摘要:

4369 4377 4387 4397 4407 4417 4427 4439 445 1 4457 447 1 4475 4487 4495 450 1 4509 Con tents A New Form of the High-temperature Isopiestic Technique and its Applica- tion to Mercury-Bismuth, Mercury-Cadmium, Mercury-Gallium, Mercury- Indium and Mercury-Tin Binary Amalgams Z-C. Wang, X-H. Zhang, Y-Z. He and Y-H. Bao The Derivation of Chemical-diffusion Coefficients of Oxygen in UO,,, over the range 180-300 "C. Spectroscopic Procedure and Preliminary Results T. R. Griffiths, H. V. St. Aubyn Hubbard, G. C. Allen and P. A. Tempest Pho tophysics at Solid Surfaces. Evidence of Dimer Formation and Polarization of Monomer and Excimer Fluorescences of Pyrene in the Adsorbed State on Silica-gel Surfaces T. Fujii, E. Shimizu and S. Suzuki Ordering in Monodispersed Polymer Latices induced by a Temperature Gradient K.Furusawa, N. Tobori and S. Hachisu X-Ray Diffraction Study of Molten Eutectic LiF-NaF-KF Mixture K. Igarashi, Y. Okamoto, J. Mochinaga and H. Ohno Viscosity Measurements of Some Tetra butylammonium, Copper( I), Silver( I) and Thallium( 1) Salts in Acetonitrile-Pyridine Mixtures at 15, 25 and 35 "C D. S. Gill and B. Singh The Ethane- 1,2-diol-Water Solvent System. The Dependence of the Dis- sociation Constant of Picric Acid on the Temperature and Composition of the Solvent Mixture Silver(1) Complexation with Tertiary Amines in Toluene M. Soledade Santos, E. F. G. Barbosa and M. Spiro Enhanced Oxygen Evolution through Electrochemical Water Oxidation mediated by Polynuclear Complexes embedded in a Polymer Film G. J. Yao, A. Kira and M. Kaneko Nature of Acid Sites in SAP05 Molecular Sieves.Part 1.-Effects of the Concentration of Incorporated Silicon C. Halik, J. A. Lercher and H. Mayer Hemimicelle Formation of Cationic Surfactants at the Silica Gel-Water Interface T. Gu, Y. Gao and L. He Nuclear Magnetic Resonance Relaxation in Micelles. Deuterium Relaxation at Three Field Strengths of Three Positions on the Alkyl Chain of Sodium Dodecyl Sulphate Studies of the Temperature Dependence of Retention in Supercritical Fluid Chromatography K. D. Bartle, A. A. Clifford, J. P. Kithinji and G. F. Shilstone Hydrogen and Muonium Atom Adducts of Trimethylsilyl Derivatives of Ethyne The Radical Cation of Formaldehyde in a Freon Matrix. An Electron Spin Resonance Study Phase Transition of the Water confined in Porous Glass studied by the Spin- probe Method H.Yoshioka G. C. Franchini, A. Marchetti, L. Tassi and G. Tosi 0. Soderman, G. Carlstrom, U. Olsson and T. C. Wong C. J. Rhodes and M. C. R. Symons C. J. Rhodes and M. C. R. Symons4369 4377 4387 4397 4407 4417 4427 4439 445 1 4457 447 1 4475 4487 4495 450 1 4509 Con tents A New Form of the High-temperature Isopiestic Technique and its Applica- tion to Mercury-Bismuth, Mercury-Cadmium, Mercury-Gallium, Mercury- Indium and Mercury-Tin Binary Amalgams Z-C. Wang, X-H. Zhang, Y-Z. He and Y-H. Bao The Derivation of Chemical-diffusion Coefficients of Oxygen in UO,,, over the range 180-300 "C. Spectroscopic Procedure and Preliminary Results T. R. Griffiths, H. V. St. Aubyn Hubbard, G. C. Allen and P. A. Tempest Pho tophysics at Solid Surfaces.Evidence of Dimer Formation and Polarization of Monomer and Excimer Fluorescences of Pyrene in the Adsorbed State on Silica-gel Surfaces T. Fujii, E. Shimizu and S. Suzuki Ordering in Monodispersed Polymer Latices induced by a Temperature Gradient K. Furusawa, N. Tobori and S. Hachisu X-Ray Diffraction Study of Molten Eutectic LiF-NaF-KF Mixture K. Igarashi, Y. Okamoto, J. Mochinaga and H. Ohno Viscosity Measurements of Some Tetra butylammonium, Copper( I), Silver( I) and Thallium( 1) Salts in Acetonitrile-Pyridine Mixtures at 15, 25 and 35 "C D. S. Gill and B. Singh The Ethane- 1,2-diol-Water Solvent System. The Dependence of the Dis- sociation Constant of Picric Acid on the Temperature and Composition of the Solvent Mixture Silver(1) Complexation with Tertiary Amines in Toluene M.Soledade Santos, E. F. G. Barbosa and M. Spiro Enhanced Oxygen Evolution through Electrochemical Water Oxidation mediated by Polynuclear Complexes embedded in a Polymer Film G. J. Yao, A. Kira and M. Kaneko Nature of Acid Sites in SAP05 Molecular Sieves. Part 1.-Effects of the Concentration of Incorporated Silicon C. Halik, J. A. Lercher and H. Mayer Hemimicelle Formation of Cationic Surfactants at the Silica Gel-Water Interface T. Gu, Y. Gao and L. He Nuclear Magnetic Resonance Relaxation in Micelles. Deuterium Relaxation at Three Field Strengths of Three Positions on the Alkyl Chain of Sodium Dodecyl Sulphate Studies of the Temperature Dependence of Retention in Supercritical Fluid Chromatography K. D. Bartle, A. A. Clifford, J. P. Kithinji and G. F. Shilstone Hydrogen and Muonium Atom Adducts of Trimethylsilyl Derivatives of Ethyne The Radical Cation of Formaldehyde in a Freon Matrix. An Electron Spin Resonance Study Phase Transition of the Water confined in Porous Glass studied by the Spin- probe Method H. Yoshioka G. C. Franchini, A. Marchetti, L. Tassi and G. Tosi 0. Soderman, G. Carlstrom, U. Olsson and T. C. Wong C. J. Rhodes and M. C. R. Symons C. J. Rhodes and M. C. R. Symons

ISSN:0300-9599    

DOI:10.1039/F198884FX009

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

NOMENCLATURE AND SYMBOLISM Units and Symbols. The Symbols Committee of The Royal Society, of which The Royal Society of Chemistry is a participating member, has produced a set of recommendations in a pamphlet 'Quantities, Units, and Symbols' (1 975) (copies of this pamphlet and further details can be obtained from the Manager, Journals, The Royal Society of Chemistry, Burlington House, London W1V OBN). These recommendations are applied by The Royal Society of Cemistry in all its publications. Their basis is the 'Systeme International d'Unit6s' (9). A more detailed treatment of units and symbols with specific application to chemistry is given in the IUPAC Manual of Symbols and Terminology for Physicochemical Quantities and Units (Pergamon, Oxford, 1979). Nomenclature. For many years the Society has actively encouraged the use of standard IUPAC nomenclature and symbolism in its publications as an aid to the accurate and unambiguous communication of chemical information between authors and readers.In order to encourage authors to use IUPAC nomenclature rules when drafting papers, attention is drawn to the following publications in which both the rules themselves and guidance on their use are given: Nomenclature of Organic Chemistry, Sections A, B, C, D, E, F, and H (Pergamon, Oxford, 1979 edn). Nomenclature of Inorganic Chemistry (Butterworths, London, 1971 , now published by Pergamon). Biochemical Nomenclature and Related Documents (The Biochemical Society, London, 1978). Compendium of Chemical Terminology: IUPAC Recommendations (Blackwells, Oxford, 1987).A complete listing of all IUPAC nomenclature publications appears in the January issues of J. Chem. SOC., Faraday Transactions. It is recommended that where there are no IUPAC rules for the naming of particular compounds or authors find difficulty in applying the existing rules, they should seek the advice of the Society's editorial staff. (xiv)NOMENCLATURE AND SYMBOLISM Units and Symbols. The Symbols Committee of The Royal Society, of which The Royal Society of Chemistry is a participating member, has produced a set of recommendations in a pamphlet 'Quantities, Units, and Symbols' (1 975) (copies of this pamphlet and further details can be obtained from the Manager, Journals, The Royal Society of Chemistry, Burlington House, London W1V OBN). These recommendations are applied by The Royal Society of Cemistry in all its publications.Their basis is the 'Systeme International d'Unit6s' (9). A more detailed treatment of units and symbols with specific application to chemistry is given in the IUPAC Manual of Symbols and Terminology for Physicochemical Quantities and Units (Pergamon, Oxford, 1979). Nomenclature. For many years the Society has actively encouraged the use of standard IUPAC nomenclature and symbolism in its publications as an aid to the accurate and unambiguous communication of chemical information between authors and readers. In order to encourage authors to use IUPAC nomenclature rules when drafting papers, attention is drawn to the following publications in which both the rules themselves and guidance on their use are given: Nomenclature of Organic Chemistry, Sections A, B, C, D, E, F, and H (Pergamon, Oxford, 1979 edn). Nomenclature of Inorganic Chemistry (Butterworths, London, 1971 , now published by Pergamon). Biochemical Nomenclature and Related Documents (The Biochemical Society, London, 1978). Compendium of Chemical Terminology: IUPAC Recommendations (Blackwells, Oxford, 1987). A complete listing of all IUPAC nomenclature publications appears in the January issues of J. Chem. SOC., Faraday Transactions. It is recommended that where there are no IUPAC rules for the naming of particular compounds or authors find difficulty in applying the existing rules, they should seek the advice of the Society's editorial staff. (xiv)

ISSN:0300-9599    

DOI:10.1039/F198884BX011

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

ISSN 0300-9599 JCFTAR 84(3) 675-883 (1 988) JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions I Physical Chemistry in Condensed Phases 675 687 703 71 3 729 739 75 1 765 773 785 80 1 815 827 84 1 85 1 23 CONTENTS Interfacial Tensions and Microemulsion Formation in Heptane-Aqueous NaCl Systems containing Aerosol OT and Sodium Dodecyl Sulphate R. Aveyard, B. P. Binks and J. Mead Multicomponent Ion Exchange in Zeolites. Part 3 .-Equilibrium Properties of the SodiumlPotassiumlCadmium-Zeolite X System K. R. Franklin and R. P. Townsend Rotational Relaxation Time and Conformation of Salt-free Sodium Poly- (styrenesulphonate) as studied by the Conductance Stopped-flow Technique T. Okubo A Study in Preferential Solvation using a Solvatochromic Pyridinium Betaine and its Relationship with Reaction Rates in Mixed Solvents J.G. Dawber, J. Ward and R. A. Williams Thermal Decomposition of y-Irradiated Silver Malonate A. K. Galwey, P. J. Herley and M. A- A. Mohamed Monolayer Adsorption of Non-spherical Molecules on Solid Surfaces. Part 2.-The Application of First-order RAM Theory to Nitrogen on Grapkte J. Penar and S. Sokolowski Photoluminescence Properties of MgO Powders with Coordinatively Un- saturated Surface Ions M. Anpo, Y. Yamada, Y. Kubokawa, S. Coluccia, A. Zecchina and M. Che Dehydrogenation of Alcohol on Hydride-forming Rare-earth Intermetallic Compounds (RFe, and R,Co,) H. Imamura, S. Kasahara, T. Takada and S. Tsuchiya Kinetic and Equilibrium Studies at the Solid-Liquid Interface. The Adsorption of Sodium Hexadecyl Sulphate to Polystyrene Latex D.Painter, D. G. Hall and E. Wyn-Jones Domain Complexions in Capillary Condensation. Part 1 .-The Ascending Boundary Curve Domain Complexions in Capillary Condensation. Part 2.-The Descending Boundary Curve and Scanning V. Mayagoitia, B. Gilot, F. Rojas and I. Kornhauser The Kinetics of the Oxidation of Hydrogen Peroxide by Bis(2,2’-bipyri- dine)manganese(m) Ions in Aqueous Perchlorate Media M. P. Heyward and C. F. Wells Absorption, Magnetic Circular Dichroism and Magnetic Circular Polarization of Luminescence Studies of Ru(bpy):+ and Complexes with a Di(ethoxy- carbony1)-substituted Bipyridine Ligand as a Probe of Rigid Environments E. Krausz Complexation of Polymer-bound Imino Diacetate-type Chelating Agents with some Transition-metal Ions. Effect of Charged Polymer Chains on Chelate Formation Reactions Thermal Stabilities of Hexacarbonyl and Subcarbonyls of Molybdenum encapsulated in NaY and NaX Zeolites Y. Okamoto, A. Maezawa, H. Kane, I. Mitsushima and T. Imanaka V. Mayagoitia, F. Rojas and I. Kornhauser Y. Kurimura and K. Takato F A R IContents 865 Hydrogen Bonding. Part 3.-Enthalpies of Transfer from 1 ,1,1 -Trichloroethane to Tetrachloromethane of Phenols, N-Methylpyrrolidinone (NMP) and Phenol-NMP Complexes M. H. Abraham, P. P. Duce, D. V. Prior, R. A. Schulz, J. J. Morris and P. J. Taylor 87 1 Coalescence Stability of Emulsion-sized Droplets at a Planar Oil-Water Interface and the Relationship to Protein Film Surface Rheology E. Dickinson, B. S. Murray and G. Stainsby

ISSN:0300-9599    

DOI:10.1039/F198884FP031

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions II, lssue3,1988 Molecular and Chemical Physics 219 227 239 247 263 273 287 299 31 1 For the benefit of readers of Faraday Transactions I, the contents list of Faraday Transactions II, Issue 3, is reproduced below Accuracy of Diatomic Potential Functions Vibrational Relaxation of C,H, and C,D, by Vibration-Rotation, Translation (V-R, T) Energy Transfer A. Miklavc and I. W. M. Smith Optical Investigation of Short-chain n-Alkanes Z. BIaszczak and P. Gauden Reactivity of the Nitrate Radical towards Alkynes and some other Molecules C. Canosa-Mas, S. J. Smith, S. Toby and R. P. Wayne Temperature Dependences of the Reactions of the Nitrate Radical with some Alkynes and with Ethylene C. Canosa-Mas, S. J. Smith, S. Toby and R. P.Wayne Study of the Reaction Li+H,O over the Temperature Range 850-1000 K by Time-resolved Laser-induced Fluorescence of Li(22P, -2,S,,,) J. M. C. Plane and B. Rajasekar Photogeneration of Hydrogen sensitised by a Water-soluble 9- Oxothioxanthene A Study of the Ground Electronic States of the Isoelectronic Ions PF+ and NC1+ by Vacuum Ultraviolet Photoelectron Spectroscopy V. Butcher, J. M. Dyke, A. E. Lewis, A. Morris and A. Ridha Reviews of Books P. Day; R. Grice; D. M. Hirst; D. Smith; M. N. R. Ash fold J. S. Wright J. Davila, A. Harriman and M. C. Richoux The following papers were accepted for publication in Faraday Transactions I during December, 1987. 7/74 1 7/908 Localized Excess Electrons in Solubilized Water Clusters in Aerosol OT-n- Heptane Solutions The Application of the Volterra Integral Equation to the Mathematical Modelling of Adsorption Kinetics at Constant-volume/Variable- concentration Conditions 7/966 Calculation of N.M.R.van der Waals Chemical Shifts based on a Generalized Polyatomic London Dispersion Theorem J. Homer and M. S. Mohammadi Thermodynamics of the Zinc Sulphide Transformation, Sphalerite-Wurtzite, by Modified Entrainment Pairwise Interaction Parameters for Sodium, Potassium and Halide Ions in Aqueous Solutions A. C. R. Antonini, M. J. Blandamer, J. Burgess, A. W. Hakin and N. D. Hall M. H. Abdel-Kader and P. Krebs M. Kocirik, G. Tschirch, P. Struve and M. Bulow 7/977 7/990 A New Conducting Polymer-coated Glucose Sensor P. C. Pandey 7/998 P. J. Gardner and P. Pang7/ 1000 Pressure Dependence of Oxygen Absorption by Metallic Zirconium with a Clean Surface H.Takeuchi, S. Naito, M. Yamamoto and T. Hashino 7/1127 7/1260 7/ 1599 7/ 1626 7/ 1634 7/ 1692 7/ 1700 7/1711 7/ 1780 7/1781 7/1810 7/ 1820 7/ 1837 7/1841 7/1871 7/ 1988 7/ 1989 The Phase Transfer of 12-Tungstanosilicate Anion across the Water/ Nitrobenzene Interface E. Wang and Y. Liu Electron Spin Resonance of a ?-Irradiated Single Crystal of Carbamylcholine Chloride F. Koksal and F. Celik Deuterium N.M.R. Lineshape Study of the Molecular Dynamics of Benzene sorbed on ZSM-5 and Faujasite Type Zeolites B. Zibrowius, E. Caro and H. Heifer N.M.R. Relaxation Study of I- and Na+ Solvation Structure in N-Methylformamide (NMF) and Preferential Solvation of these Ions in the Mixture NMF-H,O C. K.Finter and H. G. Hertz Selection of Experimental Conditions in Temperature-programmed Reduction Experiments P. Malet and A. Caballero Ammoxidation of 2-Methylpyrazine. Characterisation of Catalyst L. Forni, C. Oliva and C. Rebuscini Calorimetric and Spectrophotometric Studies of Chloro Complexes of Manganese(I1) and Cobalt(r1) Ions in N,N-Dimethylformamide S-I. Ishiguro, K. Ozutsumi and H. Ohtaki Interaction of Water with the Surface of Magnesium Oxide Y. Kuroda, E. Yasugi, H. Aoi, K. Miura and T. Morimoto The Structures and Synergistic Catalyses of the FeRu/Al,O, Catalysts derived from Fe,Ru,-,(CO) (x = 0.1,2,3). Part 1 .-The Structures of Al,O,- supported Fe,Ru,-,(CO),, Clusters K. Asakura and Y. Iwasawa Structures and Synergistic Catalyses of FeRu/ Al,O, Catalysts derived from Fe,Ru,-,(CO),, (x = 0, 1,2,3).Part 2.-Structures and Catalyses of the FeFu Catalysts reduced with H, K. Asakura, Y. Iwasawa and M. Yamada Cr3+ E.S.R. Linewidth in ZnO-ZnCr,O,-Pd Solid Mixtures L. Forni and C. Oliva Influence of Molar Mass, Concentration and Electric Field Strength on the Stationary Value of the Electric Birefringence of Aqueous Solutions of Poly(Styrene Sulphonates) in the Presence of 0.01 mol dm-, NaCl S. S. Wijmenga and M. Mandel Solvation of Thiols: An Infrared and N.M.R. Study of Ethanethiol M. C. R. Symons and G. P. Archer Amino Acids and Peptides. Part 19.-Synthesis of p-1- and 8-2- Adamantylaspartates and their Evaluation for Peptide Synthesis Y. Okada and S. Iguchi CO Adsorption on MgO and CaO: Spectroscopic Investigations of Stages prior to Cyclic Anion Cluster Formation The Influence of Crown Ethers on Cation Migration Processes. Part 6.-The Naphthoquinone Radical Anion N.J. Flint and B. J. Tabner Hydrogen and Muonium Atom Adducts of Trimethylsilyl Derivatives of Ethyne C. J. Rhodes and M. C. R. Symons E. Carrone and A. Zecchina (ii)7/2 133 Homotactic and Heterotactic Interactions in Aqueous Solutions containing some Saccharides. Experimental Results and an Empirical Relationship between Saccharide Solvation and Solute-Solute Interactions S. H. Gaffney, E. Haslam, T. H. Lilley and T. R. Ward Interaction of Divalent Catons with Polyuronates A. Cesaro, F. Delben and S. Paoletti 7/2 134 (iii)Cumulative Author Index 1988 Abe, H., 511 Abraham, M. H., 175, 865 Allen, G.C., 165, 355 Anazawa, I., 275 Anpo, M., 751 Aracil, J., 539 Aveyard, R., 675 Baba, K., 459 Baglioni, P., 467 Barna, T., 229 Bazsa, G., 215, 229 Berei, K., 367 Berroa de Ponce, H., 255 Binks, B. P., 675 Blesa, M. A., 9 Borgarello, E., 261 Breen, J., 293 Brown, M. E., 57 Brydson, R., 617, 631 Busca, G., 237 Caceres, M., 539 Carbone, A. I., 207 Cavani, F., 237 Cavasino, F. P., 207 Centi, G., 237 Chandra, H., 609 Che, M., 751 Clarke, R. J., 365 Clint, J. H., 675 Coates, J. H., 365 Coluccia, S., 751 Compton, R. G., 473, 483 Danil de Namor, A. F., 255 Dash, A. C., 75 Dash, N., 75 Davydov, A., 37 Dawber, J. C., 41 Dawber, J. G., 41, 713 de Bleijser, J., 293 Diaz Peiia, M., 539 Dickinson, E., 871 Disdier, J., 261 Domen, K., 511 Dougal, J. C., 657 Duarte, M. Y., 97, 367 DUE, P.P., 865 Egawa, C., 321 Engel, W., 617, 631 Eszterle, M., 575 FernPndez-Pineda, C., 647 FIanagan, T. B., 459 Foresti, E., 237 Foresti, M. L., 97 Forster, H., 491 Franklin, K. R., 687 Gabrail, S., 41 Galwey, A. K., 57, 729 Gans, P., 657 Geblewicz, G., 561 Gill, J. B., 657 Gilot, B., 801 Gopalakrishnan, R., 365 Grampp, G., 366 Gratzel, M., 197 Green, S. I. E., 41 Guarini, G. G. T., 331 Guidelli, R., 97, 367 Hadjiivanov, K., 37 Hall, D. G., 773 Harrer, W., 366 Hasebe, T., 187 Hashimoto, K., 87 Heatley, F., 343 Herley, P. J., 729 Herrmann, J-M., 261 Heyward, M. P., 815 Hidalgo, M. del V., 9 Hill, A., 255 Huis, D., 293 Ige, J., 1 Ikeda, S., 151 Imamura, H., 765 Imanaka, T., 851 Iwasawa, Y., 321 Jaenicke, W., 366 Johnson, G. R. A,, 501 Johnson, I., 551 Johnston, C., 309 Jorgensen, N., 309 Kane, H., 851 Kanno, T., 281 Kasahara, S., 765 Kato, S., 151 Katz, N.E., 9 Keeble, D. J., 609 Kevan, L., 467 Kirby, C., 355 Kiricsi, I., 491 Kiss, I., 367 Klissurski, D., 37 Kobayashi, M., 281 Kondo, J., 511 Kondo, Y., 1 1 1 Konishi, Y., 281 Kornhauser, I., 785, 801 Krausz, E., 827 Kubokawa, Y., 75f Kurimura, Y., 841 Kusabayashi, S., 11 1 tajtar, L., 19 Lambi, J. N., 1 Lawrence, K. G., 175 Leaist, D. G., 581 Lengyel, I., 229 Leyendekkers, J. V., 397 Leyte, J. C., 293 Lincoln, S. F., 365 Lindner, Th., 631 Maezawa, A., 851 Malanga, C., 97 Marcus, Y., 175 Maroto, A. J. G., 9 Maruya, K., 511 Mason, D., 473, 483 Matsumura, Y., 87 Mayagoitia, V., 785, 801 McAleer, J. F., 441 McMurray, N., 379 Mead, J., 675 Mensch, C. T. J., 65 Mills, A., 379 Mirti, P., 29 Mitsushima, I., 851 Mohamed, M.A-A,, 57, 72! Morris, J. J., 865 Morton, J. R., 413 Moseley, P. T., 441 Muhler, M., 631 Murray, B. S., 871 Nakamura, Y., I I1 Nakao, N., 665 Nakayama, N., 665 Narayanan, S., 521 Nazhat, N. B., 501 Nishihara, C., 433 Nishikawa, S., 665 Nomura, H., 151 Norris, J. 0. W., 441 Noszticzius, Z., 575 Nucci, L., 97 Ohtani, S., 187 Okamoto, Y., 851 Okubo, T., 703 Olofsson, G., 551 Onishi, T., 511 Oosawa, Y., 197 Painter, D., 773 Pelizzetti, E., 261 Penar, J., 739 Pezzatini, G., 367 Piccini, S., 331 Pichat, P., 261 Piekarski, H., 529, 591 P6ta, G., 215 Preston, K. F., 413 Prior, D. V., 865 Rajaram, R. R., 391 Renuncio, J. A. R., 539 Rochester, C. H., 309 Rojas, F., 785, 801 Rubio, R. G., 539AUTHOR INDEX Saadalla-Nazhat, R.A., 501 Saito, Y., 275 Sakamoto, Y., 459 Sakata, Y., 511 Sato, T., 275 Sauer, H., 617 Sawabe, K., 321 Sayari, A., 413 Sbriziolo, C., 207 Schelly, Z. A., 575 Schiffrin, D. J., 561 Schiller, R. L., 365 Schlogl, R., 631 Schulz, R. A., 865 Sellers, R. M., 355 Sermon, P. A., 391 Serpone, N., 261 Shindo, H., 433 SokoUowski, S., 739 Sokolowski, S., 19 Somsen, G., 529 Soriyan, 0. O., 1 Stainsby, G., 871 Stevens, J. C. H., 165 Stone, W. E. E., 117 Symons, M. C. R., 609 Takada, T., 765 Takato, K., 841 Tanaka, K., 601 Tanaka, K-i., 601 Taylor, P. J., 865 Thomas, J. M., 617, 631 Tofield, B. C., 441 Torres-Sanchez, R-M., 117 Townsend, R. P., 687 Trifiro, F., 237 Tsuchiya, S., 765 Uematsu, R., 11 1 Uma, K., 521 Unwin, P. R., 473, 483 van Veen, J. A. R., 65 van Wingerden, R., 65 Vasaros, L., 367 Vazquez-Gonzalez, M.I., 647 Viguria, E. C., 255 Vink, H., 133 Viswanathan, B., 365 Walker, R. A. C., 255 Ward, J., 713 Wells, C. F., 815 Williams, B. G., 617, 631 Williams, D. E., 441 Williams, R. A., 713 Wyn-Jones, E., 773 Yamada, Y., 751 Yoshida, S., 87 Zecchina, A., 751 Zeitler, E., 617, 631 Zelano, V., 29 Zielinski, R., 151NOMENCLATURE AND SYMBOLISM Units and Symbols. The Symbols Committee of The Royal Society, of which The Royal Society of Chemistry is a participating member, has produced a set of recommendations in a pamphlet 'Quantities, Units, and Symbols' (1975) (copies of this pamphlet and further details can be obtained from the Manager, Journals, The Royal Society of Chemistry, Burlington House, London WIV OBN). These recommendations are applied by The Royal Society of Cemistry in all its publications. Their basis is the 'Systeme International d'Unites' (SI).A more detailed treatment of units and symbols with specific application to chemistry is given in the IUPAC Manual of Symbols and Terminology for Physicochemical Quantities and Units (Pergamon, Oxford, 1979). Nomenclature. For many years the Society has actively encouraged the use of standard IUPAC nomenclature and symbolism in its publications as an aid to the accurate and unambiguous communication of chemical information between authors and readers. In order to encourage authors to use IUPAC nomenclature rules when drafting papers, attention is drawn to the following publications in which both the rules themselves and guidance on their use aregiven: (Pergamon, Oxford, 1979 edn).published by Pergamon). Society, London, 1978). (Blackwells, Oxford, 1987). Nomenclature of Organic Chemistry, Sections A, B, Nomenclature of Inorganic Chemistry (Buttentvorths, Biochemical Nomenclature and Related Documents Compendium of Chemical Terminology: IUPAC C, D, E, F, and H .ondon, 1971, now (The Biochemical Recommendations A complete listing of all IUPAC nomenclature publications appears in the January issues of J. Chem. SOC., Faraday Transactions. It is recommended that where there are no IUPAC rules for the naming of particular compounds or authors find difficulty in applying the existing rules, they should seek the advice of the Society's editorial staff.THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION No.85 Solvation University of Durham, 28-30 March 1988 Organising Committee: Professor M. C. R. Symons (Chairman) Professor J. S. Rowlinson Professor A. K. Covington Dr I. R. McDonald Dr J. Yarwood Dr A. D. Pethybridge Professor W. A. P. Luck Dr D. A. Young The purpose of the Discussion is to compare solvation of ionic and non-ionic species in the gas phase and in matrices with corresponding solvation in the bulk liquid phase. The aim will be to confront theory with experiment and to consider the application of these concepts to relaxation and solvolytic processes. Topics to be covered are: clusters, (c) Gas phase ionic clusters, (d) Liquid phase ionic solutions, (e) Dynamic processes including solvolysis. (a) Gas phase non-ionic clusters, (b) Liquid phase non-ionic Speakers include: H.L. Friedman, B. J. Howard, M. J. Henchman, S. Tomoda, 0. Kajimoto, M. H. Abraham, Yu Ya Efimov, J. L. Finney, P. Suppan, J. P. Devlin, D. W. James, G. W. Neilson, T. Clark, M. L. Klein, J. T. Hynes, G. A. Kenney-Wallace, G. R. Fleming, M. ,I. Blandamer and D. Chandler. The final programme and application form may be obtained from: Mr. 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. 86 Spectroscopy at Low Temperatures University of Exeter, 13-15 September 1988 Organising Committee: Professor A. C. Legon (Chairman) Dr P. B. Davies Dr B. J. Howard Dr P. R. R. Langridge-Smith Dr R. N. Perutz Dr M.Poliakoff The Discussion will focus on recent developments in spectroscopy of transient species (ions, radicals, clusters and complexes) in matrices or free jet expansions. The aim of the meeting is to bring together scientists interested in similar problems but viewed from the perspective of different environments. The Introductory Lecture will be given by G. C. Pimentel and speakers include: L. Andrews, K. H. Bowen, B. J. Howard, L. B. Knight Jr, E. Knotinger, D. H. Levy, J. P. Maier, J. Michl, M. Moskovits, A. J. Stace, M. Takami, J. J. Turner, M. Poliakoff, A. J. Barnes, J. M. Hollas, M. C. R. Symons and P. Suppan. The preliminary programme may be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBNTHE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY WITH THE ASSOCIAZIONE ITALIANA D I CHIMICA FISICA, DIVISION DE CHlMlE PHYSIQUE OF THE SOCICTC FRANCAISE DE CHlMlE AND DEUTSCHE BUNSEN GESELLSCHAFT FUR PHYSIKALISCHE CHEMIE JOINT MEETING Structure and Reactivity of Surfaces Centro Congressi, Trieste, Italy, 13-16 September 1988 Organising Committee: M. Che V.Ponec F. S. Stone G. Ertl R. Rosei A. Zecchina The conference will cover surface reactivity and characterization by physical methods: (i) Metals (both in single crystal and dispersed form) (ii) Insulators and semiconductors (oxides, sulphides, halides, both in single crystal and dispersed forms) (iii) Mixed systems (with special emphasis on metal-support interaction) The meeting aims to stimulate the comparison between the surface properties of dispersed and supported solids and the properties of single crystals, as well as the comparison and the joint use of chemical and physical methods.Further information may be obtained from: Professor C. Morterra, lnstituto di Chimica Fisica, Corso Massimo DAzeglio 48, 101 25 Torino, Italy. ~ ~~~ THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM Orientation and Polarization Effects in Reactive Collisions To be held at the Physikzentrum, Bad Honnef, West Germany, 12-14 December 1988 Organising Committee: Dr S. Stolte Professor J. P. Simons Dr K. Burnett Dr H. Loesch Professor R. N. Dixon Professor R. A. Levine The Symposium will focus on the study of vector properties in reaction dynamics and photodissociation rather than the more traditional scalar quantities such as energy disposal, integral cross-sections and branching ratios.Experimental and theoretical advances have now reached the stage where studies of Dynamical Stereochemistry can begin to map the anisotropy of chemical interactions. The Symposium will provide an impetus to the development of 3-D theories of reaction dynamics and assess the quality and scope of the experiments that are providing this impetus. Contributions for consideration by the Organising Committee are invited in the following areas: (A) Collisions of oriented or rotationally aligned molecular reagents (6) Collisions of orbitally aligned atomic reagents (C) Photoinitiated 'collisions' in van der Waals complexes (D) Polarisation of the products of full and half-collisional processes The preliminary programme may be obtained from: Mrs Y.A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN. (Viii)THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION No. 87 Catalysis by Well Characterised Materials University of Liverpool, 11-13 April 1989 Organising Committee: Professor R. W. Joyner (Chairman) Professor A. K. Cheetham Professor F. S. Stone The understanding of heterogeneous catalysis is an important academic activity, which compliments industry's continuing search for novel and more efficient catalytic processes. The emergence of relevant, in particular in situ techniques and new developments of well established experimental approaches to catalyst characterisation are making a very significant impact on our knowledge of catalyst composition, structure, morphology and their inter-relationships.Well characterised catalysts, which will be the subject of the Faraday Discussion, include single-cwstal surfaces, whether of metals, oxides or sulphides; crystalline microporous solids, such as zeolites and clays, and appropriate industrial catalysts. The elucidation of structure/function relationships and catalytic mechanism will be important aspects of the scientific programme. Contributions describing novel methods for synthesising well characterised catalysts and also reporting important advances in characterisation techniques will also be welcome. Contributions for consideration by the Organising Committee are invited and abstracts of about 300 words should be sent by 31 May 1988 to: Professor R. W.Joyner, Leverhulme Centre for Innovative Catalysis, Department of Inorganic, Physical and Industrial Chemistry, University of Liverpool, Grove Street, P. 0. Box 147, Liverpool L69 3BX. Full papers for publication in the Discussion volume will be required by December 1988. Dr. K. C. Waugh Professor P. 6. Wells~ ~~~~~ JOURNAL OF CHEMICAL RESEARCH Papers dealing with physical chemistry/chemical physics which appear currently in J. Chem. Research, The Royal Society of Chemistry's synopsis + microform journal, include the following : An E.s.r. Study of the Radiolysis of Acetylenic Acids and Esters in a Freon Matrix J. Rhodes and Martyn C. R. Symons (1 988, Issue 1 ) Imbibition of Sodium Nitrate by Zeolite Na-Y at 25 "C and Christopher Kevin R.Franklin, Barrie M. Lowe Gordon H. Walters (1988, Issue 1) The Solubility of Carbon Dioxide in Mixtures of Water and Acetone Robert W. Cargill, Donald E. MacPhee and Kenneth Patrick (1988, Issue 1) Correlation Analysis of the Reactivity in the Oxidation of Aromatic Aldehydes by An E.s.r. Study of Azoalkane Radical Cations N-Bromoacetamide Louwrier (1988, Issue 1 ) Anita Gupta, Sandhya Mathur and Kalyan K. Banerji (1988, Issue 1 ) Christopher J. Rhodes and Pieter W. F. Electrochemical Studies of some Nickel(I1) Complexes of the Type [Ni(NNS)(Heterocycle)'] and [Ni2(NNS)~-(Heterocycle)-][Cl04] (1988, Issue 1) Sanat K. Mandal, Parimal Paul and Kamalaksha Nag influence of the Acid-strength Distribution of the Zeolite Catalyst on the t-Butylation of Phenol Avelino Corma, Hermenegildo Garcia and Jaime Primo (1 988, Issue 1 ) The Effect of Nitric Oxide on the Kinetics of Decomposition of Thionitrites Garley and Geoffrey Stedman (1 988, Issue 2) Michael S.Kinetics of the Solvolysis of Chlorapenta-aminecobalt(lll) Ions in Water and in Water - Propan-2-01 Mixtures Kamal H. Halawani and Cecil F. Wells (1988, Issue 2) Evaluation of Broyden - Fletcher - Goldfarb - Shanno (BFGS) Variable Metric Method in Geometry Optimisation using Semi-empirical SCF-MO Procedures Dimitris K. Agrafiotis and Henry S. Rzepa (1988, Issue 3)FARADAY DIVISION INFORMAL AND GROUP MEETINGS ~~~ ~ Molecular Beams Group Beam-Photon Interactions To be held at University of Durham on 24-25 March 1988 Further information from Dr J.C. Whitehead, Department of Chemistry, University of Manchester, Manchester M13 9PL ~~~ ~ Polar Solids Group Computer Simulation of Defects in Polar Solids To be held at Mansfield College, Oxford on 28-30 March 1988 Further information from Professor C. R. A. Catlow, Department of Chemistry, University of Keele, Keele, Staffs ST5 4BG Colloid and interface Science Group with BRSG N.M.R. in Colloid and Interface Science To be held at the University of Bristol on 6-8 April 1988 Further information from Dr T. Cosgrove, School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS Polymer Ph ysics Group with the Plastics and Rubber lnstitue Deformation, Yield and Fracture To be held in Cambridge on 11-14 April 1988 Further information from Dr M.J. Richardson, National Physical Laboratory, Teddington, Middlesex Tw11 OLW Annual Congress: Division with Electrochemistry Group Solid State Materials in Electrochemistry To be held at the University of Kent, Canterbury on 12-1 5 April 1988 Further information from Dr J. F. Gibson, Royal Society of Chemistry, Burlington House, London W1V OBN Neutron Scattering Group Vibrational Spectroscopy To be held at Imperial College, London on 20-21 April 1988 Further information from Dr J. Howard, ICI plc, New Science Group N129, PO Box 90, Wilton, Middlesbroun h Electrochemistry Group with The Society of Chemical Industry Electrolytic Bubbles To be held at Imperial College, London on 31 May 1988 Further information from Professor W. J. Albery, Department of Chemistry, Imperial College of Science and Technology, South Kensington, London SW7 2AY Electrochemistry Group with The Society of Chemical Industry Chlorine Symposium To be held at the Tara Hotel, London on 1-3 June 1988 Further information from Dr'S. P.Tyfield, Central Electricity Generating Board, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire GL13 9BP Colloid and Interface Science Group with the Biochemical Society Dynamic Properties of Biomolecular Assemblies To be held at the University of Nottingham on 20-22 July 1988 Further information from Dr S. E. Harding, School of Agriculture, Unversity of Nottingham, Department of Applied Biochemistry, Sutton Bonington LE12 5RD Gas Kinetics Group Xth International Symposium on Gas Kinetics To be held at University College, Swansea on 24-29 July 1988 Further information from Dr G.Hancock, Physical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ Neutron Scattering Group Postgraduate Informal Neutron Conference To be held at the University of Keele on 25-27 July 1988 Further information from Professor C. R. A. Catlow, Department of Chemistry, University of Keele, Keele, Staffs ST5 5BGElectrochemistry Group with the Electroanalytical Group and the Society of Chemcial Industry Electrochemcial Dynamics To be held at the University of Strathclyde on 5-10 September 1988 Further information from Dr S. P. Tyfield, CEGB, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire GL13 9PB Statistical Mechanics and Thermodynamics Group Dense Fluids To be held at the University of Cambridge on 14-16 September 1988 Further information from Dr P. Francis, Department of Chemistry, University of Hull, Hull HU6 7RX Carbon Group with the Carbon and Graphite Group of the SCI Carbon 88 To be held at the University of Newcastle upon Tyne on 18-23 September 1988 Further information from The Conference Secretariat, Carbon 88, Society of Chemical Industry, 14/15 Belgrave Square, London SWlX 8PS Division Autumn Meeting: Polymerisation and Polymer Behaviour To be held at the University of Birmingham on 20-22 September 1988 Further information from Professor 1. W. M. Smith, Department of Chemistry, University of Birmingham, PO Box 363, Birmingham B15 21T ~~ Colloid and Interface Science Group Structure in Colloidal Systems and its Characterisation To be held at the University of Bath on 21-23 September 1988 Further information from Dr R. Buscall, ICI plc, Corporate and Colloid Science Group, PO Box 11, The Heath, Runcorn WA7 4QE (xii)

ISSN:0300-9599    

DOI:10.1039/F198884BP033

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. I , 1988, 84(3), 675-686 Interfacial Tensions and Microemulsion Formation in Heptane-Aqueous NaCl Systems containing Aerosof OT and Sodium Dodecyl Sulphate Robert Aveyard,” Bernard P. Binks? and Jeremy Mead$ Chemistry Department, University of Hull, Hull HU6 7RX John H. Clint B.P. Research Centre, Chertsey Road, Sunbury on Thames TWI6 7LN The tension, y,, between alkane and aqueous NaCl in systems containing an anionic surfactant (Aerosol OT or sodium dodecyl sulphate, SDS) at the aggregation point can be made ultralow (ca. 1 pN m-l) by adjustment of the salt concentration or the concentration of an added ‘cosurfactant ’ (e.g. a long-chain alkanol). For the twin-tailed surfactant (AOT) an ultralow minimum in 7, can be obtained in the absence of the cosurfactant, whereas the presence of the latter is required in systems containing SDS, which has only a single alkyl chain.Here we discuss how the tensions y, vary in the heptane-aqueous NaCl system containing mixtures of AOT and SDS (in the absence of cosurfactant), and consider the nature of the microemulsions which are formed in these systems. All the effects can be broadly understood in terms of well established ideas on the different molecular geometry of the two surfactants and the way in which monolayers tend to curve. The constant interfacial tension, y,, attained at the critical micelle concentration (c.m.c.) in the heptane-aqueous NaCl system containing diethylhexyl sodium sulphosuccinate (AOT), can be made to pass through an ultralow minimum as the salt concentration, m,, is varied.(1) It is suggested that the minimum tension is attained when the effective cross-sectional areas ah, of the hydrophilic head group (which depends on mJ, and a,, of the hydrophobic chain, become equal. For sodium dodecyl sulphate (SDS), however, the head group is always larger than the single alkyl chain, and therefore no such effects are observed. (2) In this paper we explore the way in which the packing of the two surfactants, AOT and SDS, in mixed adsorbed films affects the interfacial tensions, yc. The shape of surfactant molecules also has a profound effect on the formation of microemulsion phases. Under conditions where ah > a, two-phase systems consisting of an oil-in-water (o/w) microemulsion in equilibrium with an excess of the dispersed (oil) phase can be formed.Water-in-oil (w/o) microemulsions in equilibrium with an excess of the aqueous phase are found at higher salt concentrations, where a,, < a,. The droplet size in such two-phase systems is in large measure dictated by the ‘natural’ curvature favoured by the surfactant film. (3) By combination of two different surfactants we show here that both interfacial tensions and microemulsion droplet type and size can be altered in accordance with these simple principles. t Present address: $ Present address: Cedex 05, France. Laboratoire de Spectroscopie Hertzienne de l’E.N.S., 24, rue Lhomond, 7523 1 Paris Unilever Research, Colworth Laboratory, Bedford MK44 1 LQ. 675676 Systems containing AOT+ SDS Experimental Materials The heptane, AOT, SDS, water and sodium chloride used here have all been described previous1y.l.Methods Interfacial tensions were measured using a Kruss spinning-drop tensiometer (for tensions between and 20 mN m-l) and a Kruss K10 automatic tensiometer with du Nouy ring attachment (for tensions between 3 and 72 mN m-'). The light-scattering studies of the microemulsion phases were performed using a Malvern Instruments PCS 100 spectrometer and K7027 correlator. The apparatus was arranged to perform homodyne experiments. The delay times used were in the region 1-5ps, diffusion coefficients were typically lo-' cm2 s-l, and the wavelength of laser light (He-Ne) was 633 nm. The delay times were such that the autocorrelation function was determined over a time comparable with that for a droplet to diffuse over one droplet diameter.In order to obtain droplet radii, apparent diffusion coefficients D (weighted averages of self- and collective diffusion coefficients) were extrapolated to infinite dilution of droplets. Phase boundaries between single-phase microemulsions and systems in which the. microemulsion exists in equilibrium with a second phase were determined by titration. For example, a known volume of a solution of AOT in heptane was placed in a vessel thermostatted at 25 "C. A certain volume of aqueous NaCl was added under stirred conditions until the system formed two phases. This two-phase system was then titrated with SDS in aqueous NaCl until a one-phase microemulsion resulted. The composition at the w/o + (w/o +excess aqueous NaCl) transition was therefore known.The titration was then continued until a second phase (which was viscous with a bluish tinge) appeared, thus identifying the second phase boundary, and the limits of existence of the single-phase microemulsion. The nature of the viscous phase was not investigated further. Results and Discussion Interfacial Tensions We have carried out two kinds of experiment. In one case we have taken AOT at a fixed concentration, m,, close to and above its c.m.c. in systems containing heptane and aqueous NaCl and progressively increased the SDS concentration, m,. In a second set of experiments we have taken various fixed mole fractions, x = [m,/(m, + m,)], of SDS and increased the total concentration, m, = (m,+m,) from below to above the (mixed surfactant) c.m.c.We now discuss the two types of experiment, and the information they yield, separately. (a) Variation of x at Fixed m, In previous we have investigated the effects on yc of progressively adding cosurfactants (e.g. octanol and decanol), dissolved in the alkane, to heptane-aqueous NaCl systems containing AOT or SDS above the aggregation point. In these systems the cosurfactant concentrations required to produce minima in tensions are very high compared to the surfactant concentration, which is close to the c.m.c. Thus there is a cosurfactant ' reservoir ' present and we may assume that the cosurfactant activity is effectively unchanged when (the small amount of) cosurfactant is incorporated into the surfactant aggregates.As will be seen, very much smaller amounts of SDS than of cosurfactant are required to give tension minima in systems containing AOT, so that theR. Aveyard, B. P . Binks, J . Mead and J . H. Clint 677 " n ' I E z E - 2 r-" -7 W DD 3 4 0 n I " E E r-" z ' 4 s . W 2 I 3 0 0.10 0.20 0.30 0.40 X 103a Fig. 1. Effects of SDS and dodecanol on y, in systems containing AOT, heptane and aqueous NaCl. (a) Effects of x on y, at 25 "C. a, m, = 0.04 mol dm-3; and m2 = 1.5 x mol dm-3; 0 m, = 0.10 mol dm-3 and m2 = 4.59 x mol dmP3; 0, the same systems but pre-equilibrated before putting in spinning-drop tensiometer. (b) Effects of mole fraction activity, a, of dodecan-1-01 in heptane on yc at 30 "C. a, rn, = 0.017 mol dm-3; 0, m, = 0.103 mol dm-3 [see ref. (4)]. activity of SDS is unknown and the thermodynamic analysis produced for the effects of cosurfactants4 is inappropriate.Nonetheless the experiments are informative. If, as discussed, minimum y, is produced when the mean head and tail areas of surfactants in a mixed film are equal, then if we add SDS (for which in a pure film ah is always greater than a,) to a film containing AOT at low salt concentration, rn, (where in a pure AOT film ah > a,) we expect no amelioration of surfactant packing and hence no fall in y,. On the other hand if we increase rn, to a value greater than that, m,*, (0.05 mol dmP3) required to give minimum y, in a system containing AOT alone, then for AOT a, > ah and progressive addition of SDS is expected to result in a minimum in yc. In fig.1 (a) are shown results of such experiments; for both values of m,, m, is ca. twice the c.m.c. of AOT in the absence of SDS. For rn, = 0.04 mol dm-3 (i.e. nz, < m,*) addition of SDS causes only an increase in y,. For m, = 0.10 rnol dm-3 (m, > m,*) addition of SDS gives a minimum in y,. It is interesting to compare these results with the effects which addition of dodecanol has in similar system^.^ For the alkanol ah < a, (the opposite to SDS); accordingly, as seen in fig. 1 (b), addition of dodecanol when m, (0.017 mol dm-3) < m,* produces a minimum in y,, whereas for m, (0.103 mol dm-3) > m,* no such minimum is observed. (b) Variation of m, at Fixed x We consider here the variation of y at the c.m.c. (y,) with the mole fraction x of SDS in the mixtures of AOT and SDS; the salt concentration is 0.10 mol dm-3 in all cases (i.e.> m,* for AOT).? Sample plots of y against lnm,, each for a constant x, are shown in fig. 2(a). In addition to the pure surfactants (x = 0 and 1) we have studied systems for which x is 0.202,0.499,0.756,0.857 and 0.958. In the mixed surfactant systems y falls linearly with lnm, at low m, and then rises. We take the c.m.c. to be the value of rn, at the end of the linear part of the y vs. lnm, curves. For one of the mole fractions studied t These experiments are similar to those often performed on mixed aqueous surfactants in the absence of an oil phase [see ref. (5)], where the aggregates formed are normal mixed micelles. In the present systems the aggregates are microemulsion droplets, and these can be present in either the aqueous or the oil phase, depending on conditions.678 20 15 - I E z 10 E 1 * 5 0 Systems containing A 0 T + SDS -In (mJmol dm-9 10 5 -In (m,/mol dm-3) Fig.2. Variation of y with total surfactant concentration, m,, in systems containing heptane, 0.10 mol aqueous NaCl, AOT and SDS at 25 "C. (a) y us. lnm, for x = 0 (0), 0.756 (a), 0.958 (e), and 1 (0). (b) Variation of log y with lnm, for x = 0.202. Inset shows plot of y us. lnm, for same system. (x = 0.202) the tension becomes ultralow (ca. mN m-l) above the c.m.c. [fig. 2(b)J, but at the c.m.c. (as defined above) the tension is ca. 0.06 mN m-l; In (c.m.c.), obtained from the inset plot, is - 8.25, whereas the minimum tension occurs at In rn, = - 7.2. The minima in the ( y us.In m,) curves presumably result from changes in surface and micellar compositions as m, is increased above the aggregation point. We may write for changes in yc with x The Gibbs equation, written for constant T and P, is -dy = RTC r,dlna, i where a, are activities and Ti are taken to be surface excesses. The components present are water, oil, Na+, C1- and the surfactant anions of SDS (molarity m,) and AOT (molarity m,). In the presence of a constant swamping excess of NaCl we may assume that dlna,,+ = dlna,,- = 0. Further, since below the c.m.c. the surfactants and salt are present only in the aqueous phase we may also set dlna,,, = 0. The mean ionic activity coefficients of the surfactants are not expected to change significantly in the presence of swamping electrolyte, so dln a for the surfactants may be set equal to dln m.Finally, we choose a Gibbs dividing surface such that the surface excess of water is zero, so that we may write eqn (2) as - dy = RT(T, dln m, + T2 dln m,) (3) which for constant x yields (ay/aln mJZ = - RT(T, + r,) . It is also readily shown that (WWnt = RTK2/(1 - x) - w-4 (4)R. Aueyard, B. P . Binks, J. Mead and J. H. Clint 679 5 4 3 - I E z * 52 1 0 0 0.5 X 0 Fig. 3. Variation of y, with x in the system heptane-O.l mol dm-3 NaCl at 25 "C. Table 1. Variation of Tt, c.m.c., y, and x" with x in systems containing heptane, 0.1 mol dm-3 NaCl, AOT and SDS at 25 *Ca X 1 /(I?' + r,)/nm2 molecule-' dln (c.m.c.)/dx dy,/dx x" 0 0.202 0.499 0.756 0.857 0.958 1 .o 0.75 0.74 0.75 0.74 0.67 0.59 0.46 0 - - 0.82 0 0.07 1.29 1.3 0.12 1.93 4.0 0.27 2.25 7.8 0.43 2.63 17.6 0.75 - 1 .o - a c.m.c.in mol dm-3, y, in mN m-'. Combination of eqn (l), (4) and (5) then leads to f 1 -xu dln(c.m.c.) dy, dx = - RT(T, + r,) [(x-ir;). dx ]* Eqn (6) allows us to calculate the surface mole fractions of the surfactants xu = [rJ (r, + r,)] from a knowledge of dy,/dx, (r, + r,) and dln (c.m.c.)/dx, values of which may all be obtained from data of the kind shown in fig. 2. We show the variation of y, with x in the heptane-O.l mol dm-3 NaCl system at 25 "C in fig. 3. There is a shallow minimum in the region of x = 0.2, as might be expected from the data in fig. 1 (b); values of dy,/dx obtained graphically are given in table 1. Values of 1 /(r, + r,), also given in table 1, have been calculated from data of the kind presented in fig.2(a) and application of eqn (4); the values of the c.m.c. for various values of x have also been obtained from the same data and are described by (7) In (c.m.c./mol drne3) = 0.39 exp (1 S4x) - 8.76. The surface mole fractions x" of SDS, obtained by use of eqn (6) and listed in table 1, are680 Systems containing A 0 T + SDS 0 0.5 X U 0 Fig. 4. Variation of l/(rl + r,) with surface mole fraction, x, of SDS in films containing AOT and SDS at 25 “C. always lower than the mole fractions in bulk. If the two surfactants occupied the same areas in mixed and single monolayers we expect that 1/(r1+r2) = YA;+(I-X“)A; (8) where A” are areas per molecule in the saturated pure films. It is clear from fig.4, where 1 /(I?, + I?,) is plotted against xu, that 1 /(r1 + r,) exceeds the value obtained from eqn (8) for all xu. This is perhaps a surprising result if one accepts the simple ideas concerning head- and tail-group packing discussed earlier, from which one would expect the opposite to be the case. It appears therefore that the values of ah and a, operative in mixed monolayers differ from (are greater than) those in pure films. This could be a result, for example, of enhanced solvation of head and/or tail groups in mixed monola yers. The droplet composition (with respect to the two surfactants) at the c.m.c. can in principle be obtained using the approach of Funasaki and Hada.6 This gives the (changing) micellar compositions above the c.m.c., which can then be extrapolated to the c.m.c.In practice, however, our data do not lend themselves to this kind of analysis, which requires a large number of tensions above the c.m.c. An alternative method for the estimation of micellar compositions at the c.m.c. is that of Holland and Rubingh,’ based on the assumption that the surfactants form regular solutions in the micelles, characterised by the interaction parameter p”. The mole fraction of surfactant 1 in the micelles, xm, at the c.m.c. can be obtained from (xm), In (C12 x/Clxm) = (1 - xm)2 In [C,,( 1 - x)/C2( 1 - xm)] (9) where C,, C, and C,, are the critical micelle concentrations of pure 1, pure 2 and surfactant mixtures, respectively. The mixed c.m.c. values can be calculated using (10) l/C12 = x/C, e x p r ( l - ~ ~ ) ~ + ( l -x)/C, expp”(xm),.Values of C,, obtained from eqn (10) with p” = -0.35 agree reasonably well withR. Aveyard, B. P . Binks, J . Mead and J . H . Clint 68 1 1 .o m I 2 0.8 d 2 E . 2 0.6 Li 0.4 0.2 X X Fig. 5. (a) Variation of c.m.c. in AOT-SDS mixtures with mole fraction x of SDS. Points are experimental, values, full line is calculated from eqn (10) with p" = -0.35. (b) Variation of mol fraction of SDS in micelles, xm, and in plane surface, x", with x. Full line represents xm obtained using eqn (9); 0, values of x" given in table 1. The dashed line gives x" calculated using eqn (1 1) with /F' = -0.97. experiment [fig. 5(a)]; micellar mole fractions obtained from eqn (9) are compared with those in mixed plane monolayers, x", in fig. 5(b). For x > 0.2 the (oil-in-water) droplets are rich in SDS relative to the surface.For x = 0.2, x" = xm = 0.07 and y, is a minimum (fig. 3). It has recently been shown theoretically by Rosen and Murphy' that at minimum yc surface and micellar compositions should indeed be equal. Hollandg has recently extended the work on mixed micelles to non-ideal mixed plane monolayers. If it is assumed that the area per molecule of, say, surfactant 1 in a mixed monolayer in equilibrium with a solution at its c.m.c. is equal to that (A,) in a saturated monolayer of pure 1 then x" exp [p"( 1 - Y)'] = xm exp Ip"( 1 - ~")~]l/exp (Ay, Ai/kT) (1 1) where p" is the regular solution parameter for the plane monolayer. The quantity Ay, = 7; - yc, where y: is the value of y, for the system containing only surfactant 1.For our systems a value of p" can be obtained by noting that at x = 0.20, xm = x" = 0.07; in this way p" = -0.97. Using this value of p", together with p" and the value of xm already calculated, values of x" can be obtained using eqn (11). These values are in poor agreement with the values of x" obtained by use of eqn (6) [fig. 5(b)]. This must be due in part to the assumption that the areas per molecule in mixed films are equal to those in pure films (see earlier discussion and fig. 4). There is, however, a further conceptual difficulty. If it is accepted that and p" can be different in principle, the difference presumably arises from the curvature of the monolayers covering the droplets. If so, /P should change with droplet size and hence composition.As will be shown droplet size increases rapidly as the condition for minimum yc is approached. All this must cast some doubt on the values of xm obtained using eqn (9) and represented in fig. 5(b).602 Systems containing A 0 T + SDS 0 0.05 0.10 0.10 0.30 0.50 0.70 X Fig. 6. Phase boundaries in systems containing mixtures of AOT and SDS, heptane and aqueous NaCl at 25 “C. (a) I = w/o microemulsion, I1 = w/o microemulsion +excess aqueous phase, 11’ = isotropic liquid +excess viscous ‘phase ’. (b) I = o/w microemulsion, I1 = o/w micro- emulsion + excess heptane phase, 11’ = isotropic liquid + excess viscous ‘ phase’. Phase Behaviour Phase Boundaries We have determined the phase boundaries between single-phase microemulsions and two-phase systems in which a microemulsion coexists at equilibrium with a second phase, which may be an excess of the dispersed phase.The nature of the microemulsions in equilibrium with excess dispersed phase is of particular interest, since it is for such systems that we have measured interfacial tensions. Phase boundaries have been obtained as a function of x for both o/w and w/o microemulsions containing NaCl at concentrations appropriate to an excess bulk phase concentration of 0.10 mol dm-3. For w/o microemulsions the partition coefficient for distribution of NaCl between dispersed and excess phases (~aCl]disp/~aCl]e,,es,) has been shown to be reasonably constant for droplet radii > 5 nm and equal to ca. 0.4. (10) Accordingly, the titrations involving w/o microemulsions were performed using aqueous phases containing 0.04 mol dm-3 NaCl.In the two-phase rCgime the excess-phase salt concentration will be > 0.04 mol dm-3 and the dispersed phase concentration -= 0.04 mol dmV3. At the phase boundary, however, the concentration in the droplets will be 0.04 mol dm-3. If we were to add 0.10 mol dm-3 NaCl to a single-phase microemulsion at the phase boundary the microemulsion composition would not change and the excess phase salt concentration would be 0.10 mol dm-3. Results for systems containing w/o microemulsions are shown in fig. 6(a), where the ratio R, = [H,O]/([SDS] + [AOT]) in heptane is plotted against x. Equivalent results for o/w microemulsion systems are illustrated in fig. 6(b), where R, is the molar ratio of heptane in water to total surfactant available for microemulsion formation (i.e.total surfactant less the c.m.c.). The boundaries were found to be independent of total surfactant concentration in the range studied (0.03-0.20 mol dm-3). This is because the surfactant concentration, although fairly low, is considerably greater than the c.m.c., so the composition of the droplets (with respect to the two surfactants) is effectively equal to the overall x in the one-phase systems. We return to the significance of the phase diagrams following a discussion of the nature of the single-phase microemulsions.R. Aveyard, B. P. Binks, J. Mead and J. H. Clint 683 2 - 0 0.05 0.10 0.15 d Fig. 7. Dependence of diffusion coefficients, D, on volume fraction, q4, of droplets in o/w microemulsions containing heptane, 0.1 mol dm-3 aqueous NaCl and AOT-SDS at 25 "C.Points 0, 0, 0, 0 and 0 are for (x, R,) pairs of, respectively, (0.212, 5.30), (0.242,4.25), (0.288, 3.30), (0.325, 2.32), and (0.468, 1.52). Droplet Sizes In photon correlation spectroscopy (P.c.s.) the diffusion coefficient of the aggregates (droplets), D, may be obtained from the intensity autocorrelation function, g(,)(t), of the light scattered from the microemulsion using'' g(')(t) = 1 + exp (- 2DPt) (12) in which K = (4nn/A) sin (0/2), where n is the refractive index of the solution, A is the wavelength of the incident beam and 0 is the scattering angle; t is the delay time. At all finite concentrations D is perturbed from its value at infinite dilution, Do, by inter- droplet interactions.The autocorrelation function of the scattered light was evaluated as a function of angle for a number of the samples, and no systematic deviation in the apparent diffusion coefficient was observed. At low q5 the dependence of D on # is linear and can be described by D = D,(1 +a#). Here Do is kT/6nq-rh, where q- is the solvent viscosity and rh is the hydrodynamic radius of the droplets. From rh a mean area, a(p.c.s.), per surfactant molecule at the 'inner' (core) side of the surfactant monolayer coating the droplet may be calculated; on the assumption of monodisperse spheres (14) where vdisp is the molecular volume of the dispersed phase, rc is the core radius and e is the interfacial thickness. R, must be substituted for R, in the case of o/w microemulsions.The thickness e is expected to be approximately equal to the mean length of the surfactant molecules. We may, in addition to a(p.c.s.), define a mean area, rh = [(3vdisp R,/a(p.c.s.)] + e = rc + e designated a(geom), as a(geom) = a, x -i- a,( 1 - x) where a, and a, are the areas per molecule at the inner side of the monolayer in systems684 Systems containing AOT+ SDS Table 2. Data for w/o and o/w microemulsions in systems containing heptane, aqueous NaC1, AOT and SDS at 25 "C (see text) microemulsion R, or a(p.c.s.)/ a(geom)/ x type Rk rJnm t/nm nm2 nm2 0.034 0.034 0.034 0.034 0.034 0.2 12 0.242 0.288 0.325 0.403 0.468 0.497 16.0 4.2 0.8 28.8 7.2 0.8 50.3 12.1 0.8 71.8 16.6 0.8 93.2 21.3 0.8 5.3 10.2 1.9 4.3 8.6 1.9 3.3 7.3 1.9 2.3 5.7 1.9 1.8 5.8 1.9 1.5 5.1 1.9 1.4 5.4 1.9 0.41 0.41 0.41 0.41 0.41 0.47 0.47 0.45 0.44 0.34 0.34 0.30 0.50 0.50 0.50 0.50 0.50 0.46 0.45 0.43 0.42 0.40 0.38 0.37 R Fig.8. Variation of hydrodynamic radius with R for w/o microemulsions containing heptane, 0.04 mol dm-3 NaCl and AOT-SDS ( x = 0.034) at 25 "C. containing only one surfactant; we recall that in the phase work x is taken to be equal to x in the droplets. We have performed light-scattering experiments on both o/w and w/o microemulsions between the phase boundaries indicated in fig. 6. In fig. 7 we show plots of D against q5 for several o/w microemulsions. The plots are reasonably linear up to q5 x 0.15, in accordance with eqn (1 3). Similar plots (not shown) were given for w/o microemulsions. Hydrodynamic radii calculated from Do are given in table 2.For w/o microemulsions x was kept constant as R was changed; hence, from eqn (14) a plot of rh us. R should be linear if e and a (P.c.s.) are constant. Such a plot, depicted in fig. 8, is indeed linear, yielding a (P.c.s.) = 0.41 nm2 molecule-' and e = 0.8 nm.R. Aveyard, B. P . Binks, J. Mead and J . H. Clint 685 0 0.2 0.4 0.6 Fig. 9. Droplet radii in microemulsions in equilibrium with excess dispersed phase as a function of x at 25 “C: 0, w/o microemulsions; and 0, o/w microemulsions. The vertical line represents the inversion condition expected from the tension work. In the case of the o/w microemulsions, for which x was not held constant, a(p.c.s.) has been calculated from rh using eqn (14) with e = 1.9 nm, being the mean of the estimated lengths of the two surfactant molecules (1.5 nm for AOT and 2.3 nm for SDS).The geometrical areas, a(geom) [eqn (1 S)], listed in table 2 for o/w microemulsions have been obtained using a,(SDS) = 0.22 nm2 and a2(AOT) = 0.53 nm2. These are cross-sectional areas of the alkyl-chain regions obtained using molecular models ; when the alkyl chains are on the ‘inside’ of the curved surfactant films, as in o/w microemulsions, they are expected to be unsolvated. As seen, for o/w microemulsions the agreement between a(p.c.s.) and a(geom) is very good supporting the view that the droplets are reasonably monodisperse spheres. For the w/o microemulsions a(geom) was calculated using a, = 0.46 nm2 (the area per molecule in a saturated film at the heptanejwater interface; table 1) and a2 = 0.50 nm2 [the area per AOT at the droplet surface in the absence of SDS; ref.(l)]. The a(p.c.s.) and e values (0.41 nm2 and 0.8 nm, respectively) are both lower than expected on the basis of molecular geometry, but the reasons for this are not clear. We now consider droplet radii at the phase boundaries, shown in fig. 6, between microemulsion and excess dispersed phase, i.e. the 1/11 boundaries. The radii here are clearly the maximum attainable for a given value of x. They also reflect in part the ‘preferred’ curvature of the surfactant films, although the drop size is perturbed from this preferred size by effects due to the entropy of dispersion of the droplets (tending to give smaller droplets) and interactions between droplets. The radii shown in fig.9 were calculated using eqn (14), but with a(geom) rather than a(pcs) which were not available for microemulsions at the phase boundaries. Values of e of 0.8 and 1.9 nm were used for w/o and ofw droplets, respectively. Although the r h values can only be regarded as approximate, it is clear that as SDS is added to the system the w/o droplets grow in size until the system ‘inverts’. As further SDS is added the o/w droplets which now form gradually diminish in size. The role of the SDS in the phase behaviour is readily explained in terms of the preferred curvature of the mixed surfactant films. At low x the average headgroup area, ah, is686 Systems containing AOT+ SDS smaller than that, a,, of the chain. As the SDS content (hence x ) is increased the headgroup region becomes relatively more swollen than the chain region until, at the inversion condition (0.03 < x < 0.19; fig.9) the preferred curvature changes sign. We recall that in the tension studies [fig. 1 (a), 2(b) and 31 minimum y,, which is assumed on the basis of our previous work2V4 to occur at the inversion condition, occurs at x w 0.2. These systems, however, were close to the c.m.c. For x = 0.2, the surface mole fraction of SDS, x", is 0.07. Further, for minimum y, the composition of the surface and the aggregates (with respect to the surfactants) is the same.* In the phase studies (done at much higher surfactant concentrations) the bulk mole fraction x and that in the aggregates are effectively equal as discussed. Thus the inversion conditions observed in the tension and the phase work are entirely consistent. The authors thank BP Research, Sunbury, for an Extramural Research Award and for the award of a BP Research Studentship (to B. P. B.). References 1 R. Aveyard, B. P. Binks, S. Clark and J. Mead, J. Chem. SOC., Faraday Trans. I , 1986, 82, 125. 2 R. Aveyard, B. P. Binks and J. Mead, J. Chem. SOC., Faraday Trans. I , 1987, 83, 2347. 3 D. Guest and D. Langevin, J. Colloid Interface Sci., 1986, 112, 208. 4 R. Aveyard, B. P. Binks and J. Mead, J. Chem. SOC., Faraday Trans. I , 1986, 82, 1755. 5 Phenomena in Mixed Surfactant Systems, ed. J. F. Scamehorn, A.C.S. Symp. Ser. 31 1. (American 6 N. Funasaki and S. Hada, J. Phys. Chem., 1979,83,2471. 7 P. M. Holland and D. N. Rubingh, J. Phys. Chem., 1983, 87, 1984. 8 M. J. Rosen and D. S. Murphy, J. Colloid Interface Sci., 1986, 110, 224. 9 P. M. Holland, in ref. (9, p. 102. Chemical Society, Washington DC, 1986). 10 P. D. I. Fletcher, J. Chem. SOC., Faraday Trans. I , 1987, 83, 985. 11 See for example K. J. Randle, Chem. Ind., 1985, 75. Paper 612028; Received 22nd September, 1986

ISSN:0300-9599    

DOI:10.1039/F19888400675

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J. Chem. Soc., Faraday Trans. I, 1988, 84(3), 687-702 Multicomponent Ion Exchange in Zeolites Part 3.-Equilibrium Properties of the Sodium/Potassium/Cadmium-Zeolite X System Kevin R. Franklin? and Rodney P. Townsend*$ Department of Chemistry, The City University, Northampton Square, London ECl V OHB A study is reported on the ternary exchange equilibrium involving the cations sodium, potassium and cadmium in zeolite X. This system is an apt one on which to test recently developed prediction procedures since it is possible, in principle, to convert zeolite X to the 100%-exchanged homoionic forms of each of these three cations, thus avoiding the normalisation problems reported previously (K. R. Franklin and R. P. Townsend, J. Chem. Soc., Faraday Trans. I, 1985,81,3127; K. R.Franklin and R. P. Townsend, J. Chem. Soc., Furaday Trans. 1, 1985, 81, 1071). In addition, cadmium salts in solution show markedly non-ideal behaviour even in dilute solution, and therefore the procedures developed for allowing for this non-ideality can be tested adequately. Data are given not only for the ternary system but also for the three conjugate binaries, for which the thermodynamic closure rule ('triangle rule') is shown to hold. In general, successful prediction of exchange equilibria for different external solution concentrations is shown to be possible, although systematic errors develop at the highest concentration examined. In parts 1 and 2 of this series, ' 7 the equilibrium ion-exchange properties of the sodium/ calcium/magnesium-zeolite A system were discussed.Models, based on rigorous thermodynamic treatment^,^*^ were described, which enabled the prediction of both binary and ternary ion-exchange selectivities as a function of the total external solution concentration, These models were then used to predict experimental selectivity data for the Na/Ca/Mg-A system. In this paper, these studies are extended to exchange equilibria involving the ions sodium, potassium and cadmium in zeolite X. In contrast to the commercially important Na/Ca/Mg-A exchange, the Na/K/Cd-X system has less immediate practical importance, but from the viewpoint of testing theoretical predictive models it has significant advantages over the former. First, careful prior studies have shown, within the limitations of experimental uncertainty, that all three of the conjugate binary exchange systems (viz.Na/K-X, Na/ Cd-X and K/Cd-X) hold two important properties in common. These are that exchange of one ion for another can proceed to 100°/~ of the exchange capacity of zeolite X, and that the exchanges are thermodynamically reversible. 5-9 For the Na/Ca/Mg-A system, neither the Na/Mg-A nor the Ca/Mg-A conjugate binary exchanges showed 100% replacement of the former ion by the latter,' and there were, in addition, doubts as to the reversibility of the Ca/Mg-A system.' As a result, complications arose when attempts were made to test the predictive models.2 Secondly, it is appropriate to test the thermodynamic model^^.^ with systems t Present address : Chemistry Department, Edinburgh University, West Mains Road, Edinburgh EH9 355.1 Present address : Unilever Research, Quarry Road East, Bebington, Wirral, Merseyside L63 3JW. 687688 Ion-exchange Equilibrium Properties of Zeolites involving the cadmium ion, because solutions of cadmium salts show markedly non- ideal behaviour even in quite dilute aqueous solution due to ion-pairing.*-1° Since it is a basic tenet of those predictive models which are based on thermodynamic theory that solution-phase non-ideality is often the significant factor in determining selectivity changes as a function of ionic it is therefore logical to test these models with systems where marked non-ideality is expected to be manifest. The results of these tests are reported in this paper. Experiment a1 Materials Zeolite X was purchased from BDH as a fine powder, and was supplied in the sodium form.All other chemicals were AnalaR grade, and were used without further purification. Preparation of Maximally Exchanged Zeolites Although the zeolite had been supplied in the sodium form, previous experience had shown that a substantial deficiency in sodium was often present in commercial samples of zeolite X. Thus the zeolite was first exchanged several times with solutions of sodium nitrate (1 mol dm-3) in order to obtain as far as possible the homoionic sodium form of the zeolite. The zeolite was then washed briefly with water and dried at 80 “C. Finally, and before chemical analysis or use, the zeolite was equilibrated with water vapour over saturated aqueous sodium chloride in a desiccator at room temperature.Maximally exchanged samples of potassium X and cadmium X were prepared by exhaustively exchanging 20g aliquots of sodium-exchanged X with 400 cm3 portions of normalt solutions of either potassium nitrate or cadmium nitrate. The exchanges were carried out at room temperature over one day, and up to 50 one-day exchanges were used. Following the final exchanges, the zeolites were washed, dried and equilibrated over water vapour in the same manner as for the sodium form of the zeolite. Throughout these procedures, great care was taken to monitor the levels of hydronium exchange which may have occurred during the preparation of the different exchanged forms. Other studies have shown that the levels of hydronium exchange attained in aluminium-containing zeolites are critically dependent on the zeolitic framework charge,12 and on the amount of acid gases dissolved in s01ution.l~ For these reasons, extensive washing of the samples under conditions open to the atmosphere was avoided, and strict pH control of the wash water was observed.These matters, and the appropriate procedures, are all discussed in detail elsewhere. l3 Levels of hydronium exchange were determined by inference from comprehensive chemical analyses of the materials, and in appropriate cases, by complete chemical analyses of both exchanging phases (viz. zeolite and external solution) followed by mass balances. Chemical Analyses For the isotherms, both exchanging phases were analysed for all exchanging ions each time in order to construct each isotherm point.In addition, the aluminium content of the zeolite sample was analysed in some cases as a further check. t Throughout this paper, the term ‘equivalent ’ refers to one mole of unit negative or positive charges, and the term ‘normal’ refers to the concentration in solution of species in equivalents. Thus the normality of a solution with respect to ionic species i is z,ci, where zi is the valency and ci is the molarity (mol dm-s) of i in solution.K. R. Franklin and R. P . Townsend 689 For the zeolite phase, silica, alumina and water contents were determined by gravimetric methods previously described.'* The sodium, potassium and cadmium contents were determined after the zeolite had been dissolved in 30 YO nitric acid. Sodium and potassium were determined by standard methods using flame photometry, while cadmium was analysed by atomic absorption spectroscopy using an air-acetylene flame.No interferences were encountered during these analyses. For the solution phase, sodium and potassium were again determined using flame photometry. For concentrations greater than mol dm-3 a titrimetric method was employed to analyse for cadmium, using EDTAf at pH 5.15 At lower levels, cadmium concentrations were determined using an EG and G model 264 polarograph, operating in the differential pulse anodic stripping mode.16 Construction of Ion-exchange Isotherms The experimental approach was similar to that used for the construction of isotherms for the Na/Ca/Mg-A system.' All of the isotherms were constructed at a total normality of 0.1 equiv.dm-3 using a seven day exchange time, and at a temperature of 298 K. The starting material employed for both the binary and ternary exchange studies was Na-X except for the construction of the binary Cd/K-X isotherm, where the starting material was K-X. Isotherm points were obtained by contacting 0.2g aliquots of the zeolite with 50 cm3 portions of solution containing the nitrate salts of either both (binary studies) or all three (ternary studies) metal ions. To obtain high loadings of the ingoing cation(s), it was found to be necessary to use larger volumes of solution, or in some cases, even multiple exchanges. After equilibration, the two phases were separated by centrifugation, and the zeolite phase was washed carefully with water.Finally both phases were analysed for all exchanging ions. Thermodynamic reversibility of exchange was tested for the binary isotherms using the 'wet method ' described previous1y.l' For reasons given elsewhere, and concerned with the number of degrees of freedom possessed by a ternary exchange system,18 there is no simple experimental method of testing for reversibility in a ternary exchange situation. Thus overall reversibility of the ternary exchange isotherm was taken as proved if all three conjugate binary systems had been shown to be reversible within the limits of experimental error.18 All the isotherm data were plotted in terms of equivalent fractions, which were defined for the binary and ternary exchange systems, respectively, as follows. For the binary isotherms, the equivalent fraction of (say) ion A in the zeolite phase (1) is defined as whereas the corresponding function for the solution phase is (2) E A = 'A mA/(zA mA + 'I3 EA = 'A ',/('A 'A+', and z,, z , are the valencies of the exchanging ions A": and BZi.Similarly, the functions mA, mB and c,, C, are, respectively, the molalities (mol kg-l yf zeolit?) and molar concentrations in solution (mol dm-3) of the exchanging ions A'A and B'B. For the ternary isotherm, the equivalent fractions are E A = 'A mA/(zNa mNa + 'K mK + 'Cd %dl EA = 'A 'A/(',, 'Na + 'K cK + 'Cd 'Cd). (3) and (4) It should be noted that the definitions of equivalent fraction in the zeolite phase given in eqn (2) and (4) are normalised functions, i.e. they have been adjusted for any t Ethylenediamine tetra-acetic acid.690 Ion-exchange Equilibrium Properties of Zeolites Table 1.Analysis of the sodium potassium and cadmium forms of zeolite X Na-X (Yo w/w) K-X (% w/w) Cd-X (% w/w) SiO, Na,O CdO totals Si :A1 Zcation :A1 4 0 3 H2O K2O 36.71 24.70 23.60 14.76 0.00 0.00 99.77 1.261 0.983 35.23 23.54 20.18 0.29 20.54 0.00 99.78 1.270 0.963 30.31 20.25 23.12 0.24 0.00 24.10 98.02 1.270 0.965 hydronium exchange which may have occurred. The effect of hydronium exchange on equilibrium selectivities is discussed elsewhere. ‘ 9 12, 13, l9 In the present study, levels of hydronium exchange up to ca. 12% were encountered at low ECd values, but the extent of hydronium exchange decreased as Ecd increased. Results and Discussion Maximally Exchanged Zeolites The results of the chemical analyses of the three maximally exchanged forms of X are given in table 1.Unit cell compositions, derived from the data in table 1, are as follows : Na-X Na83.5 [&O]i.4 [A102184.9 [si021~07.i [H201228.4 K-X K79.9 Nai.5 [H3Oh.i [A102184.6 [sio21i07.4 [H201202.2 Cd-X : Cd40.0 Na1.6 [H3OI3.o [A O2184.6 [si021i07.4 [H20127~.5- The levels of hydronium exchange are included in the unit cell compositions. Because of the experimental precautions alluded to above (in the experimental section), these levels are not high, despite the large number of exchanges which were undertaken. In the case of the cadmium sample, the total percentage (table 1) is rather low: subsequent experiencel39 l9 has shown that aluminium and especially silicon may have been somewhat underestimated here, which means that the hydronium content of the Cd-X sample was probably slightly higher than indicated.Binary Exchange Isotherms In fig. 1-3 are given, respectively, the binary ion-exchange isotherms for the K/Na, Cd/Na and Cd/K exchange reactions in X. All three isotherms were found, within the limits of experimental uncertainty, to be reversible. In all three cases, complete replacement of the original ion by the entering one seemed to be possible, although removal of the last traces of sodium from X by either potassium or cadmium is difficult (see fig. 1 and 2, and also the discussion above regarding table 1). The potassium/sodium isotherm (fig. 1) is sigmoidal in shape, with the zeolite showing a slight preference for potassium at low loadings of this ion, and the converse preference at high loadings.Comparisons with previous studies5* 7* show very good agreement regarding the general shape. Using the procedures of Glueckauf,20v 21 a standard freeK . R. Franklin and R. P . Townsend 69 1 1 0.8 0.6 EK 0.4 0.2 Fig. 1. K/Na-X exchange isotherm obtained at 298 K and at total normality 0.1 equiv. dm-3: 0, forward points; x , reverse points. 0.2 0.4 - 0.6 0.8 1 Ecd Fig. 2. Cd/Na-X exchange isotherm obtained at 298 K and at total normality 0.1 equiv. dm-3: 0, forward points; x , reverse points. energy of exchange of 0.48 kJ equiv.-l was obtained. This value agrees well with the values previously given by Sherry5 and by Townsend et al.' (0.59 and 0.40 kJ equiv.-' respectively), but not with those of Barrer et a1.' (-0.84 kJ equiv.-l), or Ames22 ( - 0.80 kJ equiv?).A complete understanding of these discrepancies is not completely clear; however, AmesZ2 did use a pelleted sample of zeolite X, and also an external692 Ion-exchange Equilibrium Properties of Zeolites 0.2 -I 0.2 0.4 - 0.6 0.8 1 0 Em Fig. 3. Cd/K-X exchange isotherm obtained at 298 K and at total normality 0.1 equiv. dm-s: 0, forward points; x , reverse points. 14 2 - 0 0.2 0.4 - 0.6 0.8 1 EM Fig. 4. Corrected selectivity quotient plots for the K/Na, Cd/Na and Cd/K binary exchanges: A, K/Na: M = K; 0, Cd/Na: M = Cd; I, Cd/K: M = Cd. solution concentration of 1 mol dm-3. The effects that any binder may have had on the equilibrium, and the possibility of salt imbibition at such a high solution c~ncentration,~~ must both be considered in this case.The very marked preference which zeolite X displays for cadmium over either sodium or potassium is seen clearly in fig. 2 and 3. The experimental selectivity for cadmium appears to be normally higher using the sodium form of X rather than the potassium form (cf. fig. 2 and 3), and the very high negative values of the calculated standard free energies of exchange reflect the very high preference for cadmium (values of AG* were -4.92 and -5.30 kJ equiv.-' for the Cd/Na and Cd/K exchanges, respectively).K. R. Franklin and R. P. Townsend 693 However, the overall affinities (reflected in the standard free energy values) show an affinity of K-X for cadmium which is marginally higher than that shown by Na-X, in apparent contradiction to the isotherm data.There is, however, no contradiction. Fig. 4 shows plots of the logarithm of the Gaines and Thomas21 corrected selectivity quotient KG, where and K, is the mass-action quotient for the exchange reaction: The function r is the correction for non-ideality in the aqueous solution phase: its evaluation is discussed in detail el~ewhere.~ The r correction is of similar magnitude for both the Cd/Na and Cd/K exchanges, so fig. 4 reflects well the selectivities of the Na-X and K-X samples for cadmium over all compositions of the exchanger. It is seen that although the preference of Na-X for cadmium is greater than that of K-X in the region E,, = 0.5-0.9, for values of E,, < 0.5 the converse is true.Owing to the very low values of ECd in this region (fig. 2 and 3) this cannot be seen from simple inspection of the is0 t herms. Previous equilibrium studies of the Cd/Na-X system have been r e p ~ r t e d , ~ - ~ * and recently, very detailed examinations have ruled out the possibility of significant degrees of over-exchange of cadmium occurring, either through precipitation of basic salts on the external surfaces of the zeolite crystallites8-'' or through the exchange of complexed species into the zeolite.' It seems therefore that one can view the apparent reversibilities of the Cd/Na-X and Cd/K-X systems (fig. 2 and 3) as genuine, and therefore these data may be used with confidence in conjunction with thermodynamic procedures in order to predict selectivities (see below).Gal and Radovanov' obtained a value of AG* = -4.2 kJ equiv.-' for the Cd/Na exchange in X, a value close to those found by Fletcher and Townsend for the same exchange reaction using different co-anions in solution (-4.26, -4.24 and -4.48 kJ equiv.-').' The value obtained here (- 4.92 kJ equiv.-l) is slightly higher: this undoubtedly reflects a lesser degree of extrapolation to low Ec, values in this present study compared to earlier ones [cf. fig. 2, 3 and 4 with data given in ref. (6) and (S)]. Finally, reversibility of exchange can be confirmed using the ' triangle rule ',18 which (providing the standard states are consistent8) has been applied successfully bef0re.l'~~~ For the three conjugate binary exchanges described here, the triangle rule states that On inserting the values obtained experimentally for AG*, eqn (7) becomes 0.48 + 4.92 - 5.30 = 0.10 kJ equiv.-l, a result in good agreement with theory, and one which confirms that the application of thermodynamic procedures to the binary exchange data is justified.The Na/K/Cd-X Ternary Isotherm 113 sets of analyses of equilibrium compositions of the crystal phase and of the corresponding solution phase were carried out in order to construct the Na/K/Cd-X ternary isotherm. The resulting data points are shown in fig. 5 (a) and (b) for the solution and zeolite phases, respectively. It is apparent that a comprehensive coverage of the possible range of solution and zeolite phase compositions was achieved. Fig. 6 shows the resulting ternary isotherm, with the data represented by a diagram on which the solution- phase composition coordinates have been distorted with respect to the crystal phase,'.l8 so that each solution datum point lies directly on top of the corresponding datum point for the zeolite phase. The distorted plot was in part constructed by eye since the694 Ion-exchange Equilibrium Properties of Zeolites K Cd Na K Cd Fig. 5. Experimental points obtained for the construction of the Na/K/Cd-X ternary isotherm, measured at 298 K and at a total solution normality of 0.1 equiv. dm-3: (a) solution phase; (b) zeolite phase. Correspondingly numbered points on the two diagrams provide examples of tie-lines between the two diagrams.K. R. Franklin and R. P . Townsend 695 Na K Cd Fig. 6. Ternary exchange isotherm for the Na/K/Cd-X system, with the data depicted using distorted 10% grid lines to represent the solution phase (see text).mathematical procedure previously described" for the construction of such diagrams was found to fail for systems such as this, where very high selectivities occur. The selectivity which zeolite X displays for each ion separately in the presence of the other two is seen more clearly by displaying individually the appropriate subsets of those grid lines which together comprise the ternary diagram in fig. 6. The three subsets are shown in fig. 7. Thus fig. 7 (a) shows the selectivity pattern for sodium; moving along the edge corresponding to the binary potassium/sodium exchange reaction, the sigmoidal isotherm plot of fig. 1 can be traced, and the spacing of the grid lines indicates a lack of any strong preference for either ion on the part of the zeolite.In contrast, as the cadmium content of the zeolite is increased, the grid lines giving the equilibrium concentrations of sodium in solution converge strongly, emphasising the strong preference which X shows for cadmium over sodium. Fairly similar behaviour is seen in fig. 7(b), while fig. 7 ( c ) emphasises further this high and general selectivity for cadmium. As explained elsewhere," l8 these selectivity trends can be quantified using pseudo- binary' separation factors of the form where the subscripts A and B refer to any two of the three ions involved in the ternary exchange. Logarithmic plots of &&, gdi: and :&, corresponding to the data given in fig.6 and 7, are given in fig. 8.696 lon- exchange Equilibrium Properties of ZeolitesK. R. Franklin and R. P . Townsend 697 0 I698 Ion-exchange Equilibrium Properties of Zeolites Table 2. Coefficients of best-fitting polynomials of binary exchange data [after eqn (9)] coefficients of for n = system ion A 0 1 2 3 4 5 K/Na-X K 1.273 0.092 -19.04 33.74 -18.01 - Cd/Na-X Cd 9.360 -20.34 55.49 154.6 229.5 -119.9 Cd/K-X Cd 12.64 -16.07 -5.586 12.27 -2.681 - Table 3. Binary exchange predictions for the Cd/Na-X system total E C d normality lequiv. dm-3 ECd (observed) (predicted) 0.400 0.170 0.769 0.764 0.400 0.456 0.874 0.861 0.400 0.669 0.933 0.902 0.025 2.4 x 0.059 0.086 0.025 0.087 0.832 0.874 0.025 0.235 0.904 0.91 1 Table 4.Binary exchange predictions for the Cd/K-X system total E C d normality /equiv. dm-3 ECd (observed) (predicted) 0.400 0.032 0.523 0.524 0.400 0.181 0.657 0.634 0.400 0.485 0.840 0.806 0.400 0.671 0.742 0.73 1 0.025 4.1 x 10-5 0.328 0.333 0.025 0.145 0.735 0.773 0.025 0.3 15 0.835 0.838 Prediction of Exchange Selectivities The theory and procedures employed in the prediction of both binary and ternary exchange equilibria have been described in detail previously2 for the Na/Ca/Mg-A system. An iteration method was employed for all predictions, and for reasons explained before,2 the preferred procedure was to predict crystal-phase compositions from a chosen initial solution composition. Considering the binary exchange systems first, the predictions were undertaken using as base data the experimental isotherms measured at a total normality of 0.1 equiv.dm-3 (fig. 1-3). The base data were expressed as a series of polynomials, which expressed the dependence of the logarithm of the corrected selectivity quotient KG on the crystal-phase composition : ln K, = a ~ A + ~ E ~ + ~ ~ A . . + vg. (9) For the K/Na-X and Cd/Na-X exchanges, a fourth-order fit was found to beK. R. Franklin and R. P. Townsend 699 Table 5. Coefficients of best-fitting polynomials of ternary exchange data [after eqn (1 l)] coefficients (ai) of PNa for i = ~~ dependent 0 1 2 3 4 5 variable Ink,,,, -22.68 235.2 - 1165 2694 - 2797 1072 In kK/Cd - 33.95 9.063 1.45 1 39.69 -31.95 - coefficients (83) ofEjcd for j = dependent variable 1 2 3 4 5 13.79 55.21 - 152.9 155.9 - -5.194 206.4 -289.3 119.3 52.38 - satisfactory, while for the Cd/Na-X system, a fifth-order polynomial was the preferred choice.The coefficients of the polynomials used are given in table 2. As before,2 predictions were then made at total solution concentrations of 0.025 and 0.4 equiv. dm-3. Examples of some of these predictions for the Cd/Na-X and Cd/K-X exchanges are given in tables 3 and 4, and it is seen that good agreement was obtained between predicted and experimental values in most cases. These results conform with previous experience on similars or different2* la binary systems. Considering next the ternary Na/K/Cd-X system, problems arose in making the starting point of each iteration the zeolite phase composition which is in equilibrium with the chosen solution composition on the experimental isotherm (fig.6). For many solution compositions, the very high selectivity for cadmium made it impossible to read off accurately the corresponding crystal-phase compositions from the isotherm, because of the problems involved in accurately fitting such isotherms with a polynomial that covered the whole composition range. The problem was overcome by using subsets of polynomials to fit the experimental isotherm data (fig. 6 and 7). Each subset fitted only a small part of the whole isotherm, but the set overall fitted the whole composition range. Each subset comprised polynomials of the form where i = Na, K or Cd, and tl(i)-tn(i) are appropriate polynomial coefficients. pseudo-binary corrected selectivity quotients2.Next, using these equations, surface polynomials expressing the dependence of on composition were constructed : n m InkclA = C u i e + C PI,!.?; i - 0 I- 1 where k,,, may be either of two pseudo-binary quotients kKiNa or kK/Cd defined, respectively, ad8 kK/Na = (EK ‘NalENa rK/Na (12) and kK/Cd = (g ‘CdlECd ‘K2) rK/Cd (13) where rK/Na and I‘K/Cd are corrections for non-ideality in the solution phase.2*9*18 The coefficients of the best-fitting polynomials are given in table 5 . As before2 the best fits were defined for N sets of data using a sum of residuals R: (kC/A(predicted) /kC/A(rneasured))l (N-P-Q-1)700 Ion-exchange Equilibrium Properties of Zeolites Na K Cd Fig. 9. Experimental and predicted zeolite phase compositions for the Na/K/Cd-X system at 0.4 equiv.dm-a: A, solution composition at 0.1 equiv. dmV3; 0, predicted zeolite composition at 0.4 equiv. dm+; 0, experimental zeolite composition at 0.4 equiv. dm-3; W, zeolite composition at 0.1 equiv. dm+. where P and Q are the orders of the polynomial with respect to EA and EB, respectively. Care has to be taken2 in making the criterion for best fit a minimum value of R as a function of P and Q; however, contrary to the Na/Ca/Mg-A system1* high orders were required for adequate predictions because of the high selectivity of X for cadmium (table 5 ) . Using the procedures described elsewhere2 ternary equilibrium compositions were predicted for total solution concentrations of 0.025 and 0.4 equiv. dmb3, respectively. Examples are shown in fig. 9 and 10.The starting point for each prediction was a set of equilibrium compositions for each phase as determined by experiment at a total solution normality of 0.1 equiv. dm-3. Then, for the chosen solution composition, values of rK/Na and r K / C d were calculated at this composition at either 0.4 (fig. 9) or 0.025 equiv. dm-3 (fig. lo). An iteration procedure (described elsewhere2) was then applied to predict the corresponding crystal-phase composition at the chosen final solution normality. Finally, the accuracy with which the crystal-phase compositions had been predicted was tested by determining experimentally the actual final zeolite composition. All these data points are shown on fig. 9 and 10 for every prediction test. In most cases, reasonably good agreement was obtained between theory and experiment.It is, however, noteworthy that predictions seem to contain within themselves an element of systematic error. Thus if one examines first the binary predictions (tables 3 and 4), it is clearly seen that predictions at 0.4equiv. dm-3 show generally lower &,values than those observed experimentally (fig. 9), whereas the converse trend is seen at a solution concentration of 0.025 equiv. dm-3. For the ternary systems, the extra degree of compositional freedom introduced into each phase leads toK. R. Franklin and R. P . Townsend 70 1 Na K Cd Fig. 10. Experimental and predicted zeolite phase compositions for the Na/K/Cd-X system at 0.025 equiv. dm-3: A, solution composition at 0.1 equiv. dm-3; 0, predicted zeolite composition at 0.025 equiv.dm-3; 0, experimental zeolite composition at 0.025 equiv. dm-3; m, zeolite composition at 0.1 equiv dm-3. a concomitant increase in experimental error which masks to some extent these trends; nevertheless they can still be discerned (fig. 9 and 10). A possible explanation for these trends may lie in the partial failure of a basic tenet on which these prediction procedures depend, viz. that the ratio of magnitudes of activity coefficients in the zeolite phase should not vary significantly with changes in concentration in the external solution phase.2.11 If the ratio should vary, this results in the value of the corrected selectivity coefficients changing with solution concentration for a given crystal-phase composition." Causes of such variations may be changes in the intracrystalline content and activity of guest molecules such as water ;ll another possibility which is pertinent to the system considered here is the known tendency of cadmium to associate with anionic species to form complexes which may be positive, neutral or even negatively ~harged.~ This tendency is not pronounced when (as here) nitrate is the co-anion, and for the solution phase, the effects of any such association are allowed for adequately in terms of the magnitudes of rKINa and I'KICd.Also, Cd(NO,)+ species are not expected to be included within the zeolite channels or cages to any significant degree due to Donnan exclusion ;' however, neutral Cd(NO,),, the quantity of which will be greater in more concentrated solutions, could be imbibed, and within the crystal these species would certainly affect the magnitudes of the corrected selectivity coefficients.Overall, deviations between prediction and theory are small in the Na/K/ Cd-X system, but exchanges where the corrected selectivity coefficients vary markedly with solution concentration have been found, and these are discussed in part 4 of this series.702 Ion-exchange Equilibrium Properties of Zeolites Finally, while considering the accuracy of these predictive methods, it is worth comparing the procedure employed here and also in part 2 of this series2 with that used earlier. Here, values of crystal-phase compositions have been predicted from an initial solution value. In an earlier study, also on the Na/Cd-X system, the opposing approach of predicting Ecd was used which is much more time-consuming2 in terms of computing time, because r has to be recalculated for each iteration step.A comparison of data in table 3 with earlier results8 shows that both procedures give similar deviations. Irrespective of which procedure is used, errors tend to increase as ECd tends to zero. This is of course a consequence of the very high selectivity which X displays for cadmium over the alkali-metal ion ; errors incurred in analysis are exacerbated when calculating the corrected selectivity coefficients. K. R. F. gratefully acknowledges an S.E.R.C. CASE award with Unilever Research. References 1 K. R. Franklin and R. P. Townsend, J. Chem. Soc., Faraday Trans. 1, 1985, 81, 1071. 2 K. R. Franklin and R. P. Townsend, J. Chem. Soc., Faraday Trans. 1, 1985, 81, 3127. 3 P. Fletcher and R. P. Townsend, J. Chem. Soc., Faraday Trans. 2, 1981, 77, 965. 4 P. Fletcher and R. P. Townsend, J. Chem. Soc., Faraday Trans. 2, 1981, 77, 2077. 5 H. S. Sherry, J. Phys. Chem., 1966, 70, 1158. 6 I. J. Gal and P. Radovanov, J. Chem. Soc., Faraday Trans. I , 1975, 71, 1671. 7 R. M. Barrer, L. V. C. Rees and M. Shamsuzzoha, J. Inorg. Nucl. Chem., 1966, 28, 629. 8 R. P. Townsend, P. Fletcher and M. Loizidou, Proc. 6th Int. ConJ Zeolites, Reno, Nevada, 1983 9 P. Fletcher and R. P. Townsend, J. Chem. Soc., Faraday Trans. 1, 1985, 81, 1731. (Butterworths, London, 1984), p. 110. 10 M. Loizidou and R. P. Townsend, J. Chem. Soc., Dalton Trans., 1987, 191 1. 1 1 R. M. Barrer and J. Klinowski, J. Chem. Soc., Faradzy Trans. 1, 1974,70, 2080. 12 R. P. Townsend, K. R. Franklin and J. F. OConnor A h . Sci. Technol., 1984, 1, 269. 13 R. Hart and R. P. Townsend, paper in preparation. 14 R. M. Barrer and R. P. Townsend, J. Chem. Soc., Faraday Trans. 1, 1976, 72, 661. 15 A. Vogel, A Textbook of Quantitative Inorganic Analysis (Ldngmans, London, 1978), p. 324. 16 J. F. OConnor, Ph.D. Thesis (The City University, London, 1988). 17 P. Fletcher and R. P. Townsend, J. Chem. Soc., Faraday Trans. 1, 1981, 77, 497. 18 P. Fletcher, K. R. Franklin and R. P. Townsend, Philos. Trans. R. Soc. London, Ser. A, 312, 141 (see 19 C. J. Adams, K. R. Franklin, R. P. Townsend and S. J. Whelan, in New Developments in Zeolite 20 E. Glueckauf, Nature (London), 1949, 163, 414. 21 G. L. Gaines and H. C. Thomas, J. Chem. Phys., 1953, 21, 714. 22 L. L. Ames, Am. Mineral., 1964, 49, 127. 23 R. M. Barrer and A. J. Walker, Trans. Faraday Soc., 1964, 60, 171. 24 T. C. Golden and R. G. Jenkins, J. Chem. Eng. Data, 1981, 26, 366. especially pp. 150 and 160). Science and Technology, Proc. 7th Int. Con$ Zeolites, Tokyo, 1986. Paper 612 100 ; Received 29th October, I986

ISSN:0300-9599    

DOI:10.1039/F19888400687

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. 1, 1988, 84(3), 703-712 Rotational Relaxation Time and Conformation of Salt-free Sodium Poly(styrenesu1phonate) as studied by the Conductance Stopped- flo w Technique Tsuneo Okubo Department of Polymer Chemistry, Kyoto University, Kyoto 606, Japan The rotational relaxation time (z,) of sodium poly(styrenesu1phonate) (NaPSS) in aqueous media has been determined using the conductance stopped-flow technique. The effective length of a single chain may be evaluated from t,. The conformation of the macroion chain is inferred to be highly stretched at dilute concentrations lower than lop3 mol dm-3, and shrinkage of the chain occurs with increasing polymer concentration and/or with added salt. The persistence length evaluated from t, decreases sharply with polymer concentration and slightly with increasing degree of polymerization. The Debye screening length is discussed in relation to the solution properties of salt-free NaPSS.In a previous paper we measured the rotational diffusion constant (D,) of an ellipsoidal colloid by the spectrophotometric stopped-flow and conductance stopped-flow (c.s.f.) methods.' We report here the rotational Brownian motion of sodium poly(styrene- sulphonate) (NaPSS), a typical flexible polyelectrolyte, in salt-free dilute solution. D, of NaPSS was determined by the c.s.f. technique. In this method two types of solution are mixed in a cell and the reaction solution is then allowed to flow into an observation cell (a narrow tube 2 mm in diameter) under a strong shear rate. The asymmetric molecules are expected to orient themselves along the flow direction during continuous flow.When the solution flow is stopped, the molecules revert to free rotation in a Brownian random distribution. The translational diffusion is not so significant in the stopped-flow method, since the flow of the solvent molecules stops completely when observation starts. Note that the macroions must be deformed by the flow field. However, we assumed that, at the moment at which the flow is stopped the macromolecules approach their equilibrium conformation much faster, and then the macroions start to move rotationally. This assumption must be sound, because the chain dynamics are much faster than the overall rotation of macromolecules with a static conformation. The c.s.f.method has been applied to analyse various fast interionic reactions, i.e. micellar equilibria of ionic detergents,2 macroion complexations with neutral polymer^,^ metal ions4 and oppositely charged macro ion^,^ enzymatic reactions6 and other chemical reactions. 7-10 In this paper we report studies of the rotational relaxation process of the conductance anisotropy of an NaPSS solution, from which the effective rod length and the persistence length of the macroions have been evaluated. Note that the relaxation processes of the conductance anisotropy were first discussed for ionic detergents,llP l2 polyph~sphates~~-'~, graphitic acid colloids,12, l7 deoxyribonucleic acid'' and poly(methacry1ic acid)" using Couette apparatus. This work is the first reported c.s.f.determination of rotational relaxation times for flexible macroions. This new and convenient method should aid further studies of the rotational motion of a variety of flexible macroions and colloids such as deoxyribonucleic acids, tobacco mosaic virus, polyphosphates and imogolite colloids. 703704 Relaxation of Poly(styrenesu1phonate) Experimental Materials Six samples of NaPSS with molecular weights ( M ) of 4600, 18000, 74000, 400000, 780 000 and 1200 000 were obtained from Pressure Chemicals. The polydispersity indices, MJM,, were 1.07, 1.14, 1.10, 1.30, 2.10 and 1.80, which were determined using gel-permeation chromatography (type HLC8003D, Toyo Soda Co, Ube ; column, type PW) with a microcomputer (CP-8000, Toyo Soda). NaPSS with A4 6000000 was obtained from Polyscience Inc.(Warrington, PA); its polydispersity index was ca. 3. The materials were purified repeatedly using an ultrafiltration cell (model 202 membrane ; types YM-5 and PM-10, Amicon Co., Lexington, Mass) until the filtrate ceased to show U.V. absorbance above 300 nm. The solutions were then treated with mixed-bed ion- exchange resins. [AG50 1 -X8 (D), Bio-Rad Laboratories, Richmond, CAI and then freeze-dried after neutralization with NaOH. The water used for the purification and preparation of solution was obtained using a Milli-Q water system (type I, Millipore, Ltd, Bedford, MA). Potassium chloride (analytical grade) was purchased from Merck. C.S.F. Measurements The Details of the c.s.f. apparatus have been given in previous paper^.^*^ The sample solution from the mixer, which was a Teflon four-jet type, flowed through platinum- plate electrodes (2 mm x 10 mm) which were fixed on opposite walls (2 mm apart) inside the epoxy-resin observation cell.For each run ca. 0.2 cm3 of solution was required. An a.c. current of 50 kHz was applied to the Wien bridge and the applied voltage across the cell was kept at 2V (r.m.s.). The time change for the deviation of the solution conductance from its equilibrium value was amplified in two steps and monitored by a memoriscope and/or digital memory and an X-Y recorder after rectification. The dead time of the c.s.f. measurements was 1 ms. Results and Discussion Rotational Relaxation Times of NaPSS The c.s.f. method for the determination of the conformation of a polyelectrolyte is based on the phenomenon that the electrical conductivity of a solution of charged anisotropic molecules depends on the direction of measurement, if molecular orientation is induced by a velocity gradient in the rapidly streaming so1ution.13* 199 2o With charged anisotropic molecules an increase in conductivity occurs in the flow direction and a decrease is expected in the direction of the velocity gradient (perpendicular to the flow) in a capillary tube.15 This change in conductance affords a convenient means of detecting rotational movement of anisotropic polyelectrolytes. After the flow has been stopped (at time t = 0) in a c.s.f.measurement, the initial state of orientation tends to vanish until a random distribution is re-established. Consequently the initial anisotropies of the electrical conductivity, I C ~ - I C + at t = 0 decrease to zero.This process depends on the dimensions of the molecules and is described by a relaxation function: where IC, is the electrical conductivity at the initial state of orientation, caused by the shear flow in the observation cell. IC+ denotes the conductivity at a random distributionT. Okubo 705 Fig. 1. Typical traces of relaxation of NaPSS solution by c.s.f. measurements at 25 "C. M = 780000, 0.00438 mol dm-3. Full scale = (1) 100 ms, (2) 2 s. 1 Y 4 2 - Fig. 2. Typical traces of relaxation of NaPSS solution by c.s.f. measurements at 25 "C. M = 1200000. Concentrations (in mol dmP3) and full scales (in ms) as follow: (1) 9.89 x 500; (2) 0.00297, 200; (3) 0.0148, 50.at t = co, while K is the conductance at time t . z, is the rotational relaxation time. For anisotropic molecules (rod-shaped or disc-shaped, for example) of uniform size in sufficiently dilute systems, expressions (1) and (2) hold, as Schwarz'' has shown. In the case of non-uniform molecular sizes the relaxation is a superposition of processes of eqn (1) with different D, va1ues.l' From single-exponential traces of conductance values of z, are obtainable. Note that the solution conductivity will change with the measurement frequency. However, this effect may be neglected in our systems (i.e. orientation-induced relaxation), if the frequency remains constant (50 kHz in this case) during the experiment. When the same sample solutions in the two vessels of the c.s.f.apparatus were mixed, conductance relaxation curves were obtained as shown in fig. 1 and 2. Very sharp changes in conductance were observed. Note that the absolute values of conductance were not known in our experiments. The relative change in conductance was recorded on the instrument as a function of time. This situation is satisfactory for a study of relaxation phenomena. In many cases two relaxation times corresponding to fast and slow processes were observed (see curves 1 and 2, respectively, in fig. 1). Since fast (z,) 24 FAR 1706 Relaxation of Poly(styrenesulph0nate) I I I 1 0 0.005 0.01 0.01 5 [ NaPSS]/mol dm-3 Fig. 3. Relaxation times (q) of NaPSS solution obtained by c.s.f. measurements at 25 "C. A4 = 0, 6000000; A, 120000; a, 780000; x , 400000.and slow (7,) relaxation times differ more than ten-fold in magnitude, a deconvolution of the traces was not needed to obtain reliable relaxation times. The faster and slower relaxations are identified as rotational relaxation and the fluctuation of local assemblies of macroions, respectively, as will be described later in detail. In fig. 2 a sharp decrease in z, is seen with increasing polymer concentration. This is explained by a shrinkage of the macroions with concentration (see later). The z, values obtained for NaPSS samples with molecular weights in the range 400000- 6000000 are displayed in fig. 3 as a function of polymer concentration. Clearly, z, decreases with polymer concentration and decreases sharply as the molecular weight decreases. The latter tendency is reasonable, since the rotational relaxation times (7,) of rod-like molecules should be larger for longer rods.Note that the tendency for z, to increase with concentration was observed for an NaPSS sample of molecular weight 6000000 at high concentrations (see open circles in fig. 3). This is ascribed to an entanglement of the macroion chains at high concentrations. This effect was also clear for NaDNA solutions studied by the birefringence-detected stopped-flow (b.s.f.) technique,22 which was first developed by this author and is an effective technique for the study of Brownian rotation of anisotropic molecules. Recently the variation of z, and the conformation of the same NaPSS samples were studied by the b.s.f. technique.22 The c.s.f. and b.s.f. methods gave similar z, and z, values, especially in the dilute concentration region.However, the z, values from c.s.f. were slightly smaller than those from b.s.f. Note that the z, values for NaPSS from the c.s.f. method are also consistent with the reference values obtained by established techniques, e.g. electric birefringen~e,~~-~~ although the molecular weight, concentration of polymer and other experimental conditions differ. The slow relaxation processes (2,) are shown in fig. 4. Generally speaking, the z, value was a hundred times larger than z,. z, decreased with polymer concentration and with decreasing molecular weight . The z, values for M = 65000, 18000 and 4600 were tooT. Okubo 707 10 1 v) \ t-" 0.1 0.01 0.001 I 0 \ 0 -A X \ \ -- -- --- .'-----, 1 I ' - 0 0.00 5 0.01 0.01 5 [ NaPSS]/mol dm-3 Fig.4. Relaxation times (z,) of NaPSS solution by c.s.f. measurements at 25 "C; M = 0, 780000; x , 65000; 0, 18000; 0, 4600; 0, zp; M = 780000. small ( < 1 ms) to be measured by the c.s.f. method. The z, process might be due to the slow fluctuation of local assemblies of macroions, although this is speculative and remains an open question. Effective Rod Length of NaPSS estimated from the Rotational Relaxation Times The effective length (L) of rod-like NaPSS molecules can be estimated from the z, values using eqn (3)l' and eqn (4)27 for simple rods and cylinders, respectively: L = (48 kTz,/xqo)i (3) z, = (xq0L3/18 kT)[ln(L/b)- 1.57+7{[1/ln(L/b)]-0.28)2]-1 (4) The results are shown in fig. 5-8 for NaPSS samples of molecular weight 4 x lo5, 7.8 x lo5, 1.2 x lo6 and 6 x lo6, respectively.k and T are the Boltzmann constant and the absolute temperature of the solution. qo denotes the viscosity of solvent and b is the radius of the cylinder. The main results are as follows: (1) at dilute concentrations < mol dm-3, L is very close to the contour length calculated from the monomer length (0.25 nm) and the degree of polymerization. This means that the conformation of NaPSS is stretched in dilute concentrations. However, for NaPSS samples having extremely high molecular weights, the conformation is regarded as an expanded coil 24-2708 Relaxation of Poly(styrenesulphonate) 1 I I X 1000 -j - stretched rod _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _-- X - stretched rod I 500 - - ----------------- -- O x 800 - - 0 - 0 - 3 400 - - 8 \ QD 600 - 4 - X QD X - - @ @ O O 400 - - 300 - I - 1 1 0 0.0 1 0 0.0 1 0.0 2 E 1 4 800 600 400 b - I - - - - - a 1 I I I ) a a Q n I7280 2000 CI I E E a 1500 \ 4 1000 500 T.Okubo I I I I I I stretched rod _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ - _ - _ - ---- 1 I I I I 1 709 0 0.01 [NaPSS]/mol dm-3 Fig. 8. Rod length of NaPSS estimated from the c.s.f. measurements at 25 "C; A4 = 6000000; 0, cylinder (Broersma, radii = 0.8+D, nm); X, rod (Kuhn). even at low concentrations. (2) L sharply decreased with polymer concentration. The conformation of NaPSS begins to shrink with increasing concentration, which is reasonable if we take into account the increased effect of electrostatic shielding on che intra-macroion repulsion with polymer concentration.(3) The important contribution of the Debye screening length was clarified. The estimation of L values from Broersma's equation [eqn (4)] was carried out using two different radii for the macroion cylinder; one is the crystallographic size and the other is the effective size, including the thickness of the double layer around the macroions (Debye ;creening length). The Debye length, D,, in the absence of added salt is given by (4nBn)-u. Here B denotes the Bjerrum length (0.719 nm at 25 "C in H,O) given by e2/ckT, where e is the electronic charge and E is the dielectric constant of the solvent. n is the concentration of the free-state gegenions. The fraction of free gegenions was assumed to be 0.30.28 Open circles in fig.5-8 show the values of L obtained from the effective size, and triangles in fig. 7 are from the crystallographic size. The triangles are at higher values than the other symbols, which are more closely spaced to one another. This situation is consistent with other experiments on the rotational movement of anisotropic colloids, where the Debye length was important.' The significance of the Debye length has often been pointed out Fig. 7. Rod length of NaPSS estimated from the c.s.f. measurements at 25 "C; A4 = 1200000; 0, cylinder (Broersma, radii = 0.8 + D,nm); A, cylinder (Broersma, radii = 0.8 nm); x , rod (Kuhn); a, Harris-Rice (end-to-end distance) ; 1, Harris-Rice (longest length in a single chain).710 I, " ' I ' stretched rod - x x X A 0.03 -0 A A x x - A 0 O 0 v) 1 6 6 5 a X E3 X X 0.02 - 0 e 0~ d a 8 0.01 ' !! ' I A1 Relaxation of Poly(styrenesu1phonate) 1300 1200 5 \ 4 -1100 -m for the solution properties of various colloids and polyelectrolytes in salt-free systems, such as the ordered suspension 30 the conformation of flexible poly- electrolyte~~l-~~ and dynamic 35 The conformation of NaPSS is discussed here in terms of Harris-Rice the~ry,~',~' in which an equivalent chain with ionic charges carried by the monomer units concentrated at the midpoints of each statistical chain element was considered.The end-to-end distance was estimated from the statistical chain length and the bond angle using the Harris-Rice theory. The solid circles in fig. 7 show the end-to-end distance thus obtained. Bars are the longest intra-segment length in a single chain estimated by the Harris-Rice theory and by a computer simulation carried out independently 200 times.It is interesting that the L values estimated from z, agree excellently with the calculated values obtained from Harris-Rice theory at all concentrations examined. In calculations using the Harris-Rice theory, the statistical chain length was taken as 100 nm, and the number of statistical chains, the molecular weight of NaPSS, the dielectric constant of the solvent and the total charge of the statistical chain were assumed to be 14, 1.2 x lo6, 78 and 160, respectively. These values correspond to the experimental conditions in fig. 7. The conformations given in the figure are two-dimensionally projected ones and are only typical examples among many conformations.Clearly, at low polymer concentrations, NaPSS is fairly stretched near the rod, but at 0.03 mol dm-3, the molecule shrinks and is in its expanded-coil state. Fig. 9 shows z, and L in the presence of KC1 and in their absence. On addition of KCl, the MaPSS molecules shrank. In the absence of salt, the molecules are clearly highly expanded and shrink rapidly with addition of salt. Persistence Length of NaPSS estimated from the Rotational Relaxation Times For the worm-like chain model of macroions, z, is given as a function of the total persistence length (4) and the diameter of the rod:38 z, = (q0qh2M2/12 k T q ) [0.1265(hM/qM0)~+0.159 In(2q/b)-0.387+0.16(b/~i)]-~ ( 5 )T. Okubo 71 1 [NaPSS]/mol dm-3 Fig. 10. Persistence length of NaPSS in solution.M = 0, 400000; 0, 780000; x , 1200000; a, 6000000; (------) Odijk theory. Table 1. Some characteristic distances of NaPSS solution molecular concentration Dl(= K - ~ ) L L, q weight x low4 /mol dmP3 /nm /nm /nm /nm q/L, 40 2.54 x 10-3 8.45 x 10-3 78 6.13 x 10-4 1.75 x 10-3 120 6.93 x 10-4 1.98 x 10-3 2.40 x 10-3 1.31 x 1.48 x 600 2.40 x low4 13 7 27 16 6 26 15 6 44 14 530 350 920 890 470 1150 950 630 1090 900 48 5 485 947 947 947 1460 1460 1460 7280 7280 350 98 480 390 65 380 210 67 46 24 0.73 0.20 0.5 1 0.42 0.069 0.26 0.14 0.046 0.0063 0.0033 where h is the length of the monomer unit ; M, and M are the monomer molecular weight and total molecular weight of the macroion, respectively; a and b are both the Stokes's diameter of rod.The persistence lengths of NaPSS thus estimated from eqn (5) are shown in fig. 10. Remarkably large values were obtained for q, which decreased sharply with polymer concentration. A decrease of q with degree of polymerization was also observed. The total persistence length, q, is the sum of two contributions: qp is the 'intrinsic' part of the corresponding neutral chains and is reported to be 1-2 nm for NaPSS.33i 3 9 9 40 For low polyelectrolyte concentrations and in the absence of salt, the conditions KL, & 1 is verified and qe, the electrostatic persistence length, is given by Odijk's equation :41 q e = [1/(4B~~)I[1-8/(3~&)1* (7)712 Relaxation of Poly(styrenesulphonate) B is the Bjerrum length and L, denotes the contour length. Odijk32 proposed that the chain is stretched with qt/L, 1 and near the rod limit with qJL, >, 2.From table 1 it is clear that q/L, varies from 0.0033 to 0.73 and decreases with polymer concentration and increasing molecular weight. Le B ~ e t ~ ~ also discussed the values of the persistence length of a polyelectrolyte obtained theoretically, which were slightly larger than those obtained from Odijk theory. The single-chain conformation is considered to be fairly stretched, but far from fully stretched near the rod limit. This conclusion on the conformation in the absence of salt is very similar to the reports of Drifford and D ~ l b i e z ~ ~ and Tricot and Hou~sier.~~ References 1 T. Okubo, J. Am. Chem. Soc., 1987, 109, 1913. 2 T. Okubo, H. Kitano, T. Ishiwatari and N. Ise, Proc. R.Soc. London, Ser. A , 1979, 366, 81. 3 T. Okubo, Biophys. Chem., 1980, 111,425. 4 T. Okubo and A. Enokida, J. Chem. SOC., Faraday Trans. I , 1983, 79, 1639. 5 T. Okubo, K. Hongyo and A. Enokida, J. Chem. Soc., Faraday Trans. I , 1984, 80, 2087. 6 H. Kitano, J. Hasegawa, S. Iwai and T. Okubo, Polym. Bull., 1986, 16, 89. 7 M. Sawamoto, T. Higashimura, A. Enokida and T. Okubo, Polym. Bull., 1980, 2, 309. 8 T. Okubo, Dynamic Aspects of Pollelectrolytes and Biomembranes, ed. F. Oosawa (Kodansha, Tokyo, 9 T. Okubo, Makromol Chem. Suppl., 1985, 14, 161. 10 H. Kitano, J. Hasegawa, S. Iwai and T. Okubo, J. Phys. Chem., 1986,90,6281. 11 K. Heckmann, 2. Phys. Chem., N. F., 1959, 9, 318. 12 K. G. Gotz and K. Heckmann, J. Colloid Sci., 1958, 13, 266. 13 U. Schindewolf, Z.Elektrochem., 1954, 58, 697. 14 M. Eigen and G. Schwarz, J. Colloid Sci., 1957, 12, 181. 15 G. Schwarz, Z. Phys. Chem., N . F., 1959, 19, 286. 16 K. Heckmann and K. G. Gotz, Z. Elektrochem., 1958, 62, 281. 17 K. G. Gotz, J. Colloid Sci., 1965, 20, 289. 18 E. E. Kern and D. K. Anderson, J. Polym. Sci., Part AI, 1968, 6, 2765. 19 K. Heckmann, Naturwissenschaften, 1953, 40, 478. 20 B. Jacobson, Rev. Sci. Instr., 1953, 24, 949. 21 G. Schwarz, Z. Phys., 1956, 145, 563. 22 T. Okubo, J. Phys. Chem., 1987, 91, 1977. 23 K. Yamaoka and K. Ueda, J. Phys. Chem., 1980,84, 1422. 24 K. Yamaoka and K. Ueda, Bull Chem. Soc. Jpn, 1983, 56,2390. 25 K. Yamaoka and K. Ueda, Chem. Lett., 1983, 545. 26 N. Ookubo, Y. Hirai, K. Ito and R. Hayakawa, Polym. Prepr., Jpn, 1984, 33, 896. 27 S. Broersma, J. Chem. Phys., 1960, 32, 1626. 28 G. S. Manning, Q. Rev. Biophys., 1978, 2, 179. 29 S. Hachisu, Y. Kobayashi and A. Kose, J. Colloid Interface Sci., 1973, 42, 342. 30 T. Okubo, J. Chem. SOC., Faraday Trans. I , 1986, 82, 3163; 3185. 31 C. J. Barenes, D. Y. Chan, D. H. Everett and D. E. Yates, J. Chem. Soc., Faraday Trans. I , 1978, 74, 32 R. Giordano, G. Maisano, F. Mallamace, N. Micali and F. Wanderlingh, J. Chem. Phys., 1981, 75, 33 M. Drifford and P. Dalbiez, J. Phys. Chem., 1984, 88, 5368. 34 J. N. Shaw and R. H. Ottewill, Nature (London), 1984, 208, 681. 35 J. R. Goff and P. Luner, J. Colloid Interface Sci., 1984, 99, 468. 36 F. E. Harris and S. A. Rice, J. Phys. Chem., 1954,58,725. 37 S . A. Rice and F. E. Harris, J. Phys. Chem., 1954, 58, 733. 38 J. E. Hearst, J. Chem. Phys., 1963, 38, 1062. 39 G. Weill and G. Maret, Polymer, 1982, 23, 1990. 40 G. Weill, G. Maret and T. Okijk, Polymer Commm., 1984, 25, 147. 41 T. Odijk, J. Polym. Sci., Polym. Phys. Ed., 1977, 15, 477. 42 M. Le Bret, J. Chem. Phys., 1982, 76, 6243. 43 M. Tricot and L. Houssier, Macromolecules, 1982, 15, 854. 1982), p. 11 1. 136. 4770. Paper 612225; Received 18th November, 1986

ISSN:0300-9599    

DOI:10.1039/F19888400703

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I , 1988, 84(3), 713-727 A Study in Preferential Solvation using a Solvatochromic Pyridinium Betaine and its Relationship with Reaction Rates in Mixed Solvents J. Graham Dawber,* John Ward and Richard A. Williams Department of Chemistry and Biology, North Staflordshire Polytechnic, Stoke-on- Trent ST4 2DE The ET polarity values for binary liquid mixtures, derived from the solvatochromic behaviour of a pyridinium betaine, have been used to investigate the preferential solvation of the betaine by the components of 17 binary solvent mixtures. In systems involving two organic liquids the ET values have been interpreted in terms of the solvation of the betaine by the more polar component and some agreement with previous n.m.r. studies was obtained. In aqueous-organic solvent mixtures, however, the betaine was preferentially solvated by the organic component.The findings have been related to the rates of solvolysis in mixed-solvent systems, and preferential solvation is suggested as a reason for the solvent dependence of the rates of reaction. It has long been known that solvent polarity has an important effect upon chemical reactions,'-5 and there have been several attempts to correlate empirical and experimental parameters of solvents with their polarity and solvation properties.' One of the more successful approaches to solvent polarity is that of the ET values' which depend upon the solvatochromic behaviour of the betaine (I)7 [2,6-diphenyl-4-(2,4,6-triphenyl- 1 -pyri- dinio)- 1 -phenolate]. The term solvatochromism refers to the shift of an electronic absorption band when varying the polarity of the medium and the ET values (in kcal mol-l) are calculated from the absorption band position of (I) (1 in nm) by the (1) relationship The solvatochromic changes result from intermolecular solute-solvent interaction forces which usually involve alterations in the electronic ground state or may also involve the excited state of the absorbing species.Since many chemical reactions are carried out in solvent mixtures it is prudent to investigate the possible extension of polarity scales, such as the ET values, to binary ET = 28 590/1,,,. 713714 A So lva t oc h rorn ic Py r idin ium Bet a ine mixtures of solvents. Such mixtures are very useful for studying solvent effects upon reactions since the properties of various mixed solvents can be adjusted continuously by changing the composition of the mixture.Recently, an excellent compilation of ET data for binary liquid mixtures has been presented by Langhals' in which the data have been correlated in terms of a linear relation involving concentration, viz. where ET(m) is the ET value of the liquid mixture, is the ET value of the pure less-polar component, C, is the molar concentration of the more polar component, and ED and C* are empirical parameters of the equation to give a linear variation of ET(m) as a function of In (Cp/C* + 1). While a representation such as eqn (2) provides a good method of cataloguing the overall polarity of mixed solvents at various compositions; it does not represent the possibility of the occurrence of preferential solvation.Since most transition states in chemical reactions involve some form of charge separation in parts of the molecule relative to the reactants it is likely that preferential solvation of reactants or transition state will occur by one of the components of the solvent mixture, and consequently this could influence the rate of reaction. In the case of the betaine (I) we have shown by 13C n.m.r. spectroscopyQ that it is possible to construct preferential solvation profiles for four binary solvent mixtures and that changes in solvation occur near the charged sites of the molecule. The differences in solvation at these centres, which produce the extreme solvatochromic properties of the betaine, are also transmitted to other parts of the molecule.It was thought worthwhile to study the solvatochromism of (I) in the solvent mixtures previously studied by ~ ~ . m . r . ~ and other solvent mixtures using the usual visible electronic absorption spectrum in an attempt to demonstrate the presence of preferential solvation of (I) in mixed solvents. In addition, some of the data presented by Langhals' were also examined. The purpose of this approach is that other polar molecules, e.g. reactants and transition states in chemical reactions are likely to undergo preferential solvation in a parallel manner to that of (I) in mixed-solvent systems. Experimental 2,6-Diphenyl-4-(2,4,6-triphenyl-l-pyridinio)-l-phenolate (I) The synthesis of this compound involved the condensation of 2,4,6- triphenylpyrilium perchloratelo* l1 with 4-amino-2,6-diphenylphenol12 to give the perchlorate salt which is the precursor of (I); m.p.273-274 "C, (30% yield). Treatment of the perchlorate salt with sodium methoxide in methanol as described by Dimroth et al.' gives, after recrystallisation from methanol-water (1 : 3), the required betaine (I), m.p. 269-273 "C. The Visible Absorption Spectra Absorption spectra of (I) in various single solvents and their binary mixtures were measured on a Varian DMS 100 spectrophotometer which is a microprocessor- controlled instrument on which absorption peak positions are identified electronically. The solvents used were acetone, methanol, ethanol, propan- 1-01, dimethyl sulphoxide (DMSO), chloroform, formamide, tetrahydrofuran (THF), pyridine, 1,2-dimethoxy- ethane and dimethylformamide (DMF).Each solvent was dried with anhydrous MgSO, and/or MgC10, and then used immediately. Binary solvent mixtures used were DMSO-acetone, propan- 1 -01-CHCl,, methanol-CHCl,, formamide-DMSO, DMSO- CHCl,, propan- 1 -01-acetone, acetoneCHCl,, methanol-THF, water-THF, water- methanol and water-acetone. The data for the solvent mixtures of methanol-acetone,J . G. Dawber, J . Ward and R. A . Williams 715 Table 1. PT values for selected pure liquids liquid A,,,/nm E",/kcal mol-1 acetone DMSO propan- 1-01 CHCl, formamide methanol THF ethanol pyridine 1,2-dimethoxyethane DMF 673.3 630.9 563.6 695.3 505.7 512.7 759.7 550.9 693.9 718.3 648.3 42.5 (42.2) 45.3 (45.0) 50.7 (50.7) 41.0 (39.1) 56.5 (56.6) 55.8 (55.5) 37.6 (37.4) 51.9 (51.9) 41.2 (40.2) 39.8 (38.6) 44.1 (43.8) ethanol-acetone, methanol-CH,CN, water-propanol, butan- 1 -01-CS, were calculated from the compilation given by Langhak8 The ET value of water was taken as 63.1.In the case of the solvent CHC1, the absorption spectrum of (I) was measured not only on the Varian DMS 100 but also on a Varian DMS 90, a single-beam Philips PU8620 diode array spectrophotometer and also a manual digital spectrophotometer (a Cecil CE 393). The reason for this was that there was some discrepancy between the position of the band maximum (Amax) for our results and the earlier work. The values we obtained for A,,, on the four instruments used were 695.3, 696, 700, and 695 nm, respectively. Results and Discussion The values of A, and the corresponding ET values for the pure organic liquids are given in table 1.The ET values have been calculated in kcal mol-l, as is customary, rather than as kJ mol-l, and the values in the majority of cases are in quite good agreement with the older published values1* l3 (given in parentheses). One notable exception was ET for CHCl,. In an extensive and authoritative review of polarity scales, Griffiths and Pugh13 give ET = 39.1 for CHCl,, which is considerably different from our value of 41 .O. These author^'^ also show the good correlation between the ET values for a number of solvents and their corresponding 2 value [fig. 2 of ref. (13)], and ET = 41.0 for CHC1, would fall off their line. This led us to measure ET for CHC1, using four different spectrophotometers and several CHCl, samples over a period of two months, and our value of 41 .O was unchanged.The effect of dilution of solutions of (I) in CHC1, was also studied and even a solution with an absorbance as low as 0.05 in a 5 cm pathlength cell gave an unchanged A,,, value measured on the DMS 100 spectrophotometer. The fact that our value of ET for CHCl, now falls significantly off the correlation of Griffiths and Pugh13 led us to measure ET for a further three low-polarity solvents; namely, pyridine, 1,Zdimethoxyethane and DMF, all of which lie in a similar region to CHC1, with regard to the correlation of ET with 2 for s01vents.l~ Again for these solvents our values of ET are significantly higher than previous data,13 as was the case for CHC1, (table 1).Our ET values for the systems studied are plotted as a function of Z13 and compared with the earlier results (fig. 1). The correlation of the two polarity scales for our ET values, although slightly different, is equally good (correlation coefficient = 0.997) and the relationship between ET and 2 is given by ET = 0.72 2-4.9. In the subsequent work with solvent mixtures in which CHCI, was one of the components we chose to use our value of 41 .O for ET. For the mixed-solvent systems studied the values of ET were calculated from Amax and ET was plotted as a function of the mole fraction of the components in the liquid state.716 A Solvatochromic Pyridinium Betaine Fig. 1. Relationship between ET and 2: 0, ET from ref. (13); +, ET from this work.This was also done for the five solvent mixtures chosen from Langhals data,8 which involved calculating E T at selected mole fractions of liquid. If the solvation of (I) by the components of a binary solvent mixture is random, i.e. non-specific, then one would expect a linear relationship between ET of the mixture (3) [ET(m)] and mole fraction, i.e. ET(m) = .EoT(1) + q ( 2 ) X 2 where FTfl) and FT(2) are the values for the pure liquids and Xl and X2 represent the mole fractions of the component solvents in the mixture. Examples of the plots obtained are shown in fig. 2 and 3 for methanol-THF and water-methanol, respectively, where it can be seen that the graphs are non-linear. This non-linearity is likely to arise from preferential solvation of (I). The non-linear behaviour in mixed solvents of some property of a solute as a function of solvent mole fraction has been used on a number of occasions to make deductions concerning solvation.For example, the specific solvation of the iodide ion has been studied by its U.V. absorption spectral shifts in mixtures of CH,CN with the protic solvents methanol, water and ethylene glycol. The results showed14 that I- is specifically solvated by H,O and MeOH in preference to CH,CN in mixed solvents. Also, the non- linear behaviour of n.m.r. chemical shifts with solvent composition has been used on several occasions to deduce preferential solvation. Thus C1- is preferentially solvated by H20 in H,O-CH,CN mixtures, but there is fairly even competition of solvation in the H,O-DMSO mixtures.l5 Similarly, the preferential solvation of the tris-acetonyl complexes of Co"' and CrIII in CHC1,-CCl4 mixtures has been studied by n.m.r.,lS as has the preferential solvation of alkali-metal cations by H202 i n d s mixtures with water.17 More recently the roles of preferential solvation and solvent-solvent interaction on the rates of nucleophilic substitution reactions involving anions in binary mixed solvents have been studied.18 In MeOH-CH,CN mixtures it was concluded that specific solvation of Br- with MeOH was an important factor in the rate of reaction, whereas in MeOH-dimethylacetamide mixtures solvent-solvent interaction plays a significant part. Admittedly changes in solvent liquid structure from that of the pure liquid may also contribute to the observed deviation' of ET(m) from linearity in the mixed-solvent systems.Certainly changes in solvent liquid structure do occur in mixed solvents [see ref. (19) and references therein], but in this work it is assumed that the major reason for the deviation of ET(m) from a linear function of mole fraction is due to preferential solvationJ. G. Dawber, J. Ward and R. A. Williams 717 55 - 50 - I 1 1 0.2 0.4 0.6 0.8 XM~OH Fig. 2. values for methanol-THF mixtures. Fig. 3. ET(m) values for water-methanol mixtures. phenomena. In the case of fig. 2, methanol appears to be preferentially solvating (I) rather than THF, and in fig. 3 methanol appears to solvate preferentially to water. A convenient method of representing the departure from a linear relationship of ET(m) is by making use of the concept of an excess function, AET, such that ET(rn) = q ( 1 ) xl + '%(2) xZ + AET' (4) AET values were calculated for each of the solvent mixtures and these were then plotted against mole fraction.The values of AET for all the systems studied are given in table 2. From these values and the ET values given in table 1 the values of ET(m) can be re- calculated. A positive AET represents preferential solvation of (I) by the more-polar component of the solvent mixture, whereas a negative AET represents preferential solvation by the less-polar component. The AET values for binary mixtures of acetone with five more polar liquids are given in table 2. The order of preferential solvation by the more-polar component appears to be methanol > ethanol > CHCl, > DMSO.This order, however, can only be qualitative since not only is AE, a function of the preferential solvation, but also of the difference between EoT(l) and EoT(2), and an alternative method of estimation of preferential solvation is described later. The data in table 2 also show that in mixtures of CHCl, with methanol, propanol and DMSO that preferential solvation of (I) by the more-polarTable 2. AET values in solvent mixtures at various mole fractions AET/kcal mol-l solvent mole mixture fraction A-B of A: 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 acetone-CHC1, acetone-MeOH ace tone-E t OH acetone-PrOH acetone-DMSO CHC1,-MeOH CHCl,-PrOH CHCl ,-DM SO MeOH-THF MeOH-CH,CN BuOH-CS, formamide-DMSO water-MeOH wa ter-E t OH water-PrOH water-THF water-acetone 0.2 1.3 0.8 1.2 0.2 0.3 0.2 0.4 4.7 5.3 8.3 0.7 -0.5 - 0.8 - 1.0 1.4 3.0 0.6 2.4 1.4 2.4 0.4 0.5 0.4 0.8 6.5 5.6 8.4 1.3 - 1.0 - 1.6 - 2.0 2.0 2.3 0.9 3.7 2.4 3.3 0.6 0.8 0.7 1.2 6.5 5.3 7.7 1.6 - 1.4 -2.1 - 3.0 2.0 1.4 1.2 4.9 3.0 4.0 0.7 1.4 1.1 1.5 6.0 4.8 6.8 1.6 - 1.7 -2.5 - 4.0 1.3 0.3 1.5 6.2 3.6 4.6 0.75 2.4 1.4 1.8 4.6 4.2 5.8 1.6 -2.1 - 2.7 - 5.0 - 0.5 - 0.8 1.7 7.1 4.1 5.0 0.7 2.9 1.6 1.8 4.0 3.6 4.7 1.3 - 2.2 - 2.6 - 6.0 - 2.8 - 1.9 1.85 7.7 4.6 5.2 0.7 3.0 1.6 1.6 2.8 2.8 3.6 0.9 - 2.0 - 2.5 - 6.8 -4.8 -3.1 1.6 6.5 4.8 4.9 0.6 2.4 1.4 1.1 1.8 1.8 2.4 0.6 - 1.6 - 1.9 - 7.2 -6.5 - 3.6 0.95 4.0 4.5 3.5 0.4 I .5 1 .o 0.6 0.7 1 .o 1.2 0.2 - 1.0 - 1.1 - 6.0 - 5.7 - 3.3 bJ.G. Dawber, J. Ward and R. A . Williams Table 3.Preferential solvation numbers (p) in binary solvent mixtures P mole solvent mixturea fraction A-B of A : 0.25 0.50 0.75 acetone-CHC1, MeOH-acetone E tOH-ace tone PrOH-ace t one DM SO-acetone MeOH-CHCl, PrOH-CHCl, DMSO-CHCl, h4eOH-THF MeOH-CH,CN BuOH-CS, formamide-DMSO water-MeOH water-EtOH water-PrOH water-THF water-acetone not obtained 6.1 4.0 3.8 6.4 4.9 4.4 2.9 3 .O 5.2 2.4 2.0 1.3 2.0 1.8 1.3 4.7 15.7 33.0 4.6 3.6 2.4 9.6 8. I 8 .O 13.8 9.0 6.3 1.9 1.8 1.4 0.2 0.2 0.4 0.26 0.26 0.38 0.07 0.11 0.20 1.5 0.9 0.4 I .6 0.9 0.5 - - 22.2 719 a More-polar solvent listed first. component is taking place with methanol again appearing to be the most effective in solvation. It is interesting to compare the I3C n.m.r. resultsg for (I) in the binary solvent mixtures of acetone-DMSO, acetone-CHCl,, CHC1,-methanol and CHC1,-DMSO with the results obtained in this work.In the cases of acetone-DMSO, CHC1,-methanol and CHC1,-DMSO mixtures the n.m.r. resultsg showed that there was preferential solvation of the molecule near the -C-O- group by the more-polar component of the solvent mixture. This correlates perfectly with the spectral findings of the present work if one assumes that the -C-O- group of (I) plays an important role in the electronic transition responsible for the long-wavelength absorption and solvatochromic behaviour of (I). This seems an entirely reasonable assumption since the transition involved is likely to be an intramolecular charge- transfer transition involving the -C-O- group and the delocalised n-electron system of the betaine.The situation is not so clear for the CHC1,-acetone system. The n.m.r. results9 suggest preferential solvation of the -C-O- group by acetone, whereas the present work suggests preferential solvation by CHCl,. However, the graph of ET(*) against liquid composition for this mixture was unique in that there was a maximum in the curve at an acetone mole fraction of ca. 0.7. Acetone and CHCl, are known to associate strongly by hydrogen bonding, and it is possible that in some regions of composition the effective solvent for (I) is the hydrogen-bonded complex rather than either of the two components of the mixture. The FT values of acetone and CHC1, are fairly close together and although it is assumed that it is the solvation of (I) in its electronic ground state which influences the ET value, in such circumstances the excited state may be differently solvated with respect to the ground state. Such a situation would produce a different solvation profile based upon electronic absorption spectroscopy compared to that from n.m.r.spectroscopy. From table 2 the values of AET for methanol-THF and methanol-CH,CN mixtures720 A -3 - 4 Y 00 - 5 4 -6 Solvatochromic Pyridinium Betaine / / / I I I I 0.2 0.4 0.6 0.8 xHzO Fig. 4. Solvolysis of alkyl halides in water-EtOH 0, MeBr; x , EtBr; A, Pr'Br. show that in both cases the more polar methanol is preferentially solvating the betaine (positive AET values). Similarly for formamide-DMSO and butan- 1 -ol-CS, the more- polar component, i.e. fonnamide and butan- 1-01, is preferentially solvating the betaine.The AET data for the solvent mixture involving water as one of the components are interesting. In the cases of water-methanol, water-ethanol and water-propan- 1-01 the results show that over the whole solvent concentration range the betaine is preferentially solvated by the less-polar alcohol than with water (negative AET values). We assume that this must be due to the considerable hydrophobic nature of the betaine over most of its structure. However, for the water-THF and water-acetone mixtures the behaviour is biphasic, i.e. in regions of high mole fraction of water the betaine is preferentially solvated by the organic component, whereas in mixtures of low water concentration the betaine is preferentially solvated by water. A more quantitative estimate of the extent of preferential solvation of (I) at a given liquid composition X can be judged from the us.mole fraction graphs by interpolating on the linear ideal mixture line the value of mole fraction Yl corresponding to the observed ET(m) value (see fig. 1 and 2). The preferential solvation by the more- polar liquid (p) may be represented by the equation P = Yl xz/ y2 X1- ( 5 ) p = 1 corresponds to random solvation or non-specific solvation by either component of the solvent mixture. p > 1 corresponds to preferential solvation by the more-polarJ. G. Dawber, J. Ward and R. A . Williams 72 1 1 I I I 0.2 0.4 0.6 0.8 xH, 0 Fig. 5. Solvolysis of (CH,), CCl in water-MeOH ( x ) and water-EtOH (0) mixtures.22 component, while p c 1 corresponds to preferential solvation by the less-polar component.This procedure allows a more equitable comparison of the different systems to be made in that differences in the individual values of the single liquids should be minimised. The values of p calculated using eqn ( 5 ) at X I values of the more- polar component of 0.25,0.50 and 0.75 for each of the binary mixtures studied are given in table 3. The conclusions from the values of p concerning preferential solvation are the same as discussed above but they do allow a better inter-system comparison to be made. As can be seen in the systems water-methanol, water-ethanol and water-propan- 1-01 the betaine is preferentially solvated by the organic component over the whole composition range. In the case of water-THF and water-acetone mixtures the p values also show the biphasic nature of these systems.A possible application of the results of the present work is in the study of reaction rates in mixed-solvent systems and the correlation of the observed rates with properties of the mixed solvent. In a kinetic study of the solvent dependence of the rates of alkaline722 A Solvatochrornic Pyridinium Betaine 0.2 0.4 a6 0.8 XH,O Fig. 6. Solvolysis of (CH,),CBr in water-MeOH ( x ) and water-EtOH (0) mixtures. hydrolysis of quaternary phosphonium salts20 the results in 70% v/v THF-water and 70 % v/v methanol-water were quite different, the rates being faster in the THF mixture. The dielectric constants for the THF and methanol mixtures with water are ca. 2,O and 40, respectively,21 which is difficult to reconcile with the kinetic findings.However, the p values at these compositions were calculated to be 0.52 in 70 % THF and 0.26 in 70 % methanol, indicating that although (I) is preferentially solvated by the organic component, there is more water associated with the betaine in the aqueous THF than in the aqueous methanol. Thus in the case of the hydrolyses of the phosphonium salts, if the same principle applies, the effect of greater solvation by water in the THF-water mixture as compared to the methanol-water mixture may be sufficient to accelerate the reaction in THF by presenting more water to the polar species in solution. The effect is likely to differ from compound to compound (as was observed in the kinetics of hydrolysis of substrates having different numbers of phenyl groups2'), but the generalisation should be true.In view of this simple correlation with observed kinetics in mixed solvents we decided to examine other published kinetic work in mixed solvents. A very extensive study was made by Winstein and c o w ~ r k e r s ~ ~ - ~ ~ of the solvent dependence of the rates of solvolysisJ. G. Dawber, J . Ward and R. A . Williams 723 I I I I 1 1 1 0.2 0.4 0.6 0.8 XH,O Fig. 7. Solvolysis of (CH,),CBr in water-acetone mixtures.22 of several alkyl halides. We decided first to investigate the possible non-linearity of the rate data with regard to solvent mole fraction to see if the data could be interpreted in terms of preferential solvation. Fig. 4 shows such a plot for three different alkyl bromides, MeBr, EtBr, Pr'Br, in ethanol-water where some deviation from linearity in rate behaviour with solvent mole fraction is observed.The direction of the non-linearity would suggest preferential solvation of the substrate by ethanol and that this is greatest for the more hydrophobic Pr'Br which seems quite reasonable. Fig. 5 shows the corresponding plot for (CH,),CCl in two different solvent mixtures,23 namely methanol-water and ethanol-water. Preferential solvation of the substrate by the organic component of the solvent mixture is apparent, with the greatest effect being for the ethanol mixtures. Similar effects are observed with (CH,),CBr26 (fig. 6) in these two solvent mixtures, showing that the preferential solvation is not primarily724 A Solvatochromic Pyridinium Betaine I 1 I I I I I I 1 I 50 52 54 56 ~ , 5 8 60 62 64 -4 -'t -6 -i 1 55 ,n , 1 1 I I I I 56 57 58- 59 60 61 62 63 ET Fig.8. Correlation of kinetic data22-29 and ET. (a) water-EtOH and water-acetone mixtures: 0, (CH,),CBr in acetone; x , (CH,),CBr in EtOH; A, (CH,),CCl in EtOH; 0, a-phenylethyl chloride in EtOH; 0, neophyl bromide in EtOH. (b) Water-MeOH mixtures; x , (CH,),CBr; 0, a-phenylethyl chloride; A, (CH,),CCl ; 0, neophyl bromide. associated with the halogen leaving-group in the reaction. Analogous graphs were obtained (but not presented here) for the solvolysis kinetics of a-phenylethyl and neophyl bromideZ7~* in methanol-water and ethanol-water mixtures, ethanol showing the greater extent of preferential solvation, and with methanol only showing a slight extent of preferential solvation.The ET data for acetonewater mixtures were unusual in that they showed biphasic behaviour (table 2). The kinetic data for the solvolysis of (CH,),CBr in acetone-water mixtures22* 26 show different behaviour from water-methanol and water-ethanol mixtures (fig. 7). Although the rate constant is not known at zero water concentration, it is clear in fig. 7 that the behaviour is similarly biphasic as the ET data. Thus although the variation of log k with solvent mole fraction may be interpreted in terms of preferential solvation (in a similar manner to the ET and AET data), the question remains whether there is a relationship between ET and log k in solvent mixtures. In fig. 8 are * l-Bromo-2-methyl-2-phenylpropane.J .G. Dawber, J . Ward and R. A . Williams 725 Fig. 9. Correlation of ET with Y:22 0, (CH,),CCl in water-EtOH; water-acetone ; x , (CH,),CCI in water-MeOH. A, (CH,),CBr in Fig. 10. Correlation of ET with solvolysis rate of trans-dichlorobis( 1,2-diarninoethane) cobalt(III):30 (a) = 55 "C; (b) = 25 "C. plotted the kinetic data of Winstein and c o ~ o r k e r s ~ ~ - ~ ~ against our ET data. In fig. 8(a) the data for four alkyl halides in ethanol-water mixtures and one set of data in acetone-water mixtures, and in fig. 8 (b) the corresponding data for methanol-water mixtures are plotted. It can be seen that the graphs are linear for the ethanol-water mixtures and the acetone-water mixtures, whereas for the methanol-water mixtures the plots show considerable curvature.This linear relationship between the solvent polarity as measured by ET and the kinetic data could be due to preferential solvation. In the case of the methanol-water system, however, this cannot be the same since, it will be remembered the kinetic data showed little evidence of preferential solvation, whereas the ET data did show such evidence for (I). It is likely therefore that the kinetic data in methanol-water mixtures are better correlated with some overall function of polarity involving the dielectric constant of the medium.28 The thrust of the work of Winstein and c ~ w o r k e r s ~ ~ - ~ ~ was to obtain a parameter of the solvent (the Y value) which was a measure of the ionising power of the solvent and its solvating power.For many mixed solvents the relationship between Y and mole fraction in the liquid mixture was non-linear,28 but log k was linearly related to Y.29 The relationship between the Y values23 and our ET values for mixed solvents was tested in726 A Solvatochromic Pyridinium Betaine Fig. 11. Correlation of ET with solvolysis rate of 1,2-~hlorothiocyanatobis( 1,2-diarninoethane) cobalt(III):32 (a) 60 "C, (b) 40 "C. fig. 9. It can be seen that the relationship is linear for ethanol-water and acetone-water mixtures, and yet the methanol-water system strangely could be described by two linear portions and it is not clear why this is so. The correlation was also investigated between our ET results and some more recent kinetic studies of solvolysis of inorganic complexes in propan- 1 -01-water mixtures.30-32 In these studies a correlation was found between log k and Y. One might expect, therefore, a similar relationship for our ET values. The results for the solvolysis of trans- dichlorobis( 1,2-diaminoethane) cobalt(Ir1) are plotted in fig. 10, and the corresponding results for the solvolysis of 1,2-chlorothiocyanatobis( 1 ,Zdiaminoethane) cobalt(111)~~ are plotted in fig. 11. In both cases (at two temperatures) the plots are linear. The interpretation previously given31* 32 is that the structural properties of the mixed solvent are a predominant feature in the solvent dependence of the rates of solvolysis. It is possible, however, that the relative extents of preferential solvation of the reactant or the transition state may also have an important influence.Nevertheless, in solvents of low dielectric constant for reactions involving ions one must also consider the possibility of ion pairs,33 since these may lead to failure of correlation of kinetic data in mixed solvents with ET or other polarity scales. Conclusions Making use of the ET values for binary mixtures of liquids based upon the solvatochromic behaviour of (I) it is possible to demonstrate the existence of preferential solvation of the betaine by one of the solvents in the mixture. The ET values can be used to estimate the extent of preferential solvation, either from the direct measurement of ET or from the calculated values using the data of Langhak8 In the latter case the densities of the liquid mixtures need to be known in order to transpose from the molarity scale to the mole fraction scale. Although preferential solvation deduced in this way is specific to (I), it is likely to show parallel behaviour with other polar solutes, and may be used as an aid in the interpretation of kinetic studies in mixed solvents.The kinetics of solvolysis in mixed solvents studied by other workers correlate with the ET valuesJ . G . Dawber, J . Ward and R. A . Williams 727 and a possible interpretation is that preferential solvation could be an important feature in determining the solvent dependence of the reaction rate. The comments of the referees are gratefully acknowledged. References 1 E. S. Amis, Solvent Eflects on Reaction Rates and Mechanisms (Academic Press, New York, 1966).2 Solutions and Solubilities Part II, (Techniques of Chemistry, vol. VIII), ed. M. R. J. Dack (Wiley- Interscience, New York, 1976), (a) M. R. J. Dack, p. 95; (b) G. Illuminati, p. 159. 3 V. Gutmann, The Donor-Acceptor Approach to Molecular Interactions (Plenum Press, New York, 1978), chap. 1 and 9. 4 J. B. F. N. Engberts, in Water-A Comprehensive Treatise, ed. F. Franks (Plenum Press, New York, 1979), vol. 6, p. 139. 5 K. Burger, Solvation, Ionic and Complex Formation Reactions in Non-aqueous Solvents (Elsevier, Amsterdam, 1983). 6 A. Dimroth and C. Reichardt, Palette No. 11 (Sandoz AG, Basel, Switzerland); C. Reichardt, Angew. Chem., Int. Ed. Eng,, 1965, 4, 29; C. Reichardt, Solvent Effects in Organic Chemistry (Verlag Chemie, Weinheim, 1979); C.Reichardt, Liebigs Ann. Chem., 1983, 721. 7 K. Dimroth, C. Reichardt, T. Siepmann and F. Bohlmann, Liebigs Ann. Chem., 1963, 661, 1. 8 H. Langhals, Angew. Chem. Int. Ed. Engl., 1982, 21, 724. 9 J. G. Dawber and R. A. Williams, J. Chem. SOC., Faraday Trans. 1, 1986, 82, 3097. 10 K. Dimroth, G. Arnoldy, S. von Eichen and G. Schiffler, Liebigs Ann. Chem., 1957, 607, 221. 11 K. Dimroth, Angew. Chem., 1960, 72, 331. 12 H. B. Bull, C. A. Soch and G. Oenslager, J . Am. Chem. SOC., 1900, 24, 1. 13 T. R. Griffiths and D. C. h g h , Coord. Chem. Rev., 1979,29, 129. 14 M. C. R. Symons and S. E. Jackson, J. Chem. SOC., Faraday Trans. I , 1979, 75, 1919. 15 C. H. Langford and T. R. Stengle, J. Am. Chem. Soc., 1969, 91, 4014. 16 L. S. Frenkel, C. H. Langford and T. R. Stengle, J. Phys. Chem., 1970,74, 1376. 17 A. K. Covington, T. H. Lilley, K. E. Newman and G. A. Porthouse, J. Chem. SOC., Faraday Trans. 1, 18 Y. Kondo and S. Kusabayashi, J . Chem. Soc., Faraday Trans. I , 1982, 78, 109. 19 J. G. Dawber, J. Chem. SOC., Faraday Trans, 1, 1978,74,1702; 1978,74, 1709; 1979,75,370; 1982,78, 20 J. G. Dawber, J. C. Tebby and A. A. C. Waite, J. Chem. SOC., Perkin Trans. 2, 1983, 1923; Phosphorus 21 Y. Y . Akhadov, Dielectric Properties of Binary Solutions (Pergamon Press, Oxford, 1981). 22 E. Grunwald and S. Winstein, J. Am. Chem. SOC., 1948, 70, 846. 23 E. Grunwald and S. Winstein, J. Am. Chem. Soc., 1948, 70, 854. 24 S. Winstein, E. Grunwald and H. W. Jones, J. Am. Chem. SOC., 1951, 73, 2700. 25 A. H. Fainberg and S. Winstein, J. Am. Chem. SOC., 1957, 79, 1597. 26 A. H. Fainberg and S. Winstein, J. Am. Chem. SOC., 1957, 79, 1602. 27 A. H. Fainberg and S. Winstein, J. Am. Chem. Soc., 1957, 79, 1608. 28 A. H. Fainberg and S. Winstein, J. Am. Chem. SOC., 1956, 78, 2770. 29 S. Winstein, A. H. Fainberg and E. Grunwald, J. Am. Chem. SOC., 1957, 79, 4146. 30 G. S. Groves and C. F. Wells, J. Chem. SOC., Faraday Trans. I , 1982, 78, 619. 31 A. E. Eid and C. F. Wells, J. Chem. SOC., Faraday Trans. I , 1983, 79, 253. 32 A. E. Eid and C. F. Wells, J. Chem. SOC., Faraday Trans. 1, 1985, 81, 1401. 33 Ions and Ion Pairs in Organic Reactions, ed. M . Szwarc (Wiley-Interscience, New York, 1972). 1973, 69, 963; 973. 1127; 1982, 78, 2297; 1894, 80, 2133. Sulphur, 1984, 19, 99. Paper 612439; Received 18th December, 1986

ISSN:0300-9599    

DOI:10.1039/F19888400713

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. I , 1988, 84(3), 729-738 Thermal Decomposition of y-Irradiated Silver Malonate Andrew K. Galwey" Department of Pure and Applied Chemistry, The Queen's University of Belfast, Berfast BT9 5AG, Northern Ireland Patrick J. Herley Department of Materials Science and Engineering, State University of New York, Stony Brook, New York 11794, U.S.A. M. Abdel-Aziz Mohamed Department of Chemistry, Faculty of Science, Assiut University, Qena, Egypt The effects of pre-irradiation with y-rays on the isothermal decomposition kinetics of silver malonate have been studied quantitatively. Such pretreatment reduced the length of the induction period to the onset of reaction and increased the rate of subsequent salt breakdown, which has been shown' to proceed by a nucleation-and-growth mechanism.Between 6.5 x lo6 and 1 . 0 ~ log rad, the shapes of the isothermal, fractional- decomposition us. time curves were unchanged as a result of irradiation and the increase in reaction rate was directly proportional to the pray dose. It is concluded that pre-exposure to ionizing radiation increases the number of nuclei that are active in salt breakdown in direct proportion to the dose to which the sample had previously been exposed. The influence of radiation on the induction period is also discussed; the evidently greater reductions in length, for samples exposed to larger radiation doses, are attributed to radiation damage of the crystal, associated with the presence of radicals detected by an e.s.r. signal. A recent thermal decomposition study' of silver malonate concluded that this was a solid-state nucleation-and-growth process.The mechanism proposed for this crystolysis2 reaction identified the chemical changes at the interface as proceeding through the intervention of intermediates dissociatively adsorbed on the active metal surfaces of the silver product particles. The model proposed envisages heterogeneous, catalytic-type behaviour within the progressively advancing reactant-product contact zone : this is reaction propagation through functional nuclei. The experimental observation' most relevant in providing the stimulus towards undertaking the present work was that the fractional reaction (a) vs. time curves for the isothermal autocatalytic decomposition of silver malonate (which included a well demarcated induction period) were apparently insensitive to pre-irradiation with visible and near-ultraviolet photons.This may result from the presence of the constituent methylene group which hinders steric or electronic cooperative interactions between the two (unsaturated) carboxy groups of the malonate ion. It was therefore of interest to investigate whether higher intensity photons (y-rays) would affect the induction period length and to characterize quantitatively any consequences of such pre-irradiation on the subsequent thermal decomposition of silver malonate. The present work also included a test of whether the previously proposed relationship* between the isothermal induction period, ti, and the radiation dose, @, (1) ti = c, - c, log @ 729730 Thermal Decomposition of y-Irradiated Silver Malonate derived for the thermal decompositions of irradiated inorganic solids, is applicable to silver malonate (c, and c, are constants).The present study characterized quantitatively the consequences of y-pre-irradiation on the subsequent thermal decomposition of silver malonate. Silver salts differ markedly in their behaviour after exposure to ionizing radiation in their initiation of subsequent nucleation-and-growth thermal decomposition reactions, depending on the constituent anion. Silver oxide is completely ~naffected,~ whereas silver halides are extremely photosensitive.' The induction period for the reaction of silver nitrite is anomalously lengthened, while that for silver permanganate* is progressively shortened with increasing doses of y-radiation.No previous study of this type has been made for silver malonate. Experimental Reactant Silver Malonate The reactant salt used throughout the present work was prepared as described previously.' Aqueous solutions of sodium malonate and silver nitrate, in molar proportions 1 : 2, were slowly mixed with continual stirring and held at ca. 340 K for 2 h. A white precipitate settled and remained in contact with the solution at ambient temperature overnight before being filtered and dried in the atmosphere. The composition of the reactant (1 1.2 f 0.2% C and 0.59+ 0.05 % H by combustion analysis) was close to the theoretical requirements for CH,(COOAg), (11.3% C and 0.63% H), but the silver content (66.7 f 0.7 Oh Ag, determined by emission spectroscopy) was (again)' below the theoretical value (67.9 YO).Decomposition Apparatus Isothermal ( f 1 .O K) kinetic studies of salt decomposition were completed in a constant volume apparatus (ca. 1 dm3), in which the extent of reaction was measured from the pressure of evolved gaseous products, principally' CO, with ca. 3 YO CO. A cold trap at 178 K was maintained between the heated reactant and the pressure gauge. Time, temperature and volatile product pressure values were recorded at predetermined time intervals from the output of a Baratron (MKS 222B) absolute pressure gauge and a thermocouple located close to the heated sample. The apparatus has been described in detail previo~sly.~ The computer was programmed to store in its memory only values obtained following a specified minimum pressure rise, so that the frequency of readings was directly related to reaction rate.All measurements were finally obtained in the form of a printed table which also included the calculated fractional reaction, a = P/P, (where P is the pressure measured at the time t and Pf corresponds to the pressure on completion of decomposition). Silver malonate samples for y-irradiation were encapsulated under nitrogen in fused silica ampoules. Each ampoule was wrapped in aluminium foil to exclude light and irradiated at ambient temperature with 6oCo prays in the spent-fuel facility at Brookhaven National Laboratory. The y-ray dose rate was 5.3 x 10' rad h-l. Results and Discussion Influence of y-Irradiation on Subsequent Isothermal Decomposition of Silver Malonate at 466.5K Fig.1 shows a us. time plots for the isothermal decomposition at 466.5+ 1 K of seven samples of silver malonate. Six of these had been subjected to y-doses ranging from 6.5 x 10' to 1.0 x lo9 rad, each specified in the caption, together with comparative data for the unirradiated salt. The irradiation colours silver malonate yellow-brown, and its1.0- 0.8 0.6 ci 0.4 A . K. Galwey, P. J. Herley and M. A . Mohamed - . - . 1 .' . . 73 1 0 40 80 120 t/min 160 Fig. 1. Fractional decomposition a us. time curves for the isothermal decompositions at 466.5 & 1 K of seven samples of silver malonate. Six salt samples were pre-exposed to a range of y-irradiation doses and a control, unirradiated sample has also been included : (a) unirradiated, (b) 6.5 x los, (c) 2.0 x lo7, ( d ) 7.5 x lo7, (e) 2.5 x lo8, (f) 6.9 x lOS, (g) 1.0 x lo9 rad.reactivity increases systematically with progressively larger irradiation doses. The a us. time curves, however, remained sigmoid-shaped over the entire dose range investigated, and the maximum slope of each occurred after a well defined initial acceleratory process. Reproducibility of kinetic behaviour was most satisfactory (see below). Kinetic Analyses Quantitative comparisons were made between the shapes of the a vs. time plots obtained for the unirradiated salt and for two relatively highly irradiated samples (2.5 x 10' and 6.9 x 10' rad). This comparison is shown in fig. 2, where time values for the more rapid reactions of the irradiated samples have been appropriately scaled and the induction periods (discussed below) adjusted suitably to permit close juxtaposition of the three sets of data.The coincidence of the curves throughout their lengths is excellent. The differences between measurements for the unirradiated salt and the 2.5 x los rad sample, between 0.05 < cc < 0.9, were always below the estimated experimental error, i.e. < 0.5%. Similar agreement was found for the 6.9 x 10' rad sample except for a systematic deviation of ca. 1.0% during the final stages of decomposition, a > 0.8. We therefore conclude that the characteristic shape of the a vs. time decomposition curve was not detectably changed in the range 0.05 < a < 0.8 by pre-irradiation. Rate coefficients for this reaction, however, systematically increased with the pray dose to which the salt had been exposed.Activation Energy The influence of temperature on the decomposition rate of the irradiated reactant was investigated for the 7.5 x lo7 rad sample. This relatively lightly irradiated material was selected to enable the comparative measurements to extend across the same temperature interval as that studied previously.' Rate coefficients were measured from median732 Thermal Decomposition of y-Irradiated Silver Malonate a 0.6 1.I 8' . 0' 0 .d' . o , . * ' 5 8 11 14 17 I 1 1 I scaled time Fig. 2. The shapes of a us. time eurves for salt decompositions at 466.5+ 1 K are compared quantitatively for three samples of silver malonate; one was not irradiated ( a ) and the other two had been exposed to doses of 2.5 x 108 (0) and 6.9 x 108 (0) rad.The time axes for the latter two reactants have been scaled and appropriately displaced. Irradiation resulted in no appreciable change in kinetic characteristics. regions, 0.3 < a < 0.8, of the very satisfactory linear Avrami-Erofe'ev (n = 3) plots1" obtained with a' = (a-0.05)/0.95 (to allow for the 5 % gas evolution that occurred during the induction period). The conventionally calculated activation energy, 175- 10 kJ mol-' (466-480 K), agreed with the mean (169f6 kJ mol-l) of the values reported for the five salt preparations studied in our previous work.' The pre- exponential term, log (Als-') = 16.12f0.15, was appreciably larger than the mean of the values already reported, log (Als-') = 15.34f0.75.The more rapid onset of decomposition in the irradiated material cannot therefore be a consequence of a diminution of the energy barrier to reaction. The increase in rate results solely from a rise in the pre-exponential term, presumably owing to the development of a larger effective area of reaction interface as a consequence of an increase in the number of active nuclei participating in the decomposition. Product Gas Yielak The yield of gas evolved on completion of decomposition, a = 1.00, did not detectably change with an increase in the dose of y-irradiation to which the salt sample had been exposed. All yields agreed within 1 %, which is regarded as the experimental accuracy. Thus no detectable (radiolysis) gaseous products are evolved; also, because no rapid initial gas release was apparent on the initial reactant heating, we consider it to be unlikely that gaseous products remained trapped inside the solid.Radiation Damage to the Crystal E.s.r. examination of samples of silver malonate after exposure to y-irradiation show that the high-energy radiation had generated long-lived radicals that were retained within the crystal structure. A complex characteristic spectrum is illustrated in fig. 3, the form of which was independent of dose over the range studied here. This evidence of radical formation is ascribed to radiation damage because no such response, or anyA . K. Galwey, P . J . Herley and M . A . Mohamed 733 - 10 G g = 1.9884 Fig. 3. E.s.r. response trace for a sample of irradiated salt (7.5 x lo7 rad).Identical curves were found for all samples of the present y-irradiated salts but no response was found for the untreated prepared reactant. It is concluded that radiation damage results in radical formation but the identities of the species generated could not be established. other, was found in the untreated prepared salt, and only a single, broad symmetrical peak was found in partially decomposed, but unirradiated, salt. Attempts to characterize the radical species contributing to the spectrum in fig. 3 were not productive. However, this result positively demonstrates that the passage of high-energy radiation generates long-lived radical species and that these entities, or other concurrent modifications of the reactant, including crystal-structure damage (colour centres) presumably constitute the precursors to (irradiation-induced) decomposition nuclei.The Promotional Effect of Radiation Pre-irradiation of silver malonate did not change the shape of the a us. time curves for subsequent thermal decomposition; the (scaled) a us. time curves for the irradiated salt could be precisely superimposed on those for untreated reactant (fig. 2). Again' both sets of data obey the Avrami-Erofe'ev equation,1° with n = 2 or n = 3 (although over slightly different ranges of a). Thus it follows that apparent rate constants can be used for quantitative comparisons of the reaction rates. The promotional effect of radiation can be expressed through the use of a scaling factor, s, in the Avrami-Erofe'ev equation : where s(t - ti) is the scaled time, and each induction period (ti) is that (Avrami-Erofe'ev)'' value which gives the closest coincidence on scaled superimposition"? l2 of the measured data for the sample of irradiated salt with that for unirradiated salt.? The values of s and ti reported in table 1 were obtained using this method of comparative analysis applied to the data in fig.1 for the decomposition at 466.5 1 .O K of irradiated silver malonate. Values of ks measured for both applicable forms of eqn (2) (n = 2 and n = 3) were in close agreement across the median reaction interval (ca. 0.2 < a < 0.7), giving the mean values of ks and s recorded in table 1. Values of ti were, however, more sensitive to the significantly different shapes of these alternative kinetic relationships, and two values have been recorded, derived with n = 2 and with n = 3.[-In (1 - a)]''n = ks(t - ti) (2) t We thank a referee for suggesting this approach to the analysis of our results.734 +1.0 n - I 3 2 0.0- M - -1.0 Thermal Decomposition of y-Irradiated Silver Malonate - - - Table 1. Scaling factors and induction periods expressing the influence of y-pre-irradiation dose on the thermal decomposition of silver malonate at 466.5 & 1 K 0 - I I I tc/min irradiation dose/rad kslmin-'" Sb n = 2 n = 3 none 6.5 x lo6 2.0 x 107 7.5 x 107 1.0 x 109 2.5 x lo8 6.9 x lOa uncertainty 0.0154 0.0161 0.02 15 0.0345 0.0606 0.1054 0.250 +5% 1 .ooo 1.045 1.396 2.42 3.94 6.84 16.23 - +5% 68 64 66 65 51 36 31 2 46 45 53 56 46 33 30 2 a From kinetic analysis : f(a) = Ks(t - ti).Scaling factor. Induction periods from Avrami-Erofe'ev rate equation. r A plot of log (s- 1) against log,, Q, is shown in fig. 4. The slope of the line drawn is unity : the enhancement of the reaction rate is therefore directly proportional to the pre- irradiation dose across the range investigated. The largest dose (1.0 x log rad) resulted in a ca. 16-fold increase in rate of salt decomposition. Kinetic expressions formulated for nucleation-and-growth processes proceedlo from the assumption that the reaction rate is proportional to the area of the effective reactant-product contact surface. For the present system it appears that the chemistry of the main decomposition reaction is unaltered by the pre-irradiation (because this pretreatment caused no perceptible change in reaction stoichiometry, kineticA .K. Galwey, P . J. Herley and M. A . Mohamed 735 characteristics or activation energy). The observed pattern of behaviour is therefore convincingly and completely explained by the conclusion that exposure to y-rays introduces into the crystalline solid either lattice imperfections or reactive species that constitute effective precursors to growth nuclei. The transformation of these radiation- induced species into growth nuclei and their subsequent development are kinetically indistinguishable from the nucleation processes occurring in unirradiated salt. The numbers of such y-ray generated nuclei are directly proportional to the irradiation dose Re-examination of the electron micrographs obtained as part of the previous study' now lead us to estimate that decomposition of each individual, unirradiated silver malonate crystallite involved the development of a small number of nuclei, or perhaps even a single nucleus.It therefore follows that in the present work for those crystals which had been subjected to the largest y-ray dose (1.0 x lo9 rad) each crystallite, composed of perhaps 10l2 molecules, developed ca. 16-50 nuclei. Obedience to eqn (2) suggests that their spatial distribution is random, and subsequent growth is (again)' subject to the dimensional constraints imposed by the lath-shaped reactant particles and the effects of overlap.1o Attempts to estimate microscopically the numbers of nuclei generated within individual crystallites were ultimately unsuccessful.The fine texture of the residual product made it impossible for us to recognize the boundaries of individual nuclei; the features of interest were below the limits of resolution by our technique. (fig- 4). The Induction Period The induction period includes" the initial slow processes which subsequently culminate in the establishment of the growth interface that is the active participant in the main nucleation-and-growth reaction. The changes through which the reactive precursors to nucleus development are converted into active zones are slow before the autocatalytic properties of the developed nuclei are fully realized. These nucleation precursors may be identified as intrinsic imperfections in the reactant or radiation damage to the crystals.We conclude that similar, if not identical, kinetic processes result in the transformation of both types of precursor into growth nuclei. The nature of the steps by which intrinsic or radiation-induced imperfections are converted into identical growth nuclei have not been characterized, but presumably include the following. First there may be slow changes at the ionic level, in which individual components of the crystal in the immediate vicinity of the imperfection interact to evolve a minute product grouping, possibly even a single or a small cluster of a few silver atoms. Secondly, the early growth of such a small germ nucleus may be slower than that later attained; the properties of such small assemblages of silver atoms can be expected to be different from those of larger metallic crystals.The induction period in nucleation-and-growth reactions is not an unambiguous term and may be defined in several alternative, but equally acceptable ways. For the present purposes we identify ti as the time required for the precursors to nucleus generation to be transformed into fully active growth nuclei. This is identified here as the time interval prior to obedience to the Avrami-Erofe'ev equation:'O two values are listed (table 1) because two forms of this rate relation provide equally acceptable' fits to the measured data. The principal conclusion reached from the comparative consideration of the data in table 1 is that the induction period does not appear to be reduced systematically with y- ray dose until CD > 10' rad.This result accords well with the pattern of kinetic behaviour qualitatively perceived from fig. 1. For unirradiated salt, ti constitutes ca. 30% of the time required for complete decomposition (a = 1.00). Values of ti for the two most highly irradiated samples were approximately half those for which CD < los rad, and the 2.5 x 10'rad material occupies an intermediate position (fig. 1 and table 1). This is736 Thermal Decomposition of y-Irradiated Silver Malonate 106 105 E .r( Y lo2 .G 10 1 .o 0.1 Fig. 5. Plots of induction period (ti1), see definition in text) to onset of decomposition against pray dose for a variety of solid reactants. The behaviour of silver malonate is similar to that of the various inorganic salts, but promotion of the onset of this reaction is relatively insensitive to the effects of y-pre-irradiation. entirely consistent with the view that, at low dosages, y-irradiation generates prenucleation imperfections that exhibit identical reactivity with the sites of onset of reaction in unirradiated salt.The number of such nuclei is proportional to the irradiation dose (fig. 4). Extended irradiation (@ > lo8 rad) significantly reduced ti values (fig. 1 and table l), radiation evidently advanced the chemical changes through which the precursor sites were transformed into growth nuclei. The evidence was that this influence of radiation was equally effective in enhancing the onset of reaction at all nucleation sites because the decomposition rates subsequently established obeyed the same proportionality factor as those characteristic of low dosages.All data were close to the single line in fig. 4. The observed enhancement of nucleation rates when @ > lo8 cannot be ascribed to consecutive modifications of prenucleation sites by successive random y-ray inter- actions. Such generation of more reactive sites would at first yield a small proportion of growth nuclei that developed in advance of the main decomposition. This dual reactivity of nucleation would result in dose-dependent modifications of a us. time curve shapes which were not observed (fig. 2), and fig. 4 is evidence that the number of nuclei participating increases in direct proportion to dose.A . K . Galwey, P . J . Herley and M. A . Mohamed 737 The observed kinetic behaviour can be explained by the occurrence of a small amount of radiolytic decomposition at high irradiation dosage.Subsequently, at reaction temperature the crystal-retained products interact with the germ nuclei, thereby advancing their development towards growth nuclei. Such retained product quantities are expected to be very small; probably the coalescence of a few silver atoms would be sufficient to initiate an active growth nucleus. Individual silver atoms, released following irradiation damage, are expected to retain mobility, perhaps on surfaces, at reaction temperature and are therefore available to participate in nucleation. (Similar mobility of product metal atoms was envisaged to explain the growth of copper crystallites on a carbon substrate during the decomposition of copper malonate.') The data reported in table 1 do not obey eqn (1) with acceptable accuracy.An alternative definition of the induction period is possible, however, as the point of departure of the a us. time curves for irradiated samples from that of unirradiated salt. Values estimated according to this criterion (t!')) do satisfactorily fit eqn (I), and the data from fig. 1 here can be represented as tjl)/min = 158 - 14 log(O/rad). From comparisons of this line with other similar observations we conclude that silver malonate decomposition is less sensitive to the influence of pre-irradiation than several inorganic solids for which data are a~ailable,~. l3 including alkali permanganates, ammonium perchlorate, LiAlH, and BaN, (fig.5). This lower sensitivity may be due to the presence of the methylene group, which tends to inhibit interactions between the two carboxy groups within each anion. While noting that ti1) data for the present reactant are represented by eqn (l), we regard the more comprehensive kinetic analysis described earlier as providing the more satisfactory overall mechanistic representation of the observations. The effects of thermal neutron pre-irradiation on silver oxalate decomposition''! l4 are quite different from the pattern of behaviour reported and discussed above. For the former reactant apparent induction periods were negative, and there was no direct correlation between dose and acceleratory rate. Conclusions The results of the present study confirm the previous conclusion that silver malonate decomposition proceeds in the solid13 state by a nucleation-and-growth process.Anion breakdown is envisaged as proceeding through a catalytic-type process on the surfaces of metallic silver particles that constitute the active advancing interface. The promotional effect of y-irradiation is regarded as evidence that reaction is not accompanied by fusion. Melt formation would be expected to annihilate sites of crystal structure damage and fusion accounts for the absence of pre-irradiation influences on the decompositions of ammonium dichromate,15 copper(1r) malonate', and sodium perchlorate m0n0hydrate.l~ The observed promotion of decomposition by y-pre-irradiation is ascribed to the generation of additional sites of potential nucleation.These may be crystal defects or reactive radicals (fig. 3) that are not necessarily identical with intrinsic nucleation sites but are of comparable reactivity and/or (more probably) evolve by a similar sequence of steps into growth nuclei. More extensive irradiation advances the onset of reaction; this is envisaged as being due to the involvement of a small amount of decomposition products which advance the transformation of all precursor-specialized sites into active growth nuclei. The kinetics of growth of all nuclei are identical. The observed increase in reaction rate in proportion to pre-irradiation dose is ascribed to a direct relationship between the extent of salt y-irradiation and number of nuclei developed on subsequent decomposition. 25 F A R I738 Thermal Decomposition of y-Irradiated Silver Malonate We thank the High Intensity Radiation Research and Development Laboratory of the U.S. D.O.E., Brookhaven National Laboratory, for pretreating our samples. References 1 A. K. Galwey and M. A. Mohamed, J. Chem. SOC., Faraday Trans. 1, 1985, 81, 2503. 2 N. J. Carr and A. K. Galwey, Thermochim. Acta, 1984, 79, 323. 3 A. K. Galwey, Thermochim. Acta, 1985, 96, 259. 4 P. W. Levy and P. J. Herley, J. Phys. Chem., 1971, 75, 191. 5 P. J. Herley and E. G. Prout, J. Am. Chem. SOC., 1960, 82, 1540. 6 J. W. Mitchell, in Chemistry of the Solid State, ed. W . E. Garner (Butterworth, London, 1955), 7 J. W. Schneller and T. B. Flanagan, Nature (London), 1967, 215, 729. 8 E. G. Prout and M. J. Sole, J. Inorg. Nucl. Chem., 1959, 9, 232. 9 N. J. Carr and A. K. Galwey, Proc. R. SOC. London, Ser. A, 1986, 404, 101. chap. 13. 10 M. E. Brown, D. Dollimore and A. K. Galwey, Comprehensive Chemical Kinetics, vol. 22. Reactions in 1 1 D. A. Dominey, H. Morley and D. A. Young, Trans. Faraday SOC., 1965, 61, 1246. 12 R. M. Haynes and D. A. Young, Discuss. Faraday SOC., 1961, 31, 229. 13 P. J. Herley and D. Devlin, Reactivity of Solids, 1987, 3, 75. 14 D. A. Young, Decomposition of Solids (Pergamon, Oxford, 1966), p. 159. 15 A. K. Galwey, L. Poppl and S. Rajam, J. Chem. SOC., Faraday Trans. 1, 1983, 79, 2143. 16 A. K. Galwey, unpublished results. the Solid State (Elsevier, Amsterdam, 1980). Paper 71139; Received 27th January, 1987

ISSN:0300-9599    

DOI:10.1039/F19888400729

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. 1, 1988, 84(3), 739-749 Monolayer Adsorption of Non-spherical Molecules on Solid Surfaces Part 2.-The Application of First-order RAM Theory to Nitrogen on Graphite Jaros4aw Penar and Stefan Sokolowski" Department of Theoretical Chemistry, institute of Chemistry UMCS, 20031 Lublin, Nowotki 12, Poland First-order RAM (reference average Mayer) perturbational theory is used to calculate the adsorption characteristics of monolayer nitrogen on graphite. The model used for this purpose considers the adsorbed layer to be strictly two-dimensional and neglects the periodic structure of the underlying solid. However, considering the interactions in the adsorbed layer, we also discuss the effects of screening by the substrate on the interaction between a pair of admolecules.The results of theoretical calculations are compared with computer simulations and with the experimental data. In the preceding paper of this series' (henceforth referred to as Part 1) we advocated the use of a two-dimensional counterpart of the RAM2' (reference average Mayer) theory to describe monolayer adsorption of linear molecules on flat solid surfaces. According to this approach, the submonolayer adsorbed film is treated as a strictly two- dimensional phase with molecules oriented parallel to the surface. The theory was tested in the case of adsorption of diatomic molecules, and excellent results were obtained for structural and thermodynamic properties of this system. In this paper we apply the theory reported in Part 1 to a description of fluid submonolayer phases of nitrogen adsorbed on a model, structureless surface of graphite.Among other things we will consider here the effect of screening by the substrate of interactions between adsorbed particle^.^ RAM Theory for Two-dimensional Systems As in Part 1, we consider here a strictly two-dimensional system of diatomic molecule^^*^^^ interacting via a pair potential u(r, 8,, 8,) which is a function of the distance r between two selected points located within both molecules and molecular orientations 8, and 8,. Although other options can also be taken into account [cf. ref (3) and (7)], the angles describing orientations of a pair of molecules will be in this work measured with respect to the line joining centres of mass of both molecules.According to the RAM theory,'v3 the structural and thermodynamic properties of the system of interest are computed from perturbational expansions about spherically symmetric reference fluid interacting uia the potential (1) uref(r) = - kT In (exp [ - u(r, O,, S,)/kTl),l,p where (. . .) denotes an unweighted average over orientations. The first-order perturbational expansions for the two-particle background correlation function z(r, 8,, 8,) and for the Helmholtz free energy I; are given by 739 25-2740 Monolayer Adsorption of Non-spherical Molecules Afir, el, 0,) = exp [ - u(r, O,, O,)/kT-exp [ - d e P ( r ) / k ~ ] r is the two-dimensional density, N = rS,, Sa is the surface area, g(r,O1,8,) = exp[ - u(r, el, B,)/kT] z(r, el, O,),Af,,. ( r ) denotes the circular harmonic coefficient of A .0 - 9 61, 02) and the superscript ref denotes the reference system. There are several theoretical routes to evaluating the two-dimensional pressure $. In the case of bulk fluids composed of Lennard-Jones diatomic molecules, the best estimation of the pressure is obtained by differentiating the free energy.2* This approach has also been applied in our calculations. Thus The adsorption isotherm is then evaluated by using the Gibbs equation. We have where n is the number of molecules in unit volume of the gas phase and KH is the Henry constant. The average potential energy resulting from interparticle interactions can be evaluated as U=-==rc Jom ezz49 r d r ) rdr (8) where zlz’ and e,,, are the circular harmonic coefficients of the two-particle background correlation function and the function e(r, el, 0,) = u(r, O,, 0,) exp [ - u(r, O,, 8,)/kT].We note that according to the first-order RAM theory, only zl0 and z,, coefficients appear in the circular expansion of z(r, el, 0,). A knowledge of U allows one to calculate the dependence of the isosteric enthalpy of adsorption, qst, on the amount adsorbed. We have N ZZ’ where 4:‘ = qst(T = 0) is the isosteric heat of adsorption at zero coverage. The application of the equations listed above requires a detailed knowledge of the properties of reference system. As numerous calculations performed for three- dimensional fluids have indicated,,~~ the most serious errors in predictions of the RAM theory are caused by an inaccurate description of the reference system.Although the thermodynamic and local properties of adsorbed monolayers are now of great interest, only some scattered computer simulation results have been published for monomolecular fluid adsorbed phases composed of linear molecules.8* For this reason, an unequivocal choice of the best method of calculating the properties of the reference system is difficult.J. Penar and S. Sokolowski 74 1 In this work the reference-system two-particle correlation functions have been evaluated by using the two-dimensional Percus-Yevick (PY) equation, and the reference-system pressure has been calculated from the pressure equation, using gref (r) resulting from the PY approximation. Because the methods of solution of the PY equation are now quite standard,lO~ l1 they are not presented here.The Interaction Potentials In developing practical theories for the thermodynamic and structural properties of molecules adsorbed, it is crucial to know the potential energies that appear in the general statistical-mechanical equations for the problem. Thus in this section we give detailed information of the potentials used in our numerical calculations. The model which has been most frequently', used in theoretical studies of bulk nitrogen assumes that the energy of interaction between a pair of N, molecules is represented by the sum of four atom-atom Lennard-Jones (12-6) potentials : and the quadrupole-quadrupole energy uQQ(r, 8,, 6,). In two dimensions we have + 2(sin 8, sin 8, - 4 cos 8, cos 8,), - 15 cos 8, cos O,] (1 1) where Q is the quadrupole.Alternatively, the quadrupole interactions can be evaluated as the sum of nine coulombic potentials between three charges placed in each nitrogen molecule; two which equal q are located on the Lennard-Jones sites and one which is opposite in sign but equal to the sum of the other two is at the molecular centre. However, in contrast to eqn (1 l), the presence of three point charges on the molecular axis gives rise to multipoles of higher order thaa the leading quadrupole. According to Murthy et a1.l' for ~ / k = 36.4 K, 0 = 3.318 A, Q = 3.91 5 lo-*' C m2 (or q = 6.49 x 1 OP2' C ) and a molecular elongation given by I = 1.098 A, the three-dimensional analogue of the above model accurately reproduces the thermodynamic and structural properties of bulk solid and liquid nitrogen.We will refer to this model as model B. It is well known that in the presence of an adsorbing surface the intermolecular interactions between adsorbed particles become modified. Following earlier authors we allow for this effect by using the molecule-molecule version of MacLachlan theory,*! l4 according to which the free gas-phase pair potential is altered by adding the term where p = 1 +4L2/r2 and L is the height of the adsorbed layer above an effective image plane. In the Sase of nitroge! adsorbed on graphite15 C,/k = 231 288.26 K A6, C,/k = 11 726.84 K A6 and L = 5 A. We will label the model based upon these equations by the symbol S1. An attractive alternative16 to the approach based on the theory of MacLachlan is to use the potential model B with 'effective' parameters Q (or q), E and 0.The values of these parameters can be evaluated from temperature dependence of the second surface virial coefficient. The theoretical analysis of the N,-graphite experiments carried out by Bojan and Steele16 indicated that the- best agreement between computed and experimenotal values of the second surface virial coefficient is achieved for ~ / k = 28 K, 0 = 3.32 A and Q = 3.91 x loP4' C m2 (q = 6.49 C ) . The potential model outlined above will be denoted by S2.742 0.6 B2 0 - -0.6 Monolayer Adsorption of Non-spherical Molecules - - 100 80 60 c \ \ \. \. a '*\ 10 15 b X -Y E: 1 - 0.006 0.008 0.010 0.012 KIT Fig. 1. (A). The RAM reference system potentials for the models B1 (-), S1 (---) and S2 ( - -- -) of nitrogen adsorbed on graphite.T = 60 K (B). Dependence of the Henry constant and the second virial coefficient, B,* = B2u2, upon temperature. The labels S1 and S2 refer to appropriate N,-N, potentials, the points are the results of experimental determinationP and the curve marked S2-3D denotes the results of three-dimensional calculations of Bojan and Steele. l6 In fig. 1 (A) we show a comparison of the RAM reference system potentials [eqn. (l)] evaluated for the models B, S1 and S2 at T = 60 K. Both 'surface' models, S1 and S2, lead to similar results. Fig. 1(B) compares experimental and theoretical values of the second surface virial coefficient B, computed for the three models described above. According to the two-dimensional treatment B, = -nJoa (exp[-uref(r)/kT] -1)rdr.Moreover, the curve in fig. 1(B) corresponding to S2-3D denotes the results of three- dimensional calculations performed by Bojan and Steele.'' Model B does not correctlyJ . Penar and S. Sokolowski 743 reproduce the experimental quantities. We also note that models S1 and S2 lead to almost indistinguishable results. According to the two-dimensional treatment, information concerning molecule- surface interactions is contained in the Henry constant. We will identify this constant with that resulting from full, three-dimensional description. Thus KH = 1; dz exp [ - u(z, 8)/kT- 11 cos 8 do. (14) where v(z, 6) is the three-dimensional molecule-surface potential. The most widely used explicit model of molecule-solid interaction for graphite is based on a site-site approach in which each carbon atom interacts with each atom in the gas.If the site-site interactions are taken to be Lennard-Jones (12-6) functions, the potential v(z, 6) can be approximated by16 where E,, and ogs are the well-Cepth and size parameters for the nitrogenxarbo? atomic interaction and d, = 3.4 A and a, = 5.24 A2. For Egs/k = 33.4 and o,, = 3.36 A the values of the Henry constant resulting from the above model agree nicely with those evaluated experimentally16 [cf. fig. 1 (B)]. Results and Discussion In this section the theory reported above will be applied in numerical calculations of thermodynamic and structural properties of two-dimensional fluid nitrogen. Because the main purpose of this work is to propose a practical method of evaluating of properties of real monolayer adsorbed films, the theoretical predictions are confronted with the results of three-dimensional simulations,’ as well as with results of experimental determinations.”? l8 The first series of our numerical calculations has been performed for model B, investigated previously by Talbot et a1.8 using molecular-dynamics simulations.In fig. 2 we show representative examples of the circular harmonic coefficient g,1!2 (r). Fig. 2 (A) compares the results of theoretical predictions with computer simulations. Additionally, we also present the radial distribution function evaluated from the Percus-Yevick equation for spherically symmetric nitrogen molecules, lo! l1 according to which the pair potential is given by1’ u(r) = 4 & s [ ( 9 1 2 - ( y .with E,/k = 91.5 K and os = 3.68 A. Our calculations have confirmed previous findings’ concerning rapid convergence of the circular harmonic expansion of g(r, el, 02). The coefficients gllll(r) where ll and l2 are both 2 4 are very close to zero in the complete range. Remembering that the computer simulations were performed’ assuming the full, three-dimensional model, which also included the periodic variation of the adsorbing potential, the observed agreement between ‘ experimental ’ and theoretical coefficients gll ,2(r) is suprisingly good. We may expect that with an increase in the two-dimensional density r the errors introduced by the two-dimensional modelling of the system will increase, because under such conditions the adsorbed molecules are forced to tilt away from the surface.In fig. 3 (A) we present a comparison of the average potential energies of interparticle interactions computed from eqn. (8) and evaluated from molecular-dynamics simu-744 2 (a) 1 0 -1 Monolayer Adsorption of Non-spherical Molecules 1 1.5 2 2.5 rl3.332A 4 ( b ) 2 0 2 1 1.5 2 2.5 r/3.32A Fig. 2. (A) Comparison of theoretical (dashed lines) and resulting from simulationss (solid lines) circular-harmonic coefficients gZll $r). The numbers in parentheses denote the values of Zl and I, and the dotted line denotes the radi$ distribution function evaluated for spherically symmetric model of N,. T = 74.5 K, r = 0.0307 A-,. (B) The coefficients g, (r) obtained from the0 RAM theory for two state points. (a) (-) r = 0.0454 A-,, T = 745 K; (b) (---) r = 0.0272 A-2, T = 543 K.J.Penar and S . Sokolowski 745 0.02 0.03 0.04 rlK2 Fig. 3. (A) Comparison of average energies resulting from the RAM theory (the solid lines), computer simulations* (points) and the PY approximation for spherically symmetric model of N, molecules (the dashed lines). (a) T = 54.3 K, (b) T = 74.5 K. (B) Comparison of isosteric heats of adsorption at T = 74.5 K computed by using the RAM theory (a), computer simulations* (b) and the PY approximation for spherically symmetric model of N, (c). lations, whereas in fig. 3(B) we show the dependence of the isosteric heat of adsorption at T = 75.3 K on the two-dimensional density. The isosteric enthalpy of the simulation model is given by in kJ mol-1 when r is in We note that all simulation data have been treated as isothermal.The results presented here clearly demonstrate that the spherically symmetric approximation16 leads to worse results in comparison with RAM theory. qSt(r)-qit = 50.93 r746 Monolayer Adsorption of Non-spherical Molecules Fig. 4. Compressibility factors 2 = q5/TkT us. r resulting from the RAM theory for model B (temperatures in K). By way of illustration we present in fig. 4 the dependence of the two-dimensional compressibility factor 2 = $/r kT upon the density r. The critical temperature resulting from the RAM theory is close to 52 K. Since the convergence of our numerical PY procedure for the reference system two-particle background correlation function is difficult to achieve in the neighbourhood of the critical point, we have not yet completed our study in this region, and consequently cannot give accurate estimations of the critical temperature and density.Furthermore, the monolayer nitrogen adsorbed on graphite does not exhibit a gas-liquid phase transition, and its phase diagram is similar to that observed for monolayer krypton. 2o The method outlined above was next applied to analyse experimental data for nitrogen adsorbed on graphite. The interactions between adsorbed particles were described by using the model S2, and the results of numerical calculations were compared with experimental data measured at T = 79.3 K by Piper et a1.l' [system (a)] and with the data obtained at T = 7 K by Isirikyan and Kisielevl' [system (b)]. The value of the HeGry constant evaluated according to eqn (14) for both of these systems is 0.75 x 10' A.In fig. 5 we show the dependence of the isosteric heat of adsorption as a function of the amount adsorbed at T = 79.3 K. The dashed line denotes the experimental results of Piper et a1." The larger experimental values of qst at the lowest two-dimensional densities may be attributed to some heterogeneity in the energy of adsorption, but it is also likely that this effect is caused by adsorptioneon the inside wall of the calorimeter vessel. The sharp maximum in qst at r x 0.65 A-2 is connected with the transition from a two-dimensional fluid phase to the registered two-dimensional solid phase. Our theory fails completely in this region. A comparison of experimental and calculated adsorption isotherms for systems (a)J. Penar and S.Sokoiowski 747 0.0 2 OD 4 rJA-2 Fig. 5. Comparison of experimental’’ (---) and theoretical (-) values of isosteric heats of adsorption. The calculations were carried out for model S2. T = 79.3 K. and (b) is given in fig. 6(A). The error bars show the experimental error in r calculated assuming only 5% uncertainty in the surface area of both adsorbents [Sa z 21 m2 g-l for the system (a)16 and Sa z 12 m2 g-l for system (b)].’ Fig. 6(B), however, presents examples of the two-dimensional equation of state calculated for model S2. A comparison of the results presented in fig. 4 and 6(A) illustrates the screening of interactions between a pair of admolecules by the solid surface. The model considered here is very simple.In particular, we have neglected the effects of out-of-plane motion of adsorbed molecules and the role of periodicity of the gas-solid potential. The importance of both these effects depends on the temperature and surface coverage. At the lowest coverages and low temperatures, only a small fraction of the adsorbed molecules assumes non-planar configuration. Under such conditions the corrections to adsorption isotherms resulting from non-planar configuration can be evaluated by using a perturbational method, similar to that developed by Monson, Cale, Toigo and Steele for atomic adsorbed For this purpose we expand exp[ - u(RiD, R:D)/kT], where RfD denotes the three-dimensional positional and orientational coordinates of the ith molecule, into the following Taylor series about a planar configuration : exp [ - u ( R ; ~ , R,3”)/kT] = exp [ - u(ri, ri, Oi, 0,)/kT] x [ 1 - (zi - zj) u:, / k T - sli Un,/ k T - slj ud/k T + .. . ] . ( 1 7) In eqn (1 7) z denotes the coordinate perpendicular to the surface and Ri is the tilt angle of ith molecule, Moreover, for p = zi - z j , sZi and sli.748 0.06 N I 5 0.04 fi 0.02 Monolayer Adsorption of Non-spherical Molecules 1 2 n/ 10-7 A 0.02 0.04 r/K2 1.5 Z 1 Fig. 6. (A) Experimentall6, l7 (points) and theoretical (lines) adsorption isotherms. The labels (a) and (b) denote corresponding adsorption systems (see text). The solid lines were computed assuming a strictly two-dimensional model of adsorption and the dashed line was evaluated from eqn (24). (B) Compressibility factors 2 us.r for the model S2 of monolayer N, adsorbed on graphite. Temperatures in K. Substituting expansion (17) into the definition of the three-dimensional con- figurational integral for N molecules : V(RtD) + 2 u ( R , ~ ~ , Ria”)/kT). (19) i <j and retaining only the lowest-order terms, we obtain where and The symbol u ~ , ~ denotes the sites a-site p potential, &{(r) is the lowest-order circular harmonic coefficient of g(r, 0,, 0,) computed by using the site cc-site B reference frame g:{(r) = J dr, dr,(d4/24 (d0,/24&7 0,,0,) (23) (ri1,8- constant) and the sum in eqn (22) runs over all interacting sites. obtained from eqn (1). We have The lowest-order perturbational correction to the adsorption isotherm can be easily In nk, = Vplanar (r, T ) + A v ( ~ , T ) AV(r, T ) = (a/an[(r2/kn 11 4 1 (24) (25)J.Penar and S. Sokolowski 749 where v/ planar (r, T ) is the right-hand side of eqn (7). We stress that eqn (25) describes only small deviations from coplanarity and cannot be applied if the orientation of adsorbed particles differs significantly from the planar one. The dashed line in fig. 6(A) shows the adsorption isotherm (24) computed for system (a). The observed differences between adsorption isotherms (24) and (7) are relatively small, but we have not performed relevant calculations at high two-dimensional densities. Moreover, the correction term A@, T ) defined by eqn (25) is connected only with out-of-plane motion of adsorbed molecules and does not take into account the effects arising from lateral variation of the substrate potential.It is known20 that in the case of monolayers composed of spherically symmetric particles the periodic variation of the adsorbing potential may cause significant changes in the computed phase diagrams. We will return to these problems in future papers. This work was supported by the C.P.B.P. (Poland) under grant no. 01.08.E2. References 1 L. tajtar, J. Penar and S. Sokolowski, J . Chem. Soc., Faraday Trans. I , 1987, 83, 1405. 2 C. G. Gray and K. E. Gubbins, Theory of Molecular Fluids (Oxford University Press, Oxford, 1984), 3 W. R. Smith and I. Nezbeda, Adv. Chem. Ser., 1983, 209, 235. 4 S. Rauber, J. R. Klein and M. W. Cale, Phys. Rev. B, 1983, 27, 1314. 5 J. S. Rowlinson, J. Talbot and D. J. Tildesley, Mol. Phys., 1985, 54, 1065. 6 Y. P. Joshi and D. J. Tildesley, Ber. Bunsenges. Phys. Chem., 1986, 90, 217. 7 S. Sokolowski, Thin Solid Films, 1987, 147, 223. 8 J. Talbot, D. J. Tildesley and W. A. Steele, Faraday Discuss. Chem. Soc., 1985, 80, 91. 9 V. R. Bhathanabolta and W. A. Steele, Mol. Phys., in press. 10 F. Lado, J . Chem. Phys., 1968, 49, 3092. 11 E. D. Glandt and D. D. Fitts, J . Chem. Phys., 1977, 66, 4503. 12 P. A. Monson, W. A. Steele and W. B. Streett, J . Chem. Phys., 1983, 78, 4126. 13 C. S. Murthy, S. F. O’Shea and I. R. McDonald, Mol. Phys., 1983, 7, 381. 14 A. D. McLachlan, Mol. Phys., 1964, 7, 381. 15 J. Piper, J. A. Morrison and C. Peters, Mol. Phys., 1984, 53, 1463; C. Peters and M. L. Klein, Mol. 16 M. J. Bojan and W. A. Steele, Langmuir, in press. 17 J. Piper, J. A. Morrison, C. Peters and Y. Ozaki, J. Chem. Soc., Faraday Trans. I , 1983, 79, 2863. 18 A. A. Isirikyan and A. V. Kisielev, J . Phys. Chem., 1961, 65, 601. 19 J. 0. Hirschfelder, C. F. Curtis and R. B. Bird, Molecular Theory of Gases and Liquids (Wiley, New 20 D. K. Fairobent, W. F. Saam and L. M. Sander, Phys. Rev. B, 1982, 26, 1979; L. M. Sander and J. 21 P. A. Monson, M. W. Cale, F. Toigo and W. A. Steele, Surf. Sci., 1982, 122, 401. vol. 1. Phys., 1985, 54, 895. York, 1954). Hautman, Phys. Rev. B, 1984, 29, 2171. Paper 7/240; Received 9th February, 1987

ISSN:0300-9599    

DOI:10.1039/F19888400739

出版商:RSC    

年代:1988

数据来源: RSC