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/F198480FX033

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/F198480BX035

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 TRANSACTIONS, PARTS I A N D I 1 The Journalof the ChemicalSociety 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 kine tics, electrochemistry (0 t her than preparative), surface and inter facial 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 II (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 an article 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. (9NOMENCLATURE AND SYMBOLISM 1 Marlow Medal and Prize Units and Symbols. The Symbols Committee of The Royal Society, of which The Royal Society of Chemistry is a participatingmember, 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 1 V 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.Copies of the rules of the award and application forms may be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN I FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY Applications are invited for the award of the Marlow Medal for 1985 and prize of f 100. 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 made on the basis of publications (not necessarily in the Transactions) on any subject normall\ published in J. Chem. Soc., Faraday Transactions / 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. (ii)THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM NO. 1 9 Molecular Electronic Structure Calculations- Met hods 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. I t 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-moleculemesophases, 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 B. 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 W1V OBNTHE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 80 ~ Physical Interactions and Energy Exchange a t the Gas-Solid Interface 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 t o the nature of atom and molecule surface interaction potentials, including many- body effects. 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 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 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. Contributions for consideration by the Organising Committee are invited and abstracts of about 300 words should be sent by 12 October 1984 to: Professor J. S. Rowlinson, Physicat Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 302. Full papers for publication in the symposium volume will be required by August 1985. Mrs Y. A. Fish Dr N. Parsonage Dr D. A. YoungTHE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 81 Lipid Vesicles and Membranes Loughborough University of Technology, 1517 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 bitayers 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.THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY 1984 BOURKE LECTURES by Professor V. Ponec State University of Leiden, The Netherlands Monday 22 October 1984 6.00 pm Wednesday 24 October 1984 4.15 pm Thursday 25 October 1984 4.1 5 pm Friday 26 October 1984 2.00-5.30 pm (half-day Symposium Admission to the Lec Teesside Surface Science Club, Norton Hall, Stockton -on-Tees Catalysis of CO hydrogenation, and the synthesis of oxygen-containing molecules University College, Dublin Ensemble size and ligand effects in the catalysis of hydrocarbon reactions on alloys University of Bath Catalysis of CO hydrogenation, and the synthesis of oxygen-containing molecules Queen Mary College, London Particle size effects, promotion and metal-support interaction in heterogeneous catalysis ures is free and non-members will be welcome.Further information from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN~ ~ FARADAY DIVISION INFORMAL AND GROUP MEETINGS r Polymer Physics Group Fundamental Aspects of Polymer Flammability To be held in London on 29 October 1984 Further information from The Meetings Officer, The Institute of Physics, 47 Belgrave Square, London SWlX 8QX Polymer Physics Group with the Institute of Marine Engineers Polymers in a Marine Environment To be held at the Institute of Marine Engineers, London on 31 October-2 November 1984 Further information from Dr G. J. Lake, MRPRA, Brickendonbury, Hertford SG13 8ML High Resolution Spectroscopy Group CARS, Diode Laser and Microwave Spectroscopy To be held at the University of Reading on 16-18 December 1984 Further information from Dr J.M. 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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 1 &11 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 17-18 April 1985 Further information from Dr R. W. Richards, Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow G1 1 XLPolymer 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 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 4WT Division Annual Congress: Structure and Reactivity of Gas-Phase Ions To be held at the University of Warwick on 8-1 1 April 1986 Further information from Professor K. R. Jennings, Department of Molecular Sciences, University of Warwick, Coventry CV4 7AL (vii)physicochemical topics, thereby encouraging scientists of different disciplines to contr~bute their varied viewpoints to a common theme. A recent Discussion is I- The Royal Society of Chemistry- 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: The Spiers Memorial Lecture; Vibrational Redistribution within Excited Electronic States of Polyatomic Molecules Intramolecular Relaxation of txcited States Isomerization of Internal-energy-selected Ions Kinetics of Ion-Molecule Collision Complexes in the Gas Phase, Experiment and Theory Intramolecular Decay of Some Open-shell Po I y a t o mic Cations On the Theory of Intramolecular Energy Transfer Pulsed Laser Preparation and Quantum Superposition State Evolution in Regular and Irregular Systems A Quantum-mechanical Internal-coUlsion Model for State-selected Uniniolecular Decomposition The Currespondcnce Principle and Intramolecular Dynamics Intramolecular Dephasinp. Picosecond Evolution of Wavepacket States in a Molecule s i t h Intermediate-case level Structure Energy Conversion in van der Waals Coinplexes of s-Tetrazine and Argon Time-dependent Processes in Polyatoniic Molecules During and After Intense Infrared Irradiation Energy Distributions in the CN(X'I+) Fragment from the Infrared Multiplephoton Dissociation of CF7CN.A Comparison between Experimental Results and the Predictions of Statistical Theories Of ChFb + Product h e r g y Partitioning in the Decoin- position of State-selectively Excited HOON and HOOD Low-power Infrared Laser Photolysis of Tetramethyldioxetan Unimolecular Reactions Induced by Vibrational Overtone Excitation Ununolecular Decomposition of t-Butylhydro- peroxide by Direct Excitation of the 6-0 0-H Stretching Overtone Picosecond-jet Spectroscopy and Photochemistry. Energy Redistribution and its Impact'on Coherence, Isomerization, Dissociation and Solvation Energy Redistribution in Large Molecules.Direct Study of lntrainolecular Relaxation in the Gas Phase with Picosecond Gating Rotation-dependent Intramolecular Processesof SO:(A'A:) in a Supersonic Jet Role of Rotation-Vibration Interaction in Vibrational Relaxation. Energy Redistribution in Excited Singlet Formaldehyde Sub-Doppler, Spectroscopy 01' Benzene in the "Channel-t hree" Region Intramolecular Electronic Relaxation and Photo isomer Iza t 10 n Processes in the lsola ted Azabenzene Molecules Pyridine, Pyrazine and Pyrimidine Softcover 434pp 0 85186 658 1 Price f 25.00 ($48.00) Rest of the World f26.00 RSC Members f 16.25 Faraday Discussions of the Chemical Society No 7r Inrrornoleiulur Klncrlls faraday Symposia are usually held annually and are confined to more specialised topics than Discussions, with particular referenLe to recent rapidly developing lines of research.A recent Symposium is :- No.17 The Hydrophobic 1nteracKon No. 17 in the series, this publication is the result of a symposium on The Hydrophobic Interaction held at the University of Reading in December 1982. Contents: Hydrophobic Interactions--a Historical Perspective Precise Vapour-pressure Measurements of the Solubilization of Benzene by Aqueous Sodium Octylsulphate Solutions Nuclear Magnetic Resonance Relaxation Investigation of Tetrahydrofuran and Methyl iodide Clathrates Infrared and Nuclear Magnetic Resonance Studies Pertaining to the Cage Model for Solutions of Acetone in Water Isothermal Transport Properties in Solutions of Symmetrical Tetra-akylaninionium Bromides Thermodynamics of Cavity Formation in Water. A Molecular Dynamics Study Molecular Librations and Solvent Orient- ational Correlations in Hydrophobic Phenomena Monte Carlo Computer Sunulation Study of the Hydrophobic Effect. Potential of Mean Force for C(CH4)jaq at 25 and 50' C Geometric Relaxation in Water. Its Role in Hydrophobic llydration Hydrophobic Moments and Protein Structure Application of the Kirkwood-Buff Theory to the Problem of Hydrophobic Interactions Disentanglement of Hydrophoblc and tlectrostatic Contributions to the Film Pressures of Ionic Surfactants Hydrophobic Interactions in Dilute Solutions of Poly(vinyl alcohol) Conformational and Functional Properties of Haeiiioglobin in Water+Alcohol Mixtures. Dependence of Bulk Electrostatic and Hydrophobic Interactions upon pH and KCI ~oncentrdtions Softcover 240 p 0 85186 668 9 Price f36.50 ($70.001 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 SO6 1HN. England. Faraday Symposia of the Chemical Society \<, 17 The H\ drophohi' Interartion (viii)

ISSN:0300-9599    

DOI:10.1039/F198480FP069

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I , 1984,80, 2361-2374 Effect of Ionic Strength on Stability Constants A Study of the Electronic Absorption Spectra of the Mercuric Halides HgX+, HgX,, HgX; and HgXi- in Water BY TREVOR R. GRIFFITHS* AND RICHARD A. ANDERSON Department of Inorganic and Structural Chemistry, The University, Leeds LS2 9JT Received 15th June. 1983 Thermodynamic stability constants log Q and l o g e , at 20 "C, are reported for HgX, + X- + HgX;and HgX; + X- e HgXi-, respectively, in water, where X = I, Br and Cl. The electronic absorption spectra of these reactions are very sensitive to ionic strength ( I ) , and give, by a new approach, log KF = 3.79f0.01, 2.23k0.02 and 0.70+0.03 and log KT = 2.03+_0.02, 1.40+0.03 and 0.50+0.05, for X = I, Br and C1, respectively. Values determined at constant ionic strength, with added NaClO,, compare well with literature values determined by other techniques.To compute stability constants from digitised absorption spectra does not rquire constant ionic strength conditions, and thus the relations between K and I have been studied and they are discussed in terms of activity coefficients. The results are compared with the extended Debye-Huckel equation, employing appropriate distance of closest approach a parameters. The effect of added perchlorate on the reaction HgX, T HgX+ + X- is reported, and the activity coefficients of the neutral species HgX, and HgXY are found to be independent of concentration and probably essentially unity. The reaction between HgX, and added X- always involves a concentration range in which HgX,, HgX; and HgX,2- are in equilibrium.Plots of log K against I: indicate when a two-species equilibrium becomes a three-species equilibrium, and the ionic strength so identified tallies with that noted in appropriate sets of spectra. Since the KT values agree well with existing results this spectroscopic technique has great potential for determining thermodynamic stability constants in non-aqueous solvents, especially when electrochemical methods are not available. The stability constants of mercury(I1) halide complexes were among the first reported. Morsel calculated the first two-step stability constants Kl and K , for the mercury(r1) chloride system from solubility measurements in 1902, and the following year SherriP published the product K3 K4 for mercury(I1) halides, from measurements of the partition of the neutral species between benzene and aqueous halide solutions.The overall stability constants of the complexes were determined potentiometrically. For many years the preferred route to accurate (thermodynamic) stability constants has been via electrochemical measurements. When using spectroscopic procedures the approach has centred on absorbance changes at a few suitable wavelengths3 Stability constants have then been calculated directly or from a graphical technique. We have developed a new procedure (an outline is included herein) which uses complete spectra (digitised at 1 mm intervals) to give accurate concentrations of the various complex ions in solution, and hence accurate (stoichiometric) stability constants (and not under conditions of constant ionic strength). We here report our results and examine our derived (thermodynamic) stability constants and compare them with currently accepted literature values for mercuric halides.23612362 STABILITY CONSTANTS OF MERCURIC HALIDES EXPERIMENTAL SPECTROSCOPIC MEASUREMENTS Details of the Applied Physics Cary 14H spectrophotometer and its Harrison Instruments digitising system have been described previ~usly.~ Thermostattable cell holders were used, and the water circulated from a thermostat bath maintained the solutions in the cells at _+ 0.1 "C. All spectra were recorded with the appropriate medium in the reference cell. CHEMICALS AND SOLUTION PREPARATION All chemicals were of the highest purity, thoroughly dried, and stored in a vacuum desiccator.Water was distilled and deionised before use. Because of the low and slow solubility of HgI, in water, solutions were prepared by mechanically stirring in water at ca. 40 "C for several hours. After cooling and filtering the solutions were transferred to a graduated flask and made up to volume. An aliquot was mixed with KI solution such that the iodide/mercury ratio was at least 30000 and the solution approximately 1.5 mol dm-3 in KI. HgI, was thus quantitatively converted to HgIi-, which has an absorbance peak at 323 nm. The HgI, concentration was determined from a calibration plot established by dissolving weighed quantities of HgI, in 1.5 mol dm-3 KI solution. Concentrations were accurate and repeatable to within < 0.5%. Standard solutions of KI and NaI were prepared by weighing into graduated flasks.To maximise accuracy the air-conditioned laboratory was maintained at 20 & 1 "C. Solutions for spectral measurement were prepared by transferring known volumes of stock solutions to a graduated flask with a pipette or burette, as appropriate, and making up to volume. Solutions requiring constant ionic strength were prepared by adding the calculated volume of NaClO, solution required. All glassware was grade A. SOLUTION STABILITY For the different concentrations studied, HgI, solutions obeyed the Beer-Lambert law. Solutions containing added iodide were stable for several days in the dark, but for only two hours in the spectrophotometer light beam, because the reversed-beam optics of the spectrophotometer exposed the sample to the full radiation of the quartz-halogen lamp.Test solutions were measured, stored and their spectra recorded again after one, three and nine weeks. After nine weeks the peak maximum of HgI, has decreased by 3%. Fresh solutions were therefore used in all experiments so that hydrolysis was insignificant during the period of use. COMPUTING PROCEDURES The pen noise inherent in spectroscopic measurements may be reduced by multiple scanning, but this is time-consuming and unrealistic for a two hour time-window. Noise reduction was therefore achieved by mathematical smoothing5 of the digitised spectra, which were recorded at 1 nm intervals. Degradation of the original profile was avoided by using a five-point smoothing convolute. CALCULATION OF STABILITY CONSTANTS Of the many techniques available for the determination of stability constants, potentiometric, polarographic, radioactive-tracer studies and spectrophotometric measurements have all been applied to the mercuric halide systems.Direct measurement of the concentration of individual species and curve fitting for various equations are among the methods used to yield values for the stability constants under various conditions of solvent type, ionic strength and temperature. If the molar absorbances of all the species in a solution are known, then the stability constants may be calculated using a series of solutions of different ligand concentrations but constant ionic strength. For this work, the reactions occurring in solution on addition of halide were either HgX, + X- + HgX; (i> or (ii)T.R. GRIFFlTHS AND R. A. ANDERSON 2363 or both. For reaction (i) the following relationship holds, uiz. A I M = [&(HIS,) K3 X&(HgXi)I/(1 i- K3 X) (1) where A is observed absorbance, M is the total molar concentration of mercury, &(Ha,) and &(HgX;) are the molar absorbances of HgX, and H a ; , respectively, K3 is the stability constant and X is the free-ligand concentration. Similarly, for reaction (ii) we have (2) AIM = [&(HgX,)+K,Xe(HgX~-)]/(l + K4X). When both reactions are occurring the following equation is valid: ~(Hgx,) + K3 Xs(HgX;) + K3 K4 P &(Ha:-) 1 + K3 X+ K3 K4 x2 AIM = (3) However, when sets of spectra recorded at constant ionic strength were applied to eqn (1)-(3) the results were not reliable.It was found, as has been noted by that formation constants calculated in this way are very dependent on the values of the molar absorbance used. Instead, therefore, the concentrations of the individual species were calculated as follows. At any given wavelength, the absorbance of a solution will be given by A = &fEI (4) where c, and E~ are the molar concentration and molar absorbance at a given wavelength of the ith species in solution. This equation holds true for all wavelengths. Thus, if A is measured for n different wavelengths, then n linear equations of i unknowns may be derived. For n % i these linear equations may be accurately solved for ct by means of multiple linear-regression analysis.The output of the program employed was the required concentrations, with their standard error, and various parameters which indicated the accuracy of the main computation and the precision of the fitted data. These latter included the residual sum of squares, the regression sum of squares, F ratio, multiple correlation coefficient and degrees of freedom of the F ratio. A table of residuals and standardised residuals was also obtained. Stability constants, with their standard error, were computed from the regression coefficients. This method had the advantage that conditions of constant ionic strength were not necessary, and hence the variation of stability constant with ionic strength could be studied. This is thus a new approach for determining stability constants from spectroscopic data, and the accompanying statistical data ensure that all erroneous spectra can be identified and eliminated. The resulting stability constants are thus accurate and reliable.To our knowledge this is the first time stability constants have been calculated using complete spectra: previously absorbance values at fixed wavelengths have been used. An account of existing techniques is given in ref. (3). TESTING THE METHOD FOR RELIABILITY AND ACCURACY Advantage was taken of the use of complete spectra in stability constant calculations. Since eqn (4) is valid for any wavelength range, constants calculated from data covering different wavelength ranges should be the same. If they are significantly different it may be assumed that one of the reference spectra is inaccurate over part of the spectrum.The accuracy of the computation, as reflected by the correlation coefficient and standard errors, was improved upon using a correct spectrum, or part thereof. An additional check is that the total metal concentration may be computed and then compared with known concentration in the sample solution. Complete details of our method, which simultaneously generates the spectra of the species H a ; , will be published elsewhere: we restrict ourselves here to the stability constants we have determined and their implications. The symbol K will represent all experimentally determined stoichiometric stability constants and 1% will represent the thermodynamic stability constants obtained from extrapolations back to zero ionic strength.2364 STABILITY CONSTANTS OF MERCURIC HALIDES Table 1.Effect of dilution on the dissociation of HgX2 in water 10-2 0.0035 - 0.123 - 0.575 - - 10-3 0.0106 - 0.389 - 10-4 0.035 0 1.23 0.3 5.75 5 5 10-5 0.106 0 3.89 3.5 18.20 20 18.5 10-6 0.35 0.5 12.30 12.5 57.20 61-73 61.0 - - 1.82 a Calculated from % dissociation = lOO/(Km);, where logK = 10.95, 7.82 and 6.48 for HgI,, Observed % deviation from the Beer-Lambert Since deviation with HgCl, was not independent of wave- HgBr, and HgCl,, respectively [from ref. (8)) law over a wide spectral range. length, this column refers to 200 nm. RESULTS MERCURIC HALIDES AND EFFECT OF ADDED PERCHLORATE The mercuric halides have very high stability constants in but at concentrations of spectroscopic interest, typically < 1 0-4 mol dmP3, dissociation can become significant, according to HgX, + HgX+ + X-.(iii) Spectra were therefore recorded at various concentrations and the results are summarized in table 1. Essentially no reaction was found for HgI,, but below and mol dm-3 for HgBr, and HgCl,, respectively, dissociation is apparent. The percentage reduction in the molar absorbance of the peaks is comparable with the percentage dissociation. Solutions containing HgI, maintained the same spectral profile throughout the whole concentration range. Solutions of HgBr, behaved similarly, though the peak became less resolved as dilution increased, but the peak at 200 nm for HgCl, at lo--, mol dmP3 had flattened at 4 x mol dm-3 and at lower concentrations only an absorption edge was seen (fig.1). The Beer-Lambert law was thus obeyed by HgI, at all concentrations used, but by HgBr, and HgC1, only at the higher concentrations. The spectra of various concentrations of the mercuric halides in aqueous NaC10, solutions with concentrations from 1.0 to 0.0001 mol dm-3 were not significantly affected by change in ionic strength. The Beer-Lambert law was obeyed by HgI, and HgBr, for halide concentrations from lov4 to mol dm-3 and by HgCl, above mol dm3. A similar observation has been made previouslyl0 for HgBr,. At lower molarities of HgCl,, e.g. 5 x mol dm-3, as the ionic strength was increased the 200 nm peak increased slightly, so that in 0.5 mol dm-3 NaClO, solution it was 2% higher than in pure water. Thus the perchlorate was affecting the activities of the ionic species in reaction (iii) and causing the equilibrium to be displaced towards undissociated HgCl,.That perchlorate should exert a salting-out effect under these conditions was thus surprising and unexpected.4 - I E 3 - 0 E E - a 2 0, CI m . UJ 1 T. R. GRIFFITHS AND R. A. ANDERSON 2365 200 210 220 230 2 40 wavelength/nm Fig. 1. Effect of dilution on the electronic absorption spectrum of aqueous mercury@) chloride at 20 "C. 1, maximum concentration, 2 x 10-3 mol dm-3; 2, minimum concentration, 2 x mol dm-3. MERCURIC HALIDES IN PERCHLORIC ACID In aqueous perchloric acid (2 mol dm-3) the spectra of HgX, had lower molar absorbances than those in water. A plot of the percentage difference between the two sets, relative to HgX, in water, has minima at 255 and 208 nm for HgI,, at 217 nm for HgBr, and at 196 nm for HgCl, (fig. 2).It is therefore suggested that HgX+ has absorbance maxima around 255 and 208 nm. Furthermore, since for HgI, the percentage difference at 255 nm approaches zero, the molar absorbance ( E ) of HgI+ at that wavelength must be approximately equal to that of HgI, (viz. 4500). Our results for HgI+ may be compared with the value of 245 nm reported by Griffiths and Symons,ll who observed a shoulder at this wavelength on the spectrum of HgI, in 72% HClO,, and the value of 266 nm reported by van Eck,' who calculated the spectrum of HgI+ in 0.5 mol dm-3 NaC10, solution using SillCn's stability constants.s Van Eck also reported peaks at 221 and 238 nm (both with E = 4000) for HgBr+ and HgCl+, respectively.' However, if HgCl+ had a peak at 238 nm of the reported molar absorbance it should be seen to appear on dilution of HgC1, solutions.We found no peak or shoulder here, even after repeated attempts, and van Eck's results must be considered doubtful. MERCURIC IODIDE WITH ADDED IODIDE The stability constant K3 was here obtained by two different methods, but by only one for K,. Our results, at 20 "C, for ionic strength 0.5 mol dm-3, with added NaClO,, are compared with those of Sillens and Marcus,12 at 25 OC, in table 2. Our two values for log K3 are within experimental error and our findings for log K3 and log K4 compare well with those of Marcus,12 who used an extraction into benzene procedure. They do not, however, compare well with those of Sillen,s who employed solubility measurements.Each reported error limit, which is the same as or better than previous values, reflects the maximum error value obtained. Upon examining the role of ionic strength we noted that for a given R value (R = mole ratio X-/HgX,), the conversion into HgXf was greater in NaClO,2366 STABILITY CONSTANTS OF MERCURIC HALIDES 200 220 240 I , 260 1 280 300 320 200 220 240 260 200 220 24 0 wavelength/nm Fig. 2. Effect of perchloric acid on the spectra of the mercury@) halides in water at 20 "C. 1, HgX, in water; 2, HgX, in 2 mol dm-3 HC10,; 3, percentage difference. solution than in aqueous solution, The reverse was true for HgX;. This is in agreement with reported studies showing an increase in K4 with ionic strength.13 This phenomenon was studied in greater detail by recording the spectra of solutions of the same R value at different NaClO, concentrations and at different mercury concentrations [fig.3 (a) and (b)]. The two-species equilibrium between HgI; and HgIt- [fig. 3(c)] at R = 100 became a three-species equilibrium as dilution caused the ionic strength, and hence the activity of the tri- and tetra-halide species, to be reduced. Stability constants were calculated for all solutions used in the absence of NaClO,. K3 remained constant at low ionic strength, with logK3 = 3.79k0.01. Above fi GZ 1.6 x this plot deviated to higher K values [fig. 4(a)]. A plot of log K4 against @ [fig. 4(b)] gave a straight line at higher ionic strength, extrapolating toT.R. GRIFFITHS AND R. A. ANDERSON 2367 Table 2. Formation constants for HgX; and HgX,2- in water at 20 "C constant ionic X K strengtha at infinite dilutionb ref. (12)c ref. (8)d I logK3 3.69f0.03 ~ 2.38 k 0.02 Br log K3 2.34 k 0.02 1.91 f0.03 C1 IogK, 0.81 kO.10 - log K4 K4 1% K4 K4 1% K4 K4 - - - 3.67f0.02" 3.79k0.01 3.67f0.02 3.78f0.14 - 2.03 & 0.02 2.37 f 0.05 2.23 & 0.02 - 2.03 +_ 0.03 2.25f0.06" 2.23t0.02 2.27f0.02 2.41k0.11 1.87 +_ 0.02" 1.40 f 0.03 2.04 k 0.05 1.75 f 0.03 - 1.36 f 0.03 - 0.70 +_ 0.03 0.95 f 0.03 0.85 k 0.15 - 0.50 f 0.05 1.05 & 0.06 1 .OO f 0.06 - 0.49 & 0.06 - - - - - - a For I and C1, ionic strength was made up to 0.5 rnol dm-3 with NaClO,; for Br, to Extrapolation oflog K3, log K4 and K, against It plots, respectively, 0.49 mol dm-3 NaClO,+0.01 mol dm-3 HClO, 1 .O rnol dm-3 with NaC10,. for each halide, for two-species equilibria.(25 "C). 0.5 rnol dm-3 NaC10, (25 "C). Independent calculation from same data set. l o g q = 2.03k0.02 for P = 0, but this line deviated towards the origin below fi = 0.01, intercepting the abscissa at fi x 3 x MERCURIC BROMIDE WITH ADDED BROMIDE Studies at constant ionic strength (made up to 1 .O mol dm-3 with NaClO,) at 20 "C yielded values of 2.34 0.02 and 1.91 k 0.03 for log K3 and log K,, respectively. A less precise procedure gave 2.25 & 0.06 and 1.87 & 0.02, respectively. Our results are intermediate between those of SillCn6 and Marcus12 (table 2). In pure water, the formation constants again varied with ionic strength at given R values.K3 was constant only at low ionic strength, giving l o g e = 2.23 f 0.02 [fig. 4(a)] and increasing rapidly at higher ionic stren ths. The variation of log K4 with fi is shown in fig. 5(b), being linear above lfz 9 x and extrapolating to log = 1.40 0.03 at fi = 0. At lower ionic strength the plot deviates from linearity to intercept the abscissa at fi w 4 x lov2. MERCURIC CHLORIDE WITH ADDED CHLORIDE Using an ionic medium made up to 0.5 mol dmd3 with NaClO,, when R values > 500 were exceeded, salting-out occurred, thus preventing study over the whole CI- concentration range. At equal R values, with and without NaClO,, the conversion into HgCl; was more pronounced in the presence of NaClO,. The effect of ionic-strength variation at different R values is shown in fig.6. Log K3 was calculated as 0.8 I f 0.10 at ionic strength 0.5 mol dmd3 (including NaClO,). This is slightly lower than previous values8, l4 (table 2). In the absence of perchlorate, at low ionic strength log K3 remained constant at 0.70+0.03, but above 1: x 6 x it increased rapidly [fig. 7(a)]. A plot of log K4 against 14 was similar to that for HgBrg- [fig. 7(b)]. The straight line at high ionic strength gave an intercept at zero ionic strength of log KT = 0.50 k 0.05. Below 1; = 3.0 x '10-1 this line deviated towards the abscissa giving an intercept at r: w 1.5 x 10-1.2368 STABILITY CONSTANTS OF MERCURIC HALIDES 2 I , 3 2 CI 250 300 350 4 00 E E 0 U m . u, 3 2 1 0 3 2 50 3 00 3 50 400 k 2 50 300 3 50 400 260 300 3 4 0 wavelength/nm Fig. 3.Effect of ionic strength [(a) and (b)] and dilution [(c) and (d)] on the spectra of HgI, + I- in water at 20 "C. Mercury concentration 3.2 x mol dm-3. (a) and (b) Ionic strength adjusted with added NaClO, : (a) R = 20, I/mol dmP3 : 1,O.O 1 ; 2,O. 1 ; 3 , O . 5 ; 4, 1 .O. (b) R = 200, I/moldm-3: 1, 0.01; 2, 0.1;3, 0.5; 4,l.O. (c) and ( d ) Ionic strength due to added halide: (c) R = 100, I/mol dm-3: 1, 0.0071; 2, 0.005; 3, 0.0035; 4, 0.0018; 5, 0.0007. ( d ) R = 1000, I/moldm-3: 1, 0.036; 2, 0.025; 3, 0.018; 4, 0.007. DISCUSSION The thermodynamic stability constant Kr for reactions (i) and (ii), which take the form A + B + AB, is given by fl = Jww/y(A) Y(B) ( 5 ) where K is the stoichiometric stability constant, reported here, and y(A) is the activity coefficient of species A etc.KT values are more commonly obtained by extrapolating K values to infinite dilution. Plots of log K against both I and fi are used and give reliable values of KT for reactions of the type studied here.15 We chose to examine mainly the function log K(fi) because of its relation to the Debye-Hiickel equation.T. R. GRIFFITHS AND R. A. ANDERSON 2369 4.0 3.9 G 00 - 3 . 8 3.7 2.4 G ?? - 2.0 200 150 d 100 50 0 0 0.01 0.02 0.03 Ii/mol* dm-) Fig. 4. Effect of ionic strength on K3 and K4 stability constants for HgI, +I- in water at 20 "C. (a) Extrapolated thermodynamic log K3 = 3.79 f 0.01 : dashed line, K3 values calculated assuming a two-species equilibrium between HgI and HgI; ; solid line, two species equilibrium up to fi = 0.013 and a three-species equilibrium, including HgI:-, above.(b) Extrapolated thermodynamic log K4 = 2.03 k0.02. (c) K4 intercept at 106 f 7, giving thermodynamic log K4 = 2.03 k 0.03. Dashed lines in (b) and ( c ) give K4 values calculated assuming that a two-species equilibrium between HgI; and Hg1:- holds below = 0.05. log K3 VALUES AND IONIC-STRENGTH DEPENDENCE If we assume that for neutral species, here HgX,, activity coefficients are unity and independent of the ionic strength of the medium, then the relationship between log K3 and ionic strength may be explained qualitatively. Eqn ( 5 ) may be re-written as (6) log K = log KT - log y(AB) + log y(A) + log y(B) where AB = H a ; , B = X- and A = HgX,. Thus for logK3 to be essentially independent of ionic strength at low concentrations shows that the activity coefficients2370 1 .5 e 21 .o d 0 . 5 0 6 0 L 40 20 0 STABILITY CONSTANTS OF MERCURIC HALIDES d 0.3 Fig. 5. Effect of ionic strength on K3 and K4 stability constants for HgBr,+Br- in water at 20 "C. (a) Extrapolated thermodynamic log K3 = 2.23 & 0.02. (b) Extrapolated thermodynamic log K4 = 1.40 f 0.03. (c) K4 intercept at 22.4 _+ 1.6, giving thermodynamic 10gK4 = 1.35f0.03. of HgX; and X- are the same within experimental error in this region. Unfortunately this does not provide the activity coefficients of the individual ions, but the rate of decrease of the absolute activity coefficients of these two anions can be said to be the same at low ionic strengths with concentration increase. Using the extended Debye-Hiickel equation concepts implies that the mean distance of closest approach for X- is greater in water than for H a ; .The strong, and hence bulky, hydration shell of X- prevents the close approach of other ions. The larger HgX; ion is consequently expected to be less strongly solvated. log K4 VALUES AND IONIC-STRENGTH DEPENDENCE Plots of log K4 against fi (fig. 4, 5 and 7) yielded straight lines at high ionic strength, the slopes of which increased from chloride to iodide; the line for HgC1;- wasT. R. GRIFFITHS AND R. A. ANDERSON 237 1 I L 2 00 2 20 240 2 60 220 240 2 60 wavelengt h/nm Fig. 6. Effect of ionic strength upon the electronic absorption spectrum of HgC1, + CI- in water at 20 "C, with R value constant and ionic strength varied by dilution. (a) R = 1000, I/mol dm-3: 1, 0.01; 2, 0.02; 3, 0.06; 4, 0.12. (b) R = 10000, I/mol dm-3: 1, 0.06; 2, 0.12; 3, 0.29; 4.0.58. horizontal within experimental error. With decreasing ionic strength, the value at which the plots deviated from linearity towards the abscissa decreased from chloride to iodide. However, at lower ionic strengths the points at which these plots intersected the abscissa could not readily be extrapolated. We also plotted K4 against fi (fig. 4, 5 and 7). Surprisingly, although there is no theoretical justification, good linear plots were obtained, now comprising two straight lines, and the intercept on the abscissa was determined. Furthermore, our plots have many more K values than are normally determined. The results are interpreted thus. The straight line at low ionic strengths corresponds to that concentration range when HgXi- is in a three-species equilibrium with HgX; and HgXi-. After the change in slope a two-species equilibrium, between HgX; and HgX,, is present.The positive intercept on the abscissa shows that no HgXi- can form until HgX; has been formed, and HgX; will only form in the presence of an excess of halide and not by disproportionation of HgX,. The distance of the intercept from the origin affirms the ease of formation of HgXZ- and is in the order I < Br c C1 ( I = 1 x loF5, 1.6 x and 2.3 x mol dmV3, respectively). These values are also the approximate minimum ionic strengths at which formation of HgX,2- will occur and they are consistent with the spectral evidence.14 > 0.001 mol dm-3 with logK (or K ) are not expected, or common.Excluding our results for chloride, where there is more scatter, linear relationships for log K3 are here observed up to around 0.0002 and 8.0006 mol dm-3 for iodide and bromide, respectively. However, for log K4, above a critical concen- tration, when all the HgX, has reacted, a linear relationship was found up to the maximum ionic strengths used, namely 0.017 and 0.09 mol dm-3 for iodide and bromide, respectively. We thus have Interestingly, we further have where m is the slope of the line. This reduces to Linear relationships for log K4 = log - log y(HgXi-) +log y(HgX;) + log y(X-). (7) (8) y(HgX,) m-) = Y(HgX,2-)m. (9) 1% Y(HgX,) + 1% y(X-) = m 1% Y(HgX:-)2372 STABILITY CONSTANTS OF MERCURIC HALIDES I 1 I 1 I i 0 -10 0.20 0 0 0 4 J Do - 0.2 4 0 (b) I I I I I I I 0 I- 0 0 d 31- 2 0 00 0 0 0 0 1 I I 1 1 I 1- 0.4 0.8 1.2 I f / m o d dm-3 Fig.7. Effect of ionic strength on K3 and K4 stability constants for HgC1, + Cl- in water at 20 "C. (a) Extrapolated thermodynamic log K3 = 0.70 k 0.03 ; dashed line, K3 values calculated assuming a two-species equilibrium between HgCl, and HgCl; ; solid line, K3 values calculated assuming a three-species equilibrium, including HgC1,2-. (b) Extrapolated thermodynamic log K4 = 0.50 f 0.05. (c) K4 intercept at 3.1 f 0.15, giving thermodynamic log& = 0.49+0.06. Unfortunately at this time this relationship cannot be usefully employed : accurate activity coefficients for the individual halide ions are required. APPLICATION OF EXTENDED DEBYE-HUCKEL EQUATION The extended Debye-Huckel equation can be applied to the formation of HgX- in water using our data, provided appropriate closest approach parameters (a) are chosen.ls For example, for the formation of HgI,2- two sets of results were obtained,T.R. GRIFFITHS AND R. A. ANDERSON 2373 Table 3. Application of extended Debye-Huckel theory to the formation of Hg1;- in water after D-H I / mol dm-3 experimental correctiona 5.8 7.3 8.5 10.0 11.5 13.0 16.6 20.3 27.4 41.5 48.9 56.3 63.5 70.9 141.6 288.3 1.91 1.87 1.99 1.95 2.03 1.97 2.07 1.99 2.08 2.00 2.10 2.02 change of slope 2.13 2.05 2.14 2.08 2.16 2.06 2.19 2.07 2.20 2.08 2.22 2.06 2.23 2.07 2.27 2.06 2.29 2.07 2.41 2.1 1 a The distance of closest approach, a, employed for I-, HgI; and HgIq- was 4, 3 and 4.5 ( x lo-*) cm, respectively.corresponding to a three-species equilibrium where log K4 increased with increasing ionic strength, and a two-species equilibrium (between HgI; and HgIi-) where log K4 remained constant at 2.07f0.02. The a parameters chosen were (3, 4 and 4.5) x cm for HgI;, I- and HgIi-, respectively, and the results are given in table 3. Obviously a better correspondence between the experimental and Debye-Hiickel corrected log K4 values could be obtained upon varying the a values, but there would not be one unique set of a values. Note the change in slope at Z = 1.5 x low3 mol dm-3 after correction, corresponding to the observed change in the observed data, and that for the two species a straight line of approximately zero slopelS was obtained up to 0.03 mol dm-3, the limit of measurement.The extended Debye-Huckel equation is normally linear up to 0.1 mol dmm3. ACTIVITY COEFFICIENTS OF NEUTRAL HALIDES We finally note that Spiro and HumelB have claimed that the stability constant of HgXY in aqueous 0.5 rnol dm-3 NaCIO, would be 0.11 log units greater than in the absence of perchlorate. They studied the reaction of the neutral halides HgX, and HgY, and assumed that the activity coefficients of the reactants were dependent upon the ionic strength of the medium but that of the product, HgXY, was independent. We have found that the spectra of HgX, and HgYz mixtures in 0.1 rnol dm-3 NaCIO, exactly superimpose the respective mixtures in water alone. Our earlier assumption that the activity coefficients for the neutral dihalides in water are independent of concentration, and essentially unity, is thus supported.The only alternative explan- ation for our observation would be that the activity coefficients of all three species cancel, which implies that y(HgXY), x y(HgX,) y(HgY,).2374 STABILITY CONSTANTS OF MERCURIC HALIDES CONCLUSIONS The electronic absorption spectra of the various halogeno-mercury(I1) complex ions are very sensitive to the ionic strength of the medium. Stability constants calculated from digitised spectra recorded in the presence of added NaCIO, compare well with those obtained by a distribution technique. Plots of log& against fi are independent of concentration at low ionic strengths, implying equivalence of y(HgX;) and y@-) values.At high ionic strengths, the positive deviation reflects the effect of the size of the reacting halide ion and correlates qualitatively with extended Debye-Huckel theory. The concentration dependence of log& reveals the ionic strength at which the change from a three- to a two-species equilibrium occurs and parallels data derived using the extended Debye-Hiickel equation in the range where it is applicable. The activity coefficients of HgX, and HgXY are independent of concentration. Finally, this approach is equally applicable to similar spectra obtained in non-aqueous systems. Electrochemical methods are not always possible, and as the reliability of our technique is now established it has considerable potential for the future. We thank the S.E.R.C. for the provision of the Cary 14H spectrophotometer and the UKAEA for the digitising equipment (purchased under EMR 1913). R.A.A. thanks the University of Leeds for a Research Studentship. T.R.G. thanks the Chemistry Department, Michigan State University, East Lansing, Michigan 48824, U.S.A. for their hospitality while on leave from the University of Leeds. Valuable discussions with Dr S. R. Crouch, Michigan State University, are gratefully acknowledged. H. Morse, Z . Phys. Chem. Leipzig, 1902, 41, 709. M. S. Sherrill, 2. Phys. Chem. Leipzig, 1903, 43, 705. F. R. Hartley, C. Burgess and R. Alcock, Solution Equilibria (Ellis Horwood, Chichester, 1980). T. R. Griffiths and R. A. Anderson, J. Chem. SOC., Faraday Trans. 2, 1979,75, 957. A. Savitsky and M. J. E. Goley, Anal. Chem., 1964,36, 1627. S . Feldberg, P. Klotz and L. Newman, Inorg. Chem., 1972, 11, 2860. L. C. Sillen, Acta Chem. Scand., 1949, 3, 539 and references therein. 'I C. L. van Panthaleon van Eck, Thesis (Leiden, 1958). @ J. H. R. Clark and L. A. Woodward, Trans. Faraday SOC., 1965, 61,207. lo D. B. Scaife and H. J. V. Tyrrell, J. Chem. SOC., 1958, 392. l1 T. R. Griffiths and M. C. R. Symons, Trans. Faraday SOC., 1960,56, 1752. l2 Y. Marcus, Acta Chem. Scand., 1957, 11, 329, 599, 610, 81 1. l3 H. C. Moser and A. F. Voight, J. Znorg. Nucl. Chem., 1957,4, 354. l4 T. R. Griffiths and R. A. Anderson, to be published. l6 F. J . C. Rossotti and H. Rossotti, The Determination of Stability Constants (McGraw-Hill, New l6 T. G. Spiro and D. N. Hume, J . Am. Chem. SOC., 1961,83,4305. York, 1961), p. 32. (PAPER 3/1016)

ISSN:0300-9599    

DOI:10.1039/F19848002361

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J. Chem. Soc., Faraday Trans. 1, 1984,80, 2375-2393 Meniscus Curvatures in Capillaries of Uniform Cross-section B Y GEOFFREY MASON*? AND NORMAN R. MORROW New Mexico Petroleum Recovery Research Center, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, U.S.A. Received 27th June, 1983 Menisci contained in capillaries of uniform cross-section can be broadly classed according to whether wedge-like liquid structures exist, as in triangular-section tubes, or do not exist, as in circular-section tubes. In tubes which form wedge menisci the liquid in the wedge adopts a form so that a section through the liquid surface is the arc of a circle. The volume of liquid per unit length of the wedge is constant along the tube. A non-wedging meniscus, however, is locally bounded by its tube and has a curvature inversely proportional to the hydraulic radius of the tube.Mayer and Stowe (J. Colloid Interface Sci., 1965,20,893) proposed an approximate method of determining the mean surface curvature of menisci in sphere packs. It was later applied independently by Princen (J. Colloid Interface Sci., 1969,30,60) to estimating capillary rise in spaces between parallel rods. The method, which incorporates the presence of wedges, is shown to be exact for determining mean surface curvatures in systems where the meniscus is undistorted by gravity. Experimental confirmation of the theoretical predictions to within 1.5% was obtained from measurements of capillary rise of a perfectly wetting liquid in tubes formed either by a rod and a square corner or by two rods and a plate.The conditions of pore geometry and contact angles which give rise to wedge menisci are discussed and illustrated by examples which include menisci in tubes of polygonal section. A porous material partially saturated with liquid has properties that are dominated by the capillary behaviour of the liquid in the pore space. Processes such as desorption of capillary-condensed gases, mercury-intrusion porosimetry and even the water- proofing of fabrics depend upon liquid menisci in pores. An important factor determining capillary behaviour in pore spaces is the pressure differences across the interface, or meniscus, which separates two phases. This pressure difference, usually called the capillary pressure, is proportional to the interfacial tension between the two phases comprising the interface and, for a given pore shape and wetting condition, varies inversely with pore size.A fundamental property of the meniscus in a pore is the maximum mean surface curvature or displacement, which corresponds to the situation when a meniscus passes through a pore. This curvature is often normalized with respect to some characteristic length of the system such as particle radius. In general pores in porous materials have irregular shape, and the meniscus curvatures are not known for such cases. Even for relatively simple pore shapes, such as those defined by spheres, there has been no exact analysis of meniscus curvatures. Several approximate solutions have been evolved in the past. These were mostly for packings formed by equal spheres.We examine here a method put forward by Mayer and Stowel for pores formed by spheres and subsequently by P r i n ~ e n ~ - ~ for spaces given by parallel rods. t Present address: Department of Chemical Engineering, Loughborough University of Technology, Loughborough, Leicestershire LE11 3TU. 23752376 MENISCUS CURVATURES IN CAPILLARIES The basis for computations made by Mayer and Stowel and Prin~en,~-~ referred to here as the MS-P method, are identical. However, Mayer and Stowel assumed that their analysis was exact for interfaces passing through converging-diverging pores formed by spheres. While it provides a useful approximation if the contact angle is zero, it can be expected that errors in the curvatures computed by Mayer and Stowe for spheres will increase significantly with increasing contact angle.P r i n ~ e n ~ - ~ applied the same analysis to spaces of uniform cross-section given by rods in various arrays. However, the power and exactness of the method, as applied by P r i n ~ e n ~ - ~ to systems formed by rods, was partially obscured by restrictions related to deviations from constant curvature that arise from treatment of systems in terms of capillary rise against gravity. In a past investigation5 remarkable agreement was found between the predictions of the MS-P theory and experimental results. The tube geometry of those experiments involved the space enclosed by two equal parallel contacting rods in contact with a plate. Liquids having different contact angles with the solid were used.This present paper reports a rigorous derivation of the theory for systems of constant curvature together with experiments using configurations having very precisely controlled geometry. Two basic configurations were investigated: the space between a rod and a square corner and that between two rods and a plate. A perfectly wetting liquid was used. The theoretical analysis has been extended to incorporate the effect of contact angle. As an example of how changing the contact angle can alter the basic meniscus configuration, the theory is given for the variation with contact angle of the meniscus curvature in polygonal-sectioned tubes. BACKGROUND TO THEORY In general, liquid menisci in pores of complex configuration have complicated shapes.The application of the MS-P theory requires that the form of the basic meniscus in the pore be given, and for this a terminology is needed. In describing the basic form we will use the terms wedge menisci and terminal menisci. Wedge menisci exist in wedge-like spaces such a those formed by two rods in contact as shown in fig. 1 (a), by two plates contacting at an angle as in fig. 1 (b), or by two rods in contact with a plate as in fig. 1 (c). In a mathematical sense they are infinitely long, with the wedge having constant volume per unit length of the capillary. The terminal part of the meniscus, assuming for the present that the tube is perfectly wetted and ony one phase generates wedges, is the region that spans and fills the tubular space. For menisci between two contacting rods and a plate, fig.1 (c), there is a terminal meniscus which merges into three infinite wedge menisci. For two rods which are close together, but not in contact, the terminal meniscus is a saddle-shaped surface between the rods. It is easier to envisage these meniscus shapes as closed surfaces. Consider, for example, a drop of liquid between two separated rods. The drop will be elongated and there will be a terminal meniscus at each end and two wedge menisci along the centre of the drop. As another example, consider the space between two contacting rods and a plate to be totally filled with liquid and imagine a bubble in the interior space. If the bubble is much longer than its diameter it will have a terminal meniscus at each end and three wedge menisci along the central portion of the bubble. In the absence of gravity these menisci are surfaces of constant mean curvature. Note that in each case the curvature of the wedge menisci, and hence total curvature, is set by the terminal menisci.The essence of the MP-P method is the equating of the curvature of the wedge menisci to the curvature of the terminal meniscus. Together with a virtual work (orG. MASON AND N. R. MORROW 2377 A t 1 I I I I I L B ! t C t 3 Fig. 1. (a) Wedge-meniscus formation between two contacting rods. (b) Wedge-meniscus formation in the angle between two flat plates. (c) Wedge menisci and the terminal meniscus in the space between two contacting rods and a plate. force-balance) equation, this enables the curvature of the terminal meniscus to be determined.In physical terms the method centres on a particular feature of the meniscus behaviour. The curvature of a terminal meniscus is determined in part by the local boundary conditions (position of tube walls, contact angle) and in part by the wedge menisci. However, the curvature of the wedge menisci must equal that of the terminal meniscus, so by varying the radius of curvature of the wedge menisci the meniscus seeks out a position where, if possible, the two curvatures are equal. All this is general. One may, however, choose a particular position to apply the curvature conditions in the analysis. At sufficient distance (in practice, this is not far in terms of number of tube radii) from the terminal meniscus, the profile of the wedge meniscus becomes a circular arc of radius r.Its radius of curvature in the plane of the cross- section is r and its other radius of curvature is infinite. Having a position where the section through the wedge meniscus is simply a circular arc is not just convenient but is crucial to the method. As mentioned before, the systems for which the MS-P method was created obscure the simple elegance of the method. Mayer and Stowel used the analysis for pore geometries involving spheres, whereas their analysis was really for rods. P r i n ~ e n ~ - ~ was concerned with capillary rise, and this inevitably involves distortion of the menisci by gravity and wedges of changing curvature. As a consequence he introduced the condition that the meniscus dimensions must be negligible compared with the height of capillary rise.In the absence of gravity the MS-P method is exact for tubes of uniform cross-section. The principles of the application of the MS-P method are straightforward, but the details of application depend on whether or not a wedge-shaped interface exists. For example, wedge menisci never occur in the circular-section tubes but always occur in tubes made up from three cylinders in contact, irrespective of the contact angle. However, for the configuration of a rod inside a tube, there are no wedge menisci when the rod is centrally placed in the tube; when the rod touches the tube, wedge menisci2378 MENISCUS CURVATURES IN CAPILLARIES I I l l I 1 0 1 ' I I ' 1 I " I Fig. 2. Diagram showing a rod-in-a-tube configuration. When the rod touches the tube wall (a) two wedge menisci are formed.When the rod is centrally spaced (b) no wedge menisci are formed. This illustrates the need to identify the configuration of the meniscus and decide whether wedge menisci are formed. exist (fig. 2). These systems have zero contact angle but variable geometry, and the wedge menisci may appear and disappear as the geometry is changed. We shall see later that in systems with fixed geometry the presence of wedge menisci may depend on wetting properties. The method adopted for deciding on the existence, or not, of the wedge menisci is to calculate the meniscus curvature for all possibilities and then assume that the meniscus will adopt the one with the lowest curvature (minimum surface area).Both Mayer and Stowel and P r i n ~ e n ~ - ~ used this method without stating it as a principle. It is not always a reliable method, as cases are possible where metastable menisci may exist which are of physical significance. Such menisci may need to overcome an energy barrier before they adopt a configuration with a lower curvature. THEORY The aim of the theory is to calculate the curvature of the terminal meniscus. For a capillary which does not form wedges, the Gauss equation of capillarity6 relating area, perimeter, contact angle and meniscus curvature can be applied directly. Con- sider a small displacement, dx, of the terminal meniscus. In the absence of wedges, the projected area of the terminal meniscus is equal to the area of cross-section of the capillary, A'.If the capillary has a perimeter P', equating the two virtual works of displacement gives (1) where pc is the capillary pressure, 0 is the interfacial tension and 8 is the contact angle. Capillary pressure is related to surface curvature, C, by pc A' dx = a P cos 8 dx Hence pc = ac. C = P cos 8lA'G. MASON AND N. R. MORROW 2319 A Fig. 3. Cross-section of a kite-shaped pore. This section is useful for illustrating the MS-P analysis as, with a perfectly wetting liquid, it contains a single-wedge meniscus in the corner. which is simply the inverse of the hydraulic radius of the capillary, a result examined by Ca~man,~ who found that for wetting liquids the equation fitted the data of Schultze8v9 well for almost circular capillary tubes but was less adequate for other tubes. Carman’ noted that this was because ‘where capillary walls form a sharp angle, the edge of the meniscus shows a sharp local rise to a considerable height above the bottom of the meniscus’.7 Thus it was established that eqn (3) applies well to what we term non-wedging systems.The Gauss equation can also be applied to wedging systems, but now the small displacement has to allow for the effect of the wedges. For example, the area term is no longer simply the full cross-sectional area of the capillary because the wedges do not move as the meniscus is displaced. This is easily seen in an example. Consider the meniscus formed in the kite-shaped tube of fig. 3. The terminal meniscus spans the tube, and some way from the terminal meniscus a section across the tube will reveal a wedge meniscus in the corner.Let the mean meniscus curvature be C so the capillary pressure pc is aC, where CJ is the interfacial tension. Let the area of ABDE be A and the perimeter be considered in parts : Ps being the solid perimeter (AB + DE + EA) and PL being the liquid perimeter (BD). Let the contact angle with which the liquid meets the solid wall be 8 and consider a small displacement of the meniscus, dx. The virtual work balance gives (4) pc A dx = O( Ps cos 8 + PL) dx. As before, pc is related to r by Pc = a/r and so A/r = pS cos e+ pL. If we define P as p = p s c o ~ ~ + p L (7) and normalise with respect to R we obtain AIRP = r/R. (8) However, Ps, PL and A all independently depend upon r in a simple geometrical manner.It is thus possible to solve the resulting simultaneous equations numerically or graphically to find r. For some systems, the polygonal tube for example (see later), the equations can be solved analytically. For the kite pore of fig. 3, the area-to-perimeter ratio can be readily calculated for various values of r/R and 8 = 0. A graph of y = A/RP against r / R can then be plotted. The intersection of the line y = r / R with2380 MENISCUS CURVATURES IN CAPILLARIES 0.7 0.6 0.5 5 0.4 rj q 0.3 L 5 0.2 0. I 0 / 7 8 = 0 I-3R-l 0 0.5 I .o normalised radius, r/R Fig. 4. Examples of the graphical solution of the meniscus curvature for the kite-shaped pore of fig. 3. The intersection of the two lines gives the solution. The solution is always at the maximum value of A/RP.y = A/RP gives the value of r / R , which is the solution to the equations. This graphical solution is illustrated in fig. 4. Note particularly that the intersection value of A/RP is also the maximum possible value of A/RP (or r). Thus, as might be expected at equilibrium, the meniscus has minimum curvature (maximum radius of curvature) for the particular boundary conditions. This is always true, irrespective of the tube section. ROD-CORNER (8 = 0") Pore shapes involving at least one flat side are useful for experimental work because, if the flat side is transparent, the meniscus can be observed directly and measured. The general analysis will now be applied to a capillary formed by a rod in a right-angled corner (as in fig. 5) and a fluid making a zero contact angle with the solid.There are three wedge menisci, two where the rod touches the plate and the third in the right-angled corner between the two plates. A section through the meniscus some distance above the terminal meniscus will contain these wedge menisci as arcs of circles. The MS-P equation for r, the radius of curvature of the wedge menisci, is, following eqn (8), (9) area (ABCDEF) length (AB + CD + EF) + length (BC + DE + FA) r = where ABCDEF refer to fig. 5. The lengths and areas in eqn (4) are given by arcs of circles or straight lines, and straightforward geometry gives the following equations for the perimeter and area in terms of the angle a (the radius of the rods is R): P/2 = R- R sin a - r( 1 + sin a) + Rn(45 -a)/ 180 + ~ ( 2 2 5 - a)/180 (10) A/2 = R2/2 - R2z45/360 - R( R sin a + r sin a ) ( 1 - cos a) +$r2 cos a sin a + rzn( 180 - a)/360 + R2na/360 -$R2 cos a sin a - r2/2 + r2n45/360.(1 1 )G. MASON AND N. R. MORROW 238 1 Fig. 5. Cross-section of the rod-in-corner space and section through the three wedge menisci. The sections through these wedge menisci are arcs of circles of the same radius, r. 0.18 - - ROD & PLATE SYSTEM 0.14 - CORNER SYSTEM 9 = o - 0.00 0.04 0.08 0.12 0. I6 0.20 radius, r Fig. 6. Examples of the graphical solution for r / R for a perfectly wetting liquid in the rod-in-a-corner configuration and also in the two-rods-and-plate configuration. The solution is where y = r / R (the 45" line) cuts the relevant A / P line. This occurs at the maximum in A/RP. Geometry gives the relation between r and a : and we also have R(1 -cosa) 1 +cos a A / P - r = 0.r = Solving for r by a numerical method gives a = 35.122", r / R = 0.1008 and R / r = 9.985. The graphical solution for the rod-and-corner system and also for the rods-and-plate system is shown in fig. 6. As expected, the solution lies at the maximum of A / P in2382 MENISCUS CURVATURES IN CAPILLARZES INLET 4 A t (3 Q 0 1 'I A B C D E F aD 0 G3 0 I 2286 m m ES COVER PLATE SECTION A-A I+ E E (D t I VENT - Fig. 7. Diagram of the apparatus when viewed from the front. The rod pairs were in the channels labelled A-F. The precision-bore tubes were in the large central channel. Later, channel A was enlarged to accommodate 9.525 mm diameter rods. Rod pairs of such size do not show the full capillary rise because of distortion of the meniscus by gravity.both cases. Previous work5 had shown that the meniscus curvature calculated by this method for the similar geometry of two rods and a plate and zero contact angle was R / r = 6.970. EXPERIMENTAL The experiment principally involved measuring the height of capillary rise in capillaries of constant cross-section made up of rods and plates. There were two configurations, a rod in a corner and two rods and a plate. The apparatus was a compromise between using small and large rods. Small rods maximise the height of capillary rise but leave the geometry affected by the inevitable dimensional errors. Large rods minimise the dimensional errors but introduce error because the menisci are distorted by gravity. We used a series of rods of different diameters creating an apparatus in the optimum range and for which the two sources of error could be quantified.The general form of the apparatus is shown in fig. 7. The main part was an aluminium-alloy slab in which were milled a series of channels, each of precise depth and width, and a series of smaller connecting channels which carried the liquid. Each channel was machined to takeG . MASON AND N. R. MORROW 2383 a pair of precision-ground steel rods side by side. A transparent window covered and sealed the front face of the slab. The channels were dimensioned so that the rod pairs were kept in contact by the sides of the channels and kept in contact with the Lucite cover plate by the bottom of the channels. The aluminium slab also contained a compartment in which was placed a group of five precision-bore glass capillary tubes.The diameter of these tubes was determined by partly filling them with mercury and measuring both the length and weight of the mercury thread. The rod pairs and capillary tubes all had access to liquid via various channels and holes. The liquid reservoir, a 250 cm3 polytetrafluorethylene (PTFE) beaker, was connected to the slab by a length of PTFE tubing. The liquid chosen for the experiment was iso-octane, which perfectly wetted the steel rods, the aluminium slab, the Lucite window and the glass capillary tubes. By raising and lowering the beaker reservoir, the level of iso-octane in the cell could be increased or decreased. The apparatus had quite a slow response to level changes, typically taking 10min or so to reach equilibrium.This was due to the quite large (relative to the rod-plate capillaries) volume of liquid in the neighbouring region of the capillary tubes combined with the resistance to flow of the 1/8 in. 0.d. PTFE tubing and the very small liquid head. In operation, after reaching equilibrium the levels of all of the liquid menisci, both in the test capillaries and the cylindrical glass capillary tubes, were measured with a cathetometer reading to 0.01 mm. The rod diameters had been measured with a micrometer. The apparatus had two finer points. There were two rod channels which were nominally of the same dimensions at opposite ends of the slab. This gave a check on the manufacturing errors of the channels (which turned out to be smaller than expected) and also on the accuracy of horizontal travel of the cathetometer.Two capillary tubes of the same size were included for similar reasons. Before assembly the machined parts were all carefully cleaned to remove traces of coolant and swarf. Experiments were run at ambient temperature, the comparative interpretation of the measurements (see later) being such that closer control was not necessary. It was expected that almost all of the experimental error would be produced by the dimensional tolerances of manufacture. For a given configuration, the position of the meniscus was measured in both the series of calibrated glass capillary tubes and in the constructed capillaries. The levels in the calibrated glass tubes could be extrapolated to give the level of the reservoir liquid, and from this the capillary rise in the test capillaries could be determined.For rise up the glass tubes, the position of the meniscus is given by (14) where hT is the level of the tube meniscus, the suffix T being used to indicate the capillary tube, h, is the reservoir level, o is the surface tension of the iso-octane, p its density, g the acceleration due to gravity and RT the radius of the tubes. A plot of h, against 1/R, is thus a straight line through h, with gradient, g,, of 2a/pg. 20 hT = ho+- PgRT CR0 For the rods and plate we have hR = ho +- pgRR where C, is the normalised curvature of the rod-and-plate system. A plot of hR, the meniscus height, against l/RR, where RR is the radius of the rods, is thus a straight line of intercept h, and gradient CRd/pg (= gR).For the rods in the corners we have - c', d h, = h,+-. pgRR C, is the normalised curvature of the meniscus in the corner. This is also a straight line of intercept h,, but with gradient g, (= C, o/pg). It can be seen that with reference to the gradient of the tube line 2g, C, = -. and also g T2384 MENISCUS CURVATURES IN CAPILLARIES The attraction of this method is that a/pg is a constant which cancels. This makes the method relatively insensitive to temperature changes. Furthermore, the change in curvature with height within the region of the terminal meniscus caused by gravity is also largely compensated by this comparative method. An experimental run consisted of measuring the meniscus height for every meniscus in the apparatus, three menisci for each of the six pairs of rods and one each for the five capillary tubes.The bottoms of the menisci were used when measuring the meniscus height. The initial menisci were recorded again to confirm equilibrium and that there were no leaks. Runs were repeated for several levels in the apparatus, both as a check on the method and an estimate of the scatter produced by the inevitable dimensional deviations. The rods had diameters of CQ. 1/4, 1/5, 1/6, 1/7 and 1/8 in., their actual sizes being 6.243, 5.062, 4.173, 3.645 and 3.175 mm, respectively. The capillary tubes had diameters of 1.631, 1.290, 0.695 and 0.459 mm and showed, as planned, capillary rises in approximately the same range as the constructed capillaries.RESULTS The graphs for the tube meniscus height, hT, against l/RT gave excellent straight lines. Extrapolation enabled the height of the reservoir liquid relative to all the other menisci to be determined. Rather than reproduce all of the data separately, the runs for different levels in the apparatus have been condensed onto a single diagram by referring all of the menisci heights to the value of h, determined from the capillary rise in the tubes. The values of (hR - h,) and (h, -ho) plotted against 1 /RR are shown for the four separate runs in fig. 8. The deviations from the theoretical straight lines through the origin are believed to be caused mainly by the deviations of the dimensions of the apparatus from the ideal, these deviations being the main source of error in the determinations. Using each of the gradients of the separate run lines of fig.8, the values of CR and Cc were determined using eqn (1 7) and (1 8). The values obtained were C , = 6.88 & 0.02 and C, = 9.83 0.04. The theoretical values for zero contact angle are C , = 6.970 and C, = 9.985. The agreement is good, but not perfect. One possible source of error is that we have used capillary-rise measurements to test a theory which strictly applies to surfaces of constant curvature. In the analysis of this experiment, beyond using the comparative technique of determining curvatures from capillary rise, no attempt was made to correct for the gravity distortion of any of the menisci. Any severe distortion effects will increase as the size of the rods is increased and should give rise to increasing deviation from a straight line as 1 /RR decreases in a plot of 1 / R R against curvature.However, it can be seen from the results presented in fig. 8 that gravity distortion cannot be severe. In order to identify clearly the gravity effects, the slab was remachined and one of the pair of channels containing 6.243 mm rods was enlarged to accommodate 9.525 mm diameter rods. In the course of carrying out this alteration, the diameter of the capillary tubes was remeasured. Three complete runs were made with the menisci in different positions in the apparatus. The capillary rise of the meniscus in the space between the new rods was only 4 mm, and the data points fell well below the straight line generated by data obtained for the smaller rods.In capillary-rise experiments the menisci cannot satisfy the constant curvature condition of the theory because curvature varies directly with height. Measurement of the height of rise to the base of the meniscus is an experimental convenience. It also corresponds to the minimum curvature. In the present work, for the height change through the terminal meniscus to be unmeasurable with the cathetometer (< 0.01 mm) the height of capillary rise would have to be ca. 1500 mm. Observed heights of rise were in the range of 4-25 mm. Thus in practice the meniscus will alwaysG. MASON AND N. R. MORROW ( 1 /RT)/mm-’ I 2 3 4 5 6 7 1 I I I I -A.O,O I 2385 hR hC TUBES ROD IN CORNE / J ‘A,O 7 0 0. I 0.2 0.3 0.4 0.5 0.6 0.7 (1 /RR )/mm-‘ Fig.8. Graph of the heights of capillary rise against 1/R, for the capillary tubes and against 1 /RE for the rod-in-a-corner and rod-and-plate menisci. The small scatter of the points is mainly caused by dimensional variations in the machining of the cell. No correction has been made for the effects of gravity distortion on the menisci. Levels as follows: A, 57; 0, 76; 0, 95 and 0, 114 mm. have a measurably finite height. What we need to know is the level in the meniscus that corresponds to its average curvature. This can be estimated as follows. Assume first that the wedge menisci are solid wedges and that the cross-section so obtained applies to the complete length of the tube. There will be no change in shape of the terminal meniscus. Now imagine a plane across the terminal meniscus in such a position that the volume of liquid above the plane equals the volume of space below the plane.The position of such a plane could be computed or perhaps measured by some suitable experiment. The level of the plane gives a first-order correction for the 78 FAR 12386 MENISCUS CURVATURES IN CAPILLARIES effects of gravity on the meniscus height. This is the principle of the Rayleigh correction.1° For the case of menisci in circular tubes, where the meniscus can be assumed to be hemispherical, this correction leads to the addition of one-third of the tube radius to the height of the bottom of the meniscus. Using our experimental data, this correction was applied to the heights of rise measured in the capillary tubes.The height of the virtual liquid reservoir was then redetermined by linear regression of these values against 1 / R T . For the non-hemispherical menisci the correction cannot be applied exactly, but it is possible to estimate its size. For each size of each of the rod- plate and rod-corner menisci an equivalent radius was calculated from the height of capillary rise. This equivalent radius was the radius of a cylindrical tube which would give the same capillary rise. One-third of this equivalent radius was then added to the experimental heights of capillary rise. For the 6 mm rod-and-plate configuration this correction amounted to 5% of the height of rise, and for the 9.525 mm diameter rods the correction was 11 % . Plots of the adjusted heights of capillary rise against 1/R, and 1/R, (see fig.9) gave a straight line for all the points. The gradients were slightly shifted, but the ratios of the gradients changed very little. Values of the meniscus curvature so obtained were CR = 6.91 k0.02 and Cc = 9.85 f0.03. These compare almost exactly with the previous experimental determination (6.88 and 9.83, respectively) but still not perfectly with the theoretical values of CR = 6.970 and Cc = 9.985. That the two determinations, one involving a correction for the gravity distortion of the menisci and the other not, give such similar results can be attributed to the comparative method of measurement and the fact that the heights of rise in the tubes and in the rod-and-corner and rod-and-plate configurations were all roughly the same.A possible contribution to the small systematic deviation from theory is that, because of gravity, the wedge menisci in the experimental system are not precisely vertical. However, a rough estimate of the slope of the wedge in the vicinity of the terminal meniscus for the largest rods indicates that this will not reduce experimental results below theoretical values by > 0.5% at most. It is therefore concluded that the small systematic deviation from theory of ca. 1.5 % is due mainly to limitations on the tolerances of the constructed capillaries. For example, the radius of the ‘equivalent tube ’ which corresponds to the rise between the 3.175 mm diameter rod and the corner is ca. 0.32 mm. A deviation of 0.0025 mm (0.0001 in.), a typical unit of precision machining, on this radius of 0.32 mm corresponds to a change of 0.8% in the height of capillary rise. It is unlikely that our machining operations were completed to better than 0.025 mm.The overall effect of tolerance variation will always be to make the capillaries larger than their nominal values. This is consistent with observed curvatures being slightly less than theoretical values. DISCUSSION It has been shown theoretically that for systems of constant curvature the MS-P method is based on an exact analysis. Furthermore, through measurement of capillary rise, an experimental confirmation of the method has been provided. The MS-P method has great versatility and a variety of potential applications. For example, it can be used to calculate meniscus curvatures for various configurations with particular wetting conditions.Indeed, as contact angles can be notoriously irreproducible, using the MS-P method may be more reliable than experimental results for determining change in meniscus curvatures with contact angle. Furthermore, as the method is exact for uniform sections, it gives a means of calculating meniscus curvatures for complex configurations. This could be of value in systems where close control of the solidG. MASON AND N. R. MORROW (l/R,)/mm’’ 0 I 2 3 4 5 6 7 8 9 25 I I I I / I 1 I I I R, I ROD IN I I I I I I I I I 2387 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 ( 1 lR Ymm-’ Fig. 9. Graph of the heights of capillary rise plotted against the reciprocal of the tube and rod radii. These results are from the modified apparatus containing the 9.525 mm diameter rods.A correction for gravity distortion of the menisci has been applied to these results. The effect on the final ratio of line gradients is quite small as the changes are mostly self-compensatory. Levels as follows: 0, 181 ; e, 197 and A, 204 mm. surface is required, such as in contact-angle work. A pore geometry can be assembled from a particular material, and the measurement of capillary rise in some form of constructed capillary of constant cross-section gives a means of estimating the contact angle. An example of this would be a capillary formed from two rods and a plate using a material which cannot, for example, be drawn into tubes. In the application of the MS-P method, the most critical part of the theoretical analysis concerns the existence or non-existence of wedges.For the rod-and- corner example, if the contact angle is zero, wedge menisci always exist at the 78-22388 MENISCUS CURVATURES HALF ANGLE MENISCUS v IN CAPILLARIES Fig. 10. Diagram of a corner of a polygonal section tube. Depending on the contact angle, a wedge meniscus may exist in the corners. rod-and-plate contacts and in the corner. When the contact angle is a variable, the wedging behaviour is changed and the MS-P method has to be modified accordingly. This is best shown in a worked example. Two configurations will be considered: a capillary with polygonal cross-section and then a more complicated example, the rod-corner capillary. THE II-AGON TUBE (NON-ZERO CONTACT ANGLE) Princen3 has given solutions for the curvature of menisci in triangular and square cross-section tubes for zero contact angle.These cases are relatively straightforward because they always have wedge menisci. Concusll has discussed menisci in polygonal- sectioned tubes for large contact angles where the meniscus is part of a spherical surface. These also are straightforward as there are never any wedge menisci. In this section we will bring both analyses together. The results will illustrate how menisci in wedging and non-wedging systems are analysed and permit an extension to the rod-corner configuration for non-zero contact angle. They can be compared with the conclusions of Hwang,6 who did not distinguish between wedging and non-wedging systems and assumed that all systems are non-wedging.This explains why Hwang’s6 results differ from Prin~en’s~-~ results. Consider a portion of an n-sided polygonal tube as shown in fig. 10. Let the in-circle radius of the tube be R and the half-angle subtended by a side be 90 -p. Let the radius of the wedge meniscus (if any) be r and the contact angle be 6. The perimeter of the wetted solid, Ps, for the sector subtended by 90-p is given by R rcos6 ps = --- +rsin0 tan/? tan/? and the liquid perimeter, PL, is given by (90-p-6) nr. 180 PL = The total resolved perimeter is P: P=P,cosO+P, (90 - /? - 6) nr. 180 R rcos6G. MASON AND N. R. MORROW The area, A , of the section is given by R2 r2cos28 r2 zr2 2tanj? 2tanp 2 360 A = - - - + - cos 8 sin 8 + - (90 -8- 8). We also have the MS-P equation: or, rearranging A / P - r = 0 A - r P = 0.2389 (22) Substituting for A and P gives a quadratic equation in r/R: (24) cos28 cosOsin8 7t r2 cos8 r 1 -- (90-p-8)) --- -+- 2 360 R2 tan/?R 2tanB=" There are two roots, only one of which is physically realistic; the other probably corresponds to mensici on the outside of the triangle. However, there are no real roots when the (r/R)2 coefficient becomes zero, which happens when 8+/3 = 90. (25) This represents the point where the wedge menisci disappear, and so the condition (26) for wedge menisci to occur is e s 90-8. Wedge menisci always occur for 8 = 0 except when 8 = go", which corresponds to the number of sides of the polygon becoming infinite, i.e. the cylindrical tube. For 90 > 8 > 90-8, the wedge menisci do not exist and the curvature of the meniscus becomes A / P , the hydraulic radius. Thus A / P = R / ~ c o s ~ (27) becomes the solution.This corresponds to the spherical meniscus in the n-agon tube with in-circle radius Rlcos 8, a result pointed out by Concus.ll The meniscus in such a tube simply runs into the corner but does not form an infinite wedge. Concusll published photographs of menisci in such configurations. We thus have the analysis for the n-agon tube. The condition for wedge menisci to form is 8 < (90-8). This is also the condition for wedge-meniscus formation in any corner of half-angle 8, a fact which will be put to use later in the analysis of the meniscus in the rod-and-corner configuration. As an example of the effect of the wedge menisci on the total menicus curvature, fig.11 shows the meniscus curvature in 3-, 4- and co-sided tubes together with the contact angle at which the wedge menisci disappear. For zero contact angle these results duplicate those of Prin~en,~ although he normalised his results with respect to side length as compared to the in-circle radius used here. ROD-IN-A-CORNER (NON-ZERO CONTACT ANGLE) For this configuration (fig. 12) there are always wedge menisci between the rod and contacting plates. However, the corner has a 90" angle and will only have a wedge meniscus for 8 < 45". If 8 > 45" then no wedge meniscus is formed and the terminal meniscus just runs into the corner. We have two cases. (i) 8 < 45" In this case P is given by P = (R- R sin a- r[sin (0+ a)- sin 81 - r(cos 8- sin 8) + Rn(45 - a)/ 180} cos 0 + m(225 - 38 - a)/ 180 (28)2390 MENISCUS CURVATURES IN CAPILLARIES I I I I 4 1 I I I SIDES POLYGONAL TUBES 1 4 - * 12- 0 2 - 0 I0 20 30 40 50 60 70 80 90 contact angle, elo Fig.11. Curvature of the meniscus in the n-agon tube normalised relative to the radius of the in-sphere is given as a function of contact angle. The infinite side number tube is simply a cylinder and the values agree with those for meniscus curvature in cylindrical tubes. When corners exist, the menisci have wedge menisci in them for low values of contact angle, and this reduces the curvature. Fig. 12. Diagram of the section through a meniscus in the space between a rod and a corner for non-zero contact angle. Wedge menisci always exist between the rods and plate. Depending on the contact angle, a wedge meniscus may exist in the right-angled corner.G .MASON AND N. R. MORROW 2391 Table 1. Curvature ( R / r ) of the fluid meniscus in the space between a rod and a corner. The angle 0 is the angle that the fluid makes with the solid surface. In this table a wedge meniscus exists in the right-angled corner 0 35.12 0.1002 9.985 10 34.12 0.1011 9.892 20 32.95 0.1043 9.589 30 31.60 0.1105 9.049 40 30.03 0.1212 8.249 45 29.14 0.1291 7.747 where a and 0 are shown in fig. 12. The area, A , is given by A = R2/2 - R2n45/360 - R[R sin a + r sin (0 + a)] (1 - cos a) + ar2 sin 0 cos 8 +tr2 cos (0+ a) sin (O+ a) + r2n( 180 - 20 - a)/360 + R2na/360 - $R2 sin a cos a - +r2 cos2 8 + 4r2 sin O cos 0 + r2n(45 - O)/360.(29) Geometry gives the relation between r and a: R( 1 - cos a) r = cos O+cos (e+ a) ' There is also the MS-P equation: A / P - r = 0. (8) Eqn (8) and (28)-(30) can be solved numerically to give r / R as a function of 0. Some values are given in table 1. (ii) 45 d elo < 90 have P = {R - R sin a - r[sin (0+ a) -sin B] + Rn(45 - a)/ 1 80) cos O+ rn( 180 - 20- a)/180 A = R2/2 -nR2 45/360 - R[R sin a + r sin (O+a)] (1 -cos a)+gr2 sin Oeos 0 When 0 > 45", no wedge meniscus is formed in the right-angled corner. We now (31) +4r2cos(O+a) sin(O+a)+r2n(180-20-a)/360+R2na/360-~R2 sinacos a (32) R( 1 - cos a) cos 0 + cos (0 + a) r = A / P - r = 0. (8) Again eqn (8) and (30)-(32) can be solved numerically and we obtain r / R as a function of 0. (See table 2 for some typical values.) In principle the method can be applied to systems which are considerably more complex than those discussed so far.Provided the properties of the capillary are constant with respect to cross-section even variation of the wetting properties around the perimeter are permissible. Capillaries do not necessarily have to have a closed2392 MENISCUS CURVATURES IN CAPILLARIES Table 2. Curvature (Rlr) of the fluid meniscus in the space between a rod and a corner. The angle 0 is the angle that the fluid makes with the solid surface. In this table a wedge menicus does not exist in the right-angled corner 45 29.14 0.1291 7.747 50 28.14 0.1393 5.178 60 25.62 0.1706 5.861 70 21.88 0.2328 4.295 80 15.13 0.4119 2.428 perimeter, but if they do not, then attention must be paid to the effect of neighbouring capillaries.Systems where the wedge interface occurs at edges can also be treated, and the presence of the edge merely provides an additional degree of freedom with respect to minimisation of free energy. The difficult part of the application concerns the existence and position of the wedge menisci. An interesting point, and one which made our experiments easier to perform, is that the detailed surface geometry in the wedging corners is unimportant. Systems can have regions of extreme and arbitrary complexity in the corners, and provided these regions are bounded by wedges they do not affect the overall meniscus curvature. As an extreme example of this, the surface curvature of the terminal meniscus would be unchanged if the wedge menisci in wetting systems were replaced by solid material to give a capillary of reduced cross-section.SUMMARY This paper brings together several existing ideas and methods of calculating the curvature of menisci in tubes of arbitrary cross-section. When wedge menisci are not formed, then the hydraulic-radius method can be used. When wedge menisci are formed, the MS-P method must be used, with the main problem now being where the wedge menisci are formed. The MS-P theory is exact for constant curvature menisci. Nevertheless, it may be necessary to calculate the meniscus curvature for several conditions of existence or non-existence of wedge menisci and assume that the actual curvature adopted will be the one with the lowest curvature. The method has been applied here to the rod-in-a-corner configuration and the analysis tested with an experiment in which the capillary rise of a perfectly wetting liquid was measured for a range of rod sizes. The agreement was good but not perfect, most probably because of slight manufacturing errors in the apparatus which always increase the apparent size of the pore space in the cross-section. The effect of changing the contact angle in systems of fixed geometry has been discussed. In these cases there can be a contact angle at which the wedge menisci cease to exist, and this must be reflected by the analysis. This was highlighted by the n-sided polygonal tube which has a particular critical contact angle at which the wedge menisci disappear. The basic method of analysis, together with the criterion for wedging, was applied to the rod-in-corner configuration for liquids having a non-zero contact angle. The MS-P method is widely applicable in principle and can be used to estimate the meniscus curvature for any particular pore geometry with any particular contact angle.G. MASON AND N. R. MORROW 2393 We thank A. R. Romero of the TERA workshop for machining the cell and C . Lawson and Shang-shi Shu for assistance in taking the measurements. This work was jointly supported by the U.S. Department of Energy, contract no. DE-AS 19-80BC103 10, and the New Mexico Energy Research and Development Institute, project no. 2-69-3309. G. M. was on sabbatical leave from Loughborough University of Technology, Leicestershire. ' R. P. Mayer and R. A. Stowe, J . Colloid Interface Sci., 1965, 20, 893. H. M. Princen, J. Colloid Interface Sci., 1969, 30, 60. H. M. Princen, J . Colloid Interface Sci., 1969, 30, 359 H. M. Princen, J. Colloid Interface Sci., 1970, 34, 171. G. Mason, M. D. Nguyen and N . R. Morrow, J. Colloid Interface Sci., 1983, 95, 494. S. Hwang, Z . Phys. Chem. (Neue Folge), 1977 105, 225. ' P. C. Carman, Soil Sci., 1941, 52, 1. K. Schultz, Kolloid Z . , 1925, 36, 65. K. Shultze, Kolloid Z., 1925, 37, 10. P. Concus, Preprints, 48th National Colloid Symposium, Austin, Texas (June 1974), (Am. Chem. SOC., Washington, D.C., 1974), pp. 12-16. lo A. W. Adamson, Physical Chemistry of Surfaces (Interscience, New York, 1960). (PAPER 31 1 1 1 1)

ISSN:0300-9599    

DOI:10.1039/F19848002375

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J. Chem. SOC., Farahy Trans. I , 198480, 2395-2403 Reaction of Hydrogen Atoms with Cyclopropane in the Temperature Range from 628 to 779 K BY ROGER M. MARSHALL, HOWARD PURNELL* AND PAUL W. SATCHELL~ Department of Chemistry, University College of Swansea, Singleton Park, Swansea SA2 8PP Received 16th August, 1983 The reaction between cyclopropane and hydrogen atoms has been investigated in the temperature range from 628 to 779 K at a total argon pressure between 5.3 and 13.2 Torr using a discharge-flow system with gas-chromatographic product analysis. The measured products, methane, ethane, ethene, propane and propene, are shown to be consistent with a mechanism H + c-C,H, --+ H2 + c-C,H, H + c-C,H, - C,H, (propene) H + C,H,---* products wall H- 0.5H2. Measurement of k, permits evaluation of the rate constant for the abstraction process log(k,/cm3 mol-l s-l) = (13.6f 1.0)-(48.5+ 13.0 kJ mol-l/2.3RT) which is similar to that for the corresponding reaction of methane on account of the almost identical C-H bond strengths in the molecules.A composite function of the rate constants for addition to the two positions in propene is evaluated. It is shown that the flow rate of hydrogen atoms as measured by our previously described ethene titration technique gives values which are consistently only 80% of those evaluated by summation of the products of reaction with cyclopropane. Over the past few years we have investigated the reactions of hydrogen atoms with various alkene~l-~ and alkanes6-8 using flow-discharge systems equipped either for gas-chromatographic analysis of final stable products or for mass-spectroscopic measurement of reactants and products.The latter method permits the 'direct' measurement of the rate constants for the primary reactions whereas the former has permitted only ' indirect' meas~rernents~~ involving curve-fitting via computer integration of the coupled differential equations describing the overall reactions. The present paper describes a study using a flow-discharge system equipped for gas-chromatographic analysis but used in such a way as to make a 'direct ' measurement of a rate constant. The particular reaction chosen for this study is that between hydrogen atoms and cyclopropane. This reaction does not appear to have been investigated at other than room temperature.Early studieset lo suggested that there was no reaction, but Schiff and Steacie" were able to show that there was a slow reaction with a collision yield of ca. i.e. a rate constant of ca. lo6 cm3 mol-l s-l. The lack of any previous detailed investigation pro.vides the main justification for t Present address: B.P. Research Centre, Chertsey Road, Sunbury-on-Thames TW16 7LN. 23952396 REACTION OF HYDROGEN WITH CYCLOPROPANE this work, especially as the reaction is of some interest given the unusually strong C-H bond in cyclopropane.12 Moreover, it has been suggested13 that the initial step could be an addition process giving n-propyl, although this may not be relevant to the present gas-phase study since it was based on experiments at 77 K in a solid matrix of cyclopropane and hydrogen iodide.EXPERIMENTAL All experiments were carried out using a conventional discharge-flow system. The flow reactor was constructed of quartz tubing of 1.76 cm internal diameter. A heated zone, 40 cm long, was achieved using a furnace consisting of Nichrome wire wound with suitable insulation and lagging on a copper tube around the flow reactor. The temperature along the heated reactor tube was constant to +_2 K with very sharp temperature gradients at both ends of the heated zone. Hydrogen atoms were generated by passing dilute mixtures of hydrogen in argon through a microwave discharge. Hydrocarbon reactants were added to the atom flow in the heated zone via a probe which was moveable along the axis of the reactor. The temperature of the reactor was measured using a chromel-alumel thermocouple mounted in the tip of the moveable probe. All the quartz surfaces in the region of the heated zone were 'poisoned' with 10 mol dm-3 nitric acid using our previously described procedureI4 to minimise the heterogeneous loss of hydrogen atoms.All flow rates were determined from measurements of the rate of pressure change (via an electrical transducer) in calibrated volumes. Downstream of the reactor was a sampling system allowing direct injection of samples of reacted gases into the gas chromatograph, which was fitted with a flame ionisation detector. The column used for analysis consisted of a 2 m length of 5 mm 0.d. copper tubing packed with a 1 : 1 w/w mixture of benzene-washed F.20 alumina coated with 6.76% w/w of squalane and Chromosorb G (120 mesh).To improve the separation of ethane and ethene a 2.5 cm long column packed with uncoated alumina was connected in series with the main column. The column was operated at 37 "C using nitrogen carrier gas at an inlet pressure of 5 p.s.i.g. All hydrocarbon reactants contained no significant impurities and were always thoroughly outgassed before use. THEORY MECHANISM Following previous workersll we will assume that the initial reaction of H with cyclopropane is an abstraction process H + c-C3H6 -+ H, + c-C3H5 presumably followed by H + c - C ~ H ~ -+ (c-C3H6)* which, since the excited state contains ca. 425 kJ mol-l excess energy,12 will isomerise to propene, cf. the activation energy for the thermal isomerisation of cyclopropane to propeiie15 is around 270 kJ mol-l.Thus one would predict the sole, primary, stable product to be propene. However, the rate constant for reaction of H with propene, an addition process, is several orders of magnitude greater than for an abstraction process by H from a hydrocarbon on account of an activation energy difference which may be as high as 30-40 kJ mol-l. Thus, the primary product, propene, except under conditions of a vast excess of cyclopropane over hydrogen atoms, will react with hydrogen atoms generating the products characteristic of that reaction. The reactionR. M. MARSHALL, H. PURNELL AND P. W. SATCHELL 2397 of propene with hydrogen atoms at room temperature has been discussed in detail by us in a previous publication2 and a comprehensive mechanism was suggested.We assume that this mechanism applies in the present study and, where appropriate, we will also use relevant numerical values given in our earlier paper. Note that we can neglect the unimolecular isomerisation of c-C3H5 to ally1 in comparison with its bimolecular reaction with H. A recent measurement of the rate constant16 of the isomerisation shows it to be at least an order of magnitude slower in the conditions of the present experiments if we assume a rate constant of 4 x 1013 cm3 mot1 s-l for the radical-radical reaction. DETERMINATIONS OF RATE CONSTANT k, In our reaction system there is competition for hydrogen atoms between hetero- k, geneous loss and reaction with cyclopropane. Thus we can write a mechanism H + c-C3H, --+ products (1) where k, is the rate constant for the initial abstraction reaction, which is indubitably the rate-determining step in the reaction with cyclopropane.Of course, subsequent reactions of the initially produced c-C3H5 will use up more H atoms. Let us define then a stoichometry number, n, as the total number of hydrogen atoms used per molecule of cyclopropane used, i.e. the overall reaction is nH + c-C3H, + products with a rate determined by the rate of reaction (1). zone of the reactor, we derive From the above two-step mechanism, assuming that all H atoms react in the heated f= fraction of H atoms reacting with c-C3H6 = nk, FA/(nk, FA + k,) where FA is the flow rate of cyclopropane. Thus, if FA,,, is the flow rate of cyclopropane converted to products, we have where FH is the initial flow rate of hydrogen atoms.Combination of these two equations followed by, rearrangement gives =fFHln 1 k, 1 - -+-- 1 nFAcon FH k~FH nFA' Note that we have implicitly assumed that FA is constant, i.e. that only a small fraction of the cyclopropane is converted into products. It is to be anticipated that n will vary with conditions. For example, n = 2 when there is a vast excess of cyclopropane over H, since then there is no secondary reaction with the propene produced initially. Conversely n > 2 when the latter reaction does occur. Thus eqn (I) implies that we should plot l/n&,on against l/nFA, thereby obtaining a straight line of intercept l/FH and slope kw/kl&. Thus from the measured value of the ratio of intercept to slope, and knowing the value of k,, we can obtain the value of k,.2398 REACTION OF HYDROGEN WITH CYCLOPROPANE DETERMINATION OF n In order to generate the plot required by eqn (I) for the determination of k, we must derive the value of n from the experimental results.A simple consideration of the rate of reacting hydrogen atoms gives the result where eat is the total flow rate of saturated products, FcJHB is the flow rate of propane product and P is the total pressure (predominantly argon). This expression is essentially made up of three terms which account for the various ways in which H atoms are lost. The first term accounts for the 2H atoms used in converting cyclopropane to propene and the other two terms account for H atoms lost in reaction with propene, the term in eat accounting for those H atoms which end up in observable products, i.e.the alkanes, and the term in FCsHB accounting for those H atoms lost by disproportionation with i-C,H, forming H, and thus unobservable directly. The numerical values appearing in this latter term are taken from our previous paper,, and since they involve rate constants for reactions of excited C,H, molecules and the disproportionation to combination ratio for H + i-C,H, it is reasonable to assume values which are independent of temperature. Indeed, the validity of this assumption has been demonstrated for the very similar reaction of H with isob~tene.~ The expressions for the quantities required to plot eqn (I) are REACTION WITH PROPENE The initial steps of the reaction may be represented by H +c-C3H6 + c-C,H, + H, H + c-C,H, --+ C3H6 (propene) (1) (2) (3) (4) H + C3H6 -+ i-C,H,* H + C3H6 -+n-C,H,* -+ CH, + C2H4.The value of the rate constant for reaction (1) is much lower than those of reactions (2)--(4) and so the steady-state approximation may be applied to c-C,H, and to C,H,. We thus obtain kl[H1 [c-C3H61 = k2[H1 Ec-c,H,I = ( - a) kJHI [C3H61 + k4[H1 [C3H61 where a is the fraction of the propene reacting via reaction (3) which is reformed in the subsequent reactions of i-C,H,* (by disproportionation of H and i-C3H,). Our previous work2 gives a x 0.42 for the conditions of the present work and so, in terms of flow rates, we have Fc,H,, = FA k,/(0.58k3 + k4). (11) Thus a plot of Fc,.H, against F A should be a straight line of slope k,/(0.58k3 + k4) passing through the origm.R.M. MARSHALL, H. PURNELL AND P. W. SATCHELL 2399 1 1.125 0 12 24 36 48 60 FA /pmol s-* Fig. 1. Plots of Grad against FA for experiments at 677 K and 10.6 Torr total pressure with a measured FH (ethene titration) of 5.84 pmol s-l. Key to products: (a) methane, (b) ethane, (c) ethene, ( d ) propene and (e) propane. RESULTS AND DISCUSSION The reaction of hydrogen atoms with cyclopropane has been investigated in the temperature range 628-779 K at total argon pressures in the range 5.3-13.2 Torr with contact time in the heated reactor ca. 10-15 ms. The flow rate of hydrogen atoms, FH, measured using our previously described ethene titration technique,lY was in the range 1.8-6.6 pmol s-l, corresponding to a concentration of ca.5 x mol ~ m - ~ . For each value of FH experiments were conducted with ca. 15 different flow rates of cyclopropane, FA, in the approximate range 0.2-65 pmol s-l. The value of the first-order rate constant, k,, for the heterogeneous loss of hydrogen atoms was measured by monitoring the decay of the hydrogen-atom concentration along the reactor using the ethene titration technique.2 Values of k,/s-l were in the range 65-194, i.e. the walls of the reactor are not as inactive as in some of our previous worka but they are suitable for the present purposes. Complete tables of results are available. l7 MECHANISM Fig. 1 shows a typical set of results in the form of a plot of product flow rate f;lprod against FA for a constant initial FH at constant temperature and pressure. The general dependence of f;lprod on FA was independent of conditions and may be summarised thus: (i) plots of FcH, pass through broad maxima and decline at high FA, (ii) FcZH?, and FC3Hs all appear to level out at high FA and (iii) FCOHB, in contrast, constantly increases as FA increases.There is a temperature dependence of the relative amounts of the products, the most significant change being the increase in the yield of ethene as the temperature increases. All of these observations are in agreement with the mechanism assumed above. Thus the dependences of the flow rates of methane, ethane, ethene and propane on reactant flow rate at constant temperature, pressure and FH are broadly similar to those observed in our study of the H +propene reaction,2 whilst the dependence of FCIHe on FA is in accord with that predicted from eqn (11).The enhanced yield of ethene2400 REACTION OF HYDROGEN WITH CYCLOPROPANE T 0 0.006 0.012 0.018 0.024 0.030 ( I/nF, )/s prno1-l Fig. 2. Plot of l/nFAcon against l/nFA for the results shown in fig. 1. at higher temperatures is in line with the prediction based on the higher activation energy of reaction (4) in comparison with that of reaction (3).18 The significant difference between the shapes of the plots for ethene and propene yields against FA implies that reaction of ethene with H is insignificant since, otherwise, the shape of the plot for ethene would be similar to that for propene. However, even if it were significant it would make no difference to the interpretation of the present results since the reaction of H with ethene generates only alkanes as products, and thus any H atoms that had reacted in this way would be included in the calculation of n via the term in eat.We therefore conclude that eqn (I) and (11) derived above on the basis of the assumed mechanism are valid descriptions of the present results. VALUE OF k, Fig. 2 shows a typical plot of l/nFAcon against l/nFA to be an excellent straight h e , as was observed for all our sets of results. Note that for these plots we used only results derived at sufficiently high values of FA, in order to ensure that effectively all the H atoms reacted in the hot zone. Table 1 records the values of the slope, S, and intercept, I, of these plots. Also recorded in table 1 are the values of k,, k , (calculated via k , = k , I / S ) , FH (calculated both from the ethene titration and as the value of 1/1) and 2, the numerical factor converting a flow rate in pmol s-l into a concentration in mol (obtained from measurement of the flow rate of argon for a measured argon pressure in the flow tube).The error limits of the quoted values for k, are difficult to estimate but are inevitably large since the value arises via two independent plots of gas-chromatographic data, one of the plots involving a particularly complex function of experimental results. The existence of wide error limits is born out by the observed scatter of the results, j, 50% about the mean value (ignoring the slight difference in temperatures) at the lowest temperature, but, rather better, f 25 % at the other temperatures.A least-squares treatment of the data yields log (A/cm3 mo1-1 s-l) = 13.6k 1 .O and El = 48.5 j, 13.0 kJ mol-1 (95% confidence limits), which data are remarkably similar to those for H attack on methane8 which log (k/cm3 mol-1 s-l) = 13.88 -(49.9 kJ mol-l/2.3RT) yieldedTable 1. Results 628 63 1 638 676 677 683 739 737 738 779 778 773 5.3 10.6 13.2 5.3 10.6 13.2 5.3 10.6 13.2 5.3 10.6 13.2 ~~ 98 65 154 86 91 167 123 114 168 160 171 194 ~~ 25.3 17.5 28.9 11.2 10.3 12.1 9.7 8.7 10.9 9.9 9.2 8.7 0.404 0.144 0.1 14 0.409 0.156 0.128 0.391 0.170 0.144 0.398 0.170 0.139 1.83 5.88 6.47 2.03 5.84 6.57 2.14 4.35 5.41 1.99 3.78 5.17 2.48 6.94 8.77 2.44 6.41 7.8 1 2.56 5.88 6.94 2.51 5.88 7.19 2.92 2.18 1.99 2.70 1.99 1.86 2.52 1.93 1.75 2.40 1.76 1.68 5.4 2.5 3.0 11.6 6.9 9.5 19.8 11.5 12.7 26.8 17.9 18.5 3.2 1.5 1.5 3.4 2.0 2.6 4.0 1.9 2.0 3.7 2.3 2.2 a The two values quoted are those obtained (i) from the ethene titration and (ii) by taking the value of 1/Z.Z is the numerical factor to multiply flow rates in pmol s-' to give concentrations in mol ~ m - ~ .2402 REACTION OF HYDROGEN WITH CYCLOPROPANE 0.16 I I 0 10 20 30 40 50 F,/pmol s-l Fig. 3. Plot of FCSHs against FA for the results shown in fig. 1. a reflection, presumably of the near equality of the C-H bond strengths in methane and in cyclopropane.12 This result confirms that the initial step in the reaction of H with cyclopropane is a hydrogen-abstraction process, as has been assumed in this work.In a parallel study in our laboratory using a flow-discharge system with mass- spectroscopic measurement of reactants and products we have obtainedlS log (k1/cm3 mol-1 s-l) = 14.21 - (49.1 kJ mol-l/2.3RT). The agreement with the present Arrhenius parameters is good but the absolute values of k, generated differ by a factor of ca. 3 for temperatures of ca. 600 K . The reasons for this discrepancy are not known. REACTION WITH PROPENE Fig. 3 shows a typical plot of FCBH, against FA. The plots are good straight lines but only for the higher temperatures do they pass through the origin, as required by eqn (11). The reason for this is that, for the lower temperatures at low FA, some hydrogen atoms remain at the end of the hot zone and will then react with propene (and with ethene for that matter) in the cold zone.This is confirmed by calculations using data in table 1. Thus, product propene is removed and there is a small intercept on the abscissa on plots of FCBHs against FA. We have, however, simply taken the slopes of these plots as estimates of k1/(0.58k3 + k,) and thus we are underestimating the values of the denominator which are recorded in table 1. This inaccuracy is immaterial given the accumulation of errors up to this point in the calculation. The only published data which may be compared with the presently obtained values seem to be those of Wagner and Ze11ner,18 who obtained for the temperature range log(k3/cm3 mol-1 s-l) = 12.73 -(5.2 kJ mol-l/2.3RT) 195-390 K and log (k,/cm3 mol-1 s-l) = 12.64 - (1 1.7 kJ mol-l/2.3RT)R. M.MARSHALL, H. PURNELL AND P. W. SATCHELL 2403 from which we calculate (0.58k3 + k,)/1012 cm3 mol-1 s-l as 1.6, 1.8,2.0 and 2.1 at 630, 680,730 and 780 K, respectively. While the agreement with the present values appears quite reasonable, bearing in mind the very long extrapolation from 390 K, the published data suggest values of k,/k, ranging from 0.24 at 630 K to 0.30 at 780 K, which are, in fact, incompatible with the present results which require values of 2-3 at the higher temperatures if the preponderance of product ethene is to be accounted for. A clue to the possible cause of this discrepancy is given by the observation that larger values of k,/k, are required at the lowest pressure to account for the present results. This is not unreasonable since k, must be below its high-pressure limiting value in our reaction conditions. Such a pressure dependence is ignored in the results of Wagner and Zellner and thus the extrapolation to our temperature range is subject to substantial uncertainty.Any quantitative agreement between our results and theirs is thus fortuitous, but it is clear that we are in qualitative accord. ETHENE TITRATION Comparison of the values of FH obtained by the ethene titration technique and by calculation from the intercepts of plots of eqn (I) (cf. fig. 2) shows that the former is consistently only 80% of the latter (the mean ratio is 0.80, with a standard deviation of the mean of 0.02). We cannot account for a discrepancy of this magnitude. Our previous observation that the ethene titration underestimates FH by ca.3 % can only account for a minor part of the discrepancy. Fortunately, it does not affect our evaluation of k, etc. since the value of k, required depends only upon the ratios of values of FH, which are unaffected because of the consistent nature of the discrepancy. CONCLUSION We conclude that the primary step in the reaction of hydrogen atoms with cyclopropane is an abstraction reaction with a rate constant given by log(k,/cm3 mol-1 s-l) = 13.6f 1.0-(48.5+ 13.0 kJ mol-l/2.3RT). We thank the S.R.C. for the award of a research studentship to P. W. S . M. P. Halstead, D. A. Leathard, R. M. Marshall and J. H. Purnell, Proc. R. SOC. London, Ser. A, 1970,316, 575. * M. J. Lexton, R. M. Marshall and J. H. Purnell, Proc. R. SOC. London, Ser. A. 1971, 324,433. M. J. Lexton, R. M. Marshall and J. H. Purnell, Proc. R. Soc. London, Ser. A , 1971,324,447. R. M. Marshall and C. E. Canosa, J. Chem. SOC., Faraday Trans. I, 1980,76, 846. C. E. Canosa, R. M. Marshall and A. Sheppard, Znt. J. Chem. Kinet., 1981, 13, 295. D. Jones, P. A. Morgan and J. H. Purnell, J. Chem. Soc., Faraday Trans. I, 1977, 73, 131 1. ' P. Carnillei, R. M. Marshall and J. H. Purnell, J. Chem. SOC., Faraday Trans. 1 , 1974, 70, 1434. A. Sepehrad, R. M. Marshall and J. H. Purnell, J. Chem. SOC., Faraday Trans. 1, 1979, 75, 835. H. E. Gunning and E. W. R. Steacie, J. Chem. Phys., 1949, 17, 351. lo J. R. Dingle, Ph.D. Thesis (University of Toronto, 1949). l1 H. I. Schiff and E. W. R. Steacie, Can. J. Chem., 1951, 29, 1. l2 D. F. McMillen and D. M. Golden, Annu. Rev. Phys. Chem., 1982. l3 A. N. Hughes and J. H. Purnell, Nature (London), 1966, 210, 255. l4 A. Sepehrad, R. M. Marshall and J. H. Purnell, Znt. J. Chem. Kinet., 1979, 11, 411. l5 S. W. Benson and H. E. O'Neal, Kinetic Data on Gas Phase Unimolecular Reactions, (NSRDS-NBS l6 F. Trundle and R. Walsh, Seventh Znt. Symp. Gas Kinetics, Gottingen, 1982. l7 P. W. Satchell, Ph.D. Thesis (University of Wales, 1982). l8 H. Gg. Wagner and R. Zellner, BeL Bunsenges. Phys. Chem., 1972, 76, 447. l9 R. M. Marshall, J. H. Purnell and A. Sheppard, to be published. 21, U.S. Department of Commerce, Washington D.C., 1970). (PAPER 3/ 145 1)

ISSN:0300-9599    

DOI:10.1039/F19848002395

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. 1, 1984,80, 240-2415 Light-induced Electron-transfer Reactions at Electrodes Coated with Macromolecular Thionine and Ruthenium Systems BY R. TAMILARASAN, R. RAMARAJ, R. SUBRAMANIAN AND P. NATARAJAN* Department of Inorganic Chemistry, University of Madras, Guindy Campus, Madras 600 025, Tamil Nadu, India Received 22nd August, 1983 Thiazine dyes have been condensed with the macromolecules poly(N-methylolacrylamide), pol y(N-meth ylolacr ylamide-co-acrylic acid) and pol y(N-meth ylolacrylamide-co-vin ylp yridine). Cyclic voltammograms of polymeric thionine-coated electrodes indicate the formation of a complex by the dye with ferric and ferricyanide ions at the electrode. The peak potential of quinone is not affected by the polymer-dye-coated electrode.When the electrode is exposed to light cathodic behaviour at the electrode is observed, indicating a change in the polarity of the electrode in comparison with the reaction at a platinum electrode with thionine and iron(1r) present in the homogeneous solution. It is proposed that the generation of a charge-separated thionine-iron complex is stabilised by the macromolecular network. As a comparison polymeric ruthenium(I1) bipyridyl complexes have been prepared and their cyclic-voltammetric and photoelectrochemical behaviour has been investigated. The nature of the polymeric network and of the dye bound to it appears to be an important factor for efficient charge transfer at the electrode. Charge transfer across a photoconductor/liquid interface has important applications in energy-conversion systems' and in imaging processes.2 There have been several attempts to prepare electrodes coated with dyes and to use them to catalyse electron transport using Although photoconductivity is observed in several cases these systems have serious limitations5 owing to the poor light absorption exhibited by the thin films used in these studies and their instability on prolonged irradiation.Films of thicknesses of more than a few hundred Angstroms exhibit resistance to the photoinjection of charge at the electrode. Thus only a.fraction of the incident light is used for charge transport across the junction. Recently,s using macromolecular films containing dyes it has been found possible to observe a fairly appreciable current density using films of 10 pm thickness.More recently7-10 considerable effort has been made to understand electron transfer across polymer-coated electrodes. These studies reveal that charge transfer across the polymer interface is controlled by the nature of the polymer, the extent of its solvent-induced swelling and the distribution of the charge carriers in the polymer film. It is recognized that chemically modified electrode systems should facilitate the separation and stabilization of the charge carriers produced photochemically. In this report we present our results for the photoelectro- chemical properties of different types of chromophores attached to the polymeric films. 24052406 ELECTRON TRANSFER AT COATED ELECTRODES EXPERIMENTAL PREPARATION OF POLYMER SAMPLES Acrylamide (B.D.H.) obtained commercially was allowed to react with paraformaldehyde to obtain N-methylolacrylamide.N-methylolacrylamide was polymerized in aqueous solution using potassium peroxodisulphate as the initiator to obtain poly(N-methylolacrylamide), P(h1AAM). The copolymer poly(N-methylolacrylamide-co-acrylic acid), P(MAAM-co-AA), was prepared by copolymerizing N-methylolacrylamide with acrylic acid. P(MAAM-co-AA) was precipitated by adding the solution to a mixture of ethylacetate and dioxane (4: 1 v/v). The homopolymer P(MAAM) was soluble in this solvent mixture. Any poly(acry1ic acid) present in the copolymer as an impurity was removed by stirring the copolymer in dioxane in which poly(acry1ic acid) dissolved. The separated copolymer was dried and stored in a vacuum desiccator.Poly(N-methylolacrylamide-co-4-vinylpyridine), P(MAAM-co-VP), was obtained by copolymerizing N-methylolacrylamide with 4-vinylpyridine, freshly vacuum distilled from a commercial sample. The polymer in solution was precipitated using an acetone + water (4: 1 v/v) mixture. The homopolymer P(MAAM) was soluble in this solvent mixture and poly(4-vinylpyridine) was removed by stirring the precipitate in dry ethanol. The purified copolymer, P(MAAM-co-VP), was dried in a vacuum at room temperature. Since the colour of the polymer slowly changed to yellowish brown on exposure to the atmosphere it was stored in a vacuum desiccator. PREPARATION OF POLYMER-DYE COMPOUNDS Poly(acrylamidomethylt~onine-co-methylolacry1amide) (I) was prepared by the following procedure.ll Thionine obtained commercially (Riedel) was purified by repeated recrystallization in propan-2-01.Purified thionine was added to an aqueous solution of the polymer P(MAAM) in the desired molar ratio and the mixture was kept at 90 "C for 5 h. Hydroquinone was added to this mixture to prevent cross-linking of the polymer. The resulting polymer-bound thionine was precipitated in large amounts of methanol and purified by repeated precipitation. Poly(acrylamidomethylthionine-co-methylolacrylamide-co-acrylic acid) (11) and poly(acry1- amidomethylthionine-co-methylolacrylamide-co-4-vinylpyridine) (111) were prepared by follow- ing a procedure similar to that used for (I). The polymerdye complex (11) was precipitated using an ethylacetate + acetone (4: 1 v/v) mixture.The copolymers were purified by repeated precipitation. The uncondensed dye from the polymerdye compounds was removed by dialysing the solution of the copolymer in a dialysis sack for 8-10 days in water. Dialysis was continued until the solution outside the sack showed no absorption for thionine (A,,, = 600 nm). The thionine dye attached to the macromolecular chain was not removed easily even under rigorous conditions. Aqueous solutions of the polymer-dye complex were allowed to stand for several months in the laboratory and were subsequently dialysed against water; they did not show any trace of dye passing through the membrane, thus indicating the stability of the polymer-bound dye. The concentration of thionine bound to the macromolecule was estimated by a titrimetric method.I2 A solution containing a known amount of ferrous ammonium sulphate in ortho- phosphoric acid was titrated against a known volume of the polymer-bound thionine solution.The colour of the polymer-bound thionine disappeared owing to the reduction of thionine to leucothionine by ferrous ion in the presence of phosphoric acid. The end point was the appear- ance of a rose-red colour, the colour of polymer-bound leucothionine in orthophosphoric acid. From the titration data the amount of thionine present in the solution was calculated. A known volume of the original polymerdye solution was evaporated and the weight of the residue was taken as the amount of polymer-dye complex present in the original solution. The number of thionine units bound to a polymer chain consisting of a given number of monomer units (the ratio m / d ) was calculated, knowing the amounts of polymer and thionine present per unit volume of solution.The thickness of the film was measured by a micro-screw gauge.R. TAMILARASAN, R. RAMARAJ, R. SUBRAMANIAN AND P. NATARAJAN 2407 PREPARATION OF POLYMER-BOUND RUTHENIUM(II) COMPLEXES Poly(4-vinylpyridine), PVP, was obtained by the polymerization of 4-vinylpyridine using potassium peroxodisulphate as initiator. Vacuum-distilled 4-vinylpyridine (5 cm3) was dissolved in 50 cm3 of an acetone+water mixture (1 : 1 v/v) in a round-bottomed flask. Ca. 50 mg of potassium peroxodisulphate was added and the mixture was kept at 50+ 1 "C for 6 h. The resulting PVP was precipitated by pouring the viscous solution into a large amount of water.PVP, purified by being dissolved in a minimum amount of ethanol and reprecipitated in water, was dried in a vacuum oven at 25 "C and stored in a vacuum desiccator. Poly(4-vinylpyridine- co-acrylamide) was prepared by the copolymerization of 4-vinylpyridine and acrylamide. Appropriate molar ratios of 4-vinylpyridine and acrylamide were dissolved in an acetone + water mixture (1 : 1 v/v) and the solution was deaerated for 20 min by the passage of purified nitrogen. To thw solution was added 50mg of potassium peroxodisulphate and the deaeration was continued for 10 min. The solution was then allowed to remain at 45 "C for 6 h. The resulting viscous solution was poured into the appropriate non-solvents and the polymer P(VP-co-AM) was precipitated.In the case of P(V,P-co-AM) the ratio of 4-vinylpyridine to acrylamide (VP: AM) was 1 : 1.94 and the non-solvent used for precipitating the polymer was dioxane; for P(VP-co-AM) with VP:AM = 1 : 15.4 and 1 :40 the non-solvent used was acetone. The copolymer ratio (VP: AM) was found by determining the amount of vinylpyridine present in a given amount of P(VP-co-AM), taking into account that vinylpyridine absorbs at 254 nm (E = 3597 dm3 mol-1 cm-l) and that acrylamide exhibits no absorption at this wavelength. The c-dichlorobis(bipyridine)ruthenium(Ir) complex was attached to PVP13 and P(VP-co-AM) by treating stoichiometric quantities of the ruthenium complex with PVP or P(VP-co-AM) in appropriate solvents. The polymer PVP and c-dichlorobis(bipyridine)ruthenium(rI) were taken in the required stoichiometric ratio and dissolved in methanol.The methanolic solution was then refluxed for 90 h. The resultant clear solution was dialysed with cold water present outside the membrane for ca. 1 week to remove the uncondensed ruthenium complex under dark conditions. After dialysis the methanolic solution was evaporated to dryness in a thin-film evaporator at 40 "C. The film was then stored in a vacuum desiccator at room temperature. When P(VP-co-AM) was used instead of PVP, P(VP-co-AM) and Ru(bpy),Cl, were taken in methanol and water was added until P(VP-co-AM) was completely dissolved; condensation was then carried out as in the case of PVP. The products [R~(bpy)~(VP-co-AM),1~+ and [RU(~~~>,(PVP),]~+ were characterized by their visible absorption spectra, which had absorption maxima at 460 nm.The percentage ruthenium content was calculated spectrophotometrically assuming that the molar absorptivity of [Ru(bpy),(VP-co-AM),I2+ at 460 nm is equal to that of [RU(bPY)2(PY)2I2+. ANALYTICAL PROCEDURES Cyclic voltammograms were run using the PAR modules: a model 173 potentiostat, a model 175 universal programmer and a model 176 current follower. The working electrodes employed were a platinum plate (1 cm2), polymer-bound ruthenium or thionine-coated platinum (1 cm2). A platinum plate (1 cm2) was used as the counter-electrode and a saturated calomel electrode was used as the reference electrode. The latter was connected to the potentiostat through a PAR model 178 electrometer.The photovoltaic effects of the polymer-coated electrodes were studied using a cell consisting of a 1 cm2 platinum plate coated with either polymer-bound thionine or polymer-bound ruthenium and another 1 cm2 platinum electrode. The distance between the two electrodes was maintained at 1 0.1 mm. The polymer-bound thionine or polymer-bound ruthenium was coated onto the platinum by taking a known concentration of the polymer complex on the surface of the electrolytically cleaned and rinsed platinum and drying this at 90 "C in a vacuum. The evaporation of the solvent left an insoluble stable film on the electrode. In the case of polymer-bound thionine the electrodes were immersed in a cell containing 10 cm3 of 0.5 x lo-, mol dmP3 sulphuric acid and lo-, mol dm-3 ferrous solution.The solution was deaerated for 30 min by the passage of oxygen-free nitrogen. The photovoltaic effect of the polymer-bound ruthenium film was studied by keeping the electrodes in 0.1 mol dm-3 perchloric acid and 5 x mol dm-3 ferric perchlorate. The solution was deaerated by the2408 ELECTRON TRANSFER AT COATED ELECTRODES 0.6 \ 9 4 0.3 0 tlmin tlmin t/min Fig. 1. (a) Photocurrent for iron-thionine system, (b) photocurrent for iron-polymer-thionine system and (c) photocurrent for electrode coated with polymer-thionine complex for ' light on ' and ' light off' conditions. Table 1. Behaviour of electrodes coated with the polymer-dye complex substratea AE,,/mV I,,/pA Pmax/pW A 37.4b 2.84 0.027 3 1 .4c 3.70 0.029 58.gd 2.88 0.042 50.0e 3.9 0.049 SO.@ 3.7 0.046 B 32.0b 1.80 0.014 29.6c 2.76 0.020 C 4.0b - - - - 3.0c a (A) Poly(N-acrylamidomethylthionine-co-methylolacrylamide), (B) poly(N-acrylamido- methylthionine-co-methylolacrylamide-co-acrylic acid) and (C) poly(N-acrylamidomethylthio- nine-co-methylolacrylamide-co-vinylpyridine) ; stirring conditions; stationary conditions; d p H 1; e p H 2 ; f p H 3 .passage of oxygen-free nitrogen for 30 min. The irradiation source was a 300 W tungsten lamp in the case of the polymer-bound thionine film and a 1000 W tungsten lamp in the case of the polymer-bound ruthenium film. RESULTS POLY MER-BOUND THIONINE SYSTEMS The polymeric thionine film is not soluble in aqueous solution. Irradiation of the electrode coated with polymer-bound thionine immersed in a cell containing an aqueous solution of ferrous ion using a 300 W tungsten lamp shows a positive photopotential and the current flows from the uncoated platinum electrode to the coated electrode as shown in fig.1 (c). The open-circuit photopotential, AE,,, short-circuit current, Isc, and maximum power output for the coated electrode, Pmax, are given in table 1 . The photovoltaic behaviour of the electrode coated withR. TAMILARASAN, R. RAMARAJ, R. SUBRAMANIAN AND P. NATARAJAN 2409 Table 2. Behaviour of electrodes coated with polymer-dye complexes on irradiation with various reductan tsa FeSO, 32b FeSO, 1 oc K,Fe(CN), 2b H2Q 20b a Reductant concentration 1 x mol dm-3, pH 2.0; in aqueous solution ; in a 50 % water + acetonitrile mixture.Table 3. Cyclic-voltammetric behaviour at coated and uncoated platinum electrodesa reaction uncoated coated FelI1 + e- -+ FeL1 0.37 0.26 Q + 2e-+ 2H+ + H,Q 0.24 0.24 Fe(CN)g- + e- --+ Fe(Cn)4,- 0.14 0.00 a Medium 0.25 mol dm-3 H,SO,, sweep rate 50 mV s-l ; Epc, cathodic peak potential. polymer-bound thionine was studied using the reducing agents FeSO,, K,Fe(CN), and hydroquinone, and the results are summarized in table 2. The cyclic-voltammetric behaviour of the coated electrodes in the presence of these reductants is given in table 3. POLYMER-BOUND RUTHENIUM(II) COMPLEXES [Ru(bpy),(VP-co-AM),I2+ was coated onto a platinum electrode and cyclic voltam- mograms were obtained in acetonitrile + water mixtures (v/v). The wave shapes were found to be of a diffusional character, as shown for a typical case in fig. 2.For all the samples the cathodic peak is observed at 1.02 V or close to that potential. The peak current strongly depends upon the concentration of sulphuric, acid present in the medium. The cathodic peak current, peak potentials and separation of peak potentials at various acid concentrations are listed in table 4. The characteristics of these cyclic voltammograms are given in table 5 for different acetonitrile +water mixtures. The amount of RuII present in the film is determined from the known amount of RuII solution in contact with the platinum plate knowing the molar absorptivity of [Ru(bpy),(py),12+ at 460 nm. Details of cyclic voltammograms of the polymer with different amounts of ruthenium present are given in table 6 .The photopotential of polymer-bound ruthenium@) coated onto a platinum electrode dipped into ferric ion solution using a 1000 W tungsten-halogen lamp was found to be 5 mV in water or acetonitrile +water solution.2410 too 80 60 % u s" 40 20 0 ELECTRON TRANSFER AT COATED ELECTRODES I* 3 0.7 EfV vs SCE 0 3 6 9 12 I5 o-t/(mV s-+ Fig. 2. (a) Cyclic voltammogram for [(R~(bpy),(VP-co-AM,)]~+-coated electrode. 0.08 mol dm-3 H,SO,, VP:AM = 1 : 15.4, scan rates (in mV s-l) as follows: (1) 5, (2) 10, (3) 20, (4) 50, '(5) 100 and (6) 200. (b) Plot of cathodic peak current against (scan rate)&.R. TAMILARASAN, R. RAMARAJ, R. SUBRAMANIAN AND P. NATARAJAN 241 1 Table 4. Cyclic-voltammetric data for polymer-bound ruthenium complexes coated onto electrodes at various acid concentrationsa sample VP:AM H2S04 EpcIV EpaIV AEp/mV ZpcIPA l:o 0.1 0.15 0.2 1 : 1.94 0.1 0.15 1 : 15.4 0.1 0.15 1:40 0.1 0.2 1.02 1.02 1.015 1.015 1.025 1.015 1.025 1.03 1.045 1.066 1.07 1.07 1.08 1 .O n 1.08 1.085 1.09 1.09 46 50 55 65 50 65 60 60 45 470 550 570 205 297 35 50 10 15 a Sweep rate 20 mV s-l, solvent composition CH3CN:H20 = 9: 1 v/v. Table 5. Cyclic-voltammetric data for polymer-bound ruthenium complexes coated onto electrodes in various solvent mediaa solvent CH,CN : H20 (v/v) ZpcIPA EpcIV EpaIV AEp/mV 7: 3 8:2 9: 1 125 1.05 1.107 57 125 1.04 1.095 55 60 1.02 1.08 68 _ _ _ _ _ _ _ _ _ ~ ~ ~~ ~ a Sample composition VP:AM = 1 : 15.4, sweep rate 50 mV s-l, [H2S04] = 0.05 mol dm-3. Table 6. Cyclic-voltammetric data for polymer-bound ruthenium complexes coated onto electrodes amount of Ru'I sample % present VP:AM condens- in film (v/v> ZpclPA EpcIV EPa/V AEp/mV ation /mol dm-3 l:o 470 1.02 1.066 47 9.06 2.37 x lod6 1 : 15.4 35 1.015 1.08 65 1.04 2.43 x lo-' 1:40 10 1.03 1.09 60 0.58 7.98 x 1: 1.94 205 1.015 1.08 65 3.89 7.74 x 10-7 DISCUSSION Thionine dye, which absorbs strongly in the visible region with a maximum at 600 nm, and tris(2,2-bipyridine)ruthenium(11), with a maximum at 450 nm, do not undergo any net decomposition on steady photolysis (4 < 10-5).14915 In the case of both systems efficient transient charge separation processes occur on irradiation with visible light.This property makes these compounds very attractive for study as model241 2 ELECTRON TRANSFER AT COATED ELECTRODES photochemical systems for solar-energy conversion.Both dyes undergo electron transfer from the excited states. Thionine undergoes a photoinduced reduction which is followed by a disproportionation reaction to yield leucothionine and thionine via a two-electron reduction. The redox potentials are TH+/TH;+ = 0.2 V and TH,+/THi = 0.575 V us NHE. *Ru(bpy):+ ion undergoes a one-electron oxidation, with the redox potentials for the couple *Ru(bpy):+/Ru(bpy)i+ = -0.82 V and Ru(bpy):+/Ru(bpy)i+ = 1.27 V us NHE. Although no economically viable solar cells have yet been made using these compounds, attemptP1* have been made to use their derivatives in energy-conversion devices. Recentlylga other macromolecular thionine films have been prepared, and under heterogeneous conditions the dye-coated electrode behaves as a cathode.Different polymer-bound RuII-bipyridyl complexes have been prepared and their photoelectrochemical properties have also recently been reported.lgb CYCLIC-VOLTAMMETRY STUDIES OF ELECTRODES COATED WITH POLYMER-DYE COMPLEXES The cyclic-voltammetric behaviour of electrodes coated with polymeric thionine in the presence of ferrous ion reveals that the cathodic peak potential of the FeI1/Fe1I1 couple shifts cathodically. Similar behaviour has been observed for inert elec- trodes coated with monomeric thionine derivatives.20 The cathodic peak of the Fe(CN):-/Fe(CN)i- couple also shifts negatively at the polymeric thionine electrode. These results are explained by the formation of a complex between the ferric ion and dye in the former case and by the formation of an ion pair between the ferrocyanide and thionine in the latter system.The quinone system does not form an adduct with the polymeric thionine and hence its redox potential is not affected at the polymer electrode. Note that the peak currents decrease in all cases because part of the electrode surface is covered by the inert polymer network. In acidic solution pyridine groups in the PVP chain which are not coordinated to the ruthenium become protonated. Owing to quarternization of the pyridine centres the structure of the polymer chain itself is changed. With protonated pyridine units present in the macromolecule a more linear conformation is attained by the polymer. In the PVP chain, in which some of the pyridines are coordinated to the ruthenium, the film is likely to be composed of a random tangle of polymer chains.Cyclic voltammograms of ruthenium(I1)-bound polymers containing varying amounts of acrylamide in a given solvent at constant pH show a decrease in cathodic peak current (table 6). For the film with VP:AM = 1: 1.94 the cathodic peak current is decreased by a factor of ca. 2 in comparison with the ruthenium-bound homopolymer of PVP even though the total ruthenium content in the film is decreased by a factor of ca. 3. For the polymer sample with VP:AM = 1 : 15.4 the total amount of ruthenium in the film and the extent of metallation in the polymer sample are decreased by a factor of three. However, the cathodic peak current is decreased by a factor of 7 when compared with the sample in which VP: AM = 1 : 1.94.Thus it appears that below a certain ruthenium content in the polymer chain the current flow is drastically affected. Presumably as the percentage of ruthenium in the polymer chain is decreased the average distance between the ruthenium centres increases and the rate of charge transfer to the electrode through the ruthenium is decreased. This explanation is in line with the proposed mechanism for charge transfer involving electron exchange between adjacent pairs of redox centres.21R. TAMILARASAN, R. RAMARAJ, R. SUBRAMANIAN AND P. NATARAJAN 2413 PHOTOVOLTAIC EFFECT OF A POLYMER FILM CONTAINING MACROMOLECULES BOUND TO LIGHT ABSORBERS Light-induced electron transport across thick films of polymers containing light absorbers is of considerable interest and not many systems are known where this effect has been shown for film thicknesses exceeding few hundred In addition to the photoinduced current, polymeric thionine films coated onto inert electrodes also show a heterogeneous photoredox reaction not exhibited in a homogeneous solution containing macromolecular thionine.On one-electron reduction thionine produces semithionine which disproportionates in homogeneous solution. In homogeneous solution the electrode reaction is believed to involve the two-electron-reduced leu~othionine.~~ In contrast in the polymeric film it appears that disproportionation does not occur. Thus the electrode reaction occurring under heterogeneous conditions brings about a change in the polarity of the electrodes and hence a change in the direction of the current flow as shown in fig.1. The electrode reactions occurring at the electrode coated with polymeric thionine and when the dye is present in the bulk solution (at the uncoated platinum electrode) are shown in schemes 1 and 2. Scheme 1. Electrode coated with polymer-thionine complexes h Y P-TH+ -+ *P-TH+ H+ *P-TH+ + FeII + [P-TH2-FeI4+ [P-TH2-FeI4+ + e- + P-TH;+ + FeI1 (cathode) FeII -+ FelI1 + e- (anode) P-TH;+ + FeIII -+ P-TH+ + FeII + H+. Scheme 2. Polymer-thionine complex in homogeneous solution H+ *P-TH+ + FeII -+ P-TH;+ + FelI1 2P-THi+ + P-TH; + P-TH+ P-TH; -+ P-TH+ + 2H+ + 2e- (anode) FeIrl +e- -+ FeII (cathode). At the heterogeneous electrode the dye present in the macromolecular network is excited on light absorption and is reduced by the ferrous ion present in the solvent.The fact that the electrode functions as a cathode suggests that at the uncoated platinum electrode ferrous ion is oxidized to ferric ion. The species which is reduced at the polymer-dye electrode is proposed to be a complex between thionine and iron. When the dye is present in the macromolecular film attached to the electrode a new type of electrode reaction is seen which indicates a possibility of preparing chemically modified electrodes of specific character. The thionine-iron complex formed by the absorption of light is stabilized by the polymer network and at the electrode this complex is further reduced. The following reaction occurs : [P-TH2-FeI4+ + e- -+ FeII + P-TH;+ .2414 ELECTRON TRANSFER AT COATED ELECTRODES inert electro :trolyte swelled polymer film Fig, 3.Electrode coated with polymer4ye complex: 0, semithionine; 0, FeI1 and 0, FeI". The polymer-semithionine complex is oxidized by the ferric ion produced at the anode, thus completing the cycle. [R~(bpy),(PvP)~]~+ complexes show photosubstitutional and excited-state electron-transfer proce~ses.~~~ 25 Illumination of the electrode coated with [Ru(bpy),(PVP),I2+ immersed in aqueous solution containing FeIII ions does not show any appreciable photopotential. The meagre photopotential and direction of the current observed indicate that the electrode reaction is similar to that for the Ru(bpy);+-Fe*II photogalvanic cell. The absence of an appreciable photopotential when the macromolecular ruthenium complex is coated onto the electrode may be due to the following reasons.Some of the absorbed light is used in the photolabilization of the PVP ligand in the swollen film. Also, it is likely that the photoproduced ruthenium(II1) and the ferrous ion recombine at a faster rate without taking part in the electrode reaction. It thus seems that macromolecular structure alters the nature of the reactivity of the photoproducts in the case of thionine polymer, whereas in the case of the ruthenium polymer the macromolecular environment has not altered the nature of the electrode process.The dye is incorporated into the polymer-coated electrode as shown in fig. 3. The ruthenium and thionine polymers are both water soluble and the cross-linked films coated onto the electrode are swollen, allowing the free penetration of ions through the polymer network.In the case of thionine the dye centres interact to some extent with each other as shown by the absorption spectra,s whereas in the case of ruthenium no such interactions are It is apparent that the macromolecular network plays an important role in the efficient charge injection at the thionine polymer electrode. It is seen to be an essential condition since on irradiation monomeric thionine-coated electrodes do not show any change in the polarity of the thionine-coated electrode or a high current density.26 Work is underway to determine the efficiency of the thionine-coated electrode for the conversion of light and to prepare polymer-coated electrodes with different light absorbers.R.TAMILARASAN, R. RAMARAJ, R. SUBRAMANIAN AND P. NATARAJAN 2415 The work is supported by grants from the Department of Science and Technology, Government of India. R. R. is the recipient of a U.G.C. fellowship. A. Heller and B. Miller, Electrochim. Acta, 1980, 25, 29. G. C. Hartmann, L. M. Marks and C. C. Yang, J. Appl. Phys., 1976,47, 5409. L. R. Faulkner, H. Tachikawa, F. R. Fan and S . G. Fischer, A.C.S. Adu. Chem. Ser., ed. M. S . Wrighton, (A.C.S., Washington D.C., 1980), vol. 184, chap. 7. F. J. Kampas, K. Yamashita and J. Fajer, Nature (London), 1980, 2%4,40. H. Meir, Top. Curr. Chem., 1976, 61, 85. R. Tamilarasan and P. Natarajan, Nature (London), 1981, 292, 224. H. S. White, J. Leddy, and A. J. Bard, J. Am. Chem. SOC., 1982, 104, 4811 and references therein. F. C. Anson, J. M. Saveant and K. Shigehara, J. Am. Chem. Soc., 1983, 105, 1096. C. P. Andrieux and J. M. Saveant, J. Electroanal. Chem., 1982, 134, 163; 142, 1. lo W. J. Albery, Acc. Chem. Res., 1982, 15, 142. l1 H. Kamogawa, M. Kato and H. Sugiyama, J. Polym. Sci., 1968, 2967. l2 R. Tamilarasan, Ph.D. Thesis (University of Madras, 1981). l3 J. M. Clear, J. M. Kelly, D. C. Pepper and J. G. Vos, Inorg. Chem. Acta, 1979, 33, L 139. l4 S. Solar and N. Getoff, Int. J. Hydrogen Energy, 1979, 4, 403. l5 K. Kalyanasundaram, Coord. Chem. Rev., 1982,46, 159. l6 W. D. K. Clark and J. A. Eckert, Solar Energy, 1975, 17, 147. W. J. Albery and M. D. Archer, Nature (London), 1977,270, 399. D. E. Hall, P. D. Wildes and N. N. Lichtin, J. Electrochem. SOC., 1918, 125, 1365. Press, Oxford, 1981), D. 2204; (b) 0. Hass and J. G. Vos, J. Elecroanal Chem., 1980, 113, 139. (London), 1979, 282, 793. l9 (a) P. Natarajan and R. Tamilarasan, World Solar Forum, ed. D. 0. Hall and J. Morton (Pergamon 2o W. J. Albery, A. W. Foulds, K. J. Hall, A. R. Hillman, R. G. Egdell and A. F. Orchard, Nature 21 J. M. Calvert and T. J. Meyer, Znorg. Chem., 1981, 20, 27. 22 K. Doblhofer, Electrochim. Acta, 1980, 25, 871. 23 P. D. Wildes and N. N. Lichtin, J. Am. Chem. Soc., 1978, 100, 6568. 24 0. Hass, M. Knens and J. G. Vos, J. Am. Chem. Soc., 1981, 103, 1318. 26 W. J. Albery, P. N. Bartlett, J. P. Davies, A. W. Foulds, A. R. Hillman and F. S . Bachiller, Faraday R. Ramaraj and P. Natarajan, to be published. Discuss. Chem. Soc., 1980, 70, 341. (PAPER 3/1481)

ISSN:0300-9599    

DOI:10.1039/F19848002405

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans, 1, 1984, 80, 2417-2437 Effect of Organised Surfactant Systems on the Kinetics of Metal-Ligand Complex Formation and Dissociation BY PAUL D. I. FLETCHER AND BRIAN H. ROBINSON* Chemical Laboratory, University of Kent, Canterbury, Kent CT2 7NH Received 2nd September, 1983 The kinetics and mechanism of a range of metal-ligand complexation reactions have been studied in water, sodium dodecylsulphate micellar solutions and aerosol-OT-stabilised water- in-oil microemulsions. A generalised theory is developed for the interpretation of kinetic measurements in these dispersions and it is tested for metal-complex formation. In most cases the rate of complex formation is only affected by the concentration-enhancement effect of the surfactant/water interface on the reactants.Then the volume within the solution available for reaction can be deduced and reasonable values are obtained. For the reaction between Ni(phen)2+ and PADA, other factors influence the kinetics. As a result the nature of the micelle surface where reaction occurs can be investigated. For the ligand PAN there is evidence that it can partition between more than two pseudo-phases, with a consequent reduction in the rate of complex formation. The dissociation rates of the complexes at the interfaces are increased by up to a factor of ten as compared with bulk water. The presence of aqueous micelles can enhance or retard chemical reaction rates by large factors and equilibrium constants are altered in a corresponding manner. These characteristics can be exploited using a range of organised surfactant systems, which include micelles, oil-in-water and water-in-oil microemulsions and vesicular systems.Potential applications are in the diverse fields of photochemical energy storage,' novel enzyme processes2 and solvent extraction of metal ions. Previous studies of the kinetics of complex-formation reactions between Ni2+(aq) ions and hydrophobic ligands in sodium dodecylsulphate (SDS) micellar solutions3~ * have indicated that both reactants are bound in the surface layer of the micelles. The large rate enhancements observed for complex formation are shown to be caused by the reactants being localised at the micelle surface and their concentrations being effectively enhanced. There are no other significant effects on the reaction kinetics.In this paper the kinetic studies are extended to include water-in-oil microemulsion systems, and the effect of strongly bound ligands (to the metal ion) on the rate of ternary complex formation in micellar media is investigated. Accurate data for the effect of a change in the reaction environment on the kinetics of the (unimolecular) dissociation of the metal complex are also obtained. A generalised kinetic model is developed which is based on reaction in, and partitioning between, two pseudo-phases. Appropriate derived kinetic parameters are then compared for the different reaction media. The reactions investigated were all between metal-complexing agents (ligands), covering a range of hydrophobicities, and divalent metal ions.Kinetic studies included complex formation and dissociation involving Ni2+(aq), Co2+(aq) and Zn2+(aq) with the azo-dye ligands pyridine-2-azo-p-phenol (PAP), pyridine-2-azo-p-dimethylaniline (PADA) and 1 -(pyridyl-2-azo)-2-naphthol (PAN). The effects of SDS micelles on rates 79 241 7 FAR 1241 8 EFFECT OF SURFACTANTS ON COMPLEX FORMATION of ternary complex formation/dissociation involving Ni(dien)2+, where dien = di- ethylene triamine and Ni(phen)2+, where phen = 1 , 10-phenanthroline, with the ligand PADA were also investigated. The micellar system used was SDS in water. The water-in-oil microemulsion system was prepared by adding water to solutions of sodium bis(2-ethylhexyl)sulphosuccinate (AOT) in n-heptane. The microemulsion system for the composition and temperature range employed consisted of thermodynamically stable and optically clear dispersions of spherical water droplets in a continuous heptane solvent.At the oil/water interface there is an interfacial monolayer of AOT. The radii of the water droplets depend primarily on R( = [H,O]/[AOI) and are in the range 1-10 nm. For low values of R the system is essentially mon~disperse.~ Considering the micellar and microemulsion dispersions as reaction media they are similar in nature : both consist of a hydrophobic (hydrocarbon) region, a hydrophilic (aqueous) region and a charged interface region formed using a negatively charged surfactant. However, the volume fractions of the solvent domains are very different in the two systems. In addition, the properties of the dispersed water in the microemulsion system may be different from those of bulk water since the droplets contain, typically, only a few thousand water molecules.For example, there may be structural changes in the smaller droplets because of (a) the small number of water molecules present and (6) the very high ion concentration (effectively > 1 mol dm-3 Na+) in the droplets. In any case the water activity would be expected to differ from that in bulk water. The kinetic studies reported in this paper enable a direct comparison to be made of micellar and microemulsion systems insofar as they influence chemical reactivity. EXPERIMENTAL PADA was obtained from Sigma; found/calculated: C 68.8/69.0, H 6.27/6.19, N 24.7/24.8. PAP was synthesised according to the method of Betteridge and John;6 found/calculated : C 65.8/66.3, H 4.5/4.5, N 21.0/21.1.PAN was obtained from Aldrich; found/calculated: C 71.7/72.3, H 4.56/4.42, N 16.4/16.9. Phen was obtained as the analytical reagent from Fisons. Mono( 1, 10-phenanthroline)nickel(II)nitrate was prepared as described by Steinhaus and Margerum' and was used immediately after preparation; found/calculated : C 36.5/33.1, H 2.5/3.7, N 14.2/12.9, Ni 14.2/13.5. SDS was a B.D.H. specially pure reagent and was used without further purification. AOT was a Fluka purum reagent. The batch of AOT used contained a very small amount of a weak acid impurity but this did not affect the results. n-Heptane was distilled over sodium metal, stored over type 4A molecular sieve and filtered before use.Water was deionised and doubly distilled, the second time from alkaline permanganate solution. Solutions containing metal ions were prepared from AR grade nitrate salts. Solutions of Ni(dien)2+ were prepared as described by Cobb and Hague.8 Kinetic measurements were made using a small-volume stopped-flow instrument with spectrophotometric detection. This instrument has been described previou~ly.~ For reactions occurring in times shorter than a few milliseconds, a conventional Joule-heating temperature- jump instrument (Messenlagen GmbH) was employed. For aqueous micellar solutions, no inert electrolyte was added since it was found that SDS (at concentrations > 0.05 mol dm-3) provided current-carrying capacity sufficient for a rapid discharge. In all cases, kinetic traces were indistinguishable from single exponentials. Values of observed first-order rate constants, kobs, were obtained by means of an analogue curve-matching procedure.Quoted values of kobs are the mean of between 5 and 10 values, and kobs is then known to better than & 5%. The pH of all aqueous and micellar solutions was controlled manually using small additions of HCl or NaOH solutions. The pH was measured by means of a Radiometer (model 26) pHP. D. I. FLETCHER AND B. H. ROBINSON 2419 meter incorporating a dual glass/calomel electrode (Radiometer type GK 2321C). In micellar solutions the pH values measured using a glass electrode were taken to be the values corresponding to the bulk-water region and not the aqueous region in the vicinity of the micelle surface.For SDS micellar solutions the ‘ pH’ in the surface region is known to be approximately two units lower than that of the bulk ~olution.~ In microemulsion solutions u.v.-vis. spectrophotometry, using a Cary 219 spectrophotometer, was employed to check that, at the pH of the kinetic experiment, the expected ligand and metal-ligand complex species were present. Where possible, pK, values were determined to ensure that no protonation (or deprotonation) of the reacting species had occurred. lo No attempt was made to control the ionic strength of the micellar solutions as addition of excess electrolyte might be expected to alter dramatically the properties of the aggregates, e.g. shape, surface potential, c.m.c. Since the ligands were uncharged the reactions are expected to be relatively insensitive to ionic strength (I) (< 10% change in rate for a change in I of 0.1 mol dm-3).KINETIC MODEL The model employed in the analysis of the kinetic data is essentially that developed by Berezin et uZ.ll An alternative but related approach has been developed by Romsted.12 In the Berezin model, the chemical reaction is assumed to occur in two pseudo-phases, one associated with the surfactant aggregates and the other with the bulk solvent. The reaction scheme for a reversible bimolecular association reaction is where the superscript S denotes a pseudo-phase associated with the surfactant and superscript B indicates the bulk solvent pseudo-phase. Since the reactions investigated in this work occur in the time range from 0.1 ms to 10 s, it is reasonable to assume that partitioning of the reacting species between the two pseudo-phases is rapid compared with the rate of chemical reaction.Hence partitioning can be described by the equilibrium parameters KA, KB and Kc. Photochemical studies13 have shown that partitioning of divalent metal ions between SDS micelles and water occurs on the ps-ns timescale, which implies that adsorption of metal ions onto the micelle surface is close to diffusion controlled. The exit rates of uncharged, aromatic, hydrophobic species from SDS micelles have also been measured by photophysical techniques.14 Values range from cu. lo4 to lo7 s-l and rate constants for ligands used in this study will have similar values. A kinetic process unique to water-in-oil microemulsion systems is solubilisate exchange.For water- soluble species, e.g. metal ions, this process occurs following droplet c~llision.~ The kinetics have been measured by two independent method^,^^ l5 and the process is fast on the timescale of these experiments. A further point to be considered in the kinetic analysis is the dynamics of the aqueous micellar system. In particular the ‘slow relaxation ’ associated with breakdown/reformation ofmicellesmight be acomplicating factor. However, the kinetics describing complex formation depend only on the total concentration of micellised surfactant, which does not change during the timescale of the experiment. Hence eqn (1) adequately represents the process. The appropriate concentration units applicable to reactions in colloidal dispersions has recently been discussed.ls For the case where reaction is slow on the timescale 79-22420 EFFECT OF SURFACTANTS ON COMPLEX FORMATION of transport processes it is simplest to use overall concentrations as proposed originally by Berezin.ll The equilibrium constant Kx for species X partitioning between the two pseudo- phases may be defined as K X = [xlS/[xlB (2) where C is the concentration of surfactant which contributes to the surfactant pseudo-phase volume.For aqueous micellar solutions this may be taken to be ([surfactant] -c.m.c.). For AOT microemulsions [AOT] may be used, which assumes that all the AOT is bound at the interface (this may not be the case as the phase boundaries for stability are approached).17 [XI, and [XI, are the concentrations of X in the micellar and bulk pseudo-phases, respectively (expressed as mol dmP3 of total solution).K , is related to a normally defined dimensionless distribution coefficient Px( = ([x],/cv}/([x]B/( 1 - CV)}). When the volume fraction of surfactant pseudo- phase is small Kx = Px V (3) where V is the volume contributed per mole of micellised surfactant to the micellar pseudo-phase. Eqn (2) is only likely to be valid when the surfactant concentration, C, is large compared with the reactant concentrations. For eqn (l), the observed second-order rate constant for formation of complex, kfbs, is given by The observed first-order rate constant kEbs for the dissociation of the complex, C, is given by A limiting case of eqn (4) and (5) is of particular interest. When both reactants and the product partition strongly to the surfactant pseudo-phase, reaction can only occur in that pseudo-phase, and then for pseudo-first-order conditions (i.e.[Ale % [B],) the observed first-order rate constant for complexation is given by - where For C > c.m.c. and for KA and KB % C-l, this reduces to Equations of this form have been used previously in the interpretation of kinetics in micellar s01ution.~ Eqn (8) predicts kobs -+ 00 as C --* 0 for KA C and KB C B 1. In practice a maximum in kobs is observed for C slightly greater than C . ~ . C . ~ ' Eqn (2)-(8) are general and can be applied to any dispersion consisting of two (or more) pseudo-phases. Pseudo-phases present in aqueous micellar solution are the bulk (aqueous) solvent, the non-polar hydrocarbon micellar core and the micellar surface region.Pseudo-phases in the w/o microemulsion may be the central aqueous core, the charged surfactant interfacial region and the bulk oil solvent.P. D . I. FLETCHER AND B. H. ROBINSON 242 1 In practice, surfactant aggregate solutions do not generally consist of sharply defined discrete regions for the purposes of chemical reaction. For example, the distribution of reacting metal ions, attracted to a negatively charged micellar surface, may be more realistically considered to form a double-layer region surrounding the charged surface.l0 Hence, in this simplified model, I/ is defined as an 'effective' pseuso-phase volume in which reaction occurs. Obviously, a detailed interpretation of I/ would be derived from consideration of the overlap region of the radial distribution profiles of the reactants.RESULTS AND DISCUSSION REACTION OF Ni2+(aq) WITH PAP RATE PARAMETERS AND MECHANISM IN WATER At the pH used, the reaction is PA? At pH 5.0 and 25.0 "C, k, is found to be (l.OkO.1) x lo3 dm3 mol-1 s-l and k, is 0.36 k 0.02 s-l. From the temperature dependence of the rate constants, AH1 = 51 k 3 kJ mol-l and AHL = 74f 3 kJ mol-l. The mechanism for complex formation involves the rapid formation of an outer-sphere complex [with equilibrium constant Kos (in dm3 mol-l)] followed by rate-limiting loss of a water molecule from the inner solvation shell of Ni2+(aq), resulting in formation of a monodentate complex. The rate constant for water loss is identified with that for solvent exchange, kex, as measured by n.m.r.methods. This mechanism is known as the Eigen-Tamm-Wilkins mechanism. There is a ring-closure step involved in forming the final complex, which is sufficiently fast (for the similar ligand PADA) that the forward rate is still largely determined by k,,.18 KO, may be calculated using the Fuoss equation19 and values of ca. 0.1 dm3 mol-l are obtained for an uncharged ligand. Then kf = Kosk,,. (10) Measured values of k,, and AHL, (determined from 170 n.m.r. studies) are in the ranges (2.7-4.4) x lo4 s-l (at 25 "C) and 45-51 kJ mol-l, respectively.20 The experi- mental values of k, and A H ] are therefore consistent with this mechanism. The slight discrepancy between the measured and calculated values of kf may be due to a small contribution from the ring-closure step to the overall rate of complex formation.The rate constant k, for PADA complexation (1.1 x lo3 dm3 mol-1 s-l at 25 "C) is very similar to that measured for PAP. IN SDS MICELLAR SOLUTION The apparent pK, values (as determined spectrophotometrically) of PAPH+ and the protonated NiPAP complex were increased by ca. 1.5 units in the presence of SDS micelles because of an enhanced local hydrogen ion concentration in the micelle surface region.l0 The kinetics were .therefore monitored at pH 7, as measured using a glass electrode, so that the reaction was as shown in eqn (9).2422 EFFECT OF SURFACTANTS ON COMPLEX FORMATION -3 -2 -1 log,, ([SDSI -c.m.c.) Fig. 1. Plot of log,, [(A - A,)/A,] against log,, ([SDS], -c.m.c.) for the ligand PAP.The solubilities of PAP in water and SDS solutions were measured in order to determine KPAP. Using eqn (2), KpA = [PAP], / [PAP] B( [ s D s ] T - c . m . c . ) X ( S - So)/So([sDs]~ -C.m.C.) (1 1) where S and So are ligand solubilities for a given concentration of [SDS], (> c.m.c.) and for bulk water, respectively. An excess of solid' PAP in SDS solutions of various concentrations was shaken for ca. 3 days at pH ca. 7. The absorbances of the resulting filtered solutions, measured at 351 nm, are proportional to the solubilities. Fig. 1 shows a plot of log,, (A-Ao)/Ao against log,,([SDS],-c.m.c.) from which we obtain KpAp = 900 300 dm3 mol-l. PAP shows no detectable solubility in n-heptane, which, together with the pK, shift, suggests that the ligand is preferentially located in a region close to the micelle surface and is accessible for reaction with water-soluble reagents.KpAp is sufficiently large that the approximation KpAp C $= 1 may be made for C > 1W2 mol dmP3 (ie. PAP is totally associated with the micelle surfactant pseudo-phase). The Ni2+(aq) ion is known to be strongly attracted by a coulombic interaction to the negatively charged SDS micellar surface, with a valuelg of KNi of ca. lo3 dm3 mol-l. The NiPAP2+ complex, being positively charged, is also expected to be strongly bound to the micelle surface. Therefore, for C > 1 x loP2 mol dm-3, both reactants and products are located exclusively in the micelle surface region andP. D. I. FLETCHER AND B.H. ROBINSON 2423 */ 40 20 0 0 0.01 0.02 [Ni2']~/([sDs]~ -C.m.C.) Fig. 2. Plot of kobs against mi2+]T/([SDS]T-c.m.c.) for the Ni2+(aq)/PAP reaction at 25.0 "C. Table 1. Kinetic parameters for the Ni2+/PAP reaction in water, aqueous micellar and water-in-oil microemulsion media (rate-constant data refer to 25 "C) k,B*/dm3 mol-l s-l medium or kZ*/s-l kz or k;**/s-l AHi/kJ mol-1 AHL/kJ mol-l water (1 .O f 0.1) x lo3* 0.36 f 0.02* 51f3 74f3 SDS micellar (2.0 f 0.25) x lo3** 0.96k 0.09** 48k3 91 f 5 water/AOT/heptane (3.6 & 0.2) x lo3** 1.2 f 1** 50+22 93 f 25 solution microemulsions eqn (8) is applicable. A plot of kobs against [Ni2+IT/C is shown in fig. 2. Small corrections to the c.m.c. were made to allow for the changing ionic strength.19 The linear plot gives a value of k,, where k, = k f / V (12) of (2.00k0.25) x lo3 s-l.The intercept of the plot is too small for an accurate value of kt to be determined. Hence it was measured directly by mixing a micellar2424 EFFECT OF SURFACTANTS ON COMPLEX FORMATION 01 I 1 0 5 10 lo3 [NiZ'l,,/[AOT], Fig. 3. Plot of kobs against [Ni2+],,/[AOT], for the Ni2+(aq)/PAP reaction at 25.0 "C. solution of the NiPAP2+ complex with a large excess of Co2+(aq).18 Enthalpies of activation for both complex formation and dissociation were found to be 48 f 3 and 91 _+ 5 kJ mol-l, respectively. All the derived kinetic parameters are shown in table 1. Coz+(aq) is known from density measurements to show a negligible volume change on binding to SDS micelles.21 The similarity of binding constants (Kg) for Ni2+(aq) and Zn2+(aq) suggest that this is also true for these ions, and so it is concluded that the metal-ion interaction with the micelle is purely electro~tatic.~~ The value of AHf for reaction, which is unchanged from that in bulk water, and the value obtained for k , (which is interpreted later) provide evidence that the mechanism of the reaction at the micelle surface is the same as in bulk water.IN AOT/WATER/n-HEPTANE REVERSED MICELLES (MICROEMULSIONS) PAP is only slightly soluble in water [So = (2.5k0.5) x mol dm-3 at 20 "C], insoluble in n-heptane but quite soluble in reversed micellar solutions. Therefore, PAP partitions strongly to the water/AOT interface region. The Ni2+(aq) ions are assumed to be totally contained within the water droplets.Preliminary calculations suggest that the metal ions are closely associated with the interface.22 Thus, eqn (8) is again applicable. The parameter C now refers to the concentration of AOT at the interface. This is difficult to estimate but it is thought that the percentage of AOT at the interface is large (> 90%) for small values of R (< 30) at temperatures removed from the upperP. D. 1. FLETCHER AND B. H. ROBINSON 2425 and lower temperature limits for microemulsion stability. l7 Spectrophotometric measurements confirmed that the reaction was as shown in eqn (9), at the effective ‘pH’ of the water droplets. A plot of kobs against [Ni2+]/[AOT], is shown in fig. 3, from which k, ( E @ / Y ) and k: were found to be (3.6f0.2) x lo3 and 1.2+ 1 s-l, respectively, at 25.0 “C.The enthalpies of activation for the forward and reverse reactions were found to be 50 _+ 2 and 93 f 25 kJ mol-l, respectively. The assumption that partitioning and exchange processes prior to chemical reaction are rapid was tested by mixing the reactants in different ways, but producing the same concentrations after mixing. Identical kinetic results were obtained in all cases, providing excellent support for this key assumption. (In a separate series of experiments, the exchange rate constant for divalent metal ions between microemulsion droplets has been measured directly;5 the process takes place on the ps-ms timescale for the droplet concentrations used in these experiments.) The kinetic data in table 1 permit a direct comparison of the two different types of surfactant assembly on the reaction.(The parameter k, = kS/V.) The parameter V essentially describes the contribution to the reaction volume of one mole of surfactant at the interface, so that CV represents the effective reaction volume per dm3 of total solution. Any difference between and k,B would reflect the changed environment in which the reaction occurs. Since there is no change in AHf for the reaction in the three media, it would appear that kf x k,B. If they are taken to be equal, Y is readily calculated. Values of Y of 0.5 and 0.28 dm3 mol-1 are obtained for micellised SDS and interfacial AOT, respectively. The area presented by each surfactant head-group at the interface is ca. 0.6 nm2 for both types of s~rfactant.~~’ 24 The ‘thickness’ of the volume element in which reaction occurs would then be ca.1.4 nm for SDS and 0.8 nm for AOT. Visualising the reaction pseudophase in both surfactant media as a spherical shell close to the charged interface, these values are entirely reasonable, which confirms the validity of the approach. Hence, the main conclusions are (i) k: z k,B, (ii) the simple pseudophase model and eqn (8) can successfully describe the kinetics in aqueous micellar and water-in-oil microemulsion systems and (iii) both surfactant media slightly enhance the rate constant for dissociation of the complex. Since this reaction is unimolecular, the comparison may be made directly with no assumptions. REACTION OF Ni2+(aq) WITH PADA This reaction has been studied previously in water25 and in SDS micellar sol~tions.~ Precise values for the dissociation rate constant were obtained in water and SDS micellar solutions by reaction of the complex with Co2+(aq) (as used for the ligand PAP).Results are shown in table 2. The forward rate constants kf and ks are very similar for PADA and PAP, suggesting the same mechanism is operative for both ligands. The structural formula for PADA is shown in (I) below, while that for PAN is given in reaction (15) (uide infra). IN AoT/WATER/n-HEPTANE MICROEMULSIONS Unlike PAP, PADA is soluble in n-heptane. Therefore, in the microemulsion system the PADA ligand will distribute into the heptane phase. Values of the solubility of PADA at 25.0f0.5 “C in the various media of interest are shown in table 3.2426 EFFECT OF SURFACTANTS ON COMPLEX FORMATION Table 2.Kinetic parameters for the Ni2+/PADA reaction in water, aqueous micellar and water-in-oil microemulsion media (rate-constant data refer to 25 "C) k:/dm3 mol-l s-l medium or kz*/s-' kz or kF*/s-l AHr/kJ mol-l AHl/kJ mol-1 water (1.2..+0.1) x lo3* 0.06+0.004* 55+4 87f3 SDS micellar (3.0k0.3) x lo3** 0.28+0.04** 5014 69+5 water/AOT/heptane (8 2) x lo3** 0.6 & 0.2** solution microemulsions - - Table 3. Solubility of PADA in various media at 25 "C medium solubility/ mol dm-3 water (1.5f0.2) x lo-* heptane (1.9 & 0.2) x 0.1 mol dm-3 AOT in heptane (3.0k0.2) x 0.1 mol dm-3 AOT/l.O mol dm-3 H,O in heptane (3.3 kO.2) x lod3 20 " rn 1 n 0 Ado 10 0 0 5 10 15 lo3 [N"] /[AOT], Fig. 4. Plot of kobs against [Ni2+)/[AOT], for the Ni2+(aq)/PADA reaction, at 25.0 "C.[AOT]/moldm-3, R: 0, 0.1, 10; V, 0.075, 10; 0, 0.05, 5; 0, 0.05, 10; A, 0.05, 13.9; ., 0.05, 2.78; m, 0.038, 10; A, 0.025, 10.P. D. I. FLETCHER AND B. H. ROBINSON 2427 0 0 10 20 30 40 [ AOT] T-1/dm3 mol-' Fig. 5. Plot of (KPP)-l against [AOTj,' for the Ni2+(aq)/PADA reaction at 25.0 "C. A plot of kobs against [Ni2+]/[AOT], is shown in fig. 4. In contrast to the situation with PAP, a number of straight lines is observed, each corresponding to a particular microemulsion composition. The gradients increase with increasing AOT concentration at fixed R, but vary only slightly with R at fixed [AOT]. Since PADA partitions rapidly into the n-heptane phase, we can define KpADA as Then an apparent value of k, ( = kipp) may be derived using eqn (7) and (12) kkpp = ks/ 1 + (KPADA[AOTIT)-~- (14) Hence a plot of (kgPP)-l against [AOT],l should be linear with intercept kgl and slope (kSKPADA)-l.This plot is shown in fig. 5, and at 25 "C ks = (8 f 2) x lo3 s-l and KpADA = 4.8 If: 1.3 dm3 mol-l. An approximate value of KPADA may be derived from the solubility data (table 3) using eqn (1 1). A value for KpADA of 6 f 2 dm3 mol -l is obtained at R = 0, increasing slightly as R is increased. This good agreement with the value from the kinetic data shows that the pseudo-phase kinetic treatment adequately describes the system when one of the reactants is rapidly partitioning into a second phase. Table 2 gives a summary of the kinetic data for the Ni2+/PADA reaction in the three media of interest.If we again take kf = k,B then V is 0.40 dm3 mol-l in SDS micelles and 0.15 dm3 mol-1 in AOT microemulsions. These values are remarkably similar to those derived for the Ni2+/PAP reaction. Also, as for the Ni2+/PAP reaction, k,(kt) values are increased in the surfactant-containing media relative to bulk water solutions. REACTION OF Co2+(aq) WITH PADA IN AQUEOUS SOLUTION The average value for the water-exchange rate constant for Co(H20)t+ is 2.4 x lo6 s-l, with a corresponding AHI, of 46 kJ mo1-1.20 Values of kf = 7.6 x lo4 dm3 mol-1 s-l and k, = 36 s-l have been measured using the temperature-jump method.25 Values of AH: = 43 kJ mol-l and AHL = 63 kJ mol-12428 EFFECT OF SURFACTANTS ON COMPLEX FORMATION (4 4 3 - v) 1 O Z 0 2 4 6 8 lo3 ([Coz+l,, + [PADAl,,)/([SDSl -c.m.c.) Fig.6. Plot ofk,,, (z-l) against ([Cozf],, + [PADA],,)/([SDS] -c.m.c.) for the Co2+(aq)/PADA reaction at (a) 19.6, (b) 24.6, (c) 34.6 and ( d ) 44.6 "C. have also been These results are again consistent with the Eigen-Tamm mechanism (k, = K,,k,,), but with some contribution to the overall rate from the ring-closure step. IN SDS MICELLAR SOLUTION Since the Co2+/PADA reaction is similar to the Ni2+/PADA reaction, the kinetics can be described by eqn (8), provided that partitioning is fast. The temperature-jump method was used to study the kinetics as rates are too fast for the stopped-flow method. The surfactant solutions had a sufficiently low resistance at [SDS], 2 0.05 mol dm-3 for the method to be applied successfully. To test for the absence of electric-field effects, the Ni2+/PADA reaction was also studied by the temperature-jump method in order to compare it with the stopped-flow method.Identical results were obtained. A plot of kobs against ([Co2+Ies + [PADA],,/[SDS]-c.m.c.) is shown in fig. 6 . The subscript ' eq' refers to equilibrium concentrations at the final temperature. (To obtain an acceptable relaxation amplitude, it was not possible to work under pseudo-first-order conditions). The derived kinetic parameters (at 25 "C) are k, = (1.8 k0.2) x lo5 s-l, k: = 310f50 s-l, AH!' = 30+4 kJ mol-l and AH&' = 47+6 kJ mol-l. Taking k,B = k:, the value of the V parameter (= k,B/k,) of 0.42 dm3 mol-1 is in agreement with results for the Ni2+(aq)/PAP and Ni2+(aq)/PADA reactions. Eqn (8) is found to be applicable, showing that partitioning is rapid on the (ca.100 s-l) time- scale of the reaction. However, the activation enthalpy does appear to be low when compared with the value in bulk water. The dissociation rate constant is again increased in the micellar medium. REACTION OF Ni(dien)2+ WITH PADA IN AQUEOUS SOLUTION The ligand dien (diethylenetriamine) is tridentate and complexes strongly with aquo-metal ions. Bound aliphatic nitrogen ligands often exert a labilising effect onP. D. I. FLETCHER AND B. H. ROBINSON 2429 0 2 4 [Ni (dien)"] /([SDSl -c.m.c. at (a) 19.6, (b) 24.6, (c) 34.6 and ( d ) 44.6 "C. Fig. 7. Plot of kobs against [Ni(dien)]++/([SDS] - c.m.c.) for the Ni(dien)2+/PADA reaction the remaining water molecules in the inner solvation shell of the metal ion.This effect is of importance in the mode of action of metallo-enzymes.s Since many enzymes are membrane bound, it is of interest to determine the effect of micelles on complexation reactions of this type. The water-exchange rate for Ni(H,0),dien2+ has been found to be 1.2 x lo6 s-l at 25 "C, with a corresponding AH:, of 23 kJ mo1-1.20 For the reaction of Ni(dien)2+ with PADA, the kinetic parameters in waters are k,B = 4.7 x lo4 dm3 molI1 s-l, k,B(corr) = 9.4 x lo4 mol-1 s-l, k t = 36 s-l, AH! = 43 kJ mot1 and AH?, = 65 kJ mol-l. k,(corr) is the value of kf after a statistical correction has been applied for the number of exchanging water molecules in the complex. This arises since only three out of six possible coordinating positions are occupied by water molecules. The value obtained for k,B then implies either a low value for KO, or an additional complication in the mechanism, such as rate-limiting ring closure.However, the value of k,B for re- action with NH, (where no ring-closure step is present) is k,B = 7.8 x lo4 dm3 mol-1 s-l, which is similar to that for PADA. Therefore it appears that an additional effect is involved; this may be steric hindrance or an orientational effect. IN SDS MICELLAR SOLUTION Kinetic measurements were obtained using the Joule-heating temperature-jump met hod. Plots of kobs against [Ni(dien)2+]T/([SDS]-c.m.c.) at various temperatures are shown in fig. 7. Linear plots are obtained, consistent with both reactant species being strongly bound to the micelles. Derived kinetic parameters are k, = 2.2 0.1 x lo4 s-l, kt = 220 & 30 s-l, AHs = 42 5 kJ mol-1 and AGs = 56 _+ 8 kJ mol-l.2430 EFFECT OF SURFACTANTS ON COMPLEX FORMATION The derived value of Y (taking kB = kfB) is 2.1 dm3 mot1, which is significantly higher than values obtained for the aquo-metal-ion reactions ( V x 0.5 dm3 mol-l).Since there is no reason for V to change, the assumption that kp = k,B must be invalid and so it is likely that the kinetics of this reaction are specifically modified in the micelle surface region. A likely explanation is that Ni(dien)2+ and PADA are mutually orientated in the micellar surface in a manner which is unfavourable for complexation; in particular the dien ligand may tend to penetrate into the hydrocarbon interior of the micelle.This steric effect, which would be absent for the symmetrical aquo-metal ion, would be reflected by a decrease in A S l . This interpretation is supported by the unchanged value of AH! for the reaction. As is generally observed, ki is significantly increased compared with the water value. REACTION OF Ni(phen)2+ WITH PADA IN AQUEOUS SOLUTION As for Ni(dien)2+, the strongly bound 1,lO-phenanthroline (phen) might be expected to modify the kinetics of the complexation reaction. For Ni(phen)2+ the sigma-electron donation of the nitrogen atoms is largely compensated by back-bonding to the aromatic system of the ligand so the labilising effect of the bound ligand on the water molecules is As a result, complex-formation rate constants for NH, with Ni(phen)2+ and Ni2+(aq) are similar (1.5 x lo3 and 2.8 x lo3 dm3 mol-l s-l, respectively).26 Rates of ternary complex formation and dissociation were measured for Ni(phen)2+ with PADA using the stopped-flow method.The derived kinetic parameters are k,B = 8.710.8 x lo4 dm3 mol-l s-l, k t = 3.7f0.3 s-l, AH? = 46+ 5 kJ mol-l and AHL = 78 f 8 kJ mol-l. Rate constants measured at 25 "C are in good agreement with those obtained by Cayley and M a r g e r ~ m ~ ~ (kf = 9.6 x lo4 dm3 mol-1 s-l and kb = 3.6 s-'). The much larger value for k, over that for the Ni(~hen)~+/NH, reaction is attributed to a hydrophobic stacking intera~tion~~ between the incoming PADA ligand and the bound phen ligand. This hypothesis is supported by the fact that the rate enhancement is drastically reduced in water + methanol mixtures.IN SDS MICELLAR SOLUTION If it is assumed that both reactants are strongly bound to the micelles in an essentially hydrophobic environment, then it is possible that the favourable stacking interaction would be absent and kf would be reduced. In addition, the orientational factor postulated for the Ni(dien)2+/PADA reaction might be expected to further reduce the micellar rate. The kinetics were determined using both the stopped-flow and temperature-jump methods. A plot of kobs against (mi(phen)2+],, + [PADA],)/([SDS] -c.m.c.) at 26.3 "C is shown in fig. 8. Again a linear plot is obtained, consistent with eqn (8). The derived kinetic parameters at 25 "C are k, = 2.3( kO.2) x lo4 s-l, @ = 3 4 kO.7) s-l and A m s = 54( _+ 7) kJ mol-l. The parameter V (when k,B = kf) is calculated to be 3.8 dm3 mol-l. As expected, this value is unreasonably large and is a factor of 10 larger than is found for reaction of Ni2+(aq).A more likely explanation is that V has a normal value and kf x 0.1 k,B. The micellar rate reduction is fairly small when it is expected that both stacking and orientation effects are acting to decrease the rate in the micellar solution. It is expected that there will be a significant effect due to the orientational effect, as observed for the Ni(dien)2+/PADA reaction. Therefore, the data suggest that the stacking interaction may persist to some extent in the micellar solution.P. D. I. FLETCHER AND B. H. ROBINSON 243 1 100 .-( 'rn % \ 5a 0 0 1 2 3 10 [Ni hen)^'],, + [PADAleq/([SDSl,-c.m.c.) Fig.8. Plot of kobs against [Ni(phen)]zG + [PADA],,/([SDS] -c.m.c.) for the Ni(phen)2+/PADA reaction at 26.3 "C. REACTION OF Ni2+(aq) AND Znz+(aq) WITH PAN IN AQUEOUS SOLUTION The ligand PAN (see fig. 1) is markedly different from the other ligands used in this work. An intramolecular hydrogen bond can form between the napthol OH group and an azo nitrogen atom as in eqn (15) (vide infra). This has been confirmed by infrared and n.m.r. spectroscopy.6 The existence of this intramolecular hydrogen bond may explain the much lower solubility of PAN in water as compared with PADA or PAP. Also, in contrast to PADA and PAP, PAN can bind to the metal ion through two nitrogen atoms and the oxygen of the napthol group to form a tridentate complex. This has been confirmed for the Ni2+(aq) complex by infrared spectroscopy.6 As a consequence, the stability constants of metal-ion complexes with PAN are very much larger than those for bidentate complexes like PADA and PAP.28 For this reason, PAN finds application in analytical chemistry as an extractant to effect analytical separations of various metal ions.29 Even though PAN is only slightly soluble in water, the complexation kinetics with Zn2+(aq) and Ni2+(aq) have been determined at 25 "C.Values of k, for Zn2+(aq) and Ni2+(aq) were found to be (5.4 f 0.2) x lo4 dm3 mol-1 s-' and 95 If: 3 dm3 mol-l s-l,2432 EFFECT OF SURFACTANTS ON COMPLEX FORMATION Fig. 9. Plot 2 0 0 0.25 0.5 0.75 [Ni"] /([ SDS] -c.m.c.) of kobs against wi(aq)l2+/([SDS] - c.m.c.) for the Ni(aq)2+/PAN at 25.0 "C.reaction respe~tively.~~ If KO, = kf/kex, then derived values of KO, are of the order of dm3 mol-l. Since such values are far too small for KO, it was concluded that the final ring-closure step is rate determining. This is not unexpected since the formation of the tridentate complex requires rupture of the internal hydrogen bond and rotation of the naphthol ring as follows +H* A similar conclusion regarding the mechanism of the Ni2+/PAN reaction was reached following a comprehensive kinetic study by Reeves et IN SDS MICELLAR SOLUTION A pH of 7.5 was chosen for the kinetic experiments. At this pH (equivalent to a pH at the micellar surface of 1-2 pH units lower) the concentrations of charged PAN and hydroxy-metal-ion species were negligible. Since PAN is virtually water-insoluble but quite soluble in SDS micellar solutions,P.D. I. FLETCHER AND B. H. ROBINSON 2433 it is clear that PAN associates strongly with micelles. The micellar kinetics should therefore be described by eqn (8) since, although ring-closure is rate limiting for the Ni2+/PAN reaction in water, the complex-formation kinetics still obey a simple bimolecular rate law. A plot of kobs (at 25 "C) against [Ni2+]/([SDS]-c.m.c.) is shown in fig. 9. Results at high SDS concentrations were obtained using a Unicam SP8000 spectrophotometer. Otherwise the stopped-flow method was used. The plot is linear at low molar ratios of Ni2+: micellised SDS but tends towards a limiting value for a molar ratio of ca. 0.3. The limiting slope at low molar ratios is identified with k, and the plateau region corresponds to saturation of the micelle surface with Ni2+(aq) ions.It might appear that the surface would be saturated at a molar ratio of 0.5 (since Ni2+ is divalent) but the presence of Na+ counter-ions may well account for the value observed. The value of k, is found to be 9.5 f 1 s-l, which is considerably smaller than was obtained with other ligands. Consequently the derived parameter V is found to be 10 f 1 dm3 mol-l, which is anomalously large. None of the factors suggested so far to explain kp < k,W apply in this situation and a further partitioning process of PAN into the micellar core is postulated to account for the slow micellar rate. When the PAN is in the micellar core it is considered that it would be inaccessible for reaction with Ni2+(aq) ions.A parameter KkAN is defined where KbAN = [PANmS]/[PANmC] and the superscripts ms and mc refer to the micellar surface and core, respectively. If this partitioning process is rapid on the timescale of the reaction, then eqn (8) must be modified as follows: @[ Ni 2+] kobs = V([SDS] - c.m.c.) [ 1 + (K;AN)-l] ' (The reverse rate constant may be neglected as the overall stability constant is large.) When PAN is predominantly located in the micellar core (i.e. K;PAN 6 l), eqn (17) becomes K;PAN ka[Ni2+] V([SDS] - c.m.c.) ' kobs = If a value of 0.5 dm3 mol-l is taken for V and it is assumed that the water loss rate is unchanged at the micelle surface as compared with bulk water, and also that there is no change in mechanism, then KbAN is calculated to be 0.05.The following experiment was performed to test the postulate that PAN partitions to the micellar core. Addition of organic, apolar material, e.g. dodecane, to the SDS solutions should ' swell' the micelles and increase partitioning of PAN to the micellar core pseudo-phase. This would result in a decrease of kobs. An attempt was made to achieve this with dodecane as additive, but it was not sufficiently soluble in the micellar solutions. However, toluene was soluble. Table 3 and fig. 10 show the results obtained. A linear reduction in kobs with increasing amounts of toluene is noted. The same experiment performed with PADA instead of PAN showed no decrease in kobs, confirming that PADA is located exclusively at the micelle/water interface when associated with SDS micelles. In contrast, PAN is only available for reaction for 5% of the time when it is bound to SDS micelles.IN AOT/WATER/HEPTANE MICROEMULSIONS were investigated using the stopped-flow method. The kinetics of reaction of both Ni2+(aq) and Zn2+(aq) with PAN in microemulsions PAN was found to have a solubility in heptane of (3.3 f 0.3) x lov3 mol dm-3. It is2434 EFFECT OF SURFACTANTS ON COMPLEX FORMATION 6o t I I I 1 0 0.05 0.1 0.15 % v/v toluene Fig. 10. Variation of the Ni2+/PAN reaction rate in SDS solutions with added toluene at 25.0 "C ([SDS] = 0.02 mol dm-3). D, Data for PADA (see text). expected, therefore, to partition strongly to the bulk heptane pseudo-phase. We can define where the subscripts S and H indicate pseudo-phases associated with the surfactant and bulk heptane, respectively.Assuming the metal ions are located exclusively in the surfactant/water interface region over the R range studied and reaction occurs at this location, then the kinetics are described by a modified form of eqn (8): KPAN = [PANIs/[PANIH [AOTl, (19) In the case of very weak partitioning of PAN to the interface region (i.e. KpAN[AOT]T 4 l), eqn (20) reduces to and hence kobs varies linearly with metal-ion concentration and is independent of AOT concentration. Plots of kobs against [M2+] for Zn2+(aq) and Ni2+(aq) are shown in fig. 11 and 12, respectively. The gradients are (5.9 & 0.3) x lo3 and 30 f 1.5 dm3 mol-1 s-l. If kf is taken to be equal to k r and a reasonable value of V is assumed (we have take V to be 0.22 dm3mol-l, which is the mean of VpAp = 0.28 dm3mol and = 0.15 dm3 mol-l), then KPAN may be calculated for both systems.Values of 2.4 x dm3 mol-l are found for the Zn2+ and Ni2+ reactions. This general agreement is reasonable and indicates that PAN partitions very strongly away from the surfactant/water interface where reaction takes place. Solubility measurements were performed to check independently the partitioning hypothesis. The solubility of PAN in heptane in the presence of AOT and water is and 6.9 xP. D. I. FLETCHER AND B. H. ROBINSON 2435 6 - 4 'Y, Y" .I P 2 0 0 5 10 [Zn2+(aq)] / l o 4 mol dm-j Fig. 11. Plot of kobs against [Zn(aq)] for the Zn2+/PAN reaction at 25 "C. 0 5 10 [ Ni(aq)] '+/ 1 O4 mol dm" Fig. 12. Plot of kobs against [Ni(aq)12+ for the Ni2+/PAN reaction at 25 "C.2436 EFFECT OF SURFACTANTS ON COMPLEX FORMATION 0 0.1 0.2 0.3 [AOT]/mol dm'3 Fig.13. PAN solubility in various microemulsions. shown in fig. 13. A slight increase in solubility is observed. KPAN is calculated from these solubility data and is of the order of 2 dm3 mol-l. There is a large discrepancy between the two determinations of KPAN which may be rationalised by the following argument. Although PAN may partition to the interfacial region, it may not be accessible for reaction with metal ions. PAN may partition to the surfactant/heptane interface region (which would be included in the solubility estimation of KPAN) but it would have to move further (to the surfactant/water interface region) before it could react with M2+ ions.The value of KPAN measured kinetically is thus the product of two partition coefficients, and it would appear that when PAN is associated with AOT in the microemulsion system it is only available for reaction with M2+ for ca. 1-3% of the time. Hence both micellar and microemulsion surfactant assemblies significantly reduce the rate of complexation by an essentially similar mechanism. This type of process may be of importance in metal-ion extraction processes in which surfactants are employed. CONCLUSIONS The kinetics of a series of metal-ion/ligand exchange reactions have been studied in both aqueous micellar and water-in-oil microemulsion media. The kinetics of complex formation involving aquo-metal ions and simple ligands are well described in both types of media using equations derived from a simple pseudo-phase model.The large observed rate enhancements are due to localised concentration enhancements of the reactants in the volume element of solution in which the reaction occurs. If the assumption is made that rate effects are due entirely to localised concentration enhancements, then the volume of the reaction pseudo-phase in the solution can be deduced. Reasonable values are obtained in all cases where this is possible. In the limiting case of very favourable partitioning to the pseudo-phase where the reaction occurs (as in the hTi2+/PAP reaction), the activation enthalpy is unaltered from its value in bulk water and k,B = kp. Distribution of reactants between more than two pseudo-phases (as in reactions of PAN) requires that the kinetic equations be modified and, in general, smaller rate enhancements or even rate retardations are observed. Secondary rate effects are observed for reactions involving the formation of ternaryP.D. I. FLETCHER AND B. H. ROBINNSON 2437 complexes. Rates are reduced because of unfavourable mutual orientation of reactants at the surfactant/water interface. The Ni(phen)2+/PADA reaction has an unusually high rate constant in water due to a favourable stacking interaction. This interaction is still present in the micelle reaction suggesting that the reactants experience an ‘aqueous’ environment at the micelle surface. The dissociation rate constants may be compared directly in different media since they are unimolecular processes.Rate enhancements of up to a factor of 10 are observed for the interface reaction as compared with bulk water. The results may be explained in terms of stabilisation of the transition state by electric-field effects at the interface. We thank the S.E.R.C. and Shell (Thornton Research Centre) for support of this work by a CASE award (to P.D.I.F.). We also thank Dr George Brunton (T.R.C.) for useful discussions. M. Grltzel, Ber. Bunsenges. Phys. Chem., 1980,84,981. K. Martinek, A. N. Semanov and 1. Berezin, Biochim. Biophys. Acta, 1981,658.76. A. D. James and B. H. Robinson, J. Chem. Soc., Faraday Trans. I , 1978,74, 10. V. C. Reinsborough and B. H. Robinson, J. Chem. Soc., Faraday Trans. I , 1979,75, 2395. P. D. I. Fletcher and B. H. Robinson, Ber. Bunsenges. Phys. Chem., 1981,85, 863. D. Betteridge and D. John, Analyst, 1973,98, 377. M. A. Cobb and D. N. Hague, J. Chem. Soc., Faraday Trans. I , 1972,68,932. B. H. Robinson, N. C. White and C. Mateo, Adc. Mol. Relaxation Processes, 1975,7, 321. ed. W. J. Gettins and E. Wyn-Jones (D. Reidel, Dordrecht, 1979), p. 282. ’I R. K. Steinhaus and D. W. Margenun, J. Am. Chem. SOC., 1966,88,441. lo P. D. I. Fletcher and B. H. Robinson, in Techniques and Applications of Fast Reactions in Solution, l1 I. V. Berezin, K. Martinek and A. K. Yatsimirskii, Russ. Chem. Rm., 1973, 42, 787. l2 L. S. Romsted, in Micellization, Soluhilizution and Microemulsions, ed. K. L. Mittal (Plenum Press, l3 J. C. Dederen, J. Phw. Chem., 1981, 85, 1 198. l4 J. K. Thomas, F. Grieser and M. Wong, Ber. Bunsenges. Ph-vs. Chem.. 1978,82,937. l5 S . S. Atik and J. K. Thomas, J. Am. Chem. Soc., 1981,103, 3543. New York, 1977). P. D. I. Fletcher and B. H. Robinson, in Biological and Technological Relevance of Reverse Micelles and other Amphiphilic Structures in Apolar Media, ed. P. L. Luisi (Plenum Press, New York, 1983). l7 P. D. I. Fletcher, A. M. Howe, N. M. Perrins, B. H. Robinson, C. Toprakcioglu and J. C. Dore, in Proceedings-of the Third International Symposium on Surfuctants in Solution, ed. K. L. Mittal (Plenum Press, New York, 1983). B. H. Robinson and N. C . White, J. Chem. SOC., Faraday Trans. I , 1978,74, 2625. 258,733. W. Baumiiller, H. Hoffmann and W. Ulbricht, J. Colloid Interface Sci., 1978, 64, 418. lo M. Fischer, W. Knoche, P. D. I. Fletcher, B. H. Robinsonand N. C. White, ColloidPolym. Sci., 1980, 2o J. P . Hunt, Coord. Chem. Rev., 1971,7, 1. 22 N. J. Bridge, personal communication. OY D. Stigter, J. Colloid Interface Sci., 1974, 47. 473. 24 H. F. Eicke and J. Rehak, Helv. Chim. Acta, 1976, 59, 2883. 25 M. A. Cobb and D. N. Hague, Trans. Faraday Soc., 1971,67, 3069. D. W. Margerum and H. Rosen, J. Am. Chem. Soc., 1967,89, 1088. 27 G. R. Cayley and D. W. Margerum, J. Chem. SOC., Chem. Commun., 1974, 1002. 28 A. Corsini, I. M. Yih, Q. Fernando and H. Freiser, Anal. Chem., 1962,34, 1090. 2o D. Betteridge, Q. Fernando and H. Freiser, Anal. Chem., 1963,35,296. 30 C. D. Hubbard and D. Pacheco, J. Inorg. Nucl. Chem., 1977,39, 1373. 31 R. L. Reeves, G. S. Calabrese and S . A. Harkaway, Inorg. Chem., 1983, 22, 3076. (PAPER 3/1534)

ISSN:0300-9599    

DOI:10.1039/F19848002417

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J . Chem. Soc., Faraday Trans. I , 1984,80, 2439-2444 Effect of Alkyl Substituents on the Thermodynamics of the Self-association of Purine in Aqueous Solution BY HARRI LONNBERG,* JYRKI YLIKOSKI AND ANTTI VESALA Department of Chemistry and Biochemistry, University of Turku, SF-20500 Turku, Finland Received 12th September, 1983 The thermodynamic parameters for the self-association of several alkyl-substituted purines have been determined by applying the isodesmic model to the molar enthalpies of infinite dilution at different initial concentrations. Carbon-bonded alkyl groups have been shown to increase the tendency to association by making the association enthalpy more negative. In contrast, the increasing size of the N9-bonded alkyl groups appears to reduce the negative entropy contribution and thus stabilize the associated complexes.These findings are discussed in terms of dipole-induced-dipole interactions and hydrophobic bonding. Application of several experimental techniques, including o~mometry,~-~ calori- me try,8-12 ultrasonics, l3 9 l4 n.m. r . s p e c t r ~ s c o p y ~ ~ - ~ ~ and equilibrium sedimen t a t i ~ n , ~ ~ - ~ ~ has indicated that nucleic acid bases and nucleosides self-associate to a large extent in aqueous solutions. Usually the tendency to association is stronger with purines than with pyrimidines.l? 16$ 2 2 v 24 According to the widely adopted isodesmic model of Ts'O and c o ~ o r k e r s , ~ - ~ vertical stacking of several molecules occurs in a non-cooperative manner, although some evidence of both positive and negative cooperativity has also been reported.2v 26* 27 Undoubtedly the association goes far beyond the dimer stage.2q 4 9 26 Solvent water plays a decisive role in the stacking phenomenon.For example, addition of organic solvents to aqueous solutions results in marked destacking.ll9 1 5 7 2o Under non-aqueous conditions base stacking is not observed and base pairing by hydrogen bonding becomes the main interaction m e ~ h a n i s m . ~ ~ - ~ ~ The enthalpy, entropy and volume changes observed for the self-association in aqueous solutions are all negati~e.~-l~~ 1 7 - 1 9 9 2 3 7 2 5 9 3 2 9 33 This contrasts with the classical concept of hydrophobic interaction^,^^-^^ which are characterized by small or even positive enthalpy changes and positive entropy and volume changes.However, alkyl substi- tuents have been shown to enhance the association of purines, pyrimidines and their derivatives. 2-47 6 v 7 v l9 Accordingly, dipole-induced-dipole interactions have been suggested as being responsible for the while the hydrophobic contribution remains 39 The present paper reports our studies with alkyl-substituted purines. The equilibrium constants and the enthalpy and entropy changes for self-association have been calculated from the concentration dependences of the calorimetrically determined dilution enthalpies. The effects of the size and site of the alkyl groups on the thermodynamic quantities have been examined to elucidate the possible contribution of hydrophobic interactions in the base stacking. 24392440 THERMODYNAMICS OF THE SELF-ASSOCIATION OF PURINE EXPERIMENTAL MATERIALS Purine was a commercial product of Sigma Chemical Co.9-Methyl-, 6,9-dimethyl- and 8,9-dimethyl-purine were prepared and characterized as described 41 9-Ethylpurine was synthesized and separated from the corresponding N7 isomer using the method reported for 9-methylpurine40 with diethylsulphate as alkylating agent. The product was purified by sublimation at 130 "C at a pressure of mmHg. lH n.m.r. (ppm from TMS in CDCl,): 6 1.81 (CH3CH2-, t), 6 4.40 (CH3CH2-, q), 6 8.03 (H8, s), 6 8.92 (H2, s), 6 9.08 (H6, s). 13C n.m.r.: 6 152.4 (C2). Calculated for C,HsN4: C 56.74%, H 5.44% ; found: C 56.67%, H 5.56%. 9-Isopropylpurine was prepared using the alkylation method described for adenine.42 Purine (30 mmol) was converted into sodium purinide using sodium hydride (31.5 mmol) in DNF (300 cm3).Isopropyliodide (30 mmol) was added and the reaction mixture was agitated for 16 h at 30 "C in the dark. The N9 isomer was isolated chromatographically as described previo~sly.~~ The product was sublimed at 100 "C at a pressure of mmHg. lH n.m.r: 6 1.68 [(CH3),CH-, d], 6 4.97 [(CH,),CH-, m], 6 8.20 (HS, s), 6 8.73 (H2, s), 6 9.12 (H6, s). 13C n.m.r.: 6 22.5 [(CH,),CH-], 6 47.4 [(CH,),CH-1, 6 134.5 (C5), 6 143.1 (C6), 6 148.5 (C8), 6 151.1 (C4), 6 152.2 (C2). Calculated for CsHloN4: C 59.24%, H 6.21 % ; found: C 59.23%, H 6.22%. 6,8,9-Trimethylpurine was prepared as follows. 4,5-Diamino-6-methyl-2-thiopyrimidine (7 mmol), purchased from Sigma, was reduced to 4,5-diamino-6-methylpyrimidine with Raney nickel in boiling water (50cm3).The product was cyclized to 6,8-dimethylpurine by heating with acetic anhydride43 and methylated to 6,8,9-trimethylpurine as described for 9-methylp~rine.~~ The syrup obtained was crystallized from boiling n-hexane. lH n.m.r. : 6 2.67 (C8-CH3, s), 6 2.83 (C6-CH,, s), 6 3.80 (N9-CH,, s), 6 8.73 (H2, s). 13C n.m.r.: 6 14.1 6 153.4 (C8), 6 157.0 (C6). Calculated for C,H,,N,: C 59.23%, H 6.22% ; found: C 57.90%, H 6.22%. All the compounds prepared were assigned as N9 isomers on the basis of the 13C n.m.r. chemical shfts, using the method of Chenon et ~ 1 . ~ ~ The homogeneity of the products was checked by liquid chromatography with a TSK OSD5 column. Eluation (0.8 cm3 min-l) was performed with an acetic acid buffer (0.02 mol dm-3, buffer ratio 1 : 1) containing 20 vol.% acetonitrile. Distilled and degassed water was used for the calorimetric measurements. 6 15.3 (CH3CH2-, 6 38.9 (CH,CH-), 6 134.2 (C5), 6 145.0 (C6), 6 148.4 (C8), 6 151.3 (C4), (C8-CH3), 6 19.3 (C6-CH3), 6 28.7 (N9-CH3), 6 132.3 (C5), 6 151.4 (C2), 6 152.1 (C4), TREATMENT OF THE CALORIMETRIC DATA The dilution enthalpies for the alkyl-substituted purines were measured on a LKB 10700-2 batch microcalorimeter by mixing equal volumes (1.60 cm3) of aqueous purine solutions with water at 298.15 K. The reference cells were filled with equal volumes of water. The molar enthalpies of infinite dilution. AH,' d i l , were obtained by summing the molar enthalpy changes for successive dilutions from the concentration Ci to the concentration AHdil from the lowest experimentally accessible concentration to infinite dilution was estimated by linear extrapolation.The above method of obtaining the values of AH,,,il is based on the assumption that the solutes behave ideally, i.e. no volume changes take place on dilution. Previous data on the densities of aqueous solutions of 6-methylpurines lend support to this assumption. If it is further assumed that the association of monomeric purines proceeds to an infinite degree and that the association constants, K, and the enthalpy changes, AH*, for the successive steps are equal, the dependence of AHi,dil on Ci can then be expressed as AHe[ 1 + 2Ci K - (1 + 4Ci K)'] 2Ci K AHi,dil = Consequently, a two-parameter fitting procedure can be used to obtain K and AHe.Several earlier investigation^^-^. 45 show that the above assumptions are reasonable.H. LONNBERG, J. YLIKOSKI AND A. VESALA 2441 Table 1. Molar enthalpies of infinite dilution, AH,, dil, for some alkyl-substituted purines in aqueous solutions at 298.15 Ka AHi, dil/kJ mol-l 9- 9- 9- 8,9- 6,9- 6,8,9- c y 1 0 - 3 methyl- ethyl- isopropyl- dimethyl- dimethyl- trimethyl- mol dm-3 purine purine purine purine purine purine purine 1000 500 250 125 62.5 31.3 15.6 7.81 3.91 - 7.643 (- 7.680) -6.181 (- 6.159) -4.628 (-4.576) -3.150 ( - 3.1 12) - 1.888 (- 1.934) - 1.050 (- 1.1 10) - 0.530 (- 0.603) - - - - - 6.224 (- 6.243) - 5.090 (- 5.086) - 3.882 (- 3.853) -2.713 (- 2.678) - 1.679 (- 1.699) - 0.957 ( - 0.992) - 0.480 (- 0.545) - - - - - 5.538 (- 5.601) - 4.740 (-4.688) - 3.762 (- 3.675) -2.606 (- 2.657) - 1.739 (- 1.754) - 1.044 (- 1.061) -0.560 (- 0.597) - - - - - - - 3.956 ( - 4.006) - 3.247 ( - 3.1 82) - 2.372 (- 2.336) - 1.538 (- 1.568) - 0.944 (- 0.962) -0.500 (-0.548) - - - - - - - - - - -7.691 - 11.693 - (- 7.756) (- 11.769) - -6.336 - 10.145 - 12.792 (-6.292) (- 10.093) (- 12.820) -4.837 -8.221 - 10.996 (-4.741) (-8.165) (-10.968) -3.286 -6.176 -8.882 (-3.275) (-6.132) (-8.845) - 1.998 -4.206 -6.588 ( - 2.065) ( - 4.2 1 9) ( - 6.6 1 7) - 1.100 -2.589 -4.495 (- 1.201) (-2.652) (-4.534) - - 1.460 - 2.884 - (-1.537) (-2.837) - - - 1.620 - - (- 1.639) a Means of duplicate measurements.The values in parentheses are those calculated via eqn (1) using the values of K and A H e listed in table 2.RESULTS AND DISCUSSION Table 1 summarizes the molar enthalpies of infinite dilution for the alkyl purines investigated. Comparison with the values obtained by least-squares fitting indicates that eqn (1) satisfactorily describes the dependence of AHi. dil on Ci. The association constants, K, and the enthalpy changes, A@, which application of this isodesmic model yields are collected in table 2. Gill et aL9 have reported for purine the values of 2.9 & 0.2 kg mol-1 and - 15.5 f 0.8 kJ mol-1 for Kand AH*, respectively from calorimetry. The association constant of 2.80 k 0.06 dm3 mol-l obtained by equilibrium-sedimentation methodsz3 is of the same magnitude, while the concentration dependence of the molal osmotic coefficient has led to a lower value of 2.1 kg mol-l.l The osmometrically determined association constants for 9-methyl-, 9-ethyl- and 9-isopropyl-purine are also markedly lower than those obtained in the present Note that differences in the association constants measured by different experimental techniques are quite common.For example, the value for caffeine obtained calor- imetrically is 15 k 1 kg m ~ l - ' , ~ whereas n.m.r. spectr~scopy~~ and give 9.0 and 8.2 dm3 mol-l, respectively, at 298.15 K. With 6-dirnethylamino-9-@-~- ribofuranosyl) purine values of 22.2 kg mol-l, 34 dm3 mol-1 and 33.8 dm3 mol-1 have been observed at 298.15 K using osmometry,3 and equilibrium ~edimentation,~~ respectively. These differences are far greater than the experimental errors and possibly reflect the different sensitivities of the techniques to the basic2442 THERMODYNAMICS OF THE SELF-ASSOCIATION OF PURINE Table 2.Equilibrium constants and the enthalpy and entropy changes for the self-association of some alkyl-substituted purines in aqueous solutions at 298.15 Ka ~~ compound K/dm3 mol-l AHe/kJ mol-l A F / J K-l mol-l purine 3.1 fO.l - 13.42 f 0.20 - 35.6 f 1 .O 9-methylpurine 3.7 f 0.1 -10.43L0.12 - 24.1 f 0.7 9-e thylpurine 5.1 f0.3 -8.71 f0.16 - 15.7f 1.3 9-isoprop ylpurine 5.7 f 0.4 - 7.18 k 0.20 -9.6+ 1.7 8,9-dime thylpurine 7.1 f0.4 - 13.10&0.28 - 27.6 f 1.8 6,9-dimethylpurine 13.8L0.4 - 17.18k0.15 -35.8f 1.2 6,8,9-trimethylpurine 26.7 f 0.4 - 18.83 k 0.09 - 35.8 f 0.7 a Calculated from eqn (1) by least-squares fitting. assumptions of the isodesmic model.It appears reasonable to assume, however, that reliable conclusions concerning the influence of the solute structure on the thermodynamics of the association reaction can be drawn if data referring to a single experimental method are considered. The data in table 2 reveal that the associations of all the purines investigated are enthalpy driven and entropy opposed, as shown previously for a great variety of purine and pyrimidine derivati~es.~-l~$ 17-197 23 With the exception of unsubstituted purine, none of the compounds contains acidic hydrogen atoms. Accordingly, hydrogen bonding can be excluded as a possible explanation for the markedly different association abilities. It has been suggested previously38 that the dipole-induced-dipole interactions play a dominant role in the stacking of monomeric nucleic acid constituents.In other words, both the polarizing power of the polar bonds and the polarizability of the n-electron system would contribute to the stability of the associated complexes, the latter factor being more important with purine derivative^.^* 38 Part of the present results can be accounted for by this mode of interaction. The tendency to association of alkyl-substituted purines is markedly increased on going from 9-methylpurine to 8,9-dimethyl-, 6,9-dimethyl- and 6,8,9-trimethyl purine. The main reason for the stabilization of the stacks appears to be the increasingly negative enthalpy change. Part of this influence is cancelled out by the parallel changes in the entropy term, T A P . Since carbon-bonded alkyl groups are known to increase the polarizability of the purine bases, it seems possible that methyl substituents at C6 and C8 may enforce the dipole-induced-dipole interactions and thus make the association enthalpy more negative.For example, the polarizabili ties of purine and 6-methylpurine have been reported as 12.5 and 14.3 A3, re~pectively.~~ It remains obscure as to why the methyl group at C6 exerts a greater effect than the methyl group at C8. The suggested structures of the purine stacks3 offer a tentative explanation. Examination of the concentration dependences of the lH n.m.r. chemical shifts has led to the conclusion that the six-membered rings of purine nucleosides overlap in the associated complexes to a larger extent than the five-membered rings.3 Substituents in the overlapping part of the aromatic ring system may have a stronger influence on the interactions which cause the stacking phenomenon.The thermodynamic data referring to the association of 9-methyl-, 9-ethyl- and 9-isopropyl-purine are difficult to understand on the basis of the dipole-induced-dipole interactions. The ability to self-associate is again increased with increasing numberH. LONNBERG, J. YLIKOSKI AND A. VESALA 2443 of aliphatic carbon atoms, but with this series of compounds changes in the entropy term appear to be responsible for the stabilization of the associated species. Both the enthalpy and the entropy terms become less negative with increasing size of the N9 substituent, but changes in the latter are more marked.This finding is consistent with the classical concept of hydrophobic b ~ n d i n g . ~ ~ - ~ ~ Hydrophobic interactions between the N9-bonded alkyl groups may be expected to result in positive enthalpy and entropy contributions to the overall association enthalpy and entropy. On the basis of the present data these contributions appear to be 1-2 kJ mol-1 and 5-10 J K-l mol-1 for an additional methylene group. For comparison, AH* for the interaction between two methylene groups in long hydrocarbon chains has been suggested to be ca. 5 kJ mol-1 at 298.15 K and A S e is suggested to be ca. 20 J K-l m01-l.~~ Comparable evidence for the involvement of hydrophobic interactions in base stacking has been obtained from studies with alkyl-substituted uracils.’ Replacement of the N9 hydrogen atom of purine with a methyl group also exerts positive changes in both AH* and A P , the effect on AG* remaining relatively small.Analysing these influences is, however, difficult, since alkylation of the N9 site results in changes in the tautomeric composition, polarizing power, hydrogen-bonding ability and hydrophobic nature of the solute. In summary, the above discussion suggests that, besides dipole-induced-dipole interactions, hydrophobic bonding may contribute to the base stacking of purines. We thank the Academy of Finland, Council for the Natural Sciences, for financial support. P. 0. P. Ts’O, I. S. Melvin and A. C. Olson, J. Am. Chem. SOC., 1963,85, 1289. P. 0. P. Ts’O and S. I. Chan, J. Am. Chem. SOC., 1964,86,4176. A. D. Broom, M.P. Schweizer and P. 0. P. Ts’O, J. Am. Chem. SOC., 1967, 89, 3612. G. K. Helmkamp and N. S. Kondo, Biochim. Biophys. Acta, 1968, 157, 242. D. Porschke and F. Eggers, Eur. J. Biochem., 1972, 26, 490. E. Plesiewicz, E. Stepien, K. Bolewska and K. L. Wierzchowski, Nucleic Acids Res., 1976, 3, 1295. E. Plesiewicz, E. Stepien, K. Bolewska and K. L. Wierzchowski, Biophys. Chem., 1976, 4, 131. * P. R. Stoesser and S. J. Gill, J. Phys. Chem., 1967, 71, 564. S. J. Gill, M. Downing and G. F. Sheats, Biochemistry, 1967, 6, 272. lo E. L. Farquhar, M. Downing and S. J. Gill, Biochemistry, 1968, 7, 1224. l1 M. G. Marenchic and J. M. Sturtevant, J. Phys. Chem., 1973,77, 544. l2 A. Zielenkiewicz, E. Plesiewicz and K. L. Wiezchowski, Biophys. Chem., 1979, 10, 415. l3 M. P. Heyn, C.U. Nicola and G. Schwarz, J. Phys. Chem., 1977,81, 161 1. l4 P. Hemmes, A. A. Mayevski, V. A. Buckin and A. P. Sarvazyan, J. Phys. Chem., 1980, 84, 699. l5 S. I. Chan, M. P. Schweizer, P. 0. P. Ts’O and G. K. Helmkamp, J. Am. Chem. SOC., 1964,86,4182. l7 V. L. Antonovsky, A. S. Gukovskaja, G. V. Nekrasova, B. I. SukhorukovandI. I. Tchervin, Biochim. lCI W. Schimmak, H. Sapper and W. Lohman, Biophys. Struct. Mechanism, 1975, 1, 113. lCI W. Schimmak, H. Sapper and W. Lohman, Biophys. Struct. Mechanism, 1975, 1, 31 1. *O H. Sapper and W. Lohman, Biophys. Struct. Mechanism, 1978, 4, 327. *l D. M. Cheng, L. S. Kan, P. 0. P. Ts’O, C. Giessner-Prettre and B. Pullman, J. Am. Chem. SOC., 1980, 22 K. H. Scheller, F. Hofstetter, P. R. Mitchell, B. Prijs and H. Sigel, J.Am. Chem. SOC., 1981, 103, 247. 23 K. E. Van Holde and G. P. Rossetti, Biochemistry, 1967, 6, 2189. 24 T. N. Solie and J. A. Schellman, J. Mol. Biol., 1968, 33, 61. 26 R. Bretz, A. Lustig and G. Schwarz, Biophys. Chem., 1974, 1, 237. 26 F. Garland and S. D. Christian, J. Phys. Chem., 1975, 79, 1247. 27 J. Gajewska, A. Bierzynski, K. Bolewska, K. L. Wierzchowski, A. I. Petrov and B. I. Sikhorukov, 28 J. Pitha, R. N. Jones and P. Pithova, Can. J. Chem., 1966, 44, 1945. 29 Y. Kyogoko, R. C. Lord and A. Ritch, J. Am. Chem. SOC., 1967,89,496. M. P. Schweizer, S. I. Chan and P. 0. P. Ts’O, J. Am. Chem. SOC., 1965,87, 5241. Biophys. Acta, 1973, 331, 9. 102, 525. Biophys. Chem., 1982, 15, 191.2444 THERMODYNAMICS OF THE SELF-ASSOCIATION OF PURINE 30 K. A. Newmark and C. R. Cantor, J. Am. Chem. SOC., 1968,90, 5010. 31 G. G. Hammes and A. C. Park, J . Am. Chem. SOC., 1968,90,4151. 32 U. Gaarz and H. D. Liidemann, Ber. Bunsenges. Phys. Chem., 1976,80, 607. 33 D. D. Kasarda, Biochim. Biophys. Acta, 1970, 217, 535. 34 H. S. Frank and M. W. Evans, J . Phys. Chem., 1945, 13, 507. 35 W. Kauzmann, Adv. Protein Chem., 1959, 14, 1. 36 G. Nemethy and H. A. Scheraga, J . Chem. Phys., 1962, 36, 3401. 37 G. Nemethy and H. A. Scheraga, J. Phys. Chem., 1962, 66, 1773. 38 R. Lawaczeck and K. G. Wagner, Biopolymers, 1974, 13, 2003. 39 D. M. Crothers and D. I. Ratner, Biochemistry, 1968, 7, 1823. 40 J. Arpalahti and H. Lonnberg, Acta Chem. Scand., Ser. B, 1982, 36, 545. 41 J. Arpalahti and H. Lonnberg, Znorg. Chim. Acta, 1983, 78, 63. 42 M. Rasmussen and J. M. Hope, Aust. J. Chem., 1982, 35, 525. 43 A. Albert and D. J. Brown, J. Chem. SOC., 1954,2060. 44 M. T. Chenon, R. J. Pugmire, D. M. Grant, R. P. Panzica and L. B. Townsend, J . Am. Chem. SOC., 45 L. P. Vickers and G. K. Ackers, Arch. Biochem. Biophys., 1976, 174, 747. 46 B. Pullman, J. Chem. Phys., 1965,43, S233. 47 D. G. Oakenfull and D. E. Fenwick, Aust. J . Chem., 1977, 30, 741. 1975,97,4627. (PAPER 3/1601)

ISSN:0300-9599    

DOI:10.1039/F19848002439

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J. Chem. Soc., Faraday Trans. 1, 1984, 80, 2445-2458 Ionic Solvation in Water + Cosolvent Mixtures Part 9.-Free Energies of Transfer of Single Ions from Water into Water + Ethanol Mixtures BY CECIL F. WELLS Department of Chemistry, University of Birmingham, Edgbaston, P.O. Box 363, Birmingham B15 2TT Received 26th October, 1983 Values for the free energy of transfer of the proton AG,"(H+) have been determined experimentally using spectrophotometric measurements on p-nitroaniline in water +ethanol employing the method previously used for other cosolvents mixed with water. Values for AG,"(i) for other individual ions, i = anion X-, have been determined by combining these values for AG,"(H+) with values for AGF(HX) calculated from electrochemical data. These values for AG,"(X-) are subsequently used with values for AG,"(MX) and AGY(MX,) derived from electrochemical and solubility data to provide values for AG,"(M+ ) and AG,"(M2+). AG,"(X-) and AG,"(Xz-) for anions where AG,"(HX) and AG,"(H,X) are not available are calculated from AG,"(MX) and AG,"(M,X) obtained from solubility measurements and the above values for AG,"(cation).The free energy of transfer of single ions from water into water + cosolvent mixtures have been determined using a range of cosolvents each with a range of concentrations. The cosolvents investigated include methano1,lt isopropyl alcohol,2 t-butyl alcoh01,~ ethylene glycol,* glycerol,2 acetone,2$ dioxane6 and dimethyls~lphoxide.~ The method used involves first calculating the free energy of transfer of the proton AG;(H+) and separating the latter into two processes: AG;(H+), is the free energy of transfer of the aqua-proton from water into the mixture and AG(R6H2) is the free energy for rearrangement of solvent molecules in the mixed solvent which results from the intrusion of the aqua-proton.The relationship is represented as: AG,"(H+) = AG;(H+), + AG(R6H2) + where AG:(H+), c AG(ROH,) and the former is a l ~ a y s l - ~ c 10% of the latter for cosolvent concentrations not exceeding 30 wt % . The details of this method have recently been critically reviewed.* AG;(H+), for the transfer of the spherical aqua-proton on the molar scale is given by8 Ne2 AG;(H+), = '- (D;' - D;') 6 r H z 0 where e is the electronic charge, N is Avogadro's number, rHzO is the radius of the water molecule and D, and D, are the dielectric constants of the mixture and of pure water, respectively.After the transfer, the solvent molecules rearrange themselves and 24452446 IONIC SOLVATION IN H,O + COSOLVENT MIXTURES AG(ROH,) is derived by treatment of all these changes subsequent to the transfer as the solvent-sorting equilibrium : + (H2o)xH:Olv + ROHSOIV * {(H,O)Z-l ROH)H,+,,v + H2Osolv where x 3 5 to include solvent movements outside the immediate contact solvation sphere of H ~ 0 . 8 ~ 9 AG(ROH,) is givens on the molar scale by + + AG(ROH,) = - [ROH,] RT In (K,[H,O] F,) (3) where [RbH,] = {(HzO)x-l ROH}H:olv and the activity-coefficient ratio F, = YR&H,YH,O/Y~YROH. As the concentrations of all species are varying, the standard state for all species i treated as solutes in the mixture is [z] = 1 mol dm-3 and yi = 1 .O with yi 1 as [il--+ 0.8 Using eqn (1)-(3) and correcting to the mole-fraction scale,8 the total free energy of transfer on the mole-fraction scale is given bys + dsMw (4) Ne2 AG,"(H+) = - (Q1- D;l) - [ROH,] RT In (K,[H,O] Fc} + RT In ~ 6rHz0 dw Ms where ds and d, are the densities of the mixture and of pure water, respectively, and Ms and M , are the molecular weights of the mixture and pure water, respectivelys The last term on the right-hand side of eqn (4) only begins to make a significant contribution to AGF(H+) when the concentration of cosolvent exceeds 30 wt % .8 K, is determined spectrophotometrically by adding a minute concentration of p-nitroaniline (B), when the following equilibria become involved : Kl and K2 are the thermodynamic equilibria for the standard states defined as above.I f 4 = . Y ~ . Y ~ / . Y B H + Y ~ ~ O and 4 = YBYR~H~/YBH+YRoH, we can obtain' where [ROH], and Co are the total added concentrations of cosolvent and B, respectively, C and C, are the concentrations of B without added ROH and with ROH for the same Co and the same total mineral acid concentration [El+],. At constant temperature, with constant Co and [ROH],, plots of CCR/(CR-C) against C,/(Co - C,) for varying [H+], at constant ionic strength are always K, c1 = C,/(slope) [ROH],, where the slopes are derived from the linear plots of eqn (7) and F, is shown to be unity.8 At low mole fractions of cosolvent, x,, [H,O] = 55.345-[ROH], can be used in eqn (4), but at higher x,, [H,O] is more accuratelys given by (lOOOds - [ROH], MRoH)/Mw, where MROH is the molecular weight of pure ROH.All values for AG,"(H+) in water+cosolvent mixtures have been calculated8 using the latter-values for [H,O] in eqn (4): AG:(i) values have been calculated8 for all co~oIvents~-~ using these AG,"(H+) values with up-dated electro-C. F. WELLS 2447 Table 1. Values for Kc(dm3 mol-l) calculated from KzF, and of K C C 1 (dm3 mol-l) derived from the slopes at ionic strength = 1.00 mol dm3 and at 25 "C total added acidity/ concentration of ethanol (wt %) mol dm-3 4.00 8.08 16.51 25.31 34.52 44.16 54.26 0.10 0.16 0.20 0.40 0.80 K2 F, KCFi1 from slope 1 0.18 0.18 0.18 0.17 0.17 92 0.173 & 0.002 0.23 0.23 0.23 0.23 0.2 1 73 0.217 f 0.002 0.37 0.35 0.35 0.36 0.36 0.339 f 0.005 46 0.75 0.98 1.5 3.4 0.77 1.2 1.4 0.8 0.79 2.3 2.2 1 .o 4.6 0.89 53 1.2 23 14.0 10.5 10.2 0.66 1.10 1.46 1.51 kO.01 k0.02 +0.03 k0.03 - - - - + chemical and solubility data for salts and new pK, data.[ROH,] in eqn (4) is derived from: [ROH,] = 0.5{A - (A2 - 4[ROH],):) (8) EXPERIMENTAL MATERIALS p-Nitroaniline was purified as described previously. lo Water was distilled once in an all-glass still and AnalaR ethanol, HC1 and NaCl were used. PROCEDURE All mixtures were made up by mixing known volumes of solution. The contractions of the solutions on mixing were determined to allow an accurate estimation of the molar concentrations. Concentrations of the unprotonated p-nitroaniline (B) were determined spectrophotometrically at 383 nml0 using the thermostatted-cell compartment of a Unicam SP500 series 2 spectrophotometer.RESULTS AND DISCUSSION DETERMINATION OF L\G:(H+) IN WATER 4- ETHANOL Plots of CCR/(CR-C) against CR/(Co-C,) were found to be linear in 5, 10, 20, 30,40, 50 and 60% v/v ethanol, using measurements at each acidity quoted in table 1 as found previously with other cos01vents.~-~ Ionic strength was maintained at 1 .OO mol dm-3 by adding NaCl to supplement the HCl.9 Values for K, F;l determined from the slopes are given in table 1. Intercepts for these plots were calculated using C,/K;F;[ROH], where Ki = [BH+]ybH+/[B] [P]yLy& determined in water8-lo for the equilibrium K : B,, + P BH& (10)2448 IONIC SOLVATION IN H,O + COSOLVENT MIXTURES ~~ 0 10 20 30 LO 50 60 ethanol (wt %) + Fig.1. Plot of AG(ROH,) in water+ethanol mixtures at 25 "C. where K; is the thermodynamic equilibrium constant with the standard states defined as above and y; is the activity coefficient in water (Fl = y;3y;?/&+). Values from K, can also be calculated from the equation using1-l0 the value of K2 F, calculated from the slope and intercept (derived as described above) for the linear plots representing eqn (7). These values are also included in table 1, showing the invariance of Kc with varying [H+IT except possibly $t the higher mole fractions of ethanol because of the very low values for ([H+IT - [ROH,]) used in eqn (1 1): this erratic behaviour at high [ROHIT has been experiencedl-lO with all the cosolvents. The agreement between K, c1 from the slope and Kc from eqn (1 1) shows that F, = 1.0 using ethanol, as found with all the other cosolvents.l-lo This, together with the linearity of the plots representing eqn (7), supports the correctness of the assumption K; Fl = KIF,/[H,O] made in the derivation of eqn (7),* as found with all other cosolvents.l-lo Values of AG,"(H+) were calculated using eqn (l), (2) and (4).Values for AG(ROH,) were found from the experimental data for KCC1 (4 = 1 .O), [H,O] = (1000 d,-[ROH],M,,,)/M, and [ROH,] derived using eqn (8) and (9) at the concentrations of ROH used for the spectrophotometric measurements. Values for AG(ROH,) are plotted against wt % ethanol in fig. 1. Values for AG(ROH,) were then interpolated from fig. 1 for concentrations of ethanol where AG:(H+) was required and values for AG:(H+), computed8 using eqn (2).Values for the dielectric constants were interpolated graphically using the data of Akerlof," Wyman,12 Graffunder and Heymann,13 Martin and Brown14 and Hall and Phillips,15 which are all in good agreement. Values for the density were those of Bates.16 The resulting values for AGf(H+) are given in table 3. + + +C. F. WELLS 2449 AG,"(anion) IN WATER +ETHANOL A wide range of electrochemical data exist from which AG,"(HCl) can be calculated. (12) (13) and the E' values, where subscripts w and s represent water and water+ethanol, respectively. From other E" values for cell (12),21-25 AG,"(HCl), on the molality scale can be derived and converted to AG,"(HCl) on the mole-fraction scale using For the ~elll~-~O AG,"(HCl) can be calculated directly on the mole-fraction scale using Pt, H21HC1, EtOH + H,OIAgCl, Ag AG,"(HCl) = 96.5(Ei - E,O) kJ mol-l AG,"(HCl) = AG,"(HCl), + 11.41 loglo (' ___ y,l6) kJ mol-I where M, = 100/((wt % EtOH/46.07) + (wt % H20/ 18.016)).In addition, E" data for the cell26 Pt, H,IHCl, EtOH + H201Hg2C12, Hg exist, from which AG,"(HCl) can be calculated directly using eqn (13). E" data on the molar scale for the cell2' (16) produce AG,"(HCl), on the molar scale from eqn (13) which can be converted to the glasslHC1, EtOH + H201AgC1, Ag mole-fraction scale using AGF(HC1) = AG,"(HCl), + 11.41 loglo (' ~ ~ s ~ w d s ) kJ mol-l. Valus for AG,"(H+) for the appropriate composition of H20 + EtOH can then be used with AGt(HC1) to produce values for AG,"(Cl-) via AG,"(Cl-) = AG,"(HCl) - AG,"(H').(18) The resultant values are given in table 2: good mutual agreement is obtained and a smooth curve can be drawn using all the data for AG,"(Cl-) in table 2. E" values for cell (12) with C1- replaced by Br-28i 2s and I-30 have been used with eqn (1 3) to produce AG,"(HBr) and AG,"(HI) directly on the mole-fraction scale, and E" values for this cell on the molarity scale 2 4 7 31 have been used with eqn (1 3) and (14) to give AG,"(HBr) and AG,"(HI) on the mole-fraction scale. Similarly, E" for cell (1 5 ) with Cl- replaced by Br-32 on the molality scale has been used with eqn (13) and (14) to produce AG,"(HBr) on the mole-fraction scale. AG,"(HCNS) has been calculated from E" values on the mole-fraction scale for the cell: Ag, AgCNSIKCNSI IKClIAgCl, Ag (19) where allowance has been made for the liquid-junction potential.33 Values24 for AG,"(HClO,) and AG,"(HBPh,) on the molality scale have been converted to the mole-fraction scale using eqn (14).All these values for AG,"(HX) on the mole fraction scale have been used with the appropriate values for AG,"(H+) to give values for AG,"(X-) using equations of the type of eqn (18): these values for AG:(X-), where X- = Br-, I-, CNS-, C10, and BPh,, are collected in table 2. Values for AG,"(KBPh,) on the molality scale have been derived from solubility data34 using AG,"(KBPh4) = 2.303RT(pKs - pKw) (20) 80 FAR 1Table 2. Values of AG;O<-) and AG;(X2-) (kJ mol-l) in water + ethanol mixtures at 25 "C (for references see end of table 3) ethanol mole wt % fraction c1- Br- I- SCN- OH- c10, ReC1;- Pic- BPh, 3.73 5.00 7.27 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.5 13.5 15.0 16.4 20.0 20.0 20.0 20.0 20.0 20.0 20.0 20.5 21.5 25.0 26.2 30.0 30.0 30.0 0.0151 0.0202 0.0298 0.041 7 0.0417 0.0417 0.0417 0.0417 0.0417 0.0417 0.0439 0.058 0.065 0.071 0.089 0.089 0.089 0.089 0.089 0.089 0.089 0.092 0.097 0.1 15 0.122 0.144 0.144 0.144 1.03i 2.27c 2.18* 2.249 2.25h 2.26i 2.24k - 2.225 - 3.61j 5.3c 5.2d 5.39 5.3h 5.3i 5.3.5 5.3k - - 6.9 8.3c 8.3e 8.39 - 1.40 3.8 - - 0.2 - - 3.3 - - - 3.44 - - 3.94 - 1.60" - - - -4.12e -4.10"30.0 30.0 30.1 33.7 35.0 36.0 37.3 39.9 40.0 40.0 40.0 40.0 40.0 42.6 45.0 45.1 46.02 47.3 49.3 50.0 50.0 50.0 50.0 50.0 50.0 50.0 51.0 51.3 55.0 58.8 60.0 60.0 60.0 f 65.0 0.144 0.144 0.144 0.166 0.174 0.180 0.189 0.206 0.207 0.207 0.207 0.207 0.207 0.236 0.242 0.243 0.250 0.260 0.276 0.28 1 0.28 1 0.28 1 0.281 0.28 1 0.28 1 0.28 1 0.289 0.292 0.323 0.358 0.370 0.370 0.370 0.421 8.l j 8.2" - - 9.@ - - - 9.6" 9.6d 9.6i 9.g 9.6" 9.8j - - - 9.6' - - 9.9b 9.6" 10.0" 9.8f 9.98 9.93 9.9k - - 9.9 9.8f 9.9 10.1" 9.9 - - - 9.2' - - - r - 13.7" m - 13.8' - 15.3' P ul c2452 IONIC SOLVATION IN H20 + COSOLVENT MIXTURES 0 . 5 o*61 I 0 . 4 0.2 0 20 40 60 80 100 ethanol (wt %) Fig. 2. Values for the activity of water at 25 "C calculated from the vapour pressure data of Dobson (0) and Dornte (0). where pK is the negative logarithm of the solubility product. After conversion to the mole-fraction scale using eqn (14), values for AG,"(K+), determined as described later, were used in the equation AG,"(BPh,) = AG,"(KBPh4) - AG:(K+) (21) to derive further values for AGt(BPhy).A similar treatment of solubility data for potassium p i ~ r a t e ~ ~ results in values for AG,"(Pic- ) on the mole-fraction scale. Similarly, solubility data35 for Cs2ReC1, have been used to derive values for AG,"(Cs,ReCl,), on the molar scale. These have been corrected to the mole-fraction scale using and values for AG,"(ReCli-) were derived using AG:(ReCli-) = AG:(Cs2ReC1,) - 2AG:(Cs+) (23) with values for AG,"(Cs+) determined as described later. These values for AG:(X-) or AG,"(X2-) derived from solubility data are collected in table 2. The values for K = [H+] [OH-]y2,/a, for the dissociation of water3, have been converted to the ionic product for water on the molar scale, K$,, using the values of a, (fig.2) calculated from the vapour-pressure data of Dob~on.~' The vapour-pressure data of D ~ r n t e ~ ~ were not used for this purpose: these data are absent at low wt %C. F. WELLS 2453 ethanol and, as fig. 2 shows, a, calculated from these vapour pressures are more irregular than those calculated using the data of Dobson and deviate markedly from the latter data at higher wt % ethanol. These ionic products for water on the molar scale were then converted to the ionic product on the molality scale, K g , using pK$ = pK$, + 2 log,,, d,. Values for AG,"(H+) + AG,"(OH-) on the molality scale were calculated from these values of pwp AG,"(H+),+AG,"(OH-), = RT mw ms where m, and m, are the molalities of water in pure water (w) and in the mixture (s), respectively, and the activity of water in the mixture on the molality scale is derived (26) and values for a%2o interpolated from fig.2 using Dobson's vapour pressures.37 After correcting AG,"(H+), + AG,"(OH-), to the mole-fraction scale using eqn (14), values for AG,"(OH-) on the mole-fraction scale were calculated using the appropriate values for AG,"(H+) and an equation analogous to eqn (18). These values are contained in table 2. ahzO (molality) = 55.509 ahzo (mole fraction) AG,"(cation) IN WATER + ETHANOL Values for AG,"(H+)-AG,"(M+) on the molality scale have been recorded by Bax et al. for M+ = K+, Rb+, Cs+, Me,N+, (n-Pr),N+, (n-Bu),N+, Fic+ and TAB+ (where Fic+ is the ferrocinium ion and TAB+ is the tri-isoamylbutylammonium ion).The majority of these are based on solubility rneas~rements,~~ but the data for Fic+ are based on the E" measurements of Vedal.,O AG,"(H+) on the mole-fraction scale can be directly with the data to produce AG;(M+) on the mole-fraction scale: these values are recorded in table 3. Solubilities have also been used to produce free energies of transfer from water into water + ethanol mixtures for TAB Pic and TAB * BPh, on the molality scales:34 after conversion to the mole-fraction scale using eqn (14), values for AG,"(TAB+) have been calculated using (27) and the values for AG,"(Pic-) and AG,"(BPh,) in table 2. Sol~bilities~l have also been used to provide values for AG:(Ph,As - Pic) and AG,"(Ph,P * Pic) on the molar scale ; after conversion to the mole-fraction scale using eqn (17), values for AG,"(Ph,As+) and AG;(Ph,P+) were calculated using an equation analogous to eqn (27) with AG,"(Pic-) from table 2.All these values for AG,"(M+) are contained in table 3. Some E" values have been used directly to produce free energies of salts. The cellg2 AG,"(TAB+) = AG,"(TAB * X) - AG,"(X-) K(Hg)lKCl, EtOH + waterlAgC1, Ag Zn(Hg)lZnCl,, EtOH + waterlAgC1, Ag (28) (29) was used to provide AG,"(KCl)m on the molality scale with eqn (13) and the cell43 with eqn (1 3) provides values for AG,"(ZnCl,), on the molality scale. AG,"(KCl), was corrected to the mole-fraction scale using eqn (14) and AG,"(ZnCl,), using AG,"(ZnCl,) = AG;(ZnCl,), + 17.11 log,, (' - :sl ') kJ mol-l.Table 3. Values of AG,"(M+) and AG,"(MZf) (kJ mol-l) in water + ethanol mixtures at 25 "C h, P P ethanol wl mole wt % fraction H+ K+ Rb+ Cs+ Zn2+ Me,N+ Pr,N+ Bu,N+ Fic+ TAB+ Ph,As+ Ph,P+ 5.00 10.0 10.0 10.2 15.0 19.9 20.0 20.0 20.4 25.0 30.0 30.0 30.0 30.3 31.2 35.0 39.3 39.7 40.0 40.0 45.0 50.0 50.0 50.0 50.7 50.9 55.0 60.0 60.0 60.9 61 .O 65.0 0.0202 0.041 7 0.0417 0.0425 0.065 0.089 0.089 0.089 0.09 1 0.115 0.144 0.144 0.144 0.145 0.151 0.174 0.202 0.205 0.207 0.207 0.242 0.281 0.281 0.281 0.287 0.289 0.323 0.370 0.370 0.379 0.380 - -0.81 - 1.81 - - - 3.00 - -4.50 - - - 6.0 -7.2 - - - - - 7.9 - - -8.3 - - 8.4 -8.2 - - - - - 8.0 -7.5 - - - -7.1 - 0.1839 - 0.589 - - -1.109 - 1.839 - - - - 2.67' - 3.76" -3.189 - - - - 3.149 - - - 2.679 - 1.99' - 1 .33e -1.15' - - - - -0.0679 1.139 - - - 2.529 - - - 3.54 - - - - - 8.5 - - -4.37 -13.0 - - - 13.7 - - - -2.139 - - -3.13P - -3.17P - - -7.4 -11.V - -11.8P - -11.8P - - - - 14.9P - - 15.0P - - -7.1 -16.6e - 16.Y - 16.5P - - - - 4.65 -20.9 $ - - -21.8 a Ref.(21); ref. (18); ref. (17); ref. (26); ref. (24); f ref. (23); g ref. (19); ref. (22); ref. (25); i ref. (27); ref. (20); ref. (32); ref. (28); ref. (29); O ref. (30); P ref. (34) and (42); Q ref. (44).C. F. WELLS 2455 20 10 -10 -20 -30 I- Fig. 3. Values for AGF(i) for individual ions in water+ethanol mixtures at 25 "C. AG:(K+) was then calculated using eqn (27) and the smoothed values for AG,"(Cl-) in table 2 and AG,"(Zn2+) using AG,0(Zn2+) = AGF(ZnC1,) - 2AG,O(C1-) (31) and the smoothed AG,"(Cl-) values. AG,"(RbCl), on the molar scale have been calculated from the E" data for the cell44 cationic glasslRbC1, EtOH + H,OIAgCl, Ag.(32) After correction to the mole-fraction scale using eqn (1 7), values for AG,"(Rb+) were produced using an equation of the same type as eqn (27) and the smoothed AGF(Cl-) values. All these values for AGy(cation) are contained in table 3. COMPARISON OF AG:(i) IN WATER 4- ETHANOL WITH AG,"(i) IN MIXTURES OF WATER WITH OTHER COSOLVENTS In general, good agreement is obtained for values of AG,"(i) in water+ethanol mixtures for any particular species i over the data derived from various sources. Fig. 3 shows for low x, that, in general AG:(i) for i = anion are all positive and for i = cation are all negative, as found with other c 0 ~ 0 1 ~ e n t ~ , ~ - ~ the one real exception being BPh,, as found also with other cosolvents.The sequence in the anions in water+ethanol mixtures, OH- > Cl- > Br- > I- x C10, 2 SCN-, closely corre- sponds to the findings in other C O S O ~ V ~ ~ ~ S ~ - ~ where the equilibrium OH- + ROH $ RO- + H,O (33)2456 lies to the left. The only difference is found with cosolvents like methano1,ll ethylene glycol4? and glycerol2. 8, where equilibrium (33) lies to the right and OH- moves along the series of ions depending on the extent of the stabilization of OH- in the mixture resulting from the influence of equilibrium (33). The general observation of negative values for AG:(cation) at low x, shows that the effect of solvent structure on AG,"(i) is dominant over dielectric effects on transferring the charge from water.g On the Frank-Evan~~~ and Nemeth~-Sheraga~~ interpretation of water structure, involving volumes of structured water on a time-average basis, organic molecules can increase or decrease the amount of structure in the liquid.The excess change in the maximum density of water resulting from the addition of alcohols shows that the latter increase the amount of structure in water:47 the minima found in the decrease in the partial molar volume of the alcohol, K- q,48 has been ascribed2v49 to the filling of the cavities between the volumes of structured water with the alkyl groups resulting in the imposition of strain and further hydrogen-bond formation within the structured volumes; the maxima for the ultrasonic absorption50 found at the higher x, have been to the onset of structural breakdown in the water arising from this strain.Extrema in the excess enthalpy and entropy of mixing of water with the cosolvent are also associated with the formation of structure in the liquid. The extent of this strain at low x, of alcohol is shown by the depth of E- at the minimum and the point at which it is reached is indicated by the value of x2 at the minimum. Thus isopropyl alcohol and t-butyl alcohol have deep minima in K- occurring at x, < 0.1 ,48 whereas the depth is smaller for ethanol at x, x 0.1 and smaller still for methanol at x, x 0.1-0.15,4s almost non-existent for ethylene glycol and non- detectable for glycerol.48 Values for AG,"(i) show extrema or sharp changes at x, 5 0.1 for t-butyl alcohol3? and isopropyl alcohol,2* those with methanol1$ * and ethylene glycol4* show some tendency for this at x, w 0.2-0.3 and those with glycero12v show no tendency at x, up to 0.20.Ethanol shows these extrema or sharp changes in AG:(i) at x, x 0.15. However, methanol shows no peak in the ultrasonic absorption when mixed with water,50 but the peaks for ethanol, isopropyl alcohol and t-butyl alcohol are 50 at x, x 0.3, x, x 0.15-0.20 and x, x 0.1, respectively, with increasing absorption at the peak along the series, so that the broad extrema exhibited by AG,O(i) may reflect a combination of both effects. It is interesting to note that, of the various cosolvents used previously, the spread of values of AG:(i) over all i with varying x, for ethanol most closely resembles that in water + dioxane,6,8 despite some apparent differences in the physical properties of the mixtures.Thus, water + dioxane has only a small depth for the minimum in 5 - at x, z 0.0451 and only a small ultrasonic ab~orption,~, and the changes in the maximum density of water with x, that the addition of small concentrations of dioxane to water breaks structure. However, the minimum in the excess enthalpy of mixing of water + d i ~ x a n e , ~ ~ the existence of which itself suggests enhanced structure formation over that existing in water, occurs at x, z 0.15-0.2, as it does in water + the region where the extrema in AG:(i) occur in both. The maximum in the viscosity+omposition curve for water + d i ~ x a n e , ~ ~ similar to those found in water + a l c o h o l ~ , ~ ~ ~ 57 also supports the formation of structure when dioxane is added to water.Water + ethanol and water + dioxane678 both show AG,"(i) for i = an alkali metal becoming positive at x2 x 0.3: this occurs at lower x, in water+t-butyl alcohol3? and at higher x, in water + ethylene glycol.4* In both AG,"(Zn2+) is much lower than that for unipositive ions, adding support to the view8? 58 that AG,"(i) becomes increasingly negative as the charge increases for ions of approximately the same size. In both mixtures,6* anions or cations with large hydrocarbon groups have large negative values arising from the large structure-forming capacity of these IONIC SOLVATION IN H,O + COSOLVENT MIXTURESC. F. WELLS 2457 48 Interestingly, though, values of AG:(i) for BPh, and TAB+ are not equal and differ from those for Ph,P+ and Ph,As+, although the assumption of their mutual equality has been as another basis for the resolution of AG: values for salts into values for separate ions. Comparably, AG,"(Ph,As+) is also not equal to AG,"(BPh;) in water + DMSO mixtures.'? A new feature in water + ethanol mixtures is the high positive value for AG,"(ReClf), which contrasts with the negative value found for AG,"(ReO,) in water + dioxane mixtures : 6 y presumably the high positive AG,"(ReC1,2-) arises from the presentation of negatively charged chlorine atoms to the solvent. 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ISSN:0300-9599    

DOI:10.1039/F19848002445

出版商:RSC    

年代:1984

数据来源: RSC