388 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS properties of the natural and semi-synthetic gums apart from their effect in increasing viscosity of the aqueous phase ? 2. Dr. Posner regards the zeta potential as playing an important role in determining emulsion stability, especially of the water-in-oil type. To what type of emulsifier does this apply ? Is it only to a limited number like sodium dioctyl succinate ? 3. Do the methods reveal how alcohol in the aqueous phase affects emulsifi- cation and emulsion stability ? 4. Has anything been revealed of the behaviour at interfaces of "block polymers" in which a central hydrophobic zone of propylene glycol condensate is flanked by two hydrophilic zones built up by ethylene oxide condensation ? 5. What is the effect of adding isopropyl myristate to a refined white oil before emulsification ? THE LECTURER: 1. I know of no work specifically directed at determining the emulsifying properties of the natural and synthetic gums using the techniques described in my paper. I have no doubt at all that such techniques can, and probably have, been used to reveal the surface chemistry of these materials. Interactions between emulsifying agents and gums might be studied by spreading a monolayer of the emulsifying agent at an interface and injecting the gum underneath. Alternatively, the emulsifying agent could be spread on a solution of the gum. Such changes in surface pressure and surface viscosity that may occur can quite easily be measured. 2. In so far as the zeta potential plays a part in stabilising an emulsion, there is an uncorrected error in my paper it mentions its effect in stabilising water-in-oil rather than oil-in-water. Nevertheless, I feel that a zeta potential will also play a part in the water-in-oil emulsion but to a lesser extent. A double-layer will undoubtedly exist inside the water droplet and there will be an image of the double-layer in the oil phase of a water- in-oil emulsion. I think, in the case of the oil-in-water emulsion, zeta potentials will play a part in stabilising almost all emulsions, in particular, of course, where there is ionisation of the primary layer. In the case where the emulsions are non-ionic, dipoles are likely to generate small stabilising zeta potentials, but these will not be of the same order of magnitude as those coming from ionic dissociation or adsorption. 3. I am certain that the methods described will reveal how alcohol in the aqueous phase affects emulsification and emulsion stability. Alcohol will

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