MILK WHITE APPEARANCE AND ITS SIGNIFICANCE IN COSMETIC EMULSIONS By PAUL MANNHEIM, Ch.E.* The •actors governing milky white appearance are elaborated and shown to be closely connected with emulsion stability. AN ATTRACTIVE appearance in a cosmetic emulsion is as desirable as adequate stability, performance and a pleasant odour. The important factors in emulsion stability are: sufficient emulsifier of the correct type, fine particle size and correct phase-volume ratio. Foremost among properties of appear- ance is a milky whiteness. "Milky-white" has for ages been the sign of purity. In this article we Wish to point out that a milky-white appearance in cosmetic emulsions is closely associated with the factors responsible for the stability of the emulsion. Animal and vegetable milks are typical examples of the way in which Nature makes use of emulsions. The emulsion is the most logical form of the transportation of fatty substances within the aqueous environment of the living cell. The finely dispersed fat droplets form the internal phase of the thin milk emulsions. In this way the resorption and digestion of fatty bodies by the organism is well facilitated. There are other circumstances which justify the existence and use of emulsions. Man and animal alike need three basic food elements: carbohydrates, proteins and fats, and, in addition, water, various salts and vitamins. Natural milk is the ideal mixture of these hydrophylic and hydrophobic food elements. Taking into consideration the foregoing properties of milk, one can summarise the advan- tages of an emulsion as follows: 1. The possibility of combining the various hydrophylic and hydrophobic materials in one uniform liquid or cream emulsion. 2. The possibility of regulating the consistency of the emulsion according to any requirements. 3. The dilution of otherwise too concentrated an ingredient, e.g., fats. In preparing an artificial emulsion one often keeps in mind the require- ments of a natural emulsion. In general, one does not limit oneself to the low concentration and small particle size of natural emulsions: one seeks to achieve a more concentrated emulsion of uniformly dispersed particles and of long shelf life. OPAQUE APPEARANCE It is rather strange that an emulsion possesses a milky whiteness in spite of the fact that both phases are transparent and colourless. How can we * Ramat-Gan. Israel.

MILK WHITE APPEARANCE AND ITS SIGNIFICANCE 27 explain the fact that a mixture of two transparent substances such as water and oil, form an emulsion which appears milky-white ? It appears to be a physical phenomenon. It must be of great interest to the cosmetic manu- facturer to determine the causes of opacity and to apply this knowledge to the manufacture of his own preparations. Colour is an optical property of each and every materiM, whilst opacity is only the optical property of emulsions. The opposite of opacity is trans- parency, and we can prepare a range of emulsions which vary between these two extremes. The colour of any substance can be determined by colorimetric methods, by means of instruments such as colorimeters, chromo- meters, tintometers, etc. Although opacity cannot be measured in this way, I will mention something about refractometers or the idea of refraction. From an optical viewpoint, there is an obvious difference between transparent and opaque dispersion, and this is manifested by the behaviour of the passage of light. A transparent substance will allow the transmission of light with little opposition, but an emulsio• forms a definite obstacle. The opaque emulsion will in fact reflect the light due to the different optical properties of the ingredients of the emulsion. Light travels at a speed of 186,000 miles per second in a vacuum, but much more slowly in water and still slower in oils and fats. The ratio between velocity of light in a vacuum and velocity in any other medium is called Refractive Index of the medium. In industry the refractometer is used to determine the refractive index of various commercial products. This provides a rapid and easy method of identifying the purity of oils, fats, waxes, fatty acids, etc. Below is a table of refractive indices of emulsion constituents of interest to the cosmetic industry :, Materials _ Reft. indices Materials Reft. indices Vacuum 1.0000 Olive oil 1 '469 Air 1.0003 Castor oil 1 '478 Water 1.3300 Almond oil 1.471 Methylalcohol 1.329 White mineral oil 1.475 Ethylalcohol 1.362 Glycerin 1.47 Isobutylalcohol 1.3900 Benzene 1.50 Kerosene 1.4500 Gelatine 1.54 Japan Tallow 1'458 Woolwax 1.48 Coconut oil 1-449 Paraffins 1.42 Cacao Butter 1 '457 Beeswax 1.44 Palm-nut oil 1'451 Carnauba wax 1-39 Palm oil 1-454 An emulsion is a two-phase system in which each phase may contain several ingredients. When a beam of light passes at an angle from a phase having a lower refractive index (water-phase) into the oil-phase having a