摘要:

Gas Kinetics Group and Division de Chimie-Physique de la Societe Francaise de Chimie 9th International Symposium on Gas Kinetics To be held in Bordeaux, France on 20-25 July 1986 Further information from Dr R. Lasclaux, Lab. Photophys. Photochim. MolBculaire, Universite de Bordeaux I, 33405 Talence Cedex, France Poiymer Physics Group Biologically Engineered Polymers To be held at Churchill College, Cambridge on 21-23 July 1986 Further information from Dr M. J. Miles, AFRC,Food Research Institute, Colney Lane, Norwich NR4 7UA Polymer Physics Group with the British Rheological Society Deformation in Solid Polymers To be held at the University of Leeds on 9-1 1 September 1986 Further information from Dr J. V. Champion, Department of Physics, City of London Polytechnic, 31 Jewry Street, London EC3N 2EY ~~_____________ ~~~~ Carbon Group Carbon Fibres- P ro pe rt i es and A p p I i cat i o ns To be held at the University of Salford on 1 5 1 7 September 1986 Further information from The Meetings Officer, The Institute of Physics, 47 Belgrave Square, London SW1 X 8QX ~ ~~~~~~~~ ~ Division with the Surface Reactivity and Catalysis Group-Autumn Meeting Promotion in Heterogeneous Catalysis To be held at the University of Bath on 23-25 September 1986 Further information from: Professor F.S. Stone, School of Chemistry, University of Bath, Bath BA2 7AY (viii)Gas Kinetics Group and Division de Chimie-Physique de la Societe Francaise de Chimie 9th International Symposium on Gas Kinetics To be held in Bordeaux, France on 20-25 July 1986 Further information from Dr R.Lasclaux, Lab. Photophys. Photochim. MolBculaire, Universite de Bordeaux I, 33405 Talence Cedex, France Poiymer Physics Group Biologically Engineered Polymers To be held at Churchill College, Cambridge on 21-23 July 1986 Further information from Dr M. J. Miles, AFRC,Food Research Institute, Colney Lane, Norwich NR4 7UA Polymer Physics Group with the British Rheological Society Deformation in Solid Polymers To be held at the University of Leeds on 9-1 1 September 1986 Further information from Dr J. V. Champion, Department of Physics, City of London Polytechnic, 31 Jewry Street, London EC3N 2EY ~~_____________ ~~~~ Carbon Group Carbon Fibres- P ro pe rt i es and A p p I i cat i o ns To be held at the University of Salford on 1 5 1 7 September 1986 Further information from The Meetings Officer, The Institute of Physics, 47 Belgrave Square, London SW1 X 8QX ~ ~~~~~~~~ ~ Division with the Surface Reactivity and Catalysis Group-Autumn Meeting Promotion in Heterogeneous Catalysis To be held at the University of Bath on 23-25 September 1986 Further information from: Professor F. S. Stone, School of Chemistry, University of Bath, Bath BA2 7AY (viii)

ISSN:0300-9599    

DOI:10.1039/F198581FX009

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

xxxij AUTHOR INDEX Singh, Km. S., 751 Sircar, S., 1527, 1541 Slade, R. C. T., 847 Smith, I. G., 1095 Snelling, C. M., 1761 Sobczyk, L., 311 Siiderberg, D., 17 15 Solar, S., 1101 Solar, W., 1101 Soma, M., 485 Somorjai, G. A., 1263 Somsen, G., 1015 Sorek, Y., 233 Souto, F. A., 2647 Spencer, S., 2357 Spichiger-Ulmann, M., 7 13 Spoto, G., 1283 Spotswood, T. M., 1623 Srivastava, R. D., 913 Stachurski, J., 1447, 2813 Staricco, E. H., 1303 Stock, T., 2257 Stockhausen, M., 397 Stokes, R. H., 1459 Stone, F. S., 1255 Strachan, A. N, 1761 Strohbusch, F., 2021 Stuckless, J. T., 597 Su, Z., 2293 Subrahmanyam, V. S., 1655 Sugimoto, N., 1441, 2959 Suminaka, M., 2287 Suprynowicz, Z., 553 Sutcliffe, L. H., 679, 1467, 1215 Suzanne, J., 2339 Suzuki, H., 3117 Swallow, A. J., 1225 Symons, M.C. R., 433, 565, 727, 2131, 2775, 1095, 1963, 242 1 Takagi, Y., 1901 Takahashi, Y., 3 117 Takeshita, H., 2805 Tamilarasan, R., 2763 Tamura, K., 2287 Tanaka, T., 1513 Taniewska-Osinska, S., 695, Tascon, J. M. D., 939, 2399 Taylor, M. J., 1863 Taylor, N., 2357 Tejuca, L. G., 939, 2399, 1203 Teller, R. G., 1693 Tempere, J-F., 1357 Teramoto, M., 2941 Theocharis, C . R., 857 Thomas, J. K., 735 Tielen, M., 2889, 3049 Tindwa, R. M., 545 Tissier, C., 3081 Toi, K., 2835 Tokuda, T., 2835 Torrez-Mujica, T., 343 Townsend, R. P., 1071, 173 1, Trasatti, S., 2995 Treiner, C., 2513 Trenwith, A. B., 745 Trifiro, F., 1003 Troncoso, G., 1631, 1637 Tseung, A. C. C., 1883 Tuck, J. J., 833 Turner, J. E., 1263 Uemoto, M., 2333 Uma, K., 2733 Valencia, E., 1631. 1637 Valigi, M., 813 Vallmark, T., 1389 Van Oort, M.J. M., 3059 Varma, M. K., 751 Vattis, D., 2043 Vecli, A., 433 Veseli, V., 2095 Vink, H., 1677, 1725 Vliers. D. P., 2009 Vukovid, Z., 1275 3081, 1913 3127 Waghorne, W. E., 2703 Ward, A. J., 2975 Watanabe, H., 1569 Waugh, K. C., 3073 Weckstrorn, K., 2947 Weinberg, N. N., 875 Weingartner, H., 1031 Wells, C. F.. 801, 1057, 1401, White, M. A., 3059 Williams, J. O., 271 1 Williams, P. A., 2635 Williams, P. B., 3067 Williams, R. T., 847 Wojcik, D., 1037 Wood, G. L., 265 Wood, R. M., 273 Woolf, L. A., 769, 2821 Wright, C. J., 2067 Wright, J. P., 1471 Wright, T. H., 1819 Wurie, A. T., 2605 Yadav, G. D., 161 Yadava, R. D., 751 Yamaguchi, M., 1513 Yamaguti, K., 1237 Yamasaki, S., 267 Yamashita, H., 2485 Yamatera, H., 127 Yelon, W., 1693 Yoshida, S., 1513, 2485 Yoshikawa, M., 2485 Zambonin, P.G.. 621 zdanov, S. P., 2541 Zecchina, A., 1283 Zelano, V., 2365 Zhan, R. Y., 2083 Zhao, Z., 185 Zhulin, V. M., 875 Zilnyk, A., 679, 1215 Zulauf, M., 2947 Zundel, G., 1425, 2375 1985. 2145, 2475, 3091xxxij AUTHOR INDEX Singh, Km. S., 751 Sircar, S., 1527, 1541 Slade, R. C. T., 847 Smith, I. G., 1095 Snelling, C. M., 1761 Sobczyk, L., 311 Siiderberg, D., 17 15 Solar, S., 1101 Solar, W., 1101 Soma, M., 485 Somorjai, G. A., 1263 Somsen, G., 1015 Sorek, Y., 233 Souto, F. A., 2647 Spencer, S., 2357 Spichiger-Ulmann, M., 7 13 Spoto, G., 1283 Spotswood, T. M., 1623 Srivastava, R. D., 913 Stachurski, J., 1447, 2813 Staricco, E. H., 1303 Stock, T., 2257 Stockhausen, M., 397 Stokes, R. H., 1459 Stone, F. S., 1255 Strachan, A.N, 1761 Strohbusch, F., 2021 Stuckless, J. T., 597 Su, Z., 2293 Subrahmanyam, V. S., 1655 Sugimoto, N., 1441, 2959 Suminaka, M., 2287 Suprynowicz, Z., 553 Sutcliffe, L. H., 679, 1467, 1215 Suzanne, J., 2339 Suzuki, H., 3117 Swallow, A. J., 1225 Symons, M. C. R., 433, 565, 727, 2131, 2775, 1095, 1963, 242 1 Takagi, Y., 1901 Takahashi, Y., 3 117 Takeshita, H., 2805 Tamilarasan, R., 2763 Tamura, K., 2287 Tanaka, T., 1513 Taniewska-Osinska, S., 695, Tascon, J. M. D., 939, 2399 Taylor, M. J., 1863 Taylor, N., 2357 Tejuca, L. G., 939, 2399, 1203 Teller, R. G., 1693 Tempere, J-F., 1357 Teramoto, M., 2941 Theocharis, C . R., 857 Thomas, J. K., 735 Tielen, M., 2889, 3049 Tindwa, R. M., 545 Tissier, C., 3081 Toi, K., 2835 Tokuda, T., 2835 Torrez-Mujica, T., 343 Townsend, R.P., 1071, 173 1, Trasatti, S., 2995 Treiner, C., 2513 Trenwith, A. B., 745 Trifiro, F., 1003 Troncoso, G., 1631, 1637 Tseung, A. C. C., 1883 Tuck, J. J., 833 Turner, J. E., 1263 Uemoto, M., 2333 Uma, K., 2733 Valencia, E., 1631. 1637 Valigi, M., 813 Vallmark, T., 1389 Van Oort, M. J. M., 3059 Varma, M. K., 751 Vattis, D., 2043 Vecli, A., 433 Veseli, V., 2095 Vink, H., 1677, 1725 Vliers. D. P., 2009 Vukovid, Z., 1275 3081, 1913 3127 Waghorne, W. E., 2703 Ward, A. J., 2975 Watanabe, H., 1569 Waugh, K. C., 3073 Weckstrorn, K., 2947 Weinberg, N. N., 875 Weingartner, H., 1031 Wells, C. F.. 801, 1057, 1401, White, M. A., 3059 Williams, J. O., 271 1 Williams, P. A., 2635 Williams, P. B., 3067 Williams, R. T., 847 Wojcik, D., 1037 Wood, G. L., 265 Wood, R. M., 273 Woolf, L. A., 769, 2821 Wright, C. J., 2067 Wright, J. P., 1471 Wright, T. H., 1819 Wurie, A. T., 2605 Yadav, G. D., 161 Yadava, R. D., 751 Yamaguchi, M., 1513 Yamaguti, K., 1237 Yamasaki, S., 267 Yamashita, H., 2485 Yamatera, H., 127 Yelon, W., 1693 Yoshida, S., 1513, 2485 Yoshikawa, M., 2485 Zambonin, P. G.. 621 zdanov, S. P., 2541 Zecchina, A., 1283 Zelano, V., 2365 Zhan, R. Y., 2083 Zhao, Z., 185 Zhulin, V. M., 875 Zilnyk, A., 679, 1215 Zulauf, M., 2947 Zundel, G., 1425, 2375 1985. 2145, 2475, 3091

ISSN:0300-9599    

DOI:10.1039/F198581BX011

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

JOURNAL OF THE CHEMICAL SOCIETY F A R A D A Y T R A N S A C T I O N S , P A R T S I A N D I I The Journal of the Chemical Society is published in six sections, of which five are termed Transactions; these are distinguished by their subject matter, as follows: Dalton Transactions (Inorganic Chemistry). All aspects of the chemistry of inorganic and organometallic compounds; including bioinorganic chemistry and solid-state inorganic chemistry; of their structures, properties, and reactions, including kinetics and mechanisms; new or improved experimental techniques and syntheses. Faraday Transactions I (Physical Chemistry). Radiation chemistry, gas-phase kinetics, electrochemistry (other than preparative), surface and interfacial chemistry, heterogeneous catalysis, physical properties of polymers and their solutions, and kinetics of polymerization, etc.Faraday Transactions II (Chemical Physics). Theoretical chemistry, especially valence and quantum theory, statistical mechanics, intermolecular forces, relaxation phenomena, spectroscopic studies (including i.r., e.s.r., n.m.r., and kinetic spec- troscopy, etc.) leading to assignments of quantum states, and fundamental theory. Studies of impurities in solid systems. Perkin Transactions I (Organic Chemistry). All aspects of synthetic and natural product organic, organometallic and bio-organic chemistry, including aliphatic, alicyclic, and aromatic systems (carbocyclic and heterocyclic). Perkin Transactions 11 (Physical Organic Chemistry). Kinetic and mechanistic studies of organic, organometallic and bio-organic reactions.The description and application of physicochemical, spectroscopic, and theoretical procedures to organic chemistry, including structure-activity relationships. Physical aspects of bio-organic chemistry and of organic compounds, including polymers and biopolymers. Authors are requested to indicate, at the time they submit a typescript, the journal for which it is intended. Should this seem unsuitable, the Editor will inform the author. The sixth section of the Journal of the Chemical Society is Chemical Communications, which is intended as a forum for preliminary accounts of original and significant work, in any area of chemistry that is likely to prove of wide general appeal or exceptional specialist interest. Such preliminary reports should be followed up eventually by full papers in other journals (e.g.the five Transactions) providing detailed accounts of the work. NOTES It has always been the policy of the Faraday Transactions that brevity should not be a factor influencing acceptability for publication. In additioh however to full papers both sections carry at the end of each issue a section headed ‘Notes’, which are short self-contained accounts of experimental observations, results, or theory that will not require enlargement into ‘full’ papers. The Notes section is not used for preliminary communications. The layout of a Note is the same as that of a paper. Short summaries are required. The procedure for submission, administration, refereeing, editing and publication of Notes is the same as for full papers.However, Notes are published more quickly than papers since their brevity facilitates processing at all stages. The Editors endeavour to meet authors’ wishes as to whether an article is a full paper or a Note, but since there is no sharp dividing line between the one and the other, either in terms of length or character of content, the right is retained to transfer overiong Notes to the full papers section. As a guide a Note should not exceed I500 words or word-equivalents.NOMENCLATURE AND SYMBOLISM Units and Symbols. The Symbols Committee of The Royal Society, of which The Royal Society of Chemistry is a participating member, has produced a set of recommendations in a pamphlet ‘Quantities, Units, and Symbols’ (1975) (copies of this pamphlet and further details can be obtained from the Manager, Journals, The Royal Society of Chemistry, Burlington House, London W 1 V OBN).These recommendations are applied by The Royal Society of Chemistry in all its publications. Their basis is the ‘ Systeme International d’Unit6s’ (SI). A more detailed treatment of units and symbols with specific application to chemistry is given in the IUPAC Manual of Symbols and Terminology for Physicochemical Quantities and Units (Pergamon, Oxford, 1979). Nomenclature. For many years the Society has actively encouraged the use of standard IUPAC nomenclature and symbolism in its publications as an aid to the accurate and unambiguous communication of chemical information between authors and readers. In order to encourage authors to use IUPAC nomenclature rules when drafting papers, attention is drawn to the following publications in which both the rules themselves and guidance on their use are given: Nomenclature of Organic Chemistry, Sections A , B, C, D, E, F, and H (Pergamon, Oxford, 1979 edn).Nomenclature of Inorganic Chemistry (Butterworths, London, 197 1, now published by Pergamon). Biochemical Nomenclature and Related Documents (The Biochemical Society, London, 1978). A complete listing of all IUPAC nomenclature publications appears in the January issues of J. Chem. SOC., Faraday Transactions. It is recommended that where there are no IUPAC rules for the naming of particular compounds or authors find difficulty in applying the existing rules, they should seek the advice of the Society’s editorial staff. (ii)THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO.79 (in conjunction with the Polymer Physics Group) Polymer Liquid Crystals University of Cambridge, 1-3 April 1985 The object of the meeting will be to discuss all aspects of the developing subject of polymeric liquid crystals. The hope is to bring together scientists from the fields of conventional polymer science and monomeric liquid crystals who are active in this field. The discussion is aimed at understanding the following facets: (a) The chemical characteristics that give rise to polymer liquid crystalline behaviour. (b) The nature of the high local anisotropy of these systems and their structural organisation at the molecular, micron and macroscopic levels.(c) The physical properties and their industrial exploitation, with particular reference to the influence of external force fields such as flow, electric and magnetic fields. (d) The inter-relations of polymer liquid crystals with small-molecule mesophases, conventional flexible polymers and biopolymers which exhibit liquid-crystalline behaviour. The programme and application form may be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 80 Physical Interactions and Energy Exchange at the Gas-Solid Interface McMaster University, Hamilton, Ontario, Canada, 23-25 July 1985 Organising Committee: Professor J. A. Morrison (Chairman) Dr M.L. Klein Professor G. Scoles Professor W. A. Steele Professor F. S. Stone Dr R. K. Thomas The discussion will be concerned with certain aspects of current research on the gas-solid interface: elastic, inelastic and dissipative scattering of atoms and molecules from crystal surfaces, and the structure and dynamics of physisorbed species, including overlayers. Emphasis will be placed on the themes of physical interactions and energy exchange rather than on molecular-beam technology or the phenomenology of phase transitions on overlayers. The interplay between theory and experiment will be stressed as they relate t o the nature of atom and molecule surface interaction potentials, including many- body effects. The preliminary programme may be obtained from: Professor J.A. Morrison, Institute for Materials Research, McMaster University. Hamilton, Ontario, Canada L8S 4M1 or: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN, U.K. (iii)THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM NO. 20 Lipid Vesicles and Membranes ~ Loughborough University of Technology, 15-1 7 April 1986 Phase Transitions in Adsorbed ~ organising Committee: Professor D. A. Haydon (Chairman) Professor D. Chapman Mrs Y. A. Fish Dr M. J. Jaycock Dr I. G. Lyle Professor R. H. OttewiII Dr A. L. Smith Dr D. A. Young Layers University of Oxford, 17-18 December 1985 Organising Committee : Professor J. S. Rowlinson (Chairman) Dr E. Dickinson Dr R. Evans Mrs Y. A. Fish Dr N. Parsonage Dr D.A. Young The aim of the meeting is to discuss phase transitions at gas/liquid, liquid/liquid and solid/fluid interfaces, and in other systems of constrained geometry or dimensionality less than three. Emphasis will be placed on molecularly simple systems, whereby liquid crystal interfaces and chemisorption phenomena are excluded. The preliminary programme may be obtained from : Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 81 The aim of the meeting is to discuss the physical chemistry of lipid membranes and their interactions, in particular theoretical and spectroscopic studies, polymerised membranes, thermodynamics of bilayers and liposomes, mechanical properties, encapsulation and interaction forces between bilayers leading to fusion but excluding preparation and characterisation methodology.Contributions for consideration by the Organising Committee are invited and abstracts of about 300 words should be sent by 1 May 1985 to: Professor D. A. Haydon, Physiological Laboratory, Downing Street, Cambridge CB2 3EG Full papers for publication in the Discussion Volume will be required by December 1985.THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 82 Dynamics of Molecular Photof rag mentat ion University of Bristol, 15-1 7 September 1986 Organising Committee : Professor R . N. Dixon (Chairman) Dr G. G. Balint-Kurti Dr M. S. Child Professor R. Donovan Professor J. P. Simons The discussion will focus on the interaction of radiation with small molecules, molecular ions and complexes leading directly or indirectly to their dissociation. Emphasis will be given to contributions which trace the detailed dynamics of the photodissociation process.The aim will be to bring together theory and experiment and thereby stimulate important future work. Contributions for consideration by the Organising Committee are invited. Titles should be submitted as soon as possible, and abstracts of about 300 words by 30 September 1985, to: Professor R. N. Dixon, Department of Theoretical Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1 TSFARADAY DIVISION INFORMAL AND GROUP MEETINGS Electrochemistry Group Spring Informal Meeting To be held at Middlesex Polytechnic, London on 1-3 April 1985 Further information from Dr F.L. Tye, Middlesex Polytechnic, Bounds Green Road, London N11 2NQ Polymer Physics Group 6th Churchill Conference To be held at Churchill College, Cambridge on 1-4 April 1985 Further information from Professor I. M. Ward, Department of Physics, University of Leeds, Leeds LS2 9JT Statistical Mechanics and Thermodynamics Group Dense Fluids: Dynamic and Static Properties To be held at the University of Bristol on 10-1 1 April 1985 Further information from Dr D. J. Tildesley, Department of Chemistry, The University, Southampton SO9 5NH Neutron Scattering Group Small-angle Neutron Scattering from Organised Systems To be held at Imperial College, London on 17-1 8 April 1985 Further information from Dr R.W. Richards, Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow G1 1 XL Gas Kinetics Group with SERC Summer School in Gas Kinetics To be held at the University of Cambridge on 26 June to 3 July 1985 Further information from Dr I. W. M. Smith, Department of Chemistry, University Chemical Laboratory, Lensfield Road, Cambridge CB2 1 EP Industrial Physical Chemistry Group with the Food Chemistry Group Water Activity: A Credible Measure of Technological Performance and Physiological Via b i I i ty To be held at Girton College, Cambridge on 1-3 July 1985 Further information from Professor F. Franks, Department of Botany, Downing Street, Cambridge CB2 3EA Polymer Physics Group Biennial Conference To be held at the University of Reading on 11-1 3 September 1985 Further information from Professor Bassett, J.J. Thompson Physical Chemistry Laboratory, University of Reading, Whiteknights, Reading RG6 2AF Carbon Group Strength and Structure in Carbons and Graphites To be held at the University of Liverpool on 16-18 September 1985 Further information from The Meetings Officer, The Institute of Physics, 47 Belgrave Square, London SW1 X 8QX Surface Reactivity and Catalysis Group with the Catalysis Section of the KNCV Mechanism and Structure in Heterogeneous Catalysis To be held at Noordwijkerhout, The Netherlands on 18-20 September 1985 Further information from: Dr R. Joyner, BP Research Centre, Chertsey Road, Sunbury on Thames TW16 7LN Industrial Physical Chemistry Group A Molecular Approach to Lubrication and Wear To be held at Girton College, Cambridge on 23-25 September 1985 Further information from Mr M.P. Dare-Edwards, Shell Research Ltd, Thornton Research Centre, Chester CHI 3SHNeutron Scattering Group jointly with the Materials Testing Group of the Institute of Physics Industrial Uses of Particle Beams To be held at the Institute of Physics, London on 26 September 1985 further information from Dr J. G. Booth, Department of Chemistry, University of Salford, Salford M5 4WT Division Annual Congress: Structure and Reactivity of Gas-Phase Ions To be held at the University of Warwick on 8-1 1 April 1986 Further information from Professor K. R. Jennings, Department of Molecular Sciences, University of Warwick, Coventry CV4 7AL 30TH INTERNATIONAL CONGRESS OF PURE AND APPLIED CHEMISTRY Advances in Physical and Theoretical Chemistry Manchester, 9-1 3 September 1985 The Faraday Division is mounting the following symposia as part of the 30th IUPAC Congress: A.B. C. D. Reaction Dynamics in the Gas Phase and in Solution This symposium will examine the ways in which modern techniques allow detailed study of the dynamical motion of molecules which are undergoing chemical reaction or energy exchange. Micellar Systems The symposium will discuss various aspects of micellization, including size and shape factors, micellization in biological systems, chemical reactions in micellar systems, micelle structure and solubilization. Emphasis will also be given to modern techniques of examining micellar systems, including small-angle neutron scattering, neutron spin echo, photocorrelation spectroscopy, NMR and use of fluorescent probes.Surface Science of Solids The symposium will centre on recent advances in the study of kinetics and dynamics at surfaces and of phase transitions in adsorbate layers on single crystal surfaces. Both experimental and theoretical aspects will be reviewed with an emphasis on metal single crystal surfaces. New Electrochemical Sensors (in collaboration with the Electroanalytical Group of the Analytical Division) The symposium will cover such topics as the fundamentals of the subject, new gas sensors based on membrane electrodes and on ceramic oxides, the development of new ion- selective electrodes and the synthesis of new guest-host carriers, the development of CHEMFETS and other integrated devices together with the theory of the operatioh of such devices, and finally the development of biosensors including for instance enzyme electrodes, direct electron transfer to biological molecules and new potentiometric techniques for protein analysis.The second circular, giving details of all the symposia of the Congress and listing invited speakers may be obtained from: Dr J. F. Gibson, 30th IUPAC Congress, Royal Society of Chemistry, Burlington House, London W1V OBN (vii)Arthur Adamson, Editor University of Southern California Arthur Hubbard, Associate Editor University of California at Santa Barbara This new journal published by the American Chemical Society fills the void existing in current literature available today-Langmuir’s broad coverage wiN bring together authoritative papers from all aspects of this major field of chemistry! Langmuir will include fundamental and applied papers covering ultra-high vacuum surface chemistry and spectroscopy, heterogeneous catalysis, all aspects of interface chemistry involving fluids, and disperse systems.Specifically, Langmuir will publish peer-reviewed research in ‘Wet’ Surface Chemistry surface tension spread monolayers 0 wetting and contact angle 0 adsorption from solution nucleation and fundamental aspects of flotation, detergency, emulsions, foams, lubrication, etc. 4 Electrochemistry related to interfacial structure and processes r/ ‘UHV’ Surface Chemistry solid surfaces in ultra-high vacuum including surface structure, composition and spectroscopy 0 fundamental papers in heterogeneous catalysis colloidal suspensions including aerosols 0 microemulsions 0 biological and polymeric colloids 0 and membrane systems r/ Disperse Systems In bimonthly issues of Langmuir, you will find experimental and theoretical original papers, letters to the editor, and book reviews, as well as some selected symposium collections.Papers having applied aspects will be included. And, published by the American Chemical Society, Langmuir will benefit from the Society’s vast international network of scientists and editorial resources. Note to Authors: Langmuir will not have page charges. Editorial Advisory Board N.R. Armstrong, Univ. of Arizona 0 G.T. Barnes, Unrv. of Oueensland, AUSTRALlA P . Biloen. Univ.of Pittsburgh K.S. Birdi, Technical Unwersity of Denmark, DENMARK A.M. Bond, Deakrn University, AUSTRALlA B.V. Derjaguin. Academy of Science of USSR D.D. Eley, Univ. of Nottingham, ENGLAND 0 G. Ertl. Univ. of Munich, GERMANY J. Fendler, Clarkson College of Technology T . Fort, Jr.. California Polytechnic State Univ. G. Gaines. Jr.. General Electric W.A. Goddard. Ill, California Institute of Technology R.S. Hansen, lowa State Univ. 0 J . Lyklerna, Agricultural Univ., THE NETHERLANDS 0 R.J. Madix. Stanford Univ. J A Mann. J r . , Case Western Reserve Univ. P. Mukerjee. Unrv. of Wisconsin 0 K.J. Mysels, Research Consulting A.W. Neurnann. Unw. of Toronto, CANADA R . Ottewill, Univ. of Bristol. ENGLAND G D. Parfitt, Carnegie-Mellon Univ. H Reiss. Univ. of Calrfornra at Los Angeles H.A. Resing, Naval Research Laboratory T . Rhodin. Cornell Univ S Ross. Rensselaer Polytechnic Univ. J Rouquerol. Centre de Thermodynamique et de Mrcrocalorimetne du CNRS, FRANCE R.L. Rowell, Univ. of Massachusetts R. Rye, Sandia National Lab H . Seki. ISM K. Shinoda, Yokohama National Univ.. JAPAN G.A Sornorjai, Univ of Cahfornra at Berkley W.A. Steele. Pennsylvania State Univ Subscription Information 1985 Foreign Rates (Includes Air Service) ACSMembers $ 56 (Personal Use) Nonmembers $308 January-February 1985 Volume 1 No 1 One Volume Per Year ( S I X Issues) ISSN 0743-7463 Cable Address: JIECHEM Telex: 440 159 ACSPUI or 892582 ACS PUBS American Chemical Society 0 1155 Sixteenth St., N.W. 0 Washington, D.C. 20036 (viii)

ISSN:0300-9599    

DOI:10.1039/F198581FP025

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

J. Chem. Soc., Faraday Trans. I, 1985, 81, 553-563 Effects of Surface Heterogeneity on Liquid Adsorption Chromatography with Mixed Mobile Phases Analytical Approximations for Partition Coefficients BY WLADYSLAW RUDZINSKI* AND JOLANTA NARKIEWICZ-MICHALEK Department of Theoretical Chemistry, Institute of Chemistry, U.M.C.S., Nowotki 12, Lublin 20-03 1, Poland AND ZDZISLAW SUPRYNOWICZ AND KAROL PILORZ Department of Chemical Physics and Chromatography, Institute of Chemistry, U.M.C.S., Nowotki 12, Lublin 20-03 1, Poland Received 19th September, 1983 The effects of the energetic heterogeneity of solid/solution interfaces on the behaviour of liquid-solid chromatography systems have been investigated theoretically and the existing theories have been re-examined. This has led to a more rigorous explanation of the theoretical background of some equations which were developed in a semi-intuitive manner.In other cases, e.g. Soczewinski’s logarithmic relationship, a totally new theoretical interpretation has been obtained explaining why the tangent of this linear relationship is < 1 in many cases. The theory of partition coefficients in liquid-solid chromatography (1.s.c.) is only a special case of the theory of adsorption from multicomponent liquid mixtures onto solid surfaces. This is when one component (solute) is at a very low concentration, whereas the others (solvents) are at moderate concentrations. Changing the concen- tration of the solvents in the mobile phase enables a good resolution to be obtained between the various solutes.So it is no surprise that various attempts have been made to provide an equation for the partition coefficient of the solute between the surface and the mobile bulk phase as a function of the composition of the mobile phase. Several authors have proposed equations’ which have played an important role in the development of 1.s.c. The most popular are those of Snyder and Soczewinski relating the partition coefficient k, of the solute to the mole fraction y , of the more active solvent 2 in the mobile phase. Snyder’s equation is2 (1) where k,, and K,, are constants and an, is the ratio of the surface areas occupied by the molecules of the solute and of the more active solvent 2. The constant K,, describes the preferential adsorption of the more active solvent 2 over the less active solvent 1 from their binary mixture onto the solid surface.When solvent 2 is strongly preferentially adsorbed, then, except for small concentrations of that solvent in the - mobile phase, K,, y , $ 1 and eqn (1) reduces to Soczewinski’s relati~nship:~~ In kn = In kn1- a,, In (Y, + K,, v2) Ink, = C, - C, lny (2) where C, = a,,. 553554 SURFACE HETEROGENEITY AND LIQUID ADSORPTION In spite of the long time which has passed since eqn (2) was published, it is probably the most used relationship for the correlation of retention data in 1.s.c. The only explanation for its origin comes from theories of ideal adsorption on homogeneous solid surfaces. This derivation would suggest that eqn (2) is a simplified version of eqn (1). However, the interpretation of experimental data using eqn (2) leaves some intriguing questions to be answered.The good linearity which is usually found in the plot of Ink, against lny, is found at small concentrations of the preferentially adsorbed solvent 2 and fails at higher concentrations y,, contrary to the hypothetical trend predicted by the above derivation of eqn (2). Furthermore, the solute molecules are usually much larger than the solvent molecules. Thus, it is to be expected that the tangent a,, in the experimental plot of In k, against lny, should be > 1. Meanwhile, 1.s.c. systems in which an, < 1 are found experimentally at least as often as systems in which a,, > 1. The third question is why the simplified version of eqn (l), i.e. eqn (2), provides a better correlation of experimental data than eqn (1) itself.These observations suggest that eqn (2) must have a different theoretical background from that which is commonly believed. From the viewpoint of the thermodynamics of adsorption the development of the theories of the partition coefficients in 1.s.c. may be characterized as follows. In the beginning, attention was focused mainly on the interactions between the solid surface and the molecules of the solvents and and on the effects of interactions in the mobile bulk phase.5 Later on, the differences between the surface areas occupied by different admolecules were taken into account. Recently, a third important factor governing the behaviour of 1.s.c. systems has become widely realized. This is the energetic heterogeneity of the actual solid/solution interface.Because of variations in the local chemical composition and the crystallographic structure of the actual solid/solution interface, some areas of these interfaces will exhibit different adsorption features. A recent paper by Rudzinski et aZ.6 has shown that the effects of the dispersion of these adsorption features are at least as important as the effects of interactions between the adsorbed molecules. However, these effects cannot be isolated because of their mutual interference. An adequate, realistic theoretical description of 1.s.c. systems must take into account all the basic physical factors simultaneously. These are: (i) interactions between the molecules on the surface and the mobile bulk phase, (ii) differences between the molecular sizes of various molecules and the different surface areas occupied by them and (iii) the energetic heterogeneity of the actual solid/solution interface.In some systems multilayer adsorption effects may also play an essential role, especially in the case of badly mixing solvents. However, we will postpone discussion of this to future publications. Our attention in this paper is focused on factors (ii) and (iii) above, and especially on the role of the surface heterogeneity and its theoretical description in 1.s.c. We will show that it is the energetic heterogeneity of the actual solid/solution interface which is the source of the linearity of the double logarithmic plot of Ink, against lny,. Our theoretical consideration results in a new derivation of Soczewinski’s relationship which explains all the inconsistencies in its behaviour discussed above.THEORY Let xi (i = 1,2, . . ., n) denote the volume (area) fraction of component i in the adsorbed phase and yi be its volume fraction in the equilibrium mobile bulk phase.w. RUDZI-~KI et al. 555 Let ysr and y,, denote the appropriate activity coefficients in the adsorbed and the mobile bulk phase, respectively. Then, the simultaneous competitive adsorption from an n-component liquid mixture onto a hypothetical homogeneous solid surface is described by8 =Kin, i = 1,2 ,..., n-1 (4) where a,, is the ratio of the surface areas occupied by a single admolecule of ith and nth kinds. To a good approximation the equilibrium constant Kin can be written in the following form:6 Kin = ~ X P [(~i -sin En)/RTl ( 5 ) where E, and en are the adsorption energies of the single molecules i and n.We now introduce the following notation : E,, = E~ -a,, E, (6) and call E,, the ‘adsorption energy’ in the binary system (i+n). The energetic heterogeneity of the actual solid/solution interface will cause some dispersion of E,, values on various adsorption sites (surface areas associated with a local minimum in the solid-adsorbate interaction potential). The usual quantitive measure of the energetic heterogeneity of the actual interface is the so-called differential distribution of adsorption sites among various values of adsorption energy. In our case, it will be an (n - 1 )-dimensional differential distribution of adsorption sites among various sets x({E,,}) normalized to unity: r where C2 is the (n- 1)-dimensional physical domain of the variables tin.Let Xi((Ein}) denote the solution of eqn (4) for component i. In the case of a heterogeneous surface, has to be replaced by its average value xit: The fact that the different adsorption sites are characterized by different sets {E,,) is due to the many physical factors acting during the preparation of an adsorbent. It seems reasonable to assume that x({cin}) should be a Gaussian-like multi-dimensional function, since even when the single adsorption energies E, and E, have a complicated distribution (double maxima for silica gels) their difference usually has a Gaussian-like distribution.6 A more exact a priori assumption about an analytical approximation of this function seems to be difficult at our present state of knowledge.This is because both experimental and theoretical studies of adsorption from multicomponent liquid mixtures onto solid surfaces are still in their infancy. Studies of adsorption from binary mixtures are more advanced. The theoretical studies initiated by Rudzinski and coworkerss99-14 seem to confirm this a priori assumption about the Gaussian-like form of the distribution function : r where the integration is over all variables E ~ , , except the variable For the reasons556 SURFACE HETEROGENEITY AND LIQUID ADSORPTION mentioned above, and for others to be discussed below, we focus our attention on the following energy distribution : defined in the interval (- a, + a).The function xi, from eqn (10) is a Gaussian-like function of g i n , centred about E:,, whose spread is described by the heterogeneity parameter tin. When cin -+ 0, eqn (10) degenerates into a Dirac delta distribution Apart from the fact that an apriori assumption about the form ofX({Ein)) is difficult, the solution of eqn (4) and the subsequent evaluation of the (n- 1)-dimensional eqn (8) represents an extremely complicated problem. To make the problem tractable some simplifying assumptions must be made. The basic approximation accepted here lies in assuming the ratio ( x i / x , ) to be influenced only by the dispersion of the variable So, let us accept this assumption and replace the ratios ( x i / x , ) in eqn (4) by their averaged values ( x i t / x n t ) : B(&in - &&).where f+oo xit = J x ~ ( E ~ , ) x ~ , ( E ~ , ) dEin, i = 1,2, . . ., n - 1 -oo n-i i - 1 x,t = 1 - Xit. The averaging shown in eqn (1 I), together with the accompanying assumption eqn (1 2), reduces our problem to considering adsorption from binary mixtures onto heterogeneous solid surfaces. Therefore it seems necessary to involve some basic results concerning this problem. Let us first consider adsorption from the binary liquid mixture (i+n) onto a hypothetical homogeneous solid surface, characterized by an adsorption energy Let us also assume, for simplicity, that molecules i and n occupy equal surface areas, i.e. ai, = I, and that the adsorbed phase is ideal, i.e. yzi = 1 for i = 1, 2, ..., n. Then, the adsorption isotherm xi can be written as follows: xi = [ 1 +exp (Gn;;in)]-' - where ~f, is a bulk concentration function, which for the adsorption model defined above takes the following explicit form : ct, = -RT ln(ai/a,) (14) where ai and a, are the bulk activities of components i and n, equal to yi yyi and y , yyn.We will evaluate further the integral xit using the following integration by parts: +co xit = xi X i , I? g - (%) Xi, dEin --co aein whereW. RUDZINSKI et a[. 557 In the case of the infinite integration limits (- 00, + GO), the first term on the right-hand side of eqn (1 5) disappears, whereas the function (ax,/a&,,) takes the following explicit form : This is a Gaussian-like function of centred about &fn. We will evaluate the second integral on the right-hand side of eqn (15) by expanding Xi, into its Taylor series around the point = &fn, at which point the ‘sampling’ function (axi/aein) reaches its maximum.Doing so, we obtain where B, is Bernouli’s number. In the hypothetical temperature limit T + 0, when the ‘sampling’ function degenerates into a Dirac delta distribution - &), eqn (18) reduces to cn(ai/an)RTicin 1 + cn(ai/an)RTlcin ’ x . z t =-x. an (&C an ) = i = 1’2, ..., n-I where cn = exp (&/cin). (20) Note that the condition T + 0 is not the only one for eqn (19) to be valid, and it is also valid when the spread of the energy distribution x ~ ~ ( E ~ ~ ) is very large. Then, this function and its higher derivatives are small for any value of In effect, the terms under the sum in eqn (1 8) practically disappear and we have eqn (19).In other words, eqn (19) is also valid at higher temperatures for strongly heterogeneous surfaces. Let us write eqn (19) in the following compact form: RTIc,, , i = 1,2 ,..., n-1. Xnt Assuming in addition that and solving eqn (21) with respect to xit we obtain , i = 1,2 ,..., n-1. Gn<ai Ian) n-1 Xit = 1 + Z qn(aj/an)m j - 1 This equation was proposed by Jaroniec and Patrykiejew15 two years ago, following the numerical results for mixed-gas adsorption on solid surfaces obtained by Cricmore and Wojciechowski.lG Later on, Jaroniec attempted to generalize eqn (23)’ taking into account the important effect of different surface areas occupied by different molecules. He then suggested the following generalization : l7558 SURFACE HETEROGENEITY AND LIQUID ADSORPTION Let us, however, see whether the approach used by us makes a rigorous treatment of this problem possible.In the hypothetical case T - , 0, but also in the physically possible case of strongly heterogeneous surfaces, the sampling function (ax,/ae,,) behaves like a Dirac delta distribution with respect to while performing the integration on the right-hand side of eqn (15). Thus, looking for the maximum effectiveness of our approach, we should expand the function X i , into its Taylor series around the point e:,, at which the sampling function (ax,/aei,), (ain # l), reaches its maximum. This point is found from the condition (-)& = *- This means that in the general case a,, # 1 the function ern has to be replaced by &in which, according to eqn (25), takes the following explicit form: This leads us to the following generalization of eqn (19): where Eqn (27) can be rewritten in the following compact form: which is different from eqn (24).With the additional assumptions accepted when developing eqn (1 1) and (12) we arrive at the following generalization of eqn (23): RTIcjn (30) X i t = , i = 1,2, . . ., n - 1 I - 1 which should describe adsorption from a multicomponent liquid mixture onto a heterogeneous solid surface, characterized by the symmetrical dispersion of adsorption energies ein. In our case it can be used to calculate the adsorption equilibria of the competitive adsorption of solvents onto the solid stationary phase. Since in the usual chromato- graphic situation the solute (analysed substance) appears at extremely small concen- trations, its presence in the mobile phase will not affect the competitive adsorption of the solvents.Neglecting the local correlations in the adsorbed phase, one may consider the chromatographic process as the adsorption of solute molecules onto the solid surface in the molecular environment of solvent molecules, unaffected by the presence of the solute molecules. However, the competitive adsorption of solute will be governed by different rules,W. RUDZINSKI et al. 559 arising from the condition that the solute appears at very small concentrations. Let n denote the solute, whereas the indices 1,2, ,. , ., n - 1 are related to solvents. The experimentally measured partition coefficient k , is defined as follows : k , = lim (z).(31) Y n + O Since the solute n is assumed to appear at infinite dilution, its competitive adsorption with respect to any component i will be like that on a homogeneous surface, characterized by the energy E , ~ equal to &?ln. Thus, for the model of an ideal adsorbed phase considered in this work, the partition coefficient k , should be written where (33) At the same time, however, competitive adsorption of the solvents will still be described by eqn (30). This is because they usually appear at moderate concentrations. It appears that the problem lies in accepting the non-physical integration limits ( - co, + co). Their choice is reminiscent of some mathematical simplifications accepted in the theories of the adsorption of gas on heterogeneous solid surfaces.For the purpose of mathematical convenience it is often assumed there that the adsorption energy of the single molecule i, E ~ , may vary from zero to infinity. A logical consequence of this is to assume that the difference ei, may vary from minus to plus infinity. Meanwhile, for some obvious physical reasons, there must exist a minimum and a maximum energy, cFn and &fax, on an actual solid surface. Consequently, the difference We have shown in our previous publicationla* l9 that choosing the non-physical integration limits (- 00, + co) does not affect the result of integration in eqn (1 5), until the concentration of component i falls below some critical value. Below this critical value the competitive adsorption of i will be like that on a homogeneous surface characterized by the minimum value of ein found on the heterogeneous solid surface.Note that eqn (32) is different from that proposed recently by Jaroniec and will possess certain limits, &en and (Xit)1lrn ani OScik-Mendyk :20 kn = KO,~ (7) (34) where xit was assumed to be given by eqn (24). Now let us consider the explicit form of eqn (32) in which xit is evaluated by eqn (27). We also neglect the non-ideality effects in the mobile bulk phase. (Soczewinski’s linear relationship is also found in 1.s.c. systems in which the bulk solvent mixture is strongly non-ideal.) Then, from eqn (27) we have We now consider the region of small concentrations of the more active solvent, where Soczewinski’s linear relationship is usually found.Suppose that y2 + 0, y1 --+ 1. Then, from eqn (35) it follows that560 SURFACE HETEROGENEITY AND LIQUID ADSORPTION Eqn (36) combined with eqn (32) brings us to a new theoretical interpretation of Soczewinski’s relationship (see Appendix) : - Ink, = In [ e 3 & ) a n 2 ] - (37) Since for a typical heterogeneous solid/solution interface RT/c,, < 1, Ink, should decrease linearly with the logarithm of the more active (preferentially adsorbed) solvent 2. The term (1 - RT/c,,) is positive but still < 1. Furthermore, for typical solid/solution systems 0.7 < RT/c,, < 0.9, so the value of this term lies in the range 0.1-0.3. Thus, it can happen that the product (tangent) (1 - RT/c,,) a,, may be < 1 even when a,, itself is > 1. Thus, our derivation of Soczewinski’s relationship, based on the concept of energetic heterogeneity of the actual solid/solution interface, answers the three questions raised by the experimental observations of retention in 1.s.c.It also shows that energetic heterogeneity is probably the main factor governing the retention mechanism in 1.s.c. at small concentrations of the more active (preferen tially adsorbed) solvent. RESULTS AND DISCUSSION We consider eqn (37) to be the most important result obtained in this work as it provides a new theoretical background for Soczewinski’s linear relationship, eqn (2), applied so successfully by various authors. Note that eqn (37) was obtained with the simplifying assumption that both the bulk and the adsorbed phases are ideal.At the same time, however, the other two important physical factors are taken into account: i.e. the energetic heterogeneity of the actual solid/solution interfaces and the different cross-sectional areas of the different adsorbed molecules. The experimental data which have already been reported in the literature seem to provide an impressive check for the correctness of eqn (37). Let us consider for instance the work by Petrovic et al.,,‘ who chromatographed 15 mono-, di-, tri- and tetra-substituted steroid derivatives on silica-gel thin layers using benzene as the diluent (less preferentially adsorbed solvent), mixed with the following active solvents (solvent 2 in our theoretical consideration) : chloroform, diethylether, ethyl acetate, methyl acetate, methyl ethyl ketone, acetone, dioxane and propan-1-01.In all cases the dependence of the partition coefficient k , on the concentration of the more active solvent could be correlated using eqn (2). The slopes C, obtained from this linear regression have been tabulated by Petrovic et al. and their analysis is very interesting. In many cases C, < 1, although the molecules of steroid derivatives are expected to have larger cross-sectional areas on the silica surface than the above listed solvents. Moreover, for a given solute, the slopes C, for all the active solvents except propan-1 -01 are close to each other. This would suggest that the slopes C, are not very sensitive to the cross-sectional areas of the active solvents. In the case of propan-1 -01 the slopes C, are smaller by ca.20% than those of the others. It is known that propan-1-01 is more preferentially adsorbed from benzene than from the other solvents. In other words, not only geometric effects but also the chemical nature of the competitive adsorption of solvents are responsible for the value of the slope C,. This conclusion cannot be explained on the grounds of the previous interpretation of C, being equal to an2. However, the most impressive support for the validity of eqn (37) can be found in the works of Soczewinski and coworkers. Fig. 1 shows the experimental results ofw. RUDZINSKI et al. 56 1 log Y 2 Fig. 1. Experimental data of Wawrzynowicz22 for chromatography of various solutes (A, V and a) in a propan- 1-01 + n-heptane mixture on two adsorbents : (---) aluminium oxide and (---) silica gel.Wawrzynowicz,22 who chromatographed several solutes in a propan-1 -01 + n-heptane mixture on two adsorbents: silica gel and an aluminium oxide. The results in fig. 1 are presented using the experimental data RF and R,, which are obtained directly by means of thin-layer chromatography : 23 R , = In k’ = Ink, +constant (38 a) where k’ is the capacity factor. According to the previous interpretation of C, = an2, the slope C, for a given solute should be approximately the same for both adsorbents. Meanwhile, the observed slopes for silica gel are approximately twice those for aluminium oxide. This can easily be explained by eqn (37). Namely, it is known that silica surfaces are much more heterogeneous than alumina surfaces.This means that the term (RTIc,,) should, for a given binary solvent mixture, be smaller in the case of silica surfaces. Thus the term (1 - RT/c2,) should be larger and consequently the product C2[ = (1 - RT/c,,) anz] should also be larger, even if an2 is the same for both adsorbents. The trend which is observed in fig. 1 has been confirmed by Soczewinski and J ~ s i a k , ~ ~ Wawrzynowicz and D ~ i d o , ~ and Soczewinski et aLZ5 Table 1 reports some of the experimental values of C,, making a comparison between the two adsorbents possible. This selection is not yet complete, since many experimental data were reported in graphical form only. We may thus summarize our results as follows. The energetic heterogeneity of the actual solid/solution interface is one of the main, if not the main, factors governing562 SURFACE HETEROGENEITY AND LIQUID ADSORPTION Table 1.Comparison of the experimental slopes C, for silica and alumina for two binary solvent mixtures : cyclohexane + di-isopropyl ether and cyclohexane + ethyl acetate. Data selected from ref. (24) and (25). di-isopropyl ether ethyl acetate alumina silica solute alumina silica nitro benzene 2-nitro toluene 4-ni tro toluene 2-nitro-l,3-xylene 1 -nitronaphtalene 1 ,Zdinitrobenzene 1,5-dini tronap h thalene 1,3,5-trinitro benzene 0.43 0.40 0.41 0.39 0.47 0.80 0.84 1.08 1 .o 1 .o 0.7 0.8 1 .o 1 .o 1.7 1.5 0.48 0.39 0.52 0.40 0.57 1.22 0.98 1.33 1 .o 1 .o 1.2 1.1 1.2 1.4 1.9 2.0 the behaviour of the partition coefficient in solid-liquid chromatography. This fact has not been sufficiently realized in previous theoretical studies of 1.s.c.and therefore needs further extensive investigation. LIST OF SYMBOLS heterogeneity parameter in eqn (lo) constants in Soczewinski's relationship, eqn (2) capacity factor partition coefficient of solute n defined in eqn (31) partition coefficient kn in the pure solvent i equilibrium constant defined in eqn ( 5 ) generalized form of Kn, defined in eqn (28) equilibrium constant defined in eqn (20) equilibrium constant defined in eqn (33) heterogeneity parameter defined in eqn (22) and (23) gas constant temperature mole fraction of component i in the adsorbed phase average mole fraction of i in the adsorbed phase on a heterogeneous solid surface mole fraction of the component i in the bulk phase ratio of the surface areas occupied by single molecules of i and n, respectively activity coefficients of component i in the surface (x) and the bulk (y) phases adsorption energy of a single molecule of i difference between E~ and E,, defined in eqn (6) function of bulk phase composition, defined in eqn (14) generalized form of E& defined in eqn (26) most probable value of minimum and maximum values of multi-dimensional differential distribution of adsorption sites among various values of cin on a heterogeneous surfaceW. RUDZINSKI et al.563 x ~ ~ ( E ~ ~ ) Xin(cin) indefinite integral of x ~ ~ ( E ~ ~ ) a differential distribution of adsorption sites among various values of physical domain of the variables tin. APPENDIX Below we outline the derivation of Snyder’s and Soczewinski’s equations, based on the Let us neglect for the moment the non-ideality of both the bulk and the surface phase. concept of a homogeneous solid surface.26$ 27 Then, from eqn (4) it follows that Ink, = In Kn2+ an2 In (A 1) Let us assume in addition that solvent molecules occupy the same surface areas, i.e.that a,, = 1. Then (A 2) x2 Y 2 - = K,lbl+ K,, Y 2 ) - I - Combining eqn (A 1) and (A 2) one arrives at Snyder’s equation: In kn = In k,, - an2 In 01, + K,, y2) kn1 = Kn2(&Jan2- (A 4) When solvent 2 is strongly preferentially adsorbed, then, except for small concentrations of this solvent in the mobile phase, K2,y2 9 y , , and from eqn (A 1) and (A 2) we obtain In k, = In Kn2 - an2 In y 2 (A 5 ) the form of which is the same as eqn (2).M. Jaroniec and J. Okik, Journal of HRC & CC, 1982, 5, 3. L. R. Snyder, in Principles of Adsorption Chromatography (Marcel Dekker, New York, 1968). E. Soczewinski, Anal. Chem., 1969,41, 179. E. Soczewinski, J. Chromatogr., 1977, 130, 23. R. P. Scott and P. Kucera, J. Chromatogr., 1975, 112, 425. W. Rudzinski and S. Partyka, J. Chem. SOC., Faraday Trans. I , 1981,77,2577. W. Rudzinski, J. Narkiewicz-Michalek and S. Partyka, J. Chem. SOC., Faraday Trans. 1,1982,78,2361. D. H. Everett, Trans. Faraday SOC., 1965, 61, 2478. W. Rudzinski, J. Oicik and A. Dabrowski, Chem. Phys. Lett., 1973, 20, 5. lo J. Oicik, A. Dabrowski, M. Jaroniec and W. Rudzinski, J. Colloid Interface Sci., 1976, 56, 403. l1 J. OScik, A. Dabrowski and. W. Rudzinski, J. Colloid Polym. Sci., 1977, 255, 50. l 2 W. Rudzinski, A. Waksmundzki, M. Jaroniec and S. Sokolowski, Ann. SOC. Chim. Pol., 1974, 48, l 3 A. Dabrowski, J. Oicik, W. Rudzinski and M. Jaroniec, J. Colloid Interface Sci., 1978, 79, 287. l 4 A. Darowski and M. Jaroniec, J. Colloid Interface Sci., 1979,73, 475; 1980, 77, 571. M. Jaroniec and A. Patrykiejew, J. Chem. Soc., Faraday Trans. I , 1980, 76, 2486. l6 P. J. Cricmore and B. W. Wojciechowski, J. Chem. SOC., Faraday Trans. 1, 1977,73, 1216. l7 M. Jaroniec, Thin Solid Films, 1981, L97-99, 81. l8 W. Rudzinski, J. Narkiewicz-Michalek, R. Schollner, H. Herden and W. D. Einicke, Acta Chim. lo W. Rudzinski, L. Eajtar, J. Zajac, E. Wolfram and I. Paszli, J. Colloid Interface Sci., 1983, 96, 339. 2o M. Jaroniec and B. Oicik-Mendyk, J. Chem. SOC., Faraday Trans. I , 1981, 77, 1277. 21 S. M. Petrovic, L. A. Kolarov and E. S. Traljic, Anal. Chem., 1982, 54, 934 22 T. Wawrzynowicz, D.Sc. Thesis (Medical Academy, Lublin, 1980). 23 E. Soczewinski and J. Jusiak, Chromatographia, 1981, 14, 23. 24 T. Wawrzynowicz and T. Dido, Talanta, 1977, 24, 669. 25 E. Soczewinski, W. Golkiewicz and W. Markowski, Chromatographia, 1975, 8, 13. 26 D. E. Martire and R. E. Boehm, J. Liquid Chromatogr., 1980, 3, 753. 27 R. E. Boehm and D. E. Martire, J. Phys. Chem., 1980,84, 3620. 1991. Acad. Sci. Hung., 1983, 113, 207. (PAPER 3/ 1646)

ISSN:0300-9599    

DOI:10.1039/F19858100553

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. I , 1985, 81, 565-578 Radiolytic Preparation of Radical Cations of Nitroalkanes and Related Compounds BY D. N. RAMAKRISHNA RAO AND MARTYN C. R. SYMONS* Department of Chemistry, The University, Leicester LE 1 7RH Received 2nd April, 1984 Exposure of dilute solutions of a range of organic nitro derivatives in trichlorofluoromethane to 6oCo y-rays at 77 K gave radical cations thought to be formed from the expected primary species by some form of relaxation. These cations have e.s.r. spectra similar to that of nitrogen dioxide, and may be the isomeric nitrite cations, RONO+. However, the form and magnitude of the 13C hyperfine coupling for cations derived from 13CH,N0, suggests that the alternative a-structure (R'NO,)+ may be a better description. Cations derived from CH,CH,NO,, CH,CH,CH,NO,, (CH,),CHNO,, (CH,),CHO,, (CH,),C(Cl)NO,, (CH,),C(Br)NO,, (CH,),C(CO,Et)NO, and [(CH,),CNO,], all had very similar e.s.r.spectra, with indications of extra hyperfine splitting from halogen nuclei for the chloro and bromo derivatives. The ethyl derivative was unique in giving a second species in comparable yields which exhibited very large hyperfine coupling and considerable g-value variation. This is tentatively identified as CH,CH,O' radicals complexed with NO+ cations, The ethyl ester derivative gave a second species thought to be the ester cation. Several nitro aromatic compounds gave the normal n-cations initially, but all gave evidence for the rearranged NO,-like cations on annealing. A derivative ArC(Me),NO, with a p-nitro group gave high yields of this NO,-like cation even at 77 K, with no sign of the n-aromatic cation.One vinyl derivative, 1 -nitrocyclohexene, was also studied. This gave a complex spectrum which is assigned to a cation with its SOMO primarily confined to the alkene group. However, conversion into the rearranged cation was again detected on annealing. There is considerable current interest in the study of reactive intermediates of all types.'* The technique of electron spin resonance is well suited for the study of radical intermediates, but it has only recently been applied to the study of radical cations. These have, of course, been extensively studied in the gas phase, but not by magnetic resonance techniques. Two important methods have now been developed for the generation of radical cations in solid matrices.One, apparently best suited to the study of small cations, involves photoionization of the parent molecules during the process of deposition onto a cold-finger using a rare-gas matrix. This has been used to great effect by Knight et al., who have prepared species such as H,O, 'NH3,4 H,C05 and 'CHi6 and studied their structures by e.s.r. spectroscopy. The other, developed originally by Hamill et al. for optical studies,' involves exposing cold, dilute solutions of a substrate in a suitable solvent to ionizing radiation. The most commonly used solvent for e.s.r. studies is trichlorofluoromethane, partly because of its high ionization potential (ca. 1 1.8 eV) and partly because the electron-capture centres formed therefrom give very broad e.s.r.features which do not normally interfere with those 565566 RADICAL CATIONS OF NITROALKANES for the substrate cations. The essential chemistry involved is summarized by the reactions CFCl, -+ CFCli + e- (1) (2) (3) (4) These reactions have been clearly established for a wide range of solutes (S), provided their ionization potentials are -c 1 1.8 eV. In many cases the expected cations have been detected, with no evidence for major modification by the matrix,8-11 but in others hyperfine coupling to 19F l2 or 35Cl and ,'CI 1 3 9 l4 gives evidence for weak bonding to a solvent molecule. In a few cases the e.s.r. results have been unexpected, a noteworthy example being those for the radical cations of nitr0a1kanes.l~ According to simple expectation, photoelectron spectroscopy and theoretical estimates,lG the SOMO for these cations is expected to be confined to the two oxygen atoms.The three possible orbitals [(I)-(111)] CFCl, + e--+ (CFC1,)- + 'CFCl, + C1- CFC1,C + CFCl, f CFCI, + CFCl; CFC1; + S -+ CFCl, + S+. X R Y I Y I I 9 0 ;/ Q I LO, y ' D n n* are close in energy, and there is some controversy relating to their order, but that is not our main concern since such cations either were not detected or were very minor components. The major species detected had e.s.r. spectra so similar to that for 'NO, that we initially supposed this to be the product. Indeed, trapped 'NO, radicals are known to exhibit a rather variable set of e.s.r. parameters, mainly because these small molecules tend to librate in different ways in different solvents,17 so it was not possible to eliminate this theory simply because of lack of precise agreement of parameters.The aim of the present work was to establish the identity of these NO,-like species using WH,NO,, and to extend the study to a range of other nitro derivatives. EXPERIMENTAL The nitro derivatives of methane, ethane, propane and 2-methylpropane were the best commercially available and were generally used without further purification. However, because of the unique results for CH,CH,NO,, this compound was extensively purified by various standard methods. The e.s.r. spectra were unaffected by any of these procedures. Samples of CH,CD,NO, were prepared by dissolving EtNO, in D20 + NaOD solvents, neutralization and extraction after the conversion back into the nitro derivative was complete.The 2-propyl derivatives, Me,C(CI)NO,, Me,C(Br)NO,, Me,C(CO,Et)NO,, Me,C(CMe,NO, and Me,C(Ar)NO, (Ar = p-nitrophenyl) together with 1 -nitrocyclohexene were all purified com- pounds kindly supplied by Dr R. Bowman (Loughborough University). 13CH,N0, (90% enrichment) was from Brochem (B.O.C.) and was used as supplied. Dilute solutions (ca. 0.001 mole fraction) were degassed and frozen as small beads in liquid nitrogen prior to exposure to 6oCo y-rays in a Vickrad cell at 77 K to doses up to ca. 1 Mrad.D. N. R. RAO AND M. C. R. SYMONS 567 E.s.r spectra were measured at 77 K using a Varian El09 spectrometer calibrated with a Hewlett-Packard 5246L frequency counter and a Bruker B-H12E field probe, which were standardized with a sample of diphenylpicrylhydrazyl (DPPH).Samples were annealed in the insert Dewar after decanting the coolant and were recooled to 77 K whenever significant spectral changes were detected. RESULTS AND DISCUSSION In all cases good e.s.r. spectra were obtained in the g = 2 region, which we confidently assign to radical cations derived from the nitro derivatives. The resulting spectra [see fig. 1 of ref. (15) and fig. 5-7 later in this paper] are remarkably similar to the e.s.r. spectra obtained from certain irradiated metal nitrates.18 These are characterized by signals from NO; and 'NO, radicals, that characteristic of 'NO, being the asymmetric triplet with ca. 60 G splitting (B) and that characteristic of NO; being the small triplet feature (ca.3-4 G) occurring ca. 17 G downfield of the central ( M , = 0) line (species A). SPECIES A We expect the spectrum for RNOH+ cations formed by electron loss from oxygen to resemble that for NO; since both have SOMOs confined to oxygen. For NO', there is little doubt that this is an in-plane antibonding combination of oxygen 2p orbitals, but there has been controversy regarding the degree of admixture of the three available atomic orbitals.lg9 2o We therefore expect to find a small, fairly isotropic coupling to 14N in the region of 4 G and a large g-value variation. There can be little doubt that feature B is due to these unmodified cations. However, they were always overshadowed by strong features from species A, so that the full spectrum remains unknown.This means that we are unable to select a precise structure for this species, nor can we extend significantly our previous discussion.15 SPECIES B The similarity in form of the 14N hyperfine coupling and g-tensor components to those for 'NO, radicals must mean that this species has a similar structure. Also, the absence of any proton hyperfine splitting seemed to indicate low spin density on the alkyl groups. These considerations lead,us to suggest the rearranged nitrite form of the cation (IV). Although such cations are unknown, the isoelectronic carbon-centred radicals ROC0 have been As expected, the results show that these are structurally similar to 'COT radicals, and for CH,OCO the proton coupling was only 1.23 G.21 However, our new results for (l3CH3N0,)+ cations make this assignment less compelling.13CH,N0 CATIONS A typical e.s.r. spectrum is shown in fig. 1 . This can be compared with fig. 1 of ref. (1 5) for the corresponding 12CH,N0, cation spectrum. Our best computer fit for this spectrum, using the data given in table 1, was quite acceptable, accommodating all the extra features satisfactorily. The results are surprising in that they establish that the 13C coupling is remarkably large. If the isotropic and anisotropic components568 RADICAL CATIONS OF NITROALKANES 1 32505 I * H 50G , Fig. 1. First-derivative X-band e.s.r. spectrum for 13CH,N0, in CFCl, after exposure to 'j0Co y-rays at 77 K, showing features assigned to the a-radical H,C"Oi (90% enriched in 13C). are analysed in the usual way23 we obtain ca.8% 2s and 38% 2p, character for the orbital on carbon. A similar analysis of the 14N hyperfine components gives ca. 10% 2s and 34% 2pz character. Thus the SOMO seems to be rather evenly divided between carbon and nitrogen, and it is difficult to accommodate that result in terms of the nitrite structure (IV). The fact that the maximum 14N and 13C tensor components appear to be nearly colinear together with g,, which is close to the free-spin value, strongly suggests the alternative a-structure (V), in which the C-N bond has stretched but the CH, unit remains pyramidal and the NO, unit bent. This novel a-structure also accommodates the absence of detectable lH coupling since the coupling for methyl radicals is known to pass through zero as the radical bends.It accords with the a-structure for the cation (Me,CCH,)+ studied by Toriyama et al." It represents the first stage in the dissociation to give 'CH,+NOi radicals, although no such complete dissociation has been detected in the present study even for the t-butyl derivative. It is interesting to compare this a-structure with that found for the isoelectronic system, CH,COg. Lin and K e ~ a n , ~ have used electron spin-echo techniques to show that when this radical breaks down to give 'CH, +C02 in irradiated lithium acetate the two products remain close together. One of us25 has used the results to establish that the only major movements within the crystal are those of the two carbon atoms to give the planar 'CH, unit and linear CO, unit. Thus for H,C---CO, relaxation of both halves of the molecule seems to be complete, whereas for (H,CNO,)+ it is minor for both halves of the cation.Why should this arise? At first sight it might be argued that the C-C bond shouldD. N. R. RAO AND M. C. R. SYMONS 569 Table 1. E.s.r. parameters for radical cations of nitroalkanes and NO2 14N hyperfine coupling constantsa g values radical A , A?/ Az Aiso gz g, gz NO,b CH,NOz C d CH,CH,NO,+ CH,CH,CH,NO; (CH,),CHNOl (CH,),CNOl c1 / \ (35c1) / \ PBr) (CH3)2C NO,+ Br (CH3)2C NO: 46.13 52 120 53 53 52 52 52 52.5 - 0 52 - 4 C02Et 52 / \ I I (CH3)2C NO: (CH3)2C -c(CH3)2 NO,NO; Me Me, I ,NO; C 53 44.8 47 120 48 49 49 48 52 50 - 0 49 - 4 49 48 50 49 48 66.76 66.5 155 66 70 67 67 63 67 5 66 7.5 67 67 68 68 67 52.56 55.2 55.7 57.3 56 55.7 55.7 132 56.5 - - - 56 55.3 57 56.7 55 2.0062 2.0045 2.0045 2.0045 2.005 2.0045 2.005 1.999 2.003 2.004 2.005 2.0045 2.005 2.005 2.004 1.9910 1.994 1.994 1.9935 1.994 1.994 1.994 1.996 1.995 1.995 1.994 1.994 1.993 1.993 1.992 2.0020 2.002 2.002 2.002 2.002 2.002 2.002 2.002 2.002 2.002 2.002 2.002 2.002 2.002 2.002 a 1 G = T; NO, in gas phase; 13C parameters; after annealing. be stronger than the C-N bond (cf the fact that the C-C bond in C,O;- is much stronger than the N-N bond in N,O,).However, reference to the qualitative energy-level scheme in fig. 2 suggests the reverse. We are comparing the one-electron bond between CH,+ and CO, or NO;. For the former, localization on the CH, unit is clearly more favourable than for the latter.Thus we are inclined to favour the a-structure (V) for the major cationic species formed from nitromethane. However, when the irradiated 13CH,N0, sample was annealed the spectrum changed, with complete loss of the 13C features, leaving features570 RADICAL CATIONS OF NITROALKANES \ \ \ \ ,,,- CH; 0,c -\ O,N*-, ''4, I I \ '\,+; I Fig. 2. Qualitative energy-level scheme for Me'CO, and Me'NOl radicals having a single electron in the C-C or N-C a-orbitals. This suggests why the a1 bond is significant for Me'NOi but dissociates to give 'CH, + CO, for Me'CO,. LL-Juu +1 ( B I 0 (6) -l(Bll - .1 1 ( C ) L(C1 - I // ( C ) +1 1 Fig. 3. First-derivative X-band e.s.r. spectrum for CH,CH,NO, in CFCl, after exposure to s°Co y-rays at 77 K, showing features assigned to CH,CH;'NOl cations (B) together with those tentatively assigned to CH,CH26---(NO+) alkoxy radicals (C).very similar to the normal 14N triplet, as obtained from 12CH3N0, solutions. Indeed, in several instances we noticed small changes in the 14N hyperfine and g-values for these cationic species on annealing, and these results suggest that they may occur as a result of a major chemical change. It is tempting to suggest that this is the rearrangement (1) that we originally supposed to occur at 77 K: N / \ R'NO; 0 0-R+. This explains the small change in A(14N) and almost complete loss of 13C coupling. An alternative, that this secondary species is simply 'NO,, seems to us less likely since this requires the concurrent formation of CH; carbocations. We conclude that, at least for CH3N0,, the major cationic species formed at 77 K is the a-cation, H,C"O:, and that this readily rearranges (at ca.130 K) to give the nitrite cation, ONOMe+. This is supported by our results for solutions of amyl nitrite in freon. This gave a spectrum very similar to those obtained from the nitro derivatives. This result helps to support our view that ONOR+ cations should indeed give rise to such spectra.D. N. R. RAO AND M. C. R. SYMONS 57 1 Table 2. E.s.r. parameters for other radical species derived from nitro derivatives radical lH hyperfine coupling/G g values fl-("b' - L 17 (1) 25 (3) 50 (1) ca. 2.0025 ca. 2.0025 ca. 2.0025 Me,C-C 23(2H) 8(2H) ca. 2.0025 \ +OEt CH,CH,O H(l) ca. 106 2.031(11) H(2) ca. 92 2.003 (I) NITROETHANE This compound was unique in giving high yields of a second species whose outer features are shown in fig.3. The main triplet is the normal NO,-like cation, the spectrum for the new species (C) comprising either an anisotropic doublet with A z 198 G or, more probably, a doublet of doublets with A , z 106 G and A , z 92 G. The latter is supported because on annealing the outer lines were lost less rapidly than the triplet features, and two central features were revealed which are probably part of the spectrum for C. These features showed the same extra 4 G splitting seen on the outer perpendicular lines for species C. Thus we require a radical having two strongly coupled protons and strongly asymmetric g tensor (gll = 2.03 1, gl = 2.003) (table 2). All these properties point to the presence of CH,CH,6 radicals.These are expected to have a shifted gll value, strong hyperfine coupling to the two methylene protons and weak coupling to the methyl protons. Although we know of no report giving e.s.r. parameters for this radical, a range of RCH,O' radicals have been studied by e.s.r. spectroscopy, mainly by Box and coworkers, and also the CH,O radical has been studied by this technique.,' Our total proton coupling of ca. 170 G is greater than that normally found (EH = 141-156), whilst the value for g,,, of ca. 2.031 is less than that usually detected (2.054-2.093).28 Very similar results were observed for a radical recently detected by us when attempting to prepare the radical cation of s-tri~xane.,~ The major species was almost certainly the normal cation, but a secondary spectrum FA(PH) = 175 G, g,,, = 2.02861 was assigned to the alkoxy radical indicated in (VI), with weak interaction between the alkoxy oxygen and the cationic centre.In terms of this structure, such an interaction is necessary because, to have a relatively small572 RADICAL CATIONS OF NITROALKANES 1 32505 Fig. 4. First-derivative X-band e.s.r. spectrum for (CH,),CNO, in CFCl, after exposure to s°Co prays at 77 K, showing features assigned to librating (Me,CNO,)+ cations. g shift, the p-orbital degeneracy must be strongly lifted. This is normally accomplished by specific hydrogen bonding,26-28 but this cannot be invoked in the presence case. We therefore suggest structure (VII) to explain the small Ag for the CH,CH,O* radicals thought to be formed from the (EtNO,)+ cation.Note that studies of methyl nitrite and nitromethane by mass-spectrometiic techniques show that a major path for fragmentation gives MeO' radicals and NO+ ions.z8b This lends support to our postulate for species C, but we still do not understand why this species was only detected for nitroethane. In order to check the possibility that this species was formed from an impurity in the nitroethane, a variety of purification procedures was used. These made no difference to the relative yields of the two major products. Furthermore, we were unable to detect any impurities using proton resonance. Despite this identification, we have strong reservations since a rather similar but far weaker spectrum appeared for irradiated freon solutions of Me,C(NO,)C(NO,)Me,, [see fig.7(b)]. In this case, we cannot use any R O formulation to explain the results. [(CH,),CNO,] + CATIONS Although the e.6.r. spectrum differs markedly from those for other nitroalkane cations (fig. 4), the data show that the species is essentially the same, with Aiso andD. N. R. RAO AND M. C. R. SYMONS 1 3250 Ci 1 32SOG 0 1 573 Fig. 5. First-derivative X-band e.s.r. spectrum for (a) Me,CClNO, and (b) Me,CBrNO, in CFCl, after exposure to s°Co y-rays at 77 K, showing features assigned (a) to (Me,CClNO,)+ cations and (b) to (Me,CBrNO,)+ cations.574 RADICAL CATIONS OF NITROALKANES 1 3250 G 1 l u 1 0 Fig. 6. First-derivative X-band e.s.r. spectrum for Me,C(CO,Et)NO, in CFCI, after exposure to 6oCo y-rays at 77 K, showing features assigned to [Me,C(CO,Et).NO,]+ cations and to [Me,C(NO,)-C~,Et]+ cations (D).g,, close to those for the other species. We therefore invoke extensive libration of the -NO, unit about all three axes. This is presumably a consequence of the size of the t-butyl group. However, it may also reflect a change in the type of bonding between the alkyl and -NO, groups, reflecting a weaker bonding and hence greater freedom for NO,. We tentatively suggest that the Me3C- group has become planar or nearly so, the structure approaching the limiting form Me,C+ ---NO,. Very extensive libration for the small molecule NO, would then be expected. CHLORO AND BROMO DERIVATIVES We had hoped to detect well defined hypefine coupling to chlorine and especially bromine nuclei for these cations, as we did for the corresponding anion.,O In fact (fig.5) there are extra small splittings, but these are difficult to correlate with the expected quartet features from 35Cl, ,'Cl, 'OBr and *lBr, which all have Z = i. Our tentative analysis, which accommodates much of the extra splitting, is shown in fig. 5, and the suggested parameters are given in table 1. We stress that the directions of the hyperfine tensor components are unlikely to be coaxial with the 14N and g-value components, so the magnitude of the coupling can only be taken as a rough indication of delocalization. Furthermore, the coupling constants are likely to be smaller than the electrical quadrupole coupling energy, and hence the spectral features are unlikely to be quartets except for fields close to the carbon-halogen axis.Thus there is little point in our endeavouring to obtain a better spectral fit or in considering the significance of the results, except to say that, as expected for the proposed structures (IV) or (V), delocalization onto C1 or Br is small.D. N. R. RAO AND M. C. R. SYMONS 1 32506 1 3200G u - 1 575 I1 II I 1 X +1 X 0 X 1 I I I t 11 Fig. 7. First-derivative X-band e.s.r. spectrum for (a) Me,C(NO,), and (b) Me3C(NOz)- C(NO,)Me, showing features assigned to (a) [Me,C(NO,)”O,]+ cations with only one strongly coupled 14N nucleus and (6) [Me,C(NO,)-C(Me),’NO,]+ cations. [Outer features (a) in (b) are discussed in the text.] Y Z *: +r : r -1 -1576 RADICAL CATIONS OF NITROALKANES Me,C(CO,Et)NOi CATIONS These were of interest because of the potential competition between two localized cationic species, one being an ester cation (RCO,Et)+ and one a nitro cation.We have already established that, in general, ester cations are z, with major spin density on the ester oxygen and strong hyperfine coupling to the a-protons of the ester alkyl group. l4 In fact, both types of spectra were obtained (fig. 6). Analysis of the ester-cation features is difficult because of severe overlap with the 10) features of the nitro-cation spectrum. However, the spectrum was better defined at ca. 140K and gave the parameters listed in table 2. These are typical for an ethyl ester. Thus ethyl formate cations have a(2H) = 22 G and a(2H) = 10 G. The other features are clearly due to the normal form of nitroalkane cations, presumably having the a-structure (V).* Coexistence of these two structures is of considerable interest, since even if one accepts that initial electron loss should occur at either site in a statistical manner, and that no fully delocalized SOMO can be formed, one would expect subsequent electron transfer to give only the more stable cation.The ionization potentials of typical RC0,R' and RNO, molecules are ca. 10.6 and 11.2 eV, respectively. However, the value for nitroalkanes refers to loss from the n ( 0 ) orbitals, rather than from the stretched C-N a-orbital. We need either a fortuitous balance of energies or such effective orthogonality that electron transfer cannot occur despite the short path-length. DINITRO DERIVATIVES For the cation of Me,C(NO,), we find complete localization on one NO, group, with no hyperfine contribution from+the other 14N nucleus (fig.7). This accords with expectation for the a-structure 0,N 'C(Me),NO,, or the rearranged structure. The results establish that any tendency to switch from one NO, group to the other is slow, and that, as with the ester derivative, there is no available orbital which is delocalized onto both nitro groups. This also applies to the cation of tetranitromethane and to the derivative having nitro groups on two adjacent carbon atoms [Me,C(NO,)C(NO,)Me,]. The spectrum for the latter cation shows a small extra doublet splitting on the x features of ca. 4 G. This is tentatively assigned to coupling to the second 14N nucleus. This can appear as a doublet rather than a triplet under conditions of quadrupole control.Outer features {a in fig. 7(b)] are similar to those found in the spectra for EtNO, systems, but in this case they cannot be assigned to Me,C(N0,)6 radicals since there are no p protons. AROMATIC NITRO COMPOUNDS As expected, the initially formed cations have SOMOs based on the ring n - ~ r b i t a l s , ~ ~ that for nitrobenzene being the symmetric orbital with a node through nitrogen (VIII). However, this is not a powerful selection by the nitro group, since a p-methyl group I CH3 * Note added in proof: Recent work suggests that these ester cations may have rearranged structures such as //OH+ R-C ' CH,-CH,.D. N. R. RAO AND M. C. R. SYMONS 577 switches the SOMO so as to place high spin density on the methyl group (IX), as in the toluene However, the most interesting aspect of these results is the fact that, on annealing, an NO,-like spectrum appears, which we.assign either to the a-radical (V) or, more probably, the nitrite derivative, (ArONO)+. The intensities of these features vary strongly with the nature of Ar, as does the temperature at which they are first detected. However, we have not been able to discern any clear pattern. No doubt there are other pathways for decomposition besides this rearrangement. In the light of these results we thought it of interest to study the competition between nitro groups for the cation of the p-nitro derivative (X). Our results, again to our NOz surprise, show that the major product at 77 K is an NO,-like species, with an e.s.r.spectrum similar, for example, to that shown in fig. 7. Since it is formed directly, this is probably the a-radical, ArCMe,'NOz. If initial electron loss from the ring occurs as expected, transfer of the hole into this a-orbital must be facile. This suggests a conformation in which the aliphatic nitro group lies out of the plane of the ring so as to maximize a-n overlap with the C-N bond. Electron transfer then represents the limit of hyperconjugative electron release. It is a major effect rather than a minor perturbation because the C-N bond stretches extensively. We stress that in the absence of 13C data for such radicals we are unable to discover which of the two limiting structures, (IV) or (V), is actually present in these systems.NITROALKENE CATIONS One derivative, 1-nitrocyclohexene (XI), has been studied in freon. The e.s.r. spectrum after irradiation was typical of those for alkene cations and was analysed in terms of one aH at 17 G, three QH at 25 G and one PH at ca. 50 G. These results can be compared with those for the cation of cyclohexene (2aH, 9G; 2QH, 55 G; 2bH, 22 G).32 This suggests that there is a shift of spin away from the nitro group, but that the normal chair form is not adopted. It is interesting that, although the a-proton coupling indicates a high spin density on C,, this is not reflected in an increase in the Q-proton coupling constants. RCoH RADICALS In the light of these results we have searched the literature for evidence of similar processes occurring for RCO; radicals.As stressed above, however, the limiting 'a' structure is now one which resembles R---CO,, with almost no bonding between the two fragments. Although ROC0 radicals are stable in that they can be detected in 20 FAR 1578 RADICAL CATIONS OF NITROALKANES liquid-phase experiments, we found no evidence for their formation from R' and CO, formed by radiolysis of RCO; or RC0,H. Our own attempts to detect such a reaction have also failed. We thank Dr R. Bowman of Loughborough University for kindly supplying several nitro derivatives and for helpful discussions. Reactive Intermediates, ed. R. A. Abramovitch, (Plenum Press, New York, 1980, 1982, 1983), vol. 1,2 and 3. Organic Reactive Intermediates, ed. S . P. McManus (Academic Press, New York, 1973). L.B. Knight and G. Steadman, J. Chem. Phys., 1982, 77, 1750. L. B. Knight and G. Steadman, J. Chem. Phys., 1983, 78, 5940. L. B. Knight and G. Steadman, J. Chem. Phys., 1984, in press. L. B. Knight, G. Steadman, D. Feller and E. R. Davidson, J. Am. Chem. SOC., 1984, 106, 3700. chap. 9, p. 321. T. Kato and T. Shida, J. Am. Chem. SOC., 1979, 101, 6869. M. C. R. Symons and I. G. Smith, J. Chem. Res. (S), 1979, 382. ' W. H. Hamill, Radical Ions, ed. L. Kevan and B. Webster (Wiley Interscience, New York, 1968), lo J. T. Wang and F. Williams, J. Chem. Soc., Chem. Commun., 1981,666. l1 K. Toriyama, K. Nunome and M. Iwasaki, J. Chem. Phys., 1982,77, 5891. l2 A. Hasegawa and M. C. R. Symons, J. Chem. Soc., Faraday Trans. 1, 1983,79, 93. l3 D. Becker, K. Plante and M. D. Sevilla, J. Phys. Chem., 1983, 87, 1648. l4 D. N. R. Rao, J. Rideout and M. C. R. Symons, J. Chem. SOC., Perkin Trans. 2, 1984, 1221. l5 D. N. R. Rao and M. C. R. Symons, Tetrahedron Lett., 1983, 24, 1293. C. N. R. Rao, Indian J. Chem., 1976, 14A, 147. H. Sharp and M. C. R. Symons, J. Chem. Soc. A, 1970, 3075. P. W. Atkins and M. C. R. Symons, J. Chem. Soc., 1962,4794. l9 P. W. Atkins and M. C. R. Symons, The Structure of Inorganic Radicals (Elsevier, Amsterdam, 1967). 2o M. C. R. Symons, J. Chem. Soc., Dalton Trans., 1979, 423. 21 H. Hefter and H. Fischer, Ber. Bunsenges. Phys. Chem., 1970, 74, 493. 22 D. Griller and B. P. Roberts, J. Chem. Soc., Perkin Trans. 2, 1973, 1339. 23 M. C. R. Symons, Chemical and Biochemical Aspects of Electron Spin Resonance Spectroscopy (Van 24 D-P. Linad and L. Kevan, Radiat. Phys. Chem., 1981, 17, 71. 25 M. C. R. Symons, J. Phys. Chem., 1983,87, 1833. 26 J. Y. Lee and H. C. Box, J. Chem. Phys., 1973,59, 2509. 27 M. Iwasaki and K. Toriyama, J. Am. Chem. Soc., 1978, 100, 1964. Nostrand Reinhold, Wokingham, 1978). (a) W. A. Bernard, D. M. Close, J. Hiittermann and H. Zehner, J. Chem. Phys., 1977, 67, 121 1 ; (b) Y. Niwa, S. Tajima and T. Tsuchiya, Int. J. Mass Spectrom. Ion Phys., 1981, 40, 287; (c) J. P. Gilman, T. Hsieh and G. G. Meisels, J. Chem. Phys., 1983, 78, 1174. M. C. R. Symons and B. W. Wren, J. Chem. Soc., Perkin Trans. 2, 1984, 51 1. 30 M. C. R. Symons and W. R. Bowman, Tetrahedron Lett., 1981, 22,4549. 31 M. C. Symons and L. Harris, J. Chem. Res., ( S ) , 1982, 268; (M), 1982,2746. 32 M. Tabata and A. Lund, Chem. Phys., 1983,75, 379. (PAPER 4/538)

ISSN:0300-9599    

DOI:10.1039/F19858100565

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. I , 1985, 81, 579-596 Thermodynamics of Transfer of Noble Gases in Hydrophobic Solvents and in Phospholipid Membranes BY YEHUDA KATZ The National Physical Laboratory of Israel, Hebrew University Campus, Givat Ram, Jerusalem, Israel Received 2nd April, 1984 A theory of gas solubility has been formulated and tested using solubility data of noble gases in several hydrophobic solvents and in a phospholipid membrane of dimyristoyl lecithin. The theory describes solubility in terms of two independent processes: hole creation in the solvent and solute adsorption in these holes. It gives the standard thermodynamic functions of solution in terms of four independent microscopic parameters. Two parameters reflect the nature of the pure solvent and the others the nature of the solute and its interactions with the solvent.Examination reveals good agreement between theory and experiment for the solvents tested, paving the way for comparisons between membranes and bulk solvents which are more significant than the existing correlations. The Characteristics of the theory and its agreement with experiment suggest an interesting method for evaluating, from the solubility data of gases, the physical properties of hydrophobic regions of membranes and of other solvents which cannot be measured directly. Guided by our desire to engage in membrane research a theory which is both physically sound and biologically relevant, we develop here a solution theory for simple solutes and test it for bulk solvents and for phospholipid membranes. The theory is needed to evaluate the behaviour of solutions, correlating the macroscopic properties with the molecular features of the constituents. The correlations are then used to deduce solvent characteristics from measured solubilities and vice versa.The definition, characterization and analysis of biological membranes is often done by analogy with model bulk sol~ents.l-~ The long established procedure consists of measuring the extent to which solubilities in selected solvents correlate with the behaviour of the membrane investigated, assuming that the solvent which correlates best with a given property of the membrane is the one which most resembles the molecular features responsible for the pr~perty.~ This method has contributed significantly to our understanding of biological membranes but it also leads to ambiguities and controversies, because the empirical correlations were gross over- simplifications, sweeping the complexities involved in determining the behaviour of solvents into a limited number of empirical parameters.s-8 The best way to overcome this difficulty is to base the comparison between solvents on the extent to which they can be described by a given theoretical formulation instead of using the closeness of fit of an empirical correlation as a criterion.Only when a theory like this is found can the correlations be analysed in terms of the molecular parameters involved and the biological meaning of the experimental findings discussed. The formulation of a theory of solution and its test on membrane and bulk solvents, which is made here, is therefore a step toward introducing physical preciseness into the analysis of membrane phenomenology .Out of the many possible formulations of solution behaviour9 we choose a model 579 20-2580 TRANSFER OF NOBLE GASES which explains solubility as an adsorption of solute molecules in holes made in the solvent because of the thermal motion of its molecules. The choice is dictated by the simplicity of the model, by its ability to relate separately to the pure solvent and to its interactions with the solutes dissolved in it and by the fact that it is related to other successful existing models of solution. The fact that the parameters used here are closely related to some properties which are of special interest in membrane research also contributed to our choice.We limit our discussion here to the solubilization of the simple monoatomic noble gases in simple solvents. This limitation removes major theoretical obstacles and establishes firm ground on which the analysis of more complex solutes can be based. The advantages follow from the fact that the noble gases are the simplest of all solutes and yet their solubilities are determined by factors which also affect the solubilization of all known solutes.10 Simplicity is what makes the noble gases liable to quantitative theoretical analysis by simple models. The existence of a quantitative theory for the solubility of the noble gases enables one to analyse the behaviour of complex molecules in solution by comparing the experimental results for these solutes with predicted values.Noble gases absorb preferentially in the hydrophobic region of the membrane," thus making the noble gases important probes for the investigation of a membrane region which cannot otherwise be separated from the rest of the membrane and explored independently from the other parts. The theory of solution for the noble gases proves, therefore, to be beneficial for an understanding of the hydrophobic region as well as for an understanding of transport through membranes. To test the model we use literature data1'? l2 for the solubility of noble gases in bulk hydrocarbons and in a phospholipid membrane. The available data on the solubility in bulk hydrocarbons are much more abundant than those for solubilities in phos- pholipid membranes. This means that the testing of the model is better when dealing with the bulk solvents.THE MODEL The model we use considers solubility as the adsorption of a solute molecule into one of the holes which the thermal movement of the solvent molecules has created in the medium. We assume that the solute molecule recognizes the hole as a gas space in which it moves under the influence of the adhesive potential field of the environ- ment. We also assume that the two steps which constitute the process of solubility, namely hole creation and solute adsorption, are independent of each other. To simplify the treatment we limit our discussion to dilute solutions, thus neglecting interactions between the solute molecules in the solution.Consider first the number nh and the total volume & of holes which appear in one mole of a liquid whose molar volume is 5. The maximum number of holes a molar volume can accommodate is Nh. The number of holes is determined by the relative magnitudes of the cohesive energies holding the solvent molecules together and the kinetic energy, associated with the thermal fluctuations in the medium, which pulls the solvent molecules apart from each other. According to a basic law of statistical mechanics nh = Nh(vh) exp [-fll(vh)/kT] (1) where T is the absolute temperature, k is Boltzmann's constant and fll(Vh) is the reversible work required to create a hole of volume of at least vh. This work is equal in magnitude and opposite in sign to the cohesive free energy which holds the solvent molecules together.The distribution of holes and their average magnitude were calculated long and for our purpose it is sufficient to note that there is an averageY. KATZ 58 1 value of hole size and that the distribution around this size is a sharp one, meaning that Nh(Uh) andfl,(vh) refer to specific parameters determined by the average hole size. Obviously, by definition which, together with eqn (I), gives the total vacant volume, i.e. the volume free for solute adsorption vf as a function of the molar volume V, : v, = Vl exp ( - f , , / k T ) . (3) Consider now the adsorption of the solute in the free volume. The adsorption is characterized by an equation which relates the concentration of the solute in the free volume of solution n,/ V, to its concentration in the gas phase above the solution ng/ Vg.According to this equation 3 = 3 exp ( - f , , / k ~ ) v g v, where ng and n, are the number of solute molecules in the gas volume Vg and in the free volume vf, respectively, -fsl is the free energy of molecular adhesion, being the free energy gained when a solute molecule adsorbs to a hole, andf,, reflects the tendency of a solute to stick to the solvent. Combination of eqn (3) and (4) gives the expression (4) n exp [Vll -fs,)/k T3 ( 5 ) - _ 5 3 -n, v g Vl linking the experimentally measurable distribution of the solute to the microscopic parameters which determine the behaviour of solutions. Inspection of eqn ( 5 ) reveals that two independent microscopic free energies determine the phenomenological standard free energy of solution A p e .The first of the two,f,,, is independent of the solute and reflects the behaviour of the pure solvent. The other parameter, fsl, describes the interaction of a solute molecule with the solvent, dependent therefore on both solute and solvent properties. The derivation of the standard free energy of solution from eqn ( 5 ) is simple and straightforward. (a) Divide both sides of eqn ( 5 ) by Avogadro’s number, converting the number of molecules, ng and n,, into the number of moles, Ng and N,: 5 = 3 exp [Uil -f,,)/kT]. v g K (b) Substitute p/RT, where p is the partial pressure of the solute in the gas phase, for the concentration of the solute in the gas space Ng/Vg: pVg = NgRT. (7) ( c ) Insert mole fractions X , to replace N,, since in dilute solutions X , * N,.The result = x, exp [Ull -fs,)/kTl RT Vl leads directly to the standard free energy of solution ApO = RT In (RT/ V,) + NUl, -fsl) (9) since by definition N = R/k, and the standard free energy is given by A p e = RT In (p/X,). (10)582 TRANSFER OF NOBLE GASES We now turn our attention to the standard entropy A P and the standard enthalpy A H 0 of solution. These two important thermodynamic functions correlate with the chemical potential through the definition A/@ = AH@- T A P . (1 1) We analyse these functions here to understand how these are related to the microscopic details of the membrane. A review of the basic assumptions of the model reveals that we consider that the holes are detached from each other and that the solute adsorption is independent of hole formation.These features imply that each hole contributes separately to the entropy and enthalpy of the system and that this contribution has two independent parts. The two parts describe the independent contributions of hole formation and adsorption. It follows from these features of the model that the macroscopic thermo- dynamic functions are determined by four independent microscopic parameters : E,, and oll characterize hole formation and E , ~ and oS1 describe solute adsorption in holes. The parameters are characteristic entropies and energies which obey, on the microscopic scale, relations similar to the general eqn (1) : f l l = E l l - To11 (12) f S l = Es1- T%l (13) where the subscripts 11 and sl refer to hole formation and to adsorption, respectively.A combination of eqn (lo), (12) and (13) gives a relation which describes the standard free energy as a function of the microscopic entropies and energies. When the general thermodynamical relations are applied to this relation we obtain explicit relations between the microscopic and the macroscopic entropies and enthalpies : A H 0 = N(E,, - eS1) - RT (16) A S 0 = N(oll - osl) - R[ 1 + In (RT/ V,)]. The experimental finding that the dependence of entropies and enthalpies of solution on temperature are small strongly suggests that the microscopic entropies and enthalpies are also independent of temperature to a good approximation. A comment deserves to be made concerning the mechanism of hole formation in our treatment.Holes appear in our treatment as independent free entities which are formed or destroyed or interact with each other and perform Brownian motions because of the irregular thermal movement of the solvent molecules. As such they are not supposed to contribute to the free energy and to the other thermodynamic functions of the solution. These functions will then be determined by the process of solute adsorption in the holes. The reason that the work involved in hole formation contributes to the free energy of solution is that the holes disappear when adsorption of solute takes place. This disturbs the equilibrium which exists between the number of potential sites for hole formation Nh and the actual number of holes nh. The result is that a new hole must be added to the solvent to restore the disturbed equilibrium.Y.KATZ 583 This formation of a hole demands the expenditure of work against the cohesive forces of the solvent, leading to the equations given above. There is nothing new in using the convenience of considering the process of solution as consisting of two steps: (1) the formation of a hole in the liquid and (2) the accommodation of a gas molecule in this hole. The novelty is in the interpretation given to these steps and in the way of deriving them. The statistical treatment of the liquid in terms of free holes carries the concepts introduced by Uhlig14 and Eley15 into free-volume theories, allowing the simultaneous use of both: e.g. to calculate step (1) from Furth's theoryls and to evaluate step (2) using free volume considerations and obtain the properties of the pure solvent from the solubilities of the inert gases.The existence of such possibilities seems attractive, especially when analysing membranes, because it is there where holes in the form of kinks are assumed to play a dominant There should be no surprise that the equations given here appear also in other papers. The concept of splitting the process of solution into two parts has a dominant effect on the appearance of theories and ours is no exception. One must remember, however, that these steps relate to a different mechanism of solution from those commonly employed and must be considered accordingly. The test of the evaluation must be done by analysis of the thermodynamic functions of solution of the simple gases in the non-polar solvents and by independent calculations of the parameters given by the theory. Of special importance is the calculation of hole volumes, since holes play a basic role in the development of the theory and are considered as independent entities whose magnitudes depend only on the nature of the solvent.Indeed an evaluation of hole size along these lines is given in table 5 (vide infra). The relations developed thus far, although useful, are not sufficient for the description of membranes. To obtain membrane characteristics from the measured thermodynamic functions we need two additional equations. Inspection of eqn (1 6 ) and (17) shows that only two relations connect the four independent microscopic parameters which characterize the behaviour of solutions.To make useful applications of the parameters we also need an explanation which correlates them with the detailed molecularity of the solution, so as to make it possible to understand the detailed behaviour of the solution from the thermodynamical measurements. We devote the rest of this paper to the development of these two aspects. We consider in turn the enthalpies and entropies and the relation between the two, describing some of the molecular mechanisms involved in the determination of the parameters. We also develop the two extra equations needed for a complete definition of the microscopic parameters. One of the equations shows that the measured thermodynamic enthalpy of solution can be expressed in terms of the enthalpies of the pure components, and the other shows that the entropies,and the enthalpies of solution are related to each other, thus correlating the four microscopic parameters.r01e.17-19 ENTHALPIES There exists a well known relation which expresses the interactions between the solute and the solvent molecules in terms of the solute-solute and the solvent-solvent interactions. This relation XSl = O C l l X S S ~ ~ where xsl, xll and xss are energies of interaction between solute and solvent, solvent and solvent and solute and solute molecules, respectively, is well founded theoretically20 and is widely used in solution theories.21 We use it to obtain an expression which reduces the number of unknowns, because it correlates the solvent-solute interactions584 TRANSFER OF NOBLE GASES with the solvent-solvent interactions. The relation makes it possible to obtain the energy of a solvent from the measured enthalpies of the solution and of the pure solutes.This outcome is of special importance in membrane research, because membranes are heterogeneous and the different regions cannot be separated and analysed independently. The problem is solved, however, if solutes are adsorbed selectively onto a given region using the results to evaluate further the energy of the region. Consider first the energies E,, and E~~ and their relationship with the pair interaction energies xsl, xll and xSs. Inspection shows that these microscopic parameters, which specify the cohesive and adsorbing adhesive powers of the medium, respectively, reflect in their magnitude the number and strength of the pair interactions between the close neighbours which are involved in the process of solubilization. The picture leads immediately to the equations Es1 = 2zxs1 E l l = ZXll where z is the number of contacts between solvent pairs that is is made and 22 is the number of solutesolvent interactions molecule is adsorbed in a hole. abolished when a hole gained when a solute It is easy to show that similar considerations apply to the cohesive energy of the Ess = Z X S S .(21) pure solute E,, when the liquid state of the solute is considered: Combining eqn (18)-(21) and using simple algebra we obtain the expression ESl = 2(&11&ssP. (22) (23) It is through using this result with eqn (16) to give AH0 = N[E,, - 2 ( ~ , , E,,):] - RT that we discover the very important relation which links the molar energies of vaporization Ul and Us of the pure solvent and solute, respectively, to the standard molar enthalpy of solution AH*.The quantities in eqn (24), with the exception of Us and A H e , are independent of the solute. To derive eqn (24) we consider the process of evaporation in the pure liquid and compare it with the solubility of this pure entity in itself. Note that these two processes are identically opposite to each other, because evaporation is the desorption of a molecule from a medium identical to itself when the adhesive and cohesive forces are of the same nature. The energy content of a hole is because the creation of Nh holes is proportional to evaporation of a mole of solvent (note that for the solute Nh = N).We would like to stress again that eqn (24) is valid only for a solution in which both solute and solvent are hydrophobic. This is so because only for the interaction between hydrophobic molecules can we apply the assumption in eqn (18) of the geometric mean, which is characteristic of London interactions.20 Inspection of eqn (24) shows that if our theory is correct then it is possible to obtainY. KATZ 585 the cohesive energy of the membrane from solubility data and to predict the solubilities once the cohesive energy is known. This is an important and useful conclusion, because there are experimental difficulties in obtaining results which do not depend in one way or another on the solubilization of probes in the membrane.To justify further use of the model and verify its basic assumptions we measure here the extent of realization of eqn (24) using solubility data for noble gases in bulk hydrocarbons and in a phospholipid membrane. We also include solubility data for other solutes which are not as simple as the noble gases. The solvents we select are the four hydrophobic solvents n-hexane, n-dodecane, cyclohexane and benzene. The phospholipid membrane selected is a dimyristoyl lecithin lamella. This membrane can be divided into two definite regions, one hydrophilic and the other hydrophobic. The hydrophobic region is the space in the membrane into which the noble gases dissolve preferentially.ll The bulk solvents resemble each other and the hydrophobic region of the membrane in their hydrophobicity but differ from each other in the size, shape and value of the cohesive energy which holds the liquid molecules to each other.The hydrophobic region of the membrane, being a liquid crystal, differs appreciably from the bulk solvents. Solubility data in several polar solvents are also given here to establish an upper limit to deviations which may be caused by using the assumption of the geometric mean [eqn (22)] in systems which are affected by other factors beside the London forces mentioned above. The data on which we base our calculations have been collected from the literature.l1>l2 We present the results of our test on bulk solvents in fig. 1 and table 1. An examination of the validity of the theory for the description of the hydrophobic region of the phospholipid membrane is presented in fig.2 and in tables 2 and 3. The testing made for the membrane is less extensive than that for the bulk solvents because fewer experimental data are available for membranes. First we test the prediction, made in eqn (24), that there is a linear relationship between enthalpies of solution of simple solutes and the square root of their energies of vaporization. Our examination for the four hydrophobic solvents n-hexane, n-dodecane, cyclohexane and benzene is presented in fig. 1. The findings show good agreement between observation and prediction, provided that the data for helium and for hydrogen are excluded from the calculations. Comparing the calculated results with those found experimentally, we see a standard error of the mean that ranges from 1.2% in benzene to 3.2% in cyclohexane.These errors correspond to standard deviations of estimation of -t 93 cal and & 21 2 cal, respectively. One can see that the predicted linear relationship holds within experimental error, since the deviations are smaller than the experimental errors commonly found in enthalpy measurements. l2 Similar linear correlations are found when the solvents are polar solvents. These results, which are theoretically less interesting, are not presented in fig. 1 in order to prevent confusion which may result from too many data points. Inspection shows that agreement between the estimated results and the experimental results in hydrophobic solvents manifests itself in correlation coefficients of 0.99 (only for n-hexane do we find a lower correlation coefficient at 0.81).The model predicts that the slopes in fig. 1 are twice as large as the square roots of the corresponding intercepts. This prediction is seen in eqn (24) and it follows directly from the assumption eqn (18) of the geometric mean. Our examination, depicted in table 1 , validates the theory and justifies its use to describe solubilities in bulk solvents. Table 1 gives the values of the slopes, the square roots of the corresponding intercepts and the ratio between them. The theory predicts that the ratio between the two parameters is one. We find that this theoretical expectation is fulfilled for solutions in hydrophobic solvents. Larger deviations from this expectation are found for polar solvents.586 TRANSFER OF NOBLE GASES 0 He 12 H2 Ne 24 N2 36 02 CH4Kr 48 Xe C0zC#1660 (o'>:/(J rnol-')f A r Fig.1. Relations between the energies of vaporization, Us, of solutes and their standard enthalpies of solution for the solvent systems n-hexane (a), n-dodecane (O), cyclohexane (X) and benzene (0). Although data for He and H, are also given, they are not used in the calculation of the linear relations. The correlations of the lines with the experimental results are 0.81, 0.98, 0.99 and 0.99, respectively. Table 1. Values of solvent energy parameters derived from fig. 1 for different solventsa n-hexane n-dodecane cyclohexane benzene perfluorocyclohexane nitro benzene ethanol acetone 122.4 122.4 126.5 141.6 81.5 107.7 96.7 83.4 16 593 16 895 18644 23 056 7 535 18 377 11 553 14232 128.8 130.0 136.6 151.8 86.8 135.6 107.5 119.3 1.05 1.06 1.08 1.07 1.07 1.15 1.11 1.43 a The cohesive energy NE,, is in units of J mol-I.Experimental results are from ref. (12). The solvents perfluorocyclohexane, nitrobenzene, ethanol and acetone are not shown in fig. ' 1 ; the values for these solvents ar,e derived in the same way as the data for the hydrophobic solvents- l)fntercept/(NEIl)~lope *Y. KATZ 9000 - I - 8000 +7 < Q 7000 587 - - - He 0 6000 I 1 1 1 I I 10 20 30 40 50 U / J mol-' Fig. 2. Test for the applicability of the theory to solubilities of noble gases in the dimyristoyl lecithin (DMPC) membrane. A linear relation between the energies of vaporization, Us, of the solutes and their standard free energies of solution in DMPC is expected theoretically. Table 2.Energy parameters of the DMPC membrane. A test of the ability of the theory to describe the behaviour of membranesa He Ne Ar Kr Xe CHF) CHP) 33526 36125 32023 30976 28549 29553 29804 @ 7.08 40.7 76.1 89.6 l06.2 85.3 85.3 Ape A 41023 a -103.8 Nc,,(from A) 30453 B 123.7 b 0.00217 N&,,(from B) 32714 a A p e are standard free energies of solution in DMPC from ref. (1 1). Values in the last two columns are the free energies in DPPC + DPPA and DPPC + DPPA +cholesterol mixtures, respectively.22 Vaporization energies of the solutes are from ref. (2 1) and the Barclay-Butler constants a and b are from ref. (8). Units are SI. A and B are the intercept and the slope of fig. 2, respectively. The theory demands that A = [(Nell-RT)(l -bT)-aT] and B = [2(N~,,):(l -bT)].This follows from the simultaneous use of eqn (24) and (28) and the general thermodynamic equation Ap* = A H e - TAP. The molar cohesive parameters Neil from A and B agree only if the combining rule [eqn (22)] and the Barclay-Butler relation are valid in the membrane. To make sure that the special structure of the bilayer has no effect on the applicability of the theory to membranes, we have examined the validity of the combining rule eqn (22) for membranes. We have also examined the consistency between two independent calculations: one based only on energy relations and another using the Barclay-Butler correlation between entropies and enthalpies of solution.* We first test if there is a linear relationship between the free energies of solution of the noble gases in the bilayer and the square roots of the solute energies of vaporization. The results of these tests appear in fig.2 and table 2. The existence588 TRANSFER OF NOBLE GASES Table 3. Energy parameters of the DMPC membrane from enthalpies of solution of argon and krypton in the membranea solute AHe argon 795 5810 76.1 1 29430 171.55 krypton - 2470 8 040 89.60 32 280 179.50 a Enthalpies of solution AH* and vaporization energies of the pure solutes are from ref. (1 1) and (21), respectively. Units are SI. Molar cohesive energies NE,, are calculated by solving eqn (24) as a quadratic with ( N E ~ , ) ~ as the unknown. E,, = vh U,/V, is obtained from eqn (25). of such a linear relation demands the validity of eqn (24) as well as the validity of eqn (28).This means the applicability of the combining rule eqn (22) as well as the existence of a linear relation between the entropies and enthalpies of solution. Our examination is done in this way because the data on enthalpies of solution of the noble gases in phospholipids which appear in the literaturell are not sufficient for the construction of a graph, as in fig. 1. Fig. 2 shows that the theory also holds for the hydrophobic region of the membrane. It shows the linear relation between the free energies of solution of the noble gases in the bilayer and the square roots of the energies of vaporization of these gases, as from theory. Further investigation, using the Barclay-Butler coefficients for this system,8 demonstrates that the numerical values of the slope and the intercept of fig.2 correlate in accordance with the combining rule. The results which show this agreement, between the theoretical expectations and the experimental findings, are given in table 2. More support for the applicability of our theory to membranes comes from inspection of the enthalpies of solution of the noble gases argon and krypton in the bilayer. Using experimental data given in the literature for the enthalpies of solutionll and energies of vaporizationz1 we solve eqn (24) as a quadratic equation with N E ~ ~ as the unknown. Inspection of the results in table 3 shows their agreement with the results in table 2. Only if the combining rule of the geometric mean, given by eqn (22), holds will the results for argon and krypton agree with each other and with the results of table 2.ENTROPIES There are many ways to arrange the molecules of a liquid so that from the outside it looks the same. The logarithm of that number is the entropy of the liquid.23 It follows that the standard entropy of solution measures the reduction in the possible number of arrangements occurring upon solution. We express the entropy of solution by two parameters, a,, and a,,, which appear in eqn (17). These parameters represent the microscopic entropies of hole formation and of solute adsorption to the holes, respectively. We assume that the holes are rigid spherical entities, practically impermeable to either solute or solvent. The model portrayed in fig. 3 is naive and unrealistic and its success must be determined by its ability to produce results which are consistent with the experimental findings.The model asserts that the confinement of the solute molecule to a hole reduces the volume available to the movement of its centre. This happens because the centre can come no closer than one radius from the wall. It is easy to show that the volume uhs free to solute movement in a sphericalY. KATZ 589 volume in which the centre of a solvent molecule cannot be found (1) solvent (I hich the centre o f a molecule I cannot enter solvent molecule \ molecule / - \ solvent ( 2 ) molecule hole of radius fi ( 3 ) ‘cavity R in the solvent medium space from which the centre of the solute is excluded solute molecule the solute can of radius r be found Fig. 3.Schematic diagram of the model showing the effect of excluded volumes on the entropy parameters. We define an excluded volume as a volume in which centres of molecules cannot be found. We describe an excluded volume of a sphere surrounding the point of contact between two molecules having radius a. (1) The radius of the excluded sphere is a. (2) When a hole of radius r is made the radius of the excluded sphere increases to r+a. (3) The volume available to the movement of the solute molecule of radius r in a hole of radius R is VhS = @(R - r)3 hole of volume uh depends on the volume of the solute us. The dependence for a spherical solute molecule of volume us islo The reduction in volume is a constraint which causes the entropy of the system to change.24 The amount of change is To test the applicability of the model to bulk solvents we calculate hole volumes Vh from measured enthalpies of solution using eqn (25) and insert the values so obtained into eqn (27) t o evaluate the entropic parameters oSl. We then compare these calculated parameters with the experimentally measured entropies of so1ution,12 examining the agreement between the theoretical expectations and the experimental findings.The calculated parameters, the experimental values and the outcome of the comparison are given in table 4. The comparison is also shown in fig. 4. The solvents used in the examination are the four hydrophobic solvents n-hexane, n-dodecane,00'1 28'09- 19'Z€- EI'SS- €9'82- €€'PS- OZ'SZ- 8L'€€- 8€'81- LI'SP- €0'91- 8'S9 68 09€1€ 090€Z auazuaq €0.1 6L.19- 8P'ZC- 0€'9S- IS'8Z- 9Z'LS- ZI'SZ- PZ'IS- €€'81- 86'oP- 66'SI- Z'99 601 O€OO€ oP981 auexaVPd~ 86'0 LS'09- OO'€€- Z6'PS- €6'82- 8L'IS- SP'SZ- PL'€P- PS'81- 6L'SP- 91'91- L'P9 9'822 OOL8S S6891 auEmPop-u 9S'I IE'99- 90'0€- 81'LS- 8S'9Z- 98'8s- 8P'CZ- 08'9P- €€'LI- 69'IP- SI'SI - P'PL ZCI 09062 E6S91 auexay-uY. KATZ 59 1 cyclohexane and benzene, and as solutes we use the noble gases.The molecular volumes v, are values taken from the l i t e r a t ~ r e . ~ ~ The theory predicts that the entropy change caused by changing solute in a given solvent must be equal to the change in the value of the osl parameter, the reason being that the only entropic factor which is solute dependent is the parameter osl. The comparison which we present in fig.4 shows that the theoretical expectations agree well with the experimental findings. An exception is found with n-hexane, where the slope found is much larger than the slope expected theoretically. A simple application of this test to phospholipid membranes cannot be made, since it demands the use of data on the energy of evaporation of the solvent in calculating eqn (25). Because evaporation of the hydrocarbon chains of the hydrophobic region of the membrane is not possible, it follows that their energy of vaporization is not available experimentally. Estimations of the energy of vaporization of the hydrophobic region of the membrane can, however, be made using theoretical calculations of the energy of interaction between hydrocarbon chains.26 We refrain here from this kind of evaluation, because of both the complexity of the calculations and the number of assumptions involved in them.THE BARCLAY-BUTLER RULE A LINEAR RELATIONSHIP BETWEEN ENTROPIES AND ENTHALPIES Barclay and Butler found that the enthalpies of solution in a given solvent correlate with the corresponding entropies. Their empirical rule describes the correlation as a linear relation between standard entropies and enthalpies of A P = a+bAHe (28) where a and b are characteristic constants. Barclay and Butler have shown that there is a value for a and another for b which are common to many solutions and many pure liquids. Here we explore this useful relation and explain its meaning in terms of our model. Combination of eqn (28) with eqn (16) and (17) reveals that the solute-dependent parameters E , ~ and osl correlate with each other.Mathematically we formulate this correlation by the differential a is a constant which includes all the solute-independent parameters a = No,, - R[ 1 +In (RT/ V1)] - b(NE,, - RT) a = A S 0 (solute independent) - bAHe (solute independent). (30) (31) which means that Comparison between eqn (29) and (30) shows that it is much easier to understand and interpret the b constant. The numerical values found empirically for the two Barclay-Butler constants of the bulk solvents are a x 26.4 cal mol-1 K-l and b x 0.001 34 K-1.8 These coefficients constitute the ‘normal’ curve that correlates the enthalpies and entropies of the bulk solutions.2s~ 29 The numerical values of the two Barclay-Butler coefficients, found from solubility data for the noble gases in the dimyristoyl lecithin membrane, are a x 24.8 cal mol-1 K-l and b x 0.002 18 K-1.8 Inspection shows that whereas the a constant of the Barclay-Butler coefficients in the phospholipid does not differ significantly from the ‘normal’ values, the b coefficient characterizing the phospholipid is appreciably higher than ‘ normal ’.TRANSFER OF NOBLE GASES 592 -9 - 10 r( -11 - ; -12 r( & -13 3 -14 -1 5 -1 6 -1 7 b I I I I I I / / I I I I I I l l I I 1 1 L -8 -7 -6 -5 -4 -3 -8 -7 -6 -5 -4 -3 -8 -7 -6 -5 -4 -3 -8 -7 -6 -5 -4 - 3 NuSl/J K-' mol-' Fig.4. Comparison between calculated No,, parameters and experimentally found entropies of solution A 9 for the four solvents (a) n-hexane, (b) n-dodecane, (c) cyclohexane and (d) benzene.The experimental values are from ref. (1 2). Inspection of derivations of the enthalpy and the entropy shows how the correlation between these apparently independent functions occur. First notice that osl is a function of the hole volume uh [eqn (27)]. The hole volume is determined by the energy parameter E,, and by the entropy parameter o,, [eqn (2), (3), (12) and (13)]. The parameters el, and E,, relate to each other through the assumption of the geometric mean [the combining rule is given in eqn (22)]. This chain of relations establishes a correlation between the entropy parameter osl and the energy parameter E,,. This correlation reflects itself in the Barclay-Butler relation. Mathematically the argument goes as follows: osl = 41(Vh), uh = 42(~11, o,,) and E,, = 4J.sl1) E,, = 4; l(cSl), where 41, t$2 and q43 are functions and 4; is the inverse function of & Combination of these (32) into one relation gives which shows the functional relationship between the entropy and the enthalpy.This result is rather surprising because entropies and enthalpies have different origins in this model. We treat the molecules as hard spheres when calculating entropies but apply the combination rule of the geometric mean when calculating energies. It means that we derive entropies from the repulsive part of the Lennard-Jones intermolecular potential whereas to calculate the energy we apply the attractive part of the Lennard-Jones potential. Since the parts of this potential are independent, we expect the entropies to be totally independent of the energies. The difficulty vanishes when we note that the cohesive energy of the solvent affects the dimensions of the hole in which the solute moves thus affecting the entropy of solution.Employment of the osl parameters, obtained through the use of the Barclay-Butler relation, to the evaluation of the hole volumes of the solvents shown in table 5 gives values which for the bulk solvents agree with the hole volumes calculated from energy consideration using eqn (27). Inspection shows that the estimated hole volumes from entropies of solution of argon, krypton and xenon agree to within 20% or better with the values calculated from energy considerations, and that the results from entropy measurements on helium and neon deviate appreciably from the rest.This means that at least for the bulk solvents the conclusions obtained from entropic considerations are consistent with the results deduced from energy considerations. Since entropy and enthalpy arise in our discussion from different parts of the intermolecular potential U s 1 = 41 { 4 2 [ 4 , l(&sl)l>Y. KATZ 593 Table 5. Calculation of hole volumes from solubility data of noble gases in hydrophobic solvents Nu, helium neon argon krypton xenon from 13.8 83.3 156.1 183.4 217.3 solvent eqn (25) 6.9 9.2 16.7 20.7 25.3 n- hexane 74.4 21.60 12 274 165 12 230 164 11 180 170 7 574 115 93 1 n-dodecane 64.7 21.80 cyclohexane 66.2 23.10 benzene 65.8 2.55 DMPC 5.45 13.20 138.14 I .86 13.20 138.46 2.13 13.45 128.20 1.94 15.45 94.80 1.44 3 1.20 25.48 25.25 69.34 0.93 25.50 69.23 1.07 25.30 65.50 0.99 28.90 52.20 0.79 58.90 22.60 28.60 65.55 0.88 29.00 65.35 1.01 29.60 62.26 0.94 33.90 50.80 0.77 69.15 25.24 34.00 62.03 0.83 34.30 62.11 0.96 35.10 59.30 0.90 32.70 49.70 0.76 88.90 28.47 The numbers beneath the solute’s name are the square root of its vaporization energy [ref.(21)] and its molar volume [ref. (25)]. For each solvent-solute interaction we calculate the entropy parameter Nosl, the molar hole volume [from eqn (27)] and the ratio between this hole volume and hole volume obtained from energy measurements [eqn (25)]. Nos, is calculated from the data in fig. 1 and 2 using eqn (29) for the calculation. Units used are J and cm3 mol-l. they are independent of each other.The fact that similar numerical values result from the independent evaluations of the hole volumes indicates that assigning the entropy to the hard part of the intermolecular potential and the energy to its soft part agrees with the experimental findings. Further support for the validity of eqn (27) for a description of entropy in hydrophobic solvents comes from the comparison of the hole volumes obtained using different solutes in the evaluation. The comparison shows that the volumes calculated for the holes in the bulk solvents from solubility data for argon, krypton and xenon give the same result to within 5 % . Even better agreement is found for the bilayer. We find that the deviation of hole size calculated from solubility data for neon, argon, krypton and xenon does not exceed 4%.The oS1 parameters, characterizing noble-gas solubility in the membrane, are larger than the corresponding parameters found in bulk solvents by 4-12 cal mol-l K-l and they span a larger range, which increases the sensitivity of the test in this medium. Thus our findings show not only that eqn (27) is able to describe the entropy of solution but also, more importantly, that the same theoretical formulation can be applied to the description of either bulk solvents or hydrophobic regions in membranes. A possible explanation for the deviation of the results obtained from helium solubility measurements from the theoretical expectation is that the small dimensions of helium make it possible for the solute to use vacancies which are too small to be occupied by the larger solutes.Consequently, any two holes connected by a series of small vacancies will behave as one hole when the solubility of helium is considered but with larger solutes. This leads to holes which are much larger for helium than for the other solvents. If this explanation is correct then we expect smaller deviations to594 TRANSFER OF NOBLE GASES occur in the denser solvents and for the larger solutes. The first expectation is borne out by comparing the hydrophobic region with the bulk solvents. The density of the hydrophobic region of the membrane is much larger than that of the bulk giving smaller deviations in the calculated hole volumes. We also find that the deviation diminishes as the solute increases in size.DISCUSSION Our model, which describes solubilization as consisting of two independent processes, hole creation and solute adsorption in holes, conforms well' with the experimental findings on the solubility of noble gases in hydrophobic solvents and in phospholipid membranes. These features make the theory simple and flexible and offer interesting possibilities. Most of this discussion will be concerned with applications to analysis of membranes. First, note that a theory which describes bulk solvents as well as membranes constitutes a basis for comparison of the various systems. Comparison between solubilities in bulk model solvents and the behaviour of the given solutes in membranes plays an important role in the analysis of membrane phenomena.'? However, this method is based on empirical correlation^.^ The finding of a theory which describes both bulk solvents and membranes provides a theoretical basis to the correlations made.We also note that our model paves the way to the derivation of a physical interpretation of the two most important aspects of membrane transport, namely membrane permeability and membrane selectivity. Inspection shows that the model describes solubilization in terms of hole creation, which depends only on the characteristics of the solvent medium, and of solute adsorption in holes, which depends on the nature of the solvent-solute interaction. Selectivity, which describes the differences in behaviour towards different solutes, is given by the second factor. Permeability, which is strongly dependent on membrane solubility,' is defined by a combination of hole-creation and adsorption parameters.The theory offers the possibility of deducing the characteristics of the pure solvent from the properties of the solution. The ability to obtain solvent features from solubility measurements follows from the existence of relations such as eqn (24), which correlate between the properties of the pure components of the solution and the characteristics of the solution itself. This becomes important when analysing hetero- geneous systems such as the phospholipid membrane. Solutes of known properties, such as the noble gases, are adsorbed preferentially to the hydrophobic region measuring the thermodynamic functions of transfer. The thermodynamic parameters which characterize the pure solvent are then evaluated using relations such as eqn (24).Inspection shows that there are consistent deviations between experiment and theory for the small solute molecules helium and hydrogen. The deviations are seen when looking at fig. 1 and 2. We attribute this deviation to our neglect of quantum-mechanical effects on the vaporization energies of these small solute molecules.21 Further inspection8 shows that the theory applies only when the solutes under consideration are noble gases. This is especially true when considering membranes. It can be shown, however, that the construction of a theoretical framework which describes the behaviour of the simple solute is an essential step in the formulation of more general theories. We note similarities between our theory and other existing theories;l6T 31-33 this provides flexibility and the possibility of extending our treatment by combining it with an existing one.The success of our calculations of the behaviour of inert gases in non-polar liquids permits a few comments to be made concerning the solution process and the methodsY. KATZ 595 for evaluating it. Thermodynamic functions of solution of these entities have been calculated many times by considering that the process of introducing the solute consists of two consecutive steps: (1) formation of a hole in the solvent and (2) introduction of the solute into the hoie.l59 3 4 v 35 Our treatment shows that the order of these steps can be reversed : first adsorption to an existing hole and then a formation of a new hole in the solvent medium.It also shows that, contrary to ref. ( 3 9 , free holes in the liquid are determining factors in the thermodynamics of gas solubility. This means that from the point of view of thermodynamics a mechanism based on the adsorption of the solute in an existing hole is as valid as the mechanism in which the dissolving gas has to ‘dig its own hole’ in the solvent. That our approach to the solubility of gases in liquids, based on a Furth statistical treatment of the thermodynamics of liquids by the theory of holes,ls leads to a consideration made by other methods is interesting. This brings together the scaled-particle theory, as used by P i e r ~ t t i , ~ ~ and such treatments which assume the existence of holes in the liquid and treat it by a statistical theory analogous to that used for the treatment of gases,36 thus offering the possibility of relating one to the other.Indeed, our calculations give results which agree quite well with experiment. This was only achieved with difficulty using one of the existing free-volume On the other hand, one of the most significant features of the hydrophobic region, namely its high adhesive entropy parameter osl, would not have been noticed if another mechanism for hole formation [e.g. the mechanisms proposed in ref. (15), (34) and (35)] was used. Finally we define the standard states of the thermodynamic functions employed. The standard functions A S e , AH0 and A p e describe changes which occur when one mole of solute is transferred to a solution having the properties of an infinitely dilute solution.Standard free energy and standard entropy refer to a hypothetical solution obeying Henry’s rule and having the mole fraction of the solute approaching unity. I thank Professors S . Alexander, A. Ben-Naim and B. 2. Ginsburg for many helpful discussions. E. M. Wright and J. M. Diamond, Annu. Reu. Physiol., 1969, 31, 581. S. H. Roth and P. Seeman, Biochim. Biophys. Acta, 1969,255, 207. R. Collander, Acta Chem. Scand., 1950, 4, 1085. W. D. Stein, in Membrane Transport (Elsevier, Amsterdam, 1981), vol. 2, pp. 1-28. J. M. Diamond and Y. Katz J. Membrane Biol., 1974, 17, 121. A. Finkelstein, J. Gen. Physiol., 1976, 67, 45. J. M. Wolosin, H. Ginsburg, W. R. Lieb and W. D. Stein, J. Gen. Physiol., 1978, 75, 427. Y. Katz, M. E. Hoffman and R. Blumenthal, J. Theor. Biol., 1983, 105. A. Ben-Naim, Water and Aqueous Solutions (Plenum Press, New York, 1974). York, 1965). lo J. 0. Hirschfelder, C. F. Curtiss and R. B. Bird, Molecular Theory ofGases and Liquids (Wiley, New l1 Y. Katz, Biochim. Biophys. Acta, 1981, 647, 119. l2 E. Wilhelm and R. Battino, Chem. Rev., 1973, 73, 1. l3 R. C. Tolman, The Principles of Statistical Mechanics (Oxford University Press, 1950). l4 R. Furth, Proc. Cambridge Philos. SOC., 1941, 252. l5 D. D. Eley, Trans. Faraday SOC., 1939, 35, 1281. l6 H. H. Uhlig, J. Phys. Chem., 1971, 41, 1215. H. Trauble, J. Membrane Biol., 1971, 4, 193. J. F. Nagle, J. Chem. Phys., 1973,58,252. M. B. Jackson, Biochemistry, 1976, 15, 2555. 2o F. London, Trans. Faraday SOC., 1937,33, 8. 21 J. H. Hildebrand and R. L. Scott, The Solubility ofNonelectrolytes (Dover, New York, 1964). 22 K. W. Miller, L. Hammond and E. G. Porter, Chem. Phys. Lipids, 1977, 20, 229.596 TRANSFER OF NOBLE GASES 23 R. P. Feynman, Lectures on Physics (Addison-Wesley, Reading Mass., 1965), vol. I, chap. 46. 24 I. M. Klotz and R. M. Rosenberg, Chemical Thermodynamics (Benjamin Cummins, New York, 1972). 25 A. Bondi, J. Phys. Chem., 1964,68,441. 26 L. Salem, J. Chem. Phys., 1962, 37, 2100. 27 I. M. Barclay and J. A. V. Butler, Trans. Faraday Soc., 1938, 34, 1445. H. C. Longuet-Higgins, Proc. R. SOC. London, Ser. A, 1951,205, 247. 29 H. S. Frank, J . Chem. Phys., 1945, 13, 493. 30 H. Lecuyer and D. G. Dervichian, J. Mol. Biol., 1969, 45, 39. 31 I. Langmuir, Colloid Symp. Monogr., 1925, 48. 34 S. Glasstone, K. J. Laidler and H. Eyring Theory of Rate Processes (McGraw-Hill, New York, 1941). 33 H. Reiss, Adv. Chem. Phys., 1966, 9, 1. 34 P. Meares, J. Am. Chem. Soc., 1954, 76, 3415. 35 R. A. Pierotti, J. Phys. Chem., 1963, 67, 1840. 36 H. Eyring, D. Henderson, B. Jones Stover and E. M. Eyring, Statistical Mechanics and Dynamics (Wiley, New York, 1964). (PAPER 4/541)

ISSN:0300-9599    

DOI:10.1039/F19858100579

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. 1, 1985,81, 597-600 Capillary-condensed Water in Silica Gel BY WILLIAM D. MACHIN* AND J. TODD STUCKLESS Department of Chemistry, Memorial University of Newfoundland, St John’s, Newfoundland, Canada A1B 3x7 Received 6th April, 1984 Capillary forces generate large negative pressures within liquid water adsorbed in silica gel. The molar volume of the capillary water increases as the relative pressure decreases. The pressure-volume behaviour of capillary-condensed water can be described by an equation of state proposed by Speedy in 1982. The influence of this behaviour on the estimation of pore-size distributions is briefly discussed. The phenomenon of adsorption hysteresis which accompanies the capillary con- densation of adsorbates has been widely exploited to obtain the ‘ pore-size’ distri- butions of solid ads0rbents.l Pore-size distributions are derived from the change in the amount adsorbed for a small change in relative pressure, the change in the amount adsorbed being equated to the pore volume, with the radius of the pores estimated from the Kelvin equation r = - 2 VL y cos d/RT In (PIP,) where r is the radius of the pore, VL and y are the molar volume and surface tension of the liquid adsorbate, respectively, 4 is the contact angle of the liquid on the solid and P/P, is the relative pressure.Generally it is assumed that the contact angle is zero and that surface tension and molar volume are the same as those of the bulk liquid. The latter assumptions require that the liquid adsorbate be incompressible. Our results suggest that capillary-condensed water has a molar volume greater than that of ordinary water.EXPERIMENTAL A commercial silica gel (Davison, grade 03) was degassed under vacuum to constant weight at 41 3 to 423 K. The sample was subsequently exposed to water vapour at saturation pressure and ambient temperature prior to the first run. Following this treatment degassing was carried out at experimental or ambient temperatures. Distilled water was degassed by several freeze-thaw cycles and redistilled under vacuum, without ebullition, into a glass reservoir which was part of the adsorption apparatus. The apparatus consisted of a quartz spring balance, adsorbate reservoir and a capacitance manometer (MKS Baratron). The constant-temperature bath around the sample was controlled to fO.O1 K.RESULTS The isotherms determined at 273.65 and 298.15 K are shown in fig. 1. Both exhibit type E hysteresis according to the de Boer classification,2 with steep desorption at relative pressures near 0.3. B.E.T. plots are linear at relative pressures < 0.05 and 597598 WATER IN SILICA GEL , , 0.2 0.4 0.6 0.8 relative pressure Fig. 1. Adsorption isotherms for H,O on silica gel. Upper curve 298.15 K, lower curve 273.65 K. Desorption points are solid. Table 1. Summary of results B.E.T. monolayer capacity/cm3 g-' 0.085 B.E.T. surface area/mg2 g-Ia 299 B.E.T. C value 50.9 Langmuir monolayer capacity/cm3 g-l 0.086 Langmuir surface area/m2 g-la 302 amount adsorbed at saturation/mmol g-l (a) 273.65 K 18.3 (b) 298.15 K 17.5 a Area of water molecule taken as 0.105 nm2.Langmuir plots are linear at relative pressures -= 0.03. As only a limited number of points below the hysteresis loop were determined for the 298.15 K isotherm, most of the quantities reported in table 1 refer to the 273.65 K isotherm. DISCUSSION For fine-pore adsorbents it has been suggested that the lower closure point of a hysteresis loop may be determined by the tensile strength of the liquid adsorbate and not by the physical structure of the ad~orbent.~? According to this model the pressure difference across the curved liquid/vapour interface produces a negative pressure within the liquid phase, which for small pores may be sufficient to rupture the liquid adsorbate. The Kelvin equation can be combined with the Young-Laplace equation5. to yield where PL is the pressure in the liquid phase.Application of this equation is hindered by the fact that V, is a function of PL.W. D. MACHIN AND J. T. STUCKLESS 599 -150 -100 - 5 0 pressure/MPa Fig. 2. Pressurevolume behaviour of water at negative pressures: 0, eqn (4); (---) eqn (5); (---) eqn (5) with PM equal to - 130 MPa and - 120 MPa. Upper curves 273.65 K, lower curves 298.15 K. If at saturation pressure the pore volume is completely filled with liquid adsorbate, then at lower relative pressures the amount of adsorbate may decrease through the emptying of pores or the expansion of the adsorbate within a fixed pore volume. As long as the pore volume remains filled with expanded adsorbate, then AV, = AOVl (3) where A is the number of moles adsorbed per gram and A' and V i refer to these quantities at saturation pressure.The ratio A/A" (or VL/ VL) is the ratio of the density of the capillary liquid to that of normal bulk liquid. At negative pressures this ratio, the adsorbate relative density, will be < 1 . The condition that the pores be filled with adsorbate will most readily be met along the desorption branch of the isotherm. As long as the condition is satisfied eqn (2) and (3) may be combined to give PL = [(W VL") V I A " ) In (P/P,)l (4) where the quantities on the right-hand side of the equation may be determined experimentally. Values of PL calculated using desorption points from saturation pressure to the lower closure point of the hysteresis loop are shown in fig.2. The limiting negative pressure can be estimated as ca. - 130 MPa at 273 K and ca. - 120 MPa at 298 K. Recently, Speedy7 has proposed an equation of state for liquid water at negative pressures. His equation may be written as ( 5 ) where S is a constant and VM is the molar volume at the limiting negative pressure PM. Values for these quantities have been rep~rted,~ with the limiting negative pressure estimated as - 180 MPa at 273 K and - 210 MPa at 298 K. Using these values in eqn (9, PL has been estimated as a function of the adsorbate relative density. The results are shown in fig. 2 (dashed lines). 1 - PJP, = s [( V,/ V,) - 112600 WATER IN SILICA GEL Much better agreement between the experimental and calculated values of PL is obtained when PM is taken as - 130 MPa at 273 K and - 120 MPa at 298 K (fig.2, solid lines). When A/A" is < 0.75, the departure of the experimental points from eqn (5) may arise from the emptying of pores, in which case the condition imposed by eqn (3) is no longer valid and eqn (4) becomes inapplicable. Furthermore, eqn (5) predicts that PL cannot be negative when the adsorbate relative density is < 0.63, i.e. capillary-condensed liquid cannot be stable for values of A/A" < 0.63. This requires the lower closure point of the hysteresis loop to occur at or above this point. As can be seen from fig. 1, the lower closure point for both isotherms occurs for A/A" near 0.62, in good agreement with the predicted value. These results suggest that the equation of state proposed by Speedy7 provides a good description of the pressure-volume behaviour of liquid water condensed in the capillaries of porous adsorbents. CONCLUSIONS The pressure-volume behaviour of capillary-condensed water can be represented by an equation of state proposed by Speedy.' The molar volume of the adsorbed liquid increases at intermediate and low relative pressures, but the pressure within the liquid appears to pass through a minimum and then increase, becoming positive at a point corresponding to the lower closure point of the hysteresis loop.This model accounts for the shape of the desorption branch of both isotherms from saturation pressure to the steep portion of the hysteresis loop and correctly predicts the position of the lower closure point of the hysteresis loop. While a comparable equation of state has not been developed for other liquids, our adsorption isotherms for water on silica gel are similar to those reported for other liquids, including nitrogen.'? Since the general behaviour of capillary-condensed water is similar to that of other liquids then pore-size distributions determined by the analysis of desorption isotherms should include corrections for changes in the molar volume of the adsorbate. S. J. Gregg and K. S. W. Sing, Adsorption, Surface Area and Porosity (Academic Press, New York, 2nd edn, 1983), chap. 3, pp. 121 and 132. J. H. deBoer, in The Structure and Properties of Porous Materials, ed. D. H. Everett and F. S. Stone (Butterworths, London, 1958), p. 68. 0. Kadiec and M. M. Dubinin, J. Colloid Interface Sci., 1969, 31, 479. S. J. Gregg and K. S. W. Sing, Adsorption Surface Area and Porosity (Academic Press, New York, 2nd edn, 1982), chap 3, p. 157. J. C. P. Broekhoff and W. P. van Beek, J . Chem. SOC., Faraday Trans. 1, 1979,75,42. .I C. G. V. Burgess and D. H. Everett, J . Colloid Interface Sci., 1970, 33, 61 1 . ' R. J. Speedy, J . Phys. Chem., 1982, 86, 982. (PAPER 4/571)

ISSN:0300-9599    

DOI:10.1039/F19858100597

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

J. Chem. SOC. Faraday Trans. I, 1985,81, 601-608 Measurement of the Activity of Hydrogen and Oxygen Catalysts by a Photochemical Relaxation Method BY MICHAEL NEUMANN-SPALLART Institut de chimie physique, Ecole Polytechnique Fedkrale de Lausanne, CH-1015 Lausanne, Switzerland Received 17th April, 1984 A technique is described that allows the measurement of the rate and the calculation of the rate constant for the reaction of a species, R*, of appropriate redox potential with a catalyst (noble-metal colloid, metal oxide, enzyme etc.) leading to the oxidation or reduction of water. The relay, Rk, is continuously generated photochemically until at t = 0 the production is stopped. The relaxation of [R?] from its photostationary-state value is monitored by light absorption and the kinetics are analysed.Colloidal solutions of noble metals or oxides and certain enzymes are known efficiently to catalyse the reduction or oxidation of water when reacting with relays of appropriate redox potential. The application of such catalysts in photochemical systems where light energy may be stored in the form of chemical products has received much attention over the last few years.l-' Various different methods have been designed to measure the catalytic activity (see below) and have sometimes yielded conflicting results. The reason for this is the fact that catalyst activities can change with the experimental conditions because of the complicated reaction mechanisms involved. Two main reaction schemes can be identified. The first involves enzymatic reactions following Michaelis-Menton kinetics.The catalytic activity is determined by the maximal rate, omax, and K,. For concentrations of the relay [Rk] < K , (R* being the active form of the relay driving the reaction in the forward direction), the rate is linearly dependent on [enzyme] and [Rk]. If [enzyme] is expressed in mol dm-3 a bimolecular rate constant, k,, which equals iornax/(Km [enzyme]) can be calculated. The second uses a ' microelectrode model ' of the electronically conducting particles derived from corrosion theory to explain the function of a colloidal metallic catalyst.'+l' According to this the formation rate of the product, u, equals isc/F, where isc is the short-circuit current through the microsphere and its value is given by the intersection of the cathodic current against potential curve and the anodic current against potential curve.For a water reduction catalyst these curves correspond to hydrogen formation and oxidation of the reduced relay, respectively. The maximum rate for a given relay will be established if the intersection lies on the plateau of its i against E curve (diffusion control). In the following this will be called the limiting case. The rate constant (which is strictly only defined in this case) k,,, is then Zisc/(F[R*]V), where Cis, is the sum over all particles and V is the volume. By division through the molar particle concentration k, can be obtained. If the intersection does not occur on the plateau [e.g. at high pH or Eo (relay) too high (low) to drive H,O reduction or oxidation easily] isc and hence o will depend on the degree of conversion of R into R+, which itself depends on the production rate, oo, [R] and the other kinetic parameters of a particular experiment.ll 60 1602 ACTIVITY OF HYDROGEN AND OXYGEN CATALYSTS Table 1.Methods used for the determination of the activity of hydrogen-evolving catalysts variable type of experiment observed principle ref. mixing of the reactants continuous chemical generation of one reactant (R-) continuous electrochemical generation of one reactant (R-) cyclic voltammetry flash photolysis, pulse radiolysis stopped-flow mixing photostationary-state relaxation measurement of reactant decay measurement of product- formation rate at flow equilibrium measurement of product- formation rate at flow equilibrium chemical reaction dynamics decay after pulse perturbation decay from flow equilibrium measurement of reactant decay from equilibrium (multicomponent system) heterogeneous elec tro- measurement of reactant measurement of reactant 9 12 13, 14 9, 10 15, 16, 17 9, 1 1 this work Methods that have been used to measure the activity of colloidal and enzymatic hydrogen catalysts are listed in table 1.They all suffer from certain drawbacks. The first three methods are slow (because of long mixing or analysis time if hydrogen is measured by a Clarke electrode or by gas chromatography) and are thus restricted to inefficient or very dilute catalysts. Manual and stopped-flow mixing requires handling of often air-sensitive solutions of the reduced relay so that reaction with traces of oxygen may lead to wrong, i.e.too high, apparent rate constants. Electrokinetic cyclic voltammetry can be disturbed by adsorption of the catalyst or the relay on the electrode. Electrochemical continuous generation of R* creates non-uniformity of relay concentration in space precluding analysis of very efficient catalysts (inhomo- geneous kinetics in the depletion layer). Under the conditions of flash photolysis or pulse radiolysis a concentration of R* much higher than that obtained under continuous irradiation will be produced if the catalyst is very active. Since the decay time, z, of R* is only in the limiting case (for enzymes at [R+] < K,) independent of reactant concentrations and equals 1 /kcat, transient phenomena may be observed.We are thus looking for a method which allows us (i) to measure the reaction rate, u, of Rk with a catalyst at flow equilibrium (when products are continuously formed), (ii) to study the influence of several parameters such as reactant concentration, degree of conversion of the relay, p(H,) and pH on the reaction rate and (iii) to establish the limiting condition (plateau intersection of the i against E curves for colloidal metals, [R*] < Km for enzymes) for calculating kcat and k,. PRINCIPLE OF THE METHOD The rate of the catalysed reaction of species R+ or R- with water producing H, or 0, is measured by monitoring [R*] uiu its absorption of light. To generate the reduced or oxidised relay a photochemical reaction is chosen. The catalyst must beM.NEUMANN-SPALLART 603 present in optically transparent (homogeneous or colloidal) form. R+ or R- is generated by continuous illumination. When the actinic light is switched off, the decay of [R+] or [R-] relaxing from the photostationary value (flow equilibrium) by reaction with the catalyst is recorded. A characteristic decay time, z, can be calculated from the decrease of absorption with time. In the limiting cases l / z can be assigned to a first-order rate constant, kcat: d[R*]/dt = kcat [R*], kcat = k, [catalyst]. (1) If other reactions of R* (not leading to the oxidation or reduction of water) occur in the system, a measurement of z, the steady-state concentration of [Rk], and the initial production rate, Y,, when the light is switched on for the first time allows the efficiency of electron transfer, re, from R+ to the substrate (water) to be calculated: Often the main purpose of measuring catalytic activity is to be able to estimate the reaction rates in the photostationary state of a continuous-irradiation experiment.This method allows them to be measured in situ. There are some similarities to the stopped-flow technique where the reactants are continuously mixed and transported to the observation point. In the method presented here neither mixing nor transport occur. The time resolution is only restricted by the time necessary to switch off the light, i.e. by the construction of the fast shutter (an electro-optic shutter may be used). The sensitivity is limited when the concentration of the reduced or oxidised relay is so low that the relative signal changes become undetectable, as happens with very active catalysts.The setup used in these experiments allowed signal changes of a few tenths of a percent to be measured. In order to increase the sensitivity, one could envisage the use of a periodic light source. However, an important objective of the present setup was to investigate if and how the catalytic activity changed with (prolonged) irradiation time, thus requiring ' single-shot ' experiments. EXPERIMENTAL The experimental setup is shown in fig. 1. A 10 cm water filter, F,, absorbs the i.r. component of the light from the actinic light source, LA (a 250 W W-halogen or a 150 W Xe lamp). An interference filter, F,, is used to select a narrow band of the spectrum.With the neutral density filter, F,, the intensity of the light is attenuated to vary [R+] in the steady state. An He-Ne laser was used as the analysing light source to measure the absorbance of the reduced relay, methyl viologen ( E ~ ~ ~ . ~ ~ ~ = 8060 dm3 mol-l cm-l), or the absorbance of the oxidised dye, ruthenium tris(bipyridy1) ( E ~ ~ ~ . ~ ~ ~ = 620 dm3 mol-l cm-l). When a conventional light source instead of the laser is used, interference filters, F, (and FJ, must be inserted in the analysis light path. Note that in this case the observation zone is much broader and a homogeneous distribution of species within that zone must be assured by using low dye concentrations. The photodetector, PD, was a Hamamatsu Si-photodiode S780-8BQ. The light-intensity-dependent current through the photodiode was measured via the voltage drop across a 1 kG? load resis- tance using a Tektronix 5103N storage oscilloscope and a chart recorder to show the light intensity, I, at t < 0 and at t -, and also to record the decay when very high dilutions of catalysts or inefficient catalysts were investigated.The sample was contained in a 1 x 1 x 4 cm quartz cell equipped with a degassing tube. The light intensity of LA as measured with a YSI Kettering 65A radiometer was 1.88 x E s-l cm-l at 450 nm (bandwidth 60 nm) when F, was a Balzers K-45 bandpass filter. To start an experiment the actinic light was switched on by opening a second shutter (not shown in fig. 1). After an appropriate irradiation time (see Results and Discussion) a guillotine-type shutter falls from a certain hight [30 cm, which corresponds to a shutter time604 ACTIVITY OF HYDROGEN AND OXYGEN CATALYSTS LA Fig. 1.Experimental setup. LA, light source; F, water filter; F,, F, and F,, interference filters; F,, neutral density filter; S, shutter; L-PT, trigger light path (L, lamp or LED; PT, phototransistor); PD, photodetector; AL, analysing light source; OSC, oscilloscope; C, cell. of 0.4 ms, i.e. the time to fall 0.1 cm after a total fall distance of 30 cm (the thickness of the zone inspected by the laser beam was < 0.1 cm)] and switches off the actinic light beam. To observe the baseline at c < 0 on the oscilloscope, a pretrigger feature was added to the system. When the light of a lamp, L, that falls onto a phototransistor mounted opposite is interrupted by the falling shutter, the trigger signal is set.A difference of 5 cm between the laser beam (analysis zone in the cell) and the phototransistor/L gave a pretrigger time of 10ms. The distance can be changed to allow for different settings of the pretrigger time. Solutions were made up in triply distilled water and degassed with nitrogen if necessary. Chemicals were reagent grade and used without further purification. A colloidal catalyst, Pt/Carbowax, was prepared by the literature method.18 Ti0,-supported catalysts and RuO, colloids were prepared by Mr J-J. Moser in this laboratory. Desulfovibrio desulfuicans (Strain Norway) hydrogenase was a gift from Dr P. Cuendet. RESULTS AND DISCUSSION Several photochemical systems were used to determine the catalytic activity.An example of the photochemical production of R- is shown in scheme 1. A solution containing a dye, S, and a quencher, R, is continuously irradiated with light that can be absorbed by the dye. Its excited state, S*, is quenched by R to give R- and S*. hV Scheme 1.t I ( b ) t light off t light on I=O Fig. 2. Recovery of MV+ from its photostationary state. Solution composition (2-4 cm3, 0.1 mol dm-3 acetate buffer, pH 4.55): ruthenium tris(bipyridy1) chloride, Ru(bpy);+(Cl-),, (sensitiser, Amax = 452 mn), 5 x mol dm-3; methyl viologen chloride (relay), 2 x mol dm-3, in the presence of a colloidal platinum catalyst. (a) Oscilloscope trace (relative intensity of the analysis light, d = 1 cm) and ln[log(I,,/It)] as a function of time; (b) I as a function of t for a complete experiment (schematic).mol dmW3, EDTA disodium salt (donor), 1.35 x R- reacts with the catalyst and water to give hydrogen. S* is reduced by an added donor. For S = ruthenium tris(bipyridy1) and R = methyl viologen the system is well understood kinetically.6 The equation qe = 0.5 s[( 1 + 4 / ~ ) l / ~ - I] with s = kcat k/(v, kb) (3) relates qe to k,,, where the other rate constants are indicated in scheme 1 and v, is the initial production rate of R-.6 Eqn (3) also holds for enzymes if [R-] < K,. Fig. 2(a) shows a typical oscilloscope trace for an experiment with a platinum606 ACTIVITY OF HYDROGEN AND OXYGEN CATALYSTS catalyst in scheme 1. Several cycles (light-on - light-off) have been recorded to check whether any changes in the decay kinetics occur, indicating a decrease in catalytic activity caused by aging of the catalyst or accumulation of products.For the same purpose the ‘light-on’ period was finally prolonged to several minutes before recording the decay. Decay curves were analysed by plotting In [log (Io, a/It)] as a function o f t [fig. 2(a), broken line]; the slope equals l/z. in fig. 2(b) is the value of the analysis light intensity before switching on the actinic light for the first time (if the experiment is repeated with the same solution). The value of I at t + co after closing the shutter is sometimes lower than because of the non-zero equilibrium dark concentration of the reduced relay when H, is present in the system. This leads to longer decay times because a shift of the H+/H, current against potential curve to more negative potentials. This effect was only observed at high pH.If Io, a - loo is comparable to Io,?-It-o only the first points should be taken for the calculation of z since at longer times deviation from linearity occurs. If in scheme 1 the concentration of the donor can be held sufficiently high, mathematical analysis shows that the decay is almost exclusively caused by the reaction of R- with the catalyst (at high catalyst concentrations), being much faster than the back reaction with S+. Even if this is not the case (low catalyst concentrations, qe -c l), decay uia the reaction with S+ will be limited to a very short time after switching-off the light, because this reaction follows second-order kinetics and the total extent of disappearance of R- uia this reaction will be small because of the small overall concentration of S+ compared with R- in the photostationary state for low values of catalytic rate constants.Therefore the decay kinetics will not be interfered with by the back reaction. 2 h O H 2 M V Z * Scheme 2. We then considered scheme 2, without reverse electron transfer, in which acetone is excited by U.V. light in the presence of propan-2-01, its triplet state abstracts a hydrogen atom from the alcohol and two (CH,), COH radicals are formed.19 These are used in scheme 2 to reduce MV2+ to MV+, the decay of which is observed in the presence of a hydrogen catalyst. Scheme 2 allows measurements at lower pH values (pH 1) than is possible for scheme 1 (for EDTA as the donor the pH must be > 3.5).However, photolysis of MV+ is a problem when catalysts of low activity (high [MV+Istst) are investigated. For the kinetic analysis of oxygen catalysts scheme 3 was used,’ where excited ruthenium tris(bipyridy1) is quenched by persulphate ions in an irreversible reaction and the decay of the oxidised dye is observed. To some extent irreversible bleaching was observed in the case of a Si0,-supported RuO, colloid as oxygen catalystM. NEUMANN-SPALLART 607 Scheme 3. (kcat = 0.8 s-l at 0.1 gRuO, dm-3). This can be interpreted as partial destruction of the catalyst by oxidising species. No such effect was noticed for Pt sols (hydrogen formation, scheme 1). However, in the case of a Pt sol stabilised by Carbowax a gradual decrease of activity occurred upon repeating the experiments several times or prolonging the ‘light-on’ period [initial activity 24 s-l at 0.04 g dm-3, pH 4, corresponding to k’, = 600 s-l (gR dm-3)-1 or 1.3 x lo8 dm3 mol-1 s-l considering the molecular weight of the particles].Irreversible loss of activity also occurred when the solution was purged with hydrogen and subsequently with argon. l / z was found to be proportional to catalyst concentration for most catalysts. The best catalyst was a platinised TiO, so120 stabilised with PVA: k, = 280 s-I at 0.01 g,, dm-3 at pH 3.5, corresponding to k’, = 28000 s-l (gR dm-9-l or k3 = 1.7 x lo8 dm3 mo1-I s-l based on 50 A TiO, particles, these being the freely diffusing species in this case.Measurements with the acetone-based system gave similar results. The diffusion-controlled rate constant for such particles calculated using Smoluchowski’s equation,’ (kbim = 4nNDr/1000, where D is the diffusion coefficient of MV+ and r is the particle radius) is 3 x 1Olo dm3 mo1-I s-l. We conclude that the covering of the particle surface by the stabilising agent leads to a decrease of the effective collector surface into which MV+ may inject electrons. For these electrons the conduction band of TiO, acts as a collector electrode and channels electrons to Pt sites present at the surface of the particles. Since the rate of e- injection from MV+ into TiO, is pH dependent,,, the plateau of the plot of kcat against pH is reached at lower pH (3.5) than with Pt/Carbowax (4.5).The activity of hydrogenases is usually measured by analysis of H, at pH 7. High dilutions of.the enzyme are used and rates of reactions involve time scales of minutes or hours. When methyl viologen or ferredoxin reduced by dithionite12 are used as relays their reduction is not efficient at low pH. Scheme 1 allows measurements down to lower pH (3.5). Interestingly, the optimum pH value for hydrogenases can lie well below pH 7;14 for example, the activity of Desulfovibrio desuIfuricans hydrogenase as determined by the relaxation method is 0.8 s-l at 1.65 g dm-3, pH 4.55, corresponding to k’, = 0.48 s-l (gprotein dm-3)-1 or k, = 2.8 x lo4 dm3 mol-1 s-l. We conclude that this method is a useful tool for measuring and understanding the kinetics of homogeneous and colloidal catalysts, especially when they are used in photoredox catalysis.The main advantage is that measurements are obtained at the steady state, i.e. when the catalyst continuously forms the product, and not during an induction period. Thus the conditions of the kinetic experiment are exactly the same as in photochemical oxygen- or hydrogen-evolution experiments in the course of which the catalytic activity can be monitored. There is no delay caused by mixing and no product analysis is required for the measurement of enzyme activities. The608 ACTIVITY OF HYDROGEN AND OXYGEN CATALYSTS sensitivity is limited only by the fact that highly efficient catalysts lead to small relative signal changes. Because the diffusion-controlled rate constant is only defined for the limiting case, different techniques often yield different rate constants for the same catalyst (see references in table 1) when kcat is measured at high pH for hydrogen catalysts, at low pH for oxygen catalysts or at different degrees of conversion of the relay.When comparing results, extrapolations should not be made to cases where the degree of conversion is very different. This method also allows collection efficiencies in a photochemical experiment (scheme 1) to be measured without the necessity of performing actinometry and knowing the quenching efficiency. The values for a hydrogen catalyst (ranging from 10 to almost loo%, depending on catalyst and donor concentration) are in good agreement with data derived from direct measurements of hydrogen quantum yields by gas-chromatographic analysis.I thank Dr Pierre Cuendet for helpful discussions. K. Kalyanasundaram, J. Kiwi and M. Gratzel, Helv. Chim. Acta, 1978, 61, 2720. M. Kirsch, J. M. Lehn and P. Sauvage, Helv. Chim. Actu, 1979, 62, 1345. J. Kiwi and M. Gratzel, Nature (London), 1979, 281, 657. K. Kalyanasundaram and D. Dung, J. Phys. Chem., 1980,84, 2551. M. Neumann-Spallart and K. Kalyanasundaram, J. Phys. Chem., 1982,86,2681. 63, 1111. C. Wagner and W. Z. Traud, 2. Elektrochem., 1938, 44, 381. D. S. Miller and G. McLendon, J. Am. Chem. SOC., 1981, 103, 6791. a A. I. Krasna, Enzyme Microb. Technol., 1979, 1, 165. 'I M. Neumann-Spallart, K. Kalyanasundaram, C. Gratzel and M. Gratzel, Helv. Chim. Actu, 1980, lo E. Sutcliffe and M. Neumann-Spallart, Helv. Chim. Acta, 1981, 64, 2148. l1 W. J. Albery, P. N. Bartlett and A. J. McMahon, in Photogeneration of Hydrogen, ed. A. Harriman l2 Ch. D. Toai, S. D. Varfolomeev, I. N. Gogotov and I. N. Berezin, Molek. Biol., 1976, 10, 452. l3 D. S. Miller, A. J. Bard, G. McLendon and J. Ferguson, J. Am. Chem. Soc., 1981, 103, 5336. l4 V. M. Fernandez, Anal. Biochem., 1983, 130, 54. l5 J. Kiwi and M. Gratzel, J. Am. Chem. SOC., 1979, 101, 7214. l6 D. Meisel, W. A. Mulac and M. S. Matheson, J. Phys. Chem., 1981, 85, 179. l7 J. Westerhausen, A. Henglein and J. Lilie, Ber. Bunsenges. Phys. Chem. 1981, 85, 182. P.-A. Brugger, P. Cuendet and M. Gratzel, J. Am. Chem. SOC., 1981, 103, 2923. l9 A. Henglein, B. Lindig and J. Westerhausen, J. Phys. Chem., 1981,85, 1627. 2o D. Duonghong, J. Ramsden and M. Gratzel, J. Am. Chem. Soc., 1982, 104, 2977. 21 M. Smoluchowski, 2. Phys. Chem., Abt. A , 1918,92, 129. 22 M. Neumann-Spallart, work in preparation. and M. A. West (Academic Press, London, 1982). (PAPER 4/637)

ISSN:0300-9599    

DOI:10.1039/F19858100601

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. 1, 1985, 81, 609-619 Concentration Dependence of Electrokinetic Transport Coefficients of Non-aqueous Binary Mixtures through Weakly Charged Porous Plugs BY ROQUE HIDALGO-ALVAREZ,* FRANCISCO JAVIER DE LAS NIEVES AND GERARDO PARDO Physics Department, Faculty of Sciences, University of Granada, Granada, Spain Received 30th April, 1984 An experimental investigation of the concentration dependence of electrokinetic transport coefficients of methanol + ethanol mixtures through a quartz plug is described. Electro-osmotic flow and streaming-potential measurements have been carried out. The Onsager reciprocity relations have been experimentally checked for all compositions of the binary mixture used. The form of the fluidity curve indicates that the ethanol+methanol mixture is a quasi-ideal system with a small negative deviation. The hydrodynamic permeability of systems comprising methanol+ethanol mixtures and a porous plug and the fluidity of the mixture vary with the composition in a similar way.The variation of the cross-phenomenological coefficient with composition is due to an analogous variation of the term Dc/r,~ with the mole fraction of methanol. On the other hand, the ratio of streaming potential to pressure difference (in the interval 0-20cmHg) decreases as the pressure drop increases. This result agrees with the theoretical findings of Rutgers and Boumans concerning the effect of turbulence on the streaming potential. The concentration dependence of the second-order coefficients has been partially explained on the basis of Jha’s equations.In recent years several papers have been published with the main objective of studying the transport phenomena of liquid mixtures through inactive porous media and ion-exchange membranes, from the point of view of the thermodynamics of irreversible processes. Special interest has been paid to the study of the dependence of the first-order phenomenological coefficients Lik on the compositions of binary and ternary mixtures.1-8 Some investigatorsly working with the same system (methanol + water mixtures and a sintered Pyrex glass membrane) have found very different trends of variation of each phenomenological coefficient with the mixture composition. For example, Srivastava et al. obtained linear relationships between the coefficients L,, and Lik and the methanol mole fraction, these dependences being explained by means of Spiegler’s frictional model.However, using a similar porous glass membrane Singh and Singh2 obtained a non-linear dependence of Lii and Lik on the mole fraction of methanol. This different behaviour of the transport coefficients has not been satisfactorily explained by these authors. Very recently Blokhra et d3 have also employed methanol + water mixtures as permeants through a chemically treated oak wood membrane. The variations of the coefficients Lki and Lkk with the mass fraction of methanol were non-linear. These trends were attributed to the structural modifications which occur in water + methanol mixtures due to hydrogen bonding. With regard to ion-exchange membranes, Rastogi et aL4 have found that with several mixtures the electro-osmotic permeability can even change sign when potential differences beyond a determined critical value are applied.The concentration depen- dence of the transport coefficient is usually more complicated in highly charged than 21 609 FAR 1610 TRANSPORT THROUGH POROUS PLUGS in weakly charged or inactive membranes. Moreover, some investigators5 have asserted that the concentration dependence of the phenomenological coefficients can be consistent with Spiegler's model when the interactions among the components of the mixture and alterations in the membrane-permeant interaction with change in composition are negligible. Hence it seems that when a liquid mixture flows through an ion-exchange membrane the above interactions are never negligible, since in these cases the dependence of the phenomenological coefficients on composition have always been non-linear. With inactive membranes and several liquid mixtures1* 5-6 some investigators have shown that Spiegler's model is adequate to describe the transport processes of those mixtures, the relation between the phenomenological coefficients and the composition then being linear.An investigation of electrokinetic transport in liquid mixtures is thus of interest in understanding mechanisms of transport. In view of the above, we have studied the dependence of the electrokinetic and permeation coefficients on composition for a mixture of liquids (ethanol + methanol) of similar characteristics, using a plug prepared with particles of quartz as an inactive porous medium.To obtain further insight into the phenomenon the experimental results were analysed on the basis of the classical theory of electrokinetic processes. On the other hand, in order to test the thermodynamic consistency of the electro-osmosis data, streaming-potential measurements were also carried out, and the Onsager reciprocity relations were experimentally checked. In order to gain a further understanding of electrokinetic transport in weakly charged porous plugs we have also determined the second-order phenomenological coefficients that appear in streaming-potential experiments when a high pressure difference, Ap, is applied. Likewise, we have studied the effect of mixture composition on the non-linear coefficients.This aspect has been little studied since most reports refer to the variation of the linear coefficients with compositions of binary liquid mixtures. EXPERIMENTAL The experimental apparatus used in this work has been described in earlier paper~.~-ll The quartz employed was from Cerro Muriano (Cdrdoba, Spain) and its quantitative composition, obtained by spectrophotographic methods, was 95.5% SiO,, 2% Na,O, 1% Al,O, with small impurities of CaO, K,O, Fe,O, and HgO. The quartz was crushed and sieved to obtain the fraction between 150 and 200 pm. It was washed with dilute HCl(l% ) and then with deionized water until the conductivity of the washing water remained constant. Porous plugs 0.52 cm thick were prepared with quartz particles.The liquids utilized in the experiments were analytical-grade methanol and ethanol from Carlo Erba. When the electrokinetic measurements were carried out the electrical conductivity of these alcohols increased by ca. 5% of its initial value. Consequently, the quartz particles used to prepare the porous plugs did not cause any ionic contamination of the permeant liquids (methanol and ethanol). The time taken to condition the quartz in the liquid media was 48 h. All measurements were carried out in an air thermostat maintained at 20.0k0.5 "C. RESULTS AND DISCUSSION The linear phenomenological equations for the simultaneous transport of matter and electric charge are12 J = L,, Ap+LI2 A$ I = L,,Ap+L,,A$R. HIDALGO-ALVAREZ, F. J. DE LAS NIEVES AND G. PARD0 61 1 where J and I are, respectively, the total volume flux and the electric current due to both hydrostatic pressure differences, Ap, and the electric potential difference, Ad.The coefficients L,,, L,, and L,, characterize the hydrodynamic and electro-osmotic flows and the streaming current, respectively, while the electrical conductance of the plug is given by the coefficient L,,. In order to provide a suitable interpretation of the dependence of the phenomeno- logical coefficients on the composition of the permeant it is necessary to know the relation between those coefficients and the parameters characterizing the electrohydro- dynamic properties of the system. In the capillary model a porous plug is supposed to be composed of a bundle of n capillaries entering a porous medium on one face and emerging on the opposite face. Although the actual structure of any porous plug is not as simple as described by this capillary model, it has been successfuly used by many authors.According to the classical theory we have L,, = nnr4/8qZ (3) L,, = L,, = n&r2(/4qZ (4) L,, = nnr2L/Z where n is the number of capillaries, r is the equivalent pore radius, Z is the length of the capillaries, q is the absolute viscosity, E is the dielectric constant, A is the electrical conductivity of the liquid permeant and [ is the electrokinetic or zeta potential of the solid/liquid interface. From eqn (3)-(5) it can be inferred that if the geometric parameters n, r and Z are independent of the mixture composition (x) then the dependence of L,,, L,, (= L,,) and L,, on x might be explained by means of the variations that q-l, [D/q and A, respectively, undergo when the composition varies.The equivalent pore radius can be estimated by the equation r = (~~LL,,/L,,)% (6) In an earlier work9 we have found that eqn (6) provides values of r close to the true value of the equivalent pore radius of inactive porous plugs. The values of r estimated from eqn (6) are shown in table 1. In most cases there is no significant variation of r with the composition, f = 19.2f 0.4 pm being the average pore radius in the overall interval of variation of the mole fraction of methanol in the mixture. Once the equivalent pore radius of the porous plug is known, it is possible to obtain an estimation of the number of capillaries that theoretically make up the plug.From eqn (5) n can be calculated and the values are also shown in table 1. n is also reasonably constant for the different fractions of methanol, and its average value is (7.3 f 0.3) x lo4 capillaries. Note that the thickness of the porous plug was always 0.52 cm, so that the capillary length (Z) can be taken as independent of x. Therefore it might be expected that the viscosity, q, the dielectric constant, D, the electrical conductivity of the mixture, A, and the zeta potential will be the main factors accounting for the dependence of L,,, L,, and L,, on composition. DEPENDENCE OF THE FIRST-ORDER PHENOMENOLOGICAL COEFFICIENTS ON THE COMPOSITION If the relation between (J)A9-o and Ap is linear, the hydrodynamic permeability coefficient (L,,) can be obtained from the slopes of the straight lines that result when plotting (J)Ad-o against Ap.Fig. 1 shows that for each mixture the dependence of the 21-2612 TRANSPORT THROUGH POROUS PLUGS Table 1. Values of pore equivalent radius (r), number of capillaries (n), absolute viscosity (q), dielectric constant (D) and zeta potential (c) for different values of xM (methanol mole fraction) 0.0 19.7 7.3 1.206 25.8 14.5 0.2 19.6 7.0 1.038 27.3 18.0 0.4 19.1 7.5 0.9 15 28.7 19.8 0.6 19.3 6.8 0.790 30.1 22.1 0.8 18.8 7.7 0.686 32.3 19.2 1 .o 18.7 7.3 0.582 34.4 17.4 1 . 2 1 . o - 'Y) 0.8 E m Y) 0.6 \ 0 I1 t 5 0 . 4 0 . 2 0 1 I I I I I 2 4 6 8 10 12 AplN m-2 Fig. 1. Dependence of hydrodynamic flow on pressure difference for various methanol mole fractions: (a) 1.0, (b) 0.8, (c) 0.4 and (d) 0.0.hydrodynamic flow on the pressure difference is linear for the interval of pressure used, and thus the flow regime is laminar. Fig. 2 shows the variation of L,, and q-l with the mole fraction of methanol (x,). It will be observed that the dependence of both L,, and q-l on X, is linear, but for X, 2 0.6 the straight lines fitting the experimental data undergo a sharp elevation, with greater slopes than when xM < 0.6. Taking into account the close parallel that exists between both straight lines, and the constancy of the term n&/81 [see eqn (3)] when the composition of the mixture varies, one may conclude that the dependence of L,, on X, is mainly due to an analogous variation of q-l (fluidity of the mixture) According to the form of the fluidity curve, the methanol+ethanol mixture is a quasi-ideal system with a very small negative deviation, i.e.the curve practically coincides over the whole concentration with the straight line joining the fluidities of the pure components. It is clear from the values of q (see table 1) that an increase of methanol in the mixture increases the freedom of the internal molecular motion with xM.R. HIDALGO-ALVAREZ, F. J. DE LAS NIEVES AND G. PARD0 613 12 11 7 0 0.2 0.4 0.6 0.8 1.0 XM Fig. 2. Dependence of hydrodynamic permeability (L,,, a) and fluidity (q-l, 0) on methanol mole fraction. owing to an analogous decrease in the viscosity of the mixture, which is usually interpreted as a relaxation effect of the hydrogen-bonding interactions between the polar liquids (methanol and ethanol in our case).This relaxation effect seems slightly more marked when the dominant component of the mixture is methanol (x, 2 0.6). On the other hand, eqn (4) can be written in the form K being a constant equal to 4 ~ ~ , n r ~ / 4 1 and D the dielectric constant of the liquid permeant used. If eqn (7) were valid for explaining the dependence of L,, on the composition of the ethanol +methanol mixture, a close correspondence should exist between the variations of L,, and Dc/q with the mole fraction of methanol, for example. The L,, coefficient was obtained from electro-osmotic flow measurements. The value of this coefficient coincides with the slope of the straight line resulting when (J)A,co is plotted against At,$ (see fig.3). In the interval 0 < X, < 0.6 the coefficient L,, depends linearly on X, according to the equation When xM 2 0.6 the dependence of L,, on xM is also linear, but the straight line fitting the experimental data is then 101OL,, = (0.37+0.02)+(1.30+0.05)~~. (8) lolo L,, = (0.72f0.04)+(0.725+0.003)~~. (9)614 12 10 i s 4 2 TRANSPORT THROUGH POROUS PLUGS I 1 1 I I I 0 20 40 60 80 100 Fig. 3. Dependence of electro-osmotic flux on electrical potential difference for various methanol fractions: (a) 1.0, (b) 0.6 and (c) 0.4. A9/V In order to discover how the term D [ / q varies with composition it is first necessary to calculate the c potential. Values of the [ potential were obtained from streaming- potential data, using the following equation :14 c = CTW/AP)Z*O 1- (10) The constant CT depends on the liquid used and on the temperature.This equation allows the estimation of the 5 potential to be made independently of the capillary model used. The ratio ( A ~ / A P ) ~ - ~ was calculated for Ap --+ zero. In table 1 we show the values of 5 thus obtained, and the viscosities and dielectric constants of each mixture. The term D [ / q is then calculated, and its variation with composition is given by the following regression straight lines : lop2 Dl/V = (2.98 +0.04)+(8.765+0.010)~M (1 1) (12) for xM < 0.6 and D [ / q = (5.53 +0.08) + (4.65 + 0.05) .xM for xM 2 0.6. The values of Dc/q are given in m3C-l s-l. In all cases the linear correlation coefficient was always > 0.99. If the concentration dependence of the cross-phenomenological coefficient L,, was mainly due to the variation of the term D [ / q with xM, the product of the constant K [(l.44+_0.09) x 10-13 C V-l] by D [ / q should be equal to eqn (8) and (9).Thus KD5/1O1O = (0.43 _+ 0.03) + (1.26 0.08) XM (13)R. HIDALGO-ALVAREZ, F. J. DE LAS NIEVES AND G. PARD0 615 for 0 < xM < 0.6 and KDr/1O1O 71 = (0.80 f 0.05)+ (0.67 0.04) XM (14) for 0.6 < xM < 1. Comparing the eqn (8)-( 13) and (9)-( 14), respectively, it is inferred that, within experimental error, the concentration dependence of L,, is due to a similar variation of the term D [ / r with the composition of the methanol + ethanol mixture. To check the thermodynamic consistency of the electro-osmotic data, streaming- potential measurements were also carried out with the methanol + ethanol mixture.The phenomenological relation between and Ap was found to be non-linear for Ap up to 20 cmHg. DEPENDENCE OF THE SECOND-ORDER PHENOMENOLOGICAL COEFFICIENTS ON THE COMPOSITION For any Markoffian process, the local fluxes Ji depend on the local forces Xt, the intensive parameter ai and the structural factor G.15 Hence Ji = A X l , X,, . . ., a,, a,, . . ., G). (15) G is a factor which takes into account the characteristics of the system. If the intensive parameters are kept constant and Ji are expanded in the form of a Taylor’s series16 with equilibrium as the reference point denoted by subscript 0, we have Ji = (aJi/axJo XI+ (aJi/ax2)0 X2 + (1/2) [(a2Ji/aZ)o Z + (a2Ji/axz,), + . . . + 2(a2~~/ax, ax,), X , x, + .. .I (16) up to terms of second order. The derivates in eqn (16) are constants if the intensive parameters are kept fixed provided G is not altered. We may put (aJi/aXl), = Li1 (aJi/aX,), = Liz (17) (a2Ji/az), = Lil, (a2Ji/aP,), = Liz2 (a2Ji/aX1 ax2), = Li12. (18) Several authors16-21 have found that eqn (16) is valid for a great number of irreversible processes. For the particular case of flux of electric charge, eqn (16) is usually written as I = L,, AP + L,, A 4 + 45212 APA4 + &5211(AP)2 + iL222(A4)2 (19) where I = J,, X , = Ap and X , = A4. Taking into account the wide range of validity of Ohm’s law, the coefficient L,,, equals zero, and thus the streaming potential can be written as @#)I-, = - (L21/L22) AP - (L212/L22) AP(A4)1=0 - (L211/L22) @PI2.(20) Generally the constant factor of 3 is included in the coefficient L,,,. In fig. 4 we have plotted ( A ~ / A P ) ~ - , against Ap for some of the mixtures employed. The experimental results of ( A ~ / A P ) ~ - , were fitted by a multivariable regression method. The ratio of streaming potential to pressure difference decreases as the pressure drop increases. This suggests that the turbulent flow that arises at high pressure is responsible for the diminution in absolute value of ( A ~ ~ / A P ) ~ - , , in agreement with the theories of Rutgers et ~ 1 . ~ ~ and Bo~mans.,~ Rutgers et al. have indicated that the non-linear behaviour of the streaming potential with the pressure difference is a consequence of the great disturbance originating in the diffuse part of the electric double layer caused by the high speed of the liquid in the capillaries forming the porous plug.Likewise, Boumans theoretically found that the ratio ( A ~ ~ / A P ) ~ - ,616 W I TRANSPORT THROUGH POROUS PLUGS 0 4 8 12 16 20 24 Ap/ 1 O3 N m-z Fig. 4. Dependence of -(A#/AP)~-,, on Ap for various methanol mole fractions: (a) 1.0, (b) 0.4 and (c) 0.2. Table 2. Values of the coefficients L21, L22, 3, and L,,, for the different compositions of the binary mixture methanol +ethanol 0.0 0.35 kO.01 0.35 f0.02 1.56 0.90 0.76 0.2 0.68 f 0.03 0.66 f 0.03 1.18 0.72 2.09 0.4 0.81 f0.04 0.86 f 0.04 1.67 1 .oo 2.14 0.6 1.13 f 0.06 1.15 f0.04 1.82 1.20 3.72 0.8 1.30 f 0:07 1.32 f 0.04 2.27 1.40 4.5 1 1 .o 1.45 f 0.07 1.44 f 0.05 5.26 3.30 7.42 is lower for turbulent than for laminar flow.In an earlier we have found that with our plugs pressures over 5 cmHg cause turbulent flow. Table 2 shows the coefficient L,, and A (the electrical conductivity of the mixtures) plotted against the molar fraction of methanol. As expected from eqn (9, the changes of A explain the variation of L,, with xM. The ratio L,,/R is effectively a constant (1.67kO.07 cm) for the entire range of variation of xM. By considering the values of L,, (see table 2), L,,, L,,, and L,,, can be calculated. In table 2 are shown the values of L21, which are equal to L,, within experimental error. This resemblance between the values of both cross-phenomenological coefficients is a experimental verification of the Onsager reciprocity relation, and implies that the dependence of L,, on xM is also explained on the basis of the variation of D f [ / q .R.HIDALGO-ALVAREZ, F. J. DE LAS NIEVES AND G. PARD0 617 With the aim of analysing the dependences of the coefficients L,,, and L,,, on the composition of the mixture, we have employed the simple relations proposed by Jha et uI.,,~ which allow for the interpretation of the second-order coefficients in terms of known physical parameters of the system. In the kinetic interpretation of second-order phenomenological coefficients made by Jha et al., the basic hypothesis is that in transport processes the existence of coupling between the fluxes and thermodynamic forces leads to a decrease in the coefficients when restrictions are imposed on the fluxes, so that when J = 0 we obtain from eqn (1) and (2) According to Jha et al.this can be written as which represents the non-linear form of eqn (21) in a power series of A# (up to the second power). On substitution of [I -([2~2/2n2r2q;l)]A# in eqn (22) by the net potential [A#+([eAp/4nqJ.)] the net flow of current I is obtained as25 Comparing eqn (19) and (23) it can be seen that L,,, = (nr4p[3~3/256n2q4AP) (24) and L,,, = (nr4p[2~2/82nq3P) (25) where p is the density of the permeant and the other symbols have their habitual meaning. Taking into account that n, r, I and p are practically constant, it is to be expected that the concentration dependence of the coefficients L,,, and L,,, is due to the variation of r2c2/q3 and c3~3/q4;1 with x M , respectively. The L,,, and L,,, coefficients may be interpreted as a measure of the distortion caused by turbulence in the electric double layer, distortion that could be related to the orientation of the molecular dipoles, since the effective electrokinetic potential is made up of the charge distribution and dipole potential,,' and it seems that the dipole orientation in the electrical double layer is affected by the streaming pressure.According to eqn (24) and (25) the distortion effect will be more pronounced as the [ potential increases. As pointed out by Gur and Ravina,26 non-linearity of the electrokinetic processes is detected only for wide ranges of driving forces and in systems with high surface potentials. Therefore the values of L,,, and L,,, are conditioned not only by the turbulent flow in the capillaries, but also by some specific characteristics of the electric double layer (i.e.[ potential). Moreover, eqn (24) indicates that the L,,, depends on the electrical conductivity of the liquid permeant too. For all cases studied, the sign of L,,, was found to be opposite to that of L,,,. This fact is clearly explained by the eqn (24) and (25) if we keep in mind that the potential was negative (see table 1) for all compositions of the methanol+ethanol mixture. It is also noticeable that the contribution of the coefficient L,,, is greater than that In fig. 5 we have represented L,,, and c2O2/q3 as functions of the composition of the mixture. Both L,,, and C2D2/q3 show a monotonic increase as the mole fraction of L211, i*e- I L,l& I ' I L,,, AP I.618 7 - 6 - 5 - - I > z d 4 - E 2 2 ---- 3 - - I N 2 4 c.2 '- TRANSPORT THROUGH POROUS PLUGS m M X - 1 2 m E > 2 -88 ,? r m. 4 s CI Q - ' 4 - 0 0 0.2 0.4 0.6 0.8 1 .o XM Fig. 5. Variation of the coefficient L,,, (0) and r2D2/q3 (0) with the methanol mole fraction. of methanol in the mixture increases. The value of the coefficient L212, however, undergoes a sharp rise when the liquid permeant is pure methanol, whereas the corresponding increase of c2D2/q3 is less sharp. Concerning the coefficient L211, we have found that it decreases monotonicaly as the methanol mole fraction decreases (see table 2), while the term C3D3/q43L does not vary with xM in a similar way. Thus eqn (24) does not explain the behaviour of L,,, with xM sufficiently well. In conclusion, we have found that the variation of the phenomenological coefficients with the composition of the permeant is mainly determined by the variation of the electrohydrodynamic parameters of the liquid mixture and the c potential of the solid/liquid interface.Although this conclusion should not be generalized to all porous media, it seems clear that those parameters must always be taken into account when explaining the concentration dependence of the first- and second-order phenomeno- logical coefficients. When ion-exchange membranes are used, permeant-membrane interactions should probably also be considered. R. C. Srivastava, M. G. Abraham and A. K. Jain, J . Phys. Chem., 1977,81, 906. K. Singh and J. Singh, Colloid Polym. Sci., 1977, 255, 379. R. L. Blokhra, M.L. Parmar and S. Chand, Indian J . Chem., 1982, 21A, 341. R. P. Rastogi, K. Singh and J. Singh, J . Phys. Chem., 1975, 79, 2574.R. HIDALGO-ALVAREZ, F. J. DE LAS NIEVES AND G. PARD0 619 R. C. Srivastava and M. G. Abraham, J. Chem. SOC., Faraday Trans. 1, 1977,81,906. R. L. Blokhra, M. L. Parmar and S. C. Chauhan, J. Membr. Sci., 1983,14, 67. R. L. Blokhra, S. K. Agarwal and N. Arora, J. Colloid Interface Sci., 1980, 73, 88. M. L. Srivastava and S. N. Lal, Colloid Polym. Sci., 1980, 258, 877. R. Hidalgo-Alvarez, F. Gonzalez-Caballero, J. M. Bruque and G. Pardo, J. Colloid Interface Sci., 198 1, 82, 45. lo F. Gonzalez-Caballero, R. Hidalgo-Alvarez, J. M. Bruque and G. Pardo, Physicochem. Hydrodyn., 1982, 3, 15. l1 R. Hidalgo-Alvarez, F. Gonzalez-Caballero, F. J. de las Nieves, J. Non-equilibrium Thermodyn., 1982, 7, 269. l2 I. Prigogine, Introduction to the Thermodynamics of Irreversible Processes (Wiley, New York, 1968). l 3 I. Gyarmati and J. Sandor, Colloid J. USSR, 1966, 18, 305. l4 V. Smoluchowski, Handbuch der Elektrizitat und des Magnetismus (Graetz, Leipzig, 1921), vol. 2. l6 R. P. Rastogi and M. L. Srivastava, Physica, 1961, 27, 265. J. C. M. Li, J . Chem. Phys., 1958, 29, 747. l7 J. C. M. Li, J. Chem. Phys., 1962, 37, 1592. J. C. M. Li, J. Appl. Phys., 1962, 33, 616. lB I. Gyarmati, Period. Politech., 1961, 5, 219. 2o I. Gyarmati, Period. Politech., 1961, 5, 321. 21 R. P. Rastogi and R. Shabd, J. Phys. Chem., 1977,81, 1953. 22 A. J. Rutgers, M. de Smet and G. de Myer, Trans. Faraday SOC., 1957, 53, 393. 23 A. A. Boumans, Physica, 1957, 23, 1007. 24 R. Hidalgo- Alvarez, F. Gonzalez-Caballero, J. M. Bruque and G. Pardo, J. Non-equilibrium 25 K. M. Jha, M. D. Zaharia and S. P. Jha, J. Indian Chem. SOC., 1976,58, 745. 26 Y. Gur and 1. Ravina, J. Colloid Interface Sci., 1977, 72, 272. Thermodyn., 1981, 6, 295. (PAPER 4/698)

ISSN:0300-9599    

DOI:10.1039/F19858100609

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. 1, 1985,81, 621-634 Electrochemical and Surface X-Ray Photoelectron Spectroscopy Study on the Rhodium-Carbonate Electrode in Molten Nitrates BY LUIGIA SABBATINI,* ANNA G. CAVINATO, ELIO DESIMONI AND PIER GIORGIO ZAMBONIN Laboratorio di Chimica Analitica, Dipartimento di Chimica, Universith di Bad, Via Amendola 173, 70126 Bari, Italy Received 14th May, 1984 A combined potentiometric and X-ray photoelectron spectroscopic (X.P.S.) study has been performed on the system (Rh)CO,, O,/COg- in molten nitrates at two significantly different temperatures (525 and 623 K) in order to obtain further information on molten-salt carbonate electrodes involving transition metals. At both temperatures the electrode response showed Nernstian behaviour different from that relevant to the overall reaction COX- + CO, + 80, + 2e- and was strongly dependent on temperature.The parallel X.P.S. investigation showed that rhodium surfaces undergo chemical microcorrosion when in contact with the melt, with the formation of an oxide layer whose thickness is dependent on the temperature and on the time of contact between the metal and the melt; the relevant kinetics of layer formation were studied by following the modification of the spectroscopic data with time. The results may be rationalized by hypothesizing potential-determining steps involving metal oxide species present on the electrode surface. The paper shows the convenience of using X.P.S. as an addition to conventional electrochemical techniques. When placed in contact with molten nitrates, most metals undergo a spontaneous passivation process' which leads to the formation of an insoluble oxide film often involving mixed valence states of the metal.In previous potentiometric investigations2-8 it was shown that when transition-metal electrodes work in fused nitrates in the presence of the redox system CO,, O,/CO;-, two possible electrode reactions occur, both involving solid-state species (i.e. metal and oxides) : M +Cog-* MO + CO, +2e- MO + C O ; - g M +CO, +O, + 2e-. (1) (11) In reaction (I) the electrode surface is oxidized with evolution of carbon dioxide only; in reaction (11) an oxidized species present on the electrode surface is reduced with evolution of oxygen and carbon dioxide. In any case the overall reaction is an anodic oxidation process.An attempt was made by Keenan et aL8 to correlate these findings with the crystal-field stabilization energies of the carbonate-metal complexes presum- ably formed on the electrode surface. At the same time a more thorough investigation6 performed with nickel electrodes showed that the potentiometric behaviour of such a system may be strongly dependent on the working temperature. An attempt was made to rationalize these results by carrying out parallel X-ray photoelectron spectroscopic (X.P.S.) studies' on the system (Ni)CO,, O,/CO2,-. The actual composition of the electrode surface was defined and a correlation was found between electrode behaviour and surface status. 62 1622 RHODIUM-CARBONATE ELECTRODES In this paper a combined potentiometric-X.p.s. study was performed on the (Rh)CO,, O,/CO;- system at two significantly different temperatures (525 and 623 K) in order to obtain further information on carbonate electrodes involving transition metals.At the same time the kinetics of the corrosion process of rhodium electrodes by nitrate ions has been investigated by means of the X.P.S. technique. The choice of rhodium was also related to its technological importance, and in particular to the possibility of its use as an electrode material in fused-salt fuel cells. EXPERIMENTAL Potentiometric measurements were carried out using the apparatus previously des~ribed.~9 lo The solvent was an equimolar mixture of sodium and potassium nitrate at 525 and 623 K. Increasing carbonate concentrations were obtained by adding small dropsB of an Na,CO, solid solution in the specified solvent.The flowing gases N,, 0, and CO, (ultrapure grade) were further dried by a series of cooled molecular-sieve traps. The working electrodes were rhodium foils of area 15 x 5 mm2 and thickness 0.125 mm with a purity of 99.9% obtained from Goodfellow Metals (U.K.). Before each experiment the electrodes were scratched and soaked with distilled water. The reference electrode was an Ag+/Ag 0.07 mol kg-l half-cell. X.P.S. measurements were performed on rhodium foils ca. 1 cm2 in area maintained, for different lengths of time, under the same conditions as for the potentiometric investigation : contact with the melt at a controlled temperature and gaseous atmosphere. The samples were then cooled, rapidly washed with distilled and deoxygenated water, dried with argon and stored under an inert atmosphere during the transfer to the X.P.S.apparatus. All the spectra were obtained by using a Leybold-Heraeus LHS 10 X-ray photoelectron spectrometer, operated in both the FRR and FAT modes, using unmonochromatized Mg Ka radiation (1253.6 eV). The base pressure in the analysis chamber was better than Some samples were argon-etched to remove possible impurities and/or oxides in order to obtain a pure signal from the metal. Calibration was referred to the carbon 1s electron peak (284.6) owing to residual impurities on the sample surface. The experimental spectra, in the form of a time-dependent analogue signal, was traced using a digital plotter (Bascom-Turner).This instrument was fitted with the capability for analogue-to-digital conversion and with mass storage memory (in the form of a floppy disc), which was used to store a permanent digital record of the experimental curves. Data analysis was performed by using an %bit Commodore 8032 CBM microcomputer. The digital plotter was interfaced to the microcomputer through an RS232C standard. Details of the link have been published elsewhere." The analysis of the experimental spectra was performed by using peak position, height and width at half-maximum obtained from the signals recorded on pure Ar+-etched rhodium foils and pure bulk Rh,O, samples (J. T. Baker Chemicals, Holland) previously dehydrated by heating in oxygen at atmospheric pressure and ca. 1000 K for 2 h.mbar. RESULTS AND DISCUSSION POTENTIOMETRIC STUDIES POTENTIAL DEPENDENCE ON CARBONATE ADDITION TO A MELT FLUXED WITH A MIXTURE OF c0, AND 0, AT CONSTANT COMPOSITION A set of measurements was performed under constant partial pressure of 0, and CO, (Po, = 0.99 atm, pcoo = 0.01 atm)T and variable carbonate concentration in the range 4 x 10-5-1 x lo-, mol kg-l. Before any carbonate was added the electrodes took up stable potential values within 10-20 min. Over long periods of time (hours) the potentials showed a small drift toward more positive values. Plots of potential as a function of log [COi-] at 525 and 623 K are reported in t 1 atm = 101 325 Pa.L. SABBATINI, A. G. CAVINATO, E. DESIMONI AND P. G. ZAMBONIN 623 . . . . . . . . . . . I I I I -5.0 -4.0 -3.0 -2.0 log ([ CO$-]/mol kg-') Fig.1. Plots of potential against log [CO:-] for rhodium electrodes immersed in a (Na, K)NO, eutectic melt at constant partial pressure of CO2(O.01 atm) and 02(0.99 atm) at (a) 525 K, slope - 50 mV, and (b) 623 K, slope - 63 mV. The dashed lines (a') and (b') represent theoretical slopes (-52 and -62 mV, respectively) at the specified temperatures. The potentials are referred to an Ag+/Ag(0.07 mol kg-l) reference half-cell. fig. 1. In the range of concentration investigated the experimental curves show Nernstian slopes very close to 2.3RT/2F according to the relationship RT E = Kl -- In [COi-] 2F where the term Kl contains the constant concentrations of CO, and 0, which can be calculated from their partial pressures on the basis of the r e l e ~ a n t ~ ~ ? ~ ~ Henry's coefficients.POTENTIAL DEPENDENCE ON c0, AND 0, PARTIAL PRESSURE AT CONSTANT CARBONATE CONCENTRATION To test the dependence of the rhodium electrode potential on carbon dioxide and oxygen partial pressure, measurements were performed at constant carbonate concentration (2 x lo-* mol kg-l) under variable mixtures of the two gases whose total pressure was kept constant : Whenever the mixture composition was varied, the rhodium electrode showed a fast variation of the potential which reached a new steady-state value within 1 h. The PO,+PCO, = 1 atm- ( 2 )624 RHODIUM-CARBONATE ELECTRODES Fig. 2, Plots of potential as function of the percent composition of the flowing gas mixtue (Poz+pcOa = 1 atm) at constant carbonate concentration ([COi-] = 2 x lo-* mol kg-l) at 525 and 623 K [curves (a) and (b), respectively].0 and are experimental points; dashed lines are theoretical curves representative of eqn ( 4 4 (low temperature) and (4b) (high temperature). results of experiments performed at 525 and 623 K are reported in fig. 2. The relevant plots can be compared with the theoretical curves shown in fig. 3; curves (a)-(c) have been calculated from the Nernstian equation relevant to the overall process previously stated on the basis of voltammetric studies14 Cog- -+ CO, +40, + 2e-. (111) In particular, curve (a) is due to a process in which the electrode potential depends on the concentrations of both carbon dioxide and oxygen according to the equation RT 2F E = K , +- In [CO,] [O,]: (4 4 where K , contains a constant carbonate concentration.Curve (b) can be described RT by the relationship E = Kb+- ln[CO,] 2F (4 b) in which the potential is independent of oxygen; curve ( c ) is given by RT 2F E = K , +- In [O,$L. SABBATINI, A. G. CAVINATO, E. DESIMONI AND P. G. ZAMBONIN 625 I 0.05 V \ ( b ) - - - - - - ------ - - - - -- ----- --- ---__ '. 80 60 40 20 I I 20 40 60 80 inner scale: O2 (%) outer scale: C 0 2 (%) Fig. 3. (a)-(d) Theoretical curves representative of eqn (4a)-(4d), respectively, calculated by assigning arbitrary values to the constants K,, Kb, K, and Kd (see text). Since only the shape of the curves is of interest, the curves have been arbitrarily translated along the potential axis. where the potential is independent of CO,.Curve (d) describes a potential dependence on the oxygen concentration at the first power according to Since only the shape of the curves is important, K,, Kb, K, and Kd were chosen as abritrary values. A comparison between the theoretical and experimental curves illustrates the difference in behaviour of rhodium electrode at the two tested temperatures. At the higher temperature (623 K) the electrode is clearly ' oxygen indifferent' : the experi- mental plot fits the theoretical curve (b) expressed by eqn (4b) very well. At the lower temperature (525 K) the electrode shows a dependence on the concentrations of both carbon dioxide and oxygen, not according to eqn (4a), but rather according to eqn ( 4 4 . The small deviation found close to very low oxygen partial pressures can be attributed to small oxygen impurities (which are difficult to evaluate) present in the carbon dioxide cylinder.X-RAY PHOTOELECTRON SPECTROSCOPIC STUDIES The potentiometric results show different behaviour for the (Rh)CO,, O,/CO;- system at 525 and 623 K. Such a difference could be connected with different surface 'situations' on the rhodium electrode. In order to verify this possible correlation, a626 RHODIUM-CARBONATE ELECTRODES /I 31 2 309 306 binding energy/eV Fig. 4. Photoelectron spectrum relevant to the Rh 3 d doublet recorded on a pure metal sample after accurate surface cleaning by argon-ion sputtering. Analysis of the spectrum was performed as described in the Experimental section. systematicX.p.s. investigation was performed on different rhodium samples maintained under the same experimental conditions as in the potentiometric measurements.Before spectroscopic analysis the salt film was removed as described in the Experimental section. RHODIUM METAL A preliminary study was performed on rhodium metal foils to derive accurate parameters to be used in the fitting of more complex spectra recorded on rhodium samples. Before X.P.S. analysis the rhodium metal sample underwent argon-ion etching to remove any residual oxide and surface impurities. The most intense Rh signal is the 3d doublet (split by spin-orbit coupling) shown in fig. 4. The 3d5,, peak has a binding energy of 306.9kO.l eV and the 3d5,,-3d3,, splitting is 4.7 eV. The tailing of the peaks at higher binding energies could be due to conduction-band interaction effects.15-17 The peak intensity ratio is quite different from the theoretical value calculated on the basis of the multiplicity of levels involved in the transition: this feature has been observed previously and studied by Martensson and Nyholm.ls CHEMICAL OXIDATION OF RHODIUM METAL The X.p.spectra, recorded on samples maintained in contact with the melt for different time intervals at 525 and 623 K, are reported in fig. 5 and 6, respectively. The profile modifications indicate the appearance of an oxidized species whose signal falls at higher binding energy than the metal. The extent of oxidation increases with the metal-melt contact time. In particular, at the higher temperature (623 K) the oxidized species is already present as a small shoulder on the left-hand side of the metal peak after only 1 min contact with the melt, whereas after 65 h the oxide film is sufficiently thick to obscure the signal from the underlying metal.L.SABBATINI, A. G. CAVINATO, E. DESIMONI AND P. G. ZAMBONIN 627 Rho Rhoxide - - I I I 1 I 318 315 312 309 306 303 binding energy/eV Fig. 5. Examples of photoelectron spectra relevant to Rh 3 d region, recorded on rhodium foils maintained in contact with the melt for the specified time intervals (in min) : (a) t = 0, (b) t = 1, (c) t = 5 , (d) t = 35, (e) t = 300, cf) t = 480 and (g) t = 9000. The binding energies of the rhodium metal and rhodium oxide doublets are indicated. Melt temperature = 525 K, po, = 0.25 atm, pcol = 0.05 atm and [COi-] = 2 x mol kg-l.As far as the assignment of oxide species is concerned, the test is summarized in fig. 7. Curve (a) was recorded on pure Rh,O, powder; curve (b) illustrates a rhodium foil oxidized in fused nitrates to an extent sufficient to produce a thick oxide film [note that the binding energies of the doublet shown are coincident with those of curve (a)]; curve (c) was recorded on the same sample as (6) after argon-ion sputtering, which partly removed the surface oxide, showing the doublet from the underlying metal. The observed chemical shift between the oxide (overlayer) and the metal (substrate) is compared with that shown in curve (d) obtained from an Rh foil partly covered by628 RHODIUM-CARBONATE ELECTRODES Rho Rhoxide - I I 1 I I I I I 318 315 312 309 306 303 binding energy/eV Fig.6. Examples of photoelectron spectra relevant to Rh 3 d region, recorded on rhodium foils maintained in contact with the melt for the specified time intervals (in min): (a) t = 0, (6) f = 1, (c) t = 3, (4 = 15, (e) t = 30, cf) t = 780, (g) t = 1200 and (h) t = 3900. The binding energies of the rhodium metal and rhodium oxide doublets are indicated. Melt temperature = 523 K, po, = 0.25 atm, pcoz = 0.05 atm and [COE-] = 2 x lov4 mol kg-l. pure Rh,O, powder adhering to it. A value of 1.2kO.1 eV has been found in both cases. The experimental findings clearly indicate the presence of an Rh3+ species on the rhodium electrode and, in particular, they strongly suggest that the oxidized species produced by corrosion in molten nitrates is Rh,03. A slightly different chemical shift was previously reported in the 1iterat~re.l~ The Rh3+ signal was not attributed to Rh(NO,), species since the binding-energy value should be higher ( 3 ~ 4 , ~ peak atL.SABBATINI, A. G. CAVINATO, E. DESIMONI AND P. G. ZAMBONIN Rho - Rhoxide - 1 I I 1 I 318 315 312 309 306 303 binding energy/eV 629 Fig. 7. Photoelectron spectra relevant to the Rh 3 dregion recorded on: (a) a pure Rh,O, powder sample, (b) rhodium foil deeply oxidized in molten nitrates, (c) sample (b) after argon-ion sputtering (iArt = 4.5 mA, t = 2 min, pAr+ = 2 x atm) which partly removed the surface oxide and (d) rhodium metal foil with pure Rh,O, powder adhering to it. ca. 310.5 eV) and no N 1s peak could be detected even when a thick overlayer was produced on the electrode surface.The formation of rhodium carbonate was also excluded since no signal due to C0:- species could be detected in the C 1s region. A certain degree of hydration of the Rh203 film is likely (probably as Rh,03 - 5H20),20 because the samples had been washed with water (see Experimental section); in fact the 0 1s region always showed a peak due to hydrated species (531.5 eV) near to the peak due to 02- ions (529.6 eV). The same features have been630 RHODIUM-CARBONATE ELECTRODES found on a standard hydrated oxide. However, the presence of hydrated rhodium oxide species on the surface of the electrode when immersed in the melt can be excluded since no appreciable amount of water was contained in the molten system considered. No other rhodium oxide species was detected by X.P.S.whatever the temperature, metal-melt contact time and composition of the gaseous atmosphere. THE KINETICS OF Rh203 FILM GROWTH An attempt was made to study the kinetics of the formation of the oxide film chemically produced on rhodium foils as described in the previous section. The film thickness as a function of the metal-melt contact time was evaluated on the basis of X.P.S. quantitative analysis. The consistency of these results was proved by comparison with galvanostatic measurements reported in the literature [see for example ref. (21)]. When elastic scattering is negligible the intensity of ESCA electrons of a given energy obtained from a homogeneous material of path length x is given by22 I=(FaDK/a) 1-exp -- [ ( C Z J I where I is the intensity of photoelectrons, F the X-ray intensity, a the cross-section for photoionization in a given shell of a given atom for a given X-ray energy, D the density of the element in the material under investigation, K the spectrometer factor, o the reciprocal of the mean escape depth (A) and 8 the angle between the perpendicular to the sample and the analyser axis. In the spectrometer used 8 = 0 -+ cos 8 = 1.Then for a two-component system [e.g. metal oxide (ox) on metal (m)], the ratio of the intensities coming from the same atomic shell of the metal atom is given by2I assuming that a has the same value for the two species. The value of x in eqn (6) may be easily obtained if oox and om are known. Starting from the value of the mean escape depth A(Rh 3d512, A1 Ka) = 18 A reported in the l i t e r a t ~ r e , ~ ~ the value of A(Rh 3d5/2, Mg Ka) was calculated to be 16 A from a simple calibration procedure. Hence the escape depth of Rh20, species was obtained from the following relationship : A(Rh203 3d5I2, Mg Ka) = ~- IRhzo3 DRh A(Rh 3dSl2, Mg Ka) = 14 A where ZRh2O3 and IRh were derived from spectra recorded on pure samples.Fig. 8 shows the variation of the oxide film thickness as a function of the time of contact between the sample and the melt at the temperatures of 525 and 623 K. Curve (b) is dashed starting from the point at 15 min, because under these conditions the oxide thickness approximates the electron mean escape depth, so that eqn (6) may not be rigorous since Z, tends to zero and lox tends to the value for a sample of semi-infinite thickness.It is evident that the rate of oxidation of the first monolayer (ca. 3 %.) is very high at both temperatures, while the oxygen penetration process is considerably slower (vide in fra). Fig. 9 shows that at T = 525 K the kinetics of rhodium oxide formation follows a logarithmic law for a quite long period of time while at 623 K this behaviour is observed only at the beginning of the oxidation process, probably when the oxide thickness is lower than the electron mean escape depth. (7) IRh DRhz03L. SABBATINI, A. G . CAVINATO, E. DESIMONI AND P. G . ZAMBONIN 63 1 P 0 50 100 150 2b0 ZjO 3iO ’e time/min Fig. 8. Plots of the oxide film thickness as a function of the metal-melt time of contact at (a) 525 and (b) 623 K.The insert reports the first brace of the plots on an expanded scale. MECHANISTIC CONSIDERATIONS The potentiometric results indicate that the behaviour of rhodium electrodes is largely dependent on temperature. The following mechanisms can be proposed to rationalize the experimental data. At the higher temperature (T = 623 K) R h o + C0:- e Rho, + CO, + 2e- Rh203 + C0:- + 2Rh0, + CO, + 2e-. (IV) (V) Both the suggested potential-determining steps do not involve molecular oxygen : the electrode surface behaves as a temporary acceptor of oxygen which can be expelled in subsequent slow steps. The models are consistent with the potentiometric results that show, at high temperature, a complete ‘oxygen indifference’ of the rhodium electrode according to eqn (4b) where Kb contains the constant Cog- concentration and the activities of the surface rhodium oxide species.At the lower temperature ( T = 525 K) Rho, + COi- + Rho + 0, + CO, + 2e- Rh,03 + Coy2 s 2Rh0 + 0, + CO, + 2e-. (VI) (VW632 RHODIUM-CARBONATE ELECTRODES I l7 t 0 0 1 2 3 4 Fig. 9. Plots of the oxide film thickness as a function of the logarithm of the metal-melt time of contact at 1, 525 and a, 623 K. log (timelmin) The Nernst equation relevant to these potential-determining steps is eqn (44, which agrees with the experimental potentiometric findings. Once again Kd contains the constant concentration of C0:- ions and the activities of the rhodium oxide species. It may be assumed the CO, radicals act as oxygen extractors from the electrode surface, leading to the evolution of molecular oxygen.All the proposed mechanisms involve solid species formed on the electrode surface. The X.P.S. investigation, however, showed that the only detectable oxide is Rh,O,, which is present in noticeable amounts. This does not exclude the presence of other species such as Rho and Rho,, which cannot be detected by X.P.S. either because of their instability out of the melt or because they are present at levels under the detection limits of the technique. The prominent formation of Rh,O, on the samples is consistent with the fact that it is certainly the most stable rhodium oxide in the solid The large presence of Rh,O, could indicate reactions (V) and (VII) to be the most probable, but on the basis of the previous considerations one cannot exclude a priori the other suggested potential-determining steps involving as reactants species such as Rho and Rho,, certainly present in small amounts, if at all, but less stable and probably more reactive than the sesquioxide.With regard to the formation of rhodium oxides on the electrode surface by contactL. SABBATINI, A. G. CAVINATO, E. DESIMONI AND P. G . ZAMBONIN 633 with the melt, the most probable mechanisms must involve the strongly oxidizing solvent anions according to reactions such as the formation of the first oxide layer: 2(Rh+NO; + NO;+ Rho) (VTII) Rho + NO; + Rho, + NO, (IX) Rho + Rho, @ Rh203 (XI 2Rh + 3NO; e Rh,O, + 3NO; (overall) (XI) and the corrosion process penetration: 2(Rh0 + Rho, e Rh,O,) ~ 3Rh0, + Rh e 2Rh,03 (overall).W) (XIII) In reactions (VIII)-(X) the direct oxidation of the metal by melt is prevailing and it leads to the formation of the first oxide layer; reactions (X) and (XII) can explain why the penetration of oxygen into the bulk of the electrode is always accompanied by the prevailing formation of Rh,03. It can be seen (fig. 8) that while the oxidation of the first monolayer is a fast process at both temperatures, the rates of the corrosion-process penetration are much slower and different in the two cases. In particular fig. 9 shows that at the lower temperature the rapid (1 min) formation of the first oxide layer (ca. 3 A) is followed by the slow oxidation of internal layers (a total oxide thickness of ca. 8 A formed during one week). In contrast the corrosion-process penetration is much faster at the higher temperature, where a total oxide thickness of ca.8 A is formed in ca. 3 min (see fig. 8). The proposed mechanisms [with the exception of reaction (VI)] do not satisfy the classification proposed by Keenan et aL8 Other electrode reactions proposable at both temperatures, according to Keenan’s classification, appear insubstantial in the light of the present X.P.S. results. In conclusion, the correlation between the classification of the transition metals into two distinct classes of electrode response [see reactions (I) and (11)] and the crystal-field stabilization energies of the carbonato-metal complexes seems quite weak in fitting our present findings. We have also shown (in the present case and previously7 for nickel electrodes) that the same system falls in either class of electrode response, depending on the working temperature.In fact, the potentials depend on the stabilities of the various oxides which can be formed at the electrode/ionic-melt interface and on their ability to participate in electrochemical reactions which have appreciable rates under the conditions used. Either or both of these factors can be expected to depend on both the metal used and the temperature. Certainly useful indications in the field could be derived by a more extended use of electrochemical investigations in parallel with appropriate surface studies, in particular X.P.S., which is most useful in providing information on the oxidation state of surface species. Some of the present results were obtained by Mr T.R. I. Cataldi as part of his undergraduate studies. This work was carried out with the financial support of the Italian National Research Council (C.N.R. - Rome) and Minister0 Pubblica Istruzione .634 RHODIUM-CARBONATE ELECTRODES For a review see A. J. Arvia and N. R. DeTacconi, Thin Solid Films, 1977,43, 173. E. Desimoni, L. Sabbatini and P. G. Zambonin, J. Electroanal. Chem., 1976, 71, 73. L. Sabbatini, F. Palmisano, P. G. Zambonin and B. A. DeAngelis, Ann. Chim. (Rome), 1977,67,525. A. G. Keenan and C. G. Fernandez, J. Phys. Chem., 1974,78,2670. A. G. Keenan and T. R. Williamson, J. Phys. Chem., 1978, 82,46. B. Morelli, E. Desimoni, L. Sabbatini and P. G. Zambonin, J. Electroanal. Chem., 1978,94, 5. L. Sabbatini, B. Morelli, P. G. Zambonin and B. A. DeAngelis, J. Chem. SOC., Faraday Trans. 1,1979, 75, 2628. P. G. Zambonin, J. Electroanal. Chem., 1971, 33, 243. a A. G. Keenan and I. J. Ferrer-Vinent, J. Phys. Chem., 1979, 83, 358. lo P. G. Zambonin, Anal. Chem., 1969, 41, 868. l1 L. Fornari and G. Tessari, Computer Enhanced Spectroscopy, 1983, 1, 71. l2 F. Paniccia and P. G. Zambonin, J. Chem. SOC., Faraday Trans. I , 1972,68,2083. l3 F. Paniccia and P. G. Zambonin, J. Chem. SOC., Faraday Trans. I , 1973,69,2019. l4 P. G. Zambonin, Anal. Chem., 1972,44, 763. S. Doniach and M. Sunjic, J. Phys. Chem., 1970, 3, 285. S. K. Hufuer, G. K. Wertheim, Phys. Rev. B, 1975, 11, 678. G. K. Wertheim and D. N. E. Buchanan, Phys. Rev. B, 1975,16, 2613. la N. Martensson and R. Nyholm, Phys. Rev. B, 1981, 24, 7121. J. S. Brinen and A. Melera, J. Phys. Chem., 1972, 76, 2525. 2o J. W. M. Biesterbos and J. J. Horustia, J. Less-Common Metals, 1973, 30, 121. 21 T. Dickinson, A. F. Povey and P. M. A. Sherwood, J. Chem. SOC., Faraday Trans. 1, 1975,71, 298. 22 T. A. Carlson and G. E. McGuire, J. Electron Spectrosc., 1972/73, 1, 161. 23 C. R. Brundle, Surf. Sci., 1975, 48, 99. 24 W. P. Griffith, The Chemistry of the Rarer Platinum Metals (Interscience, London, 1977). (PAPER 4/788)

ISSN:0300-9599    

DOI:10.1039/F19858100621

出版商:RSC    

年代:1985

数据来源: RSC