摘要:

SPECIAL DISCUSSIONS OF THE FARADAY SOCIETY NO. 1 1970 Thin Liquid Films and Boundary Layers Published for THE FARADAY SOCIETY ACADEMIC PRESS LONDON AND NEW YORK bY ACADEMIC PRESS INC. (LONDON) LTD 24-28 Oval Road London N.W.l. 7DD. U.S. Edition published by ACADEMIC PRESS INC. 11 1 Fifth Avenue New York New York 10003 Copyright 0 1971 By THE FARADAY SOCIETY All Rights Reserved No part of this book may be reproduced in any form by photostat microfilm or any other means without written permission from the publishers Library of Congress Catalog Card Number 70-141730 ISBN 0-1 2-694550-0 Printed in Great Britain by THE ABERDEEN UNIVERSITY PRESS ABERDEEN SCOTLAND A SPECIAL DISCUSSION ON Thin Liquid Films and Bounday Layers 28th 29th and 30th September 1970 A SPECIAL DISCUSSION on Thin Films and Boundary Layers was held at the University of Cambridge on the 28th 29th and 30th September 1970.It represented the first of a new series of Discussion on physical chemistry topics of particular sig- nificance in industrial and technological research. Such Special Discussions are expected to be held on topics of scientific and industrial importance and timeliness at intervals and not necessarily on a yearly basis. The President Prof. Geoffrey Gee C.B.E. F.R.S. opened the meeting and wel- comed 220 members and others. Among the visitors from overseas were Mr. C. M. Allen U.S.A. Dr. R. De Backer Belgium Mr. Bongrand France Dr. E. Drauglis Germany Mr. F. Dumont Belgium Mr. J. G. J. Egberink Netherlands Mr. J. A. de Feijter Netherlands Dr. G. H. Findenegg Austria Dr.G. Frens Netherlands Mr. C . Guestaux France Dr. M. L. Hair U.S.A. Mr. H. Hasmonay France Dr. T. Hendrikx France Dr. E. P. Honig Netherlands Mr. J. Hougardy Belgium Dr. A. M. Joseph-Petit Belgiurn Prof. H. Lange Germany Mr. J. W. Lichtenbelt Netherlands Dr. A. Lucas Belgium Prof. J. Lyklema Netherlands Ir. A. M. Michels Netherlands Dr. K. J. Mysels U.S.A. Prof. Dr. H. van Olphen Netherlands Prof. J. Th. G. Overbeek Netherlands Dr. G. Peschel Germany Dr. A. Prins Netherlands Dr. H. E. Ries Jr. U.S.A. Mr. J. B. Rijnbout Netherlands Dr. K. Roberts Sweden Prof. J. R. Robinson New Zealand Prof. Dr. E. Ruckenstein U.S.A. Prof. Dr. G. Schay Hungary Prof. A. Scheludko Bulgaria Dr. G. Schreier Germany Dr. M. Schwuger Germany Dr. J. A. N. Scott Netherlands Prof. L. E. Scriven U.S.A. Dr. R.Senkus U.S.A. Dr. J. M. Serratosa Spain Dr. H. Sonntag Germany Dr. J. H. Stark Switzerland Dr. W. Stone Belgium Dr. B. Stuke Germany Dr. G. Szasz Switzerland Dr. M. van den Tempel Netherlands Dr. L. Ter-Minassian-Saraga France Mr. J. Thorin France Mr. P. Viaud France Prof. Miss M. Vignes France Prof. A. Vrij Netherlands Prof. A. Watillon Belgium Prof. K. G. Weil Germany A SPECIAL DISCUSSION ON Thin Liquid Films and Bounday Layers 28th 29th and 30th September 1970 A SPECIAL DISCUSSION on Thin Films and Boundary Layers was held at the University of Cambridge on the 28th 29th and 30th September 1970. It represented the first of a new series of Discussion on physical chemistry topics of particular sig- nificance in industrial and technological research. Such Special Discussions are expected to be held on topics of scientific and industrial importance and timeliness at intervals and not necessarily on a yearly basis.The President Prof. Geoffrey Gee C.B.E. F.R.S. opened the meeting and wel- comed 220 members and others. Among the visitors from overseas were Mr. C. M. Allen U.S.A. Dr. R. De Backer Belgium Mr. Bongrand France Dr. E. Drauglis Germany Mr. F. Dumont Belgium Mr. J. G. J. Egberink Netherlands Mr. J. A. de Feijter Netherlands Dr. G. H. Findenegg Austria Dr. G. Frens Netherlands Mr. C . Guestaux France Dr. M. L. Hair U.S.A. Mr. H. Hasmonay France Dr. T. Hendrikx France Dr. E. P. Honig Netherlands Mr. J. Hougardy Belgium Dr. A. M. Joseph-Petit Belgiurn Prof. H. Lange Germany Mr. J. W. Lichtenbelt Netherlands Dr. A. Lucas Belgium Prof. J. Lyklema Netherlands Ir.A. M. Michels Netherlands Dr. K. J. Mysels U.S.A. Prof. Dr. H. van Olphen Netherlands Prof. J. Th. G. Overbeek Netherlands Dr. G. Peschel Germany Dr. A. Prins Netherlands Dr. H. E. Ries Jr. U.S.A. Mr. J. B. Rijnbout Netherlands Dr. K. Roberts Sweden Prof. J. R. Robinson New Zealand Prof. Dr. E. Ruckenstein U.S.A. Prof. Dr. G. Schay Hungary Prof. A. Scheludko Bulgaria Dr. G. Schreier Germany Dr. M. Schwuger Germany Dr. J. A. N. Scott Netherlands Prof. L. E. Scriven U.S.A. Dr. R. Senkus U.S.A. Dr. J. M. Serratosa Spain Dr. H. Sonntag Germany Dr. J. H. Stark Switzerland Dr. W. Stone Belgium Dr. B. Stuke Germany Dr. G. Szasz Switzerland Dr. M. van den Tempel Netherlands Dr. L. Ter-Minassian-Saraga France Mr. J. Thorin France Mr. P. Viaud France Prof. Miss M. Vignes France Prof.A. Vrij Netherlands Prof. A. Watillon Belgium Prof. K. G. Weil Germany CONTENTS Page 7 12 20 30 37 46 57 64 75 89 98 105 112 118 128 138 General Introduction by B. A. Pethica Bursting of Soap Films. Part 4.-The Behaviour of Ions on a Crowded Surface by G. Frens Karol J. Mysels and B. R. Vijayendran Response of an Equilibrium Film to External Disturbances by A. Prins and M. van den Tempe1 Aqueous Foam Films Stabilized by a Non-Ionic Surface-Active Agent by J. S. Clunie J. M. Corkill J. F. Goodman and B. T. Ingram GENERAL DIscussIoN.-Dr. B. A. Pethica Dr. K. J. Mysels Dr. G. Frens Dr. A. T. Florence Prof. A. Vrij Dr. A. Prins Dr. M. N. Jones Dr. H. Sonntag Dr. J. M. Corkill Dr. S. Levine Prof. G. M. Bell Prof. A. Scheludko Prof. J. Lyklema Prof. R. J. Good Composition and Energy Relationships for Some Thin Lipid FiIms and the Chain Conformation in Monolayers at Liquid-Liquid Interfaces by D.M. Andrews E. D. Manev and D. A. Haydon Interaction Energy of Emulsion Droplets and the Influence of Adsorbed Layers on it by H. Sonntag J. Netzel and B. Unterberger Cohesive Properties of Thin Films of Liquids Adhering to a Solid Surface by J . F. Padday GENERAL Drscuss1oN.-Mr. D. W. J. Osmond Dr. H. E. Ries Dr. H. Sonntag Prof. J. Th. G. Overbeek Dr. D. A. Haydon Dr. G. H. Findenegg Dr. B. Vincent Prof. A. Scheludko Prof. J. Lyklema Dr. J. F. Padday Dr. R. G. Picknett Dr. L. M. Dormant Dr. G. Frens Thermodynamic Properties of Thin Films of Some Dipolar Liquids Adjacent to Fused Silica Surfaces by K. H. Adlfinger and G. Peschel Boundary Viscosity of Polydimethylsiloxane Liquids and their Binary Mixtures by B.V. Deryaguin V. V. Karasev I. A. Lavygin I. I. Skorokhodov and E. N. Khromova Boundary Layers of Pure Liquids at the Graphon Surface by S. G. Ash and G. H. Findenegg Contact between a Gas Bubble and a Solid Surface and Froth Flotation by A. Scheludko S1. Tschaljowska and A. Fabrikant Interfacial Energies of CIean Mica and of Monomolecular Films of Fatty Acids Deposited on Mica in Aqueous and Non-Aqueous Media by Anita I. Bailey Andrea G. Price and Susan M. Kay GENERAL DIscussIoN.-Prof. J. Th. G. Overbeek Dr. G. Peschel Dr. K. J. Padday Prof. L. E. Scriven Prof. B. V. Deryaguin Prof. V. V. Karasev Dr. A. Smith Dr. A. Cameron Prof. S. G. Mason Dr. S. G. Ash Dr. G. H. Findenegg Dr. J. W. White Prof. R. J. Good Mr. A. J. Groszek Dr. A. I.Bailey Dr. H. E. Ries Dr. B. A. Pethica Dr. K. W. Miller. Measurement of Forces between Colloidal Particles by L. M. Barclay and R. H. Ottewill 5 6 CONTENTS 148 158 164 175 187 194 202 213 221 231 243 251 257 269 Boundary Layers be f ween Silver Iodide crnd Aqueous Solutioris at Low Temperatures by B. Vincent and J. Lyklema Boundary Layer near the Surface of a Solid Body and Low Frequency Dielec- tric Dispersion by S. S. Dukhin GENERAL DIscussIoN.-Dr. L. M. Barclay Dr. R. H. Ottewill Prof. J. Th. G. Overbeek Prof. G. M. Bell Dr. S. Levine Prof. H. van Olphen Dr. B. A. Pethica Dr. Th. F. Tadros Dr. G. Peschel Dr. A. L. Smith Dr. B. Vincent Prof. J. Lyklema. Nuclear Magnetic Resonance Studies of Water in Disperse Systems by J. Clifford J. Oakes and G. J. T. Tiddy Interlayer Water in Vermiculite Thermodynamic Properties Packing Density Nuclear Pulse Resonance and Infra-Red Absorption by J.Hougardy J. M. Serratosa W. Stone and H. van Olphen Neutron Scattering Studies of Hydrated Layer Silicates by S. Olejnik G. C. Stirling and J. W. White GENERAL DIscussIoN.-Prof. A. Watillon Prof. S. S. Dukhin Prof. H. Pfeifer Mr. J. Clifford Dr. G. Peschel Dr. A. L. Smith Prof. J. Th. G. Overbeek Dr. E. Willis Dr. M. L. Hair Dr. J. M. Serratosa Dr. W. Stone Dr. R. G. W. Anderson Dr. J. W. White Dr. A. M. Hecht Dr. E. Geissler Dr. B. A. Pethica Dr. M. M. Breuer. Measurement of Viscosity of Liquids in Quartz Capillaries by N. V. Churayev V. D. Sobolev and Z. M. Zorin Preliminary Studies of Thick Surface Films by A. J. Smith and A. Cameron Thickness of Very Thin Films in Elastohydrodynamic Lubrication by A. Dyson Mechanical Properties of Very Thin Surface Films by A. D. Roberts and D. Tabor Smectic Model for Liquid Films on Solid Surfaces Part 1 .-Application to Monolayer Boundary Lubrication by E. Drauglis A. A. Lucas and C. M. Allen GENERAL DIscuss1oN.-Prof. J. Lyklema Dr. N. V. Churayev Dr. V. D. Sobolev Dr. Z . M. Zorin Dr. G. Peschel Dr. B. A. Pethica Dr. B. Stuke Mr. D. W. J. Osmond Dr. A. Cameron Dr. H. E. Ries Dr. D. Tabor Dr. A. J. Groszek Dr. A. J. Smith Dr. A. Dyson Dr. K. J. Mysels Dr. A. D. Roberts Dr. Th. F. Tadros Dr. E. Drauglis Dr. G. Frens. AUTHOR INDEX

ISSN:0370-9302    

DOI:10.1039/SD9700100001

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

General Introduction BY B. A. PETHICA Unilever Research Laboratory Port Sunlight Cheshire England Received 14th October 1970 These introductory remarks are intended to serve two purposes. In the first place the reasons for holding this Special Discussion of the Faraday Society are set out. Secondly a review of current problems in research into thin liquid films and boundary layers is given. During the past year the Council of the Faraday Society decided to form an Industrial Sub-Committee of the Standing Committee on Conferences. This decision was a recognition of the fact that research in industry is making a substantial contri- bution to the advance of those basic sciences which the Faraday Society is concerned to foster. It was also recognized that the motivation of industrial research towards the direct use of its results constitutes a powerful current stimulus to the advance of fundamental science in a variety of areas.The principal purpose of the Industrial Sub-committee is to propose topics for Discussions or Symposia which are of scientific merit and timeliness and of industrial significance. These topics may be adopted as part of the Society’s customary programme or may be the basis for Special Discussions or Symposia of which this meeting is the first example. This decision by the Society marks no lessening of concern with its traditional objectives and standards ; rather it marks the Society’s reassertion of the professional objectives common to academic and industrial scientists alike and its responsiveness to developments on the borders of physical chemistry.One such border is that between physical chemistry and the engineering sciences as demonstrated at this Discussion. The physico-chemical problems of industry have always been of interest to the Society as is evident from its contributions in catalysis. Thus new venture is in some ways a return to an early tradition as an examination of the titles of the Society’s Discussions back to the 1920’s will show. In 1920 the Society discussed Basic Slags and in 1921 the subject for debate was The Failure of Metals. There followed Discussions on The Physical Chemistry of the Photographic Process (1923) and Textile Fibres (1924). Coming to more recent times one of the topics for 1954 was The Physical Clzemistry of Dyeing and Tanning ; and of direct interest to us at this meeting the Society in 1948 discussed The Interaction of Water and Porous Materials.That Discussion contained some remarkable foretastes of our debates at this 1970 meeting and I can hardly do better to illustrate the joint scientific and industrial value of the Thin Liquid Film topic than to quote from the 1948 Discussion a remark by the late Prof. D. H. Bangham then Director of the British Coal Utilization Research Associa- tion and formerly Professor of Physical Chemistry in Cairo. “ In view of the large number of papers written on the subject of adsorbed water it is astonishing how little experimental work is directed towards ascertaining its properties. Most workers are content with being able to give a self-consistent account of a very small range of facts of which more than one explanation is possible.There are however awaiting solution a number of technical problems which turn upon the behaviour of these 7 8 GENERAL INTRODUCTION films and it is important that their nature should not remain merely a matter of conjecture ”.I Bangham went on to give a highly topical example of these “ technical problems ” relating to the adhesiveness of dust particles. We can easily add examples to make a formidable list of industrial problems which depend for their solutions in part at least on our understanding the properties of thin liquid layers-exampIes drawn from such areas as lubrication corrosion flotation foaming emulsion formation colloid stability wetting etc. Bangham’s remarkable pioneering work on poly- molecular liquid layers on solids is not as well known as it should be and it is sad to note that after his death the subject has languished among physical chemists in Britain until recently.There were many earlier indications that liquid layers on solids can have unexpected properties-for example in the work of Hardy in 1913-but we may regard Bangham and Razouk’s analysis of the contact angle equilibrium in 1937 as a turning point showing as it did that for non-zero contact angles the polymolecular layer adsorbed on the solid beyond the boundary of a liquid drop at saturation vapour pressure does not simply have the properties of the bulk liquid. At almost the same time Frumkin was drawing very similar conclu- sions in the U.S.S.R. The proposal that the formation of structured liquid layers near to solid surfaces is of importance in lubrication processes also has a long history.Griffiths in 1920 suggested that liquid surface layers possess a special rigidity Bastow and Bowden in 1931 among others taking the opposite viewpoint. Here again workers in the Soviet Union notably FuksY7 have interested themselves deeply in the mechanical properties of liquid boundary layers. The current broad interest in the properties of liquid boundary layers and thin films owes much to the development of quantitative theories of colloid stability particularly in the Soviet Union and Holland by Deryaguin and LandauY8 and Verwey and O~erbeek.~ These theories have facilitated a vast volume of quantitative research into surface forces and to some extent the present debate on the reality and significance of special structural properties in thin liquid films and boundary layers is the result of a growing realization that the accumulated data will not be covered by the interplay of two sets of forces alone (the electrical double-layer interactions and van der Waals’ forces) and of a certain impatience among some colloid scientists with the opacity of the now complicated corrections to a formerly elegant theory particularly so far as the electrical double-layer is concerned.On the other hand despite the many quantitative studies directed to elucidating the special properties of liquids in boundary layers and thin films particularly by Deryaguin and his co-workers considerable doubt remains as to the generality and extent of the alteration of liquids in these films. It must also be admitted that quantitative predictions of the effect of these layers in influencing colloid stability for example are not yet available.Progress towards such quantitative prediction will require further phenomenological investiga- tions and many exact studies of molecular and thermodynamic behaviour in films and boundary layers. To the extent that in dilute electrolyte systems the long-range nature of surface effects due to electrical double-layers and van der Waals’ forces is commonly accepted the issue is not simply one of the distance over which surface forces act in liquid layers but rather as to whether or not the liquid itself can play a more direct role than that of providing a fluid dielectric with convenient dispersion characteristics. In principle the situation can be covered by using more refined two-force models.Such models will take account of saturation effects ion volumes dipole terms asymmetry of polarizability etc. and hence they will include more directly the involvement of the solvent. The corresponding calculations of disjoining pressure B. A. PETHICA 9 and other measurable quantities will then include the energy and entropic contribu- tions due to the solvent. Nevertheless the role of solvent orientation is sufficiently distinct for Deryaguin to introduce the concept of “ forces of the third kind ’,.lo From the experimentalist’s point of view the importance of this concept is that it suggests new experiments designed to reveal the molecular situation at the liquid boundary thereby guiding further theory with a wider range of measurements than hitherto have been available.It is remarkable for example that until recently there has been so little interest in temperature effects in lyophobic colloid systems. An early exception is again in the work of Bangham’s group (Bond Griffiths and Maggs)? Temperature variations play a minor role in the classical DLVO theory but a major role in any model explicitly involving entropic layers and long-range ordering in the liquid. Without doubt some of these recent experiments on tempera- ture effects have been inspired by the “ordered liquid” theories and represent attempts to break away from the two-force theory. The time has come to give up our addiction to precise thermostatting at 25°C. If the phenomena are not very temperature sensitive why bother? And if they are sensitive it is the temperature coefficient that is the most interesting variable.In this Discussion we have a variety of measurements of temperature effects of direct relevance to deciding the existence and role of solvent layers and liquid structure changes in boundary layers. In the paper by Prins and van den Tempel the study of the effect of temperature changes on a free “equilibrium” soap film suggests that disjoining pressure equilibrium may not be the controlling factor in apparent film stability. The paper of Clunie and his co-workers on temperature and salt effects on a non-ionic stabilized foam system raises substantial doubts as to the meaning of the derived Hamaker constants which appear to vary by a factor of two over a 10 K range of temperature. The effects of temperature on liquid layers on quartz as described by Adlfinger and Peschel suggest a strong correlation between the disjoining pressure and ordering in the liquids as do the effects of temperature on viscosity in thin capillaries recorded by Churayev Sobolev and Zorin.in the simple well-defined graphon +liquid alkane system described in the paper of Ash and Findenegg the heats and excess volumes of wetting show temperature coefficients that indicate significant liquid structural changes at the boundary and the data for the graphon/water interface suggest extensive liquid structuring. The paper by Vincent and Lyklema deserves the special attention of colloid chemists since it gives evidence through temperature effects of some structuring of water at the surface of silver iodide sols-perhaps the most exhaustively studied colloid system.On the other hand the 1i.m.r. experiments over a range of temperatures on the aqueous polymer latex system reported by Clifford Oakes and Tiddy and those on the water + vermiculite system reported by van Olphen and his co-workers strongly suggest tight binding of small amounts of water with little evidence of extended alteration in the Brownian motion deep into the liquid layer. Clifford’s results show that the extended structural effects in water near to polyvinyl acetate latex particles reported earlier from the same laboratory,12 are most likely caused by the fibrillar and porous nature of the surfaces of those latices. The polymer latex data show clearly the importance of the precise characterization of the solid surfaces contacting the liquid layers. Of the surface layers discussed in the various papers at this meeting the mica surface studied by Bailey Price and Kay the layered vermiculites and clays discussed in several papers the silver iodide sol surface and the graphon interface are perhaps the best characterized of the solid+ liquid systems.The free-standing liquid films have been customarily regarded as a reliable model system but Prins and van den Tempel may cause us to revise this 10 GENERAL INTRODUCTION opinion,. Surfaces of solid quartz steel and rubber are less well characterized. Quartz has been widely studicd but the surface characterization is ambiguous i n most cases. An interesting illustration of this point is that the heat of immersion into water of un-annealed Aerosil silicas with a range of surface hydrations shows a striking dependence on the temperature of immersion suggesting strongly that un-annealed silica can induce significant structural effects in the local water.When the silica powders are annealed and rehydrated at the surface the temperature variation of the heat of immersion is no longer present for silicas annealed in air (in the presence of a small vapour pressure of water) but partially remains for silicas annealed in vacuo.13 Results of this kind show that short-range surface effects have a profound influence in " triggering-off" structural changes in the local liquid layers and that we should be cautious of supposing that where these effects occur they result from long-range forces entirely. This same point comes out clearly from numerous results on the thermodynamic functions for the adsorption of vapours on solid surfaces.It is well known that the B.E.T. equation as commonly applied to vapour adsorption assumes that the second and successive layers of adsorbed vapour have bulk-liquid properties and to the extent that the B.E.T. equation is successful it gives support to the view that polymolecular liquid layers on many solids have no unusual structure. Even for systems in which the B.E.T. equation gives a good fit however the experimentally determined differential heats and entropies of adsorp- tion do not usually become indistinguishable from the bulk-liquid values until several layers are adsorbed. It is almost certain that increased accuracy in the determination of these parameters would show that very many layers are necessary before true bulk-liquid properties are reached and we should never forget that even if the deviations in molar functions are small the liquid concentrations are high.The detailed preparation of the solid surface has a profound effect on the energetics of the physical adsorption of vapours and necessarily therefore on the behaviour of polymolecular layers [see e.g. Holmes 14]. Another set of considerations are provoked by taking the view that the solid/liquid and fluid/liquid interfacial systems discussed at this meeting represent extreme examples of solutions in the liquid phase. The properties of the solvent take pride of place in this approach and many suggestive correlations come to mind from considerations of solution and liquid-state thermodynamics. This approach is illustrated in part by Adlfinger and Peschel and by Ash and Findenegg.The copious data on the properties of solutions of ions inert gases polar and amphipathic molecules suggest that a variety of structure-making and -breaking effects will be manifest at surfaces. Extrapolating from thermodynamic and n.m.r. studies of surfactant micelles for example one would not expect extensive long-range solvent orientation changes at the surface of oil in water emulsions stabilized by highly charged surfactants. The variations of water-structuring effects with surface charge reported by Vincent and Lyklema are strongly reminiscent of short-range solvent polarization effects in aqueous electrolyte solutions. Perhaps most interesting of the relevant speculations that arise from considera- tions of bulk liquid thermodynamic properties is that the term " liquid " includes " liquid crystals " and a variety of systems involving the reversible aggregation of amphipathic molecules.The phase-rule relationships in these bulk nematic and smectic systems have been worked out for many examples and the use of expressions such as " surface liquid phase " may in some instances be well justified. In this meeting Drauglis Lucas and Allen develop the smectic phase analogy for lubricating films of fatty acids in hydrocarbon oils and Clifford and his co-workers remind us B. A . PETHICA 1 1 of the relationships between lamellar soap phases and thin foam films. The paper from Haydon's laboratory is a valuable contribution on several scores-it provides data on thin oil layers stabilized by a chemically defined and thermodynamically characterized solute in the oil.A striking result is that this stabilizer glyceryl mono- oleate (which is incidentally related to well-known emulsion stabilizers used in the food industry) shows reversible micellar aggregation in hydrocarbon solutions. Furthermore these black oil films form typically at concentrations near to or above the inicelle point of the stabilizer which is a common situation also in the formation of black films from aqueous surfactant solutions. These considerations suggest that the sharp salt-induced transition (recorded by Clunie and co-workers) between the first and second black films stabilized by a non-ionic surfactant is in fact a phase change which should have a direct parallel in bulk solutions of the stabilizer. These suggestions take us directly to the point that even if the forces controlling the thinning of free-standing thin films are correctly represented by the interaction of electrical double layers and van der Waals' forces in planar (or laterally extended) geometric arrangements we must still enquire as to the causes of the planar arrange- ments.Smectic and nematic phases provide one of the keys to answering this question. The thermodynamic stability of liquids which are either themselves molecularly asymmetric and flexible or contain such asymmetric solutes involves a balance of conformational entropy and energy terms deriving in part from geometrical considerations and in part from " chemical " factors (asymmetric polarizabilities of different groups etc.). This balance is necessarily modified in the vicinity of the asymmetry we call a surface and our Discussion will go far in showing whether and where these structural changes are significant in a wide range of experimental situations.' Bangham Disc. Furuday SOC. 1948,3 102. Hardy Proc. Roy. Sac. A 1913,88 313. Bangham and Razouk Trans. Faruduy SOC. 1937,33 1459. Frumkin Zhur. Fiz. Khim. 1938,12 33. Bastow and Bowden Proc. Roy. Soc. A 1931 134,404. vol. 1 p. 79. ' Griffiths Phil. Trans. A 1920 221 163. ' Fuks Research in Surface Forces ed. Deryaguin (Consultants Bureau New York 1964) Deryaguin and Landau Actu plzysicochim. 1941,14,633. Verwey and Overbeek Theory of the Stability of Lyophobic Colloids (Elsevier Amsterdam 1948). ' O Deryaguin Research in Surface Forces ed. Deryaguin (Consultants Bureau New York 1964) '' Bond Griffiths and Maggs Disc. Faruday SOC. 1948,3,29. ' l 3 Tyler Taylor Pethica and Hockey Trans. Furaduy SOC. in press. l4 Holmes The Solid-Gas Interface ed. Alison Flood (Edward Arnold & Marcel Dekker 1967) *' Clifford and Pethica Trans. Faruduy Soc. 1965 61 182. vol. 1 p. 3. Johnson Lecchini Smith Clifford and Pethica Disc. Faraday SOC. 1966,42,120. vol. 1 p. 127.

ISSN:0370-9302    

DOI:10.1039/SD9700100007

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

Bursting of Soap Films Part 4.-The Behaviour of Ions on a Crowded Surface BY G. FRENS,* KAROL J. MYSELSt AND B. R. VIJAYENDRAN $ R. J. Reynolds Tobacco Co. Research Dept. Winston-Salem North Carolina 27102 U.S.A. Received 13th April 1970 The growing hole of a bursting soap film is preceded by an aureole of accelerating contracting and thickening film. Measurements of the cross-sectional profile of such aureoles are reported and provide the basis for an interpretation in terms of surface tension changes accompanying the contrac- tion of the a m which is complete in less than a millisecond. The surface tension of sodium dodecyl sulphate solution decreases to below 15 mN/m (dynlcm) under these conditions. As desorption is negligible the data can be interpreted in terms of an extension of surface pressure-area per molecule curves towards high pressures and low areas.This extension shows an unexpected decrease of slope at low areas and indicates a rapid increase in intermolecular attractions as the surfactant ions become crowded on the surface. Once a thin liquid film specifically a soap film develops a tiny perforation the latter will keep growing and the whole film disappears rapidly. In soap films the edges of a hole recede at speeds of the order of lo3 cm s-l. The driving force is the surface tension of the two faces of the film which exerts an uncompensated force on the perimeter of the hole. It was assumed that the growing hole is surrounded by undisturbed film except for a very narrow rim which contains all the receding material. Recently however it has been found 1 * that the expanding hole is preceded by a wide zone (the aureole) of moving film material.The width of the aureole is comparable to the radius of the hole and the expansion of the hole and the aureole are approximately proportional. The thickness 6 of the film increases in the aureole region from the thickness do to approximately 2d0 at the rim of the hole. Flash photographs such as fig. 1 which show some of the complicated features of aureoles have been published.'. A theoretical analysis of the aureole phenomenon was given by Frankel and My~els.~ They showed that the existence of aureoles can be explained if there is a gradient in the surface tension of the film in the vicinity of the hole. Such a gradient is closely related to the thickness variations in the aureole. As a film element increases in thickness it decreases in surface area.If the relaxation of the surface through the desorption of surfactant molecules is slow as compared with the rapid compression of the collapsing film structure then the film surface resembles an insoluble monolayer. The surface tension of a film element becomes smaller as its surface area decreases i.e. as its thickness 6 increases. It is the purpose of the present paper to show that an interesting extension of the classical TJ-A curves (surface pressure against area per molecule) becomes available when this theory is applied to experimental data obtained for the thickness profile of the aureoles. * present address Philips Research Laboratories N.V. Philips' Gloeilampenfabrieken Eind- hoven NetherIands. -f present address Research Dept.Gulf General Atomic Inc. San Diego California 921 12 U.S.A. $ present address Research Dept. Pitney-Bowes Staniford Conn. U.S.A. 12 FIG. 1. Bursting soap film photographed using a 0.5 ys flash. The aureole surrounding the hole is relatively narrow and shows a large frontal shock. [To face page 12. G . FRENS K . J . MYSELS AND B . R. VIJAYENDRAN 13 THEORETICAL In principle it is possible to calculate values for the surface tension as a function of the increase in thickness and of the time during a burst for any arbitrary behaviour of an aureole provided that the history of the evolution of the aureole is sufficiently well known.4 In practice one would need too many and too accurate data to make such an analysis feasible. The interpretation of data is much simpler if the physical situation in a bursting soap film corresponds to a pseudo-equilibrium in which there is no desorption from the surface and where the surface pressure depends only on the area per molecule.In such a system there is no intrinsic (relaxation) timescale and the bursting becomes self-similar i.e. all features of the aureole and of the hole expand from the origin each at its own constant velocity. This assumption means that the surface tension o depends only on the relative shrinkage a or thickening p of the film defined by a = 1/p = a0/6 and not on time so that a single (a,a) curve such as that of fig. 2 characterizes the system. do I / 0 I shrinkage a FIG. 2.-Schematic (a,~) curve. The dashed lines indicate portions corresponding to shock waves in unidimensional bursting.If the burst originates from a straight line-is unidimensional-a complete analysis is then p~ssible.~ If it originates from a point which is the case in our experiments only certain features of the unidimensional analysis can be extended to this radial case and others have to be computed numerically. A fundamental velocity in this analysis is Culick’s velocity u = (2aO/p&,)* where go is the surface tension of the film at rest and p its density. Culick showed that this is the velocity of the rim of the hole but in his model there was no aureole. In the presence of an aureole it can be shown that uh is always smaller than u in the unidimensional case and is smaller or slightly larger in the radial case. As indicated in fig. 2 only certain parts of the (o,a) curve which are convex downwards are represented by the visible smooth slopes of the aureole; those that have the 14 BURSTING OF SOAP FILMS opposite curvature are hidden in shock waves the most important of which is the rim itself.4 The rim may be considered as the shock wave due to the zero surface tension within the hole.Generally there is a shock wave at the outer edge of the aureole but there niay be 0thers.l In the unidimensional case the shock waves correspond to chords exactly tangent to the (o,a) curve. In the radial case chords are not exactly tangent.6 A special case is that of a (a,a) curve lying entirely above the diagonal joining the initial state to the origin. In that case the entire aureole is compressed into the shock wave represented by that diagonal and the width of the aureole is reduced to A useful extension of the theory is an expression for the limiting properties of the aureole at its thickest point where it joins the final shock wave i.e.the rim. We denote these by the subscript 1. Conservation of mass and momentum gives for a shock wave moving with a velocity us and separating film elements moving with u and uZ respectively (us - 211)(21 - r l ) p 6 = 2(0l - Q) (1) where o- and o2 are the corresponding surface tensions on the two sides of the wave.4 For the limiting point u2 = us since the material passing through the shock wave remains in the rim o2 = 0 as explained earlier and subscript 1 is replaced by 1. Introducing 21 one obtains finally (u,-iu1)2 = U"cJ&Uo. (2) The value of a. is easily obtained and that of (rl is obtained by integration where p may be defined as to/t where to is the time for a reference feature of the aureole to reach a certain position x or Y in space and t is measured at the same point.The velocities zi arc given by eqn (5.18) of ref. (4) for the radial case and yield where I is an integral giving the amount of film material between the centre and the point considered and uo = r / t o . As there is no material in the hole 1 = 0 and combining with eqii (2) we obtain which is satisfied only at the rim. As this is derived on the assumptions of self- similarity but involves only information concerning the profile of a single aureole it can provide a criterion of self-similarity for each aureole. For the unidiinensional case the similar expression I:(uoi%)2 = Pl(Q1lQd (6) is derived similarly using eqii (5.6) of ref.(4). EXPERIMENTAL Experimentally the velocity of the rim and that of any shock waves is probably best estimated from photographs taken at different time intervals after initiation of the burst by an electric spark.' Details of the profile of an aureole are difficult to obtain precisely in this way and methods ' measuring either the variation of thickness with time at a fixed point, G . FRENS K . J . MYSELS AND B . R . VIJAYENDRAN 15 as the aureole moves by or the time required for a feature to proceed from one point to another are preferred. The results reported here were obtained with an instrument using a laser and measuring simultaneously the thickness of the film at two points along a horizontal radius as indicated in fig. 3 which also shows a typical oscilloscope result and its inter- pretation.TOMULTIPLIERS -ASS 'LATE / OSCILLOSCOPE TRACE T I M E 3 -6 t f / l I INTER P E TAT1 0 N FIG. 3.-Scheniatic diagram of the apparatus used. The laser beam is split and concentrated at two points of the film. The reflected interference pattern changing as the aureole passes these points is recorded through photomultipliers and a two-beam oscilloscope and interpreted as shown on the right. RESULTS A major difficulty discussed in detail earlier,l was that whereas the theory predicts that all velocities should vary with Culick's velocity i.e. with l/dS gross deviations were observed for thin mobile films as well as for rigid films of all thicknesses. The explanation offered earlier,l that even the apparently undisturbed film is slightly compressed was supported by direct evidence for rigid films and may be significant when applied to these but we have not been able to support it experimentally for mobile films.On the other hand we have found that frictional resistance of the atmospliere- the windage of the moving aureole and rim-greatly affects the velocity of the rapidly moving rim of thinner films and seems to account for the deviations observed with first black and thicker thin films. Fig. 4 shows rim velocities observed in air in helium and in hydrogen for films of various thicknesses of the same solution. The viscosities of these gases are 185,198 and 89 ,UP and their densities 1.20 0.165 and 0.083 g/l. respectively. The deviations from the 1 / JS behaviour indicated by the straight line become less as the inertia and viscosity of the atmospliere decreases.For thin films the deviations in hydrogen are still significant though much smaller than in air but above some 100 nm they cease to be perceptible. Hence further discussion will be restricted to films having greater thicknesses and bursting in hydrogen. That bursting is self-similar to a first approximation is shown by the constancy of rim velocities as the aureole grows which has been reported earlier for films in air. This has been supported now by experiments in hydrogen. The velocity of the frontal shock is also constant and obeys the 1/,/8 relation closely. A more sensitive criterion is given by the shape of the aureoles which should be unchangirig after correction for growth as a function of time and the corresponding spreading in space.A convenient form is to represent the thickness of the aureole as a function 16 BURSTING OF SOAP FILMS of f f / t where tf is the time for the frontal shock to reach the point of observation and t corresponds to the relative thickening B = S/S, as indicated schematically in fig. 4. The solid line of fig. 5 and the corresponding large points show the results obtained P 1.5 1.0 film thickness nm line shows the slope of ao-4. FIG. 4.-Rim velocities in various atmospheres as a function of original film thickness a0. The - ‘S* A;,& - - - ! I$\ 0 - - 1 in three experiments for the later stages of an aureole. The precision of the measure- ments defines the line quite well. The position where the rim should lie under assump- tions of self-similarity according to eqn (5) is indicated by an arrow and is close to that observed.For younger aureoles the precision gradually decreases as shown by G . FRENS K. J . MYSELS AND B . R. VIJAYENDRAN 17 the smaller points and differences in profile appear as indicated by the dashed line. These differences although systematic are not sufficiently larger than all the un- certainties of the measurement to be accepted as definitely real. Furthermore the criterion of eqn (5) indicates a lack of self-similarity for the dashed line. Fig. 6 shows the (a,@) curves corresponding to the profiles drawn in fig. 5. These were obtained by computer integration using Simpson’s approximation of the I 0 I shrinkage a FIG. 6.40,a) curves derived from the lines of fig. 5. differential expression (5.22) of ref.(4). The difference between these curves is relatively minor so that the uncertainty in a is much less than in the profile itself. This is generally true for (a,a) curves for aureoles differing only in their thicker areas and results from the complex relation Deviations such as those of fig. 5 between young and old aureoles could stem from two sources windage which presumably is greatly reduced by operating in hydrogen or relaxation of the surface by desorption of surfactant molecules under surface pressures much above the equilibrium values leading to higher a values for a given a for older aureoles. The curves of fig. 6 do indicate such an effect upon 0 and if the differences are real they could be used to study such desorption. On the other hand extrapolation to zero age of the aureole could then also be used to correct for any relaxation and serve as a basis for interpretation in terms of the self-similar theory.As shown in fig. 6 the original surface tension of the solution is reduced by about 63 % in the last stages of the aureole. In absolute terms this corresponds to a reduction from 38 to only 14 mN/m considerably below values normally encountered at the air-aqueous solution interface. between the two. DISCUSSION The (o,a) curve is closely related to the surface pressure-area per molecule ( l l A ) curve since II = t ~ ~ ~ ~ - t ~ and a = A/Ao where A . is the area per molecule 18 BURSTING OF SOAP FILMS in the original film provided that there is no desorption. Whereas oo is readily measured the direct determination of A . requires relatively delicate experiments with foaming or with tracers.Indirect determination of A . can be based on highly accurate surface tension and activity measurements through the Gibbs equation. Hence in practice literature data which are often conflicting have to be used. For sodium dodecyl sulphate (NaLS) in water a value of 0.415 nm2 (41.5A2) based on foaming seem most likely to be correct for solutions at and slightly above the c.m.c. 60 50 40 E z E \ 30 I=!! 20 10 The solid line of fig.-7 shows the (II,A) curve thus obtained \ \ I I 1 0.2 0.3 0.4 0’5 area/molecule nm2 FIG. 7.-The solid line is the (rI,A) curve corresponding to the solid ( ~ p ) curve of fig. 6. The dashed lines are some of the availableliterature data for equilibrium adsorbed monolayers of NaLS solutions R from equations of Rehfeld lo ; W data of Weil ; W + E areas of Weil 9 surface tensions of Elworthy and Mysels.from our data and based on this value. It is essentially a mirror image of the (o,cc) curve of fig. 6 and provides an extension of (II,A) curves for ionic monolayers into a new region. If the interpretation of a (o,ct) curve in terms of (n,A) is justified it should give to a first approximation a sniooth extension of the classical (IT,A) curve for higher areas. The interrupted lines of fig. 7 show some of the literature data 9-11 in this region. The values at higher areas are obtained on solutions of NaLS alone of varying concentrations and therefore varying ionic strengths. Our data pertain to a single solution and therefore presumably to a constant ionic strength of the subjacent liquid.This difference could lead to slightly different slopes. Reducing ionic strength at constant A should lead to an increase of interionic repulsions and therefore to an increased surface pressure. Hence the interrupted lines should tend to be steeper than if they had been taken at the constant ionic strength corresponding G . FRENS K . J . MYSELS A N D B. R . VIJAYENDRAN 19 to the c.m.c. As it is the data of Weil seem to give the smoothest fit with ours. Literature data particularly the surface tension values below the c.m.c. show con- siderable disagreement among themselves despite the precautions taken by each author to ensure purity and acc~racy.~-'~ Our data are obtained above the c.m.c. to reduce the effect of surface-active impurities by solubilizing them. Hence any conclusions from the intercomparison of these data must be highly tentative but is indicative of the possibilities of this approach.The striking feature of fig. 7 is that the heretofore known curves for ionic surfactants as exemplified by the interrupted lines are convex to the abscissa whereas most of our curve is concave. If the change of curvature did occur at the point where the two sets of data meet this would be a strange coincidence. In fact however practically all aureoles examined until now (the only exception being a complex mixture of alkylbenzene sulphonates) show a frontal shock. This shock corresponds to a part of the (a,a) curve which is not directly accessible because it includes a curvature opposite to that of the accessible part (fig. 2). Hence the frontal shock wave indicates an extension of the curvature observed by conventional techniques into the region where A<Ao.The surface compression and therefore the change in A during this frontal shock varies in different solutions and can reach 10 %. The aureole which follows after the frontal shock represents still lower values of A . Fig. 2 indicates that the reversal of the curvature in this part of the (I-I,A) curve is intrinsic to the existence of the aureole phen~inenon.~ Hence the reality of the observation that (Tl[,A) plots have concave portions at A < A requires only a qualita- tive validity of our experimental results and their interpretation. It does not depend on the precision of the measurements nor on the detailed outcome of the calculations. The curvature deduced from bursting experiments would be reduced by any relaxation effects since the shrunken area as measured by a would be attributed to the smaller number of molecules remaining in the surface.It does not seem possible however that any small relaxation effects occurring in our experiments could account for the observed result. An equation of state for the surface would describe the (IT,A) curve for A below as well as above Ao. Such a quantitative interpretation of the (II,A) curve in terms of intermolecular forces should become more meaningful now that the aureoles in bursting soap films give access to the metastable states of a surface where the van der Waals attraction between the molecules becomes strong enough to reverse the curvature of the (II,A) plot. Progress along these lines awaits the improvement of the experimental data including those concerning the surface tension and the area per molecule below the c.m.c.W. R. McEntee and K. J. Mysels J. Phys. Chem. 1969 73 3018. I. Liebman J. Corry and H. E. Perlee Science 1968 161 373. K. J. Mysels and J. Stikeleather J. Colloid Inferface Sci. submitted for publication. S. Frankel and K. J. Mysels J. Phys. Chem. 1969 73 3028. F. E. C. Culick J. Appl. Phys. 1969 31 1128. K. J. Mysels and B. R. Vijayendran unpublished work. ' A. T. Florence G. Frens B. R. Vijayendran and K. J. Mysels unpublished work. * A. Wilson M. B. Epstein and J. Ross J. CoZloid Sci. 1957 12 345. I. Weil J. Phys. Chem. 1966 70 133. l o S. J. Rehfeld J. Phys. Chem. 1967 71 738. P. H. Elworthy and K. J. Mysels J. Colloid Interface Sci. 1966 21 33 1. l 2 (a)B. A. Pethica and A. V. Few Disc. Faraday SOC. 1954 18,258. (b) A. P. Brady J. Phys. Chem. 1949,53 56. (c) G. D. Miles and L. Shedlovsky J. Phys. Chem. 1944 48 57. (d) E. J. Clayfield and J. B. Matthews Proc. 2nd Int. Congr. Surface Activity 1957 p. 172.

ISSN:0370-9302    

DOI:10.1039/SD9700100012

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

Response of an Equilibrium Film to External Disturbances BY A. PRINS AND M. VAN DEN TEMPEL * Unilever Research Laboratories Vlaardingen /Duiven The Netherlands Received 2nd April 1970 An equilibrium film situated in air saturated with water vapour is subjected lo a disturbance consisting of a rapid change of the temperature of the surrounding atmosphere. The resulting large change in film thickness is found to be due to exchange of water between film and atmosphere and not to expansion or contraction of the film. Pseudo-equilibrium films of widely varying thickness can be formed by means of this process. For a film with a sufficiently large area the thickness of an element far from the border is determined by the water vapour pressure equilibrium rather than by the disjoining pressure equilibrium.The thickness profile of a large vertical film in a state of apparent rest is explained on the basis of the different time scales associated with the various equili- brium processes. It was noticed by Gibbs that a thin liquid film in air need not necessarily be in complete thermodynamic equilibrium even when it is in a state of apparent rest. In a small element of the film (Le. having dimensions comparable to the film thickness) several of the equilibrium conditions will be rapidly satisfied. In particular after a disturbance the uniformity of chemical potentials and of temperature will be re-established in a time which is very short compared to the usual observation time and therefore these equilibrium conditions may be regarded as being permanently satisfied in a film element.The values of the chemical potentials in the film element may however differ from those in other parts of the system. Equilibration by transport along the film or through the adjoining bulk phases may then require a time comparable to the usual observations time or even much longer. The equilibrium thickness of the film element is determined by mechanical equilibrium of the forces acting in a direction perpendicular to the plane of the film? The mere existence of thick films slowly draining to their equilibrium thickness shows that the time required for this equilibration process must be measured in minutes. Measurements of film thickness can be carried out before this mechanical equilibrium has been established and such measurements can be used to obtain information about the rate and the nature of the other slow equilibration processes.For example the experiment of Plateau in which a soap bubble is thinned locally and reversibly by the warmth of a finger indicates the effect of small temperature variations. Recent work 3* has shown that films become thinner when exposed to an atmosphere with reduced water vapour pressure. Measurements of equilibrium film thickness 2 * require extreme caution to prevent evaporation and even then they must be carried out in close proximity to the border where complete equilibration with the contacting bulk solution may be expected to be fairly rapid. The present investigation is concerned with a more detailed investigation of the effect of small changes in temperature on the thickness of a film which is in a state of apparent rest in particular in regions well away from the film borders.* authors’ address Olivier van Noortlaan 120 Vlaardingen The Netherlands 20 A . PRINS AND M . VAN DEN TEMPEL 21 EXPERIMENTAL In this work it was necessary to take elaborate precautions to prevent uncontrolled temperature fluctuations in the system. The films were made and investigated in a completely closed glass bottle placed in a large box of double-walled glass. Water of con- stant temperature was circulated between the bottom and side-walls of the box. The box was covered with sheets of foamed plastic and placed into a constant temperature room. All the necessary manipulations with the film took place from outside the box using magnetic coupling across the wall of the glass bottle. Under these conditions the temperature in the glass bottle fluctuated over 0.02"C in a period of 10 h.After the glass bottle had been provided with about 400 cm3 of the solution the system was left to equilibrate for at least 24 h. Then a film was produced by slowly raising a frame originally submerged in the solution. The film of 4 x 4 cm was suspended between nylon wires of about 10 microns thickness weighted with a piece of glass tubing to ensure a perfectly flat film. The thinning process of the filmwas observed by following thedownward motion of the interference fringes. Fast-draining films were used in most experiments. After a few minutes a " black " region developed at the top of the film and covered the whole area in about 30min. Measurements of film thickness were started before the development of a black film and continued for at least several days.axis of Film rotation FIG. 1 .-Schematic top view of the optical apparatus used for the scanning reffectometric film thick- ness measurements. Film thickness was monitored by recording the intensity of reflected light. A light beam from a halogen lamp T (see fig. 1) passed through a layer of water a heat filter and an interference filter (5460&80&. From the beam-splitter B a part of the light moves on to the film and is reflected in the photomultiplier PI. The remaining part of the original beam is reflected in the photomultiplier Pz where it serves as a reference. The ratio of the output of the two photomultipliers was recorded. The maximum intensity of the light reflected by the film having the optical thickness of one-quarter wave-length was used for calibration.The equivalent water thickness of the film was calculated from the amount of reflected light by the usual procedure (e.g. ref. (2)). The optical apparatus was fitted on a rigid frame that was made to carry out small-amplitude oscillations around an axis perpendicular to the film. The distance between axis and film was about 1 m. In this 22 RESPONSE OF EQUILIBRIUM FILM TO DISTURBANCES way the area of the film seen by the photomultiplier PI (0.3 x 0.02 cm2) was made to oscillate over a nearly vertical line of about 2 cm length in 19 s. Black surfaces were arranged in suitable locations to absorb stray light and to reduce background illumination. Meaningful results can only be obtained if the film is perfectly flat and exactly perpen- dicular to the axis of rotation of the optical apparatus.Alignment was carried out before each measurement by means of cross-wire R and microscope M using the autocollimation principle. It was verified that the presence of the light beam did not affect the film thickness. After several hours (see fig. 2) the film came to a state of apparent rest as indicated by a thickness profile remaining constant for at least many hours. Then the temperature of the circulating water was increased by 1°C in about 2 min and the response of the film was observed. 200 E 5! final + final + l o o t I 1 I I 500 1000 1500 time/s FIG. 2.-Film thickness as a function of the time measured at the top (1) and the bottom (2) of the scanned area for a black film stabilized with 3 x mol ~ m - - ~ sodium lauryl sulphate at 250°C.The sodium dodecyl sulphate was a purified sample containing an unknown but small amount of dodecanol. No attempt at further purification was made because hydrolysis cannot be avoided during the experiments lasting several days. Moreover films of sufficient stability could only be obtained in the presence of some dodecanol and/or inorganic electro- lyte; even then the concentration of the main surfactant had to be in excess of the c.m.c. The cetyl trimethyl ammonium bromide was the same sample used in an earlier investiga- tion.6 Water and other ingredients were of the highest purity available. RESULTS AND DISCUSSION Even though elaborate precautions were taken to ensure that the system would be in complete thermodynamic equilibrium the results show that this could not be achieved.For easy comparison the " theoretical " profile has also been indicated in fig. 3 and 4. This profile was calculated on the assumption that the sum of the electrostatic repul- sion between the charged film surfaces Results of film thickness profile measurements are shown in fig. 3 and 4. PR = 64c RTy exp (- id,), A . PRINS AND M. VAN D E N TEMPEL 23 ! and the attraction due to Van der Waals' forces between the film molecules PA = - A/6niii (2) is just compensated by the hydrostatic head PH = - p g H of the film liquid. Here the electrolyte concentration c in the film liquid is expressed in mol ~ m - ~ and was assumed to be the same as in the bulk liquid. The aqueous core of thickness 6 = (6*- 20) A is bounded on both sides by a layer of 10 A thick consisting mainly of the hydrocarbon parts of the surface-active material.Using this model the equivalent water thickness h = (a,+ 8) A which is the quantity plotted in the figures. The other quantities in the above equations have their usual meaning (see e.g. ref. (2)) where y z tanh 1 and A = 5 x 10-13 erg. 747 I 385 5135 s I I I I 50 100 I50 200 film thickness/A FIG. 3.-Thickness of a film stabilized with 3 x mol ~ r n - ~ sodium lauryl sulphate in apparent rest (1) and after heating for the time indicated. Curve 2 is the calculated " equilibrium " film thickness for an electrolyte concentration of 3 x mol ~ r n - ~ . Original temperature 250°C. In all cases the thickness profile of the " equilibrium " film differed appreciably from the profile expected on the basis of the usual criterion 2* for determining equilibrium film thickness i.e.the condition for mechanical equilibrium in a direction perpendicular to the plane of the film. This means that either this equilibrium condition is not satisfied or the electrolyte content is much higher than in the bulk solution in a film element at some distance from the lower border. Evidence has already been presented to show that electrolyte is accumulated in the film liquid,' and this is confirmed by the analysis of the present results. If the equilibrium condition AP = PR+PA+PH = 0 is satisfied in every element of the film at rest it is possible to estimate the actual electrolyte concentration in the film liquid. This is because the thickness that satisfies eqn (3) is determined almost conipletely by the electrolyte concentration in (3) 24 RESPONSE OF EQUILIBRIUM FILM TO DISTURBANCES the film liquid.Results of such calculations (fig. 5) show that a very steep concentra- tion gradient in the film liquid is required to explain the thickness profile on this basis. 2400s 1500s 600 s I I 0 50 100 150 film thickness/A i I 200 FIG. 4.Thickness of a film stabilized with 1.6 x mol C M - ~ cetyltrimethylammonium bromide and 2.84 x mol C M - ~ sodium bromide in apparent rest (1) and after heating for the time indicated. Curve 2 is the calculated " equilibrium " film thickness for an electrolyte concentration of 3 x mol ~ r n - ~ curve 3 the thickness of a film after slowly draining to apparent rest in 7 h. Original temperature 29.1"C. From the response of a film to a small change in temperature however it may be concluded that eqn (3) is not in general satisfied in every film element and also that the actual concentration gradient in the film liquid is less steep in this case (see fig.5). The important observation is here that small temperature differences between the film and the bulk liquid result in immediate and reversible variations of film thickness. The greater part of the measured rate of film thinning (fig. 6) must be due to evapora- tion of water from the film surfaces which proceeds at a rate determined by the diffusion coefficient D in the gas phase and the distance x to the region of condensa- tion. The resulting rate of film thinning is dh 1 8 0 - _ - dt - R X A P ' where Ap is the excess water vapour pressure of the thin film. In a first approxima- tion the excess vapour pressure is determined by the temperature difference only AP = ( Q P ~ I R T ~ Y G ( 5 ) where Q is the heat of evaporation of water at room temperature andp its equilibrium vapour pressure.The value of AT was estimated from temperature measurements A. PRINS AND M. VAN DEN TEMPEL 25 by means of a set of thermistors placed in the gas phase a few mm from the film. The actual temperature of the film element will be somewhat lower but even then application of eqn (4) and (5) shows that the observed rate of film thinning can be explained as a result of evaporation of water from the film followed by diffusion of water vapour over a distance x of a few cm and condensation on the surface of the bulk liquid. 3 2 I I I I 2 4 6 0 10 electrolyte c~ncentration/lO-~ rnol C M - ~ FIG.5.-Electrolyte concentration as a function of the height for a film in apparent rest stabilized with 3 x mol ~ r n - ~ sodium lauryl sulphate (curve l) 1.5 x lo-' mol C M - ~ sodium lauryl sulphate and 1.5 x mol ~ m - ~ sodium bromide (curve 2) 1.6 x mol C M - ~ cetyltrimethylammonium bromide and 2 . 8 4 ~ mol ~ r n - ~ sodium bromide (curve 3) for AP = 0 (full lines) and for AP = -2gAc (dashed lines). A possible contribution from extension to the thinning of films on warming was studied by suspending a thick film from the black film using a technique that has already been described in connection with film elasticity measurements.6 The rate of descent of the border line between the black and the thick film was practically unaffected by the change in temperature.It is concluded therefrom that downward motion of the border line is only determined by the usual drainage processes in the film and there is no appreciable contribution from an extension of the black film. Fig. 7 shows a typical result of such measurements. Evaporation of water from a film element results in increased solute concentrations and therefore in decreased surface tension and water vapour pressure. The thickness of the element at any time during the establishment of the water vapour pressure equilibrium is determined not only by its decreased volume but also by expansion or contraction resulting from other equilibration processes. In order to clarify the behaviour of the film element under these circumstances it is necessary to realize that eqn (3) is not the only condition to be satisfied in complete thermodynamic 26 RESPONSE OF EQUILIBRIUM FILM TO DISTURBANCES equilibrium.There are several other conditions which may conveniently be dis- tinguished according to the characteristic time associated with the corresponding equilibration process. For any film element at any given time some of these condi- tions may already be satisfied because the equilibration processes take less time than the age of the element. Other processes may require much more time in particular because an equilibration process may proceed at very different rates in the plane of the film and perpendicular to that plane. 0 600 1200 1800 timels FIG. 6.-Increase in temperature (a) and the resulting decrease in film thickness (0) measured at three indicated heights in the film as a function of time.The film is stabilized with 1.6 x mol ~ r n - ~ cetyltrimethylammonium bromide and 2.84 x Original tempera- mol ~ r n - ~ sodium bromide. ture 29.1"C. For the present discussion the most important equilibrium conditions and the corresponding equilibration processes are (i) Uniformity of temperature and of chemical potentials of all components in a film element including the contacting gas layer. Equilibration occurs by diffusional transport between film liquid and surfaces * ; the characteristic time is much less than a milli-second. (ii) Uniformity of surface (or film) tension along the entire surface. A simple experiment demonstrating the rapidity of this process was already described by Gibbs,l and later experience has shown that the time required is of the order of milliseconds.The mechanism by means of which a thin film satisfies these two equilibrium conditions simultaneously consists of adjusting the thickness of the film elements by expansion or contraction. Flow of film liquid with respect to its surface does not contribute significantly to changes in film thickness in so short a time. (iii) Uniformity of temperature and of chemical potentials of all volatile com- ponents in the whole system. This is achieved in a time of the order of seconds by A . PRINS AND M . VAN DEN TEMPEL 27 evaporation diffusion in the gas phase and condensation. The process and the characteristic time for thermal equilibration are similar. (iv) Mechanical equilibrium in the direction perpendicular to the plane of the film i.e. between disjoining pressure and hydrostatic head.This is the condition expressed by eqn (3). The main process by means of which this equilibrium is established consists of flow of film liquid with respect to the s ~ r f a c e ~ and the charac- teristic time is of the order of many hours. (v) Uniformity of chemical potentials of non-volatile components in the whole system. Diffusion of non-volatile components along the film proceeds about lo5 times slower than diffusion in the gas phase and therefore the chemical potentials of such components may not be uniform in a large film unless it has been left undis- turbed for at least several days. During equilibration as required by conditions (iii) to (v) the equilibrium conditions (i) and (ii) remain permanently satisfied. Moreover the thickness of an element of the film of given composition is already completely determined by conditions (i) and (ii) irrespective of whether any further equilibrium conditions are satisfied.I - \ I I I i 0 100 150 200 film thickness/A FIG. 7.Thickness of a film stabilized with 1.5 x mol ~ r n - ~ sodium bromide in apparent rest (l) 30s after loading with a thick film (2) and after further draining for 760 s when the temperature is kept constant (3) or when the film is heated (4). mol ~ m - ~ sodium lauryl sulphate and 1.5 x The interpretation of the film thickness measurements reported in fig. 3 and 4 must therefore be based on the supposition that condition (v) is in general not satisfied even if the thickness profile remains unchanged during several hours. The liquid composition of the element may differ appreciably from that of the bulk liquid and even from that of other film elements.The actual composition of the element at a given time is determined by the accumulated effects of processes (i)-(v) since the birth of the elements and a full description of these effects is not possible at present. 28 RESPONSE OF EQUILIBRIUM FILM TO DISTURBANCES That the composition of a film element depends upon the states through which it has passed during its production is shown by curve 3 in fig. 4. This curve relates to a slow-draining film made from an aged solution and which required 7 h to attain a state of apparent rest. It is evident that the composition of a film element during the measurement reported in fig. 3 and 4 is determined by conditions (i)-(iii) being satisfied and this composition will in general not be such that AP of eqn (3) will vanish.The excess pressure AP gives rise to flow of film liquid with respect to the surface^.^ The resulting rate of thinning in a part of the film with area a2 is then An evaluation of the magnitude of this effect requires an estimate of AP for which we need the electrolyte content in the film liquid. The values shown in fig. 5 cannot be used for this purpose because they are based on the supposition AP = 0. If it is assumed that equilibrium condition (iv) need not necessarily be satisfied in a film element in a state of apparent rest it is still possible to estimate the electrolyte concentration in the film element from condition (iii) expressing the uniformity of the chemical potential of water. In principle it would also be possible l o use condition (ii) for this purpose.This is the condition stating that the film tension in the element at height H as determined by the composition and thickness of the element should be equal to the equilibrium surface tension of the bulk solution corrected for the weight of the film suspended from the element. The use of this condition would require extremely accurate data on the relation between surface tension and composition which are not available at present. Therefore the remaining part of this paper will be confined to a discussion of the water vapour equilibrium in a film at apparent rest. If the electrolyte content in the film element is higher than in the bulk solution by an amount of Ac this will reduce the chemical potential of water by an amount that is approximtely determined by ApW = -2RTvgAc.(7) Here g is the osmotic coefficient of the solutes and v = 18 cm3 mol-l. For a film at rest the temperature is assumed to be uniform and uniformity of the chemical potential of water requires that the pressure in the film liquid is higher than in the surrounding atmosphere by an amount of AP given by Apw = uAP. (8) Assuming that the excess pressure in the film liquid is due to the terms appearing in eqn (3) it is possible to calculate the value of Ac that satisfies eqn (7) and (8) for any film thickness h. Results are shown in fig. 5. The rate of thinning due to liquid flow can now be found by inserting the resulting AP in eqn (6). For a reasonable value of the surface area (several cm2) this gives a rate of thinning of one A in a day.It is concluded that the thickness of a film element at some distance from the border is not determined by disjoining pressure equilibrium but by vapour pressure equilibrium of volatile components. It is hoped that further work along these lines will provide information about the meaning of" pressure " in a thin film. Thanks are due to Mr. Th. C. Kouters for skilful execution of the experiments. A . PRINS AND M. V A N DEN TEMPEL J. W. Gibbs The Scientific Papers (Dover Publ. 1961) vol. 1 p. 305. J. LyMema and K. J. Mysels J. Amer. Chem. SOC. 1965 87 2539. J. S. Clunie J. F. Goodman and P. C. Symons Nature 1967,216 1203. M. N. Jones and D. A. Reed J. Colloid Interface Sci. 1969 30 577. A. Scheludko Adv. Colloid Interface Sci. 1967 1 39. A. Prim C. Arcuri and M. van den Tempel J. Colloid Interface Sci. 1967 24 84. ' J. S. Clunie J. F. Goodman and J. R. Tate Trans. Firahy SOC. 1968 64 1965. * J. Lucassen and R. S. Hansen J. Colloid Interface Sci. 1967 23 319. 29

ISSN:0370-9302    

DOI:10.1039/SD9700100020

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

Aqueous Foam Films Stabilized by a Non-Ionic Surface-Active Agent BY J. S. CLUNIE J. M. CORKILL J. F. GOODMAN AND B. T. INGRAM Procter & Gamble Limited Newcastle Technical Centre Basic Research Department Newcastle-upon-Tyne England Received 10th April 1970 The thicknesses and tensions of black films formed by aqueous solutions of a pure non-ionic surface-active agent n-decyl methyl sulphoxide (DMS) have been measured at 298 and 308 K as a function of sodium chloride concentration. In the DMS+NaCl+H20 system second black films are formed at low ionic strengths whereas first black films are formed at higher ionic strengths ( >lo0 mol m-3). Estimated values for the composite Hamaker constant for first and second black films have been obtained and compared with theoretical values. In most studies on aqueous foam films the surface-active agents used have been ionic in character.l By comparison studies using non-ionic surface-active agents are fewer in number and mostly confined to films formed by aqueous solutions of commercial alkyl phenol polyoxyethylene ethers (OP-7 to OP-20).2-4 Nevertheless some observations have been made on films formed by aqueous solutions of pure n-dodecyl hexaoxyethylene glycol rnon~ether,~ and a few results have also been reported for films stabilized by another pure non-ionic surface-active agent (a partially- fluorinated n-alkyl dimethylamine oxide 6).In the present investigation a study has been made on films formed by aqueous solutions of n-decyl methyl sulphoxide (n-CloH2 ,.SO.CH,.DMS). This lion-ionic surface-active agent has a compact hydrophilic group and unlike the n-alkyl dimethylamine oxides,' is protonated only under extremely acid conditions (PK < O).s We have determined the thicknesses and excess tensions of the foam films formed by solutions of this material in water and in the presence of added sodium chloride (to 5 kmol m-,).From these measurements estimated values for the composite Hamaker constant for first and second black films have been obtained and compared with calculated values. EXPERIMENTAL MATERIALS n-Decyl methyl sulphoxide (DMS) was prepared and purified as described previo~sly.~ The water used for preparing solutions was twice distilled in a silica vessel after initial distillation from aqueous alkaline potassium permanganate. At 298 K this water had a specific conductivity of less than A 1 krnol111-~ solution raised the surface tension of water by 1.6 mN rn-l at 298 K,1° indicating the absence of any surface-active impurities.30 R-l m-l and a surface tension of 72.0 mN m-l. A.R. sodium chloride was roasted in a platinum crucible at -900 K. J . s. CLUNIE J . M. COKKILL J . F . GOODMAN AND B . T . INGRAM 31 SURFACE TENSION MEASUREMENTS Solutions were contained in a double-walled thermostatted cell with a thermistor for temperature monitoring. Surface tensions were determined using a du Nouy tensiometer and the usual corrections were applied. l1 FILM THICKNESS MEASUREMENTS The apparatus consisted of a glass cell (3.5 x low4 m3 capacity) made vacuum tight by demountable seals liquid seals and greaseless vacuum taps (fig. 1). An auxiliary reservoir FIG.1 .-Cell used for optical reflectance measurements. (2.5 x m3 capacity) was connected to the cell by a siphon through which the surface- active solution was introduced into the evacuated cell. Both cell and reservoir were totally immersed in a constant temperature water bath controlled to fO.O1 K by a mercury-toluene regulator with two 100 W heaters and a cooling coil. The water bath was housed in a light- proof enclosure but was fitted with a window for viewing the film when necessary. Films were formed by totally withdrawing a rectangular glass frame (50 x 20 x 5.0 mm) from the surface-active SolLItioil. The frame was supported vertically from a Perspex vessel top through a liquid seal filled with the solution under investigation. Film thickness was determined by an optical reflection method similar to that described previously,12 with the addition of a beam splitter to allow continuous monitoring of the intensity of the incident monochromatic light beam.Throughout the course of any film’s lifetime the intensity of the incident beam was constant to within f 2 x. The measured background light intensity was -20 % of the reflectance from the thinnest films studied. The uncertainties in reflection coefficient measurements lead on the basis of a single homogeneous layer model for the film,12 to a corresponding imprecision in film thickness of less than 2 %. Film thicknesses were calculated from the reflectance measurements using the sym- iiietrical three-layer model previously adopted.G The parameters of this model are the re- fractive indices of the surface monolayers (1.41) and the aqueous core (1.34) and also the 32 AQUEOUS FOAM FILMS thickness of the surface monolayers (1.4 nm).The uncertainty in the calculated thickness for possible variations in these parameters is unlikely to exceed 0.4 nm. FILM TENSION MEASUREMENTS The apparatus and technique were essentially those described previously 3;b~t to increase the sensitivity in the present series of measurements an 80 mm wide glass frame made of 1 mm thick glass was used to support the film. The electronic microbalance was calibrated with standard weights at 298 and 308 K. The estimated error in the measurement of excess tension was f2.5 pN m-l. RESULTS The (surface tension log concentration) curve for aqueous solutions of DMS showed no minimum at the critical micelle concentration (c.m.c.) indicating the ab- sence of any impurity of greater surface activity.The c.1n.c. for aqueous solutions was 2.0 mol m-3 at 298 K. The effect of added sodium chloride was to lower the c.m.c. (e.g. for DMS in 1 kmol m-3 sodium chloride solution the c.m.c. was 1.0 mol m-3 at 298 K). With DMS solutions stable films could only be formed at surface-active agent concentrations above the c.m.c. Variations in surface-active agent concentrations up to 1.5 x c.m.c. had no effect on measured film thicknesses. 10 100 1000 NaCl Conc. (mol m-') FIG. 2.-Equilibrium film thickness in the DMS+NaCl+H20 system as a function of NaCl concen- tration at 298 K. Results at 308 K are identical. I t - ~ 1 * [ I I ' I * I t 1 I ' ' 1 * 1 1 1 1 1 ' 3 FILM THICKNESS Measurements were made at 298 K and 308 K.All films showed mobile drainage behaviour and rapidly reached constant thicknesses which were maintained indefin- itely and which were reproducible to +0.2 nm. Fig. 2 shows the equilibrium thick- nesses of films formed from 2.2 mol M - ~ solutions of DMS containing varying amounts of sodium chloride. For any given film the measured reflection coefficients were the J . S . CLUNIE J . M. CORKILL J . F . GOODMAN AND B . T . INGRAM 33 same at both temperatures. Since any variation in the parameters of the optical model in this small temperature interval will be very small the calculated film thick- nesses are independent of temperature. Equilibrium film thicknesses were low (4.9 nm) and independent of electrolyte concentration up to a sodium chloride concentration of 70 mol m-3 where a sharp increase in film thickness to 7.3 nm was observed.With further increases in sodium chloride concentration the equilibrium film thickness decreased continuously returning to a value of 4.9 nm at the highest concentration studied (5 kmol m-3). The abrupt transition in film thickness from 4.9 to 7.3 nm occurred over a fairly narrow range of sodium chloride concentrations (70-100 mol m-3). In this concentra- tion range two co-existing black films could be formed in the vertical film holder viz. an upper film of thickness 4.9 nm in equilibrium with a lower film of thickness 7.3 nm. FILM TENSION Measurements were made at 5 K intervals in the temperature range 298-308 K. Fig. 3 shows the excess film tension Ao (Ao = of - 2y where of is the total film tension NaCl Conc.(mol m-3) FIG. 3.-Excess film tension Aa in the DMS+NaCl+H,O system as a function of NaCl concentra- tion at 298 K (0) and 308 K (0). and y is the surface tension of the bulk solution) as a function of sodium chloride concentration at 298 and 308 K. The sharp minima in the curves at 100 mol m-3 corresponded to the abrupt transition in film thickness. Aa appeared to change linearly with temperature the extreme values of the temperature coefficient being -0.4 and + 1.1 pN m-1 K-l at sodium chloride concentrations of 50 mol M - ~ and 1 kmol m-3 respectively. DISCUSSION Mysels et aZ.149 l6 have defined " first " and " second " black films in terms of the behaviour of the equilibrium thickness with respect to the ionic strength of the bulk solution. First black films show a monotonic decrease of thickness with increasing SPI-B 34 AQUEOUS FOAM FILMS ionic strength whereas with second black films the thickness is independent of the ionic strength.It is generally considered l5 that the equilibrium thickness of the first black film is governed by the balance between the van der Waals attractive forces and the repulsion due to the overlap of the electrical double layers associated with a surface charge in the head-group planes. For the second black film the repulsion force is less well defined and is short range in nature. For films stabilized with non-ionic surface-active agents one might expect that since the head-groups are uncharged only second black films would be observed. In the DMS system second black films are indeed found at sodium chloride concentra- tions less than 70 rnol m-3 but after a restricted concentration region in which co- existing first and second black films are observed only first black films are formed.Due to salting out of the DMS the highest electrolyte concentration at which films could be formed was 5 kmol m-3 and at this composition the film thickness had returned to a value close to the electrolyte-free second black film thickness of 4.9 nm. In films formed from ionic surface-active agents,l6. l7 increasing electrolyte results in a transition from the first to the second black type ; in the present system the converse is the case. The transition from second to first black type in this system presumably results from ion adsorption in the head-group planes giving rise to a double-layer repulsion force as in first black films formed by ionic surface-active agents.This suggestion is supported by the effect on DMS films of other inorganic electrolytes having different adsorption potentials. For example with sodium hydroxide and with potassium thiocyanate the transition from second to first black film occurs at much lower electro- lyte concentrations and the first black films are correspondingly thicker (e.g. with sodium hydroxide the first black film has a thickness of 47 nm at a concentration of 0.1 mol m-3). Because of its thinness a black film has a surface tension lower than that of the adjoining bulk solution,18-20 and the tension difference ACT as defined earlier can be equated to the depth of the minimum in the potential energy-thickness curve 21 in which the film exists.For first black films the potential energy U is regarded as the sum of two terms a double layer repulsion U and a dispersion force attraction U, when the hydrostatic potential energy is negligible. For a film of total thickness h and surface layers of thickness d UE can be expressed in the form UE = (B/Ic) exp [ - K(h - 241 (1) where ic is the Debye-Huckel reciprocal length and B is a term whch includes the bulk solution concentration the Stern potential and temperature. UA is given (for non- retarded forces) by UA = -A"/12nh2 (2) where A* is the composite Hamaker constant for the film. is zero it follows from (1) and (2) that if A* is independent of h The potential energy (UE + UA) can consequently be expressed in the form Since at equilibrium the derivative of the potential energy with respect to thickness 1cU,+(2U,/h) = 0 (3) U = uA(1-2/~h) (4) or AO = (- A*/12xh2)( 1 -2/1ch).( 5 ) Since both h and AO can be directly measured eqn ( 5 ) affords a method for determining A*. J. S . CLUNIE J . M. CORKILL J . F. GOODMAN AND B. T . INGRAM 35 In fig. 4 the values of A* obtained from ACT and h for the first black films studied are shown as a function of h at 298 and 308 K. The large variation of A* with temperature 4 3 n b W 0 2 x2 t I 5 6 7 film thickness (nm) FIG. 4.-Hamaker constant A* for first (0) and salt-free second (0) black films as a function of film thickness at 298 K (closed symbols) and 308 K (open symbols). Dashed line represents the caicu- lated Hamaker constant for the non-retarded van der Waals forces. is most unexpected since the densities and polarizabilities of the film components should show only a small change for a lOK temperature difference.Taking the value of A* = 2.5 x J into eqn (3) leads to a value for U and hence the Stern potential can be calculated. At the transition concentration a potential of 20 mV is obtained which is similar to that obtained for first black films stabilized by the non-ionic surface-active agent OP-20. Approximate values of A* may be calculated by the method of Duyvis 2 2 from the Hamaker constants and thicknesses of the film surface layers and aqueous core. The Hamaker constants A for water and decane can be calculated from a form of the London equation 27 n2-1 64 ( n 2 + 2 ) A = -hv - where Iz is Planck's constant n the refractive index and v the characteristic frequency. Using the values of v given by Gregory,24 we obtain A values for water and decane of 3.9 x and 5.8 x J respectively.Inserting these into the Duyvis equation leads to A* values that are in moderate agreement with those calculated from the experimental data using eqn (5). An alternative method of estimating A' is to assume that for the second black films in the absence of electrolyte the short-range repulsion is represented by a cut-off potential and hence that U = UA at the equilibrium thickness. The value of A* for the electrolyte free films is comparable to that calculated from eqn (5) for the first black films (fig. 4). However for these films although h is independent of electrolyte concentration Aa shows large changes with electrolyte concentration passing through a maximum at - 1.0 mol M - ~ .It seems improbable that composi- tional changes could lead to variations in A'% large enough to account for these effects, 36 AQUEOUS FOAM FILMS and hence some other forces besides the van der Waals attraction and a steric (cut-off) repulsion must operate in these films. The nature of these additional short-range forces remains obscure but assuming U to be unaffected by the presence of ions then the results indicate that the only restriction on the net contribution Us of these forces to the film potential energy is that I Us I < I U I and dU,/dh>dU,/dh at all thicknesses. It is reasonable to suppose that the variations of Ao with electrolyte concentration reflect changes in solvent orientation ion adsorption and electrostatic screening. A. Scheludko Adu.Colloid Interface Sci. 1967 1 391. B. V. Deryaguin A. S. Titievskaya and V. K. Vybornova Colloid J. U.S.S.R. 1960,22,407. E. M. Duyvis and J. Th. G. Overbeek Proc. Kon. Ned. Akad. Wetens. B 1962 65,26. A. Scheludko Proc. Kon. Ned. Akad. Wetens. B 1962 65,97. J. M. Corkill J. F. Goodman D. R. Haisman and S. P. Harrold Trans. Faraday Soc. 1961 57 821. J. M. Corkill J. F. Goodman and C. P. Ogden Trans. Faraduy Soc. 1965,61 583. F. Tokiwa and K. Ohki J. Phys. Chem. 1966,70,3437. D. Landini G. Modena G. Scorrano and F. Taddei J. Amer. Chem. Soc. 1969 91,6703. J . M. Corkill J. F. Goodman and J. R. Tate Trans. Faraday Soc. 1969 65 1743. lo G. W. C. Kaye and T. H. Laby Tables of Physical and Chemical Constants 13th ed. (Longmans London 1966). W. D. Harkins and H. F. Jordan J. Amer. Chem.SOC. 1930 52 1751. l 2 J. M. Corkill J. F. Goodman C. I?. Ogden and J. R. Tate Proc. Roy. SOC. A 1963 273 84. l3 J. H. Clint J. S. Clunie J. F. Goodman and J. R. Tate Nature 1969 223 291. l4 K. J. Mysels K. Shinoda and S. Frankel Soup Films Studies of their Thinning and a Bibliography (Pergamon Press London 1959). l 5 J. Lyklema and K. J. Mysels J. Amer. Chem. SOC. 1965 87 2539. l6 M. N. Jones K. J. Mysels and P. C. Scholten Trans. Faraduy SOC. 1966 62 1336. l7 G. Ibbotson and M. N. Jones Trans. Furaday Soc. 1969 65 1146. l9 K. J. Mysels H. F. Huisman and R. Razouk J. Phys. Chem. 1966,70 1339. 2o A. Scheludko B. Radoev and T. Kolarov Trans. Furadzy Soc. 1968 64 2213. 21 F. Huisman and K. J. Mysels J. Phys. Chem. 1969,73,489. 22 E. M. Duyvis The Epdibrium Thickness of Free Liquid Films (Thesis Utrecht 1962). 23 D. Tabor and R. H. S. Winterton Proc. Roy. SOC. A 1969 312,435. 24 J. Gregory Adv. Colloid Interface Sci. 1970 2 396. R. V. Deryaguin G. A. Martinov and Yu. V. Gutop Colloid J. U.S.S.R. 1965 27,298.

ISSN:0370-9302    

DOI:10.1039/SD9700100030

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

GENERAL DISCUSSION Dr. B. A. Pethica (Unilever Res. Port Sunlight) said Frens Mysels and Vijayendran suggest that their derived (surface-pressure apparent-molecular-area) curves for a collapsing film stabilized by sodium dodecyl sulphate indicate a rapid increase in intermolecular attractions as the surface becomes very crowded. The form of the curves is however typical of those for collapsing insoluble monolayers and it is probable that a combination of solution and stacking effects in the collapsing film will explain the results. Dr. K. J. Mysels (R. J. Reynolds Winston Salem) said Pethica has raised the question whether the unexpected reversal of curvature in the ( l l A ) curves at low areas per molecule might not be due to a partial collapse of the surface monolayer. There is no way to answer that question with complete certainty on the basis of our observations.However the surfactant layer is highly ionized and not compact in our experiments (ca. 30A2/ion). This and the fact that variations in the rate of concentration-due to differences in thickness of the bursting film-by a factor of 10 do not produce significant deviations from the measured (a,@) curve are arguments against monolayer collapse as an explanation for the inversion of curvature. The reversal of curvature is also not to be explained as a time effect due to desorption. If the reverse curvature were not intrinsic for the film surface there could not be a beginning to the propagation of the aureole and therefore no such phenomenon to produce relaxation effects. In that case only a single shock-wave at the rim of the hole would propagate into an undisturbed film.In our opinion the reversal of curvature might indicate something like the incipient formation of a closely-packed vertically-oriented monolayer of long-chain ions. In such a layer important cohesive forces between chains would come into effect and act in the direction of inversion of the curvature. Dr. G. Frens (Philip Res. Lab. Eindhoven) said One would like to relate the surface tensions of super-saturated surfaces as obtained from the interpretation of the aureoles in bursting soap films to data for surface tension as a function of surface coverage below the c.m.c. We have pointed out that a problem here is the lack of agreement between the data of the different authors who have determined (17,A) curves for NaLS below the c.m.c.Mrs. Lucassen-Reynders of Unilever Research Laboratories has tackled this problem and some of her results were made available to me. She had shown earlier that two equations and relate surface pressure (ao -a) to adsorption (r) and r to the surfactant concentra- tion c for ionized surfactant.l These equations have three constants (a rW and Hs) E. H. Lucassen-Reynders J. Phys. Chem. 1966,70 1777. 38 GENERAL DISCUSSION whereas (ao - a) and c are the experimentally measured variables. Mrs. Lucassen computed which values of the constants a r" and Hs gave the best fit between theory and experimental data. The best set of constants is that for which the parameter 60 r( I E 40- ch Ei a 20- has its minimum. She analyzed some twenty sets of experimental data from the literature.Of these the surface tensions of NaLS as measured by Elworthy and MyFels gave satisfactory results. The other data led to unreasonable values for a I?" and Hs probably because there was too large a scatter in the experimental points. - - 1 I 1 1 0 20 LO 60 80 100 A (A') FIG. l.-(IT A ) curve for NaLS ; I and I1 calculated by Lucassen-Reynders ; U experimental points (ref. (2)); V " anchoring " point at c.m.c. (ref. (2)). Mrs. Lucassen's procedure will not only sort out the best set against experimental data it also produces a (l3,A) curve (or go - a against I?) through eqn (1). In fig. 1 we have combined this (l3,A) curve below the c.m.c. with the curve at high surface coverages which we obtained from OUT bursting experiments. The values ll = 30.3 mN m-l and A = 0.415 nm2 at the c.m.c.(where the soap film data are anchcred in the (&A) plane) were taken from the literature.'. The value of ll agrees with our own measurements. One would expect eqn (1) to break down near the c.m.c. as it predicts that the surface pressure would become infinite on saturation (I? = P). It is seen however that only a short interpolation is necessary in order to connect smoothly the two parts of the curve in fig. 1 into one (l3,A) plot which might then be meaningfully interpreted in a more complete equation of state than eqn (I). Dr. A. T. Florence and Dr. K. J. Mysels (Strathclyde University and R. J. Reynolds Winston SaZem) said In reply to Goodman we have been concerned with the question whether the existence of the aureole may not be due to the presence of impurities or hydrolysis in the sodium-dodecylsulphate used.As mentioned in our paper we used P. H. Elworthy K. J. Mysels J. Colloid Interface Sci. 1966 21 331. I. Weil J. Phys. Chem. 1966 71 738. GENERAL DISCUSSION 39 solutions well above the c.ni.c. so as to solubilize any surfactant impurities and thus reduce any such effects. These effects particularly those of dodecyl alcohol cannot be very significant. This is shown by some yet unpublished experiments of Dr. A. T. Florence at the R. J. Reynolds Lab. He combined the purification by foaming of a solution below the c.m.c. with photographic studies of bursting. Enough dodecyl alcohol was initially added to the solution to give rigid films. These show a peculiar bursting behaviour.2 Foaming rapidly reduced the alcohol concentration to the point where the films become mobile and bursting produces the usual aureole.This aureole persisted in the experiments without qualitative change through several days of further purification by foaming. used a similar purification method. They obtained indistinguishable results in solutions of sodium dodecyl sulphate and sulphonate. This indicates that hydroylsis of the sulphate is not a limiting factor under these conditions. Razouk and Mysels Prof. A. Vrij (Utrecht Netherlands) said I would ask Prins the following questions (i) in fig. 3 of his paper he has plotted a calculated equilibrium thickness (graph 2) for an electrolyte concentration of 3 x mol ~ m - ~ which is the total concentration of sodium lauryl sulphate. In my opinion however he should have used the con- centration of free soap ions (i.e.the c.m.c.) which is about a factor of 4 smaller. (ii) He proposes that the pressure in the film could be higher than in the surrounding atmosphere. Does this mean that in that case the van der Waals’ attraction forces exceed the double-layer repulsion forces ? (iii) The plots of calculated electrolyte concentrations as a function of height in fig. 5 are linear. Does this have any physical significance ? Dr. A. Prins (Unilever Res. Lab. Netherlands) said In reply to Vrij the activity of surfactant molecules is indeed affected by micelle formation. However since it is not known how the counter ion activity changes with the concentration in the presence of micelles we have assumed that for the calculation of the film thickness the nominal Concentration of the counter ions has to be used.This is confirmed by the agreement between the measured and the calculated film thickness at the bottom plateau border. In addition as appears from fig. 4 of our paper exchange of most of the surfactant by salt results in exactly the same profile of the film. Moreover a calculation based on a salt concentration equal to the c.m.c. would result in an even thicker film making the discrepancy between theory and experiment even bigger. The suggested explanation of the water vapour pressure equilibrium is indeed that the pressure in the film exceeds atmospheric pressure in order to compensate for the decrease in water vapour pressure caused by the higher salt concentration. The linear relation between the salt concentration and the height in the film suggests that it is perhaps possible to give a simple explanation for the observed phenomena.The physical significance however is not known at present. Dr. M. N. Jones (University of Manchester) said With regard to the paper by Prins and van den Tempel Reed and I have attempted to measure the thickness profile of films up to 10 cm in height drawn from sodium n-dodecylsulphate solutions at low concentrations ( N 0.009 M) and found that while a given film had a thickness which remained constant within -0.5 nm over a period of several hours there was considerable variation (k 10 nm) in thickness between different films drawn from K. J. Mysels and A. T. Florence in CZean Surfaces G. Goldfinger ed. (New York 1970). W. R. McEntee and K. J. Mysels J. Phys. Chem. 1969,73 3018. Razouk and K.J. Mysels J. Arner. Oil Chem. SOC. 1968,45 381. 40 GENERAL DISCUSSION the same solution. At such low ionic strength the film thicknesses were very depen- dent on the method used to humidify the atmosphere in the film chamber and it was possible to observe even with the naked eye that different elements of the film had different thicknesses. The addition of electrolyte however had a marked effect on the thickness profiles and at higher ionic strengths (-0.15 M 1 1 electrolyte) we found that for a given film although the thickness decreased with time this change was only small -0.5 nm and when the system was left to thermostat for a long period (- 70 h) the decrease was even smaller which implied that it was due to evaporation. What is perhaps more significant is that under these conditions the initial black film thickness of freshly drawn films was always constant within k0.05 nm and we concluded that these are true equilibrium thicknesses.At higher salt concentrations (-0.2 M 1 1 electrolyte) we always obtained uniform films of constant thicknesses. It would seem from these observations that the diffusion of non-volatile components along the film is dependent on ionic strength and I would like to know whether they have made any observations on thinner films. Eqn (6) of this paper predicts that the rate of thinning should be much slower for thin films and t b s is contrary to our observations. Dr. A. Prins (Unilever Res. Lab. Netherlands) said In reply to Jones a measurable thickness profile as described in our paper appears only when films axe used which contain not too much salt.By increasing the salt concentration the film profile becomes more and more parallel and a Perrin film has a uniform thickness which however is determined by other forces than are discussed in our paper. Under the usual conditions of observation the rate of thinning of large films is always deter- mined by evaporation and not by the process described by eqn (6). In reply to Lyklema Bruil carried out his film thickness measurements at 0.5 cm above the bottom plateau border. It might well be that his results were affected by the phenomena discussed in our paper which should result in slightly thinner films than predicted by the DVLO theory at that level. Dr. H. Sonntag (Deutsche Akademie der Wissenschaften Berlin) said Clunie et al. expect that films stabilized with non-ionic surface-active agents only might form second black films.I cannot agree with this opinion because (i) we only found first black films in polar systems with various non-polar surface-active agents and never obtained an alteration of film tension; (ii) aqueous films between oil droplets stabilized with nonylphenol ethylene oxide (20) formed second black films after adding K2S04 or MgS04 as electrolyte. With Ca(NO,) or BaCl e.g. we obtained only first black films at all concentrations. I think the experimental results are still insufficient to give a theoretical explanation of the formation of second black films. Moreover I would ask whether they are quite sure that there is no building-up of adsorbed multilayers of surface-active agents above the critical micelle concentration ? Dr.J. M. Corkill (Procter & Gamble Ltd. Newcastle-upon-Tyne) said In reply to Sonntag it is generally accepted that the stability of first black films is determined by a balance between van der Waals’ attraction and electrostatic double-layer repul- sion. For aqueous foam films stabilized by a non-ionizable surface-active agent the existence of any diffue double layers can only result from ion adsorption at the film-core/surfactant-monolayer interface. In the absence of ion adsorption one would therefore not expect the formation of first black films. Any stable films that GENERAL DISCUSSION 41 are formed in such systems are likely to be of the second black type. Recent radio- tracer experiments using 35S-labelled decyl methyl sulphoxide have shown the equilibrium black film radioactivity to be virtually independent of electrolyte con- centration in bulk solution (up to 4 kmol m-3).The measured activity corresponds to an area of 0.34+0.02nm2 per surfactant molecule in the film surface. This value is identical to the area per surfactant molecule at the bulk-solution/air interface obtained from surface tension measurements on the same system. Dr. S. Levine (Manchester University) and Prof. G. M. Bell (Chelsea College) said In the paper by Clunie Corkill Goodman and Ingram eqn (1) for the double- layer repulsion whch is based on the DLVO theory is applied to electrolyte concentrations as high as 5 mol/l. However this is derived from the Poisson-Boltz- mann (Gouy-Chapman) equation which in fact should not be used for concentrations of 1 1 electrolytes greater than say 0.3 mol/l.at the rather low value of the Stern potential (20 mv) quoted by the authors. The corrections to the Poisson-Boltzmann equation have been considered by many authors.2 Dr. J. M. Corkill (Procter & Gamble Ltd. Newcastle-upon-Tyne) said In reply to Levine we agree with his comment concerning the limitation of the Gouy-Chapman theory to electrolyte concentrations below -300 mol m-3. However we have found good agreement between experimental and theoretical A* values both below and above the strict limits of applicability of double-layer theory. In these film calculations the contribution to the potential energy from electrical double-layer repulsion is much smaller than that due to van der Waals’ attractive forces (maximum of 40 % for 7.2 nm films).The influence on A* of errors in calculating the double- layer repulsion term for high ionic strength films will probably be small. This may explain the consistency between calculated A* values for films at high and low ionic strengths and their agreement with the A* value calculated for the electrolyte-free second black film. Prof. A. Scheludko (Sofia) (communicated) Exerowa and Kolarov have measured the thickness of microscopic horizontal films of C1 SOCH3 aqueous solution (the compound was supplied by Dr. Goodman) the concentration of the surfactant being the same as in the paper of Clunie et al. ( 2 . 2 ~ M) and electrolyte NaCl M. White films have been obtained with thickness of 460-520A instead of 48 8 measured by Clunie et al. at low electrolyte concentration.The outer capillary pressure was 290 dyn cm-2 which yields for the potential of the diffuse electric layer 4o = 17 mV as calculated according to the DLVO theory and with a van der Waals- Hamaker constant K = A/4n = 4 x 10-14.2 The calculation has been carried out with the complete formulae and has account of all the necessary conditions as described in ref. (2). The certainty of the value obtained for 4, is diminished because of the considerable effect of the van der Waals component of the disjoining pressure with such a low value of the potential and thicknesses; the van der Waals constant is known only with limited accuracy.l It is certain however that the potential is considerably lower than that of water (30 mv) which is evident from the fact that the equilibrium thickness is smaller with the same electrolyte concentration (700 without the surfactant against - 500 A here).The significant difference in thickness corn- pared to 48 A of Clunie et al. is completely explained by the higher outer pressure of the cell in our work. J. S. Clunie J. M. Corkill and B. T. Ingram unpublished results. see e.g. S. Levine and G. M. Bell Disc. Faraday SOC. 1966 42 69. 42 GENERAL DISCUSSION In fig. 1 the curve disjoining pressure II against film thickness h at M NaCl is presented TI being calculated from the approximate expression with y = 0.164 corresponding to & = 17 mV. I1 = 64cRTy2 exp (-xh) - K/h3 I I 1 1 1 I 1 400 6 0 0 8 00 h fi FIG. 1 As is seen the force barrier corresponds to some 500 dyn cm-2 so that with a highex outer pressure this barrier would be overcome and the film would either collapse or form a more stable film (Perrin’s film) the latter not being described by the DLVO theory.The outer pressure in the vertical frame of Clunie et al. also includes the hydrostatic pressure which at the upper end of the frame is - 5000 dyn cm-2 C10HZIS03CH3 NaLS m X t 2 0 0- I 0.2 0.3 0.040 0.316 CNaCl (M) FIG. 2 so that the force barrier is definitely exceeded and a thermodynamically more stable Perrin film is formed which then covers the whole frame. Up to this point therefore the fiIms described behave normally. Exerowa and Kolarov have also studied the transition first/second (Perrin’s) black film with the same surfactant by means of the new method of foam destruction by a stream of a-particles. As shown in ref. (3) the rate of foam column reduction (dZ/dt) subjected to cr-particle bombardment possesses a sharply outlined maximum near the electrolyte concentration corresponding to the first /second black film tran- sition with microscopic films.GENERAL DISCUSSION 43 M) and for compari- son with NaLS (5 x M) are shown. As an a-ray source Pu239 with intensity 558 a-particlesls was used. In the second case the maximum is at a NaCl concentra- tion of 0.316 M very close to the value obtained using the contact angle method by us and by My~els.~ For the surfactant used by Clunie et al. a maximum at 5 x M NaCl has been observed. Therefore the first/second black film transition does not show at first sight any peculiarity compared to the above results. The striking and very interesting moment in the paper of Clunie et al.is the thicken- ing of the film from 40 to 70 A at a NaCl concentration of 7 x M. They interpret this effect as a result of the increase in 4,-potential. This explanation is based mainly we think on the fact that the 4,-potential with this surfactant has as we have shown an especially low value and therefore can easily increase on changing the conditions e.g. the electrolyte concentration. Quantitative interpretation on the basis of the DLVO theory however seems to be groundless in this case. As is well known the DLVO theory gives a maximum and a minimum for the energy and dis- joining pressure isotherms when the film thickness decreases. The maximum explains the first black films but is not able to explain Perrin films. In order to describe the latter another minimum with a consequent rise of the disjoining pressure above the outer pressure should exist at very small thicknesses.Therefore at thicknesses close to those of the Perrin films significant deviations from the DLVO theory are expected in the direction of positive disjoining pressure. Such a large deviation has been observed in the measurements of the contact angle black film/bulk liquid in the region of first/second black film tran~ition.~ Therefore the application of the DLVO theory when calculating the constants at thicknesses exceeding Perrin’s film thickness less than 30A is quite uncertain. It is appropriate to recall the measurements on films of concentrated aqueous solutions of fat acids.6 With 2 M butyric acid solution very thick equilibrium films of some lW0A have been obtained.The thickening at such high electrolyte concentration is not due to the diffuse electric layers. The high positive values of the disjoining pressure in aqueous solution of butyric acid have been confirmed by Voropaeva using the method of crossed platinum fibre^,^ i.e. with different phase surfaces. Maybe in the work of Clunie et al. a similar situation appears. Other possible reasons for the deviations from the DLVO theory are discussed in ref. (8). As far as there is no theory to explain Perrin films we suggested that all these deviations from the DLVO theory should be combined in a third component of the disjoining pressure rl[bl. This component must rise sharply at small thicknesses in order to explain the Perrin films. The object should be a study of this I&,l and in this sense we find the results in the paper of Clunie Corkill Goodman and Ingram of considerable interest.In fig. 2 the results obtained with C10H21SOCH3 (2.2 x This suggests a volume origin for the effect. Dr. J . M . Corkill (Procter & Gamble Ltd Newcastle-upon-Tyne) said In reply to Scheludko we agree with the explanation advanced by Scheludko for the Occurrence of thick (-50 nm) microscopic films below the transition concentration found by us l A. Scheludko and D. Exerowa KolloidZ. 1960 168 24. D. Exerowa Kolloid-Z. 1969,232,703 ; Proc. 5th Int. Congr. Surface Activity 1968,2 153. D. Exerowa and D. Ivanov Compt. Rend. Acad. Bulg. Sci. 1970,23 547. T. Kolarov A. Scheludko and D. Exerowa Trans. Furaday Soc. 1968,64,2864. F. Huisman and K. Mysels J . Phys. Chem. 1969,73,489.A. Scheludko and D. Exerowa Ann. Univ. Sofia Fac. Chim. 1959/60,54 205. ’ T. Voropaeva B. Deryaguin and B. Kabanov Kolloid-Z. 1962,24,398. * A. Scheludko Adv. Colloid Interface Sci. 1967 1 391. A. Scheludko Ann. Univ. Sofia Fac. Chim. 1967/68 62,47. 44 GENERAL DISCUSSION for large vertical films. For second black (Perrin) films i.e. films with thicknesses independent of electrolyte concentration we agree that the dominant repulsive force llbl does not behave simply as a cut-off potential. However we find it difficult to invoke a J&, which is not a double-layer repulsion to explain the occurrence in the decyl methyl sulphoxide system of first black films i.e. films where the thickness decreases with increasing electrolyte concentration. This difficulty becomes greater when we consider the lower concentrations and higher thicknesses involved in the transition from second to first black films with different added electrolytes.For example with NaOH a first black film of thickness 47 nm is formed at a concentration of 0.1 mol m-3. The thickness of this first black film then decreases progressively with increasing NaOH concentration. Dr. B. A. Pethica (Unilever Res. Port Sunlight) said Clunie et al. suggest that the transition from second to first black type in their films stabilized by a non-ionic surfactant is due to ion adsorption in the head-group plane. It is highly improbable that an adsorption of ions governed by a Stern adsorption potential will give such a sharp transition as is shown on their fig. 2. The adsorption of ions can be tested by electrophoretic methods using e.g.oil drops stabilized by the same non-ionic or by surface potential measurements. The film transitions are probably associated with salt effects on smectic phase transitions in solution. Bearing in mind that the film ionic composition is quite possibly different from that of the bulk associated solution it would be useful to have related data on the solution properties of the stabilizer. The determination of the apparent Hamaker constants in their paper is open to objection. The set of equations (1)-(5) rest on the improbable assumption that the term B (which contains a Stern potential) does not vary with film thickness. Assuming that the electrical repulsion is a simple exponential function of thickness the electrical term disappears from the argument leaving ACT a direct function of the Hamaker constant A thickness and the Debye-Hiickel reciprocal length.The resulting values of A vary by a factor of two over a 10 K temperature interval which leaves grave doubts as to the physical significance of the calculations. Dr. J. M. Corkill (Procter & Gamble Ltd. Newcastle-upon-Tyne) said In answer to Pethica’s comment concerning the abruptness of the film transition we refer to our reply to Lyklema; it is not necessary to postulate a corresponding lamellar mesomorphic phase transition in the bulk solution. The assumption that the Stern potential is independent of the separation between the charged surfaces is commonly employed in double-layer repulsion calculations although some authors have suggested that the surface charge is more likely to remain constant.For weakly overlapping double layers the difference between the two treatments leads to a very small difference in repulsion energy.l. The variations in A* with temperature that we have observed are hence unlikely to originate in a variation of the B term. We return to the point made in our paper that the temperature dependence of Aa for the first black films cannot be explained in simple terms such as a variation in Stern potential. Prof. J. Lyklema (Wageningen) said In fig. 2 of their paper Clunie et al. observe a steep rise in film thickness in the salt concentration range 0.07-0.1 M. An explana- tion is offered invoking ion absorption in the head-group plane of the surfactant. E. J. W. Verwey and J. Th. G. Overbeek Theory ofthe Stability ofLyophobic Colloids (Elsevier Amsterdam 1948).J. E. Jones and S. Levine J. Colloid Interface Sci. 1969,30 241. GENERAL DISCUSSION 45 Tf this were true one would also expect an analogous irregularity in the micelle formation behaviour of the used surfactant as a function of the salt concentration around the 0.07-0.1 M region. Was anything like that observed? Dr. J. M. Corkill (Procter & Gamble Ltd. Newcastle-upon-Tyne) said In reply to Lyklema the critical micelle concentration (surface tension data) of decyl methyl sulphoxide decreases with addition of salt but no abrupt discontinuities are observed. The dependence of critical micelle concentration on ionic strength is similar to that reported for other non-ionic surface-active agents. To account for the transition in film thickness it is not necessary to postulate an abrupt rise in ion adsorption.It is generally accepted that a charged film can exhibit two minima in its potential energy-thickness relationship. The secondary minimum at larger thicknesses results from superposing a double-layer repulsion term on the gravitational and van der Waals’ energy contributions. If the hydrostatic pressure is sufficiently large a secondary minimum does not occur and either a primary minimum (second black) film is formed or rupture takes place.2 In our system the hydrostatic pressure remains constant at the point of film thickness measurement (- 300 N m-2) as the solution composition is changed. The transition occurs when the surface charge of the film has risen sufficiently to create a secondary minimum rather than an inflection in the energy-thickness diagram.The observation that further increasing the ionic strength leads to a deepening of the secondary minimum in addition to moving it to lower thickness is consistent with the predictions of the DLVO theory. Prof. R. J. Good (Bristol University) said With regard to the paper by Clunie et al. it may be misleading to refer to film tension of for a second black film without distinguishing it physically from the film tension of a thick film and from surface tension. The state of stress of a second black film is a Hookean state characterized by an elastic modulus i.e. the tension in the film for small elongations is propor- tionate to the elongation. In this it is unlike the surface of a bulk liquid or of a thick film ; for these cases the static stress is independent of elongation.The dynamic stress in the latter two cases may be dependent on elongation through the Gibbs elasticity ; however this is a strongly time-dependent condition of stress. This difference between the second black film and the other surfaces exists because of the lack of a reservoir of fluid in the interior of the black film. So the black film acts mechanically simply as a linkage for transmitting the pull of the bulk-liquid surface to the Plateau border at the top of the frame and thence to the top of the frame itself. There is a contact angle formed by the surfaces of the plateau border and the film just as there is a contact angle at the bottom. I would ask whether Clunie et al. measured the contact angles directly and in particular the angle at the top of the frame? This set of measurements should enable them to tell whether the plateau border has a higher surface tension than the bulk liquid indicating appreciable depletion of surfactant. Dr. J. M. Corkill (Procter & Gumble Ltd. Newcastle-upon-Tyne) said In reply to Good we have directly measured the contact angle between the bottom of the film and the bulk solution at 298 I< using the refraction m e t h ~ d . ~ We found no significant difference between this measured equilibrium contact angle and that calculated from the film tension measurements. P. Mukerjee J . Phys. Chern. 1965 69 4038. J. Th. G. Overbeek J. Phys. Chem. 1960 64 1178. S. Frankel and H. M. Princen J. Phys. Chern. 1970,74 2580.

ISSN:0370-9302    

DOI:10.1039/SD9700100037

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

Composition and Energy Relationships for Some Thin Lipid Films and the Chain Conformation in Monolayers at Liquid- Liquid Interfaces BY D. M. ANDREWS,* E. D. MANEV t AND D. A. HAYDON$ Laboratory of Biophysical Chemistry and Colloid Science University of Cambridge Free School Lane Cambridge CB2 3RT England Received 2nd April 1970 Optically black films have been formed in aqueous media from solutions of glyceryl mono-oleate in aliphatic hydrocarbons. The thicknesscs of the hydrocarbon cores of the films were estimated from electrical capacitance measurements and the compositions from interfacial tension data. The thicknesses and the compositions were found to be interrelated in a simple way and were markedly dependent on the chain length of the hydrocarbon solvent. An electrical potential applied across a liquid film subjects it to a large compressive force under which most types of film became significantly thinner.From thickness measurenients in applied fields the strengths of the steric interactions which stabilize the films were calculated. From a knowledge of the steric interaction together with an estimate of the London-van der Waals forces from contact angle measurements the curve of potential energy against film thickness has been cal- culated for one system. The magnitude of the steric interaction at a given film thickness varies considerably for films of different solvent content. As a consequence a general picture of the time- average conformations of the hydrocarbon chains of glyceryl mono-oleate in the black films and at different hydrocarbon/water interfaces may be deduced.Detailed investigations of optically black lipid films in aqueous media have so far been inhibited by the difficulty of making simple stable films of well-characterized substances. However solutions of glyceryl mono-oleate in aliphatic hydrocarbons and other nonpolar solvents form relatively stable films in aqueous so1utions.l The thickness of these films may be measured by optical and electrical 3 9 methods and the adsorption of the oleate in the film may be estimated from interfacial tension measurements. From these data the composition of the films may be calculated. The free energy of formation of the films may be found from the contact angles between the thin film and bulk interface and may be interpreted to give the magnitude of the London-van der Waals force^.^ A uniform electric field normal to the surfaces of the black film exerts a compres- sional force which may be considerably larger than the London-van der Waals forces and which may produce appreciable thinning of the film.6 This thinning is opposed by the steric interaction of the chains of the adsorbed oleate and from the dependence of film thickness on applied field strength it is possible to deduce the magnitude of the steric interactions and to infer qualitatively the conformation of the oleate chains both in the black film and in the adjacent oil-water interfaces.The free energy against thickness reIationship has been deduced for films of n-decane in saturated sodium chloride stabilized by glyceryl mono-oleate. Films * present address Research Dept. Unilever Limited Port Sunlight Cheshire.j- present address Institute of Physical Chemistry University of Sofia Sofia Bulgaria. 2 present address Physiological Laboratory Univ. of cambridge Downing Street Cambridge. 46 D . M . ANDREWS E D. MANEV AND D . A . HAYDON 47 of glyceryl mono-oleate in other non-polar solvents have also been examined and although these studics have been less detailed they have revealed some important respects in which the solveiit may influence the oleate chain conformation and the composition of the films. EXPERIMENTAL METHODS ‘The films were fornied in a 1 mm hole in a Fluon vessel as described previ~usly.~ For experiments with volatile solvents the top of the cell was sealed by means of a glass lid. Capacitances were measured as described previo~sly.~ When a d.c.bias was applied across the film (from a potentiometer) a 1 pF capacitor was used to isolate the capacitance bridge. Interfacial tensions were determined by the drop-volume technique.’ The activity coefficients of the glyceryl mono-oleate in the more volatile solvents were obtained by vapour pressure osmometry using a Hewlett Packard type 302 Vapour Pressure Osmometer. The sensitivity of this instrument was increased by a factor of ca. 3 beyond the makers’ specifica- tion. In order to obtain good reproducibility for both tension and osmometer results it was necessary to equilibrate the apolar solutions with the appropriate aqueous phase for at least 24 h prior to the experiment. MATERIALS The glyceryl mono-oleate was obtained from Sigma and was found by thin layer chromato- graphy (kindly carried out by Dr.H van Zutphen) to be >99 % pure 1-isomer. No signifi- cant ageing of the interfacial tensions was observed except at low concentrations. The solvents were all of puriss grade and were 2 9 9 % by g.1.c. Before use they were passed through an alumina column to remove trace surface active impurities. A.R. NaCl was roasted at 700°C to remove organic impurities. The water was twice distilled first from a commercial still and then from a Pyrex still fitted with a quartz column condenser and receiver. All experiments were carried out at 20°C. RESULTS The two effects with which this paper is primarily concerned are illustrated in fig. 1 and 2. First the specific capacitance of the film increases considerably when number of carbon atoms in alkane FIG. I .-The specific capacitance of black films formed from solutions of glyceryl mono-oleate (ca.0 aqueous phase 0.1 M NaCl; El aqueous phase saturated NaCl. 12 mM) in normal alkanes. 48 THIN LIPID FILMS applied potential (V) FIG. 2.-The specific capacitance (C 0) and thickness of the hydrocarbon region (8,) of black f i l m under applied potentials. The films were formed from glyceryl mono-oleate in n-decane and the aqueous phase was saturated NaCl. the films are formed from hydrocarbon solvents longer than n-nonane (fig. 1). Secondly for a film formed from n-decane the specific capacitance increases with increasing potential across the film (fig. 2). Further results for other solvents are given in table 1. TABLE 1 - system V(V) C (nF mi-*) & 8 (nm) rp 0.1 M NaCl n-decane satd. NaCl cyclohexane n-hep t ane 2,2,4-trimethyl-pentane n-decane n-tetradecane n-hexadecane CCI 0.0141 0.106 0.0141 0.141 0.0141 0.141 0.0141 0.141 0.0141 0.1061 0.0141 0.141 0.0141 0.141 0.0141 0.141 3.83 4.14 4.42 4.94 3.98 4.47 4.1 1 4.59 4.19 4.40 5.73 5.92 6.51 6.38 5.63 5.63 2.08 2.10 2.11 2.12 2.07 2.09 2.08 2.10 2.10 2.12 2.14 2.14 2.14 2.14 2.15 2.15 4.8 4.5 4.2 3.8 4.6 4.1 4.5 4.05 4.4 3.9 3.3 3.2 2.9 2.95 3.4 3.4 0.53 0.72 0.73 0.70 0.73 1.03 1.18 0.85 D.M. ANDREWS E . D. MANEV A N D D . A . HAYDON 49 All the systems had capacitances and conductances which were frequency dependent. The dispersions were simple in form and corresponded accurately to those which would be expected for two parallel-sided isotropic layers in series. The two layers concerned are the hydrocarbon core of the film and the aqueous phase respe~tively.~~ * * Above and below the dispersion region the capacitance does not vary with frequency and the data here reported were obtained in the low frequency constant-capacitance region (at ca.1 kHz). It was confirmed that a given r.m.s. a.c. potential had a similar influence on the capacitance to the equivalent d.c. potential. When the electric field across a film was changed the re-establishment of equili- brium often took of the order of minutes or hours. This was especially so for films formed from solvents of higher molecular weight and arose evidently from the difficulty of escape of such solvents from the films. Thus when the potential was suddenly increased bright spots of surplus liquid appeared in the film after a period of seconds or minutes presumably from a squeezing-out or disproportionation process.The specific capacitance of the film was constant once the spots were visible but the diffusion of the spots to the edge of the film where they coalesced with the meniscus took much longer.1° In many of the systems the films became markedly less stable under potential differences of more than ca. 150 mV (r.m.s.) and precise capacitance measurements could not be made. The use of saturated NaCl as the aqueous phase greatly helped to minimize inaccuracies in the capacitances for films formed from low-molecular- weight solvents. Unless the apolar and aqueous phases were in perfect equilibrium the capacitances became anomalously high. In saturated NaCl this effect was considerably reduced owing presumably to the lower solubility of the solvents and a wider range of films could be examined.A film thickness was obtained from the specific capacitance C by means of the equation c = &,&/6 where E~ is the permittivity of free space E is the dielectric constant and 6 is the thickness of the hydrocarbon core of the film. The validity of this procedure and in particular the justification for ignoring contributions to the capacitance from the polar group region and the electrical double layers in the aqueous phases has been examined in previous papers.3* * * 9* The hydrocarbon core is regarded as bulk liquid hydrocarbon composed of a mixture of oleyl chains and the appropriate so1vent.12 The proportions of the two components have been estimated from the adsorption data as described below.The dielectric constants of the two components have been assumed additive on a volume fraction basis. Anisotropy in the hydro- carbon has been ignored. Even in hydrocarbon crystals this effect is small and the film structure more closely resembles a liquid than a crystal. The resulting dielectric constants are all close to 2.1 and the calculated thicknesses are shown in fig. 2 and table 1. From measurements of molecular models the thickness of the glyceride polar groups layer was estimated to be 0.45 nm. The total film thickness 2jL was therefore taken to be (6 + 0.9)nm. Such optical measurements as have been reported (e.g. films of glyceryl distearate and n-hexane 13) are consistent with the present estimated thickness. In order to estimate the composition of the black films the colligative properties of glyceryl mono-oleate and its adsorption at the various oil-water interfaces were studied.Glyceryl mono-oleate aggregates strongly in apolar media at concentra- tions above ca. moll-l. When the apolar medium is equilibrated with 0.1 M 50 THIN LIPID FILMS loglo [activity concentration (mol PI)] CC14 - 3.0 -2.5 loglo [activity concentration (mol 1-91 - 3.0 - 2.5 Iog o[activity concentration (mol 1-91 FIG. 3.-hiterfacial tensions of glyceryl mono-oleate in various solvents as a function of log, (activity) (Closed points) and or log, (concentration) (open points). (a) 0 n-decane+0.1 M NaCl ; A n-decane+ saturated NaCl ; 0 0,2,2,4trimethylpentane + saturated NaCl ; v v carbon tetrachloride+saturated NaCl (top and right hand axes). (b) All aqueous phases saturated NaCl.V n-hexadecane ; A n-tetradecane ; El m n-heptane ; 0 0 cyclohexane. NaCl the onset of the aggregation is sufficiently sharp to be described as a critical micelle concentration. When equilibrated with saturated NaC1 however the aggregation is less sharp and there is no clear c.m.c. This point is illustrated by the interfacial tension curves for decane in fig. 3(a). Black films tended to be stable D. M. ANDREWS E. D. MANEV AND D. A . HAYDON 51 only at concentrations above the c.m.c. or aggregation region although films were occasionally sufficiently stable just below this concentration range for some types of measurement to be made. All the films of table 1 were formed above the aggregation region. The adsorption of the glyceryl mono-oleate at the oil/water interface was found by means of the Gibbs equation which as the two solvents were effectively insoluble in each other and the oleate was very strongly adsorbed reduces to l4 where y is the interfacial tension T2 is the surface concentration of the glyceryl mono-oleate and a2 is the activity of the oleate in the oil phase.Curves of y against log, (concentration) are shown in fig. 3 (a) and (b) for each of the systems of table 1. For only four systems however was it possible to determine the activity co- effiicients fi. The latter were obtained from vapour pressure osmometer measure- ments via the osmotic coefficients g by means of the equation -dy = T2RTd In a2 (2) J'd lnfi = J'dg+ l(g - l)d In x2. (3) The results may be seen in fig. 3 (a) and (b). For the remaining systems the vapour pressure osmometer was insufficiently sensitive and there seemed no obvious alter- native method of obtaining the activity coefficients.The adsorption in these instances was therefore estimated by assuming that the activity correction was identical to that for the heptane system. Both determined and estimated surface concentrations are shown in table 2. TABLE 2 - system 0.1 M NaCl n-decane satd. NaCl cyclohexane n-hep t ane 2,2,4-triniethyl-pentane n-decane n-tetradecane n-hexadecane cc14 (darnax 21.8" 28.1 31 .O 27.3* 22.9" 24.8" 24.5" 26.6 r x 10-'8 (molecules m-2) 2.5t 3 .O 3.3 2.9 3.2t 3.4t 3.4.f 2.9 *-(dy/d 1ogloc)max ; t based on activity correction for n-heptane system The volume fraction (p of the oleate chains in the film was calculated as follows. The thickness 6 of a film was estimated from the specific capacitance using an arbitrarily chosen dielectric constant (say 2.1).Assuming that the partial molar volume of the oleate chain in the film was equal to the molar volume of 1-heptadecene in bulk the first approximation to was deduced from T2 the surface concentration of oleate (table 2). From this value of 40 and on the assumption that the dielectric constants of the hydrocarbons in the film were equal to their bulk values a new thickness was calculated and the procedure repeated. The second approximation to (p differed only slightly from the first. The assumption of bulk density for the hydrocarbon in the film was made by analogy with the interior of surfactant micelles in aqueous s o l u t i o ~ i ' ~ - ~ ~ and is consistent with the electrical conductances of the films.12 The assumption that the adsorption in the film was for present purposes identical to that at the bulk interface with which the film was in equilibrium has been justified by theoretical considerations,14 and has also been substantiated experi- mentally for one system by contact-angle measurements.Thus from the contact 52 THIN LIPID FILMS angles for systems below the c.m.c. the difference between the film tension and twice the bulk tension may be found and it may be shown that the variation of this difference with glyceryl mono-oleate activity is extremely small compared to the variation of either of the individual tensions. It then follows l4 that the adsorption at the film and bulk interfaces differs only very slightly (< 1 x). DISCUSSION FREE ENERGY AS A FUNCTION OF THICKNESS In the absence of an electrical potential difference across a film the forces acting normally to the film surface are assumed to be the London-van der Waals compression and the steric repulsion which originates from the interaction of the oleate chains.All other forces such as those from electrical double-layer overlap and from dipole- dipole interactions are assumed to be negligible. Experimental evidence so far available is entirely consistent with this assumption. * For unit area of film the London-van der Waals forces FL are given by F L = -A/6~c6? (4) where A is the Hamaker constant. In the present systems the retardation correction should be less than ca. 15 % and will be disregarded. At equilibrium therefore FL+Fs = 0 (5) where Fs is the steric repulsion per unit area.When a potential Vexists across the film there is an additional force Fe of compression given by Fe = -CV2/26,. (6) At equilibrium F'+&+Fe = 0 and therefore (7) Fs = (A/6n6i)+(CV2/26,). (8) The Hamaker constant for the present systems may be found from contact-angle measurements. At potentials of more than about 50 mV across the film the electrical forces exceed the London-van der Waals forces and for 370 mV applied potential the former are some 50 times the latter. From eqn (8) and a knowledge of the capacitance over a range of applied potential the variation of the steric repulsion force with film thickness may be found. This has been done for all the systems but only for the n-decane + saturated NaCl system have the measurements been taken to relatively high potentials.A plot of Fs against 6 for this system is shown in fig. 4 (inset). The change AAs in free energy of the film due to the steric interaction may be calculated by means of the relationship AAs = - [dLF,d6,. (9) Jo3 The total free energy change AA of the system as the film thins may therefore be written D. M. ANDREWS E. D. MANEV AND D. A . HAYDON 53 AA and its components are shown in fig. 4. The onset of the steric repulsion is so sharp as not to affect appreciably the depth of the minimum. This incidentally justifies the assumption which was made in the calculation of the Hamaker c~nstant.~ For the systems listed in table 1 the steric repulsion against film thickness is shown in fig. 5 ; the rise of the steric repulsion is as steep or more so than in the n-decanef saturated NaCl system.8L (m> FIG. 4.-Free energy changes as a function of film thickness for glyceryl mono-oleate + n-decane films in saturated NaCI. The dashed curves represent the separate London-van der Waals and steric interaction contributions. The inset shows the steric repulsion force Fs also as a function of film thickness. Hamaker constant = 3.48 x J. FILM COMPRESSIBILITY COMPOSITION AND CHAIN CONFORMATION It can be seen from table 1 that the thicker films contain a greater volume fraction of solvent (1 -@) than do the thinner films. In fact as the adsorption of the glyceryl mono-oleate is almost the same in each system (table 2) the film thickness is directly proportional to the amount of solvent in the film. (The finding that F > 1 for the n-tetradecane and n-hexadecane systems is attributed to the inaccuracy in the estimation of the activity correction.) The slopes of the curves in fig.5 are inversely proportional to the compressibility of the films. The thicker films where F is low are thus much more compressible than the thinner films where (Pz 1. There is therefore a direct relationship between oleate chain density and repulsive force. The nature of this force or of the related interaction free energy has been discussed by a number of authors.20 The Helmholtz free energy change of the system as the film thins is for unit area of film where 0 is the film tension y is the interfacial tension of the interfaces between the equilibrium bulk phases ni is the number of moles of i in the system and pim and p i are the chemical potentials of i in the system before and after the film has thinned.In most instances and certainly for the present systems the second term on the right-hand side is negligibly For a system in which adsorption equilibrium AA = A - A = ( ~ ~ - 2 y ) + C n i ( ~ i - ~ i > (1 1) 54 THIN LIPID FILMS with the bulk phases is maintained the first term is calculable in principle from the adsorption isotherms for the single interfaces and the thin films respectively.21 A crude attempt to do this for a thin lipid film was made previou~ly.~ An alternative approach which is theoretically less satisfactory but which more readily yields an answer is to assume the adsorption to be independent of film thickness and to estimate the osmotic pressure changes in the film produced by the overlap of the chains of the stabilizer 23 This has also been attempted for the present systems.l The important conclusion is that whichever approach is used it is found that a repulsive force sufficient to stabilize the film is generated by a very small overlap of the oleate chains of the two monolayers.Furthermore in order to explain the relatively high compressibility of the thick films the volume fraction of the 0 SL (nm) FIG. 5.-The steric repulsion force Fs as a function of film thickness for the systems of table 1. Saturated NaCl (Hamaker constant assumed to be 3.48 x J Is) ; (l) n-hexadecane ; (2) n-tetra- decane ; (3) carbon tetrachloride ; (4) cyclohexane ; (9 n-decane) ; (6) 2,2,6trimethylpentane ; (7) n-heptane. 0.1 M NaCl (Hamaker constant assumed to be 6.76 x J ’) ; (S) n-decane.oleate chain segments in the overlapping parts of the monolayers must be very small compared to the average volume fraction @ in the film. As in these particular systems the thickness of the hydrocarbon core of the film is closely similar to twice the extended chain length of the oleate it is inferred that at any given time only a small fraction of the oleate chains are fully extended. In the thinnest films (6 <3.2 nm@= 1 and the oleate chains of each monolayer are packed into a thickness of 1.5-1.6 nm. Films thicker than this must be stabilized by the greater tendency of the oleate chains to extend themselves beyond ca. 1.5 nm from the interfaces although as noted above only a small proportion of them are required to do so. The tendency to extend more fully is apparently related to the nature of the solvent.From the foregoing arguments it is possible to construct a qualitative picture of the time-average volume fraction q of oleate chain segments as a function of distance from the film interfaces (fig. 6). The curve for the thin (e.g. hexadecane) films cannot be appreciably in error as the amount of solvent present is undetectable by the present methods. For the thicker films the form of the curve is less certain, D. M . A N D R E W S E . D . MANEV AND D. A . HAYDON 55 although as any segments located outside the miniinurn volume for the chains (i.e. the region 1.45 nm thick between z = & 2.3 nm and z = & 0.85 nm) necessarily leave a similar sized hole within this region the shaded areas must be equal. -2.3 - 0.05 0 0 * 8 5 2.3 z (m) FIG.6.-A schematic representation of the time-average volume fraction 'p of oleate chain segments as a function of distance across the hydrocarbon region of a film (a) when n-hexadecane and (6) when n-heptane is used as the solvent. While the above remarks apply to the thin film it is probable that if the curves of fig. 6 are correct for the film they are also good indications of the situation in the soliated monolayers. Thus for the thick films where the individual monolayers are not compressed to significantly less than their maximum thickness this must be so. For the thin films it is difficult to give a satisfactory argument in the absence of quantitative relationships. However only a very small overlap of the monolayers at very small segment concentration is necessary to stabilize the film.If therefore the oleate chains in the monolayers were fully extended prior to film formation it is almost certain that more work would have been required to compress them into the close-packed state than to stabilize the thick film and a thick film would haveresulted. As a thick film was not formed it is concluded that the chains in the separate mono- layers must have been already contracted into the nearly close-packed state. D. M. Andrews was in receipt of a maintenance grant from Unilever Limited Port Sunlight during the course of this work. E. D. Manev thanks the British Council for the award of a Scholarship. The authors thank Mr. A. R. Taylor for considerable experimental assistance in the determination of the activity coefficients and Dr. J. L. Taylor for the determination of the interfacial tension against concentration curve for the system hexadecane + saturated NaCl.56 THIN LIPID FILMS D. M. Andrews Ph.D. Diss. (Cambridge 1970). R. J. Cherry and D. Chapman J. Mol. Biol. 1969 40 19. T. Hanai D. A. Haydon and J. L. Taylor Proc. Roy. SOC. A 1964 281 377. H. Sonntag and H. Klare Kolloid-Z. 1964 195 35. D. A. Haydon and J. L. Taylor Nature 1968,217,739. D. A. Haydon and J. Th. G. Overbeek Disc. Furuday SOC. 1966,42 76. ’ R. Aveyard and D. A. Haydon Trans. Furaday SOC. 1965 61,2255. T. Hanai D. A. Haydon and J. L. Taylor J. Theor. Biol. 1965 9 278. J. L. Taylor and D. A. Haydon Disc. Furaday SOC. 1966,42 51. lo D. M. Andrews and D. A. Haydon J. Mol. Biol. 1968 32 149. l1 C. T. Everitt and D. A. Haydon J . Theor. Biol. 1968 18 371. l 2 T. Hanai D. A. Haydon and J. L. Taylor J. Thzor. Biol. 1965 9 433. l3 H. T. Tien and E. A. Dawidowicz J. Colloidlnterfuce Sci. 1966 22,438. l4 G. M. W. Cook W. R. Redwood J. L. Taylor and D. A. Haydon Kolloid-Z. 1968,227 28. l5 G. S . Hartley Ann. Reports. 1948 45 50. l6 A. B. Scott and H. V. Tartar J. A m r . Chem. SOC. 1943,65,692. l7 J. M. Corkill J. F. Goodman and T. Walker Truns. Furuduy SOC. 1967 63 768. l8 D. F. Billett and D. A. Haydon to be published. l9 E. J. W. Verwey and J. Th. G. Overbeek Theory of the Stability of Lyophobic Colloids (Elsevier 2o R. H. Ottewill in Nonionic Surfuctunts ed. M. J. Schick (Marcel Dekker New York 1967) 21 E. L. Mackor and J. H. van der Waals J. Colloid Sci. 1952 7 535. 22 E. W. Fischer Kolloid-Z. 1958 160 120. 23 D. H. Napper Trans. Furatlay SOC. 1968 64 1701. Amsterdam 1948). vol. 1 p. 649.

ISSN:0370-9302    

DOI:10.1039/SD9700100046

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

Interaction Energy of Emulsion Droplets and the Influence of Adsorbed Layers on it BY H. SONNTAG J. NETZEL AND B. UNTERBERGER Deutsche Akademie der Wissenschaften zu Berlin Zentralinstitut fur Physikalische Chemie Berlin-Adlershof Deutsche Received 9th March 1970 The equilibrium distance contact angle and formation velocity of black films between model droplets in aqueous solution of surfaceactive agent were measured. These variables produced information about the forces of interaction between the droplets. The concentration of the surface- active agent was increased to determine the effects of surface concentration upon the above forces for small and wide particle spacings with reference to the thickness of the adsorbed layers. All experi- ments proved that by stepping up the concentration of the surface active agent the attraction between the particles was increased and consequently the stability reduced.The stability of colloids and the formation of coagulation networks in concentrated dispersions may be modified within wide limits by adsorbed layers of surface-active materials. They are capable of changing both the electrostatic repulsive forces and the dispersion forces. Other factors that have to be taken into account under conditions of small particle spacings are the dipole forces the hydrogen bonds of water molecules adsorbed according to a given orientation and the steric hindrance of the adsorbed layers likely to occur in their mutual penetration or compression. While for practical purposes there is a common use of such modification of interaction forces by adsorbed layers little quantitative work has so far been performed on this aspect.This paper will be limited to results obtained from investigations of non-ionic surface active materials which are less complex since adsorption-dependent changes of the charge may be neglected. The influence of non-ionic surface active agents on the stability of sols has already been described by some authors. 1-6 However the influence of surface concentrations of surface-active agents upon interaction has not yet been studied in connection with model tests of foams and emulsions. An attempt is made to close this gap in this paper which reports the effects undergone by the interaction forces between oil droplets which are separated by an aqueous film of surface-active agents. EXPERIMENTAL MATERIALS Spectroscopically-pure n-octane and doubledistilled water were used.The electrolytes were molten prior to use. The non-ionic surface-active agent used was nonylphenol with 20 moles of ethylene oxide (NP 20) a preparation made in the laboratory. MEASUREMENT OF THE EQUILIBRIUM DISTANCE The interaction forces between emulsion droplets may be characterized by the equilibrium spacing desn resulting from an equilibrium of repulsive with attractive forces which is reached 57 58 INTERACTION ENERGY OF EMULSION DROPLETS with a certain electrolyte concentration. The distance between microscopic emulsion droplets were measured by interferometry. The radius of the plane-parallel contact zone was 0.01 cm. The apparatus used is described in detail in ref. (7). For emulsions the calculation of droplet spacings from the intensity of light reflection proved to be easier than for foam laminae since it could be based on the single-layer model.The alkyl groups of the surface-active material were found in the oil phase. with their refractive index to first approxi- mation being identical with that of octane. The greatest change of the refractive index was found to take place at the boundary between the oily phase and the hydrated polar groups so that the percentage of the adsorbed layers formed by the polar groups will be included simultaneously by the optical measurement of oil droplet spacing. MEASUREMENT OF THE CONTACT ANGLE A porous annular glass cell g as shown in fig. 1 was used to measure the contact angle between the black film and the volume phase. The surface of the glass frit was melt-sealed t c \ 9 \ \ \ m \ \ \ 1777777772 FIG.1 .-Apparatus for contact angle determination. so that the drainage of the fluid took place only inside the conical bore 0.08 cm in diameter. The drainage proper was effected by means of the micrometer screw m. The glass frit was inserted in a cuvette c with a plane-parallel bottom which was filled with the oil phase. The glass tube t shown to be fused on to the glass frit above bore level was not needed for emulsion testing but would be required to test the heterosystem air water oil not considered in this context. The detector was mounted on an incident-light microscope and the droplets were photo- graphed prior to and after black film formation. The contact angle was calculated from the spacing of Newton’s rings or from the growth of the contact area following black film forma- tion according to the method of Sheludko and co-workers.* VELOCITY OF BLACK FILM FORMATION The velocity of black film formation was characterized by measuring the time that elapsed from the appearance of the first black spots to the completion of a homogeneous black film.g H .SONNTAG J . NETZEL AND B . UNTERBERGER RESULTS The equilibrium spacings of octane droplets in NP-20 solutions of different concentrations were measured in electrolyte concentrations of 0.001-0.01 m KCI. The results are given in fig. 2. It may be seem from the graph that up to the critical 59 500 400 - 3 0 0 s 1 2 0 0 I00 0 FIG. 2.-Equilibrium distance deqU(A) as a function of the KC1 concentration c (rnol/l) for different surface-active agent concentrations 0 7.5 x mol/l ; x 7.5 x mol/l ; v 1 x mol/l.mol/l ; 0 7.5 x mol/l ; 0 2.5 x concentration of micelle formation (c.m.c.) which was reached at 1.4 x mol/l the equilibrium films decreased in thickness with the electrolyte concentration remaining constant. The influence of the surface-active agent grows as the droplet distances are reduced. While above the c.m.c. level no further change of the equilibrium spacings takes place the influence of the surface-active agent can be observed already at lower electrolyte concentrations. These experimental findings might be explained by both a decrease of the repulsive forces or an increase of the attractive forces along with growing adsorption of surface active agent. Equilibrium spacings were measured only under conditions were they were much greater than the layer thickness of the adsorbed polar groups.Contact angle measurements were performed to find out whether with reference to the sign a further change in the influence of the surface-active agents on the interaction forces was possible even with small spacings. The droplet distances chosen for that purpose were so small that the intensity of light reflection coincided with that of the minimum range. They were smaller than 40 A. The experimental results are collected together in table 1. While the influence of the surface active agent concentration upon the contact angle was small a growth of the attractive forces rather than if the repulsive forces was evident and it was even more obvious by comparing the time intervals tA that elapsed from the appearance of the first black spots to the formation of homogeneous black films.These intervals 60 INTERACTION ENERGY OF EMULSION DROPLETS TABLE CONTACT ANGLE 8 AND TIME t OF FORMING BLACK FILMS IN EQUILIBRIUM WITH SOLUTIONS CONTAINING m AND 7.5 x lod3 m NP 20 AS A FUNCTION OF ELECTROLYTE CONCENTRATION electrolyte 10-1 m KCl 0.01 38 0.03 25 1 m KCI 0.00 30 0.04 15 2 m KCl 0.01 10 0.07 tl 5 x m MgS04 0.02 61 0.03 23 5x 10-1 m MgS04 7.5 . 1 8.0 tl show a clear-cut declining trend despite the increased concentration of the surface- active agent in the film. The result that the growth in surface-active agent concentration was accompanied for all distances by a growth (or decrease) of the attractive forces (or repulsive forces) is in a contradiction to the flocculation tests applied to silver iodide sols and latices by Ottewill 1* who found that with in the c.m.c.range any addition of surface-active agent entailed an increase of stability. Slight stability decrease was observed by Glazman in silver iodide and arsenic sulphide sols containing small amounts of surface-active agent whereas in the c.m.c. range the stability was found to grow strongly These results were confirmed by Srivastava who referred to antimonic and arsenic sulphide sols. With reference to peptization of clay with smaller quantities of surface-active agent Schott found a stability decrease for these smaller concentrations and a stability rise for higher concentrations. No change in stability after the addition of non-ionic surface-active agent was established from experi- mental sedimentation of kaolin performed by Lange.Unfortunately there have been no systematic studies into foam films as yet. However it is believed that the authors' results provide an explanation for the different layer thicknesses found by van der Waarde lo and Sheludko l1 who tested macroscopic and microscopic foam films with one and the same surface-active material. The macroscopic films were stabilized at surface-active agent concentra- tions above the c.m.c. and gave equilibrium thicknesses much smaller than those of microscopic films in which the surface-active agent concentrations were smaller by two orders of magnitude. DISCUSSION The only factors which have to be considered in calculating the interaction forces per unit area are the electrostatic repulsive forces (He,) the dispersion forces @ID) and the capillary pressure (ITu) since in all measurements the equilibrium spacings were bigger than the thicknesses of the adsorbed layers.The following equation has to be satisfied for the equilibrium spacings n, = n,+n,. The two forces may be analysed separately if the influence of the surface-active agent is attributable mainly to the change of II, or to that of II,. While nu is also dependent on the given surface-active agent concentration it can be determined separately by measurement of the interfacial tension. The inter- facial tension and consequently the capillary pressure will decrease approximately by a factor of two until the c.m.c. is reached. This will cause a reduction of the attractive forces and therefore cannot explain the reduction of the equilibrium H .SONNTAG J . NETZEL AND B. UNTERBERGER 61 spacings. Since the magnitude of the capillary pressure is identical with that of the dispersion forces for larger distances a change of the former could even lead to a compensation of the measured effect. The dispersion forces are calculated according to the model proposed by Vold.12 Two particles with the Hamaker constant Al interact in a medium with the Hamaker constant Ao. The adsorbed layer is subdivided into two parts,13 the non-polar part of thickness 6 and the Hamaker constant A2 on the one hand and the polar groups of thickness ij3 and the Hamaker constant As on the other. In earlier work evidence was produced to the effect that surface-active agents with equal alkane chain length but different polar group would affect the dispersion interaction due to the structure of this group and that the hydrocarbon chains (solute in oil) did not contribute to the energy of di~persi0n.l~ This would support the conclusion that to a first approxima- tion A is equal to A l .The dispersion forces may then be expressed by the following equation 1 (A8 - A$)2 + (A4 - 2(A$ - A$)(A$ - A t ) + d 3 (d- 6,13 n,=- 6n (d - 263)3 Y where d is the distance determined by interferometry. If A3 differs from Ao smaller distances will always result in increased attraction no matter whether A3 is smaller or bigger than A . and A l since the first and third terms of eqn (2) will be very large. A decrease of dispersion energy may result from larger spacings,13 provided that A3 is smaller than both Al and Ao. The results obtainable from the above equation cannot be evaluated unless at least A.and At are known from other measurements. erg found for microscopic foam films with low surface-active agent concentrations by Shelduko and co-workers l 5 ; A = 8.5 x 10-13 erg is substituted for the oil phase on the basis of measurements on latices l6 and emulsions with low surface-active agent concentra- tions. The adsorbed layers of surface-active agents will be capable of affecting the electrostatic forces in two ways either by changing the structure of the double layer or by changing the potential of the diffuse double layer ($8). Since structural changes would be detectable mainly along with a close approach of the particles this factor should be neglected for the purpose of equilibrium distance measurement (d> B3).The potential at the oil/water interphase may be generated by adsorption of OH’ from the water or by adsorption of the surface active-agents proper. Under the first assumption it would be possible that by increased adsorption of surface- active materials the potential-determining ions would be displaced whch would result in a reduction of the potential as such and consequently of the electrostatic interaction. This was tested by measuring equilibrium distances at constant surface- active agent concentration and variable pH values. The tests have shown that the equilibrium thicknesses remain constant in the measured pH interval of 3-7. Measurements of the interfacial tension performed concurrently have shown that within the above pH range the adsorption of surface-active agents remained un- a1 tered.The evaluation of the experimental results was to produce information as to the contribution made by each of the various factors (A3 a3 and to the influence of surface-active agents. The following equation is obtained for an 1 1-electrolyte [c(mol/l)] and 25°C by introducing in addition the various distance functions into Let us substitute for A. the Hamaker constant of 3 . 5 ~ 62 INTERACTION ENERGY OF EMULSION DROPLETS eqn (1) exp (19.83 1 exp (19.83 + 1 exp (-0.329 x 1O8deqnJc = 1.59 x 1 0 9 ~ 1 (A8 - At)' (A$ - At)2 2(AB - A$)(A$ - Af) + + + n o (3) 6.n(d,,,-263)3 dZqn (deqn - 6)3 6 and A3 may be calculated for different surface-active agent concentrations from the (desu,c) functions determined experimentally. The results are given in table 2.TABLE 2.-INTERFACIAL TENSION 8 POTENTIAL OF THE DIFFUSE DOUBLE LAYER $8 HAMAKER CONSTANT AJ AND THICKNESS 6s OF THE POLAR GROUPS AS A FUNCTION OF CONCENTRATION OF NP 20 surfactant concentration @6 A3 [mol/l.] [dyGcm] [mV] 1013 [erg] $1 7 . 5 x 19.1 32 0.7 22 7.5 x 10-5 1 0 . 5 19 0.7 22 2 . 5 ~ 10-4 10.1 12 1 . 6 38 7.5 x 10-4 9.0 12 1 . 6 38 The interfacial tension and consequently the capillary pressure decreases with increasing surfactant concentration. This will cause a reduction of the attractive forces and therefore cannot explain the decline of equilibrium spacings. The double layer potential decreases up to the saturation concentration of the adsorbed surfactant molecules. On the other hand the Hamaker constant and the thickness of the adsorbed layer increases up to the c.m.c.These results show that the influence of non-ionics on the interaction energy is complex and that each of the various factors tend to decrease equilibrium spacings. An interesting result would be obtained by comparison of the thickness of the polar groups with the structure of polyoxyethylene. Staudinger postulated two structures for polyoxyethylene the zigzag and the meander configuration. Up to a degree of polymerization of about 9 the chain exhibits in a zigzag structure whereas at higher degree of polymerization a meander structure is observed. For the oxyethylene unit in the meander-type chains a value of 2 A is obtained and therefore the length of the oxyethylene chain in NP 20 should have a value of 40 A. This value agrees well with the measured thickness of the polar groups of 38 8 in the saturated adsorbed layer.More intricate problems were faced with regard to contact angle measurement since with the small particle spacings some additional forces of interaction with their orders of magnitude and distance functions still being unknown had to be taken into account. Therefore an analysis of the contact angle measurements and fornia- tion velocity is impossible. The only possible statement is that these measurements too showed an increased attraction of the particles with increasing surface concentra- tions. The hydrate shells often postulated as the cause of stability did not play any role at least in our example. In a theoretical study by Havemann 17a evidence was produced to the effect that water molecules adsorbed by a specified orientation (the dipoles being orientated parallel and arranged vertically relative to the interphase) cause the appearance of repulsive forces almost equal to that of the dispersion forces only if the degree of orientation is above 65 "/o.Such a high degree of Orientation was not reached in our experiments. The fact that a finite particle distance is obtained may be attributed to the steric hindrance of the adsorbed layers which cannot be displaced from the interphase due H . SONNTAG J . NETZEL AND B . UNTERBERGER 63 to the insolubility of the given surface-active agent in the oil phase. Compression or mutual penetration is encountered by the adsorbed layers producing a force which can be compensated by the dispersion forces. This force was measured directly in non-polar media by compression of monomolecular and multimolecular adsorbed layers on mercury droplets.18 The reversible change of distance between two flocculated mercury droplets was determined under conditions under which the droplets were pressed against each other by different pressures.The elasticity (vertical relative to the interphase!) derived therefrom for adsorbed layers of oleic acid 54 A in thickness was lo6 dyn cm-2. Such a value was sufficient for compensa- tion of the dispersion forces. The " mechanical " strength of the adsorbed layers under load relative to the interphase in a vertical manner is believed by the authors to be the cause of the coalescence stability. The latter has often been attributed to the strength in the adsorbed layer (mechanical properties of the layer measured parallel with the interphase).Measurements of both the shear viscosity and elasticity by the method of wave damping have revealed what has become established know- ledge viz. that in monomolecular adsorbed layers of surface-active agent agreement between the coalescence stability on the one hand and the mechanical properties measured parallel with the interface on the other would occur only in few selected instances. This seems to support the conclusion that a breaking-up of the adsorbed layer i.e. the tearing open of a hole is not the decisive step towards coalescence. The question why the stability may be both increased or reduced by non-ionic surface-active agents depending on the nature of the disperse systems studied still remains unanswered. The studies conducted by Cockbain l9 and Lemberger 2o may be considered as confirmations of our own results.In emulsions 1-2p in particle size increased adsorption of ionic surface-active agents will lead to stronger aggregation as supported by the creaming volume quoted in ref. (19) and direct coulter counter determination reported in ref. (20). The same result was achieved by using macrodisperse solid di~persions.~. 21 Yet it seems not plausible that particle size alone should be responsible. K. G. Mathai and R. H. Ottewill Trans. Faraday SOC. 1966 62 759. R. H. Ottewill and T. Walker Kolloid-2.2. Polymere 1968 227 108. H. Lange Kolloid-Z. Z . Polymere 1966 211 106. M. Glazman Jr. Disc. Faraday SOC. 1966 42,255. H. Schott J. Colloid Interface Sci. 1968 26 133. K. L. Daluja and S. N. Srivastava Indian J. Chem. 1969 7 790.H. Sonntag J. Netzel and H. Klare Kolloid-2. 2. Polymere 1966 211 121. A. D. Scheludko B. Radoev and T. Kolarov Trans. Faraday Soc. 1968 64,2213. H. Sonntag and J. Netzel Tenside 1966 3 296. D. Exerowa and A. D. Scheludko Proc. IV. Int. Coizgr. Surface Active Substames (Briissell 1964) vol. 2 p. 1097. l o Van der Waarde private communication. l2 M. J. Vold J. Colloid Sci. 1961 16 1. l3 H. Sonntag and K. Strenge Koagulation und Stabilitat disperser Systertie VEB Deutscher l4 H. Sonntag Tenside 1968 5 188. l 5 A. D. Scheludko and D. Exerowa Kolloid-Z. 1960 168 24. l6 A. Watillon and A. M. Joseph-Petit Disc. Faraday SOC. 1966 42 143. l7 J. Netzel Diss. (Humboldt-Universitat Berlin 1968). l8 K. Strenge and H. Sonntag Tenside 1969 6 61. 2o A. P. Lemberger and N. Mourad J. Pharm. Sci. 1965 54 229. 21 Th. Steudel Forschung Fortschritt 1964 38 201. Verlag der Wissenschaften Berlin 1970 p. 43. 7a U. Havemann unpublished see ref. (1 3). Cockbain E. G. Trans. Faraday Soc. 1952 48 185.

ISSN:0370-9302    

DOI:10.1039/SD9700100057

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

Cohesive Properties of Thin Films of Liquids Adhering to a Solid Surface BY J. F. PADDAY Research Laboratories Kodak Limited Wealdstone Harrow Middlesex England Received 6th April 1970 The process of forming and rupturing a thin liquid film at a solid surface is described thermo- dynamically for both high and low energy solid surfaces. In part 1 the build-up of thin films on high-energy surfaces from the first monolayer is considered and reviewed. Components of the surface free energy of formation of the thin film (disjoining pressure) are defined. For curved surfaces the disjoining forces should be combined with the Laplace capillary pressure to give a correct form of the Kelvin equation. It is suggested from the early work of Bangham and Deryaguin that thin liquid layers have anomalous physical properties.These studies are discussed in relation to the thickness of the liquid films. In part 2 new experimental evidence of the critical rupture thickness of thin liquid films on low energy surface is presented. A number of pure liquids rupture spontaneously on low energy surfaces such as wax or polytetrafluoroethylene at very great thicknesses (0.01 cm). The effects of aqueous salt and surfactant solutions suggest these long-range forces are electrical in origin. We first consider the process of thinning of a thick uniform layer of liquid in contact with a pure smooth horizontal solid surface as shown in fig. 1. It is assumed \ B C FIG. 1 .-The processes of disjoining and rupture. A formation of a thin stable layer ; B formation of a metastable layer with sessile drop or lens on a wetted surface ; C unstable thin layer with the formation of a dry patch.that only vapour is present above the liquid surface and that the solid is completely wet when immersed below the surface of the liquid. If now the liquid thickness is successively reduced one of three phenomena occurs either (A) the solid surface is 64 J . F. PADDAY 65 spread with and holds a uniform thin layer of liquid which is stable at all thicknesses ; (B) or a thin layer is formed which when reduced to some critical thickness dis- proportionates to form a very thin layer in contact with a thicker lens or sessile drop; (C) or at some relatively great thickness the liquid layer in contact with the solid spontaneously breaks to leave a dry patch with a wetting meniscus in contact with it as shown in fig.1. The phenomena of fig. 1A and B are invariably associated with high-energy surfaces and are considered in part I of this paper. The phenomenon of fig. 1C is associated with low energy surfaces and is considered separately in part 2. PART I.-THIN FILMS ON HIGH ENERGY SURFACES Hardy appears to have been one of the earliest investigators of properties of thin films of liquids in contact with solids. He was concerned primarily with observa- tions with lubrication but discovered the dual thickness films (fig. 1B) and likened their behaviour to the formations of lenses of non-spreading oils on water. It became clear from this early work of Hardy and later Bangham and Fakhoury that to form stable or metastable thin films it was necessary to spread liquids on high-energy surfaces such as mica clean glass or silica and charcoal.Bangham 2-4 and his coworkers investigated the build-up of successive layers of condensed vapour to give a film of type A fig. 1. The partial vapour pressures recorded by Bangham and Mosallam did not exceed 0.7 of saturation and the film thicknesses they obtained were less than 20A. They thus never made their films thick enough to investigate the region of condensation on these surfaces where the condensed vapour was of the same thickness as a receding liquid film. Bangham also attempted to estimate the surface tension of much thicker films formed by a liquid receding on a solid surface. He claimed that the thinner of the two films in situation B of fig. 1 had a lower surface tension as measured from the angle of contact between the thick and thin However these wetting studies tended to be of a phenomenological nature and some of his experiments on the break up of liquid films on freshly cleaved mica could not be confirmed by the author .Over the same period Deryaguin and his coworkers investigated the properties of thin films of liquid on high-energy ~olids.~-lO Deryaguin introduced the term wedging-apart pressure ” later known as “ disjoining pressure ” to describe the equilibrium force required to remove a small increment of thickness of the thin liquid layer. 66 DISJOINING PRESSURE OF THIN FILMS OF LIQUIDS ON SOLIDS The process of joining a thin liquid film to a solid surface is shown in fig. 2. Rupture in effect is the “ reverse ” of this process. The disjoining process differs from rupture in that only a small increment of liquid thickness is removed and not the whole layer.The steps of the process are taken as first the formation of the free liquid film (A-B) followed by the bringing into contact of the solid surface with the thin film (B-C). Thereafter specific interactions or solution may take place between solid and thin film (C-D) and finally adsorption forces may lead to the build-up of a electric potential at each interface of the thin layer (D-E). Deryaguin and Shcherbakov Duyvis,l Kitchener and Sheludko l4 have all given thermodynamic accounts of the disjoining process. They define the dis- joining pressure as the change of free energy with thickness and break it down into SP1-C 66 COHESIVE PROPERTIES OF THIN FILMS components. Deryaguin and Obuchov expression define this disjoining pressure ll by the where y is the specific surface free energy of the thin liquid film yo is the specific surface free energy of an infinitely thick film and h is the thickness of the layer.A B C .' . .. I. - - . - + **. . . . . *. .. . .. -. . .. ' * .. .. '. ' .. I .,* 'liquid .. 4 TTLL -1 . .. * *. - . -* TT,s ' - . .. *.. . . 1 -. - - .. - . - E D I FIG. 2.-Steps in the formation of a thin film. A-B formation of thin liquid film ; B-C adhesion of thin films to solid surface ; G-D reorientation of thin liquid layer by specific interactions ; D-E adsorption of electric charge. Provided that the structure of the bulk liquid is retained in the thin layer it is possible to inter-relate disjoining pressure with changes in surface tension but Deryaguin and his coworkers have consistently maintained that thin liquid films adhering to solid surface are either totally oriented in the thin layer (Deryaguin's a- phase) or are partially oriented over a fraction of the total thickness of the film (Deryaguin's P-phase) the remainder of the layer containing unoriented or bulk liquid.Hence on Deryaguin's evidence it is incorrect to consider disjoining pressure as involving a change in surface tension alone. A more general definition of dis- joining pressure is derived by Deryaguin and Shcherbakov l1 in terms of the chemical potential of the inolecules forming the thin film and the total free energy w of the thin film. Their equation was RTln (PIP,) = ul1,(6w/6h) (2) where v is the molar volume of the substance forming the thin layer P is the vapour pressure in equilibrium with the thin film and P the saturated vapour pressure at the temperature T.The term -6w/61z is the disjoining pressure and is equivalent to TI of eqn (1). Eqn (2) applies equally to the adsorption of the first monolayer of vapour on the solid surface and to successive build-up of condensed vapour into a true liquid layer. It thus allows the vapour pressure to be used to derive the disjoining pressure when direct measurement is not possible. COMPONENTS OF THE DISJOINING PRESSURE Sheludko l4 and Kitchener l3 split the disjoining pressure ll into two components ; a term IT for the van der Waals interactions and a term 17, for the component due to electrical double layer repulsion. It is more convenient to split the van der Waals term into two components one ITLL for the disjoining pressure of the liquid film in the absence of any solid (equivalent to process A to B of fig.2); and the other, J . F . PADDAY 67 nSL (equivalent to process B to C of fig. 2) for the effect of the solid on this film. In the absence of electrical charges and of specific interactions ELL must always be negative and nSL always positive. To take account of the change in bulk properties in the film that is consistently claimed by many workers a component of the disjoining pressure n is introduced. This component represents the change in free energy of the thin layer due to orienta- tion solvation or other specific interactions between the solid and the thin liquid film. Although measurement of this component may well prove difficult its value must be positive for high energy surfaces.The contribution of the electrostatic pressure IT, has been derived by Langmuir,ls Frumkin,l6 Verwey and Overbeek,I7 Deryaguin and Landau and She1udk0.l~ Generally n, was derived for two interacting electrical double layers of the same charge sign and density such as those that stabilize thin soap films. Read and Kitchener l 9 calculated n, of a thin water film at a silica surface taking into account the difference in potential between the solid-liquid and liquid-air interfaces. They pointed out that the usual assumption of constant potentials leads to physically improbable consequences because the negative charge at one surface was much greater than at the other. The counterions of the surface of high charge would render the surface of low charge positive and thereby induce attraction.However it is certain that ITel must be positive when the two interacting surfaces of the thin film are of like charge. Similarly ITel must be negative when the two surfaces are of opposite charge. A curved liquid-air surface of radius Y will possess a capillary pressure given by 2y/r and a vapour pressure P different from the saturated value P,. The capillary pressure is associated with the liquid-air-interface and may be used as a direct measure of the disjoining pressure lo l9 when the curved surface is free from thin film perturbations. However when a curved liquid-air surface approaches a flat surface film as with a Wilhelmy plate then there will be a transition region however small where the capillary pressure diminishes and disjoining pressure increases.ln this region we suppose in the first instant that the disjoining and capillary pressures are additive so that where r and r 1 are the principal radii of curvature which possess a negative value when the liquid-air interface is concave and a positive value when convex. For thick films ll is zero and eqn (3) reduces to the Kelvin equation. The contributions ITLL ITsL IT and n, are all complex functions of /z and each will vary according to whether the thin film is flat or curved. Although 1111 and ITel are probably interrelated we assume additivity such that (4) = ~ L L + X ~ L + ~ 1 + riel. When ll is negative the thin film is unstable. positive but d2w/6h2 must be negative. For stability not only must II be REGIONS OF STABLE THIN FILMS The thin liquid film adhering to a long perfectly-wetted Wilhelmy plate will be considered following the model of Read and Kitchener.lg Such a plate is shown in fig.3 and here it will be used to distinguish certain zones of the thin films. If it is assumed that only pure vapour is in contact with the liquid and that the vapour is an 68 COHESIVE PROPERTIES OF THIN FILMS ideal gas the vapour pressure P at any height I above the free surface bulk liquid is given by (RTIu,) In (PIP,) = - I gp. ( 5 ) The hydrostatic height may thus be used as a measure of the disjoining pressure. Combining eqn (3) (4) and (9 - - At flat films the first term is zero and disjoining pressure is the main component whereas for thick films the first term predominates and all other terms are zero. ' p / CM. PS - I08 - 107 0.000; - - I d 0484 -lo5 0.930 -I& 0.993 -I$ 0.3993 -lo2 0.9999 -I0 0-99999 1 L WILNELMY PLATE l - r h rfNkt.4 10'0 lo8 I06 104 B.E.T.EANGHAM DERYACUIN a ZORIN i 20 6 0 100 400 2oC 1500 UNKNOWN REGION ZORlN L CHURAEV OERYAGUIN L KUSSAKOV READ L KITCHENER KELVIN / LAPLACE . .. FIG. 3.-Regions of stable thin films on a Wilhelmy plate. Where the Wilhelmy plate meets the free surface of liquid a cylindrical meniscus will be formed. At some height above the free surface equal to the capillary constant a the liquid meniscus will meet the flat thin film. The shape of the meniscus in this region is governed by the Laplace equation because all II terms of eqn (6) are too small to matter. This region is the meniscus zone of the " thin films " and may be located anywhere up the side of the Wilhelmy plate merely by bringing a second Wilhelmy plate into close proximity so that capillary rise is obtained.At some height say lo8 cm above this zone the partial vapour pressure is so small that the solid Wilhelmy surface may be regarded as practically " clean ',. Between these two zones lies a series of regions of different disjoining pressure in which all experimental data may be placed. The first region below the " clean'' zone may be regarded as that region in which the first monolayer of adsorbed vapour condenses on the solid surface. Descending further one enters the B.E.T. region where partial vapour pressure increases to a value of about 0.7 and multilayer adsorption sets in. J . F. PADDAY 69 Bangham's 2o system of adsorbed water vapour on glass and mica covered a range of partial vapour pressures not exceeding 0.7 and therefore falls in to this B.E.T.region. He ascertained the thickness of his adsorbed layers as being no greater than 20 8 ; hence the anomalous properties of condensed or adsorbed water claimed by Bangham refer specifically to the first few layers and not to the much thicker layers of Deryaguin and Zorin 21 and those by which anomalous water was formed as reported by Fedyakin 22 and by D e r y a g ~ i n . ~ ~ However Bangham claimed anomalous properties for the much thicker layers he obtained with receding wetting films. The results of Deryaguin and Zorin 21 on thin films of water and other liquids on glass fall into the next region of partial vapour pressure 0.7 to 0.99. This is the region where capillary condensation usually takes place on porous solid surfaces.In the B.E.T. region and the region of capillary condensation the main contribu- tion to the total disjoining pressure was believed by both Bangham and by Deryaguin to be ITI i.e. a specific but long-range interaction which is much larger than the sum of the van der Waals attractions nSL+nLL. The results of Read and Kitchener,lg Deryaguin and Kussakov l o and Zorin and Churayev 24 fall in a region of much lower disjoining pressures and refer to much thicker layers of 200 to 5000A. Read and Kitchener attribute their low disjoining pressure at these thicknesses to electrostatic repulsion IT,, because the repulsion fell as ionic strength of added salt increased. The contributions of the other terms HSL IfLL and IT in Kitchener's experiments were regarded as zero at these much greater thicknesses.Zorin and Churayev 24 measured the thickness of thin water films on quartz at low disjoining pressures. Their stable film was 100 A thick and possessed a peripheral step 400A thick. This step could well correspond to the step found when a large clean sheet of glass is withdrawn from a basin of water and allowed to e q ~ i l i b r a t e . ~ ~ * ~ ~ The step on the plane glass sheet appears at about 10 cm above the free surface and corresponds to a disjoining pressure of about lo4 dyn but is attributed by Satterly and Turnbull 25 to contamination. This step might mark a change from electrostatic to interactive disjoining forces. Each of the terms of eqn (4) are a different function of the thickness h so that II the total disjoining pressure is unlikely to fall away smoothly.Deryaguin and Zorin 21 showed that it did so for the non-polar liquids CC14 and CsH6 where the disjoining pressure decreased steadily to zero in a manner qualitatively predictable from London's theory of dispersion forces. For these liquids one may assume that where ASL and ALL are the Hamaker constants for intermolecular interactions between solid-liquid and liquid-liquid molecules respectively. K is a constant which depends on the value of n which possesses a value of 3 when the dispersion forces are unretarded and a value of 4 when retarded. Approximate expressions for ITsL and IT, are given by Kitchener l 3 and Sheludko,14 and these lead to negligible forces when 12 is greater than 500w. Thin layers of polar liquid do not show the same trend.Water and n-alcohols are known to interact with polar surfaces such as glass or mica to produce positive value of IT (Deryaguins' a-phase or Bangham's aggregated phase). The stepwise increase or decrease of the stable film has been explained by Zorin 27 as due to the difference of IT for the two phases postulated by Deryaguin. In fig. 4A the lower isotherm refers to the disjoining isotherm of Deryaguin's a-phase and the upper film on mercury. Scheludko l 4 suggests that the same data may be explained as shown in fig. 4B without recourse to the two phases supposed by Deryaguin. 70 COHESIVE PROPERTIES OF THIN FILMS Between the thick or stepped region of low disjoining pressure and the capillary condensation region of very high disjoining pressure lies a large unknown region of disjoining pressures that has never it seems been investigated.In the region of capillary condensation where 0.7 <Pips < 0.99 the Kelvin equation may be tested experimentally. The Kelvin equation without disjoining effects leads to radii of curvature between 20 and 50 A. Deryaguin and Zorin show that disjoining effects extend to lOOA in this region. This is also the region of partial vapour pressure appropriate to the formation of Deryaguin's anomalous water. Thus any test of the Kelvin equation must take into account disjoining effects as given in eqn (3) and any application of disjoining pressure to curved surfaces must include the capillary pressure. h I *O Zorin h 1.0 9 PS Shelud ko FIG. 4.-Disjoining isotherm of benzene on mercury. A Zorin; B Sheludko (taken from Sheludko 14).PART 2.-RUPTURE OF THIN FILMS AT LOW ENERGY SURFACES A third type of liquid layer in contact with a solid surface that of fig. lC is that which spontaneously ruptures when the thickness is reduced below some critical value. This type of thin liquid film is formed only at low-energy surfaces and it appears to be associated with large critical rupture thickness i.e. 0.01-0.05 cm. Although negative disjoining pressures have been measured by Sheludko and Platikanov 28 the rupture process of fig. lC does not seem to be widely studied. The experiments by which the critical thickness of these films was observed are given here together with the principal results. EXPERIMENTAL The apparatus for measuring the critical rupture thickness shown in fig. 5 is basically that described previously 29 for measuring spreading coefficients by the drop-height method.Instead of the solid surface E resting on the sample platform it was supported inside a glass container G on three glass chips broken from a microscope slide. The surface of the plane solid which was either a waxed glass microscope slide or a flat piece of polytetrafluoroethylene (Teflon) was mounted level. A 10 ml glass syringe was used to raise or lower the level of liquid in the glass container G. All apparatus was carefully cleaned and washed. Freshly distilled water checked for purity by surface-tension measurement and by shaking was poured into the vessel so as to cover the solid surface. The syringe was then J . F. PADDAY 71 used to lower the level of the liquid while at the same time its height was monitored with the pointer C.At some critical thickness the meniscus suddenly broke through to the Teflon. The critical rupture thickness was then measured as the distance between the pointer and the solid surface with an accuracy of fl .O p. The thickness at which breakdown took place was reproducible to within &3 % and was usually between 0.01 and 0.05 cin. Thick- nesses below 0.003 cm were regarded as being too small to measure to this method. The surface tension of water after disjoining was measured and found to be unaltered (within f0.05 dyn/cm) thus dispelling the possibility of artifacts caused by impurities. FIG. 5.-Apparatus for measuring the thickness of thin films on a low energy surface. A syringe B microscope with a vernier graticule ; C point adjustable in focus plane of the microscope ; D height adjustment of the microscope; E flat sample of low energy surface; F spreading liquid G glass vessel.RESULTS OF NEGATIVE RUPTURE PRESSURES It is believed that the phenomenon described measures a negative rupture pressure. In table 1 the critical rupture thickness h of a number of liquids on Teflon measured by this method are given. Even pure hydrocarbons and ethylene glycol ruptured spontaneously . TABLE CRITICAL RUPTURE THICKNESS OF PURE LIQUIDS ON TEFLON YL liquid p g/cm2 dyn/cm h cm water 1 .o 72 -0.051 benzene 0.88 29 -0.016 ethylene glycol 1.12 49 - 0.056 3 -decene 0.74 23 - 0.026 1 - tetradecene 0.775 27 - 0.027 1 -0ctadecene 0.79 28.4 - 0.027 That the process of rupture was not some artifact due to irregularities of the solid surface was shown by comparing the critical disjoining thickness of a flat and curved wax surface.The curved surface was formed by coating a watch-glass with paraffin wax. These results (table 2) indicate that the effect of curvature on the critical rupture thickness is relatively small and that therefore the effect of gross unevenness 72 COHESIVE PROPERTIES OF THIN FILMS of a solid surface cannot account for the large rupture thickness. The results of water rupturing on a Teflon and o n a wax surface (tables 1 and 2) show that the rupture thickness is critically dependent on the nature of the solid surface. TABLE 2.-cRITICAL RUPTURE THICKNESS OF WATER FILMS ON PARAFFIN WAX SURFACES h (cm) -0.032 flat surfaces - 0.03 1 curved surface (radius 6 cm) The effect of spreading aqueous solutions of KN03 tetradecyltrimethyamnionium bromide (t.t.a.b.) and the sodium salt of disec-octylsulphosuccinate (d.0.s.) on Teflon was also investigated.It was found that the critical rupture thicknesses of solution of KN03 followed closely those of solutions of t.t.a.b. as seen from fig. 6. At I I 0-5 10-4 10-3 10-2 to-’ mol/l. FIG. 6.-Critical rupture thickness of aqueous solution on teflon. x tetradecyltrimethylam- monium bromide ; 0 sodium dioctyl sulphosuccinate ; A KN03. concentration below 3 x M only small changes in thickness were observed but between 3 x and M the rupture pressure decreased suddenly and then increased to a value slightly greater than that of pure water. D.o.s. behaved differently. In the same concentration range the anionic surfactant d.0.s.produced a film that was stable at all measurable thicknesses and would not spontaneously disjoin (see fig. 6). In the concentration range 3 x to M both d.0.s. and t.t.a.b. adsorb at both the liquid-air and the low energy solid-liquid interfaces,30* 31 thus it is likely that the main contribution to these forces is the electric term arising from monolayers of adsorbed ionic surfactants. The comments of Dr. J. A. Kitchener are gratefully acknowledged. APPENDIX (ADDED IN PROOF) The following evidence was obtained from further experiments performed since writing the paper and was presented in the Discussion. SURFACE DEFORMATION PRIOR TO RUPTURE In a further series of experiments the rupture of a thin film of water at a curved wax surface was followed using a high-speed cine camera recording at approximately 10oO frames s-l while the liquid layer was continuously thinned.These sequences showed clearly that a FIG. 7.-Formation of dimple and rupture of a thin film of water on wax. Approx mag. x2. Radius of wax surface 7 cm. (a) Formation of dimple (towards left-hand edge of white patch) ; (b) rupture revealing wax surface below. To face page 73.1 J . F. PADDAY 73 local indentation of the water/air interface occurred at the position where the convex wax surface was nearest the liquid-air surface. In fig. 7 prints of two frames from this cine sequence are shown. The whitish patch is a specular reflection in the liquid/air interface produced by a powerful lamp. In the upper picture (a) the indentation is seen as a dimple in the surface near the edge of the specular reflection.Some 300 ms later the dimple broke through to the wax surface as shown in the lower picture (6). Once the wax was revealed the border expanded rapidly as a receding meniscus. Although these observations were performed on a dynamic system they served to show the formation of the dimple prior to rupture. A cross-section of it is shown in fig. 8(u). The dimple was also produced with I FIG. 8.-A apparent cross-section profile of the dimple of fig. 7(a) ; B apparent cross-section profile of deformation with wax that had been polished with cellulose fibres. the apparatus of fig. 5 but with the flat Teflon surface replaced by a convex glass surface coated with pure paraffin wax. Fig. 8(b) shows a second type of deformation obtained with a wax surface artificially polished.A dimple formed and appeared to be stable for many min- utes while observations were being made. Vibrations artificially induced at the liquid/air interface appeared to be damped out as they reached the dimple and only high energy knocks succeeded in forcing the dimple to break through prematurely. It was not found possible to estimate the depth of the dimple from the cine film but the appearance showed that the dimple penetrates a considerable part of the thickness of liquid layers Thus the thickness of the liquid film between the bottom of the dimple and the top of the convex wax-liquid interface will be considerably less than the critical disjoining thickness of tables 1 and 2. Furthermore the apparent negative disjoining forces at the bottom of the dimple will be balanced by a further restoring pressure P I given by where t is the depth of the dimple and r2 the radius of curvature (negative in value) at the bottom of the dimple.W. Hardy Proc. Roy. SOC. A 1912,86,610; 1923,104,25 ; 1925,108 1 ; 1926 112,62. D. H. Bangham and N. Fakhoury J. Chem. SOC. 1931 1324. D. H. Bangham N. Fakhoury and A. F. Mohamed Proc. Roy. SOC. A 1934 147 152 and 175. D. H. Bangham and S . Mosallam Proc. Roy. SOC. A 1938,165 552 ; 1938,166 558. D. H. Bangham and R. L. Razouk Trans. Faraday Soc. 1937,33 1459. D. H. Bangham and Z . Saweris Trans. Faraday SOC. 1938 34 554. B. V. Deryaguin J. Phys. Chem. 1932 ,3 29 ; 2. Phys. 1933 34 657 ; J. Phys. Chem. 1934 5,379 ; Sov. Phys. 1933 4,431. B. V. Deryaguin and E. Obuchov J. Colloid Chem. 1935,1,385 ; Actaphysicochim. 1936,1,5.B. V. Deryaguin and M. Kussakov Bull. Acad. Sci. U.R.S.S. 1936 471. B. V. Deryaguin and L. M. Shcherbakov Colloid J. U.S.S.R. (trans) 1961 23 33. lo B. V. Deryaguin and M. Kussakov Actuphysicochim. 1939 10,25. l2 E. M. Duyvis Thesis (Utrecht 1962). l3 J. A. Kitchener Endeavour 1962 22 118; Wetting S.C.I. Monograph no. 25 1967; (Soc. Chem. Ind. London 1967) p. 300. l4 A. Sheludko Adv. Colloid Interface Sci. 1967 1 391. l 5 I. Langmuir J. Chem. Phys. 1938 6,!873. 74 COHESIVE PROPERTIES OF THIN FILMS l 6 A. N. Frumkin Acta physicochim. 1938 9 3 13. l 7 E. J. W. Verwey and J . 1'. G. Overbeek TI2cor37 of the Stability of Lyophobic Colloids (Elsevier Amsterdam 1948). B. V. Deryaguin and L. Laundau Zhirr.. E x ~ p . Teor. Fig. 1941 11 802. l 9 A. D. Read and J. A. Kitchener J. Colloid Interface Sci. 1969 30 391. 2oD. H. Bangham J. Chem. Phys. 1946,14 352. 21 B. V. Deryaguin and Z. M. Zorin Proc. 2nd Znt. Congr. Surface Activity (London) 1957,2,145. 22 N. N. Fedyakin Colloid J. (trans.) 1962 24 425. 23 B. V. Deryaguin et al. Teor. Eksp. Khim. 1968 4 527. 24 Z. M. Zorin and N. V. Churayev Colloid J. (trans.) 1968 30 279. 2 5 J. Satterly and R. Turnbull Trans. Roy. Sac. Can. 1929 3 95. 26 R. S. Burdon Proc. Phys. SOC. 1926 38 148. 27 Z. M. Zorin KolloidZhur. 1963 25 624. 28 A. Sheludko and D. Platikanov Kolloid Zhur. 1961 175 150 ; Dokl. Akad. Nauk S.S.S.R. 29 J. F. Padday Rev. Sci. Znstr. 1959 26 256. 30 J. F. Padday Wetting S.C.1 Monograph no. 25 (SOC. Chem. Ind. London 1967) p. 234. 31 J. F. Padday Proc. 4th Int. Con.qr. Surface Active Substnnce 1964,2,299. 1961 138,415.

ISSN:0370-9302    

DOI:10.1039/SD9700100064

出版商:RSC    

年代:1970

数据来源: RSC

摘要:

GENERA L DISCUSSION Mr. D. W. J. Osmond (I.C.Z. Ltd. Slough) said My colleagues and I have been very interested in the paper of Andrews et al. We have been concerned with the behaviour of sterically stabilized particles in concentrated solutions and ‘‘ melts ”. There are now semi-quantitative theories for the action of polymeric steric barriers in low molecular weight environments. These theories suggest that a large part of the repulsion arises from the non-ideal osmotic pressure generated by the increased concentration of polymer segments in the region in which the barriers have overlapped. Clearly however this change in segment concentration and hence excess osmotic pressure is much reduced in concentrated solutions of unadsorbed stabilizing polymer in the low molecular weight solvent and disappears altogether in an environment which comprises solely molten polymer.F. A. Waite and I have considered the problem of particle stability in such cases and have concluded first that the concentra- tion of segments of stabilizing (adsorbed) polymer chains in the barrier will rise in the limiting case of the melt to 100 % and secondly that these concentrated barriers will be stiffer and stronger than the more diffuse barrier formed in low mole- cular weight solvents. We propose that where the particle surface has free access to more stabilizing molecules and there are no packing limitations of the anchor groups at the surface more stabilizer will be adsorbed to maintain the barrier of similar thickness to that of the barrier in low molecular weight solvent. Where these conditions cannot be fully met the additional rise in concentration must be met by contraction of the barrier back towards the surface.In some preliminary experiments we have demonstrated that stable dispersions of polymethyl methacrylate stabilized by polymethyl methacrylate/poly( 12 OH stearic acid) graft copolymers can be prepared in molten methyl ester of poly(l2 OH stearic acid). In these dispersions the amount of stabilizer adsorbed per unit area of particle surface appears to be several times that adsorbed from low molecular weight aliphatic hydrocarbon continuous phases. In the latter case the effective barrier thickness is believed to be about lOOA (10 nm) and the mean stabilizing segment concentration about 10-15 %. In the melt therefore the segment density may be about 50 % with a barrier still of 100 8 (10 nm) or more probably the segment density may be near 100 % with the barrier contracted to about 50A (5 nm) thickness.The experimental techniques of Dr. Haydon and his colleagues are far more refined than ours and they support our ideas closely. It would be of interest if the work could be repeated with higher molecular weight soluble chains. In this case the lower segment densities in low molecular weight solvents would allow a much larger rise in concentration as the solvent molecular weight was raised and freed from the limitation of anchor group spacing which may operate with oleyl mono- glyceride the predicted increase in adsorption might also be observed. Dr. H. E. Ries (Chicago) said Although the interesting studies of Andrews Manev and Haydon were performed on systems considerably different from those investigated in our laboratories the overall results and their implications are strikingly different from the stand-point of chain-length compatibility in mixtures of polar and non-polar compounds.In our work we have found that (a) mixed films of stearic acid and n-hexadecane at the water/air interface retain considerable hydrocarbon in the monolayer when subjected to elevated surface pressures ; and (b) radiotracer 75 76 GENERAL DISCUSSION adsorption experiments at the solid/liquid interface indicate the formation of mixed films of stearic acid and n-hexadecane. In the latter experiments radiostearic acid is adsorbed directly from n-hexadecane solutions on vapour-deposited metal films on the window of a Geiger tube.Moreover studies of rust-preventive films clearly demonstrate strong interaction and chain-length compatibility in thin films of fatty acids and hydrocarbons. Fatty acids and hydrocarbons of the same length and geometry apparently form stronger mixed films and thus provide better rust protection,2 as well as greater resistance to scuffing in four-ball studies by Cameron and other^.^ Dr. H. Sonntag (Deutsche Akademie der Wissenschaften Berlin) said With regard to the paper by Haydon et al. did the thickness of the hydrocarbon region change reversibly or irreversibly with applied potential ? We obtained with the same method the thickness of oil layers between mercury droplets reversibly up to a certain applied potential and we could calculate the elasticity of the adsorbed layer which was unaffected by the thickness and had a value for oleic acid layers (thickness 54 A) of lo6 d y n / ~ r n ~ .~ Above this potential the thickness altered irrever- sibly. From Haydon’s measurements we calculated an elasticity of 10’- lo6 dyn/cm,2 which is of the same order as our values. It it possible that the change of elasticity with applied potential could be explained by an alteration of the film diameter? We measured an increasing diameter of the film with increasing applied potentials in water/oil emulsions. Prof. J. Th. G. Overbeek (University of Utrecht) said The sharp c.m.c. in the presence of 0.1 M NaCl and the much wider transition region in the presence of saturated NaCl (see fig. 3 of the paper by Haydon et aZ.) point to small aggregates formed with saturated NaCl and larger ones with the more dilute solution.It is worth while to look for a confirmation by light scattering or ultracentrifugation. The difference in aggregate size might be connected with the difference in water activity if water is a necessary partner in at least the bigger aggregates ; or the small aggregates might be formed around ions solubilized in the oil phase. The latter explanation would require an increased conductance of the oil phase in contact with saturated NaCl. Dr. D. A. Haydon (PhysioZogicaZLab. Cambridge) said In reply to Osniond we have been interested for some time in the possibility of working with black lipid films stabilized by molecules of chain lengths both longer and shorter than that of the oleyl derivatives. A limited investigation has been reported using chain lengths between n-C, and n-C, but owing to the nature of the stabilizing molecules a detailed interpretation of the results was not possible.It is hoped however to find satisfactory stabilizers of chain length greater than CZ2. The problem of steric stabilization in one component liquids such as polymer melts to which Osmond draws attention underlines the necessity to develop further the formulation of the configurational free energy along the lines attempted by Mackor and van der Waals and now by Findenegg and Ash.‘ H. D. Cook and H. E. Ries Jr. J. Phys. Chem. 1959,63 226. H . E. Ries Jr. and J. Gabor Chem. and Ind. 1967,1561. A. Cameron and R. F. Crouch Nature 1963 198,475. Tenside 1969 6 61. J. L. Taylor and D. A. Haydon Disc. Faraday SOC. 1966,42 51. this Discussion.GENERAL DISCUSSION 77 In reply to Ries the spread films of stearic acid and n-hexadecane at the air/water interface and the adsorption of stearic acid from n-hexadecane on to metal-coated mica surfaces should perhaps not be compared too closely with the adsorbed mono- layers of glyceryl mono-oleate at the n-hexadecane/water interface. In the spread film experiment the n-hexadecane is presumably squeezed out from between the stearic acid molecules as the compression proceeds. Towards the end of the com- pression (where from Ries’s graph the surface pressure becomes constant) it is not clear where the hexadecane is. estimated that the maximum adsorption of stearic acid at the n-heptane/water interface (at ca. 0.02 mol/l.) corresponded to ca. 60 A2/molecule. The results reported in our paper for glyceryl mono-oleate and adsorption studies on n-alkanols do not indicate any great influence of the solvent on the adsorption and it seems probable that 60A2 per molecule would hold also for n-hexadecane.If this is so the space available for n-hexadecane in the monolayer would be substantially greater than in the glyceryl mono-oleate systems where thc area per molecule is ca. 30A2. A similar situation appears to exist in the metal- coated mica systems examined by Ries. The data indicate a maximum area per molecule for stearic acid adsorbed from n-hexadecane of ca. 65 A2. Again there is apparently considerable space available for solvent. The inclusion of n-hexadecane into such stearic acid monolayers does not therefore seem in any way remarkable and it would be expected that shorter chain solvents would be included to a similar or even greater extent.This does not exclude the possibility that in hexadecane the monolayers may have exceptional mechanical properties. In reply to Sonntag under the conditions described in our paper the thickness of the hydrocarbon region of the film changed reversibly with applied potential. As stated however the equilibrium condition was for some systems established only after considerable time. For higher applied potentials the films tended to rupture. Jn all membranes the application of a potential caused an increase in the diameter of the film. This effect was allowed for in the calculation of the specific capacitance of the film and it is not thought that it could account for the change of compressibility with applied potential.In reply to Overbeek no attempt was made to examine in detail the aggregation of the glyceryl mono-oleate at high concentrations in the hydrocarbons. Neverthe- less for the oleate in n-heptane at 25°C (equilibrated with saturated NaCl) the vapour pressure curve was measured to concentrations (ca. 1.2 x M) at which it became roughly linear. In this region the aggregation number was ca. 26. The interfacial tension curves certainly suggest that at lower concentrations at least the aggregates formed in systems equilibrated with saturated NaCl are smaller than in those equilibrated with 0.1 M NaCl. While no conclusive evidence is available miscellaneous observations during the course of the experiments indicated that the presence of water in the hydrocarbon phase favoured the formation of larger aggregates.Dawson Dr. G. H. Findenegg (University of Vienna Austria) said In relation to the stability of thin liquid films I would like to mention some theoretical work by Ash and myself.2 We have considered a solution of chain molecules between two plane interfaces. The chain molecules have an “ active ” end-group which is preferentially adsorbed to the interfaces. The calculations are based on a multi-layer lattice model for Dawson Ph.D. Thesis (Cambridge 1963). S. G. Ash and G. H. Findenegg Trans. Faraduy SOC. to be published. 78 GENERAL DISCUSSION adsorption from monomer + r-mer solutions. Different chain-conformations are allowed for by considering a number of configurational species of r-mer according to the sequence of their segments in the lattice 1ayers.l We have calculated for r-mers up to r = 4 the concentrations of the adsorbed configurational species the Gibbs adsorption of r-mer and the configurational Helmholtz free energy F, as a function of the following parameters the thickness of the liquid film ; the concentration of r-mers (in a bulk solution at equilibrium with the liquid film); the strength of preferential adsorption of the active end-groups ; and the segment interchange energy of active and non-active segments (monomers are energetically equivalent to non-active segments of r-rners).Many features of this model are in agreement qualitatively with the properties of non-compressed lipid films in aqueous media as reported by Haydon et al. We find that only a fraction of the r-mers are in their fully extended form normal to the interface and hence the concentration of r-mer chain segnients decreases towards the centre of the film.The surface excess of the configurational free energy F has a (shallow) minimum at a thickness of the film equal to twice the extended form of r-mer (8 layers for r = 4). F," then increases with decreasing thickness hence there exists a repulsive force. At a given film thickness F," increases with increasing chain length and concentration of r-mer as well as by an increase in the preferential adsorption of active segments and the interchange energy. For films consisting of monomers + homogeneous r-mers (all its segments preferentially adsorbed to the interfaces) we find a decrease in F," with decreasing film thickness hence such films should not be stable.Dr. B. Vincent (I.C.I. Ltd. Slough) said I would suggest some data and calcula- tions that could help resolve the apparent discrepancy between the results presented by Sonntag at this meeting on oil droplets in water and those of Ottewill and Walker on aqueous polystyrene latex dispersions both systems containing ethylene oxide (E.O.) stabilizers but showing different trends in stability as the stabilizer concentra- tion is increased. I have recently determined some Haniaker constant values by the method of Gregory using refractive index data. Values relevant to this work are material A x l O z o J octane 5.3 polystyrene 7.8 poly(ethy1ene oxide) 6.9 water 3.8 Appropriate models were set up (flat-plates for octane ; spheres for the polystyrene latices) in which the thickness of and ethylene oxide concentration in the adsorbed layer together with the surface separation of the particles were allowed to vary.The relevant equations for the attraction energy V l (flat plates) and V i (spheres) were derived from eqn (2) of Sonntag's paper and the equation of V ~ l d ~ respectively. Retardation effects were neglected for the purposes of these calculations. Fig. 1 indicates how the Hamaker constant A 3 of the adsorbed layer varies as a function of the wt fraction w of E.O. in the layer. Fig. 2 and 3 show how V i varies as a function S. G. Ash D. H. Everett and G. H. Findenegg (a) Trans. Faraday SOC. 1968,64,2645 ; (b) 1970 66 708. R. H. Ottewill and T. Walker Kolloid-Z. Polymere 1968 227 108. J . Gregory Adv. Colloid Itirerface Sci. 1969,2 396. M.J. Vold J. Colloid Sci. 1961 16 1 . GENERAL DISCUSSION 79 W FIG. 1.-Hamaker constant of adsorbed layer A3 as a function of wt fraction 10 of ethylene oxide in the adsorbed layer. 8.2 0.4 0.6 - 06 Q L- W FIG. 2.-Attraction energy between two aqueous polystyrene latex particles (radius 25 nm) covered with adsorbed layer thickness a3 as a function of ethylene oxide wt fraction w ; surface separation h = 0.2nni. 80 GENERAL DISCUSSION of w for various thicknesses d3 of the adsorbed layer. In fig. 2 the surface separation h between the particles (core and adsorbed layer) is 0.2 nm (effectively the two particles are in contact i.e. held apart by Born repulsion forces) In fig. 3 h = 10nm. The general form of fig. 2 and 3 is the same only the magnitude of V i being signifi- cantly different.The two points labelled A and B in fig. 2 indicate the values of V i when there is no surface-active agent present and when there is a monolayer of adsorbed surface-active agent present respectively. (It can be shown from the adsorption isotherm that w is about 0.7 at monolayer coverage of n-dodecyl hexa- ethylene oxide on polystyrene latex. Also the thickness of the E.O. layer is about 2.5 nm). Between these two points one would expect the actual locus of points for V; curve 111 to be between the two extreme curves I and 11 in fig. 2. Curve I I 0 2 0.4 0.6 0.8 W FIG. 3.-As for fig. 2 but h = 10 nm. corresponds to the case where the thickness of the E.O. layer gradually increases but that its E.O. content (after the initial addition) remains virtually constant. This corresponds to a gradual change from a “ horizontal ” to a “ vertical ” configuration of the E.O.molecules at the surface. Curve I1 represents the case where there is a gradual increase in E.O. concentration in the adsorbed layer the thickness remaining more or less constant (i.e. vertical configuration throughout). The real curve 111 would indicate that there is probably a gradual decrease in V i as stabilizer is added to the system as was suggested by Ottewill and Wa1ker.l Fig. 4 and 5 show the corresponding V i curves for Sonntag’s system again for Iz = 0.2 and 10 nm respectively. The maximum thickness of the E.O. layer in this case would be expected to be in the range 5-10 nm and again assuming the maximum value for w is about 0.7 the corresponding two points A and B may be plotted (fig.5). This time the case h = 10 nm is arbitrarily considered since in this system the electrical double layer repulsion will contribute to the equilibrium separation. However again the general form of the curves in fig. 4 and 5 is similar although for h = 0.2 nm the curves for d3 are virtually coincidental. (This results from the r6 dependence GENERAL DISCUSSION 81 W FIG. 4.-Attraction energy between two octane droplets in water having an adsorbed layer thickness s3 as a function of ethylene oxide wt fraction w ; surface separation h = 0.2 nm. 4 0 0.6 0.8 0.4 0 . 2 W FIG. 5.-As for fig. 4 but h = 10 nm. 82 GENERAL DISCUSSION of the inter-molecular attraction forces-neglecting retardation.) In this case unlike the polystyrene latex V (B)> V (A). The two extreme loci discussed above are represented by curves I and I1 again.Here the most probable locus (curve 111) indicates that after the initial addition of stabilizer V i increases on further addition of stabilizer as Sonntag has suggested. This would appear to resolve the apparent discrepancy. The main reason underlying these two apparently different trends in the attractive force lies in the relative Hamaker constant values for the sheaths and cores in the two cases. In the Ottewill and Walker case the core Hamaker constant is greater than that of the sheath and vice-versa in the case of Sonntag. The exact way in which V varies as stabilizer is added will depend on exactly how its configuration and concentration varies in the adsorbed layer. Dr. H. Sonntag (Berlin-Adlershof) said In reply to Vincent his calculations are correct but nevertheless incorrect.He chose as the distance between the particles the spacing from one adsorbed layer to the other. If this is done he will indeed find a reduction of dispersion forces if the Hamaker constant of the adsorbed material is less than that of the particles or vice versa. But we measured the distance from particle to particle because the polar groups are strongly solvated and the refractive index changes most rapidly at the particle surface. In this case one always obtains increasing dispersion foices independent of A3 < A or A3 > A l . Only if the Hamaker constant of the adsorbed material is less than that of the water phase can one calculate decreasing dispersion forces. Unfortunately the discrepancy between 0 ttewill's and our paper cannot be explained so easily.Dr. B. Vincent (University of Bristol) said I agree partly with Sonntag but I am also wondering if he has missed the main point of my argument. We are both looking I think for an explanation in terms of increased inter-particle attraction of why the spacings in his experiments decrease on increasing the stabilizer concentration. In terms of the Hamaker constant values I quoted it is largely irrelevant in the case of his experiments as to whether one considers constant surface-surface separation or constant adsorbed layer-adsorbed layer separation. The latter case corresponds to the situation I had previously described. The former case would correspnd to one of the fixed Even for increasing adsorbed layer thickness at constant surface-surface separation this would lead to increasing attraction.In both cases the attraction increases as stabilizer is added (in the latter case continuously in the former case probably only after some stabilizer has initially been added). In Ottewill's experiments however it is very relevant as to whether one takes constant surface-surface separation or constant adsorbed layer-adsorbed layer separation (see e.g. fig. 2). In the former case again one would again have to take a fixed 6 (again probably 25A) and this would lead to increasing attraction on adding stabilizer. My reasons for taking the constant adsorbed layer-adsorbed layer (or perhaps a better definition is constant " outer " surface-" outer " surface) separation is that Ottewill was considering particle flocculation (under conditions where the electrical double layer repulsion is eliminated) and in this case it is necessary to take the origin of the attractive forces as being at the outer surface of the particles.In this case as I showed previously (and as Mrs. Vold has shown) the most probable result is a decrease in attraction on adding stabilizer. curves in fig. 5 (say 6 = 25 A). That is what is required to explain his results. Prof. A. Scheludko (Sofia) said The paper of Sonntag Netzel and Unterberger First the use of a porous is a further step in the investigation of emulsion films. GENERAL DISCUSSION 83 plate as a modification of Mysels’ porous ring method is of considerable interest. In this particular case the role of the porous walls is to fix the film to the holder something which until now has been difficult to achieve in a smooth-walled glass cell and which represented a major obstacle.This method makes it possible to vary over a wide range the capillary pressure which although not made use of in their work in fact was its original purpose.2 Also this is the first application in real emulsion films of the microscopic method of measuring the contact angle film/bulk l i q ~ i d . ~ As with foam films the contact angle reaches significant values after transition to the Perrin films obtained in this case at sufficiently high electrolyte concentration (5 x lo-’ M MgS04). The reported values of the contact angle at low concentration (a few hundredths of a degree) are greater than the sensitivity of the method. The observed influence of the surfactant concentration on the equilibrium thickness is somewhat unexpected.According to these observations the equilibrium thickness decreases with increase of the surfactant concentration. This corresponds to the negative component of the disjoining pressure prevailing over the positive component. In fact the effect is even greater than can be seen in fig. 2 because the surface tension and therefore the outer capillary pressure decrease when the surfactant concentration is increased. We however cannot agree with their generalization of the effect to any dispersion system since in other cases as e.g. with some foam films the opposite effect of the surfactant has been observed. Thus in ref. (3) the deviations from the DVO theory were in the direction of the positive disjoining pressure prevailing over the negative component for sufficiently thin films.The influence of the surfactant on foam films was directly demonstrated in the investigation of the critical thickness of r ~ p t u r e . ~ In the latter a decrease of the critical thickness at high surfactant concentration was obtained which meant an increase of the positive disjoining pressure i.e. the reverse of the effect described in their paper. Unfortunately the curves of equilibrium thickness against electrolyte concentra- tion in the paper are not interpreted according to the DVO theory with the simplest assumption that IIvw is inversely proportional to the third power of the film thickness. Tt is hardly appropriate to take into account the thickness of the adsorption layers (in fact merely the thickness of the polar “ heads ”) when considering films with such a large total thickness (200-550 A).The thickness of the polar groups is of the order of magnitude of the experimental error. That is why their interpretation does not appear convincing. Dr. H. Sonntag (Deutsche Akademie der Wissenschaften Berlin (communicated) In reply to Scheludko one finds an alteration of the critical thickness of rupture with increasing surfactant concentration until only cb is reached. Above this concentra- tion the rupture thickness remains constant. We always measured the equilibrium thicknesses above cb and therefore a comparison of his results with ours is impossible. On the other side his coworker Exerowa also found a decrease of equilibrium thick- ness with increasing concentration of non-ionic surfactant.Prof. J. Lyklema (Wageningen Netherlands) said In looking for an explanation for the effect of surfactant concentration on the equilibrium thickness as shown in D. Exerowa and S. Scheludko Cumpt. Rend. Acad. Bulg. Sci. in press. K. Mysels J. Phys. Chem. 1964 68 3741. A. Scheludko B. Radoev and T. Kolarov Trans. Faraduy SOC. 1968 64 2213 ; T. Kolarov A. Scheludko and D. Exerowa Trans. Faruduy Suc. 1968,64,2864. I . Ivanov B. Radoev E. Manev and A. Scheludko Trans. Faraduy Soc. 1970,66,1262. 84 GENERAL DISCUSSION fig. 2 of Sonntag et aI.’s paper one could also think of a kind of “ tele-entropic ” stabilization mechanism according to the following principle. If two emulsion drop- lets each covered with surfactant molecules and bearing at least some charge approach each other the diffuse double layers start to overlap and counterions are transferred from the Gouy-layer to the Stern-layer.When the degree of occupancy of the liquid/liquid interface with surfactant molecules is low it is possible that these counterions are pressed in between these molecules which could reduce their con- figurational entropy and hence constitute a repulsion. The more compact the surfactant layer becomes the less this effect and at sufficiently high degrees of occupancy this repulsive entropic term is completely absent. Thus the model can account for a repulsion that is only operative at low surfactant concentrations. Dr. H. Sonntag (Deutsclie Akademie der Wissenschafien Berlin) said With regard to the paper by Padday the rupture of liquid films on solid surfaces is a very complicated example of hetero-coalescence.I think his measuring device is not sufficiently sensitive to decide what happens at the rupture process because the thickness of the film is inhomogeneous before breaking. We investigated the rupture process of aqueous films on cyclohexane with the apparatus described in our paper. The thickness of rupture decreased with increasing concentration of surface-active agent from several 1000 A to 300 A. Above a certain concentration the thickness of rupture remained unaltered and we obtained a stable film whose thickness depended only on the electrolyte concentration. Dr. J. F. Padday (Kodak Ltd. Harrow) In reply to Sonntag in subsequent experi- ments described in the appendix of my paper I have pointed out that high-speed cine photographs revealed an indentation or dimple in the surface of my experiments.Thus the critical thickness between the liquid-air and solid-liquid interfaces inay well be of the same order as Sonntag’s rupture thickness because the thickness I have measured is that of h in the fig. 8 whereas Sonntag measures h’. Dr. R. G. Picknett (C.D.E. Porton Down Wilts) said Padday has given a modificd form of the Kelvin equation which relates capillary pressure disjoining pressure and vapour pressure Deryaguin made it clear that for thin liquid layers the curvature is not constant but varies with a m thickness in accordance with the disjoining pressure Thus in applying eqn (I) to liquid in a capillary or at the point of contact of sphere and plane it is incorrect to calculate I3 from the total liquid thickness; instead it must be evaluated for each point on the liquid surface This has been done for water between a 720 pm dim.glass sphere and plane the work being part of an adhesion investiga- tion by Cross and myself. The surfaces were assumed to be smooth and the vertical profile of the water/air interface was calculated using I’I values derived from the work of Deryaguin and Zorin.2 The adsorbed water film far from the point of contact was so thin that it had only a negligible effect. The maximum thickness obtained for the water annulus is shown in fig. l(a) as a function of PIP it is larger than the thickness calculated from the simple Kelvin equation by a factor of 1.2-1.3. Knowing the surface profile the adhesion due to the surface tension of the water B. V. Deryaguin Proc.2nd Int. Congr. Surface Activity (London 1957) 2 153. B. V. Deryaguin and Z. M. Zorin Proc. 2nd Int. Congr. Surface Activity (London) 1957,2,145. GENERAL DISCUSSION 85 can be obtained as a function of PIP,. These adhesions are shown in fig. l(b) and l(c) as solid lines the diagrams referring to two different separations h of sphere and plate corresponding to different degrees of surface roughness. Experimental adhesions were measured using plates with artificially-induced roughness and are represented in fig. l(b) and l(c) by crosses. The experimental values lie well below the solid curves derived from eqn (I) and in fact lie more closely on the broken curves which were derived from the simple Kelvin equation. The discrepancy is thought to be significant and indicates that eqn (1) does not apply to the sphere+plane system.The explanation may well lie in the assumption used in eqn (1) that the (4 (b) (c) 40p n 0 d h E 30- s 20- --- .d 4 10- h=SO i f 0.7 - 0.8 0.9 1 0.7 0.8 0.9 0,- PIPS PIPS PIPS FIG. 1. capillary pressure and the disjoining pressure are additive. If instead of this assump- tion allowance is made for change in surface tension as well as for disjoining pressure then better agreement with experiment may well be found. Dr. J. F. Padday (Kodak Ltd. Harrow) said I am grateful to Picknett for pointing out to me that Deryaguin in 1957 proposed a modification to the Kelvin equation that took into account the effects of disjoining forces on the condensation of liquid within a pore. This equation is general in principle but is difficult to apply particularly to the system of liquid condensing between the ball and plate of Picknett's experiments.In applying either form of the Kelvin equation modified for thin film effect Picknett has made two assumptions. The first is that the surface tension is a constant value at both the thin and thick parts of the meniscus and that therefore the angle of contact between thin and thicker film is zero. This assumption is reasonably well justified because the force of adhesion is predominantly a surface pressure term and in the results quoted in the comments depends on the " bulk " surface tension and the radii of curvature at the neck of the liquid bridge. The second assumption concerns the effect of geometry of the axisymmetric liquid bridge on the disjoining pressure at any element of liquid between the solid/liquid and liquid/air interfaces.I understand that Picknett calculated the data of fig. 1 by first deriving the shape of the liquid bridge using a graphic integration method with eqn (6) of my paper. The sum of the II terms were derived from the values given by Deryaguin and Zorin and the integration process produced a shape from which shape parameters such as the two principle radii of curvature at the narrowest point at the neck of the liquid-air interface were obtained. The sum of these curvatures multiplied by the " bulk " surface tension was then it appears used to calculate both the adhesion force and the vapour pressure in equilibrium with it. In applying these calculations one must assume that the disjoining pressure measurements of Deryaguin and Zorin for disjoining of a thin layer of liquid between two flat parallel surfaces are equally suitable when the 86 GENERAL DISCUSSION two surfaces deviate appreciably from parallelism.Both van der Waals' and electric- double layer forces appear to be sufficiently sensitive to the angle between the two surfaces bounding the thin film to suggest that at the neck where the angle is nearly go" the value of the disjoining pressure would be greatly reduced and thereby explain Pickett's results. Dr. L. M. Dormant (Bristol University) said Process C of Padday's paper can be examined in a more quantitative manner. The equilibrium shape of a very large drop in a gravity field leading to the Young and Duprb equation was solved by Adamson and Ling.2 The final state of process C is just the inverse of this configura- tion so that any height less than a (1 -cos O)+ where a is the capillary constant and a' = 2y/pg will be unstable.A few values-those for which the contact angle 0 was readily obtained 3-are compared with Padday's results in table 1. All of the experimental results are in the region of predicted instability. Exact agreement cannot be expected unless the radius of the patch is very large. It is not sufficient to show that the system is unstable; one needs to discuss mechanisms for rupture. Even though the results (that the critical rupture thickness is different for Teflon and paraffin wax) seem to show that this thickness is dependent on the solid used I cannot (yet) accept the implied concept that a S/L interface can exert an influence on an L/A interface at a distance of 1 to 5 x lo6 A.lo2 A yes ; 103A maybe ; 106A NO! It would be simpler to imagine some extra-thermodynamic disturbance bringing the L/A interface to within a short (N10A) distance of the S/A interface. At this distance one could acceptably assume long-range interactions thereby forcing the L/A interface to remain there and thus forming the dimple that Padday described. TABLE 1 .-THERMODYNAMIC AND EXPERIMENTAL RUPTURE THICKNESSES benzene/Teflon 14.7 46 1.4 0.16 decane/Teflon 6.73 24 0.7 0.26 water/paraffin 6.34 106 4.3 0.31 water/Teflon 6.34 108 4.4 0.21 (a) a' = 2y/pg ; (b) from ref. (3) ; (c) from h" = a(1 -cos 813 ; ( d ) from table 1 and 2 of Padday. The mechanism which might help to explain this postulated disturbance is the formation of capillary gravity waves and the subsequent amplification of certain wavelengths due to the geometry of the system.One could make a somewhat dubious analogy to the placing of the more dense of two immiscible liquids on top of the less dense liquid. The primary mode of rupture for such bulk phases is that some of the wavelengths are selectively amplified until rupture OCCU~S.~ This system is only stable to wavelengths less than &a. The analogy is useful in so far as it poses three other questions (i) what would happen if instead of a large piece of Teflon a sample a few m2 or less was used and (ii) what would happen if his trough was only a few cm wide? These experiments could show whether Padday W. A. Zisman Ado. Chern. no. 43 (Amer. Chem. SOC. Washington D.C. 1964) p. 11. A. W. Adamson and I. Ling Adu.Chern. no. 43 (Amer. Chem. SOC. Washington. D.C.. 1964) p.72.. A. W. Adamson Physical Chemistry of Surfaces (Intersci. N . Y . 1967) p. 364. 389. R. Bellman and R. H. Bennington Quat. Appl. Math. 1954 12. 151. GENERAL DISCUSSION 87 is indeed measuring a fundamental property or only some property connected with the geometry of the system. The amplification step may take a considerable time (the time factor was not mentioned) and leads to the remaining. questions what would happen if the system was kept slightly above the critical rupture thickness but held there a factor of 10 times longer than the remainder of the experiments? Also what would happen if vibrations were deliberately induced? The question of impurity has already been brought up. Judging from fig. 6 it seems that a small concentration of impurity would have a negligible effect.Impurities should be negligible if the mechanism is a long-range effect. Yet ,if a wave pheno- menon was involved a surface tension gradient of 0.01 dynlcm (N/km) could have pronounced effects perhaps explaining the difference between Teflon and paraffin. Similar conceptual difficulties occur with the salt and surfactant solutions. The double layer thickness of a M solution is approximately lo2 A so that it would be difficult to postulate effects at distances of lo4 times the double-layer thickness regardless of the charge on the Teflon surface. On the other hand the damping of waves by the addition of a surface-active agent leading to greater stability is a common phenomenon.2 I do not believe that a negative damping coefficient has ever been previously observed.Dr. J. F. Padday (Kodak Ltd. Harrow) said I disagree with the first comment of Dormant by which it is implied that any thin film at a solid surface with a thickness less than (2y (1 - cos O)/pg)% is unstable. For water on wax this thickness is about 4.5 mm and all experiments show clearly that layers between 1 and 4.5 mm in thickness are stable over many days. Column 3 of the table in his remarks is thus not relevant as there is no instability. Layers in this region of thickness may have a degree of metastability but this will depend much more on the forces that give rise to dimple formation than to the contact angle properties of a wetting meniscus. As pointed out in the appendix the dimple once formed is resistant to low energy vibrations or ripples at the liquid-air interface.The process by which the dimple first forms could possibly be the trapping of a wave form at the point where the liquid is thinnest but the high speed cine films do not show this. The geometry of the vessel and to some extent the solid surface has been altered and this does not appear to vary the process described. As the phenomenon is also observed on a vibration-free mounting surface effects arising from very small surface tension gradients induced by waves may be discounted. The apparent electrical effects do not necessarily operate over large distances because the dimple reduces the interaction distance. Negative damping coefficients do not have to be invoked although they are well knowq4 because it is believed that the dimple is in an asymmetric force field.Dr. G. Frens (Philips Res. Lab. Eindhoven) said Padday’s work may explain why layers of (aggregated) particles form at the surface of perfectly stable gold and silver sols (and of many other colloidal dispersions). Let us suppose that Brownian motion brings a particle with a hydrophobic surface near the surface of the disper- sion. Padday shows there is a chance for the water layer between the particle and the surface to break. Then the particle will rise and despite its larger density it will be kept afloat by surface tension forces. Now let us consider the coagulation M. van den Tempel private communication. E. H. Lucassen-Reynders and J. Lucassen Adv. Colloid Znt. Sci.. 1970,2,347. J. F. Padday Proc. 2nd Znt. Congr. Surface Activity 1957,3,136 187.J. T. Davies and E. K. Rideal Znterfuciul Phenomena (Academic Press London 1961) p. 360. 88 GENERAL DISCUSSION of two such hydrophobic particles. Instead of meeting the surface they approach each other in a Brownian collision. The water layer between the two particles will break if only because the two particles come in close contact. If the water layer is broken at some point the capillary forces will push the water back from the narrow pores of the aggregate until the radius of curvature of the newly created interface becomes large enough to be in equilibrium with the hydrostatic pressure outside the aggregate. There is a close analogy between this situation inside a floc in a hydro- phobic sol and that in mercury porosimetry @ and the work of Kruyt and van Selms on dispersions in mixed solvents (one wetting the other non-wetting) comes to mind as well as that of Vanderhoff et al.on the coalescence of latex particle^.^ In general it can be concluded that an aggregate of hydrophobic particles should be a three-phase instead of a two-phase system. This will have consequences for the properties of such aggregates e.g. for their repeptization since the situation inside these aggregates will be greatly influenced by the addition of surface-active agents to the dispersion medi~m,~ and to a far lesser extent by agents which might influence the electrical properties of the interface between the original particles and the solution. Dr. J. F. Padday (Kodak Ltd. Harrow) said In reply to Frens in principle the process of rupture described in part 2 of my paper could explain the coagulation of two or more hydrophobic particles which approach each other at some critical rupturing distance when embedded in a non-wetting liquid.A similar explanation was used to account for the driving force that gives rise to the aggregation of two cyanine dye ions to form dimers against electrostatic repulsion forces. However by invoking such an explanation the very small particles may well give rise to rupture and cohesion forces that are very different from those of my experiments because the reciprocal of the radius of curvatures in the two cases differ by more than an order of magnitude. The reverse explanation i.e. that hydrophobic particles near the surface of the liquid gives rise to the dimple does not appear to be possible because the strong disjoining action of a " pillar " of small hydrophobic particles would lead to immediate rupture and not the intermediate formation of the dimple. R. P. Iczkowski Ind. Eng. Chem. Fund. 1966 516. R. P. Mayer and R. A. Stowe J . Phys. Chem. 1966,70,3867. H. R. Kruyt and F. G. van Selms Rec. Trav. Chim. 1943 62,407. J. W. Vanderhoff H. L. Tarkowski M. C. Jenkins and E. B. Bradford J . Mucvomol Clzem. 1966,1 361. E. J. Clayfield and E. C. Lumb Disc. Furaduy SOC. 1966,42,285. J. F. Padday J . Phys. Chem. 1968,72 1259.

ISSN:0370-9302    

DOI:10.1039/SD9700100075

出版商:RSC    

年代:1970

数据来源: RSC