374 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS If a third substance is dissolved in one of the phases, it will tend to concentrate at the interface to a greater or lesser degree depending on its so-called "Surface Activity." This concentration at the interface is termed "adsorption" and the extent of adsorption will depend on relative affinity of the dissolved substance for the bulk phases. Adsorption results in a lowering of the free energy of the surface and hence it becomes easier to disperse one phase in the other. Thus, surface active agents are used as emulsifiers, detergents, foaming agents, etc., because they are able to modify the properties of multiphase systems. Many of the methods of surface chemistry and physics have been developed for studying the pro- perties of adsorbed films, usually using model systems. Thus the Air/Water surface in a trough is a model for the interface existing in a foam, while the Oil/Water interface formed by placing oil on water is a model for the interface existing between the oil and water in an emulsion. This paper is a review of the experimental methods that can be used to study Gas/Liquid and Liquid/Liquid interfaces, with an illustration of the type of result that can be obtained. It is convenient to classify the experimental methods in terms of the type of interface being examined. GAs/LIQuID INTERFACE This interface and the Liquid/Liquid interface are probably the simplest to work with, since they are easy to form reproducibly. The experimental techniques are also fairly straightforward. ADSORBED FIL•4S It should be remembered when formulating a shampoo, that its ability to foam will depend on :. (1) the surface tension being lowered sufficiently, (2) a sufficiently rapid diffusion of the surface agent into the surface, especially during the rapid expansion of the surface as the foam is generated, (3) the correct mechanical properties of the adsorbed film being achieved. If the film is too fluid or too rigid, it will be unstable and an unsatis- factory foam will be obtained. The Determination of Su•3face Concentrations (i) Radio Isotopes This technique was introduced by Dixon et al • and Aniannsson and Lamm 2.

SURFACE CHEMICAL TECHNIQUES IN COSMETIC PREPARATIONS 375 The kinetics and thermodynamics of adsorption have recently been studied using radio isotopes. Nilsson g studied the adsorption of sodium dodecyl sulphate using the tritiated compound, while Flengas and Rideal 4 studied the adsorption of Cx4--tagged sodium stearate. They showed that the Gibbs Adsorption Isotherm predicted much lower surface concentrations than was found using the radio-tracer techniques. Their results suggested a molecular surface association of the solute. (ii) Surface Tension Measurements It is difficult to measure directly the surface concentration of adsorbed substances at the Gas/Liquid interface. This is usually determined in- directly by applying the Gibbs Adsorption Isotherm to the experimentally determined relationship between concentration of solute and surface tension of the solution: The Gibbs Adsorption Isotherm is of the form :-- --c d7 RT dc where F = the surface concentration. c: the bulk concentration of the solute. 7 = the surface tension of the solution. The equation is true for most practical purposes but strictly, concentra- tion should be replaced by activity. For solution of pure detergents the above equation is of the form :-- --c d7 2RT dc •Vhen there is excess neutral salt present, the equation reverts to its original form without the factor of «. The general validity of the Gibbs Adsorption Isotherm was demon- strated by McBain, who removed the surface of solution by a fast-moving microtome knife. A. Equilibrium Surface Tension Measurements The methods described here are most suitable for determining the surface tensions of solutions where the surface film has been allowed to reach equilibrium. Some of the methods can be adopted for systems, where equilibrium is attained only slowly, e.g. dilute solutions of impure detergents reach their final surface tension slowly (1--24 h.). (a) Capillary Rise Method This can be a very accurate method. It depends on the measure- ment of the rise of the liquid in a capillary tube of known radius and