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

MILK WHITE APPEARANCE AND ITS SIGNIFICANCE of oil in water results in a tremendous increase of the interfacial area. One c.c. of a fine emulsion with an average particle size of one micron pos- sesses about one thousand billions of droplets with an interracial area of 30,000 cm.• or approximately thirty-two square feet. Since the development of opacity is dependent upon particle size, the importance of the above figures can be imagined. A further advantage of the emulsified system is the possibility of intro- ducing otherwise immiscible hydrophylic and hydrophobic compounds. Cosmetics may contain water-soluble salts, sun screening agents, perfumes and water-insoluble preservatives, etc. The combination of water in oil cosmetic emulsions brings about a greater degree of cleansing action since the aqueous phase will remove the residues of perspiration and water-soluble dirt, whilst the oil phase will remove fatty excretions of the skin and previously applied cosmetics. One should be aware that certain ingredients, including inorganic salts, might be opposed to the stability of an emulsion. A breakdown in the emulsion system will naturally reduce the opacity. In certain cases a minor instability in the system may lead to reversible changes which are known as sedimentation. Below the upward creaming layer or above the downward creaming layer may appear a partial loss in opacity. The creaming layer itself, however, being the accumulation of emulsified particles, remains fully opaque. A major instability in an emulsion which results in irreversible separation will mean considerable reduction in opacity. This is evident since the breaking of the emulsion has destroyed those optical properties which were in the first place responsible for the opacity. For this reason it is necessary that cosmetic emulsions such as deodorant lotions or creams and sun screen preparations neither sediment nor break. CONCLUSIONS Opacity in an emulsion is a function of the refractive indices of the two phases, the ratio of oil and water and the size of the emulsion particles. We have seen that the factors which promote and ensure the stability of emulsions are also responsible for opacity. Opacity may therefore be regarded as a sign of stability and without a doubt lends greatly to the attractive appear- ance of an emulsion. REFERENCES Emulsionen, Dr. E. Manegold. The Theory of Emulsions, W. Clayton. Introduction to Emulsions, G. M. Sutheim. Kohlenzvasserstoj•6l• und Ferry, Prof. Dr. D. Holde.