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/F198682FX007

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/F198682BX009

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

ISSN 0300-9599 JCFTAR 82(2) 263-568 (1 986) 263 273 28 1 29 1 319 329 339 349 359 365 375 387 40 1 415 43 1 439 451 457 JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions 1 Physical Chemistry in Condensed Phases ~ ~~~ ~ ~ CONTENTS Electron Spin Resonance Studies of Free and Supported 12-Heteropoly Acids. Part 1 .-Air and Vacuum Dehydrations of H,[PMo,,O,,] *xH,O and H,(SiMo,,O,,) * xH20 Electron Spin Resonance Studies of Free and Supported 12-Heteropoly Acids. Part 2.-Reduction of H,[PMo120,,J - xH,O and H,[SiMo,,O,,] - xH,O and Oxygen Adsorption Spectroscopy of Proflavine adsorbed on Clays R. A. Schoonheydt, J. Cenens and F. C. De Schrijver The Interaction of Aromatic Compounds with Poly(vinylpyrro1idone) in Aqueous Solution. Part 5.-Binding Isotherms for Phenols and @Substituted Phenols Kinetics of the Axial Ligation of Aquocobaloximes in the Presence of Cetyltri- methylammonium Acetate Micelles A.K. Yatsimirsky, 0. I. Kavetskaya and I. V. Berezin The Effect of Tannic Acid on the Electrical Properties of the Interface and the Non-linear Streaming Potential of Cellulose in a Cationic Dye Solution M. Espinosa- JimCnez, F. Gonzalez-Caballero and C. F. Gonzdlez-Fernandez The Solvent Effect on the Electro-oxidation of 1,4-Phenylenediarnine. The Influence of the Solvent Reorientation Dynamics on the One-electron Transfer Rate M.OpaRo Alkali-metal Cryptates in Nitromethane. Thermodynamic Parameters for Complex Formation and for Transfer among Different Solvents A. F. Danil de Namor, L. Ghousseini and T. Hill A Fast Kinetic Study of the Formation and Decay of N,N,N',N'-Tetramethyl- p-phenylenediamine Radical Cation in Aqueous Solution P.Maruthamuthu, L. Venkatasubramanian and P. Dharmalingam Determination of Isotherms and Initial Heat of Adsorption of CO, and N,O in Four A Zeolites from Infrared Measurements Y. Delaval, R. Seloudoux and E. Cohen de Lara A New Analytical Solution of the Ternary Gibbs-Duhem Equation 2-C. Wang Water Spin Relaxation in Colloidal Systems. Part 1.-170 and 2H Relaxation in Dispersions of Colloidal Silica Water Spin Relaxation in Colloidal Systems. Part 2.-170 and ,H Relaxation in Protein Solutions Water Spin Relaxation in Colloidal Systems. Part 3 .-Interpretation of the Low-frequency Dispersion Non-steady-state and Transient Effects in Medium-pressure Hydrogenation of Carbon Monoxide over Rhodium Catalysts K.Gilhooley, S. D. Jackson and S. Rigby Evidence for a Thermotropic Phase Transition in Oleic Acid J. V. Champion, J. F. Crilly and R, P. Tatam An 170 Nuclear Magnetic Resonance Relaxation-time Study of Sucrose-Water Interactions Rheo-optics of Suspensions of Anisometric Particles. Part 1 .-Monodisperse Ellipsoidal Particles R. Fricke and G. Ohlmann R. Fricke and G. Ohlmann P. Molyneux and S. Vekavakayanondha L. Piculell L. Piculell and B. Halle B. Halle and L. Piculell P. S, Belton and K. M. Wright D. S. Jayasuriya and T. G. M. van de Ven 10 FAR 1Con tents 473 483 495 509 513 52 1 527 533 545 563 Rheo-optics of Suspensions of Anisometric Particles. Part 2.-Polydisperse, Brownian Ellipsoidal Particles D. S. Jayasuriya and T.G. M. van de Ven Adsorption of 4He on Spheron and Grafoil between 2 and 30K A. A. Antoniou Electrochemical Reduction of 2-Cyclohexen- 1 -ones in a Hydroethanolic Medium E. Brillas and A. Ortiz The Importance of the Electrostatic Interaction in Condensed-phase Photo- induced Electron Transfer P. Suppan Calorimetric Studies of NaI Solutions in Binary Organic Isodielectric Mixtures A. Piekarska, H. Piekarski and S. Taniewska-Osihska Electron Capture in Single Crystals of Triphenylmethylarsonium Iodide. Electron Spin Resonance Detection of Ph,AsI M. Geoffroy, A. Llinares and S. P. Mishra Electron-microscopic Observation of Photodeposited Pt on TiO, Particles in Relation to Photocatalytic Activity H. Nakamatsu, T. Kawai, A. Koreeda and S. Kawai Molecular Complexes. Part 18 .-A Nuclear Magnetic Resonance Adaptation of the Continuous Variation (Job) Method of Stoicheiometry Determination J. Homer and M. C. Perry Characterization of Clays and Clay-Organic Systems. Cation Diffusion and Dehydroxylation D. T. B. Tennakoon, J. M. Thomas, W. Jones, T. A. Carpenter and S. Ramdas The Viscosity and Structure of Solutions. Part 1.-A New Theory of the Jones-Dole B-Coefficient and the Related Activation Parameters : Application to Aqueous Solutions D. Feakins, W. E. Waghorne and K. G. Lawrence

ISSN:0300-9599    

DOI:10.1039/F198682FP025

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

JOURNAL OF THE CHEMICAL SOCIETY 115 123 135 149 151 153 165 173 183 187 20 1 209 219 235 25 1 Faraday Transactions 11, Issue 2, I986 Molecular and Chemical Physics For the benefit of readers of Faraday Transactions I, the contents list of Faraday Transactions II, Issue 2 is reproduced below. A Low-power Super-regenerative Oscillator Nuclear Quadrupole Linewidth Study of Hexamethylenetetramine from 77 to 379 K R. J. Trepanier and M. A. Whitehead Spectral Investigations of 4,5-Diamino-6-hydroxy-2-mercaptopyrimidine and 4,6-Diamino-2-mercaptopyrimidine R. K. Gael, S. P. Gupta, S. Sharma and C. Gupta Triplet-state Photophysics and Transient Photochemistry of Cyclic Enethiones. A Laser Flash Photolysis Study K. Bhattacharyya, P. K. Das, V. Ramamurthy and V. P. Rao Effect of Lateral Interactions of Adsorbed Molecules on Adsorption and Desorption Rates V.P. Zhdanov Reply to ' Effect of Lateral Interactions of Adsorbed Molecules on Adsorption and Desorption Rates' Strongly Hydrogen-bonded Molecular Complexes Studied by Matrix Isolation Vibrational Spectroscopy. Part 3.-Ammonia-Hydrogen Bromide and Amine- Hydrogen Bromide Complexes A. J. Barnes and M. P. Wright Strongly Hydrogen-bonded Molecular Complexes Studied by Matrix Isolation Vibrational Spectroscopy. Part 4.-Dimethyl Sulphoxide-Hydrogen Halide Complexes A. J. Barnes and M. P. Wright Electron Spin Resonance Study of the Salts formed by Combinations of Transition- metal Ion Chelates of Tetracationic Porphyrazines with Tetra-anionic Phthalo- cyanines A. Skorobogaty, T. D. Smith, J.R. Pilbrow and S. J. Rawlings Photoelectron Spectra of Benzo[b]biphenylene and Dibenzo[b,h]biphenylene H. Yamaguchi, M. Higashi, R. Gleiter and H. Baumann -Symmetry and the Hartree-Fock Model. Physical Interpretation and some Results for the Ally1 Radical D. B. Cook One- and Two-electron Oxidation of Lead(11) Tetrakis(N-methylpyridy1)porphyrins in Aqueous Solution M-C. Richoux, P. Neta and A. Harriman Resonance Stabilisation of Zinc Porphyrin n-Radical Cations P. Neta, M-C. Richoux, A. Harriman and L. R. Milgrom Luminescence of Porphyrins and Metalloporphyrins. Part 1 1 .-Energy Transfer in Zinc-Metal-free Porphyrin Dimers R. L. Brookfield, H. Ellul, A. Harriman and G. Porter Formation and Decay of Zinc Tetrakis(N-methyl-3-pyridy1)porphine n-Radical Cation in Water M-C.Richoux, P. Neta, P. A. Christensen and A. Harriman Addition of 2,3-Dimethylbutane to Slowly Reacting Mixtures of Hydrogen and Oxygen at 480 "C. Rate Constants of the Elementary Reactions Involved R. R. Baldwin, G. R. Dreweny and R. W. Walker 0. L. J. Gijzeman269 A Theoretical Study of the Reaction of Ca(4s4p ,P) with H, E. D. Simandiras and N. C. Handy The following papers were accepted for publication in J. Chem. Soc., Faraday Trans. I during November. 5/617 5/630 5/925 Relationship between a Synergic Effect in the Solution Power and a Change in the Solvent Complexing Capacity J. R. Quintana and I. Katime Mechanism of n-Alkane Transformations over Solid Superacids of Lewis Character Al,O,/AlCl, M. Marczewski Electron Paramagnetic Resonance Studies on Supported Rhenium Catalysts with Special Attention to Re/ZrO, System T.Komatsu, M. Komiyama and Y. Ogino and M. Iwamoto 5 / 1077 Interlayer Adsorption of Ammonia and Pyridine in V,O, Xerogel E. Ruiz-Hitzky and B. Casal 5/ 1 120 Stability of Monochloride Complexes of some Divalent Transition-metal Cations in N,N'-Dimethylformamide W. Grzybkowski and M. Pilarczyk 5/ 1177 The Electrical Conductivity of Carbon Fibre Absorbents. An Attempt to Discriminate between Chemisorption and Physisorption of Chlorin H. Tobias, H. Cohen and A. Soffer 5/ 1247 High-resolution Solid-state Magic-angle Spinning Nuclear Magnetic Resonance Investigations of Surface Hydroxyl Groups of Modified Silica Gel T. Bernstein, P. Fink, V. M. Mastikhin and A. A. Shubin 5/ 1364 Gibbs Free Energies of Transfer of Silver(I), Copper(1) and Potassium(1) G.Gritzner 5/1461 A Kinetic and Equilibrium Study of the Inclusion of Pyronine B by p- and y-Cyclodextrin R. L. Schiller, S. F. Lincoln and J. H. Coates 5 / 1466 The Reaction of Ferrimyoglobin with Methyl Hydrogen Peroxide M. L. Kremer 5 / 1489 Hydrogenolysis of Alkanes. Part 3.-Hydrogenolysis of n-Hexane and Methyl- cyclopentane over Variously Treated Ru/TiO, Catalysts R. Burch, G. C. Bond and R. Rajaram 5/1504 Superacidic Sites in Zeolites K. A. Becker and S. Kowalak 5/1513 Application of Infrared Spectroscopy to the Measurement of Surface and Bulk Oxidation/Reduction States of MOO, J. S. Chung and C. 0. Bennett 5/ 1546 Osmotic Coefficients of Aqueous NaCl and KCl Solutions. Temperature- Concentration Behaviour of the BAHE Lattice Model J.L. Gomex Estevez 5 / 1554 Polyaniline, A Novel Conducting Polymer. Morphology and Chemistry of its Oxidation and Reduction in Aqueous Electrolytes Wu-Song Huang, B. D. Humphrey and A. G. MacDiarmid 5 / 1569 The Viscosity and Structure of Solutions. Part 2.-Measurements of the &Coefficient of Viscosity for Alkali-metal Chlorides in Propan- 1 -01-Water Mixtures at 25 and 35 "C J. Crudden, G. M. Delaney, D. Feakins, P. J. O'Reilly and E. Waghorne 5 / 1 570 The Viscosity and Structure of Solutions. Part 3.-Interpretation of the Thermodynamic Activation Parameters for Propan-1-01-Water-Electrolyte Systems J. Crudden, G. M. Delaney, D. Feakins, P. J. O'Reilly and W. E. W aghorne 5/1631 Thermal Conductivity of Argon, Nitrogen and Carbon Dioxide at Elevated Temperatures and Pressures A.I. Johns, S. Rashid, J. T. R. Watson and A. A. Clifford (ii)5 / 1632 Thermodynamic Properties of Binary Alcohol-Hydrocarbon Systems A. Pettersson, P. Saris and J. B. Rosenholm 5 / 165 1 Study of Isothermal and Non-isothermal Molecular Gas Transport in Model Homogeneous Porous Adsorbents J. H. Petropoulos 5 / 1655 Entry Rate Coefficients in Emulsion Polymerization Systems I. A. Penboss, R. G. Gilbert and D. H. Napper 5/1696 The Catalysis by Colloidal Gold of the Reaction between Ferricyanide and Thiosulphate Ions M. Spiro and P. L. Freund 5 / 1698 Chemical Relaxation in Mixed Micellar Solutions Containing Surface-active Drugs and Hexadecyltrimethyl Ammonium Bromide Micelles J. Gormally and S. Sharma 5/1728 Metal Particles supported by Porous Glass R.N. Edmonds, M. R. Harrison and P. P. Edwards 5 / 1786 Remarks on Dependence on Temperature ' at Constant Volume ' P. G. Wright 5/1787 Special Features of Equilibrium Constants that are Related to Volume Fractions P. G. Wright 5/ 1789 Stochastic Models of Multi-species Kinetics in Radiation-induced Spurs P. Clifford, N. J. B. Green, M. J. Oldfield, M. J. Pilling and S. M. Pimblott 5/1947 Free-energy Parameters for the Complexation of Metal Ions and Cryptand 222 in Propan-1-01 and for the Transfer and Partition of Cryptand 222 in Water-Alcohol Systems A.F.Dani1 de Namor, H. Beroa de Ponce and E. C. Viguria 5 / 1880 Electrochemical Sensors. Theory and Experiment W. J. Albery, P. N. Bartlett, A. E. G. Cass, D. H.Craston and D. G. D. Haggett 5/1881 Trace Metal Analysis in Hydroponic Solutions C. M. A. Brett and M. M. P. M. Net0 5/1882 Anodic Detection in Flow-through Cells D. C. Johnson, J. A. Polta, T. Z. Polta, G. G. Neuburger, J. Johnson, A. P-C. Tang, In-Hyeong Yeo and J. Baur 5/1883 The Convolution of Voltammograms as a Method of Chemical Analysis K. B. Oldham 5/ 1884 Conducting-polymer Gas Sensors J. J. Miasik, A. Hooper and B. C. Tofield 5 / 1885 Semiconducing Tetrapyrrole Pigment Gas Sensors C. L. Honeybourne, J. D. Houghton, R. J. Ewen and C. A. S. Hill 5 / 1886 Aspects of the Optimization of Poly(Viny1 Chloride) Matrix Membrane Ion- selective Electrodes J. D. R. Thomas 5/1887 Ion Recognition by Macrocyclic Hosts I. 0. Sutherland 5/1888 New Host-Guest Carriers J.C. Lockhart 5 / 1889 Biosensors based on Reversible Reactions at Blocked and Unblocked Elec- trodes R. P. Buck 5 / 1890 Design of Anion-selective Membranes for Clinically Relevant Sensors U. Oesch, D. Ammann, H. V. Pham, U. Wuthier, R. Zund and W. Simon 5/ 189 1 Ion-selective Electrodes based on Siderophanes D. Midgley 5 / 1892 Solid-state Ion Sensors. Theoretical and Practical Issues R. G. Kelly and A. E. Owen 5 / 1893 Recent Advances in Microelectronic Ion-sensitive Devices (ISETs). The Operational Transducer A. C. Covington and P. D. Whalley 5 / 1894 Coated-wire Ion-selective Electrodes. Principles and Practice H. Freiser 5 / 1895 Potentiometric Monitoring of Proteins. Part 3.-Direct Potentiometry with a 5/1896 Amperometric Enzyme Electrodes M. J. Green and H.A. 0. Hill 5 / 1897 Chemically Modified Electrodes for the Electrocatalytic Oxidation of Nico- tinamide Coenzymes Lo Gorton (iii) Copper Electrode M. L. Hitchman and F. W. M. Nyasulu5 / 1955 Enzyme Entrapment in Electrically Conducting Polymers. Immobilization of Glucose Oxidase in Polypyrrole and its Application in Amperometric Glucose Sensors N. C. Foulds and C. R. Lowe 5 / 1956 Sensors from Polymer-modified Electrodes M. W. Espenscheid, A. R. Gatak- Roy, R. B. Moore 111, R. M. Penner, M. N. Szentirmay and C. R. Martin 5/20 10 Analytical Applications of Gas Membrane Electrodes S. Bruckenstein and J. S. Symanski 5/2195 Use of a Reversibly Immobilized Enzyme in the Flow Injection Amperometric Determination of Picomole Glucose Levels C. E. Lomen, U. de Alwis and G.S. Wilson (iv)Cumulative Author Index 1986 Antoniou, A. A., 483 Aveyard, R., 125 Baldwin, R. R., 89 Belton, P. S., 451 Berezin, I. V., 319 Binks, B. P., 125 Bloemendal, M., 53 Brillas, E., 495 Carpenter, T. A., 545 Cenens, J., 281 Champion, J. V., 439 Chiou, C. T., 243 Clark, S., 125 Cohen de Lara, E., 365 Crilly, J. F., 439 Danil de Namor, A. F., 349 Dawber, J. G., 119 De Schrijver, F. C., 281 Dean, C. E., 89 Delaval, Y., 365 Dharmalingam, P., 359 Espinosa-Jimenez, M., 329 Feakins, D., 563 Fisher, D. T., 119 Fricke, R., 263, 273 Funabiki, T., 35 Geoffroy, M., 521 Ghousseini, L., 349 Gilhooley, K., 431 Gonzalez-Caballero, F., 329 Gonzalez-Fernandez, C. F., 329 Gormally, J., 157 Halle, B., 401, 415 Hans&, O., 77 Heatley, F., 255 Hedges, W. M., 179 Higson, S., 157 Hill, T., 349 Homer, J., 533 Honeyman, M.R., 89 Hunt, D. J., 189 Iizuka, T., 61 Ikeda, H., 61 Jackson, S. D., 431, 189 Jaeger, N., 205 Jayasuriya, D. S., 457, 473 Jones, W., 545 Kavetskaya, 0. I., 319 Kawai, S., 527 Kawai, T., 527 Kevan, L., 213 Khoo, K. H., 1 Kleine, A., 205 Koreeda, A., 527 Lang, J., 109 Lawrence, K. G., 563 Leaist, D. G., 247 Lim, T-K., 69 Llinares, A., 521 Logan, S. R., 161 Malliaris, A., 109 Manes, M., 243 Marcus, Y., 233 Maruthamuthu, P., 359 Mead, J., 125 Mishra, S. P., 521 Miyamoto, A., 13 Molyneux, P., 291 Morgan, H., 143 Mori, K., 13 Moyes, R. B., 189 Murakami, Y., 13 Nakamatsu, H., 527 Narayana, M., 213 Ohlmann, G., 263, 273 Okazaki, S., 61 Ooe, M., 35 Opallo, M., 339 Ortiz, A., 495 Perry, M. C., 533 Pethig, R., 143 Piculell, L., 387, 401, 415 Piekarska, A., 5 13 Piekarski, H., 513 Pletcher, D., 179 Ramdas, S., 545 Rideout, J., 167 Rigby, S., 431 Rosenholm, J.B., 77 Rouw, A. C., 53 Ryder, P. L., 205 Salmon, G. A., 161 Sarkany, A., 103 Schoonheydt, R. A., 281 Seloudoux, R., 365 Shindo, H., 45 Somsen, G., 53 Suppan, P., 509 Symons, M. C. R., 167 Tanaka, T., 35 Taniewska-Osinska, S., 5 13 Tatam, R. P., 439 Tennakoon, D. T. B., 545 Thomas, J. M., 545 van de Ven, T. G. M., 457,473 Vekavakayanondha, S., 291 Venkatasubramanian, L., 359 Waghorne, W. E., 563 Walker, R. W., 89 Wang, Z-C., 375 Warhurst, P. R., 119 Wells, P. B., 189 Whyman, R., 189 Wiens, B., 247 Wren, B.'W., 167 Wright, K. M., 451 Yatsimirsky, A. K., 319 Yoshida, S., 35 Zana, R., 109 Schulz-Ekloff, G., 205NOMENCLATURE A N D SYMBOLISM Units and Symbols.The Symbols Committee of The Royal Society, of which The Royal Society of Chemistry is a participating member, has produced a set of recommendations in a pamphlet ‘Quantities, Units, and Symbols’ (1 975) (copies of this pamphlet and further details can be obtained from the Manager, Journals, The Royal Society of Chemistry, Burlington House, London WIV OBN). These recommendations are applied by The Royal Society of Chemistry in all its publications. Their basis is the ‘Systbme International d‘Unit6s’ (SI). A more detailed treatment of units and symbols with specific application to chemistry is given in the IUPAC Manual of Symbols and Terminology for Ph ysicochemical Quantities and Units (Pergamon, Oxford, 1 979). Nomenclature.For many years the Society has actively encouraged the use of standard IUPAC nomenclature and symbolism in its publications as an aid to the accurate and unambiguous communication of chemical information between authors and readers. In order to encourage authors to use IUPAC nomenclature rules when drafting papers, attention is drawn to the following publications in which both the rules themselves and guidance on their use are given: Nomenclature of Organic Chemistry, Sections A, B, C, D, El F, and H (Pergamon, Oxford, 1979 edn). Nomenclature of Inorganic Chemistry (Butterworths, London, 1971, now publis- hed by Pergamon). Biochemical Nomenclature and Related Documents (The Biochemical Society, London, 1978). A complete listing of all IUPAC nomenclature publications appears in the January issues of J.Chem. SOC., Faraday Transactions. It is recommended that where there are no IU PAC rules for the naming of particular compounds or authors find difficulty in applying the existing rules, they should seek the advice of the Society’s editorial staff. (vi)THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 81 Lipid Vesicles and Membranes Loughborough University of Technology, 15-17 April 1986 Organising Committee : Professor D. A. Haydon (Chairman) Professor D. Chapman Mrs Y. A. Fish Dr M. J. Jaycock Dr I. G. Lyle Professor R. H. Ottewill Dr A. L. Smith Dr D. A. Young The aim of the meeting is to discuss the physical chemistry of lipid membranes and their interactions, in particular theoretical and spectroscopic studies, polymerised membranes, thermodynamics of bilayers and Iiposomes, mechanical properties, encapsulation and interaction forces between bilayers leading to fusion but excluding preparation and characterisation methodology.The programme and application form may be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 82 Dynamics of Molecular Photofragmentation 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 preliminary programme may be obtained from : Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBNTHE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM NO. 21 Promotion in HeterogeneousCatalysis University of Bath, 23-25 September 1986 Organising Committee : Professor F. S. Stone (Chairman) Dr R. Burch Mrs Y. A. Fish 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 discussed . Dr R. W. Joyner Professor J. Pritchard Dr D. A. Young (Editor) Further information may be obtained from: MrsY. A. Fish,The Royal Societyof Chemistry, Burlington House, London WlVOBN. 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 C11 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 Dr 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 (viii)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 D. 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. Contributions for consideration by the Organising Committee are invited and abstracts of about 300 words should be sent by 15 June 1986 to: Dr M. Lal, Unilever Research, Port Sunlight Laboratory, Bebington, Wirral L63 3JW Full papers for publication in the Discussion volume will be required by December 1986 THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO.84 Dynamics of Elementary Gas-phase Reactions University of Birmingham, 14-16 September 1987 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 th,ermal 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. 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 Professor R. Grice, Chemistry Department, University of Manchester, Manchester M13 9PLJOURNAL 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 : Radical Cations of Di-, Tri-, and Tetra-bromoethane formed by Radiolysis: an Electron Spin Resonance Study Martyn C. R. Symons (1985, Issue 8) The Use of Deuterium N.m.r. Spectroscopy in Mechanistic Studies of Alkane-exchange Reactions on Supported Platinum and Rhodium Catalysts Ronald Brown, Charles Kemball, James A. Oliver, and Ian H. Sadler (1 985, Issue 9) Electron Spin Resonance Investigation of Environmental Effects in the Photosensitised Reaction of Uranyl Ion with Thioethers Hanna 6.AmbrozandTerence J. Kemp (1 985, Issue 9) The Iron-Vanadium-Oxygen System at 11 23, 1273 and 1373 K. Part 2. Activities in Fe,04- FeV,04 Spinel Solid Solutions Larbi Marhabi, Marie-Chantal Trinel-Dufour, and Pierre Perrot (1 985, Issue 10) Complexes of Sodium, Potassium, Magnesium, and Calcium Cations with the Lysocellin lonophore in Methanol Jean Juillard, Claude Tissier, and Georges Jeminet (1 985, Issue 10) Is Singlet Cyclopentyne a True Minimum on the C,H, Potential-energy Hypersurface? Santiago Olivella, Miquel A. Pericas, Antoni Riera, and Albert Sole (1985, Issue 10) Richard W. McCabe, John M .Adams, and Keith Martin (1 985, Issue 11 ) Clay- and Zeolite-catalysed Cyclic Anhydride Formation Predicted Binding Energies of Dihydrofolate Reductase Inhibitors Alistair F. Cuthbertson and W. Graham Richards (1 985, Issue 11 ) FARADAY DIVISION INFORMAL AND GROUP MEETINGS Theoretical Chemistry Group Post graduate Students' Meeting To be held at University College, London on 5 March 1986 Further information from Dr G. Doggett, Department of Chemistry, University of York, York YO1 5 0 0 Electrochemistry Group Graduate Students' Meeting To be held at Imperial College, London on 5 March 1986 Further information from Dr G. H. Kelsall, Department of Mineral Resources, Imperial College, London SW7 2BP Neutron Scattering Group Workshop on Neutron Scattering Data Reduction To be held at the Rutherford Appleton Laboratory, Didcot, on 13-1 4 March 1986 Further information from Mrs M.Sherwin, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX1 1 OQXMolecular Beams Group with CCP6 Molecular Scattering-Theory and Experiment To be held at the University of Sussex on 19-21 March 1986 Further information from Dr A. Stace,. School of Molecular Sciences, University of Sussex, Falmer, Brighton BN1 9QJ Division with the Institute of Physics, Institute of Mechanical Engineers, Plastic and Rubber Institute and the Institute of Chemical Engineers Tribology in Powder Conveying and Processing : Powder Compaction and Interface Shear To be held at the University of Bradford on 26 March 1986 Further information from Dr B.Briscoe, Department of Chemical Engineering, Imperial College, London SW7 2BY Electrochemistry Group New Techniques for the Characterisation of Electrodes and their Reactions To be held at St Catherine's College, Oxford on 7-9 April 1986 Further information from Dr S. P. Tyefield, CEGB, Rs Dept, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire GL13 9PB Divisio-Annual Congress Structure and Reactivity of Gas Phase Ions To be held at ihe University of Warwick on 8-1 1 April 1986 Further information from Professor K. R. Jennings, Department of Molecular Sciences, University of Warwick, Coventry CV4 7AL Polymer Physics Group with the Statistical Mechanics and Thermodynamics Group Macromolecular Flexibility and Behaviour in Solution To be held at the University of Bristol on 16-1 8 April 1986 Further information from the The Meetings Officer, The Institute of Physics, 47 Belgrave Square, London SWlX 8QX Colloid and Interface Science Group with the Colloid and Surface Science Group of the SCI Dispersions (G.0. Parfitt Memorial Meeting) To be held at the Society for Chemical Industry, London on 15 May 1986 Further information from Dr R. Aveyard, Department of Chemistry, The University, Hull HU6 7RX Division with the Societe Franpaise de Chimie, Deutsche Bunsen Gesellschaft fur Ph ysikalische Chemie and Associazione ltaliana di Chimica Fisica Dynamics of Molecular Crystals To be held at Grenoble, France on 30 June to 4 July 1986 Further information from Dr C. Troyanowsky, 10 rue Vauquelin, 75005 Paris, France Industrial Physical Chemistry Group Physical Chemistry of Water Soluble Polymers To be held at Girton College, Cambridge on 1-3 July 1986 Further information from Dr I.D. Robb, Unilever Research Laboratory, Port Sunlight, Bebington, Wirral, L63 3JW Division with the Institute of Ph ysics, Institute of Mechanical Engineers, Plastic and Rubber Institute and Institute of Chemical Engineers Tribology in Powder Conveying and Processing : Wear Attrition in Powder Flows To be held at the University of Birmingham on 2 July 1986 Further information from Dr B. Briscoe, Department of Chemical Engineering, Imperial College, London SW7 2BYGas Kinetics Group and Division de Chimie-Physique de la Societe Franqaise de Chimie 9th International Symposium on Gas Kinetics To be held in Bordeaux, France on 20-25 July 1986 Further information from Dr R.Lasclaux, Lab. Photophys. Photochim. Moleculaire, Universite de Bordeaux I, 33405 Talence Cedex, France Polymer Physics Group Biologically Engineered Polymers To be held at Churchill College, Cambridge on 21-23 July 1986 Further information from Dr M. J. Miles, AFRC Food Research Institute, Colney Lane, Norwich NR4 7UA Polymer Physics Group with the British Rheological Society Deformation in Solid Polymers To be held at the University of Leeds on 9-1 1 September 1986 Further information from Dr J. V. Champion, Department of Physics, City of London Polytechnic, 31 Jewry Street, London EC3N 2EY Colloid and Interface Science Grobp Surfactant Systems with Liquid-Liquid 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: a New Development in Physical 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 7RX Carbon 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 SWlX 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 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 (xii)

ISSN:0300-9599    

DOI:10.1039/F198682BP027

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1986, 82, 263-271 aa 12-Heteropoly Acids Part 1 .-Air and Vacuum Dehydrations of H,[PMo,,O,,] - xH,O and H,[SiMo,,O,,] - xH,O Rolf Fricke and Gerhard Ohlmann Central Institute for Physical Chemistry, Academy of Sciences of the G.D. R., DDR-1 I99 Berlin-Adlershof, German Democratic Republic Air and vacuum dehydrations of SiMo,, and PMo,, heteropoly acids (HPA) have been studied, the sequence of Mo5+ signals detected under various conditions allowing the evaluation of different stages in the dehydration and destruction of the HPA. The stability of the HPA is shown to decrease when they are supported on Aerosil. The strong interaction of the HPA with an alumina support both influences and modifies properties characteristic of the HPA. Heteropoly acids (HPA) have attracted especial attention due mainly to their involvement in catalysi~,l-~ but paralleled by an increasing number of non-catalytic studies using a variety of physico-chemical methods.Of major concern in catalysis, and not yet clearly understood, is whether or not the structure of the heteropoly acid is preserved during dehydration, reduction and the catalytic reaction. Of similar importance is the still unknown influence of various supports on the properties of the HPA, although it has been suggested1, that an HPA is necessarily formed from an ordinary impregnation catalyst owing to the tendency of molybdenum ions to polymerise. This electron spin resonance (e.s.r.) study continues some recent chemical, X-ray diffraction, d. t.a.-t.g. and u.v.-visible spectroscopic investigations1' of H3+,[PV,Mo12-,0,,] - xH,O ( n = 0-3) HPA.E.s.r. spectroscopy is shown to be a useful technique for the characterisation of different states of the solid HPA during the course of its dehydration and destruction and of the influence of various supports on these properties. Experimental Preparation 12-molybdophosphoric acid (PMo,,) and 12-silicomolybdic acid (SiMo,,) were prepared according to the methods of Tsigdinos12 and Strickland,13 respectively. The supports used were Degussa Aerosil-200, surface area, S = 200 m2 g-l, and Degussa Alumina-C, S = 100 m2 g-l. After pasting in water, drying at 403 K and crushing, the 0.6-1.0 nm mesh fraction was used as the support for the HPA. Following impregnation with the desired quantity of aqueous HPA, the solids were dried at 403 K for 4 h yielding catalysts with good homogeneity over the range 5 4 0 % HPA.ll Samples are designated as PMo,,-HPA for unsupported HPA and PMo,,/SiO, and PMol,/A1203 for the supported HPA, each containing 20% HPA.E .S .R . Measurements The treatment of samples was carried out under static conditions in the e.s.r. sample tube, dehydration being performed in air and under vacuum, lo-, Torr.? The duration of t 1 Torr = 101 325/760 Pa. 263 10-2264 E.S.R. Studies of Heteropoly Acids sample treatment was 1 h at each temperature at 25 K intervals within the range 293-773 K. Rehydration from the atmosphere was carefully avoided. For comparison, a conventional Mo/SiO, monolayer catalyst, 4.3 % Mo on Aerosil, described elsewhere,14 was treated and analysed in the same manner as the supported SiMo,,-HPA samples.E.s.r. measurements were carried out using a ZWG ERS-220 spectrometer at 77 K and at room temperature, operating at X-band wavelength. Diphenylpicrylhydrazyl (DPPH, g = 2.0036) and n.m.r. markers were used for the calculation of e.s.r. parameters and calibration of the magnetic field. Selected single or superimposed signals were simulated using the computer program COMPAR.~~ Magnetic-field values are given in gauss (1 G = 10-4 T). Results Heat Treatment in Air Both the supported and the unsupported HPA show only a weak signal or none at all when dehydrated below 373 K. At temperatures between ca. 373 and 423 K the PMo,,-HPA is liquid and shows a single, symmetric line at 77 K (signal A*; g = 1.951 ; peak-to-peak width, AH,, = 36 G).At room temperature no defined signal could be detected. After treatment at 448 K a new signal was observed at 77 K, showing axial symmetry [signal A (fig. 1); gl = 1.957, gI1 = 1.9311, but with no discernible hyperfine structure (h.f.s.). For the PMo,,-HPA, signal A is the most characteristic up to a dehydration temperature of ca. 648 K and shows a maximum intensity at ca. 500 K. At 673 K this signal had completely disappeared and was replaced by signal B [fig. 2(a); gl = 1.948, gI1 = 1.8641, which again does not show h.f.s. While signal B is stable up to heat-treatment temperatures of 773 K, at and above 723 K signal B becomes superimposed on signal C, which does exhibit hyperfine structure.Fig. 2(b) shows the superposition of resonances B and C, with C predominating in intensity. Computer simulation of C yielded the following parameters: gl = 1.930, gI1 = 1.873, Al = 44.0 G and All = 93.6 G. The SiMo,,-HPA sample shows similar behaviour when dehydrated in air under the same conditions, but signal C was not detected. This situation may have arisen because, under most of the conditions used, the e.s.r. signal intensities for PMo,, samples were significantly higher (sometimes by a factor of 10) than for SiMo,, samples. The resonances observed during these dehydration treatments are summarised in fig. 3. In a second series of experiments the PMo,,-HPA sample was calcined in air after pre-dehydration in vacuum at room temperature, to avoid liquefaction of the sample.Three observations have significance : (a) signal A is observed immediately after evacuation and before air calcination; (b) from ca. 548 to 648 K the e.s.r. spectrum consists of two overlapping Mo5+ signals, previously unobserved and denoted here as signals E and H (fig. 4, shown for PMo,,/SiO, because of its better resolution); ( c ) again, signal B characterises the spectra for samples dehydrated in the temperature range 673-773 K, signal C being superimposed as described previously. On consideration of the aerosil- and alumina-supported HPA samples, treated under identical conditions, it becomes obvious that only PMo,,/SiO, behaves in a manner substantially similar to that of unsupported HPA. Supporting PMo,,-HPA on silica does, however, influence the critical temperatures at which characteristic e.s.r.signals appear or disappear. These transition temperatures decrease by ca. 50-125 K for PMo,,/SiO, In contrast, the PMo,,/Al,O, sample exhibits completely different behaviour (fig. 5). At dehydration temperatures up to 473 K, the room-temperature spectra show no or only a very weak signal, but at 77 K an intense, single line was detected (signal A* ; g = 1.955, AHpp = 22 G). After dehydration at 473 K the resonance becomes slightly asymmetric and is moved to lower magnetic fields (g = 1.963). Increasing the dehydration temperature (fig. 3).‘I DPPH I ; I I I / I , l <---, c- /-- --L 265 I I I I I ‘91 Fig. 1. PMo,,-HPA, dehydrated at 523 K in air for 1 h (signal A) (-) 77 K, (---) 293 K. Fig.2. PMo,,-HPA, dehydrated in air for 1 h at (a) 673 and (b) 773 K. (c-smr. = 77 K for both spectra.)266 E.S.R. Studies of Heteropoly Acids d.t.a. ,d.t.a. -, 2 93 373 473 573 %$ 873 I I 1 1 I I I 1 I I I P M O , ~ - HPA: ! I I air+;^ A A A A A A A A A B B 6 B E @ ) I I E E E E E ( C I C c c C ? I I I I I v a c u u m I I C C I A F F F I I , 1 I I I I I 1 I I I I I I I I I I v a c u u m 1 A I I I F F @ F F F F ? E E E I 1 I I I a i r + I ? ( - 1 ( - 1 I I c c c c c c c c c c c c c c c ! I I I I _ _ _ _ I - - - - _ _ _ _ _ - - - _ - - - - - - - - - - - - - - - - - - - - - - - - - . I I I I I v a c u u m I A* A* (A*) I 1 (D) ( D) D (D 1 I I (C 1 I I [ I I I I I ' 1 I 293 373 4 73 5 73 6 73 773 873 T/K Fig. 3. Schematic representation of the dehydration results of HPA in air and vacuum.Letters denote e.s.r. Mo5+ signals (circles mark spectra shown separately; for theg values see table 1) CJ ref. (23).t Sample pre-evacuated at 293 K. causes a decrease in the resonance intensity, but not to zero, while a new, temperature- independent resonance with axial symmetry appears, identified as signal C . Over a limited temperature range (573-673 K) a new signal, D (gl = 1.940, gI1 = 1.895), is superimposed to C . After pre-evacuation only signal C is observed up to 873 K. Heat Treatment in Vacuum Vacuum treatment of HPA samples at temperatures up to 773 K produces a completely different set of resonances, especially after calcination at high temperatures (fig. 3). After heating in vacuum at 373 K, a characteristic and well resolved signal F (fig.6) appearsR. Fricke and G . Ohlmann 267 Fig. 4. PMo,,/SiO, Fig. and HPA) dehydrated in air for 1 h at various and (c) 573 K. (q.s.r. = 77 K.) temperatures : 1 (a) 523, (b) 548 r --__--.-.-,-a- __--- _-___. *’ f iw! 5. PMo,,/Al,O, (20% HPA) dehydrated in air for 1 h at various temperatures: (a) 373, (b) 473 and (c) 573 K. (q.s.r. = 77 K. Dashed lines show the spectra, registered for the amplification see data on the right-hand side.) untreated at 295 K;268 D PPH I E.S.R. Studies of Heteropoly Acids 100 G - g1 d Al- g , , All ‘ I I I I I Fig. 6. PMo,,-HPA, dehydrated by evacuation at 473 K for 1 h (q.s.re = 77 K, signal F.) for which computer simulation yields gl = 1.958, gll = 1.852, Al = 45.6 G and All = 101.6 G. This resonance characterises the behaviour of both unsupported PMo,,-HPA and supported PMo,,/SiO, up to ca.573-623 K (fig. 3). The unsupported PMo,,-HPA alone shows an additional signal E from 523 to 623 K whose peak maximum corresponds to an approximate g value of 1.942, differentiating the resonance from signal F. The sequence of Mo5+ signals observed for PMo,,/Al,O, after vacuum heat-treatment is also given in fig. 3. The spectra are essentially similar to those measured after treatment in air without evacuation, but, owing to line-broadening effects, exact assignment and parametrisation proved to be difficult. For example, the poor spectral resolution observed prevented the identification of separate signals C or D or a superposition of both for high- temper ature-evacuated samples. Discussion It would be helpful to make some general remarks before considering the experimental results in detail.First, although the results are generally reproducible, irreproducibility was experienced for samples heat-treated in the vicinity of the ‘transition temperatures’. These difficulties are probably the consequence of the strong dependence of the properties of HPA on its water content, a feature very clearly shown on comparing the spectra of air-treated samples, with and without pre-evacuation (fig. 3). Secondly, the e.s.r. spectra exhibit a great variety of line shapes, so that their analysis is sometimes only speculative. It would appear that the transitions between different states of the HPA pass through a variety of intermediates, each of which generates a ‘ typical’ spectrum.Spectrum simulation, carried out for a limited number of cases, was undoubtedly helpful, but was not able to solve all of these problems. Its difficulties wereR. Fricke and G. Ohlmann 269 often connected with poorly or unresolved hyperfine structure of the Mo5+ resonance of the isotopes 9 5 M ~ and 9 7 M ~ ( I = 5/2) with a natural abundance of 25.3%. When poorly resolved, the resulting envelope could incorrectly suggest the presence of an additional Mo5+ signal or, at the very least, disturb the position of gI1. The e.s.r. evidence suggests that for dehydration in air, the heat-treatment temperature range can be subdivided into four regions in which identifiably different effects occur. At low temperatures, up to cu. 423 K, the HPA is dissolved in its water of crystallisation; its colour is yellow-green.The single, narrow line (signal A*), according to Prados and Pope,lG is characteristic of strong delocalisation of the corresponding electrons in the Keggin ion, so that fast hopping of the electron occurs between the 12 Mo ions, with a hopping rate > los s-l. The medium-temperature range, 400-650 K, in which the HPA has lost all of its water of crystallisation,ll? l7 is dominated by signal A, which has been described previously by Cuvaev et a1.18 Signal A also appears after evacuation at room temperature for both the unsupported and the silica-supported HPA. The axial symmetry of the resonance indicates that the odd electron is now localised at the molybdenum positions. The loss of water of crystallisation now limits the mobility of the electron or drastically changes the hopping rate.Considering the variety of heat-treatment conditions after which signal A occurs (fig. 3) it may be supposed that signal A is the most important Mo5+ resonance characteristic of the dehydrated but structure-preserved PMo,,-HPA samples. Note that under these conditions the line shapes of the spectra do not show evidence for interactions between Mo5+ species. The resonance intensity more than doubles on increasing the dehydration temperature, reaching a maximum at ca. 523 K. It appears that the dehydration process is related to a loss of oxygen, leading to a partial reduction of Mo ions. The third dehydration step, which was not observed for the unsupported PMo12-HPA (fig.3, first row), is characferised by the low-intensity e.s.r. spectrum shown in fig. 4. This was originally only observed at 77 K, but becomes visible at room temperature as the dehydration temperature is increased. Precise analysis of the resonances is especially difficult, as the extrema appear to have no clear relationship to each other (see for example the second maximum in the sequence of these spectra), but the most plausible combination recognising this restriction yields the following values : gl = 1.943, gI1 = 1.917 (signal E) and gl = 1.966, gI1 = 1.893 (signal H). The parameters of resonance E are similar to those reported by Che et al.19 for the brown Mo,Oi; polyanion (gl = 1.930, gll = 1.919) and by Altenau et aL20 for PMo5+ - Wl,Oi; (gl = 1.9395, gI1 = 1.91 38).However, the differences in sample state and in other conditions, in addition to the relatively low intensity of the spectrum, do not permit an unambiguous assignment to an ionic structure representative of the whole Keggin anion. Kravets et uL21 observed an Mo5+ resonance with g1 = 1.95 and gll = 1.92 for Ti-Mo-HPA supported on TiO, and assigned the signal to Mo5+ ions in a slightly distorted HPA structure possessing a weak molybdenyl bond. D.t.a.4.g. measure- ments combined with determinations of water content for PMo12-HPA suggested to Jerschkewitz et al.ll that, at calcination temperatures up to 633 K, the HPA has released all of its constituent water. It is possible to conclude therefore that signal E, observed after heat treatment in the medium temperature range, is in some way connected with this intermediate state.The HPA in this condition can be fully rehydrated, and X-ray diffraction analysisll of PV,M,,-HPA reveals all of the characteristic lines of the original anion structure. It is therefore suggested that the intermediate state, represented by signal E, allows a reversible ' reconstruction ' of the Keggin anion structure. In contrast, signal B, observed at the higher dehydration temperature of ca. 673 K for PMo,,-HPA, and at 623 K for PMo,,/SiO,, indicates the irreversible destruction of the270 E.S.R. Studies of Heteropoly Acids heteropoly anion. This has already been established through the observation by several authors of an exothermic d.t.a. peak at 687 K1l or 705 K.22 Resonance C , whose g values are only slightly different from those of signal B, may result from a final destruction product of the HPA.Usually this product is MOO,, necessarily partially reduced to account for species detected by e.s.r. Indeed, X-ray diffraction measurements for the PV2Molo-HPA treated at 773 K for several hoursll show reflections attributable to MOO,. However, partially reduced MOO, itself shows a variety of different resonances, depending upon the nature of the sample and its treatment, and thus it is impossible to reach a conclusion based on e.s.r. evidence alone. If, however, it is assumed that signal C alone represents the destroyed Keggin anion structure, then it is possible to conclude that loading HPA onto alumina leads to the immediate destruction of the anion structure at low dehydration temperatures (fig, 3).Summarising the dehydration studies in air, one concludes that, in general, the different types of Mo5+ e.s.r. signals do characterise the different states of PMo,,-HPA with sufficient accuracy. This conclusion is emphasised on comparing the e.s.r. ' transition temperatures' with the d.t.a. peaks for the appropriate HPA which indicate important structural transitions in the state of the HPA (see the arrows in fig. 3). This is further illustrated by the results of thermal treatment after predehydration in vacuum at room temperature; the dissolution of the HPA is avoided, yet the transition temperature characterising the onset of the destruction of anionic structure remains unchanged, as shown by the e.s.r.spectra. Although vacuum treatment over the whole temperature range changes these assign- ments in some respects (fig. 3), the transition temperature for the destruction of the HPA anion in vacuum occurs again at ca. 673 K. Between room temperature and 673 K, signal F clearly dominates the e.s.r. spectra. This resonance exhibits the greatest axial symmetry of all of the observed resonances and is a feature of high-temperature, hydrogen or CO reduced PMo,,-HPA~~ and Mo/SiO, 25 The vacuum-thermal treatment of HPA, which easily removes some oxygen after treatment in air, appears to have a similar effect on the molybdenum oxidation state as a hydrogen atmosphere. Although the extent of reduction after vacuum treatment has not been measured, the greater changes in colour (from green-brown to blue at 673 K), which are not observed when HPA is heat-treated in air, demonstrate that the HPA is reduced to a greater extent in vacuum.Signal F, mainly observed below 673 K, therefore probably reflects a strong distortion of the primary structure of the Keggin anions, rather than its complete destruction. A summary of the Mo5+ signals of air-treated and reduced HPA samples related to their structure is given in Part 2.23 We thank Dr A. Ellison, Humberside College of Higher Education, Hull, for valuable discussions and kind support with the revision of the article. Thanks are also due to Dr H-G. Jerschkewitz for discussion and supplying the HPA samples and Dr U. Ewert, Center for Scientific Instruments, for making his computer program COMPAR available.The technical assistance of Mr U. Marx is gratefully acknowledged. References 1 S. Nakamura and H. Ichihashi, Proc. 7th Int. Congr. Catal., Tokyo 1980 (Kodansha, Tokyo, 1981), 2 M. Otake and T. Onoda, Proc. 7th Int. Congr. Catal., Tokyo 1980 (Kodansha, Tokyo, 1981), part B, 3 M. Misono, K. Sakahata and Y. Yoneda, Proc. 7th Int. Congr. Catal., Tokyo 1980 (Kodansha, Tokyo, 4 T. Okuhara, A. Kasai, N. Hayahawa, Y. Yoneda and M. Misono, J. Catal., 1983, 83, 121 and 5 H. Niiyama, Y. Saito and E. Echigoya, Proc. 7th Int. Congr. Catal., Tokyo 1980 (Kodansha, Tokyo, part B, p. 755. p. 780. 1981), part B, p. 1047. references therein. 1981), part B, p. 1416.R. Fricke and G . Ohlmann 27 1 6 M. Ai, J. Catal., 1981, 71, 88. 7 M.Akimoto, Y. Tsuchida, K. Sat0 and E. Echigoya, J. Catal., 1981, 72, 83. 8 H. Hayashi and J. B. Moffat, J. Catal., 1982, 77, 473. 9 Y. Izumi, R. Hasebe and K. Urabe, J. Catal., 1983, 84, 402. 10 N. N. Chumachenko, T. M. Yurieva, D. V. Tarasova and G. I. Aleshina, React. Kinet. Catal. Lett., 1 1 H-G. Jerschkewitz, E. Alsdorf, H. Fichtner, W. Hanke, K. Jancke and G. Ohlmann, Z . Anorg. Allg. 12 G. A. Tsigdinos, Znd. Eng. Chem., Prod. Res. Dev., 1974, 13, 267. 13 J. D. H. Strickland, J. Am. Chem. Soc., 1962, 74, 862. 14 R. Fricke, W. Hanke and G. Ohlmann, J. Catal., 1983,79, 1. 15 U. Ewert, Thesis (Humboldt University, Berlin, 1979). 16 R. A. Prados and M. T. Pope, Znorg. Chem., 1976, 15, 2574. 17 M. Misono, N. Mizuno, K. Katamura, A. Kasai, Y. Konishi, K. Sakata, T. Okuhara and Y. Yoneda, 18 V. F. Cuvaev, T. A. Karpukhina, G. K. Mailieva, K. I. Popov and V. I. Spicyn, Zzv. Akad. Nauk SSSR, 19 M. Che, M. Fournier and J. P. Launay, J. Chem. Phys., 1979,71, 1954. 20 J. J. Altenau, M. T. Pope, R. A. Prados and H. So, Znorg. Chem., 1975,14,417. 21 G. A. Kravets, T. Kh. Shokhireva, V. F. Anufrienko and T. M. Yurieva, React. Kinet. Catal. Lett., 22 H. Niiyama, H. Tsuneki and E. Echigoya, Nippon Kagaku Kaishi, 1979, 996. 23 R. Fricke and G. Ohlmann, J. Chem. Soc., Faraday Trans. 1, 1986,82, 273. 24 R. F. Howe and I. R. Leith, J. Chem. Soc., Faraday Trans. 1, 1973,69, 1967. 25 M. Che, F. Figueras, M. Forissier, J. McAteer, M. Perrin, J. L. Portfaix and H. Praliaud, Proc. 6th Znt. 1980, 14, 87. Chem., 1985, 526, 73. Bull. Chem. Soc. Jpn, 1982, 55, 400. Ser. Khim., 1981, 4, 717. 1982, 19, 85. Congr. Catal., London, 1976 (The Chemical Society, London, 1977), vol. 1, p. 261. Paper 41 1969; Received 19th November, 1984

ISSN:0300-9599    

DOI:10.1039/F19868200263

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1986, 82, 273-280 Electron Spin Resonance Studies of Free and Supported 12-Heteropoly Acids Part 2.-Reduction of H,[PMo,,O,,] - xH,O and H,[SiMo,,O,,] xH,O and Oxygen Adsorption Rolf Fricke and Gerhard Ohlmann Central Institute .for Physical Chemistry, Academy of Sciences of the G.D.R., DDR- 1 199 Berlin-Adlershof, German Democratic Republic Hydrogen and carbon monoxide reductions of SiMo,, and PMo,, heteropoly acids (HPA) have been studied, the various Mo5+ signals detected being identified as arising from stages in the dehydration and destruction of the HPA in a similar manner to vacuum heat treatment. Oxygen adsorption studies revealed the formation of 0; radicals on the supported HPA, prereduced at high temperatures, a phase in which the Keggin structure is already destroyed.In contrast, HPA in which the Keggin structure is preserved proved unable to stabilise 0; species. The stability of the supported HPA was shown to be markedly dependent on the nature of the support. Changes in the structure of heteropoly acids during dehydration in air and in vacuum have been identifiedl through an analysis of the sequence of e.s.r. signals arising from different stages in the dehydration process. The dehydration of the HPA is apparently accompanied by the reduction of the molybdenum ions and eventual destruction of the Keggin anion structure. This electron spin resonance (e.s.r.) study continues these investigations and evaluates the differences and similarities in behaviour of free and supported HPA during hydrogen or carbon monoxide reduction and in their responses to oxygen adsorption. Experimental General details of materials preparation and e.s.r.techniques are given in Part 1.' Reduction with hydrogen or carbon monoxide was carried out at 30 Torr? pressure in situ within e.s.r. tubes under static conditions and avoiding subsequent exposure to the atmosphere. For the oxygen adsorption studies three catalysts were compared : SiMo,,/SiO, and SiMol,/Al,O,, each containing 5 % HPA, and an Mo/SiO, monolayer catalyst (4.3% M o ) ; ~ all samples contained no phosphorus. The samples were prereduced with CO at different temperatures (623, 673, 723 and 773 K) and evacuated for 10 min at the temperature chosen. Molecular oxygen was adsorbed on the prereduced catalyst at room temperature and the excess was pumped off for several minutes.For a given temperature the oxygen pressure was varied (5, 30 and 100 Torr). t 1 Torr = 101 325/760 Pa. 273274 E.S.R. Studies of Heteropoly Acids 293 373 473 573 673 773 8 73 f I I I I I ]PMO,~ - HPA I I I I - - I co H2 f I 290 373 473 573 673 773 873 T/K Fig. 1. Schematic representation of Mo5+ e.s.r. signals obtained after reduction of PMo,, samples with H, or CO. (Circles denote spectra shown separately; for the g values see table 1.) Results Reduction in H, or CO The resonance signals observed after reduction in hydrogen or carbon monoxide are shown in fig. 1. (Most are described in detail in Part 1 .)l Some additional remarks may be helpful. (a) In a manner similar to that observed for dehydrated HPA (fig.4 in Part 1) the spectra of PMo,,-HPA reduced at 573 K show a superposition of two signals, one of which has identical parameters to signal F.I The second signal has a maximum intensity when measured at room temperature [fig. 2(a)] and shows an enhanced intensity after further evacuation of the sample at the reduction temperature [fig. 2(b)]. The g value assessed at the peak maximum (g = 1.944) suggests an assignment as signal E.l (b) A PMo,,-HPA sample whose Keggin anion structure was destroyed by heating in air at 773 K was reduced in a stepwise manner beginning at room temperature. After each reduction step the e.s.r. spectrum remained identical to that obtained immediately after the 773 K air treatment. The sequence of signals characterising the reduction states (fig.1) could not be observed. ( c ) The marked variation in line shape observed for PMo,,-HPA and PMo,,/SiO, samples reduced between 623 and 773 K (fig. 3) is a result of the superposition of threeR . Fricke and G. Ohlmann 275 Fig. 2. PMo,,-HPA, reduced with hydrogen at 573 K for 2 h (a) without further treatment and (b) after additional evaluation of sample a for 1 h at 573 K. (Dashed lines show the spectra registered at 295 K; full lines show those at 77 K.) Fig. 3. PMo,,/SiO,, reduced with hydrogen for 1 hat (a) 623, (b) 673 and (c) 723 K. [q.s.r. = 77 K (full lines), 293 K (dashed lines).] pairs of spectra. The spectral variation is a function of the temperature dependence of individual spectra of species formed at the same reduction temperature and also a function of the large variation in their line shapes on increasing the reduction temperature.The most remarkable resonance in this series is a single line (signal G) with g = 1.923, peak-to-peak width AHpp = 40 G [fig. 3 ( c ) ] , which becomes slightly asymmetric at 773 K. ( d ) The PMo,,/Al,O, sample behaves differently from both the unsupported and silica-supported PMo,, samples (fig. l), as found during the dehydration studies., (e) A change in the reduction medium (hydrogen or carbon monoxide) had no significant effect on the type of Mo5+ e.s.r. signal observed, with the exception that at high reduction temperatures the gross changes in spectra experienced for hydrogen reduction (fig. 3 ) were not observed for the samples reduced with CO.Oxygen Adsorption Adsorption of oxygen on unsupported SiMo,,-HPA produced no oxygen radicals detectable by e.s.r. spectroscopy at either room temperature or 77 K, after varying the pressure of oxygen and the reduction temperature between 673 and 773 K. In contrast, the SiMo,,/SiO, sample showed an e.s.r. signal attributable to an oxygen radical (gl = 2.015, g, = 2.009 and g, = 2.004), the resonance intensity depending upon the experimental conditions. The signal becomes broadened in the presence of an excess276 E.S.R. Studies of Heteropoly Acids 91 1 20 G - ' g3 Fig. 4. SiMo,,/SiO, (5% HPA), 0; signal recorded after prereduction at 773 K with 30 Torr CO and adsorption of 30 Torr 0, (the excess of 0, has been pumped off). (c.s.r. = 77 K.) I 5 O L," 1 I I I I I I 1 I a I l I I 673 773 672 773 673 773 reduction temperature/K Fig.5. Intensity I of the 0; radical signal plotted against temperature of prereduction with CO: (a) SiMo,,/SiO,, (b) SiMo,,/Al,O, and (c) Mo/SiO,. Different curves represent changes in the pressure of adsorbed 0,: x 5; 0, 30 and 0, 100 Torr. Note different calibration for (c). of oxygen, but is well resolved after evacuation (fig. 4). In the case of SiMo,,/Al,O, a similar signal was observed for which g, = 2.016, g, = 2.010 and g , = 2.005. Both sets of parameters clearly identify the radical as O,., Quantitative comparisons of resonance intensities were obtained for the three samples studied under identical conditions (fig. 5). The results show that the two silica-supported samples have the same tendency [fig.5(a) and (c)] to exhibit a marked increase in the 0; concentration when the samples are prereduced at higher temperatures. A markedR. Fricke and G . Ohlmann 277 influence of oxygen pressure was only observed for SiMo,,/SiO,. In contrast, for SiMo,,/Al,O, the 0; concentration was generally low and almost independent of the prereduction conditions or the oxygen pressure [fig. 5 (b)]. The formation of 0; species was also observed for PMo,,/SiO, samples. In all cases the Mo5+ signal observed after prereduction did not change during oxygen adsorption. The formation of 0- radicals, which sometimes accompanies the appearance of 0; species on transition metal catalysts, was not observed for these heteropoly acid samples. Discussion H, or CO Reduction The Mo5+ e.s.r.signals observed for samples after reduction in hydrogen or carbon monoxide are similar in some respects to those obtained during vacuum dehydration of HPA., Thus reduction also causes a transition temperature at ca. 673 K, associated previously1 with signal B and with irreversible destruction of the Keggin-anion structure. The intensities of resonances produced after reduction were, however, 5-10 times greater than after vacuum dehydration. The colour changes of samples also occurred in the same order: yellow-green --+ brown-green --+ blue --+ black (black was not observed for the vacuum-treated HPA). However, H,- or CO-reduced HPA had already changed colour to blue at ca. 550-600 K, 150 K lower than was observed for vacuum-treated HPA.l This transition temperature compares favourably with t.p.r. results which show that hydrogen uptake begins at ca.550-600 The Mo5+ spectra obtained appear similar to several reported by Konishi et aL6 after various stages of reduction at 523 K in a hydrogen atmosphere, although g values for the species were not given. On the other hand, the Mo5+ resonance observed by Otake et al.’ for a PMo,,-HPA sample, reduced at 553 K in hydrogen, could not be reproduced even for reduction in a flow system. Infrared spectra have shown that the ternary Mo-0 bond of the MOO, octahedron is relatively stable during the reduction of K,PMo,,O,, with hydrogen.8 In contrast, the intensities of Mo-0-Mo infrared bands at 800 and 870cm-l have been shown to decrease as the reduction process progressed.This phenomenon has been confirmed for the PMo,,-HPA sample discussed in these s t ~ d i e s . ~ It is therefore suggested that signal F, observed after reduction in hydrogen (or after vacuum treatment) is due to Keggin anions which have lost some of their bridging Mo-0-Mo oxygens. The large Ag value (gl-gll = 0.106) suggests a relatively large distortion of the MOO, octahedra, while the Keggin-anion structure is not irreversibly destroyed. This is in accord with the X-ray diffraction analysis by Eguchi8 of K,PMol,O,,, reduced at 673 K in hydrogen, reported to have the same crystal structure before and after reduction. In addition, the absence of signal F when reducing PMo,,-HPA, already destroyed by air treatment at 773 K, confirms the above assignment of this signal.An increase in the reduction temperature between 623 and 773 K produced marked changes in the e.s.r. spectra, shown in fig. 3. The greater the degree of reduction of the sample, the smaller the linewidth of the relevant signal. This suggests that as the anion structure is destroyed during the reduction process, the exchange interactions between neighbouring Mo5+ ions become more probable, leading to the decrease in linewidth. As depicted in fig. 3, the interaction depends upon the measurement temperature, producing a variation in e.s.r. linewidth. The temperature dependence of the spectra observed for the sample reduced at 623 K [fig. 3 (a)] cannot be explained on this basis alone. Furthermore, spin-lattice relaxation effects can be excluded because the e.s.r.spectral intensities vary with temperature approximately according to the Curie law. It is therefore assumed that the structure278 E.S.R. Studies of Heteropoly Acids Table 1. Mo5+ signals obtained after various treatments of PMo,,-HPA with a probable assignment to the HPA structure signal g l gii origin A* A B C D E F G H 1.957 1.948 1.930 (44. O)u 1.940 1.944 1.958 (45.6)a 1.966 1.95 1 1.93 1 1.864 1.873 (93.6)b 1.895 1.917? 1.852 (101.6)b 1.893 1.923 electron hopping Keggin anion dehydrated but undestroyed irreversible destruction of the Keggin-anion final destruction product (MOO,) structure reduced Al-0-Mo 'phase' (?) loss of constitution water, not irreversibly strong distortion, loss of bridging oxygen destroyed destroyed anion structure, interacting Mo5+ unknown a A l ; A,, (in G).produced on extraction of bridging oxygens in the first reduction step and terminal oxygens in the second step is stable, and can be changed easily by decreasing the measurement temperature to 77 K. The g values of the Mo5+ resonances observed under oxidisingl and reducing conditions, together with their proposed assignments, are listed in table 1 . It must, however, be clearly understood that the appearance of one or more e.s.r. signals does not alone constitute conclusive evidence for the presence of an HPA. Oxygen Adsorption Oxygen radicals were not observed after oxygen adsorption on the unsupported SiMo,,-HPA samples. Variation in the conditions of prereduction with CO or during the adsorption of oxygen did not change the result.In contrast, the supported HPA is able to stabilise 0; species in high concentrations, provided that the samples have been prereduced at 723 K or above (fig. 5). In the region of prereduction temperature in which the Keggin-anion structure is at least partly maintained, 0; species were again not detected. One may conclude that the distorted but undestroyed Keggin anion is unable to form, or at least to stabilise, oxygen radicals. Two mechanisms may be responsible for this behaviour. (a) It is accepted3 that, for supported transition-metal catalysts, the transfer of an electron from the metal atom to the adsorbed oxygen molecule may proceed according to the following scheme: M(n-l)+ + O,(ads) + Mn+ + O;(ads) requiring a special coordination symmetry around M which is probably tetrahedral.Low-temperature prereduction does not obviously lead to such coordinatively unsaturated adsorption sites of the Keggin anion at which molecular oxygen could be adsorbed and transformed into 0; species. The similar behaviour of the SiMo,,/SiO, and the monolayer Mo/SiO, catalysts with respect to oxygen adsorption would appear to confirm this mechanism. (b) If oxygen were adsorbed on the preserved Keggin anion it could be quickly incorporated into the lattice according to the following scheme : O,(gas) 4 O;(ads) -+ 0-(ads) -+ 02-(lattice)R. Fricke and G. Ohlmann 279 without stabilisation of 0- or 0;. This rapid electron transfer may well be an important mechanism for the unsupported HPA, as also noted by Akimoto et ai.,l0 but it has limited applications to the case of supported HPA.Dismissing a discussion of the properties of oxygen radicals, which are irrelevant in this context, it is possible to conclude in general that the detection of the 0; radicals is closely linked to the structure of the HPA. Thus it is suggested that the existence of 0; may be taken as evidence for the destruction of the Keggin-anion structure. For alumina-supported HPA the concentration of 0; species is relatively low and almost independent of the experimental conditions. It is therefore assumed that on SiMol,/A1203 the radicals originate from small parts of the sample which characterise neither the Keggin structure nor the Al-0-Mo ‘phase’ suggested in Part 1 (see table 1 of the present paper).This phase is obviously unable to form or stabilise oxygen species under the conditions used in these studies. Influence of the Support It is now possible to draw two conclusions regarding the influence of the supports on HPA from the results presented here and in Part 1.l (a) Under almost all of the heat-treatment conditions studied, the aerosil-supported HPA behaves in a manner very similar to that of the unsupported, powdered HPA. In accordance with other studiesll it would appear that the HPA is not chemisorbed by the aerosil, which serves merely as an inert support, leaving the HPA undistorted. After having lost almost all of its water of crystallisation, the supported HPA becomes more strongly chemisorbed on the aerosil surface, with a consequent increased interaction between the surface and the HPA.This interaction accelerates the destruction of the Keggin anion on the support surface, as shown by the results in fig. 3 of ref. (1) and concluded from the results of d.t.a. and u.v.-visible spectroscopic measurements.11 This, however, is in contradiction to the proposals of Chumachenko et a1.,12 who have suggested that the silica support should stabilise the HPA structure during thermal treatment. (b) In contrast, for alumina-supported HPA, the e.s.r. spectra of PMo12/Al,03 samples show no analogies with PMo12-HPA or with PMo12/Si0,, especially when comparing the temperatures at which characteristic signals appear. Typical transitions between resonances were obtained in some cases, but under different conditions of heat treatment.Thus signal C clearly dominates the air-treated samples, whereas signal D is characteristic for high-temperature reduced samples. At low temperatures signal A* indicates an electron-hopping process. Jerschke~itzl~ has observed that impregnation of alumina with up to 13.5 % HPA, immediately leads to a strong interaction between HPA and the support. In this context the appearance of signal A*l is taken as evidence that, assuming the sample is not heated at high temperatures, the Keggin structure is preserved after impregnation. Under oxidising conditions the HPA may be easily destroyed at low temperatures, as shown by the appearance of signal C. At elevated temperatures a new HPA ‘phase’, represented by signal D, is formed. As signal D is formed exclusively on the alumina-supported material, it must be assigned to the formation of a reduced Al-0-Mo phase.Evidence to support this supposition is afforded by the results from the oxygen-adsorption experiments, which again indicate a strong interaction between the alumina and the HPA. As this conclusion is in disagreement with the results of Eguchi et aZ.14 for reduced but unsupported PMl,-HPA, further studies of the origin of signal D would appear to be necessary. We thank Dr A. Ellison, Humberside College of Higher Education, Hull, for valuable discussions and kind support. Thanks are also due to Dr H-G, Jerschkewitz for advice and for supplying the HPA samples and Dr U. Ewert, Center for Scientific Instruments, for making his computer program COMPAR available. The technical assistance of Mr U. Marx is gratefully acknowledged.280 E.S.R. Studies of Heteropoly Acids References 1 (Part 1) R. Fricke and G. Ohlmann, J. Chem. SOC. Faraday Trans. 1, 1986,82, 263. 2 R. Fricke, W. Hanke and G . Ohlmann, J. Catal., 1983, 79, 1. 3 M. Che and A. J. Tench, Adv. Catal., 1983, 32, 1. 4 K. Katamura, T. Nakamura, K. Sakata, M. Misono and Y. Yoneda, Chem. Lett., 1981, 89. 5 S. Yoshida, H. Niiyama and E. Echigoya, J. Phys. Chem., 1982, 86, 3150. 6 Y. Konishi, K. Sakata, M. Misono and Y. Yoneda, J. Catal., 1982, 77, 169. 7 M. Otake, Y. Komiyama and T. Otaki, J. Phys. Chem., 1973, 77, 2896. 8 K. Eguchi, Y. Toyazawa, K. Furuta, N. Yamazoe and T. Seiyama, Chem. Lett., 1981, 1253. 9 E. Schreier, unpublished results. 10 M. Akimoto, Y. Tsuchida, K. Sat0 and E. Echigoya, J . Catal., 1981, 72, 83:. 11 H-G. Jerschkewitz, E. Alsdorf, H. Fichtner, W. Hanke, K. Jancke and G. Ohlmann, 2. Anorg. Allg. 12 N. N. Chumachenko, T. M. Yurieva, D. V. Tarasova and G. I. Aleshina, React. Kinet. Catal. Lett., 13 H-G. Jerschkewitz, unpublished results. 14 K. Eguchi, N. Yamazoe and T. Seiyama, Chern. Lett., 1982, 1341. Chem., 1985, 526, 73. 1980, 14, 87. Paper 5/963; Received 7th June, 1985

ISSN:0300-9599    

DOI:10.1039/F19868200273

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I , 1986,82, 281-289 Spectroscopy of Proflavine adsorbed on Clays Robert A. Schoonheydt and Jos Cenens Laboratorium voor Oppervlaktechemie, K . U. Leuven, Kurd. Mercierlaan 92, B-3030 Leuven (Heverlee), Belgium Frans C. De Schrijver Departement Scheikunde, K. U. Leuven, Celestijnenlaan 200F, B-3030 Leuven (Heverlee), Belgium The ion exchange of monoprotonated proflavine on Wyoming bentonite, Camp Berteau montmorillonite and a synthetic mica-type montmorillonite, Barasym SSM-100, results in the presence of monoprotonated and diproto- nated proflavine and dimers on the surface after freeze-drying. On Barasym SSM-100, below a loading of 500 pmol g-l, the molecules are exclusively adsorbed on the external surface. For the other minerals the interlamellar surface is also available for adsorption.Protonation to the diprotonated proflavine is favoured the higher the negative charge density of the minerals: BS > CB > WB. The extent of dimerization is proportional to the available surface area, but sets in far below monolayer coverage. These three different species of proflavine on the surface have been characterized by their typical absorption and fluorescence spectra. For the first time fluorescence of the proflavine dimers has been detected at 580 nm. The fluorescence intensity is quenched by FeII', by protonation to diprotonated proflavine and by dimerization. Surface photochemistry of organic molecules is receiving increasing attention from the scientific c0mrnunity.l The reduced mobilities of adsorbed molecules, with respect to their mobility in solution, the possibility to retard the dark back reactions and the increase of the selectivity of photochemical reactions are among the most important benefits expected from surface photochemistry. Clay minerals are interesting supports for organic photochemical systems because of their ion exchange, swelling and adsorptive properties.In addition, they offer the possibility to organize the adsorbed molecules in mono1ayers,2 which can be studied spectroscopically. One might also hope to gain insight into the surface properties of the clays by a study of the excited state properties of the adsorbed molecules. We present here data for proflavine (3,6-diaminoacridine) adsorbed on clays. Proflavine is a photosensitizer which has been studied in solution and in micellar ~ystems.~-~ Its typical spectroscopic properties in aqueous solution are summarized in table 1.The dimer spectrum is calculated with a dimerization constant of 500 dm3 mot1 and an extinction coefficient of 30000 dm3 mol-1 cm-1.4 The presence of an intense (allowed) band at 428 nm and a weak (forbidden) band at 473 nm is typical for molecules with interacting n-electron systems such as the proflavine dimer. The blue shift of the adsorption maximum from 445nm for the monomer PFH+ to 428 nm for the dimer is called metachromasy. This phenomenon has been described suspensions. In this paper are affected by for methylene blue6 and acridine orange7 in aqueous montmorillonite we show how both the absorption and fluorescence spectra of proflavine the clays.28 1282 Proflavine on Clays Table 1. Spectroscopy of proflavine in the 15000-30000 cm-l region molar extinction band maxima coefficient emission quantum species /nm (cm-l) /dm3 mol-1 cm-l maxima/nm yield - - proflavine (PF)a 395 (25300) - PFH;+a, b 455 (22000) 20 000 545 0.18 PFH+a. 445 (22470) 40 000 506 0.34 490 (20400) - - 360 (27800) - 345 (29000) - - - - - dimerb 428 (23 360) 30 000 - - - 473 (21 140) a Ref. (5). Ref. (4). Table 2. Characteristics of the clays cation exchange capacity /pmol g-l mean charge density Fe20, surface area clay 22Na Ag(tu)$ / e - per Si,Al2O1,(OH), (wt % ) /m2 g-' BS 364 835 CB 880 1506 WB 649 1308 0.387 0.339 0.270 0.05 134 3.45 86 5.18 32 Experimental Clays The following clays from the Source Repository of the Mineral Society were used : Camp Berteau montmorillonite (CB), Wyoming bentonite (WB) and Barasym SSM-100 (BS), a synthetic mica-type montmorillonite.The clays were saturated with Na+ by repeated (3 x ) exchange with 1 mol dm-3 NaCl solutions. The fraction < 2 ,um was separated by centrifugation, concentrated, washed ClF-free and freeze-dried. The cation exchange capacities (c.e.c.) were determined by the 22Na-method and by the Ag(tu)i-method (tu = thioureua).* They are given in table 2 together with the external surface areas taken from the literatures and the total Fe contents. determined previously.1° The ion exchange of proflavine on the three clays was followed in dialysis membranes containing 0.1 g clay in 10 cm3 distilled water, immersed in 200 cm3 aqueous solution with known amounts of proflavine.The exchanges were performed overnight by continuously shaking the bottle with solution and membrane. After exchange proflavine- and Na+-contents of the equilibrium solutions were determined spectrophotometrically (3, = 445 nm, E = 4 x lo4) and by atomic absorption spectrometry, respectively. Samples for spectroscopy were prepared as follows. 1 or 2 g of clay was suspended in 40 cm3 distilled water in a centrifuge cup. The quantity of proflavine necessary for the desired exchange level was added and the cup was shaken on an end-over-end shaker overnight for exchange. After exchange the clay was centrifuged, washed twice with distilled water and freeze-dried. Preparation and storage of samples was always done in the dark.283 0 120 240 360 480 600 720 840 960 1080 1200 loading/pmol g-' Fig.1. doe, spacing as a function of the loading of proflavine for freeze-dried Camp Berteau (0) and Wyoming Bentonite (+). Techniques Reflectance Spectroscopy Reflectance spectra in the range 2200-210 nm were taken on a Cary 17 instrument with type I diffuse reflectance attachment. The spectra were computer-processed and plotted as the Kubelka-Munk function against wavenumber.ll The reflectance standard was the Eastman Kodak white reflectance standard. Fluorescence Spectroscopy All the spectra were taken in the range 460-730 nm with a Spex fluorolog spectrofluori- meter. The excitation wavelength was 445 nm. The sample holders were the same as those for reflectance spectroscopy.The detection of the emitted light was done in front-face arrangement. The emitted signal was corrected with the Spex emission corrector. The spectral measurements were controlled with a Tectronix desktop computer and transmitted to a PDP 11 /23 computer for storage on tape, integration and plotting. X- Ray Diflract ion The do,,, spacings of air-dried samples with different loadings of proflavine were determined with a Seifert-Scintag PAD I11 X-ray powder diffractometer using Cu Ka rays. Results PFH+ is taken up very selectively by the three minerals. For CB and WB the maximum capacities approach the c.e.c. as determined by the Ag(tu)i method. For BS ca. 500 pmol g-l is taken up selectively, and this is followed by a continuous increase of the loading up to 750 pmol g-l.0.961 5000 15000 25000 35000 45000 w ave nu m be r/c m - Fig.2. Reflectance spectra of proflavine on Barasym SSM-100: (a) 0.4, (b) 21.8 and (c) 87.0 pmol g-I. The variation of the do,,, spacing with the loading is shown in fig. 1 for CB and WB. The maximum spacing at full occupancy is 1.53-1.55 nm. For CB there is an initial sharp rise with increasing loading and a levelling off towards 1.55 mm. For WB the expansion at small loadings is limited, but the clay expands rapidly above 200 pmol 8-l to attain 1.53 mm. BS does not expand with adsorption of proflavine in the range 196779 pmol g-I. Reflectance Spectroscopy Representative spectra of proflavine on BS are shown in fig. 2. These spectra are plotted after subtraction of the Na+-BS spectrum, which shows significant absorption in the U.V.R. A .Schoonheydt, J . Cenens and F. C. De Schrijver 1.55 1.24 - 0.93 - c n 8 s 5 0.62 - 0.31 - 0.00 5000 15 000 2 5 000 35000 45000 wave nu in ber/cm-' Fig. 3. Reflectance spectra of proflavine on Wyoming bentonite: (a) 129.8, (b) 30.7 and (c) 6.2 pmol ggl. 2.50 2 .oo 1.50 - h 8 s 5 1.00 0.50 n 28 5 5000 15000 25000 35000 45000 w avenumber/cm-' Fig. 4. Reflectance spectra of proflavine on Camp Berteau montmorillonite: (a) 176.0, (b) 41.7 and (c) 4.2 pmol g-l. only. As the reflectance cells of Na+-BS and PF+-BS cannot be loaded in an identical manner, the bands and their intensities in the U.V. range (v > 32000 cm-l) are strongly deformed. However, the bands of interest in this study are in the range 15 OO(b30000 cm-l.These are not deformed, because the Na-clay does not absorb in this range. There are two band systems, one in the range 20000-25000 cm-l, the other286 ProJavine on Clays wavelength/ nm Fig. 5. Fluorescence spectra of proflavine on Barasym SSM-100: (a) 0.4, (b) 2.2, (c) 4.4, (d) 21.8 and (e) 87.0 pmol g-l. The relative intensities are: (a) 1.00, (b) 0.73, (c) 0.44, (d) 0.36 and (e) 0.01. between 26000 and 31 000 cm-l. The first consists of three bands at 20000 (500), at 22400+_400 (445) and at 23400+400 cm-l (427 nm). In view of the assignments of table 1, the 23400 cm-l band is due to the dimer, the 22400 cm-l band to PFH+ and the 20000 cm-l band to PFHi+ and/or the dimer. The second band system comprises two bands at 27 175 (368) and at 28330 cm-l(353 nm).These are due to PFHi+. The following qualitative evolution of the band maxima can be seen. At the smallest loading (0.4 pmol gel) PFH+ is predominant, but small amounts of PFHi+ are visible by weak bands at 27500 and 20000 cm-l. As the loading increases, the ratio of the 22400 cm-l band intensity of PFH+ to the 27 175 cm-l band intensity (PFHi+) decreases from ca. 4 to 2, indicating that the relative amount of PFHi+ increases with loading. A shoulder around 23000 cm-l, due to the dimer, becomes visible. At the highest loadings investigated (87 pmol g-l), the dimer band at 23400 cm-l is dominant. The 27 175 and 28 330 cm-l bands are barely visible on the high-frequency side of the dimer band, while the 22400 cm-l band of PFH+ has completely disappeared.If it still exists, it must be completely buried under the bands of the dimer. 100 cm-l (452 nm) is predominant whatever the loading (fig. 3). The presence of dimers is indicated by a band broadening towards higher wavenumbers. The PFHi+ bands are barely visible at a loading of 30.7 pmol g-l. The spectra of proflavine on CB are shown in fig. 4. Again, the PFH+ band is predominant with a high-frequency shoulder due to the dimer. There is a clearly visible PFHi+ band system at intermediate loadings. At the highest loading investigated only a broad band envelope is seen, centred at 22000 cm-l, which encompasses PFH+, PFHi+ and dimer bands. On WB the PFH+ band at 22 100 Luminescence Spectra Luminescence spectra of proflavine on BS, CB and WB are shown in fig. 5, 6 and 7, respectively.The relative intensities are indicated in the figure captions with spectrum (a) of fig. 5 as the reference spectrum. The numbers show that, in the absence of FeIII, the emission intensities strongly decrease with loading. On the Fe-containing clays, CB and WB, emission intensities are small and almost independent of the loading, except for CB at the lowest loadings.R. A . Schoonheydt, J. Cenens and I;. C. De Schrijver 287 500 600 700 wavelength/nm Fig. 6. Fluorescence spectra of proflavine onCamp Berteau montmorillonite: (a) 0.9, (b) 8.3, (c) 41.5 and (d) 176.0pmolg-l. The relative intensities are: (a) 0.11, (b) 0.02, (c) 0.02 and (d) 0.01. I - . . . 1 . . . . . . . . . . . . . . . . . . . . . 5 00 6 00 7 00 wavelength/nm Fig.7. Fluorescence spectra of proflavine on Wyoming Bentonite: (a) 0.7, (b) 3.1, (c), 30.7 and (d) 129.8 pmol g-l. The relative intensities are: (a) 0.006, (b) 0.005, (c) 0.005 and (d) 0.004. For BS with 0.4 pmol g-l the fluorescence maximum is at 495 nm with shoulders around 530 and 580 nm. As the loading increases the 530 nm band becomes predominant and shifts to 552 nm, while at the highest loading (87 pmol g-l) the 580 nm fluorescence band is predominant. The same qualitative evolution with loading is found for CB, but the band maximum at the lowest loadings is at 503 nm. For WB at the smallest loadings the 490 nm band is predominant. It shifts to 500 nm at 3.1 pmol g-l. It is not the 530 nm band which becomes predominant at intermediate loadings but the 590 nm band, and this remains so up to the highest loading investigated.288 Projlauine on Clays Discussion Adsorption of Proflavine Proflavine is very selectively exchanged onto the smectite-type clays, CB and WB, from PFH+ solutions.This is derived from the observation that the maximum capacities are very close to those of Ag(tu)l, a complex which is also very selectively adsorbed.8 It is therefore also a true ion-exchange process with no co-adsorption of proflavine hemi- sulphate or neutral proflavine molecules. The increase of the doe, spacing with increasing loading is indicative of an interlamellar adsorption process. This process sets in at smaller loadings for CB than for WB. This is somewhat surprising in view of the higher charge density and larger external surface area of CB. It may have to do with the average dimensions of the clay platelets; the larger ones being more difficult to intercalate than the smaller ones.It follows that, at the small loadings of the spectroscopic observations, the proflavine molecules are mainly located at the external surface, certainly in the case of WB. The do,,, spacing at full loading (1.53-1.55 nm) can be explained by a double layer of flat-lying proflavine molecules. Indeed a monolayer is expected to give the same spacing as the pyridinium ion (1.25 nm). The second layer adds another 0.3 nm to result in a spacing of 1.55 nm as observed. BS does not swell upon adsorption of proflavine. The majority of the molecules is therefore adsorbed on the external surface at all loadings.Spectroscopic Characterization of Proflavine Species on Clays There are three species of proflavine on the surface of the clays : PFH+, PFHi+ and dimers. The relative concentration of these species depends on the loading and the type of clay. PFH+ is the dominant species at the lowest loadings of BS, but is the main form of proflavine on CB and WB at all exchange levels investigated. This is derived from the observation that the band maximum remains at 22 100 cm-l, whatever the loading for CB and WB. The corresponding emission maximum is at 495-503 nm. The emission in the range 530-552 nm is attributed to PFHi+. The spectra show that the concentration of PFHi+ on the surface at equal loadings decreases with the decrease of the negative charge density of the minerals: BS > CB > WB (table 2).97 12, l3 Such a relation was already found for the exchange of ethylenediamine cations.The phenomenon is related to the lower hydration state of the organic cation on the surface with respect to solution. On decreasing the charge density, the cation-water interactions on the surface tend more to the corresponding process in s01ution.l~ It is remarkable that our spectroscopic results are in qualitative agreement with those of the ion exchange of ethylenediamine cations; our freeze-dried clays have a low water content and the majority of proflavine molecules are adsorbed on the external surface only. We think that, besides charge density effects, small numbers of strongly acidic sites must be present at the external surface which selectively adsorb PFH+, and transform it to PFHg+.The number of these sites is proportional to the external surface area. Experiments are under way to determine the number of these sites and, if possible, their nature. With the majority of the proflavine molecules on the external surface, dimerization is expected to be more pronounced on clays with the smallest external surface area. This is not the case; dimerization sets in at smaller loadings for BS than for CB and WB. There are two possible explanations: (i) on the smectite-type clays, CB and WB, some interlamellar adsorption takes place, extending the available surface area beyond the external surface area; (ii) the PFHi+ are preferred adsorption sites of incoming molecules, giving the dimeric structure, shown below with a charge + 3 or + 4.R.A . Schoonheydt, J . Cenens and F. C. De Schrijver 289 H I + H Whatever the explanation, dimerization sets in far below monolayer coverage. This can be visualized as a simple concentration effect. For example, 1 pmol PFH+, randomly distributed over the external and interlamellar surfaces (750 m2 g-l) in a monolayer, is equivalent to a solution concentration of 3.3 mmol dmP3. This is a concentration at which dimerization is clearly visible.* The dimer emits at 580 nm; the excitation band is the weak forbidden band around 20000 cm-l. As far as the authors are aware, this is the first time the dimer emission has been reported for proflavine. In any case, protonation, dimerization and quenching by structural FelI1 are all processes which suppress the luminescence intensity, the last factor being predominant.R.A.S. acknowledges a position as senior research associate of the National Fund of Scientific Research (N.F.W.O., Belgium). J. C. thanks the Instituut voor Wetenschappelijk Onderzoek in Nijverheid en Landbouw (I.W.O.N.L., Belgium) for a Ph.D. grant. This work was supported by the National Fund for Scientific Research (N.F.W.O., Belgium). References 1 P. de Mayo, Pure Appl. Chem., 1982, 54, 1623. 2 B. K. G. Theng, The Chemistry of Clay-Organic Reactions (A. Hilger, London, 1974). 3 M. P. Pileni and M. Gratzel, J. Phys. Chem., 1980, 84, 2042. 4 G. R. Haugen and W. Melhuish, Trans. Faraday Soc., 1964, 60, 386. 5 K. Yamaoka and M. Shimadzu, Bull, Chem. Soc. Jpn, 1983,56, 55. 6 K. Bergmann and C. T. O'Konski, J . Phys. Chem., 1963, 67, 21 69. 7 R. Cohen and S. Yariv, J. Chem. Soc., Faraday Trans. 1, 1984, 80, 1705. 8 R. Chabra, J. Pleysier and A. Cremers, Proc. 5th Znt. Clay Conf, 1973, 439. 9 H. Van Olphen and J. J. Fripiat, Data Handbook for Clay Minerals and other Non-metallic Minerals 10 R. A. Schoonheydt, Diffuse Reflectance Spectroscopy, in Characterization of Heterogeneous Catalysts, 11 R. A. Schoonheydt, P. De Pauw, D. Vliers and F. C. De Schrijver, J . Phys. Chem., 1984, 88, 51 13. 12 A. Maes, M. S. Stul and A. Cremers, Clays Clay Miner., 1979, 27, 387. 13 M. S. Stul, L. Van Leemput, L. Leplat and J. B. Uytterhoeven, J. Colloid Interface Sci., 1983,94, 154. 14 A. Maes and A. Cremers, J. Chem. Soc., Faraday Trans. 1, 1981, 77, 1553. (Pergamon, New York, 1979), p. 13. ed. F. Delannay (M. Dekker, New York, 1984), p. 125. Paper 4/2194; Received 31st December, 1984

ISSN:0300-9599    

DOI:10.1039/F19868200281

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J. Chem. Soc., Faraday Trans. I , 1986,82, 291-317 The Interaction of Aromatic Compounds with Poly(vinylpyrro1idone) in Aqueous Solution Part 5.-Binding Isotherms for Phenols and @Substituted Phenols Philip Molyneux" Macrophile Associates, 53 Crestway, Roehampton, London S W15 5DB Sudhe Vekavakayanondha Faculty of Pharmaceutical Sciences, Chulalongkorn University, Bangkok 10500, Thailand In an extension of previous work, and as part of an investigation of the mechanisms by which phenols precipitate water-soluble polymers from aqueous solution, the binding isotherms have been determined for poly- (vinylpyrrolidone) (PVP) in aqueous solution at 25 "C with the following 13 cosolutes: PhOH; PhOMe; PhOEOH; HOPhNO,; PhOGl; HOPhOH; HOPhOMe; MeOPhOMe; HOEOPhOEOH ; HOPhOGl; Ph(OEOH), ; PHPhOH ; HOPhPhOH [where Ph = phenyl or phenylene (1,6substitution in XPhY and 1,3-substitution in PhXY), E = ethylidene (CH,CH,) and G1 = 1 -deoxy, 1 -P-D-glucopyranosyl (' glucosyl ')I.The three 0-substituents used (i.e. Me, EOH and G1) have been chosen to reveal the contribution to the binding from the phenolic hydrogen without affecting that from the phenoxy or phenylenedioxy part of the cosolute molecule. The two com- plementary experimental techniques of equilibrium dialysis and cosolute solubility have been used for these measurements. With most of the cosolutes the binding could be interpreted in terms of the uniform site-binding model which leads to the hyperbolic (Langmuir) form of binding isotherm; with HOPhNO,, and possibly with HOPhOMe, the binding involves cooperative effects resulting from attractive interactions between bound cosolute mol- ecules.The binding parameters are discussed on the basis of group- contribution theory, in relation to the four main types of interaction forces in these aqueous systems, i.e. hydrophobic effects, dipole/induced dipole (van der Waals-Debye) forces, dipole/dipole (van der Waals-Keesom) forces and hydrogen bonds. The correlations observed have enabled a group-contribution table to be drawn up for the binding constants of six aromatic cosolutes: PhH, PhOH, HOPhOH, PhPh, PhPhOH and HOPhPhOH. The results for the glucosides, PhOGl and HOPhOGl, showed there to be a net repulsion between the glucose ring and the PVP chain in aqueous solution. The diverse interactions that take place in aqueous solution between water-soluble polymers and the small-molecule solutes that accompany them (i.e.cosolutes) have many practical and theoretical implications. In many cases significant effects are seen at fairly low cosolute concentrations, i.e. < ca. 0.1 mol dm-3; in such cases, the effects may be attributed to specific binding (complexing) of the molecules of the cosolute by the polymer chain. In previous parts of this series we have reported studies on the interactions between poly(vinylpyrro1idone) [PVP;f- monomer-unit structure (I)] and various aromatic cosol~tes;~-~ we have also reported parallel studies on the binding of aliphatic bolaform t See the Appendix for the symbols and abbreviations used in this paper. 29 1292 Binding of Phenols by Poly(viny1pyrrolidone) compounds (a, cu-disubstituted alkane^).^ PVP is a water-soluble synthetic polymer which is important pharmaceutically and in numerous other technical applications,l09 l1 and which is also interesting from the viewpoint of its physical chemistry.2* 3 9 l2 Its cosolute binding properties continue to attract scientific attention.l1, 13-16 In an extension of our previous work we now report the results of a more detailed examination of the behaviour of phenolic compounds as cosolutes. One reason for choosing these compounds is that they include many non-ionic, simple-structured cosolutes, so that a closer study of their binding isotherms and their mode of binding may reveal the forces involved in the binding, as well as the reasons for the relatively large size for the binding site (ca.10 monomer units) observed with aromatic cosolutes and PVP. A second reason for the choice of these cosolutes is that many small-molecule compounds used in aqueous pharmaceutical and other technical formulations (e.g. drugs, antioxidants and antimicrobial agents) are phenolic in type.17 When such a formulation also contains a water-soluble polymer (e.g. as a dispersing agent, stabiliser or thickener), the polymer-cosolute binding can lead to the depletion of the (free) cosolute in solution, and hence may lead to its activity being reduced below the level intended. The particular ability of PVP to bind phenolic compounds has been applied, for example, in the chromatographic analysis of these compounds,1s-21 in the removal of phenolic glucosides from plant extract^^^-^^ and in the use of tannin (tannic acid), which is a polyphenol g l u c ~ s i d e , ~ ~ ~ 25 in the determination of cross-linked PVP.26 Phenolic cosolutes are also interesting because they can precipitate water-soluble polymers such as PVP from aqueous solution.This precipitation may play a part in the applications of PVP-phenol interactions listed above. Indeed, such precipitation represents one of the most simple, striking and important examples of a visible effect resulting from the interaction between a polymer and a cosolute. It seems to be quite a general phenomenon, since in the case of phenol itself it occurs not only with PVP27-31 but also with other synthetic polymers such as poly(ethy1ene 2 8 j 3 2 7 33 and the alkyl ethers of its oligomers (although these are micellar rather than p~lymeric),~~~ 35 poly(propy1ene oxide) (i.e.its water-soluble ~ligomers),~~ block copolymers of ethylene oxide and propylene oxide (' Pluronics ', ' Poloxamers ')36 and poly(vinylmethyloxazo1i- done),37 as well as with natural polymers such as the proteins (e.g. egg albumen17 and serum albumin3*), and also with modified natural polymers such as methylcellulose.17~ 28 With PVP, precipitation occurs also with many substituted p h e n ~ l s , ~ ~ ~ 2 9 7 3 0 9 3 9 9 40 and especially with polyhydric phenols such as tannin, which indeed shows a very marked general precipitating ability towards these polymers.17* 2 8 ~ 3 3 7 3 6 9 41-44 This precipitating ability of phenols has been applied in identification tests for proteins such as gelatin and albumen with tannin (and conversely for tannin itself),45 in the turbi- dimetric assay of PVP31 and other water-soluble synthetic polymers41 and more generally as a method for differentiating between various types of these p01ymers,~~9 3 6 9 41+ 42 as well as in the applications to plant extracts and analysis already noted.It is also pre- sumably the mechanism by which PVP and other water-soluble polymers work in their application to the clarification of beer and other beverages, i.e. by precipitating out the tannin.44 On the other hand, such precipitation is an effect to be avoided or circumvented when it is desired to incorporate a phenolic small-molecule substance along with a water-soluble polymer in the same water-based or water-borne technical formulation.P .Molyneux and S . Vekavakayanondha 293 The evident practical and theoretical importance of this precipitating ability provides the third reason for choosing phenols to study as cosolutes with PVP. The previously cited studies have shown that the precipitation of the polymer takes place when the concentration of the cosolute attains a certain level, the critical precipitation concentration, c*, which depends on the structures of the cosolute and the polymer, and the molecular weight of the latter. This behaviour, and the fact that the value of c* is generally low (often < 0.1 mol dm-3), confirms that the precipitation is a consequence of the specific binding of the cosolute by the p~lymer.~ At concentrations below c*, phenolic cosolutes cause a shrinkage in the encompassed volume of the PVP molecule, as shown by reductions in the intrinsic viscosity6~39 and in the molecular dimensions observed from light-scat tering.These conformational changes may be used to study the effects underlying the precipitation seen at higher cosolute concentrations. The changes may be attributed to the ability of the bound cosolute molecule to form a transient, reversible non-covalent cross-link with another section of polymer chain. If this section is on the same polymer molecule (intrachain cross-linking) then it leads to the observed reduction in encompassed volume, while if it is on another polymer molecule (interchain cross-linking) it can lead eventually (i.e.with a sufficiently high degree of cross-linking) to the precipitation that is observed. The reality of the interchain effect is supported by the accompanying increases that are seen in the second virial coefficient A , (from light-scattering)6 and in the Huggins viscosity slope parameter kH61 39 at cosolute concentrations below c*. In a wider context, systematic studies of the present type should give more information on the effective strengths of the component non-covalent interactions, such as van der Waals forces, hydrogen bonds and hydrophobic effects, that are involved in these phenomena (binding, cross-linking and precipitation) in aqueous solution, where the solvent water plays such an important 47 In molecular biology such information is essential if there is to be a valid link between the structural information that has been obtained for proteins and other biopolymers and their behaviour in the aqueous environment of the living cell.More specifically, the similarities in structure between the chain units of PVP (I) and of the proteins suggests that the present type of study may clarify the interactions occurring in cells and in organisms between phenols and proteins. Thus the antimicrobial action of phenols seems at least in part to be the consequence of a rather general denaturing effect on the cell but protein binding and denaturing effects by phenols has been studied relatively little compared with those of other denaturing 49 Similarly, the effects of bound phenols on the conformation of PVP may clarify the extent to which the aromatic side-groups of phenylalanine and tyrosine residues are involved in the folding and the conformational stability of protein chains.In choosing the range of compounds to be studied in the present work we were guided by the desire to correlate the effects observed with the molecular structures of the cosolutes, since this should then give us fundamental information and enable predictions to be made about the behaviour of related systems. Interaction phenomena of these types are currently interpreted on the basis of the group-contribution method, whereby in the ideal case the standard free-energy change of a particular process (here cosolute binding or reversible cross-linking) is the sum of independent contributions from the interacting groups on the molecules 51 However, the success of the method often depends on a judicious choice of the combination of atoms that go to make up the ‘groups’ involved.For example, in the present case if we consider phenol as the prototype molecule, then clearly it would not be appropriate to divide this into phenyl and hydroxy as the two component groups, since these interact strongly with one another rather than being independent; this is shown by its electronic spectrum being markedly different from (say) benzene or benzyl alcohol, and by the acidity of its hydroxy group being much higher than when it is attached to the ring through an alkyl group as in benzyl alcohol (see table 2 later). These intramolecular interactions can be represented formally by 11 FAR 1294 Binding of Phenols by Poly(uinylpyrro1idone) resonance between the canonical forms (11), which therefore leads the charge contributions shown, i.e. a fractional positive charge on the oxygen atom and fractional negative charges on the ortho and para positions of the ring.In the present context this will make the hydroxy group a stronger donor group in hydrogen bonding but a weaker acceptor group, while the charges on the ring must also alter its ‘hydration’ state in aqueous solution. A more suitable division of phenol is that into the phenoxy group and the remaining hydrogen atom, with the proviso that the latter is a phenolic hydrogen. Accepting this proviso, this division has the merit that the behaviour of the phenoxy group should be relatively insensitive to the replacement of the hydrogen atom by a variety of other groups, so that the consequences of such replacement can potentially give us information on the part played by the phenolic hydrogen atom in the binding, precipitation and conformation effects.It is for these reasons that we have studied a number of 0-substituted phenols alongside the parent phenols themselves, both in the binding studies reported in this paper and in the polymer precipitation and viscosity studies reported in the following Part.52 For the parent phenols we have used the related quartet of PhOH (phenol), HOPhOH (hydroquinone), PhPHOH (biphenyl-4-01) and HOPhPhOH (biphenyl-4,4’-diol). From the group-contribution viewpoint this quartet provides two distinct ways of introducing a phenolic hydroxy group into a cosolute molecule (PhOH + HOPhOH; PhPhOH -+ HOPhPhOH), and two ways in which a phenylene group is introduced (PhOH -+ PhPhOH; HOPhOH + HOPhPhOH). In previous work with aromatic car- boxylate ions the introduction of a second phenyl ring (i.e.PhCO; + PhPhCO;) was found to give rise to a marked increase in the binding ~trength.~ This effect is further investigated for the non-ionic phenolic cosolutes in the present work. For the substituent groups three types have been used : (i) Methyl (Me). The structural change here is small, which should make it easier to interpret the results. Unfortunately the derived cosolutes have a greatly reduced solubility, which makes the experimental work more difficult and also reduces the observable span of the binding isotherm so that the binding parameters are lesscertain.(ii) 2-Hydroxyethyl (CH,CH,OH, here abbreviated to EOH) : This gives derivatives with a much higher solubility than with Me. However, the greater complexity of the group means that the results are more difficult to interpret from the molecular viewpoint. At the same time, the new hydroxy group will be much less acidic than the phenolic one it replaces, and hence presumably forms weaker hydrogen bonds. In a wider context, this group is present in certain pharmaceuticals and other technically important The substituted compounds, such as phenoxyethanol (PhOEOH), can themselves be pharmaceutically important and also may be viewed as the prototypes of the condensates of alkylphenols and ethylene oxide (‘Octoxinol ’, ‘ Nonoxinol’, etc.) which are used as spermicides, surface-active agents and in allied app1i~ations.l~ (iii) 1 -Deoxy, 1 -B-D-glucopyranosyl (‘ glucosyl’, G1) : This group gives cosolutes with high solubility.At the same time, the substituent group is obviously very important biologically. Thus, in view of the similarities already pointed out between the structures of PVP and of the proteins, the results of the present studies with such cosolutes may cast light on protein/polysaccharide interactions in the aqueous environment of the living cell. In addition, we have studied one nitro derivative, HOPhNO, (4-nitrophenol). HereP. Molyneux and S. Vekavakayanondha 295 the introduction of the nitro group greatly enhances the acidity of the phenolic hydrogen, and hence presumably increases its hydrogen-bonding strength.In the present paper we report our work on the binding equilibria for these cosolutes with PVP. Our parallel work on the precipitation and viscosity effects will be reported separately in the following Part,52 since these effects are presumed to relate to a secondary effect of the bound cosolute molecules, although some of the results of this parallel work are anticipated in the present paper. To obtain the binding isotherms we have used two complementary methods, equilibrium dialysis (e.d.) and cosolute solubility (c.s.). The first of these methods was that used in our previous ~ t u d i e s , ~ ~ ~ ~ ~ but we have modified the technique to enable the maximum observable span of the binding isotherm to be obtained.The second method provides the highest point that is experimentally attainable on the binding isotherm, corresponding to a free cosolute concentration equal to the saturation solubility; however, this method evidently cannot be applied to those cosolutes that precipitate the polymer at a lower concentration. C”3 To obtain information on the relation between binding and the PVP, i.e. to see whether it is necessary for the monomer units in a chain for binding to occur, we have also carried out some the polymeric nature of to be connected together studies using in place of PVP the small-molecule monomer-unit analogues N-methylpyrrolidone, NMP (111), and sarcosine anhydride, SA (IV).2 For this purpose we have used the C.S.method (the e.d. method is obviously inapplicable, since these compounds diffuse through the cellophane membrane used), with HOPhOMe as a typical phenolic cosolute and PhOEOH as a typical non-phenolic cosolute. Experimental Materials Poly(vinylpyrro1idone) Batches of PVP of the three molecular-weight grades K-30, K-60 and K-90 were obtained from the Chemical Division of GAF (Great Britain) Ltd, Manchester. Their characteristics are listed in table 1 .? With K-60, the material was supplied as a 45 % solution which was evaporated to give the polymer in flake form. With K-90, two lots (designated here K-90A and K-90B) were purchased at separate dates, the first being used in the early part of the work and the second later; these were subsequently found to have markedly different t Throughout this paper, numbers in parentheses indicate the spread or uncertainty in the last decimal place of the main number. 11-2296 Binding of Phenols by Poly(uinylpyrro1idone) Table 1.Characteristics of the poly(viny1- pyrrolidone) samples used K-30 22 4.0 x 104 3.6(6) x 104 K-60 62 1.6 x lo5 1.8(3) x lo5 K-90Ad,e 135 3 . 6 ~ lo5 6.1(11)~ lo5 K-90Bd.e 200 3.6 x lo5 1.1(2) x lo6 a Intrinsic viscosity, [q]/cm3 g-' (water at 25 "C); see Part 6.52 Manufacturer's nominal values, presumed to be un [ref. 53)]. Calculated from the Mark-Houwink-Sakurada equation : [q] = KM;, using a = 0.63(6) and K = 3.0(4) cm3 g-' [ref. (2)]. A and B represent separately purchased batches of K-90 grade. The sample of K-90 used previously for binding studies had [q] = 138 cm3 g-' (water, 30 "C), giving M, = 6.4(12) x lo5 [footnote (c)] and Mw = 1 .O x lo6 (light-scattering) [ref.( 5 ) and (6)]. intrinsic viscosities (table 1), although both of these values lay within the manufacturer's specification (which allows the viscosity-related Fikentscher K-value to vary between 80 and 100 for this grade).53 The water contents of the polymers were determined by vacuum drying at 105 "C; their solutions were assayed by evaporation followed by such vacuum drying. Cosolu tes The thirteen phenols and related compound used as cosolutes with PVP are listed with some of their physical characteristics in table 2. These characteristics comprise the following: U.V. spectral data (A, and e), obtained either in the course of this work, or from the 1iteratu1-e;~~ the aqueous solubility s, obtained either in the course of this work, or from the 56 the octanol/water partition coefficient P, either from the literature or e ~ t i m a t e d ; ~ ~ , ~ ~ and the pK, value, either from the literature or e ~ t i m a t e d .~ ~ - ~ ~ These parameters are in many cases directly relevant to the present experimental work, while they are also potentially useful in correlating these interaction phenomena in aqueous solution with the molecular structures of the cosolutes (see Discussion). The table also gives the abbreviated formulae which are used for these compounds throughout the rest of the paper because they represent the cosolute structures more clearly and succinctly than the trivial or the IUPAC names (see also Appendix). From the pK, values in table 2 it is clear that it is only with HOPhNO, (pK, = 7.15) that the acidity is sufficiently pronounced for its ionisation to require suppression in the binding studies.Even with this cosolute, the pH of the (central-London air-equilibrated) distilled water that was used was always near to 5.0, so that strict control of the pH was not required, and indeed the extent of binding did not change appreciably when either 1 mmol dm-3 hydrochloric acid (i.e. pH ca. 3) or pH 4.7 acetate buffer was used in place of distilled water; however, in the spectrophotometric analysis the acetate buffer was used throughout for consistency. The materials used had a minimum purity of 99% as obtained from the supplier, or were purified by standard methods (recrystallisation and fractional distillation) to this level, as confirmed by physical measurements (m.p., b.p., U.V.spectrum, i.r. spectrum and refractive index).Table 2. Characteristics of the small-molecule solutes (cosolutes) U.V. spectral datab serial abbreviated letter compound formulaa Arnax/nm 10-3 &C s/mmol dm-3d log pe PKaf A B C D E F G H I J K L M phenol methoxybenzene (anisole) 2-phenox ye t hanol 4-nitrophenol phenyl P-D-glucopyranoside 1,4-dihydroxybenzene (hydroquinone) 4-met hoxyphenol 1,4-dimethoxybenzene 1,4-bis(2-hydroxyethoxy)- benzene hydroquinone P-D-gluco- pyranoside (arbutin) 1,3-bis(2-hydroxyethoxy)- benzene P biphenyl-4-olq biphenyl-4,4-diolq PhOH PhOMe PhOEOH HOPhNO, PhOGl HOPhOH HOPhOMe MeOPhOMe [OEOPhOEOH HOPhOGl Ph(OEOH), PhPhOH HOPhPhOH 270 (269) 269 (268) 270 (270) 318 (317)" 267 (26QZ 287 (288) 287 285 285 282 (285) 273P 258 (259) 260 1.5 (1.48) 1.2 (1.6) 1.3 (0.33)i 9.8 (9.8)k 0.7 (1.6)z 2.6 (3.2) 2.5 2.5 2.3 2.0 (2.1) 1.9 19 (18) 20 (894)g 18 (1.3)h 2 10 (200) -(llO) 62 - (655) 350 6 59 176 96 0.25 0.45 1.49 (2) 2.08 1.16 1.95 (8)" 0.55 (6) 1.36 (2) 2.04n -0.71 - 0.06n - 1.35 O.lsn,p 3.20 2.70n 10.0 14.7i 7.15 12% 10.9 10.2 14.4j - p $ %- 10.2O E s 9.2' % - fb 14.4j.P A, 9 9.5 -T a See Appendix for the abbreviations used in these formulae. Aqueous solution: values in parentheses are literature data.54*55 Note that for PhH (benzene), A,,, = 254 nm, E = 0.148 x lo3 dm3 mol-l cm-l [ref. (55)]. Aqueous solubility, 25 "C; values in parentheses are literature data, from ref.(55) or (56) except where otherwise indicated. Note that for PhH (benzene), s = 23 mmol dm-3 [ref. (55)]. P is the octanol/water partition coefficient, either literature value from ref. (57) or (58), or estimated as specified in the footnotes. Note that for PhH (benzene), log P = 2.10 (6). f Aqueous solution, 25 "C, either literature value taken from ref. (59), or estimated as specified in the footnotes. Note that for water itself, pK, = p(K,/[H,O]) = 14.0+log (55.4) = 15.7. g Liquid-liquid equilibrium, with saturated aqueous solution a water-rich liquid in equilibrium with the phenol-rich phase containing 7.2 mol dm-3 phenol. The solubility value of 1.3 mmol dm-3 reported by Andrews and Keeferso is apparently too small by a factor of ten, as is evident from their other experimental solubility values, e.g.1.6 mmol dmP3 for the isoelectronic but more hydrophobic solute, ethylbenzene.61 The single literature values2 is evidently erroneous. i Estimated using values of the Taft polar substituent parameter o* [ref. (63) and (64)] and the correlation between pK, and o* for alcohols established by Long and Ballir~ger;~~ see also Murtossa and Takahashi et dssb Experimental data, acetate buffer (pH 4.7); literature data for pH < 5. The parent compound, PhNO, (nitrobenzene), has log P = 1.84 (5). Literature experimental pK, for glucose (extrapolated to 25 "C) is 12.3.67 Estimated value of log P from literature data for related compounds using group-contribution method^.^^^^^ Note that for both 1,2- and 1,3-Ph(OMe),, log P = 2.21.O Value of pK, taken to be the same as that for 4-methoxyphenol (HOPhOMe); the glucose ring should not make any appreciable direct or indirect contribution to the acidity of this compound. P The parent phenolic compound resorcinol [Ph(OH),] has A,,, = 273 nm, E = 1.9 x lo3 dm3 mol-1 ~ 3 n - l ; ~ ~ log P = 0.78 (2);579 58 pK, = 9.8.59 Q The parent hydrocarbon, biphenyl (PhPh) has A,,, = 247 nm, E = 1.7 x lo3 dm3 mol-l cm-1;55 log P = 4.08 (9);579 58 s = 0.0485 mmol dm-3.55 Using an intrinsic pK, assumed to be the same as that for PhPhOH, together with a factor of 0.3 as the statistical contribution for the two hydroxy groups. Molar absorption coefficient, in dm3 mol-l cm-l. Literature data for phenyl-P-D-galactopyranoside.298 Binding of Phenols by Poly(viny1pyrrolidone) Binding Measurements Equilibrium Dialysis (e .d. ) The ' immersed-bag ' technique was employed as in other studies of polymer/cosolute binding.4 In the conventional application of this technique, when the binding systems are set up initially the solution inside the bag contains polymer only and that outside cosolute only, so that (with the same volume inside and outside) the maximum possible final cosolute concentration is only one-half that used initially. In the present work, to avoid this dilution effect and hence to maximise the span of the isotherm that could be studied, the polymer was introduced into the bag as a pre-weighed amount of the solid, followed by the measured volume of the cosolute solution, with the same volume of the same cosolute solution outside.Further details are as follows. The bags were prepared from 20 cm lengths of 19 mm Visking cellophane (regenerated cellulose) tubing (Scientific Supplies Ltd, London) and were sealed by knotting. The inside and outside volumes were 10 cm3, so that 100 mg of solid polymer was required for the final polymer concentration of 10 g dm-3 (1 % w/v) used in most of the studies. The systems were set up in 30 cm3 Sovirel screw-cap tubes (V. A. Howe & Co. Ltd, London), with the Teflon/synthetic rubber cap-liner rroviding an inert and essentially impermeable seal. Each cosolute was generally studied in triplicate at five concentrations up to the highest attainable, which was limited either by the value of c* (as was the case with the four precipitant cosolutes: PhOH, HOPhNO,, HOPhOH and HOPhOMe) or by its solubility.With PhOH, measurements were carried out on all four grades of PVP (table 1) to look for any effect of variation in polymer molecular weight on the binding. With the other cosolutes, the studies were confined to either K-90A or K-90B. With each cosolute a number of blank tubes were also set up with distilled water in place of cosolute solution, to correct for irrelevant absorption (arising from the polymer and the cellophane bag) in the subsequent spectrophotometric assay. After equilibration (1 8 h on a rotating mixer) at 25 "C, the tubes were opened and separate samples taken from inside and outside each bag; it was found convenient to transfer the bag to a separate empty Sovirel tube before cutting it open for sampling, to prevent intercontamination of the two phases..f- The samples were diluted by an appropriate factor and the absorbance at 3,,,, for the cosolute (table 2) was determined on a manual spectrophotometer.The absorbance was corrected for irrelevant absorption by subtracting the corresponding value from the similarly diluted blank tube contents; these corrected absorbances were then converted into the corresponding cosolute concentration in the tube using the dilution factor and the molar absorption coefficient for the cosolute (table 2). At the final dilutions used there should be no appreciable binding to interfere with the spectrophotometric assay. The measured total cosolute concentration, c, from the binding system in the bag, is the sum of the free concentration, a, and the bound concentration, b: c = a+b.(1) Also, at dialytic equilibrium the measured cosolute concentration outside the bag is equal to the free concentration inside (a). Thus the corresponding value of the bound concentration (b) may be obtained by difference using eqn (1). Previous work5 has shown that, for a fixed free cosolute concentration, b is proportional to the polymer concentration t Sampling both phases obviates the effect of any sorption of the cosolute by the bag. Sorption of polymer by the bag should be negligible for this system (PVP/regenerated cell~lose).~ Separate experiments to check for permeation of the polymer through the bag, by making up tubes in the same way with polymer, but using distilled water in place of cosolute solution, showed that < 0.3% of the K-30, and < 0.1 % of the K-60 and the K-90, had permeated through.P.Molyneux and S. Vekauakayanondha 299 in the binding system; the amount bound can therefore be conveniently expressed as the binding ratio, r, defined by r = b / p . (2) Thus with b in mol dm-3 and p in base mol (moles of monomer units) dm-3, r then represents the average number of molecules of cosolute bound to each monomer unit at that free cosolute concentration. Cosolute Solubility Method (c.s.) The basis of this method is that when the pure cosolute (solid, liquid or gas/vapour) is equilibrated with the polymer solution, any increase in the saturation solubility above the intrinsic solubility (i.e. that in pure water) can be attributed to binding to the polymer chains; this is valid only if the presence of the polymer does not itself affect the value of the intrinsic solubility (i.e.by changing the nature of the solvent medium), but with the low polymer concentration (10 g dmP3) used here it is unlikely that the effect is appreciable.? With liquid cosolutes (PhOMe and PhOEOH) the equilibrium is with the water-saturated cosolute, and the extent of this water content is likely to vary with the amount of the other solute (i.e. polymer) in the aqueous phase, which will in turn affect the aqueous solubility, but again with the low polymer concentration used this effect should not be appreciable. It should also be evident that this method cannot be used for solutes that precipitate the polymer, which in the present work is the case with PhOH, HOPhNO,, HOPhOH and HOPhOMe. In any solubility determination two problems have to be overcome: (i) the presence of more soluble impurities which would be assayed with the main solute and (ii) the need to isolate a sample of the solution free of solute for the assay (which is particularly difficult with liquid solutes).In the present work these problems were overcome by using a progressive extraction technique with a cellophane bag to contain the saturating cosolute. The details of the method (which was conveniently carried out alongside the e.d. on the same cosolute) are as follows. A bag was prepared in the same manner as for e.d. An amount of pure cosolute sufficient to saturate at least 50 cm3 of polymer solution was placed inside the bag, and then 10 cm3 volumes either of distilled water (for the intrinsic solubility) or of 10 g dm-3 PVP solution (for the polymer solubility) were pipetted inside the bag, and outside after sealing it.The systems were set up in 30 cm3 Sovirel screw-cap tubes as for e.d., and again ‘blanks’ were made up containing no cosolute in order to correct for irrelevant absorption in the subsequent spectrophotometric assay. After equilibration, as with e.d., only the outside phase of each tube was assayed. The bag (still sealed) was then immersed in a fresh 10 cm3 volume of the same liquid (i.e. water or polymer solution, as appropriate), equilibrated as before, and the outside phase assayed again. The saturation procedure was then repeated a third time.The spectrophotometric assays from the second and third saturations always agreed with one another, while there was invariably a higher value from the first saturation which results from the presence of traces of more soluble, u.v.-absorbing impurities. The polymer- solution value is the total concentration c for the binding system, while the distilled-water value is the intrinsic solubility corresponding to the free concentration a in the binding system, so that the bound concentration b and the binding ratio r can be calculated as for the e.d. method [eqn (1) and (2)]. In the case of PhOGl the method gave irreproducible results, probably owing to the partial hydrolysis of the cosolute by residual enzymes in the cellophane. The solubilities t Some support for this assumption comes from the results of Eliassaf et ~ l ., ~ ~ who found no detectable changes in the solubility of the (presumably non-bound) aliphatic amino acids leucine and aspartic acid in the presence of 36 g dmh3 PVP, whereas increases were found as expected for the aromatic compounds benzoic acid and phenylalanine, which are known to be bound by the p~lymer.~300 Binding of Phenols by Poly(vinylpyrro1idone) were therefore determined instead with a piston filter-tube device previously described.69 The solubility was essentially the same even when the amount of cosolute used was doubled, showing that it was free of more soluble u.v.-absorbing impurities. The U.V. spectra of the final (diluted) cosolute solutions were not detectably changed on making them alkaline, showing the absence of any appreciable hydrolysis of the cosolute during the 18 h equilibration period.Results The binding data were interpreted on the basis of the uniform site-binding (u.s.b.) m0de1,~ which in statistical-thermodynamic terms corresponds to the formation of an ideal localised one-dimensional monolayer of cosolute on the polymer chain.70 This leads to the hyperbolic (Langmuir) form of binding isotherm, which applies to many aqueous synthetic polymer/cosolute binding systems :4 r = aKn/( 1 + aK) (3) where K is the binding constant, i.e. the equilibrium constant for the attachment of a molecule of cosolute A onto a site S by a specific combination of non-covalent forces (see Discussion) : A+S = A-S (4) and n is the site density, i.e.the limiting value of r for ‘monolayer’ coverage, which is thus a measure of the density of the sites S along the polymer chain.? The reciprocal of n is the site-size, u, which may be taken to represent either the average number of monomer units occupied by the bound cosolute molecule, or more generally the average spacing of bound cosolute molecules when the chain is saturated. To get the best values of the binding parameters from the experimental data we have used a computer program which gives appropriately weighted least-squares fits to each of the three linearised forms currently used :* (i) the Klotz equation : l / r = l/aKn+ l / n ( 5 ) (ii) the Scatchard equation : r/a = Kn- Kr (iii) the Langmuir equation : a l r = a/n+ l/Kn.(7) Test runs with model data having either a fixed or a proportional scatter in the values of r showed that the average values assumed initially for the parameters K and n were reproduced satisfactorily in the computer outputs. Out of the thirteen cosolutes studied, the uniform site-binding eqn ( 3 ) was obeyed by eight of them: PhOH, PhOEOH, HOPhOH, HOEOPhOEOH, HOPhOG1, Ph(OEOH),, PhPhOH and HOPhPhOH. The goodness of fit in the case of PhOEOH j. The use of the concentration of the free cosolute, a, rather than its activity, in eqn (3), which implies that its activity coefficient has been omitted in formulating the equilibrium constant K, is necessary because of the paucity of information on the activity behaviour of these aromatic cosolute~.~~* 72 However, there is an equally implicit omission of the activity coefficients for the free sites S and for the occupied sites AS which may lead to some cancellation of errors.Moreover, in the case of PhOH, results from its salting-out behaviouP indicate that at the maximum concentration used here (80 mmol dm-3) the activity coefficient is 0.975, i.e. there is only a 2.5% deviation from ideality. This is supported by the solute vapour-pressure measurements made by Good and Mil10y’~ on PhOH solutions, which showed little deviation from Henry’s law up to 200 mmol dm-3. The amphiphilic character and high solubility of cosolutes such as PhOGl suggests that they may associate into micelles, but comparison with the known micellisation behaviour of the related gl~cosylalkylbenzenes~~~ 75 shows that such association is unlikely to occur at these concentrations.P. Molyneux and S.Vekavakayanondha 0-15 I ", 0.10 E z -3 0.05- 0 .d c b0 - 30 1 0 50 100 150 200 250 free cosolute concentration, almmol dm-3 Fig. 1. Binding isotherm for 2-phenoxyethanol (PhOEOH) with 10 g dm-3 PVP K-90 at 25 "C. 0, equilibrium dialysis results; a, cosolute solubility result. The full line is the uniform site-binding isotherm [eqn (3)] with binding constant K = 3.0 dm3 mol-1 and site density n = 0.3 1 ; the broken lines show the effect of increasing or decreasing both parameters by 3%. Table 3. Binding parameters for phenol with different grades of PVP: 25 *Ca gradeb p/g dmV3 Blmmol dmP3 Ki/dm3 mol-1 K/dm3 mol-1 n U P K-30C' 10 75.3 1.87 (11) 13.0 (17) 0.144 (16) 6.9 (8) 0.49 K-60' 10 75.5 1.82 (8) 11.9 (12) 0.153 (12) 6.5 (5) 0.47 K-90A" 10 74.5 2.20 (10) 13.2 (18) 0.167 (19) 6.0 (7) 0.50 K-90BC1 15 76.5 1.94 (14) 12.1 (21) 0.160 (22) 6.2 (8) 0.48 averages - 75.4 1.96 12.6 0.156 6.4 0.48 a Equilibrium dialysis results, with ci (maximum concentration of free cosolute) limited by the precipitation of the polymer.See text for the significance of the other symbols. See table 1 for characteristics of the polymer samples used. ' An uncharacterised sample of PVP (molecular weight estimated to be between 2 x lo4 and 6 x lo4) studied by PackteP gave: K = 11 dm3 m o P , n = 0.2 (1); however, it is not clear whether the isotherm was hyperbolic for this particular system. A sample of K-30 studied by Horn and Ditter gave Ki = 2.1 (2) dm3 mo1-I ( K and n were not evaluated).16 The previously studied sample of PVP K-90 gave Ki = 1.4 dm3 mol-l, K = 14 (1) dm3 mol-', n = 0.10 (3), u = 10 (3).5 is shown in fig.1, which also illustrates the concordance between the e.d. and the C.S. results generally observed in this work. The derived values of the binding parameters K and n are listed in table 3 for PhOH with the various grades of PVP, and in table 4 for all cosolutes with K-90A or K-90B. The final column of each table contains the derived values of the 8, the maximum fractional occupancy attained experimentally, calculated from the definition of fractional occupancy 8: 8 = r / n (8) using the value of r at the maximum experimental free cosolute concentration, and with the site-density obtained for the u.s.b.model. It should be evident that the larger the value of 8, the greater is our confidence that the u.s.b. model does apply to the polymer/cosolute binding system; for these eight cosolutes the average value of 8 is 0.48( 12). Columns three of tables 2 and 3 list the values of the initial binding constant, Ki, which is the initial slope of the binding isotherm, and thus the average binding strength of a302 Binding of Phenols by Poly(vinylpyrro1idone) cosolute molecule by a single monomer unit on an unoccupied chain. In the u.s.b. model it is equal to the product Kn, for in the limit as a --+ 0 the isotherm (3) reduces to r = aKn = aKi. (9) The values of Ki are significant for several reasons. In the first place, they are more precisely defined than the component values K and n.Secondly, and more fundamentally, it is possible to take the sceptical chemist’s viewpoint that the apparent good fit of the binding data to the u.s.b. eqn (3) is fortuitous, with the fall-off in slope beyond the initial linear section being the result of effects other than (or in addkion to) the simple occupation of sites of the u.s.b. model. Such effects might be a loss of sites arising from folding up of the polymer molecule (see the following Part5,), or interactions between bound cosolute molecules (see below); moreover, inasmuch as the forces involved in the binding are largely non-localised in kind (see Discussion) then we might expect the more appropriate form of isotherm to be that for non-localised (mobile) binding, which nevertheless approximates to the hyperbolic form over quite a wide range of degrees of occ~pancy.~~ Indeed, the simple linear form (9) is preferred by certain workers,13 who view the binding as more akin to a partition of the cosolute between the domain of the polymer molecule and the bulk of the solution.Other workersl59l6 prefer also to work in terms of Ki alone, mainly because the site-density parameter n is difficult to define precisely for some systems. However, note that the limiting linear form (9) will be obtained if a 4 1/K, i.e. if through choice or necessity the maximum free cosolute concentration is not made sufficiently high; this applies to the chromatographic method used by this second group,15? l6 where measurements must be limited to the initial part of the binding isotherm to keep the chromatographic peaks symmetrical.We now consider the treatment of the results for the five cosolutes (PhOMe, HOPhNO,, PhOGl, HOPhOMe and MeOPhOMe) that apparently do not conform to the u.s.b. model. 12%) up to the solubility value. This is still consistent with the u.s.b. model, where because of the cosolute’s low solubility (18 mmol dm-3) we are limited to the initial linear section of the-binding isotherm and to the initial binding constant Ki [eqn (9)]. (ii) HOPhNO, : Here, as fig. 2 shows, the binding isotherm is curved upwards, rather than downwards as the u.s.b. model requires. We attribute this to a cooperative effect resulting from hydrogen bonding between adjacent bound cosolute molecules : (i) PhOMe: Here the binding isotherm is essentially linear (scatter HOPhNO, * - * HOPhNO,.Hydrogen bonding of this kind was previously proposed by us as the basis for the cooperative effect seen in the binding of the alkyl 4-hydroxybenzoates to PVP.a Cooperative effects have also been seen in the binding of HxPh(OH), (hexylresorcinol), although they were attributed in that case to hydrophobic attractions between the alkyl chains.l59 l6 In the present case we have followed our previous procedurea and fitted the data for HOPhNO, to the Schwarz binding 8( 1 - 8)/( 1 - 28)2 = aKo/( 1 - UUK,)~ (10) where 8 is the fractional occupancy of the sites as defined by eqn (8), the intrinsic binding constant KO is the association constant for the attachment of a cosolute molecule onto a site having unoccupied sites on either side of it [cf. eqn (4)] and the cooperativity parameter a is the factor by which this association constant is raised when one of these neighbouring sites is occupied by a bound cosolute molecule.This was tested as beforeaP. Molyneux and S. Vekavakayanondha ~ 0.02 303 0 4 8 12 16 free cosolute concentration, almmol dm-3 Fig. 2. Binding isotherm for 4-nitrophenol (HOPhNO,) with 10 g dm-3 PVP K-90 at 25 “C (pH < 5). The full line is the cooperative site-binding isotherm [eqn (lo)] with site density = 0.10 (assumed value), intrinsic binding constant KO = 1 1.2 (10) dm3 mol-1 and cooperativity parameter a = 2.7. The side-shaded vertical line represents the precipitation point (c* = 17 mmol dmd3) for this system. by calculating the 8 values from Y assuming the site-density value n = 0.10 (3),t and then applying the Schwarz isotherm (10) in the rearranged form (1 - 20) [a/8( 1 - 0)]1/2 = K;II2 - aaK;I2 so that a plot of the function of 8 and a on the left-hand side against a should be a straight line of ordinate intercept Kill2 and of slope - This procedure gave an essentially linear test plot, leading to the parameters: KO = 11.2(10) dm3 mol-l and a = 2.7(4); the goodness of fit to the experimental data is shown in fig.2. (iii) PhOGl: Here, as already noted in the Experimental Section, the e.d. method did not give satisfactory results, and only the C.S. method could be used. In this case the solubility was reduced by 4% compared with that in the absence of the polymer, indicative of a net repulsion between the cosolute molecule and the polymer chain leading to its exclusion from the binding system.$ (iv) HOPhOMe: As with PhOMe, the binding isotherm is linear (scatter 8%) up to the maximum free cosolute concentration (limited by precipitation of the polymer at c* = 69 mmol dm-3), but here this corresponded to binding ratio Y = 0.096, which is t This is the average value obtained previously for a range of aromatic C O S O ~ U ~ ~ S , ~ and is also consistent with the average value of 0.15(9) from the present work for the six single-ring derivatives: PhOH, PhOEOH, HOPhOH, HOEOPhOEOH, HOPhOGl and Ph(OEOH), (tables 3 and 4).$ This behaviour may be compared with the e.d. results obtained by Eliassaf et a1.@ for ammonium sulphate with PVP, where the total cosolute concentration was lower in the ‘binding’ system than in the polymer-free phase.A certain degree of exclusion is indeed to be expected for any binding system because of the finite volume of the polymer chains, although the fact that the same workerP found there to be no change in the solubility of the (presumably non-bound) aliphatic amino acids leucine and aspartic acid in the presence of P W (see footnote, p. 299) suggests that any ‘excluded volume’ effect is negligible with small-molecule solutes. To interpret such ‘negative’ binding, the equilibria would have to be considered in terms of the ‘degree of sociation ’ (representing the deviation from a random distribution of the molecules) proposed by Guggenheim,Eo and correspondingly as an expansion in cosolute activity in the manner developed by Wood and coworkers.E1w 0 P Table 4.Binding parameters for the cosolutes with 10 g dm-3 PVP K-90 at 25 "Ca serial letter cosoluteb ii/mmol dm-3c Ki/dm3 mol-1 K/dm3 mol-l n U pd A B C D E F G H I J K L M PhOHe PhOMd PhOEOHf9 g HOPhN0,j PhOGlz HOPhOHi HOPhOMdi ' MeOPhOMef HOEOPhOEOHf HOPhOGlf Ph(OEOH),fy' PhPhOHft HOPhPhOHf? 75.4e 1 8f 21of 14i 62l 4 1 j 56i 6W 179 1oo-f 6.W 0.25 0.44 1.96 (17)e 0.17 (2)" 0.94 (3) 1.12 (10)" m - 3.2 (2)n 1.73 (1 3)P 0.95 (15)q 0.91 (1) 1.6 (1) 1.5 (1) 14.4 (7) 38 (1) 12.6 (6)'? 1.7 (2)" 3.0 (3) 11.2 (10)'E m - 26 (4)n -P 9.5 (15)g 11 (2) 10 (2) 16 (2) 1150 (50) 2500 (20) 0.156 0.10 (3y 0.32 (3) 0.10 (3)i 0.12 (1)n 0.10 (3)i - m -P 0.08 (1) 0.16 (2) 0.09 (1) 0.012 (5) 0.015 (1) 6.4 (4)'? 3.1 (3) 10 (3)i 10 (3)i m - 8 -P 10 (3)i 12 (2) 6.2 (8) 11 (1) 80 (30) 65 (4) 0.48e 0.03h 0.38 0.18k - m bJ 52 0.50n % G % 2 -P 0.064 0.39 0.63 0.62 0.28 ;;;I 0.53 $ (0 x 0 ~~ a Results from PVP batches K-90A and K-90B not differentiated.See table 2 for the cosolute names; the techniques used [equilibrium dialysis (e.d.) and/or cosolute solubility (c.s.)] are specified in the footnotes. Maximum free cosolute concentration used, limited either by cosolute solubility or polymer precipitation, as specified in the footnotes. Maximum degree of saturation of sites, assuming the uniform site binding model [eqn (3)] and the derived binding parameters, unless otherwise indicated. E.d. only: mean values for all PVP samples from table 3. f E.d. and c.s.: B limited by the cosolute solubility.Q With the small-molecule analogues NMP (111) and SA (IV) in place of PVP, this cosolute showed no change in solubility, i.e. i d 0.003. " Isotherm essentially linear up to B (giving i = 0.003); K and 4 calculated assuming n = 0.10; see footnote (i). Average value for aromatic cosolutes from previous e.d. ~tudies.~ j E.d. only; ci limited by polymer precipitation. " Binding isotherm curved upwards [see fig. (2)]; data fitted to simple cooperative site binding isotherm eqn (10) assuming n = 0.1 [footnote (i)], giving K as the intrinsic binding constant KO and with cooperativity parameter a = 2.7 (4) (see Results section). Cosolute solubility reduced by 4% in presence of the polymer; see text. Literature data: K-90 [see table 1, footnote (d)],30 "C, Ki = 3.8 dm3 mol-l, K = 38 dm3 mol-l, n = 0.10 (3), u = 10 (3);5 cross-linked PVP, 25 "C, Ki = 5.9 dm3 mol-l, K = 14 dm3 mol-l, n = 0.42, u = 2.4;39 cross-linked PVP, 25"C, Ki 8.0 dm3 mol-l (sorption) or 8.3 dm3 mol-1 (chromatography) ( K and n not evaluated).16 With the small-molecule analogue NMP (111) in place of PVP, the C.S.method gave a 10% reduction in solubility with this cosolute; with SA (IV) there was a small increase corresponding to r = 0.003. P Binding isotherm linear (scatter +8%) up to r = 0.097; see text for discussion. Q Binding isotherm linear (scatterf 16%) up to r = 0.006; K and 4 calculated assuming n = 0.10; see footnote (i). ' Literature data for the parent compound, Ph(OH), (resorcinol): K-90 [see table 1, footnote (d)],25 "C (interpolated), Ki = 2.9 dm3 mol-l, K = 29 dm3 mol-l, n = 0.10 (3), u = 10 (3);5 cross-linked PVP, 25 "C, Ki = 13.1 dm3 mol-1 (sorption) or 14.0 cm3 mol-l (chromatography) (n and K not evaluated).16 See Discussion for the binding of the parent compound, PhPh (biphenyl), evaluated from the solubility measurements of Okubo and Ise." 2 3.& 3 $ 3 C.S. only; see text.P . Molyneux and S . Vekauakayanondha 305 around the level for saturation of the sites by simple aromatic compounds observed both previously5 and in this work (tables 3 and 4). Even if the value of n were several times this value, there would still be expected to be some curvature of the isotherm; possibly there is a cooperative effect occurring as with HOPhNO, (see above) which would flatten out the isotherm, although it is difficult to put this on a quantitative basis.Never- theless, the slope of the line will still be the initial binding constant Ki, although again this is the limit of the information obtainable. (v) MeOPhOMe: Once again, as with PhOMe and HOPhOMe, the isotherm is linear (scatter f 8 % ) up to the maximum level attainable experimentally. As with PhOMe, we presume the u.s.b. model applies but that the isotherm is confined to its initial linear section because of the low cosolute solubility (6 mmol dm-3), so that only Ki can be evaluated. Monomer-unit Analogues The C.S. results obtained for HOPhOMe and PhOEOH using the monomer-unit analogues N-methylpyrrolidone [NMP, (111)] and sarcosine anhydride [SA, (IV)] at 10 g dm-3 (i.e.ca. 0.1 mol dmP3) in place of PVP were as follows. With HOPhOMe, NMP gave an 11% reduction in solubility; this possibly indicates the formation of a less soluble solid-state complex between the two,2 although naturally this cannot be used as evidence for (or against) the complexing of the two in aqueous solution. With this same cosolute, SA caused only a 0.6% increase in solubility, equivalent to a binding ratio r = 0.003. These results may be contrasted with the binding ratio Y = 0.60(5) expected for this cosolute with PVP using Ki = 1.73(13) dm3 mol-1 (table 4) and assuming the linear isotherm to continue up to the solubility (350 mmol dmP3). With PhOEOH as cosolute, neither NMP nor SA gave any detectable change in solubility, which corresponds to essentially the same upper limit of 0.003 for the value of r.This again may be contrasted with the binding ratio r = 0.115 obtained for this cosolute with PVP by the C.S. method (fig. 1). These results indicate that it is necessary for the monomer units to be linked in a chain for appreciable binding to occur. Discussion In interpreting binding parameters, one important goal is to derive correlations with the molecular structures of the cosolutes so that we can arrive at a better understanding of the interaction forces involved, and also so that we can predict the binding behaviour of related systems. However, for predictive purposes it would be equally useful if correlations of a more indirect type could be established, i.e. with such extensively measured physical properties as the U.V.absorption parameters A, and E , the octanol/water partition coefficient P and the acid dissociation constant K, (table 2). Both types of correlation are explored in this Discussion.? Types of Interaction Force For uniform site-binding [eqn (3)] the standard free-energy change of binding, AGg, will be related as usual to the binding constant K by AGF = -RTln K. (12) 7 Parallel studies on the binding behaviour of these same cosolutes with other water-soluble polymers can give information on the relation between binding and the structure of the polymer.* In this connection, analogies have been drawn39 between the PVP/phenol interactions and the sorption of phenols by solid polyamides such as poly(caprolactam).82 There will indeed be certain basic parallels between the types of forces involved; however, unlike the situation with PVP, in the polyamides the nitrogen atom in the amide group is unsubstituted and hence able to act as a hydrogen-bond donor, while the environment involved is presumably the amorphous regions of the solid polymer, which will call for a quite different combination of forces from those in the present wholly aqueous systems.306 Binding of Phenols by Poly(uinylpyrro1idone) polymer bound chain cosolute units molecule I HCH ,---I- ------ ! P-CH H I / / types of interaction interacting groups on polymer cosolute hydrophobic methylene and methyl effects methine groups group dipole/dipole amide group phenolic forces in P dipole induction amide group benzene forces i n P ring and/or hydrophobic methylene and benzene effects methine groups ring hydrogen carbonyl phenolic bonding group in P hydroxy group HCH I Fig.3. Schematic representation of non-covalent interactions in the binding of a typical phenolic cosolute, 4-methoxyphenol (HOPhOMe) by poly(vinylpyrro1idone) where P represents the pyrrolidone ring. See text for discussion. This quantity will be the sum of enthalpy and entropy contributions from four main types of interaction, as shown schematically in fig. 3, where HOPhOMe is used as a ‘typical’ cosolute. This representation is of course highly simplified. For example, it necessarily omits the neighbouring and surrounding water molecules, whose presence profoundly affects all the interactions listed, as discussed below. Furthermore, it is likely that the polymer chain is folded around the cosolute molecule, so that several monomer units can ‘chelate’ onto this same cosolute molecule to increase the number of contributions from these interactions, although there will at the same time be an entropy penalty from the loss of conformational freedom of the section of chain involved.Fig. 3 also shows only one cosolute molecule, but cosolute/cosolute interactions may also be significant. Taking place between neighbouring bound molecules, they seem to be the basis of the cooperative effects seen previously with the alkyl 4-hydroxybenzoate~~ and hexylres~rcinol,~~~ l6 and in the present work with HOPhNO, and possibly also HOPhOMe. Between cosolute molecules attached to different sections of polymer chain, they are the ‘cross-links’ that we propose as the basis for the precipitation and viscosity effects seen with the phenols, although as shown in the following Part5, these involve only a small fraction of the bound cosolute molecules. These four types of interaction forces are now considered briefly in relation to the present systems.(i) Hydrophobic effects.46* 47$ 83 These are specifically aqueous-solution interactions, which in the present case will involve the aromatic ring and the methyl and methylene groups on the cosolute molecule and the methine and methylene groups on the PVP chain. The importance of these effects in polymer/cosolute binding is indicated by the increase in binding strength with increase in the size of the non-polar part of the cosolute molecule, which is seen in general with synthetic polymer^,^ and more specifically for PVP with simple aromatic cos~lutes,~ bolaform aliphatic cosolutesg and the alkyl 4-hydroxy-P .Molyneux and S. Vekavakayanondha 307 benzoates.* However, in the present case of PVP with aromatic cosolutes this effect is difficult to disentangle from dipole/induced-dipole forces [see (ii) below]. One common method of estimating the hydrophobic character of a molecule is from the octanol/water partition coefficient (table 2 ) . 5 7 9 58 However, hydrophilic groups will reduce the value of this coefficient without necessarily interfering directly with the hydrophobic hydration around the non-polar group which is involved in the hydrophobic effects.? (ii) Dipole/induced-dipole (van der Waals-Debye) force^.^^-*^ These will occur between the highly dipolar amide group on the PVP monomer unit (I) and the highly polarisable aromatic group on the cosolute molecule.The importance of this effect is shown by the fact that PVP in general binds aromatic compounds more strongly than it does the analogous aliphatic ones.4 Furthermore, PVP produces greater increases in the solubilities of the aromatic hydrocarbons biphenyl (PhPh) and naphthalene than other water-soluble polymers [poly(ethylene glycol), poly(acrylamide), poly(viny1 alcohol) etc.1” - whereas on this same criterion of solubility increase, its binding strength towards alkyl bromides (ethyl to butyl) is less even than that of the highly hydrophilic polymer poly(acrylamide).8s However, it is difficult to make a confident estimate of the actual strength of these forces, since as with other van der Waals forces the energy is inversely proportional to the sixth power of the internuclear distance, so that they depend very critically on the relative orientation and separation of the interacting groups.At the same time there will be an important but uncertain extent of shielding by the neighbouring water molecules. These will involve the amide group on the monomer unit (I) and the phenolic dipole. Although they represent another type of van der Waals force that must be expected to contribute to the binding strength, for the same reasons as with dipole/induced-dipole forces [see (ii) above] it is difficult to make a confident estimate of their actual magnitude.(iv) Hydrogen bonds.8s Bonding of this type will be expected to occur between hydroxy groups on the cosolute molecule and the oxygen atom on the monomer unit (I). (The nitrogen atom is probably prevented from acting as a hydrogen-bond acceptor by steric hindrance and its fractional positive charge.) However, the situation is complicated by the fact that these locations will already have water molecules hydrogen- bonded to them in aqueous solution, while when they are released these water molecules bond again to others in the bulk solvent. The net result is a competition between four different types of hydrogen bond, with equilibria being set up (involving the exchange of hydrogen bonds) of the type (iii) Dipole/dipole (van der Waals-Keesom) ROH - * * OH, + HOH - - - O=CR’R’’e ROH - - - O=CR’R + HOH ..* OH, (13) where ROH is the cosolute and R’RC=O represents the PVP carbonyl group.The balance of such equilibria is not necessarily in favour of cosolute-polymer bonding. Furthermore, although it is plausible to assume that the strength of this bond will increase with increase in acidity of the ROH group, by the same token there would be expected to be a parallel increase in the strength of the cosolute-water bond, so that it is difficult to judge the final effect on the position of the equilibrium (13). Although all these four types of interaction are expected to occur, in each case their t Some support for the significance of hydrophobic effects in the present systems is obtained from . measurements of the solubility of HOPhOH in aqueous solutions of salts, carried out more than 50 years ago by Linderstr~m-Lang.~~ In his work, with ammonium chloride and other inorganic halides there was the usual ‘ salting-out ’, i.e.reduction in solubility, but alkylammonium chlorides gave increases which outweighed any salting-out. The effects rose linearly with chain length for the monoalkylammonium chlorides RNH,Cl (R = H, Me, Et and Bu), while proportionately even greater effects were seen with the quaternary ammonium salts Me,EtNCl and Et,NCl. The analogy with the present systems lies in the fact that the nitrogen atom on the PVP monomer unit (I) will have a fractional positive charge through resonance in the amide group, and hence will act in a way as an alkyl-substituted ammonium ion.This feature of PVP is also shown by the fact that it binds anionic cosolutes much more readily than it does the analogous cationic ones.4308 Binding of Phenols by Poly(uinylpyrro1idone) part in aqueous polymer/cosolute binding systems is neither well understood nor predictable with any certainty. Furthermore, although they have been discussed separately as if they were independent and essentially additive, in practice where they are operative in the same system they must be expected to influence one another. For example, even with PhOH there must be expected to be some interference between the hydrophobic hydration around the benzene ring and ‘ hydrophilic hydration’ on the hydroxy group. One of the aims of this Discussion is therefore to see how the present work on related cosolutes can give us a better understanding of how these interactions behave in practice.Site Density and Site Size The other aspect of binding behaviour is the interpretation of the values of the site density n, or equivalently its reciprocal the site size u, for the different cosolutes. In conformity with the earlier less detailed data,5 for the present single-ring cosolutes n has an average value of 0.15(9), equivalent to a site size u of 8(3) monomer units (table 4). On one viewpoint, this may be taken to imply that each site involves ca. eight successive monomer units which ‘envelop’ or ‘chelate’ the cosolute molecule, as already discussed above in connection with the interaction forces. An alternative viewpoint is that the binding requires a particular conformation, or a particular configuration (such as an isotactic or a syndiotactic sequence), of a few monomer units, which on the average occurs this frequently along the polymer chain.1 5 9 l6 as to whether there really are discrete binding sites on the polymer chain. Nevertheless, their existence is supported by the fact that, as a recent listing for aqueous synthetic polymer/cosolute systems with a number of understandable exceptions (such as micelle-forming cosolutes) the binding isotherms generally conform either to the u.s.b. model, eqn (3), or to a cooperative model such as the Schwarz form, eqn (10); furthermore, the values of the site size u are generally quite large (in line with the present results), although there does not seem to be any direct correlation with the size of the cosolute molecule itself.However, doubts have been expressed recently by several groups of Effect of Polymer Molecular Weight on the Binding of PhOH The data summarized in table 3 for PhOH with the four different batches of PVP show that the binding parameters are essentially independent of the molecular weight of the polymer, so that the binding behaviour is characteristic only of the sequence of chain units. This therefore gives us confidence in comparing the data obtained for other cosolutes with the two batches of K-90, which will therefore not be differentiated in this paper, as well as in comparing the present work with that in the literature for other grades of PVP, such as clinical materials that have lower molecular weights and narrower molecular-weight distributions than these commercial grades of PVP.Binding Behaviour of Specific Cosolutes We now consider the binding parameters for individual cosolutes and groups of cosolutes with 10 gdmP3 PVP K-90 (table 4) in the light of the previous discussion of the interaction forces in these systems. For this purpose, fig. 4 is a display plot of log Kagainst log u for the binding data from this work, and also from earlier together with that for PhPh (see below). This type of plot is useful in that it shows up graphically any regularities in the binding parameters (thus a change in K by a constant factor results in a fixed displacement in the ordinate value). Also, on this plot the lines of unit slope are loci of fixed values of the initial binding constant, Ki, which is the limit of the information obtainable for some of the present systems, and for some of those reported in the literat~re.~P .Molyneux and S. Vekauakayanondha 309 0.5 1.0 1.5 2.0 2.5 log u Fig. 4. Double-logarithmic display plot of binding constant K against site size u (reciprocal of site density n ) for phenolic and related cosolutes with PVP (grade or M as indicated) at 25 "C. The lines of unit slope represent fixed values (labelled in dm3 mol-I) of the product Kn, i.e. the initial binding constant Ki for those systems where only this value is known; the vertical chain dotted line represents the previously reported average site size u = 10 (3) for simple aromatic cosolute~.~ Open circles, and single hexagons, this work (K-90 - see table 4); filled circles, results from ref.(5) (K-90); double hexagon, CS results for PhPh (biphenyl) from ref. (77) (M? = 2.4 x 104). For each cosolute or group of cosolutes we look first at its state alone in aqueous solution, since this affects the binding behaviour, as revealed by three properties (table 2): the U.V. parameters for its electronic structure, where because these relate to the transition to an excited state, only similarities in spectra can be used to suggest similarities in the actual (i.e. ground state); the pK,, which gives an indication of the hydrogen-bond donating ability (i.e. viewing this as akin to the complete donation to the solvent that takes place in the ionisation of the hydroxy group); and the octanol/water partition coefficient, which in default of more extensive data on the thermodynamics of hydration of these aromatic corn pound^^^ gives us some idea of the hydration state and the hydrophilic/hydrophobic balance of the c o ~ o l u t e .~ ~ ~ 58 PhH, PhOH, HOPhOH, PhPh, PhPhOH and HOPhPhOH We naturally start with the basic quartet, PhOH, HOPhOH, PhPhOH and HOPhPhOH, together with the parent hydrocarbons PhH and PhPh, for which data are available as discussed below. For their states alone in aqueous solution, the marked differences in their U.V. spectral parameters presumably reflect corresponding differences in their (ground-state) electronic structures arising from conjugation effects represented in resonance terms for PhOH by (11). The pK, values are also markedly different: PhOI-I, 10.0; HOPhOH, 10.9 (corresponding to an intrinsic value per hydroxy group of 11.2, but note that the value of 10.2 for HOPhOMe is much closer to that for PhOH); and310 Binding of Phenols by Poly(vinylpyrro1idone) PhPhOH, 9.5 (there are no data for HOPhPhOH).For the partition coefficients (where data are available for all of the cosolutes except HOPhPhOH), hydroxy substitution produces irregular decreases in log P: PhH -+ PhOH, 0.65; PhOH -+ HOPhOH, 0.95; and PhPh .+ PhPhOH, 0.88 [cf. PhOMe -+ HOPhOMe, 0.74 and PhOGl -+ HOPhOGl, 0.64, and also the meta-substitution series: PhH -+ PhOH -+ Ph(OH), --+ 1,3,5-Ph(OH),, where there is an essentially constant fall of 0.65(6) at each stage]. The increase of log P on phenyl substitution is also not consistent: PhH -+ PhPh, 1.98 and PhOH -+ PhPhOH, 1.71.In contrast to the rather irregular behaviour of these characteristic parameters, we find marked regularities in the binding parameters. Considering first the site size u, the values of 6.4(4) for PhOH and 8(1) for HOPhOH are similar, and comparable to the overall average of lO(3) obtained for simple aromatic compounds in earlier work (where, however, no value was obtainable for the simplest such compound, PhH, because it could only be studied by the C.S. rneth~d).~ The values for PhPhOh [80(30)] and HOPhPhOH [65(4)] are also similar, and about the square of that for the single-ring compounds. It therefore appears that there is some particular ‘ phenyl-binding ’ conformation or configuration which occurs at intervals of ca.8 monomer units in a random fashion, so that the adjacent pairs of such species required for the binding of the PhPh derivatives occur at intervals of ca. 64 ( i t . S2) units along the chain. Considering the binding constants, data are also available for the parent hydrocarbons. For PhH, previous work gave K = 3.3 dm3 mol-1 (extrapolated to 25 “C); the limits of error on this value are not well defined because it is a C.S. result depending on an assumed value of 0.1 for the site density.5 For PhPh the binding constant may be estimated from the work of Okubo and Ise” on the effects of water-soluble polymers on the solubility of this cosolute and also naphthalene. They found that with each cosolute, the greatest increase in solubility occurred with PVP.The results were interpreted on the basis of the Setschenow equation In (sp/s) = kp (14) where sp is the aqueous solubility in the presence of polymer at concentration p , and s that in the absence of polymer; for PVP/PhPh at 25 “C, the Setschenow constant k was 5.2 dm3 mol-1 [where p in eqn (14) is in base-molar, that is, moles of monomer units dm-3]. This interpretation presumed that the effects observed are non-specific, but we prefer to regard them as the result of the specific site binding of the cosolute by the polymer, particularly because the overall superiority of PVP parallels its binding behaviour with other aromatic co~olutes.~ Since with PhPh the free cosolute concentration is so low (a = s = 0.0485 mmol dm-3), this means that it is only the initial linear section of the binding isotherm that is involved, where a 6 1/K [cf.eqn (9) and the value of K obtained below]. On this basis it may be readily shown that the Setschenow constant k is equal to the initial binding constant, Ki, i.e. the product Kn. If we choose the average of the site-size values for PhPhOh and HOPhPhOH, i.e. u = 72, as that for PhPh, then this gives K = 374 dm3 mol-1 for this cosolute. Alternatively, if we ‘extrapolate’ the trend with these derivatives to give u = 95, then we obtain K = 494 dm3 mo1-l. Taking the average of these two estimates gives K = 434(60) dm3 mol-l. Correlating the values of the binding constant for these six cosolutes, on the group contribution approach we would expect it to change by a constant factor on the introduction of a particular group into the cosolute molecule, corresponding to a constant additive contribution to the standard free-energy change of binding [eqn (1 2)].For hydroxy substitution, the K ratios are: PhH -+ PhOH -+ HOHPhOH, 3.8 and 2.1 and PhPh -+ PhPhOH -+ HOPhPhOH, 2.6 and 3.2. For phenyl-group substitution the ratios are: PhH -+ PhPh, 132; PhOH -+ PhPhOH, 91 and HOPhOH -+ HOPhPhOH, 96. In view, particularly, of the uncertainties in the values for PhH and PhPh, these ratiosP. Molyneux and S. Vekauakayanondha 31 1 may be taken to be essentially the same in each case. These correlations are considered again after the discussion of the other cosolutes. HOPhNO, With this cosolute alone in aqueous solution, there will be strong interactions between the ring and the two substituent groups, which may be represented in resonance terms by (V).This enhances the fractional positive charge on the oxygen atom compared with OH 6l-i PhOH, and the fractional negative charge on the oxygen atoms of the nitro group compared with PhNO,. As table 2 shows, this results in the U.V. spectrum being markedly different from these parent compounds, and its pK, (7.15) lower than that of PhOH (10.0). To get some idea of the effect of this on the hydration of the molecule, we can use the octanol/water partition coefficient, P, for this and other nitro-compounds; examination of the tabulated data57p58 reveals the following changes in the value of log P on nitro-substitution: (i) aliphatic compounds, - 1.5 units (i.e.more hydrophilic, as expected); (ii) non-phenolic aromatic compounds, - 0.32 (i.e. less of a hydrophilic effect than with aliphatic compounds) and (iii) phenols, 0.4(1) (i.e. the substitution makes it more hydrophobic). This anomalous behaviour with nitrophenols is evidently not a through-ring effect,89 since the 3- and 4-isomers of HOPhNO, have essentially the same value of P, whereas (for example) their pK, values are different (8.38 and 7.15). Possibly it arises from the two types of group interfering synergistically with the hydrophobic hydration around the benzene ring. Considering the binding behaviour, we have interpreted the upward curvature in the binding isotherm (fig. 2) as arising from a cooperative effect involving hydrogen bonding between adjacent bound cosolute molecules (see Results section and table 4).Nevertheless, the intrinsic binding constant, KO, is the exact equivalent of the binding constant, K, for systems not showing cooperative effects. However, the value of 1 1.1( 1 .O) dm3 mol-1 has been obtained assuming a site density of 0.1, since the cooperative effect and the precipitation of the polymer (at c* = 17 mmol dm-3) prevent this being estimated directly. Using group-contribution theory and the K values for PhH, PhOH and PhNO, (table 4), we would expect a value of ca. 305 dm3 mol-1 for HOPhNO,, which is ca. 27 times the experimental value. This ‘neutralising’ effect of the OH and NO, groups in binding seems to parallel that seen in the partition behaviour discussed above. Looking at the cooperative effect, the value of the cooperativity parameter cc is 2.7(4) (table 4), which is smaller than the value of 9.6(12) obtained for the alkyl 4- hydroxybenzoates HOPhC0,R (R = Me, Et, Pr and B U ) .~ This is surprising, since the postulated hydrogen t xding might be expected to be stronger with HOPhNO,, both because it is a stronger acid (pK, = 7.15) than HOPhC0,R (pK, = 8.35) and also because the accepting oxygen atoms would be expected to have higher fractional negative charges. However, all these interactions are by definition between cosolute molecules bound to the polymer chain, and they must therefore be modified both by this involvement in binding and by the proximity of the polymer chain itself.312 Binding of Phenols by Poly(vinylpyrro1idone) PhOMe, HOPhOMe and MeOPhOMe For these methoxy compounds alone in aqueous solution, the U.V.parameters are in each case similar to those of the parent phenols, indicating that (as anticipated) the @substitution by the Me group does not seriously affect the electronic structure of the phenoxy or phenylendioxy group. The pK, of HOPhOMe (10.2) might be expected to be close to the intrinsic value for HOPhOH ( i e . 1 1.2, when we bring in the statistical-factor contribution of 0.3 for the two groups), but it is in fact much closer to that of PhOH (10.0); if this value is correct, it suggests that HOPhOMe should have a hydrogen-bond donating ability as high as that of PhOH. From octanollwater partition-coefficient data, the effects of 0-methyl, methoxy- or hydroxy-substitution all indicate that the 0-methyl group has a consistent effect on the hydration of these and other such solutes.Despite these promising regularities, the binding behaviour is less simple. In the first place, as already indicated in the Results section we are limited to a discussion of the Ki value in each case because the isotherms are linear over the observable span; this parameter is not necessarily correlated with molecular structure because it is the product of the two separate parameters K and n [eqn (9)]. Whether we compare these cosolutes with others on the basis of methylation of the phenolic hydroxy group, or of methoxy substitution on the ring, no clearcut regularities in the Ki values are apparent. PhOGl and HOPhOGl With these glucosides alone in aqueous solution, the evidence from the physical characteristics indicates little interaction, either direct (through-bond) or indirect (through-space), between the benzene ring and the glucose ring.The absence of direct interactions is shown by the fact that the U.V. spectra are similar to those of the parent phenols and other @substituted derivatives, although with PhOGl there is more ‘fine structure’ (i.e. resolved secondary peaks in the main band) than with the other compounds. The similarity in U.V. spectra also suggests that the pK, for the phenolic hydroxy group in HOPhOGl should be much the same as the intrinsic value for HOPhOH, and that these hydroxy groups should have much the same hydrogen-bond reactivity towards the PVP chain. Indirect interactions would involve interference between the hydrophobic hydration around the benzene ring and the hydrophilic hydration around the glucose ring.The evidence for their absence comes from octanol- water partition coefficient data, where for a range of substituted phenyl glucosides there is an essentially constant difference between log P for ROGlS0 and the value for the parent phenol ROH.577 58 Looking at the binding data, it is notable that the binding behaviour of HOPhOGl is quite similar to that of PhOH, both as regards Ki [1.6(1) us. 1.96(17) dm3 mol-l], u [6.2(8) us. 6.4(4)] and K [10(2) us. 12.6(6) dm3 mol-l]. It is evident that the introduction of the glucose ring has a marked inhibitory effect on the binding, since the substitution PhOH --+ PhOGl abolishes binding and indeed gives rise to ‘negative’ binding, while for HOPhOH + HOPhOGl the value of K is reduced by a factor of 2.6(10) and that of Ki by 2.0(3).Since we have already shown that the component groups behave essentially independently, it therefore appears that effectively there is a net repulsion between the glucose ring and the PVP chain in aqueous solution. PhOEOH, HOEOPhOEOH and Ph(OEOH), For these three hydroxyethyl compounds alone in aqueous solution, the fact that the U.V. spectral parameters are similar to those of the parent phenols and the other 0-substituted derivatives (table 2) shows that (as anticipated) the hydroxyethyl group does not greatly affect the electronic structure of the phenoxy or phenylenedioxy part of the molecule.The absence of any marked interference between the hydration aroundP . Molyneux and S. Vekavakayanondha 313 the benzene ring and that on the hydroxy group is indicated by the octanol/water partition coefficient data, where the difference between log P for PhOEOH and for PhOCH,CH, (1.35 units) is very close to that for the parallel aliphatic pair CH,CH,OCH,CH,OH and CH,CH,OCH,CH, (1.37 58 The main distinction between these compounds and the parent phenols is the greatly reduced acidity of the hydroxy group, the estimated ‘intrinsic’ pK, value (table 2) for the group being 14.7, although it is still more acidic than water (pK, 15.7). This results, of course, from the insulation of the hydroxy group from the ring by the two intervening methylene groups. This suggests that the hydroxy group is likely to be correspondingly less active in hydrogen bonding than a phenolic hydroxy group.At the same time, there would be expected to be a hydrophobic contribution to binding from the methylene groups, although as already discussed this will be modified by the presence of the adjacent oxygen atoms. Looking at the actual binding behaviour of these three compounds, it is notable that the site size u is only 3.1(3) units for PhOEOH, compared with the more normal values of 12(2) for HOEOPhOEOH and 11( 1) for Ph(OEOH),. It appears that for some reason PhOEOH is not so fastidious as the other cosolutes, with its binding less sensitive to the conformation or the configuration of the sequence of units in the PVP chain. Considering the binding constants, on the group contribution approach their ratio for complete hydroxyethyl substitution for a dihydric compound would be expected to be the square of that for the monohydric compound.Looking at both the Ki and the K values, the ratios are: PhOH --+ PhOEOH, 0.48(6) and 0.24(4); HOPhOH + HOEOPhOEOH, 0.28(2) and 0.42( 14); Ph(OH), -+ Ph(OEOH),, 0.52(7) and 0.55(10) (the binding data for the first of these two cosolutes are taken from an earlier s t ~ d y , ~ so that the comparisons are less certain). It is only with the Ki values for the first two pairs that the expected behaviour is seen, the overall average ratio for monosubstitution being 0.50(4). By contrast, with the K values, on the basis of the behaviour with the dihydric compounds the ratio for the monohydric would be expected to be 0.7(5), compared with the actual value of 0.24(4).In interpreting the binding behaviour of these hydroxyethyl compounds, it is relevant that Bahal and Ko~tenbauder~l were not able to detect any binding with either of the aromatic alcohols PhCH,OH (benzyl alcohol) or PhEOH (2-phenylethanol),t although subsequently Horn and Ditter16 showed there to be rather weak binding for PhCH,OH with cross-linked PVP, the initial binding constant Ki being either 1.3 dm3 mol-l (sorption studies) or 0.3 dm3 mol-1 (column chromatography). The absence (or weakness) of binding of PhEOH, contrasted with the definite binding seen in this work with its analogue PhOEOH, clearly demonstrates that the phenolic oxygen has an activating effect on cosolute binding to PVP.However, it is difficult to quantify this, both because of the apparent failure of the group-contribution approach with these compounds, and also because the presence of the adjacent oxygen atoms means that the methylene group contribution cannot be expected to be the same as that (for example) determined for the alkyl group in the alkyl 4-hydro~ybenzoates.~ Group Contributions to Binding Using the correlations already noted between the values of the binding constant, K , for the six cosolutes: PhH, PhPh, PhOH, HOPhOH,PhPhOH and HOPhPhOH, it is possible to derive a consistent set of free-energy group contributions g which can be used to ‘predict’ the standard free energy of binding [eqn (12)] from the relation (15) AGF = g(core) + Cg(substituents) t It is also notable that these same workers observed definite binding with the chlorinated aliphatic alcohol CCl,CMe,OH (chlorbutanol), with the u.s.b.parameters u = 20 and K = 22 dm3 mol-l (interpolated for 25 “C). This binding activity presumably arises from the sizeable hydrophobic part of the molecule, along with an enhanced acidity of the hydroxy group from the neighbouring chlorine atoms.314 Binding of Phenols by Poly(vinylpyrro1idone) Table 5. Group contributions, g (kJmol-l), to the standard free-energy change, AGF, for the binding of p-substituted aromatic compounds by poly(vinylpyrro1idone) in aqueous solution [eqn (1 2) and (1 5)J" Phb - 3.3, Ph" - 11.2, Hod - 2.9, CH,f - 1.00 (16) Hoe - 1.8, a Results from this work, at 25 "C, unless otherwise indicated. The tabulated values of g also apply to certain cosolutes studied in earlier work at 30 "C, [ref.(5)], as indicated in the footnotes. However, it seems that the group-contribution method does not apply to certain substituent groups in this work (MeO, HOEO, GLO and NO,; see Discussion) or in earlier work 1e.g. CO,H, CO;; ref. (5)]. Benzene [30 "C; ref. ( 5 ) ] , i.e. with g(H) = 0 by convention, and for the first ('core') phenyl group. Second phenyl group (i.e. for PhH -P PhPh, PhOH + PhPhOH and HOPhOH -, HOPhPhOH); but note that for PhCO; -P PhPhCO; [30 "C; ref. (5)], g = -2.1 kJ mol-l. First phenolic hydroxy group, i.e. for PhH + PhOH only. Phenolic hydroxy groups other than in footnote (d), para to: HO (PhOH-HOPhOH); Ph (PhPh+PhPhOH and PhPhOH-, HOPhPhOH); CO,H [PhCO,H -P HOPhC0,H; 30 "C, ref.( 5 ) ] ; CO, [PhCO, + HOPhCO;; 30 "C, ref. (5)]. f For the alkyl p-hydroxybenzoates, HOPhCO,[CH,],H, with rn = 1-4 [ref. (S)]; however, this does not apply to the CH, group in the aliphatic bolaform cos~lutes.~ where 'core' refers to the benzene ring; since these contributions relate to the substitution of a hydrogen atom by the group involved, then on this basis g is zero for the hydrogen atom. The values ofg listed in table 5 allow us to 'predict' Kto within 1 or 2% for PhOH, HOPhOH, PhPhOH and HOPhPhOH, and within 14 or 15% for PhH and PhPh, which is satisfactory agreement in the light of the experimental uncertainties. As the footnotes to table 5 indicate, these values also apply reasonably well for the groups listed with some (but not all) cosolutes to data obtained in earlier w ~ r k .~ ~ ~ However, it should be clear from the previous discussion that we have not been able to verify the group contribution approach for either the methyl, hydroxethyl, glucosyl or nitro groups in the present work. Conclusion The present work has given us the quantitative information on the binding characteristics of the present thirteen cosolutes with PVP (table 4) which is necessary for the interpretation of the polymer precipitation and viscosity behaviour, as discussed in the next Part.52 The absence of any well defined complex formation by the monomer-unit analogues N-methylpyrrolidone (111) or sarcosine anhydride (IV) shows that binding by PVP is associated with the chain structure, which is confirmed by the insensitivity of the binding parameters for phenol to the molecular weight of the polymer.At the same time it has been possible to derive group contributions to the binding strength (table 5) to predict the binding strength for some of these, and related, cosolutes. The well defined binding observed with the hydroxyethyl compounds shows that a phenolic hydrogen atom is not necessary for binding to occur, although the phenolic oxygen does seem to enhance the binding strength. The binding behaviour of the glucosides reveals a net repulsion between the glucose ring and the PVP chain in aqueous solution. The binding of HOPhNO, (and possibly also HOPhOMe) provides a further example of cooperative effects in these systems. Finally, the data obtained for these systems should be applicableP.Molyneux and S. Vekavakayanondha 315 to the quantitative interpretation of the non-covalent interactions for aqueous polymer/ cosolute binding in general, including that in biological systems. We thank the Department of Pharmacy, Chelsea College, University of London, for providing facilities for this work. a B A A-S b C" E e.d. G1 AGF Hx K Ki KO n NMP P Ph PVP r i SP S SA u.s.b. a 0 0 c C.S. S U N Appendix : Abbreviations and Symbols concentration of free cosolute maximum value of a attained experimentally cosolute molecule binding site occupied by a non-covalently bound cosolute molecule concentration of bound cosolute total cosolute concentration value of c for incipient precipitation of the polymer cosolute solubility method ethylidene (CH,CH,) equilibrium dialysis method 1 -deoxy, 1 -P-D-glucopyranosyl (' glucosyl ') standard free-energy change of binding; eqn (12) hexyl binding constant for the u.s.b. model; eqn (3) initial binding constant; eqn (9) intrinsic binding constant for cooperative site-binding ; eqn (10) limiting value of r (site density in the u.s.b.model); eqn (3) N-methylpyrrolidone ; structure (111) polymer Concentration? phenyl, phenylene (1,4-substitution in XPhY, 1,3-~ubstitution in PhXY) or higher substitution as indicated poly(vinylpyrro1idone); monomer-unit structure (I) binding ratio, i.e. average number of cosolute molecules bound per monomer unit; eqn (2) maximum value of r attained experimentally aqueous solubility of the cosolute value of s in the presence of polymer binding site on the polymer chain sarcosine anhydride; structure (IV) reciprocal of n (site size on the u.s.b.model) uniform site-binding model ; eqn (3) cooperativity parameter; eqn (10) fractional occupancy of sites; eqn (8) maximum value of 0 attained experimentally non-covalent interactions References 1 P. Molyneux, in Chemistry and Technology of Water-Soluble Polymers, ed. C. A. Finch (Plenum, 2 P. Molyneux, Water-Soluble Synthetic Polymers: Properties and Behavior (CRC Press, Boca Raton, New York, 1983), p. 1. FL, 1983), vol. 1. -f For general descriptions of experimental conditions it is convenient to use a mass-in-volume basis for p (e.g. g dmP3), but for equilibrium calculations [as in eqn (2) and (14)] it must be put on a monomer-unit- in-volume basis, i.e.base mole (moles of monomer units) dm-3.316 Binding of Phenols by Poly(vinylpyrro1idone) 3 P. Molyneux, Water-Soluble Synthetic Polymers: Properties and Behavior (CRC Press, Boca Raton, 4 P. Molyneux, Water-Soluble Synthetic Polymers : Properties and Behavior (CRC Press, Boca Raton, 5 P. Molyneux and H. P. Frank, J. Am. Chem. SOC., 1961, 83, 3169. 6 P. Molyneux and H. P. Frank, J . Am. Chem. Soc., 1961, 83, 3175. 7 P. Molyneux and H. P. Frank, J . Am. Chem. Soc., 1964, 86, 4753. 8 P. Molyneux and M. Cornarakis-Lentzos, Colloid Polym. Sci., 1979, 257, 855. 9 P. Molyneux and G. S. Ahmed, Kolloid-Z. Z. Polym., 1973, 251, 310. FL, 1984), vol. 11. FL, 1984), vol. IT, chap. 2. 10 P. Blecher, D. H. Lorenz, H. L.Lowd, A. S. Wood and D. P. Wyman, in Handbook of Water-Soluble Gums and Resins, ed. R. L. Davidson (McGraw-Hill Book Co., New York, 1980), chap. 21. 11 Proc. Znt. Symp. Povidone, University of Kentucky, Lexington, KY, 1983, ed. G. A. Digenis and J. Ansell. 12 P. Molyneux, in Proc. Int. Symp. Povidone, University of Kentucky, Lexington, KY, 1983, ed. G. A. Digenis and J. Ansell, p. 1. 13 J. A. Plaizier-Vercammen and R. E. De Neve, J . Pharm. Sci., 1980, 69, 1403; 1981,70, 1252; 1982,71, 552. R. E. De Neve and J. A. Plaizier-Vercammen, in, Proc. Znt. Symp. Povidone, University of Kentucky, Lexington, KY, 1983, ed. G. A. Digenis and J. Ansell, p. 39. 14 R. L. Reeves, S. A. Harkaway and A. R. Sochor, J . Polym. Sci., Polym. Chem. Ed., 1981, 19, 2427; T. Takagishi, T.Sugimoto, H. Hamano, Y-J. Lim, N. Kuroki and H. Kozuka, J . Polym. Sci., Polym. Chem. Ed., 1984, 22, 4035, and earlier papers. 15 K. H. Fromming, W. Ditter and D. Horn, J . Pharm. Sci., 1981,70, 738. 16 D. Horn and W. Ditter, J. Pharm. Sci., 1982, 71, 1021. 17 Martindale: The Extra Pharmacopoeia, ed. J. E. F. Reynolds and A. B. Prasad (The Pharmaceutical 18 M. N. Clifford, J. Chromatogr., 1974, 94, 261. 19 L. Olsson and 0. Samuelson, J . Chromatogr., 1974, 93, 189. 20 A. Carpenter, S. Siggia and S. Carter, Anal. Chem., 1976, 48, 225. 21 G. Goldstein, J . Chromatogr., 1976, 129, 61. 22 W. D. Loomis and J. Battaile, Phytochem., 1966, 5, 423. 23 N. R. Siege1 and R. K. Enns, Plant Physiol., 1979, 63, 206. 24 F. Bjorksten, L. Halmepuro, M. Hannuksela and A. Lahti, Allergy, 1980, 35, 671; S.A. Kutz, M. S. Mentnech, S. A. Olenchok and P. C. Major, Chest, 1981,79, 4th suppl., 56s. 25 E. Haslam, Chemistry of Vegetable Tannins (Academic Press, London, 1966). 26 F. Dazzo and D. Hubbell, Plant Soil, 1974,40, 435. 27 D. Guttman and T. Higuchi, J. Am. Pharm. Assoc., Sci. Ed., 1956,45, 659. 28 B. N. Kabadi and E. R. Hammarlund, J. Pharm. Sci., 1966, 55, 1069. 29 B. SCbille and J. NGel, J. Chim. Phys., 1963, 60, 475. 30 H. Sekikawa, R. Hori, T. Arita, K. Ito and M. Nakano, Chem. Pharm. Bull., 1978,26, 2489. 31 C. A. J. von Frijtag Drabbe and J. G. Reinhold, J. Lab. Clin. Med., 1952, 40, 616. 32 T. Higuchi and J. L. Lach, J . Am. Pharm. ASSOC., Sci. Ed., 1954, 43, 465. 33 B. N. Kabadi and E. R. Hammarlund, J . Pharm. Sci., 1966,55, 1072.34 E. Azaz and M. Donbrow, J. Colloid Interface Sci., 1976, 57, 11. 35 M. Donbrow and E. Azaz, J . Colloid Interface Sci., 1976, 57, 20. 36 A. D. Marcus, E. Wetstein and M. Ruderman, J . Am. Pharm. ASSOC., Pract. Pharm. Ed., 1956,17,453. 37 Anon., Znd. Eng. Chem., 1969, 52(12), 34A; 37A. 38 J. P. McManus, K. G. Davis, J. E. Beart, S. H. Gaffney, T. H. Lilley and E. Haslam, J. Chem. Soc., 39 P. Bandyopadhyay and F. Rodriguez, Polymer, 1972, 13, 119. 40 W. B. Breuninger and R. W. Goettsch, J. Pharm. Sci., 1965, 54, 1487. 41 Y. A. Attia and J. Rubio, Br. Polym. J., 1975, 7, 135. 42 H. Deuel and J. Solms, Kolloid-Z., 1951, 124, 65; H. Deuel and H. Neukom, in Natural Plant Hydrocolloids, Advances in Chemistry Series No. 1 1 (American Chemical Society, Washington, D.C., 1954), p.51. Press, London, 28th edn, 1982). Perkin Trans. 2, 1985, 1429. 43 B. Jirgensons, Makromol. Chem., 1951, 6, 30. 44 W. D. McFarlane, H. L. Grant and H. R. Held, Proc. Am. SOC. Brewing Chem., 1958, 119. 45 British Pharmacopoeia Commission, British Pharmacopoeia 1980 (H.M.S.O., London, 1980). 46 W. Kauzmann, Adv. Protein Chem., 1959, 14, 1. 47 F. Franks and D. Eagland, CRC Crit. Rev. Biochem., 1975, 3, 165. 48 Pharmaceutical Microbiology, ed. W. B. Hugo and A. D. Russell (Blackwell, Oxford, 2nd edn, 1980). 49 C. Hansch, K. Kiehs and G. L. Lawrence, J . Am. Chem. Soc., 1965,87,5770; F. Helmer, K. Kiehs and 50 S. S. Davis, T. Higuchi and J. H. Rytting, Ado. Pharm. Sci., 1974, 4, 73. 51 B. Y . Okamoto, R. H. Wood and P. T. Thompson, J. Chem. Soc., Faraday Trans. I , 1978, 74, 1990. C . Hansch, Biochemistry, 1968,7, 2858.P. Molyneux and S. Vekavakayanondha 52 P. Molyneux and S. Vekevakayanondha, J. Chem. Soc., Faraday Trans. I , (Part IV), 1986,82, 635. 53 Anon., ‘PVP: Polyvinylpyrrolidone’, Technical Bulletin 2550406 (GAF Corp., New York). 54 Organic Electronic Spectral Data (Wiley-Interscience, New York, 1960-1982), vol. 1-1 8. 55 R. L. Bohon and W. F . Claussen, J. Am. Chem. SOC., 1951,73, 1571. 56 (a) A. Seidell, Solubilities of Organic Compounds (Van Nostrand, New York, 3rd edn, 1941), vol. 11; (b) H. Stephen and T. Stephen, Solubilities of Inorganic and Organic Compounds (Pergamon, Oxford, 1963 and 1964), vol. 1 and 2. 317 57 A. Leo, C. Hansch and D. Elkins, Chem. Rev., 1971, 71, 525. 58 C. Hansch and A. Leo, Substituent Constants for Correlation Analysis in Chemistry and Biology (Wiley, New York, 1979). 59 E. P. Serjeant and B. Dempsey, Ionisation Constants of Organic Acids in Aqueous Solution (Pergamon, Oxford, 1979). 60 L. J. Andrews and R. M . Keefer, J. Am. Chem. SOC., 1950,72, 31 13. 61 L. J. Andrews and R. M. Keefer, J. Am. Chem. SOC., 1950, 72, 5034. 62 G. Parker, R. J. Cox and D. Richards, J . Pharm. Pharmacol., 1955,7, 683. 63 G. B. Barlin and D. D. Perrin, Q. Rev. Chem. SOC., 1966, 20, 75. 64 D. D. Perrin, B. Dempsey and E. P. Serjeant, pK, Prediction for Organic Acids and Bases (Chapman & Hall, London, 1981). 65 P. Ballinger and F . A. Long, J. Am. Chem. SOC., 1959, 81, 1050; 1960, 82, 795; F. A. Long and P. Ballinger, in Electrolytes, ed. B. Pesce (Pergamon, Oxford, 1962), p. 152. 66 (a) J. Murto, Acta Chem. Scand., 1964, 18, 1043; (b) S. Takahashi, L. A. Cohen, H. K. Miller and E. G. Peake, J. Org. Chem., 1971, 36, 1205. 67 J. Thamsen, Acta Chem. Scand., 1952, 6, 270. 68 J. Eliassaf, F. Eriksson and F. R. Eirich, J. Polym. Sci., 1960, 47, 193. 69 P. Molyneux, Lab. Pract., 1984, 33, 86. 70 R. H. Fowler and E. A. Guggenheim, Statistical Thermodynamics (Cambridge University Press, 71 F. A. Long and W. F . McDevit, Chem. Rev., 1952,52, 119. 72 M. M. Breuer, J. Phys. Chem., 1964,68, 2074. 73 W. Good and M. H. Milloy, Chem. Ind., 1956, 872. 74 E. Hutchinson, V. E. Sheaffer and F. Tokiwa, J. Phys. Chem., 1964,68, 2818; F. Tokiwa, Bull. Chem. 75 P. Mukerjee and K. J . Mysels, Critical Micelle Concentrations of Aqueous Surfactant Systems, 76 A. Packter, Kolloid-Z. 2. Polym., 1963, 189, 125. 77 T. Okubo and N. Ise, J . Phys. Chem., 1969,73, 1488. 78 P. Molyneux, Nature (London), 1964, 202, 368. 79 G. Schwarz, Eur. J. Biochem., 1970, 12, 442. 80 E. A. Guggenheim, Trans. Faraday Soc., 1960, 56, 11 59. 81 R. H. Wood, T. H. Lilley and P. T. Thompson, J. Chem. SOC., Faraday Trans. I , 1978,74, 1301. 82 H. Endres and H . Hormann, Angew. Chem., Int. Ed. Engl., 1963, 2, 254. 83 C. Tanford, The Hydrophobic Effect: Formation of Micelles and Biological Membranes (Wiley, 84 K. Linderstrerm-Lang, C.R. Lab. Carlsberg, 1924, 15, 1; 1929, 17, 1. 85 J. H. Hildebrand and R. L. Scott, The Solubility of Non-electrolytes (Dover, New York, 3rd edn, 1964). 86 J. C. Speakman, The Hydrogen Bond and Other Intermolecular Forces, Monographs for Teachers 87 R. F. Blanks and J . M. Prausnitz, Znd. Eng. Chem., Fundam., 1964, 3, 1. 88 T. Okubo, S-X. Chen and N. Ise, Bull. Chem. SOC. Jpn, 1973, 46, 397. 89 R. Wolfenden, Proc. Indian Acad. Sci. Chem. Sci., 1985, 84, 121. 90 R. D. Poretz and I. J . Goldstein, Arch. Biochem. Biophys., 1968, 125, 1034. 91 C. K. Bahal and H. B. Kostenbauder, J. Pharm. Sci., 1964,53, 1027. Cambridge, 1960), chap. 10. SOC. Jpn, 1965, 38, 75 1. NSRDS-NBS 36 (National Bureau of Standards, Washington, D.C., 1971). New York, 2nd edn, 1980). no. 27 (The Chemical Society, London, 1975). Paper 5/024; Received 2nd January, 1985

ISSN:0300-9599    

DOI:10.1039/F19868200291

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1986,82, 319-327 Kinetics of the Axial Ligation of Aquocobaloximes in the Presence of Cetyltrimethylammonium Acetate Micelles Anatoly K. Yatsimirsky,* Ol'ga I. Kavetskaya and Ilya V. Berezin Department of Chemistry, Moscow State Wniuersity, 119899, GSP, Moscow, W.S.S.R. The kinetics of the axial ligation of trans-Co(DH),(R)(H,O), where R = NO,, I, SO, or CH, and DH is dimethylglyoxime anion, by anionic (NCS-, Br-) or neutral (pyridine, benzimidazole) ligands was investigated in the presence of hexadecylammonium acetate micelles using the stopped- flow method or conventional spectrophotometry. Notable accelerations induced by micelles were observed in the reactions between NCS- and the complexes trans-CO(DH),(I)(H,O) and trans-CO(DH),(SO,)(H,O)-. The competition between negatively charged reactants for positively charged micelle surface was manifested in the reaction kinetics in the latter case.In view of this effect, the theory of ionic reactions in micellar systems is discussed. The binding constants of reactants as well as the ratios of true rate constants in the micellar pseudo-phase (ky) and in the aqueous solution (ky) were estimated. Since ky/k! < 1 in all cases, it is concluded that the catalytic micellar effects are due to increasing concentrations of reactants in the micellar pseudo-phase. The micellar effects in the kinetics of the axial ligation of coenzyme B12a (aquocobalamine, Bzm-Co-OH,) according to the equation 4 Bzm-Co-OH, + L f Bzm-Co-L + H,O k-1 have been studied by Fendler and cow~rkers.~-~ Minor rate variations or pronounced inhibitory effects were observed in most cases.However, appreciable micelle-induced accelerations were found in the reactions of model cobaloxime compounds (trans- Co(DH),(R)(X), where DH = dimethylglyoxime anion) ; namely in their dealkylation by mer~ury(I1)~ and alkylation of reduced cobaloximes by alkyl halide^.^ Axial ligation of aquocobaloximes kl k- 1 trans-Co(DH),(R)(H,O) + L trans-Co(DH),(R)(L) + H20 (11) is being studied widely as a model for reaction (I) with natural coenzyme.6-11 The rate of ligation for R = C1, I, NO, is known to be very low, k, being about dm3 mol-l s-l at room temperature,12 which is typical of substitution in cobalt(II1) octahedral c~mplexes.~~ When R is a strong trans-influencing ligand, the rate of reaction (11) approaches that of reaction (I).So, the value of k, is about 10 dm3 mol-l s-l, for R = and about 10, dm3 mol-1 s-l for R = CH,,' whereas in reaction (I) k, is about lo3 dm3 mol-1 s-l.15 In view of this, an attempt to stimulate reaction (11) by introducing surfactant micelles could be promising in elaboration of a better model of reaction (I). Another aspect of this study involves the kinetic features of micellar catalysis of ionic reactions including metal complex formation. The latter was the subject of some recent publications16-18 devoted to development of a general theory of micellar effects in ligand substitution reactions. 319320 Kinetics of Ligation of Cobaloximes 8 L r I I I 1 ' I - 0 . 2 0.4 0.6 0.8 1.0 2.0 Fig.1. Dependence of the observed rate constant, kobs for the ligation of trans-Co(DH),(I)(H,O) by Br- as a function of [Br-] added as KBr (curve 1) or CTAB (curve 2) at pH 2.9 and 25.5 "C. [ Br-]/mol dm-3 Experiment a1 The aquocobaloximes were synthesized according to published methods : trans- CO(DH),(NO,)(H,O),~~ trans-Co(DH),(I)(H,0),20 trans-[C0(DH),(S0,)((H,0)]Na~~ and trans-Co(DH),(CH,)(H20).21 Cetyltrimethylammonium acetate (CTAA) was pre- pared by the reaction of cetyltrimethylammonium bromide (CTAB, ' Chemapol ') with silver acetate. All other chemicals were analytical grade reagents of Reakhim. The critical micelle concentration (c.m.c.) of CTAA determined conductimetrically was 2.6 lop3 mol dm-3. In the presence of reactants, especially NCS- ions, the c.m.c.was lowered significantly and precise values were determined from the breaks on the rate vs. [ surfac tant] concentration profiles. Kinetic runs were made under pseudo-first-order conditions with ligand concentration in at least a tenfold excess over aquocobaloxime concentration. Reaction progress was monitored by measuring the changing absorbance on mixing the reactants. Conventional spectrophotometry (Hitachi-356 instrument) was used for slow reactions when R = NO, or I. Fast reactions (R = SO,, CH,) were studied with an RA-401 stopped-flow reaction analyser. All solutions were buffered with 0.1 mol dm-3 acetate to pH 2.9-6.0. In preliminary experiments it was verified that the acetate anion does not participate in reaction (11). Results Kinetics of the reactions of trans-Co(DH),(NO,)(H,O) and trans-Co(DH),(I)(H,O) The rates of ligation of trans-Co(DH),(NO,)(H,O) by Br- and NCS- do not change on addition of CTAA to the reaction mixture.However, the ligation of trans- Co(DH),(I)(H,O) by the same anions is accelerated in the presence of cationic micelles. Fig. 1 shows the dependence of the observed rate constant (k&) for this reaction on Br- concentration introduced as KBr or CTAB. Clearly Br- reacts faster in micellar solution. A more pronounced effect was found in the reaction with NCS-. The k1,obs (the slope of the kobs us. NCS- concentration, see Discussion) variation with CTAA concentration is shown in fig. 2. The maximum rate increase is 40-fold at the optimum surfactant concentration.A . K. Yatsimirsky, 0.I. Kavetskaya and I. V. Berezin 0.06 0 . 0 4 0.02 321 Fig. 2. kl,obs for the ligation 0.04 0.08 [CTAA]/mol dm-3 31 "C. of trans-Co(DH),(I)(H,O) by NCS- against [CTAA] at pH 2.9 and 3 .O 2 .o e I cn \ n 0 Ato 1 . o 2 1 0.004 0.008 [ SCN-]/mol dm-3 Fig. 3. kobs for the ligation of trans-Co(DH),(SO,)(H,0)- by NCS- as a function of [NCS-] in aqueous (curve 1) and CTAA solution (curves 2-6) at pH 6.0 and 45.5 "C. CTAA concentrations: (2) 0.002, (3) 0.0032, (4) 0.005, (5) 0.01 and (6) 0.015 mol dm-3.322 Kinetics of Ligation of Cobaloximes 0.00 L 0.008 0.012 [CTAA]/mol dm-3 Fig. 4. kl,obs for the ligation of trans-Co(DH),(SO,)(H,O)- (curve 1) and trans-Co(DH),(CH,)(H,O) (curve 2) by NCS- against [CTAA] at pH 6.0 and 44.5 "C (for sulphitocobaloxime) and 39.5 "C (for methylcobaloxime).1 m E a I- I I I I 0.002 0.OOL [CTAAl/mol dm-3 Fig. 5. kl,obs for the ligation of trans-Co(DH),(SO,)(H,O)- by benzimidazole as a function of [CTAA] at pH 6.0 and 40 "C. Kinetics of the reactions of ~w~s-CO(DH),(SO,)(H,O) - and truns-Co(DH),(CH,)(H20) The reactions of the sulphito-complex were studied more thoroughly. Fig. 3 shows the kobs - [NCS-] dependence at various CTAA concentrations. It is seen that in the presence of the surfactant these dependences become non-linear or even extremal. The values of kl,obs were calculated from the initial slopes of these curves at low CTAA concentrations. Fig. 4 shows more than a 10-fold increase in kl,obs in the presence of CTAA. Less effective micellar catalysis was observed in the ligation of trans-Co(DH),(SO,)(H,O)- by benzimidazole (fig.5). The reaction with pyridine was inhibited by CTAA (table 1). The rate of ligation of trans-Co(DH),(CH,)(H,O) by benzimidazole was independent of CTAA (table 1). The reaction with NCS- was accelerated by CTAA (fig. 4), but less effectively than in the case of sulphito-complex.Table 1. Rate and activation parameters for the axial ligation of aquocobaloximes in aqueous micellar systems at 25 "C 2 2 complex in aqueous solution at optimal CTAA concentration 3. z* kl AH? AS! kl,obs AH!,QbS Asi,obs Y ?? 0 ligand /dm3 mol-l s-l /kJ mol-1 /J mol-l K-l /dm3 mol-l s-' /kJ mol-l /J mo1-1 K-1 aquocobalaminea NCS- trans-Co(DH),(NO,)(H2O) Br- tran.~-Co(DH)2(I)(H20) NCS- Br- p yridine benzimidazoleb benzimidazole ~~~w-CO(DH),(SO,)(H,O)- NCS- ~ ~ ~ ~ ~ - C O ( D H ) ~ ( C H , ) ( H ~ O ) NCS- 2.3 x 103 71.5 2.1 x 10-4 - 1.03 x 10-3 72.8 4.4x 10-4 - 8.1 73.2 3.4 11.5 98.7 1 .7 7 ~ lo2 57.7 2.04 x lo2 61 - - - 58.6 - 2.1 x 10-4 - - 9.3 x 10-4 - 18.8 1.2 x 102 96.3 - - 56.5 6.2 x - 1.8 106 62.0 - 8.0 2.53 x lo2 63 - - 2.0 2.06 x lo2 - - 117.2 - - 10.5 - fr S tn 2 3 a From ref. (15). At 30 "C. ch N w h) w324 Kinetics of Ligation of Cobaloximes Table 1 contains the rate constants together with respective activation parameters for all investigated reactions both in pure aqueous solution and at optimal surfactant concentration. Discussion Reaction (11) is appreciably reversible.,, In the pseudo-first-order condition it follows the kinetic equation In the presence of surfactant, the same form of kinetic equation was found to be operative (with exception of sulphito-complex ligation by NCS-, fig.3, see below), but with k, and k-, being dependent on surfactant concentration The values of k-l,obs were usually determined with low precision and so only kl,obs will be discussed below. According to the general kinetic theory of micellar effects,,? 23 the change in k , on surfactant addition is due to complex and ligand partition between micelles and the bulk aqueous region as well as being due to the change of reaction rate constant in the micellar pseudo-phase. The most pronounced effects were observed for ionic reactants (table 1). However, the description of ionic distribution between bulk water and micellar surface regions is very compli~ated.~~ In a semiquantitative approach a distinction between ionic and non-ionic reactants can be neglzcted.In this case eqn (3), deduced for a bimolecular reaction in micellar 23 may be used: where KCoR and KL are the binding constants (i.e. partition constants multiplied by surfactant molar volume) for aquocobaloxime and ligand, respectively; k y and ky are the respective rate constants in micellar pseudo-phase and bulk solvent; C is the surfactant concentration minus the c.m.c. and V is the surfactant molar volume. The absence of any micellar effects in ligation of trans-Co(DH),(NO,)(H,O) is obviously due to the highly hydrophilic nature of this complex preventing its incorporation into micelles (see below). The ligation of the more hydrophobic trans-Co(DH),(I)(H,O) is more sensitive to the presence of surfactant.In the reaction with Br- only a small rate increase was observed (fig. l), while for more tightly bound NCS-, the micellar catalytic effect was fairly high (fig. 2). The surfactant concentration us. rate profile has a typical passing through a maximum according to eqn (3). The calculated values of the constants are KcoI = 35 dm3 rnol-l, KNcs = 130 dm3 mol-1 and k p / k t = 1.2 (assuming V = 0.35 cm3 m ~ l - l ) . ~ ~ Since the ratio kF/kF is close to unity the observed acceleration should be assigned to the increasing concentrations of reactants in the micellar pseudo-phase. Attempts to find the binding constants KCoR for trans-Co(DH),(I)(H,O) and other cobaloximes from independent spectral measurements were unsuccessful owing to the very low spectral changes observed on addition of surfactant. On passing to anionic cobaloxime, trans-Co(DH),(SO,)(H,O)-, an increase in binding constant KcoR with positive micelles of CTAA and hence increased catalytic micellar effect are expected.However, the ligation of trans-Co(DH),(SO,)(H,O)- by pyridine is slightly inhibited by CTAA (table 1). In terms of eqn (3) this is due to a low binding constant for pyridine and a low ky/k? ratio. When more hydrophobic benzimidazole is used as ligand, an appreciable acceleration instead of inhibition is observed (fig. 4). From these results and using eqn (3) and KL = 35 dm3 mol-1 for ben~imidazole,~~ the values of Kcos (550 dm3 mol-l) and kp/kF = 0.032 were calculated for theA . K. Yatsimirsky, 0.I. Kavetskaya and I, V. Berezin 325 sulphito-complex. Thus, the ligation of anionic sulphito-complex is strongly retarded in a micellular medium, unlike ligation of neutral cobaloxime. The retardation effect is obviously responsible for the observed inhibition of the reaction with weakly bound pyridine by CTAA. A more pronounced catalytic effect of CTAA was found in the ligation of trans- Co(DH),(SO,)(H,O)- by NCS-. In a pure aqueous solution the reaction kinetics follows eqn (1). In the presence of CTAA the linear dependence of kobs on [NCS-] was observed only at a low [NCS-] (fig. 3). At higher NCS- concentration, the dependences level off or even pass through a maximum. The slope of the dependence of kobs on [NCS-] reaches that for aqueous solution at a high [NCS-] (curves 1 and 2 in fig.3). These observations can be interpreted as follows. Both reactants are counterions for positively charged CTAA micelles. As [NCS-] increases, the competition between reactants for micellar surface becomes important. Owing to this process the cobaloxime is displaced from micelles so the reaction rate is lowered. At a sufficiently high NCS- concentration, all cobaloxime anions appear in the bulk aqueous phase and the values of kobs become close to those in a pure aqueous solution. The theory of ionic reactions in the presence of surfactant micelles was developed by Romsted.26 His model describes the competition between the ionic reactant and non-reacting inert counterions existing in the solution for micellar surface. Consider this approach to our case of bimolecular reaction of two ionic reactants : k2 A + B -+ products.(4) The total concentration of ions in the micellar pseudo-phase is where X is an inert counterion (CH,COO- in our case), is the constant26 degree of counterion binding to the Stern layer and S is the molar density of the micellar pseudo-phase. The inclusion of reactants into the micellar pseudo-phase is described as their exchange with inert counterions,26 with respective distribution constants Ka and K& [primed in order to distinguish them from the binding constants in eqn (3)]. The binding of CH,COO- with polycations is known to be very weak.27 On the other hand, the binding constants Kcos and KNcs found above are fairly high. Both distribution constants are thus expected to be much higher than unity.Using the same procedure as described in ref. (22), (23) and (26), with this and some other reasonable simplifications, the following equation can be derived : for the case when [A], >> [B], in reaction (4). Eqn (6) shows that the increased ionic reactant A concentration reduces the contribution of the micellar reaction and increases the contribution of aqueous reaction in the observed rate. If the micellar reaction is faster than the aqueous reaction, then k, decreases. An example of such a situation is cited by Corsaro28 on the micellar effects in the ionic reaction k2 BrCH,COO- + S,O:- -+ Br- + S,0,CH,C002-. (7) The value of k, in the presence of cationic surfactant decreases approximately in proportion to the total concentrations of reactants.28 Eqn (6) is applicable to description of the results in fig.3 and 4 assuming k, to be kl,obs and with A = NCS-, B = Co(DH),(SO,)(H,O)-, and X = CH,COO-. It is evident that kl,obs (the slope of curves in fig. 3) should decrease with increasing [NCS-1. Moreover, at high [NCS-1, kl,obs 12 FAR 1326 Kinetics of Ligation of Cobaloximes should approach k! (the slope of the dependence without surfactant). Both predictions agree with the observed behaviour. At a low [NCS-1, eqn (6) is simplified and takes the form Eqn (8) evidently coincides with eqn (3) at K = K~/[CH,COO-],. So, the simple eqn (3) is suitable to describe the kl,obs us. [CTAA] profile in fig. 4 also the case of the ligation of trans-Co(DH),(SO,)(H,O)- by NCS-.Fitting these results to eqn (3) gives kF/ky = 0.04 with the values of Kcos and KNcs found above. The ligation of sulphito-complex by NCS- is inhibited by the micellar medium as observed for the ligation by benzimidazole. Micellar effects in the ligation of trans-Co(DH),(CH,)(H,O) are rather low. This cobaloxime is expected to bind more weakly to CTAA micelles than does Co(DH),(I)(H,), but stronger than Co(DH),(NO,)(H,O). This follows from the com- parison of ‘hydrophobicity increments’ 7c29 of these substituents. For I, 7c a 1 ; for CH,, 7c x 0.5; and for NO,, 7c ranges from -0.85 to -0.35.29 KCoNOz should thus be 30-100 times lower than KcoI and so, KCoNOz < 1 dm3 mol-l. This value of the binding constant shows that Co(DH),(NO,)(H,O) practically does not bind to CTAA micelles.This explains the absence of any effects for the nitrocobaloxime. The same estimation based on n values predicts KCoCH3 to be three times lower than KcoI and so KCoCH3 x 10 dm3 mol-l. The high binding constant for the sulphito-complex does not fit this series owing to the negative charge of the complex. Let us confine ourselves to a qualitative discussion of micellar effects on the ligation of trans-Co(DH),(CH,)(H,O) on the basis of eqn (3). The kl,obs vs. [CTAA] profile shown in fig. 4 for the ligation by NCS- is in accord with the binding constants given above for reactants, if ky/kF is close to the 0.1. The same ratio of the rate constants in two phases explains the absence of any effect in the ligation by benzimidazole (table 1). So, for the ligation of a highly reactive aquocobaloximes, trans-Co(DH),(S0,)(H20)-- and trans-Co(DH),(CH,)(H,O), the micellar environment is unfavourable, ky/k! 6 1, whereas the ligation rate of the inert trans-Co(DH),(I)(H,O) is essentially insensitive to the micellar medium, ky/k: = 1.The inhibition of the former reactions in micelles is not due to the decreased dielectric constant since we found no retardation of the ligation of trans-Co(DH),(S0,)(H20)- by NCS- in water-dioxane mixtures containing up to 40% v/v of the organic solvent. The generally accepted mechanism of reaction (11), for both labile and inert complexes, involves the dissociative (D or Id) activation with the rate-limiting step being the loss of a coordinated water molec~le.~? 1 2 ~ l4 It was found previouslyls that water is lost from Ni;; at the same rate in aqueous solution and the micellar region.So the low kF/kF values for highly reactive sulphito- and methylaquocobaloximes may reflect some peculiarities of their activation. Costa and coworkers7 pointed out that the reactions of reactive complexes are characterized by positive A S while those of inert complexes have negative A S (see also table 1). Micellar reactions show even more positive A S for reactive complexes (table l), but these values may also involve ASo for binding reactants,, and hence are hard to discuss. So, these reactions proceed through a rather disordered transition state and the probable reason for ky/kF being much less than unity is some restriction on molecular motion caused by the organized micellar medium.A .K. Yatsimirsky, 0. I. Kavetskaya and I. V. Berezin 327 References 1 J. H. Fendler, F. Nome and C. Van Woert, J. Am. Chem. Soc., 1974,%, 6745. 2 F. Nome and J. H. Fendler, J. Am. Chem. SOC., 1977,99, 1557. 3 G. C. Robinson, F. Nome and J. H. Fendler, J. Am. Chem. SOC., 1977,99,4969. 4 R. J. Allen and C. A. Bunton, Bioinorg. Chem., 1976, 5, 31 1. 5 R. J. Allen and C. A. Bunton, Bioinorg. Chem., 1976, 5, 241. 6 A. 0. Hill, in Inorganic Biochemistry, ed. G. B. Eichhorn (Elsevier, Amsterdam 1975), vol. 2. 7 R. Dreas-Garlatti, G. Tauzher, G. Costa and M. Green, Znorg. Chim. Acta, 1981, 50, 95. 8 R. D. Garlatti, G. Tauzher and G. Costa, Inorg. Chim. Acta, 1983, 70, 83. 9 K. L. Broun and R. G. Kallen, J. Am. Chem. SOC., 1972, 94, 1894.10 A. L. Crumbliss and W. K. Wilmarth, J. Am. Chem. SOC., 1970,92,2593. 11 T. Sakurai, J. P. Fox and L. L. Ingraham, Znorg. Chem., 1971, 10, 1105. 12 D. N. Hague and J. Halpern, Znorg. Chem., 1967,6,2059. 13 N. M. Samus’, 0. N. Damaskina and T. S. Lukianez, Substitution Reactions in Cobalt Coordination 14 H. G. Tsiang and W. K. Wilmarth, Znorg. Chem., 1968,7, 2535. 15 D. Thusius, J. Am. Chem. SOC., 1971,93, 2629. 16 V. C. Reinsborough and B. H. Robinson, J. Chem. SOC., Faraday Trans. I , 1979, 75, 2395. 17 H. D. Burrows, J. Ige and S. A. Umon, J. Chem. Soc., Faraday Trans. I , 1982, 78, 947. 18 S. Diekmann and J. Frahm, J. Chem. SOC., Faraday Trans. I , 1979.75. 2199. 19 A. V. Ablov and G. P. Sirtsova, Zh. Neorg. Khim., 1956, 1, 687. 20 A. V. Ablov, Dokl. Akad. Nauk. SSSR, 1954,97, 1019. 21 G. N. Schrauzer and R. J. Windgassen, J. Am. Chem. SOC., 1966,88, 3738. 22 K. Martinek, A. K. Yatsimirsky, A. V. Levashov and I. V. Berezin, in Micellization, Solubilization, and 23 I. V. Berezin, K. Martinek and A. K. Yatsimirsky, Russ. Chem. Rev. (Usp. Khim.), 1973, 42, 787. 24 D. G. Hall, J. Chem. SOC., Faraday Trans. I , 198 1,77, 1 121. 25 A. K. Yatsimirsky, A. P. Osipov, K. Martinek and I. V. Berezin, Kolloidn. Zh., 1975, 37, 526. 26 L. S. Romsted, in Micellization, Solubilization, and Microemulsions, ed. K. L. Mittal (Plenum Press, 27 H. P. Gregor, J. Belle and R. A. Marcus, J. Am. Chem. SOC., 1955, 77, 2713. 28 G. Corsaro, J. Chem. Educ., 1980, 57, 225. 29 A. Leo, C. Hansch and D. Elkins, Chem. Rev., 1971, 71, 525. Compounds (Shtiinza, Kishinev, 1979). Microemulsions, ed. K. L. Mittal (Plenum Press, New York, 1977), vol. 2, p. 489. New York, 1977), vol. 2, p. 509. Paper 51050; Received 7th January, 1985 12-2

ISSN:0300-9599    

DOI:10.1039/F19868200319

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. 1, 1986,82, 329-337 The Effect of Tannic Acid on the Electrical Properties of the Interface and the Non-linear Streaming Potential of Cellulose in a Cationic Dye Solution Manuel Espinosa- JimCnez Department of Physics, University College of JaPn, JaPn, Spain Fernando Gonzalez-Caballero" and Carlos F. Gonzalez-Fernandez Department of Physics, Faculty of Sciences, University of Granada, Granada, Spain An experimental investigation on streaming potentials of porous plugs of cellulose in both the linear and the non-linear range is described. The variation of electrokinetic coefficients with the concentration of a cationic dye in solution has been studied and the influence of mordant on the electrical properties of the interface of cellulose/cationic dye solution systems has been analysed.The second-order electrokinetic coefficients have been interpreted in terms of modifications of generalized coefficients produced by changes in thermodynamic forces. The study of the adsorption processes of different dyes on the surface of textile fabrics together with the analysis of the electric properties of the fibre-solution interface is fundamental to understanding the mechanism of dyeing and finishing textile materials.lT Although the electrokinetic properties of cellulosic fibres have been largely investigated to gain insight into the dyeing process, there are some aspects which have been studied less. One of them is the influence of tannic acid, used as mordant in dyeing processes, on the adsorption of the cationic dyes on to the cellulosic fibres.This is a very interesting aspect in textile physicochemistry. Electrokinetic phenomena are concerned with the transport of electric charge in porous media under the influence of an electric field and a pressure gradient. The analysis of these phenomena falls into the domain of non-equilibrium thermodynamic^.^ In the last fifteen years, non-linear effects in electrokinetic phenomena have become more systematically studied in order to gain a wider knowledge of the interfacial properties in different ~ystems.~-l~ However, research has been confined to a few Pyrex/solution and some ion-exchange membranes, and very few studies have been devoted to investigate the electrokinetic behaviour of fibrous systems under a wide range of applied electrokinetic forces.ll The streaming potential effect has been shown to be most appropriate for the study of electrokinetic properties of fibrous systems.l2 We have carried out an experimental investigation of the streaming potential of a fibrous system of hydro p hilic cotton. The purpose of the present study is to investigate (1) the influence of tannic acid on the adsorption of a cationic dye onto the cellulosic fibres and (2) the electrokinetic behaviour of the system in this process by means of first and second-order electrokinetic coefficients. 329330 40 0 350 300 2 00 150 Non-linear Streaming Potential of Cellulose 5 10 15 AP/cmHg 20 Fig. 1. Streaming potential (Am,) dependence on the applied pressure difference (AP). 0, Cotton treated with mol dm-3 tannic acid; 0, cotton treated with loM4 mol dmP3 tannic acid in the mol dm-3 Rhodamine B.presence of Materials and Methods The fibrous material was 100 % pure hydrophilic cotton obtained from Cotonificio Badalona S.A. (Spain). The standard procedure for preparation of this fibre implies (i) an alkaline scouring treatment of the cellulose, followed by repeated washing with acidified water to neutralize the alkali ; (ii) bleaching with sodium hypochlorite and washing with acidic water and then with water until constant conductivity is achieved. Our samples were washed again repeatedly with deionized water, until the conductivity of the washing water remained constant. Finally, they were dried in an oven at 40 "C. Solutions of Rhodamine B (C.I. 45170) and tannic acid were of A.R. quality, from Carlo Erba, and were used without further purification.Water having conductivity of ca. low6 0-l cm-1 was used for preparing the different solutions. Porous plugs with constant cotton content and equal thickness (2.90 cm) were used in the streaming potential experiments. The density of fibre in the pad was ca. 257 kg mP3. The samples were always conditioned with a solution of tannic acid at a constant temperature of 30.0f0.5 "C for 24 h. They were dried in an oven and finally they were conditioned with a solution of Rhodamine B at the same constant temperature as above for 24 h, this time being sufficient to attain equilibrium. The streaming cell and potential measurement procedure have been described e1~ewhere.l~ A streaming flux was always started by the successive application, in opposite directions, of a pressure difference (20-200 mmHg)? to both sides of the plug.t 1 mmHg % 133.3 Pa.M. Espinosa-Jimtnez et al. 33 1 Fig. 2. Dependence of phenomenological coefficient L,, on the concentration of Rhodamine B in solution. 0, Untreated cotton; cotton treated with tannic acid: 0, A, and A, mol dm-3. "1 Fig. 3. Dependence of the amount of cationic dye absorbed on the cotton, Am, on the final concentration of cationic dye in solution. 0, Untreated cotton; cotton treated with tannic acid: x , loF6 and 0, lop5 mol dm-3.332 Non-linear Streaming Potential of Cellulose Fig. 4. Dependence of phenomenological coefficient L,,, on the concentration of Rhodamine B in solution. 0, Untreated cotton; cotton treated with tannic acid: e, A, and A, mol drnp3.Electric potential differences (streaming potentials) were thus obtained between the two perforated platinum electrodes in contact with the two faces of the plug. These were measured with a Keithley (model 61 6) digital electrometer. The electrical resistance of the pad, equilibrated with the liquid, as well as the conductivity of the liquids, were measured with a Philips conductivity bridge at 50 Hz. The amount of dye adsorbed on the fibre was determined from the difference between final and initial concentrations of dye solution, provided that adsorption equilibrium was already attained. Results were expressed in terms of millimoles of dye per kg of dried fibre. A Beckmann spectrophotometer, model DB-GT, was used for the determination of dye in solution.Results and Discussion Streaming potential experiments were conducted with plugs of cotton with a definite conformation. The streaming potentials developed for each value of AP were always the same for both directions of the flow. Thus, the systems can be considered as isotropic. A transport equation relating the density of the electric current, I, the pressure difference, AP, and the electrical potential difference, A@, can be written in the form: I = L,, AP+ L,, A@ + L211(AP)2 + L,,, AP A 0 (1) were L , and L,,,, ( i , j , k = 1,2) are called first- and second-order electrokinetic coefficients, respectively . The streaming potential effect is described by the quotient (A@s/AP)z=o. Rearrangement of eqn (1) gives for this quantity the expression L21 L21, L221 L22 L22 L22 (A@s/AP)Z-o = ---- AP-- Ams.Plots of (AOs)I-o us. AP have been produced for the different cotton/tannic acid- Rhodamine B solution systems considered in this study. The streaming potentials were found to vary non-linearly with streaming pressure (fig. 1). The experimental data, AQS and AP, were fitted to eqn ( 2 ) by the multivariable least-squares method, from which the electrokinetic coefficients were obtained. The coefficient of determination r2, charac-M. Espinosa-Jimknez et al. 333 10-6 10-5 I O - ~ c/mol dm-3 10-3 10-2 Fig. 5. Dependence of electrical conductance, L,,, on the concentration of Rhodamine B in solution. 0, Untreated cotton; cotton treated with tannic acid: a, A, lop4 and A, mol dm-3.I I " . 10-6 10-5 10-4 10-3 10-2 c/mol dm-3 Fig. 6. Dependence of surface conductance, L,,(s), on the concentration of Rhodamine B in solution. 0, Untreated cotton; cotton treated with tannic acid: a, lop5; A, and A, mol dm-3. terizing the quality of the fit, was always higher than 0.95. Thus, all coefficients in eqn (2) are statistically significant at a confidence level of 95%, which suggest the presence of non-linear effects in the streaming potential. According to the classical theory of electrokinetic effects,14 the following relationship can be considered between the phenomenological electrokinetic coefficient L,, and the parameters characterizing the electrical double layer properties : nr2ec L,, = - 4171 (3) where n is the equivalent number of capillaries in the plug, r is the capillary radius and I is its length: is the zeta potential and E and 17 are the dielectric constant and the absolute viscosity of the liquid, respectively.The behaviour of the electrokinetic coefficient L,, is shown in fig. 2 for cotton immersed in aqueous solutions of Rhodamine B of increasing concentration. Previously, the fibre was treated with mol dm-3 solutions of tannic acid, in this way andNon-linear Streaming Potential of Cellulose 1 334 d \ N N r 4 I 10-6 10-5 10-4 I O - ~ c/mol dm-3 10-2 Fig. 7. Dependence of phenomenological coefficient LtZ1 on the concentration of Rhodamine B in solution. 0, Untreated cotton; cotton treated with tannic acid: 0, ,A, lo-* and A, mol dm-3. varying the amount of mordant present in the solid phase.Adsorption experiments with the cationic dye on the fibrous system were carried out simultaneously, also varying the amount of mordant present on the surface of the fibre. The results are shown in fig. 3, where it can be observed that the amount of dye adsorbed increases in all cases with the final concentration of dye solution. On the other hand, the adsorption of Rhodamine B increases greatly with the amount of mordant present on the fibre; the increase being notably marked when the tannic acid concentration is ca. mol dm-3. This behaviour could be explained, probably, by the hydrogen bonds formed between the carboxy groups of cellulose and the phenolic hydroxy groups of tannic acid.15 This effect increases the negative surface charge of the cellulose, making possible the adsorption of dye by electrostatic attraction between the dye cation and the surface of the fibre.It can be observed in fig. 2 that the coefficient L,, shows a maximum in the range of low concentrations and a marked decrease in the range of higher concentrations. The sign of L,, changes for high concentrations of Rhodamine B in the liquid phase. When the amount of mordant on the solid phase increases, a displacement of this sign inversion towards lesser concentrations may be observed. Assuming the proportionality between L,, and the zeta potential, eqn (3), the adsorption of dye cations produces a corresponding diminution and the displacement of the sign inversion of the zeta potential towards lesser concentrations. Furthermore, this sign inversion suggests some additional chemical affinity of the dye towards the fibre-mordant/solution interface, this being in accordance with the trend of adsorption curves of fig.3. Fig. 4 shows the dependence of second-order electrokinetic coefficient L,,, on the concentration of aqueous solutions of Rhodamine B, varying the amount of tannic acid on the surface of the fibre. The values for this coefficient are always of the order of m4 A N-,, analogous to those obtained with mineral systems,lo whereas ion- exchange membranes have afforded values ten orders of magnitude greater than those reported here.* With regard to the sign of L2,,, note that generally it has been foundM . Espinosa-JimPnez et al. 335 to be positive. Both positive and negative signs were found with the systems used in ref.(8)-( 10). Upon establishing the general characteristics of the curves, we observe the presence of a maximum for the range of low concentrations and a marked decrease as the concentration of Rhodamine B increases. When the amount of mordant on the fibre increases, a sign inversion of L,,, and a displacement of this sign inversion towards lesser concentrations may be observed. It is remarkable the similarity of the curves obtained for the electrokinetic coefficients L,, and L,,, (fig. 2 and 4). Not only is their shape very similar, but the relative values of concentration for maxima and points of inversion of sign are almost coincident. Such similarity suggests a proportionality between both coefficients. The interpretation of results shown in fig.4 can be made in terms of a generalized electrokinetic coefficient, Lf,, proposed in ref. (1 6) : 8I aAP Lf1= L!jl (AP, A@) = - (4) i.e., Lf, being a function dependent on the thermodynamic forces applied to the system. The linear electrokinetic coefficient L,, of eqn (1) is then the generalized one, evaluated at the thermodynamic equilibrium, and the second-order coeffcient, is given by :16 L211 = (g-$)o i.e., L,,, gives information on the extent of the influence of pressure on the generalized electrokinetic coefficient, Lf,. Lf, can be related to double layer properties, and its derivative with respect to AP, L,,, [eqn (6)], can be taken as an estimate of the modifications originated in those properties by such a thermodynamic force.According to eqn (2), the ratio L,,,/L,, represents the slope with changed sign of the dependence of (A@s/AP)I-o on AP. Since the electrical conductance L,, > 0, and L,,I has opposite sign to that of L,,, the non-linear effect of pressure on streaming potential is of a decreasing nature. From the microscopic point of view, Rutgers et al.17 pointed out that the disturbance caused in the diffuse part of the electric double layer by the high speed of the liquid in the capillaries gives rise to non-linearities in electrokinetic experiments. According to this, L,,, could be interpreted as a measure of the distortion caused by the pressure on the electric double layer. On the other hand, according to eqn ( 5 ) and (6), Lf, always decreases in absolute value with pressure, on the account of the signs of the coefficients L,, and L,,, (fig.2 and 4). This might be interpreted as a decrease of zeta potential brought about by the distortion exerted by the hydrodynamic flow on the electric double layer, if we suppose that L$, is related to the zeta potential in a analogous way as L,, is. On the other hand, the proportionality between L,, and L,,, could be explained because the higher the zeta potential, the greater the alteration brought about on the electric double layer by a hydrodynamic perturbation in the capillaries. Fig. 5 shows the behaviour of L,,, the electrical conductance of the system, with the concentration of Rhodamine B in solution, on varying the amount of mordant on the surface of the fibre. The increase of L,, is specially marked from mol dmP3. It can be seen in fig.5 that when the amount of tannic acid adsorbed on the solid phase increases, the electrical conductance of the system also increases. This is consistent with both the increase of conductivity of the solutions of Rhodamine B and with the increase of surface charge. The latter is mainly determined by tannic acid, which facilitates the adsorption of dye cation on the substratum.336 relationll Non-linear Streaming Potential of Cellulose One can define L,,(s), the surface conductance of the fibre, by means of the with L,,(v), the bulk conductance, equal to A/C, where A is the conductivity of the liquid and C is the streaming cell constant. Fig. 6 shows L,,(s) is dependent on the concentration of Rhodamine B in solution and on the different amounts of mordant used in this work.Both fig. 5 and 6 show that the greatest influence on the total conductance of the system is produced by the surface conductance in the range of low concentrations. For concentrations higher than ca. lo-* mol dm-3 there is a greater influence of the bulk conductance. In the absence of tannic acid the surface conductance is almost constant. This shows that dissociation of surface carboxy and hydroxy groups of cellulose contributes mainly to the conductivity of the system under these conditions. On the other hand, when the amount of tannic acid increases, coefficient L,,(s) also increases. The strong surface conductance manifested by the cotton/Rhodamine B solutions system when the concentration of tannic acid is lop3 mol dm-3 can be justified by the strong adsorption of the cationic dye on such a hydrophilic surface in this condition, according to curves of adsorption in fig.3. The curves showing the dependence of second-order coefficient L,,, on the cationic dye concentration are represented in fig. 7. The values are in the range 10-10-10-8 m2 R-l N-l and the sign is always negative, in agreement with those found in mineral systems.1° Values of L,,, greater than ours were found, however, by Rastogi et a1.8 in membrane systems, as well as both positive and negative signs. According to the data of fig. 7, L,21 has lower values in the range of concentration up to lo-* mol dm-3. Above this concentration, L,,, increases abruptly, the tendency becoming even more marked as the amount of mordant is increased. This behaviour is similar to that shown in fig.3 for the adsorption of the cationic dye at the cotton-solution interface, varying the amount of tannic acid on the surface of the fibre. The curves of fig. 7 clearly show a similarity to those shown in fig. 5 and 6. Thus, the similarity of the behaviour of L,, and L,,, is now reproduced between L,, and L,,,, indicating a proportionality between the latter coefficients, or better still, between L2,(s) and L,,,. In the same way as with coefficient L,,,, it is possible to give an interpretation of coefficient L,,, on the basis of a generalized coefficient Lf2, defined as16 whose physical significance is that of a ‘generalized electrical conductance’. In using such a coefficient, L,,, is given by L,,, = (*) aAP (9) where the zero subscript means that the derivative is evaluated at the thermodynamic equilibrium.Thus, L,,, gives the extent of the changes of the generalized electrical conductance of the system as a consequence of the applied pressure gradient, which gives rise to alterations in the hydrodynamic flow into the capillaries. The negative sign of L2,, obtained in this work means that the effect of an increase of pressure on cottonsationic dye solution systems is the diminution of its electrical conductance. This modification is also influenced by the amount of mordant added to the fibre. The similarity of the curves of both fig. 6 and 7 proves that the greater the surface conductance, the more pronounced is the alteration caused by the hydrodynamic flow on it.The magnitude of L,,, would be determined, therefore, not only by the hydrodynamic rate of flow but also by the value of L,,(s).M . Espinosa-Jimknez et al. 337 References 1 H. T. Lokhande, V. F. Androsov and E. N. Golovanov, Indian J. Technol., 1970,8, 89. 2 V. I. Filonenko, V. F. Androsov and L. A. Chursina, Izv. Vyssh. Uchebn. Zaved., Tekhnol. Tekst. 3 H. Vink, J. Chem. Soc., Faraday Trans. I , 1982, 78, 31 15. 4 R. P. Rastogi and K. M. Jha, Trans. Faraday Soc., 1966, 62, 585. 5 R. P. Rastogi, K. Singh and M. L. Srivastava, J. Phys. Chem., 1969, 73, 46. 6 R. P. Rastogi, K. Singh and S. N. Singh, J. Phys. Chem., 1969, 73, 1593. 7 R. P. Rastogi, M. L. Srivastava and S. N. Singh, J. Phys. Chem., 1970,74, 2960. 8 R. P. Rastogi, K. Singh, R. Kumar and S. A. Khan, J. Phys. Chem., 1977, 81, 21 14. 9 F. GonzAez-Caballero, R. Hidalgo-Alvarez, J. Morales-Bruque and G. Pardo-Sanchez, J. Non- 10 R. Hidalgo-Alvarez, F. Gonzalez-Caballero, J. Morales-Bruque and G. Pardo-Sanchez, J. Non- 1 1 C. F. Gonzalez-Fernandez, M. Espinosa-Jiminez and F. Gonzalez-Caballero, Colloid Polym. Sci., 12 W. Aichele, E. Schollmeyer and H. Herlinger, Makromol. Chem., 1977, 178, 2011. 13 F. Gonzalez-Caballero, G. Pardo and J. M. Bruque, An. Fis., 1975, 71, 41. 14 R. P. Rastogi, J. Sci. Ind. Rex, 1969, 28, 284. 15 Linus Pauling, Uniones Quimicas (Kapelusz, Buenos Aires, 1965), p. 508. 16 C. F. Gonzilez-Fernandez, J. M. Bruque, F. Gonzalez-Caballero and A. Hayas, J. Non-Equilib. 17 A. J. Rutgers, M. De Smet and G. De Myer, Trans. Faraday Soc., 1957, 53, 393. Prom., 1978, 1, 73. Equilibrium Thermodyn., 1980, 5, 30 1. Equilibrium Thermodyn., 198 1, 6, 295. 1983, 261, 688. Thermodyn., 1984, 9, 147. Paper 51054; Received 8th January, 1985

ISSN:0300-9599    

DOI:10.1039/F19868200329

出版商:RSC    

年代:1986

数据来源: RSC