SURFACE CHEMICAL TECHNIQUES IN COSMETIC PREPARATIONS 387 Mechanical Properties of Insoluble and Adsorbed Films Surface Viscosities The mechanical properties of the surface film often parallel the stability of a foam or emulsion. If the film is too rigid, then the system will be unable to resist mechanical or thermal shock. If it is too fluid, rapid drainage of a foam or coalescence of the emulsion will take place. Davies has recently introduced two new surface viscometers which rely on the measurement of the speed of rotation of talc particles placed in a narrow circular channel placed at the interface, when the vessel containing the interface is rotated. All the different rheological types known in three-dimensional systems can be shown in two dimensions. The Oil/Water Interface Many of the methods described for the air/water interface may be adapted to the oil/water interface. Zeta Potential When one phase is dispersed in another, electrical charge separation occurs either by adsorption of ions from one of the phases, ionic dissociation, or by an electrical dipole at the interface. This charge plays an important role in determining emulsion stability, especially of the oil-in-water type. The presence of such charges may promote or hinder penetration of active ingredients through the skin. The potential may be determined by measur- ing the speed with which the dispersed phase migrates under an electrical potential. Particles are observed with an ultra-microscope. The technique has been successfully used to gain insight into the mechanism of the salt precipitation of various colloids and dispersions. It has been shown that coagulation is due to the reduction in the thickness of the double layer and potential. You may well ask what use have these types of measurements for the cosmetic chemist ? Many emulsifying systems depend on structure of the primary emulsifying layer. It might be possible to learn something about the physio-chemica! properties and structures of the diverse emulsify- ing agents by spreading them at the oil/water and air/water interface as has been done by Schulman and Cockbain. Similar remarks apply to the study of adsorbed films. The above descriptions are by no means exhaustive. They have been collected together to direct thinking along such lines as might be found useful in determining some of the basic surface chemical and physical problems of cosmetic formulation and application. DISCUSSION DR. W. W. MYm)L•TON: 1. Has anything been revealed of the emulsifying

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