SURFACE CHEMICAL TECHNIQUES IN COSMETIC PREPARATIONS 389 obviously affect the solubility of the emulsifying system and this in turn will be reflected in the surface activity of the emulsifying agent. There will also be a change in the mechanical properties of both adsorbed and insoluble films. In the case of synthetic detergents, however, small amounts of long- chained alcohol form surface complexes which have considerable surface activity and viscosity. These complexes influence the behaviour of shampoos and other commercial detergents in which there is always some free fatty alcohol present. Thus, pure detergents have a poorer deterging power than the commercial ones containing free fatty alcohol. 4. A considerable amount of work has been done by Alexander and co- workers and many others on the behaviour of insoluble polymers at both the Air/Water and Oil/Water interfaces. I know of no specifically pub- lished work on the "block polymers" mentioned. Again, I know of no reason why those interested in examining surface chemical properties of these "block polymers" should not set up the type of experiments described. 5. If there is an effect due to isopropyl myristate, you will pick this up as an interfacial complex formation. MR. G. E. NEU: The surface balance using a Wilhelmy plate is accurate for gaseous films as well as for more rigid films. I have worked with Pro- fessor Alexander using this type of balance, and spreading extremely rigid monolayers of polymer succrates and polyacrylates nitriles which were, up to that time, too rigid to spread. The surface tension results obtained are similar to the surface potential results and the theoretical results expected. Professor Alexander has used alrnost exclusively for the last 6-7 years Perspex troughs and Perspex barriers which eliminate waxing the sides of the bath and the barriers. The float used is a piece of mica sanded in a horizontal direction to eliminate any creep of the film. THE LECTURER: I use a Teflon trough which has similar properties to those mentioned. MR. H. CARTER: IS there an analogy between the Vant Hoff and Langrnuir isotherms and the equation you gave for charged liquid surface phenomenon ? THE LECTURER: The equations I have given are those of "state" and do not relate to isotherms. The latter determine the extent of adsorption, from one phase to another while the former defines the relationship between area and pressure in a single (surface) phase. MR. J. B. WILKINSON: The normal touchstone of a theory is its ability to

390 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS make varifiable or refutable predictions. The techniques discussed assist in predicting short-term emulsion stability, but the practical cosmetic chemist is vitally interested in long-term stability, not minutes or hours, but weeks or months. At present he cannot find out empirically if prophetic judg- ment can be made this would be most valuable. It is appreciated, of course, that such long-term stability may be affected in complex systems by migra- tion of trace impurities to the interface, but could these techniques be of assistance ? THE LECTURER: The techniques I have described could give you the information you require provided the change taking place occurs at the interface. It is my experience that there is no substitute for time and one must therefore be prepared to set up the experiment to last for the required period. It may be that surface chemical techniques will detect changes much earlier than those detected by storage tests on the product. For instance, migration of divalent ions to a surface stabilised by a sodium soap will be manifest as a change in rigidity of fhe interface. TECHNIQUES OF FOAM MEASUREMENT G. E. NEU, B.Sc.* Delivered at the Summer Confere•ce of the Society on 25th August 1960. The physical properties of shampoo and toothpaste foams are character- ised using seven different measurements. These measurements are easy to perform and yield reproducible results. A subjective assessment of the foam is correlated with the measurements described. INTRODUCTION FOAMINC IS of major importance in shampoos and toothpastes, and the nature of the foam may be critical in determining the acceptability of these products. With toothpaste, the volume must be sufficient but controlled, and one of the chief qualities required is easy rinsing away the foam is also important to the flavour which it disperses throughout the mouth to allow maximum contact with the taste buds. The actual requirements of these two products are very different. A shampoo is normally expected to give a thick, creamy, voluminous foam, which is associated by the user with cleansing power and emolliency. Even if a shampoo has excellent cleansing properties and leaves the hair in very good condition afterwards but only provides a small amount of a thick, *Unilever Ltd., Isleworth, Middlesex.