he second Symposium on the Physics and Chemistry T of Interfaces, l i e the first one held in includes reviews of liquid-gas, liquid-liquid, solid-gas, solid1964,
liquid, and solid-solid interfaces. The sympmium starts with a paper on the theory and practice of capillarity by Dr. A. M. Schwartz. The basic principles of capillarity have long been known, but their application to modem problems is far from being a simple or swaightforward exercise, because of a multiplicity of variables as well as nonequilibrium situations. The next paper, also on the liquid-air interface, deals with a topic whose scientifichistory is almost as hoary as that of capillarity-namely, adsorbed insoluble filmson a water surface. A revival of interest in this subject is welcome, as the 20 years of activity that followed Langmuir’s development of the &-balance technique failed to interest biochemists in the power and promise of the method. With modern instrumentation, however, both precision and rapidity have been improved to the point where we are now witnessing a second period of intense activity in this subject. Professor Cadenhead, as the fruits of his patient labors, has developed an instrument of the first rank at the University of Buffalo. From insoluble filmsto soluble films of a surface-active agent requires but a slight hydrophilic addition to the hydrophile-lipophile balance of the molecule. The concept of that balance is the key factor in determining which agent to select for a given purpose, but it has found only limited practical application, due in part to the crudely empirical way in which it has heretofore been treated. Dr. (Satin’s paper in this symposium is the latest effort to relate the concept of a physical property that can be readily measured and quantitatively expressed. The next contribution to the program also deals with soluble films and with a complex property of mixed films that is most manifest in foam lamellae-namely, the plasticity that is sometimes found at the liquid-air interface. The presence of a plastic or gelatinous layer at this interface greatly enhances both the stability and the bulk plasticity of the foam produced from such a solution. Where stable and plastic foams are desired, as in fire fighting or in the production of polyurethane foam, the effect is welcomed; where foams are a nuisance, it 12
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
is not. But the chemistry of the combination of solutes that can produce or destroy surface-plastic films remains a mystery. Dr. Kanner has reviewed the present state of knowledge of this fascinating subject and its practical application to foam stability. The same phenomenon can be investigated by the study of a single lamella. Dr. Leo Shedlovsky has been in the forefront of research on this aspect of the subject, and his contribution to the symposium is therefore authoritative and provides a timely epilog to Dr. Kanner’s review. The study of the liquid-liquid interface is initiated by Dr. R. J. Good with a system of one component distributed between an oil and a water phase. This, and the following paper by Dr. Cratin, again form a related pair. Where Dr. Cratin discusses the system from the viewpoint of the diswibution coefficient, Dr. G o d attends to the interfacial tension between the immiscible phases. Those two contributions are independent, but it may well prove possible to combine them, thus attaining the quantitative prediction of interfacial activity from the molecular structure of a solute. But even such an advance in theory looks quite inadequate when one faces the complexity of a technological application, as is soon made evident by Dr. c. P. Roe on emulsion polymerization. The monomer can be present in the form of gas bubbles or liquid droplets (under pressure) dissolved in micelles or dissolved in water. Where is polymerization initiated? Dr. Roe suggests that surface chemists have given too prominent a role to the surface-active agents in the system, and ought to look elsewhere for the main locus of particle initiation. I t is well to be thus reminded of the limitations of the theory of surface forces and the unbridged gap between science and practical know-how in the field of interfaces. A lot of useful products are made long before we know what we are really doing, however untidy such a way of going about it may seem. We cannot wait for science to explain, but neither can we afford to remit our efforts to bring scientific principles to bear on our practical achievements. The remainder of the symposium is devoted to solidgas, solid-solid, and, more particularly, solid-liquid interfaces. Dr.H. C. Van Ness starts off with a discussion of the function of thermodynamics in research, and neatly
solids, degree of wetting by the medium, particle size and shape, and even the previous history of the sample. The cause of the widespread neglect of the rheological behavior of lyophobic suspensions may, therefore, be accounted for by the same excuse as that given by Dr. Johnson . . . for a wrone ” definition of a word in his dietionary : “Ignorance, Madam, ignorance!” The electrophoresis of suspended particles has recently come back into favor because of our better understanding of the phenomenon, for which a great part of the credit must go to the Dutch colloid scientist, J. Th. G. Overbe&. Dr. R.P. Lone and Dr. S.Rws review techniaues and their application to the stability of the suspension. Although impressive calculations of potential energy wells can be obtained from electrophoretic data, the question of stability is often best answered experimentally-how much electrolyte can be added to the point of flocculation. When this procedure is not practicable, calculations of electrostatic repulsions may be based on a measurement of the electrophoretic mobility. The symposium concludes by re-opening the question posed by Dr. W. Drost-Hansen in our first symposium: What water really is. Apart from the structure of water in its liquid phase, clear evidence points to a different structure of water at a solid-water or oil-water interface. The alteration of physical properties, such as viscosity or dielectric constant, when water is subjected to a strong electric field is already well known and has been considered by many authors as a possible effect during electrophoresis of particles in water. A few molecular layers of water next to a phase discontinuity can logically be equated to the condition of water in a strong electic field. Dr. Drost-Hansen discusses the most likely structure of water at interfaces, on the basis of a variety of relevant facts. His present paper is a sequel to his previous contribution to the first symposium, and also provides an excellent bonw bouche with which to conclude the second symposium. After the papers are successively published in I&EC in the coming months, it is planned to have them collected in one bound volume for republication by the American Chemical Society. We hope the book will be of value to engineers, scientists, educators, and students. ~
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illustrates his thesis with an application to the adsorption of gas mixtures by solids. The solid-solid interface is less amenable to a thermodynamic analysis, but its practical importance in the effectsof friction, wear, and lubrication is much in evidence. Dr. M. J. Furey informs us of how those subjects are investigated by scientists, and how far from simple are the various effects encountered, Nevertheless one must start from a fundamental viewpoint, and Dr. F. M. Fowkes has gone as far as anyone in the calculation of dispersion force interactions between dissimilar liquids or solids. His paper on the theoretical calculation of adhesion points out once again the now familiar contrast: Here is the complexity of what we have to live and work with and, on the other hand, here is the crudity of the concepts and models that we think with. If the gap between them disgusts the devotees of rigor, they are commended to a more highly developed science. The techniques that are available for the study of solid-liquid systems are few. The most frequently employed are light scattering, rheological measurements, and the electrokinetic phenomena. Dr. Milton Kerker gives us a masterly summary of the theory and applications of light scattering. Dr. J. P. Olivier confines his discussion to the rheology of lyophobic suspensions, but those are precisely the systems that are most neglected in comprehensive treatises on rheology. All possible variations of flow with the applied shear stress can be exhibited, depending on factors such as concentration of
AUTHOR D r . Sydtuy Ross, mganim and chairman of the @mposium on the Chemistry and Physics of Interfaces II, is professor of colloid science at Rensselam Polytechnic Institute, a position he has held sime 1951. Dr. Ross has published nearly 100 p a w s , including a long series on physical adsorption and another on inhibition of foaming. Dr. Ross also chaired the ItYECSummn Symposium, The Chemistry and Physics of Interfaces-I, in 7964. The chairman wishes to acknowledge, with thanks, the contributions of the speakers and attmdees to this symposium. Thanks are also due to Mrs. Bonnie Watkins and Mrs. Gloria Jones of the stag of INDUSTRIAL tY ENGINEERING CHEMISTRY f m their help in organizing all physical arrangementsf m the symposium.
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