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/F198480FX001

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/F198480BX003

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. I , 1984,80, 13-27 Small-angle Neutron-scattering Study of Microemulsions Stabilised by Aerosol-OT Part 1 .-Solvent and Concentration Variation BY BRIAN H. ROBINSON,*? CHRISTO TOPRAKCIOGLU~~ AND JOHN C. DORE$ ?Chemistry and $Physics Laboratories, University of Kent, Canterbury CT2 7NR AND PIERRE CHIEUX Institut Laue-Langevin, Grenoble, France Received 1st November, 1982 Small-angle neutron-scattering (SANS) measurements have been made for a series of aerosol-OT (A0T)-stabilised water-in-oil microemulsions. The intensity pattern has been used to extract a value for the radius of the water core, rw, using D,O to provide the required contrast profile. In heptane the radii are found to follow an approximately linear relationship with respect to the [D,O]/[AOT] concentration ratio, R.At 20 OC, and R = 20, the structure of the water-droplet system is dependent on the hydrocarbon chain length of the oil medium. The experimental SANS patterns show increasing discrepancies with a fitted function based on monodisperse spheres as the length of the alkane chain is increased from n-heptane to n-dodecane. This effect is attributed to polydispersity and indicates that the droplet-size distribution within these microemulsion systems is much larger than had previously been thought. In this paper we report a small-angle neutron-scattering (SANS) study of water-in-oil microemulsions stabilised by aerosol-OT (AOT) as surfactant. This system was chosen for detailed study since no cosurfactant is required to form a stable microemulsion and the behaviour is therefore considerably simplified.Structural studies of four- component microemulsions pose various problems which have not as yet been fully resolved. On adding water to a solution of AOT in a hydrocarbon (e.g. heptane), the microemulsion is formed spontaneously after being shaken for a few seconds. It is generally accepted that the system is thermodynamically stable and the equilibrium sizes are established rapidly following dispersion. However, evidence has recently been obtained which suggests that water-in-oil microemulsions formed by the system AOT/water/decane can separate into two phases on standing for a few m0nths.l have clearly indicated the existence of discrete water droplets dispersed in a continuous oil phase for the AOT systems.The effect of temperature was studied by Zulauf and Eicke.2 These results taken together show unambiguously that the size of the water droplets increases with R (or coo), the molar ratio of water to AOT in the system. For R < 20 the size did not vary significantly over a fairly wide temperature range. Neutron scattering offers several advantages over other techniques since deuteration of the various chemical constituents of the system enables different structural parameters to be obtained. In the present paper the main emphasis of the investigation concerns the size of the water core. This is obtained by using fully deuterated water Previous studies by photon correlation spectroscopy 2-4 and 1314 SANS FOR AOT MICROEMULSIONS in a hydrogenated surfactant and solvent medium.The information is complementary to that obtained in PCS experiments in which the overall droplet size, as characterised by the hydrodynamic radius, is determined from the translational diffusion coefficient. Further considerations related to polydispersity and surfactant-partitioning effects are also considered and related to the detailed behaviour of the SANS intensity profile. This work builds upon the pioneering neutron study of AOT-based microemulsions reported by Cabos and de Lord.5 THEORY OF THE SANS METHOD Small-angle scattering occurs when a beam of neutrons passes through a sample containing regions of different mean coherent scattering lengths. The intensity profile for the scattered neutrons depends on the size distribution of these regions as well as the difference (or contrast) in their relative scattering amplitudes, which are determined by the nature of the nuclei contained in the regions concerned.The coherent neutron scattering lengths of hydrogen and deuterium are -3.74 and 6.67 fm, respectively.' These values are significantly different, so that selective deuteration of the various components of the system conveniently adjusts the contrast profile to provide the required information. The scattered intensity for a system consisting of an infinitely dilute solution of non-interacting, monodisperse spherical particles of radius r may be expressed as where p and ps are the mean scattering lengths per unit volume of the particle and solvent, respectively, V is the volume of the particle, n is the number of particles per unit volume and Q is the scattering vector, whose magnitude is given by 4n Q = sin 8/2 where 8 denotes the angle of the scattered beam with respect to an incident beam of wavelength A.The function I@, r ) is essentially the form factor representing the interference pattern for scattering from a single droplet. In a real system of finite concentration there is also interference between droplets, so that the observed intensity distribution is given by (3) where S(Q) is the structure factor for the droplet distribution. At sufficiently low concentrations S(Q) approximates to unity over the Q-range of interesta and the intensity pattern may be used directly to determine the structural parameters of the droplet.In practice, however, this is not necessarily the case for all low-concentration dispersion^.^ In the case of charge-stabilised dispersions, such as aqueous micelles, the interaction between different aggregates can be represented by electrostatic forces with a screened Coulomb potential so that some estimate can be made of S(Q). For microemulsions there is a much weaker electrostatic interaction due to charge cancellation of counter-ions in the region of the surfactant head-group at the water interface. Under these circumstances there is less overall structure in the particle distribution and the system can be represented in first order as a system of hard spheres with negligible long-range interaction. The second virial coefficient, B,, is thought to have a smallB.H. ROBINSON, C. TOPRAKCIOGLU, J. C. DORE AND P. CHIEUX 15 negative value,l0 which indicates a residual attractive force. In the present measurements a low droplet concentration (typically ca. 4% by volume) is used and the system is not normally close to the phase boundary defining the stability of the microemulsion (fig. I). Under these conditions the structure factor will approximate to unity for the @range of interest. 100- R 50 - 0 70 0 70 0 70 T/" C Fig. 1. Phase behaviour of the systems : (a) water/AOT/heptane, (b) water/AOT/decane and (c) water/AOT/dodecane, with [AOT] = 0.1 mol dm-3 in each case. The area under each curve corresponds to the 'clear' microemulsion region. In the case of heptane, in the upper part of the 'clear' region where the two phase boundaries converge, the system acquires a light blue appearance due to Tyndall scattering.EXPERIMENTAL MATERIALS The AOT (fig. 2) was obtained from Sigma Chemicals. The samples obtained from this source were found to be consistently pure and contained minimum amounts of carboxylic acid and alcohol, which form as a result of self-hydrolysis of the ester AOT.ll The heavy water (D,O) was obtained from Prochem (B.O.C.) and B.D.H. and had a quoted isotopic purity of > 99.8%. The microemulsion samples were prepared by adding D,O (with the aid of a microsyringe) to a solution of AOT in one of the various hydrocarbons used and shaking the mixture vigorously for a few seconds. The hydrocarbon solvents used in the present experiments (hexane, heptane, octane, decane and dodecane) were Fisons or B.D.H.materials and were distilled before use. APPARATUS Neutron-scattering measurements were performed on the D 17 small-angle diffractometer at the Institut Laue-Langevin (ILL), Grenoble, and also on the Pluto small-angle instrument at A.E.R.E., Harwell. The wavelength of the monochromatic neutron beam was varied in the range 6-9 A and corresponded to scattering vectors, Q, of 0.02-0.20 A-l for most measurements. The samples were contained in stoppered quartz cells and had a neutron path length of 1 mm. The temperature was maintained constant to kO.1 K throughout each run. The small-angle intensity profile was recorded using a two-dimensional multidetector. The separate channels were grouped to provide a radial summation and the usual corrections applied to the data in order to produce a SANS intensity profile, lob&?).16 SANS FOR AOT MICROEMULSIONS o .1 P 0 ter AOT n - h e / ptane 0 Na \ \ Fig. 2. Structure of the AOT molecule and the spherical microemulsion droplet with the corresponding contrast in the mean coherent-scattering length per unit volume, p , which is given in units of 10-12 cm 81-3. RESULTS PHASE BEHAVIOUR OF MICROEMULSION SYSTEMS Since the dimensions of water-in-oil microemulsion droplets are typically in the range 20-100 A these systems have a visually clear appearance. If at a given temperature increasingly larger amounts of water are added to a solution of AOT in heptane (or other hydrocarbon), the surfactant concentration being kept constant, a stage is eventually reached at which the system undergoes what appears to be a phase transformation, and becomes visually turbid.Denoting the molar ratio of water to surfactant by R, it is possible to construct phase diagrams by measuring the value of R at which the transition occurs as a function of temperature. The phase behaviour of water/AOT/heptane is shown in fig. 1 ; when D,O is substituted for H,O the phase diagram shifts to higher temperature by ca. 6 K.12 The transition involves the formation of two phases separated by a well defined interface. The nature of the two phases has yet to be fully characterized, and the behaviour does not appear to correspond to simple separation of the oil and water components. The results reported in this investigation correspond to the clear microemulsion region of the phase diagram for all the systems studied; data for conditions close to the transition region will be reported in a later paper.SANS MEASUREMENTS The constant profile for a typical AOT-stabilised microemulsion droplet is shown in fig. 2, for a water core consisting of D,O surrounded by hydrogenated surfactantB. H. ROBINSON, C. TOPRAKCIOGLU, J. C. DORE AND P. CHIEUX 17 1.00 0.75 h 2 + 0.50 0.25 0.05 0.10 0.15 0 -75 h 0.50 O a Z 5 i 1 0.05 0.10 0.15 QI A I I I 0.05 0.10 0.15 i -\. 1 L I I I I I I I 0.05 0.10 0.15 0.05 0.10 0.15 0.25 QIA Fig. 3. Fit to the observed SANS intensity pattern for R = 20 microemulsions in various solvent media: (a) hexane, (b) heptane, (c) octane, (d) decane and (e) dodecane. The solid line is the fit using the monodisperse formalism [eqn (l)].and solvent. The surfactant head-groups are hydrated and protrude into the aqueous region, so that the overall profile includes the sulphosuccinate portion of the AOT molecule in addition to the water core. The hydrocarbon chains of the surfactant are well matched to the n-alkane, which constitutes the continuous phase. The SANS measurement is therefore dependent on the radius, rw, which includes the volume occupied by the head-group. If it is assumed that the interface region is relatively sharp, the observed SANS intensity may be characterised by eqn (1) and used to extract a value for the radius parameter, rw. Typical results for various hydrocarbon media with R = 20 are shown in fig. 3. The curves represent least-squares fits based on the intensity function for monodisperse18 SANS FOR AOT MICROEMULSIONS Table 1.The core radius, rw, obtained from the fit to the SANS intensity profile for various hydrocarbon mediaa ~~ ~ hydrocarbon medium rw/A n-hexane 33.1 & 1 .O n-heptane 35.8 k 1 .O n-octane 36.8 & 1 .O n-decane 34.4 & 1 .o n-dodecane > 50 a [AOT] = 0.1 mol dm-3; R = 20; T = 293f 1 K. spherical particles [eqn (l)]. The value of rw for each system is shown in table 1, and the quoted error incorporates the variation for several runs; the calculated statistical error from a single observation is much smaller. With the exception of dodecane, both the shape of the intensity pattern and the extracted value for rw remain virtually independent of the hydrocarbon solvent. Although the particle size in dodecane appears to be considerably larger than in the other solvents, it is clear that eqn (1) fails to produce an adequate fit to the experimental data.Closer examination of the results for hexane to decane reveals small but significant systematic deviations in the higher region of Q, these being most pronounced in the case of decane. In particular the minimum predicted at Q x 4.49/r [eqn (111 is never actually observed in the experimental data. The absence of this minimum can be attributed to various factors, although the effects due to polydispersity are expected to dominate; these features are discussed more fully later in the paper. SIZE DEPENDENCE ON R It is possible to predict an approximate relationship between rw and R on the basis of certain simplifying assumptions.13 The amount of water in a microemulsion system consisting of monodisperse spheres is equal to jn(rw - ?-k)3 pw n, where n is the number of droplets, rk is the effective size of the head-group protruding into the aqueous phase and pw is the density (in moles per unit volume) of water in the droplet.If all of the surfactant present in the system is located at the interface, the total amount of surfactant is equal to 47t(rw - rk)2 a, n, where a, is the density (in moles per unit area) of the surfactant at the interface. The molar ratio, R, of water to surfactant is then given by R = in(rw - r,)3 pw n/4n(rw - rk)2 a, n which establishes a linear relationship between R and rw provided that pw and a, are independent of droplet size. Experimental results2v seem to support this simple relationship quite well for AOT systems, and this has contributed to the view that the systematic variation of R has a well defined effect on the structure of the droplets.There is now increasing evidenceff to suggest that the assumptions are probably not justified and that the linear relationship is a result of various competing factors which conceal the underlying complexity of the structural changes.B. H. ROBINSON, C. TOPRAKCIOGLU, J. C. DORE AND P. CHIEUX 19 I 50 I i 10 20 30 40 R = [ DzO] /[ AOT] Fig. 4. Radius parameter, rw, as a function of R for water/AOT/heptane at 293 k 1 K. ([AOT] = 0.1 mol dm-3). The results of the present measurements for water/AOT/heptane are shown in fig.4 for the range 5 < R < 40 and are qualitatively similar to those given by micro- emulsions prepared with different hydrocarbon media ; the error bars again include variations from several different runs and therefore incorporate any uncertainties arising from sample preparations. Note that at low values of R the errors are mainly associated with sample preparation as well as unfavourable statistics (due to low count rates) while at high R there are increasingly larger uncertainties arising from the least-squares fits (due to the fact that the droplets are larger, and consequently most of the intensity profile falls outside the accessible Q-range). Tests were made at R = 20 with samples that had been freshly prepared before the run and other microemulsion samples that had been measured in an earlier experiment, two months previously.The two data sets were in good agreement and did not indicate any change in droplet size. The results are also in satisfactory agreement with those reported by Cabos and de Lord,5 although their radius values were extracted from a simpler analysis based on Guinier-radius formalism and therefore restricted to a much smaller Q-range . The overall features are therefore well represented by the linear relationship between rw and R which can be used to predict approximate values that are in accordance with a comprehensive photon correlation spectroscopic study of AOT-stabilised micro- emulsions in iso-octane.2 For larger R values (> 40) the available data suggest that there are significant deviations from this simple relationship which are probably due to the partitioning of the surfactant between the interface and the oil medium." Furthermore the current analysis is based on monodisperse size of the droplets and there is increasing evidence (see next section) to indicate that the spread in droplet size is much greater than has previously been thought.If this is the case there is no reason to expect a linear relation between rw and R, and it may be dangerous to assign too much significance to the value for the effective head-group area of the AOT molecule at the interface which may be extracted from the data. 2 FAR 120 SANS FOR AOT MICROEMULSIONS TREATMENT OF POLYDISPERSITY The results of the previous section show that radius values can be readily extracted from the observations using a simple formulation based on the assumption of a monodisperse distribution. However, a close examination of the experimental data shows that there are systematic differences particularly in the region where there is a minimum in the fitting function.This indicates that the detailed shape of the SANS pattern contains further information about the contrast profile in the sample material. For spherically symmetric particles the intensity can be more generally expressed as where QA(Q) = s" rp(r) sin Qr dr 0 and p(r) is the contrast profile for the particle. A constant value for p ( r ) with a sharp cut-off at the interface leads directly to eqn (1). In a real system it would be expected that the interface region will not be sharp and the presence of the head-group and associated counter-ions may alter the shape of the profile.However, the resulting A(Q) function will still have an oscillatory form, so that I(Q) will always have a zero at points where the curve crosses the axis. Hence the introduction of a diffuse interface is unlikely to explain the deviations from eqn (1). The introduction of elliptical distortion for the droplet shape changes this situation slightly but does not have a large effect on the results unless large eccentricities are used. The uncertainty in neutron wavelength associated with the velocity selectors used in our experiments was of the order of f 10%. This can be expected to cause a certain amount of flattening of the intensity profile in the higher region of Q, but it cannot account for the systematic changes observed in the profile as a function of the hydrocarbon medium.It is clear from the experimental data that there is no zero-scattering region [i.e. I(Q) = 01 in the high-Q part of the SANS intensity pattern. This immediately implies a variation in the sizes of the particles such that the minimum occurs at different Q values for different particle sizes and is smeared out in the total pattern. The observed intensity therefore becomes a weighted mean of independent contributions from the different sizes, i.e. where w(r) is an appropriate weighting function. In order to study the effects of droplet-size variation on the SANS profile, some assumptions must be made about the shape of the distribution which defines the weighting function w(r).Unfortunately there are no theoretical predictions based on statistical-mechanical principles, and it has been necessary to choose three simple models based on empirical distribution functions to study the effects on the SANS pattern. The mathematical basis of the three models is given in the Appendix for symmetric, triangular and concave shapes of the radius distribution function, p(r). An index of polydispersity, A, is also introduced which is linearly related to o/f where o is the spread and F is the mean for the distribution. The effect of polydispersity on the shape of the intensity pattern is illustrated in fig. 5 for the triangular distribution [eqn (6)]. As the index of polydispersity, A7 is increased, the minimum in I(Q) for the monodisperse system [curve (a)] becomes lessB.H. ROBINSON, C. TOPRAKCIOGLU, J. C. DORE AND P. CHIEUX 21 1 .oo 0.75 0.50 0.2 5 0.10 0.15 0.20 0.05 0.10 0.15 0.20 QIA Fig. 5. Effect of increasing polydispersity on the SANS intensity pattern using a triangular distribution (model 2). Curves are shown for fixed P and for different values of I : (a) 0, (b) 0.2, (c) 0.4 and ( d ) 0.6. The region of the minimum is shown on a larger scale. pronounced [curve (b)] and is eventually smoothed to give a monotonically decreasing curve [curves (c) and (41. There is also a change in the value of I(O), which eventually increases with il. It is immediately apparent that the experimental data are more closely related in general shape to curves (c) and (d) than to curves (a) and (b).The overall behaviour is similar for the other two models [eqn (5) and (7)] and this suggests that the level of polydispersity in AOT microemulsions is much larger than is normally assumed. EVALUATION OF POLYDISPERSITY MODELS Although the three models of polydispersity have been introduced in an empirical way in the absence of any detailed theoretical formulation, it is convenient to compare the results directly with the experimental data in order to investigate the most likely shape of the distribution function, p(r). This has been carried out by means of a x2 fit for the data on water/AOT/decane at R = 20, using il as an adjustable parameter. As expected, there is a substantial improvement in the fit as shown by the x2((a) dependence given in fig. 6.The results are found to be strongly model-dependent and the best fit is obtained with the concave parabolic distribution. In all cases the polydispersity index is large and it is therefore clear that there are substantial departures from the monodisperse condition (13. = 0). The results are summarised in table 2 and the improvement to the fit is shown in fig. 7. 2-222 10 x / \ \ , 0.2 0.L 0.6 0.8 1.0 h Fig. 6. Variation of x2 for water/AOT/decane (R = 20) using different models of polydispersity (see Appendix for details). The arrows indicate the optimum values of ,I corresponding to the fits shown in fig. 7. Table 2. Parameter values obtained from the fit to Iobs(Q) for D,O/AOT/decane" 0 monodisperse [eqn (l)] 0 123 - symmetric (eqn (5)] 0.2 66 31 31 3 0.09 triangular [eqn (6)] 0.8 15 25 19 9.6 0.50 concave [eqn (7)] 0.6 9 32 22 7.5 0.34 - - a [AOT] = 0.1 mol dm-3, R = 20, using different models for polydispersity where rm = (rl + r2)/2.DISCUSSION The present SANS study of AOT-stabilised microemulsions suggests that the structural composition of the droplets is not as simple as had been previously thought. It seems clear that polydispersity is strongly linked to the dynamics of droplet collisions. This implies a continuous process in which two droplets collide and formB. H. ROBINSON, C. TOPRAKCIOGLU, J. C. DORE AND P. CHIEUX 23 0.75 0.50 0.2 5 0.05 0.10 0.15 0.05 0.10 0.15 0.20 Q/A Fig. 7. Experimental data and optimised fits corresponding to different models of polydispersity for water/AOT/decane ( R = 20); [AOg = 0.1 mol dm-s: (a) monodisperse, R = 0, (6) sym- metric (model l), il = 0.2, (c) triangular (model 2), 1 = 0.8, ( d ) concave (model 3), il = 0.6.an aggregate which subsequently disintegrates. l1 Reaction kinetics can give a valuable insight into these processe~.~*~ l5 There is some evidence4? l1, l6 to suggest that for a given value of R, a certain fraction of the surfactant molecules reside in the oil and continuous phase in dynamic equilibrium with those associated with the interface. Since droplet collisions leading to aggregation are expected to result in the expulsion of some surfactant molecules from the interface in view of the reduced surface-to-volume ratio of the aggregate, surfactant partitioning may be linked to po1ydispersity.l' Calculations based on the droplet-size distribution models considered here show that the total interfacial area of a microemulsion system is a decreasing function of polydispersity. Several lines of further investigation will be required to elaborate the findings of the present measurements.For low-R values the AOT/heptane system appears to follow24 SANS FOR AOT MICROEMULSIONS the idealised behaviour reasonably well, and this can be readily understood by reference to the photon correlation spectroscopy results for AOT/iso-octane obtained by Zulauf and Eicke.2 The deviations from ideal behaviour are most pronounced for systems close to the phase-transition region. From fig. 1 it is clear that at R = 20 and T = 293 K the higher-temperature phase boundary is progressively approached as the solvent hydrocarbon chain length is increased in the order heptane, decane, dodecane. The decane system can be analysed by the polydispersity approach discussed in this paper, but in the dodecane system [fig.3(a)] critical phenomenal7-lg are already becoming apparent in the low-Q region. In this connection it is also of interest to extend the study to R values > 40. As the droplets increase in size it becomes necessary to cover a much lower Q-range, typically down to 5 x A-l, which is accessible using the D11 instrument at ILL. Another method of approaching the transition region is by temperature variation. It is already known that there are small droplets present even when the system is visibly turbid,20 but it is not known for how long these persist.In addition, the investigation of structural properties over lengthy time periods to check for possible changes is now an important aspect of both the stability and polydispersity features. Unfortunately the beam schedules for neutron experiments do not make these time-dependent measurements very easy, but it would now seem desirable to extend these studies, particularly for conditions close to the transition. Another aspect of the neutron-scattering technique has not yet been utilised in the work presented here. This concerns the ability to vary the contrast profile H/D substitution in both the water and solvent media. In addition to providing information on the overall droplet size, including the surfactant thickness, it can be shown that the shape of the SANS profile is very sensitive to polydispersity effects as well as shape variation and solvent penetration into the surfactant coat.The neutron measurements are therefore capable of providing a more detailed picture than has been achieved in this initial work. It may also be possible to refine the method of fitting the data. It is feasible that more complex forms for the distribution could give a better fit, and there is some indication that a bimodal distribution may be more representative for the system. Other studies4 also support this viewpoint, but our present results do not justify such a treatment. APPENDIX: MODELS FOR POLYDISPERSITY In order to investigate the effects of polydispersity it is convenient to define a radius dis- tribution function, p(r), which will determine the weighting function w(r) of eqn (4).The form of this function is not known a priori and therefore several simple relationships may be chosen to illustrate the sensitivity of lobs(Q) to the parameters defining the distribution. The integral runs over all possible values of r (i.e. 0-co) but in practice there must be a range which is defined by the physical constraints of the system. In order to provide a mathematically convenient formalism which is correctly normalised, it is useful to consider models with a well defined range of r values from rl to r2. Three models have been chosen for p(r) where p(r) dr is the probability of finding a droplet of radius between r and r+dr. The peak probability is represented by po and the mean value is with a variance, 0, given by where the integrals are evaluated between the limits rl and r2.B.H. ROBINSON, C. TOPRAKCIOGLU, J. C. DORE AND P. CHIEUX 25 It is also useful to introduce a dimensionless parameter A as an index of polydispersity. It is defined as a ratio of the spread to the mid-point of the range, i.e. r2-r1 A=- r2 + rl and is linearly related to the variance for each model distribution. ( a ) Model 1 : symmetric j p " r - (r - r2) , r , < r < r , otherwise. This has the form of an inverted parabola with a mean value of P k - 1 = (r-r2I2 and a variance such that (b) Model 2 : triangular ( 5 ) I 0 otherwise. This has a linear distribution with a maximum at the lowest value of the range. The mean value and variance are - r2+2r1 r = - 3 (c) Model 3: concave I 0 otherwise.This gives a more strongly peaked distribution at lower r values in the range. It is therefore asymmetric with a parabolic shape. The mean value and variance are - r2+3r, r=------- 4 0 = (r2 -rl). All the curves are modelled as simple power-law relationships and are schematically illustrated in fig. 8. Since the droplet volume is also of interest the corresponding distribution, A V ) , is shown where p(r)dr =f(V)dV and A V ) = -. 4nr226 SANS FOR AOT MICROEMULSIONS m o d e l 1 I m o d e l 2 30 60 X f IX 1: I -j( I 30 50 i "!$& i- Fig. 8. Shapes of the p ( r ) andfTV) distribution functions for the three models of polydispersity (see text for details). The total volume of water in the system may be expressed as 7 2 V, = n jn r2p(r) dr 7-1 where n is the number of droplets.This provides a relationship between po, V, and n for each model and shows that p o may be treated as a scaling parameter. For a fixed volume of water, V,, the number of droplets will be dependent on the shape of the p(r) distribution function. The observed SANS pattern [eqn (4)] can therefore be expressed as [l ~ ( r ) W , r ) dr W ) d r L s ( Q ) = V, ,.,.* rl where V(r) = %nr3. For a case where the total water volume is V,. Strictly the volume V, appropriate to the present experimental studies should also include a contribution from the surfactant head-groups but this is a minor correction in the present treatment. Each model therefore defines a function which represents the SANS profile and is dependent on the chosen range given by the parameters rl and rz.A value for A can readily be obtained by comparing the intensity pattern for the model distribution with the observed data, Zobs(Q). The results are presented in the main text.B. H. ROBINSON, C. TOPRAKCIOGLU, J. C. DORE AND P. CHIEUX 27 The work was carried out at A.E.R.E., Harwell and Institut Laue-Langevin, Grenoble. We thank Vic Rainey (A.E.R.E.) for assistance during the experiments and the Neutron Beam Research Committee of the S.E.R.C. for financial support. Thanks are also due to Mr A. M. Howe for fig. 1. T. Assih, P. DeLord and F. C. Larche, in Biological and Technological Relevance of Reverse Micelles and other Amphiphilie Structures in Apolur Media, ed. P. L. Luisi (Plenum Press, New York, 1983, to be published). M. Zulauf and H-F. Eicke, J. Phys. Chem., 1979, 83, 480. R. A. Day, B. H. Robinson, J. H. R. Clarke and J. V. Doherty, J. Chem. Soc., Faraday Trans. I, 1979, 75, 132. E. Gulari, B. Bedwell and A. Alkafaji, J. Colloid Interface Sci., 1980, 77, 202. C. Cabos and P. DeLord, J. Appl. Crystallogr., 1979, 12, 502. C. Cabos and P. DeLord, J. Phys. Lett., 1980,41, 455. B. Jacrot, Rep. Prog. Phys., 1976, 39, 91 1. D. J. Cebula, R. H. Ottewill, J. Ralston and P. Pusey, J. Chem. Soc., Furaday Trans. I, 1981,77,2585. J. C. Brown, P. N. Pusey, J. W. Goodwin and R. H. Ottewill, J. Phys. A, 1975,8, 66. lo A. Vrij, E. A. Nieuwenhuis, H. M. Fighaut and W. G. M. Agterof, Furaday Discuss. Chem. Soc., 1978, 65, 101. l 1 P. D. I. Fletcher, A. M. Howe, N. M. Perrins, B. H. Robinson, C. Toprakcioglu and J. C. Dore, Proc. 3rd Int. Symp. Surfactants in Solution, ed. K. Mittal, (Plenum Press, New York, 1983). l2 A. M. Howe, Ph.D. Thesis (University of Kent at Canterbury, 1983). l3 D. G. Oakenfull, J. Chem. Soc., Furuduy Trans. 1, 1980, 76, 1875. l4 P. D. I. Fletcher and B. H. Robinson, Ber. Bunsenges. Phys. Chem., 1981, 85, 867. l5 S. S. Atik and J. K. Thomas, Chem. Phys. Lett., 1981, 79, 351. l6 T. Assih, F. Larche and P. DeLord, J. Colloid Interface Sci., 1982, 89, 35. l7 R. Triolo, L. J. Magid, J. S. Johnson Jr and H. R. Child, J. Phys. Chem., 1982,86, 3689. l9 M. W. Kim and J. S. Huang, Phys. Rev. B, 1982, 26, 2703. 2o J. C. Dore, B. H. Robinson and C. Toprakcioglu, unpublished results. J. S. Huang and M. W. Kim, Phys. Rev. Lett., 1981, 47, 1462. (PAPER 2/1841)

ISSN:0300-9599    

DOI:10.1039/F19848000013

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J. Chem. Soc., Faraday Trans. I, 1984, 80, 29-35 Fourier-transform Infrared Spectroscopic Study of Adsorption and Decomposition of Ammonia over Magnesium Oxide BY SETSUKO KAGAMI, TAKAHARU ON IS HI*^ AND KENZI TAMARU Department of Chemistry, Faculty of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan Received 24th January, 1983 The adsorption and decomposition of ammonia over magnesium oxide have been studied by means of infrared spectroscopy. The decomposition of ammonia proceeds at 573 K to form N, and H,. The following mechanism of decomposition of ammonia over MgO has been elucidated : (1) (2) (3) at room temperature 1 NH,(g)=NH,(a) NH,(a)eNH,(a)+H(a) NH,(a)+NH(a)+H(a) at 473 K NH(a) s N(a) + H(a) ') (4) Many studies have been carried out on the synthesis and decomposition of ammonia over supported transition metals by means of infrared spectroscopic methods.Nakata and Matsushita found the NH, adsorbed species on the iron-supported silica catalyst . I Pozdnyakov and Filimonov2 investigated the adsorption of ammonia on metal catalysts such as Fe, Ni, Pt, Pd and Ru supported on SiO, or MgO and reported that ammonia was adsorbed on metal surfaces at least in two forms; one of the adsorbed forms is coordinated ammonia and the other is dissociatively chemisorbed. Brill et aZ.3 studied ammonia synthesis from nitrogen and hydrogen over an iron cata- lyst supported on magnesia and found that the chemisorbed species on the Fe catalyst has a hydrazine-like structure. Okawa et aL4 reported that after catalytic decomposition of ammonia on iron catalysts supported on magnesia, two infrared bands were observed at 2220 and 2050 cm-l; these were both assigned to the N-N stretching vibration of molecularly adsorbed nitrogen.However, they did not study the chemisorption and decomposition of ammonia on the MgO carrier in detail. The adsorption of ammonia on various oxides such as al~mina,~ silica-alumina,8 zinc oxideg and magnesium oxidelo* l1 has been studied by infrared spectroscopy in connection with Lewis- and Brarnsted-acid sites. However, no studies were carried out on the decomposition of ammonia over these oxides. Nagatsuta, Yokohama 227, Japan. t Present address: Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 2930 I.R. STUDY OF AMMONIA DECOMPOSITION ON MgO In this paper the adsorption and decomposition of ammonia over magnesium oxide studied by means of infrared spectroscopy are reported.In this study nitrogen species adsorbed on the oxide were observed for the first time. EXPERIMENTAL The high-purity MgO used in this experiment was obtained from Merck. The major impurities were Na (ca. 2.079, Ca (ca. 0.0279, carbonate (ca. 1.5%) and Fe (ca. 0.005%), and its surface area was 89 m2 g-l. Ca. 40 mg of MgO was pressed into a self-supporting disc 20 mm in diameter. The sample was placed in an i.r. cell which was attached to a closed gas-circulation system as described previously.'* The oxidized catalyst was obtained by evacuating MgO at 773 K for 1 h and subsequently oxidized under 13 kPa of oxygen for 5 h in the cell; the reduced catalyst was prepared by reducing the oxidized sample under 13 kPa of hydrogen at 773 K for 1 h.15N-substituted ammonia (96 at. % purity) used was obtained from M.S.D. Canada Ltd. These gases were used without purification. Infrared spectra were recorded with a JEOL JIR 10 Fourier-transform infrared spectrometer using a liquid-nitrogen-cooled HgCdTe detector. Infrared spectra of the adsorbed species were recorded at room temperature in the region 3600-800 cm-l, after gaseous ammonia had been removed at the reaction temperature by a liquid-nitrogen trap. 1.r. spectra were usually obtained with 256 scans and the spectral resolution was 2 cm-l. Ratio-recorded spectra were obtained by first measuring the background produced by MgO stored as a reference and then the adsorbed gas samples.3600 3200 2800 2400 2000 1600 1200 800 wavenum berlcrn-' Fig. 1. Fourier-transform infrared spectra of ammonia adsorbed on MgO. (a) Ammonia (1.6 kPa) was introduced at 298 K for 20 min and the gas phase was removed by a liquid-nitrogen trap; (b) after heating for 1 h at 473 K in gaseous ammonia; (c) at 573 K for 1 h; (d) at 673 K for 1 h; (e) at 773 K for 1 h.S. KAGAMI, T. ONISHI AND K. TAMARU 31 RESULTS AND DISCUSSION Fourier-transform infrared spectra of species adsorbed over reduced MgO were obtained after the adsorption and decomposition of ammonia (and deuterated ammonia) at various temperatures and are shown in fig. 1 and 2. At room temperature characteristic absorption bands were observed at 3370 (2610), 3230 (2510), 1600 (I 170) and 1160 (875) cm-l as shown in fig.1 (a) and 2(a). 3600 3200 2800 2LOO 2000 1600 1200 800 wavenum ber/cm -' Fig. 2. Fourier-transform infrared spectra of deuterated ammonia adsorbed on MgO. (a) ND, was introduced at room temperature for 20min and the gas phase was removed by a liquid-nitrogen trap; (b) after heating for 1 h at 473 K in gaseous ND,; (c) at 573 K for 1 h; ( d ) at 673 K for 1 h; (e) at 773 K for 1 h. The assignments of these bands are given in table 1 together with related data. As shown in the table, most ammonia was molecularly adsorbed over MgO at room temperature. The i.r. spectra of ammonia adsorbed over oxidized MgO at room temperature are shown in fig. 3. In this case characteristic absorption bands were observed at 3310 and 1550 cm-l in addition to absorption bands of the species NH,(a).The intensity of the band at 3750 cm-l due to an OH stretching vibration increased as the temperature was raised. The 33 10 and 1550 cm-l bands can be assigned to the NH,(a) species on the basis of the data given in table 2. It may be concluded that over oxidized MgO a small amount of ammonia is dissociatively adsorbed to produce NH,(a) even at room temperature : NH, -+ NH,(a) (at room temperature) (1) NH, + NH,(a) + H(a) (partial). (2)32 I.R. STUDY OF AMMONIA DECOMPOSITION ON MgO Table 1. Comparison of vibrational frequencies (cm-l) of adsorbed NH, (ND,) NH,(ND,)/ dehydrated NH,/ NH,/ NH,/ assignment MgOa NH3(s)13 MgO'O A120,, Zn09 Fe-Mg02 Pt"'l4* vd(NH3) 3370 3378 v, (NH,) 3230 3223 sd(NH3) 1600 1646 6, (NH,) 1160 1060 (2610) (2510) (1 170) (875) 3340 3400 3320 3270 3340 (2500) 3280 3335 3270 3200 3240 3210 (2350) 1603 1620 - 1610 1600 (1180) - 1140 1140 1110 - (930) a This work: over the reduced surface at room temperature; * at low coverage (8 < 0.4).A 3600 3200 2800 2LOO 2000 1600 1200 800 wavenum ber/cm -* Fig. 3. Fourier-transform infrared spectra of ammonia adsorbed on oxidized MgO. (a) Ammonia was introduced at room temperature for 20 min and the gas phase was removed by a liquid-nitrogen cold trap; (b) after heating for 40 min at 473 K in gaseous ammonia. Table 2. Comparison of vibrational frequencies (cm-l) of amido compounds ~~~ ~~ assignment NH3/MgOa Hg(NH2)+Cl-l5 NH,/A1,0,5 Co complex(I)lsv Co complex(II)16* v[NH,(a)] 3370 - 3386 3200 - v[NH,(s)] 3310 - 3335 - - W H , 1 Pw (NH2 1 Pt (NH,) Pr (NH, 1 1550 1534 1510 1560 1560 1110 1022 - 1049 1068 - - - - - - (978) - 668 - a This work: over the oxidized surface at room temperature; [Co(NH,) (NX3)lo] (NO,),; K O , (NH,) (NH, )8 CW2O)I c1,.S. KAGAMI, T.ONISHI AND K. TAMARU 33 Over the reduced catalyst these bands were weak and appeared as a shoulder on the NH,(a) band. When the adsorbed ammonia was heated to 473 K the absorption bands gradually decreased and a band at 1410 cm-1 appeared which was tentatively assigned to the deformation vibration of NH(a) on the basis of reference spectra of imino metal complexes, as shown in table 3: NH,(a) -+ NH(a) + H(a) (at 473 K). Table 3. Comparison of vibrational frequencies (cm-l) of imido compounds (3) assignment NH3/MgOa 0 s [Fe(NO), NH],18 V(NH) 3370 2994 3376,3304 W H ) 1410 1414 1458 a This work at 473 K; [Os,(NH)Cl,biPY,] (ClO,).At 573 K two sharp bands at 2220 and 2195 cm-l and a weak broad band at 2080 cm-l appeared; at 673 K these three bands became strong in intensity, as shown in fig. 1 ( c ) and (d) and 2(c) and (d). At 773 K the former two bands became weaker and the 2080 cm-l band became stronger. When gaseous ammonia was introduced to this surface at room temperature, the former two peaks were reduced in intensity while the 2080 cm-l peak was unchanged. Three bands at 2220,2195 and 2080 cm-l were observed at the same position when ND, was used instead of NH,. When the decomposition of 15NH, proceeded at 773 K, these bands shifted to the lower-frequency region, as shown in table 4.Table 4. Isotope shift of adsorbed N, species after ammonia decomposition at 773 K on reduced MgO gases used wavenumber/cm-l NH3 2220 2 195 2080 ND3 2220 2 195 2080 2200 2 1 d0 2060 15NH3 The surface preadsorbed species was obtained by decomposition of 14NH, at 773 K; after the gas phase was evacuated, 15NH, was introduced into the system at the same temperature for 30min. The absorption bands of adsorbed species due to 14NH, disappeared and those due to 15NH, appeared. Infrared spectra of dinitrogen adsorbed on metal and oxide catalysts have been studied and it was found that i.r. bands due to adsorbed dinitrogen species appear in the region 1900-2300 cm-I.l9 The three bands observed are related to dinitrogen species adsorbed on the magnesium oxide.However, the isotope shifts of these bands are smaller than the calculated values. In table 5 i.r. data for the isotope shift of dinitrogen adsorbed on various catalysts are shown. In the case of dinitrogen species adsorbed on a K-Al,O, catalyst the isotope shift is also too while the same species adsorbed on a34 I.R. STUDY OF AMMONIA DECOMPOSITION ON MgO Table 5. Infrared data of dinitrogen species adsorbed on various catalysts wavenumber/cm-' Ni/Si021g 2195 2123 K-A120,20 2030 2010 2220 2200 (present work) 2195 2180 2080 2060 MgO Ni/SiO, catalyst was observed at the expected wavenumber.20 The reason for the small isotope shift is not clear at present. The former two sharp bands at 2220 and 2195 cm-1 can be assigned tentatively to the N E N stretching vibrations of dinitrogen species weakly adsorbed on MgO.The broad and stable band at 2080 cm-' was due to dinitrogen species adsorbed strongly on a magnesium metal site. The decomposition of ammonia over MgO proceeded slowly at 573 K to form nitrogen and hydrogen and more rapidly at high temperature: NH(a) -+ N(a) + H(a) ) (4) l (at 573-773 K). These nitrogen adsorbates disappeared readily when hydrogen was introduced at 773 K and ammonia was observed in the gas phase. When hydrogen was introduced onto the preadsorbed nitrogen [fig. 1 (e)] absorption bands at 1600 and 1160 cm-l corresponding to N-H stretching vibrations appeared again. These results show that the decomposition and hydrogenation processes are reversible in this system.However, we found that no ammonia was formed when a mixture of hydrogen and nitrogen was introduced over the reduced catalyst at 773 K. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 T. Nakata and S. Matsushita, J. Phys. Chem., 1968, 72, 458. D. V. Pozdnyakov and V. N. Filimonov, Kinet. Katal., 1972, 13, 522. R. Brill, P. Jiru and G. Schulz, 2. Phys. Chem. (NF), 1969,64, 215. T. Okawa, T. Onishi and K. Tamaru, 2. Phys. Chem. (NF), 1977, 107, 239. J. B. Peri, J. Phys. Chem., 1965, 69, 231. M. J. D. Low, N. Ramasubramanian and V. V. Subba Rao, J. Phys. Chem., 1967,71,467. B. A. Morrow, I. A. Cody and L. S. M. Lee, J . Phys. Chem., 1975, 79, 2405. M. R. Basila and T. R. Kantner, J. Phys. Chem., 1967, 71,467. T. Morimoto, H. Yanai and M. Nagao, J. Phys. Chem., 1976, 80, 471. A. J. Tench and D. Giles, J. Chem. Soc., Faraday Trans. I , 1972,68, 193. A. J. Tench, J. Chem. SOC., Faraday Trans. I , 1972,68, 197. K. Tamaru and T. Onishi, Appl. Spectrosc. Rev., 1975, 9, 133. K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds (Wiley, New York, 1978), p. 122. B. A. Sexton and G. E. Mitchell, Surf. Sci., 1980, 99, 523. I. Nakagawa, R. R. Penland, S. Mizushima, T. J. Lane and J. V. Runaglinno, Spectrochim. Acta, 1957, 9, 199.S. KAGAMI, T. ONISHI AND K. TAMARU 35 l6 D. J. Hewkin and W. P. Griffith, J. Chem. Soc., 1966,472. l7 N. Hieber and H. Beutner, 2. Anorg. Allg. Chem., 1962, 317, 63. l8 E. G. Brame Jr, J. L. Margrave and V. W. Meloche, J. Inorg. Nucl. Chem., 1957, 5, 48. l9 Catalysis - Science and Technology, ed. J. R. Anderson and M. Boudart (Springer Verlag, Berlin, 'O R. P. Eischens and J. Jacknow, Proc. 3rd Int. Congr. Catal. (North Holland, Amsterdam, 1965), 1981), vol. I, p. 102. p. 627. K. Aika, H. Midonkawa and A. Ozaki, J. Phys. Chem., 1982,86, 3263. (PAPER 3/102)

ISSN:0300-9599    

DOI:10.1039/F19848000029

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I , 1984,80, 37-45 Profile and Contact Angle of Small Sessile Drops A More General Approximate Solution BY MARTIN E. R. SHANAHAN Centre de Recherches sur la Physico-chimie des Surfaces Solides, 24 avenue du President Kennedy, 68200 Mulhouse, France and Laboratoire de Recherches sur la Physico-chimie des Interfaces de 1'Ecole Nationale Superieure de Chimie de Mulhouse, 3 rue Alfred Werner, 68093 Mulhouse-Cedex, France Received 9th February, 1983 The second-order differential equation describing the profile of a sessile drop has been derived in polar coordinates using the criterion of minimum free energy and applying the calculus of variations. Application of a form of perturbation theory leads to an approximate solution valid for drops of sufficiently small maximum diameter.In the case of drops of contact angle > 90°, this solution can be exploited directly to obtain the contact angle from a knowledge of drop height, maximum diameter and diameter at the plane of contact with the solid. If this last datum is lacking, the contact angle can still be obtained by a reiterative method or graphically. For contact angles c 90" this last procedure must be used and thus little advantage is gained over a solution previously obtained in cartesian coordinates. Although the solution is less accurate than data obtained from numerical integration, its relative simplicity should prove useful for the objective determination of contact angles. The well known analytically insoluble second-order differential equation describing the profile of an axisymmetric drop (or related meniscus) can be derived either from the Laplace equation1 for the equilibrium of forces at a liquid/fluid interface [e.g.ref. (2)-(7)] or by exploiting the calculus of variations to minimise the free energy of the system in question [e.g. ref. (8)-(lo)]. The most frequently employed methods of approximate solution of this equation involve numerical integration by computer, and some very accurate data have been obtained and t a b ~ l a t e d . ~ ? ~ Another approach for obtaining approximate solutions is to employ perturbation theory. This is generally less accurate (except for small drops) than the numerical techniques but has the advantage that continuous functions are obtained to describe drop profiles rather than tabulated point values.The first reference to the use of perturbation theory in this context that the author has encountered is that of Ehrlich,ll in which contact angles Bo > 90' can be evaluated from sessile drops from a knowledge of maximum drop diameter and diameter at the plane of contact with the solid. Chesters12 employed perturbation theory to obtain the approximate profile of a pendant (and by extension, sessile) drop. Two solutions were found necessary, one describing drop profile in the region near the maximum diameter and the other representing (most of) the rest of the profile. In recent work by the author13 an approximate solution was obtained by using a novel form of perturbation theory originally suggested by Roth.l* This solution bears a marked resemblance to one of Chesters' equations,12 although both its derivation and method of application are quite different.The resulting equation is useful in the evaluation of contact-angle data, but, being based on a cartesian- coordinate system, is only valid for sessile (or pendant) drops of 8, < 90' in the form 3738 PROFILE AND CONTACT ANGLE OF SMALL SESSILE DROPS presented. A solution for drops of 8, >/ 90° can be obtained but is rather too involved in its application to be of any practical use.15 The present study involves the application of the same basic perturbation method as that of ref. (13), but by using a polar-coordinate system drops of 9, > 90" may be treated. Although the resulting equations to be solved are slightly more complicated than in the previous study, the final approximate solution for the drop profile is in fact simpler.With a knowledge of drop height, maximum diameter and diameter at the plane of contact with the solid, the solution can easily be used to obtain 8,. Even if the contact diameter is not available, the contact angle can be evaluated by a reiterative met hod. THEORY The usual implicit assumptions in axisymmetric meniscus calculations are made here (solid and fluids homogeneous energetically, immiscibility of phases etc.). Consider fig. 1, which represents an axisymmetric drop of liquid 1 resting on a solid surface S of area A in the presence of a less dense fluid phase 2. The contact angle, 8,, is assumed > 90°, and the free energy of the system stems from the three free S Fig. 1.Liquid drop (phase 1) lying on flat solid surface (phase S) in presence of fluid (phase 2) and coordinate system adopted. interfacial energies, yI2, ysl and ysz, and the potential, gravitational energy; the latter is calculated in the present case, for the sake of convenience, with respect to the height of the origin, 0, which corresponds to the height of the maximum drop diameter. (In the previous paper13 the gravitational energy was evaluated with respect to the solid surface, but in fact this difference is of no consequence, introducing only an additive constant which disappears later in the calculation.) A polar-coordinate system [r(8), 81 is applied to the drop as shown. (Unfortunately, convention has it that 8 is used both for the polar angle and the contact angle, but confusion should not arise since in the present context 8, is adopted as the symbol for contact angle.)M.E. R. SHANAHAN The free energy of the drop to be minimised for equilibrium is given by 39 = loec Ee d8 + constant where r =f(8) is the function describing drop profile (0 < 8 < O,), re = dr/dO, p is the density difference of the fluids (pl -p2) and g is gravitational acceleration. The volume of the drop is a constant, V : nr3(8~) sin2 8, cos e, V = -jOecr3 sin Ode-- 2n 3 3 = s,” VedO + constant. (2) The Euler equation from the calculus of variations which constitutes a necessary condition for E to be stationary with the given volume constraint is16* where A is a Lagrange multiplier. Application of eqn (3) to eqn (1) and (2) leads to Ar2 Y12 sin e+c9 sin ocos o+- sin 8 (4) where c = p g / ~ , , .~ , 1 3 9 la* l9 Eqn (4) is an analytically insoluble second-order differential equation whose solution describes the profile of a sessile drop for a given value of c and a certain fixed, although undetermined, value of A. A perturbation method will be used to solve eqn (4) considering that c is sufficiently small. Radius r then becomes a function of both 8 and C; r(8, c). We consider eqn (4) from a purely mathematical standpoint for the moment and wish (hypothetically) to vary c from zero to its ‘real’ value. For each value of c in this range, we can choose a value of A such that we have a solution r(8, c) of eqn (4) satisfying the imposed boundary conditions r ( x / 2 , c ) = R and (Ck/aO)l(n,2, c ) = 0.These boundary conditions are thus compatible with eqn (4) provided that A is a function of c. The constant R represents the radius of the unperturbed circular profile as shown by the dotted line in fig. 1. Consideration of eqn (4) for the unperturbed case when c = 0 leads to the relation r(8, 0) = R = -2y12/A(0). ( 5 ) The significance of imposing the above boundary conditions is to ensure that the family of solutions to be considered, of which our final result will be a member, all pass through the point on the profile corresponding to the maximum horizontal40 PROFILE AND CONTACT ANGLE OF SMALL SESSILE DROPS diameter of the unperturbed drop (point P in fig. 1) and have a vertical tangent there. (Other boundary conditions could have been chosen of course, but as shown below those adopted lead to a tractable final solution.) Having established the desired boundary conditions, we assume that the profile of the drop, when c is non-zero, may be expressed as a Maclaurin expansion.Thus Terms O(c2) will be ignored in this first-order perturbation approximation. We define z(e) = r/ ac ( e , o ) whence Differentiation of eqn (4) with respect to c followed by evaluation at c = 0 (where r = R and re = 0) and substitution of the terms z and zo leads to "(. sin 8) = R3 sin 8 cos 8-22 sin d8 has been made of the fact that A = -2y,,/R when c = 0 [eqn (5)]. can be rewritten zeo+ze cot 8+22 = R3 cos 8+-- (7) where 208 = d2z/d02. the complementary function is thus readily found [AP,(cos 8) + BQ,(cos O)].With the right-hand side of eqn (8) equal to zero, Legendre's equation results and By making the substitutions x = cos 8 and eqn (8) can be solved by the method of variation of parameters20 and the particular integral obtained is - R3 z = -- - - cos 8 In (1 - C O S ~ 8). (9) The final general solution of eqn (8) is then (10) where the three constants A , B and (R2/2y12) (dA/dc)(,-, remain to be found. Since z(0) must be finite, terms in In (1 - cos 8) must be removed. This leads to B = - R3/3. The principal term in eqn (6), i.e. r(8,0), is equal to R. Bearing in mind the boundary conditions imposed above, we can thus deduce that both z(n/2) and ~e(n/2) are zero. These conditions imply respectively that both the total additive constant and A in eqn (10) are zero.The final first-order perturbation term is thus R3 3 z(8) = -- cos 8 In (1 +cos 8)for 0 d 8 -= Z, approximation M. E. R. SHANAHAN and the radius, r, of the sessile drop is thus by 41 given to a first provided the drop is sufficiently small. Note that in the above solution the unperturbed circular profile of radius R shown in fig. 1 does not correspond to the same physical drop where c is zero, since both volume and contact angle are incorrect. This profile is only an artefact leading to the obtention of eqn (12). [A similar procedure was used in ref. (1 3).] We now consider an approximate upper limit to the validity of eqn (12). At 8 = 0, we have This corresponds to the height of the liquid drop above 0. Now this height should increase with drop size such that a maximum drop height is attained.According to Padday and Pitt,21 the maximum (overall) drop height is attained at a value of the drop parameter B( = pgb2/y12, where b is the radius of curvature at the drop apex) between ca. lo3 and lo4 for drops possessing contact angles in the range 90-180°. Only after this maximum does a continuous increase in drop volume lead to a slow reduction in drop height reaching an asymptotic value towards values of /3 of ca. 10l2. However, it is clear from eqn (13) that r(O,pg/y,,) starts to decrease for drops of R > (y,,/pg In 2);. Now the radius of curvature, b, in the polar coordinate system Using eqn (12) and (13) and the limiting value, R = (y12/pg In 2)4 in eqn (14), it is easy to show that r(O,pg/yl2) starts to decrease at a value of of ca.33, well below the values obtained by Padday and Pitt. It is therefore reasonable to consider the perturbation solution to be valid for the range of maximum drop radii of Conslder fig. 2, which represents schematically the contact area of the drop on the solid surface. In the limit that the increment of angle, 68, tends to zero the contact angle of the drop, e,, is given by R 5 (Yl?/Pg In 214. e, = e,+v where 0 Fig. 2. Schematic representation of contact area of drop.42 PROFILE AND CONTACT ANGLE OF SMALL SESSILE DROPS From eqn (1 2) we have pgR2 sin 8,[( 1 +cos 8,) In (1 + cos 8,) + cos 8,] (1 +cos 8,) [3y12-pgR2 cos 8, In (1 +COS 6,)J Thus combining eqn (1 5)-( 17) leads to the final expression for contact angle, 8,. the shaded cone, K, at the base of the drop is given by The volume of the drop, V , can be calculated as follows.In fig. 1 the volume of cos 8, In (1 + cos 8,) --" ( 8, -nR3 3 3 sin2 8, cos g c z ____ K = The volume of the rest of the drop, &, is & = $Cc r3 sin 8 d 8 x 2"3R3 - Ioe' (1 -? cos 8 In (1 +cos 0) However, the perturbation approach used to obtain eqn (1 2) ignored terms of O(c2) and thus the volume, V = V = - 2 + V2, should be written as [l - cR2 cos 8 In (1 +cos O)] sin 8d8 - sin2 8, cos 8,[ 1 - cR2 cos 8, In (1 +COS O,)] + O(c2). (20) 1 nR3( 3 Joec Reintroducing the notation c = pg/y12 and introducing q = (1 +cos O,), we have (21) nR3 3 V = - (q - 2)2 [q + 1 - cR2(q2 In q + $)I + O(c2). APPLICATION OF THE THEORY The application of the above theory is basically straightforward and it can be seen that for a sessile drop of 8, > 90" the profile can be immediately assessed from a knowledge of p, g, y12 and the maximum radius, R.Although the method could be applied to drops of 8, < 90°, a reiterative or graphical technique would need to be employed in order to obtain the parameter R (which would no longer have any physical meaning). Thus for 8, < 90" the method presents little advantage over the previously developed theory in Cartesian coordinates. l3 However, assuming that 8, > 90°, the method can be used successfully to obtain contact-angle data in an objective manner. Let us first assume that we are in possession of the overall drop height, H, the maximum radius, R, and the radius of the circle of contact with the solid surface, R, (see fig.1). The position of the origin, 0, can be immediately obtained from eqn (13). The value of 8, can then be found since Ro tan$, = ____ r(0) - H ' With p, g, yI2, R and 0, known, eqn (1 5)-( 17) can be used directly to evaluate the contact angle, 0,. If, however, as is sometimes the case experimentally, R, cannot be easily measured, all is not lost (by contrast with Ehrlich's method1'). As before, the position of the origin, 0, can be determined from eqn (1 3 ) . From fig. 1 (23) H - ~ ( o ) = r(e,) cos (n-e,) = -r(oc) cos 8,.M. E. R. SHANAHAN 43 Therefore, using eqn (1 2) H- r(0) = pE cos2 8, In (1 + cos 6,) - R cos 6,. (24) 3Y12 Now eqn (24) can be used either reiteratively or graphically to obtain 6,. As before, the contact angle, O,, is then calculated from eqn (15)-(17).This method is clearly more tedious, but may be of practical use. RESULTS AND DISCUSSION The above theory was tested experimentally using a system known to give contact , angles > 90’. Photographs in profile were taken of drops of freshly distilled mercury on glass and these were enlarged to x 30 for analysis. Fig. 3 represents a tracing of one of the larger drops studied together with two half-profiles corresponding to the perturbation solution from this study and that from the ‘best’ Bashforth and Adams solution available in the tables.2 The circular profile corresponds to a sessile drop of Fig. 3. Drop of mercury on glass (drop 6); 0, calculated profile; 0, profile of Bashforth and Adams for p = 1.5.Circular profile represents drop of equal V and 0, in absence of gravity. the same volume and contact angle but in the absence of gravity as calculated from the well known equation where R, represents the radius (R, # R). (This circular profile is not to be confused with that of fig. 1, which does not correspond to a ‘real’ drop.) As can be seen from fig. 3, there is good agreement between the experimental profile and the perturbation solution. The agreement shown by the tabular results of Bashforth and Adams is perhaps less convincing (near the contact region), but this is hardly surprising since clearly the value of /? for this drop does not correspond exactly to the nearest value for which profile data have been published (B = 1.5). It was found experimentally that the limiting value of R for the validity of the theory, as calculated above [ s(y,,/pg In 2):], was a little optimistic. The theoretical value in the present case corresponds to ca.2.3 mm whereas in practice the perturbation technique is valid up to ca. 90% of this value. Of course, this criterion is fairly arbitrary since the judgement of acceptability of approximate solutions is a subjective matter.44 PROFILE AND CONTACT ANGLE OF SMALL SESSILE DROPS Owing to the difficulty of estimating volumes of drops of mercury from syringe measurements, another technique was employed to investigate the validity of eqn (21). Small drops of arbitrary volume were formed and individually photographed for analysis. These were then brought together to coalesce and photographs of the final relatively large drops were taken.Fig. 3 represents such a final drop, and data concerning it (drop 6) and its constitutive smaller drops (1-5) are given in table 1. Table 1. Data for mercury drops on glassa measured values contact angle drop no. H/mm R/mm R,/mm 9Jo V/mm3 B O O / " wo 1 1.010 0.627 0.400 135 0.94 0.11 141 139 2 1.103 0.700 0.493 131 1.27 0.14 137 136 3 1.430 0.965 0.705 127 3.16 0.29 137 138 4 1.677 1.130 0.890 124 4.85 0.42 136 136 5 1.950 1.435 1.157 118 9.00 0.79 134 134 - total 19.22 6 2.277 1.917 1.550 118 19.17 2.68 144 141 a yI2 = yT, = 480 mJ m-2; p = 13.55 g cme3. Using eqn (21), the volume of each drop was calculated. It can be seen that the sum of the volumes of the constitutive drops 1-5 as calculated is in good agreement with the value obtained for the final drop 6.The error is of the order of 0.05 mm3, or 0.3%. The good agreement obtained is considered evidence for the validity of eqn (21). Using the analysis of Bashforth and Adams, in which volume intervenes,2 a value of ca. 17.5 mm3 was obtained; however, since clearly the assumption of /? = 1.5 is very approximate, too much importance should not be attached to this figure. Values of /? in table 1 were calculated from eqn (12) and (14) and are probably slightly larger than the actual values. Considering again fig. 3, it can be seen that the perturbation solution tends to overestimate the value of the radius of curvature at the drop apex, b, and any such error is magnified in /? (B = pgb2/yI2). The sense of the discrepancy between the value obtained for drop 6 and the equivalent figure of Bashforth and Adams of 1.5 tends to confirm this.Comparison ofmeasured contact angles, OM, and those calculated from eqn (1 5)-( 17) also shows a good correlation between theory and experiment. The above theory provides a reliable technique for obtaining contact-angle data from axisymmetric drops of 8, > 90° as long as the drop is sufficiently small. Clearly the measurement of lengths (H, R and R,, if possible) is less subjective than the estimation of a tangent at the contact line. A point of interest is that the above does not require physical measurement of the height of the drop at its maximum diameter: the diameter itself suffices. This is useful since it is well known that precise determination of the former is often difficult experimentally.There is no theoretical reason why the above should not be applied to pendant drops simply by changing the sign of c. Although no experimental results were available, a comparison was made using the drawings of pendant-drop profiles of PaddayS3 The agreement was good from the drop apex up to a polar angle, 0, of ca. 140° (depending on the drop), but the perturbation approach broke down near the neck.M. E. R. SHANAHAN 45 CONCLUSIONS The differential equation describing the profile of an axisymmetric sessile drop has been studied in polar coordinates using a form of perturbation theory. An approximate solution has been found which is valid for drops of sufficiently small maximum radius.In the case of mercury, this radius is ca. 2 mm. From measurements of drop height, maximum diameter and diameter at the contact plane with the solid surface, the contact angle can be calculated directly. Even if the diameter at contact is unavailable, a reiterative (or graphical) method may be employed to calculate the contact angle. The method can in principle be applied to pendant drops, although the solution would seem to be inadequate near the drop neck. I this 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 thank one of the referees for useful constructive criticism of the first version of paper. P. S. Laplace, Mkcanique Celeste, Suppl. au X Iivre (Coureier, Paris, 1805). F. Bashforth and J. C. Adams, An Attempt to Test the Theory of Capillary Action (Cambridge University Press and Deighton, Bell and Co, 1892), described by J. F. Padday in Surface and Colloid Science, ed. E. Matijevid. (Wiley, London, 1969), vol. 1, chap. 2. J. F. Padday, Philos. Trans. R. Soc. London, Ser. A, 1971, 269, 265. J. F. Padday, J. Electroanal. Chem., 1972, 37, 3 13. S. Hartland and R. W. Hartley, Axisymmetric Fluih-Liquid Interfaces (Elsevier, Amsterdam, 1976). E. A. Boucher, Rep. Prog. Phys., 1980,43, 497. E. A. Boucher and T. G. J. Jones, J. Chem. Soc., Faraday Trans. I , 1981,77, 1183. E. Pitts, J. Fluid Mech., 1974, 63, 487. E. Pitts, J. Inst. Math. Its Appl., 1976, 17, 387. M. E. R. Shanahan, in Adhesion 6, ed. K. W. Allen (Applied Science Publishers, London, 1982), chap. 5. R. Ehrlich, J. Colloid Interface Sci., 1968, 28, 5 . A. K. Chesters, J. Fluid Mech., 1977, 81, 609. M. E. R. Shanahan, J. Chem. Soc., Faraday Trans. I , 1982,78, 2701. J. P. Roth, personal communication, 198 1. M. E. R. Shanahan, unpublished work. C. Ray Wylie, Advanced Engineering Mathematics (McGraw-Hill, New York, 4th edn, 1961), chap. 12. V. I. Smirnov, A Course of Higher Mathematics, transl. D. E. Brown (Pergamon Press, Oxford, 3rd edn, 1964), vol. IV, chap. 11. S. Hartland and S. Ramakrishnan, Chimia, 1975, 29, 314. S. Ramakrishnan, P. Scholten and S. Hartland, Ind. J. Pure Appl. Phys., 1976, 14, 633. E. L. Ince, Ordinary Differential Equations (Dover, New York, 1956), chap. v. J. F. Padday and A. R. Pitt, Proc. R. Soc. London, Ser. A , 1972,329,421. H. S . W. Massey and H. Kestelman, Ancillary Mathematics (Pitman, London, 2nd edn, 1964), chap. 8. (PAPER 3/208)

ISSN:0300-9599    

DOI:10.1039/F19848000037

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1984, 80,47-54 Pre-heat Treatment Study of Recoil 1281 in Lithium and Copper Iodates S. P. MISHRA,* ARCHITA PATNAIK AND D. P. WAGLEY Nuclear and Radiation Chemistry Laboratory, Department of Chemistry, Banaras Hindu University, Varanasi - 22 1005, India Received 22nd February, 1983 The effect of pre-heat treatment on the (n, y ) reaction of LiIO, and Cu(IO,), targets is an increase in the initial retention in Cu(IO,), and decrease in LiIO,. This has been ascribed to the competitive participation of oxidizing and reducing inherent crystal defects. Annealing kinetic studies reveal two apparent first-order processes for both the pre-heated and untreated iodates. An exciton mechanism is proposed to explain the higher plateau values and annealing kinetics in pre-heated Cu(IO,), and LiIO,.The formation of precursors in an irradiated matrix has not yet been established by methods in which the crystals are dissolved for analysis, since on dissolution these species may be changed considerably. However, the nature of these precursors may be investigated by adopting indirect means such as pre-neutron irradiation heat treatment and post-irradiation perturbation, e.g. thermal annealing. When samples whose thermal annealing has been interrupted in a plateau region are crushed or irradiated with a low y-dose and subsequently annealed at the same temperature, further rapid annealing occurs, thereby providing direct evidence for the effect of dislocations or radiation-produced defects on annealing. The role of inherent crystal defects has been elucidated by Jones and Warren.2* The deliberate introduction of defects of different kinds into the crystals before neu- tron activation may affect the transient reactions of the precursor(s)-forming events after the recoil and thus may influence both the initial yields and pseudo-plateau values.Various attempts have been made in the past to find the effect of pre-heat treatment on recoil stabilization in different halogenate^.^ Bearing in mind that the defects produced by a concomitant y-dose may mask5 the role played by inherent crystal defects, the present studies were carried out with a low concomitant y-dose (ca. 172 rad h-l) associated with the neutron source. The effect of pre-irradiation thermal treatment on the retention of lZ8I and subsequent thermal annealing in LiIO, and Cu (103)2 has been investigated.Such studies should provide information about the role of precursors which are stable in lattices and hence the mechanism of recoil reactions. EXPERIMENTAL 300 mg of reagent-grade LiIO, and Cu(IO,), (anhydrous) (Ventron) targets contained in very thin boron-free sealed glass ampoules were irradiated at ambient temperatures for 3 h by paraffin-thermalized neutrons from a 300 mCi Ra-Be neutron source with an integrated flux of 3.2 x lo6 neutron cm-, s-l. Irradiation at - 196 "C was carried out dipping the neutron source and sample into a Dewar flask containing liquid nitrogen, which was periodically replenished. The whole assembly was surrounded by a paraffin block. 4748 PRE-HEAT TREATMENT STUDY OF R E C O I L ~ ~ ~ I IN IODATES 90 Pre-heat treatments and thermal annealing were performed in an electronically controlled oven maintained to within & 1 "C.Bulk amounts of target materials were pre-heated [LiIO, at 200 "C and Cu(IO,), at 175 "C] for 1 h and thereafter transferred to a desiccator to cool them slowly. For post-recoil thermal annealing the irradiated LiIO, and Cu(IO,), targets were heated for various lengths of time in an oven maintained at the desired temperature. The irradiated materials were dissolved in 10 cm3 of ammoniacal solution. Chemical analysis was made by the fractional precipitation procedures and the radioactivities of the different fractions as their silver salts were counted with the help of an end-window Geiger-Muller counter under conditions of constant geometry.Retention values were computed after the usual necessary corrections. RESULTS The retention values of lZaI reported are an average of at least three independent experiments with an accuracy of k 2%. The initial retentions, R,, of lZ8I in untreated LiIO, and Cu(IO,), are found to be 63.28 and 38.5%, respectively, while for pre- heated targets the respective values are 54.7 and 41.5%. Our value for the initial retention of LiIO, is nearly in agreement with the values of 67 and 61 % reported by Cleary et al.' and Ambe and Saito,a respectively. - rn 60 h E 90 80 70 60 - (a 1 d 50' I I I I I 0 2 5 10 15 20 25 time of heatiiig/rnin Fig. 1. Annealing isotherms for LiIO, irradiated at room temperature (31 "C) by thermal neutrons: (a) untreated, (b) pre-treated (pre-heated at 200 "C for 1 h).0, 125; x , 150; A, 175 and @, 200 "C.60 50 40 h S. P. MISHRA, A. PATNAIK AND D. P. WAGLEY (a 1 I I 49 Y 70 - (b 1 30 I I I I I I time of heating/min Fig. 2. Annealing isotherms for Cu(IO,), irradiated at room temperature (31 "C) by thermal neutrons: (a) untreated, (b) pre-treated (pre-heated at 175 O C for 1 h). 0, 70; x , 100; A, 125 and 0, 150 OC. 0 5 10 15 20 25 30 An inspection of the thermal annealing isotherms (fig. 1 and 2) reveals the usual trend, i.e. a fast initial rise followed by temperature-dependent pseudo-plateau regions. At any annealing temperature the plateau values, R,, for both the pre-heated Cu(IO,), and LiIO, were higher in comparison with the untreated samples.Pre-heating causes a decrease in initial retention for LiIO, whereas the reverse is the case for Cu(IO,),. Pre-heat treatment shifted the time required for saturation from ca. 5 to ca. 2 min in the case of LiIO, whereas no such trend was observed for Cu(IO,),. The annealing rate constants for both the pre-heated and untreated lithium and copper iodates (tables 1 and 2) were computed from the slope of plots of log (R, - R,) against time of heating, R, being the retention at a particular time of the annealing isotherm. The plot revealed the presence of a combination of two apparent first-order processes, one being much faster than the other. The rate constant at a particular temperature is greater for both the untreated samples (cf tables 1 and 2) than the corresponding pre-treated ones.Activation energies were computed from the classical Arrhenius plot (cf. tables 1 and 2). The lower activation-energy values obtained for both the pre-heated targets in comparison with the untreated ones indicate the increased propensity for annealing reactions in the pre-heated iodates. We have also refined the value of the50 PRE-HEAT TREATMENT STUDY OF RECOIL'~*I IN IODATES Table 1. Isothermal annealing dataa for LiIO, irradiated by thermal neutrons at 3 1 "C [R, = 63.28 k 2% (54.70 f 2%)] ~~ slow component T/OC R , R,-R, t:/min k/ 1 OP2 min-l 200 88.5 (9 1 .O) 175 87.0 (89.0) 150 82.5 (87.0) 125 79.5 (84.0) 25.22 (3 6.30) 23.72 (34.30) 19.22 (32.30) 16.22 (29.30) 2.0570 2.8795 (3.3284) 3.1 134 (4.4239) 3.7010 (4.2066) (3.45 5 5) 33.69 f 0.82 (20.05 f 0.83) 24.07 f 0.37 (20.82 & 0.58) 22.26 & 0.63 (15.66f0.28) 18.72 & 0.14 (16.47 & 0.27) activation energyb ~ ~~~~ Fle tcher-Brown Arrhenius kinetics model /kcal mol-l /eV /kcal mol-l /eV 2.7209 & 0.207 1 0.1 180 rf: 8.984 x 16.36 0.70 (1.30 18 +_ 0.2378) (0.0564 +0.0103) (14.80) (0.64) a Values in parenthesis are for the pre-heated sample.1 kcal = 4.1840 kJ. Table 2. Isothermal annealing dataa for Cu(IO,), irradiated by thermal neutrons at 31 "C. [R, = 38.50+2% (41.50&2%)] slow component T/OC R , R,-R, ti/min k/lO+ min-l 150 65.0 (7 1 .O) 125 61.0 (65.0) 100 58.0 (59.0) 70 54.0 (49.5) 26.5 (29.5) 22.5 (23.5) 19.5 (1 7.5) 15.5 (8.0) 3.8253 (6.2428) 4.0238 (6.7673) 4.4428 (6.5432) 4.95 19 (7.3428) 18.12 k0.33 (1 1.10k0.62) 17.22 f 0.1 3 (10.24& 0.15) 15.60 & 0.16 [ 1 0.59 f 0.1 4) 13.99 +_ 0.20 (9.44 f 0.66) activation energyb Fle tcher-Brown Arrhenius kinetics model /kcal mo1-I /ev /kcal mol-1 /eV 0.9548 k0.0186 0.0414k8.069 x lo-*) 10.12 0.43 (0.5043 k0.0659) (0.0218 f2.859 x (7.60) (0.32) a Values in parenthesis are for the pre-heated sample; 1 kcal E 4.1840 kJ.S. P.MISHRA, A. PATNAIK AND D. P. WAGLEY 51 30 30 20 10 I I I I I I I l l I I I I I I I I I 1 I I 0.1 1.0 10.0 40 equivalent annealing time at 200 "C t'lmin Fig. 3. Fletcher-Brown composite annealing curves for various recoil fragments in (n, 7)- irradiated LiIO,: (a) untreated, (b) pre-treated (pre-heated at 200 "C for 1 h). 0, 70; x , 100; A, 125 and 0, 15OOC. annealing rate constant and energy of activation by a least-squares technique by which the probable associated errors involved in these parameters were also calculated (cf. tables 1 and 2).These trends have been confirmed by fitting our annealing data to the Fletcher-Browne model. The composite annealing curves are shown in fig. 3 and 4. Activation energies were obtained from the slope of a linear plot of log t'/t (where t' is the equivalent annealing time) against I/T (cf. tables 1 and 2). DISCUSSION The low initial retention values observed in the present work for Cu(IO,), (28.46_+2%) and LiIO, (39.5 +2%) activated at - 196 "C compared with ambient- temperature irradiation values show that intrinsic thermal annealing is occurring even at lower temperatures. Since radiation annealing is thermally activated, the effect of the 6Li(n, a) T reaction is blocked at - 196 "C.Moreover, since retention values at - 196 "C are too high to be accounted for by the failure of bond rupture during the initial act of recoil, the reaction should be diffusion-controlled second-order intrinsic annealing as suggested by Andersen:1° (1) Fast intrinsic annealing might be the reason for the inability of previous workers to obtain evidence for 10, ion formation in irradiated iodates. 10, + 0 -+ 10,. 3 FAR 152 PRE-HEAT TREATMENT STUDY OF RECOILI~~I IN IODATES 20 5 1 0 . +j 0 ' X s '0 c rd c .- + m & 20 10 30 t ' . ' 01 I I 1 I I 1 I l l I I I I I l l 1 1 I I 0.1 1.0 10.0 40 equivalent annealing time a t 150 O C , t'lmin Fig. 4. Fletcher-Brown composite annealing curves for various recoil fragments in (n, 7)- irradiated Cu(IO,),: (a) untreated, (b) pre-treated (pre-heated at 175 "C for 1 h).0, 125; x , 150; a, 175 and @, 200 O C . The initial retention data in the present work may be explained by considering the roles played by inherent crystal defects in direct and competitive oxidation and reduction. Pre-heat treatment increases the initial retention of Cu(IO,), from 38.5 & 2 to 41.5 & 2% due to the removal of reducing defects which would otherwise stabilize recoil iodine in the reduced form as in the untreated case. Arnikar et al.' have also found an increase in the retention of lZsI on pre-heating HIO,. However, the decrease in the initial retention for LiIO, from 63.28 a 2 to 54.7 a 2% on pre-heat treatment may be ascribed to the fact that the reducing defects are situated at deep trapping levels and are unable to stabilize the recoil atom in its reduced form.On pre-heat treatment they are either promoted to less deep trapping levels to stabilize recoil iodine in its reduced state or are promoted to the conduction band and are annihilated with the positive holes which would otherwise stabilize, as in the untreated case, the recoil atom in a higher-valent form. Thus both possibilities permit a decrease in the initial retention of LiIO, on pre-heat treatment. Andersen12 has proved by thermoluminescence and electrical-conductivity measure- ments that recoil annealing is an electronic phenomenon which originates through the release of charge carriers. In an irradiated crystal the number of defects is normally far greater than the number of recoil atoms.If thermal annealing were due to diffusion of ionic species in the rate-determining step its activation energy would be in the range 1-2 eV.13 In fact the activation energies in all the present investigations are less than this range, and we agree with the defect mechanism proposed by Lin andS. P. MISHRA, A. PATNAIK AND D. P. WAGLEY 53 Wiles14 involving exciton capture at the recoil site in the rate-determining step. The following mechanism is proposed : (2) slow I (recoil) + exciton +- I* (excited) fast I* -+ 10, ( 3 ) 10; + q5 (phonon) + IO:(hole) + e- (4) IO;+Lv -+ IO:+e- weak ( 5 ) I* + 10; ++I* + 10, *I+ + 10; -+ *IO- + 10;. (6) (7) The production and annihilation of holes and electrons [reaction (4)] would be an important step in thermal annealing.A sequence of reactions (6), (6), (7), (6), (6), (7) etc. would give rise to retention in the parent form. Our data on thermal annealing may be further explained on the basis of the exciton mechanism. Since the defects produced by internal-conversion phenomena (as the recoil atom dissipates its electronic and translational excitation energy) and self- radiolysis in both LiIO, and CUIO, samples and by sLi(n, a) T in LiI0,15 are common to both pre-heated and untreated targets we can assume that, by virtue of annealing-out inherent crystal defects in pre-heat treatments, the lattice of a pre-heated sample is more ordered than in the untreated case. Thus the probability that excitons reach recoil iodine sites in order to deposit their energy and thus allow annealing to occur in pre-heated samples is greater than in the more disordered untreated iodates.In untreated samples, owing to the relatively large number of inherent crystal defects some of the excitons produced are stopped or captured and are thus not able to reach the recoil sites. We have obtained (as expected from the exciton mechanism) higher pseudo-plateau values and lower values of the activation energy for both pre-heated iodates than for the untreated ones. However, we face a difficulty in explaining the lower value of the annealing rate constant for both pre-heated targets than for the corresponding untreated ones. The apparent discrepancies may be due to the dangers of formally applying Arrhenius kinetics to describe processes in solid systems where the compensation effect may apply.16 Such an effect reveals that the pre-exponential factor in the Arrhenius equation varies exponentially with the energy of activation.Furthermore, a process cannot be described as being strictly unimolecular or first order when the amount of material reacting is a function of the temperature at which the experiment is conducted. Fletcher and Brown9 showed that annealing should depend upon a factor t / z where z is the average jump time and t is the time of isothermal annealing, provided that the vacancy moves through the crystal by jumping to an adjacent side. As z entirely governs the temperature dependence of annealing, the isothermal annealing data at different temperatures can be combined to obtain (cf.fig. 3 and 4) a single curve of equivalent annealing at a single reference temperature by multiplying the time scale of each isothermal curve by an appropriate factor. The resultant composite annealing curves may be fitted by unimolecular, bimolecular or error-function expressions or any combination of these. The gradations in activation energy for the untreated and pre-heated targets are found to be similar from both the Fletcher-Brown model and Arrhenius kinetics, which indicates the success of the present analysis. Thus it can safely be concluded that the annealing propensity for recoil stabilisation 3-254 PRE-HEAT TREATMENT STUDY OF RECOIL12*I IN IODATES is enhanced on pre-heating Cu(IO,), and LiIO, in accordance with the exciton mechanism, bearing in mind that plateau values are greater for the pre-heated iodates than for those that are untreated.We thank the Head of the Department of Chemistry, Banaras Hindu University for providing the necessary facilities, the C.S.I.R. for the award of a junior research fellowship to Archita Patnaik and the Ministry of Education, New Delhi for a general cultural scholarship to D. P. Wagley. We also thank Drs R. N. Singh and R. A. Singh for useful discussions. T. Andersen and A. G. Maddock, Nature (London), 1962, 194, 371. C. H. W. Jones and J. L. Wanen, J. Inorg. Nucl. Chem., 1968,30, 2289. C. H. W. Jones and J. L. Warren, J. Inorg. Nucl. Chem., 1970, 32, 21 19. C. W. Owens, in Chemical Effects of Nuclear Transformations in Inorganic Systems, ed. G. Harbottle and A. G. Maddock (North Holland, Amsterdam, 1979), p. 145. I. G. Campbell, Radiochim. Acta, 1968, 9, 7. G:E. Boyd and Q. V. Larson, J. Am. Chem. SOC., 1969,91,4639. F. Ambe and N. Saito, Radiochim. Acta, 1970, 13, 105. R. C. Fletcher and W. L. Brown., Phys. Rev., 1953,92, 5115. lo T. Andersen, Experimental Investigations of Chemical Effects Associated with Nuclear Transformations in some Inorganic Solids (Institute of Chemistry,. University of Aarhus, Denmark, 1968), chap. 4, p. 65. H. J. Amikar, V. G. Dedgaonkar and K. K. Shrestha, J. Univ. Poona, Sci. Technol., 1970, 38, 177. l2 T. Andersen and K. Olesen, Trans. Faraday Soc., 1965, 61, 781. l3 G. Harbottle and N. Sutin, in Advances in Inorganic Chemistry and Radio Chemistry, ed. H. J. Emeleus l4 Y. C. Lin and D. R. Wiles, Radiochim. Acta, 1979, 13, 43. l5 G. E. Boyd and T. G. Ward, J. Phys. Chem., 1964, 68, 3809. l6 V. Talroze, Proc. Chem. Effect of Nuclear Transformations (I.A.E.A., Vienna, 1961), vol. 1, p. 464. ' R. E. Cleary, W. H. Hamill and R. R. Williams, J. Am. Chem. SOC., 1952, 74,4675. and A. G. Sharpe (Academic Press, New York, 1958), vol. 1, p. 279. (PAPER 3/284)

ISSN:0300-9599    

DOI:10.1039/F19848000047

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I , 1984,80, 55-60 A Consideration of Pitzer’s Equations for Activity and Osmotic Coefficients in Mixed Electrolytes BY ROBERTO IALENTI Istituto Universitario Navale, Via Amm. Acton 38, 80133 Napoli, Italy AND RAFFAELE CARAMAZZA* Facolta di Farmacia, Universita di Napoli, Via L. Rodino 22, 80138 Napoli, Italy Received 25th February, 1983 The equations developed by Pitzer and coworkers in order to calculate the activity coefficients of two mixed electrolytes in aqueous solutions are transformed into more simple equations. It is demonstrated that these new equations do not satisfy one of the necessary requirements of thermodynamic arguments, the only exception being when there are two electrolytes that have a common ion and are of the same type. Many authors have already dealt extensively with the theoretical aspects of the ionic interactions of electrolytes in aqueous s~lutions.~-~ Much work has been done by Pitzer and coworkers,8-11 who, on the basis of statistical-mechanical considerations and accounting also for the effects of short-range forces, obtained equations which reproduce, accurately and within a rather wide composition range, experimentally measurable activity and osmotic coefficients of single electrolytes.The most interesting part of their work is the possibility of predicting the properties of mixtures of electrolytes starting from those of the single components, by means of analytic relations whose compliance with thermodynamic principles have never been challenged. However, as will be seen in this paper, when Pitzer’s equations for evaluating the activity coefficients of mixtures of two electrolytes are transformed into more simple and compact analytical expressions, a necessary condition derived by a thermodynamic argument is not always satisfied by the new relations.For simplicity we will consider here only pairs of electrolytes of the types 1 : 1, 1 : 2 and 2: 1, for which it is possible to set 2, = vx and 2, = vM. THEORY First we rearrange Pitzer’s equations8 for the activity and osmotic coefficients of a single electrolyte MX as a function of ionic strength I 5556 CONSIDERATION OF PITZER'S EQUATIONS At 25 OC and 1 atm we have (3) lny,, = -0.39212 2 I x ( +:.2rh+ 1.667 In (1 + 1.211) Btx = PMx +Ptx exp ( - 21;) Ckx = 1.5CtX where BhX, PAx and C& are characteristic parameters for any single electrolyte and are reported elsewhere.12a For mixtures of electrolytes, if we neglect the effect of interactions among ions of the same charge, the general equation for calculating the activity coefficient of one electrolyte in the mixture, written in molal terms, is 2VM lnyMX = lnyEL+- mu [BMu+@mcZc) cMul vMX a 2VX +- Z mc [Bcx + (Zmc 2,) Ccxl vMX c where MX is the electrolyte selected and the sums run over all cations (including M) denoted by the subscript c and all anions (including X) denoted by the subscript a.The coefficients Bca, BLa and C,, are given by the following expressions In the case of mixtures of two electrolytes, MX = A and NY = B, if 1 A and 1, are the respective ionic strengths, it is possible to demonstrate that, bearing in mind eqn (l), we can obtain from eqn (7) the following two equations where y i and y g are the activity coefficients of the single electrolytes in separate solutions at ionic strength I = IA+IB.The transformation is performed by means of the following devices: (a) substitution of 2, with vu and 2, with v, as noted earlier, (b) substitution of the ionic molalities in terms of the ionic strengths I A and I B using the following equations : 24, m, = - vu vcu 24, ma=- vc vc,R. IALENTI AND R. CARAMAZZA 57 where a = X when c = M and a = Y when c = N, and (c) expression of the resulting equation for the electrolyte A as a function of I and IB and that for the electrolyte B as a function of I and I A . In this way we obtain the following relationships between A , and A , and Pitzer's parameters: (BNX + B&X I ) (BMY + BMY I ) + - vA vB vY 4vx (16) where BCu, BEu and Ccu are the same as in eqn (8)-( 10).The B, and B, parameters are obtained from eqn (15) and (16) by substituting subscript A for B, M for N and X for Y and vice versa. We now demonstrate, by a thermodynamic argument, what conditions must be satisfied by the parameters Ai and Bj of eqn (1 1) and (12). Let us suppose that the activity coefficients of the two electrolytes may be expressed by equations of the following type lny, = lnyO,+C. AjIh (17) lny, = lnyL+C. BjZJ (18) i i with j = 1, 2, . . . k. First we will show that for each eqn (1 7) and (1 8) the maximum number of parameters cannot be more than two (j < 2) and then we will deduce the relationships that must hold between them.We now apply to our system the condition of cross-differentiation which must be obeyed by the chemical potentials, and so obtain, with the ionic strength as an independent variable, the following equation: vN vY (T)IA a In YA = vM vX (I) a In YB * ZB From this equation, applied to eqn (17) and (18), following a treatment reported elsewherel3 and bearing in mind that IB = I - I , and that the coefficients A, functions of I only, we can obtain the following equality: Bi are (20) This cannot be satisfied identically for all I, IA, if the left-hand side contains any terms of the form ImIz, since the right-hand side contains no such terms. This means that ( I - I*) cannot in fact take a power greater than unity, which implies that A, = 0 for all j >, 3 and (dAj/dI) = 0 for all j > 2; replacement of IA by ( I - I B ) in eqn (20) allows an exactly analogous conclusion to be drawn about the coefficient Bi.58 CONSIDERATION OF PITZER'S EQUATIONS Eqn (20) therefore reduces to where A , and B, are constants, but A , and B, may be functions of I.However, eqn (21) is satisfied identically for all IA only if which, on integration, yields where K is an integration constant. However, there is another relationship between parameters of eqn (1 7) and ( 1 8) that is obtained if the Gibbs-Duhem equation is applied to our ternary system. It is possible to demonstrate that if (a) I = IA + IB is held constant, (b) two limiting cases are considered, that is IA = 0 and IB = 0, and (c) the corresponding activities of the solvent are expressed as a function of osmotic coefficients, then we can obtain the following relationship 2 vM vX (B1+$B2 I ) - v N vY I ) = 7 i V N v Y ( 4 A - l)-vM vX(dB- l)1 (24) where 4A and 4B are the osmotic coefficients in the two limiting cases corresponding to the single electrolytes separately dissolved at ionic strength I.We may summarize the discussion thus far by stating that when activity coefficients of binary mixed electrolytes obey eqn (1 7) and (1 8), it is possible to conclude : (a) there are not more than two non-zero parameters A , B in each sum, (b) only A , and B, are functions of I, while A , and B, must be constant and (c) the parameters are inter-related by eqn (23) and (24).(All these conditions are in fact satisfied in the systems NaCl + CoCl, and CaCl, + CoCl, studied by Dowries.'*) Returning to eqn (1 5) and (16), it is easy to see that the parameters A , and A , are both functions of I, as Pitzer's Hca values, which according to eqn (9), are functions of I, appear in eqn (16). The same is also valid for B, and B,. So, there is evident disagreement between Pitzer's equations, rewritten in the form of eqn (1 1 ) and (12), and the thermodynamic requirement derived from the cross- differentiation condition, eqn (19), which implies the constancy of A , and B,. These parameters are zero only if the mixtures are formed by two electrolytes of the same type and having one common ion, i.e. a system for which Hunde's rule is valid.It is possible to verify this result bearing in mind that, in eqn (15) and (16), if the electrolytes have a common anion, MX = MY = A and NY = NX = B, or, if they have a common cation, MX = NX = A and NY = MY = B. For such mixtures it also follows that A , = -B,. Therefore only in such 'common-ion' cases is the above- mentioned requirement satisfied. As to the two relationships that must hold between the parameters Aj and Bj, expressed by eqn (23) and (24), it is possible to verify that the second one is satisfied,R. IALENTI AND R. CARAMAZZA 59 but the first one is not. In fact, bearing in mind eqn (15) and (16), it can be shown that the left-hand side of eqn (23) is equal to ( Bb I - BB) 8v v v 8vA vM vX Y ( ~ ; ~ - ~ A ) + V A VB + 8vM vy(BMy - B d y I ) + 8vN vX(BNX -B&XI).(25) On the basis of the eqn (8) and (9) this sum must be a function of Z and so is not constant, as required by eqn (23), except for mixtures of electrolytes of the same type and with a common ion, because in this case it is equal to zero. Eqn (24) is, however, satisfied by Pitzer’s equations. Its left-hand side may be rewritten as Using eqn (8) and (9) again and by comparison with eqn (2), ( 4 ) and (9, it is possible to show that for any type of mixture this expression is indeed identical with the right-hand side of eqn (24). CONCLUSION It follows from the preceding analysis that the equations of Pitzer and coworkers may be considered valid for evaluating the activity and osmotic coefficients of single electrolytes. However, they are in disagreement with thermodynamic theory for mixed electrolytes, except for the above-mentioned special cases which are related to Hunde’s rule.The same conclusions are obtained if additional parameters such as 8, 8‘ and v/, which are related to the differences in the interactions between ions of the same sign, are considered in our previous treatment. According to the Bronsted principle of specific interactions3 these terms should be equal to zero, and Pitzer12b affirms that ‘ they have only a small effect, if any, on mixing electrolytes, the principal effects arising from differences in the pure-electrolyte parameters $, B1 and 0. Nevertheless, if the 8, 8’ and ly parameters are introduced into eqn (7), as rarely occurs, they will appear in eqn (17) or (16) as additional terms for A , and A,.These last two coefficients will remain as functions of I, in contrast with the thermodynamic requirement previously demonstrated. Moreover, we need to remember that it is common practice to obtain the 8 and v/ values for a mixture of two electrolytes by calculating the differences between the experimental and theoretical values of 4 or lny calculated using the appropriate Pitzer’s equations with zero values for 8 and v / . Therefore, such terms seems to be more useful for an accurate reproduction of the experimental data than for the extension of the ionic interaction theory to mixed electrolytes. S. R. Milner, Philos. Mag., 1912, 23, 551 ; 1913, 25, 742. * G. N. Lewis and G. A. Linhart, J. Am. Chem. SOC., 1919,41, 1952. J. N. Bronsted, J. Am. Chem. Soc., 1922,44, 877, 938. G. N. Lewis and M. Randall, J. Am. Chem. Soc., 1921,43, 1 112. P. Debye and E. Huckel, Phys. Z., 1923, 24, 185; 344; 1924,25, 97. E. A. Guggenheim, Philos. Mag., 1935, 19, 588. K. S. Pitzer, J. Phys. Chem., 1973, 77, 268. K. S. Pitzer and G. Mayorga, J. Phys. Chem., 1973,77, 2300. ’ E. A. Guggenheim and J. C. Tuegeon, Trans. Faraday SOC., 1955,51, 747.60 CONSIDERATION OF PITZER’S EQUATIONS lo K. S. Pitzer and G. Mayorga, J. Solution Chem., 1974, 3, 539. l1 K. S. Pitzer and G. Mayorga, J. Am. Chem. SOC., 1974,%, 5701. l2 M. R. Pytkowicz, Activity Coeficients in Electrolyte Solutions (C.R.C. Press, Boca Raton, Florida, l3 H. S. Harned and B. B. Owen, The Physical Chemistry of Electrolytic Solutions (Reinhold, New York, l4 C. J. Downes, J . Solution Chem., 1979, 3, 191. 1979), chap. 7, (a) p. 157, (b) p. 187. 1958), chap. 14, p. 620. (PAPER 3/302)

ISSN:0300-9599    

DOI:10.1039/F19848000055

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. I, 1984, 80, 61-71 Reaction of Modulated-molecular-beam Chlorine with Polycrystalline Iron BY MEHDI BALOOCH AND DONALD R. OLANDER* Materials and Molecular Research Division of the Lawrence Berkeley Laboratory and the Department of Nuclear Engineering, University of California, Berkeley, California 94720, U.S.A. AND WIGBERT J. SIEKHAUS Chemistry Division of the Lawrence Livermore National Laboratory, University of California, P.O. Box 808, Livermore, California 94550, U.S.A. Received 7th March, 1983 The volatilization of polycrystalline iron by chlorine gas has been studied by modulated- molecular-beam-mass-spectrometric methods. The reaction was investigated in the temperature range 300-1250 K at equivalent chlorine pressures from 2 x Torr. FeC1, was the only detectable volatile reaction product; its production rate increased rapidly with surface temperature and levelled off at ca.1100 K. Studies of the composition of the reacting surface by laser-stimulated desorption and by ESCA indicated the presence of a thin scale of a sub-stoichiometric iron chloride at the spot struck by the molecular beam. A reaction model based on diffusion of chlorine in the scale and production of gaseous FeC1, from parallel Eley-Rideal and Langmuir-Hinshelwood processes was developed from the molecular-beam data. to 3 x The chemical aspects of metal-halogen reactions have many technological impli- cations. The high volatility of metal halides has long been utilized in halogen lamp design and in the production and purification of metals' and may play an important role in the water-splitting cycle for hydrogen production.2 The reaction of chlorine with an iron surface has been studied by Frueharn, in a conventional reaction tube in the temperature range 530-920 K and at a chlorine partial pressure of several Torr.The dimer (FeCl,), was reported to be the dominant reaction product. Below 620 K the rate was controlled by diffusion through a surface scale (presumed to be FeCl,). Above 620 K the reaction depended on the properties and the flow rate of the inert carrier gas, indicating rate control by gas-phase mass transfer. Kishi and Ikeda4 studied the reaction of evaporated films of iron exposed to 10 Torr pressure of C1, for 10 s by X-ray photoelectron spectroscopy. From the chemical shift of the 2p3I2 state of iron, they concluded that an FeC1, scale was formed on the surface of iron.RHEED and Auger studies by Dagoury et aL5 of chlorine reactions on various low-index faces of iron confirmed the existence of FeCl, islands and chemisorbed chlorine on the surface for a gas pressure of ca. 1.3 x lo-, Torr and temperature ranging from 400 to 600 K. In the present work, the kinetics of the iron-chlorine reaction are investigated by modulated-molecular-beam techniques utilizing phase-sensitive detection of reaction products with an in situ mass spectrometer. All gas flows are collision-free, so that mass-transfer limitations are avoided and only surface reactions are detected. The data obtained by this method consist of signals of scattered and desorbed volatile reaction products which, in conjunction with surface analysis, provided the principal means of deducing a surface reaction model.6162 M. BALOOCH, D. R. OLANDER AND W. J. SIEKHAUS SOURCE bT b’. : . .. .. . TA R TO PUMP + DIFFUSION TO PUMP SCATTERED R EACTAN AND DESO RBED PRODUCT DIFFUSION+ t TO ION PUMP \ Fig. 1. Schematic representation of the modulated-beam reactive scattering apparatus. EXPERIMENTAL A detailed description of the molecular-beam apparatus used for the kinetic studies is given elsewheres but the principles of operation are shown in fig. 1. The apparatus consists of three differentially pumped chambers separated by collimating orifices. A beam of chlorine is formed by effusion and mechanically modulated in the source chamber.The incident chlorine beam is not heated and the temperature of the gas in the source is CQ. 300 K. A collimator shapes the beam to a thin pencil prior to striking the iron target in the second chamber. The intensity of the molecular beam of chlorine at the solid surface can be calculated for the fixed source-to-target distance from the gas pressure and the conductance of the hole in the source tube. The incident beam intensity can, if desired, be converted to an equivalent reactant gas pressure by standard gas kinetic theory form~lae.~ Beam-formation and vacuum-pump limitations result in a maximum achievable equivalent pressure at the target surface of ca. 3 x lo-* Torr. The high-purity polycrystalline target, previously polished and washed with alcohol, is heated to temperatures up to 1250 K by radiation and by electron bombardment from a hot filament.Portions of the scattered chorine and desorbed reaction products are detected by a quadrupole mass spectrometer mounted in a third chamber, which communicates with the target chamber via a 1 mm diameter orifice. The mass-spectrometer ionizer has a direct line-of-sight view of the beam spot on the target. The modulated signals from the mass spectrometer are processed by a lock-in amplifier to yield the apparent reaction probability, E (the ratio of the amplitudes of the product and reactant signals, corrected for ionization efficiencies of the mass spectrometer) and the phase lag, 4, which is the difference between the product and the reactant phase angle^.^ Phase-sensitive detection responds only to the fundamental mode of the periodic input signals at the frequency of modulation provided by the beam chopper.Thus the modulation frequency becomes the third controllable experimental variable, for which a range of l&103 Hz is achievable. RESULTS MOLECULAR-BEAM DATA The iron-containing ions observed in the mass spectrometer using 70 V ionizing electrons were FeCQ (20%), FeCl+ (50%) and Fe+ (30%). All had the same phase angle and the same dependence on surface temperature. Therefore it was concluded that FeC1, was the sole volatile product of reaction under low-pressure/high- temperature experimental conditions.J . Chem. SOC., Faraday Trans. 1, Vol. 80, part 1 Plate 1 Plate 1. Scanning electron micrograph of an iron surface after reaction with a chlorine beam.M. BALOOCH, D. R. OLANDER AND W. J. SIEKHAUS (Facing p . 63)REACTION OF MOLECULAR-BEAM CHLORINE WITH IRON 63 0 7 I I 15 lo4 KIT Fig. 2. Apparent reaction probability and phase lag of FeCl, as a function of target temperature; I, = 4.8 x l0ls molecule cm-, s-l,f= 20 Hz. Fig. 2 shows the measured reaction probabilities and the conjugate phase lags for this product as functions of surface temperature. The apparent reaction probability increases rapidly with temperature up to ca. 1100 K and then levels off. Below ca. 850 K the phase lag approaches a limiting value of approximately 45O. The beam-intensity dependence of the molecular-beam data is shown in fig. 3 and 4 for three different surface temperatures. At the highest temperature (1 150 K) the reaction appears to be linear, while at low temperature (817 and 585 K) higher-order reaction behaviour with respect to the incident chlorine molecule beam is observed.The frequency dependence of the apparent reaction probability and phase lag is shown in fig. 5 and 6, again for three different temperatures. At 1233 K, the surface chemical process is fast with respect to the time scale of primary-beam modulation so that the apparent reaction probability does not change appreciably with frequency and the phase lag is nearly zero. At 952 K, the phase lag is substantial and goes through a maximum at ca. 250 Hz. At 622 K, the phase remains constant at ca. 45O up to 250 Hz and decreases at higher chopping frequencies. SURFACE CHARACTERIZATION Information on the chemical state of the surface at the beam spot is essential for understanding the mechanism of the chlorine-iron reaction.To the unaided eye, the post-reaction surface at the beam spot appeared bluish-black in colour. Plate 1 shows a scanning electron micrograph of this surface. When analysed by AES in a different64 M. BALOOCH, D. R. OLANDER AND W. J . SIEKHAUS A " 1 A 6 585 K 1 Fig. 3. Effect of chlorine beam intensity on the apparent reaction probability of FeC1, for three different target temperatures at a fixed modulation frequency; f = 20 Hz. vacuum system, however, no chlorine was detected on the reacted spot. This element may have been lost by hydrolysis due to moisture in the air to which the specimen was exposed during transfer.To detect a non-volatile chloride coating on the reacted surface, a specimen was heated to 700 K and exposed to a chlorine pressure of Torr in a system equipped with ESCA. The spectrum was recorded after one hour exposure to C1, under the above conditions. The width and position of the chlorine peak corresponded to a mixture of FeCl,, FeCl and adsorbed C1, confirming the observations of Kishi and Ikeda.4 Finally, laser-stimulated desorption was applied to characterize the surface during exposure to chlorine. An iron target, held at 900 K in an environment of Torr of chlorine, was rapidly heated to a high temperature by a Nd glass laser pulse. Species desorbed from the surface were detected by an in situ quadrupole mass spectrometer. Both FeCl+ and FeCl; ions were detected, with the integrated signal from the former about twice that of the latter.In separate tests, it was determined that ca. 20% of FeCl, fragmented to FeCl+ at the ionizer electron energy of 15 eV used in these experiments. Thus, the surface composition probably has a chlorine-to-iron ratio of less than two. However, thermal dissociation of FeCl, by the laser pulse could have contributed the FeCP signal detected by the mass spectrometer. The visual and instrumental evidence described above, although qualitative, strongly suggests that the surface exposed to the reactant molecular beam was covered by an iron chlorine scale of undetermined stoichiometry. The coating must have contained more than a monolayer of chlorine for so small a quantity could not have provided the strong ESCA and mass-spectrometer signals in the laser pulsing tests.REACTION OF MOLECULAR-BEAM CHLORINE WITH IRON 8om 60 65 - 585 K I 2 5 0 0.5 Clz beam intensity, Io/1016 molecule cm-2 s-1 Fig.4. FeCl, phase-lag dependence on chlorine beam intensity for three different target temperatures at a fixed modulation frequency; f = 20 Hz. The scale could not have been pure FeCl, because the vapour pressure of this compound at 700 and 900 K (0.01 and 4 Torr, respectively) is far greater than the lo-' Torr pressure of C1, which supplied this element to the surface. Even if all of the incident chlorine molecules reacted to form FeCl,, the high volatility of this substance at the test temperatures precludes the presence of a scale of pure FeCl,.The Cl/Fe ratio of the scale must be sufficiently larger than zero to contain significant quantities of chlorine but sufficiently smaller than two to prevent it from vaporizing into the vacuum. DISCUSSION KCAL 1 IUN MUUEL The evidence cited above indicates that the reaction mechanism should include the sub-stoichiometric chloride scale on the iron surface during corrosion. Incident molecular chlorine from the beam chemisorbs dissociatively on the exposed surface of this scale and reacts to produce the volatile FeCl, species which is ultimately detected by the mass spectrometer. Inclusion in the model of a scale on the reacting surface is consistent with many previous metal + halogen studies. McKinley6 reported a NiF, scale produced in the Ni+F, reaction. Machiels and Olanders assumed a fluoride scale on tantalum to explain their molecular beam results on the Ta+F, system.This inference was subsequently confirmed by N ~ r d i n e . ~ Paullo showed that during the reaction between iron and chlorine at low pressure (ca. Torr), a solid reaction product was present at the surface, growing as a thin continuous layer. The molecular-beam data in fig. 2-6 exhibit characteristic signatures which indicate the presence of certain elementary processes in the mechanism of the surface reaction. The relationship of distinctive features of the apparent reaction probability and the66 M. BALOOCH, D. R. OLANDER AND W. J. SIEKHAUS I I I I I I I I I I modulation frequency, f/Hz 50 100 200 500 1041 10 2 0 K) Fig. 5. Modulation-frequency dependence of the apparent reaction probability for three target temperatures and fixed chlorine-beam intensity; I, = 4.8 x 1Ols molecule cm-2 s-l.chopping frequency, f/Hz Fig. 6. Modulation-frequency dependence of the FeCl, phase lag for three target temperatures and fixed chlorine-beam intensity; I, = 4.8 x 10l6 molecule cm-2 s-l.REACTION OF MOLECULAR-BEAM CHLORINE WITH IRON 67 phase lag of the product to particular steps in the mechanism have been catalogued previ~usly.~? l1 One such feature is the near 4 5 O phase lag which is observed at low temperatures and low frequencies (fig. 2 and 6). This behaviour is strongly indicative of a diffusion-controlled step in the mechanism. When coupled with the previously discussed likelihood of an FeCl, layer (0 < x < 2) on the reacting surface, the most probable diffusional process is migration of chlorine in this scale.Another aspect of the data which serves as a guide to modelling the reaction is the behaviour of the apparent reaction probability and the phase lag as the incident-beam intensity is varied. Except at very high temperatures, the reaction is non-linear with respect to chlorine-beam intensity (fig. 3 and 4). The most likely source of this non-linearity is reaction of adsorbed chlorine with the top of the FeCl, scale to produce adsorbed FeC1,. This is a Langmuir-Hinshelwood type of surface reaction, which was also observed in the Ta+ F, system.6 The third salient point of the data which suggests a particular surface step is the variation of the phase lag with modulation frequency.As seen in fig. 6, the phase lag decreases with increasing frequency at 622 K and exhibits a maximum at 952 K. This phenomenon can only occur if two parallel reaction paths are available for the production of the same species.’ Basically, the phase lag is a measure of the time between the arrival of a chlorine molecule at the surface and the emission of an FeCl, product molecule. In a simple adsorption-desorption mechanism, the phase lag directly measures the mean lifetime of the adsorbed species and 4 increases with increasing modulation frequency. In a branched process, the interaction of the two reaction-product vectors can result in a maximum of the type observed in fig. 6 if the branching ratio favours the channel with the longer residence time.In a steady-state experiment, the fast but low-probability channel contributes little to the total product rate. The dominance of the high-probability but slow channel occurs at low frequencies in a modulated-beam experiment as well. As the modulation frequency is increased, the slow branch becomes demodulated, so that its contribution to the apparent reaction probability decreases and its contribution to the phase lag increases. At high frequencies, the slow channel is immeasurable by phase-sensitive detection and only the fast, low-probability branch provides a signal. If this branch is very fast, the phase lag can decrease with increasing frequency for a significant range of modulation frequencies. Assuming that the slow branch is the Langmuir-Hinshelwood step discussed above, the most likely candidate for the fast branch is a reaction of the Eley-Rideal type, which occurs with zero delay and hence induces no phase lag in the product signal. A final direct hint of the reaction mechanism is obtained from the behaviour of the apparent reaction probability at high temperature.Fig. 2 indicates that for T > 1100 K the reaction phase lag is almost zero and the apparent reaction probability is insensitive to temperature. This behaviour suggests that the production of FeC1, is limited solely by the rate of supply of reactant C1, to the surface; all other steps in the mechanism are rapid compared with the millisecond time scale of beam modulation. Therefore, the apparent reaction probability in this limit is identical to the sticking probability of C1, on the coated surface.Large, temperature-independent sticking probabilities have been found in other halogen-metal 8 v l2 SURFACE STEPS The overall reaction is driven by dissociative chemisorption of molecular chlorine on the surface of the FeCl, scale exposed to the impinging reactant beam. This process is characterized by a sticking probability, q, which, from the high-temperature limit68 M. BALOOCH, D. R. OLANDER AND W. J. SIEKHAUS of the apparent reaction probability shown in fig. 2, is approximately 0.03. The rate of adsorption is expressed by (1) where I is the intensity of the CI, molecular beam striking the surface. Modulation of the incident beam is represented by Rads = 2q1( 1 - 6) where I, is the amplitude of the beam and g ( t ) is the gating function of the modulator, which is a square wave of frequencyf Hz.The quantity 8 in eqn (1) is the fraction of the FeCl, surface occupied by adsorbed FeCl,. This portion of the surface is inactive for chlorine chemisorption. Some sort of coverage-dependence of chlorine sticking on the surface is needed in order to match the rapid drop of the apparent reaction probability with decreasing temperature (fig. 2). The Langmuir model in which sticking varies linearly with surface coverage has been assumed since this type of behaviour is frequently exhibited by heterogeneous reacting systems. Following the qualitative discussion of the reaction mechanism given earlier, FeCl, is formed by two parallel channels. The dominant path is the reaction of surface- adsorbed chlorine with the scale. This Langmuir-Hinshelwood step is characterized by a rate constant kLH, and to reflect the observed non-linearity of the reaction discussed previously the kinetics are assumed to be mth order with respect to the chlorine adatom concentration, n.The rate of this step is given by R,, = kI,Hrzm. (3) In the Eley-Rideal process, an incident C1, molecule strikes the FeCl, surface where it directly produces FeCl, and a chlorine adatom. The rate of this step is described where qER is the probability that an incident C1, molecule undergoes reactive adsorption when it strikes the portion of the surface not covered with adsorbed FeCl,. Adsorbed FeCl, produced at the rate given by the sum of eqn (3) and (4) is assumed to leave the surface by simple desorption Rdes = kdes ( 5 ) where kdes is the first-order rate constant for FeC1, desorption.The analogous process of desorption of adsorbed chlorine is not considered in the model because no experimental evidence was found for atomic chlorine leaving the surface. The ab- sence of halogen adatom desorption is consistent with previous halogen-metal studies.s* l2 DIFFUSION IN THE SCALE A scale of approximately constant thickness is maintained by the balance of two processes. Removal occurs at the top by desorption of FeCl, and regeneration takes place at the interface with the substrate metal at a rate governed by chlorine diffusion through the scale. Upon arriving at the metal interface, chlorine converts Fe to FeCl, and thus scale growth occurs.Except for the volatilization process occurring at the top, the mechanism is equivalent to the classical Wagner model of scaling. The modification involving simultaneous removal by volatilization and growth by diffusion- controlled scaling was first analysed by Rosner and A1lend0rf.l~ The model has been applied to molybdenum oxidation wherein MOO, is volatile14 and most recently to the Ta + F, reaction.6 While the time-averaged properties of the reacting system determine the thicknessREACTION OF MOLECULAR-BEAM CHLORINE WITH IRON 69 of the scale, the modulated portions of the surface reactions (to which the detection system responds) are influenced by the periodic change in dissolution and diffusion of chlorine into the scale.As the chlorine concentration on the surface increases during the beam-on portion of a modulation cycle, not only do surface reactions proceed faster but more chlorine is lost to the solid beneath the surface. Conversely, when reactant supply ceases abruptly during the beam-off part of the cycle, the scale can continue to act as a source of chlorine to the surface adatom population. When this flywheel effect of bulk diffusion dominates the purely surface steps, the response of the chemical system is identical to the thermal response of the earth’s surface to diurnal heating by the sun,15 in which the surface temperature lags the driving heat flux by 45O. The discussion at the beginning of this section pointed out regions of the experimental variables which elicited a phase-lag response near 45O.Thus, the action of the bulk as a source or sink of chlorine atoms must be considered in assessing the response of the surface reactions to the incident modulated beam. The diffusion process is described by Fick’s law where C and D are, respectively, the concentration and diffusion coefficient of chlorine in the scale and z is the depth beneath the surface. The connection between the surface concentration of chlorine adatoms, n, and the bulk concentration of chlorine in the scale at the surface, C(z = 0), is assumed to be linear C(z = 0) = Hn (7) where H is the surface-to-bulk solubility coefficient. This formalism for including the effect ofbulk diffusion on surface kinetics hasin the past been applied to molecular-beam studies of other gas+ solid reaction^.^^ 11$ l6 QUANTITATIVE MODELING The net effect of the elementary processes acting in concert is determined by mass balances on the surface species contained in the model.For chlorine adatoms, the balance is and for FeC1, on the scale surface the analogous balance n = &ds - RLH + - Rdiff NS8= R,,+R,,-Rdes where N , is the number of surface sites per unit area (ca. (8) (9) is 1015 crn-,). The mathematical treatment of this type of molecular-beam analysis has been elaborated 16, l7 Basically, the component rates on the right-hand sides of eqn (8) and (9) are expressed by eqn (1) and (3)-(6). The driving force for the reaction is the periodic supply of chlorine represented by eqn (2). These equations are solved by Fourier expansion7 or other16 techniques.Part of the solution involves treatment of Fick’s second law describing chlorine diffusion in the scale, for which eqn (7) is a boundary condition. The end result of the analysis is the reaction-product vector, which is the ratio of the rate of FeCl, desorption to the rate of C1, supply to the surface (i.e. Rdes/Z). This vector exhibits a phase lag 4 in addition to an amplitude factor, which is the apparent reaction probability E. The quantities are directly comparable to the experimental measurements. The detailed treatment of the analytical model of the Fe + C1, reaction outlined here is presented in ref. (1 8). The model contains 10 adjustable parameters. The sticking probability is directly70 M. BALOOCH, D. R. OLANDER AND W. J.SIEKHAUS determinable from the high-temperature limit of the apparent reaction probability shown in fig. 2. The reaction order of the Langmuir-Hinshelwood step [the exponent m in eqn (3)] must be greater than unity to reflect the increase in E with I,, shown in fig. 3; the choice m = 2 adequately fits the data, although this number cannot be fixed with high precision. The remaining eight parameters, representing the pre-exponential factors and activation energies of the Langmuir-Hinshelwood rate constant kLH, the direct reaction probability qER, the mean lifetime of FeCl, on the scale surface, N,/k,,,, and the combined solution-diffusion parameter H2D, were determined by fitting the entirety of the data to the model. The best-fitting theoretical curves shown along with the data points in fig.2-6 correspond to the following surface rate constants kLH/cm2 s-l= 300 exp (-46/RT) qER = 2 x exp ( - l / R T ) (kdeS/Ns)/ss1 = 2 x 1013 exp (- 36/RT) where the activation energies are in kcal mol-1 and R is the gas constant. CONCLUSIONS Although the large number of parameters used in the data fitting precludes a claim to uniqueness of the model, several features lend credence to its general form. First, inclusion of a sub-stoichiometric FeC1, scale in the model is supported by qualitative surface analysis and is consistent with similar scale formation observed in other gas-solid reactions which produce a volatile product. Secondly, the conjugate pairs of data points ( E , 4) cover ranges of the three experimental variables (f, I, and T ) that are sufficiently broad to circumscribe closely the nature of the surface mechanism.These data exhibit features that strongly indicate certain elementary steps in the overall process; it would not be possible to remove from the model solution diffusion in a scale, a branch mechanism for FeCl, production, or non-linearity in one of the branches and still obtain reasonable agreement with the ensemble of the data. Thirdly, none of the pre-exponential factors or activation energies of the surface steps are outside the theoretically acceptable ranges for the processes they claim to represent. The type of surface reaction leading to the production of adsorbed FeCl, by the Langmuir-Hinshelwood branch of the mechanism has been analysed by Baetzold and Somorjai.l9 Using transition-state and hard-sphere reaction models to estimate pre-exponential factors, they find that low values such as that observed here for kLH are not uncommon for many bimolecular surface reactions. The pre-exponential factor of kdes is consistent with a simple desorption process; it approximates the frequency of adatom vibration normal to the surface. FeCl,.production by the Eley-Rideal branch is small with respect to that of the Langmuir-Hinshelwood step and is significant only at high frequencies. At 1100 K, the direct reaction probability, qER, is ca. 1.5 x los3, which is ca. 5% of the sticking probability. The low activation energy of the direct reaction step (ca. 1 kcal mol-l) is consistent with values reported for similar Eley-Rideal processes in other reaction systems.Bernasek and Somorjai20 found an activation energy of 0.6 kcal mol-l at high temperatures for the D,+H + DH+D surface reaction on stepped Pt surfaces, and McCarty et d 2 l estimated 2.5 kcal mol-l for the decomposition of acetic acid on nickel surfaces at elevated temperature.REACTION OF MOLECULAR-BEAM CHLORINE WITH IRON 71 This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U.S. Department of Energy under contract no. DE-AC03-76SF00098 and U.S. Department of Energy under contract no. W-7405-Eng-48, and by the U.S. Army Research Office, Research Triangle Park, North Carolina under contract no. 158 12-MS. Proc. Symp. High Temperature Metal Halide Chemistry, ed. D. L. Hildenbrand and D. Cubicciotti (The Electrochemical Society, Princeton, New Jersey, 1978). K. F. Kroche, H. Cremer, G. Steinborn and W. Schneider, Int. J. Hydrogen Energy, 1977, 2, 269. R. T. Frueham, Metal. Trans., 1972, 3, 2585. K. Kishi and S. Ikeda, J. Phys. Chem., 1974,78, 107. G. Dagoury, D. Vigner, M. C. Paul and J. Rousseau, Proc. 7th Znr. Vac. Congr. and 3rd Int. Conf. on Solid Surfaces, 1977, 1093. A. J. Machiels and D. R. Olander, Surf. Sci., 1977, 65, 325. R. H. Jones, D. R. Olander, W. J. Siekhaus and J. A. Schwarz, J. Vac. Sci. Technol., 1972, 9, 1429. J. D. McKinley, J. Chem. Phys., 1966, 45, 1966. @ P. C. Nordine, J. Electrochem. Soc., 1978, 125, 498. lo M.-C. Paul, Docteur Cycle (Universitb de Pans Sud, Centre D’Orsay, 1975). l1 J. A. Schwarz and R. J. Madix, Surf. Sci., 1974, 46, 317. l2 J. D. McKinley, J. Chem. Phys., 1964, 40, 120. l3 D. Rosner and H. D. Allendorf, J. Phys. Chem., 1970,74, 1829. l4 D. R. Olander and J. L. Schofill Jr, Metal. Trans., 1970, 1, 2775. l5 H. S. Carslaw and J. C. Jaeger, Conduction of Heat in Solidr (Oxford University Press, Oxford, 2nd l6 H. C. Chang and W. H. Weinberg, Surf. Sci., 1977, 65, 153. l7 H. C. Chang and W. H. Weinberg, Surf. Sci., 1978, 72, 617. edn, 1959), p. 65. M. Balooch, W. J. Siekhaus and D. R. Olander, Investigation of the Iron-Chlorine Reaction by Modulated Molecular Beam Mass Spectrometry, U.S.D.O.E. Report UCRL-88 163, 1982. R. C. Baetzold and G. A. Somorjai, J. Catal., 1976, 45, 94. 2o S. L. Bernasek and G. A. Somorjai, J. Chem. Phys., 1975,62, 3149. J. McCarty, J. Falconer and R. J. Madix, J. Catal., 1973, 30, 235. (PAPER 3/367)

ISSN:0300-9599    

DOI:10.1039/F19848000061

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J . Chem. Soc., Faraday Trans. I, 1984, 80, 73-86 Raman Studies in Micellar Sodium Octyl Sulphate Solutions BY MURRAY H. BROOKER* Chemistry Department, Memorial University of Newfoundland, St John's, Newfoundland, Canada A1B 3x7 AND DAVID J. JOBE AND VINCENT C . REINSBOROUGH Chemistry Department, Mount Allison University, Sackville, New Brunswick, Canada EOA 3CO Received 13th April, 1983 Raman spectra of micelle-forming sodium octyl sulphate have been measured as a function of concentration over the critical micelle concentration. Accurately measured frequency shifts of the C-H stretching region and relative intensities at 1080 and 3125 cm-l characteristic of gauche isomeric forms can be used as indicators of micelle formation. Quantitative Raman measurements can be used to measure solubilization of organic additives to micellar solutions.Studies of the effects of cations on the halfwidth of the 0-SO, symmetric stretching vibration indicate a strong interaction with Ca2+, perhaps even complex formation. This blocking effect may account for the decreased catalytic ability of micelles in the presence of Ca2+. Investigations in micellar systems have utilized almost every physicochemical technique available to throw light upon the properties and structures of micelles and the process by which micelles effect the solubilization of a wide variety of substances. One method that has received scant attention in this regard is Raman spectroscopy: yet in its latest refinements this technique is proving to be a sensitive and versatile indicator of changes in the microenvironments of solutes.Accurate measurements of peak maxima, peak halfwidths and band intensities can provide information about the average local field, the vibrational relaxation time (as affected by the dynamic local field) and species concentrations, respective1y.l Current techniques are largely limited to concentrations above 0.05 mol dm-3 but improved instrumentation and computer- based signal-averaging methods could lower this limit by two orders of magnitude into the range of concentrations most useful for micelle studies. The present study was initiated with three goals in mind. (i) Micelle formation: studies were performed to determine which, if any, parameters of the Raman spectrum could serve as a sensitive probe of micelle formation. Previous studies had indicated that relative intensity changes between the solid and aqueous states or as a function of temperature could be employed to follow conformational changes of the surfactant alkyl (ii) Solubilization of additives: it is well known that the solubility of organic solutes in water is significantly increased by the presence of micelles, and previous Raman studies7t8 were able to detect solute peaks.In the present study the effect of sodium octyl sulphate on the solubility of benzene in water was measured quantitatively. (iii) Effect of cations : the presence of anionic micelles has been shown to have a catalytic effect on certain ligand exchange reactions which are believed to be associated with the high charge density at the anion head group. The effects of several cations on the Raman spectra were measured to investigate this phenomenon.7374 RAMAN STUDIES OF MICELLAR SDS EXPERIMENTAL Sodium n-octyl sulphate (SOS) and sodium n-lauryl sulphate (SLS) were obtained from Eastman Kodak and were reported to be 99% pure with respect to other alkyl isomers. Reagent-grade CaC1, - 4H,O, NiC1, * 6H,O, NaCl and MgC1,. 6H,O and spectroscopic-grade benzene, n-hexane and butanol were used without further purification. Solutions were prepared by weighing the appropriate amount of solid into volumetric flasks. Solutions were treated with small amounts of activated charcoal to remove fluorescent impurities and filtered through fine frits to remove particular matter which would cause a bothersome stray-light background. The benzene-saturated water and benzene-saturated 0.50 mol dm-3 SOS solution were prepared by vigorously shaking a mixture which contained an excess of benzene and allowing the two phases several hours to separate before drawing off the appropriate layer with a syringe and recording the spectrum immediately.Solids were measured in thin-walled capillary tubes at 77 and 298 K while solutions were measured in 1 cm diameter Pyrex test-tubes at 298 K. Raman spectra were obtained with a Coderg PHO spectrophotometer equipped with photon counting and digital output. Sample excitation was achieved with either the 488.0 or 514.5 nm line from a Control model 553A argon-ion laser operating at ca. 800 mW. Narrow-bandpass interference filters were employed to remove unwanted plasma lines but these same plasma lines were used as required for frequency calibration. The standard 90° angle between the incident and scattered light was employed throughout.Two polarization techniques were used. Method one, which involved changing the polarization of the incident light with a half-wave plate, was used for rough depolarization measurements, X ( Z 5 ) Y and X( Yg) Y orientations. Method two involved placement of Polaroid analysers to accept scattered light polarized parallel or perpendicular to the incident light, X(Zz> Y and X(ZX) Y orientations. A quarter-wave plate before the slit compensated for the different response of the gratings for the two polarizations of light. Method two was employed for accurate depolarization studies.Most spectra were recorded in the normal recorder mode, but selected spectra in the 900-1 160 and 2800-3000 cm-l ranges were collected digitally and stored on a PDP 11/70 computer. Spectra obtained in this manner could be signal-averaged through repetitive scans, smoothed with a 7 point Savitsky-Golay smoothing program, baseline-corrected and curve-resolved as required. RESULTS AND DISCUSSION Raman frequency, polarization data and tentative assignments are collected in table 1 for the SOS and SLS salts in both the solid and aqueous phases. Data are presented for 0.5 mol dmP3 micellar solutions; the effect of dilution will be discussed below. Peak positions are in reasonable agreement with previously published although a number of discrepancies were noted.Surprisingly, previous workers did not report depolarization measurements. Both the polarized and depolarized spectra of SOS are shown in fig. 1. Most of the peaks of the alkyl chain are strongly polarized and must be due to symmetric vibrations. The peak at 1128 cm-l has previously been assigned to an antisymmetric C-C stretch but the depolarization ratio of ca. 0.4 puts this assignment in doubt. Peaks at ca. 1080 and ca. 1300 cm-l and the 1400-1470 cm-l envelope are clearly depolarized. The depolarized peaks at 420-460 and 580-630 cm-l and the strongly polarized peak at 1063 cm-' are due to the sulphate head groups and are similar to observations of HSO;.l. The polarized peak at ca. 827 cm-l appears to be due to the C-0-SO; stretch. Except for the symmetric stretching mode of -0-SO; the peak frequencies do not differ significantly from the solid to aqueous phase.The symmetric stretching mode of -0-SO; occurs at ca. 1063 cm-l in the aqueous phase but at 1084 cm-l in the solid salts. Attempts to measure the effects of micelle formation on the C-C stretching vibrations in the 1020-1 150 cm-l region were complicated by the presence of the intense -0-SO; peak and special efforts were required to identify and measure peaks in this region.75 M. H. BROOKER, D. J. JOBE AND V. C. REINSBOROUGH Raman studies of n-hexane indicate that the -CH,-CH, chain has peaks at ca. 1010 p, 1040 p, 1066 dp, 1080 dp and 1140 p (fig. 2). In solid SOS and SLS the -O-SO, peak tends to mask the C-C stretching peak at ca. 1080 cm-l, although peaks at ca.1076 (sh), SOS, and 1081 cm-l, SLS, could be resolved in the solid at 77K along with a peak at 1063 cm-l due to another C-C stretching vibration (fig. 2). For the aqueous micellar solution, the 1081 cm-l C-C stretching peak appears as a shoulder on the 1063 cm-l 0-SO, stretching peak in the polarized spectrum but as a distinct peak in the depolarized spectrum (fig. 1 and 3). Intensity from a C-C vibration probably contributes to the 1063 cm-l peak intensity and may provide most of the intensity in the depolarized spectrum (fig. 3), since the depolarization ratio of the 0-SO, symmetric stretching vibration is expected to be much smaller than the measured value of 0.10. The presence of the 1045 cm-l peak for the aqueous micellar solution can only be inferred from attempts to curve-fit the 1000-1 150 cm-l envelope (table 1).The C-H stretching regions of aqueous SOS and SLS were essentially totally polarized and must be due to symmetric stretching vibrations (fig. 4). Previouslys the peak at 2930cm-l was assigned to an antisymmetric stretching vibration, but this cannot be correct. In the depolarized spectrum the most intense peak (2897 cm-l) does not even coincide with a peak in the polarized spectrum. Several sharp peaks on the C-H profile of SLS reported by Kalyanasundaram and Thomas3 were not observed in this work (fig. 5) and would appear to be due to plasma lines superimposed on their spectra. Studies of the effect of SOS and SLS on the water spectrum revealed almost no change from the spectrum of pure water.The hydrogen-bond peak at ca. 180 cm-l is reasonably sensitive to structure-breaking effects;' therefore the present results indicate that the micelle does not disrupt the water structure significantly. MICELLE FORMATION The critical micelle concentration (c.m.c.) for SOS is 0.1 15 mol drn-,,1° so the concentration range chosen for this study was 0.05-0.50 mol dm-3. The 0.05 mol dm-3 concentration represented the practical lower limit for reasonably resolved spectra (4 cm-l resolution) recorded with a single scan. Studies of micelles with lower c.m.c. would require signal averaged spectra. The spectra of SOS above and below the c.m.c. showed only subtle differences. The most significant change in the spectrum occurred in the C-H stretching region from 2850 to 2960 cm-l, where all the peak maxima shifted by 5 cm-l to lower values from the dilute solution of monomers to the micellar solution (fig.4 and 6). The plot of the peak maximum against concentration for several of the peaks indicated that the shift occurred fairly abruptly over a 0.1 mol dm-, concentration range (fig. 7). Similar results were obtained for the shift which occurred for the C-H stretching region of butanol between pure butanol and a saturated butanol +water solution. These results suggest that in general one should expect a ca. 5 cm-l shift to lower frequency for the C-H stretch when an aqueous environment is replaced by a hydrocarbon environment. Small shifts of 1 or 2 cm-l in the same direction may also occur in the 1 128 and 108 1 cm-l peaks over the same concentration range (fig.3). Previous workers have not measured the peak frequencies with significant precision to detect the above shifts. It would appear that even these small changes could be used to follow micelle formation. In the context of the present controversy regarding the degree of hydration of surfactant chains within micellesll* l2 it would thus appear that in the picosecond observation range of the Raman technique the carbons of the octyl sulphate chain are predominantly not wetted in the micelle. It must be stressed that sodium octyl sulphate micelles are relatively unstable andTable 1. Raman peak positions (cm-l) for sodium octyl sulphate and sodium lauryl sulphatea sodium octyl sulphate sodium lauryl sulphate 0.5 mol dm-, (aq) solid, 298 K 0.5 mol dm-3 (as) solid, 298 K assignment ca.180 br, dp 350 w, p 422 m, dp ca. 460sh,dp 510, 526 w, p 582 m, dp 626 w, dp 735 w, p 767 w, p 810 m, p 826 m, p 846 m, p 877 m, p 895 m, p 930 w, p 968 w, p 1016 m, p ( 1045)b w, p (1060y w, dp 1063 s, p 1081 w, dp - ca. 360w 422 m ca. 430 - 579 m 595 w, 629 w 760 br 812 m 846 m 892 m 928 m 964 w 1014 w 1040 w 1063 m 1084 s ca. 1076 sh - - - ca. 180 br, dp 360 w, p 420 m, dp - - 584 m, dp ca. 620w,dp 735 w, p 765 w, p 827 m, p 867 m, p 890 m, p - - - - ca. 1015w,p - (1060)c w, dp 1063 s, p ca. 1080 sh, dp - ca. 360w 406 w, 420 m 503 w ca. 575 m, 598 m ca. 630 w 770 w - - - - 837 m 867 890 m - - 1020 w 1063 m 1085 s 1081 - H,O hydrogen bond CCO deformation -SO; rock -SO,- deform C-O-S03- str CH, rock CH, rock CH,-0 CH, twist C-C sym str 0-SO,- sym str C-C sym str1128 m, p (partly) 1152 w, p 1190 w, p 1218 w, dp 1250 w, dp 1305 m, dp 1370 w, p 1392 w, p 1441 m, dp 1455 m, dp 1470 sh, dp 1637 br, p 2859 s, p 2875 vs, p 2904 s, p 2934 s, p 2963 m, p ca.3250vs,p CQ. 3410vs,p - - 1128 m 1150 sh, w 1196 w 1218 w 1262 w 1300 m - ca. 1400 w 1440 m 1456 m 1470 sh 2850 s 2862 s 2876 s 2885 s 2902 s 2914 s 2934 s 2962 m - 1128 m, p 1145 sh, p 1129 m ca. 1150 w C-C str - 1294 m, dp 1370 w, p 1395 w, p 1437 m, dp 1453 m, dp 1468 sh, dp 1637 br, p 2854 vs, p 2875 s, p 2901 s, p 2931 s, p 2962 m, p ca. 3250vs, p ca. 3410 vs, p - - - - 1296 m 1373 w 1436 m 1455 m - - 2862 m 2882 vs 2897 s 2934 m 2960 m - - - 1 0-SO; antisym str CH, twist CH2 scissor v 2 H2O CH2 C-H sym str and C-H sym str of CH, P ? a !z ?i U 5 0 a p, polarized peak; dp, depolarized peak; w, weak; m, medium; s, strong; br, broad; sh, shoulder.Inferred from curve-resolution attempt. Inferred from studies of other phases.78 RAMAN STUDIES OF MICELLAR SDS loosely packed, so that in micelles where the chains are longer there should be even less hydration within the micelle. Previous studies of alkyl carboxylates and alcohols have shown that the relative intensity changes between solid and aqueous solution and as a function of temperature for the 1040-1 140 cm-l region and that the 2800-3000 cm-l region can be used to 1305 1081 I I I I r I I I I I I I I 1 I I 1600 1400 1200 1000 800 600 LOO 200 wavenumber/crn-' Fig. 1. Raman spectra of 0.50 mol dm-3 SOS obtained with the 514.5 nm line at 4.0 cm-' slit width for the parallel (a) and perpendicular (b) polarizations. estimate the relative concentrations of trans and gauche isomer~.~-~ For instance a decrease in the relative intensities of peaks at ca.2925 and 1080cm-l has been associated with an increase in the trans isomer and a decrease in the gauche isomer. Comparison of the dilute SOS monomer solution (0.070 mol dm-3) to that of the SOS micellar solution (0.50 mol dm-3) indicated a decrease in relative intensities at 2925 and 1081 cm-l for the micellar solution (fig. 6). These results indicate that the more randomly ordered gauche form of t5e alkyl chain is more stable in a dilute aqueous environment and that formation of micelle aggregates favours the more ordered trans isomers.For both the studies it was necessary to record the spectra digitally and to apply a baseline correction to the data so that the spectra could be directly super- imposed for comparison. The computer-collected spectra for the C-H region also show the ca. 5 cm-1 offset required to overlay the maxima (fig. 6). Unfortunately the symmetric stretching frequency of 0-SO; at 1063 cm-l dominates the polarized spectrum in the C-C region and, although the 1081 cm-l peak can be seen as a shoulder on the 1063 cm-l peak (fig. 8), it was difficult to measure relative intensity changes with concentration. Fortunately the 0-SO; band is completely polarized while the 1081 cm-l band is depolarized, and better resolution is possible in the depolarized spectrum (fig.1 and 3). Because the intensities of the peaks were weak the measurements were made for 10 scans smoothed and averaged for both the dilute and concentrated SOS solutions. The results (fig. 3) clearly indicated that the 108 1 cm-l peak decreased in relative intensity for the 0.50 mol dm-3 micellar solution.M. H. BROOKER, D. J. JOBE AND V. C. REINSBOROUGH 19 1200 1100 1000 w avenum berlcrn-' Fig. 2. Raman spectra of (a) n-hexane liquid at room temperature, (b) solid SOS at 77 K and (c) solid SLS at 77 K. The 514.5 nm laser line and 2.0 cm-l slit widths were employed. SOLUBILIZATION OF BENZENE Organic molecules can have greatly enhanced solubility in micellar solutions com- pared with pure water, It is desirable to establish techniques to measure the solubility directly and to establish the micellar site of solubilization.To obtain information on this subject studies were performed on saturated solutions of aqueous benzene and aqueous benzene in 0.50 mol dm-3 SOS. At 25 OC the solubility of benzene in water is 0.01 1 mol dm-3 and the two major peaks of benzene, the 991 cm-l ring-breathing80 RAMAN STUDIES OF MICELLAR SDS and the 3070 cm-l C-H stretch, were easily observed in the Raman spectrum (fig. 9). The 991 cm-I peak occurs at the same frequency for benzene in water as for pure benzene but the C-H stretching peak at 3072 cm-l is ca. 8 cm-l higher for benzene in water than for pure benzene (3062 cm-l). Again the C-H stretching frequency is sensitive to whether the environment is aqueous or hydrocarbon.The presence of wavenumber/cm-' Fig. 3. Depolarized Raman spectra, X(ZX) Y orientation, for (---) 0.07 mol dm-3 and (-) 0.50 mol dm-3 SOS to illustrate the intensity decrease of the 1081 cm-' peak and slight frequency shifts which accompany micelle formation. Spectra are smoothed averages of 10 scans which have been baseline corrected. Spectra were obtained with the 488.0 nm line and 2.0 cm-l slits. 0.5 mol dm-3 SOS greatly enhanced the solubility of benzene (fig. 9). Measurement of the relative intensity of the benzene peaks to the water peaks at 3400 and 1637 cm-l were used to measure a 7.0 times increase in the solubility of benzene in 0.50 mol dm-3 SOS over that of pure water. Jobe et all3 found a roughly ten-fold increase in benzene solubility at the same SOS concentration from ultraviolet studies but only after several days of equilibrium.It usually requires at least a day before maximum uptake of solubilizate is achieved in micellar solutions. Since we were not particularly interested in benzene solubilities, this aspect of the work was not pursued further. However, it is clear that measurements of relative intensities of Raman lines could easily be utilized to monitor solubilizate concentrations. More precise quantitative measure- ments could be achieved if an internal standard anion such as NO; or ClO, were employed since these ions have peaks which are more easily measured than those of water. Evidence that the benzene dissolves in the micelle and is removed from bulk water comes from the fact that the C-H stretching frequency of benzene in 0.50 mol dmP3 SOS at 3062 cm-l corresponds to the value in pure benzene and not to the value for benzene in pure water (3072 cm-I).Other techniques which throw light on solubiliza- tion sites in micelles confirm that benzene in concentrations > 0.01 mole fraction is principally located in the micellar ~ o r e . ' ~ - ' ~ The Raman results are in agreement. Since similar results were obtained for both the 488.0 and 514.5 nm laser lines it wouldM. H. BROOKER, D. J. JOBE AND V. C. REINSBOROUGH 81 6 I - - cm-' 2897 x 2 3 000 2900 wavenumber/cm-' 2 000 Fig. 4. Raman spectra for the C-H stretching region for SOS solutions of (a) 0.07 and (6) 0.50 mol dm-3 concentration to illustrate frequency shifts and small depolarization ratio. Spectra were obtained with the 488.0 nm line and 2.0 cm-l slits.appear that the Raman signal due to benzene was not resonance enhanced by interactions with the hydrocarbon environment as has been reported for anthracene by Beck and Brus.ls EFFECTS OF IONS ON sos It has been shown that micelles serve as catalysts in ligand exchange reactions and it is generally believed that the high charge density at the polar micelle surface enhances the ligand exchange of reacting ~ati0ns.l~ In order to investigate this phenomenon we have studied the effect of various cations on the spectrum of SOS. Particular emphasis was placed on the 0-SO; symmetric stretch because studies of aqueous HSO, and SO:- have demonstrated that the frequency shifts, halfwidth82 500 - 400 - 300- 200 - 100- I 0 RAMAN STUDIES OF MICELLAR SDS I I 2882 2897 9 I , l , , , , l , , , , ~ ( , L 2950 2850 2700 wavenum berlcm-' Fig.5. Raman spectra of the C-H stretching region for (a) solid SLS and (b) solid SOS, at room temperature. Spectra were obtained with the 514.5 nm line and 2.0 cm-l slits. 600 4 IM. H. BROOKER, D . J. JOBE AND V. C. REINSBOROUGH -2.0 IB I!, -L.O -6.0 83 + * 0 A + - 1 5 + - 0 0 - + A + Q 0 0 Q * I - I 0.1 0.2 0.3 0.4 concentrationlmol dm-3 Fig. 7. The frequency difference (v-v,,) between the peak maxima of C-H vibrations from the dilute solution value, vD, and the value observed at a specific concentration, V, plotted against concentration of SOS: 0, 2864; +, 2879; *, 2941; A, 2069 cm-l; 0, all points. I ~ I I I I ~ I I I I ~ I I I I ~ I I I 1150 1100 1050 1000 wavenum ber/cm -l Fig.8. Raman spectra of the 1000-1150 cm-l region of (a) 0.070 mol dm-3 SOS, (b) 0.50 mol dm-3 SOS and (c) 0.50 mol dm-3 SOS + 2.5 mol dm-3 CaC1,. Spectra were obtained with the 488.0 nm line and 2.0 cm-l slit. 4 FAR 184 RAMAN STUDIES OF MICELLAR SDS 3062 'L A I t , , , I , , , , l , , , , I , , , , I , , 31 00 3050 11 00 1000 wavenumber/cm-' Fig. 9. Portions of the Raman spectra for (a) benzene-saturated and 0.50 rnol dm-3 SOS and (b) benzene-saturated water. Spectra were obtained with the 488.0 nm line at 2.0 cm-l slits. Table 2. Peak positions and halfwidths for the symmetric stretching frequency of aqueous sodium octyl sulphate with different added salts (A) SOS, 0.070 mol dmP3 1063.0 13.5 B+2.5 mol dmP3 HC1 1063.0 14.5 NaCl 1 064.5 14.0 MgCl2 1064.0 12.5 NiC1, 1064.0 13.2 CaCl, 1065 .O 17.0 (B) SOS, 0.50 rnol dm-3 1063 .O 12.0 changes and even doubling of peaks due to complex formation can be used to gain information about the ionic interactions.' In the present study the halfwidth and peak frequencies were measured for solutions of 0.070 mol dm-3 SOS, 0.5 mol dm-3 SOS and 0.5 mol dm-3 SOS plus 2.5 mol dm-3 HCl, NaCl, MgCl,, NiCl, and CaCl,.The results are presented in table 2. Only the symmetric stretching 0-SO; peak at 1063 cm-l exhibited a measurable change due to the addition of salt. The presence of added salt caused a small increase in peak position and halfwidth. In studies of other systems it has been found that frequencies of concentrated solutions tend toward the values of the solid salts, i.e.1084 cm-l for pure SOS solid. Halfwidth changes ofM. H. BROOKER, D. J. JOBE AND V. C. REINSBOROUGH 85 the isotropic polarized symmetric stretching frequency will be primarily due to the vibrational dephasing effects of elastic collisions due to changes in the environment about the 0-SO; group. Note that the concentration increase from 0.070 to 0.50 mol dm-3 was accompanied by a 1.5 cm-l halfwidth decrease from 13.5 to 12.0 cm-l. This could be interpreted to indicate a less dynamic environment about the 0-SO; micellar head groups. The magnitude of the effect of cation on the 0-SO; head group as measured by the peak frequencies and halfwidth changes follows the order: Mg2+ < Ni2+ < H+ < Na+ < Ca2+.Calcium exhibits the greatest influence on both the halfwidth and peak maximum and attests to the strong interaction between Ca2+ and the 0-SO; head group. The high-frequency asymmetry of the peak (fig. 8) may be an indication that the Ca2+ resides on the micelle head group in an inner-sphere-type interaction long enough to create a second type of 0-SO; environment in equilibrium with the uncomplexed site. Hicks and Reinsboro~ghl~ have found that the presence of small amounts of Ca2+ significantly reduced the catalytic effect of micelles for the ligand exchange reaction of Ni(H,O);+ with pyridine-2-azo-p-dimethylaniline (PADA), whereas Mg2+ and Na+ additions of the same scale had little effect. Calcium ion apparently competes much more effectively than nickel ion for adsorption into the Stern layer of the micelles.The present Raman results corroborate this view. The very small effect of Mg(H20)2+ and Ni(H,O);+ is not unexpected since the ions are strongly hydrated and will not readily lose the hydrated water, which would be a necessity for the Mg2+ or Ni2+ to form an inner-sphere complex with the head group. Peaks due to the M-0 symmetric stretching vibration of the M(H,O);+ complex were observed in the present study at 355 and 395 cm-l for the Mg(H,O);+ and Ni(H,O);+ complexes. No similar bands have been reported for aquated Ca2+ and Na+ ions, a fact which attests to the weaker hydration of these i0ns.l These results are similar to complex formation studies of NO; and NO; where Ca2+ accepts these weak ligands much more readily than does Mg(H,O);+ or Ni(H,O);+.l The effect of HCl(aq) was very small but one could predict a greater effect if the H+ concentration were increased to > 10 mol dm-3 since at this concentration the proton would be forced onto the 0-SO; group.The effect of Ca2+ on the SOS solutions was also studied as a function of concentration of SOS to concentrations below the c.m.c. of the pure SOS (0.1 15 mol dm-3). The peak positions and halfwidths of the SOS for a 2.5 rnol dm-3 CaCl, concentration were unaffected by decreasing SOS concentrations as low as 0.08 mol dm-3. These results indicated that the Ca2+ - - - OSO; interaction was not affected by the SOS concentration but it was not possible to determine whether the Ca2+ - * 0-SO; interaction was the same for monomer and micellar forms of SOS because as indicated by the C-H stretching frequencies the presence of 2.5 mol dm-3 CaCl, had lowered the c.m.c.below 0.08 mol dm-3 and prevented the study of Ca2+ with monomer, Generous support of the National Science and Engineering Research Council of Canada for separate grants to M. H. B. and V. C. R. is gratefully acknowledged. 4-286 RAMAN STUDIES OF MICELLAR SDS D. E. Irish and M. H. Brooker, in Advances in Infrared and Raman Spectroscopy, ed. R. J. H. Clarke and R. E. Hester (Heyden, New York, 1976), vol. 2, chap. 6, p. 212. J. L. Lippert and W. L. Peticolas, Proc. Nut1 Acad. Sci. USA, 1971, 68, 1572. K. Kalyanasundaram and J. K. Thomas, J. Phys. Chem., 1976,80, 1462. H. Okabayashi, M. Okuyama, T. Kitagawa and T. Miyazawa, Bull. Chem. SOC. Jpn, 1974,47, 1075. H. Okabayashi, M. Okuyama and T. Kitagawa, Bull. Chem. SOC. Jpn, 1975,48, 2264. K. Larsson, Chem. Phys. Lipids, 1972,9, 181. J. B. Rosenholm, K. Larsson and N. Dinh-Nguyen, Colloid Polym. Sci., 1977, 255, 1098. T. Takenaka, K. Harada and T. Nakanaga, Bull. Inst. Chem. Res. Kyoto Univ., 1975, 53, 173. H. Chen and D. E. Irish, J. Phys. Chem., 1971, 75, 2672; D. E. Irish and H. Chen, J. Phys. Chem., 1970, 74, 3796. lo P. D. I. Fletcher and V. C. Reinsborough, Can. J. Chem., 1981, 59, 1361. l1 H. Wennerstrom and B. Lindman, J. Phys. Chem., 1979,83, 2931. l* F. M. Menger and B. J. Boyer, J. Am. Chem. SOC., 1980, 102, 5936. l3 D. J. Jobe, V. C. Reinsborough and P. J. White, Can. J. Chem., 1982, 60, 279. l4 J. R. Hicks and V. C. Reinsborough, Surfactants in Solution, ed. K. L. Mittal and B. Lindman l5 S. J. Rehfeld, J. Phys. Chem., 1970, 74, 117. l6 S. M. Beck and L. E. Brus, J. Chem. Phys., 1981, 75, 1031. l7 J. R. Hicks and V. C. Reinsborough, unpublished results. (Plenum Press, New York, in press). (PAPER 3/590)

ISSN:0300-9599    

DOI:10.1039/F19848000073

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1984,80, 87-97 Interaction of Hydrogen Chloride with a Molybdena-Silica Catalyst BY S. RAZI SEYEDMONIR Department of Chemical Engineering, Pennsylvania State University, University Park, Pennsylvania 16802, U.S.A. AND RUSSELL F. Horn* Department of Chemistry, University of Auckland, New Zealand Received 18th April, 1983 The interaction of HCl with a molybdena-silica catalyst has been investigated using e.p.r. and infrared spectroscopy. Exposure of reduced catalysts to HCl at room temperature causes a large (up to 20-fold) increase in the amount of MoV detected by e.p.r. which is reversed on subsequent outgassing. The accompanying changes in g- and the 9 5 M ~ hyperfine-tensor components indicate that chloride ions replace oxide ions in the coordination sphere of MoV, and the increase in MoV spin concentration is attributed to substitution of bridging oxide ligands between magnetically coupled MoV ions.Oxidized catalysts are reduced by exposure to HC1. The reaction of HC1 with molybdena-silica is compared with that previously reported with molybdena-alumina catalysts. Supported molybdena catalysts are widely used for reactions such as hydrogenation of olefins and aromatics, polymerization, metathesis and oxidation of olefins, dehydrogenation and dehydrocyclization of paraffins, hydrodesulphurization and hydrodenitrogenation of petroleum fractions and conversion of synthesis gas to methane and Fischer-Tropsch products.'q Many characterization studies have been undertaken of such catalysts with the objectives of identifying the molybdenum- containing phases present in the catalysts, determining the coordination and valence states of molybdenum in oxidized and reduced forms of the catalysts and ultimately identifying the active sites for various reactions.E.p.r. spectroscopy is a technique which has been employed to follow the reduction of molybdenum from the + 6 to the + 5 valence state during catalyst activation. In the case of alumina-supported molybdena catalysts, the amounts of MoV measured by e.p.r. are significantly lower than those determined by chemical or X-ray photoelectron spectroscopic method^.^-^ This discrepancy was first attributed by Hall et al. to magnetic coupling between adjacent MoV ions on the catalyst surface, such that only isolated MoV ions contributed to the observed e.p.r.spectrum. We have recently presented direct evidence for coupling between MoV ions involving bridging oxide ligands on alumina-supported molybdena catalysts. Reaction of the reduced catalysts with gaseous hydrogen chloride or hydrogen bromide caused a 3-fold increase in the MoV spin concentration determined by double integration of the first- derivative e.p.r. signal. The accompanying changes in the MoV g and 9 5 M ~ hyperfine- tensor components indicated that halide ions were replacing oxide ions in the coordination sphere of MoV, reducing the magnitude of the MoV: MoV interactions and allowing previously invisible MoV to be detected by e.p.r. 8788 INTERACTION OF HCl WITH A MOLYBDENA-SILICA CATALYST The purpose of the present study was to investigate the magnetic interactions between MoV ions in silica-supported molybdena catalysts, using the reaction with hydrogen chloride as a probe. Silica-supported molybdena catalysts differ from the more widely studied molybdena-alumina catalysts in several important respects.In general, the interaction of molybdena with silica is much weaker than that with alumina.8-11 A 'free' MOO, phase is detected on silica at molybdenum loadings well below that corresponding to monolayer coverage of the support, whereas on alumina the MOO, phase appears only after the monolayer capacity of the support has been exceeded. The various molybdenum phases on both supports have similar structures, according to diffuse reflectance9 and laser Ramanlo studies, but temperature- programmed reduction measurements have shown that at least part of the molybdenum on alumina is more difficult to reduce than that on silica.12 Some differences in the extent and magnitude of MoV : MoV interactions on silica- and alumina-supported catalysts were thus anticipated.EXPERIMENTAL The molybdena-silica catalyst contained 6.2 % molybdenum by weight on silica gel (Davison grade 950, 700 m2 g-l) and was prepared by impregnation of the support with an aqueous solution of ammonium dimolybdate, drying at 423 K and calcination in air at 773 K for 18 h. A v5Mo-enriched catalyst containing 5.5% molybdenum by weight was prepared as follows: 9 5 M ~ metal (97% enriched, Oak Ridge National Laboratory) was dissolved in 50% nitric acid and the solution evaporated to dryness. The resulting precipitate was dissolved in 5% ammonium hydroxide at 340 K and again evaporated, followed by dissolution in distilled water and addition of the silica support.The catalyst was subsequently dried and calcined as above. Standard pretreatment of the catalysts involved heating in uacuo to 723 K followed by calcination in oxygen at this temperature and further evacuation. Reduced catalysts were prepared by subsequent exposure to hydrogen (100 Ton*) for 30 min at the desired temperature and further evacuation at this temperature for 30 min. Research grade hydrogen was purified by passage through a Pd-Ag thimble at 673 K. Anhydrous hydrogen chloride (Matheson) was subjected to repeated freeze-pumpthaw cycles before use.E.p.r. spectra were recorded (at room temperature unless otherwise stated) with a Varian El 15 spectrometer at 9.5 or 35 GHz. Catalyst samples were prepared in a Pyrex high-vacuum cell fitted with a quartz sidearm for e.p.r. measurements. Spin concentrations were determined by numerical double integrationI3 of the MoV signals and comparison with a Varian strong pitch sample which had in turn been calibrated against a fresh single crystal of CuSO, - 5H,O (using a dual sample cavity). Relative spin concentrations are considered to be accurate to & 10% and absolute spin concentrations to & 30%. Simulated e.p.r. spectra were obtained with the program ~ 1 ~ 1 3 . ' ~ Infrared spectra were recorded with a Nicolet MX1 Fourier-transform infrared spectrophotometer, using a conventional high-vacuum cell allowing in situ treatment of pressed wafers of catalyst.RESULTS Reduction of the molybdena-silica catalyst in hydrogen (100 Torr at 773 K for typically 30 min) produced an intense e.p.r. signal characteristic of MoV.15-17 Fig. 1 (a) shows this signal recorded at 9.5 GHz and 298 K. Identical line shapes were observed in spectra recorded at 77 and 20 K. The integrated intensity of the signal in this particular experiment corresponded to 8 x 10l8 spin g-l of the catalyst. Subsequent exposure to HCl (100 Torr, 298 K) caused no change in the colour of the reduced catalyst (dark grey), but the e.p.r. spectrum gave the new signal shown in fig. l(b). The changes in line shape resulting from HC1 addition were accompanied by an * 1 Torr = 101 325/760 Pa.S.RAZI SEYEDMONIR AND R. F. HOWE 89 Fig. 1. X-band e.p.r. spectra of reduced catalyst: (a) before exposure to HC1 and (b) after exposure to HCl. Relative spectrometer gains are indicated on each spectrum. Table 1. Integrated MoV signal intensities MoV spin concentration catalyst treatment /1Ols spin g-l oxidized catalyst reduced in H,, 773 K + HCl outgassed at: 300 K 373 K 473 K 673 K oxidized catalyst + HCI outgassed at: 300 K 373 K 473 K 0.3 8.2 186 77 38 30 9.1 1.7 1.7 1.8 1.7 approximately 20-fold increase in integrated signal intensity (relative spectrometer gain settings are indicated on each spectrum). The new signal appears to have an almost symmetric line shape at 9.5 GHz (which was unchanged on cooling to 77 K), but a spectrum measured at 35 GHz (not shown) revealed that the g tensor of the new signal is in fact orthorhombic, with principal components 1.967, 1.955 and 1.942.Subsequent outgassing of the catalyst, beginning at a temperature of 298 K and increasing in 100 K steps (30 min at each temperature), caused a gradual return to the original spectrum. Table 1 gives the measured signal intensities (expressed as spin concentrations) at several points in this experiment. A significant decrease in intensity90 INTERACTION OF HCl WITH A MOLYBDENA-SILICA CATALYST 100 G - g= 2.0028 '~ Fig. 2. X-band e.p.r. spectra of reduced g5Mo-enriched catalyst: (a) before exposure to HCI and (b) after exposure to HCl. Relative spectrometer gains are indicated on each spectrum. I 1 V I V g ='2.002 8 Fig 3.X-band e.p.r. spectra of reduced g5Mo-enriched catalyst exposed to HCl then evacuated at (a) 300 (b) 473 and (c) 773 K. Relative spectrometer gains are indicated on each spectrum.S. RAZI SEYEDMONIR AND R. F. HOWE 91 was caused immediately by evacuation at 300 K, and the original spin concentration and signal shape were restored after evacuation at 673 K. The spectra obtained at outgassing temperatures between 300 and 673 K showed a signal shape intermediate between those in fig. 1 ( a ) and (b). Spectra obtained from a similar experiment with a 95Mo-enriched catalyst are shown in fig. 2 and 3. The MoV signal of the reduced catalyst shows in this case 2 overlapping sets of 6 lines due to the parallel and perpendicular components of the 9 5 M ~ hyperfine 1 1 1 I I 1 M 11 Fig.4. X-band e.p.r. spectra of oxidized catalyst: (a) before exposure to HC1 and (b) after exposure to HCl. Identical spectrometer gains in both spectra (Mn indicates manganese impurity). tensor. (A similar spectrum of a 95Mo-enriched molybdena-silica catalyst has been published by Che el aZ.17) Addition of HCl to the reduced catalyst at room temperature gave the spectrum in fig. 2(b). The increase in signal intensity appears in the first- derivative spectra to be less than that observed for the normal catalyst, but double integration of the signals revealed that the increase in spin concentration is in fact of similar magnitude to that found for the unenriched catalyst (the increased line width of the new signal partially obscures the intensity increase).Fig. 3 shows a series of spectra recorded following subsequent outgassing at successively higher temperatures. Evacuation at 300 K significantly altered the signal shape [compare fig. 3 ( a ) with fig. 2(b)], and the original shape and intensity were completely restored after evacuation at 673 K. Catalysts given the standard pretreatment and not reduced in hydrogen are white in colour and contain molybdenum almost entirely in the +6 valence state. Fig. 4(a) shows an e.p.r. spectrum of an oxidized catalyst; the intensity of the weak residual MoV signal corresponds to a spin concentration of 3 x IOl7 g-l. Exposure of this sample to 150 Torr of HCl gave the spectrum shown in fig. 4(b) and caused a colour change to orange-yellow. The new signal has identical g-tensor components to that obtained from reduced catalysts treated with HCl, although noticeably better92 INTERACTION OF HCl WITH A MOLYBDENA-SILICA CATALYST Fig.5. X-band e.p.r. spectra of oxidized 95Mo-enriched catalyst: (a) exposed to HCI and evacuated at (b) 300, (c) 373, (d) 473 and (e) 773 K. Relative spectrometer gains are indicated on each spectrum. resolved. The MoV spin concentration increased approximately 6-fold. Subsequent outgassing did not in this case restore the original spectrum. Evacuation at 473 K gave a signal identical in shape to that of a catalyst reduced in hydrogen, without any change in integrated intensity, and no further changes occurred on outgassing at higher temperatures (table 1). Fig. 5 shows spectra obtained in a similar experiment with a 95Mo-enriched catalyst.S.RAZI SEYEDMONIR AND R. F. H O W 93 Exposure of the oxidized catalyst to HCl gave the spectrum in fig. 5(a). This shows 9 5 M ~ hyperfine components at identical positions to those for a reduced catalyst treated with HC1 [fig. 2(b)]; however, the signal shape is significantly different. Subsequent outgassing progressively converted the new signal to that of a reduced catalyst, without causing any change in spin concentration. Fig. 5 (d) illustrates that after outgassing at 473 K the new signal was almost completely removed. The effect of HC1 adsorption on the infrared spectrum of the oxidized catalyst was also investigated. Exposure to HCl at 298 K caused a significant change in the v(0H) region (3800-3400 cm-l).Intense new bands appeared at 3720 and 3560 cm-l, superimposed on the existing bands at 3747 and 3650 cm-l due to hydroxyl groups of the silica support. Similar changes in the v(0H) region could be induced by exposing the oxidized catalyst to water vapour at 298 K. In this case, however, a weak band also appeared at around 1624 cm-l, whereas the 1624 cm-l band was not detected following exposure to HC1. Outgassing above 473 K restored the original spectrum in the v(0H) region. In the region 800-1000 cm-l the infrared spectrum of the oxidized catalyst shows three bands due to v(Mo-0) vibrations of the MOO, and surface molybdate phases.18 Exposure to HCl caused the complete removal of all three bands. On subsequent outgassing the Mo-0 bands were only partially restored.DISCUSSION Exposure of reduced catalysts to HCl caused significant changes in both the intensity and e.p.r. parameters of the MoV signal. The g- and 9 5 M ~ hyperfine-tensor components of the observed signals are listed in table 2. The g-tensor components of the new signal were obtained by inspection from the Q-band spectrum. Two of the 9 5 M ~ hyperfine-tensor components of the new signal were obtained by inspection from the X-band spectrum of the 95Mo-enriched catalyst [fig. 2 (b)] and the third determined by computer simulation. Fig. 6 (b) shows a computer-simulated spectrum generated with the parameters listed for the new signal in table 2. Agreement between the observed and calculated peak positions was found to be extremely sensitive to the val- ues given to the 9 5 M ~ hyperfine-tensor components.The perpendicular components (Azz and A,,), which are not resolved in the experimental spectrum, were adjusted to give the best fit of the experimental peak positions. Note, however, that the simulated spectrum in fig. 6(b) does not match at all the observed intensity distribution [fig. 2 (b)]. The observed intensity distribution could be satisfactorily simulated only by adding a second signal to the simulation, as illustrated in fig. 6. The composite spectrum in fig. 6(a) [which should be compared with fig. 2(b)] was obtained by adding the simulated spectrum in fig. 6(b) to the broad symmetric signal in fig. 6(c), in the ratio 1 : 9. The parameters of the broad signal and its quantitative contribution to the total integrated intensity cannot be accurately determined from the simulations, since the broad signal could not be observed independently, and the intensities of simulated signals are extremely sensitive to the line widths and line-shape functions employed.For the purposes of simulation the broad signal was generated with the following parameters: g, = 2.030, g, = 1.952, g, = 1.870 and Lorentzian linewidths of 80 G. Nevertheless, the increase in total integrated signal intensity occurring on exposure to HCl must be attributed largely to the contribution from the broad signal. Comparison of the e.p.r. parameters of the signals observed before and after exposure of reduced catalysts to HCI (table 2) reveals that the g-tensor components are reversed by HCI treatment.The normal MoV signal obtained from thermally re- duced catalysts has g,, (associated with the largest 9 5 M ~ hyperfine-tensor component)94 INTERACTION OF HCl WITH A MOLYBDENA-SILICA CATALYST Table 2. E.p.r. parameters of MoV signals catalyst or A x x l A y y l A z z / compound ref. g x x g y p g,, cm-l cm-l lop4 cm-l reduced molybdena- this work 1.947 1.947 1.892 41 41 85 (19) 1.940 1.940 1.882 41 41 91 silica reduced molybdena- this work 1.954 1.944 1.967 36 30 73 oxidized molybdena- this work 1.954 1.944 1.967 36 30 73 silica + HCl silica + HCl alumina + HCl reduced molybdena- (7) 1.951 1.943 1.962 34 34 76 MoOC1,2- (19) 1.94 1.94 1.963 33 33 75 MoOC14- (19) 1.950 1.950 1.967 35 35 73 Fig. 6. Simulated e.p.r. spectra: (Q) superposition of signals (b) and (c) in the ratio 1 : 9; (b) signal simulated with the parameters given for the HC1-treated catalyst in table 2, and Lorentzian line widths of 12.5 G; (c) broad signal simulated with the parameters described in the text.less than g,, and gyy, whereas this order is reversed in the HCl-treated catalysts. A similar reversal of the g-tensor components was observed previously for alumina- supported catalysts treated with HCl,' and is commonly found in coordination compounds of MoV containing chloride ligands. l9 Manoharan and Rogers20 first attributed the g-tensor reversal to spin-orbit coupling from the ligandsS. RAZI SEYEDMONIR AND R. F. HOWE 95 [A(Cl) = 586 cm-l compared with A ( 0 ) = 152 cm-l], although it has been suggested more recently that the dominant g-shift mechanism in chloromolybdenum complexes involves low-lying charge-transfer states rather than ligand spin-orbit coupling.21 The evidence against the spin-orbit coupling model is that no correlation exists between g,, and the number of chloride ligands coordinated to Mo,.The reversal of the g tensor of MoV on silica following treatment with HCl is thus due to replacement of oxide ions in the coordination sphere of molybdenum by chloride ions, but the number of chloride ligands cannot be determined from the e.p.r. parameters. The changes in the e.p.r. spectra of reduced catalysts following exposure to HCl indicate that two processes are occurring: ligand substitution around existing paramagnetic MoV ions [reaction (l)] and formation of a new paramagnetic species responsible for the broad symmetric signal.The second process is attributed to the ‘ uncoupling’ of magnetic interactions between adjacent MoV ions through removal of bridging oxide ligands [reaction (2)], as described previously for molybdena-alumina catalysts : (1) (2) No detailed chemical measurements of the MoV concentrations in silica-supported catalysts are available, but X.P.S.~~ and reduction-isothermg data suggest that the total amount of MoV in reduced catalysts considerably exceeds the 3% of the total Mo content observed by e.p.r. prior to HCl treatment. Rapid spin-lattice relaxation of the ‘missing’ MoV species at 77 K or above can be ruled out, since no new signals were observed at 20 K. In particular the second MoV signal reported by Kazanski et aZ.15 and also observed by Che et aZ.16 in catalysts prepared from MoCl, was not detected here.The missing MoV ions must therefore be strongly magnetically ,coupled, evidently via bridging oxide ligands, since reaction with HCl reduces the extent of coupling. The uncoupled MoV ions are still in close proximity, and their e.p.r. signal should be strongly dipolar broadened, as is observed. The increase in MoV spin concentration in reduced catalysts on treatment with HC1 cannot be due to reduction of remaining MoV1, since outgassing restored the original spin concentration. The reversal of reactions (1) and (2)’ i.e. reaction of H,O with halide ligands to restore the original oxomolybdenum species, accounts for the reversibility of the spectral changes in reduced catalysts.The temperature range over which the original spectra are restored on outgassing corresponds to that needed to remove adsorbed water from the catalyst. In contrast, the increase in MoV spin concentration observed on exposure of oxidized catalysts to HC1 was not reversible (table l), and the spectra obtained contained no contribution from the broad symmetric signal. This is particularly clear from the 95Mo-enriched catalyst; the observed signal [fig. 5(a)] could be simulated with the parameters of a single MoV signal [fig. 6(b)]. Since the oxidized catalyst contains molybdenum almost exclusively in the + 6 valence state, the increase in MoV concentration must in this case be due to a redox reaction [reaction (3)]: MoVO, + 2y HCl = M O ~ O , - ~ C12y + y H,O MoVO MoV + 2 HCl = MoVCl C1 MoV + H20.MoV1O + 2HC1= MoVCl + H,O + iC12. (3) MoVr in molybdena-alumina catalysts is known to be capable of oxidizing polynuclear aromatic compounds to the corresponding radical cations.23 The MoV species produced in reaction (3) are magnetically isolated, giving a single well resolved e.p.r. signal. On subsequent outgassing the chloride ligands are replaced by oxide ions without any change in oxidation state and e.p.r. signal intensity. The extent of reduction achieved by treating oxidized catalysts with HCl is low; the MoV spin concentration (table 1) corresponds to 0.5% of the total molybdenum content, and96 INTERACTION OF HCl WITH A MOLYBDENA-SILICA CATALYST no coupled MoV species are produced. The replacement of chloride ligands by oxide on outgassing is consistent with the observations of Che et all6* l7 that silica-supported molybdenum catalysts prepared from MoCl, and (NH,),MoOCl, give MoV signals identical to that of a conventional catalyst prepared from chlorine-free precursors.The infrared spectra of oxidized catalysts exposed to HCl support the above description of reduction and ligand substitution. Exposure to HCl completely removes the 3 bands due to Mo-0 stretching vibrations.ls This does not necessarily mean that all oxide ions in the catalyst are replaced by chloride, since adsorption of water on the oxidized catalyst also causes a significant reduction in intensity of the Mo-0 bands,24 and Mo-Cl stretching vibrations lie outside the frequency range of the MX1 spectrophotometer used.The effects of chloride substitution and adsorption of water as a reaction product cannot be separated clearly from the infrared result. On subsequent outgassing, the infrared spectrum resembles that of a catalyst slightly reduced in hydrogen, consistent with the e.p.r. observations. Although the reactions of HCl with molybdena-silica catalysts resemble in many respects those with molybdena-alumina, some significant differences also exist between the two systems. The increase in MoV spin concentration obtained by treating reduced molybdena-silica catalysts with HCl is much larger than the threefold increase found with molybdena-alumina,' which suggests that a larger fraction of the MoV produced by reduction is magnetically coupled on silica.The ligand substitution reaction in reduced catalysts is more readily reversed on silica and the fully oxidized silica- supported catalyst is reduced to a lesser extent on exposure to HCl. These differences may reflect differences in the distribution of molybdenum between monomeric and polymeric species on the two supports. This work was undertaken in part in the Chemistry Department of the University of Wisconsin-Milwaukee. Partial support by the Laboratory for Surface Studies is gratefully acknowledged. F. Massoth, Ado. Catal., 1978, 27, 265. G. C. A. Schuit and B. C. Gates, AIChE J., 1973, 19, 417. W. K. Hall and M. LoJacono, Proc. 6th Int. Congr. Catal., London, 1976, ed. G. C. Bond, P. B. Wells and F. C. Tompkins (The Chemical Society, London, 1976), p.246. T. A. Patterson, J. C. Carver, D. E. Layden and D. M. Hercules, J. Phys. Chem., 1976, 80, 1700. L. Petrakis, P. L. Meyer and T. P. Debies, J. Phys. Chem., 1980, 84, 1020. S. Abdo, R. B. Clarkson and W. K. Hall, J. Phys. Chem., 1976, 80, 2431. 'I S. Abdo, A. Kazusaka and R. F. Howe, J. Phys. Chem., 1981,85, 1380. P. Gajardo, P. Grange and B. Delmon, J. Phys. Chem., 1979, 83, 1771. P. Gajardo, D. Pirotte, P. Grange and B. Delmon, J. Phys. Chem., 1979, 83, 1780. lo H. Jeziorowski, H. Knozinger, P. Grange and P. Gajardo, J. Phys. Chem., 1980, 84, 1825. l1 J. Medema, C. van Stam, V. H. J. de Beer, A. J. A. Konings and D. C. Koningsberger, J. Catal., 1978, 53, 386. l2 R. Thomas, M. C. Mittelmeijer-Hazelberger, F. P. J. Kerkhof, J. A. Moulijn, J. Medema and V. H. J. de Beer, Proc. 3rd Inr. Con5 Chemistry and Uses of Molybdenum, 1979, ed. P. C. H. Mitchell and H. F. Barry (Climax Molybdenum Co, Ann Arbor, Michigan, 1979), p. 85. l3 P. B. Ayscough, Electron Spin Resonance in Chemistry (Methuen, London, 1967). l4 G. P. Lozos, B. M. Hoffman and C. G. Franz, Quantum Chemistry Program Exchange, 1973, 265. l5 V. M. Vorotyntshev, V. A. Shvets and V. B. Kazanskii, Kinet. Catal., 1971, 12, 1108. l6 M. Che, F. Figueras, M. Forissier, J. C. McAteer, M. Perrin, J. L. Portelfaix and H. Praliaud, Proc. 6th Int. Congr. Catal., London, 1976, ed. G. C. Bond, P. B. Wells and F. C. Tompkins (The Chemical Society, London, 1976), p. 261. M. Che, J. McAteer and A. J. Tench, J. Chem. Soc., Faraday Trans. I , 1978, 74, 2378. l8 S. R. Seyedmonir, S. Abdo and R. F. Howe, J. Phys. Chem., 1982, 86, 1233. lo M. Che, M. Fournier and J. P. Launay, J. Chem. Phys., 1979, 71, 1954.S. RAZI SEYEDMONIR AND R. F. HOWE 97 2o P. T. Manoharan and M. T. Rogers, J . Chem. Phys., 1968,49, 5510. z1 M. I. Scullane, R. D. Taylor, M. Minelli, J. T. Spence, K. Yamanouchi, J. H. Enemark and N. D. ** M. B. Ward, M. J. Lin and J. H. Lunsford, J . Catal., 1977, 50, 306. 23 C . Naccache, J. Bandiera and M. Dufaux, J . Catal., 1972, 25, 334. 24 S. R. Seyedmonir, Ph.D. Thesis (University of Wisconsin-Milwaukee, 1982). Chasteen, Inorg. Chem., 1979, 18, 3212. (PAPER 3/618)

ISSN:0300-9599    

DOI:10.1039/F19848000087

出版商:RSC    

年代:1984

数据来源: RSC