecular ReIa\atton uI 1:\ciled Statcs 1 w meriza t io n 0 1 I nt ern3 I-e nerg y-se lectcd lo ns Kinetics of Ion-Molecule Collision Coiiiplc\cs in thc Gas Phasc, E\perinlcnt and Theory Intramolecular Decay o l Sonic Open-shell Polyat o mic Pat tons On the ‘Theory 01 Intraiiiolecular 1:iicrpr Transfer Pulsed Laser Preparation and Vuantuni Superposition State 1:voluticin in Kegubr and Irregular Systellls A Ouantutii-iitcclianical Inlcriial-collision Model for State-sclcctcd Unitiiolccular 1)econipositio n The Correspondence Principle and Intramolccular Dynamics Intranio~eculdr DephdSlilg. ~ ’ i ~ ~ l \ ~ ~ O l l d Evolution ot Wavepacket Stales in a Molecule with Intrriiicdiare-ca.ie lcvcl Structure Energy Conversion in van dcr Waals (~oiiiplc\c\ 01 s-TeuaLinc and Argun Tiinc-dependent Processes in PoiyJtoiiiic Molecules During and Alter Intense Infrared lrradial ion tnergy 1)istributions in the (“(X ‘I+) 1.ragnicnt froin the lntrarcd Multiplephoton Dissociation 0 1 (3 ?CN.A Coinpariwn betwccn L:\periiiicntal Results and t h e Predictions oi. St;~tisticaI Theories of ChpO + Product tnergy Partitioning in the Decorii- position 01 Statc-selectively k-ucited tIOON and 11001) Low-po%cr Inl.rarcd 1 . a ~ Pliotolyris 0 1 Tetrantctliyldioxctan Untinolerukr Reactions Induced by Vibrational Overtone t x c i t a t i o n Uniiiiolecular 1)ccoriiposition 01 t-Butylhydrw pero\ide by Direct l:\cilation 0 1 the 6-0 0-11 Stretching Ovrrtone I’icosccond-jet Spectroscopy and Pliotoclieniistry. 1 nergy Redistribution and i t s Iritpact’on Coherence, I s o i n c r i n t i o n , I h w c i a t i o n and Solv:ition 1:nergy Redistribution in Large Molecules. Direct Siudy 01 1ntr.iiiiolccular Rehution iii tlic Gas I’Iiase with I’ico\econd Gating Ki)tation-dcpcndeiit Intrdniolccular Prucrsscs (11 SO: ( A ’ A ~) in a Supersonic Jet Kolc ot Rotation Vibration Interaction in Vibrational Keh\ation.1:ncrgy Redistribution in I . ~ c i t c d Singlet l~orinaldcliydc Sub-1)opplcr. Spectroscopy 01 Benrcnc in the “Channel-t hree” Kegion Intrainolccular 1;lectronic Kcla \ation and l’li‘)t(iisoiiirrirati(in I’rocer\cs in the Isvliilcd Azahenrene Molcciilcs Pyridinr. I ’ y r ~ r i n c and I’yrimidinc Softcover 434pp 0 85186 658 1 Price f25.00 ($48.00) Rest of the World f26.00 RSC Members f 16.25 of Chemistry- Farada) Discussions of the Chemical Societ) 75 I n i r t i n i o I c ~ u l c i r hrn‘ric I Faraday Symposia are usually held annually and are confined to inore specialised topics than Discussions, with particular reference to recent rapidly developing bnes of research.A recent Symposium is : No.17 The Hydrophobic Interaction No, 17 in the series, this publication is the result of a symposium on The Hydrophobic Interaction held at the Uiiiversity of Reading in December 1982. Contents: Hydrophobic Int rract io iis J I list o r ica I Perspective Precise Vapour-pressure Measurciiicnts 01 the Solubillzation 01 Benzene by Aqueous Sodium Octylsulphate Solutions Nuclear Magnetic Resonance Keh\atton Investigation of Tetrahydroluran and Methyl Iodide Clathratea Infrared and Nuckdr Magnrlic Kesonancc Studies Pertaining to the Cage Model l o r Solutions o f Acetone in Water Isothermal Transport Properties in Solutions of Symmetrical Tetra-a~yhiiinioniuni Bromides Thermodynamics of Cavity I.’oriiiation in Water. A Molecular Dynamics Study Molecular Librations and Solvent Oricnt- ational Correlations in Hydrophobic Phenomena Monte Carlo Computer Sunuhtion Study ol t h e Hydrophobic Effect. Potential 0 1 Mean Force for C(CI{.r)]aq at 25 and 50‘ C Geonietric Kcla\atiun in Water. It.; Role in tlydropliobir~ Ilydration Hydr~ipluibic Moinciits and Protein Structure Application o f the Knkwood-Buff Tlieury 10 tlic Problem of Hydrophobic Interactions Disentanglcinent CI! Hydrophobtc and blcctrostatic Contributions to t h e t.11ni Pressures 01 lunic Surldctani\ tll drophobic Inreracttcina i n Dilute Solutions 01 I’oly(viny1 alcohol) (’onforiiiational and l‘unct ional Propertic\ 01 I l a e i t i ~ i ~ l o b i n in W‘ater+Alcohol Mi\turcs. Dependence 01 Bulk I’lectrostatic and Ii>dropl~obic Interaction\ upon ptl and KCl i‘u nc‘ent r a t ion\ Softcover 240pp 0 85186 668 9 Price f 36.50 ($70.00) Rest of the World f38.50 RSC Members f 23.75 ORDERING RSC Members should send their orders to: The Royal Society of Chemistry, The Membership Officer, 30 Russell Square, Non-RSC Members The Royal Society of Chemistry, Distribution Centre, Blackhorse Road, L London WClB 5DT. Letchworth, Herts SG6 1HN. England. (viii)

ISSN:0300-9599    

DOI:10.1039/F198480FP085

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. 1, 1984,80, 2889-2910 Salt Effects on the Cloud Point of the Poly(ethy1ene oxide)+ Water System BY EBBA FLORIN,* ROLAND KJELLANDER AND JAN CHRISTER ERIKSSON Department of Physical Chemistry, The Royal Institute of Technology, S-100 44 Stockholm, Sweden Received 19th September, 1983 The cloud point of high-molecular-weight poly(ethy1ene oxide), PEO, in aqueous salt solution has been determined as a function of the salt concentration for all potassium halides, alkali-metal chlorides and alkali-metal hydroxides. Our theoretical model for the pure PEO +water system ( J . Chem. Soc., Faraday Trans. I, 1981, 77, 2053) has been extended to include the effects of salt on the phase separation. Basic features of the present model are a hydration shell with enhanced structuring of water as well as a zone with decreased salt concentration surround each chain.Overlaps of such regions are involved in polymer-polymer contacts and imply transfer of water molecules and ions from the proximity of the chains to the bulk solution, which gives important contributions to the free energy of interaction. The existence of the salt-deficient zone is explained as a consequence of asymmetric hydration of the ions near the polymer. The effects of the zone are large enough to account for the influence of salts on the clouding. The experimental differences found for the alkali-metal halides have been rationalized mainly in terms of varying degrees of salt penetration into the region around the chain. Poly(ethy1ene oxide), PEO, is water soluble in all proportions at moderate temperatures.l This system is a very interesting model for the study of water-structure effects in aqueous solutions of macromolecules, since PEO has a simple molecular structure and is uncharged. In a previous study2 we made an analysis of the phase behaviour of the PEO+water system, which shows a solubility gap at high tempera- tures.We found that the phase separation can be accounted for if there is enhanced structuring of the water around the PEO chain, as compared with bulk water. At high temperatures the enhanced structure is unfavourable for entropic reasons and the system phase-separates in order to decrease its extent. Introduction of a salt into an aqueous solution of PEO changes the phase-separation temperature.This alteration varies considerably in magnitude depetlding on the kind of electrolyte introduced and on its c0ncentration.l Since the location of the solubility gap is determined by a rather delicate free-energy balance, the system is sensitive to disturbances. This makes the salt effects large. The fact that PEO does not contain any charged groups (except perhaps in acidic solution) makes it necessary to consider the effects of the salt on the water structure when analysing the behaviour of the system. Bailey and'Callard3 have studied the influence of a great number of salts on the phase separation of high-molecular-weight (4 x lo6) PEO solutions, but their cloud point against salt concentration curves are based on at most two experimental points per salt, apart from the point for the salt-free system.Adamcova and Tao4 have determined the phase diagrams at 25 "C of low-molecular-weight PEO in aqueous solution with some different salts. Boucher et aZ.596 have determined the theta temperatures in the same kind of system. However, the number of 1 : 1 electrolytes investigated was limited in both cases. We know of no study that has been made on 28892890 CLOUD POINT OF POLY(ETHYLENE OXIDE) + WATER + SALT the effects of Rb+ and Cs+ salts on the pure PEOfwater system, although theta temperatures for hydrosols formed by poly(viny1 acetate) latex particles stabilized by attached PEO chains and containing RbCl or CsCl have been r e p ~ r t e d . ~ In order to obtain a better picture of the dependence on salt concentration and to study the effects of the whole series of alkali-metal ions, we have determined the cloud-point temperatures of high-molecular-weight PEO solutions in the presence of alkali-metal halides and hydroxides.The cloud-point data are interpreted in molecular terms with the help of a theoretical model, here derived, which is an extension of our previous PEO + water We first present the experimental procedures and results, followed by the derivation and evaluation of the theoretical model for phase separation in the three-component system polymer + salt + water. It is shown there that a salt-deficient zone exists around the polymer chain and has a substantial influence on the phase behaviour. Finally, the model is further analysed for a step-wise salt-concentration profile around the chains, which allows a simplified treatment.The effect of the salt-deficient zone is shown to be sufficient to explain the influence of salt on the clouding temperature. EXPERIMENTAL The poly(ethy1ene oxide) used in this investigation was Polyox WSR-301 (weight-average molecular weight of 4 x lo6) from the Union Carbide Corporation. Since it showed the same cloud point in aqueous solution as found for high-molecular-weight PEO in other it was used without further purification. The alkali-metal halides were Merck suprapure grade, except for KF which was pro analysi grade. They were all, except for LiC1, dried before use at a minimum temperature of 200 "C for at least 24 h. To avoid weighing errors from the considerable hygroscopicity of LiC1, a stock solution was prepared in this case.Its concentration was determined by potentiometric titration with AgNO,. The alkali-metal hydroxides came from different sources (the purity is shown in parenthesis) : LiOH (98 % ) and NaOH (99% ) from Merck, KOH (99.5%) from EKA and RbOH (99.9%) and CsOH (99.5%) from Ventron, Alfa Division. Stock solutions of the hydroxides were prepared and were used fresh. Their concentrations were determined by titration. Doubly distilled water which had been treated with active carbon and bubbled with nitrogen, free from carbon dioxide, was used. To prepare the samples the salt was added to a part of the water and the solution was then heated to 100 "C. The PEO was dispersed in the hot sample, which was repeatedly shaken during cooling.The final amount of water was added and the samples were equilibrated in darkness for 5-7 days. The solutions were prepared and kept in glass tubes with tight screw caps, and the measurements were always performed on samples that were less than three weeeks old. The solutions containing > 1 mol kg(water)-l KF were prepared cold, equilibrated in a refrigerator and used within four days. The samples had, uTess otherwise specified, a constant pdymer-to-water ratio of 1 : 99 by weight, which we will refer to as a concentration of 1 % . To determine the cloud-point temperatures the glass tubes were suspended in a well stirred heating bath. The tubes were gently shaken mechanically, to perform mixing of each sample. The temperature was raised at a constant rate of 0.01 K min-l (0.1 K min-' for the samples containing KF or alkali-metal hydroxide) by a Heto 02 PG 623 thermostat.The cloud-point temperature was defined as the temperature when one could visually observe that the sample started to get cloudy. The clouding '-'as closely followed by macroscopic phase separation. For the slow gradient, the cloud point occurred sharply within < 0.05 K and reproducibly within & 0.10 K, without showing hysteresis effects. The advantage of using a very small temperature gradient, 0.01 K min-', must be stressed, as a larger gradient diminished the reproducibility and made the error limits apprecip bly wider. Two measurements were made for the samples containing hydroxide, while for all the others the results quoted are the mean of at least three, but generally more, different measurements on (usually) the same sample.The error introduced by the increased pressure in the sealed tubesE. FLORIN, R. KJELLANDER AND J. C. ERIKSSON 289 1 during measurements at elevated temperatures should be negligible. The pressure dependence of the cloud-point temperatures in the salt-free system has been shown to be 0.004 K atm-l up to a pressure of 50 atm.8 High-molecular-weight PEO is known to be sensitive to degradation.' Our samples containing salts showed no apparent signs of degradation, but the salt-free ones that were heated to the highest temperatures did. However, by taking special precautions for the latter, i.e. avoiding oxygen and minimizing the time in the heating bath, we managed to obtain reproducible results.Since the cloud-point temperature is nearly independent of chain length for very long polymers, the results would not be affected by limited degradation. Thus, the results obtained are valid for high-molecular-weight PEO. In order to establish tie lines in the three-component phase diagram, the salt content of the polymer-poor phase, obtained after phase separation, was determined in a few cases. Since the polymer content ought to .be vanishingly low in that phase (cf. the salt-free system with high-molecular-weight PE08) and since the initial composition of the sample is known, this measurement should be sufficient to specify the tie line to a reasonable approximation, except for its polymer-rich end point. Samples with cloud points at 80.0 "C were prepared and kept at 83.4 "C for at least two hours to equilibrate before the polymer-poor phases were sucked off.Their salt concentrations were determined as moles of salt per 1000 g of solution by potentiometric titration with AgNO,. Since the PEO content ought to be negligible, these values equal the salt molalities (m3). Three titrations were made on each sample and the standard deviation in the m3 determinations was *0.2%. To this systematic errors are added. Furthermore, the preparation of samples for titration is another source of error. It was, for example, difficult both to prevent evaporation from the hot sample and to avoid completely contamination from the polymer-rich phase. We therefore estimate that a more reasonable uncertainty in the m3 determination should be &0.4%.The increase in salt concentration as compared with the concentration (mi) of the originally prepared sample was evaluated as 6 = (m,-mi)/mi. For the S values, the error should be the same as for m3 (i.e. &0.4%), since mi is determined with much greater precision (it is only affected by weighing errors, 5 0.01 % , in the original sample preparation). RESULTS AND DISCUSSION CLOUD-POINT MEASUREMENTS The lower phase-separation temperatures for 1 % PEO solutions containing different alkali-metal halides are presented in table 1 and fig. 1-4. From fig. 1 and 2 it can be seen that both the type of electrolyte introduced and its concentration affect the cloud-point temperature. The relative effects of the different alkali-metal ions can be distinguished in fig.1, where only results for chlorides are shown (see also table 1). Apparently K+ has the greatest effect, but it is only slightly larger than that of Rb+, while Li+ lowers the cloud-point temperature the least from the salt-free case. This pattern is followed in practically the whole concentration range (table 1) and is illustrated in fig. 3, where a comparison has been made between the systems with 2 and 0.2 mol kg(water)-l alkali-metal chlorides. The difference in clouding temperature for samples with equal concentrations of potassium and rubidium chloride is small and should have been within the uncertainty of the experiments, unless the samples were always observed simultaneously, under identical conditions.Because of the small temperature gradient there is a substantial time lag between the onset of clouding in the two samples [e.g. 8 min when the salt concentration is 0.6 mol kg(water)-l]. Furthermore, even a slightly clouded sample is easily distinguished from a clear one when they are compared directly. For the samples with high salt concentrations, those containing KCl were always very cloudy before those containing RbCl started to become turbid. We thus claim that the small differences in cloud-point temperature for solutions containing KCl and RbCl of the2892 CLOUD POINT OF POLY(ETHYLENE OXIDE) + WATER + SALT Table 1. Cloud-point temperatures for PEO + water + salt solutions with various salt concen- trations but with a constant PEO: water ratio (1 .OO wt % PEO, see text).The last digit of the temperature is not significant (as regards KC1 and RbCl, see text). salt concentration cloud-point salt /mol kg(water)-' temperature /"C - LiCl LiCl LiCl LiCl LiCl NaCl NaCl NaCl NaCl NaCl NaCl NaCl KC1 KC1 KCI KCI KC1 KCl KCI RbCl RbCl RbCl RbCl R bC1 RbCl RbCl CSCl CSCl CSCl CSCl CSCl CSCl CSCl KF KF KF KF" KBr KBr KBr KBr KI KI KI KI 0.00 0.40 0.80 1 .oo 1.20 2.00 0.10 0.20 0.40 0.60 0.80 1 .oo 2.00 0.10 0.20 0.40 0.60 0.80 1 .oo 2.00 0.10 0.20 0.40 0.60 0.80 1 .oo 2.00 0.10 0.20 0.40 0.60 0.80 1 .oo 2.00 0.50 1 .oo 1 S O 1.93 0.50 1 .oo 1 S O 2.00 0.50 1 .oo 1 S O 2.00 96.24 88.73 83.73 8 1.68 79.98 74.63 93.30 90.24 85.25 80.80 76.95 73.38 58.94 92.92 89.88 84.54 79.9 1 75.70 71.85 55.55 92.92 89.89 84.59 79.99 75.78 71.98 55.64 93.59 90.43 85.44 81.00 77.03 73.52 58.09 63.93 39.15 18.38 3.44 90.25 84.5 1 79.20 73.38 97.58 95.75 92.83 87.78 a PEO content: 0.97 wt %E.FLORIN, R. KJELLANDER AND J. C. ERIKSSON 2893 9 0 8 0 0 i2 70 6 0 50 0 0.4 0.8 1.2 1.6 2.0 salt concentration/mol kg (water)-' Fig. 1. Cloud-point temperatures for the various PEO + water + alkali-metal chloride systems as a function of salt concentration (1% PEO solution with added salt). +, LiC1; A, NaCl; A, KC1, 0, RbCl; 0, CsC1. 100 8 0 60 U 0 L 4 0 20 0 0 0.5 1 .o 1.5 2 .o salt concentration/mol kg (water)-' potassium halides. A, KI; 0, KBr; W, KCI; +, KF. Fig. 2. Cloud-point temperatures for PEO + water solutions (1 ) with different contents of2894 100 ,O 5 0 - k 0- CLOUD POINT OF POLY(ETHYLENE OXIDE) + WATER +SALT I I I I I 92.5 92.c - - I k +-I h $ 91.5 E W M 24 - c'! 91.0 2 c V 9 0 .5 9 0 . 0 75 70 -: 3 h c.) $ W M .x 6 5 3 E V N Y ii2 60 55 0.5 1 11 .o I 11.5 I L i* Na* K* Rb' CS* radiusl.4 same concentration are significant, at least for concentrations of 0.4 mol kg(water)-l and higher (table 1). The relative effect of the halide ions on the PEO+water system is shown in fig. 2, where the clouding temperatures for samples containing potassium halides areE. FLORIN, R. KJELLANDER AND J. C. ERIKSSON 2895 100 90 80 70 u c 6 0 5 0 4 0 30 0 0.5 1.0 hydroxide concentration/mol kg (water)-' Fig. 5. Cloud-point temperatures for the various PEO + water + alkali-metal hydroxide systems as a function of salt concentration (1 % PEO solution with added salt).+, LiOH; A, NaOH; A, KOH; e, RbOH; 0, CsOH. shown. It is apparent that for a fixed salt concentration the differences between the clouding temperatures caused by the various halide ions are larger than the analogous alkali-metal-ion effects (fig. 1 and table 1). It is remarkable that at a salt concentration of, for example, 2 mol kg(water)-l, KI only lowers the cloud-point temperature by 8.5 "C, compared with the salt-free case, while the lowering by KF is as large as 92.9 "C. In fig. 4 the cloud points are plotted against the ionic radii of the halide ions. It is seen that a small halide ion causes a lower phase-separation temperature than a larger one, a result found for all salt concentrations investigated. The phase-separation temperature is also lowered when alkali-metal hydroxides are added (fig.5), but at high salt concentrations (fig. 6 ) the relative order of the effects of the alkali-metal hydroxides is very different from that found for the alkali-metal chlorides (fig. 3). This shows that there is an appreciable influence on the phase separation from specific anion5ation interactions (i.e. not a purely electrostatic interaction between point charges). At lower hydroxide concentrations, where this effect should be smaller, the order of the cations (fig. 7) approaches that found for the alkali-metal chlorides. However, RbOH decreases the cloud-point temperature slightly more than KOH for salt concentrations 2 0.1 mol kg(water)-l. This difference between the alkali-metal hydroxides and chlorides may also disappear at even lower concentrations.In any case, K+ and Rb+ have the greatest effect and Li+ the smallest. In the case of the alkali-metal chZorides the relative cation order, common for both2896 CLOUD POINT OF POLY(ETHYLENE OXIDE) + WATER + SALT 50 4 5 V & 40 35 30 \ \ 0.5 1 11 .o 1 11.5 I Li+ Na' K * Rb* Cs* radius1 A Fig. 6. Cloud-point temperatures for 1 % PEO + water solutions containing 1 mol kg(water)-' alkali-metal hydroxides as a function of the cationic radii. 70 6 5 V 5 60 8 8 ' , 0.5 I 11.0 I 11.5 I L i + Na' K* Rb* cSi radiuslA Fig. 7. Cloud-point temperatures for 1 % PEO+water solutions containing 0.5 (m) and 0.1 mol kg(water)-l (A) alkali-metal hydroxides as functions of the cationic radii.E. FLORIN, R. KJELLANDER AND J. C. ERIKSSON 2897 Table 2.Effect of polymer concentration on the cloud-point temperature of PEO +water + NaCl [0.60 mol kg(water)-l] PEO cloud-poin t con ten t(a) temperature (% 1 "C 0.10 82.04 0.50 8 1.48 1 .oo 80.80 2.00 81.14 3.00 81.58 a Percentage counted on PEO and water only, see text. halides and hydroxides at low concentrations, is retained at higher concentrations, most notably for KCl and RbCl. This makes it reasonable that the influence of specific anion-cation interactions is small in this case. (The concentration dependence of different interactions is usually unequal.) Then the cation order of fig. 3 would reflect the differences in the alkali-metal-ion effects on the PEO + water system. Finally, a few experiments were made when the polymer concentration had been changed.Some properties, e.g. the viscosity,l, vary considerably with the PEO content. Table 2 shows the cloud points of PEO + water + NaCl[O.60 mol kg(water)-l] for PEO concentrations in the range 0.1-3%. It is apparent that the phase-separation temperature is only slightly affected by the polymer content in the range investigated. The PEO concentration that gives a minimum cloud-point temperature is close to 1 % . According to our previous analysis2 the enhanced water structure around the polymer chains gives rise to phase separation in the PEO + water system. Since ions affect the structure of water, one would expect that their influence on the phase separation is partly due to the effect of the ion on the hydration shell of PEO. This contribution is certainly different for the various ions.The ions would also be distinguished by unequal penetration into the interfacial region between the polymer and the bulk water. It is less reasonable that specific interactions between PEO, which is uncharged, and the ions give rise to the differences in cloud-point temperatures. N.m.r. relaxation measurementsg have shown, however, that the alkali-metal ions interact directly with PEO in aqueous solutions but that the extent of these contacts is very small. Hence, they have only minor thermodynamic implications. This agrees with the fact that the haZide ions have much larger effects on the cloud point than the alkali-metal ions. The relaxation measurementsg on alkali-metal and halide nuclei in these systems also support the basic idea of this work and that of ref.(2), namely that PEO in aqueous solution is surrounded by an extended region with enhanced water structure. It is illustrative to compare PEO + water + salt systems with other related systems. For different non-ionic surfactants with ethylene oxide oligomers as polar groups, analogous patterns for the effects of salt on the cloud point are found as for PEO [ref. (lo)-( 13) give a representative choice]. Since the mechanism behind the phase separation in this case is the same as that for PEO+water systems,l* it is very reasonable that the surfactant systems are affected by addition of salts in approximately the same way as pure PEO + water systems. Furthermore, a similar alkali-metal ion-order as that in fig. 3 is also found for the effects of salt on the cloud-point temperature of aqueous solutions of poly(viny1 methyl2898 CLOUD POINT OF POLY(ETHYLENE OXIDE) + WATER + SALT ether).15 The phase-separation mechanism should be very similar to that of PEO + water + salt, since this polymer is water-soluble for similar reasons as PE0.2 Note that fluoride has a large influence on PEO + water + salt and that small amounts of iodide even raises the cloud-point temperature slightly, compared with the PEO+water system.This is found also for poly(viny1 methyl ether) for both potassium and sodium halides. l5 THEORETICAL MODEL SALT EFFECTS Since our statistical-thermodynamical model for the PEO + water system2 has proved to be useful, we have attempted to extend it to include the effects of salts.The assumption of random mixing between polymer chains and water shall be retained. Thus, the Flory-Huggins expression for the combinatorial contribution from the chains to the entropy of the system will be used. The volume fractions for polymer and for solvent (aqueous salt solution) in the Flory-Huggins can, since water is in large excess, be reasonably well approximated by 4 2 = n2/(n, +n, +n3) = x2 and 4 S O l V e I l t = (nl+n3)/(nl+n2+n3) = xl+x3* Therefore 1 In ~ , + ( n , + n ~ ) In (xl+.x,)-nn,k where n, is the number of moles of water, n, is the number of basemoles of polymer (moles of monomer units), n3 is the number of moles of salt, rn is the degree of polymerization and k is a constant which is dependent on rn and the coordination number only.In our previous model for the PEO+water system2 the aqueous solution was divided into two parts : (a) the hydration shells, surrounding the polymer chains, where the water structure, because of the interactions with each chain, is more developed than in (b) the bulk, where the water is not influenced by the presence of the polymer. We are going to retain this subdivision in the three-component case, but here we will also have to include the possibility that the salt concentration near the chains may be affected by their presence. There are several reasons to believe that there exist zones with a decreased salt concentration, as compared with the bulk, around the chains. One effect that may give rise to this is the weaker electrostatic interaction of the ions with the poorly polarizable polymer chain, compared with that between the ions and water (repulsive image charge forces).The importance of this has been stressed by Garvey and R0bb.l’ We include this effect and the change in water coordination around the ion upon close approach to PEO in the concept of asymmetric hydration. The increased structuring of the water around the chain may also affect the salt concentration there. Some experimental results support the existence of salt-concentration gradients. We found an increase in the salt concentration in the polymer-poor phase after phase separation for several PEO +water + alkali-metal halide systems (table 3). From similar measurements on the analogous alkali-metal hydroxide systems we obtained 6 values of 1-2%.As we shall see, these observations support the existence of zones with decreased salt concentrations. Recent n.m.r. results also show the existence of these zones.lS Furthermore, similar salt-concentration gradients are known to exist at macroscopic phase boundaries in other systems, as will be discussed below. To estimate the contributions to the Gibbs free energy of the solution fromE. FLORIN, R. KJELLANDER AND J. C. ERIKSSON 2899 Table 3. Relative difference between the salt concentration in the polymer-poor phase after phase separation and the original homogeneous sample (see text). The phase separations were performed at 83.4 "C for PEO + water + salt systems with cloud points at 80 "C. All samples contained 1% PEO (counted on PEO and water only, see text).salt Concentration of the water-rich phase total salt concentration before phase separation after phase separation aa salt mi/mol kg(water)-l m,/mol kg(water)-l (% ) LiCl NaCl KC1 CSCl KBr 1.200 0.640 0.598 0.656 1.408 1.202 0.2 f 0.4 0.648 1.3f0.4 0.600 0.5 i- 0.4 0.662 0.8 f 0.4 1.417 0.7 & 0.4 a For definition, see text. intermolecular interactions, we thus assume that the polymer chains are surrounded by zones of decreased salt concentration. Furthermore, we assume that a region of increased structuring of water still exists around the chain (hydration shell) when salt is present. The size of the salt-deficient zone is described by the parameter h, the number of excess water molecules per monomer unit. The existence of such zones, of course, diminishes the number of water molecules in the bulk.Overlap of the zones upon polymer-polymer contact gives rise to the opposite effect; water molecules are forced back into the bulk solution from the overlap regions. Using the same suppositions as before,, the total number of such water molecules is proportional to x,n,. The factor of proportionality, b, can be obtained as follows. For each possible position of a chain segment in the neighbourhood of another, a certain number of excess water molecules are in the overlap region. An equal amount of water has been transferred to the bulk, as compared with the situation before overlap. The sum of these transferred water molecules for all possible types of overlap, i.e. for all possible positions of two segments of neighbouring chains, constitutes the factor b.In the evaluation of b from h we have for simplicity assumed that the salt concentration changes in a step-wise or linear manner close to the chain (fig. 8). We have neglected that further changes in the concentration profile occur when two chains approach each other and have only considered one relative orientation of the chain segments at each position. The results are given in fig. 9. The number of water molecules in the bulk is n, - n,(h - bx,). The mole fractions of salt and water in the bulk solution are I n3 x; = 1 -xi. x, = n, - n,(h - bx,) + n3 We now assume that the bulk has the same thermodynamic properties as an equivalent pure salt + water solution. The contribution to G from intermolecular interactions is gSn1- n,(h - bx,)l+ g, n3 + g, n, + @x2 n, Ginter = = gdn1- hn,) + g3 n3 + g, n!Z + wxzn, (2) where g, and g3 are the chemical potential of water and salt, respectively, in a pure aqueous salt solution with the composition xi, xi (bulk concentrations), g, is the2900 CLOUD POINT OF POLY(ETHYLENE OXIDE) + WATER + SALT 0 R distance from PEO chain (arb.units) Fig. 8. Schematic picture of a stepwise (a) and linear (b) salt-concentration profile in the vicinity of a PEO chain. At the distance R from the chain the salt concentration have in both cases reached its bulk value (xi). The value close to the chain is assumed to be zero. basemolar 'excess' free energy of a single, extended polymer chain in aqueous salt solution combined with various contributions from the region around the chain (analogous to g: in the salt-free system,) and w = @+bgl is the molar-free-energy contribution from polymer-polymer interactions and overlaps of hydration shells and of salt-deficient zones.The free energy of the h excess water molecules near a polymer unit is included in g,, which also contains the corrections to the two first terms in eqn (2) from the increased structuring of water and the remaining contributions from the salt gradient. The effects of PEO on the long-range ion-ion interactions are also included as a further correction term in g,. This is done rather than changing g , and g , from the pure salt + water values, since the PEO concentration is fairly low. If we now combine eqn (1) and (2) we obtain G for the solution as a function of n,, n, and n3.It is advantageous, however, to express G as a function of only one variable. We then choose a starting point with a fixed composition determined by, say, ny, ni and ni. The addition ofn moles of a salt + water solution (composition xr, x,*) will move the total composition of the system from the starting point along a straight line in the phase diagram. The Gibbs free energy of the system as a function of n can thus be written: G(n) = RT[(ni/rn) In x, + (ny + ni + n) In (xl + x,) - ni k] +g,(n; + xf n - hni) +g,(ni + x,* n) + g , ni + wx, ni (3) ni+x,* n n, +ni+ni+n' where x3= 0 ny+xrn 4 x, = x, = ny + nz + ni + n n: + ni + ni + nE. FLORIN, R. KJELLANDER AND J. C. ERIKSSON 2 ‘0 R ‘0 0 290 1 20 b 10 0 0 1 2 3 h Fig. 9. Extension R, of the salt-deficient zone and values of the proportionality factor b as functions of the parameter h, the number of excess water molecules per monomer unit.Full lines indicate linear and broken lines step-wise salt-concentration profiles. R is given in multiples of r,,, the effective radius of a PEO chain. Eqn ( 3 ) gives G in the one-phase region of the three-component system. Let us consider the boundary of the two-phase region in the phase diagram at constant temperature. For one very special choice of x: and x,*, the line along which the total composition is moving upon changes in n is the tangent to the boundary curve at the critical point. We call this the critical tangent. The values of x: and x: are given by the polymer-free end point of that line, and we must take ny, ni and nz such that the starting point lies on the line.Then the critical composition can be computed from the zero values of the second and third derivatives of G(n), obtained from eqn (3). The calculation can be simplified by the use of the Gibbs-Duhem equation for the pure aqueous salt solution. Unfortunately, we do not know the variation of h, b, g, and 6 with the concentration of salt. However, in our applications of the model this concentration will vary very little with n (since xk x x,*). We therefore assume that h and b are independent of n. The salt-concentration-dependent contributions to g, and 6 are composed mainly of differences in free energies for states with and without PEO. This indicates that it is reasonable as a first approximation to neglect the n dependence of g , and 6 in comparison with that of g, and 8,.Thus, we obtain two equations which determine the critical composition :2902 CLOUD POINT OF POLY(ETHYLENE OXIDE) + WATER +SALT where c = n;+ni+ni e = ny+ni. In cases where we do not have access to experimental values of both g , and g,, we can calculate ag,/an and a2g,/an2 from gl(n) using the Gibbs-Duhem equation. We shall evaluate the model by making use of the cloud-point data given above. A basis for this is that eqn (4) and (5) for the critical point can also be used for conditions that are not too remote from the critical. Arguments supporting this extension are given in Appendix A, where the relationship between the critical tangent and tie lines in its vicinity is discussed.For the present evaluation we simply take ny, ni and ni from the composition of a sample which phase separates at a given temperature, set x,* and x f equal to their values in the water-rich phase obtained after phase separation and vary n accordingly. At the phase-separation temperature for the sample eqn (4) and (5) will be approximately satisfied for some n. Lacking the complete three-component phase diagrams, we can estimate values of x: and x: from the S values in table 3: x: = ny/[ny+ni(l+d)] and xf = 1-x,*. If the critical compositions of the PEO + water + salt system were available at the temperatures of interest, the n values that satisfy eqn (4) and (5) would be known for every case and thereby the other parameters could be settled. However, this is not the case, although from our measurements on the influence of PEO content on the cloud point in the NaC1-containing system (table 2) we can estimate some approximate limits for n. The cloud points for compositions along a line of constant salt molality are described in table 2.If we had instead moved along a tie line (extended outside the two-phase region), the minimum cloud-point temperature along that line would have practically coincided with a critical point (cf. the general two-component case: if the tie lines were independent of temperature, the coincidence would be exact). From our measurements (table 3) we know that the changes in salt concentration along the tie lines of interest are small. The data of table 2 thus indicate that the critical point in the NaC1-containing system should appear at PEO concentrations around 1 % .We shall assume that this is also true for the other alkali-metal halides at low concentrations. The value is close to that of the salt-free case w: "N 0.8% .l We therefore first choose the n value that gives a critical polymer concentration equal to the latter, i.e. n = -0.20 ny. Later we shall see what happens if the polymer content is allowed to vary from 0.8% to 1.25% (i.e. -0.20 ny d n ,< 0.20 n:). In eqn (4) and (9, which constitute the basis of our calculations, derivatives of g, and g3 at the high temperature used in the evaluation (80 "C) are needed. The compilation of these data is given in Appendix B. We are now ready to give the results obtained from the calculations.We have limited ourselves to those PEO + water + salt solutions which have cloud points at 80 "C and which contain any of the salts mentioned in Appendix B. The results are presented in table 4, where mi (fig. 1 and 2) and S (table 3 ) are experimentally determined. The values of h, b = b(h) and w satisfy the system of eqn (4) and (5) for the specified n (n = -0.20 ny) and the salt-concentration profile chosen. The remaining quantity R = R(h) is then computed (see fig. 9). Our calculations (table 4) show that the values of h and w are not very sensitiveE. FLORIN, R. KJELLANDER AND J. C. ERIKSSON 2903 Table 4. Results of evaluation from eqn (4) and (5), see text salt total salt concentration mi s W /mol kg(water)-' (% ) ha b R/ro /cal mol-I KBr LiCl NaCl KCl CSCl KBr LiCl NaCl KCl CSCl linear salt-concentration profile 1.42 0.7 1.7 2.9 1.20 0.2 0.4 0.1 0.64 1.3 3.1 11.5 0.60 0.5 1.2 1.4 0.66 0.8 2.0 4.3 1.42 0.7 1.7 1.8 1.20 0.2 0.4 0.2 0.64 1.3 3.1 7.1 0.60 0.5 1.2 0.9 0.66 0.8 2.0 2.7 step-wise salt-concentration profile 1.2 0.3 1.9 0.9 1.4 0.6 0.2 1 .o 0.5 0.7 - 354 - 353 - 357 - 353 - 354 - 353 - 353 - 356 - 353 - 353 a Error limits are f 1.2, mainly from experimental uncertainty in 6.to the magnitude of b. Generally speaking, this means that neither the type of gradient nor the concentration near the chain is crucial in these cases. Hence we can conclude that a value of h can be attributed to each salt and that this value characterizes the size of the salt-deficient zone, irrespective of the detailed features of the gradient.This also shows that it is permissible to neglect the changes in the zones when two chain segments approach each other. From the columns for w in table 4 it can be seen that these values are very similar, independent of the salt present. The total contribution to the interaction parameter w from electrolyte effects is ca. - 55 cal mol-l since w", the corresponding parameter in the pure PEO+ water system, is ca. - 300 cal mol-1 at this temperature.2 The interesting point is that this decrease of -55 cal mol-l, which is necessary for phase separation to occur, is induced by different amounts of salt in the various cases. This will be discussed further below. The w values are close to that predicted by the Flory-Huggins theory16 for polymers in a pure (one-component) solvent : w z 0.5 RT, = -351 cal molt1, valid for a high-molecular-weight polymer with T, = 353 K (80 "C).The quantities h, b and R are of reasonable magnitude. However, a variation of 6 within its error limits of f0.4 in table 3 implies a change in h of f 1.0. f i e uncertainty in the location of the critical point adds less to the error. If it occurs at a PEO concentration of, say, 1.25% (corresponding to n = +0.20 ny, see above) instead of 0.8 % , the h values change by at most 0.1. The other approximations also contribute to the overall error. Thus we may conclude that our results so far show the existence of a salt-deficient zone (except possibly for LiCl, but see below), and that it has a profound influence on the phase behaviour of the system.However, the relative order of the sizes of the zones according to table 4 is not signigcant. Recently, the existence of these zones around PEO in the presence of LiCl and KF has been shown by n.m.r. spectroscopy.l*2904 CLOUD POINT OF POLY(ETHYLENE OXIDE) + WATER + SALT THE ORIGIN OF THE SALT EFFECT According to our assumptions and the results given above, the occurrence of a salt-deficient zone gives a substantial contribution to the salt effect in the PEO+ water system. Here we shall show that this contribution is large enough to generate the observed lowering of the cloud-point temperatures. An overview of different treatments of salt effects on non-electrolytes in aqueous solution is given by Garvey and R0bb.l' They suggest that negative adsorption of the ions from some polymers causes the salting-out effect, which is supported by our findings.As we have seen, the general application of our theoretical model to interpret the differences in ion effect requires more accurate data regarding the three-component phase diagram than those available. However, we have shown the existence of the salt-deficient zones around PEO. Another useful result is that w z - 0.5 RTindepen- dent of the other parameters, provided that we are not too far away from the critical point. Fortunately, this makes it possible to continue the evaluation of the model for a step-wise salt-concentration profile, without using the uncertain 6 values and critical points. Provided that this is a reasonable approximation, our results here will be more accurate than the previous ones.This will be judged by a comparison with some other data. The free-energy changes caused by contacts between two polymer segments consist of several parts : the polymer-polymer interaction and the contributions from overlaps of hydration shells and of salt-deficient zones. The relevant differences between the interaction parameters w (in the presence of salt) and w0 (in the absence of salt) are that in the former case the overlap region contains salt as well as water and that the bulk is changed: molecules from the region around the chains are transferred to an aqueous salt solution instead of to pure water upon overlap. By expressing w as w = w"+b(g,-&)+A we take the difference in bulk composition into account through the second term on the right.The remaining salt effects are then included in A. If we now approximate the salt-concentration profile with a step function [fig. 8 (a)], we must choose 3, = 0 for consistency. This can be seen as follows. A deviation of the ion concentration near a PEO segment from the bulk value implies a non-zero potential of mean force between the ion and the segment and vice versa. (We have set the potential to zero for large separations.) Consider now two interacting PEO segments. As soon as an approaching ion affects this interaction, the free energy is changed. This implies a non-zero potential of mean force for the ion. Since the ion concentration is assumed to be equal to the bulk concentration (i.e. the potential is zero) up to the step inside which no ions are allowed, the absence or presence of the ion near the segments does not affect their interaction.Hence it equals the interaction in pure water unless the salt-free zones overlap; the latter effect is expressed by the second term in eqn (6). Thus we can conclude that putting A = 0 is consistent with the assumption of a step-wise change in the salt concentration from zero to the bulk concentration at a certain distance from the polymer chain. Of course, this situation is hypothetical, since a strictly step-wise profile does not occur in reality. The salt concentration further away from the chain deviates from the bulk value because of the ion-PEO-water interactions. However, some support for the applicability of the A = 0 approximation is provided by the thermodynamic study of the air/electrolyte- solution interface described in ref.(19). I t shows that only minor free-energy changes arise for the salt-deficient surface phase when the salt concentration is varied. ThisE. FLORIN, R. KJELLANDER AND J. C. ERIKSSON 2905 Table 5. Results of evaluation assuming a step-wise salt concentration profile and A = 0, see text ~~ cloud-point salt temperature W W0 concentration - (8, -p;) /"C /cal mol-1 /cal mol-1 salt /mol kg(water)-l /cal mol-l b h R / r , 90 - 361 -344 LiCl NaCl KCl RbCl CSCl KF KBr KI 80 -351 -299 LiCl NaCl KC1 RbCl CSCl KF KBr 70 -341 -252 NaCl KC1 RbCl CSCl KF 60 -331 -203 NaCl KCI RbCl CSCl KF 0.32 0.21 0.19 0.19 0.22 0.09 0.52 1.81 1.20 0.64 0.60 0.60 0.66 0.24 1.42 1.20 1.10 1.1 1 1.20 0.39 1.91 1.70 1.72 1.86 0.58 7.8 5.1 4.5 4.6 5.2 2.1 12.5 48.4 30.6 15.1 13.6 13.6 14.6 5.5 34.4 28.3 24.7 24.8 26.2 8.8 45.7 37.6 37.6 39.7 12.4 2.1 1.8 0.7 3.3 2.2 0.8 3.7 2.3 0.8 3.6 2.3 0.8 3.2 2.2 0.8 7.8 3.2 1.1 1.3 1.7 0.6 0.3 0.8 0.3 1.7 1.6 0.6 3.5 2.3 0.8 3.8 2.3 0.8 3.8 2.3 0.8 3.6 2.3 0.8 9.5 3.5 1.1 1.5 1.8 0.6 3.2 2.2 0.8 3.6 2.3 0.8 3.6 2.3 0.8 3.4 2.2 0.8 10.2 3.7 1.2 2.8 2.1 0.8 3.4 2.2 0.8 3.4 2.2 0.8 3.2 2.2 0.8 10.4 3.7 1.2 behaviour is probably the result of compensating energy and entropy effects caused by the ions close to the interface.Since we know w and the chemical potentials of the salt+water solutions, we can now calculate the values of b and hence of h and R . Since -the calculation is approximate we may use the semi-empirical equations for the osmotic coefficients from Silverster and Pitzer20 to calculate g , - py.Unfortunately, the upper-temperature limit for these equations is not given, but the agreement at an elevated temperature with the measurements discussed in Appendix B is good, though not excellent. However, Silvester and Pitzer give data for all the alkali-metal halides that we have used. The values of b obtained from these calculations at four temperatures are presented in table 5, where the corresponding values of h and R for a step-wise profile are also given. A comparison with the values in table 4 shows that the quantities are of the same order of magnitude in both cases. The differences are due to the relatively large errors involved in table 4, as discussed above, and our assumption of a step-wise profile for table 5.However, both series of results are compatible with 6 values of ca. 1 % . In table 5, the relative order of h, b and R for the different salts is essentially maintained for all four temperatures. The negative ions give larger contributions to these2906 CLOUD POINT OF POLY(ETHYLENE OXIDE) + WATER + SALT Table 6. Surface excesses of water, r1(3), relative to the Gibbs dividing surface where the surface excess of salt is zero, for interfaces between aqueous electrolyte solutions [ 1 mol kg(water)-l] and air, dodecane and decanol, at 20 "C [calculated from data in ref. (23)] ~~~ ~~~ interface LiCl NaCl KC1 KBr KI air /elec trol y te C,,H,,/electrolyte C,,H,,OH/electrolyte air /elec trol y te C,,H,,/electrolyte C1,H,,OH/electrolyte surface excess of water r1(3,/10-6 mol mP2) 18.67 19.61 18.67 15.89 14.11 16.78 16.39 16.17 10.00 -0.83 6.83 8.72 9.06 4.67 -5.83 excess water molecules per 20 Biz 2.25 2.36 2.25 1.91 1.70 2.02 1.97 1.95 1.20 -0.10 0.82 01.05 1.09 0.56 -0.70 quantities than the positive ions (cf.the cloud points in fig. 1 and 2). The smaller the anion the greater the effect, while among the cations potassium and rubidium give the largest effect. The sizes of the salt-deficient zones for LiCl and KF (table 5 ) are in agreement with recent n.m.r. measurements1* using the intermolecular nuclear Overhauser enhance- ment effect. They show that the distances of closest approach to the protons in a PEO segment are ca. 1.9 Before proceeding further with the analysis of the PEO +water + salt system, we will compare these results with some other systems.In the literature, surface excesses of water are reported for interfaces between aqueous electrolyte solutions and e.g. 21v 22 dodecaneZ2 and decan01.~~ The surface or interfacial tension for these systems increases, with one exception to be discussed later, when salt is added. Using surface thermodynamics, these increments can be transformed to values of the surface excess of water for the different salts [cf ref. (19)] given in table 6. The major effects can always be attributed to the influence of the negative ions. Furthermore, iodide is connected with the smallest surface excess of water and chloride with the largest. In fact, iodide is even positively adsorbed in several cases.The difference in the effects of Li+, Na+ and K+ is substantially smaller for measurements on the same interface. The kind of alkali-metal chloride that gives the maximum surface excess of water varies for the different interfaces. Although the results for PEO in table 5 are not fully accurate because of the assumption about the profile, the qualitative agreement with the interfacial systems is good. (See the numbers of excess water molecules per 20 A2 in table 6, which are of approximately the same magnitudes as the h-values in table 5. The choice of the area 20 Hi2 is arbitrary, but it is of similar magnitude to the cylindrical area of a segment in a PEO chain.) In all cases, the anions give the largest effects and the excess water increases with decreasing size of the anion.For the decanol/water interface,23 both the relative influence of the negative and positive ions agree with the findings in table 5 , although this may perhaps be fortuitous. Nevertheless, under the same assumptions as above, a calculation for PEO+water+KI at high temperatures gives positive adsorption of the ions to PEO [e.g. at 96 "C and a KI concentration of 0.958 mol kg(water)-l we get b = -0.18, where the negative sign means depletion of water near the polymer]. At the alcohol/water interface KI also adsorbs positively. Thus, at least the magnitudes and trends in table 5 appear reasonable. Thus, we conclude that the effect of salt on the clouding can be explained in terms of a salt-deficient zone near the PEO chains. The different magnitude of the effects for Li+ and ca.2.6 A for F-.E. FLORIN, R. KJELLANDER AND J. C. ERIKSSON 2907 for the various salts depends, for this approximation, only on the unequal sizes of the zones. To gain some insight into this variation we must consider the ion-PEO interaction in water. To start with, we review some results regarding the ion-water interactions. The different influences of the alkali-metal ions on water have given rise to the following classification: Li+ and Na+ are ‘ structure makers’ (Frank’s terminology2*) or are ‘positively hydrated ’ (Samoilov’s termin~logy~~), while K+, Rb+ and Cs+ are ‘ structure breakers’ or ‘negatively hydrated’, respectively. K+ can also be regarded as an intermediate case. The meaning of ‘structure maker’ is not that structure is created from unstructured water, but that the normal water structure is rearranged in the electric field of the ion, which leads to a state where the thermal motion of water in the neighbourhood of the ion is less than in the bulk.Samoilov’s terminology is perhaps better in this case. He uses bulk water as a reference state and compares the water close to the ion with that in the bulk. The mobility of the water around the negatively hydrated alkali-metal ions is larger than in the bulk. This is also the case for C1-, Br- and I-. One possible explanation for this behaviour is that these ions are too large, so that the structuring caused by the water-water interaction becomes less effective than in the 27 In addition, the electrostatic field close to each ion is less than for the small ions.The F- ion is exceptional, since it may participate in hydrogen bonding with water, replacing a water molecule. Note that those ions that do not have strong enough fields to become positively hydrated, and that are not big enough to be strong structure breakers, namely K+ and F-, have the largest effects on the cloud point of PEO. The water excesses for the alcohol/water interface23 and the results in table 5 also follow this pattern. We are now ready to discuss the PEO-ion interaction in water. First, we note that the water surrounding the ion is polarized by the ionic field. This lowers the free energy. On the other hand, the water in the PEO hydration shell is in a high-free-energy state because of an unfavourable entropy contribution.2 When an ion and a PEO segment approach each other, the amount of intervening water decreases.Since PEO is far less polarizable than water, the removal of water polarized by the ion leads to a repulsive force (image-charge force) between the ion and PEO. The removal of water from the hydration shell of PEO, on the other hand, leads to an attractive force (cf. the PEO-PEO interaction). We must here include changes in the water coordination and structure around the ion and PEO, respectively, because of the presence of the other species. All of these effects can be described-as asymmetric hydration. The direct ion-PEO interactions are probably less important. The balance of these contributions to the total force will depend on the ion.Since we have a salt-deficient zone around PEO (in most cases) the repulsive component must be dominant. The attractive component of the force is greater the larger the ion, since more structured water around PEO is expelled. Furthermore, the big ions break down some of the remaining water structure. These effects make the total force less repulsive. For the large anions the polarizability of the ion itself would decrease the repulsive component, which makes the total repulsion still smaller. In the case of iodide, the force may become weakly attractive, which would explain the likely positive adsorption of this ion at low concentrations. Since the size and the polarizability vary on going from F- to I-, while they vary less for the cations, it is reasonable that the anion effect is larger than the cation effect.In the case of the small fluoride ion, the water structure around PEO may be enforced energetically because of hydrogen bonding, and we would expect only a small attractive component. Therefore, F- is effectively expelled and the lowering of the cloud point is large. Thus, the halide-ion effect is explained. The difference between the sizes of the salt-deficient zones when we vary the cation is small. Since the cation order is unequal for different surfaces,23 see table 6, it is hard2908 CLOUD POINT OF POLY(ETHYLENE OXIDE) + WATER + SALT to judge whether the approximation with a step-wise profile gives the correct cation order for PEO (table 5). The effects of ions on the PEO-PEO interaction that we neglect in this approximation may contribute significantly to the differences between the cations.Therefore, we shall not attempt to discuss the ion order in this case. There remains the temperature effect on the size of the salt-deficient zones, as shown in table 5, to be discussed. To minimize complications from the variation in salt concentration, we shall limit ourselves to KF, for which the molality varies the least and is lowest. From table 5 it can be seen that the zone decreases as the temperature is raised, which means that the total repulsive force on the ions decreases. This is reasonable on the following grounds. The repulsive image-charge force should decrease with increasing temperature, since the polarizability of the water becomes smaller (cf. the decrease in dielectric constant of water).Futhermore, the attractive component of the force between an ion and PEO should increase with temperature, since the water structure around PEO becomes increasingly unfavourable (cf. the interaction between two PEO segments;2 the free energy increases with T because of an unfavourable entropy contribution). This work has received financial support from Bengt Lundqvist Memorial Founda- tion (E.F.) and the Swedish Natural Science Research Council. APPENDIX A As shown in fig. 10, we have determined approximate phase boundaries of the PEO + water +NaCl system for temperatures between 82.0 and 84.5 "C [PEO content: 0.6&1.98% ; NaCl concentration: 0.42-0.58 mol kg(water)-l]. These boundaries are compared in the figure with a tie line determined at 83.4 "C.Fig. 10 shows only a very small part of the phase diagram, but we conclude that the phase boundaries for the temperatures investigated are approximately parallel to the tie line shown. For continuity reasons, this must be the case for all tie lines in the neighbourhood. Hence, a tangent to the phase boundary at one temperature is nearly coincident with a tie line at a slightly higher temperature. This applies specifically to the critical tangent. Further support for this conclusion can be obtained as follows. From the (T, w2) phase diagrams for the pure PEO + water ~ y s t e m ~ . ~ ~ 28 it is known that the lower phase boundary of the solubility gap is very flat near the critical point, i.e. the phase-separation temperature is insensitive to the polymer concentration there.The water-rich phase found after phase separation has a very low polymer concentration for high-molecular-weight PEO. This ought to be true even when salt has been added, at least in small amounts. It is therefore likely that at a fixed temperature ( T ) the tie lines (in the two-phase region of the three-component phase diagram) for salt concentrations close to that of the critical composition span a wide range of polymer concentration. For continuity reasons these lines must be nearly parallel to the critical tangent. Furthermore, if we focus on one of the tie lines at T,, there must exist a slightly lower temperature (T,) which gives a critical point with composition equal to one of the points on that line. Consequently, the line is nearly coincident with the critical tangent for the system at temperature T,.APPENDIX B Activity data for aqueous alkali-metal halide solutions at elevated temperatures are sparse. Furthermore, the need for the derivatives of g, and g,, and not only the functions themselves, has made it necessary to choose the most accurate measurements available for the different salts. For NaCl and KBr these derivatives were obtained by fitting suitable simple functions numerically to the activity data of Robinson and Stokes for aqueous salt and by calculating the derivatives with respect to the salt concentration of these functions. Activity data for LiC130 were interpolated to 80 "C and fitted to simple functions. For KC1 only osmoticE. FLORIN, R.KJELLANDER AND J. C. ERIKSSON 2909 Fig. 10. Phase boundaries for PEO+water+NaCl at temperatures from 82.0 to 84.5 "C compared with a tie line at 83.4 "C. The concentration unit is mole fractions. Note that the figure only contains a very small part of the complete three-component phase diagram. Full lines show phase boundaries at different temperatures (in "C): A: 82.0; B: 82.5; C: 83.0; D: 83.5; E: 84.0; F: 84.5. The tie line (-----) and the phase boundary (-.-.-) at 83.4 "C are also given. The composition of the sample from which the tie line has been determined is indicated by 0. coefficients (in the form of an empirical equation) at 80 "C and m3 2 0.8 are a~ailable,~, while we need data for 0.6 mol kg(water)-'. The dependence of g , on salt molality could be fitted as before and with the Gibbs-Duhem equation we can calculate i3g3/drn3 and compare it with the same derivative obtained from activity-coefficient measurements at 70 "C and low KCl concen- trations (0 < m3 < 2).32 This comparison showed that it is reasonable to extend the range of salt concentration for wluch the empirical equation at 80°C is valid towards lower values.Analogous considerations have also been made for CsCl to diminish the lower salt concentration boundary for the validity of an empirical fit of osmotic coefficients at 80 0C.33 In this case a comparison was made of 3g3/i3m3 at 50 "C (where activity coefficients had been measured for low salt concentration^^^), which justified the extension of the validity range. For the remaining salts, no sufficiently accurate activity data at a suitable temperature could be found. F. E. Bailey Jr and J. V. Koleske, Poly(ethy1ene oxide) (Academic Press, New York, 1976). F. E. Bailey Jr and R. W. Callard, J. Appl. Polym. Sci., 1959, 1, 56. Z. Adamcova and D. D. Tao, Sb. Vys. Sk. Chem.-Technol. Praze, Anorg. Chem. Technol., 1973, B17, 217. E. A. Boucher and P. M. Hines, J. Polym. Sci., Polym. Phys. Ed., 1976, 14, 2241. M. Ataman and E. A. Boucher, J. Polym. Sci., Polym. Phys. Ed., 1982, 20, 1585. D. H. Napper, J. Colloid Interface Sci., 1970, 33, 384. S. Saeki, N. Kuwahara, M. Nakata and M. Kaneko, Polymer, 1976, 17, 685. E. Florin, to be published. lo W. Luck, Proc. 3rd Int. Congr. Surface Activity, Cologne 1960, p. 264. l 1 M. J. Schick, J. Colloid Sci., 1962, 17, 801. l 2 K. Deguchi and K. Meguro, J. Colloid Interface Sci., 1975, 50, 223. l 3 H. Schott and S. K. Han, J. Pharm. Sci., 1975,64, 658. l4 R. Kjellander, J. Chem. SOC., Faraday Trans. 2, 1982, 78, 2025. l5 R. A. Horne, J. P. Almeida, A. F. Day and N-T. Yu, J. Colloid Interface Sci., 1971, 35, 77. l6 T. L. Hill, An Introduction to Statistical Thermodynamics (Addison-Wesley, Reading, Mass., 1962), * R. Kjellander and E. Florin, J. Chem. SOC., Faraday Trans. I, 1981, 77, 2053. p. 401 ff.2910 CLOUD POINT OF POLY(ETHYLENE OXIDE) +WATER + SALT l7 M. J. Garvey and I. D. Robb, J. Chem. Soc., Faraday Trans. I , 1979, 75, 993. l8 E. Florin and U. Henriksson, to be published. 2o L. F. Silvester and K. S. Pitzer, J. Solution Chem., 1978, 7, 327. 21 J. Ralston and T. W. Healy, J. Colloid Interface Sci., 1973, 42, 629. 22 R. Aveyard and S. M. Saleem, J. Chem. Soc., Faraday Trans. 1, 1976, 72, 1609. 23 R. Aveyard, S. M. Saleem and R. Heselden, J. Chem. Soc., Faraday Trans. I , 1977, 73, 84. 24 H. S. Frank and M. W. Evans, J. Chem. Phys., 1945,13, 507; H. S. Frank and W. Y. Wen, Discuss. 25 0. Ya. Samoilov, in Water and Aqueous Solutions, ed. R. A. Home (Wiley-Interscience, New York, 26 M. S. Bergqvist and E. Forslind, Acta Chem. Scand., 1962, 16, 2069. 27 E. Forslind and A. Jacobsson, in Water, A Comprehensive Treatise, ed. F. Franks (Plenum Press, New 28 G. N. Malcolm and J. S. Rowlinson, Trans. Faraday Soc., 1957, 53, 921. 29 R. A. Robinson and R. H. Stokes, Electrolyte Solutions (Butterworths, London, 1959). 30 H. F. Gibbard Jr and G. Scatchard, J. Chem. Eng. Data, 1973, 18, 293. 31 J. T. Moore, W. T. Humphries and C. S. Patterson, J. Chem. Eng. Data, 1972, 17, 180. 32 T. M. Herrington and R. J. Jackson, J. Chem. Soc., Faraday Trans. I, 1973, 69, 1635. 33 J. Pupezin, G. Jakli, G. Jancso and W. A. van Hook, J. Phys. Chem., 1972,76, 743. 34 R. Caramazza, Ann. Chim. Ital., 1963, 53, 481. K. Johansson and J. C. Eriksson, J. Colloid Interface Sci., 1974, 49, 469. Faraday Soc., 1957, 24, 133. 1972), p. 597. York, 1975), vol. 5 , p. 173. (PAPER 3/1647)

ISSN:0300-9599    

DOI:10.1039/F19848002889

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. I , 1984,80, 2911-2928 Effects of Hydrocarbon Diluents on the Kinetics of the Seeded Emulsion Polymerization of Styrene BY GOTTFRIED LICHTI-~ AND DAVID F. SANGSTER AINSE and CSIRO Division of Chemical Physics, Lucas Heights Research Laboratories, Private Mail Bag, Sutherland, New South Wales 2232, Australia AND BARRY C. Y. WHANG, DONALD H. NAPPER* AND ROBERT G. GILBERT Departments of Physical and Theoretical Chemistry, The University of Sydney, New South Wales 2006, Australia Received 22nd November, 1983 Kinetic relaxation studies (using y-rays from a 6oCo source as initiator) of the seeded emulsion polymerization of styrene have shown that the presence of hydrocarbon diluents, such as ethylbenzene, toluene and cyclohexane, can significantly increase the rate coefficient for the exit of free radicals from the latex particles.This increase was evident even when the chain-transfer constant for the diluent was less than that for styrene. Such an increase may arise from the low reactivity of the diluent free radicals with the monomer molecules. This allows the diluent free radicals additional time, compared with the monomer free radical, to undergo diffusional exit from the particles before reaction with the monomer confines the species to the latex particles. The free-radical exit rate coefficient is thus determined inter uliu by both the chain-transfer constant for the diluent and the reactivity of the diluent free radicals, as demanded theoretically. Large increases in the exit rate coefficient are predicted to occur with certain diluents at high replacements of the styrene by diluent. Seeded emulsion polymerizations of styrene initiated by potassium peroxydisulphate con- firmed that certain diluents may increase the exit rate coefficient significantly.Surprisingly, it was found that the entry rate coefficient may also be increased significantly. This was attributed to the presence of the diluent promoting the stabilization of those free-radical species that are capable of entering the latex particles at lower degrees of aggregation. This reduces the rate of bimolecular termination of the free-radical species located outside the latex particles. At higher monomer replacement levels, however, it is possible that the rate of entry of free radicals into the latex particles may be reduced as a consequence of the depletion of the monomer concentration in the aqueous phase.Thus the entry rate coefficient in the presence of diluents is determined inter alia by two processes whose effects tend to be opposed: on the one hand, enhanced stabilization of the coagulating colloidal free-radical species and, on the other, depletion of the monomer concentration in the aqueous phase. Seymour and coworkers' were the first to show that certain hydrocarbon diluents (e.g. benzene, cyclohexane and octane) retard the ab initio emulsion polymerization of styrene. They proposed that a decrease in viscosity within the latex particles was responsible for the rate reduction. Subsequently, Blackley and Haynes2 confirmed that four hydrocarbon diluents (benzene, toluene, ethylbenzene and cyclohexane) reduced the rate of emulsion polymerization in ab initio systems more than could be accounted for by the dilution of monomer.They showed experimentally that the decrease in rate was accompanied by a reduction in both the size of the latex particles produced and t Present address: ICI Australia, Central Research Laboratories, Ascot Vale, Victoria, Australia. 291 12912 SEEDED EMULSION POLYMERIZATION OF STYRENE the molecular weight of the polymer formed. These results were also interpreted as indicating that the hydrocarbon diluents diminished the Trommsdorff gel effect at the reaction sites within the latex particles. Azad et al.3 subsequently pointed out that any explanation for the retardation observed by Blackley and Haynes that invokes the Trommsdorff effect must be invalid because the average number of free radicals per particle (a) was always very small (in the range 0.002-0.02) in their experiments.Multiple occupancy of the particles by the radicals (i.e. values of n comparable to, say, one-half) is required for a significant Trommsdorff effect to manifest itself. These authors proposed instead that the reduction in rate observed by Blackley and Haynes2 was a consequence, at least in part, of the smaller size of the particles generated in the presence of hydrocarbon diluents. This increased the rate of exit of free radicals from the particles, leading to a smaller value of n and a slower rate of polymerization. In order to obviate the effects arising from a change in particle size, Azad et al.3 studied experimentally the effects of hydrocarbon diluents on the seeded emulsion polymerization of styrene.However, even under conditions of constant particle size all of the hydrocarbons studied decreased the polymerization rate more than could be accounted for by the dilution of monomer. These authors distinguished between two classes of diluents, which were postulated to reduce the rate of polymerization by quite different mechanisms. First, water-insoluble alkanes (e.g. n-octadecane and n-tetracosane), which are presumably not transported through the aqueous phase, appeared to act by lowering the thermodynamic activity of the monomer in the emulsion droplets and thus reducing the monomer concentration in the swollen latex particles.Secondly, lower-molecular-weight hydrocarbons (e.g. n-pentane and ethyl benzene), which by virtue of their solubility in water are capable of being transported through the aqueous phase, were considered to decrease the rate of polymerization by a chain-transfer mechanism. Chain transfer to the diluent was postulated to result in a low-molecular-weight free radical that was capable of diffusing from the particle. This reduced fi to below the value of one-half that was claimed to be operative in these experiments in the absence of diluents. There are several unresolved difficulties associated with the explanation proposed by Azad et aL3 First, if n were truly close to one-half, its value could only be reduced to the smaller values (calculated to be in the range 0.25-0.38) in the presence of diluents if the exit rate coefficient k were increased dramatically (e.g.fifty-fold). This follows directly from the steady state (ss) nss = 1 /(2 + k / p ) , where p is the rate coefficient for the entry of free radicals into the latex particles. Such a massive increase in k is not expected since the propagating polystyryl free radical has similar rate constants for chain transfer to styrene and ethylbenzene; moreover, both of the hydrocarbons have comparable solubilities in water. Secondly, although the chain- transfer constant for ethylbenzene is probably greater than that for styrene, those for benzene, toluene and cyclohexane are all significantly smaller than that for styrene. Yet Blackley and Haynes found that all four hydrocarbon diluents reduced the rate of polymerization per particle.In this paper we describe the results of additional experiments on the seeded emulsion polymerization of styrene in the presence of certain hydrocarbon diluents. These experiments were initiated by both y-radiolysis and a chemical initiator. The former allowed the exit rate coefficient to be measured directly by relaxation studies after removal of the system from the 6oCo initiating s o ~ r c e ; ~ the latter permitted the value of n to be varied fi~e-fold.~ The combination of these two types of experiment allowed the effects of diluents to be more clearly discerned. To do this we exploited recently developed technique^^-^ that enable the observed overall polymerizationLICHTI, SANGSTER, WHANG, NAPPER AND GILBERT 2913 kinetics to be resolved into the rate coefficients for individual microscopic events : specifically, those for free-radical entry into the latex particles and those for free-radical exit (desorption) therefrom.As these two events are mechanistically different, the presence of hydrocarbon diluents might be expected to affect them differently. Mechanistic deductions can be made more reliably from the effects of diluents on these microscopic rate coefficients than from the effects on the overall rate. EXPERIMENTAL All studies were performed using seeded latex systems in Interval I1 at a temperature of 50 "C. The methods and recipes used for both the relaxation studies (in which y-rays from a 6oCo source constituted the readily removed initiating source) and the chemically initiated studies have been described in detail previou~ly.~.The sizes of the seed latex particles, which were prepared by the methods described el~ewhere,~ were determined by both ultracentrifugation and calibrated electron microscopy. The radius of the particles used in the relaxation studies was 44 nm; those of the latices used in the chemically initiated studies were 29 and 51 nm for the systems with low and high initiator concentrations, respectively. Electron microscopy established the absence of nucleation in all of systems reported here. Note that the total weight of monomer plus hydrocarbon diluent (if present) at the commencement of each type of experiment was held constant so that the effects of the substitution of the monomer by hydrocarbon diluent could be explored.The precision of the data for the runs containing diluent was estimated by error analysis using the slope and intercept method. That for the runs in the absence of diluent was found from the reproducibility of some 6 runs. The relatively poor precision of the kinetic parameters obtained in the presence of diluents arose from the rapid attainment of the steady state. RESULTS AND DISCUSSION RELAXATION STUDIES Table 1 lists the values of the exit rate coefficient k determined by relaxation studies for the seeded emulsion polymerization of styrene, both when the monomer was pure and when the weight ratio of monomer to diluent present in the dilatometer at the commencement of polymerization was 70 : 30. These results were all calculated from the experimental data by assuming a value for the exited-free-radical rate parameter of a = + 1.This was the value of a found to be appropriate in previous relaxation studiess using the chain-transfer agents carbon tetrabromide and carbon tetrachloride and it corresponds to complete re-entry of the exited free radicals.6 Note, however, that the values calculated for k were relatively insensitive to the value of a adopted and they were also insensitive to the value assumed for A (provided A < 0.5) in the radiation cavity. Under these conditions, A was set equal to one-half, as demanded by previous kinetic5? and particle-size-distribution studiesg It is apparent from table 1 that the substitution of 30 wt% of the monomer by either ethylbenzene or toluene caused an approximately four-fold increase in the exit rate coefficient.Cyclohexane at the same level of substitution only doubled the value of k. Benzene, in contrast to the other hydrocarbons, induced no apparent change in k within the limits of experimental error. The simplest explanation for the effects of hydrocarbon diluents on the exit rate coefficient would assume that the diluents function as chain-transfer agents, thereby increasing the rate of production of small free-radical species. These small free radicals could then diffuse out of the latex particles, leading to an increase in the value of k. This simple model would imply that the value of k in the presence of diluents, whose 95 FAR 12914 SEEDED EMULSION POLYMERIZATION OF STYRENE Table 1.Measured exit rate coefficients for various diluents (particle radius, 44 nm; particle number, 5 x 10l6 dm-3) diluent k / 10-3 s-1 ~ k / 10-3 s-1 1.8 & 0.4 0 - et hy 1 benzene 6.6 1 .O +4.8& 1.1 benzene 1.8 0.4 0.0 f. 0.6 cyclohexane 3.6 l- 0.7 + 1.8k0.8 toluene 6.7 & 1 .O +4.9& 1.1 solubility in water is comparable to that of the monomer, could be expressed approximately as where ktr, refer to the chain-transfer constants for the monomer (M) and diluent (D), respectively, CM and CD are their respective concentrations inside the latex particles and Q is simply a constant of proportionality. It is readily evaluated because in the absence of diluents eqn (1) reduces to k M Q(ktr, M CM + ktr, n CD) (1) and ktr, k" = Qktr, M C M Q = k"/ktr, M C M (2) where the superscript " denotes the relevant values in the absence of diluents.Accordingly so that a revised version of eqn (1) is (3) Eqn (4) allows the values of k predicted by this simple theory to be calculated from the literature valueslO for ktr, M, ktr, and CMy5 together with the values calculated for CM and C, assuming ideal mixing. The latter assumption has been confirmed experimentally for ethylbenzene by Azad et aZ.3 The above analysis permits the theoretical change in the exit rate coefficient Ak arising from the presence of diluent to be calculated according to this simple model. The results so obtained are compared with those measured experimentally in table 2. It is apparent from table 2 that according to this model of the diluents studied only ethylbenzene is expected to increase the exit rate coefficient.This follows from ethylbenzene being the only diluent studied which had a chain-transfer constant that is apparently greater than that for styrene. The other diluents would be expected to decrease k. With the notable exception of benzene, however, all the diluents were found experimentally to give rise to significant increases in k . It is apparent that the simple theory outlined above is inadequate to explain the observed results. The failure of the simple concepts embodied in eqn (4) has been noted previously8 in relation to strong chain-transfer agents. There it was proposed that the reactivity with monomer of the small free-radical species generated by chain transfer was important in determining the magnitude of k.Reaction of these free radicals with monomer was postulated to reduce their solubility in water. This rendered them less able to diffuse from the latex particles. The importance of this mechanistic step in the process of exit even in the absence of diluents has been recognized previously byLICHTI , SANGSTER, WHANG, NAPPER AND GILBERT 2915 Table 2. Comparison of the predictions of the simple transfer theory with the results of experiment Ak/ lop3 s-l k r , D C D diluent /lo+ dm3 mol-l s-l /mol dm-3 simple theory experiment 1 .oa et hy 1 benzene 1.5 toluene 0.25 benzene 0.03 cyclohexane 0.05 - - - - 1.8 + 0.3 +4.8+ 1.1 2.0 - 0.4 +4.9+ 1 . 1 2.2 - 0.6 0.0 f 0.6 2.1 - 0.6 + 1.8k0.8 Ugelstad and Hansen,ll Nomura and Haradal29 13 and Hawkett et aL5 The value of k would be expected to be proportional to the ratio ktr, CM/kb CM, where k;, is the propagation rate constant for the reaction between the monomeric styryl free radical and ~ t y r e n e .~ ' ~ ~ . ~ ~ Note that, as pointed out by Ugelstad and Hansen,ll k; is not necessarily equal to the conventional propagation rate constant k, relevant to the polymeric free radical. The physical insight implicit in the above ratio is that the rate of free-radical exit depends not only on the frequency (= ktr, C,) with which new free radicals are generated by chain transfer but also on the average time spent by these free radicals before adding monomer units, which results in their confinement to the latex particles. It is possible to formalize the above concepts for diluents whose solubility in water is not too different from that of the monomer using the relationship where R is the constant of proportionality and kpD is the rate constant for the reaction of the diluent free radical with monomer.Eqn (5) is obtained readily from consideration of the total exit rate of the separate free-radical species that arise from transfer to monomer and diluent. It thus embodies the notion that exit of free-radical activity from the latex particles can occur by the diffusion of both the monomeric styryl free radicals (M') and the diluent free radical (DO) from the latex particles. Further, the free radicals M' and D' are assumed to be of comparable solubility in water. In the absence of diluents, the exit rate constant can be written as k" = Rktr,,/kb so that eqn ( 5 ) can be recast as It follows that according to this approach Eqn (8) implies that the change in k is directly dependent upon the ratio of the rates of transfer to diluent and monomer, and inversely related to the relative propagation 95-22916 SEEDED EMULSION POLYMERIZATION OF STYRENE Table 3.Calculated values for the relative reactivities of the diluent and monomeric free radicals diluent 1 .o - ethylbenzene 0.25 toluene 0.05 cyclohexane 0.03 benzene - rates of D' and M'. Note that, in contrast to eqn (I), one essential prediction of eqn (8) is that the value of k must always increase or remain constant, as was indeed observed experimentally. Eqn (8) can be used to calculate the values of the ratio k,,/kL from the measured values of Ak and the literature values for the chain-transfer constants.The values so calculated are presented in table 3. It is apparent that, according to this approach, the free radicals produced by chain transfer to the diluents ethylbenzene, toluene and cyclohexane are all less reactive towards styrene monomer than is the free radical (presumably CH&H Ph and/or CH,=tPh,l* where Ph = phenyl) generated by chain transfer to styrene. The reduced reactivity of the diluent free radicals with monomer means that such free radicals can undergo diffusion within the latex particles for a longer period of time than styryl free radicals before monomer addition occurs. The latter results in the free radicals being confined to the domains of the latex particles.The lengthened period of diffusional motion by the unreacted free radical increases the probability of escape of the free radical from the particle and so leads to the observed increase in k . Note that the data presented in table 2 suggest that the more difficult it is to generate a free radical by chain transfer, the slower is its reaction with monomer. This conforms with the general rule that governs the reactivity of free radicals with a given monomer in copolymerizations. One possible explanation for this rule lies in the stabilization of the transition states involved in the two proces~es.~~ Any mechanism (e.g. electron delocalization) that stabilizes the transition state of the hydrogen-abstraction reaction between the free radical and diluent necessarily promotes the rate of chain transfer.A very similar mechanism is likely to be operative in stabilizing the transition state of the addition reaction between the diluent free radical and monomer, thus promoting that reaction as well. CHEMICALLY INITIATED STUDIES In the experiments to be described below, the seeded emulsion polymerization of styrene, in both the presence and absence of diluents, was initiated by potassium peroxydisulphate. It was possible in favourable circumstances to measure both the steady-state rate and the approach to the steady state. This allowed the slope and intercept method5* to be used to give values for the rate coefficients for both the entry of free radicals into the latex particles and the exit of free radicals therefrom.Previous studies6T8 have shown that the value of the exited-free-radical rate parameter a that is appropriate in the presence of chemical initiators lies in the range - 1 < a < 0. This corresponds physically to significant heterotermination of the exited free radicals in the aqueous phase.6 The values obtained for the entry and exit rate coefficients wereLICHTI, SANGSTER, WHANG, NAPPER AND GILBERT 2917 relatively insensitive to the precise value chosen for a in this range and so a was set equal to - 1. The extent of substitution of the styrene by the hydrocarbon diluent will be expressed by the initial diluent weight fraction, d,", defined such that d,"m, is the weight of diluent in the dilatometer at the beginning of the experiment.(m, is the corresponding total weight of monomer plus diluent in the dilatometer.) Note that as polymerization proceeds the value of the overall diluent weight fraction increases; its actual value is given by &[l -x(l -&)I, where x is the fraction of monomer converted to polymer. In the quantitative determinations of the rate coefficients to be presented below the numerical data were all derived for x < 0.2. Accordingly, changes in the diluent weight fraction were relatively small and have been neglected. The kinetic results at different initial diluent weight fractions will be presented as plots of the percentage conversion of monomer to polymer as a function of time. This form of presentation was adopted because if simple dilution effects alone were operative, the rate curves would all be identical with that observed in the absence of diluent. Moreover, at constant particle number, the gradient of such curves at any point is proportional to the rate of polymerization per particle expressed relative to the initial amount of monomer present in the system.It is convenient to discuss the results in terms of this relative rate of polymerization per particle rather than the absolute rate of polymerization per particle. The latter will necessarily vary with the extent of the substitution of monomer by diluent. Note that given sufficient time (usually between 24 and 72 h), all polymerizations were found to proceed to complete conversion. Note also that the conversion was measured at intervals of 1 min. This resulted in the data points being too close together to be presented individually in the kinetic curves.A smooth curve has therefore been drawn through the observed points. The results obtained with ethylbenzene as the hydrocarbon diluent were both qualitatively and quantitatively unambiguous. These will therefore be discussed first and in some detail. The results obtained for toluene, benzene, cyclohexane and 11-heptane, whilst being qualitatively unambiguous, proved to be less clear cut quantitatively. Therefore only differences from the effects observed with ethyibenzene will be discussed in these cases. E:THYLBENZENE The effects of ethylbenzene at various initial diluent weight fractions on the kinetics of the seeded emulsion polymerization of styrene at two different initiator concentrations (1.2 x lop2 and 1.2 x mol dmP3) are displayed in fig.1 and 2. It is apparent that the influence of the diluent was different at the higher and the lower initiator concentrations. At the higher concentration, the curves do not coincide, as would be expected if the only factor to be taken into account was dilution of monomer. The results imply that an additional mechanism which reduces the rate of polymerization to below that expected for dilution alone was operative. The magnitude of this additional reduction in rate increased monotonically with increasing mono- mer dilution. At the lower initiator concentration, however, the relative rate of polymerization first increased and then decreased as the monomer became progressively more dilute (see fig.2). Stated differently, this meant that at relatively low levels of replacement of styrene by ethylbenzene, the rate of polymerization was not reduced to the extent expected by simple dilution considerations. This was despite the fact that Azad et al.3 have shown experimentally that the swelling of polystyrene latex particles by styrene + ethylbenzene mixtures closely follows the behaviour expected for the ideal mixing of diluent and monomer. Fortunately, at these low monomer-replacementSEEDED EMULSION POLYMERIZATION OF STYRENE 2918 100 80 8 60 E: .- L W 40 V 20 I I I 0 50 150 200 Fig. 1. Effect of ethylbenzene on the rate of polymerization of styrene in seeded systems at a high initator concentration (1.2 x mol dm-9. Diluent weight fraction 4: curve 1, 0.00; 2, 0.15; 3, 0.26; 4, 0.50; 5, 0.75.loo t/min 80 - h rf 60 - 0 150 200 loo t/min 50 Fig. 2. Effect of ethylbenzene on the rate of emulsion polymerization of styrene at a low initiator concentration (1.2 x lop4 mol dm-9. Diluent weight fraction 4: curve 1,O.OO; 2,0.15; 3,0.26; 4, 0.50; 5 , 0.75. levels the slope and intercept method allows the free-radical entry and exit rate coefficients to be determined. The results obtained are listed in table 4. The values of k determined from these chemically initiated experiments show that the presence of ethylbenzene increased the exit rate coefficient. This is in accord with the relaxation results presented above. What is surprising, however, is that the entry rate coefficient pA( = p + kfi, when a = - l), relating to the production from the initiator of free-radical species that could enter the latex particles, was also increasedLICHTI, SANGSTER, WHANG, NAPPER AND GILBERT 2919 Table 4.Effects of diluents on the entry and exit rate coefficients (particle concentration, 3 x 10'' dmP3) particle [I1 diluent radius/nm d; mol dm-3 nss s-la k/lOP3 s-' - - ethylbenzene 5 1 0.00 120 0.43 51 0.26 119 0.38 51 0.50 120 0.33 51 0.75 119 0.24 29 0.00 1.2 0.08 2.3f0.3 1.4 f 0.5 29 0.15 1.2 0.10 6.4f1.5 3.0 + 1 .O 29 0.26 1.2 0.10 8.1 f 2 . 0 3.6f 1.5 - - - - - - toluene - - 51 0.00 120 0.43 51 0.26 118 0.38 - - 51 0.50 118 0.37 51 0.75 116 0.34 29 0.00 1.2 0.08 2.3f0.3 1.4 f 0.5 29 0.15 1.2 0.08 3.9k 1.0 2.1 f 1.0 29 0.26 1.2 0.08 2.5+ 1.5 1.5+ 1.5 - - - - benzene 29 0.00 1.2 0.08 2.3k0.3 1.4 f 0.5 29 0.15 1.2 0.10 3.2f 1.0 l.5f 1.0 29 0.26 1.2 0.07 3.4+ 1.5 2.3f 1.5 cyclohexane 29 0.00 1.2 0.08 2.3k0.3 1.4 f 0.5 29 0.15 1.2 0.09 4.5f1.0 2.2 f 1 .o 29 0.26 1.2 0.08 3.2+ 1.5 1.9+ 1.5 n-heptane 29 0.00 1.2 0.08 2.3f0.3 1.4 f 0.5 29 0.15 1.2 0.08 2.4f 1.0 1.4+ 1.0 29 0.26 1.2 0.07 2.3k 1.5 1.6f 1.5 significantly (e.g.by a factor of ca. 3). The reality of this increase was supported by a significant decrease (from ca. 30 to ca. 10 min) in the induction period in the presence of ethylbenzene. This increase in the rate of production of free-radical species capable of entering the latex particles is responsible for the rate of polymerization not decreasing in accordance with simple dilution requirements. The explanation for the dramatic increase in the entry rate coefficient in the presence of ethylbenzene is not immediately apparent.It appears to be unlikely that a sparingly water-soluble substance, such as ethylbenzene, could promote molecule-induced homolysis of the initiator and so cause a large increase in the rate of production of primary free radicals. It seems more likely that ethylbenzene reduces the rate of bimolecular termination of primary and oligomeric free radicals located outside of the latex particles. Such bimolecular termination reduces the radical capture efficiency to < 100% . There are several possible mechanisms by which bimolecular termination could be reduced but, whatever the mechanism, it is presumably closely related to the mechanism by which ethylbenzene, and other diluent hydrocarbons, reduced the particle size in the ab initio emulsion polymerizations of styrene reported by Blackley and Hayne.s2 Our studies16 of the nucleation mechanism in the emulsion polymerization of2920 SEEDED EMULSION POLYMERIZATION OF STYRENE styrene suggest that there is a range of oligomeric species that can enter the seed latex particles.These include colloidal precursor (or primary) particles, which are aggregates of ‘insoluble’ oligomeric species that are characterized by a very slow growth rate. This probably arises from the poor swelling of such aggregates by monomer as a consequence of their residual hydrophilic character and/or their relatively small size. Free radicals associated with such precursor particles propagate more slowly than those in mature latex particles.They would also be expected, on diffusional considera- tions, to enter the seed particles more slowly than surfactant-like oligomeric species. In the context of the preceding initiation mechanism, one possible way by which ethylbenzene could increase the rate of free-radical entry into the seed particles is by promoting the stabilization of the precursor particles by the surfactant at relatively low degrees of aggregation. This appears to be a synergistic phenomenon in that the effect of surfactant plus ethylbenzene seems to be greater than the sum of the effects of surfactant and diluent acting separately. This synergism may arise from the ethylbenzene rendering the precursor particles more lipophilic because, unlike the monomer, the diluent molecules do not polymerize.Futhermore, ethylbenzene is a good solvent for polystyrene. In these circumstances the surfactant could be adsorbed more readily onto the precursor particles than in the absence of diluent. Such a mechanism would explain the increase in the number of particles produced in the presence of diluents in the ab initio emulsion polymerizations reported by Blackley and Haynes.2 In addition, the above mechanism is consistent with the large increase in the free-radical entry rate observed in seeded systems in the presence of ethylbenzene, since bimolecular termination, which accompanies the coagulation of precursor particles, would be significantly reduced. It might at first sight be thought that the explanation proposed above for the mechanism whereby ethylbenzene increased the free-radical entry rate is in conflict with the results of Piirma and Chen.18 These authors found that, at equilibrium, the saturation amounts of anionic alkyl surfactants adsorbed onto unit surface area of polystyrene latex particles decreased when the particles were swollen by hydrocarbons.It should be recalled, however, that precursor particles are postulated to be different from mature polystyrene latex particles inasmuch as they are more hydrophilic in character. Their behaviour might be expected to resemble more closely that of more polar polymers, such as poly(methy1 methacrylate). Piirma and Chen indeed showed that, in contrast to the results for polystryene, the adsorption of sodium dodecyl sulphate increases significantly when poly(methy1 methacrylate) latex particles are swollen by a hydrocarbon (benzene).This observation is in accord with the concepts postulated above for precursor particles. Note, however, that kinetic factors may also contribute to any increase in surfactant adsorption by the precursor particles in the presence of hydrocarbons. It is also possible that the increase in the free-radical exit rate coefficient induced by the presence ofethylbenzene may contribute to the increase in the overall free-radical entry rate coefficient. It was mentioned above that precursor particles would be expected to enter the seed latex particles more slowly than oligomeric surfactant-like species. If the free radicals trapped inside precursor particles were released by chain transfer and exit to form additional aligomeric species in the aqueous phase, the rate of entry of free radicals into the seed particles could be enhanced.As noted previously, inspection of the rate curves shown in fig. 2 reveals that the rate of polymerization, expressed relative to the amount of monomer originally present in the system, first increased on replacement of monomer by ethylbenzene. However, as the extent of monomer substitution increased, the relative rate of polymerization dropped significantly so that the relative rates at 50% and 75%LICHTI, SANGSTER, WHANG, NAPPER AND GILBERT 292 1 substitution levels were both considerably smaller than that in the absence of diluent. Note that the same pattern of behaviour was observed at all degrees of monomer replacement at the higher initiator concentration (fig.1). Consideration must therefore be given to possible effects, other than dilution, that may retard the polymerization rate. The most obvious reason why the rate of polymerization is reduced to below that expected from dilution is an increase in the exit rate coefficient induced by the presence of ethylbenzene (see tables 1 and 4). It should be recognized that high levels of substitution of the monomer by ethylbenzene can result in very large increases in k according to eqn (8). This can be rewritten as Ak/ko = A(CD/CM) (9) where A is a constant. From the relaxation data recorded in table 1 for 30% replacement of styrene by ethylbenzene, A is found to have the value 6.2.Alternatively, the data for the chemically initiated reaction with 15% substitution of styrene by ethylbenzene gives A = 6.5, in good agreement with the relaxation value. Using the former value for A , it is readily predicted from eqn (9) that the exit rate coefficient at 75% replacement level should be ca. 20 times larger than that in the absence of diluent. This is a sufficiently large increase in the value of k to explain the observed decrease in the relative rate of polymerization at high dilutions, irrespective of the initiator concentration. There is, however, an additional mechanism which might be implicated in the reduction in the rate of polymerization at high levels of substitution of monomer by ethylbenzene. This is a significant reduction in the rate of production in the aqueous phase of the oligomeric free-radical species that are capable of entering the latex particles.It would be expected that at very high dilutions of monomer the reduction in the chemical potential of the monomer in the emulsion droplets must be accompanied by a significant decrease in the equilibrium concentration of monomer in the aqueous phase. This could reduce the rate of addition of monomer to the primary free radicals in the aqueous phase to such an extent that the rate of production of oligomeric species is markedly reduced. This decrease in the rate of production of oligomeric free radicals could occur at high degrees of replacement of the monomer by the hydrocarbon, despite the fact that the entry rate coefficient is increased at lower levels of replacement.It is not possible from the data presented in fig. 2 to discriminate between the two possible causes of the reduction in rate at high levels of substitution: a large increase in k and/or a reduction in p. Note, however, that at high replacement levels the kinetic curves display virtually no approach to the steady state. The time to achieve the steady state is of the order of (2p+k)-l. The rapid attainment of the steady state, which precludes the determination of the entry and exit rate coefficients in these cases, suggests that either k and/or p must have been relatively large in these systems. The above results might seem at first sight to be in conflict with those previously reported by Azad et aL2 and discussed in the introduction.It is apparent that since the value of k at low replacement levels is not dramatically increased by the presence of ethylbenzene, it is unlikely that the value of iiss could have been reduced from 0.5 in the absence of ethylbenzene to ca. 0.38 in the presence of ethylbenzene (4 = 0.23), as was claimed by those authors. This apparent discrepancy may be readily resolved, however, by recognizing that the calculation of A from rate data involves an assumption as to the value of the propagation rate constant for styrene, kp. Azad et aL2 assumed that k, = 206 dm3 mol-1 s-l, whereas our previous studies5y l7 have shown that for the emulsion polymerization of styrene at 50 "C the appropriate value2922 SEEDED EMULSION POLYMERIZATION OF STYRENE loo 1 80 60 c 0 .- E $ 40 > 20 0 50 100 150 200 t/min Fig.3. Effect of toluene on the rate of emulsion polymerization of styrene at a high initiator concentration (1.2 x lop2 mol dm-3). Diluent weight fraction 4: curve 1,O.OO; 2,0.15; 3,0.26; 4, 0.50; 5, 0.75. is significantly greater (kp = 255 10 dm3 mol-l s-l). It follows that the correct value for A,, in the absence of diluent in the experiments reported by Azad et al. was ca. 0.40, not 0.50 as claimed. The corresponding revised value of A,, calculated from their rate data in the presence of ethylbenzene at 4 = 0.23 is 0.30. This measured change in A,, can be compared with that predicted from the data presented in table 1 for the increase in k on adding ethylbenzene to this level of replacement, provided it is assumed that the rate coefficient for the entry of free radicals into the particles remains constant.This latter assumption appears reasonable at the relatively high initiator concentration (1.2 x mol dm-3) used in these studies. It would be predicted from the measured increase in k that A,, should change from 0.40 to 0.26, in fair agreement with the revised experimental range (from 0.40 to 0.30). It is clear that the data reported by Azad et al. are in reasonable accord, both qualitatively and quantitatively, with those reported here provided that the value for kp that we previously determined from rates of emulsion polymerization is adopted. TOLUENE The data obtained with toluene as diluent at both high and low initiator concen- trations are presented in fig. 3 and 4. The results for the higher initiator concentration closely parallel those obtained with ethylbenzene as diluent and so will not be discussed further.At the lower initiator concentration, however, a different pattern of behaviour to that obtained with ethylbenzene was observed. At low diluent weight fractions the entry rate coefficient first increased and then decreased (see table 4). This pattern of behaviour is confirmed qualitatively by the sequence of the curves displayed in fig. 4 with increasing degrees of monomer substitution. This observation of a maximum in the entry rate coefficient can be ascribed to the competing effects discussed in relation to ethylbenzene as diluent: on the one hand, the toluene may allow the surfactant to stabilize the precursor particles at a smaller particle size, thus increasing the entry rate coefficient, and on the other, the presence of toluene mayLICHTI, SANGSTER, WHANG, NAPPER AND GILBERT loo 4 80 60 c .- v1 e, 40 8 20 t 2923 0 50 100 150 200 Fig.4. Effects of toluene on the rate of emulsion polymerization of styrene at a low initiator concentration (1.2 x mol dm-3). Diluent weight fraction 4: curve 1,O.OO; 2,0.15; 3,0.26; 4, 0.50; 5, 0.75. t/min decrease the styrene concentration in the aqueous phase, thus reducing the rate of production of oligomeric free-radical species that can enter the latex particles. Futhermore, as styrene is consumed by the polymerization reaction, so the rate of production of free-radical species that can enter the latex particles should decline. BENZENE The effects of benzene on the seeded emulsion polymerization of styrene at a relatively low initiator concentration (1.2 x mol dm-3) are displayed in fig.5. The entry and exit rate coefficients determined at low initial diluent weight fractions (see table 4) show that the exit rate coefficient was not significantly changed by the presence of benzene at these dilution levels. This is in conformity with the results of the relaxation studies presented above. It is apparent from the sequence of the curves shown in fig. 5 that the relative rate of polymerization per particle increased at low levels of substitution of the monomer by benzene. The quantitative data presented in table 4 suggest that this increase resulted from an increase in the entry rate coefficient.CYCLOHEXANE The effects of cyclohexane as a diluent at a low initiator concentration are shown in fig. 6. Again at the lowest initial diluent weight fraction studied the value of the entry rate coefficient was significantly increased (see table 4), but as the level of substitution increased, the rate coefficient again declined. Note, however, that cyclohexane did not exert as large an effect on the free-radical entry rate parameter as did ethylbenzene. This may be a consequence of the fact that cyclohexane at 50 "C is only slightly better than a0-solvent for polystyrene (0 = 34 "C), whereas ethylbenzene is a significantly better solvent for polystyrene.2924 SEEDED EMULSION POLYMERIZATION OF STYRENE - 8o t 50 100 150 200 t/min 0 Fig. 6. Effect of cyclohexane on the rate of emulsion polymerization of styrene at a low initiator concentration (1.2 x mol dmP3).Diluent weight fraction 4: curve 1,O.OO; 2,0.15; 3,0.26; 4, 0.50; 5, 0.75. n-HEPTANE The kinetic curves obtained at a low initiator concentration with n-heptane as a diluent are displayed in fig. 7. The entry and exit rate coefficients determined from these curves at low initial diluent weight fractions are listed in table 4. These results show that for 4 < 0.28 the rate coefficients were not dramatically changed by the replacement of styrene by n-heptane.100 80 h 60 0 .- E 0, 40 > LICHTI, SANGSTER, WHANG, NAPPER AND GILBERT I I I 50 100 150 200 t/min 0 2925 Fig. 7. Effects of n-heptane on the rate of emulsion polymerization of styrene at a low initiator concentration (1.2 x mol dm-3).Diluent weight fraction d;: curve 1,O.OO; 2,O. 15; 3,0.26; 4, 0.50; 5, 0.75. Note that n-heptane is a non-solvent for polystyrene at 50 "C. As a result, it might be expected on free-energy considerations that n-heptane would be located preferentially in the emulsion droplets rather than in the particles. This could result in more of the monomer being present in the droplets at equilibrium than predicted by dilution considerations. As a consequence, the monomer concentration in the particles could well be reduced to below that expected for ideal mixing. Any such depletion of the monomer concentration in the latex particles would necessarily lead to a reduction in the rate of polymerization. CONCLUSIONS The kinetic relaxation studies described above imply that the effect of hydrocarbon diluents on the exit rate coefficient for styrene is determined not only by the chain-transfer constant of the diluent but also by the reactivity of the diluent free radicals with monomer.The latter appears to follow the rule that the more difficult it is to form a free radical, the slower is the reaction of that free radical with monomer. If a diluent free radical reacts only slowly with monomer, there is increased probability of that free radical escaping by diffusion from the latex particle before it adds on monomer and is rendered incapable of undergoing exit. This can result in an increase in the observed exit rate coefficient, even if the constant for chain transfer to the diluent is less than that for chain transfer to monomer.Very large increases in the exit rate coefficient are predicted theoretically to be operative with certain diluents at high monomer replacement levels. The effects of two diluents (ethylbenzene and toluene) at various monomer- replacement levels on the relative rate of polymerization at the higher concentration of potassium peroxydisulphate are summarized in fig. 8. The rate of polymerization was reduced in both cases more than expected on the basis of dilution of monomer alone. This decrease may arise from at least two possible causes: an increase in the2926 SEEDED EMULSION POLYMERIZATION OF STYRENE I 1 c .- 0 0.25 0.50 0 0.25 0.50 0.75 diluent weight fraction Fig. 8. Effect of increasing diluent weight fraction on the relative rate of emulsion polymerization of styrene at high initiator concentration (1.2 x mol dm-3).Diluent: (a) ethylbenzene and (b) toluene. exit rate coefficient and/or a change in the entry rate coefficient. The latter may either increase or decrease in the presence of diluent. The increase, when it occurs, appears to arise from the ability of the diluent to render the surfactant more effective in stabilizing the oligomeric and colloidal free-radical species that can enter the latex particles. Stabilization at lower degrees of aggregation leads to an increase in the rate of entry of free radicals into the latex particles. Any decrease in the entry rate coefficient may be postulated to arise from the reduction in the monomer concentration in the aqueous phase.This depletion necessarily accompanies the reduction in the chemical potential of the styrene in the emulsion droplets on dilution with the hydrocarbon. The results obtained for the relative rates of polymerization of styrene in the steady state in the presence of five diluents at the lower initiator concentration are summarized in fig. 9. It is apparent that a complex pattern of behaviour is observed, which is scarcely surprising in the light of the subtle interplay of the various competing effects operative in these systems. These include the mechanisms which were outlined above that cause an increase in the exit rate coefficient and may either increase or decrease the entry rate coefficient. Ethylbenzene, benzene and cyclohexane, which are all good solvents for polystyrene at 50 "C, produced significant increases in the relative rate of polymerization at low initial diluent weight fractions.These apparently resulted from an increase in the entry rate coefficient, presumably as a consequence of a synergistic interaction between the diluent and the surfactant. This allowed the surfactant to stabilize the entering species at relatively low degrees of aggregation and so reduced the rate of biomolecular termination of the free radicals located outside the latex particles. More free radicals were thus available for entry. n-Heptane, which is a non-solvent for polystyrene, had little effect on the entry or exit rate parameters. At higher hydrocarbon weight fractions, the rate of polymerization was reduced in the presence of all diluents significantly more than that expected from dilution considerations alone. Several factors may contribute to this reduction in rate.First, at high dilutions some hydrocarbons may increase the exit rate coefficient massively. Secondly depletion of the monomer concentration in the aqueous phase may reduceC 0 m .- Y .g 1.0 9) - E 0 a rr QJ 0 5 0.5 0 0.5 0 0.5 0 0.5 0 0.5 0 0.5 diluent weight fraction Fig. 9. Effect of increasing diluent weight fraction on the relative rate of emulsion polymerization of styrene at low initiator concentration (1.2 x mol dm-"). Diluent: (a) ethylbenzene, (6) toluene, (c) benzene, ( d ) cyclohexane and (e) n-heptane. E Q2928 SEEDED EMULSION POLYMERIZATION OF STYRENE the rate of production of species capable of entering the latex particles and thus lower the rate coefficient for entry of the free radicals into the particles. Thirdly, the concentration of monomer in the latex particles may be reduced below that expected for ideal mixing. We thank the Australian Research Grants Committee (B. C. Y . W.) and AINSE (G.L.) for financial support. The Electron Microscope Unit of the University of Sydney is also thanked for their generous provision of facilities. D. R. Owen, D. McLemore, Liu Wan-Li, R. B. Seymour and W. N. Tinnerman, in ACS Symp. Ser. no. 24, Emulsion Polymerization, ed. I. Piirma and J. L. Gardon (American Chemical Society, Washington, D.C., 1976), p. 299. D. C. Blackley and A. C. Haynes, Br. Polym. J., 1977, 9, 312. A. R. M. Azad, R. M. Fitch and M. Nomura, in ACS Symp. Ser. no. 165, Emulsion Polymers and Emulsion Polymerization, ed. D. R. Bassett and A. E. Hamielec (American Chemical Society, Wash- ington, D.C., 1981), p. 357. B. S. Hawkett, D. H. Napper and R. G. Gilbert, J. Chem. SOC., Faraday Trans. I , 1975, 71, 2288. B. S. Hawkett, D. H. Napper and R. G. Gilbert, J. Chem. SOC., Faraday Trans. I , 1980,76, 1323. B. C. Y. Whang, D. H. Napper, M. J. Ballard and R. G. Gilbert, J. Chem. SOC., Faraday Trans. I , 1982, 78, 11 17. S. W. Lansdowne, R. G. Gilbert, D. H. Napper and D. F. Sangster, J. Chem. SOC., Faraday Trans. I , 1980, 76, 1344. G. Lichti, D. F. Sangster, B. C. Y. Whang, D. H. Napper and R. G. Gilbert, J. Chem. SOC., Faraday Trans. I , 1982, 78, 2129. G. Lichti, B. S. Hawkett, R. G. Gilbert and D. H. Napper, J. Polym. Sci., Polvm. Chem. Ed., 1981, 19, 925. lo Polymer Handbook, ed. J. Brandrup and E. H. Immergut (Wiley, New York, 2nd edn, 1975). l1 J. Ugelstad and J. K. Hansen, Rubber Chem. Technol., 1976,49, 536. l2 M. Nomura and M. Harada, J. Appl. Polym. Sci., 1981, 26, 17. l3 M. Nomura, in Emulsion Polymerization, ed. I. Piirma (Academic Press, New York, 1982), chap. 5. l4 M. H. George, in Vinyl Polymerization, ed. G. E. Ham (Marcel Dekker, New York, 1969), vol. 1, l5 P. J. Flory, Principles of Polymer Chemistry (Cornell University Press, Ithaca, New York, 1953), l6 G. Lichti, R. G. Gilbert and D. H. Napper, J. Polym. Sci., Polym. Chem. Ed., 1983, 21, 269. l7 B. S. Hawkett, D. H. Napper and R. G. Gilbert, J. Chem. SOC., Faraday Trans. I , 1981,77, 2395. 18 I. Piirma and S-R. Chen, J. Colloid Interface Sci., 1980, 74, 90. chap. 3. pp. 189-195. (PAPER 3/2078)

ISSN:0300-9599    

DOI:10.1039/F19848002911

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1984, 80, 2929-2934 Hydrogen-atom Attack on Methyl Viologen in Aqueous Solution Studied by Pulse Radiolysis BY S. SOLAR, W. SOLAR AND N. GETOFF* Institut fur Theoretische Chemie und Strahlenchemie der Universitat Wien and Ludwig Boltzmann Institut fur Strahlenchemie, A- 1090 Wien, Wahringerstrasse 38, Austria AND J. HOLCMAN AND K. SEHESTED Accelerator Department, RIS0 National Laboratory, DK-4000 Roskilde, P.O. Box 49, Denmark Received 25th November, 1983 Using hydrogen at high pressures of up to 150 bar (0.12 mol dmP3 H,) as an OH scavenger in aqueous MV2+ solutions (pH 1) it is possible to differentiate between two kinds of transient formed simultaneously by H-atom attack on methyl viologen. One of them is assigned to an H adduct on the N atom, MV'+H+ ( k = 3.1 x los dm3 mol-l s-l), with absorption bands identical to those of the radical cation, MV'+, but with E,,,., = 3200 m2 mo1-I and E ~ , , = 1100 m2 mol-I.The MV'+H+ species deprotonates with k = 2 x lo4 s-l, forming the long-lived radical cation, MV'+. The second type of transient produced, with k = 2.9 x lo8 dm3 mol-l s-l, is attributed to an H-adduct on the ring carbon, MVa2+H, with E,,, = 570 m2 mol-l and E,,, = 920 m2 mol-l, decaying by second-order kinetics with 2k = (6.0 1) x lo8 dm3 mol-l s-l. The formation of MV'+ by electron transfer from the propan-2-01 radical has been reinvestigated (pH 0-7); its absorption spectrum does not change in this pH range. Recently we have reported1 that hydrogen atoms react with methyl viologen (MV2+) in acidic aqueous solution (pH 1) forming two types of transient, one attributed to an adduct on the nitrogen atom (A,,, at 390 and 595 nm) and the other to an adduct on the ring carbon of the MV2+ molecule (A,,, at 310 and 470 nm).Both their superimposed bands and the kinetics of their formation and decay were resolved by means of a semi-linear optimization procedure.2 Both chloride ion and butan-2-01 were used as efficient OH scavengers in two separate series of experiments, and appropriate corrections for the simultaneously produced C1; species and the alcohol transients have been made. Since it is well known that alcohol and acetone radicals can also reduce MV2+ to MV'+,3-5 the additional formation of MV'+ by the t-butyl alcohol radical was taken into consideration. The absorption spectra of the above-mentioned two H adducts of MV2+ obtained in both series of experiments were in good agreement.Based on the measured dependence of the optical-density (O.D.) values at 392.5 and 600 nm in the pH range 0.5-4.5 a pKvalue for the protonation of MV'+ was 0btained.l Very recently, however, Venturi et a1.6 observed no significant change in the O.D. values at these absorption maxima for MV'+ produced by one-electron transfer from propan-2-01 and acetone radicals to MV2+ in the pH range 0-14. These authors have also questioned the formation of H adducts absorbing at 470 nm. Hence both the kinetics of formation and decay and the absorption spectra of these intermediates have been reinvestigated. Pulse-radiolysis studies using H, under high pressure (140-1 50 atm)? to convert the OH radicals into H atoms were performed. f I atm = I01 325 Pa.29292930 H-ATOM ATTACK ON METHYL VIOLOGEN EXPERIMENTAL The pulse-radiolysis equipment (10 MeV Linac, Haimson Research Corp., HRC-712, Perkin-Elmer double monochromator with a band-pass of f 1 nm, combined with a Nicolet Explorer I11 digital-storage oscilloscope and PDP-8 computer on-line) has been described previ~usly.~ Aqueous solutions of methyl viologen dichloride (1,l '-dimethyL4,4'-bipyridinium, p.a. quality, B.D.H. Chemicals Ltd) were prepared using triply distilled water. All other chemicals (propan-2-01, acetone, HClO, and NaOH, E. Merck) were also of p.a. purity. The MV2+ solutions (at pH 1 and 6.5) were irradiated in a special pressure cells under 140-1 50 bar H, (0.1 1-0.12 mol dm-3 H,).They were first deoxygenated by purging with high-purity argon for ca. 1 h (syringe technique) and then transferred into the pressure cell avoiding any contact with air. The solutions were equilibrated under H, pressure by intense stirring for ca. 30 min before irradiation. The dose rate was determined by means of a ferrocyanide d~simeter.~ The applied dose was 5-10Gy (0.5-1 krad) per 1 ,us pulse. The measured O.D. values per cm were normalized to 10 Gy. RESULTS AND DISCUSSION EXPERIMENTS IN THE PRESSURE CELL On pulse radiolysis of 4 x lop4 mol dm-, MV2+ (containing 8 x mol dm-, C1-) in the presence of 0.11 mol dm-, H, at pH 1 in the pressure cell, eiq is converted into H-atom within 0.1 ,us, since k(eiq + H+) = 2.3 x 1O1O dm3 mol-l s-l.l0 Under these conditions the OH radicals are involved in reactions (1)-(4) initiating the formation of various chlorine species : OH+H, + H+H,O k, = 3.5 x lo7 dm3 mol-l s-l l1 OH + MV2+ --+ (MV2+0H) k, = 2.8 x los dm3 mol-1 s-l l2 OH + C1- + ClOH- k, = 4.2 x lo9 dm3 mo1-1 s-l l3 C10H- -+ OH + C1- k, = 6.1 x lo9 s-' l3 ClOH- + H+ --+ C1' + H,O k, = 2.1 x 1O1O dm3 mol-l s-l l3 c1- + C1' -+ c1;- k, = 2.1 x 1010 dm3 mol-l s-l l3 Clg- + Clg- -+ Cl; + C1- 2k, = 7.8 x lo9 dm3 mol-1 s-l.l3 (1) (2) (3) (4) ( 5 ) (6) (7) As a result of these processes 79.5% of the hydroxy radicals are converted into hydrogen atoms, 18% produce Clg- and 2.5% form adducts of methyl viologen (MV2+0H). The lapse of the transient concentration change for H, Cl', Cli- and MV2+OH as a function of time is displayed in fig.1. mol dm-, MV2+ and different dose rates (5-12 Gy) it was found that the conversion of OH into H varies from 75 to 81 % and the share of MV2+0H transients is -c 5 % . Since the Cli- species absorbs in the range from 250 to ca. 450 nm with a maximum at 340 nm (E,,, = 880 m2 m01-l),l3 its contribution was subtracted from the measured total absorption spectrum of methyl-viologen transients, obtained from 4 x low4 mol dm-, MV2+ (fig. 2). The concentration of CK- is very small and hence it does not have any significant influence on the absorption and kinetics of the other intermediates. The transient absorption spectrum exhibits four pronounced absorption maxima at 310, 392.5, 470 and 600 nm. The O.D. values per cm around the peaks were measured every 2.5 nm.Using 2 x 10-*-8 xs. SOLAR et al. 293 1 0 0.2 0.4 0.6 0.8 1.0 1.2 time after pulse/ps Fig. 1. OH conversion in the pressure cell (0.1 1 mol dm-3 H2) as a function of time in the presence of methyl viologen dichloride at pH 1 (see text). Initial concentrations/mol dm-3: [MV2+], 4 x lov4; [Cl-1, 8 x [H,], 0.1 1 ; [OH], 3 x [H+], 0.1. A/ nm Fig. 2. Total absorption spectrum of transients produced by reaction of methyl viologen with H atoms in deoxygenated solution at pH 1 ; (a) measured and (b) corrected for Cl;; (solution: 4 x low4 mol dm-3 MV2+, 0.11 mol dm-3 H,; applied dose: 4-5 Gy; O.D. values normalized to 10 Gy per 1 ps pulse). Insert: Mean value for subsequent absorption increase (A0.D.) at 392.5 nm as a function of G(H) (solution: 4 x mol dm-3 MV2+; applied dose: 5 Gy per 1 ps pulse; for further details see text).The absorption band at 470 nm, similar to that of the OH adduct,12y14 has been ascribed earlier to the H-atom adduct (MV2+H) on the ring carbon of methyl viologen.' Note that this absorption band at 470 nm was also observed at pH 6.5 in an H, pressurized system, and at pH 1 in an Ar-saturated solution, with C1- added as OH scavenger. In all these systems O.D.,,, is found to be proportional to the corresponding G(H), which is additional support for our previous assignment. The MV+H transients disappear according to a second-order reaction with 2k = (6.0f 1) x lo8 dm3 mol-1 s-l. The absorption bands at 392.5 and 600 nm (fig. 2) show features characteristic of2932 H-ATOM ATTACK ON METHYL VIOLOGEN the well known methyl viologen radical cation (MV'+).lq 3 9 5* 15-17 These two absorptions develop in two steps, a relatively fast one, with overall k = 6.0 x 10, dm3 mol-1 s-l, followed by a slower absorption increase, which is complete ca.150 p s after the pulse. The same build-up was also observed at pH 1 in deoxygenated MV2+ solutions (4 x mol dm-3) in the presence of C1- ions added as OH scavenger, as well as in H,-pressurized (0.12 mol dm-3 H,) MV2+ solutions at the same concentration but at pH 6.5. The transient absorption spectrum obtained 150 ps after the pulse is essentially not influenced by reactions of Cli- species over this time-scale. In all systems investigated the A0.D. values (representing the slow increase at 392.5 and 600 nm) are found to be proportional to the corresponding G(H) (fig.2, insert). This is an indication of a consecutive reaction of the primary produced transients, which can be interpreted as deprotonation of the H adduct. The kinetics of this process was found to be first order with k,, = 2 x lo4 s-l, independent of the solute concentration and pH. Based on these findings the following probable reaction mechanism is suggested: H H The H adduct on the N atom of MV2+ (MV'+H+, equivalent to the protonated form of MV+, absorbing at 392.5 and 600 nm) is produced simultaneously with the H adduct on the ring carbon, having an absorption band at 470 nm. The absorptions that finally developed (392.5 and 600 nm) could be assigned exclusively to the radical cation MV'+, owing to the similarity of its spectrum and stability.This assign- ment allowed an estimation of the final distribution of the products of H attack on MV2+ as 51% MV'+ [reaction ( 8 a ) followed by reaction (8b)l and 49% MV',+H [reaction (9)]. Based on these results and taking into consideration the overall k = 6 x 10, dm3 mol-l s-' it was possible to calculate k,, as 3.1 x 10, dm3 mol-1 s-l and k,, as 2.9 x 10, dm3 mol-1 s-l. These data are in good agreement with previously reported va1ues.l Additionally, E(MV'~+H),,, = 920 m2 mol-1 was estimated, taking into account the small contributions of MV'+ at this wavelength. FORMATION OF MV' + BY AN ELECTRON-TRANSFER PROCESS The well known formation of MV'+ by the reaction of the propan-2-01 radical (CH3),COH with MV2+ 3-6 was now reinvestigated under our experimental condi- tions in the pH range 0-7 (4 x mol dm-3 MV2+, 0.1 mol dm-3 propan-2-01, 0.1 mol dm-3 acetone, 5 Gy per 1 p s pulse) in order to check the previously reported pK value for the protonation of MV'+.l In the entire pH range the primary radicals (eiq, H and OH) result in the formation of the (CH,),COH species, which reacts with MV2+ by electron transfer: MV2+ + (CH,),COH -+ MV'+ + H+ + (CH3),C0 (10) k,, = (2.9 to 3.5) x lo9 dm3 mol-1s.SOLAR et al. 2933 0 X/nm Fig. 3. Absorption spectrum of MV'+ obtained by reaction of 4 x mol dm-3 MV2+ with propan-2-01 radicals in the pH range 0-7 (see text). Applied dose: 5 Gy per 1 ps pulse. Insert: absorption spectrum of H adduct on ring carbon. Table 1.Kinetic and spectroscopic characteristics of the transients formed by H attack on methyl viologen in aqueous solution (pH l), using H, at high pressure as an OH scavenger rate constant, molar extinction coefficients, Elm2 mol-' k/dm3 mol-' s-' at A/nm transient formation decay (2k) 310 392.5 470 600 H adduct on 3.1 x lo8 2 x - 3200 - 1100 radical 2~ 104~ long-lived 420 4200 80 1450 H adduct on 2.9 x lo8 (6.0k 1) x lo8 570 920 ~ N atom cation C atom - First-order reaction (k' in s-l). The resulting absorption spectrum (fig. 3) is independent of pH, showing the characteristic absorption maxima of MV'+ and is in very good agreement with previously reported data.lY 3-63 15-17 By subtracting the absorption spectrum of MV+ (fig. 3) from the total spectrum (fig.2) resulting from experiments in the high-pressure cell, the spectrum of the H adduct on the ring carbon (MVo2+H) was obtained (see insert to fig. 3). It exhibits two maxima at 310 and 470 nm. The kinetic and spectroscopic data obtained for the transients produced by H attack on MV2+ are summarized in table 1 . Under more definite conditions of the experiments on MV2+ at high H, pressures (pH 1-6.5) as well as using the propan-2-01 radical for the production of MV2+ (pH 0-7) no indication of the previously proposed pK value1 for the deprotonation of MV'+H+ could be found down to pH 0.5. This is in agreement with the observations of Venturi et aL62934 H-ATOM ATTACK ON METHYL VIOLOGEN We thank the authors of ref. (6) for making their paper available prior to publica- tion. The technical assistance of Miss H.Corfitzen and Mr T. Johansen is greatly acknowledged. S. S. and N. G. express thanks for the financial support awarded by the RIS0 National Laboratory. S. Solar, W. Solar, N. Getoff, J. Holcman and K. Sehested, J. Chem. Soc., Faraday Trans. 1, 1982, 78, 2467. S. Solar, W. Solar and N. Getoff, Radiat. Phys. Chem., 1983, 21, 129. L. K. Patterson, R. D. Small and M. S. Matheson, Radiat. Res., 1977, 72, 218. M. Kaneko, H. Araki and A. Yamada, Sci. Pap. Znst. Phys. Chem. Res. Jpn, 1979, 73, 67. D. Meisel, W. A. Mulac and M. S. Matheson, J. Phys. Chem., 1981, 85, 179. M. Venturi, 0. G. Mulazzani and M. Z. Hoffman, Radiat. Phys. Chem., 1984, 23, 229. ' K. Sehested, H. Corfitzen, H. C. Christensen and E. J. Hart, J. Phys. Chem., 1975, 79, 310. * H. Christensen and K. Sehested, Radiat. Phys. Chem., 1980, 16, 183. R. H. Schuler, A. L. Hartzell and B. Behar, J. Phys. Chem., 1981,85, 192. lo M. Anbar, M. Bambenek and A. R. Ross, Selected Specific Rates of Reactions of Transients from Water in Aqueous Solutions, Part Z, The Hydrated Electron (National Bureau of Standards, Washington, D.C., 1973). l1 M. Anbar, Farhataziz and A. B. Ross, Selected Specific Rates of Reactions of Transients from Water in Aqueous Solutions. Part IZ, The Hydrogen Atom (National Bureau of Standards, Washington, D.C., 1975). l2 S. Solar, W. Solar, N. Getoff, J. Holcman and K. Sehested, to be published. la G. G. Jayson, B. J. Parsons and A. J. Swallow, J. Chem. Soc., Faraday Trans. I , 1973, 69, 1597. l4 L. K. Patterson, R. D. Small Jr and J. Scaiano, Radiat. Res., 1977, 72, 218. l5 E. M. Kosower and J. L. Cotter, J. Am. Chem. Soc., 1964, 86, 5524. I6 J. A. Farrington, M. Ebert and E. J. Land, J. Chem. Soc., Faraday Trans. I , 1968, 74, 665. l7 J. A. Farrington, M. Ebert, E. J. Land and K. Fletcher, Biochim. Biophys. Acta 1973, 314, 371. (PAPER 3/2097)

ISSN:0300-9599    

DOI:10.1039/F19848002929

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J. Chem. Soc., Faraday Trans. 1, 1984,80, 2935-2941 Electrical Conductivity and Enthalpies of Transformation in Li,UBr, between 500 and 800 K Comparison with Na,UBr, BY ALEKSANDER BOGACZ* AND WLODZIMIERZ SZCZEPANIAK Institute of Inorganic Chemistry and Metallurgy of Rare Elements, Wroclaw Polytechnic, Wybrzeze Wyspianskiego 27, 50-370 Wrodaw, Poland AND JEAN-PIERRE BROS, YVONNE FOUQUE AND MARCELLE GAUNE-ESCWD Laboratoire de Dynamique et Thermodynamique associe au CNRS (L.A. 72), Universite de Provence, Centre de St Jerbme, 13397 Marseille 13, France Received 20th December, 1983 As with Na,UBr,, previously investigated, Li,UBr, undergoes several transitions before melting. These were studied using electrical-conductivity measurements, differential thermal analysis and differential enthalpic analysis performed in the temperature range 500-800 K.For Li,UBr, the thermal effects before fusion are insignificant and are much smaller than those found for Na,UBr, (1.7 f 0.4 kJ mol-l at 505 & 2 K and 0.35 f 0.15 kJ mol-1 at 740 2 K), while the enthalpy of fusion is larger (48 f 2 kJ mol-' at 781 k 2 K). For both compounds, comparable values were found by summing the entropies of transformation (61.9 J mol-l K-' for Na,UBr, and 65.2 J mol-1 K-l for Li,UBr,). The variation of electrical conductivity with temperature is very different for these compounds. While continuous evolution is observed for Li,UBr,, each transition of Na,UBr, corresponds to a jump in conductivity. The results obtained for Li,UBr, are discussed by comparison with those previously obtained for Na,UBr,.In spite of the scarcity of information concerning the compounds M,UBr, (where M = Li, Na, K, Rb, Cs), it seems from the melting temperatures and the room- temperature str~ctures~-~ that this series of compounds can be divided into two groups containing Li,UBr, and Na,UBr, on the one hand and K,UBr,, Rb,UBr, and Cs,UBr, on the other. Following our recent thermodynamic, electrical and structural investigations on Na,UBr6,4-6 similar experiments were undertaken on Li,UBr,. Here we give the results of electrical conductivity (e.c.), differential thermal analysis (d.t.a.) and differential enthalpic analysis (d.e.a.) and these are compared with results already obtained for Na,UBr,. EXPERIMENTAL Details of the preparation of this type of compound and the experimental methods used (e.c., d.t.a.and d.e.a.) have been given previ~usly,~ so that only points specific to Li,UBr, will be mentioned here. 29352936 ELECTRICAL CONDUCTIVITY OF Li,UBr, SYNTHESIS Li,UBr, was prepared from lithium bromide (LiBr) and uranium tetrabromide (UBr,). LiBr (from B.D.H.) was recrystallized, dried and then melted under an HBr atmosphere. UBr, was synthesized from uranium oxide (UO,) by the successive action of carbon and bromine;, it was then purified by sublimation in a vacuum of ca. lo-, Pa. Its chemical analysis corresponded to the formula UBr,~,,,,~,,. Li,UBr, was obtained by the direct action of stoichiometric amounts of LiBr and UBr,, sealed under vacuum in a silica ampoule and heated to 970 K. The liquid mixture was maintained at this temperature for 1 h in order to ensure homogeneity of the sample, and was then cooled very slowly.The compound was stored in sealed ampoules and handled in a dry-box filled with purified argon. ELECTRICAL CONDUCTIVITY A silica capillary cell was used in measuring the electrical conductivity of Li,UBr,. Details of the experimental arrangement and its operation have been reported previ~usly.~ The electrodes and junction wires were made of platinum. The experimental temperature, detected on the cell wall, was known to within + 2 K using a Pt/Pt + 10% Rh thermocouple connected to a Meratronic V534 digital voltmeter. The electrical resistance of the sample was measured with a BM484 Tesla alternating-current bridge. The resistance values were obtained to within four significant figures.Molten KCl was used for the calibration of the experimental cell.7 We found for the cell constant 6669k9 m-l. DIFFERENTIAL THERMAL ANALYSIS Semi-quantitative differential thermal analysis of Li,UBr, was performed with a Paulik-Erdey derivatograph. The sample used was contained in a silica ampoule, sealed under a vacuum of 0.01 Pa, and had a mass of 6.09 g. The features of the cell and the details concerning the calibration of the apparatus were described previ~usly.~ Experimental runs were performed at two different sensitivities and at heating rates of 5 and 10 K min-'. DIFFERENTIAL ENTHALPIC ANALYSIS Three samples with masses ranging from 5 to 7 g were investigated by differential enthalpic analysis. A high-temperature Calvet microcalorimeter was used for this purpose; as stressed in previous paper^^,^^^ its great sensitivity (each of the two thermopiles including many hundreds of thermocouples) and its slow heating rate (< 15 K h-l) are well suited to measuring accurately the magnitudes of weak and close thermal effects.RESULTS ELECTRICAL CONDUCTIVITY Our experimental results are shown in fig. 1 (a) as plots of log 0 against 1 / T; there are two breaks in the heating curve at 505 and 778 K. While the break at 778 K (corresponding to the melting process) is reproducible that at 505 K appears only in the heating curve. Fortunately, its participation in the total increase of the conductivity is very small. The electrical conductivity of Li,UBr, in the solid state increases rapidly but does not exhibit as many dislocations as seen in the d.t.a.and d.e.a. curves. At low temperatures, the electrical conductivity of solid Li,UBr, corresponds to an activation energy of ca. 1 10 kJ mol-l; this activation energy decreases with increasing temperatures and reaches a value between 30 and 40 kJ mol-l in the vicinity of the melting temperature, 778 K.A. BOGACZ, w. SZCZEPANIAK, J-P. BROS, Y. FOUQUE AND M. GAUNE-ESCARD 2937 1 . 5 0 -0.5 h - I f - I \ c: - 1 . 5 0 M v - - 2 . 5 -3.5 1 . 1 1 1 . 1 .o 1.4 1.8 103 K I T Fig. 1. (a) Temperature dependence of the electrical conductivity of solid and liquid Li,UBr,: x , heating; 0, cooling. (b) D.t.a. thermogram. DIFFERENTIAL THERMAL ANALYSIS AND DIFFERENTIAL ENTHALPIC ANALYSIS Curve (b) in fig.1 is a thermogram obtained from differential thermal analysis; it should be compared with that given by differential enthalpic analysis (fig. 2), although the deformation of the d.t.a. thermogram caused by the choice of units (reciprocal temperature) in fig. 1 should be taken into account. Large dissimilarities in both the sensitivities and the heating or cooling rates make these two thermograms appear fairly different; however, in each case two effects caused by low thermicity were obtained in the solid phase at 496 and 505 K (first transition) and at 736 and 740 K (second transition). The melting point was found at 773 K (d.t.a.) and 781 K (d.e.a.). These results were obtained using one sample in the case of d.t.a. and three samples from separate syntheses for d.e.a.(table 1). Taking into account the specific shapes of both thermograms and the respective uncertainties caused by the apparatus used, we selected the following temperatures : 505f2 K for the first transition, a value identical to that detected from electrical- conductivity measurements, 740 * 2 K for the second transition and 780 * 2 K for fusion. For the enthalpies of transformation we selected (table 1) 48 kJ mol-l for the enthalpy of fusion, a mean value for the three samples investigated by d.e.a. The enthalpy results obtained for the second transition were very different since, on the one hand, the two thermal effects (fusion and second transition) were very difficult to separate and, on the other, the phenomenon itself was not dependent on temperature: indeed, in order to obtain better separation the heating rate was2938 ELECTRICAL CONDUCTIVITY OF Li,UBr, L .t 505 740 781 TIK Fig. 2. D.e.a. thermogram of Li,UBr, at a heating rate of 5 K h-l. (4 is the thermal flux, T the temperature and t the time). Table 1. Temperatures and enthalpies of transformations of Li,UBr, method 1 st transformation 2nd transformation fusion e.c. 505 - 778 + 2 d.t.a. 496 736 773f 10 d.e.a. 505 740 781 + 2 AH/kJ mol-' d.t.a. 0.9 1.9 45+ 12 d.e.a. 1.7 0.35 4 8 f 2 T / K decreased and this resulted in an increased duration of the small effect and a resulting increased uncertainty. For the first transformation the thermal effect ranges between 0.9 and 1.70 kJ mol-l, the difference being due to the same reasons as mentioned above.In addition, whichever apparatus was used, good agreement was obtained for the total thermal effect (50 f 3 kJ mol-l). DISCUSSION Using the results obtained with the same procedures for the electrical conductivities and variations in enthalpy, we attempted to establish a comparison between the two compounds Li,UBr, and Na,UBr,. We have plotted in fig. 3 logo = f ( l / T ) for each compound; these curves exhibit the following features. In the liquid state the electrical conductivities of both compounds are very close and are ca. 100 R-l m-l. However, on melting the electrical conductivity of Li,UBr, increases, while that of Na,UBr, decreases (at the lowest temperatures, e.g. 600 K, Li,UBr, is a much better conductor than Na,UBr,). The general shapes of the electrical-conductivity curves for the two compounds in the solid state differ noticeably; the electrical conductivity of Li,UBr, has a continuous variation while that of Na,UBr, proceeds by jumps.Since a quantitative explanation of the process of ionic conduction from these compounds is not available, only a qualitative approach, taking into account the enthalpy results, will be given. The compounds Li,UBr, and Na,UBr, are ionic crystals with an anionic sublattice consisting of UBri- units and a cationic sublattice containing Li+ or Na+ ions. TheA. BOGACZ, w. SZCZEPANIAK, J-P. BROS, Y. FOUQUE AND M. GAUNE-ESCARD 2939 2.00 1.50 n I - E - . k 0 w W 2 1.00 0.50 0.00 T/K 833 714 625 555 I I I Fig. 3. Temperature dependence of the electrical conductivities of (a) Na,UBr, (values of the electrical conductivity were recorded at 1 kHz for the upper curve and at 6 kHz for the lower curve) and (b) Li,UBr,.room-temperature structure of Na,UBr, is trigonal P3m 1 (type Na,SiF,)5 related to that of UCl,10 by the partial filling of the vacant octahedral sites with Na+. The structure of Li,UBr, is unknown. With regard to analogous compounds it has been found that Na,UCl,, like Na,UBr,, crystallizes in the space group P3ml and that Li,UCl, belongs to the space group P6,/mmc.'l Up to 652 K the electrical conductivity of Na,UBr, increases with temperature but has moderate values: for instance, at 625 K it is only 1 !2-l m-l. At the same temperature the electrical conductivity of Li,UBr, is 30 times that of Na,UBr, and the two small thermal effects visible in the d.e.a.curve at 505 and 740 K have only a weak influence on the curve log 0 = f ( l / T ) : at 740 K no irregularity can be detected, while in the vicinity of 505 K a small and smooth kink appears in the conductivity curve only on heating. In contrast, for Na,UBr, large enthalpy changes are associated with substantial conductivity jumps and correspond to phase transitions. In the solid state, Li,UBr, has a high electrical conductivity with a continuous temperature dependence (a = 75 !2-' m-' at 50 K below the melting point and 0 = 151 0-l m-l at the melting point). The activation energy of the electrical con- ductivity varies from 110 kJ mol-' at ca. 500 K to 30-40 kJ mol-1 before melting. The structure of Na,UBr, above 652 K is orthorhombic, Pmnn, allowing Na+ ions to jump unidirectionally between equivalent holes in accordance with a change in2940 ELECTRICAL CONDUCTIVITY OF Li,UBr, electrical conductivity.6 In this temperature range the electrical conductivity is of the same order of magnitude as that of Li,UBr,.At 663 K, Na,UBr, undergoes a new phase transition to a cubic structure Im3m, enabling the free motion of Na+ ions in all directions;6 the electrical conductivity is then higher than for Li,UBr,: Na,UBr, is a superionic conductor with a corresponding activation energy (9.34 If: 0.06 kJ mol-l). Just before the melting point its electrical conductivity is 112 0-l m-l, exceeding that of the liquid by 13 0-l m-l. In the liquid state Na,UBr, has an electrical conductivity similar to that of Li,UBr,. Very different entropy changes are associated with the fusion of Li,UBr, and Na,UBr, (61.4 J mol-l K-l at 781 K and 13.8 J mol-l K-l at 787 K, respectively.) For Na,UBr,, if we assume from the electrical-conductivity results that the Na+ ions are fully delocalized after the second thermal effect ( T = 684 K), we obtain good agreement between the experimental entropy of melting and that predicted by the theory of independent melting of anionic and cationic sublattices. l2 Therefore for Na,UBr, only the anionic sublattice melts at 787 K, resulting in the melting of the compound and the disruption of the motion of the Na+ ions (the decrease in electrical conductivity).This simple relation between the entropy of melting and the electrical conductivity is not valid for Li,UBr,.Its high melting entropy is in contrast to its high electrical conductivity, suggesting the probable degradation of UBri- anions during melting. This conclusion may be compared with Morrey's observation,13 obtained from absorption spectra in liquid MCl+ UC1, systems, of the increasing stability of the UCli- anion with the size of the alkali-metal ion. It can also be seen that the sum of entropies associated with the detected phase transitions is comparable for these two compounds (65.3 J mol-1 K-l for Li,UBr, and 61.9 J mol-l K-l for Na,UBr,). The lack of high-entropy phase transitions in solid Li,UBr, and the smooth increase in its electrical conductivity (fig. 1) may indicate a higher-order phase transition associated with the disordering of Li+ ions.For Na,UBr, this is realized during two first-order phase transitions. CONCLUSION These joint electrical and calorimetric investigations of the apparently very similar compounds Li,UBr, and Na,UBr, have shown the following. Disordering of the cationic sublattice before melting occurs in both Na,UBr, and Li,UBr, but only for Na,UBr, is this seen as a first-order transition. Partial dissociation of the UBri- ions might occur in liquid Li,UBr,, while the anionic species remain stable in liquid Na,UBr, near the melting point. Other experimental information (in particular a structural investigation) is necessary for a better explanation of the electrical and calorimetric results obtained for Li,UBr,. V. M. Vdovenko, I. I. Kozhina, I. T. Suglobova and D. E. Chirkst, Radiokhimiya, 1973, 15, 172. W. Szczepaniak and A, Bogacz, Mater. Sci., 1978, IV, 117. V. M. Vdovenko, 1. I. Kozhina, I. T. Suglobova and D. E. Chirkst, Radiokhimiya, 1973, 15, 54. Y. Fouque, M. Gaune-Escard, W. Szczepaniak and A. Bogacz, J . Chim. Phys., 1978,75, 360. A. Bogacz, J. P. Bros, M. Gaune-Escard, A. W. Hewat and J. C. Taylor, J . Phys. C, 1980, 13, 5273. A. W. Hewat, J. C. Taylor, M. Gaune-Escard, J. P. Bros, W. Szczepaniak and A. Bogacz, J . Phys. C, to be published. E. R. Buckle and P. E. Tsaoussoglou, J . Chem. SOC., Furaday Trans. I , 1972, 68, 1024. Y. Fouque, J. P. Bros, M. Gaune-Escard, W. Szczepaniak and A. Bogacz, J . Inorg. Nucl. Chem., 1980, 42, 257.A. BOGACZ, w. SZCZEPANIAK, J-P. BROS, Y. FOUQUE AND M. GAUNE-ESCARD 2941 J. P. Bros, Y. Fouque, M. Gaune-Escard, W. Szczepaniak and A. Bogacz, J. Chim. Phys., 1982, 79, 71 5. lo J. C. Taylor and P. W. Wilson, Acta Crystallogr., Sect. B, 1974, 30, 1481. l 1 P. J. Bendall, A. N. Fitch and B. E. F. Fender, J. Appl. Crystallogr., 1983, 16, 164. l2 A. R. Ubbelohde, The Molten State of Matter (John Wiley, New York, 1978). l 3 I. R. Morrey, Znorg. Chem., 1963, 2, 163. (PAPER 3/2241)

ISSN:0300-9599    

DOI:10.1039/F19848002935

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. 1, 1984, 80, 2943-2956 Iodide Islands in Iodine The Electrical Properties of Iodides in Liquid Iodine BY JAN LUDWIG,? KWS DUPPEN AND JAN KOMMANDEUR* Laboratory for Physical Chemistry, University of Groningen, Nijenborgh 16, 9747 AG Groningen, The Netherlands Received 5th January, 1984 Liquid iodine is a good solvent for potassium and tetrabutylammonium iodide and for organic molecules of low ionization potential, such as p-phenylenediamine. Their dissolution leads to a considerable conductivity, mainly (95%) carried by the iodide ions. The dependence of the equivalent (per particle) conductivity from 0 to 10 mol % can be described quantitatively by assuming chemical equilibrium between associated and dissociated ion pairs. At infinite dilution the ions are dissociated, at finite concentrations considerable association occurs, but at higher concentrations (> 1 % ) redissociation takes place, because ail ions are then in the (Debye-Huckel) field of all other ions.Thermoelectric measurements show that the iodide species should at low and intermediate concentrations be described as an electron delocalized over ca. 100 iodine molecules. These ‘ iodide islands’ move by density fluctuations, which accounts for the very small temperature dependence of the conductivity. At concentrations < 1% the solution becomes ‘full’, and on increasing the concentration further the islands shrink in size until only four iodine molecules participate in island formation and the solubility limit is reached. The liquid range of elemental iodine at atmospheric pressure extends from 113.6 to 184 “C.It is curious that up to now very little attention has been paid to this liquid, particularly in view of an early report by Lewis and Wheeler,l who found that solutions of KI in liquid I, showed considerable electrical conductivity with a metal-like (i.e. negative) temperature dependence ! These results were later substantiated by Russian workers,, who also studied I, solutions of other alkali-metal iodides. Steacie and Johnson3 noticed the high conductivity of iodide +iodine solutions and ascribed it to an unusually high mobility. Rabinowitsch4 reported that even pure liquid iodine showed the ‘metallic behaviour ’ observed for the iodide + iodine solutions. Most of these phenomena were reviewed by J a n d e ~ .~ and at the time we started our work no clear understanding of the conductivity mechanism of iodine + iodide solutions appeared to exist, although there were suggestions by Gorenbein et aL99 lo that the high conductivity of iodine + iodide solutions was due to the ‘relay mechanism’. Our interest arose from earlier work on the electrical properties of doped single crystals of iodine,ll? l2 where it was found that I; was responsible for the conductivity by a Grotthus mechanism. An e.s.r. investigation of various organic dopants showed that compounds with a low ionization potential were partially ionized in these crystals, and it appeared that complete ionization was prevented by the lack of free volume In more recent years, only incidental reports have t Present address: College of Education Ubbo Emmius, Grouwelerie 6, 9747 AK Groningen, The Netherlands.29432944 IODIDE ISLANDS IN IODINE needed for the accommodation of the large I; ions, which are the first products of the process. It appeared logical then to provide free volume by melting the crystals, and indeed such molecules as p-phenylenediamine (PPD) do ionize completely, as shown by the intensity of the e.s.r. spectrum of PPD+. The I; ions formed will react with the excess of iodine to form polyiodide ions, and it thus seemed obvious that the accommodation of electrons in the melt could also be brought about by simply adding KI or other iodides. This paper reports an extensive study of the electrical properties of solutions of KI, PPD and TBAI (tetrabutylammonium iodide) in liquid iodine, substances chosen because of the increase in their cationic radii, which will be shown to have a substantial effect on the electrical properties.Following a short review of the experimental aspects we give the results and their interpretation. It will become clear that the iodide ion in liquid I, is almost as mobile as the proton in water and that it constitutes an intermediate between metallic and ionic conductivity; as a side benefit we found that the Debye-Huckel theory for ionic solutions can be extended to much higher concentrations in as apolar a solvent as liquid iodine. EXPERIMENTAL PURITY AND STABILITY OF THE SOLUTIONS In contrast to what might be expected, liquid iodine constitutes a fairly inert substance which is a good solvent for many organic materials.Solutions of the rather reactive cation PPD+ in liquid iodine are stable for several days at temperatures of ca. 120 "C, as shown by the invariance of the intensity of its e.s.r. spectrum. The same holds for other properties of this solution. Because it is such a good solvent, liquid iodine might leach out ions from glass. We therefore first established whether this effect could influence our measurements. Electrolysis for ca. 1 h reduced the conductivity of sublimed and then molten iodine by about a factor of five. The conductivity of 3 x f2-l cm-l thus reached was sufficiently low as to have no effect on our measurements. As reported previously'? the temperature dependence of this background conductivity is still negative (-0.46f0.02% K-l), as it is at higher levels of doping.It seems safe to assume that the same mechanism is operative at both low and high concentrations, the conductivity still being produced by iodide ions. Earlier suggestions6 that the conductivity is produced by the reaction I, + I+ + I- would appear to be ruled out by the negative temperature dependence. Because of the very limited conductivity, if self-ionization were to take place it would have to happen to a very small extent. The equilibrium would then have to have a large positive molar enthalpy, the partial molar entropy being large in any case for a dissociation. The number of ionic species would then have a strong positive temperature dependence, and it seems extremely unlikely that this exponential dependence would be compensated by such a strong negative temperature dependence of the mobility so as to lead to a total negative temperature dependence for the overall conductivity. For the purposes of this work it is safe to assume that the background conductivity is due to ionic species that cannot be removed by repeated sublimation or electrolysis, such as protons.The impurity concentration inferred from the extrapolated equivalent conductivity at infinite dilution is 0.75 ppm, which is a factor of five lower than in the same iodine used in our single-crystal work," in which the impurity concentration was arrived at by very different means. In any case this background conductivity can be neglected completely in comparison with the conductivities which we discuss below.An electrode material which was unaffected by the iodine was found to be tungsten, showing no weight increase or loss, even after prolonged usage. This was not so for platinum, but a 10% iridium-platinum alloy could also be used. As vessel material we usually used glass; experiments with quartz vessels gave identical results.J. LUDWIG, K. DUPPEN AND J. KOMMANDEUR 2945 CONDUCTIVITY, DIELECTRIC CONSTANT, DENSITY AND VISCOSITY Conductivity measurements were carried out in a conventional electrolytic cell. Even though no irreversibility of the electrode arrangement was noted, we measured a.c. conductivities with a Wayne-Kerr universal bridge, type B224, operating at a frequency of 1593 Hz with an accuracy of ca. 0.2%. The cells were suspended in a silicone oil bath held at constant temperature.The accuracy of the temperature measurements was ca. 0.2 "C. The resulting conductivities were corrected for the density change upon dissolution of the solute. The densities were measured by the buoyancy method, in which a calibrated sinker is weighed after submersion in the iodine melt. The results for solutions of KI, PPD and TBAI in iodine are given in fig. 1. Good agreement was obtained with Saito's value7 for pure iodine of 3.784 g at 150 "C, ours being 3.796 g cm-3 at the same temperature. The dielectric constant was measured with two parallel Pt (+ 10% Ir) plates inserted into liquid iodine. Owing to the density variation of liquid iodine (ap/dT z 0.0035) and the dependence of E on the density via the Clausius relation (&lap z 3.9, the dielectric constant was found to depend slightly on temperature, E = 13.3 -0.016 (kO.002) T, yielding a value of 7 at 120 "C.Jagielski13 reported a value of E x 11 with a positive temperature dependence, but we feel sure that the system we measured was of higher purity. The dielectric constant depends only slightly on frequency between lo4 and lo7 Hz. Viscosities were measured with a slightly modified glass viscometer as used by Steacie and J o h n s ~ n , ~ which allowed electrical determination of the run-out time. The apparatus was calibrated with water over a range of 35 "C. The cell constant, which changed by only 2% over this range, was then extrapolated to temperatures between 120 and 150 "C, where viscosities were determined at 10 "C intervals.With a value for qo of 1.8 cP* at 120 "C, the results for KI in iodine could be fitted to the relation q/qo- 1 = 0.38c, where c is the concentration in mol dm-3. Via the Jones-Dole and Einstein equations the volume of the solvated ion could be inferred to be 149 & 5 cm3 mol-l, or 248 A3 per dissolved molecule or spherical particle with a radius of 3.9 A. The temperature dependence of the viscosity followed the Andrade equation 17 = A exp E,/kT, with E, = 0.12f0.03 eV. 6 3.60 $ 1 3.L 0 \ t 10' 1 cfrnol dm-3 10 Fig. 1. Density of the solutions at 120 "C plotted as a function of concentration: 0, KI; A, PPD; 0, TBAI; (---) pure liquid. * 1 P = lo-' Pa s. 96 PAR 12946 IODIDE ISLANDS IN IODINE TRANSFERENCE MEASUREMENTS 'l'ransference measurements were also undertaken.These could conveniently be carried out in the case of PPD+, where the increase or decrease of that ion could be followed using an e.s.r. spectrometer (Varian E-3). For this purpose one half of a Hittorf-type cell was inserted into the e.s.r. cavity. Much higher accuracies were obtained when previously determined specific conductivites were used to determine the increase or decrease in concentration in each half of the Hittorf cell. To this end two auxiliary electrodes were brought into each half, and the conductivities were measured as a function of time by the a.c. method. Comparison with the previously determined concentration dependence of the specific conductivity immediately yields the transport numbers for PPD', in good agreement with the e.s.r.result. For the cations we found t+(K+) = 0.06; t+(PPD+) = 0.07; t+(TBA+) = 0.04 all accurate to within ca. 0.01 and essentially independent ofconcentration. It can be concluded that the current is largely (i.e. for 95%) carried by the negative ions. whatever their nature. THERMOELECTRIC MEASUREMENTS Thermoelectric measurements were carried out in a quartz tube with platinum-iridium electrodes. The temperature gradient could be applied very quickly by means of a Leeds and Northrup Speedomax H-PID regulator. The mean temperature, T,, the gradient, AT, and the Seebeck voltage, A V, (after d.c. amplification), were registered on a Kipp four-channel recorder, together with a reference voltage. The immediate and full response of A & to A T assured the absence of thermal diffusion (the Soret effect) or convection. The measurements were carried out at various concentrations of donors and temperatures.A typical plot of AV, against A T is shown in fig. 2, the value of the Seebeck coefficient being determined from the slope. Within an accuracy of ca. 3% it was found to be independent of temperature for all solutions between 115 and 170 "C. ATlK Fig. 2. Typical plot of Seebeck voltage against applied temperature difference. a = -295 pV K-I.J. LUDWIG, K. DUPPEN AND J. KOMMANDEUR 2947 RESULTS THE NATURE OF THE NEGATIVE CHARGE CARRIER Although pure liquid iodine appears to be an insulator, the conductivities of iodide + iodine solutions are substantial, particularly at concentrations > 1 mol dm-3. Therefore in fig.3 we display the equivalent conductivity, i.e. A = a/c, as a function of log c, where a is the specific conductivity and c the concentration of added iodide. All iodides show the same behaviour: after a considerable decrease, at ca. 10-2-10-1 mol dmV3 the equivalent conductivity again increases, reaching a value almost equal to the extrapolated value at c = 0. After that it decreases sharply again. v ti3 ?I2 1d 1 O - L 10 c/mol dm-' Fig. 3. Equivalent conductivity (a/c) at 125 "C plotted against the logarithm of the concentration for three different iodides: 0, KI; A, PPD; 0, TBAI. The latter appears to be a viscosity effect. In fig. 4 the product 1111 (where 11 is the viscosity) is shown as a function of the concentration for a KI+I, solution.This product becomes constant at ca. 2 mol dm-3, indicating that Walden's rule is obtained. The results for the thermoelectric measurements as a function of concentration (now given in mol%) are shown in fig. 5 . The Seebeck coefficient at first increases linearly with the logarithm of the concentration, but at ca. 1% it slowly changes slope. In general the Seebeck coefficient is given by a = S*/q, where S* is the entropy of transport (per particle) in the medium and q is the value of the charge carried. A logarithmic concentration dependence indicates some sort of distributive entropy. Remembering that 95% of the current in iodide+iodine solutions is carried by the negative species, it would seem that it is their distributive entropy which is responsible for the thermoelectric effect.If this charge carrier needs rn iodine molecules for its 96-22948 -LOO IODIDE ISLANDS IN IODINE - I I I I I I I I I 0 I I I 0 1.0 2 .o 3.0 clmol dm-3 Fig. 4. Walden product Av plotted against the concentration for KI+I, at 125 "C. / / / / ''Or / / Fig. 5. Seebeck coefficient plotted against the logarithm of the concentration. Slope 195 ,uV K-I. accommodation, there are N / m sites for the n carriers in the system, where N is the total number of iodine molecules. The distributive entropy then is S = k l n W with w = (N;m). For the thermoelectric effect we need the partial molar entropy S* = (dS/dn),, T, = k In [(Nlm - n ) / n ] where N is Avogadro's number. With the concentration x = n/N (in mol per mol) a = -k/e In ( l l m x - 1) this yields for aJ. LUDWIG, K.DUPPEN AND J . KOMMANDEUR 100 I s g m - 10 2949 - - I \ 1 ‘ I I , I \ \ , ‘ lo-& lo- 10’ 1 mole fraction/mol per mol Fig. 6. Logarithm of the accommodation coefficient plotted against the logarithm of the concentration. (a) Close packing, rn = n/32/2x; (b) solubility limit. which for very low concentrations (x < l/rn) reduces to a z k/e In mx, predicting a slope of a against log,, x of k/e = 198.4 pV K-l. This is in good agreement with the value of 195 pV K-l as found from our experiments depicted in fig. 5 . At 1 mol % the value of a reaches zero, indicating a value of rn of ca. 100. It appears that the negative charge carrier needs at least 100 molecules of iodine for its accommodation! the logarithmic term in a then diverges and something has to happen.Within the model, the only parameter that can change is rn, according to rn = l / x (1 +exp-ea/k)-l. Fig. 6 shows the variation of rn with concentration. As pointed out above, at low concentrations rn = 100, starting to decrease at x = 0.01 and reaching a value of ca. 4 at x = 0.1, the solubility limit of KI in I,. A reasonable interpretation appears to be the following. In an infinitely dilute system the charge carrier occupies 100 iodine molecules. Around a concentration of 1 % the solution is ‘full’, and as more carriers are added to the solvent the solution remains full, but the size of the ‘islands’ on which the carriers are accommodated decreases until a further decrease is energetically impossible and the solubility limit is reached.Fig. 6 also gives a further illustration of this phenomenon. In the range x = 0.01-0.1 the solution can be considered as a close packing of spheres. A sphere of radius r will contain rn = #nr3/Vm iodine molecules, where Y is the radius of the sphere and V, the molecular volume of iodine. In hexagonal close packing a sphere needs on average 42/2r3 space. Therefore, in a molar volume we can pack N Vm/42/2r3 spheres, which equals the number concentration, if we assume the spheres to fill all space. Then, NV,/4d2r3 = Nx. Substituting V, from the previous equation we find n2 = 4n2-8x-1 . This line has been drawn in fig. 6, and it is seen that our results for rn clearly approach this limit. We can conclude that the mutual repulsion of the ‘islands’ determines their size, until they become too small at rn = 4.The temperature dependence of the conductivity was also measured at various concentrations. It is very small. A nearly linear relation between ts and T was always found. For KI +I, the conductivity slowly decreased with increasing temperature, for PPD + I, it was almost constant, while for TBAI +I, it slowly increased. Fig. 7 gives a plot of (1 /a) da/d T - (1 / p ) dp/dT against log c for the various solutions, the term Clearly at x z2950 IODIDE ISLANDS IN IODINE Fig. 7. Relative temperature dependence of the conductivity, corrected for the density variation, plotted against the logarithm of the concentration at 125 "C. Symbols as fig. 3. (l/p) dp/dTbeing present to correct for the trivial change due to the density variation.The high-concentration rise in the conductivity is obviously due to the temperature dependence of the viscosity, which yields a value of ca. 1 % K-l, which all three curves approach. MECHANISM OF THE CONDUCTIVITY The iodide ion in liquid iodine is an extremely fast ion. From its transference number (0.95) and Ao, the equivalent conductivity at very low concentration (Ao = 210 cm2 R-l mol-l), a value for the mobility of p- M (2.07k0.2) x 10-3cm2 V-l s-l is found, which is comparable to that of the proton (,u = 3.5 x in water. Since the cm2 V-l s-l ) the high value cannot be due to a low local viscosity of liquid iodine; rather we must consider a Grotthus mechanism (as for the proton) or an ' iodinated' electron (as for the hydrated electron).The iodine melt is highly structured, as shown by its extended radial distribution function.l* Also, a large number of polyiodides are known15 where one electron is responsible for holding many iodine nuclei together. An iodide ion in an iodine melt will therefore share its electron with many iodine molecules. Our thermoelectric measurements indicate that at low concentrations this number is ca. 100, which amounts to a sphere of ca. 13 A in radius. The negative charge carrier in an iodine + iodide melt should therefore probably be viewed as an electron accommodated on an island, i.e. a structured part of the melt with a diameter of ca. 26 A. If it is to stay on this island, it will move with it. The island will fluctuate in magnitude and position because of 'phonons' in the liquid, and the conductivity will follow the fluctuation in position.At present it is unknown how fast these 'structures' move in liquid iodine, but the similarity of the mobilities of I- in I, and e- in H20, whose mobility certainly depends on fluctuations in the water structure, strongly points in that direction. or the hydrated electron (p = 1.98 x mobility of the positive counterions seems to be normal (p: M 1 xJ. LUDWIG, K. DUPPEN AND J. KOMMANDEUR 295 1 There is an alternative manner of describing this process. As an electron enters an iodine melt it will try to become delocalized to lower its kinetic energy. (Iodine is close to being a metal: even fairly low pressure turns it into one.) Delocalization requires a periodic potential and this is offered by the structured regions of the melt.One therefore expects the electrons to end up in these structured parts, where they perform their wave-like motion as in a metal. These waves, however, will be scattered where the correlation is lost, where irregularities occur or, in our previous description, at the edge of the island. The electron will be scattered, preferably into other structured regions, since these are closest in energy. Since they should also be spatially close, the electron will usually scatter in the structured region, i.e. on the island and, as the latter moves, move with it. It will be clear that the two descriptions are entirely equivalent, if not the same. The electron itself will contribute to the formation of structure; it will tend to orient I, molecules favourably to attain its delocalization, since that lowers the energy of the whole system.However, this will cost some orientational entropy and the balance of these two effects will determine the magnitude of the fluctuating structured region. In this language the charged carrier can be called a polaron, since it is responsible for creating the nuclear structure with which it moves. These views of the negative charge carrier in liquid iodine permit a general understanding of the temperature dependence of its motion. Since it is entirely dominated by phonon scattering and not by nuclear transport as for normal ions in solution, we expect a very minor temperature dependence, as was indeed obserued. THE CONCENTRATION DEPENDENCE So far we have discussed the low-concentration limit.The concentration dependence of A, however, clearly shows association to occur at higher concentrations, as should be expected for low-dielectric-constant solvents. Chemically, we can symbolize this association by M+ +I; e M+I; where M+ stands for the positive ion involved and rn denotes the size of the island discussed above. To calculate the conductivity we should realize that there is no contribution from the associated species. We shall therefore take it to be proportional to the concentration of the dissociated species, and since almost all current is carried by the 1, ion, in particular to its concentration. We can now apply standard ionic theory to calculate the concentration of I; as a function of the concentration of dopant (say KI) added.Neglecting solution-entropy changes, the equilibrium constant for the dissociation can be written as where y is the degree of dissociation, c the concentration (in mol dm-3), a, the (average) distance in the associated ion pair and hd and ha the molar enthalpies of the dissociated and the associated ions.16 For ha we can then write for a system of singly charged ions Ne2 ~ X E , &a, + h, ha = -~ where E is the dielectric constant of liquid iodine and h, is the molar enthalpy of the positive and negative ions in liquid iodine, apart from their mutual Coulomb interaction.2952 IODIDE ISLANDS IN IODINE For the electrostatic part of the molar enthalpy of the dissociated species we should realize that the ions are still in a potential, i.e. the potential due to all the other ions.When the system is very dilute this potential will, of course, tend to zero, but as the system becomes more concentrated it increases in importance until at high concen- tration it outweighs the pair potential of the associated species. We may therefore expect that the association caused by the increase of concentration will be overcome at high concentrations, and our system will first associate and then dissociate again. Quantitatively then we can use Debye-Hiickel theory,17 which gives for the average potential at a distance r from any central ion in a neutral solution of other ions: e exp[-b(r-a)] 4(r) = ~ 4m,~r 1 + ba the 1 / r part of the potential clearly being shielded with the characteristic Debye length b-' = (e2 Z ni/~,~k7')-lI2 i in which n, is the number of ions i per unit volume and a is a phenomenological parameter, denoting the distance from the central ion over which the potential should be considered constant or the hard-sphere diameter of the ions.The potential due to the central ion is e/4ne0 Er; the potential solely due to the ion atmosphere is therefore The ion-atmosphere potential at the central ion (r = a) is then given by To calculate the electrostatic energy e(a) of the central ion due to the ionic atmosphere we have to place a charge q at that site and let it increase to the magnitude q = e e2 b 8 ~ ~ 0 E 1 + ba * dq = - -bq Joe 4mo E( 1 + ba) e(a) = For c mole of added salt we then have E(a) = cNe(a), and the partial molar electrostatic enthalpy then becomes e2 b'~3I2 h , d ( - N - ac 8 m , E 1 + ab'c1I2 where b' = bc-lI2, c = (2N)-lX ni and i Ne2 b 1 he = -~ 2+- 47t.5,~ 4(1 -tab)( 1 +ab).For the partial molar enthalpy of the dissociated ionic species we then have h, = he + h,, where we assume that the only contribution to the enthalpy change upon dissociation is due to the mutual electrostatic interaction of the ions. The dissociation constant then becomes, with k = R / N , Y 2 3 x 10-3 e2 K&) = -c = ~ 1 - y 4nNa:J. LUDWIG, K. DUPPEN AND J . KOMMANDEUR 2953 where 1 2+- b = 4(1 +ab)( 1 +oh) where p = p + + p - , the sum of the positive and negative ion mobilities. It will be clear that the exponential will, through the term l / a , , depend on concentration.At low concentration it is small, and the equilibrium is then determined by the energy of association, so that we expect normal associative behaviour. For c + 0 the dissociation is complete ( y z 1). As the concentration increases y will decrease, the number of free ions contributing to the equivalent conductivity will decrease, and we expect A to decrease, as found in experiment. At still higher concentration l/al becomes larger, and therefore y becomes larger again. Depending on the values of a, and a,, y may reach unity and A will then rise to its value at c = 0, as is observed in most experiments. It remains to fit the parameters a, and a to the experimental results. At this point it is probably worthwhile to point out that our result is reminiscent of the calculations of Mott,l* who showed that for electrons and holes in a solid at T = 0 K there is no binding energy for qa, > 1, where q is the ‘shielding’ parameter (cJ b in our calculation) and aH is the Bohr radius of the electron and the hole.Requiring in our calculation the enthalpy ofdissociation to be zero yields a, = a,, and neglecting a gives ba, = 4/3, similar to Mott’s result. We measured conductivity, and if the Debye length is used in the expression for the energy one should investigate whether the electrostatic interaction has an effect on the transport. In ionic solutions one normally considers the following two effects. THE RELAXATION OR ASYMMETRY EFFECT If a positive ion is displaced, the negative ions around it will in general lag behind, leading to a reduction in the applied electric field.This manifests itself as a reduction in the mobility, depending on the concentration. However, if one of the ions is very fast and the other slow the ion atmosphere will always be relaxed, no such lag will occur and no opposing field will be exerted. With the large differences of mobilities in our system we can therefore neglect this effect. THE ELECTROPHORESIS EFFECT If an ion is displaced it takes its solvation sphere with it. This creates a viscous flow into which the other ions have to move. Because of the probable nature of the negative charge carrier in our system (an electron delocalized on a molecular cluster, which moves by fluctuations) no such viscous flow will occur, and since the negative carriers are responsible for 95% of the conductivity we may neglect the electrophoresis effect as well.However, there is at high concentrations a viscosity effect. As shown above, this range obeys Walden’s rule, i.e. the macroscopic viscosity determines the conductivity completely. Phenomenologically, this can be taken into account by writing A0 A = 1 +0.38c2954 IODIDE ISLANDS IN IODINE where A, is the equivalent conductivity at infinite dilution and the term (1 +0.38c) is obtained from the experimentally determined concentration dependence of the viscosity. It is only of importance at the highest concentrations. Substituting the expression for y into the equation for A we find This expression, with A,, a, and a as parameters, can be fitted to the experimental results. We expect A, to be independent of the cation, a,, to be of the order of the sum of the van der Waals radii of the cation and I-, and a to be of the order of an ionic radius or smaller.DISCUSSION As is shown in fig. 8 a surprisingly good fit to the equivalent conductivity of KI in I, is obtained for the parameters a, = 2.33 A and a = 0.23 A, using E = 13.3, A, = 210 cm2 R-l mol-l and the viscosity constant 0.38 as determined by ourselves. Fig. 8 shows fits to the equivalent conductivities of PPD in I, and TBAI in I, for values of a, = 2.41 and 2.95 A and a = 0.28 and 0.05 A, respectively. Since no viscosity measurements were available for these compounds the viscosity constant was also taken as a parameter, yielding 0.65 and 0.9 1, respectively. These only affect the few measurements at the highest concentrations.Our choice of E = 13.3, the dielectric constant extrapolated to T = 0 K, was dictated by the fit we required. Considerably lower values would not yield the desired result. It may be that higher-order polarizabilities play a role or that considerable electrostriction occurs, removing the 'free space' between the molecules and therefore increasing the actual value of E and making it equal to E at T = 0 K. The values of a, seem reasonable. At least they are 160 120 - I - E c 80 " I PI 5 --. < LO I I I I 1 -4 - 3 - 2 1 1 01 10 10 10 10 1 10 c/mol dm-3 Fig. 8. Calculated (solid line) and measured (points) equivalent conductivity plotted as a function of the logarithm of the concentration for KI (a), PPD (A) and TBAI (m) at 125 "C.The parameters used for the fits were as follows. KI: a, = 2.33 A, a = 0.23 A; PPD: a, = 2.41 A; a = 0.28 A; TBAI: a, = 2.95 A, a = 0.05 A. For all solutions the equivalent conductivity at infinite dilution was taken as A. = 210 cm2 W1 mol-'.J. LUDWIG, K. DUPPEN AND J. KOMMANDEUR 2955 Table 1. Occurrence of redissociation in iodine +iodine mixtures solute redissociation reference KI, TBAI, PPD RbI CsI, TMPD,a TMAI,b ferrocene Te “i(NH3)JIz [Z~(NH~)~IIZ, [Cd(NH3)4II, LiI, NaI InI,, GaI,, SbI, a Tetramethyl-p-phenylenediamine ; tetramethylammonium iodide. in the order one would expect for the ionic radii a,(KI) < a,(PPD) < a,(TBAI). The fact that a,(KI) is less than the sum of the K+ and I- radii (1.3+2.2 = 3.5 A) may well point to the delocalization of the electron on the I, molecules around the K+, which might allow a closer approach of the electron to the positive ion than would simply be given by the sum of these ionic radii.The ‘hard-sphere diameter’ a appears to be very small, if not zero. There appears to be no such quantity, and this can again be understood if the electron is delocalized. However, a is basically an ad hoc parameter, introduced by Debye to prevent the divergence of the Coulomb integral at r = 0. It may be without much physical significance in these systems. Introduction of chemical equilibrium leads to a temperature dependence of the degree of dissociation. Given the parameters this could be numerically investigated. Calculations showed that this effect leads to a temperature dependence of ca.1 % K-l in the concentration range below 1 % , which is of the same order of magnitude as was found experimentally. However, it can also be shown that the probable temperature dependence of the (albeit small) contribution of the slow cationic species also gives a similar order of magnitude. Therefore, no further conclusions about the temperature dependence of the mobility of the anionic species could be drawn. It is clear, however, that the temperature dependence of the association equilibrium in itself does not dominate the experimental observations. At this stage it is expendient to consider the question of whether the established association - redissociation behaviour is typical. Table 1 summarizes a number of other solutes for which the dependence of A on the concentration has been measured.In general, redissociation occurs. There are, however, a few exceptions, which may be associated with the small ionic radii of these ions. As mentioned in the introduction, in these solutions one can carry the Debye-Huckel theory apparently up to the unusually high concentration of 1 mol dm-3, as shown by the excellence of our fits to experiment. This probably originates in the molecular nature of the iodine melt. CONCLUSIONS The iodide ion in liquid iodine is a very fast ion with a mobility of 2.07 x cm2 V-l s-l. The electron is probably delocalized over ca. 100 molecules if the concentration is sufficiently low. The ‘ions’ then move by density fluctuations2956 IODIDE ISLANDS IN IODINE (phonons) and therefore their mobility is practically independent of temperature.When the concentration rises above 1 % the solution becomes full and the delocalization decreases until, when < 4 molecules are available to the electron, the solubility limit is reached. The concentration dependence of the conductivity can be understood by taking into account ion association at low concentrations and ion redissociation at higher concentrations; it can even be described quantitatively by Debye-Huckel theory for reasonable values of the interionic distance. We thank Mr B. Van Meurs for help with some of the computations. G. N. Lewis, and P. Wheeler Z. Phys. Chem., 1906, 56, 179. E. W. R. Steacie and F. M. G. Johnson, J. Am. Chem. Soc., 1925,47, 754. M. Rabinowitsch, Z. Phys. Chem., 1926, 119, 79. G. Jander Die Chemie in wasserahnlichen Losungsmitteln (Springer Verlag, Berlin, 1949, pp. 192-209. D. J. Bearcroft and N. H. Nachtrie, J. Phys. Chem., 1967,71, 316; 4400. K. Saito, K. Ichikawa and M. Shimoji, Bull. Chem. SOC. Jpn, 1968, 41, 1104. * K. Ichikawa, T. Okubo and M. Shimoji, Trans. Faraduy Soc., 1971, 67, 1426. E. Ya Gorenbein, 0. K. Kudra and A. E. Gorenbein, Zh. Fiz. Khim., 1972,46, 996. lo A. E. Gorenbein and E. Ya Gorenbein, Zh. Fiz. Khzm., 1975,49, 371. I i D. Bargeman and J. Kommandeur, J. Chem. Phys., 1968, 49, 4069. J. Ludwig and J. Kommandeur, J. Chem. Phys., 1970,52, 2302. l3 A. Jagielski, Bull. Int. Acad. Pol. Sci. Lett., Ser. A , 1932, 327. l4 P. Bosi, F. Cilloco and M. A. Ricci, Mol. Phys., 1980, 40, 1285. l5 E. H. Wiebenga E. E. Havinga and K. H. Boswijk, Adv. Inorg. Chem. Radiochem., 1961, 3, 133. l6 N. Bjerrum, K. Dan. Vidensk. Selsk. Mat. Fys. Medd, vrr, 1926, 9. l7 See for instance R. S. Berry, S. A. Rice and J. Ross, Physical Chemistry (John Wiley, New York, * W. A. Plotnikow, J. A. Fialkow and W. P. Tschalij, Z. Phys. Chem., Teil A , 1935, 172, 304. 1980), p. 997. See for instance N. F. Mott and Z. Zinamon, Rep. Prog. Phys., 1970, 33, 905. (a) Ya A. Fialkov and F. D. Shevchenko, J. Gen. Chem., 1950, 20, 1413; (b) Ya A. Fialkov and F. D. Shevchenko, J. Gen. Chem., 1952, 22, 1143. (PAPER 4/025)

ISSN:0300-9599    

DOI:10.1039/F19848002943

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. I , 1984, 80, 2957-2968 The Defect Structure of Nickel Oxide Surfaces as Revealed by Photoelectron Spectroscopy BY M. WYN ROBERTS* AND ROGER ST C. SMART? Department of Chemistry, University College, P.O. Box 78, Cardiff CFl 1XL Received 10th January, 1984 Nickel oxides characterised by electron microscopy and with different defect concentrations have been studied by X-ray photoelectron spectroscopy. Oxides preannealed in air at 700, 1 100 and 1450 "C were examined after heating in uacuo in the spectrometer between 20 and 500 "C. High-binding-energy components of O( 1s) spectra at 531.4 eV and Ni(2p3,,) spectra at 856.1 eV can be correlated with the oxide defect structure. Loss of electron-acceptor surface hydroxyl groups as water during evacuation and heating results in the development of surface charge owing to increased band bending at the surface.Above 300 "C, carbon impurity present at < 0.15 of a monolayer is removed predominantly as CO. With the defective ' 700 "C annealed' nickel oxide this results in a major increase in the intensity of the O(ls) peak at 531.4 eV, but the other two samples show a continuous decrease in this peak's intensity. The O(ls) peak at 53 1.4 eV is attributed initially to OH, and after evacuation at 500 "C to an 0- surface species. O/Ni atom ratios > 1.0 are found on all surfaces, even after evacuation at 500 "C, but higher values, up to 2.5, are found after evacuation at 25 "C. Spectra from a NiO(100) single-crystal surface evacuated at 600°C and heated in oxygen at 450 "C show enhancement of the high-binding-energy O( 1s) and Ni(2p3,?) peaks at low electron take-off angles, indicating stabilisation of defect 0- and Ni3+ species at the solid/vacuum interface.The electronic structure of semiconducting oxides is of fundamental importance in determining the surface properties that influence reactions with species from the gas phase1 and from solution.2 The details of defect structure and charge transport have been investigated for over 25 years, but some problems related to these properties have yet to be res01ved.~-~ Surface reactions, including catalytic and chemisorptive properties, have been shown to depend strongly on the pretreatment of the oxide because of differences in the stoichiometry6. and surface perfection.89 An under- standing of the chemistry of these surface processes must depend on defining, in some detail, the electronic configurations of metal and oxygen species in the surface region.Thus it has been suggested in much of the catalytic w0rk~7~O and in studies of dissolution kineticss. that the presence of Ni3+ ions, and of their associated modified oxygen species, is responsible for the reactivity of nickel oxide surfaces. This contention is strongly supported by recent theoretical work11-13 which shows that the presence of low-coordination sites at steps, ledges, cavities etc. or compositional imperfections is critical in adsorption and catalytic processes. oxi- dised n i ~ k e l l * - ~ ~ and nickel hydroxide1* have been concerned with the identification of species having high-binding-energy components in the O( 1s) spectra at 53 1.4 eV and in the N i ( 2 ~ ~ , ~ ) spectra at 856.1 eV.This O(1s) species has been variously X-ray photoelectron spectroscopic (X.P.S.) investigations of nickel t Permanent address: School of Science, Griffith University, Nathan, Queensland 41 1 1 , Australia. 29572958 DEFECT STRUCTURE OF NICKEL OXIDE SURFACES attributed to OH and 0- species and to a Ni203 gross defect structure. The N i ( 2 ~ ~ / ~ ) peak at 856.1 eV has been assigned to Ni3+, and Norton et al.24a suggested a possible correlation between the O( 1s) peak at a binding energy of 53 1.4 eV and the formation of Ni3+ ions in oxide overlayers at nickel surfaces. A more complete discussion of the evidence is given below.On the other hand Srinivason et al.24b conclude that Ni20, does not form at an oxidised nickel surface and that there is no evidence for Ni3+. These authors assign the N i ( 2 ~ ~ / ~ ) components at 852.4 and 854.2 eV to NiO and Ni2+ species, respectively . In our work we have been interested in the magnitude of the surface potential as a means of monitoring changes in the electronic structure in the surface of the p-type nickel oxide. The magnitude of the surface potential after evacuation of the sample in the spectrometer at 500 "C has been correlated with the defect concentration in the oxide.ls* l7 The effect of adsorption of electron-acceptor and electron-donor molecules on the free-hole concentration at the semiconductor surface is reflected by changes in the surface potential monitored through the kinetic-energy shifts of core-level peaks.Thus nitric oxide17 and oxygen2s adsorption, as NO- and 02- species (accumulative), substantially reduces the surface potential, whereas CO adsorption,17 as C08+ (depletive), increases surface potential on all NiO surfaces. An e~planation~~ has been given of the dependence of the surface potential on band bending ( Vs) and values of Vs calculated. In this paper we examine several questions relevant to the understanding of the electronic structure of the surface of nickel oxide. First, whether there is a correlation of the high-binding-energy O(ls) and Ni(2p3/,) peaks with each other and with the defect structure arising from different pretreatments and oxide preparation. The implication of such a correlation is that there is present a very high defect concentration (up to 50 atomic% ) in the surface,26 where bulk defect concentrations up to only 0.02% are expected even for the more defective ' 700 "C annealed ' oxide.6 An excess of oxygen is also found in all NiO samples with O/Ni atomic ratios up to 2.4.Secondly, we will attempt to define the chemical nature of the O(ls) peak at 531.4 eV after different treatments in the spectrometer. We will see that the use of surface-potential changes in following electron transfer at the interface is particularly valuable. Finally, we conclude that 0- species are stabilised at the solid/vacuum interface of a NiO(100) crystal. EXPERIMENTAL SAMPLE PREPARATION The purity, surface area, surface morphology and defect properties of the polycrystalline nickel oxides have been discussed previously,a* transmission electron microscopy being useda to study the morphology of the NiO crystals.The particles from powders annealed in air at temperatures below ca. 900 "C are generally transparent to 100 keV electrons, but those resulting from the higher-temperature treatments are generally so thick that only at edges are internal details discernible. The majority of particles annealed in air at 700 "C are individual highly perfect single crystals, with typically a smoothly rounded equiaxed shape and mean diameters of ca. 200 nm. Some particles form part of a well sintered polycrystal, but most are either separate or sintered to adjacent crystals. There is evidence that the powder contains some large clusters of such crystals, sintered in this way, into an open-structured aggregate.The crystals resulting from annealing in air at 1100 and 1450 "C are far larger, are still approximately equiaxed and appear to have developed into large particles of only a few grains, which are typically 1 pm in diameter for the 1450 "C material. Both the 1100 and the 1450 "C crystals show some areas with steps and ledges which are not observed with NiO prepared at 700 "C. Preparation of the samples by heating the oxide for 4 h in air at 700 (NiO,oo), 1100 (NiOlloo) and 1450 "C (NiO1450) produces surfaces with no impurities detectable by X.P.S. other than carbon contamination as described previ0us1y.l~M. W. ROBERTS AND R. ST C. SMART 2959 The (100) orientation of the single-crystal NiO surface was verified by in situ (low-energy electron diffraction) (LEED) facilities.The LEED pattern was checked after each treatment. A perfect (100) pattern, with < 5% monolayer carbon contamination, could be obtained after AT+-ion bombardment (400 eV, 2.8 pA for 15 min) at 500 "C, followed by oxidation (3 x lo4 L 0,) at 300 OC.* PHOTOELECTRON-SPECTROSCOPIC STUDIES Spectra from polycrystalline powders were obtained using a Vacuum Generators model ESCA-3 spectrometer, fitted with a VG 4 7 mass spectrometer for use in monitoring the gas phase during evacuation and thermal treatment. Details of the spectrometer, sample preparation and mounting, base pressures and operating conditions can be found in previous publications.26i The NiO( 100) crystal was examined in a Vacuum Generators photoelectron spectrometer with 4-grid LEED facilitie~.,~ For both spectrometers, digital data were collected using a Digico Micro 16 V minicomputer, sampling the spectrometer output every 500 ps, and storing the average value every 0.5 s.Analysis of the data included30 both curve-fitting procedures, using Gaussian peaks, a least-squares fitting procedure and deconvolution, to remove line broadening due to the X-ray line source and the analyser slit width. The former procedure was used for comparison of intensities of species contributing to different peaks and for comparison of intensities of oxygen and nickel spectra. The latter procedure gives reliable information, normally only obtainable at very high resolution, when sensitivity is usually limiting, on the number of species contributing to a broad, complex envelope and on the correct binding energy of each species.The deconvolution, together with experience of a variety of charged and uncharged samples, allows values of kinetic energy and binding energy to be obtained with an accuracy better than f 0.2 eV. Binding energies were calculated, as previously discussed,16* l7 and are relative to uncharged carbon, C(1s) at 284.9 eV, present on the sample holder and uncharged Au (4f7,,) at 83.8 eV. RESULTS INFLUENCE OF VACUUM PRETREATMENT TEMPERATURE ON NiO,,,, NiO,,,, AND NiO,,,, Ni0700 Fig. 1 shows the O( 1s) and Ni(2p3,,) spectral regions of NiO,,, evacuated at (a) room temperature, (b) 125, (c) 250 and ( d ) 500 "C for 1-1.5 h to pressures below 1 x Pa in each case.In (6)-(d) the sample was cooled to room temperature, after evacuation at the specified temperature, before spectra were recorded. The C(ls) spectra in each case give C/O ratios (corrected for emission cross-sections and analyser energy response but neglecting escape-depth variation) as described in our previous work," decreasing from 0.15 for (a)-(c) to 0.07 for ( d ) . The method of determining the surface potential from the difference in kinetic energy between carbon contamination on the uncharged clip holding the sample to the probe (kinetic energy 1203.5 eV) and the sample surface carbon contamination has been discussed in detail.l67 l7 The potentials determined were (a) 0.0, (b) 0.2, (c) 1 .1 and ( d ) ca. 1.5 eV. The full width at half maximum (f.w.h.m.) of the C( 1s) spectra increases from 2.4 eV in (a) and (b) to 3.0 eV in (c) and 4.0 eV in (d). Consequently the position of the peak maximum is rather uncertain in the very broad, weak C(ls) spectrum from sample (d), and the value of surface charge lies between 1.0 and 2.0 eV. The O( 1s) spectra in fig. 1 also show shifts with increasing temperature in the kinetic energy of the main peak compared with that observed at 958.5 eV with the uncharged surface (a) at room temperature at increasing evacuation temperature. The sharp, intense peak in (a) and (b) corresponds to a binding energy of 529.7 eV. The second peak in (a) is sharp and at a binding energy of 531.5 eV, whilst in (b) it is a broad * 1 L (langmuir) = Torr s.2960 529.7 5 DEFECT STRUCTURE OF NICKEL OXIDE SURFACES 960 954 638 630 622 kinetic energy/eV kinetic energy/eV Fig.1. (A) O(1s) and (B) Ni(2p3,,) spectra (pass energy 20 eV, full-scale deflection, i.e. f.s.d., 3 x lo3 counts s-l) from NiO,,, evacuated for 1-1.5 h to < 1 x Pa at (a) room temperature, (b) 125, (c) 250 and (d) 500 "C. All spectra were recorded at room temperature after cooling. The spectra are displaced to lower kinetic energy for two reasons: (i) surface charging and (ii) change in the nature of the surface species, i.e. the generation of increased concentrations of 0- and Ni3+ from (a) to (d). Surface charges were calculated to be (a) 0.0, (b) 0.2, (c) 1.1 and (d) 1.5 eV. The absolute binding energies (854.6, 856.1 and 861.6 eV) are shown (4) on spectra (a) and ( d ) only.shoulder centred near 53 1.4 eV. In fig. 1 (c) and (d) deconvolution of the O( 1s) spectra gave peaks at binding energies of 529.7 and 53 1.4 eV after taking account of surface charge. Curve-fitting all four O( 1s) spectra gave the intensity ratios for the 53 1.4 and 529.7 eV peaks and f.w.h.m. values shown in table 1. There has been a considerable shift of intensity from the 529.7 eV peak into the 531.4 eV peak after evacuation at 500 "C. On the other hand, evacuation at temperatures up to 250 "C produces only a reduction in the 531.4 eV peak intensity. This is consistent with mass spectra recorded during evacuation. At temperatures up to 250 "C loss of water is observed with < 10% CO, and CO, whilst between 300 and 500 "C loss of CO predominates with, at 500 "C, H,O and CO, loss each ca.50% of the CO loss. It would appear that the reduction in intensity of the 531.4 eV peak at temperatures up to 250 "C is primarily a result of surface hydroxyls by dehydroxylation. Changes in the Ni(2p3/,) peaks (fig. 1) at binding energies 854.6 and 856.1 eV parallel the O( 1s) spectral changes. The high-binding-energy Ni(2p3,,) component in (a) may well be made up of two contributions, one at 856.1 eV and the other at 856.5 eV. As with the O( 1s) spectrum, the Ni(2p3,,) spectrum in (b) also shows a broadening of the high-binding-energy component. Spectrum (d) has the majority of its intensity in the 856 eV peak, similar to the predominance of the high-binding-energy O(ls) peak in this spectrum [fig.1 (A) spectrum (d)]. There is some evidence for NiO in the shoulder at 852.6 eV. Comparing the Ni(2p3,,) spectra in ( c ) and ( d ) it is clear that in (c) there is a kinetic-energy shift due to charging and a considerable broadening of the envelope of peaks, but the difference between (c) and ( d ) is mainly the result of a chemical shift towards the high- binding-energy components.M. W. ROBERTS AND R. ST C. SMART 296 1 Table 1. Data obtained from analysis of O(1s) spectra for NiO,,, 53 1.4 eV evacuation surface deconvolution intensity ~ temp./"C charge/eV peaks /eV f.w.h.m./eV 529.7 eV 0.80 0.72 0.50 ca. 3.0 1.3 1.8 2.8 3.0 529.7 20 0.0 (531.5 529.7 529.7 250 1.1 (531.4 500 1.5 125 0.2 (531.4 Table 2. O/Ni atomic ratios calculated from O( 1s) and Ni(2p,,,) intensities evacuation sample temp./"C O/Ni ratio NiOm 20 125 250 500 160 300 500 150 500 NiO,,",, 20 NiOidm 20 1.6 1.7 1.8 1.1 2.5 2.0 2.4 1.8 2.4 2.2 1.7 The surface corresponding to the spectrum in fig.1 ( d ) was bombarded with Ar+ ions at 400 eV and 1.5 pA for 10 min but still shows a predominance of the high-binding-energy components, although there was some shift (< 20%) to the O( 1s) 529.7 eV and Ni(2p3,,) 854.6 eV components. There was no apparent increase in the NiO shoulder at 852.6 eV. The defective layer is obviously relatively deep and involves most of the oxygen in the 'surface layer' (ie. within the escape depth of the photoelectrons). Spectra from other NiO,,, samples, similarly evacuated at increasing temperature, show similar qualititative changes, but after evacuation at 500 "C the intensity ratio of the 531.4 eV/529.7 eV peaks varies from 1.8 to 3.0.The shift of the O( 1s) spectrum to the high-binding-energy peak is always reflected in a similar shift of the Ni(2p3,,) intensity into the high-binding-energy 856.1 eV peak. Table 2 contains estimates of the ratio of oxygen to nickel atoms for each of the spectra (a)-(d). The estimates are based on intensities of the O( 1s) and Ni(2p3,,) peaks calculated using a corrected background that compensates for increasing scattering of high-kinetic energy e l e c t r o n ~ . ~ ~ The areas are then compared by correcting for differences in photoionisation cross-sections, using Scofield's data,32 escape depth (proportional to Eg8, where Ek is the kinetic energy) and the kinetic-energy response of the analyser (proportional to ,Ti1).This gives a 'correction factor' for2962 DEFECT STRUCTURE OF NICKEL OXIDE SURFACES 529.7 ( A ) 531.4 854.6 854.6 Q60 952 kinetic energy/eV 638 630 kinetic energy /e V Fig. 2. (A) O(1s) and (B) Ni(2p3,J spectra (pass energy 50 eV, f.s.d. 1 x lo4 counts s-l) from NiO,,,,, evacuated for 1-1.5 h to < 1 x lop6 Pa at (a) room temperature, (b) 160, (c) 300 and ( d ) 500 "C. All spectra were recorded at room temperature. Surface charges were calculated to be (a) 0.0, (b) 2.3, (c) 3.5 and ( d ) 4.0 eV. The absolute binding energies (854.6 and 856.1 eV) are displayed (4) on spectra (a) and (d). N i ( 2 ~ ~ / ~ ) / 0 ( Is) intensities of 5.42. This was checked by using a nickel nitrate sample, evacuated at 100 "C, which gave a ratio of oxygen to nickel atoms of 5.82 using this factor.Although we do not suggest that the values of the O/Ni atom ratios are absolute, the error is not likely to be > f 20%. Consequently, there appears to be a large initial excess of oxygen in the surface region, increasing on evacuation at temperatures up to 250 "C, but decreasing abruptly after evacuation at 500 "C. NiOllOO Fig. 2 shows the O(ls) and Ni(2p3i2) spectra observed with NiOll,, evacuated at (a) room temperature, (b) 160, ( c ) 300 and ( d ) 500 "C. From the C(ls) spectra it can be concluded that the surface charge in each case is (a) 0.0, (b) 2.3, (c) 3.5 and ( d ) 4.0 eV. The O( 1s) spectra have two components at 529.7 and 53 1.4 eV.However, with NiOll,, and in contrast to Ni,,, the intensity of the high-binding-energy 53 1.4 eV peak decreased continuously during evacuation from 20 to 500 "C. The f.w.h.m. value of the 529.7 eV peak increases from 1.2 to 2.0 eV. The absence of the broad 531.4 eV component, as occurs in (d), results in an almost symmetrical O( 1s) peak. The Ni(2p3,,) spectrum in (a) has a main component at 854.6 eV with the high-binding-energy shoulder at 856 eV, corresponding to the high-binding-energy O( 1s) shoulder observed in (a). In (b) the broadening of both N i ( 2 ~ ~ / ~ ) peaks, but particularly the 856 eV peak, leads to increased overlap. The 856 eV peak decreases in intensity from (a) to ( d ) , as also has the O(ls) peak at 531.4 eV. Spectra in (c) and ( d ) clearly show increasing relative predominance sharpening of the 854.6 eV peak as the high-binding-energy component's intensity decreases.The O/Ni atomic ratios obtained by the method described above for each evacuation temperature are shown in table 2. Again there appears to be a large excessM. W. ROBERTS AND R. ST C. SMART 2963 of oxygen in the surface, which is reduced by evacuation at 500 "C, but there is no clear pattern evident in the ratio changes at intermediate temperatures. Ni01450 O( 1s) and Ni(2p3/,) spectra from NiO,,,, evacuated at temperatures between room temperature and 500 "C are very similar to those for NiO,,,, surfaces shown in fig. 2. At room temperature the NiO,,,, surface is also uncharged but becomes increasingly charged during evacuation at increasing temperature.Spectra of NiO,,,, after evacuation at 500 "C have been published in our previous work.l69 l7 The NiO,,,, spectra exhibit the same relationships as those found for NiO,,,, surfaces, namely (i) correspondence of the O( 1s) 53 I .4 eV and Ni(2p3/,) 856.1 eV peak intensity changes, (ii) broadening of the O(1.s) peak at 529.7 eV and N i ( 2 ~ ~ , ~ ) peak at 854.6 eV with increasing surface charge and (iii) loss of intensity in the high- binding-energy O(1s) and Ni(2p3,,) peaks at 531.4 and 856.1 eV, respectively, with evacuation at increasing temperature. A Ni,,,, surface evacuated at 25 "C to 1 x Pa over 12 h, showing a 531.4/529.7 intensity ratio of ca. 0.4, was bombarded with Ar+ ions at 400 eV and 1.7 pA for 10 min. The high-binding-energy components of the O( Is) and Ni(2p3/,) spectra were almost entirely removed, i.e.the 53 1.4/529.7 ratio is now < 0.05. Some reduction to NiO also occurs and the O/Ni ratio approaches 1.0. ANGULAR DEPENDENCE OF PHOTOELECTRON INTENSITIES FROM SPECIES AT A NiO( 100) We have studied the angular variation of the O(ls) and Ni(2p3/,) spectra from a NiO( 100) single-crystal surface. Krishnan et a1.l9 have previously shown a relative increase of the 531.4 eV O(1.s) intensity at low take-off angles on oxidised nickel surfaces, while Evans et aL2, using the same approach found no relative intensity change for oxide present on sputtered nickel. However, Brundle and Hopster2, reported enhancement of the O( 1s) peak at 53 1.4 eV at low angles on oxidised Ni( 100) and correlated this peak with secondary-ion mass spectrometry (SIMS) observations of OH- species. Preparation conditions were chosen in our work to minimise surface hydroxyl-group formation.The crystal was prepared by heating in a vacuum (4 x Pa) at 600 "C for 2.5 h followed by heating in oxygen at a pressure of 133 Pa at 450 "C for 2 h. Carbon contamination remaining after this treatment is < 0.1 of a monolayer, and a perfect LEED pattern with (100) symmetry is observed. Fig. 3 shows the O(ls) spectra from this surface at various electron take-off angles (ca. 10, 15, 45 and 85" relative to the analyser entrance slit). The intensity of the high-binding-energy O( 1s) peak is enhanced relative to the low-binding-energy peak at low angles. This is also found for Ni(2p3/,) spectra, where the 856.1 eV peak is enhanced at low angles relative to the 854.6 eV peak. Conversely the high-binding-energy peaks are reduced by comparison with the main 529.7 and 854.6 eV peaks observed at high angles, and the spectrum at 85" is similar to that normally obtained after oxygen adsorption above 250 0C.26 This confirms that the species responsible for the high-binding-energy peaks are at the solid/vacuum interface.DISCUSSION DEFECT STRUCTURE OF NiO SURFACES Three aspects of our results are interesting for the information that they provide on the defect structure at, or close to, the surface: (a) the change in surface charge as surface dehydroxylation and water desorption occur during evacuation at increasing SURFACE2964 DEFECT STRUCTURE OF NICKEL OXIDE SURFACES 85 45 1s 10 960 956 952 kinetic energy/eV Fig.3.0( 1s) spectra (pass energy 50 eV, f.s.d. 1 x lo4 counts s-l) from a (100) surface of a NiO single crystal prepared by evacuation at 600 "C and heating in oxygen ( 1 33 Pa) for 2 h at 450 "C. The spectra are from take-off angles of 10, 15,45 and 85" relative to the analyser entrance slit. J. Represent absolute binding energies (in eV). temperature; (b) the correlation of the high-binding-energy O( 1s) peak at 53 1.4 eV with the N i ( 2 ~ ~ , ~ ) peak at 856.1 eV under a variety of conditions, and the relative proportions of the species responsible for these peaks; (c) the excess of oxygen over nickel and the changes in the O/Ni atom ratio under various conditions. We have shown that, at constant X-ray flux, conductivity to the surface is changed by band bending V, (from the flat-band potential) as a result of electron-donor or electron-acceptor adsorption.l7? 27 The initial surfaces of NiO,,,, NiO,,,, and NiO,,,, after evacuation at room temperature all show no surface potential, i.e.Vch = 0. During evacuation at temperatures up to 300 "C all samples acquire surface charge, while the mass spectra show that water is the major desorbing species. > 75% of the final potential (after evacuation at 500 "C) is present after evacuation at 300 "C. This is consistent with the loss of an electron-accepting species since the free-hole concentration of the p-type semiconductor surface will be reduced, leading to increased band bending and lower conductivity in the space-charge region.The magnitude of the potential depends on the defect concentration of the oxide, as shown previo~s1y.l~ For X-ray power above ca. 200 W, the surface potential V,h depends27 on the band bending V, and the bulk conductivity 0, throughM. W. ROBERTS AND R. ST C. SMART 2965 where B(a,) represents a functional dependence on the bulk conductivity, Kl and K3 are constants representing, respectively, electron currents arising from photon-induced emission and secondary electron discharging from the X-ray tube window. It is apparent that the more defective oxide NiO,,,, with an estimated bulk conductivity16 of 3.1 x lop2 R - m-l, is expected to have lower V,, than the NiO,,,, with much lower defect concentration (cf. 5.0 x lo-, R-l m-l) even if Vs is the same in each case.Surface hydroxyl groups appear to be acting as electron acceptors at the initial surface since all three surfaces have reasonably intense O( 1s) components at 53 1.5 eV and Ni(2p3/,) components at 856.4 eV, values corresponding closely to those found for bulk Ni(OH),.14 The removal of hydroxyl species as water releases electrons trapped at the interface causing an increase in Vs due to free-hole annihilation in the p-type oxide surface: (1) 20H-(a) = H,O(g) + 0-(a) +e-. In the later stages of evacuation at temperatures between 300 and 500 "C it is apparent that a second reaction occurs. It is in this temperature range that the C(1.s) intensity is markedly reduced and the mass spectra show a predominance of CO as the desorption product.We propose that the predominant reaction on all NiO surfaces in this temperature range is C(a) + 0-(a) 3 CO(g) + e- (2) again involving the loss of an electron-accepting species and further increase in surface charge. With NiO,,, surfaces, but not with NiO,,,, or NiO,,,, surfaces, it is also evident that a third reaction is important, namely NiO(s)+ CO(g) + Ni(s) + CO,(g) (3) since Nio is clearly observed in the Ni(2p3/,) spectra (fig. 1) after evacuation at 500 "C. Cochran and Larkins', have shown the importance of this reaction in the reduction of high-surface-area (or low-temperature-calcined) nickel oxide surfaces by carbon impurities. It is interesting to consider the possible influence of defect concentration on surface hydroxylation and consequent band-bending after high-temperature evacuation.Recent theoretical simulation of reactions at oxide surfacesf2, l3 have revealed that hydroxylation of surface oxide ions will only occur at defect sites (e.g. sites with less than five-fold coordination). Electron-microscopic characterisation of these surfaces8 shows that the concentration of such sites is greatly increased from the NiO,,,, surfaces, with quite extensive flat (100) facets, to the rounded equiaxed NiO,,, particles. This is confirmed in infrared spectra of such surfaces evacuated at 25 "C, where NiO,,, has strong v(0H) absorption but NiO,,,, shows barely detectable v(0H) after due allowance for surface-area differences. X.P.S. data for NiO,,,, are in accord with this observation in having much reduced intensity of the O(1s) peak at 53 1.4 eV.As a consequence of this difference between samples, we might expect less effect due to loss of the electron-accepting OH species on the NiOll,, and Nil450 samples and this is evident in the spectra (fig. 2), where the high-binding-energy O(1s) peak at 53 1.4 eV is progressively reduced by evacuation at increasing temperature. Conversely, we have observed that water, adsorbing slowly over 24 h on a NiO,,,, surface pre-evacuated at 500 "C, gradually decreases the surface charge from ca. 4.0 to 0.0 eV and is accompanied by an increase in the intensity of the O(1s) peak at 53 1.4 eV peak. Under the same conditions NiO,,, is very reactive, the surface charge being removed instantaneously.2966 DEFECT STRUCTURE OF NICKEL OXIDE SURFACES The NiO,,, sample behaves very differently after evacuation at 500 "C, giving a very large increase in the O( 1s) peak at 53 1.4 eV [fig.1 ( d ) and table 13. This, coupled with the large reduction in intensity of the Ni(2p3,,) shake-up satellite at 861.6 eV [figure 1 (d)], strongly suggests structural disorder, less well defined band structure and a defective, partially reduced surface. This view of the NiO,,, surface is in accord with the known reactivity of high-surface-area oxides prepared by decomposition at low temperature.'9 lo NATURE OF THE HIGH-BINDING-ENERGY O( 1 s) AND Ni(2p3/,) COMPONENTS The second observation is a close correlation in all spectra between the high- binding-energy O( 1s) and N i ( 2 ~ ~ / ~ ) peaks at 53 1.4 and 856.1 eV, respectively.Other 24 have commented on this possibility. The existing information on the chemical identity of the peaks is, however, not unambiguous. Oxidation of clean, evaporated nickel films14920-22 up to 250 "C shows both O(1s) peaks at 529.7 and 53 1.4 eV with considerable intensity in the 53 1.4 eV peak at sub-monolayer coverage. Oxidation of (1 1 l), (100) and (1 10) faces of nickel single crystalslg9 22, 2 4 9 25 also produces both high-binding-energy O( 1s) and Ni(2p3,,) peaks but with reduced 53 1.4 eV/529.7 eV intensity ratios for the O( 1s) components compared with nickel films. Argon-ion sputtering of a (100) face followed by oxidation24 gave marked enhancement of the high-binding-energy components consistent with the difference between evaporated films and single-crystal faces.Brundle and Hopster2, have correlated the 53 1.4 eV/529.7 eV O( 1s) ratio with the OH-/O- ratio observed in SIMS and conclude that the 53 1.4 eV peak is caused by OH groups on the oxidised Ni( 100) surface. They have also found the same angular variation of the q l s ) spectra as we have observed for the single-crystal NiO(l00) surface showing that this species is confined to the first one or two atomic layers. It is also interesting that 0; ion bombardment of nickel films,14 single-crystal faces24 and heavily oxidised nickell, at 25 "C produces both the high-binding-energy O( 1s) and Ni(2p3I2) components with high intensity. At -70 "C nickel films subject to 400 eV 0; bombardment give intensities almost entirely in these peaks, whereas at 200 "C only ca.10% of the O(ls) peak is in the 531.4 eV component. These results, taken together with our results, illustrate that the O(ls) peak at 531.4 eV cannot be assigned uniquely to hydroxyl groups, although with oxide surfaces before evacuation at high temperature it is likely to be associated mainby with hydroxyl species. The third observation from our work is that there is an excess of oxygen on all surfaces even after evacuation at 500 "C. This strongly suggests a high concentration of metal-ion vacancies at the interface, such cation vacancies are known to induce adjacent Ni3+ and 0- l2 We suggest that the most reasonable explanation of our results is that the high-binding-energy O( 1s) and Ni(2p3/,) peaks observed with the NiO surfaces evacuated at 25 "C arise from the OH groups with values close to those observed with nickel hydroxide,14 i.e.53 1.5 and 856.4 eV, respectively. As water is removed [reaction (1) above], 0- species are formed together with nickel vacancies, so that the binding energies O( 1s) at 53 1.4 eV and Ni(2p3/,) at 856.1 eV now correspond to 0- and Ni3+ species, respectively. Concurrently the surface charge increases. Evacuation at 500 "C reduces the 0- and Ni3+ concentration on NiOll,, and Nil,,, by reaction with carbon [reaction (2)]. The same treatment of NiO,,, leads to a reduced surface [reaction (3)] dominated by 0- species, with a significant Nio concentration. This interpretation is consistent with the other studies described above, with the change of surface charge, O( 1s) and Ni(2p3,,) spectra, and with the significantM.W. ROBERTS AND R. ST C. SMART 2967 reduction in the O/Ni ratio observed after evacuation at 500 "C (table 2). It is surprising that the O/Ni atom ratio does not appear to decrease after evacuation at 300 "C, unless there is some surface reconstruction with oxygen ions moving to the outer surface whilst nickel ions move inwards. The spectra from the angular variation of O(ls) (fig. 3) and N i ( 2 ~ ~ ~ ~ ) features with the single/crystal NiO(100) surface indicate that 0- species are stabilised at the interface. This result is significant in the description of surface-defect structure because bulk defect concentrations, (i.e. nickel vacancies and associated holes) are of the order of 0.02 atom% whereas at the NiO(100) surface-defect concentrations of up to 70% are evident.CONCLUSIONS The object of this study of four different oxides of nickel was to identify specific surface species and to search, through temperature-dependent photoelectron spectro- scopy, for possible correlations between core-electron binding energies, the chemistry of surface defects, bulk defect concentration and structural data. Mass-spectrometric analysis of the gas phase and infrared spectroscopy have also helped to characterise the oxides. Such information is relevant to understanding the catalytic chemistry of nickel oxide, and the following conclusions emerge. (a) In addition to O(1s) and Ni(2p3/,).peaks at 529.7 and 854.6 eV, respectively, all the samples exhibited initially high-binding-energy components at 53 1.4 and 856.1 eV.The latter are associated with surface hydroxyls and Ni3+, possibly present as an oxyhydroxide overlayer, for which similar photoelectron peaks have been The concentration of these species depends on the defect structure of the oxide preparation, with NiO,,, showing more intensity in these peaks than Ni01450. The surface species are electron-accepting, resulting in increased surface conductivity and therefore a lower surface potential. (b) On heating the oxides in a vacuum (< lop6 Pa), dehydroxylation occurs leaving 0- species and nickel vacancies at the surface. Surface charging increased while mass-spectrometric analysis of the gas phase established that desorption of water was extensive. The O( 1s) peak associated with 0- is at ca.53 1.4 eV. Above 300 "C reaction with surface carbon (impurity) results in desorption of carbon monoxide and a decrease in the 0- concentration. The defective, high-surface-area NiO,,, surfaces are partially reduced by the CO formed and exhibit predominantly 0- and Ni3+ species, with O( 1s) and Ni(2p3,& peaks at 531.4 and 856.1 eV, respectively, after evacuation at 500°C. The presence of Nio is also evident in the Ni(2p312) spectrum. The high-temperature, low-surface-area oxides, NiOll,, and Ni01450, show merely a loss of carbon and 0- species during evacuation. (c) After evacuation at 600 "C and heating in oxygen at 450 "C a NiO( 100) single crystal has 0- and Ni3+ species located at the surface. These defects are present at concentrations of up to 70% of the surface.M. W. R. acknowledges the support of the S.E.R.C., while R. St C. S. is grateful to Griffith University for study leave. F. F. Vol'kenstein, The Electronic Theory of Catalysis on Semiconductors (Pergamon, Oxford, 1963). Z. M. Jarzebski, Oxide Semiconductors (Pergamon, Oxford, 1973). J. B. Goodenough, Prog. Solid State Chern., 1971, 5, 145. H. J. van Daal and A. J. Bosman, Phys. Rec., 1967, 158, 736. J. Deren and J. Stoch, J. Catal., 1970, 18, 249. * R. L. Segall, R. St C. Smart and P. S. Turner, Chem. Aust., 1982, 49, 241. ' P. C. Gravelle and S. J. Teichner, Adz,). Catal., 1969, 20, 167.2968 DEFECT STRUCTURE OF NICKEL OXIDE SURFACES * C. F. Jones, R. L. Segall, R. St C. Smart and P. S. Turner, J. Chem. Soc., Furaduy Trans. 1 , 1977,73, 1710. C. F. Jones, R. L. Segall, R. St C. Smart and P. S. Turner, J. Chem. Soc., Furaday Trans. 1 , 1978,74, 1615. lo S. J. Cochran and F. P. Larkins, personal communication. l1 V. M. Bermudez, Prog. Surf. Sci., 1981, 11, 1 . l2 G. T. Surratt and A. B. Kunz, Phys. Rev. B, 1979, 19, 2352. l3 E. A. Colbourn and W. C. Mackrodt, Surf. Sci., 1982, 117, 571. l4 K. S. Kim and N. Winograd, Surf. Sci., 1974, 43, 625. l5 T. Robert, M. Bartel and M. Offergeld, Surf. Sci., 1972, 33, 123. l6 M. W. Roberts and R. St C. Smart, Chem. Phys. Lett., 1980,69, 234. l7 M. W. Roberts and R. St C. Smart, Surf. Sci., 1980, 100, 590. l* P. R. Norton and R. L. Tapping, Furuday Discuss. Chem. SOC., 1975, 60, 71. l9 N. G. Krishnan, W. M. Delgass and W. D. Robertson, Surf. Sci., 1976, 57, 1 . 2o C. R. Brundle and A. F. Carley, Chem. Phys. Lett., 1975, 31, 423. 21 C. R. Brundle and A. F. Carley, Furuday Discuss. Chem. Soc., 1975, 60, 51. 22 S. Evans, J. Pielaszek and J. M. Thomas, Surf. Sci., 1976, 56, 644. 23 G. Schiin and S. T. Lundin, J. Electron Spectrosc., 1972, 1, 105. 24 (a) P. R. Norton, R. L. Tapping and J. W. Goodale, Surf. Sci., 1977, 65, 13. (b) A. Srinivason, K. Jagannathan, M. S. Hegde and C. N. R. Rao, Ind. J. Chem., 1979, 18A, 463. 25 C. R. Brundle and H. Hopster, in Secondary Ion Muss Spectrometry, SIMS II, ed. A. Benninghoven, C. A. Evans Jr, R. A. Powell, R. Shimizu and H. A. Storms (Springer Verlag, Berlin, 1979), p. 272. 26 M. W. Roberts and R. St C. Smart, Surf. Sci., 1981, 108, 271. 27 R. St C. Smart, Surf. Sci., 1982, 122, 1643. ** C. R. Brundle, M. W. Roberts, D. Latham and K. Yates, J. Electron Spectrosc., 1974, 3, 241. 29 M. W. Braithwaite, R. W. Joyner and M. W. Roberts, Furuday Discuss. Chem. Soc., 1975, 60, 89. 30 A. F. Carley and R. W. Joyner, J. Electron Spectrosc., 1978, 13, 41 1 . 31 D. A. Shirley, Phys. Rev., 1972, 85, 4709. 32 J. H. Scofield, J. Electron Spectrosc., 1976, 8, 129. 33 L. M. Moroney, R. St C. Smart and M. W. Roberts, J. Chem. SOC., Faruduy Trans. 1, 1983,79, 1769. (PAPER 4/043)

ISSN:0300-9599    

DOI:10.1039/F19848002957

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

S. Chem. SOC., Faraday Trans. 1, 1984,80, 2969-2979 Ozone Formation in Laser Flash Photolysis of Oxoacids and Oxoanions of Chlorine and Bromine? BY ULRIK K. KLANING* Department of Chemistry, University of Aarhus, Langelandsgade 140, DK-8000 Aarhus C, Denmark AND KNUD SEHESTED Accelerator Department, Rispr National Laboratory, DK-4000 Roskilde, Denmark AND THOMAS WOLFF University of Siegen, D-5900 Siegen 21, Federal Republic of Germany Received 1 1 th Januury, 1984 The kinetics of ozone formation in the photolysis of oxygen-containing solutions of HC10, C10-, ClO;, ClO;, HBrO, BrO- and BrO; has been studied by laser flash photolysis and conventional flash photolysis. The usual assumption, that ozone only forms in the reaction of oxygen atoms in the spin-triplet ground state with molecular oxygen: o+o,+o, subsequent to the primary process h v hv XO; -+ XO;-,+O(HXO --+ H++X-+O) is found to be valid for solutions at pH < 10 but not for more strongly alkaline solutions.The rate constants of the reactions of 0 with 0, and BrO; are found to be 4 . 0 ~ lo9 and 1.5 x lo7 dm3 mol-1 s-l, respectively. The formation of ozone in strongly alkaline solutions can be accounted for by the additional processes o;+Xo,~, +o,+xo,-, 0- +o, GO, xo, -+ xo,-, + 0-. with rate constants in the range 5 x loH to 1.5 x lo9 dm3 rno1-I s-' subsequent to and the primary process hv Previous inve~tigationsl-~ have shown that ozone is a product in the photolysis of oxygen-containing aqueous solutions of the oxoanions C10-, Clog, BrO-, BrO;, BrO, and 10;. The formation of ozone has been assumed to take place in the reaction of oxygen atoms in the spin-triplet ground state with molecular oxygen o+o, + 0, (1) subsequent to the primary photochemical process in which 0 is formed: hv xo, + xo,-, + 0. 7 Presented in part at the 1 lth International Conference on Photochemistry, Maryland, U.S.A.29692970 OZONE FORMATION IN PHOTOLYSIS OF OXOANIONS The ozone produced can be identified in flash-photolytic experiments by its strong absorption band centred at 260 nm ( E , , ~ = 3300 dm3 mol-l ~ r n - l ) ~ and by its decay kinetics in alkaline s o l ~ t i o n . ~ ~ To verify the mechanism of ozone formation we have measured the kinetics of absorbance change in the region 240 < A/nm < 600 with a time resolution of lo-* s in laser flash photolysis of acid, neutral and alkaline 0,-free and 0,-containing solutions of HClO, C10-, ClO;, Clog, HBrO, BrO- and BrOg.The laser-flash-photolysis experiments were supplemented with conventional flash photolysis and pulse-radiolysis measurements of alkaline solutions of BrO;. EXPERIMENTAL The apparatus for laser flash photolysis was as described previously7 except for interchange in the laser flash photolysis apparatus of glass lenses with quartz lenses and of the RCA 1P28 photomultiplier with a RCA 4840 photomultiplier. The 193, 248.5 and 308 nm emissions of the Lambda Physik EMG 500 excimer laser were collimated in a 2 cm wide beam into the 2 cm long and 0.3 cm wide rectangular quartz cell perpendicular to the monitoring light. Owing to absorption of laser light in the solution, the concentration of photoproducts decreases with the distance from the entrance window of the laser beam.The concentration of the solute was adjusted to an absorption of laser light in the cell of < 90%. In most experiments the absorption was < 50%. Calculations8 show that the effect of absorption of laser light in the cell on the measured kinetics of a second-order reaction is negligible for an absorption of laser light < 50%. At 90% absorption the calculations show that measured rate constants for second-order reactions are 10-20 % higher than those measured for solutions with uniform concentration of reactants. The light intensity of the pulsed monitoring light was sufficiently constant for measurements up to 2 x lo-* s after the laser flash.To reduce photolysis by the monitoring light, 'cut off' filters were used. The solutions were saturated with 0, or air by bubbling N, through the solution for 3 min. No effect on the experimental results could be detected by prolonged bubbling. Spectra were measured point-by-point, changing the monochromator setting after the flash. The solutions were replaced after each flash. Attenuation (8 x ) of the 100-200 mJ flash showed proportionality between absorbance change and laser light intensity. Thus no effects of multiphoton absorption or of secondary photolysis of photoproducts were detected. Clog and BrO; solutions with [BrO;] < lo-, mol dmP3 were irradiated with laser light of wavelength 193 nm; BrO; solutions with 0.025 < [BrO;]/mol dm-, < 0.1 and solutions of Clog, HClO and HBrO with 248.5 nm and solutions of C10- and BrO- with 308 nm laser light.Water used for the preparation of solutions for the pulse-radiolysis experiments was purified as described el~ewhere.~ Water used in the photochemical experiments was deionized and destilled twice in an all-quartz apparatus. Gases were N, 99.999 % and .O, 99.995 "/, . KClO, and KBrO, were Merck p.a., NaC10, was Matheson, Coleman and Bell analytical grade dried over P,O,, assay 99% (iodometrically determined). NaClO . 5H,O, and NaBrO . 5H,O were prepared as described,1° recrystallized from 2% aqueous NaOH solution and stored at - 70 "C. Solutions of NaBrO .5H,O and of NaClO * 5H,O were analysed as follows. The concentration of hypohalite was determined by potentiometric titration with As"' in alkaline solution.The sum of the concentrations of hypohalite and halide was determined by potentiometric titration with Ag+ of solutions in which hypohalite had been reduced with As"'. The analysis showed a content of halide in hypohalite of 1-2%, a content which did not have any effect on the results. The content of BrO; and BrO; in hypobromite determined iodometrically as BrO; was < 0.5 % . BrO- solutions were freed from Br- by acidifying the solution and removing the Br, formed by bubbling N, through the solution for 10 min. Pulse radiolysis and conventional flash photolysis were carried out as described previ~usly.~ Computations were made as described el~ewhere.~ All measurements were made at ambient temperature (21 2 "C).u .K. KLANING, K. SEHESTED AND T. WOLFF 297 1 I 1 I I I I I 260 300 350 400 4 50 500 540 Xlnm Fig. 1. Transient absorbance, OD, plotted against wavelength, A/nm, recorded 2 ps after laser flash irradiation (193 nm) of solutions containing (0 and 0 ) lo-, mol dm-, KBrO, and (A, 0 and A) 10-3mol dm-, KC10,. Open symbols, 0,-free solutions; solid symbols, [O,] = 1.27 x lo-, mol drn-,. All solutions neutral except for 0, [HClO,] = 2 x lo-, mol dm-,. RESULTS AND DISCUSSION 0,-FREE SOLUTIONS The primary process (2) cannot be observed by photolysis of 0,-free solutions. The absorbance changes are assigned to the primary processes hv xo, --* XO,-, +o- hv HXO+X+OH hv xo, -+ xo- + 0, and to secondary processes in agreement with previous results.Transient absorption bands centred at 350 nm in solutions of C10; and at 475 nm in solutions of BrO, were observed (fig. 1). These bands were assigned to C10, and Br02 [reaction (3)], respectively, in accordance with previous studies.,T l2 A band centred at 330 nm in BrO; solutions is assigned to BrO- formed in the primary process (5). Similarly, the observation that the transient absorbance at 270 < I/nm < 330 in Clog solutions after flash excitation is smaller in acid than in neutral solution is ascribed to the difference in absorbance of HClO and C10- formed in the primary process (5).2 A transient absorption band centred at 280 nm observed in flash-irradiated C10; solutions is assigned to formation of C1014 in the primary process (3). The band decays2972 OZONE FORMATION IN PHOTOLYSIS OF OXOANIONS 030 - 8 0.05 - 25 0 300 350 LOO X/nm Fig.2. Transient absorbance, OD, plotted against wavelength, A/nm, recorded (A) ca. lop2 ,us and (0) 4ps after laser flash irradiation (248.5 nm) of a solution containing 1.65 x mol dmP3 NaClO,. into a band centred at 360 nm (fig. 2) which we ascribe to C10, formed in the reactions15 (6) C10; + OH -+C10, + OH- H,O ClO; + O-+ClO, + 20H- (7) c10, + c10 +ClO- + CIO,. (8) The transient absorption bands at 3 10 and 280 nm observed by laser flash photolysis of hypohalite solutions at pH < 10 are assigned to halogen atoms,16 formed in reactions (3) and (4). These bands, which are thought to be due to charge transfer from decay into two new bands centred at 280 and 340 nm (fig.3), which we assign to ClOl and BrO17 formed in the reactions8 (9) x+xo-+xo+x- H++OH OH + { HXo --+ XO+{ H2O xo- In strongly alkaline solutions of C10- and BrO-, the absorption bands of C1 and Br are not observed owing to the fast reaction O,-CONTAINING SOLUTIONS (PH < 10) The increase in absorbance by addition of O,, AOD, observed at A < 300 nm after laser irradiation is ascribed to the formation of ozone. The yield and kinetics of the formation of 0, depend on pH. At pH < 10,0, is formed in 0,-containing HClO, C10; and BrO; solutions in a first-order process. AOD measured at 240 < A/nm < 300 increases with time to a limiting value AOD,. Fig. 3 (a) shows that AOD, of a solution of HClO plotted against wavelength fits the absorption spectrum of 0,.5 We assign the formation of 0, to the reaction (1) subsequent to the primary process (2).Fig. 4 shows a plot of in (AOD, - AOD) measured at 260 nm againstu. K. KLANING, K. SEHESTED AND T. WOLFF 2973 A/ nm 0 300 350 Fig. 3. Transient absorbance, OD, plotted against wavelength, A/nm, after laser flash irradia- tion (308 nm) of a solution containing (a) 4.85 x rnol dm-3 HClO and 2.5 x lop2 rnol dmP3 HC10, and (b) 1.53 x rnol dm-3 HBrO and lop3 mol dmP3 HC10,. 0, Recorded ca. pus after flash; and A, recorded 1 ,us after flash. Open symbols, 0,-free solutions; solid symbols, [O,] = 1.27 x mol dm-3; x , AOD, (difference between OD of 0,- containing and 0,-free solutions). 10 s o a8 9 -1 c 0 4 I 0 E: + -2 c I I I I I I I I I I I 1 2 3 4 5 6 7 8 9 10 11 t / lo-' s Fig.4. First-order plot of increase of absorbance at 260 nm in 0,-containing solutions after laser flash irradiation. x and 0, [BrO;] = 5 x lov4 mol dmP3 ( x , [O,] = 2.54 x lo-,; 0, [O,] = 1.27 x lov3 mol dmp3), laser light 193 nm. W, [ClO;] = mol dm-3, l.O,] = 2.54 x mol dm-3, laser light 193 nm; A, [HClO] = 4.85 x rnol drnp3, [O,] = 2.54 x mol dm-3, laser light 308 nm. time in 0,-saturated solutions of 4.85 x mol dm-3 HC10, 1.0 x rnol dmP3 C10; and 5.0 x mol dm-3 BrO; and in air-saturated solutions of 5.0 x mol dm-3 BrO;. From the slopes of the straight lines in fig. 4, which are proportional to [O,], we find, by taking [O,] = 1.27 x mol dm-3 in 0,-saturated so1utions18 and 2.54 x mol dm-3 in air-saturated solutions,2974 OZONE FORMATION IN PHOTOLYSIS OF OXOANIONS I I 1 I I 2 4 6 8 10 [ BrO;] / 1 0-2 mol dm'3 Fig.5. Yield of BrO, relative to yield of 0,, yRro2/yOs, in laser-flash-irradiated BrO; solutions plotted against [BrO;] (laser light: 0, 193; A, 248.5 nm). k, = 4.0 x lo9 dm3 mol-l s-l. The fact that the slopes in fig. 4 are proportional to [O,] indicates that the reactions of oxygen atom with the solutes HClO, C10; and BrO; are slow compared with the reaction of oxygen atom with molecular oxygen. By photolysis of solutions containing 0.025 < [BrO;]/mol dm-3 < 0.1 with laser light of wavelength 248.5 nm we were able to detect a reaction which we assume is BrO; + 0 -, BrO; + 0,. (12) Assuming that 0, and BrO, are formed by reactions (1) and (3) only their relative yields yBrO2/yo3 may be expressed by where is the branching ratio for the photochemical processes (3) and (2).We find k,,/k, = 3.8 x from a plot (fig. 5) of the ratio yBrO2/yO2 against [BrO;]. Values of yBr02/yo, were obtained from yBrO2/yo3 = AOD475 x ( E ~ : + E ~ : ~ ; ) / ( E & " ~ ~ x AODgO). (14) AOD475 is the absorbance at 475 nm measured just after the flash irradiation and ~ g : ~ ~ , EE:~; and E?: are the corresponding extinction coefficients of BrO,, BrO; and 0, at the given wavelengths.5* l7, l9 The fact that no reaction with water is observed is in keeping with the assignment of the ozone precursor to an oxygen atom in a spin-triplet state since the route to H,O, is forbidden because it involves a change of spin multiplicity and the route to 20H is not feasible thermodynamically.20 The reason for the relative sluggishness of the reactions with the solutes HClO, CIO; and BrO; might be that the pathway to products in these reactions also involves a spin-forbidden step (e.g.with the intermediate formation of peroxy compounds). Table 1 shows yields of ozone relative to yields of the products formed in the primary process (3). The yields were obtained from measured absorbance by usingu . K. KLANING, K. SEHESTED AND T. WOLFF 2975 Table 1. Ratio of yields at pH < 10, yo3/yxOn-,, and at pH > 12, Yo3/ Yxon-l - HClO 0.2a c10- O.Osa 0.3a HBrO < 0.01 BrO- < O.Olb 0.3,b 0.4c - c10, < 0.01 <0.01 c10, 0.1 - BrO; 0.062,d 0.064" 0.5,f0.6c a Calculated from Yclo by taking Yclo = 2 Y,, ; calculated from YBpo by taking YBro = 2 YBr ; laser flash wavelength 193 nm; laser flash wavelength 248.5 nm, determined by extrapolation to [Broil = 0 (fig.5); f conventional flash pho tolysis. calculated from values in ref. (3) (see text); equations analogous to eqn (14). Note that yo,/y,on-l < 1 for all solutes in accordance with the view that process (2) is a spin-forbidden process. Further, note that the branching ratio, y03/yBrOz, between processes (2) and (3) appears to be the same for photolysis with light of wavelengths 193 and 248.5 nm. O,-CONTAINING SOLUTIONS (PH > 12) On photolysis of 0,-containing solutions of ClO-, BrO- and BrO; at pH > 12 we observe the formation of 0, with a much higher yield and at a much slower rate than observed in solutions at pH < 10. On photolysis of 0,-containing solutions of C10-, ClO;, BrO- and BrO; at pH > 12, 0; is observed in addition to species observed at pH < 10.0; is formed in alkaline solution by the reactions OH=O-+H+, pK= 11.9,l (15) 0-+o, e o ; (16, -16) subsequent to the primary reaction (3). 0; is not observed at pH < 10 owing to fast reactions of OH: [20H --+ H,O,, reactions (6) and (1 l)]. The following observations indicate that most of the ozone formed at pH > 12 is produced in reactions of 0; with primary or secondary formed halogen oxides. Fig. 6 shows transient spectra measured 0.5 and 45 ,us after laser flash irradiation of BrO; solutions at pH 13, saturated with 0, or rendered 0,-free before irradiation. The difference in absorbance at 240 < A/nm < 280 between 0,-saturated and 0,-free solutions 0.5 ps after the flash is assigned to the formation of 0, in reaction (1) subsequent to the primary process (2), in agreement with the assignment at pH < 10.In 0,-free solutions an additional band at 470 nm due to BrO, is observed. The unsymmetrical band at A z 450 nm seen in 0,-saturated solutions is ascribed to a composite of the band at 470 nm due to BrO, and the band at 430 nm due to O;.,, The difference in absorbance at 240 < A/nm < 280 between 0,-saturated and 0,-free solutions increases with time simultaneously with a decrease in absorbance at 350 < l/nm < 550. The rate of relative decrease of absorbance at 375 nm (0;) equals the rate of relative increase of absorbance at 260 nm (0,) (fig. 7). 45ps after the flash the absorption band at 470 nm has disappeared and the2976 OZONE FORMATION IN PHOTOLYSIS OF OXOANIONS n 0 2LO 300 400 500 600 Fig.6. Transient absorbance, OD, plotted against wavelength, A/nm, after laser flash irradiation (248.5 nm) of solutions containing 0.05 mol dmP3 KBrO, and 0.1 mol dmW3 NaOH. Open symbols represent 0,-free solutions; solid symbols, [O,] = 1.27 x mol dmP3. A and A, recorded 0.5 ps after flash; 0 and 0, recorded 45 ,us after flash. A/ nm Fig. 7. Relative 0 5 10 15 20 change in absorbance, OD$ and ODE' calculated as (OD-OD,)/(OD,-OD,), plotted against time. 0 ~ and ., 5 x lo-, mol dm-, BrO;, 0.1 mol dm-, NaOH, laser light 248.5 nm; 0 and 0, 0.1 mol dm-3 BrO;, 0.1 mol dm-3 NaOH, pulse radiolysis, 30 Gy per pulse. Open symbols, measured at 260 nm; closed symbols, measured at 375 nm.0 and 0, [O,] = 2.5 x lop4 mol dm-3; 0 and ., [O,] = 1.27 x loP3 mol dmP3. spectrum in the 0,-saturated solution now contains an absorption band at 260 nm due to 0, and one at 430 nm due to remaining 0; (fig. 6). After the disappearance of BrO, no further changes in [O,] and [O;] were detected in the next 150 ps, which is in agreement with the rates of decay of 0,6 and of O;,, measured in solutions at the same pH and 0, concentration. The small band at 260 nm observed after 45 ps in solutions which were freed of 0, before the flash irradiation is probably due to 0,, formed from traces of 0, produced in primary process (5).u. K. KLANING, K. SEHESTED AND T. WOLFF 2977 These observations suggest that 0, is formed by a reaction of 0; with BrO,.Formation of ozone by pulse radiolysis of alkaline air-saturated BrO; solutions corroborates this suggestion. In such experiments 0; and BrO, are formed from the primary radiolytical species 0- and eiq, 0; by reaction (16) and BrO, by the reaction %O BrO; + eiq--+BrO2 + 20H-. In the present experiments [BrO;] and [O,] were chosen to avoid formation of O;,23 and thus it was possible to assign the increase of absorbance observed at A < 300 nm to the formation of ozone. The kinetics of the formation of ozone was similar to that observed in laser flash photolysis. The formation took place simultaneously with a decrease in the absorbance of 0; and BrO,, and the rate of formation of 0, was equal to the rate of disappearance of 0; (fig.7). Kinetics similar to that for the formation of ozone was observed in 0,-containing ClO- and BrO- solutions at pH > 12. We suggest that ozone is formed by the reaction XO,+O; +O,+XO, (18) in competition with the reactions* l7 ,OH- 2XO,--+XO; +XO;+,+H,O in which XO, denotes the halogen oxides CIO, BrO and BrO,. Reaction (18) is analogous to the reactions by which 0; is oxidized to 0, by OH and the carbonate radical ion, COY.^^^ 25 Ozone was not detected in the photolysis of 0,-containing alkaline ClO; solutions. In the present experiments this means that the-rate constant, kI8, for XO, = C10, cannot be greater than ca. lo7 mol-l dm3 s-l and furthermore fits the observation that CIO, which is formed in the primary process (3), reacts fast with the solute, C10; [reaction (8)].l5 The rate constants, k18, were estimated by assuming that the yield of O;, Yo;, is constant during the decay of XO,.After XO, had disappeared the solution contained 02; and 0,, the rate of decay of which was small compared with the rate of reaction (18). The yield of 0; measured after the decay of XO,, Y$;, was about half the yield measured initially, Yo;. With the approximation of a constant value of Yo, the rate equations for reactions (18) and (19) may be integrated and eqn (20) is obtained: ( '0, -YO,)/ yXo,o' = k18 y0,/(2k19 yX0,02) In + 2k19 yXOmo2/(k18 (20) This equation relates the ratio of the yield of 0, formed in reaction (18), Yo, -yo3, and yield of XO,, YxOmo2, in 0,-containing solutions with YxOmo2 and Yo;, which was taken equal to 0.5 x (Yo,+ Y$;).Using values of k,, equal to 2.5 x 2.8 x lo9 and 7 x los dm3 mo1-1 s-l l7 for C10, BrO and BrO,, respectively, we find from determination of Yos-yo3, Yo, and YxOmo2, k,, = 1 x lo9, 1.5 x lo9 and 5 x lo8 dm3 mo1-1 s-l for the reaction of 0; with C10, BrO and BrO,, respectively. The yields were calculated from absorbance changes measured at the band maxima of X0,,26 0c2, and 0,.6 For C10 and BrO, YxOmo2 = Yxo, - Po;. Yxo, is the yield of XO, measured initially in 0,-free solutions. In accordance with the stoichiometry of reactions (3), (9)-(11) and (19) Yxo, was calculated from yxo, = (AOD, - AOD,)/[r(&XO, - $XO, -&xo,+J (21) where AOD, is the initial absorbance change and AOD, the absorbance change after 97 F A R 12978 OZONE FORMATION IN PHOTOLYSIS OF OXOANIONS 0.30- -A- *-.---= --- -- - -- n 0 1 - I I 2 4 6 a 10 t / l P s Fig.8. Abs rbance measured at (A) 260 nm, (0) 430 nm and (m) 475 nm plotted gainst time (t/10-4 s) after flash irradiation of a solution containing lov3 mol dm-3 BrO;, 1.27 x l OP3 mol dm-3 0, and 0.2 mol dm-3 NaOH. Flash light from a conventional flash- photolysis apparatus, flash energy 400 J. Solid curves are computed by numerical integration (see text). XO, has decayed. Z is the optical length of the cell and eXo, and E ~ ~ , + ~ are the extinction coefficients of the hypohalite 27 and halite 27 respectively. YBrO,Oz was calculated from AOD475 = [O;] + E ~ ~ ~ ~ [ B ~ O , ] ) AOD430 = Z(E~: [O;] + ~ ~ ~ ~ , [ B r 0 , ] ) . which relate the absorbance changes AOD475 and AOD4,0 to the concentrations (i.e.yields) of 0; and BrO,. The kinetics of 0, formation was further studied by conventional flash photolysis of 0,-saturated solutions containing lo-, mol dmV3 KBrO, and 0.2 mol dm-, NaOH. Fig. 8 shows the absorbance change measured at 475,430 and 260 nm plotted against time. Solid curves are calculated from eqn (22) and (23) and from AOD260 = Z E ~ : [O,]. [BrO,], [O;] and [O,] are computed by numerical integration of the rate equations for At pH 13.3 k,, = 9.3 x los mol-1 dm3 ss1l7 was used. k,, was determined by the method of trial and error in such a way that the calculated curves for AOD475 and AOD430 fitted the experimental points. The curves in fig. 8 were computed with k18 = 7 x los dm3 mol-1 s-l. Note that this value agrees fairly well with the value for k,, estimated from laser-flash-photolysis measurements (k18 = 5 x los dm3 mol-1 s-l).Note also that the difference between the measured and computed values of the absorbance at 260 nm (AOD, z 0.03, solid and broken curves in fig. 8) corresponds to yo3/yBrO1 z 0.07, in agreement with the value determined at pH < 10. Table 1 shows that our measured ratios of Yo3/ Yxo,-l are in agreement with those calculated from values of Yo3/ Yo- determined previ~usly.~ In these calculations we have taken Yo- = Ygi and Yo- = Yxo,-l - Yo3. [BrO,], [O,] and [OJ-u. K. KLANING, K. SEHESTED AND T. WOLFF 2979 We thank Prof. A. Weller, Max-Planck Institut fur biophysikalische Chemie, Gottingen, and Dr Jerzy Holcman, Riss National Laboratory, for helpful discussions.Financial support from Statens naturvidenskabelige ForskningsrAd to U. K. K. is grate fully acknowledged. G. V. Buxton and M. S. Subhani, J. Chem. SOC., Faraday Trans. I , 1972, 68, 970. F. Barat, L. Gilles, B. Hickel and B. Lesigne, J. Phys. Chem., 1971, 75, 2177. 0. Amichai and A. Treinin, Chem. Phys. Lett., 1969, 3, 61 1. U. K. Kilning and K. Sehested, J. Chem. SOC., Faraday Trans. 1, 1978, 74, 2818. E. J. Hart, K. Sehested and J. Holcman, Anal. Chem., 1983, 55, 46. L. Forni, D. Bahnemann and Edwin J. Hart, J. Phys. Chem., 1982,86, 255. U. K. Klaning and T. Wolff, to be published. E. Bjergbakke, in Manual of Dosimetry, ed. N. W. Holm and R. J. Berry (Marcel Dekker, New York, 1970). lo Handbuch der praparativen anorganischen Chemie, ed. G. Brauer (Ferdinand Enke Verlag, Stuttgart, 1962), p. 282. l1 G. V. Buxton and M. S. Subhani, J. Chem. Soc., Faraday Trans. I , 1972,68, 970. l 2 0. Amichai, G. Czapski and A. Treinin, Zsr. J. Chem., 1969, 7, 351. l 3 A. Treinin and M. Yaacobi, J. Phys. Chem., 1964, 68, 2487. l4 G. V. Buxton and M. S. Subhani, J. Chem. SOC., Faraday Trans. 1, 1972, 68, 947. l5 U. K. Kilning and T. Wolff, to be published. l7 G. V. Buxton and F. S. Dainton, Proc. R . Soc. London, Ser. A , 1968, 304, 427. l9 0. Amichai and A. Treinin, J. Phys. Chem., 1970, 74, 3670. 2o W. H. Koppenol and J. F. Liebman, J. Phys. Chem., 1984,88,99. ' T. Wolff, J. Photochem., 1979, 11, 215. A. Treinin and E. Hayon, J. Am. Chem. SOC., 1975,97, 1716. R. Battino and H. L. Clever, Chem. Rev., 1966, 66, 39. D. D. Wagman, W. H. Evans, V. B. Parker, R. J. Schumm, I. Halow, S. M. Bailey, K. L. Churney and R. L. Nattall, J. Phys. Chem. Ref: Data, 1982, 11, supplement no. 2. 21 Reactivity of the Hydroxyl Radical in Aqueous Solution, NSRDS-NBS46 (U.S. Department of Commerce, National Bureau of Standards, Washington D.C., 1973). 22 K. Sehested, J. Holcman, E. Bjerbakke and E. J. Hart, J. Phys. Chem., 1982, 86, 2066. 23 Selected Specific Rates of Reactions of Transients from Water in Aqueous Solution: Hydrated Electron, NSRDS-NBS43 (U.S. Department of Commerce, National Bureau of Standards, Washington D.C. 1973 and 1975). 24 K. Sehested, J. Holcman, E. Bjergbakke and Edwin J. Hart, J. Phys. Chem., 1984,88, 269. 25 .J. Holcman, K. Sehested, E. Bjergbakke and Edwin J. Hart, J. Phys. Chem., 1982, 86, 2069. 26 Optical Spectra of Nonmetallic Inorganic Transient Species in Aqueous Solution, NSRDS-NBS69 (U.S. 27 'T. Chen, Anal. Chem., 1967, 39, 804. Department of Commerce, National Bureau of Standards, Washington D.C., 198 1). (PAPER 4/052) 97-2

ISSN:0300-9599    

DOI:10.1039/F19848002969

出版商:RSC    

年代:1984

数据来源: RSC