摘要:

Contents 4259 4269 4277 4287 4295 431 1 4321 4335 Protonation Constant of Caffeine in Aqueous Solution M. Spiro, D. M. Grandoso and W. E. Price Ionic Equilibria in Acetonitrile Solutions of 2-, 3- and 4-Picoline N-oxide Perchlorates, studied by Potentiometry and Conductometry L. Chmurzynski, A. Wawrzyn6w and Z. Pawlak Liquid-phase Adsorption of Binary Ethanol-Water Mixtures on NaZSM-5 Zeolites with Different Silicon/Aluminium Ratios W-D. Einicke, M. Heuchel, M. v.Szombathely, P. Brauer, R. Schollner and 0. Rademacher Influence of Oxidation/Reduction Pretreatment on Hydrogen Adsorption on Rh/TiO, Catalysts. An lH Nuclear Magnetic Resonance Study J. P. Belzunegui, J. M. Rojo and J. Sanz Volumetric Properties of Mixtures of Simple Molecular Fluids A. C. Colin, E. G. Lezcano, A.Compostizo, R. G. Rubio and M. D. Peiia Study of Ultramicroporous Carbons by High-pressure Sorption. Part 4.-Iso- thems and Kinetic Transport in Activated Carbons J. E. Koresh, T. H. Kim, D. R. B. Walker and W. J. Koros Kinetic and Equilibrium Studies associated with the Solubilisation of n- Pentanol in Micellar Surfactants G. Kelly, N. Takisawa, D. M. Bloor, D. G. Hall and E. Wyn-Jones The effect of Carboxylic Acids on the Dissolution of Calcite in Aqueous Solution. Part 1 .-Maleic and Fumaric Acids R. G. Compton, K. L. Pritchard, P. R. Unwin, G. Grigg, P. Silvester, M. Lees and W. A. House 130-2Contents 4259 4269 4277 4287 4295 431 1 4321 4335 Protonation Constant of Caffeine in Aqueous Solution M. Spiro, D. M. Grandoso and W. E. Price Ionic Equilibria in Acetonitrile Solutions of 2-, 3- and 4-Picoline N-oxide Perchlorates, studied by Potentiometry and Conductometry L.Chmurzynski, A. Wawrzyn6w and Z. Pawlak Liquid-phase Adsorption of Binary Ethanol-Water Mixtures on NaZSM-5 Zeolites with Different Silicon/Aluminium Ratios W-D. Einicke, M. Heuchel, M. v.Szombathely, P. Brauer, R. Schollner and 0. Rademacher Influence of Oxidation/Reduction Pretreatment on Hydrogen Adsorption on Rh/TiO, Catalysts. An lH Nuclear Magnetic Resonance Study J. P. Belzunegui, J. M. Rojo and J. Sanz Volumetric Properties of Mixtures of Simple Molecular Fluids A. C. Colin, E. G. Lezcano, A. Compostizo, R. G. Rubio and M. D. Peiia Study of Ultramicroporous Carbons by High-pressure Sorption. Part 4.-Iso- thems and Kinetic Transport in Activated Carbons J. E. Koresh, T. H. Kim, D. R. B. Walker and W. J. Koros Kinetic and Equilibrium Studies associated with the Solubilisation of n- Pentanol in Micellar Surfactants G. Kelly, N. Takisawa, D. M. Bloor, D. G. Hall and E. Wyn-Jones The effect of Carboxylic Acids on the Dissolution of Calcite in Aqueous Solution. Part 1 .-Maleic and Fumaric Acids R. G. Compton, K. L. Pritchard, P. R. Unwin, G. Grigg, P. Silvester, M. Lees and W. A. House 130-2

ISSN:0300-9599    

DOI:10.1039/F198985FX017

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY ASSOCIAZIONE ITALIANA DI CHIMICA FlSlCA DEUTSCHE BUNSEN-GESELLSCHAFT FUR PHYSIKALISCHE CHEMIE KONINKLIJKE NEDERLANDS CHEMISCHE VERElNlGlNG SOCIETE FRANGAISE DE CHIMIE, DIVISION DE CHlMlE PHYSIQUE FARADAY DIVISION GENERAL DISCUSSION No. 90 Colloidal Dispersions University of Bristol, 10-12 September 1990 Orga nising Com mitte e Professor R. H. Ottewill (Chairman) Professor P. Botherol Professor E. Ferroni Or J. W. Goodwin Professor H. Hoff mann Professor A.L. Smith Professor P. Stenius Dr Th. F. Tadros Professor A. Vrij Dr D. A. Young The joint meeting of the Societies will be directed towards examining current understanding of the behaviour of colloidal dispersions. In particular, stability and instability, short range interactions, dynamic effects, non-equilibrium interaction, concentrated dispersions and order-disorder phenomena will form topics for discussion.The preliminary programme is now availablemay be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN. THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM No. 26 Molecular Transport in Confined Regions and Membranes Oxford, 17-18 December 1990 Experimental, theoretical and simulation studies which address fundamental aspects of molecular transport will be discussed in the following main areas: a) Transport of atoms and molecules in pores, zeolite networks and other cavities; time-dependent statistical mechanics of small systems in confined geometries b) Molecular transport through synthetic membranes, biological membranes, smectic liquid crystalline phases and Langmuir Blodgett films; the dynamics of the molecules forming the membrane c) Diffusion, reorientation, conformational dynamics, viscosity and conductivity of polymer melts, to include papers dealing with bulk systems since the segments of the polymer will move in the anisotropic environment of the complete chain d) Applications of Brownian dynamics to the study of diffusion in porous media and across membranes including the transport of flexible aggregates such as microemulsions e ) The growth of crystals, colloidal aggregates and droplets on irregular surfaces and in pores Contributions for consideration by the Organising Committee are invited and abstracts of about 300 words should be sent by 31 December 1989 to: Dr D.J. Tildesley, Department of Chemistry, The University, Southampton SO9 SNH. Full papers for publication in the Symposium Volume will be required by August 1990.THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY ASSOCIAZIONE ITALIANA DI CHIMICA FlSlCA DEUTSCHE BUNSEN-GESELLSCHAFT FUR PHYSIKALISCHE CHEMIE KONINKLIJKE NEDERLANDS CHEMISCHE VERElNlGlNG SOCIETE FRANGAISE DE CHIMIE, DIVISION DE CHlMlE PHYSIQUE FARADAY DIVISION GENERAL DISCUSSION No. 90 Colloidal Dispersions University of Bristol, 10-12 September 1990 Orga nising Com mitte e Professor R. H. Ottewill (Chairman) Professor P. Botherol Professor E. Ferroni Or J. W. Goodwin Professor H. Hoff mann Professor A.L. Smith Professor P. Stenius Dr Th.F. Tadros Professor A. Vrij Dr D. A. Young The joint meeting of the Societies will be directed towards examining current understanding of the behaviour of colloidal dispersions. In particular, stability and instability, short range interactions, dynamic effects, non-equilibrium interaction, concentrated dispersions and order-disorder phenomena will form topics for discussion. The preliminary programme is now availablemay be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN. THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM No. 26 Molecular Transport in Confined Regions and Membranes Oxford, 17-18 December 1990 Experimental, theoretical and simulation studies which address fundamental aspects of molecular transport will be discussed in the following main areas: a) Transport of atoms and molecules in pores, zeolite networks and other cavities; time-dependent statistical mechanics of small systems in confined geometries b) Molecular transport through synthetic membranes, biological membranes, smectic liquid crystalline phases and Langmuir Blodgett films; the dynamics of the molecules forming the membrane c) Diffusion, reorientation, conformational dynamics, viscosity and conductivity of polymer melts, to include papers dealing with bulk systems since the segments of the polymer will move in the anisotropic environment of the complete chain d) Applications of Brownian dynamics to the study of diffusion in porous media and across membranes including the transport of flexible aggregates such as microemulsions e ) The growth of crystals, colloidal aggregates and droplets on irregular surfaces and in pores Contributions for consideration by the Organising Committee are invited and abstracts of about 300 words should be sent by 31 December 1989 to: Dr D.J. Tildesley, Department of Chemistry, The University, Southampton SO9 SNH. Full papers for publication in the Symposium Volume will be required by August 1990.

ISSN:0300-9599    

DOI:10.1039/F198985BX019

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

ISSN 0300-9599 JCFTAR 85( 5) 101 9-1 1 98 (1 989) JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions I Physical Chemistry in Condensed Phases 1019 1027 1043 1049 1065 1075 1083 1091 1099 1111 1117 I129 1139 1149 1159 1169 CONTENTS Temperature Dependence of Thermo-osmosis. A Solution Model C. Fernandez-Pineda and M. I. Vazquez-Gonzalez Intramolecular Photochemical Electron Transfer. Part 5.-Solvent Dependence of Electron Transfer in a Porphyrin-Amide-Quinone Molecule J. A. Schmidt, J.-Y. Liu, J. R. Bolton, M. D. Archer and V. P. Y. Gadzekpo Preparation and Characterization of Ti0,-SiO, Aerosil Colloidal Mixed Dispersions C. Morrison and J. Kiwi The State of Water in Non-ionic Surfactant Solutions and Lyotropic Phases. Oxygen- 17 Magnetic Relaxation Study Dielectric Relaxation in Concentrated Solutions of cis-Polyisoprene.Part 1.-Effect of Entanglement on the Normal-mode Process K. Adachi, Y. Imanishi and T. Kotaka Dielectric Relaxation in Concentrated Solutions of cis-Polyisoprene. Part 2.-Motions of Local Segments and Solvent Molecules K. Adachi, Y. Imanishi and T. Kotaka Dielectric Relaxation in Concentrated Solutions of cis-Polyisoprene. Part 3.-Relationship between Friction Coefficient for Dielectric Normal-mode Process and Local Segmental Motions K. Adachi, Y. Imanishi and T. Kotaka Diaphragm Cell for High-temperature Diffusion Measurements. Tracer Diffusion Coefficients for Water to 363 K A. J. Easteal, W. E. Price and L. A. Woolf Fourier-transform Nuclear Magnetic Resonance Studies of the Effects of 2- Chloroethanol on the Association of N- Acetyl-L-amino Acid N',N'-Dimethyl- amides in Aqueous Solutions K.Mizuno, T. Takagi, Y. Ikeda and Y. Shindo Infrared Study of NH,/CO Reactions on Fe/A1,0, and Promoted Fe/SiO, Catalysts Infrared Study of the Adsorption of Acetone, Acrolein, Ethanoic Acid and Propene-NO Mixtures on Rh/AI,O, Catalysts J. A. Anderson and C. H. Rochester Infrared Study of the Adsorption of Ethanoic Acid, Acrylic Acid, Acetone, Acrolein and Propene-NO Mixtures on Rh/SiO, Catalysts J. A. Anderson and C. H. Rochester X-Ray Emission Spectra and Electronic Structure of the Sulphate and Methyl Sulphonate Anions, Dimethyl Sulphone and the Trimethylsulphoxonium Cation [(CH3)nS04-n]n-2, (n = 0, 1, 2 and 3) Infrared Spectroscopic Studies on Carbon Dioxide Adsorption in Alkali-metal and Alkaline-earth-metal Ion-exchanged A- type Zeolites. Part I .-General Features of CO, Interaction with A-type Zeolites H. Forster and M. Schumann Surface-enhanced Raman Spectroscopy of NAD+ and related Compounds J. C. Austin and R. E. Hester Mechanistic Study of P-Hydroxy Elimination from [Tetra sulphophthal- ocyanine Co1"-CR,R,CR,R4OH] in Aqueous Solutions. A Pulse Radiolysis Study G. Carlstrom and B. Halle C. Johnston, N. Jorgensen and C. H. Rochester R. Foerch and D. S. Urch Y. Sorek, H. Cohen and D. Meyerstein 35 FAR 1Con tents 1 18 1 1 189 Ring-Disc Electrodes. Part 22.-Theory of the Measurement of Proton Fluxes at the Disc W. J. Albery and A. R. Mount Ring-Disc Electrodes. Part 23.-Studies of Proton Fluxes at a Thionine-coated Electrode W. J. Albery and A. R. Mount

ISSN:0300-9599    

DOI:10.1039/F198985FP061

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions 11, Issue 5,1989 Molecular and Chemical Physics For the benefit of readers of Faraday Transactions I , the contents list of Faraday Transactions 11, issue 5, is reproduced below. 335 341 35 1 367 383 407 415 431 457 467 471 485 491 505 517 52 1 537 541 555 565 Mott-Littleton Calculations in Solid-state Chemistry and Physics C. R. A. Catlow The Mott-Littleton Method: An Introductory Survey A. B. Lidiard Calculation of the Entropy of Defect Processes in Ionic Solids J. H. Harding Atomistic Simulation Studies of Technologically Important Oxides S. M. Tomlinson, C. M. Freeman, C. R. A. Catlow, H. Donnerberg and M. Leslie Calculated Lattice and Dynamic Properties and Defect Chemistry of Ternary and Quaternary Cuprates related to High-Tc Superconductivity N.L. Allan and W. C. Mackrodt Calculation of the Energies of Point Defects iri Quartz M. Leslie Defects in Alkali-metal Halide Crystals P. M. W. Jacobs Calculated and Experimental Defect Parameters for Silver Halides J. Corish Ionic Defects and the Photographic Process Y. T. Tan Theoretical Studies of the Blue Luminescence in a-Quartz A. J. Fisher, W. Hayes and A. M. Stoneham Quantum-mechanical Calculations of Defect Energies A, H. Harker Quantum-mechanical Cluster Calculations and the Mott-Littleton Methodology R, W. Grimes, C. R. A. Catlow and A. M. Stoneham Simulation Studies of Reactive Molecules in Zeolites R. Vetrivel, C. R. A. Catlow and E. A. Colbourn Small Polarons and Polaron Transitions A. M. Stoneham Defect Physics and Chemistry: Some Problems awaiting a Solution N.F. Mott A6 Initio Calculation of the Energy and Structure of Solids M. J. Gillan Self-consistent Molecular Relaxation by Simulated Annealing D. S. Wallace Atomistic Simulation of the Surfaces of Oxides W. C. Mackrodt Impurity Segregation to the Surfaces of Corundum-structured Oxides M. J. Davies, P. Kenway, P. J. Lawrence, S. C. Parker, W. C. Mackrodt and P. W. Tasker Appendix: Conduction in Polar Crystals. I . Electrolytic Conduction in Solid Salts N. F. Mott and M. J. LittletonThe following papers were accepted for publication in Faraaby Transactions I during February, 1989. 8/0317OC 8/03180K 8103450H 8/03891K 8/03947J 8/04068K 8M70B W 7 1 K 81043995 81044286 8/04448A 8W69D 8/04470H 8/04472D Kinetic Salt Effects on the Reaction between [1,3,6,8,10,13,16,19-0cta- azabic y clo( 6,6,6)icosane] cobal t( m) Ion , Co( ~ep)~', and Hexa-aquochromium (Q Ion Indelli, A.and Ferranti, F. Redox Property of Borate Radical and Effect of Ionic Strength on its Reactivity. A Flash Photolysis Study Padmaja, S., Ramakrishnan, V., Rajaram, J. and Kuriacose, J. C. Study of Ultramicroporous Carbons by High-pressure Sorption. Part 4.-Isotherms and Kinetic Transport in Activated Carbons Koresh, J.E.,Kim, T.H.,Walker, D. R. B. and Koros, W. J. The Effect of Glucose on the Crystallization of Hydroxyapatite in Aqueous Solutions Dalas, E. and Koutsoukos, P. G. Partial Oxidation of Methane over Oxide Catalysts. Comments on the Reaction Mechanism Hutchings, G. J., Scurrell, M. S. and Woodhouse, J.R. AlPOeTia Catalysts. Part 2.--Structure, Texture and Catalytic Activity of Systems Precipitated with Ammonia or Ethene Oxide Campelo Perez, J. M., Garcia, A., Lunda, D., Marinas, J. M. andMoreno, M. S. N.M.R. Studies of Preferential Solvation. Part 6.-Application of Blander's Coordinated Cluster Theory to the Methanol-Water Solvent System Covington, A. K. and Dunn, M. N.M.R. Studies of Preferential Solvation. Part 7.4odium Iodide in Ethene Glycol (EG)-Acetonitrile (AN) and in Propene Glycol (PG)-Acetonitrile Mixtures Covington, A. K. and Dunn, M. Enthalpic Interaction Coefficients of some Dipeptides Dissolved in N&-Dimethylformamide Sijpkes, A. H. and Somsen, G. Activity Coefficients for Aqueous Solutions of Potassium Succinate at 25°C Esteso, M. A., Fernandez-Merida, L., Gonzalez-Diaz, 0.M. and Hernandez-Luis, F. F. Active Structures and Electronic States for Adsorption of CO2 and NO on Nmi02(110) Surface Onishi, H., Aruga, T., Egawa, C. and Iwasawa, Y. Polarographic and Spectrophotometric Investigation of the Reduction and Deprotonation of [ 1,3,6,8,10 , 13 , 16,19-Octa-azabicyclo(6,6~6)icosane]cobalt(111) Ion, Co(~ep)~', in the Presence of Different Anions Indelli, A., Lanza, P. and Duatti, A. The Enthalpies of Interaction of Sodium Chloride and Potassium Chloride with some Amides in Water at 25°C Davis, K. G., Gallardo-Jiminez, M. A. and Lilley, T. H. Application of Radiation and E.S.R. Spectroscopy to the Study of Few1 Myoglobin Symons, M. C. R., Petersen, R. L. andTaiwo, F. A. A Phase Separation caused by the Solubility of Butane into Mixtures of 2-Methylpropan-2-01 with Water Cargill, R.W. and MacPhee, D. E, Solubility of Hex-l-ene, Hexane and Cyclohexane in Liquid Nitrogen Szczepaniec-Cieciak, E. and Kurdziel, M. Transfer and Partition Free Energies of 1: 1 Electrolytes in the Water-Dichloromethane Solvent System at 298.15 K Danil de Namor, A. F., TkabouIssi, R., Fernandez Salazar, F., Dianderas de Acmta, V, and Fernandez de Vizcardo, I. Evidence for Adduct Formation in the Solubilisation of Hydrophobic Compounds by Aqueous Solutions of Urea Byfield, M. P., Frost, V, L., Pemberton, J. L. J. and Pratt, J. M, (ii)8/04482A 8104591G 8M34D 8/04639E 811)4663H 8/04713H 81048811 8/04884C 9/00260J 8/05048A Detection and Quantitative Determination of the Composition of Bismuth Molybdate Phases by various Spectroscopic Techniques Vedrine, J.C., Olier, R., Coudurier, G., El Jamal, M. and Forissier, M. Oxygen Adsorption on Ag Powder Bowker, M., Pudney, P. and Roberts, G. Induction of Mesoporosity in AlP05-. Treatment with Silicon Tetrachloride Theocharis, C. R., Gelsthorpe, M. R. and Yeates, D. The Enthalpies of Interaction of some Alkali-metal Halides with N-Methylacetamide and with NJ-Dimethylformamide in Water at 25'C Gallardo-Jiminez, M. A. and Lilley, T. H. Adsorption of Polystyrene and Poly(methy1 methacrylate) on to Silica Surface by I.R. Technique: Comparison with Theory Kawaguchi, M., Yamagiwa, S., Takahashi, A. and Kato, T. Dielectric Studies of Association in Phenol and l-Alkyl Substituted Phenols Habibullah, M.and Walker, S. Shape-selective Adsorption of Aromatic Molecules from Water by Tetrmethylammonium Smectite Boyd, S. A., Lee, J. F., Mortland, M. M. and Chiou, C. T. Separation of Small-particle Dispersions by the Preferential Accumulation in One of Two Liquid Phases, or by Static Flotation at their Interface Boucher, E. A. Crystallochemical Characterization of Magnetic Spinels Prepared from Aqueous Solution Mann, S., Sparks, N. H. C., Couling, S. B., Larcombe, M. C. and Frankel, R. B. The Interaction of Different Molecules with Cu2+ Cations in CUH-ZSM-5. E.S.R. Study and Quantum-chemical Calculations Kucherov, A. V., Slinkin, A. A., Chuvylkin, N. D., Kliachko, A. L. and Nikishenko, S. B. (iii)Cumulative Author Index 1989 Adachi, K., 1065, 1075, 1083 Aguilella, V.M., 223 Akitt, J. W., 121 Albery, W. J., 1181, 1189 Albuquerque, L. M. P. C., 207 Allen, G. C., 55 Amodeo, P., 621 Anderson, J. A., 1117, 1129 Anpo, M., 609 Apelblat, A., 373 Arai, T., 929 Archer, M. D., 1027 Asakura, K., 441 Austin. J. C., 1159 Baiker, A., 999 Bald. A., 479 Barone, G., 621 Barone, V., 621 Beckett, M. A., 727 Bellotto, M., 895 Bengtsson, L., 305, 317 Berry, F. J., 467 Bertoldi, M., 237 Blandamer, M. J., 735 Bolis, V., 855 Bolton, J. R., 1027 Bond, G. C., 168 Borowko, M., 343 Boss, R. D., 1 1 Bowker, M., 165 Brimblecome, P., 157 Burgess, J., 735 Busca, G., 137, 237 Campbell, J. A., 843 Carlstrom, G., 1049 Cattania, M. G., 801 Chadwick, A. V., 166 Che, M., 609 Chen, J., 829 Cheii, L-f., 33 Clegg, S. L., 157 Cohen, H., 1169 Coluccia, S., 609 Comninos, H., 633 Compton, R.G., 761, 773, 977 Conway, S. J., 71, 79 Copperthwaite, R. G., 633 Cox, B. G., 187 Cristiani, C., 895 Cristinziano, P., 62 I da Costa, M. A., 907 Datka, J., 47, 837 Dawber, J. G., 727 De Giglio, A., 23 Dell’Atti, A., 23 Domen, K., 929 Donini, J. C., 91 Drummond, C. J., 521, 537, 551, Ishida, H., 1 1 1 Itoh, N., 493 56 1 Easteal, A. J., 1091 Eden, J., 991 el Torki, F. M., 349 Falconer, J. W., 71, 79 Fernandez-Pineda, C., 1019 Finch, J. A., 91 Fletcher, P. D. I., 147 Foerch, R., 1139 Foo, C. H., 65 Forster, H., I149 Forzatti, P., 895 Frey, H. M., 167 Fubini, B., 237, 855 Gabriel, C. J., 11 Gabrys, B., 168 Gadzekpo, V. P. Y., 1027 Garrone, E., 585 Gasser, D., 999 Gervasini, A., 801 Geus, J. W., 269, 279, 293 Giamello, E., 237, 855 Gilbert, P.J., 147 Girault, H. H., 843 Gottschalk, F., 363 Grieser, F., 521, 537, 551, 561 Guardado, P., 735 Gutierrez, C . , 907 Halle, B., I049 Hampton, S . , 773 Han, S., 829 Handreck, G. P., 645 Harland, R. G., 761 Hasted, J. B., 99 Hatano, M., 199 Healy, T. W., 521, 537, 551, 561 Heatley, F., 917 Hesselink, W. H., 389 Hester, R. E., 171, 1159 Higgins, J. S . , 170 Higuchi, A., 127 Hill, W., 691 Hirai, T., 969 Holmberg, B., 305, 317 Hong, C . T., 65 Howarth, 0. W., 121 Hubbard, C. D., 735 Hummel, A., 991 Hunter, R., 363, 633 Hutchings, G. J., 363, 633 Ichikawa, K., 175 Ikeda, R., 1 1 1 Ikeda, Y., 1099 Imanishi, Y., 1065, 1075, 1083 Iwasawa, Y., 441 Jin, T., 175 Johnson, G. R. A., 677 Johnston, C., 1 I I 1 Jonkers, G., 389 Jorgensen, N., 11 11 Jutson, J. A,, 55 Kaneko, K., 869 Kanno, T., 579 Katoh, T., 127 Keeler, J.H., 163 Kelebek, $., 91 Kishi, R., 655 Kiwi, J., 1043 Knijff, L. M.. 269, 293 Kobayashi, M., 579 Koda, S . , 957 Kosugi, N., 869 Kotaka, T., 1065, 1075, 1083 Kozlowski, Z . , 479 Kuroda, H., 869 Kuwabata, S., 969 Larramona, G., 907 Laschi, F., 601 Lawrence, K. G., 23 Lelj, F., 621 Levy, O., 373 Lewis, T. J., 1009 Leyendekkers, J. V., 663 Li, C., 929 Liu, J.-Y., 1027 Lorenzelli, V., 137 Loudon, R., 169 Lowe, B. M., 945 Lund, A., 421 Mafe, S . , 223 Maignan, A., 783 Manzurola, E., 373 Marcus, Y., 381 Markovits, G., 373 Maruya, K., 929 Masiakowski, J. T., 421 Matsuhashi, N., 1 I 1 Matsui, H., 957 Matsumoto, T., 175 McAleer, J. F., 783 Meima, G. R., 269, 279, 293 Meyerstein, D., 1169 Miessner, H., 691 Mills, A., 503 Mizuno, K., 1099 Morazzoni, F., 801 Morrison, C., 1043 Moseley, P.T., 783AUTHOR INDEX Mosier-Boss, P. A., 1 1 Mount, A. R., 1181, 1189 Nakagawa, T., 127 Nakamura, D., 1 1 1 Nakamura, T., 493 Natarajan, P., 8 13 Nazhat, N. B., 677 Neagle, W., 429, 719 Newman, K. E., 485 Nicholas, A., 773 Nomura, H., 957 Nowak, R. J., 11 Nowicka, B., 479 Nunes, M. R., 907 Ohlmann, G., 691 Ohyama, Y., 749 Okubo, T., 455, 749 Oliveira Jr, 0. N., 1009 Onishi, T., 929 Orchard, S. W., 363 Osada, Y., 655 Otsuka, K., 199 Pandey, J. D., 331 Pellicer, J., 223 Pereira, I., 907 Piwowarska, Z., 47, 837 Pope, C. G., 945 Portwood, L., 71 1 Price, W. E., 415, 1091 Rai, R. D., 331 Ramaraj, R., 813 Ramis, G., 137 Rao, K. J., 251 Reed, W. F., 349 Rees, L. V. C., 33 Reis, J. C. R., 207 Reller, A., 855 Rhodes, C.J., 71 1 Rochester, C. H., 71, 79, 429, Rosen, D., 99 Rossi, C., 601 Rowlinson, J. S., 171, 172 Saadalla-Nazhat, R. A,, 677 Sacco, A., 23 Said, M., 99 Sakata, Y., 929 Salvagno, S.. 1009 Schmehl, R. H., 349 Schmidt, J. A., 1027 Schneider, H., 187 Schneider, I., 187 Schumann, M., 1149 Scotti, R., 801 Selvaraj, U., 251 Shido, T., 441 Shindo, Y., 1099 Shukla, R. K., 331 Smith, G. W., 91 Smith, J. J., 1 1 Smith, M. R., 467 Smith, T. D., 645 Sorek, Y., 1169 Stewart, A. A., 843 Stirling, C. J. M., 1009 Stroka, J., 187 Strumolo, D., 801 Sundar, H. G. K., 251 Sutton, H. C., 883 Symons, M. C. R., 71 1 719, 1111, 1117, 1129 Szejgis, A,, 479 Szpak, S., I 1 Takagi, T., 1099 Takagi, Y., 493 Taniewska-Osinska, S., 479 Taylor, D. M., 1009 Thamm, H., 1 Themistocleous, T., 633 Ugliengo, P., 585 Urch, D.S., 1139 Vaccari, A., 237 van Buren, F. R., 269, 279, 293 van Dillen, A. J., 269, 279, 293 van Leur, M. G. J., 279 van Lith, D., 991 van Rensburg, L. J., 633 van Veen, J. A. R., 389 Vazquez-Gonzalez, M. I., 1019 Vink, H., 699 Vis, R. J., 269, 279 Wacker, T., 33 Waller, A. M., 773, 977 Warman, J. M., 991 Waugh, K. C., 163 Weale, K. E., 165 Williams, D. E., 783 Williams, G., 503 Woolf, L. A., 1091 Yamada, Y., 609 Yeh, C-t., 65 Yoneyama, H., 969 You, X., 829 Young, D. A., 173 Zecchina, A., 609Special Issues of Faraday Transactions Readers of the Faraday Transactions will be aware that we have now established Special Issues which fall broadly into two categories: Collections of the refereed papers that have been presented at a Scientific Conference, normally run by a Subject Group of the RSC, and approved in advance by the Fur- Editorial Board.We insist, with varying degrees of success, that the papers published under this scheme shall describe original work which fully meets the n o d requirements for submittedpapers. Keynote Issues which are opened by a paper written by an acknowledged authority in a particular field of interest. The Keynote author then invites colleagues known to him to contribute original papers on cognate topics, to appear in the same Keynote Issue. These papers are refereed and, as intended, have led to issues of an exceptionally high scientific standard. The initial stage of this development having been completed, Farakzy Editorial Board has directed that wider invitations should be issued to the physicochemical and chemical physics community to send us origrnal work which they would like to appear alongside papers on similar topics. Special issues in the planning stage include the topics listed below.If you think that you have some good work coming along which could be ready for submission around the indicated date, please do write to me and let me know, when I will make the necessary arrangements. For my part, I see no objection to adding such papers to Special Issues in both categories. David Young Scientific Editor spcial Issue Reactive and Inelastic Scattering Concentrated Colloidal I)lspersions Structure and Activity of Adsorbed Species (with special emphasis on surface science) Closing Date 30th September 1989 15th November 1989 TobeannouncedTHE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION No.88 Charge Transfer in Polymeric Systems University of Oxford, 11-13 September 1989 This Discussion aims to bring together physicists and chemists interested in the mechanism of electron and ion transport in polymeric systems. The systems include conducting polymers, redox polymers, ion exchange membranes and modified electrodes. Discussion topics will cover experimental evidence from spectroscopy, electrochemistry and new techniques such as the quartz microbalance. Theoretical models ranging from band theory through polarons to localised chemical structures will be critically evaluated and compared with experiment. The following have agreed to participate in the Discussion: R.Murray W. J. Albery M. D. lngram M. 6. Armand D. Bloor H. Cheradame P. G. Bruce R. Friend R. J. Latham A. J. Heeger A. R. Hillman P. V. Wright A. G. MacDiarmid M. Ratner M. E. G. Lyons S. Roth 0. Haas T. J. Lewis G. Tourillon C. Vincent P. G. Pickup G. Wegner A. Hamnett L. M. Peter K. Doblhofer The final programme and application form may be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN. DEUTSCHE BUNSEN-GESELLSCHAFT FUR PHYSIKALISCHE CHEMIE ASSOCIAZIONE ITALIANA DI CHIMICA FlSlCA FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SOCIETE FRANCAISE DE CHIMIE, DIVISION DE CHlMlE PHYSIQUE JOINT DISCUSSION MEETING 1989 Transport Processes in Fluids and in Mobile Phases Aachen, 25-27 September 1989 Organised by: H.Versmold (F.R.G.) Al.Weiss (F.R.G.) M. Zeidler (F.R.G.) G. R. Luckhurst (U.K.) The purpose of the meeting is to bring together scientists working on transport and related phenomena in simple and complex fluids, colloidal and micellar systems, and surface phases. Experimental techniques considered include classical methods, optical spectroscopy, light scattering, nuclear magnetic resonance, and neutron scattering. The following persons have accepted invitations to present talks: D. Evans, Canberra; B. U. Felderhof, Aachen; D. Frenkel, Amsterdam; A. Geiger, Dortmund; W. Gliiser, Grenoble; H. G. Hertz, Karlsruhe; S. Hess, Berlin; J. Jonas, Urbana; R. Klein, Konstanz; K. Lucas, Duisburg; H.-D. Ludermann, Regensburg; H. Posch, Wien; P.Pusey, Malvern; J. P. Ryckaert, Brussels; W. A. Steele, Penn State; D. J. Tildesley, Southampton; H. Weingartner, Karlsruhe. Further details may be obtained from: Professor H. Versmold, lnstitut fur Physikalische Chemie, RWTH Aachen, Templergraben 59, D-5100 Aachen. Federal Republic of Germany. P. Turq (France) (vii)THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM No. 25 Large Gas Phase Clusters University of Warwick, 12-14 December 1989 Organising Committee: Professor K. R. Jennings (Chairman) Professor P. J. Derrick Professor D. Phillips Dr N. Quirk Dr R. P. H. Rettschnick Dr A. J. Stace The Symposium will focus on recent developments in the rapidly expanding field of large gas phase clusters, including the preparation, structure and reaction of both neutral and ionic clusters.It is hoped that the meeting will bring together scientists working on many different types of cluster, e.g. rare gas atoms, metals, inorganic and organic species, and biomolecules, to discuss the chemistry and physics of clusters from different viewpoints. The preliminary programme is now available and may be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN. THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION No. 89 Structure of Surfaces and Interfaces as Studied using Synchrotron Radiation University of Manchester, 4-6 April 1990 Organising Committee: Professor J. N. Sherwood (Chairman) Professor D. A. King Dr G. King Dr C. Norris Dr R. Oldman Dr G.Thomton The Discussion will focus on the wealth of novel information which can be obtained on the nature and structure of surfaces using the full spectral range of synchroton radiation. Emphasis will be placed on the scientific results of recent investigations rather than on technical aspects of experimentation. Papers will be welcome which shed new light on the structure of the complete range of interfaces: solid/solid, solidlgas, solifliquid, gasniquid and "dean" surfaces including both static and dynamic in situ examinations. It is hoped that the discussion will define the utility of synchroton radiation examinations in surface science studies at a time of expansion of the availability of such sources and the inauguration of new and more powerful sources. Further information may be obtained from: Professor J.N. Shemod, Department of Pure and Applied Chemistry, University of Strathclyde, Thomas Graham Building, 295 Cathedral Street, Glasgow G1 1 X L (viii)THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY ASSOCIAZIONE ITALIANA DI CHIMICA FlSlCA KONIKNLIJKE NEDERLANDS CHEMISCHE VERElNlGlNG SOCICTC FRANGAISE DE CHIMIE, DIVISION DE CHlMlE PHYSIQUE FARADAY DIVISION GENERAL DISCUSSION No. 90 DEUTSC H E BU NSEN-GESELLSCHAFT FUR PHYSI KALISCHE CH EMI E Colloidal Dispersions University of Bristol, 10-12 September 1990 Organising Committee Professor R. H. Ottewill (Chairman) Professor P. Botherol Professor E. Ferroni Dr J. W. Goodwin Professor H. Hoffmann Professor A.L. Smith Professor P. Stenius Dr Th. F. Tadros Professor A.Vrij Dr D. A. Young The joint meeting of the Societies will be directed towards examining current understanding of the behaviour of colloidal dispersions. In particular, stability and instability, short range interactions, dynamic effects, non-equilibrium interaction, concentrated dispersions and orderdisorder phenomena will form topics for discussion. Contributions for amsideration by the Organising Committee are invited. Titles and abstracts of about 300 words should be submitted by 30 September 1989 to: Professor R. H. Ottewill, School of Chemlstry, University of Bristol, Bristol BS8 lTS, England. Full papers for publication in the Discussion Volume will be required by May 1990. JOURNAL OF CHEMICAL RESEARCH Papers dealing with physical chemistry or chemical physics which appear currently in J.Chem. Research, The Royal Society of Chemistry's synopsis + microform journal, include the following: An Electron Spin Resonance Investigation of the Trimethylsilyl h i d e Radical Cation: MwSiN3+' The Versatile Reactivity of a-Acetylenic Aldehydes towards Furan: an Attempted Rationalization Andre Simon, Franpis Corre and Alain Christopher J. Rhodes (1989, Issue 1) using Sustmann's Variation-Perturbation Method Gorgues (1 989, Issue 2) Refinement Program PROTAF to the Copper(ti) Complexes of N,N-Dimethyl-l,2diaminoethane Robert Fournaise and Christian Petitfaux (1 989, Issue 2) Study of Complex Formation in Aqueous Solution. Part 4. Application of the Multiparametrii Geminal Carbon-Carbon Coupling Constants in Ketones A Fast and Accurate Estimation of Amide-lminol Tautomerization Energies by the AM1 Method Electron Spin Resonance Evidence for the Radical Cation of s-Triazine Anthony G.Avent, Penny A. Chaloner and Alison Jones (1 989, Issue 2) Andrzej Sygula (1 989, Issue 2) (1 989, Issue 3) Christopher J. Rhodes~~ FARADAY DIVISION INFORMAL AND GROUP MEETINGS Gas Kinetics Group Developments in Gas Kinetics: New Techniques, Results and their Interpretation To be held at the University of York on 34 July 1989 Further information from Professor R. J. Donovan, Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ Industrial Physical Chemistry Group with the Thin Films and Surfaces Group of the IOP Materials for Non-linear and Electrosptics To be held at Girton College, Cambndge on 4-7 July 1989 Further information from The Meetings officer, Institute of Physics, 47 Belgrave Square, London SWlX 8QX ~ Electrochemistry Group with Elecfroanalytical Group Graduate Students' Meeting To be held at Imperial College, London on 12 July 1989 Further information from Dr G.H. Kelsall, Department of Mineral Resources Engineering, Imperial College, London SW7 2BP Polymer Physics Group Biologically Engineered Polymers 89 To be held at Churchill College, Cambndge, on 31 July to 2 August 1989 Further information from Dr M. J. Miles, AFRC Institute of Food Research, Colney Lane, Nonvich NFM 7UA Polymer Physics Group Biennial Meeting: Physical Aspects of Polymer Science 25th Anniversary To be held at the University of Reading on 13-1 5 September 1989 Further information from Dr G. R. Mitchell, Polymer Physics Laboratory, University of Readng, Whiteknights, Reading RG6 2AF. Colloid and Interface Science Group Inorganic Particulates To be held at Chester College on 1921 September 1989 Further information from Dr R. Buscall, ICI plc, Corporate Colloid Science Group, PO Box 11 , The Heath, Runcorn, Cheshire WA7 4QE Division with the Institute of Physics Sensors and their Applications To be held at the University of Kent at Canterbury on 1922 September 1989 Further information from The Meetings officer, Institute of Physics, 47 Belgrave Square, London SWlX 8QX Division with the Deutsche Bunsen Gesellschaft, Division de Chimie Physique of the Societe Franpise de Chimie and Associazione ltaliana di Chimica Fisica Transport Processes in Fluids and Mobile Phases To be held at the Physikalische Institiit, Aachen, West Germany on 2528 September 1989 Further information from Professor G. Luckhurst, Department of Chemistry, University of Southampton, Southampton SO9 5NH Division Autumn Meeting: Chemistry at Interfaces To be held at Loughborough University of Technology on 26-28 September 1989 Further information from Professor F. Wilkinson, Department of Chemistry, Loughborwgh University of Technology, Loughborwgh El 1 3TU

ISSN:0300-9599    

DOI:10.1039/F198985BP063

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

J. Chern. Soc., Faraday Trans. I , 1989, 85(5), 1019-1025 Temperature Dependence of Thermo-osmosis A Solution Model Cristobal Fernandez-Pineda and M. Isabel Vazquez-Gonzalez Department of Physics, Faculty of Sciences, University of Malaga, Malaga, Spain A model has been proposed to study the temperature dependence of the permeation and the thermo-osmosis of a single substance through a membrane. The model takes into account the dependence on temperature of the molar volume of permeant, the enthalpy of transfer from the permeant phase to the membrane phase, and the phenomenological coefficients defined for a thin slab of membrane. The empirical phenomenological coefficients, permeability and thermo-osmotic permeability of the dis- continuous system are obtained as integral formulae of the abovementioned properties.A method has been proposed to evaluate the transport phenomenological coefficients, true for a thin slab of membrane, from the experimental data of the enthalpies of dissolution of permeant in the membrane, of the permeability and of the thermo-osmotic permeability. The experimental data reported in the literature and those obtained from our previous work fit well with this theoretical model, but it is recognized that new experimental results are needed to apply this model completely. Thermo-osmosis, the flow of matter through a membrane due to a difference in temperature, has been studied in a wide variety of experimental situations.'-8 The common feature shown by all the results of the experiments is that thermo-osmotic permeability is a function of temperature, changing with the average temperature and difference in temperature of the external solutions.From a theoretical point of view thermo-osmosis has been given considerable attention. The mechanisms of thermo-osmosis in charged membranes were studied by different a u t h o r ~ . ~ - l ~ In non-charged membranes they were discussed by others, using (completely or in part) the local formulation of thermodynamics of irreversible processes. 1* 2 * 1 3 9 l4 There are also authors who assume an unconventional point of view in their theoretical analysis of the physical nature of the forces driving non-isothermal fluids transport in porous membranes.8. 15-17 The works more related to the present paper1' 2, 1 3 7 l4 study thermo-osmosis only under the effect of small and constant pressure and temperature gradients, not explaining, therefore, any dependence of thermo-osmotic permeability on temperature. In addition, they do not include in the dissipation function the term due to viscous forces.The proposed model explains the dependence of permeability and thermo-osmotic permeability on temperature reported el~ewhere.~-~, 6-7 These data do not allow an entire applicability of the model. Therefore, new experimental results are needed to apply it completely. The Model Particularizing eqn (26) of ref. (18) to the case of a system composed of the membrane matrix and a monocomponent permeant fluid (which will be taken as water), on which no external forces will act (4 = 0), we obtain for the average dissipation function 1019 35-21020 Temperature Dependence of Thermo-osmosis where the angular brackets indicate the average values for the cross-section of the membrane, (for example, where z is the volume of a thin cross-sectional slab of membrane of thickness 6z at position z , and A is the area of cross-section of the membrane), ( J Z ) is the molar flux of water relative to the membrane matrix, whose velocity is vm = 0, (4) is the reduced heat flux, (gradp;), is the gradient of chemical potential of water at constant temperature and grad Tm is the gradient of the thermodynamic temperature.The superscript m indicates that the quantity affected by it is within the membrane. for an isotropic, non-reacting system containing two species i (water and membrane matrix) under steady-state conditions, with inertial terms neglected (terms in u2).Even then, viscous contributions to the fluxes can still occur, but they have become implicit ones, so that dissipation is absorbed by the force required to keep the membrane stationary. Eqn (1) can be applied to a continuous membrane phase, where the distances between adjacent parts of the membrane matrix are of the same order of magnitude as the diameter of a permeating molecule, which gives us reason to suppose that the membrane component is interspersed at a molecular level among the components of the permeating fluid and therefore partakes intimately in the processes occurring. If we suppose that the membrane has plane-parallel faces and that all the gradients in the membrane are in the z direction, which is perpendicular to the lateral surfaces of the membrane, then eqn (1) would be Eqn (1) is ( J i ) dT" ( @ ) = - ( J C ) - -- - (d:~Z)T T" ( dz ) where ( J Z ) and ( J i ) are the components in the z axis of the corresponding fluxes ( J c ) and (4).The phenomenological equations corresponding to eqn (2) are where I,, IT, ZwT and I,, are the phenomenological transport coefficients for a thin slab of the membrane parallel to the faces of the membrane and perpendicular to the direction of the average flow (the z direction). The coefficient Z, is a pseudo-coefficient of diffusion in which the viscosity coefficient is concealed. Integrating eqn (3), taking into account that the flow (J,") is constant in the steady state, between the surfaces of the membrane (i.e.between z = 0 and z = Az) we obtain In order to carry out the integrations which appear in the above equation it is necessary to know the dependence of the phenomenological coefficients on the intensive state variables (which characterize the intensive thermodynamic state of each slab of the membrane) and the profile of these in the membrane, that is the dependence of the intensive variables on z, as well as the gradients and their dependence on z. The solution to this general problem is almost impossible, therefore in practice it recurs to postulate a model of behaviour of the membrane. The model which we areC. Fernandez- Pineda and M. I. Vazquez-Gonzalez 1021 going to use to integrate eqn ( 5 ) is based on the ideas of those already established,'.2 v 13* l4 but allows for the strong dependence of thermo-osmotic permeability on temperature. Imagine our membrane as a continuous system with macroscopic overall dimensions and well defined properties. The system is formed by two components: the structural substance of macromolecules of the membrane, which form the matrix, and which we consider as the solvent, and the permeant substance, which we take as the solute. We take the pressure, P", the temperature, T", and the composition of the binary system, which is characterized by the mole fraction, x, of the permeant, varying continuously from point to point and in the perpendicular direction of the lateral surfaces of the membrane. Under steady-state conditions it is possible to imagine the binary system divided into slabs, characterized by P"(z), T"(z) and x(z), each of which is found in a separate saturation equilibrium with the pure permeant liquid with the same values of pressure and temperature (P, T ) .This implies that temperature, pressure and chemical potential of the permeant are the same inside and outside of the slab. Considering all this and using the expression for the differential of the chemical potential, we obtain - where enthalpy change for the dissolution of permeant in the slab, which is by definition is the partial molar volume of the pure permeant (water) and ms is the molar - def - AHs = HC-Rw (7) and Hw and HT are the partial molar enthalpies of the water and of the water in the binary system, respectively. Substituting eqn (6) into eqn ( 5 ) gives (J,") = -:[ s,"lw V w ( ~ ) d z + s , " ( T + ~ ) ( ~ ) d z ] .l w m ' lwT In this equation Az = 6, the thickness of the membrane. In order to solve the integrals which appear in eqn (8) it is necessary to know the functions l,[T(z), P(z)], Vw[T(z), P(z)], AH'[ T(z), P(z)], lwT[ T(z), P(z)] as well as the gradients (dP/dz) and (dT/dz). The problem is simplified if we take into account the fact that in condensed phases the thermodynamic properties are practically independent of the pressure, except for very high values. In the study of thermo-osmosis the differences in pressure usually reach only a few centimetres of mercury, in a way that the above statement is true in the present case. Consequently, eqn (8) can be expressed as ~ Two particular cases can be presented.(1) If the temperature is kept constant eqn (9) becomes where we have put AP = P(Az)-P(0).1022 Temperature Dependence of Thermo-osmosis (2) With a constant pressure we obtain ( J E ) = -- JT(*’) (l.(T)m’(T)+m)dT. T T * T(O) Comparing eqn (9), or its particular cases which are given by eqn (10) and (I l), with the global empirical description given by Haase2’ we obtain for the permeability A(T) = Iw(T) Vw(n (12) and for the thermo-osmotic permeability where & and T, are the temperatures at the surfaces of the membrane. Eqn (1 2) and (1 3) relate the phenomenological coefficients, which are determined experimentally, with the average coefficients 2, and lwT. Eqn (12) allows us to determine the function I,( T ) from measurements of permeation at different temperatures-From eqn (1 3) it follows that if we know I,( T ) from measurements of permeation and AHs( T ) after calorimetric measurements, we can calculate fwT( T ) from thermo-osmotic measurements.Application of the Theoretical Analysis As stated above, eqn (12) evaluates the I,( T ) coefficient from the permeability values at different temperatures. The values obtained from Iw(T) can be fitted to a development in the form of power series: (14) where a,, a,, a,, . . . are constants. The values of ms for different temperatures can be fitted to another power series (15) I,( T ) = a, +a, T+ a2 T2 + . . . A F ( T ) = b,+b, T+b2T2+ ... where b,, b,, b,, . . . are constants. form of power series of the absolute temperature in such a way that It is also possible to suppose that the IJT) coefficient can allow a development in the (16) where c,, c,, c,, , .. are constants. All the constants ai, bi, and ci are characteristics of the permeant-membrane system. Substituting eqn (14H16) in eqn (1 3) and after carrying out the integration, we obtain ZwT(T) = C , + C , T+c2T2+ ... where B, = a, b, + c, B, = a,b,+a,b,+c, B2 = (~)(aob2+a,b,+a,b,+c2) B3 = ($)(a0b3 + a, b, + a , b, +a3 b, + c,) etc. Eqn ( I 7) is a general expression for the thermo-osmotic permeability as a function of the surface temperature of the membrane. The experimental data, in the systems where the hypothesis of our model are true, should be well described by such an equation.C. Fernandez- Pineda and M. I. Vazquez- Gonzalez 1023 Table 1.Optimal fit to eqn (17) for the thermo-osmotic permeability divided by membrane thickness, B / 6 , for the different membranes membrane [B(T,, T,)/q/rnol mP2 s-' K-' cellophane 6003 cellophane 6004 cellophane 600P' cellophane 500P7 C.A. mem. no. 16a - 2.42 x lo-' In (</T,)/( q - T,) + 1.7 x 4.8 In(&/?)/(q-- TJ-2.96 x 10-2+2.3 x 10-5(T,+ 7'J 3.4 x 2.1 In (q/ T,)/( T, - q) - 1.34 x -4.9 x 10-21n(T,/T,)/(T,-T,)+3.0x 10-4-2.2x 10-7(T,+T,) - 1.47 x T, + 7J In (T,/T,)/(T, - T,) - 1 .O2 x 1 0 - ~ + 1.06 x T, + TJ C.A. mem. no. 26a -7.7 x 10-31n(~,/~,)1(~,-~,)+3.16x 10-5 ~ a The values reported for this membrane have been obtained from steady state. The experimental results given by Haase et al.,3 Rastogi et al.* and Fernandez-Pineda and Va~quez-GonzBlez,~ who used different cellophane membranes and water as the permeant, like those of Mengual et a1.,6 who used cellulose acetate membranes with 2.7 degree of acetylation and water as permeant, have been analysed using eqn (17).The procedure followed consisted of fitting the experimental results to an equation with the form of eqn (17) by multiple regression analysis. To select the optimum fits, the same procedure as indicated in ref. (7) was followed. The results of the optimal fits are shown in table 1, where the functions B(T,, T,)/6 appear, in mol m-2 s-' K-' to permit a comparative analysis. Inspection of table 1 suggests that in all the fits, terms with coefficients B, and B, appear to be statistically different from zero; this indicates that the sum (lw ms + ZwT) depends, at least linearly, on the thermodynamic temperature.In four of the six fits, terms with coefficients B,, B, and B, appear to be statistically different from zero; this indicates that the sum (1, ms + IwT) depends quadratically on the thermodynamic temperature for the stated experimental systems. This quadratic dependence on temperature is notable in the fit of the experimental results obtained by Fernandez- Pineda and Vazquez-Gonzalez7 for the membrane 500P, where the average temperature and the difference in temperature between the faces of the membrane vary within small intervals (between 308 and 317 K and between 2 and 6 K, respectively). In order to complete the application of the theoretical analysis to the experimental results, the dependence of the coefficient I, and the heat of dissolution ms on temperature must be determined.However, values for AHs have not been reported for the experimental systems described above. Experimental measurements of the heat of dissolution and its dependence on temperature are of great importance for the application of the present model. Values of m: have been estimated for the 600P and 500P membranes in ref. (7) from microcalorimetric meas~rements~~ and determinations of membrane water content at saturation. Only two temperatures were studied (33 and 45 "C) and the results obtained were fitted to a linear dependence on T. and - AH",J mol-1 = 374.6 - 1.973 T rH",J mol-' = 97.7- 1.104T for the 600P and 500P membranes, respectively. Eqn (12), the values of permeability A,7 the values of the specific volume of water,22 and the values of the molecular mass of the water have been used to determine the values of lw(T).These values have been fitted to a power series of absolute temperature. The best fits obtained, within the temperature range studied, are lW/mol2 s kg-' m-3 = 1.254 x 10-7-4.13 x 10-l'T and lw/mo12 s kg-' mP3 = 2.208 x 10a7-7.71 x 10-llT for the 600P and 500P membranes, respectively. (20)1024 Temperature Dependence of Thermo-osmosis The dependence on temperature of the transport coefficient, lwT, is calculated by substituting eqn (1 9) and (20) into eqn (1 8) and by solving the system of equations found for ci. Note that the values of B,, B,, . . . etc. used here were obtained from the direct fit of the experimental data to eqn (17), and not those which would be obtained from the values appearing in table 1 multiplied by the thickness of the membranes, since a small discrepancy exists between these, this being caused by the arithmetic precision of the microcomputer we have used.The dependences on temperature found are : 1,,(600P)/mol m-'s-' = -4.5 x 10-5+2.56 x 10-7T-8.15 x 10-"T2 and IW,(500P)/mol m-' s-' = 8.6 x 10-5-4.30 x 10-7T+9.91 x 10-'"T2. The constant coefficients a,, b, and c, which appear in eqn (19), (20) and (21) do not allow a simple interpretation. For this reason the dependence on temperature has been referred to its lowest value in the range in which the experimental data are available. The selected temperature is = 306.15 K. By using some elemental algebraic manipulations eqn (19)-(21) are transformed for the 600P membrane into } (21) Iw/mo12 s kg-' m-3 = 1.127 x 10-7-4.13 x lO-''(T- T,) - AHi/J mol-' = - 229.5 - 1.973( T- T,) ] (22) ] (23) I,,/mol m-' s-' = 2.6 x + 2.06 x lop7( T- T,) - 8.15 x 1O-l1( T - and for the 500P membrane into Iw/mo12 s kg-' mP3 = 1.971 x 10-7-7.71 x lO-''(T- T,) - AHt/J mol-' = - 240.3 - 1.104( T- T,,) I,,/mol m-'s-' = 4.8 x 1.76 x 10-7(T- &)+9.91 x 10-lo(T- The constant terms which appear in eqn (22) and (23) are now the values of I,, mi and IwT for the temperature of 306.15 K (33 "C). For the 600P membrane the term c;(T- Q2 i n the coefficient IwT can be neglected when looking at the other two within the temperature range in which there is experimental data.For the 500P membrane this term can also be neglected, in the abovementioned conditions, but the errors are greater than those for the 600P membrane (in this one the difference ranges between 1 x and 1 x while in the other it varies between - 1 x and 2 x In both cases outside of the range of temperature indicated above, the discrepancies can be noticed.The behaviour of the numerator of the integrand in eqn (1 3) for the 600P membrane, from the procedure followed in obtaining eqn (22), is reduced to a linear dependence on temperature. The extrapolation, with all the risks it entails, permits us to expect its annulment at ca. 60 O C , this would be the temperature at which the direction of the thermo-osmotic flow would be inverted, i.e. instead of going from hot to cold it would go from cold to hot.With a 600P membrane different from the previous one the thermo- osmotic coefficient has been determined at a stirring rate of 260 rev min-l and at 65 and 70 "C bulk temperatures. The flow observed was from cold to the hot side, obtaining avaluefor thecorrected thermo-osmotic permeability Po" = 2.4 x lo-', mol m-' s-' K-l. Taking this value and the BCorr values obtained at the highest average temperatures from ref. (7) and following the procedure mentioned there, the differential thermo- osmotic coefficient b(t) is calculated. This coefficient is annulled at ca. 57.4 "C. In contrast to the above, the numerator of the integrand in eqn (1 3) for the 500P membrane is a quadratic function of the temperature; the extrapolation does not reveal a real value of the intercept with the axis of temperatures; however, this function has a minimum value at a temperature of ca.43.5 "C. This temperature is approximately equal to the mean temperature at which the experimental data present the lowest value. In summary, a model has been developed to describe the dependence of the thermo- osmotic permeability on temperature. The model is applied to the transport underC. Fernandez- Pineda and M . I . Vazquez-Gonzalez 1025 steady-state conditions of one solute, driven by a temperature gradient, assuming that the membrane-permeant system can be described as a non-electrolyte solution. The experimental data reported in the references were analysed and fitted with this model, but it is recognized that new studies are needed to apply it completely : to measure equilibrium parameters (like those which allow us to know the states and the enthalpies of dissolution of the permeant into the membrane) and likewise to measure transport parameters (like the permeability and the thermo-osmotic permeability in a broad temperature range, with the aim of determining with accuracy their dependencies on temperature).We are indebted to Professor M. I. Paz-Andrade and her team of collaborators of the Department of Applied Physics of the University of Santiago de Compostela (Spain) for performing the microcalorimetric experiments. References 1 M. S. Dariel and 0. Kedem, J. Phys. Chem., 1975, 79, 336. 2 H. Vink and S. A. A. Chisthi, J. Membrane Sci., 1976, 1, 149. 3 R. Haase, H.J. De Greiff and H. J. Buchner, Z . Naturforsch., Teil A , 1970, 25, 1080. 4 R. P. Rastogi, R. L. Blokhra and R. K. Agarwal, J . Electrochem. Soc., 1962, 109, 616. 5 J. I. Mengual, J. Aguilar and C . Fernandez-Pineda, J . Membrane Sci., 1978, 4, 209. 6 J. I. Mengual, F. Garcia-Lopez and C . Fernandez-Pineda, J. Membrane Sci., 1986, 26, 21 1. 7 C. Fernandez-Pineda and M. I. VBzquez-Gonzalez, J . Chem. Soc., Faraday Trans. 1 , 1988, 84, 647. 8 N. Pagliuca, U. Bencivenga, D. G. Mita, G. Perna and F. S . Gaeta, J . Membrane Sci., 1987, 33, 1. 9 Y. Kobatake and H. Fujita, J. Chem. Phys., 1964, 41, 2963. 10 N. V. Churaev, B. V. Deryagin and P. P. Zolotarev, Dokl. Akad. Nauk SSSR, 1968, 183, 1139 (Dokl. 11 J. W. Lorimer, in Charged Gels and Membranes ZI, ed. E. Sdegny (Reidel, Dordrecht, 1976), p. 76. 12 M. Tasaka, Pure Appl. Chem., 1986, 58, 1637. 13 R. Haase, 2. Phys. Chem. (Frankfurt), 1966, 51, 315. 14 R. Haase and E. 0. Timmermann, J. Membrane Sci., 1982, 10, 57. 15 F. S. Gaeta and D. G. Mita, J. Phys. Chem., 1979, 83, 2276. 16 M. Imai, S. Furusaki and T. Miyauchi, Ind. Eng. Chem., Process Des. Dev., 1982, 21, 421. 17 E. Drioli and Y. Wu, Desalination, 1985, 53, 339. 18 J. W. Lorimer, J . Membrane Sci., 1983, 14, 275. 19 D. C . Mikulecky and S. R. Caplan, J . Phys. Chem., 1966, 70, 3049. 20 E. A. Mason and L. F. del Castillo, J . Membrane Sci., 1985, 23, 199. 2 1 R. Haase, Thermodynamics of Irreversible Processes (Addison- Wesley, London, 1969). 22 Handbook of Chemistry and Physics, ed. R. C. Weast (CRC Press, Boca Raton, Fla, 1974), vol. 55. 23 M. I. Paz-Andrade, personal communication. Phys. Chem., 1968, 183, 935). Paper 8/01038B; Received 14th March, 1988

ISSN:0300-9599    

DOI:10.1039/F19898501019

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

J. Chem. Sac., Faraday Trans. I , 1989, 85(5), 1027-1041 Intramolecular Photochemical Electron Transfer Part 5.-Solvent Dependence of Electron Transfer in a Porphyrin-Amide-Quinone Molecule John A. Schmidt, Jing-Yao Liu and James R. Bolton" Photochemistry Unit, Department of Chemistry, The University of Western Ontario, London, Ontario, Canada N6A 5B7 Mary D. Archer* and Victor P. Y. Gadzekpo Department of Physical Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 IEP The compound PAQ, which consists of a tetra-arylporphine attached to methyl-p-benzoquinone via a single amide linkage, exhibits light-induced intramolecular electron transfer from the porphyrin excited-singlet state to the quinone at a rate which is strongly solvent-dependent. The rate constants are found to correlate well with the semiclassical Marcus theory of electron transfer, provided that the solvent effect on both the Gibbs energy change, AGO, for the electron-transfer reaction 'P*AQ -+ P'+AQ'- and the Marcus reorganisation energy, A, are considered.The AGO values are obtained from direct measurement of redox potentials in each solvent with various work-term corrections for Coulombic interaction in P'+AQ'-, and 1 is calculated from the optical and dielectric properties of each solvent. For fourteen solvents, reasonable agreement with Marcus theory is found using this approach on uncorrected AGO values and those corrected with a solvent- dependent work term ; a solvent-independent correction is not successful. For two solvent mixtures (acetonitrile-benzonitrile and acetonitrile- chloroform), excellent agreement with Marcus theory is found using uncorrected AGO values and those corrected with a solvent-dependent work term.We have found that Weller's method for calculating A G O in various solvents from a single measurement in a reference solvent gives a poor correlation with Marcus theory, primarily because of a poor prediction of the solvent dependence of AGO. Covalently linked porphyrin-quinone molecules are important as models of photo- synthetic electron-transfer reactions. Increasingly complex synthetic strategies have been devised'-5 to mimic the molecular architecture of photosynthetic reaction centres, which has recently been revealed by X-ray crystal-structure ' We have studied the relatively simple porphyrin-quinone compound PAQ' 6 PAQ 10271028 Intramolecular Photochemical Electron Transfer in which a free-base tetra-arylporphine is linked via a single amide group to methyl-p- benzoquinone.In an earlier communications we reported that the porphyrin excited singlet state of PAQ is quenched by electron transfer to the attached quinone group: and that the rate constant k,, is strongly solvent-dependent. We showed a qualitative correlation between log k,, and the difference in solvation energy between the thermally equilibrated non-polar precursor state 'P*AQ and the corresponding high-dipole- moment Franck-Condon successor state P"AQ'-. In a subsequent paperlo we compared this analysis with the semiclassical Marcus theory but with a constant, solvent- independent Gibbs energy change AGO for reaction (1); this gave a very poor correlation. However, in a recent preliminary report'l we showed that when AGO is estimated from electrochemical measurements in each solvent, the agreement with Marcus theory improves dramatically.A complete characterisation of the photophysics of PAQ in benzonitrile solutions has recently been reported. l2 In this paper we report a further analysis of the solvent effect on reaction (1). When the solvent dependence of AGO, and especially the reorganisation energy A, are considered, the electron-transfer rate constants in fourteen solvents correlate reasonably well with semiclassical Marcus theory. 13-15 Good agreement is obtained when the solvent properties are varied by using different compositions of binary solvent mixtures (acetonitrile-benzonitrile and acetonitrile-chloroform). We analyse our results in terms of the high-temperature limit of semiclassical Marcus theory, which gives the electron-transfer rate constant k,, as14* l5 where Hps is the electronic coupling between the precursor (lP*AQ) and successor (P'+AQ'-), II is the reorganisation energy and AGO is the Gibbs energy change for the electron transfer.? We shall use eqn (2) in a logarithmic form (2n Wps ) (AGo+A)2 log (ke, 2;) = log - h (471kB T): -(2.303) 4AkB T or where Y = c - x (3 b) Y = log (ket A;) C = l o g ( z % ) and X = (AGO + A)' h (4nkBT); (2.303) 4Ak, T' The reorganisation energy II has two components, an inner term ,Ii, arising from internal reorganisation (primarily bond-length changes) within the reacting molecule(s), and an outer term, A,, arising from changes in the surrounding medium: t As long as 1 > IAC"l (this is the case for almost all our data), then the more complex semiclassical equation^,'^ which do not require the high-temperature approximation, reduce to eqn (2).J .A . Schmidt et al. 1029 In semiclassical Marcus theory the solvent is assumed to behave as a dielectric continuum and A, is given by13 = B ( i - 3 where e is the electronic charge, E, is the permittivity of free space, and E,, and E, are the optical and static dielectric constants of the solvent: E,, is given by the square of the refractive index.16 In eqn (5a) it is assumed that the donor (P) and acceptor (Q) are spheres of radii a p and a, separated by a centre-to-centre distance apQ 4 (ap+aQ); this model is hardly appropriate for PAQ, but a more realistic ellipsoidal model also yields an equation which closely approximates to the form of eqn (5b), and B turns out to be fairly model-insensitive.l7 The use of eqn (2) and ( 5 ) implies that the precursor 'P*AQ is sufficiently long-lived to equilibrate both internally and with the surrounding solvent. The minimum observed lifetime was ca. 1 ns, sufficient for this condition to be fulfilled. Evaluation of eqn (2) requires values of both 2 and AGO for reaction (1) in each solvent. It is not uncommon in work of this type either to ignore any solvent dependence in AGO, or to calculate AGO in different solvents from the value in a single reference solvent using Weller's equation.'' We consider these procedures to be unsatisfactory, for the reasons adduced in the Discussion section.Therefore we have measured AGO from the difference A P = Ep - EQ between the experimental redox potentials for the reactions P"AQ + e- -+ PAQ and PAQ+ e- -, PAQ*- in each solvent and the energy 4, of the equilibrated porphyrin excited singlet state relative to the ground ~ t a t e . ' ~ This quantity we denote AG;: This expression is not unexceptionable. It implies no work term, i.e. no Coulombic interaction or difference in interaction, in P*+AQ*-, P*+AQ and PAQ*-. However, there is likely to be some Coulombic stabilisation of the ion pair P*+AQ'- relative to the ions P'+AQ and PAQ'- produced separately in the electrochemical measurements, albeit in the presence of supporting electrolyte ions which afford some stabilisation.Therefore we have examined the effect of two methods of 'correcting' AG,". Our second estimate AGY assumes a Coulombic interaction through the solvent, exhibiting its bulk dielectric constant E,, between two point charges at the centre-to-centre distance apQ : e2 ~ Z E , E, apQ ' AGY = AGi- Coulombic interaction in P*+AQ*- must occur, at least in part, through the molecule itself rather than through the solvent. Moreover, insofar as this occurs through the solvent, local dielectric saturation may lower the effective static dielectric constant. As Suppan2' has pointed out, a solvent-independent correction term w would then be appropriate, and our third estimate is where w was taken to be 0.14.4 eV; w = 0.4 eV is equivalent ,to assuming Coulombic interaction between two point charges a distance apQ = 14 A apart separated by a medium of E, z 2.5.1030 Intramolecular Photochemical Electron Transfer (acetonitrile) to estimate AGO in other solvents from Weller's equation" Finally, we have used our AG: measurement for one reference (ref) solvent where apQ is again the porphyrin-quinone centre-to-centre separation and a is the average of the radii of P'+ and Q'-.Use of eqn ( 6 4 implies that the Born equation for individual ion solvation is applicable and that the work term correction of eqn (6b) is valid. Experimental The synthesis and characterisation of PAQ have been described,'Vg as have the procedures for determining the redox potentials of PAQ by differential pulse voltammetry in each solvent system in a mini-cell under vacuum.''- Experiment showed that the influence of i.r.drop on peak positions is negligible, so i.r. compensation was not used. All redox potentials were measured us. ferrocene (Fc), added in small concentrations as an internal reference. Tetrabutylammonium hexafluorophosphate (TBAH) at a concentration of 0.1 mol dm-3, if soluble in the solvent, was used as the supporting electrolyte because the large size of its ions should minimise ion association with P'+AQ or PAQO-. Electron-transfer rate constants k,, were determined as previously described'. ' 9 22 from the relation k,, = l / ~ , - l / ~ , (7) where z, and Z, are the fluorescence lifetimes of PAQ and the corresponding hydro- quinone compound PAQH,.It is assumed that the shorter lifetime of PAQ relative to that of PAQH, is entirely a consequence of reaction (I), and that all other decay pathways (fluorescence, internal conversion and intersystem crossing) have the same rate constants in PAQ and in PAQH,. This assumption has been verified by picosecond absorption studies of PAQ in benzonitrile.12 All solvents were dried and distilled prior to use. Binary solvent mixtures were prepared by mixing appropriate volumes of each solvent. The percentage composition on a molar basis was calculated using densities of 1.4832, 1.0102 and 0.7857 g ~ r n - ~ for chloroform, benzonitrile and acetonitrile, respectively. 23 Refractive indices for solvent mixtures were measured at 20 "C with a Bausch and Lomb refractometer.Static dielectric constants for solvent mixtures were measured using a General Radio model 1689 a.c. capacitance bridge, operated at 100 kHz.' Results Fluorescence Measurements The fluorescence lifetimes of PAQ in 22 solvents have been reported previously:9 the relevant ones are repeated in table 1 in more detail. In some solvents, single exponential decays were observed, but in others, particularly for PAQ, the data could be fitted only with two exponential decays, as indicated in table 1. This we attribute to the simultaneous presence of both PAQ (shorter-lifetime component) and PAQH, (longer- lifetime component) in solvents in which complete reduction or oxidation was not achieved. In a few solvents z, and z, were rather close. To check the integrity of the compound in these cases, the solvent was evaporated after the lifetime measurements had been made and the precipitate was taken up in CH,Cl,.In all cases the fluorescence lifetimes observed for this recovered solution were those expected for PAQ in CH,Cl,.J. A . Schmidt et al. Table 1. Analysis of fluorescence lifetimes for PAQ and PAQH, 1031 solvent compound 7,/nsa t,/nsb x 2 c Itet/ loR s-' CH,CN C,H,CN C,H,CN acetone Bu"0H 1,2-DCE" CH,CI, 2-MTHFf 1 ,l , 1 -TCE' 1 ,2-DMEh EtOAc' CI-BZ' Cl-Naphk CHCl, 1,2-DBE' anisole 7.6 7.2 2.1 8.9 3.1 1.5 1.1 9.4 1.7 10.0 9.5 1.1 1 .o 1.2 1.4 0.42 0.52 0.53 2.4 2.5 - - - - - - - - - - - - - 12.0 12.0 11.5 (2.2 Yo) 11.5 12.0 12.0 (4.1 Yo) 12.0 9.6 8.8 (1.6%) 9.1 12.0 10.5 (6.1 Yo) 10.5 12.5 12.0 10.5 (1.2%) 1 1 .O (89 Yo) 10.0 (29 Yo) 10.5 (91 Yo) - - - - - - 7.6 (2.9 %) 9.1 1.3 (4.6 Yo) 1.5 (84%) 11.0 (1.5Yo) 12.0 (93 Yo) 1.10 1.25 1.31 1.15 1.14 1.18 1.02 1.38 1.17 I .02 1.20 1.14 1.01 0.97 0.48 0.56 3.9 0.29 2.4 5.6 8.0 0.23 4.9 0.20 0.22 8.2 7.4 22.7 12.6 3.3 a Fluorescence lifetime; 7, is the lifetime of PAQ and t, is the lifetime of PAQH,; errors are & 0.1 ns for 7, in PAQ and 7, in PAQH, and & 0.5 ns for z2 in PAQ.The figure in parenthesis, given only for a decay described by two exponential components rather than a single component, is the percentage of the component of lifetime t, (i.e. PAQH,). "Reduced x2 for the fit. Calculated using eqn (7); error & 5 %. 1,2-Dichloroethane. 2-Methyltetrahydrofuran. l , l , 1-Trichloroethane.1,2-Dimethoxyethane. Ethyl acetate. j Chlorobenzene. 1-Chloro- naphthalene. ' 1,2-Dibromoethane. Electrochemical Measurements The potentials R, and R, of the first and second reductions of the porphyrin ring of PAQ, Q, the first quinone reduction potential, and 0, and O,, the first and second porphyrin ring oxidations, are given in table 2.1032 Intramolecular Photochemical Electron Transfer Table 2. Redox potentials of PAQ (V us. Fc) solvent a electrolyteb R, R, Q, CH,CN C,H,CN C,H,CN acetone Bu"0H CH,Cl, 1,2-DCE 2-MTHF 1,l ,I-TCE 1,2-DME Cl-Bz EtOAc C1-Naph CHC1, anisole 1,2-DBE TBAHd TBAH TBAH TBAH TMACe TBAH TBAH TBAFBf TBAFB TBAH TBAHg TBACh TBAC TBAH TBAFB TBAPi ? ? - 1.530 ? ? ? ? - 1.536 ? ? ? - 1.742 -2.112 ? ? - 1.872 ? - 1.482 - 1.290 ? - 1.568 - 1.512 - 1.526 - 1.304 - 1.440 - 1.572 - 1.739 - 1.542 - 1.536 ? ? - 1.386 - 0.985 -0.830 -0.852 -0.908 - 0.920 -0.912 - 0.906 - 0.948 -0.816 - 0.936 - 1.175 - 0.987 - 0.696 - 1.014 - 0.940 - 1.056 0, AE'" 0.425 0.582 0.596 0.600 0.496 0.420 0.496 0.468 0.648 0.636 0.364 0.524 0.780 0.326 0.480 0.516 ? ? ? ? ? 0.996 ? 0.636 0.888 ? ? 0.950 ? ? 0.980 ? 1.41 1.41 1.45 1.51 1.42 1.33 1.40 1.42 1.46 1.57 1.54 1.51 1.48 1.34 1.42 1.57 a See footnotes to table 1 for definition of solvent symbols.0.1 mol dm-, unless otherwise indicated. AF/V = 0, - Q, = E',--E",. The error in the difference between any pair of potentials = kO.040 V; individual values us. Fc are less precise than this because the potential of the quasi-reference electrode was time-dependent.Tetrabutylammonium hexafluorophosphate. Tetrabutylammonium fluoroborate ; TBAH is insoluble in 2-MTHF, l,l,l-TCE, anisole and 1,2-DBE. Saturated solution (< 0.1 mol dm-,) TBAH in ethyl acetate. Tetrabutylammonium chloride. Tetrabutylammonium perchlorate. Tetramethylammonium chloride. Table 3. Effect of supporting electrolyte on electron-transfer rate constants for PAQ in three solvents CH,CN 0 7.5k0.1 14.7 f 1.3 7.82 k 0.05 0.1 7.9kO.l 13.6k0.6 7.73 & 0.04 C,H,CN 0 2.1 1 f 0.02 11.95 0.1 8.59 f 0.0 1 0.1 2.01 f0.03 1 1.67 f 0.05 8.62k0.01 2-MTHF 0 6.4 f 0.3 11.8 k0.3 7.85 f 0.06 0.1 5.7 f 0.5 11.8k0.2 7.96 k 0.08 Influence of the Supporting Electrolyte Most of the fluorescence lifetimes and corresponding rate constants were determined using ca. mol dm-3 PAQ solution without any other components present.On the other hand, most of the electrochemical measurements were carried out on ca. mol dm-3 PAQ in the presence of ca. 0.1 mol dm-3 supporting electrolyte. Thus we were concerned as to whether the presence of supporting electrolyte would affect our photophysical measurements. The data collected in table 3 show that the effect of supporting electrolyte on k,, is small compared with the overall solvent effects. However, the effect of supporting electrolyte on the measured redox potentials may be more significant. Previously we showed'' that varying the con- centration of our favoured supporting electrolyte, TBAH, had little effect in the rangeJ . A . Schmidt et al. 1033 Table 4. Rate constants, reorganisation energies and Gibbs energies for electron transfer in PAQ CH,CN C,H,CN C,H,CN acetone Bu"0H CH,CI, 1,2-DCE 2-MTHF I ,1,1-TCE 1,2-DME EtOAc C1-Naph CHCl, anisole CI-BZ 1,2-DBE 7.7 1 7.72 8.60 7.47 8.37 8.73 8.91 7.3 1 8.70 7.28 7.32 8.89 8.89 9.35 9.10 8.51 1 .800 1.910 2.328 1.839 1.953 2.080 2.020 1.974 2.062 1.899 1.876 2.3 16 2.667 2.082 2.369 2.293 35.94 24.83 25.20 20.56 17.51 10.37 8.93 7.60 7.25 7.20 6.02 5.62 5.04 4.8 1 4.78 4.33 1.15 1.07 0.90 1.09 1.02 0.89 0.89 0.87 0.83 0.90 0.86 0.66 0.52 0.69 0.59 0.57 0.49 0.49 0.45 0.39 0.48 0.57 0.50 0.48 0.44 0.33 0.36 0.39 0.42 0.56 0.48 0.33 0.52 0.54 0.49 0.44 0.54 0.67 0.6 I 0.62 0.57 0.47 0.53 0.57 0.63 0.77 0.70 0.57 0.69 0.69 0.65 0.59 0.68 0.77 0.70 0.68 0.64 0.53 0.56 0.59 0.62 0.76 0.68 0.53 0.52 0.48 0.50 0.48 0.46 0.40 0.38 0.34 0.33 0.32 0.28 0.25 0.22 0.20 0.20 0.16 See footnotes to table 1 for definition of solvent symbols.log ( k J s - l ) calculated from eqn (7) (values from ref 9); error in log k,,f0.04. Optical dielectric constant (square of the refractive index). Static dielectric constants and refractive indices were taken from J. A. Riddick, W. B. Bunger and T. K. Sakano, Techniques of Chemistry ZZ, Organic Solvents (Wiley, New York, 4th edn, 1986). Reorganisation energy in eV calculated from eqn (4) and (5b), with B = 1.80 eV and Ai = 0.2 eV. Calculated in eV from eqn (6a) using A F from table 2 and 4, = 1.90 eV as obtained from the overlap of the absorption and fluorescence spectra;29 this value is not solvent-sensitive to more than k0.02 eV.* Error in AG; k0.04 eV.Calculated in eV from eqn (6b); error k0.04 eV. Calculated in eV from eqn (6c) taking w = 0.2 eV; error f0.04 eV. Calculated in eV from eqn ( 6 4 using electrochemical data for PAQ in CH,CN as the reference solvent; a = 5.2 A; apQ = 14 A. Static dielectric constant. 0.05-0.3 mol dmP3, but that varying the electrolyte itself had a perceptible effect. Some ion pairing of P"AQ and PAQ'- with the ions of the supporting electrolyte is inevitable for solvents of lower E,. We found, for example, that tetrabutylammonium fluoroborate (TBAFB) lowered A F values by ca. 0. I5 V in dichloromethane." Marcus Analysis of Results for Different Solvents The rate constants k,, for reaction (1) were calFulated fromoeqn (7) and are given in table 1. To estimate I,, [eqn (5 a)] we took up = 7 A and aQ = 4 A.Computer modelling of extreme conforyations of PAQ" showed a,,g to lie in the range 12.7-14.3 A and we have taken up& = 14 A. Using these values, B in eqn (5b) was calculated to be 1.80 eV. For each of the 16 solvents for which we have electrochemical measurements, calculated values of Rw are given in table 4. Measurement errors in E , and eOp result in an uncertainty in Lo of k0.01 eV, small compared with the uncertainty in ili, which we assumed to lie in the range M . 2 eV because bond-length differences between the precursor state 'P*AQ and the successor state P'+AQ'- are thought to be ~ r n a l l . ~ ~ ~ 25 Brunschwig et af." used Ri = 0.1 eV for a series of transition-metal complexes. If ili is in this range, then the values of il listed in table 4 are reasonably consistent with other estimates of A z 0.7 eV26 and 3, z 0.9 eVZ7 for similar porphyrinquinone molecules. Closs et af." estimate il = 0.75 eV in methyltetrahydrofuran for a biphenyl anion linked to an acceptor via a steroid bridge.1034 Intramolecular Photochemical Electron Transfer 0 -0.5 - 1 .o Y -1.5 -2 .o I 014 10 0 11 t 8 0 0 -2.51 I 1 1 0 0.5 1 .o 1.5 2 .o X 0 -0.5 -1.0 Y - 1 .5 -2 .o - 2 . 5 ( b ) 0 14 0 15 \ I ? -.I \ 1 I I I 0 0.5 1.0 1.5 2 .O X Fig. 1. For legend see opposite. Also given in table 4 are the kinetic and solvent data and four representative sets of AGO values calculated from eqn (6aH6d). Fig. 1 (a)-(d) show plots of Y us. X [eqn (3 b)] for the four alternative sets of AGO values and representative values of w and Ai.On each plot is given the least-squares slope for a linear fit to the data for 14 solvents and the correlation coefficient r for that fit. The data for two solvents, 2-MTHF and EtOAc, were excluded from all fits in fig. 1 : EtOAc because cyclic voltammetry showed it to be attacked by PAQ*-, and 2-MTHF because TBAFB had to be used as supporting electrolyte. Both AG: and AGY (which differ little at higher E,) yield reasonable agreement with Marcus theory. In particular, it is satisfactory that the least-squares slopes are very closeJ. A . Schmidt et al. 1035 -0 - 1 Y - 1 - 2 - 2 X 0 -0.5 -1.0 Y - 1 . 5 -2.0 - 2 . 5 0 0.5 1.0 1.5 2.0 X Fig. 1. (a)--(d) Plots of Y = log (ket &/Jt s-l) us. X = (AGE + R)2/(2.303) 4Rk, T [see eqn (3)] using the AGE and R, data of table 4, Ri = 0.2 eV and w = 0.2 eV in eqn (6c), where (a) n = 0 [eqn (6a)], (b) n = 1 [eqn (6b)], (c) n = 2 [eqn (6c)l and ( d ) n = 3 [eqn (641.The solid lines in fig. 1 (a)-(c) are least-squares fits to the solid points: (a) slope = -0.90, r = 0.893; (b) slope = -0.89; r = 0.8 17; (c) slope = - 1.80, r = 0.9 1 1. The open points have been excluded for the reasons stated in the text. The dotted line in fig. 1 ( d ) is a line with the expected slope of - 1 for comparison. The error bars show the effect of changing A by fO.1 eV. The number code for the solvents is: (1) CH,CN, (2) C3H,CN, ( 3 ) C,H,CN, (4) acetone, (5) n-butanol, (6) 1,2-dichloroethane, (7) CH,CI,, (8) 2-methyltetrahydrofuran, (9) 1,l ,I-trichloroethane, (10) dimethoxyethane, (1 1) ethyl acetate, (12) chlorobenzene, (1 3) 1-chloronaphthalene, (14) CHCl,, (1 5) 1,2-dibromoethane, (16) anisole.1036 Intramolecular Photochemical Electron Transfer to the expected value of - 1.Also, the high rate constant observed in chloroform can be explained by the approximate equality of the calculated h and predicted -AGO values. Moreover, the rate constants for solvents with widely different properties and functional groups correlate satisfactorily with Marcus theory. There is little to choose between AGO" and AGY in terms of fit. We have explored the sensitivity of the correlation in fig. 1 (a) to holding either AGO" or 3, constant. When AGO" is held constant at -0.48 eV (the value for acetonitrile) the slope is -0.78 but the correlation coefficient drops from 0.893 to 0.690.When A is held constant at 0.80 eV, the slope is - 1.03 but r drops to 0.477. Clearly the best correlation is found when the solvent dependence of both AGO and A are taken into account. The uncertainty in 3, dominates the errors in X and Y, and the representative X error bars in the figures show the effect of changing h by kO.1 eV; the error in Y produced by this variation in h is negligible. As the X error bars indicate, a change in the chosen value of hi changes the absolute value of X and hence the intercept, but we found that the correlations of fig. 1 (a) and (6) were little affected in the chosen range Izi = 0 4 . 2 eV, although higher Ai values increase the range of h and reduce the best-fit slope.Both the constant work-term expression AG; and Weller's AG: are markedly less successful in correlating the data in different solvents [fig. 1 (c) and (41. In fig. 1 ( c ) (in which w was taken as 0.2 eV), the correlation is good but the slope (- 1.80) is too large in magnitude. For w = 0.1 eV, the slope is - 1.33, while w = 0.4 eV brings the predicted rate constants largely into the inverted region and produces poor correlation. As fig. l(d) shows, Weller's equation [eqn (6d)l is not successful in correlating the data in different solvents. The trends in AG," and A, with increasing E, partly cancel in the calculation of X [eqn (3 c)], leading to the small range of Xevident in fig. 1 (d). Moreover, examination of table 4 shows that Weller's equation does not predict the experimental values AGO" well in low dielectric solvents, in which -AG: is small in magnitude because of the predicted low Born solvation energies of ions in such solvents.Furthermore, the predicted trend in AG: cannot explain the observed trend in ke.. Any reasonable variant of transition-state theory, whether a linear or a quadratic free-energy relationship is employed, predicts that log k,, would increase as AG: becomes more negative, but the actual trend is opposite to that predicted by Weller's AG:. Marcus Analysis of Results for Mixed Solvents The use of binary solvent mixtures minimises abrupt changes in short-range solvent properties, while still providing a wide range of calculated 3, values. We chose two binary mixtures for study : the acetonitrile-chloroform system because these two solvents span a wide range of experimental rate constants, and the acetonitrile-benzonitrile system because these two solvents have the same functional group but also exhibit quite different rate constants.We chose acetonitrile as the common component to minimise the extrapolation required to calculate AG; [eqn (6d)l. Table 5 gives the measured dielectric properties of these solvent mixtures, the fluorescence lifetimes and electron-transfer rate constants and four representative sets of AGO values calculated from eqn (6). Electrochemical measurements on some of these mixed solvents indicated that, within experimental error, A F ( = E", - Eo,) varied smoothly between the values for the two pure solvents. We therefore assumed a linear dependence of AE" on mole fraction to effect both interpolations.The A F values do not span a wide range in either mixture, so the interpolation is not expected to introduce significant error into the analysis. In order to emphasise the overall correlation we have plotted the data for the two solvent mixtures together in fig. 2 for the four alternative AGO values. For the acetonitrile-benzonitrile solvent mixture, the agreement of experiment with Marcus theory is very good when either AG: or AGY [fig. 2(a or b)] is selected. AG," also gives a good correlation [fig. 2(d)]. This lack of discrimination results from the small range andJ . A . Schmidt et al. 1037 Table 5. Rate constants, reorganisation energies and Gibbs energies for acetonitrile-benzonitrile and acetonitrile-chloroform mixtures v:va Xb t,/nsc r2/nsc -AG,"h -AGY 0: 1 1:8 1:4 1:2 1 : l 2: 1 4: 1 8: 1 1:o 0: 1 1:8 1:4 1:2 1 : l 2: 1 4: 1 8; 1 l : o 0.000 0.196 0.328 0.494 0.661 0.796 0.887 0.940 1 .ooo 0.000 0.161 0.278 0.435 0.606 0.755 0.860 0.925 1 .ooo 2.06 2.2 1 2.44 2.77 3.38 4.16 4.96 5.68 7.44 0.42 1.05 1.41 2.0 1 3.36 4.45 5.40 5.90 7.44 11.74 11.82 11.82 1 I .82 11.97 11.99 11.99 12.00 11.95 9.05 9.37 9.25 9.83 9.80 10.66 11.15 10.37 11.95 acetonitrile-benzonitrile 8.60 8.57 8.5 1 8.44 8.33 8.20 8.07 7.97 7.71 25.20 26.70 27.74 29.41 3 1.60 33.61 35.26 36.23 35.94 2.328 0.90 2.275 0.92 2.238 0.94 2.167 0.97 2.085 1.01 1.986 1.05 1.907 1.09 1.863 1.12 1.800 1.15 acetonitrileechloroform 9.36 4.81 2.082 0.69 8.93 10.10 2.047 0.90 8.78 12.40 2.029 0.94 8.60 17.00 1.989 1.00 8.29 22.50 1.937 1.05 8.12 27.10 1.888 1.09 7.98 31.00 1.850 1.12 7.86 33.00 1.833 1.13 7.71 35.94 1.800 1.15 0.45 0.49 0.46 0.50 0.46 0.50 0.47 0.51 0.48 0.51 0.48 0.51 0.49 0.52 0.49 0.52 0.49 0.52 0.56 0.77 0.55 0.64 0.54 0.60 0.53 0.57 0.52 0.55 0.51 0.53 0.50 0.53 0.50 0.52 0.49 0.52 -AGii -AGi 0.65 0.50 0.66 0.50 0.66 0.50 0.67 0.51 0.68 0.51 0.68 0.51 0.69 0.52 0.69 0.52 0.69 0.52 0.76 0.20 0.75 0.39 0.74 0.42 0.74 0.46 0.72 0.49 0.71 0.50 0.70 0.51 0.70 0.51 0.69 0.52 a Volume ratio CH,CN: C,H,CN or CH,CN : CHCl,.Mole fraction of CH,CN. f 5 %. Calculated from T~ and 5, using eqn (7); error in log k,, k0.04. k0.05. f f0.005. Reorganisation energy in eV calculated from eqn (4) and (5b) with B = 1.80 eV and Ai = 0.2 eV.All AGO in eV. Calculated from eqn (6c) with w = 0.2 eV. close similarity of the three AGZ sets, which in turn derives from the similar A P values in the two pure solvents and the relatively large static dielectric constants of the whole range of mixtures. The solvent reorganisation energy A. is high, and the fit and the slope are rather insensitive to the choice of Ai in the range M . 2 eV. It is evident that the measured dielectric properties account very well for the effect of solvent on k,, in these mixtures. For acetonitrile-chloroform mixtures, good correlation with Marcus theory is obtained using either AGO" or AG; [fig. 2(a) and (b)], although here the ranges of A and AG; are larger and the best-fit slope is more sensitive to the choice of Li, changing from -1.04 for Ai = 0.2 eV to - 1.17 for Ai = 0.1 eV and - 1.58 for Ai = 0.For both solvent mixtures in the case of AG; calculated with w = 0.2eV, the correlation is linear but the slope is too large ( - 1.78) in magnitude. It is reduced to ca. - 1.0 by the choice of w = 0.1 eV. However, this would require a choice of E , z 10 for the dielectrically saturated linkage, a value which we feel is unreasonable. For the acetonitrileechloroform mixtures, the use of Weller's AG; values yields a small range of X due to partial cancellation of trends in AG; and A,, with increasing E,, and hence no correlation with Marcus theory [fig. 2(d)]. Again, this can be ascribed to the low ion solvation energies predicted by the Born equation. It is notable that the correlation is reasonable for those mixtures in which E , 2 15.The intercepts in fig. 1 (a) and 2(a) each give C z 0 [eqn 3(c)]. From eqn (3a) we can thus estimate that the electronic coupling term Hps = 4 x lop4 eV at 300 K ; this low value is consistent with a non-adiabatic electron-transfer p~ocess.'~0.5, I I I I I I \ -2.0 I -2.0 -0.5 t- t\_ s Q t 0.5 I 0 -0.5 Y -1 . o - 1 . 5 X 0 . 5 1 0 ' -0.5 Y -1 . 0 - 1 . 5 -2.0 X 0 . 5 0 - 0 . 5 Y - 1 . o -1 . 5 . \ I - " 4 . CH CI, . '. - 2 . 0 0 0.5 1 .o 1.5 2 .o 0 0.5 1 .o 1 . 5 2.0 X X Fig. 2. Same as for fig. 1 except for the solvent mixtures acetonitrile-benzonitrile and acetonitrile-chloroform. The points were calculated from the data in table 5. The open circles refer to the pure solvents; the open triangles to the acetonitrile-chloroform mixtures and the open squares to the acetonitrile-benzonitrile mixtures.(a) Slope = - 1.04, r = 0.988; (b) slope = - 1.02, r = 0.989, (c) slope = - 1.78; r = 0.988.J. A . Schmidt et al. 1039 Discussion Our results show that the semiclassical Marcus theory provides a satisfactory explanation of the large solvent effects on the rates of intramolecular electron transfer observed in PAQ, provided that the solvent dependence of the Gibbs energy change and, more importantly, the reorganisation energy, are considered. The data for most of the 16 solvents studied correlate well with the predictions of Marcus theory. The correlation is even better when the bulk solvent dielectric properties are manipulated by varying the composition of binary mixtures.We have looked at four methods of estimating AGO for the electron-transfer reaction from the excited singlet state. The first three (AG:, AG: and AG;) utilise redox potentials measured in each of the solvents with, respectively, no work correction, a solvent- dependent work correction and a constant solvent-independent work correction. The fourth estimate (AG;) utilises Weller’s equation for estimating AG; in solvents for which electrochemical measurements are not available. We have found that both AGE and AG; give a good correlation with Marcus theory, and in particular yield a slope, in plots based on eqn (3 b), very close to the predicted value of - 1. AG; yields a good linear correlation but generates incorrect slopes for w = 0.1-0.4 eV. The use of Weller’s equation to estimate AGO in various solvents does not provide a good correlation of our data with the Marcus theory, primarily because the predicted AGO values do not agree well with our experimental values.We therefore consider the use of the Weller equation, as for example by Irvine et and Wasielewski et aZ.,59 27 to be a risky procedure. It is clear from the data of table 4 that the experimental AGE values vary less, and less systematically, with solvent e, than do Weller’s AGZ values. Moreover, the range of A, is greater than the range of AG;, particularly for the solvent mixtures, so 1, is the major determinant of Xin eqn (36). We have placed considerable faith in eqn (5a) as a method of calculating A, while we have been critical of Weller’s equation, eqn ( 6 4 .Since the expressions for A, and AG: both derive from Coulomb’s law and the behaviour of charges in dielectric media, a comment as to why we think eqn (5b) to be more acceptable than eqn ( 6 4 is appropriate. The solvent reorganisation energy lo is linearly related to the energy that would be required to effect a Franck-Condon transition from the almost non-polar precursor ‘P*AQ in its equilibrium solvation state to P’+AQ’- surrounded by solvent which retains the pre-existing nuclear coordinates (i.e. orientation polarisation) but which has adjusted its electronic polarisation in response to the transfer of the electron. Hence Lo is more sensitive to E,, and the solvent polarisability than to E, and the solvent polarity. Indeed, if E, + E,,, A, is insensitive to E , [eqn (5b)l.The higher cop, the lower R, because the transferring electron is stabilised by simultaneous adjustment in the electronic polarisation of the medium, which is well described by the continuum property cop. The Born equation, and hence Weller’s equation, reflect the difference in equilibrium orientation polarisation of the solvent. If the solvent is viewed as a continuum, this is sensitive only to E,. The Born equation is well known to be quantitatively unreliable even for spherical ions in water and the situation is worse for non-aqueous solvents. S t r e h l ~ w ~ ~ quotes extensive data ‘to show conclusively that the free energy of transfer of electrolytes from one solvent to another is not simply proportional to the difference of the reciprocal dielectric constants.’ In the case of water-methanol and water-dioxane mixtures, ‘application of the Born equation could do no more than correctly predict the sign of the free energy of tran~fer’.~’ Successful correlation of the thermodynamics of charge-transfer reactions with solvent properties commonly requires three or four parameters, not Although some dependence on E, is likely, there are also specific solvent effects. For example,1040 Intramolecular Photochemical Electron Transfer Q'- is probably stabilised by Lewis acidity in the solvent. On the other hand, the porphyrin moiety of PAQ is sufficiently large that the equivalent of the 'ferrocene assumption '30 might well be made in respect of P*+/P, i.e. its redox potential might well be taken to be insensitive to the solvent rather than varying systematically with E, in the manner suggested by the Weller equation.The acetonitrile-chloroform data provide further evidence of the failure of the Weller equation. The experimentally determined rate constant k,, decreases systematically as the solvent is varied from pure chloroform to pure acetonitrile, while the polarity (E,) rises and the polarisability (cop) falls over the same range. Weller's equation, responsive to E ~ , predicts AG; to become strongly more favourable (negative) over this range. The observed trend in k,, is thus inexplicable in terms of AG; and any linear or quadratic free energy relationship between log k,, and AGO in the non-inverted region. In our interpretation, the decrease in k,, is caused by the trend in A.from lower to higher values as the solvent polarisability decreases. Trends similar to those for acetonitrile-chloroform are observed for k,,, E ~ , eOp and A in the acetonitrile-benzonitrile mixtures. However, the ranges of both AG: and AG; are so small that A. controls the range of the predicted rate constants, and the data do not discriminate between the various expressions for AGO. Finally, we note that the electron-transfer rate constant and the position of equilibrium in a reaction such as eqn (1) are controlled by different parameters. Writing k,, as kLt for the forward electron transfer and kEt for the back electron transfer to 'P*AQ (AGO + A)2 ( -AGO + A)2 - AGO - -- + 4Ak,T k, T' (2:) 4Ak,T In, = - Thus while A may dominate the predicted range of k:, and k:,, it has of itself no influence on AGO.To conclude, we therefore think that, while eqn (5a) for A. is reasonable, eqn ( 6 4 is unreliable as a means of estimating AGO This work was supported by a Strategic Grant in Energy and an Operating Grant from the Canadian Natural Sciences and Engineering Research Council, by a NATO Research Grant to JRB and MDA for International Research Collaboration and by financial support of VPYG by LKB Biochrom Ltd and Trinity College, Cambridge. We are grateful to Dr John S. Connolly of the Solar Energy Research Institute, Golden, Colorado for helpful discussions regarding the data analysis and a critical review of the manuscript. References 1 2 3 4 5 6 7 8 9 10 S. Gaspard, C. Giannotti, P. Maillard, C.Schaeffer and T-M. Trin-Thi, J. Chem. SOC., Chem. Commun., 1986, 1239. G. M. Dubowchik and A. D. Hamilton, J. Chem. SOC., Chem. Commun., 1986, 1391. G . M. Sanders, M. van Difk, A. van Veldhuizen and H. C. van der Plas, J. Chem. SOC., Chem. Commun., 1986, 1311. P. Seta, E. Bienvenue, A. L. Moore, P. Mathis, R. V. Bensasson, P. Liddell, P. J. Pessiki, A. Joy, T. A. Moore and D. Gust, Nature (London), 1985, 316, 653. M. R. Wasielewski, M. P. Niemczyk, W. A. Svec and E. B. Pewitt, J. Am. Chem. SOC., 1985, 107, 5562. J. Deisenhofer, 0. Epp, K. Miki, R. Huber and H. Michel, J. Mol. Bio,'. 1984, 80, 385. C. H. Chang, D. Tiede, J. Tang, U. Smith, J. R. Norris and M. Schiffer, FEBS Lett., 1986, 205, 82. J. A. Schmidt, Ph.D. Thesis (The University of Western Ontario, London, Canada, 1986).Part 3: J. A. Schmidt, A. Siemiarczuk, A. C. Weedon and J. R. Bolton, J. Am. Chem. Soc., 1985,107, 61 12. J. R. Bolton, J. A. Schmidt, A. Siemiarczuk, M. D. Archer and J. H. Wilford, in Homogeneous and Heterogeneous Photocatalysis, ed. E. Pelizzetti and N. Serpone, NATO Advanced Study Institute (Reidel, Dordrecht, 1986); ser. C, vol. 174, pp. 175-187.J . A . Schmidt et al. 1041 11 M. D. Archer, V. P. Y. Gadzekpo, J. R. Bolton, J. A. Schmidt and A. C. Weedon, J . Chem. Soc., 12 Part 4: J. A. Schmidt, A. R. McIntosh, A. C. Weedon, J. R. Bolton, J. S . Connolly, J. K. Hurley and 13 R. A. Marcus, J . Chem. Phys., 1956, 24, 966. 14 P. Siders and R. A. Marcus, J . Am. Chem. Soc., 1981, 103, 748. 15 R. A. Marcus and N. Sutin, Biochim. Biophys. Acta, 1985, 811, 265. 16 M. Born and E. Wolf, Principles of Optics (Pergamon Press, London, 1975); pp. 10-13. 17 B. S. Brunschwig, S. Ehrenson and N. Sutin, J. Phys. Chem., 1986, 90, 3657. 18 A. Weller, 2. Phys. Chem., 1982, 133, 93. 19 D. Rehm and A. Weller, Ber. Bunsenges. Phys. Chem., 1969, 73, 834. 20 P. Suppan, J . Chem. Soc., Faraday Trans. I, 1986, 82, 509. 21 J. H. Wilford, M. D. Archer, J. R. Bolton, T-F. Ho, J. A. Schmidt and A. C. Weedon, J . Phys. Chem., 22 Part 2: A. Siemiarczuk, A. R. McIntosh, T-F. Ho, M. J. Stillman, K. J. Roach, A. C. Weedon, J. R. 23 The Handbook ofchemistry and Physics, ed. R. C. Weast, (CRC Press, Cleveland, 1985) pp. C52, C136 24 D. Huppert, H. Kanety and E. M. Kosower, Faraday Discuss. Chem. Soc., 1982, 74, 161. 25 M. Bixon and J. Jortner, Faraday Discuss. Chem. Soc., 1982, 74, 17. 26 M. P. Irvine, R. J. Harrison, G. S . Beddard, P. Leighton and J. K. M. Sanders, Chem. Phys., 1986,104, 27 M. R. Wasielewski, M. P. Niemczyk, W. A. Svec and E. B. Pewitt, J . Am. Chem. Soc., 1985, 107, 28 G. L. Closs, L. T. Calcaterra, N. J. Green, K. W. Penfield and J. R. Miller, J . Phys. Chem., 1986, 90, 29 D. J. Quimby and F. R. Longo, J. Am. Chem. Soc., 1976, 97, 51 1 1. 30 H. Strehlow, in The Chemistry of Non-aqueous Solvents, ed. J. Lagowski, (Academic Press, New York, 31 J. I. Padova, in Modern Aspects of Electrochemistry, ed. B. E. Conway and J. O’M. Bockris 32 J. Shorter, Correlation Analysis of Organic Reactivity, with Particular Reference to Multiple Regression Faraday Trans. 2, 1986, 82, 2305. M. R. Wasielewski, J. Am. Chem. Soc., 1988, 110, 1733. 1985, 89, 5395. Bolton and J. S. Connolly, J . Am. Chem. Soc., 1983, 105, 7224. and C350. 315. 1080. 3673. 1966); V O ~ . 1, pp. 129-171. (Butterworths, London, 1972); no. 7, chap. 1. (Research Studies Press, Chichester, 1982). Paper 8/01345D; Received 6th April, 1988

ISSN:0300-9599    

DOI:10.1039/F19898501027

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

J . Chenz. SOC., Faraduy Trans. I, 1989, 85(5), 1043-1048 Preparation and Characterization of Ti0,-SO, Aerosil Colloidal Mixed Dispersions Colin Morrison and John Kiwi* Institut de Chimie Physique, Ecole Polytechnique Fe'de'rale, CH-1015 Lausanne, Sw itzerland TiO, sol adsorption on Si0,-Aerosil has been carried out by three different methods to produce Ti0,-SiO, Aerosil sols having different mole ratios. Characterization of these particles by electrophoresis, spectroscopy and electron microscopy is presented. Corrosion of Si0,-Aerosil was not observed under photolytic conditions at pH 3 when loaded with 10 atom YO TiO,. The purpose of this study is to elucidate the role of SiO, as a substrate when TiO, colloid is dispersed on its surface via different techniques and at different loading levels.TiO, is the electron-donating species in a colloidal or a particulate semiconductor-SiO, system.' Ti0,-SiO, obtained by the precipitation method has been reported to be active in isomerization of butanes and in amination reactions., The properties of these coprecipitated silica-titania gels have also been Ti0,-SiO, glasses have recently been a subject of much attention in the ceramic field.6i ' The reverse situation to ours, namely the deposition of SiO, on TiO, surfaces, has been a subject of wide interest in material science.'. Photocatalytic studies on organic reactions1'. l1 and water-based systems over titania-silica oxide catalysts have been more recently described. l3 Experimental Hydrolysis of TiCl, was carried out by methods that have been well documented in the 1iterat~re.l~ Freeze-drying of Ti0,-SiO, systems was carried out according to work reported previously.l5 U.v.-visible and diffuse reflectance spectra were recorded on a Hitachi-Perkin-Elmer 340 recording spectrophotometer, equipped with an integrating sphere. The electro- phoretic mobility was measured with a commercial mark I1 microelectrophoresis apparatus (Rank Bros, Cambridge). The average velocity was calculated by dividing the velocity by the field strength (potential/distance between the electrodes). Electron microscopy (t.e.m.) was carried out wit! a Philips 300s instrument. The limit for resolution for such an instrument is 3 A. Continuous photolysis experiments were carried out with a Rofin Xe lamp (100 mW crn-,). Prior to being photolysed, the samples were flushed with 48Ar for the removal of oxygen.Hydrogen was analysed by gas chromatography using a Carbosieve 5 A column at 40 "C and a Gow-Mac conductivity detector. The following methods were used to deposit TiO, on Aerosils. Method A TiO, was prepared by hydrolysis of TiC1, in 1 : 1 HCl pH 0 (5 g dmP3 TiO, equiv.) at 0 "C and was subsequently dialysed to pH 3. SiO, (Degussa, Aerosil) suspension was prepared by addition of 1 g of the solid to 100 cm3 water at pH 3 and sonicating. An appropriate amount of TiO, stock was added to water at pH 3 and diluted to 100 cm3 10431044 Ti0,-SiO, Aerosil Colloidal Mixed Dispersions with water at pH 3 (adjusted with HCl). This was rapidly added to the sonicating SiO, suspension and then allowed to sonicate for a further 5 min.The suspensions were stored in plastic bottles and were periodically shaken. Method B TiO, was prepared at pH 0 and 0 "C as above and an appropriate amount was added to water at pH 0 and adjusted to 100 cm3. A suspension of 1 g SiO, in water at pH 0 was sonicated while the TiO, was added rapidly. Sonication was continued for 5 min. Later, this solution was dialysed to pH 3. Method C Method A was followed and the freshly prepared suspensions were rapidly frozen in liquid N, followed by freeze-drying. Results and Discussion SiO, of different surface areas and particle sizes were covered with TiO, (prepared from TiC1, and used in sol form) in order to optimize surface coverage for u.v.-photoinduced H, production. As shown above we have chosen three methods: (A) TiO, sol dialysed to pH 3 was mixed with a suspension of SiO, at pH 3.This first method was chosen because at pH 3 SiO, and TiO, are oppositely charged and should attract one another. Uniform deposition on the SiO, particles would be very dependent6-' on how the two components were mixed. To overcome the mixing problem, method B was devised. At pH 0, the TiO, would exist as Ti02+ ions8 and would use the SiO, substrate' for nucleation and growth. Taking into account this observation, undialysed TiO, sol at pH 0 was mixed with a suspension of SiO, at pH 0 followed by dialysis to pH 3. A third approach was also tried (C), in which the suspension formed in method A was freeze-dried to a powder. Freeze-drying was performed in order to deposit the TiO, onto the SiO, surface and to form a powder which could be resuspended in water.In such a powder it is less likely that particles would agglomerate in suspension on storage. It would also be more convenient to heat-treat. Aerosil 90 (A 90) has been reported to have a mean size of 20 nm with particle sizes varying from 12 to 40 nm. The B.E.T. surface area has been reported to be 90 m2 g-'. Aerosil 200 and Aerosil 380 had mean sizes of 12 and 7 nm with 200 and 380 m2 8-l B.E.T. surface areas, respectively. This information was kindly supplied by Degussa AG (Frankfurt). Some samples were heated in air at 400 "C for 24 h.? Solvent evaporation gives rise to surface modifications of catalyst prepared by method B on heating. This is not the case for the heating of the freeze-dried samples as prepared by method C.15 Fig.1 compares the u.v.-visible absorption characteristics of TiO, sol alone (3.5 cm3 of 5 g dm-3 in 25 cm3 at pH 3) with that of a solution phase of 10 atom O/O Ti-SiO, Aerosil 380 sols prepared by methods A and B. The solution phase of the suspensions shows only slight absorption below 300 nm, whereas the TiO, sol has a sharp absorption above 300nm. This observation confirms that the TiO, has been adsorbed onto the surface of Aerosil 380. This was found to be the case for all the samples prepared. At this point we would like to estimate the monolayer coverage for the three SiO, Aerosils used assuming that (a) the particles are spherical and (b) the TiO, is close- packed with a cross-sectional area of m2 molecule-'.SiO, A 90 (Aerosil with t The Degussa literature indicates that the surface area remains constant up to 500 "C before becoming smaller.C. Morrison and J . Kiwi 1.0 0.90 0.80 0.70 1045 1 I I 1 - - - - 0.0 1 I 20 0 300 4 00 500 600 700 800 Alnm Fig. 1. Absorption characteristics of TiO, sol alone (a) (for details see text) and of 10 atom % Ti Aerosil 380 (b). 90 m2 g-') having a mean particle size of 20 nm (ref. 16) will have a surface particle of 12.56 lo-'' m2. This renders 7.16 x 10l6 particles SiO, A 90 per gram and 1256 molecules per SiO, particle. Therefore, 1 g SiO, A 90 can hold 0.9 x 10,' molecules of TiO, (or 0.15 mmol). This is equivalent to a 0.90 atom % TiO, coverage for one monolayer. In the same way, for SiO, A 200 and SiO, A 380 monolayer coverage is estimated to be 2.0 and 3.8 atom YO TiO,.It is clear that TiO, loadings (5 and 10 atom YO) were chosen to give more than a monolayer coverage, although in one case, SiO, A 380 at 2 atom YO TiO, only a submonolayer was available. The mechanism of Ti0,-SiO, sol formation during the preparation of the catalyst involves terminal hydroxyl groups on the colloidal silica" interacting with titania hydrates so that they can be uniformly distributed during sol preparation. More important is that hydroxyl groups provide a mechanism for Ti-OH hydrates to attach to the colloidal silica particle via hydrogen bonding and condensation with surface silanol groups." Therefore, the pore spaces of the Aerosils become filled with the added titania.The heated samples (500 "C) show the tendency of these gels to undergo crystallization during sintering. l9 The presence of internal surfaces on the colloidal microstructure is a contributing factor when heat-treating. Following the absorption spectrum of heteropolyblue for silicon1' we tested SiO, dissolution after 24 h photolysis at pH 3 in Ti0,-loaded and Ti0,-unloaded materials under the conditions described in the Experimental section. This pH has been selected for our studies since corrosion to silicates and hydroxysilicates sets in only at ca. pH 10.7. Good stability for Aerosil materials in the presence of electrolytes up to pH 9 has been reported." There was no difference between the SiO, dissolution in the presence or absence of 10 atm O h TiO, at pH 3, the overall effect being rather small.Therefore, no photosensitization of SiO, induced by TiO, took place. Fig. 2 shows the diffuse reflectance spectra (d.r.s.) for the titania-silica oxide catalysts.1046 Ti0,-SiO, Aerosil Colloidal Mixed Dispersions 0.8 0.6 8 5 -e 0, 4 0.4 0.2 I I I I I I I 2 00 3 00 400 500 Ahm Fig. 2. U.v.-visible diffuse reflectance spectra of (a) SiO, Aerosil 380, (b) 2 atom % Ti-Aerosil 380, ( c ) 5 atom YO Ti-Aerosol 380, ( d ) 10 atom YO Ti-Aerosil 380, ( e ) TiO, sol. All materials were freeze-dried. Spectra were recorded at 293 K. MgCO, was used as a reference. It is clearly seen that with the increasing SiO, A 380 content the absorption band of these catalysts shifts toward shorter wavelengths. Fig. 2 therefore shows the effects of titania interactions with the silica surfaces.The hypsochromic shift increases with TiO, loadings up to a Ti0,-SiO, A 380 content of 10 atom YO. The inflection observed in the spectra around 400 nm (3.2 eV) for TiO, sol [fig. 2(e)] is due to the band gap of this material. Fig. 2 shows that higher-energy transitions take place as the silica content of the catalyst increases. Therefore, in a highly dispersed state the TiO, particles coat the SiO, colloidal substrate incompletely and the spectra reflect the dual character of these powders. TiO, anchored on Vycor glass2’ also has been reported to induce a hypsochromic shift in Ti0,-SO, catalysts analogous to results obtained in fig. 2. Fig. 3 ( a ) (trace 1) shows the electrophoretic mobility and an isoelectric point (IEP) for TiO, Degussa P-25 of pH 6.4.This value agrees well with the value of pH 6.6 reported by the manufacturer.,l Trace 2 shows the electrophoretic mobility of TiO, prepared by hydrolysis of TiCl, (see Experimental). The value found for the IEP is shifted considerably towards more acid values, showing that, despite the dialysis carried out (pH 3), the low residual chloride content plays a significant role on the TiO, surface. The mobility of SO, Aerosil 380 is shown in trace 3 as function of pH. This curve is similar to the values found for Cab-0-Sil.”. 22 An IEP of pH 2.3 is found in our studies. OtherJ. Chem. SOC., Faraday Trans. I , Vol. 85, part 5 Plate 1. For legend see over. C. Morrison and J. Kiwi Plate I (Facing p . 1046)J. Chem.SOC., Faraday Trans. I , Vol. 85, part 5 Plate 1 Plate 1. Electron micrographs of samples with a magnification of 194000~ : (a) Aerosil 380, (6) TiO, sol dried at 130 "C, ( c ) 10 atom % Ti-Aerosil 380 dialysed together (method B). C. Morrison and J. KiwiC. Morrison and J . Kiwi 1047 c -0.9 Fig. 3. (a) Electrophoretic mobility as a function of pH for mol dm-3 (NaOH + HC1) solutions: (1) TiO, Degussa P-25, (2) TiO, sol freeze-dried, (3) Aerosil 380. (b) Same conditions as in (a): (1) 10 atom YO Ti-Aerosil380, (2) 5 atom YO Ti-Aerosil 380, (3) 2 atom YO Ti-Aerosil380. Aerosils, e.g. A 90 and A 380, were found to have IEP values between pH 2.5 and 3.2. The production of these surface species depends, therefore, on the manner of preparation and thermal pretreatment of the samples. Such IEP values indicate fairly acidic oxides.It was difficult to observe the positive mobilities in the low-pH region for pure Aerosil samples, since these values were in general quite small. To obtain these data the dispersions were left overnight at the lowest pH. Equilibration times at each subsequent pH were at least 1 h and varied depending on the time required for the pH to stabilize. In this way the electrophoretic mobilities found will reflect precisely the adsorption capacity for each catalyst at a given pH. Fig. 3 (b) shows the effect of TiO, deposition on the electrophoretic mobility of Aerosil 380 as a function of pH. The IEP increases to higher pH values as more titania is added to Aerosil 380. This behaviour indicates that interaction is taking place between titania and the silica surface.The electrophoretic behaviour of both components are pH-dependent and the shape of traces 1-3 in fig. 3(b) reflect some complex phenomena at the catalyst surface. A possible explanation for this behaviour is that the Aerosil 380, when loaded with titania, is affected in its average surface charge/pH b e h a ~ i o u r . ~ ~ This is shown by the evolution of the polarization curves as a function of pH in traces 1-3. The binding links (Ti-0-Si) are readily accessible to protons at pH values between 2 and 6 as seen in fig. 3(b). The very low positive mobilities observed in fig. 3(b) seem to be due to an opening of the silica structure at low pH values and probable disruption of the (Ti-0-Si) links.* Plate 1 shows the results of electron microscopy studies.Work was carried out at 100 kV. Plate 1 (a) presents Aerosil380 powders with a mean size for the primary particle1048 Ti0,-SiO, Aerosil Colloidal Mixed Dispersions (aggregate size of the crystal) of 170 A and particle sizes between 70 aFd 400 A. Plate 1 ( b ) shows dried TiO, sol (1 30 "C), with a primary particle size of 170 A. A 10 atom % Ti/A 380 sample prepared according to method B is shown in plate 1 (c). As seen from this electron micrograph, as the samples of titania and silica mix together, the primary particles conserve their original sizes. These samples were dried at 130 "C for observation purposes. A 10 atom %/A 3800freeze-dried sample (method C ) showed particles with a primary particle size of ca.60 A. The smaller size found in this preparation agrees well with other materials prepared by this method and reported recently in our 1ab0ratory.l~ In conclusion, a few methods have been developed wherein colloidal silica is loaded with titania in aqueous solution. The representative system studied consisted of TiO, particles immobilized on negatively charged SiO, colloids (pH 3). Particle-particle interactions in aqueous dispersions containing SiO, and TiO, have been characterized by diffuse reflectance spectroscopy, electron microscopy and microelectrophoresis. These methods have been useful in reporting the loading and degree of stoichiometry of the surfaces formed. Photoinduced hydrogen evolution at pH 3 proceeds without silica corrosion. References 1 H.Hattori, M. Itoh and K. Tanabe, J. Catal., 1975, 38, 172. 2 M. Itoh, H. Hattori and K. Tanabe, J. Catal., 1974, 35, 225. 3 S. Kaneko and K. Tsukamoto, Chem. Lett., 1983, 1425. 4 H. Morikawa, T. Osuka, F. Marumo, A. Yasumori and M. Yamane, J. Non-cryst. Solids, 1986, 82, 5 R. Butz and H. Wagner, Phys. Stat. Sol., 1986, 94, 71. 6 W. Beier, A. Goktas and G. Frischat, J. Am. Ceram. Soc., 1986, 69, C148. 7 C. Sherer and C. Pantano, J. Non-cryst. solids, 1986, 82, 246. 8 D. Furlong, K. Sing and G. Parfitt, J. Colloid Interface Sci., 1979, 69, 409. 9 R. Iler, US Patent 2885366, 1959. 97. 10 S. Kodama, H. Nakaya, M. Anpo and Y. Kubokaba, Bull. Chem. SOC. Jpn, 1985, 58, 3645. 11 V. Nikisha, B. Shelimov and V. Kazanskii, Kinet. Catal., 599, 15, 676. 12 M. Anpo, H. Nakaya, S. Kodama, Y. Kubokawa, K. Domen and T. Onishi, J. Phys. Chem., 1986,90, 13 A. Frank, I. Willner, Z. Goren and Y. Degani, J. Am. Chem. SOC., 1987, 109, 3568. 14 J. Kiwi, Chem. Phys. Lett., 1981, 83, 594. 15 K. R. Thampi, M. Subba Rao, W. Schwarz, M. Gratzel and J. Kiwi, J. Chem. Soc., Faraday Trans. 1, 16 D. Boltz and J. Howell, Colorimetric Determination of Non-metals (John Wiley, New York, 1978), p. 17 Aerosil Pigments, Descriptive no. 1 I, 23, Degussa AG, Frankfurt, Federal Republic of Germany. 18 Colloid Science, ed. H. Kruyt (Elsevier, Amsterdam, 1952). 19 G. Parfitt, Progr. Surf. Membr. Sci., 1976, 11, 181. 20 M. Anpo, N. Aikawa, Y. Kubokawa, M. Che, C. Louis and E. Giamello, J , Phys. Chem., 1985, 89, 21 Degussa Pigments, Aerosol Process, Hanau 1, Federal Republic of Germany, 1977. 22 R. Harding, J. Colloid Interface Sci., 1972, 40, 164. 23 R. James and G. Parks, in Surface and Colloid Science, ed. E. Matijevic (Plenum Press, New York, 1633. 1988,84, 1703. 430. 5689. 1983), vol. 12, p. 119. Paper 8/01390J; Received 1 lth April, 1988

ISSN:0300-9599    

DOI:10.1039/F19898501043

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

J. Chern. Suc., Faraday Trans. I, 1989, 85(5), 1049-1063 The State of Water in Non-ionic Surfactant Solutions and Lyotropic Phases Oxygen- 17 Magnetic Relaxation Study Goran Carlstrom and Bertil Halle* Physical Chemistry I , University of Lund, Chemical Center, P.O. Box 124, S-221 00 Lund, Sweden Water "0 longitudinal and transverse relaxation rates have been measured in aqueous solutions and mesophases of the non-ionic alkyl oligo(ethy1ene oxide) surfactants C,,E, (n = 4,5,8) over wide ranges of temperature and concentration. In addition, two reference systems have been investigated : high molecular weight poly(ethy1ene oxide) and the dimer 1,2-dimethoxy ethane. The relaxation data are consistent with the following picture of the state of water in the headgroup shell of CI2E, aggregates.The shell is compact, containing less than 5 and possibly as little as 2-3 water molecules per EO group. The shell exhibits substantial structural integrity; its water content is essentially invariant with respect to changes in concentration. The headgroup shell is highly dynamic, the rate of water rotation in the shell being reduced by at most a factor of 5 (at room temperature) as compared to bulk water. Alkyl oligo(ethy1ene oxide) surfactants of the type n-C,H,,,,(OCH,CH,),OH (abbreviated C,E,) in water form micellar aggregates in solution as well as a variety of lyotropic liquid-crystalline phases.' In common with poly(ethy1ene oxide) (PEO) polymers,' the isotropic solution phase of these non-ionic surfactants exhibits a lower critical consolute point (cloud point).The macroscopic properties of C,E,-water systems depend sensitively on composition and temperature as well as on the chemical structure of the surfactant (m, n).' This rich variety of macroscopic behaviour reflects a delicate balance in the intra- and inter-aggregate interactions. Prominent among these is the solvent-mediated interaction between the EO headgroups, the temperature dependence of which is thought to be decisive for the phase behaviour.1-8 It is therefore of interest to examine the state of water in aqueous C,E, solutions and mesophases, and its dependence on the system variables (temperature, concentration, headgroup size). A great variety of experimental techniques have been used to extract information about the state of water in C,E,-water systems.This information is usually quantified in terms of 'hydration numbers'. Such hydration numbers are not only operational (hydration numbers obtained by different experimental methods cannot be directly compared), but furthermore, are often derived from the experimental observables in a highly model-dependent way. The model dependence is particularly severe for experimental techniques that probe the collective response of the system, e.g. light scattering and v i ~ c o s i t y , ~ - ~ ~ or diele~tric'~, l5 and ultrasonic16 relaxation. More recently, hydration numbers have been deduced from water self-diffu~ion'~ and small-angle neutron scattering." While these results may be somewhat less model-dependent, it is clear that the specification of an (operational) hydration number amounts to a rather incomplete description of the spatial distribution and molecular interactions of water in C,E, solutions and mesophases. In order to arrive at a more complete picture, it is necessary to draw on experimental evidence from several sources.Rather than 36 I049 FAR 11050 Water in Non-ionic Surfactant Micelles indiscriminately comparing hydration numbers obtained by different techniques, one should aim at interpreting the experimental data in terms of unambiguously defined quantities, such as local particle densities, order parameters and correlation times. Nuclear magnetic resonance techniques are among the most direct sources of molecular-level information about water behaviour in complex aqueous systems.l9 Valuable insight can be gained from chemical shifts, spin relaxation rates and (in non-cubic mesophases) static line splittings. These n.m.r. observables report primarily on chemical environment, molecular reorientation rate and orientational order, respectively. When these local properties vary within the system, the n.m.r. experiment usually yields a spatial average. Of the three stable and magnetic water isotopes 'H, ,H and 1 7 0 , only the former two have to our knowledge been used in water n.m.r. studies of C,E,-water systems. Corkill et a1.20 reported water 'H longitudinal relaxation rates from the system C,,E,-H,O in the micellar solution as well as in the hexagonal and lamellar mesophases, while Clemett,' measured the same quantity in dilute micellar solutions of C,,E,.While the qualitative conclusions from these early 'H n.m.r. studies are valuable (see below), the relaxation rates cannot be interpreted quantitatively because of the contribution from inter- molecular magnetic dipole couplings between water molecules and EO-headgroup protons.22, 23 Further, at high surfactant concentrations,20 there is a significant contribution to the observed relaxation rate from the terminal hydroxyl proton, which exchanges rapidly (on the n.m.r. time scale) with the water More recently, ,H quadrupolar line splittings have been reported for the hexagonal and lamellar mesophases in the C,,E,-D,O system: for n = 4 and 6 by Klason and HenrikssonZ5 and for n = 3-8 by Rendall and Tiddy.26 On account of the configurational flexibility of the EO chains, as suggested by the 'H and 13C relaxation rates of the EO chain^^'-^' and by the quadrupolar splittings of the deuterated EO chains in a lamellar phase,30 the orientational order parameter for water in the headgroup region must be quite small. In fact, Rendall and Tiddy concluded that only water molecules associated with the first EO group (proximal to the hydrocarbon core) contribute significantly to the observed splitting.With the aim of further characterizing the state of water in C,E,-water systems, we present here the results of a water oxygen-17 spin relaxation study. The 1 7 0 relaxation rates report on the reorientation of individual water molecules and are free from complications due to intermolecular couplings and proton-exchange averaging, which prevent a straightforward interpretation of 'H relaxation rates.20* 21 We have measured the 1 7 0 longitudinal and transverse relaxation rates in the Cl,E,-H20 system (n = 4,5 and 8) over a range of surfactant concentrations (micellar solution, cubic and hexagonal mesophases) and temperatures (approaching the lower consolute boundary). To aid the interpretation, we also performed measurements on two reference systems : PEO polymer solutions (for which more extensive results have appeared recently24) and the 1,2-dimethoxy ethane-water system.Experiment a1 Materials The EO surfactants C12E4, C1,E, and C,,E, (stated purity ca. 98 YO) were obtained from Nikko Chemicals. PEO polymer (M.W. 2 x lo4 g mol-', gas chromatography quality) and 1,2-dimethoxy ethane ( > 99 YO) were obtained from Merck. These chemicals were used without further purification.The samples containing H,O were prepared from doubly distilled water (quartz apparatus) to which was added a small amount of enriched H,O (21.7 YO 1 7 0 , 62.2 % "0) from Norsk Hydro. The C,,E,-D,O samples were made with D,O (99.7% ,H, Ciba-Geigy) with addition of enriched D,O (10% 1 7 0 , Ventron). The final 1 7 0 abundance in the water was 1-2%.G . Carlstrom and B. Halle 1051 The solutions were prepared by weighing all components. Samples in the cubic and hexagonal mesophases were weighed directly into 12 mm n.m.r. tubes and were homogenized by shaking at a high enough temperature to be in the isotropic micellar phase. Between measurements, the samples were stored at 6 "C.To avoid a contribution to the transverse relaxation rate from scalar rela~ation,~' the pH of the final solutions was adjusted to 3-4 by addition of hydrochloric acid. At these pH values, the scalar contribution is negligible.32- 33 Relaxation Measurements Water 1 7 0 relaxation rates were measured for all systems, except C,,E,-D,O, using a home-built spectrometer equipped with a 6 T wide-bore superconducting magnet. The 1 7 0 resonance frequency at this field is 34.6 MHz. 1 7 0 transverse relaxation rates for the C,,E,-D,O system were measured at 13.6 MHz using a Varian XL 100 instrument, where also some additional measurements on the C,,E,-H,O system were performed. No significant difference in the transverse relaxation rate between the two resonance frequencies was observed.Longitudinal relaxation rates (R,) were determined by the conventional inversion recovery method and transverse relaxation rates (R,) were obtained from the linewidth at half height (Av) of the absorption spectrum according to the relation R, = nAv. For samples in the anisotropic hexagonal mesophase, R, was determined from the inversion recovery of the central peak in the powder spectrum, while R, was obtained from the central linewidth at half height, Avc. In the extreme narrowing limit, where the reorientational motion that induces spin relaxation is fast compared to the resonance frequency, relaxation is exponential and R, = R, provided that the system is isotropic. For anisotropic systems in the extreme narrowing limit, all spectral lines have exponential inversion recovery with the same R,, but Ri = nAvc = (9/4)R, for the ''0 spin-5/2 nucleus.34 To simplify the comparison of isotropic and anisotropic samples, we therefore define R, as 4nAvc/9 for samples in the hexagonal phase.As a reference we also measured the relaxation rates for slightly acidic solutions of bulk water, enriched to 1% in "0 and having the same water ,H enrichment as the sample. The excess relaxation rates, Ri, ex, were calculated from where is the relaxation rate for the reference bulk water solution. We assume that the contribution to R, from magnetic field inhomogeneity is the same for the sample as for the reference, so that R2,ex is free from the effect of inhomogeneity broadening. During each experiment the probe temperature was kept constant to within k0.5 "C by the passage of thermostated air.The temperature was measured with a calibrated thermocouple. From the reproducibility of the relaxation rates, we estimated the uncertainty in the determined rates as 3 % for the isotropic phases and 4 % for the hexagonal mesophase. Results The samples investigated by water "0 n.m.r. in the C,,E,-water and C,,E,-water systems are indicated in the phase diagrams of fig. 1 and 2. In the C,,E,-water system, relaxation measurements were performed on samples in the micellar solution phase (L,) and in the cubic (I,) and hexagonal (H,) mesophases. In the C,,E,-water system, measurements were confined to the L, phase and some of the samples were made with heavy water. Some of the micellar solution sample points are quite close to the lower consolute boundary (C,,E,) or to a mesophase boundary (C,,E,).In addition, we investigated three samples in the C,,E,-water system. These were all in the L, phase, 2-5 "C below the lower consolute boundary or the lamellar phase boundary. 36-21052 20- 10 Water in Non-ionic Surfactant Micelles I 0 e O 0 I I 0 @O 0 0 A. I ---4,. . I I ;/ "' "1 - I I I I I I I ! I I I I - @ . . . L1 . . . . 60 - 40 - . 0 . I \ 20 - /// s - . . . II' 0: I I ; '1 1 / / O O ' 20 40 I I 60 80 I I - )O Fig. 1. Phase diagram of the C,,E,-H,O system' with indication of samples investigated by H,"O n.m.r. (a). 0 h \ Fig. 2. Partial phase diagram of the C,,E,-H,O system' with indication of samples investigated by H,"O (a) and D,"O (0) n.m.r.(For the D,O samples, the actual wt % C,,E, has been corrected so as to correspond to the same mole fraction in H,O.) For all samples where both longitudinal and transverse relaxation rates were measured, it was found that R1,ex = within experimental uncertainty. This was the case for the isotropic phases (L, and I,) as well as for the anisotropic phase (HI). In the following, we will thus refer simply to the excess relaxation rate Rex. Fig. 3 and 4 show how Re, varies with composition (at several temperatures) in the systems C,,E,-H,O and C,,E,-D,O. Here, and in the following, we use as the composition variable the molar ratio x of water (H,O or D,O) to EO groups. The most noteworthy feature in these figures is the extensive composition range where Re, is inversely proportional to x.G.Carlstrom and B. Halle I053 X 0 1oO/x Fig. 3. Water 1 7 0 excess relaxation us. composition (x =mole H,O/mole EO) at different temperatures in the C,,E,-H,O system. Open symbols refer to the micellar solution phase, filled symbols to the hexagonal phase and the half-filled symbol to the cubic phase. The lines (constrained to pass through the origin) are least-squares fits to the data points connected by the unbroken part of the lines. X Fig. 4. Water "0 excess relaxation rate us. composition (x = mole D,O/mole EO) at different temperatures in the C,,E,-D,O system. Filled symbols refer to sample points that are within 5 "C of the lower consolute boundary (cf fig. 2). The lines (constrained to pass through the origin) were obtained from least-squares fits.1054 Water in Non-ionic Surfactant Micelles X 1oo/x Fig. 5.Water 1 7 0 excess relaxation rate us. composition ( x = mole H,O/mole EO) in solutions of C,,E, micelles at 30.1 "C (0) and of PEO polymer of degree of polymerization 2700 at 25.0 "C (0). The PEO data were obtained from Breen et using R,,, = 142 s-'. For CI2E8, the line (constrained to pass through the origin) is a least-squares fit to the data points connected by the unbroken part of the line, while for PEO the line is calculated from data in ref. (24), using the value of Rref given above. If correction is made for the temperature difference, the two lines nearly coincide. Such a behaviour indicates (as discussed more fully in the following section) that water added to the system in the linear regime is unaffected by the presence of surfactant in the sense that its relaxation properties are indistinguishable from those of bulk water.The linearity in the Re, us. l / x plots is all the more remarkable as the composition range comprises several phases (fig. 3). This strongly suggests that the "0 spin relaxation is induced by local molecular motions (cf. next section). The absence of relaxation rate discontinuities at the phase boundaries was noted also by Corkill et al. in their 'H n.m.r. study of the C,,E,-H,O systemz0 and by Bozonnet-Frenot et al. in their 'H and 170 study of alkylammonium chloride-D,O systems. 35 Since we are primarily concerned here with the state of water in the headgroup shell under conditions where the shells of different aggregates do not interpenetrate extensively, we have not systematically examined samples with extremely low water content (x < 1).While such data may be interesting in their own right, their interpretation would be complicated by the difficulty of disentangling effects of an inhomogeneous water distribution from those of reorientational dynamics and electric field gradient perturbations. It is informative to compare the water relaxation behaviour in EO surfactant systems with that in PEO polymer solutions. In fig. 5 we thus show our data from the L, phase in the C,,E,-H,O system (at 30 "C) together with comparable data, reported by Breen et al.,'* for PEO-H,O solutions (at 25 "C). After a correction for the temperature difference, the Re, us.I/x slopes are closely similar for the two systems, while the onset of non-linearity occurs at higher water content in the polymer system. Another useful comparison is with the 1,2-dimethoxy ethane (DMEFwater system. DME may be regarded as an EO dimer. Fig. 6 shows, as expected, that Re, increases linearly with l / x in dilute DME solutions. The slope is similar to that obtained in EO-surfactant systems at the same temperature.G. Curlstrorn and B. Halle 1055 1 Oolx Fig. 6. Water 1 7 0 excess relaxation rate at 28.5 "C us. composition ( x = mole H,O/mole EO) in solutions of 1,2-dimethoxy ethane. The line (constrained to pass through the origin) is a least- squares fit to the data points. Interpretation Water Distribution and EO-chain Configuration Before attempting a detailed interpretation of our spin relaxation data, we shall briefly discuss some relevant structural features of the surfactant aggregates in C,E,-water systems.To be specific, we consider a dilute micellar solution. In the classical model, the micellar aggregate comprises a spherical hydrocarbon core of radius R, containing N surfactant tails, surrounded by a concentric shell of thickness L, containing all the Nn EO groups along with some water (x, water molecules per EO group). The notion of a water-free core is supported by bulk solubility considerations as well as by n.m.r. studies. 21, 27. 28 However, the composition and structure of the shell region has not been unambiguously characterized. In particular, we would like to know the amount and distribution of water in the shell or, which essentially amounts to the same thing, the configurational statistics of the EO chains.These issues are usually discussed in terms of concepts such as ' hydration ' and 'trapped water '. This terminology reflects the peculiar nature of the C , En-water interface, where intermolecular forces may elicit a non-local response (mediated by the EO-chain configuration) that alters the spatial extent of the shell region. Despite this underlying complexity, the quantity x, has a straightforward (and non-operational) interpretation as the number of water molecules (on an EO-group basis) in the headgroup shell. (We assume that the shell region is well defined, i.e. that the EO density falls sharply at the shell boundaries.) An estimate of x, can be obtained from the small-angle neutron scattering study of Zulauf et d .1 8 We consider a sample in the L, phase of the C12E,-D,O system far away from any phase boundary (x = 130, T = 30 "C) so that the scattering is not significantly affected by intermicellar correlations. Analysing the scattering dFta in terms of thg form factor for the classical miFelle model, one obtains" R, = 19.1 Aoand L, = 12.0 A. The micelle radius R, = 3 1.1 A coincides with the Stokes' radius 3 1 A deduced (assuming a non-draining shell) from surfactant self-diffusion studies of the same The fact that the core radius R, exceeds the length of the fully extended C,, chain,1056 Water in Non-ionic Surfactant Micelles 1 = 16.7 need not imply a non-spherical ~ h a p e , ' ~ but can be explained by a very slight EO penetration of the core.From simple volume considerations, it follows that the mean number n, of EO groups per surfactant that reside in the core is given by To obtain this result, one can note that the EO groups penetrating the peripheral core region must occupy a volume 47r(R, - 1)3/3, equal to the volume of the void (at the centre of the core) that would occur in the absence of EO pepetration, for a given core radius R, > 1. Taking the C,,-chain volume3' as vhc = 350 A3 and the EO-group volume as vLo = 64.6 A3 (as in the neat surfactant melt38), one obtains n, = 0.01 1. Thus, to increase the aggregation number (which is proportional to RE) by 50%, only 0.2% of the core volume (less than one EO group) has to be occupied by EO groups.A more substantial EO penetration may occur in micelles of the alkylaryl EO-surfactant Triton X-100, due to the presence of polarizable phenyl groups in the outer core region.39 If the C,,E8 micelle can be regarded as spherical with a sharp core/shell interface (this was assumed in the analysis of the SANS data"), then the number x, of D,O molecules per EO group in the shell is given by With anoaggregation nuFber N = 83 (which follows from R, and v,,,) and taking v, = 30 A3 and v,, = 61 A3 (this value of the apparent EO volume in the shell of C,E, micelles was deduced4' assuming bulk water density), one obtains x, = 2.8. (Zulauf et a1.,18 using slightly different values for the molecular volumes, obtained x, = 2.4.) Such a small x, value implies extensive coiling of !he EO chains, as seen more directly froom the fact that the shell thickness (L, = 12.0 A) is much smaller than the length (28.8 A) of a fully extended E, chain (which would correspond to x, = 19.6). It should be noted that the value of x, deduced from eqn (3) is quite sensitive to the core radius R,.Origin of Water "0 Spin Relaxation The interpretation of spin relaxation rates from water nuclei in locally heterogeneous systems can be a complicated task, involving molecular motions on several time scales and spatial averaging over different microenvironments. l9 However, for the C,E,- water systems, several simplifications are possible. The persistence of linearity in the Re, uersus l/x plots (fig. 3-5) down to very low water contents suggests that the surfactant-induced perturbation of the water spin relaxation is local and of short range.Similar behaviour has been found for water in clay suspension^,^' in alkylammonium chloride-water solutions and mesophases, 35 and in AOT-stabilized microemulsion droplets.*' On account of the short range of the perturbation, we may introdlice a two-state approximation. A fraction x,/x of the water molecules are thus regarded as perturbed, while the remaining water molecules have the relaxation properties of the bulk liquid. While the perturbed water molecules are likely to reside in the headgroup shell (see below), it is not necessarily the case that all water molecules in the shell (as given by x,) are significantly perturbed in their I7O relaxation behaviour (i.e., we may have x, < xs).The monotonic decrease of the relaxation rate with increasing temperature and the observation of Lorentzian lineshapes suggest that the diffusive water exchange between the perturbed and bulk-like regions is fast compared to the difference in relaxation rate (of the order 100 s-l) between these regions. (This is also expected on the basis of the relatively weak interactions and small spatialG. Carlstrom and B. Halle 1057 regions involved.) The excess relaxation rate in the linear regime (x 2 x,) may then be expressed as X Re, = "(R, - RF) (4) where R, is the (known) bulk water relaxation rate [R1,,ef in eqn (l)]. The relaxation rate RB refers to the perturbed water molecules; it is assumed to be independent of x (for x 2 x,) but it may vary within the perturbed region and should thus be regarded as an average. The observed equality of the longitudinal and transverse "0 relaxation rates implies that molecular motions on the Larmor precession time scale (2nvJ' "N 5 x s, or slower, do not contribute to the spin relaxation.The observation of quadrupolar line splittings from water in the non-cubic me so phase^^^- 26 demonstrates that some of the water in the headgroup shell has a locally anisotropic environment, There is thus the possibility that relatively slow motions (correlation time 5 lop's), such as lateral water diffusion and micelle rotation, contribute to the re1a~ation.l~ However, any such contribution will be weighted by a factor A 2 , where A is the residual ani~otropy.'~ We have measured the ''0 splitting for several samples in the hexagonal phase of the C,,E,-water system, obtaining A z 0.01.Since the observed correlation time is of order lo-" s (see below), it follows that contributions from any such slow motions are entirely negligible. This conclusion is further supported by the absence of a frequency dependence in the relaxation rates (v,, = 13.6-34.6 MHz), by the absence of discontinuities in Rex at mesophase boundaries (fig. 3), and by the independence of Rex on micellar size36 in the C,,E,-water system (fig. 4). The relaxation in the perturbed region is thus induced by local water reorientation. The relaxation rate in this region may be expressed as43 X where all quantities should be regarded as averages over the perturbed region.X, is the effective water ''0 quadrupole coupling constant (qcc), which incorporates the effect of asymmetry in the electric field gradient tensor as well as the effect of partial motional averaging by subpicosecond librations and intermolecular vibration^.^^ In the absence of where vB is the electric field gradient asymmetry parameter. The correlation time 7, in eqn (5) should be regarded as an effective second-rank rotational correlation time for the perturbed water molecules, defined as the time integral of the (reduced) non-exponential rotational correlation function. State of Water in Headgroup Shell According to eqn (4), a plot of Re, vs. l / x should be linear in the concentration range x 3 x B . Fig. 3 and 4 demonstrate that this is the case at least down to x = 5 in the investigated C,,E,-water systems.But even for the most concentrated sample, with x z 1, the deviation from linearity is barely significant. In the range x < x,, where eqn (4) is not valid, we may write formally where R J x ) is an average over the x (< x,) perturbed water molecules per EO group. At x = x,, eqn (4) and (7) must coincide, whence R , = R,(x,). Now if the effect of reducing x below x, were simply to strip the headgroup shell from water without1058 Water in Non-ionic Surfactant Micelles affecting RB(x) (which thus equals R,), then Re, would level off at a constant value R,- R , at x < x,. This is clearly not observed in the investigated range x 2 1. More probably, RB(x) increases as x is reduced below x,.The deviation from linearity can then go either way depending on whether x[R,(x) - RF] is smaller or larger than x,(RB - R,). If these two quantities happen to be nearly equal over a range of x values below x,, then the linearity in the Rex us. l / x plot will persist into the perturbed regime x < x,. While this may be the case at the lowest x values in fig. 3, it seems unlikely that a fortuitous linearity should exist over more than a narrow concentration range. Indeed, in the C,,E,-H,O system at 25 "C, we obtained R, = R, = 3300 s-l at x = 0.123, which is far below the line extrapolated from the large-x data, but still a factor of 24 larger than the relaxation rate of water in DME at infinite dilution (x-+O). In view of these considerations, the data in fig.3 and 4 suggest that X, is, in fact, small. We believe that x, < 5 is a conservative estimate. The uncertainty in this estimate and the possibility that there is some bulk-like water in the headgroup shell (x, > x,) means that we cannot, solely on the basis of the data in fig. 3 and 4, determine the amount of water (x,) in the headgroup shell of C&, aggregates, nor whether this quantity changes with concentration. Turning now to fig. 5, we see that (in contrast to the C,,E, data) the PEO data exhibit an increasing deviation from linearity as water molecules are removed from the primary hydration shells of the EO chains (on geometrical grounds, we estimate the coordination number to about 6 water molecules per EO group). This is precisely what one would expect on the basis of previous 1 7 0 n.m.r.~ t ~ d i e ~ ~ ~ , ~ ~ * ~ ~ of heterogeneous aqueous systems, which have shown that the amount of water (x,) whose reorientational dynamics is measurably perturbed from that of bulk water corresponds closely to the primary hydration 'shell', i.e. to the water in direct contact with the solute. The stronger than linear increase of Rex with l / x seen in the PEO solutions shows that the average excess relaxation rate R,(x) - R, for the remaining water increases faster than l / x as water is removed from the primary hydration shell. Further, Breen et al.24 found that Rex depends little on the length of the polymer chain; even an E, polymer shows a large non-linearity as the primary hydration shell is disrupted.This indicates that the observed behaviour of Re, depends primarily on the number of water molecules in contact with EO groups rather than on the long-ranged features of the EO chain (i.e. if an EO group belongs to a long or short chain or to a chain which is motionally constrained at one end, as in the micellar headgroup shell). The fact that the C,,E, data do not show any non-linearity in the region where the primary hydration would have been disrupted if it existed thus indicates that a full primary hydration is not present in the headgroup shell. Since the PEO data show that perturbation extends to the full primary hydration, it follows that there is no bulk-like water in the headgroup shell, i.e. we have x, = x,. Furthermore, this must be true at all concentrations, for if the water content of the headgroup shell was higher in the more dilute (larger x) micellar solutions, then, not only x,, but also X, would be concentration dependent (in contrast to what is seen in fig.3 and 4). As noted above, the SANS data of Zulauf et al." indicate that x, is in the range 2-3 for C,,E, micelles in dilute solution (x = 130, T = 30 "C). Our results are consistent with an x, value in this range and, moreover, suggest that this x, value applies also at high concentrations (as x approaches x,). Our n.m.r. data thus lend support to a picture of a compact headgroup shell with less than 5 and possibly as little as 2-3 water molecules per EO group. The constancy of x, and, thus, of x, (cf. above) over a wide range of concentrations (including different phases), as implied by the extensive linear regimes in fig.3-5, suggests that the shell exhibits substantial structural integrity (statistically speaking : we are not implying a static structure). In contrast, the SANS data', for C,,E, yield a reduction of x, from 2.4 to 1.0 as x goes from 30 to 6 at 30 "C, while at 60 "C x, is reduced from 0.7 to 0.1 as x goes from 70 to 15. Such a strong concentration dependence of the water content inG. Carlstrom and B. Halle 1059 Table 1. Effect of ethylene oxide groups in different systems on water 170 relaxation rate system T/"C x,(R,- RF)/102 s-Ia t , / t F b 12E5-H20 20.1 24.3 30.1 C12E5-DZ0 19.6 24.8 29.8 34.8 C12E,-H2O 10.0 21.1 24.7 28.2 30.1 50.5 64.3 66.9 69.7 PEO-H,Od 10.6 14.2 20.1 24.3 30.0 49.5 PEO-H20e 25.0 DME-H20 28.5 15.2 13.0' 11.8' 8.5' 15.5 (11.5)f 12.0 (9.0)f 9.4 (7.l)f 8.1 (6.2)f 22.9 12.6 11.3" 9.2 8.4 3.7 2.4" 2.2" 1.4 17.9 15.0 12.5 11.7 8.6 4.3 10.1 8.3 5.2 5.2 5.2 4.4 4.5 4.2 3.8 3.8 6.3 4.9 5.1 4.7 4.3 3.3 2.8 2.8 2.2 5.5 5.1 5 .O 5.2 4.5 3.8 4.6 4.1 a From slopes in Rex us.l / x plots. Estimated uncertainty less than 10%. Assuming x, = 2 and 2, =zF. 'From one concentration only. From one concentration (x = 28.6) well within the linear regime of the plot of Rex us. 1 / ~ . ' ~ From ref. (24). f Corrected for isotope effects to a value valid for H,O. the headgroup shell does not seem to be compatible with our data. We believe that the low x, values are artefacts of the model used to interpret the scattering curves, which at higher surfactant concentrations are dominated by the effects of intermicellar correlations.Water self-diffusion measurements have also been used to deduce information on non-ionic micelle h~drati0n.l~ In that work, the water molecules were divided into two groups: 'free' water molecules having a translation mobility as in bulk water (but subject to obstruction effects) and 'water of hydration' having the same mobility as the micelles (determined separately). However, as pointed out by Jonsson et aZ.,45 the assumption that dynamically perturbed water molecules are completely immobilized with respect to the micelle is questionable and can lead to an artificial concentration dependence in the deduced hydration number (x,). In fact, using the diffusion model of Jonsson et aZ.,45 which contains the two parameters x, and D , (water diffusion coefficient in the shell), we have found that the x-dependence of the macroscopic water diffusion coefficient in the C,,E,-D,O system" can be quantitatively accounted for with constant (independent of x) values of x, and D,.Further, we find that x, and D , cannot be1060 Water in Non-ionic Surfactant Miceiles 3 Fig. 7. Effect of ethylene oxide groups in different systems on water 170 relaxation rate. Data from table 1. The curve resulted from a least-squares fit lo an Arrhenius-type temperature dependence, yielding an apparent activation energy of 33 kJ mol-’. A C,,E,-H,O, (0) C,,E,-H,O, (0) C,,E,-D,O, (V) C,,E,-H20, (0) PEO-H,O, (B) PEO-H,O from ref. (24), (*) DME-H,O. separately determined from the data ; virtually identical theoretical curves can be generated (by adjusting D,) for any x, 2 3. Not unexpectedly, the D, value so deduced for a given x, is within a factor of 2 of Dobs in a PEO-D,O solution with the total water content (x) matching that of the micelle shell ( x B ) .In conclusion, we believe that the present relaxation data, as well as the earlier diffusion data,17 are compatible with a low and virtually concentration independent water content in the headgroup shell. The quantity x,(R,-R,), as obtained from the slope at x > xH in a plot of Re, US. l / x , is given in table I for the investigated systems at several temperatures. When these data are plotted together (fig. 7), it is seen that the effect of EO groups on water 1 7 0 relaxation is insensitive to whether the EO groups reside in Cl,E, micelles or mesophases, in PEO polymers, or in the DME ‘dimer’.This suggests that the hydration structure and dynamics around EO groups are similar in these systems. However, since we have seen (cf. above) that more water molecules per EO group are dynamically perturbed in PEO solutions than in C,,E,-water systems, the near-invariance with respect to system as seen in fig. 7 must be partly fortuitous, i.e. small differences in x,, and R, between PEO and Cl,E, tend to cancel out. It is noteworthy that the water self- diffusion exhibits the same kind of system invariance; the diffusion coefficient is virtually the same in C,,E,-water and PEO-water systems at the same x value.I7 In view of the different obstruction volumes and different local water environments (cf.above) in these two kinds of system, this result is unexpected. It is possible that cancellation effects are involved here also. Having estimated the total amount of water in the headgroup shell, we would also like to know its distribution. It is conceivable, for example, that the innermost EO groups are less exposed to water than EO groups further along the chain. This is not a geometric necessity, however, since the geometrically allowed volume fraction of water within a distance of one EO group from the core surface (74%) is larger than the mean volume fraction of water in the whole shell (50-71 O/O for x, = 2-5). Our relaxation data suggest that all EO groups in the headgroup shell are exposed to water to roughly the same extent.First, the linearity in fig. 3 indicates that x, is the same in the micellarG. Carlstrorn and B. Halle 7 6- 5- & .4- @ 3- 2 - '0 1061 V 0 $ % 0 vg 0 * O . v v V I I I I I I I 20 40 60 E I3 v Fig. 8. Relative increase of rotational correlation time for water in contact with ethylene oxide groups in different systems, assuming x, = 2 and xR = xF: (A) C,,E,-H,O, (0) C,,E,-H,O, (@) C,,E5-D,0, (V) C,,E,-H,O, (0) PEO-H,O, (m) PEO-H,O from ref. (241, (*) DME-H,O. solution phase as in the liquid crystalline phases, despite the different headgroup areas at the surface of spherical and cylindrical hydrocarbon cores. Secondly, the value of x,(R,-R,) is nearly the same for C1,E4, Cl,E5 and C12E, micelles at a given temperature (fig.7). This argument may also be taken as evidence against a substantial penetration of the hydrocarbon core by EO groups. Although the hydration structure in the headgroup shell may be strongly hydrogen- bonded, the 1 7 0 relaxation rate shows that it is highly dynamic. To quantify this statement, we shall use eqn ( 5 ) to estimate the rotational correlation time ratio T ~ / z ~ . In order to calculate this quantity from x,,(R,-R,) (table 1) and the known bulk water relaxation rate R,, we need to know x, and the qcc ratio zB/x,. Since the 1 7 0 electric field gradient in water depends partly on the extent of hydr~gen-bonding,~~ it is possible that xB # zF. However, if, as seems reasonable, water in the headgroup shell is extensively hydrogen-bonded, then the difference should be small.To answer this question, we measured both 2H and 1 7 0 relaxation rates in two samples (Cl,E4 and C12E5, both 25 wt %) with 10% ,H in the water. For the C,,E, sample at 14.4 "C we obtained R(170)/R(2H) = 71 and for the C,,E, sample at 20.1 "C R("O)/R(,H) = 75. Within experimental uncertainty these ratios are the same as for bulk water at these temperatures (72.5 and 74, respectively). Since the 2H qcc depends primarily on the 0-H bond length (rather than on the extent of hydr~gen-bonding)~~, these results suggest that X, = zF to a good approximation. Taking x, = 2 (at all temperatures) we thus obtained the correlation time ratios z,/z, given in table 1 and plotted in fig. 8. The reduction in rotational mobility for water in the headgroup shell is seen to be modest: a factor of 2-6 in the investigated temperature range.We take x, = 2 to be a lower limit; hence, the deduced z,/z, ratios should be regarded as upper bounds. Our results for water rotation happen to agree closely with the factor 2 reduction in local translational water mobility deduced from quasi-elastic neutron scattering on the PEO-H,O system at 65 "C and x < 4.461062 Water in Non-ionic Surfactant Micelles Conclusions Water 1 7 0 spin relaxation is a powerful method for probing (more directly than by most other techniques) the state of water in the headgroup shell of EO-surfactant aggregates. Valuable molecular-level information about structure and dynamics can be extracted without invoking detailed models of uncertain validity.Our 1 7 0 relaxation data support the following picture of the headgroup shell in C,,E, aggregates. (1) The core/shell boundary is sharp. There is negligible penetration of the core by EO groups or water. (2) The headgroup shell is compact. It contains less than 5 and possibly as little as 2-3 water molecules per EO group, with an essentially uniform distribution of EO groups and water. (3) The shell exhibits substantial structural integrity. Its water content is essentially invariant over a large concentration range. (4) The headgroup shell is highly dynamic. The reorientational motion of water molecules is reduced by at most a factor of 5 (at room temperature) compared to bulk water. (5) The dramatic changes, induced by small variations in the system parameters, in the macroscopic phase behaviour do not reflect major changes in water behaviour at the molecular level.We thank Per-Gunnar Nilsson for his kindness to let us use his results on the C,,E,- D,O system and Gunnar Karlstrom, Bjorn Lindman and Hikan Wennerstrom for useful comments on the manuscript. Grants from the Swedish Natural Science Research Council and Kungliga Fysiografiska Sallskapet i Lund are gratefully acknowledged. This work was not supported by any military agency. References 1 D. J. Mitchell, G. J. T. Tiddy, L. Waring, T. Bostock and M. P. McDonald, J. Chem. Soc., Faraday 2 S. Saeki, N. Kuwahara, M. Nakata and M. Kaneko, Polymer, 1976, 17, 685. 3 J. C. Lang and R. D. Morgan, J. Chem. Phys., 1980, 73, 5849. 4 R. Kjellander, J.Chem. SOC., Faraday Trans. 2, 1982, 78, 2025. 5 P-G. Nilsson, H. Wennerstrom and B. Lindman, Chem. Scr., 1985, 25, 67. 6 P. M. Claesson, R. Kjellander, P. Stenius and H. K. Christenson, J. Chem. SOC., Faraduy Trans. 1 , 7 H. Evans, D. J. Tildesley and C. A. Leng, J. Chem. Soc., Faraduy Trans. 2, 1987, 83, 1525. 8 G. Karlstrom, J. Phys. Chem., 1985, 89, 4962. 9 P. H. Elworthy and C. B. Macfarlane, J. Chem. SOC., 1963, 907. Trans. 1, 1983, 79, 975. 1986, 82, 2735. 10 P. Becher and H. Arai, J. Colloid Interface Sci., 1968, 27, 634. I 1 D. Attwood, J. Phys. Chem., 1968, 72, 339. 12 J. M. Corkill and T. Walker, J. Colloid Interface Sci., 1972, 39, 621. 13 C. Tanford, Y. Nozaki and M. F. Rohde, J. Phys. Chem., 1977,81, 1555. 14 U. Kaatze, 0. Gottmann, R. Podbielski, R.Pottel and U. Terveer, J. Phys. Chem., 1978, 82, 112. 15 U. Kaatze, Ber. Bunsenges. Phys. Chem., 1978, 82, 690. 16 G. J. T. Tiddy, M. F. Walsh and E. Wyn-Jones, J. Chem. SOC., Faraday Trans. 1, 1982, 78, 389. 17 P-G. Nilsson and B. Lindman, J. Phys. Chem., 1983, 87, 4756. 18 M. Zulauf, K. Weckstrom, J. B. Hayter, V. Degiorgio and M. Corti, J. Phys. Chem., 1985, 89, 341 1. 19 B. Halle and H. Wennerstrom, J. Chem. Phys., 1981, 75, 1928. 20 J. M. Corkill, J. F. Goodman and J. Wyer, Trans. Faraday Soc., 1969, 65, 9. 21 C. J. Clemett, J. Chem. Sac. A , 1970, 2251. 22 A. Llor and P. Rigny, J. Am. Chem. SOC., 1986, 108, 7533. 23 S. Conti, Mol. Phys., 1986, 59, 449. 24 J. Breen, D. Huis, J. de Bleijser and J. C. Leyte, J. Chem. SOC., Faraday Trans. 1, 1988, 84, 293. 25 T. Klason and U. Henriksson, in Surfactants in Solution, ed. K. Mittal and B. Lindman (Plenum Press, New York, 1984), vol. 1, p. 93.G. Carlstrorn and B. Halle 1063 26 K. Rendall and G. J. T. Tiddy, J. Chem. SOC., Faraday Trans. I , 1984, 80, 3339. 27 F. Podo, A. Ray and G. Nimethy, J. Am. Chem. SOC., 1973,95, 6164. 28 A. A. Ribeiro and E. A. Dennis, J. Phys. Chem., 1977, 81, 957. 29 T. Ahlnas, G. Karlstrom and B. Lindman, J. Phys. Chem., 1987, 91, 4030. 30 A. J. Ward, H. Ku, M. A. Phillippi and C. Marie, Mol. Cryst. Liq. Cryst., 1988, 154, 55. 31 S. Meiboom, J. Chem. Phys., 1961, 34, 375. 32 Z. Luz and S. Meiboom, J. Chem. Phys., 1963, 39, 366. 33 B. Halle and G. Karlstrom, J. Chem. SOC., Faraday Trans. 2, 1983, 79, 1031. 34 I. Fur6, B. Halle and T. C. Wong, J. Chem. Phys., 1988, 89, 5382. 35 M. P. Bozonnet-Frenot, J. P. Marchal and D. Canet, J. Phys. Chem., 1987, 91, 89. 36 P-G. Nilsson, H. Wennerstrom and B. Lindman, J. Phys. Chem., 1983,87, 1377. 37 C. Tanford, The Hydrophobic Efect, (Wiley, New York, 2nd edn, 1980). 38 S. Kucharski, A. Sokolowski and B. Burczyk, Roczn. Chem., 1973, 47, 2045. 39 R. J. Robson and E. A. Dennis, J. Phys. Chem., 1977, 81, 1075. 40 S. Kaneshina, M. Yoshimoto, H. Kobayashi, N. Nishikido, G. Sugihara and M. Tanaka, J. Colloid 41 D. E. Woessner, J. Magn. Reson., 1980, 39, 297. 42 G. Carlstrom and B. Halle, Langmuzr, 1988, 4, 1346. 43 A. Abragam, The Principles of Nuclear Magnetism (Clarendon, Oxford, 1961). 44 P. L. Cummins, G. B. Bacskay, N. S. Hush, B. Halle and S. Engstrom, J. Chem. Phys., 1985, 82, 45 B. Jonsson, H. Wennerstrom, P-G. Nilsson and P. Linse, Colloid Polym. Sci., 1986, 264, 77. 46 A. Maconnachie, P. Vasudevan and G. Allen, Polymer, 1978, 19, 33. Interface Sci., 1980, 73, 124. 2002. Paper 81022645; Received 8th August, 1988

ISSN:0300-9599    

DOI:10.1039/F19898501049

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

J. Chern. Soc., Furaduy Trans. I, 1989, 85(5), 1065-1074 Dielectric Relaxation in Concentrated Solutions of cis-Pol yiso prene Part 1 .-Effect of Entanglement on the Normal-mode Process Keiichiro Adachi," Y asuo Imanishi and Tadao Kotaka Department of Macromolecular Science, Faculty of Science, Osaka University, Toyonaka, Osaka 560, Japan Dielectric measurements were carried out on concentrated toluene solutions of narrow molecular-weight distribution cis-polyisoprenes (cis-PI) with molecular weight of 1.6 x lo3 to 5 x 10'. Two loss maxima were observed in the temperature range 200-400 K. The low-frequency process was assigned to the normal mode process due to the fluctuation of the end-to-end distance of the cis-PI molecules; and the high-frequency process to the segmental mode process related to the glass transition of the polymer.The molecular- weight dependence of the relaxation time T, for the normal mode process at a fixed concentration was found to be similar to the viscoelastic relaxations : below the characteristic molecular weight M(,, 5, cc M2 but in the range above M,, t, cc M43. The M , varied in proportion to the inverse of concentration. The high-frequency segmental mode process exhibited the relaxation time t, almost independent of the molecular weight. The time-temperature superposition principle was applied to construct master curves of the dielectric loss factor E" over a wide frequency range. The E" curves broadened with increasing molecular weight and concentration in the high frequency side of the curves.However, the half width of the loss curves was almost independent of the molecular weight and concentration. Each monomeric unit of cis-polyisoprene (cis-PI) has two different types of components of the dipole moment: one is the parallel component aligned parallel to the chain and the other perpendicular to the chain contour.'*2 The former causes the dielectric normal mode process due to the fluctuation of the end-to-end vector, and the latter the segmental mode process known as the primary or cc relaxation process near the glass transition temperature. We have already reported the dielectric relaxation behaviour of cis-PI in dilute and semi-dilute ~ o l u t i o n s ~ - ~ as well as in bulk.'V2 For bulk cis-PI, we found that the relaxation time z, for the dielectric normal mode process shows the molecular weight M dependence similar to the viscoelastic longest relaxation time.'T2 In the range of A4 lower than the characteristic molecular weight M,, z, was proportional to M2 in accordance with the Rouse theory.6 In the range above M,, however, zn was proportional to kP7 due to entanglement of the molecules.This behaviour is similar to the 3.4 power law'.8 for the viscoelastic longest relaxation time and was explained semi-quantitatively in terms of the tube theory proposed by de Gennes' and Doi and Edwards." In dilute solutions, z, was proportional to M'.' and M'.74 in a theta solvent, dioxane, and a good solvent, benzene, re~pectively.~ This behaviour was essentially in agreement with the Zimm theory. l 1 On the other hand, the relaxation time z, for the segmental mode process is almost independent of M .We pointed out that for cis-PI with a certain M the ratio ZJZ, is independent of temperature T, although z, and z, themselves depend strongly on T. This behaviour suggests that the local segmental motions relate intimately with the 10651066 Entanglement in cis- Polyisoprene Solutions friction coefficient [ for the large scale motions. However, as far as we know, no experimental work on the relationship between t, and t, have been reported. As is well known, [ and the molecular weight Me ( I I MJ2) between entanglements are the key parameters to describe the dynamic properties of polymers. They depend strongly on the content of a diluent. To clarify these problems, we studied in detail the dielectric normal mode and the segmental mode processes in concentrated toluene solutions of cis-PI.The results are reported in this article and the succeeding two papers.12 This first paper is concerned with the effect of diluent on the normal mode process of cis-PI. The first purpose of this study is to clarify the M dependence of z, in concentrated toluene solutions of cis-PI by comparing the results with the data for the bulk cis-PI reported previously. The second purpose is to clarify the concentration C and M dependences of the distribution of relaxation times. In our previous study on bulk cis-PI,2 we observed that the width of the E” curves increased with M. We attempted to explain this behaviour based on the tube model.’O On the other hand, the width of the E” curves of cis-PI in dilute solutions was almost independent of M.3 In these experiments, we used cis-PI with the polydispersity factor, M,/M,, of 1.2 to 1.4.Thus, the broadening of the E” curves may be attributed in part to the distribution of molecular weight (MWD). In this study, we examined the effect of MWD on the width of the E” curves, using narrower distribution samples. Theory The theories of the dielectric normal mode process were described by Zimm,” Stockmayer and Baur,13* l4 and by o~rselves.~ Stockmayer and his coworkers carried out pioneering work on poly(propy1ene oxide) and called this process the low frequency process of type A polymer^.^^"^ Here, we briefly describe the basic equations of the process which we called the normal mode process.When a polymer has a dipole moment p proportional to the end-to-end vector r, the contribution of the parallel dipoles to the complex dielectric constant E*(w) at angular frequency o is given by4 where Cis the concentration in wt/vol., N A is the Avogadro constant, E , is the unrelaxed dielectric constant, p is the dipole moment per unit contour length (p = p-) and ( r 2 ) is the mean-square end-to-end distance. The relaxation strength, A E ~ [ = ~ ( 0 ) - E,], for the normal mode process is given by16 (2) Ae,/C = 4xN,p2 (r2)F/(3k, T M ) where ~(0) is the E’ value at zero frequency and F is the ratio of internal to external electric field. F i s close to unity.16 Both the Rouse theory for a non-entangled polymer and the tube theory for an entangled system predict an autocorrelation function # = (r(0) - r ( t ) ) / ( r 2 ) of the same form : # = C ( 1 / p 2 ) exp ( -p21/tl) p = 1 , 3 , 5 , .. . (3) where p is the number of the normal mode and t,, the relaxation time for the first mode. Since the first mode has the dominant contribution, the relaxation time z, for the normal mode process is approximately described by 7,. The difference between the theories is only in the M dependence in the expression of z,. The longest relaxation time 2, of the free-draining model proposed by Rouse is written as‘K. Adachi, Y. Imanishi and T. Kotaka 1067 where [ is the monomeric friction coefficient, and x, the number of the repeat unit. Since in the unperturbed state ( r 2 ) is proportional to the molecular weight M , zR is proportional to W even in bulk and in concentrated solutions as long as the polymer chains are not entangling.It is noted that zR for the dielectric normal mode is twice the viscoelastic longest relaxation time. Zimm’l described z, for the non-draining model and predicted that z, cc W 5 . However, we will not use the Zimm theory since the hydrodynamic interactions are screened in condensed systems. According to the tube model, z1 is equal to the relaxation time z, for the tube disengagement process :’ where L is the contour length of the tube, and D, the diffusion coefficient of the chain along the tube. According to the Doi-Edwards theory,1° eqn ( 5 ) is rewritten as z, = L2/D ( 5 ) where Me is the molecular weight between entanglements. Experimental Samples of cis-PI were prepared by anionic polymerization with sec-butyllithium and n-butyllithium as the initiator, and characterized by a gel permeation chromatography (GPC) as described previously.’+ The weight average molecular weight M , was determined with a low-angle light scattering monitor installed in GPC.The number- average molecular weight M , was determined for some low molecular weight samples by freezing point depression with benzene and cyclohexane as the solvent. The polydispersity index, M,/M,,, was determined from GPC chromatograms. The characteristics of the samples are listed in table 1 where the samples already reported are also listed.2 The number of the sample code indicates the weight average molecular weight in units of kg mol-’.The solvent toluene was dried with calcium hydride and then distilled by vacuum distillation. Sample solutions were prepared and stored under dry argon atmosphere to prevent contamination by moisture. Measurements of the dielectric constant E’ and loss factor E” were made in the frequency range from 20 Hz to 100 kHz with transformer bridges (General Radio 1615- A and Showa Denki). A capacitance cell was the same type as described previously.233 Results and Discussion Frequency Dependence of the Complex Dielectric Constants The representative frequency-dependence curves of E” are shown in fig. 1 and 2 for 50 O/O solutions of PI-05 and PI-53, respectively. In fig. 1 and 2, two loss maxima are seen: for example see the E” curve at I89 K in fig. 1. Comparing the loss curves for PI-53 and PI- 05 at 179 K, we find that the loss maximum frequency for the high-frequency process is almost independent of M,.On the other hand, the loss curves at 244 2 K indicate that the low-frequency process depends quite strongly on M,. Thus, the high- and low- frequency processes were assigned to the segmental mode process and the normal mode process, respectively. Separation of the two loss maxima for the 52% solution of PI-53 is more than 5 decades. Since the available frequency range was limited to only 4 decades, we could not observe the whole E’ and E” curves including both relaxation regions by one measurement at a fixed temperature. To avoid this difficulty, we constructed master curves of E” assuming the time-temperature superposition principle. We reported for bulk cis-P12 that the relaxation time z, for the normal mode process and that z, for the segmental mode process shift by the same amount parallel with temperature.In concentrated1068 code Entanglement in cis-Polyisoprene Solutions Table 1. Characteristics of cis-polyisoprene PI-02 PI-03 PI-05 PI-14 PI-32 PI-53 PI- I74 PI-09 PI-1 I PI- 16 PI-20 PI-24 PI-42 PI-59 PI-74 PI- 102 PI- 128 PI- 160 MW - - - 13.5 31.6 52.9 8.6 11.0 16.2 20.4 23.5 41.9 58.9 74.0 174 102 128 164 M , Mw/M, initiator" 1.1 1 1.09 1.07 1.08 1.05 1.08 1.13 1.29 1.17 1.23 1.20 1.20 1.34 1.33 1.20 1.18 1.37 1.17 S S S S S S S n n n n n n n n n n n " s: sec-butyllithium, n: n-butyllithium. l # ~ ~ l ~ ~ ~ l l l I l l 1 , n I 4r 179K 189 K log Cf/Hz) Fig. 1. Frequency, f, dependence of the dielectric loss factor E" at various temperatures indicated in the figure for 53 wt % of PI-05 in toluene.solutions, the same behaviour may be expected. This is supported by the Arrhenius plot for 7" and 7, reported in part 2 of this series of studies.12 We took 273 K as the reference temperature. The E" curve at a temperature T was multiplied by 273/ Tand then shifted in the horizontal direction to get best superposition. When the temperatures for the two loss curves were separated widely, vertical shift was also needed t o get best superposition. This is probably due to the change in density,K. Adachi, Y . Imanishi and T. Kotaka 1069 log CflHZ) Fig. 2. Frequency dependence of the dielectric loss factor E" for 52 wt YO of PI-53 in toluene.L 6 A 2 2 4 6 a 0 v l " ~ ' ' ' ' ~ ' ' ' ~ ' ' ' ~ log ( f l W Fig. 3. (a) Master curve for the 53 YO solution of PI-05 in toluene and (h) that for bulk PI-05, both at 273 K . Thus, the log E" us. log f plot was shifted both in the horizontal and vertical directions. Fig. 3 shows two examples of the master curves thus chnstructed for PI-05. We see that superposition is quite good in the region where the normal mode process was observed, while in the region of the segmental mode process, some scattering of data points is seen. Smoothed curves were drawn as shown in fig. 3 for the master curves. Fig. 4 and 5 show examples of the smoothed master curves all reduced to 273 K for solutions1070 Entanglement in cis- Polyisoprene Solutions log CflW Fig. 4.Smoothed master curve of the loss factor E" for solutions of PI-32 in toluene at 273 K. Dashed curve corresponds to eqn (9). 0, 41.4; 0, 52.2; A, 61.9; ., 85.5; 0, 100 wt YO. log Cf/Hz) Fig, 5. Smoothed master curve of the loss factor E" for toluene solutions of &PI with various molecular weights at 273 K. 0, PI-05, 52.2%; 0, PI-14, 49.2%; A, PI-32, 52.2%; 0 , PI-53, 52.3%. of PI-32 from the bulk to 41.4% concentration and for approximately 50 YO solutions of cis-PI with varying M,. Molecular Weight Dependence of the Relaxation Time The relaxation times z, and z, at 273 K were determined from the loss maximum frequency f, by z = l/(2;nfm). Fig. 6 shows the double logarithmic plots of z, and z, against M,. The data for the bulk samples reported previously are also plotted.2, l7 The slope of the double logarithmic plot of z, vs.M , changes at the characteristic molecular weight Me. On the other hand, z, is almost independent of M,. A slight M dependence of z, seen for the bulk samples with M , less than 5000 is ascribed to the increase in the free volume content as is generally known for oligomers.K . Adachi, Y. Irnanishi and T. Kotaka 1071 -2- -4- n \ W M - s -6- r -A- --A-- 50% -30% . - 10 Fig. 6. Weight average molecular weight M , dependence of the relaxation time z for the normal mode process (open symbols) and those for the segmental mode process (filled symbols) for the solutions and bulk. Dashed curve corresponds to the theoretical z given by eqn (6) and (7), and dot-dash curve to the one given by eqn (4), (6) and (8).According to the Rouse‘ and Zimmll theories, t, is expected to be proportional to ikP and W 5 , respectively. Although the experimental error is relatively high, the slope of the plot in the range of M, < M , is determined to be 2.0f0.2. Thus, the hydrodynamic effect does not contribute in the concentrated solutions with C > 0.2. In the range of M , > M,, the slopes of the tn vs. M , plots for 50, 30 and 20% solutions are 4.3 0.2. Generally, the viscoelastic relaxation time z, in the entangled state is proportional to the 3.5 f 0.2 power of Mw.73 * The present values of the power in concentrated solutions are significantly higher than the well- known empirical value of 3.5. Doi18 explained the deviation of the observed exponent of 3.5 from the theoretical value of 3.0 predicted by the tube theory considering the fluctuation of the contour length of the primitive chain confined in a tube.The theory predicts (7) where t, is given by eqn (6). Assuming Me = 5000 and using the observed zn at M , = 5000 for t,(M,), we calculated the theoretical t as given by the dashed line in fig. 6. It t = [ 1 - 1 .47(Me/M)fl2 t,1072 Entanglement in cis-Polyisoprene Solutions I ‘ I 1 1 1 I I I I I- -I 0 015 I log (C/g ~ r n - ~ ) Fig. 7. Concentration C (in g ~ r n - ~ ) dependence of the characteristic molecular weight M,. is seen that agreement is fairly good. Since M, increases with decreasing C, the data covered in the present study are limited in the range close to M,. Therefore, the high value of the exponent in the M dependence of z may be partly attributed to the fluctuation of the contour length.As the origin of the discrepancy between the theoretical and observed powers, we should consider the so-called tube renewal effect due to disintegration of tube-like constraint with time arising from the motions of the chains surrounding the test chain. Kleinlg proposed the relaxation time z for the tube disengagement process by taking the tube renewal effect into account. He assumed that the tube itself moves similarly to the Rouse chain and that an entangled chain relaxes through the two mechanisms, i.e. reptation and tube renewal : where 7, and z, are given by eqn (4) and (6), respectively. This equation is plotted by a dash-dot line in fig. 6. As seen in the figure, eqn (8) predicts a power close to 3.0 in the range M > M, and is not satisfactory to explain the present results.Concentration Dependence of the Characteristic Molecular Weight Although the experimental uncertainty is relatively large, we determined the M,, for the solutions at various C and plotted them in .Fig. 7. It is seen that M, is nearly proportional to the inverse of C . This behaviour is the same as the M , for viscoelastic relaxation in concentrated polymer solutions.* Distribution of Relaxation Time for the Normal-mode Process In fig. 3-5, we see that E” curves of the segmental and normal modes overlap partly. In order to see the concentration and molecular weight dependence of the shape of the E”K. Adachi, Y. Imanishi and T. Kotaka Table 2.The parameters of Negami-Havriliak equation for the segmental mode process and the parameter K for the normal mode process conc. code (wt %) a @ ~ , / l O - ~ s 102A& K PI-32 41.4 0.32 0.25 2.1 4.95 -0.40 PI-32 52.2 0.32 0.20 3.4 6.36 -0.37 PI-32 61.9 0.25 0.24 2.8 7.35 -0.34 PI-32 85.5 0.22 0.33 19 7.52 -0.32 PI-32 100 0.18 0.38 93 7.68 -0.31 - - - - -0.31 - - - - -0.41 PI-53 52.3 PI-14 49.2 I, I I . 1 I I I I . I . 1 ) . I - 2 0 2 4 1% (flfm) \ \ I I I . I , 1 , I \ , I -2 0 2 4 1073 Fig. 8. Double logarithmic plot of the normalized loss curve: (a) 50 YO solutions of cis-PI in toluene with various molecular weights and (b) solutions of PI-32 with various concentrations. The arrow indicates the half width. The dashed line indicates the results of eqn (3).curves for the normal mode process, we have to subtract the contribution of the segmental mode process from the observed E" curves. We estimated the contribution of the segmental mode process using the Hav- E*(w) - E , = A&/[ 1 - (iws,)'-']B (9) riliak-Negami equation :*O where 7, is the nominal relaxation time, and a and p are the constants. The dashed line in fig. 4 corresponds to a = 0.32, p = 0.20, to = 3.4 x lo-', and A& = 0.065. The same parameters were used for the 50% solutions shown in fig. 5. For the solutions of PI-32, we used the parameters listed in table 2. The E" curves thus estimated for the normal mode process are compared in fig. 8 where E" and frequencyfare normalized by the maximum value 8: and the loss maximum frequency f,, respectively. We recognize that the loss curve broadens with increasing M and C.Obviously, this may be attributed to the effect of entanglement. However, the broadening occurred only in the high frequency side of the loss peak. In the low frequency side, we see that the observed 8' agrees rather well with the theoretical curve calculated by eqn (3). This result does not agree with the theoretical prediction by either the bead-spring1074 En tanglement in cis- Polvisoprene Solutions model or the tube model. Both theories predict the same distribution of relaxation times given by eqn (3) and hence the same shape of the E” curve as given by the dashed line in fig. 8. In our previous studies,21 we discussed the distribution of the relaxation times for the normal mode process in bulk samples2 and dilute solutions3 of cis-PI, using the half width of the E” curves and the Davidson-Cole parameter.22 It was demonstrated that the MWD affects strongly the half width.In the present study, we used cis-PI samples with M,,/M,, less than 1.08 to discuss the shape of the E” curves. As is seen in fig. 8, the half width indicated by arrows depends little on M and C . Thus the half-width is an insensitive measure of the distribution of the relaxation times. On the other hand, the slope K of the double logarithmic plot of E” vs.fin the high frequency side is considered to be a better parameter than the half width to describe the broadening of the E” curve. The values of K for the present samples are listed in table 2. We see that the absolute value of K increases with decreasing M and C.It is noted that both the Rouse model for nonentangled systems and the tube model for entangled systems predict the slope of -0.50. At present there exists no molecular theory to explain the relaxation spectrum for the normal mode process. Summary 1. The relaxation time z,, for the normal mode process in concentrated solutions of cis-polyisoprene is proportional to Ww in the range below the characteristic molecular weight M , and to Mk3 above M,. 2. The characteristic molecular weight M , is inversely proportional to the concentration. 3. The distribution of the relaxation times for the normal mode process increases with increasing molecular weight and concentration. This work was supported in part by the Grant-in-Aid (6055062) for Scientific Research by the Ministry of Education, Science and Culture.Support from the Institute of Polymer Research, Osaka University is also gratefully acknowledged. References 1 K. Adachi and T. Kotaka, Macromolecules, 1984, 17, 120. 2 K. Adachi and T. Kotaka, Macromolecules, 1985, 18, 466. 3 K. Adachi and T. Kotaka, Macromolecules, 1987, 20, 2018. 4 K. Adachi and T. Kotaka, Macromolecules, 1988, 21, 157. 5 K. Adachi, H. Okazaki and T. Kotaka, Macromolecules, 1985, 18, 1687. 6 P. E. Rouse, J . Chem. Phys., 1953, 21, 1272. 7 J. D. Ferry, Viscoelastic Properties of Polymers (Wiley, New York, 3rd edn, 1980), chap. 10, p. 224. 8 W. W. Graessley, Adv. Polym. Sci., 1982, 47, 67. 9 P. G . de Gennes, J . Chem. Phys., 1971, 55, 572. 10 M. Doi and S. F. Edwards. J . Chem. Soc., Faraday Trans. 2, 1978, 74, 1789; 1802; 1818. 1 1 B. H. Zimm, J . Chem. Phys., 1956, 24, 269. 12 K. Adachi, Y. Imanishi and T. Kotaka, J. Chem. Soc., Faraday Trans. I, 1989, 85, 1075; 1083. 13 W. H. Stockmayer, Pure Appl. Chem., 1967, 15, 539. 14 W. H. Stockmayer and M. E. Baur, J . Am. Chem. SOC., 1964, 86, 3485. 15 M. E. Baur and W. H. Stockmayer, J. Ph-vs. Chem. 1965, 43, 12. 16 K. Adachi, H. Okazaki and T. Kotaka, Macromolecules, 1985, 18, 1486. 17 Y. Imanishi, K. Adachi and T. Kotaka, J . Chem. Phys., 1988, 89, 7585; 7593. 18 M. Doi, J . Polym. Sci., Polym. Phys. Ed., 1983, 21, 667. 19 J. Klein, Macromolecules, 1978, 11, 852. 20 S. Havriliak and S. Negami, J . Polym. Sci., Part C , 1966, 14, 99. 21 K. Adachi and T. Kotaka, J . Mol. Liquids, 1987, 36, 7 5 . 22 D. W. Davidson and R. H. Cole, J . Chem. Phys., 1951, 19, 1484. Paper 8/02267D; Received 6th June, 1988

ISSN:0300-9599    

DOI:10.1039/F19898501065

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

J. Chern. SOC., Furuday Trans. I, 1989, 85(5), 1075-1082 Dielectric Relaxation in Concentrated Solutions of cis-Pol yisoprene Part 2.-Motions of Local Segments and Solvent Molecules Keiichiro Adachi," Yasuo Imanishi and Tadao Kotaka Department of Macromolecular Science, Faculty of Science, Osaka University, Toyonaka, Osaka 560, Japan Dielectric relaxations in concentrated solutions of cis-polyisoprene (&-PI) in toluene were studied over a wide temperature range. Four loss maxima were observed in the temperature dependence curves of the loss at a given frequency and termed a,, a,,, /?, and y processes from the high temperature side. The a, and a,, processes were assigned to the normal mode process and the segmental mode processes of cis-PI, respectively. The intensity of the p process decreased with increasing PI concentration and the loss maximum due to this process disappeared in the range above 50 wt % concentration. Thus, the /? process was assigned to the rotation of toluene molecules.The loss maximum of the y process was observed below the glass transition temperature of toluene and its intensity decreased slightly with increasing PI concentration. We assigned the y process to the rotational oscillation of toluene molecules in the glassy matrix. The Arrhenius plots of the a,, a,,, and /I processes conformed to the Vogel-Tamman equation. The ratio of the relaxation times of the a, and a,, processes was approximately independent of temperature. Multi-relaxation phenomena are commonly observed in dielectric and mechanical relaxations of polymers since flexible polymers have a large number of degrees of freedom.' In polymer/diluent systems, additional relaxations due to interactions between the polymer and solvent molecules are expected.Addition of a small amount of a diluent is known to cause a drastic decrease in the glass transition temperature.'v2 This is explained by considering that the solvent molecules provide extra free volumes to the ~ y s t e m . ~ On the other hand, in dilute or semidilute solution^,^ detailed molecular motions or the interactions between the polymer chain and the solvent molecules have not been ~larified.~ Usually, solvent molecules are smeared and regarded as a continuous m e d i ~ m , ~ and the molecular interactions are represented by the friction coefficient as proposed by Debye' as early as in 1929 to explain the dielectric relaxation of simple spherical molecules.In part 1 of this series,' we reported the dielectric normal mode process in concentrated toluene solutions of cis-polyisoprene (cis-PI). In this article, we report the dielectric relaxation processes due to the local segmental motions of cis-PI and the motions of the solvent toluene molecules. Experimental Preparation and characteristics of the cis-PI samples used were reported in part 1.' Measurements of the dielectric constant E' and loss factor E" were also described in part 1. The number in the sample code indicates the weight average molecular weight M , in kg mol-l. 10751076 Motions in cis-Polyisoprene Solutions Fig. 1. Temperature dependence of E‘ and E” for 30.6% solution of PI-24 in toluene.0, 100 Hz; A, 1 kHz; 0, 10 kHz; V, 100 kHz. Results and Discussion Temperature Dependence of Complex Dielectric Constant The representative temperature dependence curves of the dielectric constant E’ and loss factor E” for toluene solutions of PI-24 with the concentration of 30 and 50 wt Yo are shown in fig. 1 and 2, respectively. As is is seen in fig. 1, four loss maxima were observed in the 30% solution, but only three loss maxima were seen in the 50% solution. The same behaviour was observed for solutions of cis-PI with different molecular weight (PI-74 and PI-102). The loss maxima seen in the 30% solution are termed a,, a,,, p, and y from the high temperature side. The lowest temperature process in the 50% solution was termed the /3-y process since the p and y processes appeared to merge in the range above 50 YO.The loss maximum temperatures Tmax at 1 kHz for these processes are plotted in fig. 3. In part 1 of this series of study, the a, and a,, processes are assigned to the normal mode and segmental mode processes, respectively. Since the a, process depended quite strongly on molecular weight, only the T,,, for PI-24 is shown. The other processes were independent of molecular weight indicating that they are due to local motions of cis-PI or motions of the toluene molecules. As observed in the concentrated solutions of polystyrene,’ poly(methy1 metha~rylate)~ and other vinyl polymers,1o* the T,,, for theK. Adachi, Y. Irnanishi and T. Kotaka 1077 T/K Fig.2. Temperature dependence of E' and E" for 50.8 70 solution of PI-24 in toluene. 0, 100 Hz; A, 1 kHz; lJ, 10 kHz. 200 - 5 - e - 150 - 10 0 05 I W Fig. 3. Loss maximum temperature TnaX us. weight fraction M' of cis-PI for solutions of PI-24 (O), PI-74 (A) and PI-I02 (a) in toluene (a). The error of q,,,, is estimated to be less thank3 K.1078 Motions in cis-Polyisoprene Solutions Fig. 4. Arrhenius plots for 30.6 (0) and 50.8 (@) wt % solutions of PI-24. The solid lines indicate eqn (2) with the parameters given in table 1. segmental mode process (aII) changed monotonously with concentration. In the solutions of vinyl polymers, the normal mode process was not observed since they do not have the component of dipole moment aligned in the same direction parallel to the chain contour.The monotonous change in the Tmsx of the a,, process reflects the change in the glass transition temperature, Tp, in the so1utions.8-'o On the other hand, the concentration dependences of the Tmax for the p and y processes are complex in the concentration around 40 to 50 %. The E" peak for the p process becomes very weak in this range and the Tmax for the /I process appears to decrease to merge with the y peak. This suggests that the structure of the solution transforms from the semi-dilute to concentrated state in this concentration range. In fig. 3, the filled square symbols indicate the Tmax for the dielectric primary and secondary relaxations in toluene. Obviously, the values of the Tmax for the p and y processes extrapolated to w = 0 coincide well with the Tmax of toluene. The intensities of these processes decreased with increasing concentration. Therefore, the p and y processes are assigned to the motions of the toluene molecules rather than to cis-PI.In polystyrene/toluene systems,' similar behaviour was found. The y process was assigned to the local mode process due to the motions of both the polymer and solvent molecules from the line width of nuclear magnetic resonance.8 It is not clear whether cis-PI molecules also contribute to the y process in the present system. Arrhenius Plots Arrhenius plots for 30 and 50% solutions of PI-24 are shown in fig. 4, where the loss maximum frequency of E' and E" is plotted against the inverse of the loss maximum temperature. It is seen that the plots for the aI, aI1 and processes are curved.Thus, we attempted to cast these plots to the Vogel-Tamman equation : 1 2 * 1 3 logf, = A - B / ( T - &) (1) where A, B and T, are adjustable parameters and determined as listed in table 1 so that the best fit of the data was achieved. These plots conform well to eqn ( I ) as shown by the solid lines in fig. 4. The Arrhenius plot for the y process conforms to a straight line. From the slope, the activation energy EY was calculated as listed in table 1.K . Adachi, Y. Imanishi and T. Kotaka Table 1. The parameters of the Vogel-Tamman equation A, B and T, and the activation energy E;. for the y process code conc. mode A B T I T ~~~ PI-24 30.6% a, 8.18 510.2 95.4 - 10.22 287.9 117.6 - 11.32 151.4 120.4 - PI-24 50.8% aI 8.44 701.7 94.1 - Y 29.9 a,, 10.00 252.0 135.0 - Y 21.2 1079 a in kJ mol-’.Table 2. Logarithm of the ratio of relaxation times for the aI, aII and p processes in solution of PI-24 W ratio 170K 180K 200K 220K 240K 30.6 % log [ ~ ( a , ) / r ( a J ] 3.38 3.45 3.42 3.32 3.21 1% [r(a,, )/ rW)I 3.53 3.16 2.68 2.38 2.18 50.8 % log [r(a,)/t(a,,)] 3.60 4.13 4.31 4.17 3.97 In the study of dielectric relaxations in bulk cis-PI, we pointed out that the ratio of the relaxation times t for the a, and a,, process was almost independent of temperature. 14,15 We tested this relation for the 30% solution by extrapolating the Arrhenius plots using eqn (1). Table 2 shows the ratio of z for the a, and a,, processes and that for the aII and p processes at several selected temperatures from 170 to 240 K.We see that the ratio of t(a,)/t(a,,) changes only slightly compared with the strong temperature dependence of the relaxation times themselves. This fact indicates that the rate of large scale motions such as the fluctuation of the end-to-end distance is governed by the rate of local segmental motions. In this sense, we may regard the segmental motions as the elementary process for large scale motions of the cis-PI chain. On the other hand, the value of t(a,)/t(P) varied more than ten times in the range from 170 to 220 K. However the zu) extrapolated to 220 K is estimated to include an error ofk0.5 decades, since the extrapolated t value is very sensitive to the parameters of the Vogel equation. Within this error, we may also regard that the rate of the segmental motions is approximately proportional to the mobility of the solvent molecules.The rate of the local jump of the polymer segments is governed by the frequency of collision of the solvent molecules to the segments.16 The Primary all Process In part 1,’ the E” curve for the a,, process in solutions of PI-32 was cast to the Havriliak-Negami equation” and the Havriliak-Negami parameters a and p were determined. It is known that this equation is approximately equivalent to the Fourier-Laplace transform of the empirical correlation function proposed by Williams and Ngai and his coworkers20*21 explained eqn (2) based on the concept that the relaxation time increases with time. Budimir and Skinner explained the correlation1080 Motions in cis-Polyisoprene Solutions Table 3.The half width A of the E” curve for the aII process in solutions of PI-32 concentration (%) width 52 3.6 62 3.3 86 2.7 100 2.5 function by an Ising model incorporating the nearest neighbour dipole-dipole interactions. 22 Since the E” curve for the a,, process overlaps partly with the E” curves for the a, and P processes, the detailed analysis of the full E” curve is difficult. Here, we only discuss the concentration dependence of the half width A for the a I I process. Table 3 shows the A in solutions of PI-32 indicating that A increases with decreasing concentration. This trend is opposite to that of the concentration dependences in solutions of poly- (vinylchloride)lo and poly(methy1 metha~rylate).~~ It seems that the A for the primary relaxation process depends on the specific interactions between the polymer segments and the solvent molecules.We note that the A for the aII process is much broader than that for the a, process which exhibited A of ca. 1.7 decades.’ The a,, process appears to involve the local segmental motions of the variety of sizes. The /3 and y Processes The relaxation strengths AE for the B and y processes are determined from the jumps of the E’ curves as shown in fig. 1 and 2. The unrelaxed value of E’ was estimated by extrapolating the E’ curve at 1 MHz measured in the range from 90 to 120 K as shown by the dotted line D-B in fig. 1. The relaxed E’ curve for the P process was estimated by extrapolating the E’ curve in the range 150 to 160 K. The sum of the relaxation strengths for the p and y processes is taken as the difference of E’ between the points A and B.Similarly, the relaxation strength for the y process is given by the difference of the points C and D. The relaxation strengths multiplied by the temperature Tare plotted in fig. 5. It is seen that the intensity of the /3 process decreases rapidly with the increasing concentration. The intensity of the y process also decreases with increasing concentration in the range below 40 YO. The intensity of the /3-y process for the 50 % solution is slightly higher than that of the y process of 40% solutions. This suggests that the p and y processes merge in the range of polymer concentration higher than 50 YO. From an E’ curve for toluene including 6.1 O h xylene, the relaxation strength for the primary process of toluene at 140 K was estimated to be 0.39.8 From this value, we expect that the relaxation strength of the P-y relaxation for the 50 YO solution is ca.0.20 at 140 K if the motions of the toluene molecules are not restricted. The observed A& for the /&y process was only 0.05 at 130 K, indicating that the motions of toluene in the 50 % solution are restricted. Johari proposed two possibilities for the mechanism of the secondary process of simple molecules in the glassy state : (1) All molecules oscillate within an angle allowed to rotate or (2) only a small number of molecules rotate fully in the glassy matrix and the others do not.24 If case 1 is assumed for the present system, the toluene molecules oscillate with an amplitude of 30”.K.Adachi, Y. Imanishi and T. Kotaka 1081 W fraction N’ dependence of the relaxation strength multiplied p, y , and a-y processes. The error of TAt. is less than Fig. 5. Weight by temperature for the & 2. On the other hand, the process may originate from the motions of toluene similar to the motions in the pure liquid state since the /? process corresponds to the primary process of toluene. Summary 1. Four relaxation processes termed a,, a,,, p and y from the high temperature side were observed in the solutions of cis-PI with concentration less than 50 %. Above 50 YO, the /? process was not observed. 2. The a , process is assigned to the normal mode process; a,,, the segmental mode process; p, the non-restricted rotation of toluene molecules; and y, the restricted motions of the toluene molecules.3. The relaxation times for the a,, a,, and p processes varied almost parallel with temperature. This work was supported in part by the Grant-in-Aid for Scientific Research by the Ministry of Education, Science and Culture (6055062). Support from the Institute of Polymer Research, Osaka University is also gratefully acknowledged. References I N. G. McCrum, B. E. Read and G. Williams, Anelustic and Dielecrric Effects in Polymeric Solids (Wiley, New York 1967). 2 M. C. Shen and A. Eisenberg, Prog. Solid Stute Chem., 1966, 3. 407. 3 J. D. Ferry, Viscoelusric Properries of Polymers (Wiley, New York, 3rd edn, 1980) chap. 17. p. 486. 4 B. H. Zimm, J . Cltem. Phys., 1956, 24, 269. 5 A. M.North and T. G. Parker, Trans. Faraduy Soc., 1971, 67, 2234. 6 P. Debye. Polar Molecules, (New York, 1929). 37 F A R I1082 Motions in cis-Polyisoprene Solutions 7 K. Adachi, Y. Imanishi and T. Kotaka, J. Chem. Soc., Faraday Trans. I, 1989, 85, 1065. 8 K. Adachi, I. Fujihara and Y. Ishida, J. Polym. Sci., Polym. Phys. Ed., 1975, 13, 2155. 9 K. Adachi and T. Kotaka, Polym. J., 1981, 13, 687. 10 K. Adachi and Y. Ishida, J. Polym. Sci., Polym. Phys. Ed., 1976, 14, 2219. I 1 K. Adachi, M. Hattori and Y. Ishida, J. Polym. Sci., Polym. Phys. Ed., 1977, 15, 693. 12 H. Vogel, Phys. Z., 1921, 22, 645. 13 G. Tamman and W. Hesse, Z. Anorg. Allg. Chem., 1926, 156, 245. 14 K. Adachi and T. Kotaka, Macromolecules, 1984, 17, 120. 15 K. Adachi and T. Kotaka, Macromolecules, 1985, 18, 466. 16 M. M. Omar, A. M. North, T. G. Parker and R. A. Pethrick, J. Chem. Soc., Faraday Trans. 2, 1983, 17 S. Havriliak and S. Negami, J. Polym. Sci., Part C, 1966, 14, 99. 18 G. Williams and D. C. Watts, Trans. Faraday Soc., 1970, 66, 80. 19 G. Williams, M. Cook and P. J. Hains, J. Chem. SOC., Faraday Trans. 2, 1972, 68, 1045. 20 K. L. Ngai, Comments Solid State Phys., 1979, 9, 127. 21 K. L. Ngai, R. W. Kendell, A. K. Pajagopal and S. Teiler, Ann. N. Y. Acad. Sci., 1986, 484, 150. 22 J. Budimir and J. L. Skinner, J. Chem. Phys., 1985, 82, 5232. 23 P. J. Phillips and G. Singh, J. Polym. Sci., Polym. Phys. Ed. 1975, 13, 1377. 24 G. P. Johari, J. Chem. Phys., 1982, 77, 4619. 79, 687. Paper 8/02268B; Received 6th June, 1988

ISSN:0300-9599    

DOI:10.1039/F19898501075

出版商:RSC    

年代:1989

数据来源: RSC