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-

30 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS sions. To the best of the writer's knowledge, the reason for this behaviour has not yet been explained in the literature. MOUTHWASH PREPARATIONS Such preparations contain a moderately dilute alcohol, flavour compound and antiseptics. Ii we pour a few drops of the mouthwash solution into a beaker, we observe a development of a milky opalescence. The ingredients of the mouthwash are intimately dispersed in the alcohol forming a molecular solution. However, they are insoluble in water. When a few drops of the mouthwash are added to water, the alcohol becomes considerably diluted, and since the flavour oils and preservatives are not soluble in the Water/Alco- hol mixture, they appear in the form of exceedingly fine droplets. Without sufficient time to coalesce, they remain dispersed in the water and do not rise to the surface as large droplets. The opalescent fluid can be by no means considered as a true emulsion. Oil hydrosols are highly diluted emulsions of the O/W type with extremely fine particle sizes. The feature which characterises the oil hydrosols is that they do not contain emulsifiers and, in the main, they have little practical value. They are, however, of the highest theoretical interest. The lack of opacity in an oil hydrosol system is obviously in accordance with the previously expressed principles of opacity formation. FLUID EMULSIONS The lotion or milk type of cosmetic may be either O/W or W/O liquid emulsions. Normally they contain small amounts of the oil phase. They are sold as hand lotions, facial milks, cleansing lotions, sunburn lotions, etc. Lacking the favourable advantage of high viscosity, they require more care in formulation of manufacture than do the firm creams. Nevertheless, there are, on the market, many lotions of excellent stability and high opacity. This latter property is due to the employment of efficient emulsifiers and homogenisers. Oils, fats, waxes and other modern hydrophobic materials are used in cosmetics for cleansing, softening, protecting and nourishing human skin. The human skin is basically of a hydrophylic nature and shows little affmity for hydrophobic substances, and it is therefore difficult to introduce these latter materials into the skin. However, in the form of an emulsion, hydro- phobic materials are more easy to apply and in addition are more economical in use. In an O/%V emulsion the fatty materials are enveloped in the hydro- phylic or external phase. This results in good wetting properties and facilitates spreading and penetration into the skin. Despite the distinction between oil and water, they tend to lose their independent properties when in common emulsion. They form an homogeneous system and the dispersion