376 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS relies upon a zero contact angie between the tube and test liquid. This is sometimes difficult to achieve with dilute solutions of surface active materials. (b) Maximum Bubble Pressure Method This method depends on a measurement of the force required to blow a bubble of gas from a tube of known radius immersed in the test solution. This bubble must be blown sufficiently slowly to allow the equilibrium film to be established. The technique can be rendered simpler, by comparing the pressures for two tubes of differing diameters. (c) Drop Volume and Drop Number Method This is the most common and simplest method for determining surface tensions. The volume or weight of drops which fall from the tip of a vertical tube is determined. It is usual to calibrate the tip with solutions of known surface tension, e.g. ethyl alcohol, although absolute values may be determined using published correction factors. An Agla micro-syringe is a most convenient device with which to carry out Drop Volume determinations. (d) Ring Detachrnent Method The surface tension may be determined by measuring the force required to detach objects of various shapes from a liquid surface. If the object is wetted by the liquid (zero contact angle) some of the liquid comes up with it as it is lifted. At a certain height the liquid breaks away. The force at the break point is a function of the surface tension. The most commonly used shape for this method is a ring in conjunction with a du Nuoy tensiometer. It is always necessary to apply a cor- rection factor, a table of which has been published by Harkins and Jordan 5. The pull on a thin plate (Wilhelmy) is often quite satisfactory, especially when making quick comparative studies. (e) Sessile and Pendant Drop The shape of a large bubble or drop of liquid at rest on a plate or hanging from a suitable tip, is related quite simply to the surface tension of the liquid. The surface tension may be calculated from the height and maximum diameter of the drop or bubble. A typical surface tension-concentration curve is shown in Figure 1 for n-butyl alcohol. The Gibbs Adsorption Isotherm may be applied to this and the force (surface tension lowering)--area curve shown in Figure 2 derived.

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.