SURFACE CHEMICAL TECHNIQUES IN COSMETIC PREPARATIONS 385 Addison, C. C. J. Chem. Soc. 535 (1943). Rideal, E. K., and Schulman, J.H. Proc. Roy. Soc., London A 130 259 (1931). Posner, A.M., and Alexander, A.E. Trans. Faraday Soc. 45 651 (1949). Padday, J.F. Proc. Second International Congress of Surface Activity 1 1 (1957). Alexander, A.E. Bordeaux Conference on Surface Activity 123 (1949) (Butterworth's Scientific Publications, London). Matalon, R., and Schulman, J.H. J. Colloid Sci. 4 89 (1949). Alexander, A. E., and Cureper, C. W.N. Trans. Faraday Soc. 46 235 (1950). Blakey, B.C., and Lawrence, A. S.C. Disc. Faraday Soc. 18 268 (1954). Davies, J.T. Proc. Second International Congress of Surface Activity 1 220 (1957). Davies, J. T., and Mayers, G. R.A. Trans. Faraday Soc. 56 691 (1960). Monquin, H., and Rideal, E. K. Proc. Roy. Soc. London A, 114 690 (1927). Cumper, C. W. N., and Alexander, A.E. Australian J. Sci. Research, Ser. A 5 189 (1952). Alexander, A. E., and Teorell, L. Trans. Faraday Soc. 35 727 (1939). Davies, J.T. Nature 167 193 (1951). Bradbury, F. R., and Jordan, D.D. Blochem. J. 36 23 (1942). INTRODUCTION BY THE LECTURER THE CLASSICAL division of matter into three forms or phases, namely gas or vapour, liquid and solid, is satisfactory as long as one is concerned only with a continuous phase or the gross properties of a number of phases in contact with one another. If, however, the region of transition from one phase to another is examined closely, one finds what are in effect new forms of matter with special properties existing at these boundaries. These regions of transition are termed surfaces or interfaces. The simplest interfaces are liquid/liquid and gas/liquid. This paper is primarily concerned with review of the methods of examining these latter two interfaces, although there are many features in common amongst the various types of interface. Consider a pure water phase in which the molecules are all one kind. Each molecule will exert, as a result of its internal energy content, the same average force on its nearest neighbour. If an air/water interface is now formed, as for example by blowing a bubble or forming an Aerosol, those molecules at the surface will no longer be surrounded by molecules all exerting the same force. There will, in fact, be a resultant pull on the molecules at the surface directed towards the bulk tending to cause the surface to contract and take up a position of least energy content in which the area/volume ratio is at a minimum. Thus, a small drop of liquid or gas bubble will try to assume a spherical shape. The surface force involved here is known as the "Surface Tension" In the case of solutions of one or more substances, whose molecules differ in the magnitude of the forces exerted on each other, the unbalanced forces at the surface may be diminished in another way. The molecules with the greatest affinity for one another will pass into the bulk, while those with the

386 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS smaller affinities will pass into the surface. This concentration of constitu- ents is called "adsorption". Both positive and negative adsorption are possible, although the cosmetic chemist is almost always concerned with the positive adsorption of organic substances dissolved either in oil or in water, or in both. Considering the various methods we have-- Static Methods at the air/water interface 1. The use of radio-isotopes. 2. Surface tension measurements. Dynamic Methods The formation of an adsorbed film at the usual concentrations of surface active material takes place very rapidly, often in less than a second. However, in the case of an aqueous formulation in pressurized or squeeze pack, the surface tension of the water is very high and therefore a great deal of energy is necessary to break the material into a satisfactorily fine spray. Surface active agents may be added but they must be of the type, and be present in sufficiently large amounts, to bring the surface tension down to a low value in a few milli-seconds. Where a high concentration of alcohol or mineral oil is used, the surface tension is naturally low and the problem is not so acute. In order to investigate the surface tension of surfaces during the early moments of formation, special techniques have been used. 1. Oscillating Jet 2. Surface Potention Measurements Along a Jet Surface Results are then interpolated on a previously determined calibration curve of surface potential against surface tension. All dynamic methods of studying surface tension suffer from the fact that the true age of the surface is unknown because of the complex hydro- dynamic conditions at the surface. The methods are therefore best used for making comparison between different surface active agents and the effect of concentration. The results should therefore always quote the method used. Insoluble Films Surface Pressure Measurements at the Gas/Liquid Interface The Langmuir trough is used to measure the force/area of curves for insoluble mono-layers, although recently the hanging plate has been used in place of the barrier of the Langmuir trough which obviates the difficulty of leaks past the edges of the barrier. The hanging plate is probably satis- factory for all films except the most rigid.