摘要:

ISSN 0300-9599 JCFTAR 82 ( I 2 ) 3525-371 9 (1 986) JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions I Physical Chemistry in Condensed Phases 3525 3535 3553 3561 3569 3587 360 1 361 1 3625 3635 3647 3657 3667 368 1 3697 3709 3717 CONTENTS Fourier Transform Infrared Studies of the Irreversible Oxidation of Cyanide at Platinum Electrodes A. S. Hinman, R. A. Kydd and R. P. Cooney The Dielectric Properties of Zeolites in Variable Temperature and Humidity A. R. Haidar and A. K. Jonscher The Time-domain Response of Humid Zeolites A. K. Jonscher and A. R. Haidar Molecular-orbital Studies of C-H Bond Scission induced by Ionizing Radia- tion T. Tada Zeolites treated with Silicon Tetrachloride Vapour. Part 2.-Sorption Studies M. W. Anderson and J. Klinowski Studies of Propene Oxidation over Mixed Uranium-Antimony Oxides F.J. Farrell, T. G. Nevell and D. J. Hucknall Adsorption and Desorption Kinetics of Oxygen on Tin-Antimony Oxide Cat a1 y st by Quasi -cons tan t Coverage Met hods M-C. Bacchus-Montabonel and CO Adsorption at 77 K on KCl Films. An Infrared Investigation D. Scarano and A. Zecchina The Utilization of Time-resolved Dielectric Loss to probe the Role of the Surface in Heterogeneous Photochemistry C. J. Dobbin, A. R. McIntosh, J. R. Bolton, Z. D. Popovic and J. R. Harbour Polysilicate Equilibria in Concentrated Sodium Silicate Solutions I. L. Svensson, S. Sjoberg and L-0. Ohman Potentials of Ion-exchanged Synthetic Zeolite-Polymer Membranes M. Demertzis and N. P. Evmiridis Adsorption and Reduction of Nitrogen Monoxide by Potassium-doped Carbon T.Okuhara and K. Tanaka Physicochemical Properties and Isomerization Activity of Chlorinated Pt/Al,O, Catalysts A. Melchor, E. Garbowski, M-V. Mathieu and M. Primet Excess Pressures for Aqueous Solutions M. J. Blandamer, J. Burgess and A. W. Hakin Effect of Temperature on the Point of Zero Charge and Surface Dissociation Constants of Aqueous Suspensions of y-Al,O, K. Ch. Akratopulu, L. Vordonis and A. Lycourghiotis Thermal Decomposition of Solid Sodium Bicarbonate M. C. Ball, C. M. Snelling, A. N. Strachan and R. M. Strachan Reviews of Books J-P. Joly I I7ISSN 0300-9599 JCFTAR 82 ( I 2 ) 3525-371 9 (1 986) JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions I Physical Chemistry in Condensed Phases 3525 3535 3553 3561 3569 3587 360 1 361 1 3625 3635 3647 3657 3667 368 1 3697 3709 3717 CONTENTS Fourier Transform Infrared Studies of the Irreversible Oxidation of Cyanide at Platinum Electrodes A.S. Hinman, R. A. Kydd and R. P. Cooney The Dielectric Properties of Zeolites in Variable Temperature and Humidity A. R. Haidar and A. K. Jonscher The Time-domain Response of Humid Zeolites A. K. Jonscher and A. R. Haidar Molecular-orbital Studies of C-H Bond Scission induced by Ionizing Radia- tion T. Tada Zeolites treated with Silicon Tetrachloride Vapour. Part 2.-Sorption Studies M. W. Anderson and J. Klinowski Studies of Propene Oxidation over Mixed Uranium-Antimony Oxides F. J. Farrell, T. G. Nevell and D. J. Hucknall Adsorption and Desorption Kinetics of Oxygen on Tin-Antimony Oxide Cat a1 y st by Quasi -cons tan t Coverage Met hods M-C.Bacchus-Montabonel and CO Adsorption at 77 K on KCl Films. An Infrared Investigation D. Scarano and A. Zecchina The Utilization of Time-resolved Dielectric Loss to probe the Role of the Surface in Heterogeneous Photochemistry C. J. Dobbin, A. R. McIntosh, J. R. Bolton, Z. D. Popovic and J. R. Harbour Polysilicate Equilibria in Concentrated Sodium Silicate Solutions I. L. Svensson, S. Sjoberg and L-0. Ohman Potentials of Ion-exchanged Synthetic Zeolite-Polymer Membranes M. Demertzis and N. P. Evmiridis Adsorption and Reduction of Nitrogen Monoxide by Potassium-doped Carbon T. Okuhara and K. Tanaka Physicochemical Properties and Isomerization Activity of Chlorinated Pt/Al,O, Catalysts A. Melchor, E. Garbowski, M-V. Mathieu and M. Primet Excess Pressures for Aqueous Solutions M. J. Blandamer, J. Burgess and A. W. Hakin Effect of Temperature on the Point of Zero Charge and Surface Dissociation Constants of Aqueous Suspensions of y-Al,O, K. Ch. Akratopulu, L. Vordonis and A. Lycourghiotis Thermal Decomposition of Solid Sodium Bicarbonate M. C. Ball, C. M. Snelling, A. N. Strachan and R. M. Strachan Reviews of Books J-P. Joly I I7

ISSN:0300-9599    

DOI:10.1039/F198682FX027

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

FA RA DA Y 1RA N SA CTlO N S AND SYMPOSIA From the Royal Society of Chemistry FARADAY TRANSACTIONS II Molecular and Chemical Physics SPECIAL ISSUE - AUGUST 1986 Professor Alan Carrlngton delbred the 1985 Faraday lecture at the Royal lnsmutlon on 10th December, 1985. As a compliment to Professor Carrington, a group of his colleagues and Mends submitted original papers on the general theme of Molecular Dynamics and Spectroscopy. These papers are collected in the present Issue. CONTENTs: The Faraday Lecture: Spectroscopy of Molecular Ions at thelr Dissociation Limits A. Carrington The Spectroscopy, Photophysics and Photochemistry of Clusters of Metrazlne D. H. Levy Spectroscopy of Transient Species produced by Photodissociation or Photoionizatlon in a Supersonlc Free-jet Expansion T.A. Mlller Molecukr-beam Infrared Spectroscopy of the Ar-N,O van der Waals Molecule J. Hodge, G. D. Hayman, T. R. Dykeand B. J. Howard The Estimation of Vlbrational Predissociation Ufetimes M. S. Chlld The Infrared Spectrum of H; and its Isotopomers. A Challenge to Theory and Experlment J. Tennyson and B. T. Sutcliffe The Augmented Secular Equation Method for calculating Spectra of van der Waals Complexes. Application to the Infrared Spechum of Ar-HCI J. M. Hutson Quantum-mechanical Wavepacket Dynamics of the CH Group in Symmetric- top X,CH Compounds using Effecttve Hamiltonians from Hlgh-resolution Spectroscopy R. Marguardt, M. Quack, J. Stohner and f. Sutcllffe Internal Dynamics of Subunits and Bondlng Force Constants In Weakly Bound Dlmers P. Cope, D.J. Mlllerand A. C. fegon Internal Dynamics and HF Bond Lengthenlng In the Hydrogen-bonded Heterodimer CH,CN . . . HF determined from Nuclear Hyper-fine Structure in its Rotational Spectrum P. Cope, D. J. Miller, L. C. Willoughbyand A. C. fegon Pumping and Rshing. Double-resonance Measurements on Molecular Jets U. Veeken, N. Damand J. Reuss nme-resow Fluorescence of Jet-cooled Carbazoles and thelr Weak Complexes A. R. Auty, A. C. Jones and D. Philllps Prediction of the Ct(*P, J/CI(P,J Branching Ratio in the Photodissociation of HCI S. C. GIvertzand 6. C. Ballnt-Kurt/ Hlgh-resolution Laser Photofragment Spectroscopy of CH' P. J. Sam, J. M. Walmsleyand C. J. Whitham Asymmetric Uneshapes associated with Predissociating levels M. N. R. Ashfold, R. N. Dixon, J. D. Prince, B.Tutcherand C. M. Westem A Threshold-photoelectron Ruorescence-Photon Coincidence Study of Radlatlonless TransMons in the B ,iI State of BCN+ €. Castellucci, G. Dulardin and S. leach Cornpetthe Channels in the Interaction of Xe('P,) with CI,, Br, and I,. Atom Transfer, Excitation Transfer, Energy Disposal and Product Alignment K. Johnson, R. Pease, J. P. Simons, P. A. Smith and A. Kvaran Mco Non-RSC Momkn P14.30 ($27.70) RSC Momkn M.00 Payment should accompany ordon for lhlr Ihm. RSC Members should send their orders to: Membershlp Manager, The Royal Society of Chemistry, 30 Russell Square, London WC1 B SDT. Non-RSC Members should send their orders to: The Royal Socleiy of Chemistry, Distribution Centre, Blackhorse Road. Letchwotth, Herts SG6 1 HN. Faraday Discussions No.80 Physical Interactions and Energy Exchange af the Gas-Solid Interface [his publication discusses aspects of current research on the gas-solid Interface: elastic, inelastic and dlssipattve scattering of atoms and molecules from cfystal surfaces; the structure and dynamics of physisorbed species, including overlayers. Emphasis Is placed on the themes of physical interactions and energy exchange rather than on molecular beam technology or the phenomenology of phase tmnsmons in overlayen. The interplay between theory and experlment is stressed as they relate to the nature of atom and molecule-surface interaction potentials including many body effects. Faraday Discussions No. 80 (1986) Softcevor Prico 531 .OO ($60.00) RSC Mombon J66.25 ROYAL SOCIETY OF CHEMISTRY Information Services (xiii)FA RA DA Y 1RA N SA CTlO N S AND SYMPOSIA From the Royal Society of Chemistry FARADAY TRANSACTIONS II Molecular and Chemical Physics SPECIAL ISSUE - AUGUST 1986 Professor Alan Carrlngton delbred the 1985 Faraday lecture at the Royal lnsmutlon on 10th December, 1985.As a compliment to Professor Carrington, a group of his colleagues and Mends submitted original papers on the general theme of Molecular Dynamics and Spectroscopy. These papers are collected in the present Issue. CONTENTs: The Faraday Lecture: Spectroscopy of Molecular Ions at thelr Dissociation Limits A. Carrington The Spectroscopy, Photophysics and Photochemistry of Clusters of Metrazlne D. H. Levy Spectroscopy of Transient Species produced by Photodissociation or Photoionizatlon in a Supersonlc Free-jet Expansion T.A. Mlller Molecukr-beam Infrared Spectroscopy of the Ar-N,O van der Waals Molecule J. Hodge, G. D. Hayman, T. R. Dykeand B. J. Howard The Estimation of Vlbrational Predissociation Ufetimes M. S. Chlld The Infrared Spectrum of H; and its Isotopomers. A Challenge to Theory and Experlment J. Tennyson and B. T. Sutcliffe The Augmented Secular Equation Method for calculating Spectra of van der Waals Complexes. Application to the Infrared Spechum of Ar-HCI J. M. Hutson Quantum-mechanical Wavepacket Dynamics of the CH Group in Symmetric- top X,CH Compounds using Effecttve Hamiltonians from Hlgh-resolution Spectroscopy R. Marguardt, M. Quack, J. Stohner and f. Sutcllffe Internal Dynamics of Subunits and Bondlng Force Constants In Weakly Bound Dlmers P.Cope, D. J. Mlllerand A. C. fegon Internal Dynamics and HF Bond Lengthenlng In the Hydrogen-bonded Heterodimer CH,CN . . . HF determined from Nuclear Hyper-fine Structure in its Rotational Spectrum P. Cope, D. J. Miller, L. C. Willoughbyand A. C. fegon Pumping and Rshing. Double-resonance Measurements on Molecular Jets U. Veeken, N. Damand J. Reuss nme-resow Fluorescence of Jet-cooled Carbazoles and thelr Weak Complexes A. R. Auty, A. C. Jones and D. Philllps Prediction of the Ct(*P, J/CI(P,J Branching Ratio in the Photodissociation of HCI S. C. GIvertzand 6. C. Ballnt-Kurt/ Hlgh-resolution Laser Photofragment Spectroscopy of CH' P. J. Sam, J. M. Walmsleyand C. J. Whitham Asymmetric Uneshapes associated with Predissociating levels M.N. R. Ashfold, R. N. Dixon, J. D. Prince, B. Tutcherand C. M. Westem A Threshold-photoelectron Ruorescence-Photon Coincidence Study of Radlatlonless TransMons in the B ,iI State of BCN+ €. Castellucci, G. Dulardin and S. leach Cornpetthe Channels in the Interaction of Xe('P,) with CI,, Br, and I,. Atom Transfer, Excitation Transfer, Energy Disposal and Product Alignment K. Johnson, R. Pease, J. P. Simons, P. A. Smith and A. Kvaran Mco Non-RSC Momkn P14.30 ($27.70) RSC Momkn M.00 Payment should accompany ordon for lhlr Ihm. RSC Members should send their orders to: Membershlp Manager, The Royal Society of Chemistry, 30 Russell Square, London WC1 B SDT. Non-RSC Members should send their orders to: The Royal Socleiy of Chemistry, Distribution Centre, Blackhorse Road. Letchwotth, Herts SG6 1 HN. Faraday Discussions No. 80 Physical Interactions and Energy Exchange af the Gas-Solid Interface [his publication discusses aspects of current research on the gas-solid Interface: elastic, inelastic and dlssipattve scattering of atoms and molecules from cfystal surfaces; the structure and dynamics of physisorbed species, including overlayers. Emphasis Is placed on the themes of physical interactions and energy exchange rather than on molecular beam technology or the phenomenology of phase tmnsmons in overlayen. The interplay between theory and experlment is stressed as they relate to the nature of atom and molecule-surface interaction potentials including many body effects. Faraday Discussions No. 80 (1986) Softcevor Prico 531 .OO ($60.00) RSC Mombon J66.25 ROYAL SOCIETY OF CHEMISTRY Information Services (xiii)

ISSN:0300-9599    

DOI:10.1039/F198682BX029

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

lSSN 0300-9599 JCFTBS 82(8) 2283-2620 (I 986) 2283 230 I 231 1 2323 2333 2345 2353 2367 2377 2385 240 1 241 1 2423 243 5 2459 2473 248 1 2497 2505 JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions I Physical Chemistry in Condensed Phases CONTENTS Solvent Structural Constant and Solvation Behaviour applied to the Descrip- tion of Aqueous Electrolytes at 25-300 "C W. L. Marshall Modification of the Order of Reaction and Reaction Rate of Nucleophilic Aromatic Substitution in Micellar Solutions J. Lelikvre, 0. Haddad-Fahed and R. Gaboriaud A Quasi-elastic Neutron Scattering Study of Water-in-oil Microemulsions stabilised by Aerosol-OT. Effect of Additives including Solubilised Protein on Molecular Motions P. D. I. Fletcher, B. €3. Robinson and J. Tabony The Electrochemical Reduction of Polyacetylene with Selected Reducing Agents R.B. Kaner, S. J. Porter and A. G. MacDiarmid Complexation of Rocellin by p- and y-Cyclodextrin R. J. Clarke, J, H. Coates and S. F. Lincoln Dissolution of Cobalt Ferrites by Thioglycolic Acid M. A. Blesa, A. J. G. Maroto and P. J. Morando Acid-Base Equilibria in Polyelectrolyte Systems H. Vink Radical Cations of Organic Carbonates, Trimethyl Borate and Methyl Nitrate. A Radiation-Electron Spin Resonance Study N. S. Ganghi, D. N. Rama- krishna Rao and M. C. R. Symons The Hydrophobic Behaviour of Orange IV in Water and in Aqueous Electrolyte Solutions M. De Vijlder Polyaniline, a Novel Conducting Polymer. Morphology and Chemistry of its Oxidation and Reduction in Aqueous Electrolytes W-S. Huang, B. D. Humphrey and A.G. MacDiarmid Fourier-transform Infrared Spectroscopy of Colloidal a-, p- and y-Ferric Oxide Hydroxides T. Ishikawa, S. Nitta and S. Kondo Small-angle Neutron Scattering Studies of Microemulsions stabilised by Aerosol-OT. Part 3.-The Effect of Additives on Phase Stability and Droplet Structure A. M. Howe, C. Toprakcioglu, J. C. Dore and B. H. Robinson Correlation between Hydrodesulphurization Activity and Reducibility of Unsupported MoS,-based Catalysts promoted by Group VIII Metals S. Gobolos, Q. Wu, 0. And& F. Delannay and B. Delmon Thermodynamic Properties of Binary Alcohol-Hydrocarbon Systems A. Pettersson, P. Saris and J. 33. Rosenholm Isothermal and Non-isothermal Molecular Gas Transport in Model Non- homogeneous Porous Adsorbents J. H. Petropoulos The Dubinin-Radushkevich-Kasganer Equation J.Cort6s and P. Araya Adsorption of Organic Substances at the Mercury/Ethylene Glycol Interface. Part 2.-Aromatic Compounds J. I. Japaridze, S. S. Japaridze, N. A. Abuladze, A. De Battisti and S. Trasatti Chemical Relaxation in Mixed Micellar Solutions containing Surface-active Drugs and Hexadecyltrimethylammonium Bromide Micelles J. Gormally and S. Sharma Thermal Desorption and Infrared Studies of Primary Aliphatic Amines adsorbed on Haematite (a-Fe,O,) U. Marx, R. Sokoll and H. Hobert 76 FAR2515 2521 253 1 2547 2557 2565 2569 2577 2589 2605 261 5 Metal Particles supported by Porous Glass R. N. Edmonds, M. R. Harrison and P. P. Edwards Reactivity of Solvated Electrons in Tetrahydrofuran A. A. H. Kadhum and G.A. Salmon On the Fundamental Concepts underlying Henry-law Adsorption and Adsorbed Gas Transport in Porous Solids J. H, Petropoulos and V. I, Havredaki Thermodynamic Study of Organic Compounds in Octan-1-01. Processes of Transfer from Gas and from Dilute Aqueous Solution P. Berti, S. Cabani, G. Conti and V. Mollica Remarks on Dependence on Temperature ' at Constant Volume' P. G. Wright Special Features of Equilibrium Constants that are based on Volume Fractions P. G. Wright Infrared Study of Pyridine Adsorption on Rutile and Silica-coated Rutile Immersed in Heptane C. H. Rochester and D. G. Smith Ionic Solvation in Water-Cosolvent Mixtures. Part 12.-Free Energies of Transfer of Single Ions from Water into Water-Propan-1-01 Mixtures I. M. Sidahmed and C. F. Wells Isobaric and Isothermal Hysteresis in Metal Hydrides and Oxides T. B. Flanagan, J, D, Ciewley, T. Kuji, C-N. Park and D. H, Everett Thermodynamics of the Adsorption from Aqueous Alcohol Solutions by Graphitised Carbon (Graphon) D. H. Everett and A. J. P. Fletcher Studies of Alkylphenothiazinesulphonate Micellar Assemblies in Aqueous Solution H. Hidaka, T. Onai, M. Murata, T. Ishii and M. Gratzel

ISSN:0300-9599    

DOI:10.1039/F198682FP095

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

JOURNAL OF THE CHEMICAL SOCIETY 1089 1107 1123 1137 1143 1151 1163 1173 1189 1197 1207 1219 1231 1243 1257 Faraday Transactions 11, Issue 8, I986 Molecular and Chemical Physics Professor Alan Carrington delivered the 1985 Faraday Lecture at the Royal Institution on 10 December 1985. As a compliment to Professor Carrington, a group of his colleagues and friends submitted original papers on the general theme of Molecular Dynamics and Spectroscopy. These papers are collected in Issue 8 of Faraday Transactions 11, whose contents list is reproduced below. The Faraday Lecture : Spectroscopy of Molecular Ions at their Dissociation Limits A. Carrington The Spectroscopy, Photophysics and Photochemistry of Clusters of Tetrazine D. H. Levy Spectroscopy of Transient Species produced by Photodissociation or Photoion- ization in a Supersonic Free-jet Expansion T.A. Miller Molecular-beam Infrared Spectroscopy of the Ar-N,O van der Waals Molecule J. Hodge, G. D. Hayman, T. R. Dyke and B. J. Howard The Estimation of Vibrational Predissociation Lifetimes M. S. Child The Infrared Spectrum of HZ and its Isotopomers. A Challenge to Theory and Experiment J. Tennyson and B. T. Sutcliffe The Augmented Secular Equation Method for calculating Spectra of van der Waals Complexes. Application to the Infrared Spectrum of Ar-HCl J. M. Hutson Quantum-mechanical Wavepacket Dynamics of the CH Group in Symmetric-top X,CH Compounds using Effective Hamiltonians from High-resolution Spectro- scopy R. Marquardt, M. Quack, J. Stohner and E. Sutcliffe Internal Dynamics of Subunits and Bonding Force Constants in Weakly Bound Dimers P.Cope, D. J. Miller and A. C. Legon Internal Dynamics and HF Bond Lengthening in the Hydrogen-bonded Heter- odimer CH,CN - - - HF determined from Nuclear Hyperfine Structure in its Rotational Spectrum P. Cope, D. J. Millen, L. C. Willoughby and A. C. Legon Pumping and Fishing. Double-resonance Measurements on Molecular Jets K. Veeken, N. Dam and J. Reuss Time-resolved Fluorescence of Jet-cooled Carbazoles and their Weak Com- plexes A. R. Auty, A. C. Jones and D. Phillips Prediction of the Cl(2P3/2)/Cl(2P3/2) Branching Ratio in the Photodissociation of HCl S. C. Givertz and G. C. Balint-Kurti High-resolution Laser Photofragment Spectroscopy of CH+ P. J. Sarre, J. M. Walmsley and C. J. Whitham Asymmetric Lineshapes associated with Predissociating Levels M.N. R. Ashfold, R. N. Dixon, J. D. Prince, B. Tutcher and C. M. Western127 1 A Threshold-photoelectron Fluorescence-Photon Coincidence Study of Radia- tionless Transitions in the 211 State of BrCN+ E. Castellucci, G. Dujardin and S. Leach Competitive Channels in the Interaction of Xe(3P2) with Cl,, Br, and I,. Atom Transfer, Excitation Transfer, Energy Disposal and Product Alignment K. Johnson, R. Pease, J. P. Simons, P. A. Smith and A. Kvaran 1281 The following papers were accepted for publication in J . Chem. SOC., Faraday Trans. I during May 1986. 51 1829 512123 512160 51226 1 512262 61 126 61260 61328 61370 61405 61427 61434 6/45 I 61479 61480 6/550 61585 Viscosity of Na,SO, and Solutions in Ethanol-Water Mixtures at 15, 25 and 25 "C The Hydrogen Evolution Reaction under Mixed Kinetic Control A.Saraby-Reintjes The Electrical Conductance of Molten Lead@) 9,lO-Dihydroxyoctadecanoate and some Binary Mixtures with Lead(I1) Octadecanoate M. S. Akanni and P. C. Mbaneme Local Structure of Nickel Oxide Growth at High Temperatures in Ceramic Electrolyte Cells M. Tomellini, D. Gozzi, A. Bianconi and I. Davoli Zeolites treated with Silicon Tetrachloride Vapour. Part 2.-Sorption Studies M. W. Anderson and J. Klinowski Nature of Oxide-supported Cupric Ions and Copper derived from Cupric Chloride P. A. Sermon, K. Rollins, P. N. Reyes, S. Lawrence and M. A. M. Luengo Metal-Organic Chemical Vapour Deposition (MOVCD) of Compound Semi- conductors. Part 2.-Preparation of ZnSe Expitaxial Layers on (100) Orientated GaAs Single Crystalline Substrates G.Fan and J. 0. Williams An Electron Nuclear Double Resonance Study in Glassy Matrix of Nitroxide Radicals with Delocalized Spin Density M. Brustolon, A. L. Maniero, U. Segre and L. Greci The Identification and Characterisation of Mixed Oxidation States at Oxidised Titanium Surfaces A. F. Carley, P. R. Chalker, J. C. Riviere and M. W. Roberts Radiation Damage in Phosphated Sugar: An E.S.R. Study of Phosphorus Centred Radicals Trapped in an X-Irradiated Single Crystal of a Phenoxy- phosphoryl Zylofuranose Derivative A. Celalyan-Berthier, T. Berclaz and M. Geoffroy Exchanges of Oxygen Isotopes between Carbon Dioxide and Ion-exchanged Zeolites A T. Takaishi and A. Endoh Solvent Exchange Reactions of Metal Ions.Diagnosis of Mechanisms in Terms of the Bond Order of the Activated Complexes S. J. Formosinho Aspects of Temperature-programmed Analysis of Some Gas-Solid Reactions. Part 2.-Hydrogen Temperature-programmed Desorption from Silica- supported Platinum M. s. W. Vong and P. A. Sermon N.M.R. Study of 129Xe Adsorbed on Alkali and Alkaline-Earth Y Zeolites. Influence of the Chemical Shift T. Ito and J. Fraissard CH Bond Activation and Radical-Surface Reactions for Propylene and Methane over a-Bi,O, A lH and 13C N.M.R. Study of the Conformation of Aerosol OT in Water and Hydrocarbon Solutions F. Heatley Catalytic Properties of Synthetic Faujastites Modified with Fluoride Anions K. A. Becker and S. Kowalak C. Quintana, C. Moran, M. Sanchez and A.Vivo S. P. Mehandru, A. B. Anderson and J. F. Brazdil (ii)61615 6/68 1 61756 61822 61823 61824 61825 61826 61849 61850 61851 61852 61853 61854 61998 61999 Acrylonitrile Polymerization from Aqueous Solution. The Role of Particle Area E. Elbing, S. J. McCarthy, B. A. W. Coller and I. R. Wilson Influence of Lithium on Reduction, Dispersion and Hydrogenation Activity of Nickel on Alumina Catalysts S. Narayanan and K. Uma Smectite Molecular Sieves. Part 2.-Expanded Flurohectorite Sorbents R. M. Barrer and R. J. B. Craven Formation of Superoxide during the Autoxidation of Anthralin (1,s- Dihydroxyl-9-anthrone) J. M. Bruce, A. J. F. Dodd and C. W. Kerr Two-dimensional Transient Electron Spin Resonance Spectroscopy K. A. McLauchlan and D. G. Stevens Paramagnetic Adducts in the Reaction of 4-Substituted Pyridines and Phosphorus-centred Radicals A.AIberti, A. Hudson and G. F. Pedulli Direct Observation of 1,4-Hydrogen Shift in Vinyl Radicals derived from the Reaction of Alkynes with Thiyl Radicals B. C. Gilbert, D. J. Parry and L. Grossi Reactions of Ozonate and Superoxide Radical Anions A. R. Forrester and V. Purushotham Electron Nuclear Double Resonance of S = 112 Defects in a Single Crystal of a Morpholinium-TCNQ 1 : 1 Complex A. L. Maniero, 0. Priolisi and C. Corvaja Selective Formation and Conformational Analysis of Carbohydrate Derived Radicals R. Sustamann and H.-G. Korth Linewidth Alternation, as a Result of Intramolecular Cation Migration, in the E.S.R. Spectrum of the 1,4-Naphthoquinone Radical Anion N. J. Flint and B.J. Tabner E.S.R. and Saturation Transfer E.S.R. of Virus and Membrane Systems M. A. Hemminga Electron Spin Resonance, ENDOR and TRIPLE Resonance of Some 9,lO- Anthraquinone and 9,lO-Anthraquinol Radicals in Solution M. Vuolle and R. Makela The Role of Solvent Reorganisation Dynamics in Homogeneous Electron Self-exchange Reactions The Use of E.S.R., ENDOR and TRIPLE Resonance Methods for Structural Elucidations. Isomeric 10,19-Diphenylphenanthrones B. J. Herold, M. J. Romao, J. M. A. Empis, J. C. Evans and C. C. Rowlands A Single-crystal ENDOR Study of y-Irradiated Pyridoxine Hydrochloride N. M. Atherton and W. A. Crossland G. Grampp, W. Harrer and W. Jaenicke 61 1000 Magnetic Resonance of Ultrafast Chemical Reactions. Examples from Photo- synthesis J. R. Norris, C.P. Lin and D. E. Budil 611001 An E.S.R. Study of the Radicals formed on U.V. Irradiation of the Photo- allergens Fentichlor and Bithionol J. N. Delahanty, J. C. Evans, C. C. Row- lands and M. D. Barratt 61 1002 Electron Paramagnetic Resonance Spectroscopy as an Analytical Tool V. Axelsen and J. A. Pedersen 611003 H.P.L.C.1E.S.R. Analysis of Sugars irradiated in the Solid and Liquid Phase J. J. Raffi, P. B. Vincent, J.-P. L. Angel, C. M. Battesti and C. L. Thiery 611004 Application of Electron Spin Resonance Spectroscopy to the Study of the Effects of Ionizing Radiation on DNA and DNA Complexes M. C. R. Symons 61 1005 Generation of Radicals from Antioxidant-type Molecules by Polyunsaturated Lipids P. Lambelet, F. Ducret, F. Saucy, M-C. Savoy and J. Lollinger (iii)Cumulative Author Index 1986 Abu-Gharib, E.-E.A., 1471 Abuladze, N. A., 2481 Adams, D. M., 1020 Adams, M., 1979 Aida, M., 1619 Aika, K-i., 2269 Al-Hakim, M., 1575 Albery, W. J., 1033 Allen, G. C., 1367 Alwis, U. de, 1265 Ammann, D., 1179 Anderson, J. A., 1911 Anderson, M. W., 569, 1449 Andersson, S. L. T., 1537 Andersson, T., 767 AndrC, O., 2423 Antoniou, A. A., 483 Araya, P., 1351, 2473 Attwood, D., 1903 Avent, A. G., 1589 Aveyard, R., 125, 1031, 1755 Baldwin, R. R., 89 Balk, R. W., 933 Barone, G., 2089 Bartlett, J. R., 597 Bartlett, P. N., 1033 Battisti, A. De, 2481 Baur, J., 1081 Becker, K. A., 2151 Belton, P. S., 451 Benecke, J. I., 1945 Bennett, C. O., 2155 Berezin, I. V., 319 Bernstein, T., 1879 Berry, F. J., 1023 Berti, P., 2547 Bhattacharyya, S.N., 2103 Bieth, H., 1935 Binks, B. P., 125, 1031, 1755 Biswas, P. K., 1973 Blake, P. G., 723 Blandamer, M. J., 1022, 1471 Blesa, M. A., 2345 Bloemendal, M., 53 Bloor, D., 21 11 Boelhouwer, C., 1945 Bond, G. C., 1985 Booth, B. L., 2007 Booth, C., 1865 Boucher, E. A., 1589 Bozonnet-Frenot, M-P., 2185 Brereton, I. M., 1999 Brett, C. M. A., 1071 Brigandi, P. W., 1032 Brillas, E., 495, 1781 Bruckenstein, S., 1105 Buck, R. P., 1169 Bui, V. T., 899 Burch, R., 1985 Burgess, J., 1471 B. Nagy, O., 1789 Cabani, S., 2547 Cameron, P., 1389 Canet, D., 2185 Carley, A. F., 723 Carpenter, T. A., 545 Casal, B., 1597 Cass, A. E. G., 1033 Castro, V. Di, 723 Castronuovo, G., 2089 Cenens, J., 281 Cesteros, L. C., 1321 Champion, J. V., 439 Chang, C. D., 1032 Chiou, C. T., 243 Chitale, S.M., 663 Chung, J. S., 2155 Clark, B., 1471 Clark, S., 125 Clarke, R. J., 2333 Clewley, J. D., 2589 Clifford, A. A., 2235 Coates, J. H., 2123, 2333 Cochran, S. J., 1721 Cohen de Lara, E., 365 Coller, B. A. W., 943 Compostizo, A., 1839 Conti, G., 2547 Cooney, R. P., 597 Copperthwaite, R. G., 1007 Cortks, J., 2473 Cortes, J., 1351 Corti, H. R., 921 Covington, A. K., 1209 Craston, D. H., 1033 Craven, J. R., 1865 Crespo Colin, A., 1839 Crilly, J. F., 439 Crudden, J., 2195, 2207 Danil de Namor, A. F., 349 Das, M. N., 1973 Dawber, J. G., 119 De Schrijver, F. C., 281 Dean, C. E., 89 Dearden, S. J., 1627 Del Vecchio, P., 2089 Delaney, G. M., 2195, 2207 Delannay, F., 2423 Delaval, Y., 365 Delmon, B., 2423 Dharmalingam, P., 359 Dias Peiia, M., 1839 Domen, K., 2269 DomCnech, J., 1781 Dore, J.C., 2411 Duatti, A., 1429 Duce, P. P., 1471 Eagland, D., 2008 Ebeid, E-Z. M., 909 Edmonds, R. N., 2515 Edwards, P. P., 2515 Egdell, R. G., 2003 Ekechukwu, A. D., 1965 El-Daly, S. A., 909 Elbing, The Late E., 943 Elia, V., 2089 Elworthy, P. H., 1903 Espenscheid, M. W., 1051 Espinosa-JimCnez, M., 329 Evans, D. F., 1829 Everett, D. H., 2589, 2605 Ewen, R. J., 1127 Farnia, G., 1885 Feakins, D., 563, 2195, 2207 Fegan, S. G., 785, 801 Fernandez-Prini, R., 921 Findenegg, G. H., 2001 Fink, P., 1879 Fisher, D. T., 119 Flanagan, T. B., 2175, 2589 Fletcher, A. J. P., 2605 Fletcher, P. D. I., 231 1 Folman, M., 2025 Foulds, N. C., 1259 Fraser, I. M., 607 Freiser, H., 1217 Freund, P. L., 2277 Fricke, R., 263, 273 Fukuda, H., 1561 Funabiki, T., 35, 707, 1771 Fyles, T. M., 617 Gaboriaud, R., 2301 Gabrys, B., 1923, 1929 Ganghi, N.S., 2367 Garbassi, F., 2043 Garbowski, E., 1893 Garrido, J. A., 1781 Gellan, A., 953 Geoffroy, M., 521 Gervasini, A., 1795 Ghatak-Roy, A. R., 1051 Ghoneim, M. M., 909 Ghousseini, L., 349 Gilbert, R. G., 1979, 2247 Gilhooley, K., 431 Gobolos, S., 2423 G6mez-EstCvez, J. L., 2167 Gonzalez-Caballero, F., 329AUTHOR INDEX Gonzalez-Elipe, A. R., 739 Gonzilez-Fernandez, C. F., 329 Gormally, J., 157, 2497 Gorton, L., 1245 Gosal, N., 1471 Green, M. J., 1237 Grieser, F., 1813, 1829 Gritzner, G., 1955 Grzybkowski, W., 1381, 1703, Guardado, P., 1471 Haddad-Fahed, O., 2301 Haggett, B. G. D., 1033 Hakin, A. W., 1471 Hall, D., 2111 Halle, B., 401, 415 HansCn, O., 77 Harrison, M. R., 2515 Havredaki, V.I., 2531 Heatley, F., 255 Hedges, W. M., 179 Hellring, S. D., 1032 Hemfrey, J. P., 1589 Hersey, A., 1271 Hewitt, E. A., 869 Hey, M. J., 1805 HeyrovskL, M., 585 Hidaka, H., 2615 Higgins, J. S., 1923, 1929, 2004 Higson, S., 157 Hill, C. A. S., 1127 Hill, H. A. O., 1237 Hill, T., 349 Hitchman, M. L., 1223 Hobert, H., 1527, 2505 Hobson, D. B., 869 Homer, J., 533 Honeybourne, C. L., 1127 Honeyman, M. R., 89 Hooper, A., 11 17 Houghton, J. D., 1127 Howe, A. M., 241 1 Hronec, M., 1405 Hsu, W. P., 851 Huang, W-S., 2385 Hubbard, C. D., 1471 Humphrey, B. D., 2385 Humphreys, F. J., 1020, 2006 Hunt, D. J., 189 Hussian, S. M., 2221 Hutchings, G. J., 1007 Ige, J., 2011 Iizuka, T., 1681, 61 Ikeda, H., 61 Ikeda, O., 1561 Indelli, A., 1429 Inomata, S., 1733 Inoue, M., 2175 Inoue, T., 168 1 Ishii, T., 2615 Ishikawa, T., 2401 Issa, R.M., 909 Iwamoto, M., 1713 Jackson, S. D., 189, 431 Jaeger, N., 205 1745 Japaridze, J. I., 2481 Japaridze, S. S., 2481 Jayasuriya, D. S., 457,473 Jensen, M., 1351 Johns, A. I., 2235 Johnson, D. C., 1081 Johnson, J., 1081 Johnston, P., 1007 Jones, W., 545 Jonson, B., 767 Jose, C. I., 663, 681, 691 Kadhum, A. A. H., 2521 Kakuta, N., 1553 Kamat, P. V., 1031 Kaminade, T., 707 Kaner, R. B., 2323 Katime, I., 1321, 1333 Kavetskaya, 0. I., 319 Kawaguchi, T., 1441 Kawai, S., 527 Kawai, T., 527 Kazusaka, A., 1553 Kelly, H. C., 1271 Kelly, R. G., 1195 Kevan, L., 213 Khoo, K. H., 1 Kido, K., 2269 Kinoshita, N., 2269 Kishimoto, S., 2175 Kleine, A., 205 Klinowski, J., 569, 1449 KodejS, Z., 1853 Komatsu, T., 1713 Komiyama, M., 1713 Kondo, S., 2401 Kondo, Y., 2141 Koreeda, A., 527 Koresh, J.E., 2057 Kowalak, S., 2151 Kremer, M. L., 2133 Kuji, T., 2589 Kusabayashi, S., 2141 Kuzuya, M., 1441 Lancz, M., 883 Lang, J., 109 Langevin, D., 2001 Larkins, F. P., 1721 Larsson, R., 767 Lawless, T. A., 1031 Lawrence, K. G., 563, 2195, Lawrence, M. J., 1903 Leaist, D. G., 247 Lelikvre, J., 2301 LConard, J., 899 Lim, T.-K., 69 Lincoln, S. F., 1999, 2123, 2333 Llars, S., 767 Llinares, A., 521 Lockhart, J. C., 1161 Loewenschuss, A., 993 Logan, S. R., 161 Lomen, C. E., 1265 Lowe, B. M., 785, 801 Lowe, C. R., 1259 2207 (v) Lundin, S. T., 767 Mactaggart, J. W., 1805 MacCallum, J. R., 607 MacDiarmid, A. G., 2323, 2385 Mahnke, R., 1413 Malliaris, A., 109 Mandal, P. C., 2103 Manes, M., 243 Marabini, A.M., 2043 Maran, F., 1885 Marchal, J-P., 2185 Marcus, Y., 233, 993 Marczewski, M., 1687 Maroto, A. J. G., 2345 Marshall, W. L., 2283 Martin, C. R., 1051 Maruthamuthu, P., 359 Marx, U., 2505 Mastikhin, V. M., 1879 Mathieu, M-V., 1893 Matsuda, T., 1357 McCarthy, S., 943 Mead, J., 125, 1031, 1755 Melchor, A., 1893 Miale, J. N., 1032 Miasik, J. J., 11 17 Midgley, D., 1187 Minami, Z., 1357 Mishima, S., 1307 Mishra, S. P., 521 Miura, H., 1357 Miyake, Y., 1515 Miyamoto, A., 13 Mobbs, R. H., 1865 Mol, J. C., 1945 Mollett, C. C., 1589 Mollica, V., 2547 Molyneux, P., 291, 635 Moore 111, R. B., 1051 Morando, P. J., 2345 Morazzoni, F., 1795 Morgan, H., 143 Mori, K., 13 Moyes, R. B., 189 Mulla, S. T., 681, 691 Murakami, Y., 13 Murata, M., 2615 Muscetta, M., 2089 Nagano, S., 1357 Najbar, M., 1673 Nakajima, T., 1307 Nakamatsu, H., 527 Nakanishi, M., 1441 Nakano, A., 2141 Napper, D.H., 1979, 2247 Narayana, M., 213 Neto, M. M. P. M., 1071 Neuburger, G. G., 1081 Nikitas, P., 977 Nitta, S., 2401 Nyasulu, F. W. M., 1223 Oakes, J., 2079 Oesch, U., 1179 Ogino, Y., 1713 Ohlmann, G., 263, 273AUTHOR INDEX Okazaki, S., 61 Okuda, T., 1441 Oldham, K. B., 1099 Onai, T., 26 15 Onishi, T., 2269 Ooe, M., 35 Opallo, M., 339 Orchard, S. W., 1007 Oref, I., 1289 Ortiz, A., 495 Owen, A. E., 1195 O’Reilly, P. J., 2195, 2207 Parbhoo, B., 1789 Park, C-N., 2589 Parsons, B. J., 1575 Pease, W. R., 747, 759 Peeters, G., 963 Peeters, S., 963 Penboss, I. A., 2247 Penner, R. M., 1051 Perry, M. C . , 533 Pethig, R., 143 Petropoulos, J. H., 2459, 2531 Pettersson, A., 2435 Pham, H.V., 1179 Phillips, G. O., 1575 Piculell, L., 387, 401, 415 Piekarska, A., 513 Piekarski, H., 513 Pilarczyk, M., 1703, 1745 Pinna, F., 1795 Pletcher, D., 179 Polta, J. A., 1081 Polta, T. Z., 1081 Porter, S. J., 2323 Pouchly, J., 1605 Primet, M., 1893 Puchalska, D., 1381 Quintana, J. R., 1333 Radulovic, S., 1471 Rajaram, R. R., 1985 Ramakrishna Rao, D. N., 2367 Ramdas, S., 545 Rashid, S., 2235 Rebenstorf, B., 767 Richardson, P. J., 869 Rideout, J., 167 Rigby, S., 431 Rizkallah, P. J., 1589 Roberts, M. W., 723 Robinson, B. H., 1271, 2311, Robinson, P. J., 869 Rochester, C. H., 953, 1805, Rodriguez, R. M., 1781 Rooney, J. J., 2005 Rosenholm, J. B., 77, 2435 Rouw, A. C., 53 Rubio, R. G., 1839 Ruiz-Hitzky, E., 1597 Ryder, P. L., 205 Sacchetto, G .A., 1853 Saez, C., 1839 Saleh, J. M., 2221 241 1 1911, 2569 Salmon, G. A., 161,2521 Sanchez, F., 1471 Sandona, G., 1885 Sangster, D. F., 1979 Saris, P., 2435 Sarkany, A., 103 Sawada, K., 1733 Scharpf, O., 1923, 1929 Schiller, R. L., 2123 Schlosserova, J., 1405 Schmelzer, J., 1413, 1421 Schmitt, K. D., 1032 Schoonheydt, R. A., 281 Scott, R. P., 1389 Segall, R. L., 747, 759 Seloudoux, R., 365 Sen& M., 2065 Sharma, S., 2497 Shibata, Y., 1357 Shigeto, M., 1515 Shindo, H., 45 Shubin, A. A., 1879 Sidahmed, I. M., 2577 Siiman, O., 851 Simmons, R. F., 1965 Simon, W., 1179 Sircar, S., 831, 843 Smallridge, M. J., 1589 Smart, R. St C., 747, 759 Smith, D. G., 2569 Smith, I., 869 Smith, J. A. S., 2004 Snowdon, S., 943 Soffer, A., 2057 Sokoll, R., 1527, 2505 Solymosi, F., 883 Somsen, G., 53, 933 Soria, J., 739 Soriyan, O., 2011 Spiess, B., 1935 Spiro, M., 2277 Spotswood, T.M., 1999 Strazielle, C., 1321 Strukul, G., 1795 Strumolo, D., 1795 Sugiyama, K., 1357 Suppan, P., 509 Sutherland, I. O., 1145 Suzuki, T., 1733 Swallow, A. J., 1575 Symanski, J. S., 1105 Symons, M. C. R., 167, 2367 Szentirmay, M. N., 1051 Tabony, J., 231 1 Tamura, H., 1561 Tamura, K., 1619 Tanaka, T., 35 Tanaka, Y., 2065 Tang, A. P-C., 1081 Taniewska-Osinska, S., 1299 Taniewska-Osinska, S., 5 13 Tatam, R. P., 439 Tawarah, K., 21 11 Schulz-Ekloff, G., 205 Tear, S. P., 1022 Tennakoon, D. T. B., 545 Teramoto, M., 1515 Thijs, A., 963 Thomas, J. D. R., 1135 Thomas, J. M., 545 Thomson, A. J., 2009 Tofield, B. C., 11 17 Toprakcioglu, C . , 241 1 Townsend, R. P., 1019 Trasatti, S., 2481 Turner, P.S., 747, 759 Tyler, J. W., 1367 van de Ven, T. G. M., 457,473 Vansant, E. F., 963 Vekavakayanondha, S., 291,635 Venkatasubramanian, L., 359 Verhaert, I., 963 Vesely, V., 1405 Vijlder, M. De, 2377 Vink, H., 2353 Volkov, A. I., 815 Vonk, D., 1945 Waghorne, W. E., 563, 2195, Walker, R. W., 89 Wallwork, S. C., 1589 Walton, A. J., 1023 Wang, Z-C., 375 Warhurst, P. R., 119 Warr, G. G., 1813, 1829 Watson, J. T. R., 2235 Watts, P., 1389 Weale, K. E., 1020, 2002 Weiss, E., 2025 Wells, C . F., 2577 Wells, P. B., 189 Whalley, P. D., 1209 Whyman, R., 189 Wiens, B., 247 Wilson, G. S., 1265 Wilson, I. R., 943 Wbjcik, D., 1381 Woinicka, J., 1299 Wren, B. W., 167 Wright, K. M., 451 Wright, P. G., 2557, 2565 Wu, E. L., 1032 Wu, Q., 2423 Wuthier, U., 1179 Wyn-Jones, E., 21 11 Wysocki, S., 715 Xiaoding, X., 1945 Yamashita, H., 707, 1771 Yamazaki, A., 1553 Yatsimirsky, A.K., 319 Yeates, S. G., 1865 Yeo, I-H., 1081 Yoshida, N., 2175 Yoshida, S., 35, 707, 1771 Yoshikawa, M., 707, 1771 Zana, R., 109 Zanderighi, L., 1795 Zund, R., 1179 2207THE 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.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 SYMPOSIUM NO. 21 Promotion in HeterogeneousCatalysis University of Bath, 23-26 September 1986 Organising Committee : Professor F. S. Stone (Chairman) Dr R. Burch Mrs Y. A. Fish Dr R. W. Joyner Professor J. Pritchard Dr D. A. Young (Editor) The symposium will form the Faraday Division Programme at the 1986 Autumn meeting of the Royal Society of Chemistry, however, it will be conducted as a discussion meeting, with pre-printed papers and subsequent publication, following the style of the traditional Faraday discussions and symposia.The role of promoters is of intrinsic interest as well as being important for many industrial processes. Promoters are used for three purposes, to improve catalyst activity, to increase selectivity for the desired reaction, and to prolong catalyst life at high activity and selectivity. There are current advances in both exprimental and theoretical aspects of promoter action, making this an opportune time for a Faraday symposium. Attention will be focussed on the role of promoters in enhancing activity and selectivity. Three areas will be highlighted - model studies using well-defined surfaces such as single crystals, characterization of promoter function in real catalysts, and theoretical aspects of promotion.The mechanisms of promoter action in metal, oxide and sulphide catalysts will be d i sc u ssed . The programme and application form may be obtained from : MrsY. A. Fish,The Royal Society of Chemistry, Burlington House, London W1 VOBN. (vii)THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM NO. 22 Interaction-induced Spectra in Dense Fluids and Disordered Solids University of Cambridge, 1 &11 December 1986 Organising Committee : Professor A. D. Buckingham (Chairman) Dr R. M. Lynden-Bell Dr P. A. Madden Professor E. W. J. Mitchell Dr J. Yarwood Or D. A, Young Mrs Y. A. Fish Whilst interaction-induced spectra have been studied in the gas phase for many years, their importance in the spectroscopy of condensed matter has been appreciated only relatively recently.At present a considerable number of studies of induced spectra are taking place in what are (nominally) widely separated fields of study. It is highly desirable to bring these communities together so that common issues can be identified and the progress of one field appreciated in another. 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. 83 Brownian Motion University of Cambridge, 7-9 April 1987 Organising Committee Dr M. La1 (Chairman) Dr R. Ball Dr E. Dickinson Dr J. S. Higgins Dr P.N. Pusey Dr 0. A. Young Mrs Y. A. Fish The aim of the meeting is to discuss new developments in the experimental and theoretical studies of Brownian motion of colloidal particles and macromolecules, with particular emphasis on the dynamics of aggregate formation and breakdown, computer simulation and many-body hydrodynamic interactions. Further information may be obtained from: Or M. Lal, Unilever Research, Port Sunlight Laboratory, Bebington, Wirral L63 3JW (viii)THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 84 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 1986 to ~ Dynamics of Elementary Gas-phase React ions University of Birmingham, 1&16 September 1987 Professor R.Grice, Chemistry Department, University of Manchester, ~ Manchester M13 9PL Organising Committee: Professor R. Grice (Chairman) Dr M. S. Child Dr J. N. L. Connor Dr M. J. Pilling Professor I. W. M. Smith Professor J. P. Simons The Discussion will focus on the development of experimental and theoretical approaches to the detailed description of elementary gas-phase reaction dynamics. Studies of reactions at high collision energy, state-to-state kinetics, non-adiabatic processes and thermal energy reactions will be included. Emphasis will be placed on systems exhibiting kinetic and dynamical behaviour which can be related to the structure of the reaction potential-energy surface or surfaces.JOURNAL OF CHEMICAL RESEARCH Papers dealing with physical chemistry/chemical physics which have appeared recently in J.Chem.Research, The Royal Society of Chemistry’s synopsis+microform journal, include the following : Electron Spin Resonance Study of Anatase-supported Vanadia-Molybdena Catalysts Guido Busca and Leonard0 Marchetti (1986, Issue 5) A Comparison of Some Linear Substituent-free-energy Relationships Martien C.Spanjer and C. Leo de Ligny (1986, lssue 5) Interception of the Electron-transport Chain in Bacteria with Hydrophilic Redox Mediators. Part 1. Selective Improvement of the Performance of Biofuel Cells with 2,6-Disulphonated Thionine as Mediator Anna M.Lithgow, Lorraine Romero, lvelisse C. Sanchez, Fernando A. Suoto and Carmen A. Vega (1 986, Issue 5) Ionic Strength Dependence of Complex-formation Enthalpies: a Literature Data Analysis Alessandro de Robertis, Concetta de Stefano, Carmelo Rigano and Silvio Sammartano (1 986, Issue 5) Phenanthrene Hydroconversion over Nickel and Molybdenum Sulphides Supported on Alumina: Effect of the Sulphidation Method Jean-Louis Lemberton and Michel Guisnet (1986, Issue 6) Deposition of Platinum onto CdS Aqueous Supensions under Ultraviolet Illumination Javier Domenech, John Curran, Nicole Jaffrezic-Renault and Robert Philippe (1 986, Issue 6) A Prototype Model for Artificial Photosynthatic Membranes: Water-swollen Chelate Filter Paper with Adsorbed Tris(2,2’-bipyridine)ruthenium(2+) and Methyl Viologen Yoshimi Kurimura, Noriko Matsuo, Etsuko Kokuta, Yasuyuki Takagi and Yoshiharu Usui (1 986, Issue 7) ln situ Electrochemical Electron Spin Resonance Spectrometry: the Anodic Oxidation of Triphenylmethanol Richard G. Compton, Barry A.Coles and Michael J. Day (1 986, Issue 7) Racemization of Peptides. An MNDO Study of the c-(Gly-Gly) Anion Miguel Pons, Josep M. Bofill and Ernest Giralt (1 986, Issue 7) FARADAY DIVISION INFORMAL AND GROUP MEETINGS 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 Colloid and interface Science Group Surf acta nt Systems with Liq u id-Liq u id Interfaces To be held at the University of Hull on 9-1 0 September 1986 Further information from Dr R.Aveyard, Department of Chemistry, The University, Hull HU6 7RX Statistical Mechanics and Thermodynamics Group Fractals in Physics and Chemistry To be held at the University of Salford on 10-1 2 September 1986 Further information from Dr P. Francis, Department of Chemistry, The University, Hull HU6 7RXCarbon Group Carbon Fibres-Properties and Applications To be held at the University of Salford on 15-1 7 September 1986 Further information from The Meetings Officer, The Institute of Physics, 47 Belgrave Square, London SW1X 8QX Electrochemistry Group with the Electroanalytical Group New Electrode Materials for Electrochemistry and Electroanalytical Applications To be held at Imperial College, London on 15-1 7 September 1986 Further information from Professor W.J. Albery, Department of Chemistry, Imperial College, London SW7 2AZ Neutron Scattering Group Neutron Scattering Summer School To be held at the Rutherford Appleton Laboratory, Chilton on 1 5 2 5 September 1986 Further information from Dr R. J. Newport, Physics Laboratory, The University, Canterbury, Kent CT2 7NR Theoretical Chemistry Group Lennard-Jones Lecture by Professor A. D. Buckingham To be held at the University of Exeter on 17 September 1986 Further information from Dr G. Doggett, Department of Chemistry, University of York, York YO1 5DD ~ ~~ ~~~ Neutron Scattering Group Waddington Memorial Lecture To be held at the Rutherford Appleton Laboratory, Chilton on 23 September 1986 Further information from Dr R.J. Newport, Physics Laboratory, The University, Canterbury, Kent CT2 7NR 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 Industrial Physical Chemistry Group Water Soluble Polymers and their Industrial Application To be held at Girton College, Cambridge on 24-26 September 1986 Further information from Dr I. D. Robb, Unilever Research Laboratory, Port Sunlight, Bebington, Wirral L63 3JW Division Half-day Endowed Lecture Symposium including the Tilden Lecture by Professor J. Pritchard and the Meldola Lecture by Dr J. S. Foord To be held at the Scientific Societies Lecture Theatre, London on 4 November 1986 Further information from Mrs Y.A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN Neutron Scattering Group Neutron Crystallography To be held at Imperial College, London on 17-1 9 December 1986 Further information from Dr R. J. Newport, Physics Laboratory, The University, Canterbury, Kent CT2 7NR Colloid and Interface Science Group with Macrogroup UK Polymer-Polymer Interfaces To be held at the Scientific Societies Lecture Theatre, London on 15 December 1986 Further information from Dr R. Aveyard, Department of Chemistry, The University, Hull HU6 7RXColloid and Interface Science Group with the Colloid and Surface Group of the SCI Nucleation and Growth in Colloidal Systems To be held at the Society of Chemical Industry, 14 Belgrave Square, London on 16 December 1986 Further information from Dr R. Aveyard, Department of Chemistry, The University, Hull HU6 7RX Electrochemistry Group The Photoelectrochemical Properties of Colloids To be held at the University of Southampton on 7-8 January 1987 Further information from Dr S. P. Tyefield, CEGB Berkeley Laboratories, Berkeley, Gloucestershire Neutron Scattering Group Neutron Scattering and Phase Transitions To be held at the University of Warwick on 30-31 March 1987 Further information from Dr D. McK. Paul, Department of Physics, University of Warwick, Coventry CV4 7AL Electrochemistry Group Spring Informal Meeting To be held at the University of Bristol on 1-3 April 1987 Further information from Dr A. R. Hillman, School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS Polymer Physics Group Biennial Meeting To be held at the University of Reading on 9-1 1 September 1987 Further information from Dr D. Bassett, Department of Physics, University of Reading, Reading RG7 2AD Neutron Scattering Group Applications of Neutron and X-Ray Optics To be held at the University of Oxford on 14-1 5 September 1987 Further information from Dr R. K. Thomas, Physical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ Polymer Physics Group New Materials To be held at the University of Warwick on 22-25 September 1987 Further information from Dr M. J. Richardson, Division of Materials Applications, National Physical Laboratory, Queens Road, Teddington, Middlesex TWl 1 OLW (xii)

ISSN:0300-9599    

DOI:10.1039/F198682BP097

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. I , 1986, 82, 2283-2299 Solvent Structural Constant and Solvation Behaviour Applied to the Description of Aqueous Electrolytes at 25-300 OC William L. Marshall Chemistry Division, Oak Ridge National Laboratorj?, Oak Ridge, Tennessee 37831, U.S.A. From the application of a solvent structural constant and calculations of ' free' water remaining after solute solvation, water activities are described at temperatures up to 300 "C for NaCl solutions and at 25 "C for other electrolyte solutions. Ionization constants for NaCl are also obtained. The concentration of free (non-solvating) water is proportional to the activity of water (derived from vapour pressures) raised to a power B, which is a temperature independent constant (6.67) believed to represent a structural property of the solvent.The solute-solvent mass-action equilibrium appears to be predominantly a function of the density of water in the immediate vicinity of each solvated solute species, rather than of the bulk concentration of free water. Relative solvation numbers obtained for univalent ions quantitatively agree with expectations. Thus a model with all parameters defined to have chemical significance describes reasonably well the activity of water over aqueous electrolyte solutions. ____ ~ _ _ __ _ _ ~ _ _ _ _ Previously, application of a complete constant (P), which includes the solvent as a reactant, has described ionization behaviour of many aqueous electrolytes over wide ranges of temperature, pressure and concentration of ~ a t e r .l - ~ This constant was applied only where the concentration of electrolyte approached zero ; therefore the concentration or density of pure water was always used. This concentration could be varied by hydrostatic pressure or dilution with an assumed ' inert' soivent (dioxane), thus providing a value of K" that was isothermally a constant. In the present study, the behaviour of aqueous electrolytes at moderate to high concentrations is evaluated through a consideration of their extents of solvation and through the activity of water [a(H,O)] (derived from vapour pressures) which is related to the concentration of free water [C'(H,O)] in the solution. At a given molarity ( M ) of electrolyte, the concentration of ' free' (non-solvating) water [C'(H,O)] should equal the analytical concentration of total water [C(H,O)] minus the concentrations removed by solvating each of the electrolyte species.Thus for NaCl C'(H,O) = C(H,0)-jC(NaC1"~jH20)-pC(Na+~pH,0)-qC(C1--qH,0) ( I ) wherej, p , and q are average solvation numbers and C is the molarity of the designated species. For each electrolyte, an ionization constant ( K ) that is needed to obtain the fraction of ionization (xi, a value ofj, and a term n (which equals p + q -j, the average increase in solvation number in the ionization process) can be applied to a description of a(H,O) as a function of C'(H,O). A question arises, however, in the evaluation of K in the electrolyte solution given the acceptance of the behaviour of a complete constant. The complete constant is expressed as = K/[H,OIn 76-2 2283 (3)2284 Solution of Aqueous Electrolytes where the brackets signify absolute concentration (in mol dmP3) and y+ is the mean molar activity coefficient of NaCl.In the previous studies, at constant temperature and at the approach to infinite dilution of electrolyte, the concentration of H,O was varied either by hydrostatic pressure or by the dilution of H,O by 'inert' dioxane.lPs The value of K" was found to remain a constant, with K and [H,O] varying in a manner to yield a constant value of n. The question is whether [H,O] in eqn (3) represents the concentration (or density) of free water in the immediate vicinity (the microscopic concentration) of each electrolyte species (concept 1) or the bulk free water concentration, which should equal C'(H,O) (concept 2).At infinite dilution of electrolyte these two concentrations are identical. Up to a moderately high range of electrolyte concentration at saturated vapour pressures, the microscopic concentration of water should be relatively constant and, if accepted for application, provides a nearly constant value of K in accordance with eqn (3). This microscopic concentration is assumed to equal approximately the concentration or density of pure water at the given temperature and total, relatively constant, pressure. [In solvent4ectrolyte equilibria adhering to eqn (3), the microscopic concentration can be changed significantly only by large changes in hydrostatic pressure or by dilution with an inert solvent, as observed in the many studies at the approach to infinite dilution of ele~trolyte.l-~ Decreases in vapour pressure are thus assumed to cause insignificant changes in the microscopic concentration of H,O.] Conversely, if the bulk concentration of free water is the important variable in eqn (3), then this concentration decreases significantly with increasing electrolyte concentration owing to both the volume occupied by the solvated electrolyte species and to the water removed by solvation.Its concentration is expressed by eqn (1). Accordingly, under this condition K would decrease sharply with increasing electrolyte concentration in order to maintain K" as a constant. Evaluation of the fits of the activity of water over electrolyte solutions by both of these approaches has been applied in this study to determine whether concept 1 or concept 2 should be favoured.Customarily for a stoichiometric equi-valent electrolyte, the ionization constant can be expressed : from which x, the fraction of electrolyte ionized, may be calculated by a quadratic equation. For comparison of fits to the subsequent eqn (6), the value of K is taken either as the usual constant, independent of electrolyte concentration {where [H,O] in eqn (3) is taken as a constant (concept l)), or as a variable (concept 2). The introduction of x into eqn (1) and rearrangement yields the following expression for C'(H,O) for an equi-valent, incompletely ionized, electrolyte, K = x2y$M/( 1 -x) (4) C'(H,O) = C(H,O) - M(nx +j). ( 5 ) An equation that appears to represent a(H,O) up to high solute concentrations may be written as a(H,O) = [C'(H,O)/C(H,O-~U~~)]~'~ (6) where C(H,O-pure) equals 55.51 x density (in g ~ m - ~ ) and B is believed to represent a structural term for the solvent relating to a solvent structural unit.Thus when a solvent vaporizes, its liquid structure must be destroyed to form perhaps B separate moles of vapour from each mole of (free solvent) structural unit, and a mass-action expression can be applied. The following equations describe this behaviour : [H20], (in liquid) =+ B H,O (in vapour) (7)W. L. Marshall 2285 where Ef(H20)] (vapour) is the vapour fugacity (f), [(H,O),] (liquid) is the free concen- tration of the structural unit of solvent, which equals C'(H,O)/B, and K(vaporization) is taken to be a constant at low to moderate electrolyte concentrations.By normalizing eqn (8) to the concentration of solvent at infinite dilution and to the corresponding fugacity, and substituting a(H,O) for f(H,O)/f(H,O-pure), we obtain, [a(H2O>IR K'(vaporization) = [C'(H,O)/C(H,O-pure)] ' (9) At a given temperature at saturated vapour pressure, values of a(H,O) and C'(H,O) can be varied only by the dilution of the solvent by solutes or other solvents. K'(vaporization) is unity for pure solvent and, if it remains a constant upon dilution of solvent, then eqn (9) is simply equivalent to eqn (6). Constants and Data Used The objective was to test adherence to the model (with a comparison of fits by the use of concepts 1 and 2) by electrolytes for which values of the conventional K and of n could be obtained or estimated and for which very accurate values of a(H,O) and activity coefficients were available.Previous papersl-s have listed and illustrated several electrolytes that adhere to a straight-line relationship of log K us. log C'(H,O) in dioxane-water solutions, but always at the approach to infinite dilution of electrolyte. By extrapolating these straight lines to log C(H,O-pure), values of Kat this limit are obtained. The relevant values of n are obtained from the slopes. Values of K for the strong acids were approximated as that for NaCl, and their values of n were set equal to that for acetic acid in dioxane-water solutions.2 The activities of water, or the osmotic coefficients (+), and the mean molal activity coefficients ( y + ) [which were converted to molar activity coefficients b+) through application of4ensityl were collected from the critically evaluated tables at 25 "C given by Robinson and Stokes.loa Some more recent absolute vapour pressure measurements at 25 "C for NaCl-H,O solutionsll compared favourably with those compiled by Robinson and Stokes.(These latter measurements show graphically the difficulty in obtaining directly measured solvent activities at 25 "C to much better than 0.5% .) For NaC1-H,O solutions at temperatures from 25 to 300 "C values were obtained from the activity measurements of Liu and Lindsay1,* l3 and the compiled tables of Pitzer, Peiper and Busey.14 Since the model is developed on the basis of absolute concentration, tables of densities given e l ~ e w h e r e ~ ~ - ~ ~ were used for conversions from molality to molarity, and these separate sets of densities were fitted to quadratic equations for easy application. Acceptance of Concept 1 over Concept 2 Computer programs were written to evaluate solvent activities by concept 1 (C(H,O-pure) is used for [H,O] in eqn (3) whereby K" and K are both constants up to moderate electrolyte concentrations} and by concept 2, where C'(H,O) from eqn (1) is incorporated into eqn (3). The non-linear least-squares (NLLS) program of LietzkelR was used for the evaluations in these and all subsequent least-squares evaluations.The NLLS determination for a constant value of Kand values o f j and B by concept 1 was relatively easy. However, for concept 2, where C'(H,O) is used, K must vary with C'(H,O) in order to maintain the required constant value of K".This behaviour was incorporated into an iterative procedure together with a consecutive series of values for B in order to obtain the best- fit values of B and j for a given set of electrolyte solutions. The fits by concept 2 were somewhat poorer over a molality range 0-2 mol kg-I than those obtained by concept 1 [0.32% (2) versus 0.0604 (1) average deviations in calculated a(H,O)]. The fits when2286 Solution ojq Aqueous Elec frolytes extended to much greater molalities than 2 mol kg-l were vastly poorer with concept 2 than those by application of concept 1. With the much better fits by concept 1, there seemed to be no further reason to continue with concept 2. Therefore, it was concluded that the concentration of solvent water to be applied in solvent-solute equilibrium is that concentration (or density) of solvent in the immediate vicinity of the solute species, which is the microscopic concentration and not the bulk concentration. Over moderate ranges of electrolyte concentration, it thus would appear that K remains constant with the use of this microscopic water concentration for [H,O] in eqn (3).[Again, large changes in hydrostatic pressure or dilution with 'inert' dioxane will sharply change the microscopic concentration and accordingly K. The present, separate evaluations are all made isothermally at saturated vapour pressure, and therefore under these conditions (concept 1) K remains essentially a constant.] All further discussions in this paper consider only concept 1 as the most likely or predominant description of behaviour.Aqueous Sodium Chloride Behaviour, 25-300 "C Simultaneous Calculation for K, j and B up to 300 "C By ming published values of n (25-800 OC),l? 2 j 4-69 8. with interpolations for intermediate temperatures, simultaneously determined best fit values of K, j , and B were obtained by application of eqn (4)-(6) for describing the activities of water over aqueous NaCl solutions at temperatures from 25-300 "C. For these particular calculations, the range of molality of NaCl was limited to 2 mol kg-l so that values of K, j , B and n would not be expected to change significantly with electrolyte concentration at constant temperature. Moreover, y + was set equal to unity, as discussed later, since there were insignificant differences in fit, and obtained values of K and j with the use of y+ were not reasonable.Table l ( a ) gives the simultaneously determined values of logx, the solvation number of the neutral ion-pair (jZrn) and the structural constant (B). Included also are the values of n used, the sum (n +j2m), which should equal the sum of the average solvation numbers of the Na+ and C1- ions, and the percentage average deviations in fit from the published smoothed activities of water. It should be noted that the fit to the mathematically smoothed table values of Pitzer et al.14 allows an almost perfect fit at each temperature. This fit, however, may not reflect the inherent uncertainty in the 'raw' data evaluated in tabulating or calculating the smoothed values.The fits described in this study in reality simply compare calculated values of a(H,O) by the present model with the particular model of Pitzer et al. applied at 25-300 "C and, for the later comparisons, of Robinson and Stokes at 25 "C used to obtain their particular smoothed table values.loa To compare the many sets of original raw data at this time would have been prohibitive. Fig. 1 shows the best-fit values of log K,j,,, and B, and of the resulting (n +j,,) [table 1 (a)] plotted against temperature. The curves drawn represent the smoothed values given in table 1 (b) and discussed below. We observe that: (i) the separate values of log K make a relatively smooth curve, showing the usually observed maximum for aqueous electrolyte constants in the vicinity of 50-100 "C; (ii) j,, decreases with increasing temperature and approaches the constant value of 1.8 at 40&800 "C as observed previously;g (iii) B is essentially a constant, with an average value of 6.60k0.15; and (iv) the sum (n +j2m) is almost constant with an average value of 1 1.3, which is very close to the constant value of 12.0 given earlier for the range 400-800 0C.9 The S-shape of an assumed plot exactly through a set of best-fit values (log K, j2,, or B) or of (n +j2m) as a function of temperature is most certainly due to the fitting not of the 'raw data', but to the Pitzer et al.mathematical model that calculates a(H,O) as a function of both the temperature and the concentration of NaCl, as mentioned above.W. L. Marshall 2287 Table 1.Simultaneously obtained values of log K, j and B together with smoothed values for the ionization-solvation behaviour of NaCl at 25-300 "Ca (a) simultaneously obtained average n deviation T/"C usedb log K j,, B (n+kl> E% a(H2O)I" 25 6.43 0.693 ( f 0.034) 4.79 (f 0.19) 6.63 (k 0.03) 50 6.98 0.946 (k0.058) 3.58 (k0.35) 6.37 (f0.17) 100 7.87 1.004 (k0.071) 2.52 (20.46) 6.28 (k0.24) 150 8.55 0.784 (f0.048) 2.52 (fO.37) 6.50 (k0.19) 200 9.16 0.644 (k0.046) 2.38 (k0.37) 6.78 (k0.19) 250 9.66 0.416 (k0.038) 2.72 (f0.30) 7.19 (rfi0.14) 300 10.00 0.176 (f0.034) 2.07 (10.24) 6.46 (kO.10) average 6.60 ( & 0.15) ~ ~_________ ~ ~~ _ _ ~ ~ ~ ~ (b) smoothed values (B = 6.67) average deviation log K" T/"C log K j,, PA a(H,O>IC (n+jz,) ( c a W d i 1.22 10.56 10.39 11.07 11.54 12.38 12.07 11.32 0.002 0.003 0.004 0.004 0.005 0.005 0.005 0.004% from literatureb log K log K" 0 25 50 100 150 200 250 300 0.352" 0.724 0.823 0.864 0.801 0.640 0.446 0.220 5.85" 4.78 4.20 3.30 2.70 2.30 2.00 1.90 average : - 11.70" - 9.90 - - 0.005 11.21 - 10.48 1.179f - 10.00 0.004 11.19 -11.32 1.043 -11.13 0.005 11.17 -12.72 0.882 -12.70 0.0 10 11.25 -13.79 - 0.020 11.46 - 14.78 - - 0.01 7 11.66 -15.46 - 0.040 11.90 -15.96 0.210 -16.00 0.014% 11.44 - - a 0-2 mol kg-l NaCl; K based on molarity. The values in parentheses represent one standard deviation.Average deviations from reported smoothed values (an almost perfect fit). They do not reflect a fit to the 'raw' data (see text). From log K(2nd col), n(part a ; 5.85, extrapolated for 0 "C), and densities of water, with use of eqn (3); Extrapolated values at 0 "C.f Slightly revised from 1.155.l. 2i References in text. Fig. 2 shows representative plots of the deviations in fit, expressed as A log[a(H,O)], at 25 "C (part a) and 300 "C (part b) in solving forj,, versus a series of preselected values of B, with values of K at the two temperatures from table 1 (b). These plots are shown to emphasize the sharp minima in the deviations of fit for B at or near 6.67. A (universal) value of 6.67 was selected for the solvent constant for application up to moderate con- centrations since it was close to the average value found from the similar treatment of the series of electrolytes listed in table 2. Also, in the evaluation of a vaporization property, AE/(PAV), of liquids at the approach to the critical temperature, this property approaches asymptotically a constant value of 6.67 f 0.02 for water.lg The quantitative similarity seemed striking, and it was hypothesized that there might be a correlative relationship with respect to solvent structure. In another series of calculations, B was held constant at 6.67 and other values of log K and j,, were obtained simultaneously.The smoothed values of log K and of j,, from these latter calculations are given in table l(b), with percentage deviations in fit for a(H,O) by their use over molalities of 0-2 mol kg-l NaCl. Included also are extrapolated values for log K andj,, at 0 "C. From the values of log Kin table 1 (b) and of n in table 1, complete ionization constants (A") were calculated as defined by eqn (3) and where2288 Solution of Aqueous Electrolytes I I 1 I T 4 1 .o 0 .5 OD -- 0 f (b) 1 2 '\ 1 2 2 4 B 10 0 100 200 30 0 4 00 T/O c Fig. 1. Simultaneously determined values of (a) log K(based on molarity), (b)j2m and (c) B, and ( d ) the sum (n +j2m); aqueous sodium chloride, 0-2 mol kg-l, 25-300 "C. Triangles indicate results from ref. (9). the concentration of NaCl approaches infinite dilution. Again, at this limit concepts 1 and 2 are identical. These calculated values of log K" are included in table 1 (b) together with published 4-69 8* both of log K and of log K" obtained by extrapolation of values in dioxane-water solutions to pure water at saturated vapour pressure. Fig. 3 shows a plot of the presently calculated values of log K" and those from the literature at temperatures from 0 to 800 "C.The agreement is excellent, with the excep- tion of small divergences at 25 and 50 "C. The extrapolated value of log K at 25 "C from logarithms of the ionization constants in dioxane-water solutions is 1.179 (based on molarity) whereas the best-fit smoothed value from the present study is 0.724 as given in table 1 (b). Although this difference might appear to be seriously large, it should be recognized that ionization constants of 5, 15 or greater are very difficult, if not impossible, to obtain accurately by experiment. In this light, the moderate differences observed only at 25 and 50 "C should not be considered too seriously. However, any other reason for these differences is not obvious since the literature values at these two temperatures were obtained by extrapolation just like those at the higher temperatures.W.L. Marshall 2289 0.0010 0.0008 0.0006 * m .- 5 0.0010 e, 2 2 0 . 0 0 0 8 0.0006 0 . 0 0 0 t 0.0002 I \ ' I ' I ' I ' I ' I 1 1 1 1 1 1 1 1 1 1 1 1 1 - 0 2 4 6 8 10 12 B Fig. 2. The rerage deviation [A log a(H,O)] versus B for the solvent-ionization equilibria fit t 1 the observed activities of water over 0-2 mol kg-l sodium chloride solutions, 25 "C (part a> and 300 "C (part b). Solvation Number of the Ion Pair, NaCl" The values ofj,, given in table 1 (b) were plotted as logj,, against l/T(K) to produce a (pseudo) van't Hoff type of plot as shown in fig. 4. An approximately straight line is obtained that provides an 'enthalpy' (AH) of solvation or formation of 1.20 kcal mo1-l.j- This low value for AH is reasonable for a very weak interaction of water with the dipole of the ion pair. Thus, at moderately low salt concentrations, ca.4.78 water molecules would appear to associate on average with NaCl" at 25 "C; this value decreasing to ca. 1.9 at 300 "C and approaching a constant value of ca. 1.8 at higher temperature^.^ The sum of the average (primary) numbers of water molecules attached to the sodium and chloride ions, however, appears to remain constant from 25 to at least 800 "C. This proposal, given previously for the behaviour at 40&800 "C9 would now appear to be supported experimentally in the present study over the much larger range of temperature from 0 to at least 800 "C.Note that the value ofj,, at 0 "C, given in table 1 (b) and obtained by straight-line extrapolation as shown in fig. 4, is 5.85. This value is close to 6, which is certainly a t 1 cal z 4.18 J.2290 Solution of Aqueous Electrolytes Table 2. Ionization constants and solvation numbers for aqueous salts used to describe the activity of water at 25 "Ca _ _ _ _ _ _ _ _ _ _ ~ _ _ _ ~ _ _ ~ ~ ~ ~ ~~ ~ ~~ salt LiCl NaCl KC1 RbCl CSCl LiBr RbBr CsBr LiI RbI HC1 HBr HI (from elsewhere) Kh nc fit, (from fit) &2 mol dmP3 jSm (% deviation)" 10.0 6.40 15.1f 6.43 7.91 6.24 6.03 6.24 5.34 6.24 10.0 6.40 10.3 6.24 10.0 6.40 8.35 6.41 10.0 6.20 15.09 7.69 15.09 7.69 15.09 7.69 5.50 0.05 4.12f 0.05 3.70 0.05 3.53 0.03 3.00 0.02 5.59 0.1 1 2.85 0.05 2.09 0.04 5.00 0.07 2.43 0.03 4.5 1 0.07 4.78 0.13 4.78 0.09 average = 0.067; a &2 mol kg-' salt, B = 6.67, activity coefficients set at unity.Ionization constant, molar units (text). Increase in average solvation number upon ionization (text). Average solvation number of ion pair from data over a 0-2 moldmP3 range; standard errors in jzm are 0.02-0.04. Average ;< deviation in a(H,O) calculated from reported smoothed values. f Values of 5.30 and 4.78 for K and j Z m , respectively, obtained directly from activities (table lb). 9 Estimated (see text). reasonable number for a primary coordination shell around a small-sized solute species. The extrapolated value of n at 0 "C is also 5.85. The sum is 11.70 and closely agrees with the sums at higher temperatures given in table 1.It might appear that water molecules at the lowest temperature of liquid-phase existence (for pure water) solvate to essentially the maximum amount as a primary shell around the NaCI" ion pair. At higher temperatures, because of the weak NaCl"-H,O interactions, solvated water on the ion pair is removed rather easily. In contrast, H,O in the primary shells of Na+ and C1- ions appears to be held strongly over a wide range of temperature because of the electrical charge of the ions, which is present on the neutral ion pair only as a dipole separation. Concentration versus Activity for Behaviour of Vapour At temperatures up to 100 "C, the water vapour pressure is sufficiently low that the system can be treated as ideal. At higher temperatures there is a moderately large divergence from ideality.Since, in this description, concentration is used to describe the behaviour of 'free' water in the liquid phase, it was of interest to test whether a concentration ratio [rather than activity used in applying eqn (6)] of the water vapour would provide an acceptable description. Fits in determining K , j,, and B either simultaneously or separately therefore were made using a vapour concentration ratio for comparison with the use of activity. Vapour concentrations are easily calculated from the reported equilibrium vapour pressures12y l3 and the steam tables.,O The vapour concentration ratio (Cr) was taken to equal C(vap)/C(vap-pure) where C(vap) is the saturated concentration of water vapour over the solution and C(vap-pure)W.L. Marshall 229 1 -9 -10 -11 -12 h - u 2" -13 v 5 -14 -15 w -16 -17 18 TI0C 800 400 200 100 25 0 I 0 ,'- - / 1 .O 1.5 2.0 2.5 3.0 3.5 K/ T Fig. 3. Log K" for aqueous sodium chloride uersus l/T(K), 0-800 "C. (a), Results from this study. (O), Results from: ref. (9) (300-800°C); ref. (4) (300°C); ref. ( 6 ) (100°C); ref. (8) (50 "C); ref. (l), (2) and (3b) (25 "C). 0.80 0.75 0.70 0.65 0.60 e 0.55 .-. TI" C 300 200 100 50 25 0 M 2 0.50 :::I / , , , , , , 4 0.30 0.2 5 1.5 2 .o 2.5 3.0 3.5 4 .O KI T Fig. 4. Logj,, for NaCl" uersus l/T(K); 0-2 mol kg-I NaCl, 6300 "C. Logj,, = - 0.192 + 262/T. AH (solvation) = 1.2 kcal mol-l.2292 Solution of Aqueous Electrolytes is that over pure water at the same temperature. By substituting C, for activity on the left-hand side of eqn (6), the comparisons at 200 and 300 "C showed that only the use of activity [in eqn (6)] as given by the values of Liu and Lindsayl2, l3 and those compiled by Pitzer et al.14 produced consistent behaviour with the other measurements together with the good fits described above. Although solving simultaneously for j,,, K, and B with the use of C, [substituted for activity in eqn (6)] gave relatively good fits (but not so good as with activity), the values of K obtained differed widely from literature values. The simultaneously obtained values of B changed sharply with temperature and at 300 "C differed greatly from 6.67.Best-fit values for j,, did not allow the consistent behaviour obtained by the fits to activity. When for a molality range of 0-2 mol kg-l NaCl B was taken as 6.67, and either the literature or present values of K were used in solving for jZm, the average deviations in fit at 300 "C increased from 0.01 by use of activity to 0.5% with concentration. Thus, in the description given, the concentration of 'free' liquid solution water on the right-hand side of eqn (6) allows the good fits, but activity [in eqn (6)] is necessary for the best description of vapour.In our previous studies of ionization equilibria at the approach to infinite dilution of electrolyte, we had shown that concentration of the solvent water from 25 "C to supercritical temperatures, but always at densities higher than about 0.3 g ~ r n - ~ , provided very good fits to a model of mass-action solvation equilibria. The present observations appear to confirm experimentally (by the model) that activity of solvent is the proper quantity to use in gas-like regions.However, upon reaching liquid, or high density, regions the use of concentration greatly simplifies the description, and its use under these conditions would appear to be significant. Other Electrolytes and Extension to Concentrated Solutions Values o f j at 25 "C were calculated over six ranges of electrolyte molality for several salts and acids with the values of K and n given in table 2 and also with the assumption that K was equal to infinity. Attempts to obtain values of K, j and B simultaneously or of K and j over the 0-2 mol kg-l range for electrolytes other than NaCl were not satisfactory in that, while the fits were good, the best-fit values for the constants ( K , j and B or K andj) varied widely from consistent behaviour and reasonable expectations.As mentioned above, solving simultaneously for B andjzm for the 13 electrolytes at 25 "C did, however, produce an average value of B close to 6.67. It was concluded that the smoothed activities of NaCl represent the most extensively studied and evaluated set of activity measurements and presumably the most accurate, thus allowing the simultaneous attainment of relatively reasonable values of K, j , and B by the fitting procedure described above. For the other electrolytes, independently determined values of K and n were needed for obtaining reasonable values o f j . The determined values o f j over the 0-2 mol kg-* range (izm) and 0-full range OF) for the several 1 : 1 electrolytes by this procedure, together with the fits, are given in tables 2 and 3.Because the value of j for a given electrolyte depended partly upon the range of electrolyte concentration used for its calculation, the separately determined values were plotted against molality of electrolyte and extrapolated to infinite dilution to obtain j,. These values are included in table 3. In fitting over the entire range of molality, how- ever, an average best-fit value for B was found to be 5.50, which is perhaps reasonable if one considers that a high concentration of solute will certainly partly destroy solvent structure. When n for NaCl behaviour was set equal to (1 1.3-j,,) for the fits over the full range of molality at 25 to 300 OC, and with B kept equal to 6.67, best-fit values of j , were unreasonable and sometimes negative, with slightly poorer fits than by the procedure taken above. For this reason, the particular approach with the assumption of a constant value for n was used, although some ambiguity exists.However, attainment of a bestW. L. Marshall 2293 Table 3. The extrapolated solvation numbers for the ion pair at infinite dilution (j,,) and average values over a full range of electrolyte concentration OF) at 25 "C" full of over full range salt J o j , molality ("4 deviation)b range fit using j , LiCl NaCl KCl RbCl CSCl LiBr RbBr CsBr LiI RbI HCl HBr HI 4.82 4.90 4.54 4.14 3.66 4.50 3.72 3.22 4.09 3.16 3.68 3.37 4.0 1 3.57 3.06 1.94 1.89 1.52 3.77 1.06 0.53 3.45 0.82 2.34 3.36 3.35 0-6.0 C6.0 0 4 .8 0-5.0 c 6 . 0 0-6.0 0-5.0 0-5.0 G3.0 0-5.0 C6.0 0-3.0 b3.0 average 1.09 0.38 0.1 1 0.06 0.09 1.48 1.01 0.03 0.25 0.03 0.14 0.32 0.34 0.41 a B = 5.50, y , = 1; standard errors i n j are 0.02-0.04 % deviation in a(H,O-calcd) from reported smoothed values. Average fit for an average solvation number G when B is near 5.5, as described next, supports a decrease in the value of B at the high molalities. With K assumed to be infinity, x becomes unity and (nx+j) in eqn ( 5 ) can be replaced by an average (overall) solvation number (G), with no distinction made for solvated ions or ion pairs. Fits to a(H,O) in obtaining B and G simultaneously were made over the full range of molalities (producing GF) of the aqueous electrolytes listed in table 2.As expected, these values of G, fell between those of (n+jF) and j , while the average best- fit value of B was still in the vicinity of 5.5. The introduction of activity coefficients did not significantly change the fits or determined values o f j , GF or B, and therefore the activity coefficients were taken to be unity for the calculations in table 3, as discussed below. Consideration of a Debye-Huckel Contribution and Electrolyte Activity Coefficients Since the description of the water activities to high concentrations of electrolyte is described predominantly by the simple single term of eqn (6), the addition of an extended Debye-Huckel term (DHT) to this model can only be significant at moderately low concentrations. In order, therefore, to test its application in the descriptions, a DHT was divided by (1 + m2) that allowed the decrease in the Debye-Huckel contribution (DHC) to moderately low levels at high molalities (m) of electrolyte. The simple form of this denominator was taken after best-fit evaluations were made of several functions as discussed elsewhere.21 The introduction of DHC as an additional term in eqn (6), however, gave poorer fits to the measurements. Also, solving simultaneously for K, j and B [for NaCl solutions (25-300 "C)] gave values of K that were far removed from literature values.The addition of DHT alone (rather than DHC) sharply decreased the fit. The value of DHT becomes large at high ionic strengths and requires an offsetting term that eqn ( 5 ) and (6) cannot satisfy by best-fit adjustments in the values of K, j or B.2294 Solution of Aqueous Electrolytes - 0.0°2 t 1 NaCl/mol dm-3 Fig. 5.Deviation plot for the log activity of water at 25 "C over sodium chloride solutions from 0 concentration to saturation (5.3 mol dm-3) by the solvation model using activity Coefficients (y*) and solving simultaneously for B and jk. K = 15.1, n = 6.428, B = 5.455 and j , = 2.083. The introduction of electrolyte activity coefficients into eqn (2) or (4) did not significantly change the fits when values of K and n were used for the calculations ofj. However, when the measurements were fitted simultaneously for j and K, with activity coefficients, the resulting values of Kwere far removed from literature values. Nevertheless, when NaCl solutions at 25 "C were fitted over the entire range of concentration, the introduction of activity coefficients did moderately improve the fit as shown in fig.5 [0.05 % deviation cf. 0.13 % without activity coefficients]. Therefore it is difficult to decide unambiguously upon the significance of the electrolyte activity coefficients in this mass-action solvation model except that they do not make a large contribution. Simultaneous calculations for j , K and B, with activity coefficients, do not give reasonable values of j and K, and values of B are temperature and salt dependent. Lastly, a combination of both activity coefficients and DHC or DHT did not produce a better description or realistic values of K. In contrast, the model with activity coefficients taken as unity provides chemically reasonable quantities as shown in table I and fig.1 and a good, overall fit to the measured activities. Discussion Sodium Chloride Solutions At infinite dilution for NaCl at 25 "C, j o equals 4.90 (table 3) with n equal to 6.43 (table 2); the total solvation number for the two ions (n +j,) is therefore 1 1.33. Previously, j , was estimated to equal between 2 and 4 at 25 "C19 and 1.8 at 40&800 0C.9 The value of n increases from 6.43 at 25 "C to 10.2 at 400-800 0 C . 1 ~ 2 ~ 4 - 6 ~ 8 ~ 9 As noted above, if it is reasoned that water of solvation is strongly held by Na+ and Cl- ions even as the temperature rises, but weakly by the dipolar NaC1" ion pair, thenj, could decrease, n increase, and (p+ q) remain equal to 11 or 12 over the range 25-800 "C.Fig. 1 shows the decrease with increasing temperature in the simultaneously calculated values ofjBm, and the approximate constance of (n +jZm) over the 0-300 "C range. By accepting a valueW. L. Marshall 2295 of 4.9 forj, at 25 "C, @+q) would equal 11.3 at this temperature and be approximately the same at 400-800 "C (10.2+ 1.8 = 12). The present observations as described in table 1 and fig. 1 and 3 correlate extremely well with the earlier studies. At 25 "C there is excellent agreement of calculated activities of water with reported values (fig. 5) for NaC1-H,O solutions over the entire range of molality [0-6 mol kg-l (near saturation)] obtained by using the simultaneously determined values of B (5.455) andj, (2.083) together with values of y+ . The larger number of significant figures given for a(H,O) of NaCl solutions than forthose of other salt implies greater accuracy.Activity coefficients appear to be significant for NaCl when applied at high concentrations, but not for the other salts at 25 "C, possibly owing to the presumed lower accuracies for the other values. Nevertheless, by setting y+ equal to unity and again solving simultaneously for B and j F , the average deviation Tor NaCl(0-6.0 mol kg-l) is still low at 0.13 "/o in a(H,O). The effect of using y , - in the calculations seems to be small. Additivity of Obtained Solvation Numbers Table 4 compares differences in total solvation numbers (n+j,) between the several electrolytes for testing the consistency of the model in obtaining relative solvation numbers of ions.Column 2 shows that the several differences in each of the several sets are relatively close together, and in column 3 the differences are averaged. Recently Biggin et al.', have reported a coordination number of 5.5 & 0.4 for the C1- ion in NaCl solutions (obtained by neutron scattering structural studies for a molality of 5.32 at ambient temperature). If the solvation number ( q ) of C1- is assigned a value of 5.66 from the present study, based on (n+jo) for NaCl of 11.33 where 11.33/2 equals 5.66, and the values of (n+j,) for LiCI, NaCl and KCl are used in obtaining relative values ofp(Li+), p(Na+) and p(K+), then relative solvation numbers for the other cations and anions can be calculated, again by difference. The complete set of values are given in table 4, and they generally show an expected decrease in relative solvation number with an increase in ionic size for both the cations and the anions separately.The overall consistency would appear to be surprisingly good. Free Water and Solubility Calculated concentrations of free water by eqn (9, with K, n andj,, from table 2, and with B and y* equal to 6.67 and unity, respectively, are plotted against M(sa1t) in fig. 6 for the several salts except those of lithium. Extrapolations to C'(H,O) equal to zero provide values of M(sa1t) in the vicinity of the solubility for each salt. There is very close agreement for NaC1, RbBr and RbI. If the rationale is significant, strong divergences for the other salts must occur at C'(H,O) below ca. 15 mol dm-3.The high solubilities of the lithium salts do not justify this reasoning, and it can only be conjectured that C'(H,O) reaching 'zero' is meaningful with respect to solubility for the other salts. Conclusions There are many experimental and theoretical studies aimed toward describing concen- trated aqueous electrolyte behaviour. The majority of studies are concerned with calculation of activity coefficients and, except for those ions that strongly complex, do not include ionization (or association) constants or solvation numbers. The present study provides chemical insight into the actual behaviour of aqueous electrolytes and thereby shows that their behaviour may be described predominantly by a chemical model, with a chemical significance for all terms used. The correlative behaviour of additional parameters and terms, while providing better fits at high concentrations to the reported smoothed values (including any inaccuracies introduced2296 Solution of Aqueous Electrolytes Table 4.Relative solvation differences and numbers for ions at 25 "Ca relative differences ionic in total solvation electrolyte solvation average number, pair numberb difference 25 "C HC1-LiC1 HBr-LiBr HI-LiI HCI-RbBr HBr-RbBr HI-RbI HCl-CsCl HBr-CsBr LiCl -RbC1 LiBr-RbBr LiI-RbI RbC1-CsCl RbBr-CsBr LiCl-LiBr RbC1-RbBr CsC1-CsBr LiCI-LiI RbC1-RbI LiBr-LiI RbBr-RbI ~~~ 0.06 0.07 1.11 0.90 1.01 1.1 1 1.38 1.35 0.84 0.94 1.14 0.48 0.34 0.32 0.42 0.28 0.72 1.02 0.40 0.60 ~~ ~~ H+ 5.97 Lif 5.56 0.41 (H+-Li+) Na' 5.66 Ki 5.12 1.39 (H+ - Rb+) 1.36 (H' - CS') Rb+ 4.59' Cs+ 4.40' 0.97 (Li+-Rb+) - Cl- 5.66* Br- 5.32 0.41(Rb+-Csf) I- 4.80' 0.34 (ClP - Br-) 0.87 (C1- - I-) 0.50 (Br ~ -I-) a Values for n andj, from tables 2 and 3 ; ionic solvation numbers based on q(C1F) = 5.66 and (n+j,) for LiCl, NaCl and KC1 and averages in column 3 ; T = 25 "C.Number represents ( n +j,) (1 st electrolyte) minus (n +j,) (2nd electrolyte), values from table 2. Averaged from two values from column 3. Arbitrarily assigned a value of 5.66. by smoothing), would effectively reduce the quantitative significance of the chemically defined constants in the model. The earlier model of Robinson and Stokes10bt23 includes solvation numbers, but not association constants. GlueckauP* emphasized that solvation numbers would decrease with increasing concentration of electrolyte, which is the observed behaviour in this study with respect to average solvation numbers.(The observed average decrease evidently reflects a lessening reliance on concept 1 as the range of electrolyte concentration is extended since concept 1 assumes a constant microscopic concentration of solvent, thereby assuming a constant solvation number for each species. However, full reliance on concept 2 is not indicated, as discussed above.) The Robinson-Stokes-Glueckauf model describes activity coefficients in terms of solvation numbers of ions, with application of Debye-Huckel theory, but without allowance for some association to form ion pairs.W. L. Marshall 2297 I I I I I 60 m I c 40 e 0 E 0- u . - z 20 0 salt concentration/mol dm-3 Fig.6. The approach of the free water concentration [C'(H,O)] to zero at the solubility limit for several selected salts; solvation model with B = 6.67 and j z m ; T = 25 "C. Solubilities: (O), NaCl; (a), KC1; (O), RbCl; (a), CsC1; (m), RbBr; (B), CsBr; (A), RbI; and (---), extrapolated values. The electrostatic-interaction model of Pitzer et aZ.25-28 has been applied widely for fitting aqueous electrolyte behaviour over a broad range of temperature to obtain activity coefficients and solution thermodynamic properties. Helgeson et al. 29 9 30 have presented an electrostatic-interaction model also for application to aqueous electrolytes at higher temperatures. In another approach, the Lietzke-St~ughton-Fuoss~~ two-structure model for aqueous electrolytes demonstrates good accuracy in calculating electrolyte activity coefficients by decreasing the Debye-Huckel contribution as the concentration of electrolyte increases.The present model incorporates solvation numbers, an ionization constant, and a universal structural constant (B) relating the activity of water to the concentration of free (non-solvating) water in the liquid phase. It must be noted that, for a given electrolyte, values of K and of n can be obtained from independent and completely different types of studies, e.g. from interpretations of conductance rneas~rements.~-~ If B is takefl to have a universal value of 6.67 (for water), then there is only one necessary parameter (j) for fitting the activities. Since j is believed to be the average solvation number of the ion pair, then it too can, in principle, be obtained independently for application with eqn (6), thus providing a description of solvent activity requiring no chemically undefined parameters. There does not seem to be any inconsistency in this approach and the earlier approaches taken for descriptions at infinite dilution.lP9 Perhaps it would appear that acceptance of concept 1 is not consistent with application of a complete constant at infinite dilution of electrolyte, where C(H,O) varying as the nth power allows K" to remain a constant. However, at infinite dilution concept 1 equals concept 2, and there is no contradiction. The mass-action equilibria between solvent and electrolyte species of finite concentration with respect to the microscopic concentration of solvent would seem to be reasonable chemical behaviour (as borne out by better fits compared to application of concept 2).Again, the microscopic concentration can be easily changed by hydrostatic pressure or by dilution with an 'inert' solvent in the same manner as C(H,O) was changed at the limit of infinite dilution. The apparently constant value of the defined solvent structural constant B at moderately low electrolyte concentrations over the wide range of temperature (25-300 "C)2298 Solution of Aqueous Electrolytes might be considered inconsistent since some may feel that water structure changes markedly with widely changing temperature. Narten et al.,32 however, have shown by X-ray diffraction that for H 2 0 the average number of nearest (primary) neighbours remains constant at 4.4 from 4 to 200 "C.Thus 'structure', as related to the number of nearest neighbours, is independent of temperature over this range. The conclusions in the present paper agree with Narten et al. By accepting the conclusion of Narten et al. a coordination number for secondary neighbours must also remain constant with changing temperature (from 4 to 200 "C). The value of B is defined to represent a coordination number for a solvent molecule in a unit structural cell, and this number may represent both nearest neighbour and other near neighbours. As temperature increases, the liquid density decreases because of increasing kinetic energy that increases intermolecular distances. It does not necessarily follow that the liquid structure must change.The presumed evidence from this indicates that an average coordination number, representing ' bonds ' destroyed in the vaporization (including nearest and possibly some secondary neighbours) of a water molecule remains essentially a constant with increasing temperature, and may remain so even as the critical temperature is ap~r0ached.l~ A structural constant for H 2 0 of 6.7 was first given by this author in 197033 and equations representing successive solvent-solute mass-action equilibria in 1 972.3b The present paper describes the application to concentrated solutions from extensions, revisions and refinements of that earlier work. This paper thus presents a chemical model using mass-action equilibria that closely describes a particular experimental behaviour of aqueous electrolyte systems.The model produces a greater insight into the chemical equilibria that are necessarily involved in these systems. The observations describe the activity of water to be proportional to the concentration of free (non-solvating) water raised to the reciprocal power of the structural constant, rather than to unity as might be generally assumed. The model produces ionization constants (for NaC1) and average solvation numbers for electrolytes in excellent agreement with expectations and values obtained independently, and provides possible insights into the structural nature of the solvent and its behaviour with temperature. Another paper describes the behaviour of aqueous electrolytes up to 300 "C by a modification of Raoult's law in accordance with the same rationale.21 I thank R.H. Busey, M. F. Holmes, M. H. Lietzke, H. F. McDufEe, R. E. Mesmer and J. E. Ricci for their helpful comments. This work was sponsored by the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy, under contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc. References 1 W. L. Marshall and A. S. Quist, Proc. Natl. Acad. Sci., USA, 1967, 58, 901. 2 A. S. Quist and W. L. Marshall, J. Phys. Chem., 1968, 72, 1536. 3 W. L. Marshall, J . Phys. Chem., (a) 1970, 74, 346; (b) 1972, 76, 720. 4 L. B. Yeatts and W. L. Marshall, J. Phys. Chem., 1972, 76, 1053. 5 L. A. Dunn and W. L. Marshall, J . Phys. Chem., 1969, 73, 723. 6 L. B. Yeatts, L. A. Dunn and W. L. Marshall, J . Phys. Chem., 1971, 75, 1099. 7 W. L. Marshall, Rec. Chem. Prog., 1969, 30, 61. 8 T. H. Leong and L. A. Dunn, J . Phys. Chem., 1972,76,2294. 9 A. S. Quist and W. L. Marshall: J. Phys. Chem., 1968, 72, 684. 10 R. .4. Robinson and R. H. Stokes, Electrolyte Solutions (Butterworths, London, 2nd edn, 1965); (a) 11 C. N. Pepela and P. J. Dunlop, J . Chem. Thermodyn., 1972, 4, 255. 12 C. Liu and W. T. Lindsay Jr, J . Phys. Chem., 1970, 74, 341. 13 C. Liu and W. T. Lindsay Jr, J . Solution Chew., 1972, 1, 45. 14 K. S. Pitzer, J. C. Peiper and R. H. Busey, J . Phys. Chem. Ref. Data, 1984. 13, 1. 15 International Critical Tables, ed. E. W. Washburn (McGraw-Hill, New York, 1928), vol. 111. appendices 8.3-8.10; (b) pp. 238-252.W. L. Marshall 2299 16 R. W. Potter and D. L. Brown, Geological Survey Bulletin 1421-C (U.S. Government Printing Office, 17 P. S. Z. Rogers and K. S. Pitzer, J . Phys. Chem. Ref. Data, 1982, 11, 15. 18 M. H. Lietzke, A Generalized Least Squares Program for the IBM 7090 Computer, Oak Ridge National Laboratory Report ORNL-3259 (April 1962). 19 W. L. Marshall, J . Phys. Chem., 1985,89,4128. 20 International Formulation Committee 1984 Steam Tables of the International Association for the Properties of Steam, 1984. 21 W. L. Marshall, J . Solution Chem., 1986, 15. 439. 22 S. Biggin, J. E. Enderby, R. L. Hahn, and A. H. Narten, J . Phys. Chem., 1984, 88, 3634. 23 R. H. Stokes and R. A. Robinson, J . Am. Chem. Soc., 1970, 70, 1870. 24 E. Gleuckauf, Trans. Faraday Soc., 1955, 51, 1235. 25 K. S. Pitzer, J . Phys. Chem., 1973, 77, 263. 26 K. S. Pitzer and G. Mayorga, J . Phys. Chem., 1973, 77, 2300. 27 K. S. Pitzer and G. Mayorga, J . Solution Chem., 1974, 3, 539. 28 K. S. Pitzer and J. J. Kim, J. Am. Chem. Soc., 1974, 96, 5701. 29 H. C. Helgeson and D. H. Kirkham, Am. J . Sci., 1974, 274, 1089; 1199; 1976, 276, 97. 30 H. C. Helgeson, D. H. Kirkham, and G. C. Flowers, Am. J . Sci., 1249, 281, 1249. 31 M. H. Lietzke, R. W. Stoughton, and R. M. Fuoss, Proc. Natl Acad. Sci. US, 1968, 59, 39. 32 A. H. Narten, M. D. Danford, and H. A. Levy, Discuss. Faraday Soc., 1967, 43, 97. 33 W. L. Marshall, Complete Equilibrium Constants andh'ew Relationships to Electrolyte-Solvent Equilibria, Published Book of Abstracts, 160th Natl. Meeting of the American Chemical Society, Abstract 91, Div. of Phys. Chem. (American Chemical Society, Chicago, 1970). Washington 1977). Paper 5/61 1 ; Received 1 Ith April, 1985

ISSN:0300-9599    

DOI:10.1039/F19868202283

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. I, 1986,82, 2301-2309 Modification of the Order of Reaction and Reaction Rate of Nucleophilic Aromatic Substitution in Micellar Solutions Joel Leli'evre," Oma'ima Haddad-Fahed and RenC Gaboriaud Laboratoire de Physicochimie des Solutions E.N.S.C.P., 11, rue Pierre et Marie Curie, 75005 Paris, France The rate of formation of 2,4,6-trinitrodiphenylamine following the attack of aniline on 1 -methoxy-2,4,6-trinitrobenzene has been studied in micellar media. The partial order with reference to the nucleophilic reagent (aniline) is unity in solutions of cationic detergents (positive micelles) and 3/2 in water or solutions of anionic detergents. For such reactions there are two main steps in the reaction scheme: first, the formation of an adduct between reagents, favoured by the effect of local higher concentration in the two kinds of micellar solutions and secondly, ejection of a proton from the adduct formed.The latter reaction is catalysed largely by positive micelles and in this case the kinetics are not limited by the deprotonation step. On the other hand, negative micelles inhibit the ejection of a proton and this opposes, in part, the effect of higher local concentrations. __ ~ ~ - _ _ _ _ ~ ~ _ _ ~ ~ ~ _ _ _ _ ~ ~ ~ _ _ _ _ ~~ ~ ~ _ _ _ _ Aqueous solutions of surfactants often form micelles and the kinetics of numerous chemical reactions have been shown to be modified in the presence of these micelles. The catalytic properties of such solutions have recently been the subject of many publications and several workers have shown the complex nature of such phenomena.1-24 We propose to recall briefly the main proposals made to explain the observed features micellar catalysis.Fundamental Elements of Micellar Catalysis When a chemical reaction is set up in the presence of micelles, the reagents can remain in the aqueous phase or may be extracted partly or wholly into the core of the micelles and they can be adsorbed into the polar sheath. Many different situations have been described, according to the nature of the reagents and substrates. For organic substrates, extraction into the micellar bulk is favoured and the catalysis usually results from a local higher concentration in this phase.25 30 This description is applied generally to neutral solutes and non-ionic micelles and in this case the rate equation of micellar catalysis is calculated on the enzymatic m ~ d e l ." ~ - ~ ~ However, if we use ionic micelles, the aqueous and micellar phases both contain an excess charge and there is an interphase potential A 4 between the two phases. When one of the reagents is an ion whose charge is opposite to the micellar charge and the other is a neutral species dissolved in the organic phase, we can expect a catalytic effect that can be explained in two complementary ways: (i) a local higher concentration of reagents occurs and therefore there is an increase of probability of reaction; or (ii) the reaction implies a charge transfer of the ionic reagent from the aqueous phase to the core of the micelle through the potential difference A4.Very often these two explanations are proposed separately or have been proposed to be opposed,s whereas they are completely c~mplementary.~~~ 36 The first explanation is attractive because it uses the same terminology and the same formulation used in the case of neutral systems. 230 12302 Nucleophilic Aromatic Substitution in Micelles In order to include in the theory the decrease in the rate of reaction observed on the addition of background electrolyte in situ, we have to introduce competition between the several ions having opposite charge to the micelles for the occupation of the superficial sites. If we consider two ions (i) and (i) having the same charge z [z(i) = z('j)], we can write an exchange equilibrium: i(mic) + j(aq) + i(aq) + j(mic) with a constant K([i]/b]). If ion j is the reagent, it is obvious that the introduction of an ion i will decrease more or less strongly [according to the value of K ] the number of j ions bound to the micelles and consequently will decrease the potential reactivity.This explanation has been used in many paper^^-^^^ 37-39 and proves generally to be very satisfactory. However, difficulties appear in some s y ~ t e m s ~ ~ - ~ ~ * 40 and when the ionic reagent has the same sign of charge as the micelle. The interphase potential appears in the second explanation and it was used in the earliest p a p e r ~ . ~ l - ~ ~ We have already shown that it was not incompatible with the above interpretation and that these two explanations are complementary in a simple mode1,34-36'45-47 which leads to the same formulation in every case, independent of whether the reagents are molecules or ions.The previous theories can be applied directly for systems in which the reaction scheme has only one determinant process. The aim of this work is to investigate whether the previous methods can be applied to examples of reactions involving several processes. We have chosen nucleophilic aromatic substitution in which the kinetics may or may not be controlled by the pH, according to whether the step involving ejection of a proton from the intermediate complex is the predominate one or not. Mechanism of the Reaction The whole mechanism of the reaction of nucleophilic substitution of aniline (AnH,) on 1 -methoxy-2,4,6-trinitrobenzene (TNA) has been described in previous papers4*9 49 and it follows thegeneral mechanism established by Bunnett et al.and in ref. (5 1). The reaction scheme is explained in detail in fig. 1. A zwitterionic intermediate complex (HI) is formed by attack of aniline on TNA according to equilibrium (1) of the reaction scheme. This complex can rapidly eject a proton to form an anion I- that decomposes to 2,4,6- trinitrodiphenylamine (TNDPA) and CH30-. The two intermediate complexes HI and I- never accumulate in the system (stationary state) and the general equation established from the reaction scheme is written: KK'k, k , - dt = kdt dx (a - x) (b - x) - [(H+) + K ] [k-,(H+) + K'k21 where a and b are the initial concentrations of TNA and AnH,, respectively, and x is the concentration of the final product TNDPA.Generally we used very low concentrations of TNA in comparison with other reagents because the TNDPA formed is a dye that allows us to study the kinetics spectrometrically. Consequently, the kinetic curves always degenerated to first order and the equation of rate is written : where kapp is the apparent rate constant. We have already studied this reaction in non-micellar solutions and the results show that this system can be interpreted in accordance with two kinetic laws. (i) When the medium is basic, ( K $- [H+] 4 K'k,/k-,) the zwitterionic complex HI ejects the proton completely and instantaneously. Then the reaction (1) (fig. 1) is the determinant step because k, (loss of the leaving group) is very high. kapp is dependent only on the concentration of aniline: u = dx/dt = kb(a - x) = kapP [TNA] (2)J.Lelihre, 0. Huddud-Fahed and R. Gaboriaud 2303 NH2 / H+ + a + AnHz NO;! NO, I - Fig. 1. Reaction scheme of nucleophilic substitution of aniline (AnH,) on 1 -methoxy-2,4,6- trinitrobenzene (TNA). The complexes HI and I- are not accumulated during the reaction and 2,4,6-trinitrodiphenylamine (TNDPA) formation is followed by means of a spectrophotometer. (ii) On the contrary, for a less basic range ( K % [H+] % K'k,/k-,) the intermediate complex exists mainly in the HI form and dissociates principally to give the initial reagents because it is very unstable [process (- l)]. The rate of formation of TNDPA is thus determined by means of the ratio of the rates corresponding to the two processes of dissociation of the intermediate complex HI, and this ratio is controlled by the pH : If we use only TNA and AnH, as reagents, the pH value of the solution is fixed by the aniline concentration and [H+] is calculated as : [H+] = (K"K,/c)'/~ [PH = O.S(pKW + pK, + log c)] [AnHz]3/2.k , k , K' and eqn (4) becomes: k P P = - k-, (KwK,)1/2 ( 5 ) The partial order of the reaction according to aniline is 1 for case (i) and 3/2 for case (ii). Using different systems (see below) we can obtain one or other type of behaviour. Nevertheless the experimental range in which this occurs is restricted by parasitic reactions. (i) When the medium is acid, the aniline is more or less in the form of the anilinium cation AnH: which is unreactive. The best range for study is where pH values are higher than the pKvalue of the couple AnH:/AnH, in the micellar solution.(ii) When the medium is basic, two other reactions can occur: (a) the product formed (TNDPA) is a weak acid and is thus able to ionize in solution, but this ionization has no effect on the kinetics of the reaction. (b) The nucleophilic attack of aniline [process (l), fig. 11 can be in competition with reaction with OH- ions, which indeed predominates when the pH value is high enough. The product of this reaction is not TNDPA (the molecule or the corresponding ion) but the picrate anion formed from 2,4,6-trinitrophenol (picric acid). (iii) If we use buffer solutions for controlling the pH value, the basic species in the buffer can compete with the aniline.2304 Nucleophilic Aromatic Substitution in Micelles Table 1.C.m.c. values for surfactants, concentrations of stock solutions and minimum concentrations for plateau rate surfactan t c.m.c. /mol dmP3 HDTABr TDTABr DDTABr DTABr NaDDS NaDS NaOS 9.2 x 10-4 3.5 x 10-3 8.1 x 10-3 1.5 x low2 6.5 x 3.3 x 10-2 1.4 x 10-1 concentration of stock solution /mol dmP3 5 x 10-2 5 x 10- 2 x 10-2 2.5 x 10-1 2 x 10-1 2 x lo-' 2.5 x 10-1 ~~ ~~ plateau-rate concentration /mol dm-3 10-2 2 x lop2 2.5 x 10P 10-1 1.5 x 5 x Experiment a1 1 -Methoxy-2,4,6-trinitrobenzene (TNA) was prepared in our laboratory from picryl chloride and sodium hydroxide in methanol solution. This aromatic substrate is not very soluble in water and it was used in the form of stock micellar solutions. The concentration of TNA was in the range (4-5) x mol dm-3.The aniline was a commercial reagent, purified by distillation and the stock aqueous solutions ([aniline] = 0.2 mol dm-3) were stored at low temperature. We have already studied the reaction in water and several H,O-CH,OH mixtures4*3 49 and shown that a plot of logk,,, =f(pH) was principally composed of two linear parts with slopes 1 and 0, corresponding to the limiting laws (4) and (5). The system discussed here has been investigated in aqueous solutions of several surfactants. The kinetics were carried out in a micellar solution of TNA and an aqueous solution of aniline without any other compound. In those solutions the pH value is determined by the concentration of aniline and though the medium is basic, the pH is not high enough and there is no other nucleophilic reagent to be in competition with AnH,.We used (Sigma Chemical Co.) for hexadecyl-, tetradecyl- and dodecyl-trimethyl- ammonium bromide (HDTARr, TDTABr and DDTABr, respectively) and decyltri- methylammonium bromide and sodium dodecyl-, decyl- and octyl-sulphate (DTABr, NaDDS, NaDS, NaOS, respectively, from Eastman Kodak). Table 1 shows the values of critical micelle concentrations (c.m.c.) and the concentrations of stock solutions used. An Acta I11 Beckman spectrophotometer was used with the measurement cells thermostatted at 25 0.1 "C. The maximum of absorption of TNDPA is at 3, = 400 nm and all the kinetic data have been carried out at this wavelength and were extracted using an Apple IIe microcomputer and a program written in our laboratory. We checked, in every case, that the kinetic development was always pseudo-first-order in terms of aniline.Results By using several surfactants with different aliphatic chainlengths we were able to study a variety of solutions with different c.m.c. Below the c.m.c. the kinetics of reaction were modified little by surfactants. For concentrations higher than the c.m.c., the rate increased in every case and we found a flat maximum of micellar catalysis. The increase in rate of reaction is illustrated in fig. 2 for a fixed concentration of aniline. The micellar catalysis occurs, in every case, for a concentration close to the value of the c.m.c. of every detergent; we could not illustrate the catalytic effect for NaOS in this figure because this effect is observed for a very high concentration (ca.0.2 mol dm-3). The progressive increase in the number of micelles allows an increased extraction ofJ. Leliivre, 0. Haddad-Fahed and R. Gaboriaud T 2305 5 x 10-2 lo-' [ surfactant ]/mol dm-3 Fig. 2. Plot of the pseudo-first-order rate constant kapp over concentration of aniline us. concentration of several detergents. The concentration of aniline is fixed (4 x lop3 mol drnp3) and T = 25 "C. x , HDTABr; 0, TDTABr; +, DDTABr; 0, DTABr; 0 , NaDDS; ., NaDS. the two reagents (AnH, and TNA) from the aqueous phase and the rate of reaction reaches a maximum value when extraction is practically complete. For higher concen- tration of surfactants, the observed rate (for fixed concentration of AnH,) remains constant and it is depicted in fig. 2 by a plateau.The increased rate of reaction obtained for the conditions of maximum extraction (when the concentration of aniline is 4 x lop3 mol dmP3) is greater than in water by a factor of: 4 for NaDS solutions, 10 for NaDDS solutions, 60 for DTABr solutions, 100 for DDTABr solutions and 400 for TDTABr solutions and HDTABr solutions. This increase is obviously the result of a local higher concentration of reagents in the micelles. But, in fact, it is difficult to interpret these values in terms of actual concentrations for several reasons: (i) the volume of the micellar phase is not properly defined because the boundary of the reaction between the aqueous phase and the micelle is not clearly specified. (ii) The increase of rate is the result not only of the change of concentration but also the modification of free enthalpy of each reagent by transfer from the water phase to the organic phase.We would be able to take into account the change of medium only if we knew the free enthalpies of extraction of the two reagents (deduced from the partition constants) and this information is not available.? (iii) Finally, the main reason why the rate ratios are not significant in an absolute way is because their value is dependent upon the concentration of aniline: any other value of [AnH,] would give similar behaviour, but other numerical values (see below). The results obtained show that the rate of reaction depends on the pH according to the kind of surfactants used. As the pH is determined by the concentration of aniline in our experimental conditions, this concentration modifies the overall rate in a different 1- A binding constant K = 14 has been proposed by Bunton et aL50 for AnH, with NaDDS, but we have no corresponding value for TNA.2306 Nucleophilic Aromatic Substitution in Micelles slope = 1 -- slope = 1.5 1 log( [ AnH, ]/mol dm-3) Fig.3. Plot of the pseudo-first-order rate constant kapp in several solutions of detergents: 0, HDTABr (0.015 mol dmP3); 0, TDTABr (3 x mol dmP3); 0, DTABr (0.15 mol dm-3); +, NaDDS (4 x rnol dm-3); x , water. For cationic detergents the plots are represented by straight lines whose slopes are unity and thus kapp is given by eqn (3); for anionic detergents and aqueous solutions plots are represented by straight lines whose slopes are 1.5.For these experimental cases, kapp is given by eqn (5). mol dmP3); A, DDTABr (4 x mol dm-3); D, NaDS (8 x way in every situation. For a constant concentration of AnH,, a comparison between the different kinds of surfactants by use of fig. 2 is inevitably not significant. For comparing the catalytic efficiencies of every surfactant we have proceeded as follows: we have chosen to use for every surfactant a concentration such that the rate of the reaction had attained its maximum constant value. Under these conditions the effect of concentration of amphiphile is eliminated and the influence of variations of the concentration of nucleophilic reagent AnH, can be investigated without interference from this effect (fig. 3). Then we notice that we find again a first-order kinetic law in terms of aniline when salts of alkyltrimethylammonium are used and formed positive micelles. In contrast, when the reaction is carried out in aqueous media (or in H,O-CH,OH) or using negative micelles formed by alkylsulphate ions, the kinetic law has experimentally an order of 3/2 in terms of aniline.The previous experiments in water-methanol mixtures have proved the validity of the reaction scheme. Consequently, these two different orders are explained by the two kinetic equations (3) and (5). The last result is significant because it is a demonstration that micelles are able to modify not only the rate of the process but also the kinetic laws; that is to say the intervention of several processes contributing to the total rate.We have shown in the previous paragraph that the reaction is controlled by process (l), fig. 1, if the intermediate species HI ejects a proton rapidly and totally while this control is modulated by the ratio of rates of dissociation of the intermediate complex HI into I- and H+, or alternatively TNA and AnH, if the ejection of proton occurs with more difficulty. Consequently, the plateau shown in fig. 2 does not have the same meaning for the two cases: in the presence of cationic detergents the plateau value is equal to log(k,) and the micelles greatly increase the first kinetic step. On the contrary, for an aqueous solution or anionicJ . Lelihre, 0. Haddad-Fahed and R. Gaboriaud 2307 detergent solutions, the plateau value depends on the concentration of aniline and increases with this concentration : the k , value cannot be obtained straightforwardly from the graph and the influence of anionic and cationic detergents upon the first step cannot be compared directly from this figure.These findings are confirmed in another way. It is well known that an addition of background electrolyte is almost ineffective on the rate of reaction between two neutral species enclosed in the micellar core but, in comparison, such an addition can be very effective on the reaction rate if there is charge transfer between the micelle and the bulk. In our case, if we add background electrolyte to our solutions (TNA-AnH,), two experimental trends are observed. When the micellar system in use gives an order of 1 iii terms of aniline there is no modification of the reaction rate, but when the order is 3/2 there is a decrease of the reaction rate.These effects are in good agreement with the proposed mechanism. Finally, the part played by each type of surfactant is very clear: when the micelles are positive, the charge of the medium thermodynamically favours the ejection of H+ from HI and this process is easier than in aqueous solution; the rate of reaction is controlled simply by the process of formation of the HI complex with a rate constant k,. On the contrary, in water the ejection of Hf is more difficult and the order in terms of aniline is 3/2, which is a demonstration of the influence of pH. When we use anionic micelles this effect is reinforced, the charge of the micelle is opposed to the ejection of a proton from HI and this process becomes even more difficult (the pKof the intermediate complex is lowered by cationic micelles and enhanced by anionic ones).Thus, in the case of anionic micelles, the influence of surfactant is felt in two opposing ways: the extraction. of reagents into the micelles creates an increased probability of encounter between reagents, but the complex appears in a region of negative potential that is opposed to the reaction path which leads to the usual final product. However, the first effect is the dominating one and the reaction is faster than in water (10 times higher for NaDDS solutions) but slower than in cationic surfactants. For positive micelles the alkyl chain length seems to influence the degree of extraction and the rates are 5 times less for C,, chains than for C,, and C,, chains and 10 times less for C,, chains.The results carried out in the presence of the same detergents, but for concentrations where the reaction has not reached its maximum value are not reported here. In this case, the concentration of the detergent cannot be eliminated and the values of the slopes of the experimental lines giving the apparent order of the reaction in terms of aniline are not necessarily in the range 1-1.5, owing to the progressive incorporation of AnH, into the micelles. Conclusion Investigation of the reaction between aniline and 1 -methoxy-2,4,6-trinitrobenzene shows a more complex influence of a micellar system than in ordinary cases. For such a system, with several determining steps, the kinetic study in micellar solutions can be used to confirm the reaction scheme. In the present case, for the first process of the kinetic scheme where two molecules are reacting (TNA + AnH,), the micellar catalysis is only the effect of a higher concentration in the micelles and consequently a larger probability for the reaction.The adduct formed during this first process is able to react later according to two different ways, either splitting up into two molecules (return to the initial components) or losing a proton. The charge of the micellar system has a large influence upon this last process and the cationic detergents are good catalysts of this transformation because they favour the ejection of the proton. On the contrary, this ejection is largely inhibited by the use of anionic detergents: the effect of a higher concentration in the micelles is partially compensated and the concentration of the nucleophilic reagent becomes essential (order 3/2) because the ejection of the proton is favoured by the increase of this concentration.2308 Nucleophilic Aromatic Substitution in Micelles These conclusions are confirmed by experiments carried out in other conditions (additions of background electrolytes and buffer solutions). These experiments imply the intervention of other parameters and of some parasitic reactions; they will be described in a later paper.However, it is interesting to report now that the whole set of experimental data can be interpreted using the theoretical model which has been recalled in the first paragraph. References 1 Micellization, Solubilization and Microemulsions, ed.K . L. Mittal (Plenum Press, New York, 1977), vol. 2 Solution Chemistry of Surfactants, ed. K . L. Mittal (Plenum Press, New York, 1979), vol. 1 and 2. 3 Solution behavior of Surfactants. Theoretical and Applied Aspects, ed. K . L. Mittal and E. J. Fendler 4 Surfactants in Solution, ed. K . L. Mittal and B. Lindman (Plenum Press, New York, 1984), vol. 1, 2 5 C. A. Bunton and S. Diaz, J. Am. Chem. Soc., 1976,98, 5663. 6 C. A. Bunton, F. H. Hamed and L. S . Romsted, J. Phys. Chem., 1982,86,2103. 7 P. Linda, A. Stener, A. Cicipiani and G. Savelli, J. Chem. Soc., Perkin Trans. 2, 1983, 821. 8 C. A. Bunton, L. S. Romsted and L. Sepulveda, J. Phys. Chem., 1980, 84, 261 1.9 C. A. Bunton, L. H. Gan, J. R. Moffatt and L. S . Romsted, J . Phys. Chem., 1981, 85,4118. 1 and 2. (Plenum Press, New York, 1982), vol. 1 and 2. and 3. 10 I. M. Cuccovia, R. M. V. Aleixo,N. E. Erismann,N. T. E. vanderzee, S. Schreierand H. Chaimovich, 11 J . B. S. Bonilha, G. Chiericato Jr., S . M. Martins-Franchetti, E. J. Ribald0 and F. H. Quina, J. Phys. 12 F. H. Quina, M. J. Politi, I. M. Cuccovia, S. M. Martins-Franchetti and H. Chaimovich, in ref. (3), vol. 13 C. A. Bunton, Y. S. Hong and L. S. Romsted, in ref. (3), vol. 2, p. 1137. 14 H. Chaimovich, R. M. V. Aleixo, I. M. Cuccovia, D. Zanette and F. H. Quina, in ref. (3), vol. 2, p. 949. 15 H. Al-Lohedan, C. A. Bunton and J. R. Moffatt, J. Phys. Chem., 1983,87, 332. 16 A. Cicipiani, P. Linda, G.Savelli and C. A. Bunton, J. Phys. Chem., 1983, 87, 5262. 17 C. A. Bunton, L. H. Gan, F. H. Hamed and J. R. Moffatt, J . Phys. Chem., 1983, 87, 336. 18 C. A. Bunton, L. H. Gan and G. Savelli, J. Phys. Chem., 1983,87, 5491. 19 C. A. Bunton and L. S. Romsted, in ref. (3), vol. 2, p. 975. 20 C. A. Bunton, J. Frankson and L. S. Romsted, J . Phys. Chem., 1980,84, 2607. 21 C. A. Bunton, L. H. Gan, J. R. Moffatt, L. S. Romsted and G. Savelli, J. Phys. Chem., 1981,85,4118. 22 F . Nome, A. F. Rubira, C. Franco and L. G. Ionescu, J. Phys. Chem., 1982,86, 1881. 23 L. S. Romsted, in ref. (4), vol. 2, p. 1015. 24 C. A. Bunton, L. S. Romsted and H. J. Smith, J. Org. Chern., 1978, 43, 4299. 25 C. A. Bunton, A. Kamego and M . J. Minch, J. Org. Chem., 1972,37, 1388. 26 I. V. Berezin, K.Martinek and A, K. Yatsimirskii, Russ. Chem. Rev., 1973, 42, 787. 27 C. A. Bunton and J. L. Wright, Tetrahedron, 1975,31, 3013. 28 H. Chaimovich, A. Blanco, L. Chayet, L. M. Costa, P. M. Monteiro, C. A. Bunton and C. Paik, 29 I. M. Cuccovia, E. H. Schroter, P. M. Monteiro and H. Chaimovich, J. Org. Chem., 1978, 43, 2248. 30 K. Martinek, A. P. Osipov, A. K. Yatsimirskii and I . V . Berezin, Tetrahedron, 1975, 31, '709. 31 J. H. Fendler and E. J. Fendler, Catalysis in Micellar and Macromolecular Systems (Academic Press, 32 C. A. Bunton, Pure Appl. Chem., 1977, 49, 969. 33 T. Eiki and W. Tagaki, Bull. Chem. Soc. Jpn, 1982,55, 1102. 34 R. Gaboriaud, G. Charbit and F. Dorion, in ref. (4), vol. 2, p. 1191. 35 F. Dorion, G. Charbit and R. Gaboriaud, J. Colloid Interface Scz., 1984, 101, 27. 36 G. Charbit, F. Dorion and R. Gaboriaud, J . Chim. Phys., 1984, 81, 187. 37 F. H. Quina and H. Chaimovich, J . Phys. Chem., 1979,83, 1844. 38 H. Chaimovich, J. B. S. Bonhila, M. J. Politi and F. H. Quina, J . Phys. Chem., 1979, 83, 1851. 39 M. Almgren and R. Rydholm, J . Phys. Chem., 1979, 83, 360. 40 C. A. Bunton, L. S. Romsted and G. Savelli, J. Am. Chem. Soc., 1979, 101, 1253. 41 K. Shirahama, Bull. Chem. SOC. Jpn, 1975, 48, 2673. 42 K. Shirahama, Bull. Chem. Soc. Jpn, 1976, 49, 2731. 43 N. Funasaki, J. Colloid Interface Sci., 1978, 64, 461. 44 N. Funasaki, J. Phys. Chem., 1979, 83, 237; 1998. 45 J. Lelievre, Thesis (Paris), 1982. J. Am. Chem. Soc., 1982, 104, 4544. Chem., 1982,86,4941. 2, p. 1125. Tetrahedron, 1975, 31, 1139. New York, 1975).J . Lelihre, 0. Haddad-Fahed and R. Gaboriaud 2309 46 J. Lelievre and R. Gaboriaud, J. Chem. Soc., Furaday Trans. 1, 1985, 81, 335. 47 R. Gaboriaud, J. Lelievre, G. Charbit and F. Dorion, Proc. 5th Symp. (Znt.) on Surfuctunts, ed. K. L. Mittal and P. Bothorel (Plenum Press, New York, to be published). 48 J. Lelievre, R. Gaboriaud and R. Schaal, C. R. Acud. Sci., 1971, 272 C, 1780. 49 J. Lelievre, P. Letellier and R. Gaboriaud, C. R. Acad. Sci., 1972, 274 C, 748. 50 C. A. Bunton, G. Cerichelli, Y . Ihara and L. Sepulveda, J . Am. Chem. Soc., 1979, 101, 2425. 51 F. Terrier, Chem. Reti., 1982, 82, 78; 127. Paper 5 / 12 12; Received 16th July, 1985

ISSN:0300-9599    

DOI:10.1039/F19868202301

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. 1, 1986, 82, 23‘11-2321 A Quasi-elastic Neutron Scattering Study of Water-in-oil Microemulsions stabilised by Aerosol-OT Effect of Additives including Solubilised Protein on Molecular Motions Paul D. I. Fletcher? and Brian H. Robinson* University Chemical Laboratory, University of Kent at Canterbury, Canterbury, Kent CT2 7NH James Tabony Departement de Physico-Chimie, Centre d’Etudes Nucleaires de Saclay, 91 191 Gif-sur- Yvette, France The quasi-elastic incoherent neutron scattering method has been used to investigate the mobility of surfactant and water molecules in single-phase oil-continuous microemulsions stabilised by sodium bis(2-ethyl hexy1)sulpho- succinate (AOT). Addition of benzyl alcohol, which is adsorbed at the interface, or toluene effects little change in the lateral translational diffusion of AOT within the interface.Replacement of the dispersed water by glycerol results in a two-fold reduction in surfactant mobility. Solubilisation of a-chymotrypsin withm the water droplet core of the microemulsion system causes no significant change in AOT mobility. However, spectra associated with water mobility in the enzyme-containing system clearly reveal the presence of 250-500 water molecules per protein molecule, which are ‘bound’ on the time scale of the experiment (6 x 1O1O rad s-l). The mobility of the remainder of the solubilised water in the system is unaffected by the presence of the enzyme. Quasi-elastic incoherent neutron scattering has recently been used to study molecular motions in micellar and microemulsion systems.3 For water-in-oil microemulsions stabilised by sodium bis(2-ethylhexyl)sulphosuccinate (AOT), such studies have indicated that the AOT molecules undergo lateral diffusion along the curved water/oil interface of the water droplets with a characteristic diffusion coefficient of 8 x m2 s-l. The motion of solubilised water within the droplet core is characterised by a diffusion coefficient of 1.2 x m2 s-l, which is comparable to the value obtained in high- ionic-strength bulk aqueous rnedia.l? * It is easily possible to prepare glycerol-in-oil microemulsions stabilised by AOT which are analogous to the more conventional water-in-oil type. The oil-continuous microemulsion system formed by glycerol-heptane-AOT, where the water core has been replaced by glycerol, has been extensively characterised using the techniques of photon correlation spectroscopy and viscometry and it behaves in a similar way to the corresponding water-containing microem~lsion.~ In the present study we compare quasi-elastic incoherent neutron scattering profiles obtained from both systems.By this means the local lateral molecular motions of AOT can be determined and it can be established whether these are controlled primarily by the viscosity of the non-continuous dispersed phase, the continuous apolar oil phase or the viscosity of the interfacial region. It is known that in the AOT-stabilised microemulsion system, water-solubilised hydrophilic reagents such as metal ions can readily be exchanged between water t Present address: Chemistry Department, University of Hull, Hidl HU6 7RX.2312 Neutron Scattering Study of Microemulsions droplets.4> This process occurs with a second-order rate constant of 106-107 dm3 mol-1 s-l and is thought to proceed via a transient coalescence of the water droplets, followed by reseparation of the droplets. This results in random exchange of species contained within the water droplets. This is an important kinetic process in microemulsions and is thought to be the mechanism whereby the microemulsion establishes its equilibrium properties (e.g.particle size, polydispersity etc.), which are then maintained in rapid, dynamic equilibrium. The kinetics of this exchange process are known to be affected by the addition of various additives to the system.For example, addition of 0.1 mol dm3 benzyl alcohol to the AOT-heptane-water system ([H,O]/[AOT] = 20; [AOT] = 0.1 mol dm-3) causes a 50% increase in the rate, whereas addition of 10% v/v toluene causes a 50% decrease in the rate.6 In particular, species located in the interface region, such as Ru(bipy)g+, are readily transferred between two droplets in c0ntact.j It is of interest, therefore, to determine whether the changes observed in the inter-droplet exchange rate are paralleled by changes in the local motions of the AOT molecules at the oil/water interface. Finally, there is much current interest in the solubilisation of enzymes in water-in-oil microemulsions, since these systems have interesting synthetic po~sibilities.~ The hydro- philic protein a-chymotrypsin retains its activity when solubilised in water-in-oil microemulsions stabilised by AOT.Measurements of the catalytic activity of this enzyme in AOT microemulsions towards a variety of substrates have shown that the turnover number of the protein (kcat) is little affected upon solubilisation, but that the binding affinity between the enzyme and various substrates is reduced by a factor of ca. one-hundred-fold.8-11 A possible explanation of this effect for AOT is that it binds to the enzyme, thus reducing its substrate affinity. In this study, we compare the AOT spectra obtained from microemulsions with and without a-chymotrypsin in order to observe any changes in molecular mobility of the AOT caused by the presence of enzyme in the core. Also, selective deuteration permits a comparison to be made of the diffusion characteristics of the solubilised water in the two systems.Experimental AOT was obtained from Sigma (as sodium dioctylsulphosuccinate) and was used without further purification. Its purity and consistency were checked by interfacial tension measurements. The phase behaviour of the sample, in particular the upper temperature cloud point, was also checked. It contained negligible amounts of an acidic impurity which is often present in samples of AOT.12 Hydrolysis of AOT is known to produce a carboxylic acid and 2-ethylhexanol, the rate of hydrolysis being quite rapid under basic conditions. l2 However, under the conditions of sample preparation and recording of spectra such hydrolysis was negligible.The enzyme a-chymotrypsin was purchased from Sigma (bovine pancreas type 11). Deuterated octane and D,O were obtained from CEA, France. A sample of deuterated glycerol was generously donated by Drs J. C. Dore and A. Angell. Benzyl alcohol and toluene were of analytical grade and were used without further purification. Triply distilled water was employed throughout and all neutron measurements were made within 24 h of preparation of the microemulsions. Water-in-oil microemulsions were prepared by adding the correct volume of water to an AOT solution in octane. Clear solutions were obtained after gentle shaking for ca. 30 s. Glycerol microemulsions were prepared by weighing the required amount of glycerol into a flask, adding an octane solution of AOT, followed by gentle shaking.a-Chymotrypsin was weighed into the flask, the correct volume of water was added and the flask shaken to produce a solution. As before, addition of an AOT solution in octane followed by gentle shaking produced clear microemulsion solutions. Neutron measurements on the enzyme-contaning microemulsions were made within 3 h of preparation in order to obviate, as far as possible, the effects of enzyme inacti~ation.~? l1P. D. I. Fletcher, B. H. Robinson and J . Tabony 2313 Neutron incoherent quasi-elastic spectra were recorded at ambient temperature ( 1 9 _+ 2 "C) on the high-resolution, time focusing, time-of-flight spectrometer (IN6) located at the high-flux reactor of the Institut Laue-Langevin, Grenoble. The incident wavelength (A) was 5.1 A and counting times were of the order of 1 h.The samples were good scatterers, with ca. 20000 counts at the peak maximum, and ca. 4000 counts in the wings. Energy spectra at nineteen different scattering angles (6) between 12 and 112" were recorded for each sample. Spectra were taken with the sample at 45 and 135" to the incident beam. The energy resolution (half-width at half height) varied with both the scattering angle and the sample angle and was 40-80 peV. This resolution was determined using a vanadium sample. Samples were contained in flat aluminium holders of sample thickness 0.5 or 1 mm. Ca. 5 cm3 of the sample was required in order to obtain a spectrum. Neutron transmissions were greater than 90% in all cases. Data were corrected for transmission, detector efficiency and background scattering by means of standard procedures and programs available at the ILL.No corrections were made for multiple scattering. The incoherent neutron scattering spectrum S(Q, co) is related to diffusional motion (Fickian translation) by where Q = (412/A) sin 6 / 2 , D is the translation diffusion coefficient and w is the neutron energy. The quasi-elastic spectra are then described by Lorentzian curves with a full-width at half-height (2Aw) of 2DQ2. In contrast, for rotational motion, a superposition of a quasi-elastic peak, which is independent of Q, and an 'elastic' peak is observed. This approach has been used to obtain the translational diffusion coefficient (D) of AOT and water in oil-continuous microemulsions in cyclohexane by means of plots of the half width Aco us.Q2.1 The spectrum corresponding to one particular chemical component in the microemul- sion solutions was obtained by subtraction of the various experimental spectra. For example, the spectrum due to AOT motion in the system D,O-AOT-D(octane) was obtained by subtracting the deuterated octane solvent spectrum from the spectrum of the AOT microemulsion containing D20. (Necessary corrections were made in this procedure for the volume fraction of dispersed material.) The resulting spectrum contains a negligible scattering contribution from D 2 0 (for the concentrations used here where the volume fraction of D 2 0 is of the order of 2%) in comparison with the scattering arising from the proteated AOT. A check was in any case made that the integrated intensities were proportional to the concentration of proteated material.The resulting spectra was then fitted to a convolution of a Lorentzian curve with the experimentally determined resoluton function. Results and Discussion Effect on Surfactant Motion of the Substitution of Water by Glycerol in the Droplet Core Spectra were recorded for the following samples: (i) 0.2 mol dm-3 AOT, 4 mol dm-3 D,O in deuterated octane and (ii) 0.2 mol dm-3 AOT, 0.550 mol dm-3 deuterated glycerol in deuterated octane. The spectra corresponding to AOT alone were obtained by subtraction of the solvent [D(octane)] spectrum. Both microemulsion systems are known to contain discrete, approximately spherical droplets of dispersed phase [D,O or D(glycerol)] surrounded by an interfacial layer of the surfactant.A small amount of the total surfactant may be present in the form of reversed mi~e1les.l~ Both microemulsion systems have been2314 Neutron Scattering Study of Microemulsions -0.5 0 0.5 - 0.5 0 0.5 O / 0 0 0 0003000000300 -0.5 0 0.5 energy/meV 1.250 h 3 9 0.625 0.0 -1.0 -0.5 0 0.5 1 .o energylmev Fig. 1. Quasi-elastic incoherent spectra due to AOT in (a) the water microemulsion, (b) the glycerol microemulsion and the (c) resolution function. The scattering angle 8 was 97.3", sample at 45" to incident beam, continuous phase is n-octane. ( d ) Matching of the experimentally determined curve to the theoretical line. AOT in the aaueous microemulsion at 8 = 97.3".P. D. I. Fletcher, B. H. Robinson aiid J .Tabony 200 r 2315 0 2 4 Q2/8-2 Fig. 2. Plots of linewidth us. Q2 for AOT spectra in the water microemulsion (0) and the glycerol microemulsions (a). The dashed and solid lines refer to the resolution function width for sample mounting at 135 and 45", respectively. Error is ca. 15% in Am. All points refer to spectra recorded at 45" except Q2 values in the range 1-2 k2. Continuous phase is n-octane. characterised by a variety of methods including small-angle neutron scattering and photon correlation spectro~copy.~~ 14-16 The droplet core radii (ie. not including the surfactant shell) of the water and glycerol samples are 3.5 and 2.4 nm, respectively. The overall hydrodynamic radii (i.e. including the surfactant shell and any associated solvent) are of the order of 5.0 and 4.1 nm.Therefore, for the quasi-elastic neutron scattering experiments reported in this paper the spectra of the interfacial AOT in the water and glycerol dispersions are compared in droplet systems of comparable particle sizes and hence curvature of the surfactant interface. We do not expect a significant variation with droplet size or droplet concentration, based on previous experience with the water droplet dispersion.lt Fig. 1 shows the AOT spectra in both systems and the resolution function obtained under identical conditions. It can be clearly seen that the energy broadening is considerably reduced for the glycerol system [fig. 1 (b)] as compared with the water system (a). In addition, there is clearly some broadening in (b) as compared with the instrument resolution (c).The spectra were analysed as described previously to obtain a quasi-elastic scattering width (Am) as a function of the scattering vector Q. A typical fit of the experimental data to the theoretical curve based on eqn (1) is shown as fig. 1 (d). Plots of Am us. Q2 for both systems are shown in fig. 2. For the dispersed water system, the plot closely resembles that previously observed for AOT motion when cyclohexane was used as the oil-continuous so1vent.l There is an approximately linear relationship between Am and Q2 and from the slope the diffusion coefficient D is obtained. (The gap in the region of Q2 = 1-2 k2 is due to the fact that for samples mounted at 45" to the beam the scattering is attenuated by the sample container edge. Mounting the sample at 135" yields a full data set, but the energy resolution is less favourable.For certain samples, measurements were made at both 45 and 135".) The measurements made at 135" show a plateau in the plots of Am us. Q2 for values of Q2 between 1 and 2 k2, This could indicate that the observed motion is more complex than a simple translational motion. However, the plateau could also result from the fitting procedure used. The random error in the derived broadening is ca. 15 % , and this covers most of the difference between the values of the broadening plotted and a linear least-squares fit through the experimental points. In the plateau region the broadening is comparable with the instrument resolution, which may introduce some difficulties into the fitting procedure.When the broadening is significantly different from the instrument resolution, no major problems are encountered with the fitting routine. Broadenings less than the resolution are detected (as indicated for example in fig. 2) because a Lorentzian broadening has wings which extend much further out than those of the instrument resolution, which is Gaussian-shaped [fig. 1 (c)]. However, when the broadening is comparable with the resolution, it may be2316 Neutron Scattering Study of Microemulsions that the fitting criteria used cannot so readily distinguish between slightly different broadenings. Hence, for broadenings which are slightly different the derived value could be essentially invariant, which might account for the plateau behaviour observed between Q2 = 1-2 k2.The discussion in this paper will be concentrated on possible interpretations of the Q-dependent diffusion parameter that we associate with an essentially translational diffusion mode, which for AOT represents lateral molecular diffusion within the interface. Drawing straight lines passing through the origin for the Am z's. Q2 plots yields values for the diffusion coefficient of AOT in the water and glycerol systems of 6.1 and 3.8 x 10-lo m2 s-l, respectively, with n-octane as dispersed phase. The value determined in the corresponding AOT-water-cyclohexane system was 8 x 10-lo m2 s-l. It is important to note that this was essentially independent of the amount of water in the microemulsion system.l As the water concentration is increased at constant AOT concentration the size of the droplets increa~es.~ Since a different fitting procedure was used in the cyclohexane work, the difference between the two water-containing systems as the solvent is changed is not thought to be of significance.However, there is a very clear reduction in translational diffusion when glycerol is substituted for water. The observed translational motion of AOT may arise from motion of AOT around a microemulsion droplet or from motion of the whole microemulsion droplet through the solution. The translational diffusion coefficient (DT) of the total microemulsion droplet may be calculated from the known droplet hydrodynamic radius and the solvent viscosity using the Stokes-Einstein equation.2 It is readily determined experimentally, for low concentrations of droplets, using photon correlation spectroscopy : D, = kT16nrq.(2) In eqn (2), r is the droplet hydrodynamic radius and q is the solvent viscosity (5.42 x kg m-l s-l for octane at 20 "C). Using eqn (2), values of the translational diffusion coefficient for the water and glycerol droplets of 0.8 x loplo and 1 .O x m2 s-l, respectively, may be calculated. Thus the overall translational motion of the droplets can account at most for only 1&20% of the observed effect. The surface motion of the AOT surfactant molecules may arise from two causes. First, the AOT molecules may diffuse laterally along the interface. Secondly, since the characteristic length scale of our neutron scattering observation is in the range 2 to 10 A (0.2-1 nm), dimensions much smaller than the droplet radius (ca.3.5 nm) are probed. On this length scale, the overall rotation of the droplet would cause the AOT molecules, if rigidly located on the surface of the sphere, to move with an apparent translational motion. The magnitude of this effect may be calculated using eqn (3) for the rotational diffusion coefficient (8) of a sphere:17 8 = kT/8nyr3. (3) The apparent diffusion coefficient caused by rotation of the whole particle [Dr(app)] is then given by the product of 8 and the mean-square jump distance. Taking the latter as being approximately the droplet radius, gives : (4) kT Dr(app) = - 8nqr. As might be expected, this diffusion coefficient is very similar to that derived on the basis of the Stokes-Einstein law and yields values for Dr(app) similar to those calculated for D,.For both translational and rotational displacement of the droplet, the diffusion rate will vary with the inverse of the radius. Previous experiments of AOT dispersions in cyclohexane, in which the droplet size was increased by a factor of three, resulted in no change in the Q-dependent broadening with increasing droplet size.l The sum of the evidence therefore suggests that our observed diffusion effect is dominated by theP. D. I. Fletcher, B. H. Robinson and J . Tabony 300 r 200 I l o o t 0 o o 2 Q2/A-2 2317 Fig. 3. Plots of linewidth us. Q2 for the AOT spectra in AOT-octane-water microemulsions containingno additive (O), 10% v/v deuterated toluene (0) and 0.2 mol dmP3 benzyl alcohol (A).Error is ca. 15 % in Am. Continuous phase is n-octane. translational diffusion of the surfactant molecules laterally along the interface. Moreover since the water and glycerol droplets have sizes which differ by only 20%, and the glycerol droplet is smaller than the water droplet, the reduction in the AOT line broadening when glycerol is substituted for water must arise from a reduction in the translational mobility of the AOT molecules in the interface. Since the viscosity of glycerol is ca. 1500 times greater than that of water, this would imply little or no ‘anchoring’ of the AOT within the dispersed phase in either system. Effect of Surfactant Motion of Addition of Benzyl Alcohol and Toluene Spectra were recorded for the following samples : (iii) 0.2 mol dm3 AOT, 4 mol dmP3 D20, 10% v/v deuterated toluene in deuterated octane and (iv) 0.2 mol dmP3 AOT, 4 mol dm-3 D20, 0.2 mol dm-3 benzyl alcohol in deuterated octane.As discussed previously, for sample solution (iii) the spectrum of the AOT alone was obtained by subtraction of the solvent (deuterated octane) spectrum. Toluene, being deuterated, made a negligible contribution to the observed scattering. For sample solution (iv), deuterated benzyl alcohol was not available, hence the difference spectrum will contain a contribution from both AOT and the alcohol. The ratio of integrated intensities from AOT and the alcohol is approximately given by the ratio of protons in each molecule; this ratio is 37: 8. Plots of Aco us. Q2 for samples (i), (iii) and (iv) are shown in fig.3. Values of the apparent diffusion coefficient calculated from the slopes for samples (i), (iii) and (iv) are (6.1, 7 and 8) x 10-lo m2 s-I, respectively. Since the benzyl alcohol-containing microemulsion spectra contain contributions from the alcohol, it seems that the AOT mobility is essentially unchanged with and without the additives. Certainly there appears to be no correlation between the local AOT mobility and the rate of solubilisate exchange, which involves droplet coalescence. In addition, the upper phase-transition temperature is affected differently by the two additives : benzyl alcohol induces phase separation at a lower temperature, whereas toluene increases the tem- perature corresponding to instability of the single-phase microemulsion. The implication is that the interactions are more attractive in the benzyl alcohol containing system.6 It has been observed previously that addition of pentanol to aqueous micelles of tetradecyltrimethylammonium bromide causes no change in the quasi-elastic broadening of the surfactant spectrum.2 Also, in the case of glycerol microemulsions, in which we have now shown that the AOT mobility is reduced considerably, recent measurements2318 Neutron Scattering Study of Microenzulsions 0 2 I, @/A -2 Fig.4. Plots of linewidth us. Qz for AOT spectra in microemulsions with (@) and without (0) a-chymotrypsin. Error is ca. 15 % in Am. Continuous phase is n-octane. of the kinetics of exchange of ions confined within the glycerol droplet indicate that there is no large reduction in the rate of solubilisate exchange between glycerol droplets as compared with water droplets.lR This again would seem to imply little or no correlation between the local AOT mobility and the kinetics of droplet exchange.Effect of Solubilised a-Chymotrypsin in Water Droplets on the Dynamics of Surfactant Motion and Motion of Solubilised Water within the Droplet Core a-Chymotrypsin (aCT) is a highly water-soluble hydrolytic enzyme with a molecular weight of 24800 daltons. Its external dimensions are ca. 4.0 x 4.0 x 5.1 nm, which corresponds approximately to a sphere of radius 2.2 nm.9 The enzyme was solubilised in a droplet system in which the radius of the water cores before solubilisation is ca, 3.5 nm and which contains ca. 6000 water molecules. Ultracentrifugation studies, together with studies by small-angle neutron scattering and photon correlation spectros~opy,~~ have shown that a-chymotrypsin is solubilised into this microemulsion system with no appreciable change in droplet size.In contrast, in some systems, there is evidence that large droplets are formed, containing the majority of the protein molecules, and these are in equilibrium with a population of smaller dr0p1ets.l~~ 2o Ultracentrifuge results, confirmed by small-angle neutron results, show that at the solution composition used here the size and stoichiometry of the AOT-stabilised droplets is not significantly affected by the presence of a-chymotrypsin.lg The overall concentration of a-chymotrypsin used was 0.81 x lop3 mol dm-". The initial concentration of water droplets, measured using a fluorescence technique, is 0.82 x lop3 mol drnP3.,l Therefore, the solutions used in this work are thought to correspond to a situation in which each water droplet contains approximately one a-chymotrypsin molecule.The following samples were prepared and spectra recorded : (v) 0.2 mol dmV3 AOT, 4 rnol dm-3 D20, 0.81 x lop3 mol dmP3 aCT in deuterated octane; (vi) 0.2 mol dmP3 AOT, 4 mol H,O, 0.81 x lop3 mol dm-3 aCT in deuterated octane; and (vii) 0.81 x The spectrum due to aCT was obtained by subtraction of the D,O solvent spectrum from the spectrum of (vii). The spectrum of AOT was obtained by subtraction of the 'elastic' aCT spectrum and the octane solvent spectrum from spectrum (v). The spectrum of the water in the aCT-containing microemulsion was obtained by the subtraction of spectrum (v) from spectrum (vi).The water spectrum of the non-aCT-containing microemulsion was obtained by a similar H,O/D,O difference procedure. Fig. 4 shows the results obtained for the AOT linewidths in microemulsions with and without the protein. The results are virtually identical indicating that the protein has no significant effect on the surfactant mobility. Fig. 5 shows the spectrum due to the solubilised water in the microemulsion droplets mol dm-3 aCT in D,O.P. D. I. Fletcher, B. H. Robinson and J . Tabony > 3 d g 2 0 0 . . 2319 w/meV Fig. 5. Spectra due to water at 8 = 11 1.6” for (a) microemulsion containing aCT and (b) microemulsion without xCT. Continuous phase is n-octane.300 a 0 0 0 0. 0 'oat***@. , , , , 0 2 L Q2/A-2 Fig. 6. Plots of linewidth us. Q2 for the water spectra (quasi-elastic portion) in microemulsions with (0) and without (0) aCT. Continuous phase is n-octane. in the presence of aCT (a) and in the absence of aCT (b). The total integrated intensities are equal under both peaks, but an ‘elastic’ peak can clearly be observed in the spectrum from the enzyme-containing solution, suggesting that some fraction of the total solubilised water has been ‘iimmobilised’ or has a drastically reduced mobility. The energy resolution of the IN6 experiment is such that any translational diffusion slower than 3 x loplo m2 s-l and rotational motions slower than loplo s will appear as ‘immob- lised’ and it should be borne in mind that the term ‘immobilised’ can only be considered with reference to the resolution of our experiment (i.e.40 peV or 6 x 1O1O rad s-l). The fact that this ‘elastic’ peak is only observed when enzyme is present indicates the presence of some ‘immobilised’ water associated with the enzyme. The analysis procedure involved fitting the spectrum in fig. 5(a) to the sum of an ‘elastic’ peak and2320 Neutron Scattering Study of Microemulsions a Lorentzian curve. Linewidths were derived from the quasi-elastic portions of the curves. Fig. 6 shows a comparison of the quasi-elastic linewidths as a function of Q2 for water in microemulsions with and without aCT. As observed previously, the plot is linear and a value for the diffusion coefficient of 1.3 x lop9 m2 s-l is obtained.This value is approximately half that observed for water self-diffusion in bulk water (D = 2.5 x m2 s-l), but is comparable to that observed in ca. 5 mol dm+ lithium chloride in aqueous solution.22 This result is not unexpected in view of the high concentration of Na+ counter ions in the water pools. For [H,O]/[AOT] = 20, “a+],, z 1.4 mol dm-3. The result for the enzyme-containing droplet system shows that there is virtually no change in the mobility of a major fraction of the solubilised water on addition of aCT. From the integrated areas of the elastic and quasi-elastic portions of the scattering curves, it can be concluded that ca. 5-10% of the solubilised water is ‘immobilised’ in the vicinity of the enzyme. This corresponds to 250-500 water molecules for each solubilised protein molecule, or 0.2-0.4 g of water per gram of protein. This level of ‘bound’ water is comparable with that observed for many proteins, as determined by various techniques including dielectric rnea~urements,~~~ 24 i.r.spectros~opy~~ and calorimetry.26 [For a fuller discussion, see ref. (27).] Thus, when the enzyme aCT is solubilised within the microemulsion water cores, there is clearly an interaction with the core water. Since only a small amount of water is present, it is easily possible to distinguish between ‘free’ and ‘immobilised ’ or bound water. In conclusion, the results described in this paper suggest that the mobility of the AOT surfactant in oil-continuous microemulsions is not much affected by addition of toluene or benzyl alcohol.Likewise, solubilisation of a-chymotrypsin within the water pools has no effect. Substitution of the water by glycerol, however, produces a significant reduction in AOT mobility. Measurements of the solubilised-water mobility clearly reveal the presence of 5-10% of ‘ bound’ water around the solubilised enzyme in the microemulsion system. The mobility of the remaining 90-95% of the solubilised water in the core is apparently unaffected by the addition of the enzyme. We thank the ILL, Grenoble and the S.E.R.C. (Neutron Beams) for provision of beam time and financial support. We also thank Dr A. J. Dianoux for his assistance on IN6, and Drs A. Angel1 and J. C . Dore for a gift of a sample of deuterated glycerol. B. H. R. and P. D. I. F. thank the S.E.R.C.(Biotechnology Directorate) and Tate and Lyle (Reading) for support of this work through a Cooperative award. References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 J. Tabony, A. Llor and M. Drifford, Colloid Polym. Sci., 1983, 261, 938. J. Tabony, Chem. Phys. Lett., 1985, 113, 75. P. D. I. Fletcher, M. F. Gala1 and B. H. Robinson, J. Chem. Sac., Furuduy Trans. I , 1984, 80, 3307. P. D. I. Fletcher and B. H. Robinson, Ber. Bunsenges. Phys. Chem., 1981, 85, 863. S. S. Atik and J. K. Thomas, J. Am. Chem. Soc., 1981, 103, 3543. P. D. I. Fletcher, A. M. Howe and B. H. Robinson, J. Chem. Soc., Furuduy Trans. I , in press. P. L. Luisi, Angew. Chem., Znt. Ed. Engl., 1985, 24, 439. F. M. Menger and K. Yamada, J. Am. Chem. Soc., 1979, 101, 6731. S. Barbaric and P. L. Luisi, J. Am. Chem.SOC., 1981,103, 4239. K. Martinek, A. V. Levashov, N. L. Klyachko, V. I. Pantin and I. V. Berezin, Biochim. Biophys. Acta, 1981, 657, 277. P. D. I. Fletcher, R. B. Freedman, J. Mead, C. Oldfield and B. H. Robinson, Colloids Surf., 1984, 10, 193. P. D. I. Fletcher,N. M. Perrins, B. H. RobinsonandC. ToprakciogluinReverseMicelles,ed. P. L. Luisi and B. E. Straub (Plenum Press, New York, 1984), p. 69. P. D. I. Fletcher, A. M. Howe, B. H. Robinson, J. C. Dore, N. M. Perrins and C. Toprakcioglu, in Surfuctunts in Solution, ed. K. Mittal and B. Lindman (Plenum Press, New York, 1983), vol. 3, p. 1745. M. Zulauf and H. F. Eicke, J. Phys. Chem., 1979, 83, 480.P. D. I . Fletcher, B. H. Robinson and J . Tabony 2321 15 B. H. Robinson, C. Toprakcioglu, J. C. Dore and P. Chieux, J. Chem. Soc., Faraday Trans. 1, 1984, 16 J. D. Nicholson and J. H. R. Clarke in Surfactants in Solutions, ed. K. Mittal (Plenum Press, New York, 17 K. E. van Holde, Physical Biochemistry, (Prentice-Hall, New Jersey, 1971). 18 N. Z. Atay and B. H. Robinson, unpublished results. 19 P. D. I. Fletcher, R. B. Freedman and B. H. Robinson, to be published. 20 P. L. Luisi and L. J. Magid, Solubilisation of Enzymes and Nucleic Acids in Hydrocarbon Micellar 21 N. J. Bridge and P. D. I. Fletcher, J . Chem. SOC., Furaday Trans. I , 1983, 79, 2161. 22 J. W. White, Ber. Bunsenges. Phys. Chem. 1971, 75, 379. 23 E. H. Grant, R. J. Sheppard and G. P. South, Dielectric Behazjiour of Biological Molecules in Solution 24 J. T. Koide and E. L. Carstensen, J. Phys. Chem., 1976, 80, 2526. 25 P. L. Poole and J. L. Finney, Biopolymers, 1983, 22, 255. 26 P-H. Yang and J. A. Rupley, Biochemistry, 1979, 18, 2654. 27 G. NCmethy, W. J. Peer and H. A. Scheraga, Annu. Rev. Biophys. Bioeng., 1981, 10, 459. 80, 13. 1984), Vol. 3. Solutions, in CRC Critical Reviews in Biochemistry (CRC Press, Boca Raton, Florida), in press. (Oxford University Press, Oxford, 1978). Paper 5/1232; Received 19th July, 1985

ISSN:0300-9599    

DOI:10.1039/F19868202311

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. 1, 1986, 82, 2323-2332 The Electrochemical Reduction of Polyacetylene with Selected Reducing Agents Richard B. Kaner,? Simon J. Porter$ and Alan G. MacDiarmid* Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, U.S.A. The Coulombic efficiency, stability, constant-current discharge characteris- tics, energy density and the relation of cell potential to degree of reduction of a partly reduced polyacetylene cathode, [Na,t(CH)Y--], ( y d 0. lo), in a cell of the type NalNaPF,(tetrahydrofuran)l[Naj(CH)Y-1, have been investi- gated. By comparison with data obtained with a (Lit(CH)Q-], electrode, thermodynamic properties, such as the relation of cell potential to degree of reduction at diffusion equilibrium, appear to be intrinsic t o the reduced polyacetylene and independent of the countercation, whereas kinetic prop- erties, such as the cell potential during constant-current electrochemical reduction, vary with the countercation.The electrochemical reduction of polyacetylene with the incorporation of potassium countercations has been accomplished in a KIKClO,(tetrahydrofuran)l[K,+(CH)Y-], cell using a complexing agent to solubilize the KC10, in the tetrahydrofuran. A convenient electrochemical method for incorporating organic countercations into polyacetylene is discussed, together with selected properties of a [(Bu,N)i(CH)y-], electrode. In a previous paper we have reported the properties of polyacetylene, (CH),, reduced electrochemically with the incorporation of lithium countercations.l In this paper, we investigate the electrochemical reduction of polyacetylene with the incorporation of a variety of different countercations to examine which of the observed properties are intrinsic to the reduced polyacetylene and which vary with the counterion.Experiment a1 Polyacetylene film was synthesized as described previously.2 All electrochemical experi- ments were performed using cells constructed in a purified argon dry box and then sealed on a vacuum line as described previous1y.l Lump sodium or potassium metals (J. T. Baker Chemical Co.) were scraped with a knife and pressed into 189 nickel grid (Delker Corp.) to serve as electrodes. The electrolytes consisted of 1 .O mol dm-3 NaPF, (Aldrich Chemical Co.) in tetrahydrofuran (THF), 1 .O mol dm-3 KClO, (Fisher Scientific Co.) with 1 .O mol dm-3 dicyclohexano[ 181- crown-6 (Alfa Ventron Corp.) in THF and 1.0 mol dm-3 Bu,NClO, (Eastman Kodak Co.) in THF.Anhydrous NaPF,, KClO, and Bu,NClO, were heated at 120 "C under dynamic vacuum for 48 h prior to use. Reduction of the (CH), electrodes (cell discharging studies) was performed with a Princeton Applied Research potentiostat/galvanostat model I73 at constant currents. Reoxidation to convert the reduced polyacetylene back to neutral (CH), (cell charging studies) were carried out by oxidizing the [M,+(CH)u-], (ha = Na, K or Bu,N) first at t Permanent address : Department of Chemistry and Biochemistry, University of California, Los Angeles, $ Permanent address : University Chemical Laboratory, Canterbury, Kent, CT2 7NH.California 90024, U.S.A. 23232324 Electrochemical Reduction of Poli~acetylene a constant current and then at a constant applied potential. The amount of charge passed during reduction or reoxidation was recorded using a PAR coulometer, model 179. Voltages, resistances and currents were measured with a Keithley 177 microvolt digital multimeter. Voltage-charge, voltage-time and current-time curves were recorded with a Houston Instruments Corp. Omnigraphics xy recorder. All potentials are given with respect to either a lithium, sodium or potassium reference electrode which was also used as the counter electrode. It .must be stressed that the electrochemical characteristics of (CH), electrodes are extremely sensitive to the method of cell construction, presence of impurities (especially water and oxygen), rate of reduction, etc.The results given in this report were obtained by following the described procedures exactly. Results and Discussion Relation between the Degree of Reduction, y , in [(CHY,-], and Cell Potential A cell was constructed from a sodium anode and a (CH), cathode, both immersed in an electrolyte consisting of 1 .O mol dm-3 NaPF, in THF. The open-circuit voltage, Voc, of such a cell falls in the range 1.5-2.8 V. The reason for this variation has been discussed previously. When the electrodes were connected by an external wire a spontaneous electrochemical reaction occurred in which the sodium was oxidized and the (CH), was reduced according to the following equations : anode reaction: xyNa + xyNa+ + xye- (1) cathode reaction : (CH),+xye- -+ [(CH)Y-], (2) giving the overall net reaction: xyNa + (CH), -+ [Nai(CH)Y-], (3) where y < 0.10.The reaction given by eqn ( 3 ) is the discharge reaction of a voltaic cell, which in its charged state consists of parent, neutral (CH), and metallic sodium. It is completely analogous to the electrochemical reaction which occurs between polyacetylene and 1ithium.l To obtain the relationship between the V,, and percent reduction given in fig. 1, two different cells were reduced using a constant current method. Each data point given in fig. 1 involved: (a) a constant current reduction of the (CH), at a 100 A kg-l of (CH), rate, equivalent to 5 mol% (CH), reduction per hour; followed by (b) a 48 h stand period to promote equilibration of the Na+ ions within the ca.200 A diameter polyacetylene fibrils, followed by (c) a constant current reoxidation of the polyacetylene at a 100 A kg-l of (CH), rate, equivalent to 5 mol% (CH), oxidation per hour, until a cell potential of 2.0 V was reached, at which point the cell was held at a constant potential of 2.0 V for 16 h to promote complete reoxidation of the [Naj(CH)Y-],. The data points given in fig. 1 show the relationship between Voc, 48 h and the percent reduction of the (CH),. The arrows in fig. 1 represent the increase in Yo, of each cell on standing for 48 h, while the circle and square represent the final VOc, 48 h values for the NalNaPF,(THF)I(CH), cells containing 2.7 mg (0.8 cm2) and 3.0 mg (0.9 cm2) of (CH),, respectively.After 48 h, no further diffusion equilibration within the (CH), fibrils as measured by changes in the open circuit voltage, could be observed. In order to see if the data plotted in fig. 1 for the equilibrium cell potential of [Naj(CH)Y-], us. Na are intrinsic to the reduced polyacetylene, the empirical equation used to relate the equilibrium potential of [Lii(CH)g-], us. Li [ref. (l)] was drawn in fig. 1 with a correction for the difference in electrochemical reduction potential between Na and Li. This difference is calculated by taking Ked for Na (2.71 1 V us. the standard hydrogen electrode, SHE) and subtracting it from Ged for Li (3.045 V us. SHE) to giveI .5 I .o 0.5 voc I I 1 I I I I I 2325 0 1 2 3 4 5 6 7 8 9 1 0 reduction (%) Fig.1. Relationship between the open-circuit voltage, V,,,, 4R h and the percent reduction of (CH), in an NalNaPFG(THF)IINai(CH)u-], cell. The empirical relationship Voc, 4R = I. 17 - (0.13 + 0.02q) In q, where q (q = 1OOy) is the percent reduction was used to draw the curve. The two different cells used in this study employed the following amounts of polyacetylene: 0, 3.0 mg (0.9 cm2); and 0, 2.7 mg (0.8 cm2). A g e d = 0.334 V. This difference was then subtracted from the empirical relationship between Voc (us. Li) at approximate diffusion equilibrium and the percent reduction of [Li$(CH)g-], to give Voc = (1.50 - 0.334) - (0.13 + 0.02q) In q = 1.17-(0.13+0.02q) lnq (4) where q (q = 1OOy) is the percent reduction. A fairly good agreement can be seen in fig.1 between the curve drawn based on eqn (4) and the experimental data points for reduction levels between 1.0 and 10.0 mol%. Thus the thermodynamic function of V,, us. percent reduction at apparent diffusion equilibrium for [M,f(CH)y-], (us. M) is essentially identical for all values of y < 0.10, for M = Li or Na. Therefore, the potential of the [Na$(CH)g-], electrode under these conditions reflects only the degree of reduction of the polyacetylene and is essentially independent of the countercation. The 48 h stand period is a condition needed to promote diffusion equilibration of dopant cations within the (CH), fibrils as is demonstrated in fig. 2. The upper solid line in fig. 2 is based on the best fit through the [Na$(CH)y-], us. Na data points from two different cells as given in fig.1. The middle dashed line was obtained by Shacklette et aZ.3 by measuring immediate Voc values of an Nal(CH), cell after stepping the voltage in small increments and allowing the current to decay to 10 pA cm+. This is theoretically equivalent to a constant current discharge at 10 pA cm-2 [2 A kg-l of (CH),]. The lower dotted line was obtained by discharging a Nal(CH), cell at a constant current of 0.135 mA [50 A kg-l or 175 pA cm-2 of (CH),], as described in a later section (see fig. 5, later). Note the similarity of the dotted and dashed lines, both of which exhibit a marked plateau effect which is much more pronounced than that observed in the solid line. This strongly suggests significant continuing equilibration of dopant ions during the 48 h stand period.The change in potential is not due to loss of charge by the reduced polyacetylene, since, as shown in a later section, the Coulombic recovery obtained on electrochemical oxidation of the reduced material back to neutral (CH), was ca. 100%. Baughman et aZ. have reported that alkali-metal doping of partially oriented (CH), involves ' staging' analogous to that observed when graphite is intercalated with alkali2326 - - Electrochemical Reduction of Polyacetylene 0.41 metals4* We suggest that alkali-metal doping of (CH), may involve the formation of the kinetically favoured structure exhibiting staging. Once a deformation has been introduced into the (CH), lattice by the insertion of a metal ion, further insertion of metal ions is favoured at this defect site to give a staging effect.If solvent is then removed, diffusion of metal ions to give a more homogeneous distribution of dopant ions is inhibited, resulting in the observed staging. If, however, solvent is present it promotes attainment of a more homogeneous thermodynamically favourable distribution of dopant, although some staging, but less pronounced, is still evident after the 48 h equilibration period. It would be of interest to ascertain whether analogous X-ray studies of polyacetylene, of the type previously reported by Baughman et al., if carried out on electrochemically reduced material as described here with a 48 h stand period, would lead to a more homogeneous distribution of dopant ions throughout the polymer. Diffusion of Na+ Ions in Polyacetylene Fibrils The change in Voc (us.Na) during the 48 h stand period after completion of each constant current reduction step was monitored periodically. Four typical curves showing the change in V,, with time are shown in fig. 3 for polyacetylene reduced to 1.7,4.0, 5.8 and 7.0 mol%. The increase in cell potential during the 48 h stand period is consistent with a decrease in the degree of reduction on the outside of the [(CH)Y-], fibrils as the counter Na+ ions diffuse toward the interior of the fibril together with their attendant negative charge on the polyacetylene. Exactly the opposite effect is observed after a partial electrochemical oxidation of the [(CH)Y-Iz to a less-reduced state. In this case, the Voc falls on standing as Na+ ions, which now have a greater concentration in the interior of the [(CH)Y-], fibrils, diffuse toward the surface of the (CH), fibrils.As discussed previously,' the diffusion constant, D, for the diffusion of ions within the (CH), fibrils and the time constant, z, can be obtained from the V,, us. time data given in fig. 3. Graphs of In [( - &)/( 5 - vf)] against time, t, for the essentially linearR. B. Kaner, S. J . Porter and A . G. MacDiarmid 2327 0.4 1 &-0- 0 0 0 0- 0-0 - - - - t parts of the Voc us. time curves in fig. 3 give a straight line with a slope of z-l. Here, 5 and V, are the initial and final open-circuit voltages, respectively, and V, is the open-circuit voltage at any time t. For 4.0 mol% reduced (CH),, a z value of 11.3 h was found, while for 7.0 mol% reduced (CH),, a z value of 14.9 h was obtained.Using = (2.405)2 D for diffusion into or out of a cylinder, the diffusion constant, D, can be calculated from the z values obtained.6 Assuming a radius of 100 A for the polyacetylene fibril^,^ the T value of 11.3 h, obtained for 4.0 mol% reduced (CH),, gives a D value of 4.2 x cm2 s-l and the z value of 14.9 h, obtained for 7.0 mol% reduced (CH),, gives a D value of 3.2 x 10-ls cm2 s-l. The values obtained for z and D are for intrafibrillar diffusion of Na+ ions from the outside to the inside of the 200 A diameter fibrils under a concentration gradient only. It should be stressed that these diffusion studies involve no external applied electric potential and hence involve no diffusion of ions within the electrolyte between the (CH), fibrils. These values may be compared to the corresponding z values obtained for the diffusion of Li+ ions in [Li&,3(CH)-0.03], (10.9 h) and in [Li,'.0,(CH)-o-06], (12.4 h).l It should be noted that the diffusion of the M+ ions discussed above involves only a redistribution of these ions within the (CH), fibrils and does not necessitate any migration of (PF,)- ions.The diffusion of NaS cations into or out of the polyacetylene fibrils will be 99% complete after a time equal to 4z. Since the V,, values used in the Voc us. percent reduction curve (fig. 1) were taken after 48 h, a time close to 42, these values may be considered to be equilibrium values. Coulombic Efficiency and Stability The two Na(NaPF,(THF)((CH), cells employed in the last section were also used to determine the reversibility of the reaction given by eqn (3).After each reduction of the polyacetylene followed by the 48 h stand period, the polyacetylene was reoxidized, first at a constant current until 2.0 V (us. Na) was reached and then at a constant applied potential of 2.0 V (us. Na) for 16 h to remove residual dopant ions. The total amount2328 0.5 - - 0 I I I I 1 1 I Electrochemical Reduction of Polyacetylene Y Y ( 6 ) 1 A n X- 2.0 t h-x-x-x - voc 1.0 1.51 time/ d a y s Fig. 4. V,, us. stand time, demonstrating the stability in potential (vs. Na) of (a) 7.0 mol% reduced polyacetylene and (b) neutral polyacetylene in an NalNaPF,(THF)I(CH), cell. of charge, Q (in, total) obtained in each oxidation process was recorded and was used in the relation [Q(out, total)/Q(in, total)] x 100 to calculate the Coulombic efficiency, Q,,,.Up to reduction levels of ca. 10 mol%, the Coulombic efficiency associated with each data point was ca. 100% (e.g. at 1.1 mol% reduction, Qeff = 101.6%, at 5.8 mol% reduction, Qeff = 100.4% and at 9.5 mol% reduction, Qeff = 100.8%). Since the reduction and reoxidation of polyacetylene is completely reversible up to ca. 10 mol%, one would expect the polymer to be stable on standing in this electrolyte. This is indeed the case, as is demonstrated in fig. 4. A 7.0 mol% reduced polyacetylene electrode in a Na) 1 .O dm3 mol-l NaPF6(THF)I[Na$~07(CH)-o.07], cell maintained a constant potential of 0.64 V (us. Na) during a 40-day period, as shown in the lower curve of fig.4. The slight rise in potential during the first two days is consistent with diffusion of the Na+ ions from the surface of the fibrils to their interior. After the 40-day stand period, oxidation to 2.0 V [neutral (CH),] gave a Coulombic efficiency of 100.2%. The cell potential then remained constant at 1.65 V for the following 40-day period, as shown in the upper curve of fig. 4. The slight decrease in potential during the first two days is consistent with diffusion of residual Na+ ions from the interior of the fibrils to their surface. These studies demonstrate the very great stability of polyacetylene in both its neutral and reduced forms in an appropriate electrolyte such as 1.0 dm3 mol-1 N aP F 6 (TH F) . In a further attempt to determine the maximum level of polyacetylene reduction which is stable in the NaPF,(THF) electrolyte, the following chemical reduction experiment was carried out.Two pieces of (CH),, one of which was encased in a Pt mesh, were placed in a 0.5 dm3 mol-1 sodium naphthalide solution. A spontaneous reaction took place as given by (CH), + xyNa+Nphth.- + [Na$(CH)y-], + xyNphth. (6) The reaction was allowed to proceed for 48 h to promote approximate diffusion equilibrium of the dopant ions within the polyacetylene fibrils. To evaluate the reduction level of the [Nai(CH)y-], produced, one sample was placed in methanol for 24 h to remove all the Na+ dopant ions in the form of sodium methoxide. The NaOMe-MeOH solution was then titrated with 0.1 dm3 mol-1 HC1. The results indicated a 17 mol% reduction level, i.e.the formation of [Na&7(CH)-0-17],. The second piece of washed and dried reduced polyacetylene was used an an electrode. Its potential us. Na, 0.246 V, wasR. B. Kaner, S . J . Porter and A . G. MacDiarmid 2329 0.5 time/min Fig. 5. Cell potential, V,, duringconstant-current discharge of polyacetylene reduced to 10.0 moly;, i.e. [Na~~,o(CH)-o.lo],, at (a) 0.135 mA, (b) 0.27 mA, (c) 0.54 mA and (d) 1.08 mA in an NalNaPF,(THF)I(CH), cell employing 2.7 mg (0.8 cm2) of (CH),. Table 1. Discharge characteristics of an NalNaPF,(THF)I[Na'(CH)Y-], cell discharge curvea. . constant discharge/mA applied current/A kg-l discharge time/h final cell potential/V overall Coulombic efficiency (%) average cell potential/V energy densityb/W h kg-l average power densityb/W kg-l energy efficiency (%) 0.135 50 ca.4.0 101.0 115.4 28.1 76.6 0.23 0.66 0.27 100 ca. 2.0 100.4 113.2 55.1 74.2 0.22 0.65 0.54 200 ca. 1.0 c 0.20 0.63 101.1 109.7 106.7 70.8 1.08 400 'a. 0.5 101.5 105.7 205.6 66.5 0.18 0.6 1 a Discharge curves (a), (b), (c) and (d) are given in fig. 5. employed and on the weight of Na consumed in the discharge reaction. Based only on the weight of (CH), measured immediately in the drybox after immersion in a 1.0 dm3 mol-l NaPF,(THF) electrolyte. This potential corresponds to an equilibrium reduction level of ca. 12 mol%, i.e. [Na:.12(CH)-0.12],, based on eqn (4). A sealed rectangular glass NalNaPF,(THF)I[Na$.l,(CH)-0.12], cell was then constructed using this same sample of reduced polyacetylene. By the time the cell construction was finished (ca.30 min), the cell potential was 0.4 V, corresponding to an equilibrium reduction level of ca. 10 mol% . This was confirmed by applying a constant potential of 2.0 V to the cell to oxidize the reduced polyacetylene to neutral (CH),. The amount of charge passed (1.85 C) corresponded to a reduction level of 10.3 mol%. The above results demonstrate that a higher level of reduction of polyacetylene can be attained in a sodium naphthalide-THF solution than in a NaPF,(THF) solution. In the presence of the latter solution the [Nat.,7(CH)-o.17], is oxidized rapidly to [Na$.lo(CH)-o-lo],, presumably with the concomitant reduction of the PF; ions. Hence, the stability of the reduced polyacetylene in the THF electrolyte is limited by the nature of the solute.A recent report* has shown that reduction levels in THF of ca. 18% can be obtained when alkali-metal alkyl borate salts, M+(BR,)-, in THF are used as the elec tr ol yte, Constant-current Discharge (Reduction) Studies The constant-current discharge characteristics of an NalNaPF,(THF)I(CH), cell em- ploying 2.7 mg (0.8 cm2) of (CH), are shown in fig. 5. In each of the four studies the (CH), was reduced to a 10.0 mol% reduction level. The results are given in table 1. Inte-23 30 Electrochemical Reduction of Polyacetylene grating the area, (V,Q), under a discharge curve and dividing by the charge (Q) invo!ved, gives the average cell potential during discharge. The ratio of the area under the discharge curve to that under the charge curve (not shown) gives a value for the energy efficiency, Eeff.After each constant-current discharge (reduction) the cell was charged to 2.0 V to oxidize the [Na~,,o(CH)-o.lO], to neutral (CH),. A constant current, identical to that used during reduction, was employed. When a potential of 2.0 V was reached, oxidation was completed at a constant potential of 2.0 V for 16 h. The constant-current oxidation step removed ca. 90% of the Na+ countercations and the constant potential step removed the remaining ca. 10%. The overall Coulombic efficiency in each case was close to 100% as given in table 1. A theoretical energy density for the reduction of polyacetylene to a 10.0 mol% level with Na+ countercations can be calculated from the area under the equilibrium curve in fig.1 . The value of 143.9 W h kg-’ obtained is based only on the weight of (CH), employed and the weight of sodium consumed in the reduction process. A comparison of this theoretical value to the experimental value of 115.4 W h kg-l obtained above during the 0.135 mA constant-current reduction shows that 80.2% of the theoretical energy capacity can be utilized at this ca. 4 h reduction rate. The polarization of the NalNaPF,(THF)J(CH), cell, i.e. change in voltage with increasing magnitude of the applied constant current, is remarkably low. The average cell potential during reduction changed from only 0.66 to 0.61 V when the current was increased eight-fold, i.e. from 0.135 to 1.08 mA. Similarly, the energy density decreased only very slightly from 115.4 to 105.7 W h kg-’.Electrochemical Reduction of Polyacetylene with the Incorporation of K + Countercations At the time the present studies were being performed, the electrochemical synthesis of [K,+(CH)y-], could not be carried out readily in THF since potassium salts such as KClO, are essentially insoluble in this solvent. This problem was overcome by the use of the crown ether complexing agent, dicyclohexano[ 181-crown-6, which rendered KClO, soluble in THF. However, recent studies have demonstrated that certain potassium tetra-alkyl borate salts are soluble in THF and can be used in the electrochemical reduction of (CH)2.8~ A KIKClO,(THF)I(CH), cell, employing 5.6 mg (1.6 cm2) of (CH), and ca. 0.5 cm3 ofan electrolyteconsisting of 1 .O dm3 mol-1 KClO, and 1 .O dm3 mol-l dicyclohexano[ 181- crown-6(THF), was constructed.The cell was discharged at a constant current of 0.056 mA [lo A kg-l of (CH),] for ca. 6 h to a (CH), reduction level of 3.2 mol%, i.e. [K,’~.,3,(CH)-o~032]x, based on the amount of charge passed. The current used corresponded to a reduction rate of ca. 0.5 molx h-l. The cell potential during discharge was recorded as a function of time and is given as curve ( d ) in fig. 6. A relatively small current was used, since at higher currents, e.g. at 100 A kg-’ (0.56 mA), the cell potential fell rapidly (below 0.5 V at ca. 1 mol% reduction), indicating a diffusion-limited reaction. This may be due to low conductivity of the electrolyte, slow diffusion of the large complexed K+ ions into the (CH), fibrils or slow dissociation of the K+ ion from the crown ether complex.This is in contrast to LiILiClO,(THF)I(CH), cells, Li~LiC10,-dicyclohexano[ 181- crown-6(THF)I(CH), cells and NalNaPF,(THF)I(CH), cells, all of which can be discharged at high current densities [at least 400 A kg-l of (CH),] with only a small decrease (< 0.1 V) in cell potential. Electrochemical Reduction of (CH), with the Incorporation of Organic Countercations If the electrolyte in which (CH), is reduced electrochemically contains an organic cation instead of an alkali-metal cation, then the organic cation will act as the ‘dopant’ cation.R. B. Kaner, S. J. Porter and A . G. MacDiarmid 233 1 1.5 In i I I I 1 I I I I I 1 0 I 2 3 4 5 6 7 8 9 10 average reduction (%) Fig. 6. Cell potential, V,, during reduction (discharge) as a function of the average percent reduction of polyacetylene in the following cel!s : (a) LilBu,NClO,(THF)I(CH), ; (b) LilLiClO,(THF)I(CH), ; (c) NalNaPF,(THF)I(CH), ; and ( d ) KIKClO,-dicyclohexano[ 1 81-crown- 6(THF)I(CH),.Cells (a)-(c) were reduced (discharged) at a rate of 100 A kg-l of (CH),; cell (d) was reduced at a rate of 10 A kg-'. For example, if tetra-n-butylammonium cations, (Bu,N)+, are present in a LilBu,NClO,(THF)!(CH), cell, then on electrochemical reduction of the polyacetylene, these countercations will be incorporated into the reduced (CH), to give [(Bu,N)t(CH)v-],, as given in eqn (8), assuming that diffusion of Li+ ions, liberated from the Li anode during reduction, to the (CII), cathode is prevented by means of, for example, a semipermeable membrane : (8) It should be noted that the oxidation of the reduced polyacetylene formed in this reaction is not the reverse of that given by eqn (8).Instead, as the [(Bu,N),+(CH)y-], is electrochemically oxidized back to neutral (CH),, unstable ' (Bu,N)O' is formed at the Li anode, as given by eqn (9): (9) (CH), + xy(Bu,N)+ + xye- + [(Bu,N),+(CH)v-I,. [(Bu,N);(CH)y-], + (CH), + xy' (Bu,N)O '. Spontaneous decomposition of the (Bu,N)O occurs to produce Bu,N, butane and but- 1 -ene.l0 Therefore, a cell such as Li(Bu,NClO,(THF)((CH),, employing organic cations, is not infinitely rechargeable per se. Instead, on each charge cycle some of the tetrabutylammonium ions are decomposed and replaced in solution by Li+ ions. As this cell is continually cycled the concentration of LiClO, in the THF increases. Eventually, on continued cycling, this cell will become equivalent to a LiILiClO,(THF)1(CH), cell.An Li(Bu,NClO,(THF)[(CH), cell, employing 4.9 mg (1.4 cm2) of (CH),, was con- structed. The cell was discharged (reduced) at a constant current of 0.49 mA [ 100 A kg- of (CH),], for ca. 2 h to a (CH), reduction level of ca. 10 molod , i.e. [ ( B U ~ N ) ~ ~ ~ ~ ( C H ) - ~ ~ ~ ~ The cell potential during reduction was recorded as a function of time and is given as curve (a) in fig. 6. The [(Bu,N)c(CH)u-], can be oxidized to give (CH), with ca. 100% Coulombic efficiency up to a reduction level of ca. 6.5 mol% if oxidation is carried out immediately after the reduction cycle. However, on standing in the electrolyte, the [(Bu,N)$(CH)y-], is somewhat unstable.For example, a [ ( B U ~ N ) ~ , , ~ , ( C H ) - ~ . ~ ~ ~ ] ~ elec- trode with a Voc, 24 h of 1.19 V (us. Li) increased to 1.25 V during a 10-day stand period. From the amount ofchargeinvolved in its oxidation back to neutral (CH), it was found that the reduction level had decreased to 3.8 mol%. The slight decrease in the reduction level may be due to reaction of the [(Bu,N)t(CH).v-], with degradation products formed from reducing (Bu,N)+ during the previous recycling studies. The reaction which produced [(Bu,N),+(CH)v-],, in an Li(Bu,NClO,(THF)I(CH), cell, can be generalized to incorporate other organic cations into (CM),. For example, if2332 Electrochemical Reduction of Polyacetylene tetraphenylphosphonium perchlorate, Ph,PC10,, is used in place of Bu,NC10,, [(Ph,P)t(CH)y-], is formed.ll The discharge curves of the Li(Bu,NClO,(THF)I(CH!, cell [fig.6(a)] and the K IKClO,(THF)I(CH), cell [fig. 6 (d)] may be compared with typical constant-current dis- charge curves to a 10.0 mol% (CH), reduction level for an LiJLiClO,QTHF)J(CH), cell’ and for an NaJNaPF,(THF)I(CH), cell at rates of 100 A kg-l [fig.6(b) and (c), respectively]. Note that the general shape of reduction curves (a), (b) and (c) is comparable. There is very little difference between the reduction curves of an LilBu,NClO,(THF)I(CH), cell [curve (a)] and an LilLiClO,(THF)I(CH), cell [curve (b)]. However, the reduction curve of an NalNaPF,(THF)I(CH), cell [curve (c)] possesses a larger ‘plateau’ region, extending to a (CH), reduction level of ca.5 mol%. The reduction curve of a K~KC10,-dicyclohexano[ 18]-crown-6(THF)I(CH), cell [curve (d)] exhibits a lower voltage, presumably owing to diffusion effects discussed previously. These data demonstrate that the kinetic properties of [Mi(CH)y-], electrodes vary with the countercation. The thermodynamic equilibrium potential of reduced polyacetylene, [(CH)g-],, as a function of y (y 6 0.10) is essentially independent of the nature of the countercation, M+, at least when M is Li or Na. Electrochemical properties of polyacetylene which depend on kinetic factors, such as cell potential during reduction are, however, dependent on the nature of countercation. This study was supported by the U.S. Department of Energy, contract no. DE-AC02-81-ER10832 and the S.E.R.C. References 1 R. B. Kaner and A. G. MacDiarmid, J . Chem. Soc., Faraday Trans. 1, 1984, 80, 2109. 2 H. Shirakawa and S. Ikeda, Polym. J., 1971,2, 231 ; H. Shirakawa, T. Ito and S. Ikeda, Die Makromoi. Chem., 1978, 179, 1565; H. Shirakawa, T. Ito and S. Ikeda, Polym. J., 1973,4, 460. 3 L. W. Shacklette, R. L. Elsenbaumer and R. H. Baughman, J. Phys. (Paris), 1983, 44, C3-559. 4 R. H. Baughman, N. S. Murthy and G. G. Miller, J. Chem. Phys., 1983, 79, 515. 5 R. H. Baughman, N. S. Murthy, G. G. Miller, L. W. Shacklette and R. M. Metzger, J . Phys. (Paris), 6 W. Jost, Diffusion (Academic Press, New York, 1960), p. 45. 7 A. G. MacDiarmid and A. J. Heeger, Synth. Met., 1979/80, 1, 101. 8 L. W. Shacklette, J. E. Toth, N. S. Murthy and R. H. Baughman, J . Electrochem. SOC., 1985,132, 1529. 9 T. R. Jow, L. W. Shacklette and M. Maxfield, The Electrochem. Soc. Extended Abstracts, Vol. 1984-2, 1983, 44, C3-53. No. 620, p. 902, New Orleans, Louisiana, Oct. 7-12 (1984). 10 J. E. Dubois, A. Monvernay and P. C. Lacaze, Electrochim. Acta, 1970, 15, 315. 11 D. MacInnes, Jr and A. G. MacDiarmid, 1981, unpublished results. Paper 5/1249; Receiued 22nd July, 1985

ISSN:0300-9599    

DOI:10.1039/F19868202323

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. I, 1986,82, 2333-2343 Complexation of Roccellin by B- and y-Cyclodextrin Ronald J. Clarke, John H. Coates* and Stephen F. Lincoln* Department of Physical and Inorganic Chemistry, Uniuersity of Adelaide, South Australia 5001, Australiu Measurements of the u.v.-visible, fluorescence and circular dichroic spectra of roccellin (RO) in the presence of a-, p- and y-cyclodextrin (aCD, PCD and yCD) have been carried out. From equilibrium u.v.--visible spectral measurements, in the case of PCD, a single 1 : 1 complex was observed (298.2 K): RO+/ICD+RO*PCD ( K l ) where Kl = (7.20k0.88) x 10, dm3 mol-'. In the case of yCD a single 2: 1 complex was observed (298.2 K): 2RO+yCDe(RO),.yCD ( K J where K,, = (9.0+ 1.8) x 1Olo dm6 rnolk2. No complexation was observed between roccellin and aCD.Measurements of induced circular dichroism and of fluorescence were consistent with the reaction schemes proposed above for both /ICD and yCD, but no interaction was observed with aCD. Measurements of the u.v.-visible spectra of aqueous solutions of roccellin allowed characterisation of the dimerisation equilibrium : 2R0 G (RO), (Kd) where K , is (1.64 & 0.12) x 1 O4 dm3 mol-l. Measurements of the u.v.-visible and fluorescence spectra of roccellin, in increasing concentrations of lithium sulphate up to 0.5 mol dm-3, showed marked changes, consistent with the presence of increasing concentrations of aggregates of dye molecules with increasing electrolyte concentration. There has been considerable interest recently in the interactions between cyclodextrins and a variety of aromatic molecules, which form complexes by inclusion within the cyclodextrin ~avity.l-~ Such studies are of importance as there are analogies between this series of inclusion compounds and both drug-receptor and enzyme-substrate inter- actions.Indeed a number of attempts have been made to simulate enzyme behaviour by chemical modification of cyclodextrin molecules.6 In addition, the possibility exists for controlled sterically directed chemical syntheses by including one or more reactant molecules within a cyclodextrin cavity. Finally, it should be noted that a variety of drugs are suitable for inclusion within cyclodextrins, thus enabling improvement of their pharmacological proper tie^.^ In view of the above possibilities, it is worthwhile examining the selectivity of aCD, PCD and yCD, for a variety of included molecules.aCD, PCD and yCD are a-l,4-linked cyclic oligomers of D-glucopyranose with internal radii of 5-6, 7-8 and 9-10 A, respectively. We have investigated the diazo dyes methyl orange (MO)7 and tropaeolin 000 No. 2 (TR)8 in terms of both their kinetic and equilibrium properties on inclusions by the three cyclodextrins. A degree of size selectivity is shown7 by these molecule. Methyl orange, with two benzene rings, has been shown to form inclusion complexes with all three cyclodextrins, whereas TR,8 which possesses one naphthalene and one benzene ring, and roccellin, with two naphthalene rings, do not form any inclusion 23332334 Complexation of Roccellin by Cyclodextrins complexes with aCD, the smallest of the three cyclodextrins.Tropaeolin has been shown to be included as a 1 : 1, a 2: 1 and as a 2: 2 (dye-cyclodextrin) complex with yCD, in the case of DCD the 2 : 2 complex is not formed. Since roccellin possesses two naphthalene rings, but is otherwise quite similar to tropaeolin, it was decided to investigate the inclusion complexes which it will form with DCD and with yCD. It is a notable feature of the inclusion of azo dyes by the larger cyclodextrins that any tendency which they may have to form dimers in solution is enhanced by the possibility of inclusion in a suitably large cavity. In the case of roccellin, the dye itself forms dimers and higher oligomers readily, particularly in the presence of electrolytes.This property is also associated with the dye having a very low solubility in the presence of added salts, such as those used to provide a conductive solution for electrical Joule heating in a temperature-jump experiment. Consequently, measurements of the kinetics of the inclusion process were not possible for this dye using the usual temperature-jump methods. The kinetics of reactions occurring in non-conducting solutions can be studied using a pressure jump rather than a temperature jump, provided that the reaction of interest or some coupled reaction has a suitably large associated volume ~ h a n g e . ~ Experiments were attempted using a pressure-jump apparatus, but it was found that the amplitudes of the pressure-induced perturbations were too small and too rapid for satisfactory evaluation.However, it has been found possible to characterise the equilibrium properties of the RO-cyclodextrin complexes quantitatively. Experimental The a-, D- and y-cyclodextrins were obtained from the Sigma Chemical Co. and were used without further purification. They were stored as the anhydrous materials over phosphorus pentoxide in a vacuum desiccator. Rocellin (Sigma) was purified by salting out from hot distilled water with sodium acetate, recrystallised twice from distilled water and the final crystals were rinsed with distilled water. Elemental analysis was consistent with the dye being present as the monohydrate. The analytical grade salts potassium sulphate (B.D.H.), sodium chloride (Univar) and lithium sulphate (B.D.H.),, were used without further purification. All measurements were made on freshly pre ared dye solutions and exposure to light was kept to a minimum.No adsorption o f t f e dye to glass or quartz was detected. All volumetric glassware was cleaned by prolonged soaking in Decon 90 solution, followed by extensive rinsing in distilled water. The quartz spectrophotometer cells were carefully rinsed with each solution to be studied, prior to the recording of its spectrum. All solutions were diluted by weight from stock solutions prepared using A-grade volumetric apparatus. Visible spectra were measured in quartz cellsusingazeiss DMRlOdouble-beam spectrophotometer equipped witha thermostatted ( f 0.1 K) cell block. Spectra were run in duplicate at 298.2 K, recorded digitally at 2 nm intervals in the range 350-650 nm, punched onto paper tape and analysed using a Cyber 173 computer.Circular dichroic spectra were measured on a JASCO J40-CS spectropolarimeter, equipped with a microprocessor for averaging repeated measurements made at each wavelength. Linear dichroic spectra were determined on stretched poly(viny1 alcohol) films using a modified Zeiss PMQII spectrophotometer. Fluorescence spectra were measured using a Perkin-Elmer 3000 fluorescence spectrometer, equipped with a thennostatted cell holder. All measurements were made in a 1 cm path-length quartz fluorescence cell at 298.2 f 0.1 K. Results and Discussion Aqueous solutions of roccellin exhibit a red colour which deepens on the addition of a small amount of 1 mol dm-3 sodium hydroxide solution.This colour change occurs in the pH range 11-12. Thus, by analogy with tropaeolin8 it is likely that the pK, of theR. J. Clarke, J . H. Coates and S . F. Lincoln 2335 hydroxy group of roccellin is close to 1 1.4. Since this value is higher than that ofp-naphthol (pK, = 9.51), it is likely that the OH group is involved in a hydrogen bond in a similar fashion to the equivalent group in tropaeolin. The results described in this paper, except where specifically noted, were obtained in aqueous solution at ca. pH 7. Thus the monoanion is normally assumed to be the species present throughout. The similarity between the structures of roccellin and tropaeolin, shown below, suggests that the former, as well as containing an intramolecular hydrogen bond, should also undergo azo- hydrazone tautomerism (scheme 1).When in aqueous solution the hydrazone is probably the predominant species.lO tropaeolin d 7 roccellin azo hydrazone Scheme 1. Measurements of the u.v.-visible spectra of aqueous solutions of roccellin showed marked departure from the Beer-Lambert law. Determination of the dependence of the apparent molar absorbance on concentration for solutionscontaining no added electrolyte allowed characterisation of the dimerisation equilibrium : 2RO (RO), (&) (1) where Kd = (1.6450.12) x lo4 dm3 mol-l at 298.2 K. In view of the strong tendency for dimerisation, and in order to avoid contributions to the total absorbance from dimers, subsequent experiments involving cyclodextrins were carried out at roccellin concentrations of ca.2 x lop6 mol dm-3. At these concen- trations the bulk of the dye was in the monomer form. The nature of the aggregation in roccellin solutions was further investigated by measuring the visible absorption spectra of a series of roccellin solutions ([RO] = 5 x lop5 mol dm-3) containing increasing concentrations of lithium sulphate ( c 0 . 5 mol dm-") to increase the ionic strength. The results are shown in fig. 1. It can be seen that there is a large decrease in the intensity of the band at 505 nm, and a subsequent increase in the intensity of a band at 440 nm, as the ionic strength is increased. The curves in fig. 1 show that several spectroscopically distinct species must be present, since there is no isosbestic point. The diminution of the absorption band at longer wavelength, associated with increase in intensity of a band at shorter wavelength, is characteristic of the association of planar molecules into stacked aggregates accompanied by excition interaction.ll According to a number of workers,12' l3 higher cation concentrations aid aggregation in such systems by promoting ion-pair formation with negatively charged groups, thus decreasing intermolecular repulsions.Roccellin solutions in water do not give rise to any fluorescence emission. However,2336 Complexation of Roccellin by Cyclodextrins - I 20 " I - z m E 2 * 10 8 2 P m --- i 4 c( z 0 4 0 0 500 600 h/nm Fig. 1. Visible absorption spectrum of roccellin ( 5 x loe5 mol dmP3) in the presence of increasing concentrations of lithium sulphate at 298.2 K [Li,SO,]: (a) 0, (b) 0.01, (c) 0.05, ( d ) 0.075, ( e ) 0.10, v) 0.15, ( g ) 0.20, ( h ) 0.225, (i) 0.25, ( j ) 0.30 and ( k ) 0.50 mol dm-3.120 100 n +., -r( 5 80 d 3 v x c.' .4 Y 60 5 3 2 40 2 E: .r( Q) 2 0 0 1 I I (dl 600 700 h/nm Fig. 2. Fluorescence spectrum (Aex = 420 nm) of roccellin (4.9 x mol drnp3) in the presence of increasing concentrations of lithium sulphate at 298.2 K [Li,SO,]: (a) 0.15, (b) 0.20, (c) 0.30 and (d) 0.50 mol dmP3R. J . Clarke, J . H. Coates and S . F. Lincoln 2.0 1 . 5 1 . o 0 . 5 R . . . ...............:..* ..... . . . . . ...-.*a . - . . ... . . . . .. 0 . 0 2 50 4 50 hlnm 650 2337 Fig. 3. Linear dichroic spectrum of roccellin in stretched poly(viny1 alcohol) film (left-hand ordinate). (-) Light polarised parallel to stretch.(---) Light polarised perpendicular to stretch. The ratios of the parallel and perpendicular absorbances, R, are shown as individual points and refer to the right-hand ordinate. in the presence of increasing concentrations of lithium sulphate, a band appears at 600 nm, which increases in intensity and moves towards 620 nm (fig. 2). Since this behaviour parallels the u.v.-visible spectral behaviour in the presence of lithium sulphate, the fluorescence may be attributed to the presence of dye aggregates, within which quenching by oxygen or water is attenuated. In order to estimate the manner in which planar aromatic molecules aggregate, it is of interest to determine the geometrical relationship between the transition moment for an absorption band and the long axis of the molecule.This may be achieved by measuring the linear dichroism of a dye in a stretched poly(viny1 alcohol) film.14 It can be seen from fig. 3 that over the range 40s580 nm the absorbance parallel to the direction of stretch is substantially greater than that perpendicular to the direction of stretch and that the dichroic ratio is approximately constant. Assuming that the roccellin molecules align with the molecular long axis along the direction of stretch, these results indicate that the absorption bands in this wavelength range are polarised approximately along a line interconnecting the centres of the two naphthyl groups. Before proceeding to discuss the interaction between roccellin and the cyclodextrins in terms of molecular inclusion, it must be established that interaction between the glucose residues and the dye does not lead to spectral changes in the absence of inclusion.To this end the spectra of roccellin (2.0 x lop6 mol dmp3) in water and in a 0.01 mol dmP3 glucose solution were determined. It was found that only very small increases in molecular absorbance were obtained compared with those induced by the cyclodextrins, which are described later.2338 20 10 0:. Complexation of Roccellin by Cyclodextrins - . . . - L . - . . Eo - I 0 - E “E 0, -0 m . . d . I - . 20 5 . E * I - . m . E 2 - 10 s 5 -fl 0, P, . E ‘0 -u m . . h - . 0 . 400 50 0 600 A/ nm Fig. 4. Visible absorption spectrum of roccellin (2.0 x mol drnp3) in the presence of ( a ) BCD and (6) yCD at 298.2 K.In both cases the molar absorbance at 500 nm decreases systematically as the cyclodextrin concentration increases. The B-and y-CD concentration ranges were 0-2 x 10- and 0-5 x rnol dm-3, respectively. a-Cyclodextrin Interactions The addition of aCD (4.04 x lop3 mol dmp3) to an aqueous solution of roccellin (4.01 x mol dm-3) resulted in no significant changes in the dye’s absorption spectrum and no measurable fluorescence, neither was any induced circular dichroism apparent. Thus, it appears that aCD is not able to include the roccellin anion to any significant degree. This conclusion is in agreement with the results of attempts to construct possible inclusion of complexes using space-filling molecular models, where it is apparent that the naphthyl rings are too large to fit into the aCD cavity. b-Cyclodextrin Interactions The u.v.-visible absorption spectrum of roccellin (2.0 x lop6 rnol dm-3) alone, and in the presence of PCD concentrations from 5 x lop5 to 2 x lop3 mol dm-3, is shown in fig.4. A small hypochromic effect was observed on complexation, but no wavelength shift was detectable. The data are adequately described by the 1: 1 complex formation equilibrium : RO +pCD + RO *PCD (Kl). For this scheme the observed absorbance is given by A = &RO[RO] +EEO.BCD[RO*PCD]. (3) The equilibrium spectra of fig. 5 were fitted to eqn (3) by using the non-linear least-squares data fitting routine DATA FIT,^^ at all measured wavelengths, except thoseR. J. Clarke, J . H. Coates and S . F. Lincoln 2339 l-l d 0 E Fig.5. I I 10 - - 0 1 I 500 600 h/nm spectrum of RO alone (a). Derived spectra of the RO -BCD (b) and (RO), . yCD (c) complexes compared to the 24 2 0 I -12 - 1 6 300 50 0 700 X/nm Fig. 6. Induced circular dichroic spectrum of roccellin in the presence of p- and y-CD at 298.2 K. (---) [RO] = 4.0 x lop5, [DCD] = 4.0 x mol drnp3. (-) [RO] = 8.0 x [yCD] = 8.0 x mol dm-3.2340 Complexation of Roccellin by Cyclodextrins 100 8 0 h + .3 r: e 2 6 0 5 3 2 4 0 x 2 2 x Y .r( 2 0 0 550 6 50 X/nm 750 Fig. 7. Fluorescence spectrum (Aex = 420 nm) of roccellin (4.0 x lop5 mol dm-3) in the presence of 4.0 x lop3 mol dm-3 PCD (---) and yCD (--) at 298.2 K. where small changes in absorbance prevented DATAFIT converging to a best-fit value. The Kl values, calculated at 2 nm intervals in the range 462-570 nm, were weighted according to their estimated uncertainties, and averaged to give Kl = (7.20 & 0.88) x lo2 dm3 mol-l.Using this value of Kl, together with the directly determined molar absorptivities of roccellin, the spectrum of the RO.QCD complex was derived (fig. 5 ) . Comparison of the Kl values of roccellin and tropaeolin (Kl = 7.1 x lo2 mol dm-3 from temperature-jump measurements)* suggests that a similar process may be occurring in both cases, possibly the preferential encapsulation of the a-naphthol moiety. Com- parison of the spectrum of roccellin with that of PCD encapsulated roccellin, shows negligible shift in the wavelength of the absorption maximum, coupled with a diminution of the absorbances across the band. This behaviour is consistent with a change in the local environment of the chromophore, such as would be experienced on encapsulation.Fig. 6 shows the induced circular dichroic spectrum of roccellin in a 100-fold excess of Q-cyclodextrin. The spectrum exhibits only positive signals and there is no evidence of splitting due to exciton interaction. Thus, the spectrum appears to be consistent with the formation of a simple 1 : 1 complex and, since the CD signals are all positive, the transition moments of the dye molecule across the wavelength range studied must, according to Kajtar et a1.,16 lie within a 30" cone centred on the axis of symmetry of the QCD. The fluorescence spectrum of roccellin in a 100-fold excess of /3-cyclodextrin is shown in fig. 7. In the absence of PCD the dye exhibits no fluorescence.The fluorescence in the presence ofQCD no doubt arises as a consequence of the protection which the PCD cavity confers against quenching caused by solvent water and dissolved oxygen.17R. J . Clarke, J . H . Coates and S . F. Lincoln 234 1 y-CyclodextrieRoccellin Interaction The visible absorption spectra of roccellin (2.0 x mol dm-3) alone and in the presence of yCD concentrations ranging from 2.5 x lop7 to 5 x lop4 mol dmp3 are shown in fig. 4. A large hypochromic effect and a blue shift of cu. 15 nm were observed. In contrast to the roccellin-PCD system, significant changes are observed in the spectrum even at concentrations at which the dye is in excess, indicating that the affinity of the dye for the yCD is much greater than for the PCD.In addition, the form of the spectral changes suggests that the dye is included as a dimer rather than a monomer. In fact the data are best described by the equilibrium: 2 R 0 + yCD + (RO), - yCD (Kl,). (4) The methodology used previously for fitting the spectroscopic data was found to be inappropriate, since the very large value of K , , resulted in a shallow minimum which prevented convergence. Instead it was assumed that the system could be described by eqn (4), and a computer program MOLABS~~ was devised to calculate the absorbance of the dimer at every wavelength of interest by extrapolation of a double reciprocal plot of the change in the apparent molar absorptivity of the dye Germs the cyclodextrin concentration, to infinite cyclodextrin concentration.Program ROCEQU~~ was then used to calculate first the equilibrium concentrations of dye, cyclodextrin and complex, from the molar absorbances of the complex and of the free dye. The equilibrium concentrations were then used to obtain a value of the equilibrium constant for each wavelength of interest. Finally, these values were averaged over the range 580-464 nm to give the quoted value: K12 = (9.0 1.8) x 1Olo dm6 mol-,. Using this, and the directly determined values of the molar absorbances of roccellin, the spectrum of the (RO), yCD complex, shown in fig. 5, was derived. The form of the spectrum, which is characterised by a pronounced shift to shorter wavelengths of the maximum and the appearance of a shoulder at 540 nm, is consistent with the formation of an included dimer, accompanied by exciton interaction.The induced circular dichroic spectrum of roccellin in a 1 00-fold excess of yCD is shown in fig. 6. The positive and negative signals observed are characteristic of exciton splitting caused by dimerisation of the dye within a chiral environment, since the dye itself was shown by linear dichroism studies to have only long-axis polarised transitions associated with its visible absorption band. The necessary chiral environment is presumably provided by inclusion within the cyclodextrin. Two degenerate transition moments in close proximity and in the appropriate orientation are known to produce high-intensity exci ton spectra. The fluorescence spectrum of roccellin in a 100-fold excess of yCD is shown in fig.7. The intensity is almost twice that observed for PCD under the same conditions, and is presumably enhanced by the presence of the dye in the dimer form since, as has been observed earlier, aggregation of the dye is associated with fluorescence enhancement. The results presented here suggest that, where a complex is formed between any of the three cyclodextrins and roccellin, its stoichiometry and stability are determined mainly by the relative sizes of the guest molecule and the host cavity. It appears that the naphthyl groups of roccellin are too large to allow encapsulation by aCD. On the other hand, PCD is able to encapsulate one naphthyl group, whereas yCD allows two naphthyl groups to be encapsulated simultaneously, since the complex has the spectroscopic properties of a dimer.Furthermore, the tendency for dimerisa- tion of the dye is considerably enhanced by the presence of yCD in solution. Thus, the effective dimerisation constant (Kl, JyCD]) is ca. 4.5 x lo7 dm3 mol-1 at [yCD] = 5 x lop4 mol dmp3, compared with Kd = 1.64 x lo4 dm3 mol-l. The results des- cribed here, particularly when considered in the light of our previously published data7 for similar azo-dye systems, show that larger stability constants are associated with guests2342 Complexat ion of Roccellin by Cyclodextrins Table 1. Log Ka values for selected azo dyes and cyclodextrins (298.2 K) - methyl orange tropaeolin roccellin Kl, K2, 4 . 2 4, Kz, K1,Z K,, K . 2 _ - - - _ aCD 3.9,-, -b 9 7 , DCD 3.3,-, -b 2.8,6.6,9.4" 2.8,d ,- - 7 yCD 1.6,6.3, 7.9e 2.6,6.2,8.8" -, 10.9d a K , refers to DYEi-CDeDYE-CD.K2 refers to DYE * CD + DYE + (DYE), * CD. K l , refers to 2(DYE) + CD=(DYE);CD. Ref. (18). " Ref. (8). This study. Ref. (7). which fit most closely into the host. The results for three dyes are shown in table 1. It can be seen that MO, the dye with the smallest aromatic groups, is included by all three cyclodextrins. Although for a- and P-cyclodextrins the equilibrium spectra are not perfectly consistent with the total absence of other species, their spectra in the presence of MO suggest that the predominant species is the 1 : 1 complex and that the smaller aCD forms a more stable complex than the larger PCD. In the case of yCD, its 1 : 1 MO complex is considerably less stable than the 1 : 1 complexes of MO with the two smaller cyclodextrins.However, yCD is sufficiently large to allow the formation of an included MO dimer species of considerable stability. Tropaeolin is apparently unable to form an inclusion complex with aCD, probably because of the size of the naphthyl group. However, a dimer is included in both yCD and PCD. It is also notable that in the examples illustrated in table 1, the stability constants for the formation of a 1 : 1 complex (K,) are markedly less than for the corresponding 2: 1 complex (&), no doubt a consequence of the looseness of fit between the monomer and the cyclodextrin, which contrasts with the tightness of fit between the dimer and the cyclodextrin. Roccellin, with two naphthyl groups, is not surprisingly unable to form any inclusion complex with aCD, and forms only a 1 : 1 complex with PCD.The stability constants for the 1 : 1 complexes of both tropaeolin and roccellin with PCD are very similar, possibly indicating that in both cases it is the naphthyl moiety which is entering the cyclodextrin torus. It appears that there is insufficient room for a second roccellin molecule to enter the PCD molecule. y-Cyclodextrin is large enough to allow the entry of two roccellin molecules and the resulting complex has the greatest stability constant of those in table 1, again probably as a consequence of the closeness of fit between the large dimer and the large yCD cavity. We thank the Australian Research Grants Scheme for partial support of this research, and Dr Tom Kurucsev for the use of his program DATAFIT and for advice on some spectroscopic aspects of this study. References 1 M. L. Bender and M. Komiyama, Cyclodextrin Chemistry (Springer, Berlin, 1978). 2 W. Saenger, Angew. Chem., Int. Ed. Engl., 1980, 19, 344. 3 I. Tabushi, Acc. Chem. Res., 1982, 15, 66. 4 J. Szejtli, Cyclodextrins and their Inclusion Complexes (Akademiai Kiado, Budapest, 1982). 5 R. Breslow, Chem. Br., 1983, 126. 6 I. Tabushi and Y. Kuroda, J . Am. Chem. Soc., 1984, 106,4580.R. J. Clarke, J. H. Coates and S. F. Lincoln 2343 7 R. J. Clarke, J. H. Coates and S. F. Lincoln, Carbohydr. Res., 1984, 127, 18 1. 8 R. J. Clarke, J. H. Coates and S. F. Lincoln, J.Chem. Soc., Faraday Trans. 1, 1984, 80, 3 1 19. 9 J. S. Davis and H. Gutfreund, FEBS Lett., 1976, 72, 199. 10 R. L. Reeves and R. S. Kaiser, J . Org. Chem., 1970, 35, 3670. 11 M. Kasha, H. R. Rawls and M. Ashraf El Bayoumi, Pure Appl. Chem., 1965, 11, 371. 12 C. H. Giles, V. G. Agnihotri and K. McIver, J . Colloid Interface Sci., 1975, 50, 24. 13 B. R. Craven, J. C. Griffith and J. G. Kennedy, Aust. J. Chem., 1975, 28, 1971. 14 C. C. Bott and T. Kurucsev, J . Chem. Soc.. Faraday Trans. 2, 1975, 71. 749. I5 M. E. Gal, G. R. Kelly and T. Kurucsev, J . Chem. Soc., Faraday Trans. 2, 1973, 69, 395. 16 M. Kajtar, Cs. Horvath-Toro, E. Kuthi and J. Szejtli, Acta. Chim. Acad. Sci. Hung., 1982, 110, 327. 17 J. R. Lakowicz, Principles of Fluorescence Spectroscopy (Plenum Press, New York, 1983). 18 R. J. Clarke, Ph.D. Thesis (University of Adelaide, 1984). Paper 5 / 1373, Received 6th August, 1985

ISSN:0300-9599    

DOI:10.1039/F19868202333

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1986,82, 2345-2352 Dissolution of Cobalt Ferrites by Thioglycolic Acid Miguel A. Blesa," Albert0 J. G. Maroto and Pedro J. Morando Departamento Quimica de Reactores, Comision Nacional de Energia A tbrnica, Avenida del Lihertador 8250, 1429 Buenos Aires, Argentina The dissolution of cobalt ferrites CoZFe3-Z04 by thioglycolic acid inxiolves the chemisorption of thioglycolate anion onto Fe"I ions of the solid, followed by an electron transfer from the ligand to the metal ion and subsequent release of FeII. Kinetic data suggest that two adjacent Fe"'-L sites evolve to two FelI+L,. Substitution of Co" for Fe" does not bring about any noticeable change in the kinetics for x < 0.6. For larger values of x, the early mechanism of dissolution changes, suggesting that electron hopping within the octahedral sites may produce a chain dissolution of FeI' for each single original electron transfer from thioglycolate.Rate data in the presence of exogenous FeI' are also discussed. ~~~ The affinity of the thioglycolate anion towards FelI and FeIII is well known. In the former case the complexes have been characterized structurally and thermodynamically [ref. ( 1) and references therein] ; Felll-thioglycolate complexes, on the other hand, are unstable, and decompose to FeII and disulphur compounds.i The high stability of the complexes, as well as the deep colour characteristic of some of them, was the basis of the use of thioglycolic acid in the determination of iron in ores and insoluble oxides.2 Implicit in this use was the high dissolving potency of thioglycolic acid for iron1I1 o x i d e ~ .~ In a former paper4 we reported on the influence of solution variables (pH and acid concentration) on the dissolution of magnetite in thioglycolic acid solutions, thus putting forward evidence for the role of surface Fe-thioglycolate species in the dissolution mechanism. Now we have explored the characteristics of the dissolution process in the cobalt ferrite-thioglycolic acid system. By using ferrites of formula Cox Fe3-x04 (0 < x < 0.9) we have now been able to put forward evidence for some subtle characteristics of the dissolution process that are dependent on the structure of the solid. Since our original paper on Fe,O,-thi~glycolate,~ we have shown several instances of the 'reductive dissolution' mechanism of magnetite, mainly by FelI cornplexe~~-~~ and similar reductive mechanisms have been investigated and clarified through the work of Sellers and c o ~ o r k e r s .~ ~ - ~ ~ The mechanism involves an interfacial electron transfer from an adsorbed species to an FerIr surface state as the crucial point in the mechanism.lo Elegant electron microscopy by Sellers and coworkersi5 has shown that the FeI1 surface states thus formed are highly reactive and dissolve readily, thus justifying the assumption of rate control by the electron-transfer process. The abovementioned work has not put forward evidence of any kinetic manifestation of the semiconducting properties of magnetite. The dissolution process is akin to phenomena such as photocurrents at the semiconductor-electrolyte solution interfacei83 l9 that can be understood only on the basis of an adequate description of the electronic surface structure of the semiconductor.Segall and coworkersz0 have advanced evidence of the importance of the semiconducting characteristics of NiO in its oxidative dissolution mechanism. Similar considerations were put forward by NiiZ1 and DiggleZ2 These authors have shown that the availability of minority or majority carriers at the interface may control the dissolution rate. It is well know that both the crystal and electronic structures of magnetite can suffer 78 2345 FAR 12346 Dissolution of Cobalt Ferrites important changes upon substitution. Thus, magnetite presents an inverse spinel ~tructure,,~ with FelI1 ions distributed evenly in tetrahedral and octahedral sites and FerT located in octahedral sites only.There is a fast electron-hopping between [FeIII], and [FeTT], that is responsible for the high electrical conductivity of magnetite.24 Upon substitution of ColI for Feil, cobalt distributes unevenly in octahedral and tetrahedral sitesz5 and when x reaches ca. 0.7 the electrical conductivity drops to very low values26 because there are no close-neighbour pairs [Fe1i1-Fe11]o.27 Our previous results had given some clues in the sense that electrical conductivity might have some non-trivial consequences in the dissolution p r o ~ e s s . ~ We now report data on the dissolution of cobalt ferrites that prove the involvement of this property in the dissolution mechanism.Experimental All reagents employed were of analytical purity and were used as provided. Ferrites were prepared as described for magnetite,2s but partially replacing Coil for Fe**. Mixtures of FeCl, * 4H,O and CoCl, * 6H,O were treated in boiling aqueous solution with ammonia in the presence of hydrazine and NaNO,; the resulting slurry was aged under continuous stirring for 30 min. The solid thus formed was rinsed several times with water, filtered off and dried in a desiccator at room temperature. The samples were characterized by chemical analysis, X-ray diffraction, scanning electron microscopy and specific surface area measurements. The solids were in all cases composed of cubic particles of average edge 0.1 ,urn and with a rather small extent of polydispersion.The specific surface area values ranged between 7 and 9 m2 g-l, in good agreement with the calculated geometric area for the particles. Kinetics experiments were performed as described previ~usly.~ The ferrite (usually 40 mg) was suspended in doubly distilled water in a magnetically stirred cylindrical beaker provided with a water jacket. The reaction was started by adding solutions of thioglycolic acid and sodium hydroxide (to obtain the desired pH); the volume of the reacting slurry was 170 cm3. Samples were taken periodically and poured into a large volume of water containing thioglycolic acid (TGA) and excess ammonia. This solution was filtered through a Nuclepore membrane (pore size 0.45pm) and the absorbance at 530 nm was measured in a Shimadzu UV-210A spectrophotometer.The amount of dissolved iron was then calculated from the calibration curve. Experiments carried out at various stirring rates were performed to ensure that the reaction was not under diffusional control. Results and Discussion The Shape of a vs. t Plots Plots of the fraction of ferrite dissolved as a function of time are shown in fig. 1 for various x values. In the range 0 6 x 6.0.6 neither the shape nor the time scale is noticeably dependent on x. On further increasing x, an induction period ensues. The change in the shape is not simply of the type that can be accounted for by shifting the time scale [e.g. plotting a =flt/lo.5)]zg and suggests, therefore, a change in the reaction mechanism, i.e. in the relative importance of the nucleation and growth stages.3o Our previous results on magnetite dissolution by TGA were interpreted contracting-sphere kinetics, i.e.assuming that nucleation was fast over all the solid s ~ r f a c e . ~ The cubic-root law holds well for cobalt ferrites with x < 0.6: see fig. 2. Except when noted, all quoted k values refer to the expression 1 -(1 -a)$ = k t . (1) Almost identical rate constants were obtained for x < 0.6: k = (8.2f0.4) x lop2 s-l.M. A . Blesa, A . J , G. Maroto and P. J . Morando 2347 1 .o a 0 . 5 0 5 10 15 20 25 tlmin Fig. 1. Dissolution fraction a us time profiles for various Co,Fe,-,O, at 70 "C and total thioglycolic acidconcentrationC,,, = 6.5 x mol dm-"pH 3.70; .,x = 0.16; 0 , x = 0.25; 0 , x = 0.50; B, x = 0.69. 0.6 m - h a I 0.4 & v I - 0 .2 0 A / i / 2 4 6 8 1 0 r/min Fig. 2. Linear plot of 1 -(1 -a); as function of time at 70 "C; A, pH 3.90; e, pH 3.32; C,,, = 6.5 x mol drn-,. Electron Microscopic Characterization of the Dissolution Process SEM was used in an attempt to distinguish between massive and localized attack on the surface of ferrite crystals. Massive dissolution is apparent from the photographs, however, because of the small size of the crystal and the rather low magnification available (30000), pitting cannot be ruled out. 78-22348 Dissolution of Cobalt Ferrites 7.0 d I N I WY 4: Y 5 -0 3.0 1 I I 3.0 1.0 5.0 PH Fig. 3. Influence of pH on the specific rate constant k ; C,,, = 6.5 x mol d ~ n - ~ ; t = 70 “C; 0, experimental; (-a -) calculated assuming first order on surface complex concentration, k = kE”,”[C, FeTGA)/ qwaFeTGA)] ; (---) calculated assuming second-order dependence on surface complex concentration, k = kg;[Cq_ FrTGA)/.q-a$&GA)].Surface complex concentration ratios were calculated according to ref. (4). Superscript ‘max’ in the above relations refers to maximum rate, i.e. pH 3.8. The Influence of Solution Variables on the Rate of Dissolution pH influences the rate in the same way as described before for magnetite.4 Fig. 3 shows the values of the dissolution rate constant ( k ) us. pH at 70 “C. The interpretation is as follows in a modified version of the reasoning given in ref. (4). The basic idea is that the data in fig. 3 simply describe the pH dependence of the adsorption equilibrium, e.g. the change in the concentration of surface complexes FeIII-TGA with pH.or its equivalent, &ds, calculated from Langmuir-type expression^.^ 9 31-33 AGids has been shown to exhibit maxima (or humps) at pH values close to the pK of the conjugate acid of the complexing anion.7* 34-36 In fact, the shape of the curve can be very asymmetrical, showing a not very pronounced decrease in the acidic branch and this can be modelled by assuming that the complexing anion replaces both OH- ions and H,O molecules bound to metal ions located in kinks of the solid surface :37 Adsorption of complexing anions onto metal oxides can be characterized by M(OH),-r(H,O) + A- + (1 - 5)H’ + MAC- + H 2 0 . (2) In eqn (2), 5 is the fraction of exposed M sites accounted for by H,O ligands.For high coverages, 5 -+ 1, a very shallow maximum and a very asymmetrical curve are expected, as actually found in several instance^.^^^^^^^ Thus, the pH dependence of the rate (fig. 3) is controlled by the dependence of the concentration of surface complexes on pH. In our former paper on the dissolution of magnetite,4 the pH dependence of adsorption was assumed according to a very simple model that predicts a symmetric curve, the width of which was determined by the acidity constants of the surface and of thioglycolic acid. Even on this assumption, a much better fit is obtained if the rate is assumed to be second-order with respect to surface complex concentration R = khom[ - Fe1I1-TGAI2. (3)M. A . Blesa, A . J . G. Maroto and P. J . Morando 2349 This is shown in the calculated profiles included in fig.3. Following the procedure outlined in ref. (4), a term of the form: was assumed to account for the pH dependence of surface Fe-TGA complexes; Ka, and Ki are the first acidity constants of thioglycolic acid and of Fe-OH: surface sites, respectively. The outermost profile in fig. 3 represents precisely the behaviour of the above expression, whilst the inner profiles represents the behaviour of the squared expression. In both cases the height of the maximum was chosen to fit the kinetic data. This result agrees well with mechanistic information from homogeneous aqueous systems; depending upon the experimental conditions, the rate of the internal redox reaction in Fe'II-thioglycolate complexes has been found to be or ~ e c o n d - o r d e r ~ ~ - ~ ~ in the complex.In every case, however, two ligand anions were required to form the activated state, e.g. FelI1 (TGA)g (first-order in complex), or either of the dimers I H I H2 (second-order in complex). All these species involve deprotonated R-S- groups and sulphur complexation. The data in the heterogeneous system do not agree with this: the maximum in rate is found at pH 3.8, which is reasonable only if the RCO,H/RCO,- protolytic equilibrium is considered (Kal = 3.72 x 10-4),44 but not if -SH deprotonation is required (Ka2 = 7.9 x 10-9).44 Furthermore, the basicity of -S- groups should be larger under heterogeneous and electrostatic considerations prevent the acceptance of //O - F-0-c highly charged - Fe species as dominant.Dimer sites, such as I are ' O T O \ S-C-H - Fe-S-CH, I H not excluded by this reasoning, but at high TGA concentration these species are expected to dissociate to two monomeric complexed sites. We therefore postulate that the ensuing redox reaction is of the type: 2 FeI11-O-C(0)CH2SH- 2 2 The close proximity of FelI1 centres provides FeII + R--S-S-R (4) 1 fast FeIIaq the adequate vicinity of the two organic molecules,- fulfilling the same role as the previously mentioned homogeneous precursors. It is probable that the involvement of adjacent sites may be a general reason for rate enhancement in redox reactions when more than one electron must be exchanged. A similar case was suggested to be the oxidation of hydrazine by solid barium The influence of total thioglycolic acid concentration is shown in fig.4. This influence is well modelled by the above mentioned assumption of Langmuir adsorption as a pre-equili brium.2350 Dissolution of Cobalt Ferrites 7.5 5.0 I vl N 0, . 2.5 0 0.2 0 . L 0.6 CT,-,/mol dm-3 Fig. 4. Specific rate constant k for the dissolution of Co,~,5Fe,~,,0, as a function of total thioglycolic acid concentration at 70 "C; pH,, 2.9. 0.3 m , I n 7 0.2 - v I * 0 . 1 0 1 I I I I I 2 r, 6 f/min Fig. 5. Contracting sphere plots for the dissolution by TGA 6.5 x lop2 mol dmp3 of Co,,,,Fe,,,,O, at 70 "C in presence of ferrous ion; 0, [Fe2+] = 0; a, [Fe2+] = 9.0 x mol dm-3; a, [Fe2+] = 2.5 x mol dm-3; pH,, 2.9. The role of thioglycolic acid can be performed by other complexing anions that can be engaged in an internal electron-transfer process with FelI1.These are potentially all ligands featuring ligand-to-metal charge-transfer bands, the simplest example being SCN-. Thiocyanate dissolves magnetite in acidic solutions, both in a thermal and a photochemical processes.47 Other reported examples are I-,47 oxalate17 and itr rate.^^ At high x, addition of ferrous salts supresses the lnduction period, and gives rise to a faster dissolution, see fig. 5. This is an important result that shows (a) that the reaction is not diffusion controlled, (b) the subtleties of the mechanism to be discussed below.M . A . Blesa, A . J . G. Maroto and P. J . Morando 2351 The Influence of Solid Composition on the Dissolution Rate When discussing the influence of solution variables, the possible importance of CoI1 substitution for FeII was ignored.For x d 0.6 this is consistent with the experimental results, and can be understood easily on the basis of a fast release of MI1 ions as compared to FeIII. This has been abundantly documented in the literature [see ref. (lo), (49) and (50)] and points to an essentially identical FeTrl surface in all cobalt-ferrites with x 6 0.6. Because of this reasoning, and because of the sharp change in behaviour observed at x x 0.6, it is not reasonable to attribute the onset of an induction period to different phase-transfer rates of CoII and FeII.? On the other hand, it is well known that the electronic structure of the solid suffers a similar drastic change in the same composition range, owing to the isolation of FelI ions from other FeT1 centres.26327 It is not at all obvious why the two changes should be related, but we shall offer a tentative rationale.The steps in the overall dissolution process after electron transfer to Fe"I are:51 Ferlkink --+ FelIads (Stern plane) --+ FelIbulk. ( 5 ) As long as there is a possible fast electron hopping, the intermediate FelIads can be an excellent reductant for further FeIII ions,33 and successive electron-transfer processes can give rise to an appreciable 'chain length' for the dissolution process originated from one single FerI1 reduction. This is in agreement with the recent report by Tronc et ~ l . , ~ ~ who have shown that FeII adsorbed onto magnetic iron oxides can in fact pump electrons into the colloid core within the octahedral sub-lattice of the spinel structure.Exogenous FeII can play the same role, i.e. dissolve magnetite in the presence of complexing ions through an outer-sphere heterogeneous electron transfer : Kads FeIII-TGA + FeII-TGA,, -+ - Felll-TGA..*F e I1 -TGAads FE11-TGAa9 + Fe1I1-TGAaq t - FeII-TGA. ..Felrl-TGA,,,. This is similar to the reductive dissolution of magnetite by FeII in ~ x a l i c , ~ ethylendiaminetetra-acetic6T 52 and nitrilotriaceticll acid solution^.^^ 11, 47 This interpreta- tion suggests that the overall reaction rate measured at high conversion contains, in every case, a substantial contribution from the FeIT pathway. Even the initial rate in the induction-period-free systems does not give a direct measure of the rate of reaction (4) because of the 'chain-length' effect mentioned above.Under the conditions of the present work, this effect did not give rise to noticeable localized attack, and dissolution proceeded in an essentially isotropic fashion, with only rounding off of edges. However, the possibility of a pitting attack must be taken into account; such pitting was observed by Segal and Sellers13 in the case of the dissolution of nickel ferrite by tris(picolinato)vanadium(Ir). L E T (6) M. A. B and P. J. M. are members of CONICET. Partial support through grants from SECYT-CONICET and CICPBA is gratefully acknowledged. References 1 D. L. Leussing and I. M. Kolthoff, J . Am. Chem. Soc., 1953, 75, 3904. 2 R. A. Hummel and E. B. Sandele, Anal. Chim. Acta, 1952, 7 , 308. 3 D. Bradbury, in Water Chemistry in Nuclear Reactor Systems 1 (British Nuclear Energy Society, London, 1978), p.373. i. 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ISSN:0300-9599    

DOI:10.1039/F19868202345

出版商:RSC    

年代:1986

数据来源: RSC