SURFACE CHEMICAL TECHNIQUES IN COSMETIC PREPARATIONS 377 6o 4.0 • • • • '• '• '• '4- Figure 1 The surface tension concentration relationship for n-butyl alcohol at 12øC. • •_ BuT¾_•L • L_•0HOL Figure oe Force-area curve for n-butyl alcohol at 12øC calculated from _Figure 1, using the Gibbs Adsorption Isotherm.

378 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS B. Dynamic Surface Tension Measurements Where the ageing of a surface takes place over a long period of time, the hanging plate, Sessile Drop or Pendant Drop methods are probably the most suitable for studying the formation of adsorbed films. One of the difficulties involved in formulating an aqueous spray form- ulation using a squeeze pack, say a wave-set, is the fact that the surface tension of water is very high and therefore a great deal of energy is required to be concentrated to break up the material into a satisfactorily fine spray. Additions of surface active agents may be made but they must be of the type, and be present in sufficiently large amounts, to bring the surface tension down to a low value (25--30 dynes/cm.) in a few milli-seconds. That is in the time required to form the droplet. Where high concentration of alcohol or mineral oil systems are 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 to be used. 1. The Oscillating Jet One of the earliest methods developed for determining dynamic surface tensions is due to Rayleigh. He showed that when a jet of liquid issues from an elliptical orifice, oscillations are set up in the liquid, the wavelength of which is related to the surface tension at that point. Assuming that the jet moves en bloc, the age of the surface is related simply to the distance along the jet surface. The relationship between wavelength and surface tension is a complex one and has not been fully resolved. Addison 6 attempted to resolve these difficulties but without complete success. Sutherland carried the mathematical analysis a stage further but considered that the whole system was complicated by indeterminate diffusion conditions under the surface, which rendered absolute interpretation of the restfits difficult. 2. Surface Potential Measurements When two dissimilar surfaces are in contact with one another, an electrical potential difference known as contact potential is set up. If now a third material is introduced into the interface either by adsorption or by some other means, the potential changes by an amount known as the Surface Potential. The surface potential is related to the concen- tration of molecules at the surface, their orientation, dipole moment and the dielectric constant of the interface. If all the factors are constant then the surface potential is proportional to the surface conc4ntration.