28 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS higher refractive index, the light changes in velocity, i.e., it is refracted. In addition, a certain amount of light will also be reflected. The reflection occurs at the interface between the two phases. The phenomenon of di,•use reflection is the cause of opacity. Now the greater the distance between the refractive indices of the two phases, the greater will be the opacity of the emulsion. It can be anticipated and indeed confirmed by practical experiences that a large amount of glycerin in the aqueous phase will cause a decrease in opacity. Glycerin itself pos- sesses a high refractive index and the water-phase containing it will there- fore be of higher refractive index. The result is that the refractive indices of the two phases are now closer to one another. In an O/W emulsion, for instance, there is an immense number of oil droplets finely dispersed in the external aqueous phase. Light will meet with opposition to its passage from individual oil droplets and it strains to penetrate deeper and deeper through the mass of droplets. As light impinges on an emulsion droplet, only a very tiny proportion will be reflected, the majority of the light continuing deeper into the emulsion. On its way, the light strikes a multitude of droplets of the internal phase until at a certain stage there is a total reflection, i.e., full opacity is developed. A transparent emulsion on the other hand occurs if the refractive indices of the two phases are identical or very dose together. Moreover, the develop- ment of the opacity may be caused by other factors. A concentrated emul- sion with an internal phase of over 50 per cent, that is, high phase-volume ratio, appears extremely opaque. This is because the particles are close together. In very dilute emulsions, that is, low phase-volume ratio, there exists only relatively few droplets and these are separated by considerable distances. On its way, the light meets only a small number of droplets and therefore a heavier layer of emulsion will be necessary to reflect all the light. It is also possible to realise the higher degree of opacity with favourable conditions such as an increased degree of dispersion. Dispersion aims at breaking the cohesive forces that keep the larger particles together. Such an object can be attained either by mechanical or chemical means or a com- bination of both. The chemical method depends upon the use of suitable and sufficient emulsifiers. Mechanical dispersion may be brought about by the use of the colloid mill or homogeniser. If we use in an emulsion two phases whose refractive indices are fairly close together, we are left with two opportunities to correct or improve the opacity of that emulsion. These are: (a) The use of a high phase-volume ratio (concentrated emulsion). (b) A decrease in the particle size of the droplets. Regarding particle size, emulsions are rather coarse with particle sizes from 5-10 micron, therefore, less stable. To produce a finer emulsion, we

MILK WHITE APPEARANCE AND ITS SIGNIFICANCE 29 aim at an average droplet size in the neighbourhood of one micron. Naturally, coarse emulsions are less opaque than fine emulsions. Opacity in colloidal dispersions with particle sizes between 0.1 microns and one millimicron is not observed. It can be seen that opacity depends upon particle size and that maximum opacity occurs within a certain range of fine particles. OPACITY AND COSMETIC ,PRACTICE Water in oil emulsions are at present popular in cosmetic practice, especially those prepared from absorption bases. These absorption bases consist essentially of 7-10 per cent woolwax alcohols and woolwax esters dissolved in paraffin hydrocarbons (mixtures of liquid soft and hard paraffins). The internal phase of a W/O all-purpose c•eam consists of 50-55 per cent of water or of about 50 per cent water and 5 per cent glycerin. In this way, the correct phase-volume ratio is ensured. The chief characteristic of W/O emulsions prepared from lanolin wax alcohols is ,that they develop coarse primary emulsions. The process of emulsification usually consists of two separate operations. The initial coarse primary emulsion can be produced by relatively simple stirring. The resulting emulsion will be lacking in both opacity and stability. In this operation it is necessary to overcome powerful forces of interfacial tension, forces which oppose adequate dispersion of one phase in the other. Although the presence of the emulsifier reduces the tension, a second step is still necessary. Interfacial tension has still to be reduced to a minimum and a protective film must be formed around the droplets by mechanical means such as violent treatment in a homogeniser or colloid mill. Either of these two machines will produce emulsions of very small particle size of adequate opacity and great stability. The breaking down of coarse particles into fme globules requires a con- siderable amount of mechanical energy. The energy required for complete stability will be inversely proportional to the square of the diameter of the particle of the emulsion globules. Homogenisers and colloid mills are de- signed chiefly for the emulsification of W/O creams. Here the particle size of the globules must be of the smallest so as to ensure lasting stability of the emulsion. HAND LOTIONS AND HAND CREAMS Changes in opacity can be established in cosmetic preparations in which the water phase contains a high proportion of glycerin. As previously mentioned, the addition of glycerin increases the refractive index of the aqueous phase. Cosmetic emulsions for the care of the hands usually contain large amounts of glycerin and, therefore, tend to lack opacity. It should be mentioned in'passing that some O/W emulsifiers such as glyceryl mono- stearate, cetyl sulphate, cetyl alcohol, etc., tend to reduce opacity in emul-