22 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS aromatic compounds with glycerin increases with the number of hydroxyl and amine groups, decreasing with the introduction of alkyl groups. Glycerin's insolubility in oils further adds to its value, especially in the formulation of two-layer fluids and, on a much wider scale, in that of emul- sions. Many cosmetic and pharmaceutical materials have greater solubility in glycerin than in water or alcohol. TM In general, however, glycerin is inferior to propylene glycol both as a solvent and an inhibitor of mould growth. The solubility characteristics of such three-component systems as glycerin-phenol-water, glycerin-alcohol-water and glycerin-benzene-alcohol have been determined as a result of glycerin's long history of industrial utilisation, and the existence of such data somewhat simplifies laboratory assessment of the potentialities of glycerin as a co-solvent. Glycerin is, in fact, frequently included in physical chemistry studies of ternary systems --whether in relation to mutual solubility or to miscibility, density contrac- tion, homogenising action, selectivity or co-solvency. Glycerin acts, by the way, as the conventional "third party" in lubricating greases, where it tends to form, with traces of water, stable gels of improved consistency. Many inter-related explanations have been put forward for this phenomenon, some writers concentrating on the increased solubilisation of soap in hydrocarbons, while others refer to the co-ordinating capacity of glycerin's hydroxyl groups or glycerin's capacity for lowering the temperature of phase trans- formations. •7 We here approach the territory of transparent soap manu- facture and the broad study of soap phases and their behaviour. One of the most striking features of glycerin is the sharp decline shown in the viscosity of its aqueous solutions. At 20 ø C. the viscosity of 100 per cent glycerin is 1410 centipoises. At 70 per cent concentration this drops to 22.5 c.p., at 50 per cent to 6 c.p. and at 10 per cent to 1-3 c.p. This pro- perty is used to advantage 'in the formulation of various skin lotions, where a clear, free-flowing liquid is required that has a pleasantly smooth, emollient effect on the skin, is free from stickiness, and has just enough "body" to prevent its being taken for mere coloured water. The relatively low surface tension of glycerin also plays some part in the behaviour of this type of cosmetic. While still on the subject of viscosity, we may note that the effect of this polyol on viscosity is not always in proportion to the amount added or to its viscosity in the pure state. Cosmetic formul,e contain so many variables that nobody can at present predict, with any great degree of confidence, the exact effect that glycerin will have on the viscosity or stability of any parti- cular emulsion. Sometimes maximum "body" may be achieved with a low proportion of glycerin and decrease upon further addition. In some emulsions (vide Kalish) substantial amounts of glycerin are compatible with the maintenane of a desired and reasonably high viscosity while in others (vide de Navarre •8) the presence of smaller amounts of glycerin ensure

GLYCERIN AS A CONSTITUENT OF COSMETICS 23 fluidity whereas replacement of glycerin with other humectants gives unpourable emulsions. In certain systems glycerin can markedly promote viscosity, either as so-called "local viscosity" or, in the more usual way, by forming highly viscous aqueous dispersions with very small amounts of gums and gum-like polymers. Segur and Miner, in their studies on a series of vanishing creams, 18 showed how choice of polyol may influence the consistency of such creams. With glycerin, the maximum stiffness was obtained at 7.5 per cent concen- tration, whereas with sorbitol syrup this maximum effect necessitated the use of 15 per cent. I am currently making comparisons of the effect on the viscosity of certain simple emulsions of varying proportions of glycerin, sorbitol and propylene glycol, and hope to publish my conclusions at a later date. Meanwhile, I recommend readers to study the interesting observations on humectants in hand creams by S. J. Strianse.'-8 The plasticising action of glycerin is made use of in such products as solid cologne sticks (not more than about 5 per cent), deodorant sticks, wave sets and lacquers (to plasticise the residual film left on the hair), and in many other cosmetics that are designed to leave a thin, smooth film on the skin. Closely related to this property of conferring plasticity is glycerin's lubricant, softening and spreading action. In a typical face cream, glycerin may first of all act as an aid to emulsifica- tion and as a promoter of emulsion stability. (This is particularly the case in o/w emulsions based on the polyoxyethylene alkyl ethers.) Secondly, it will assist in the maintenance of viscosity at a desired level. Thirdly, it will function as a humectant, preventing the cream from drying out and pro- moting the maintenance of consistency. On application to the skin, it will enable the film to spread easily, exerting in the process a combined lubricant and plasticising action, preventing "drag" or "bailing up." Fifthly, it may soften the skin or (in the case of nasal creams and jellies) the mucous surface to which it is applied. Sixthly, when the emulsion breaks down or tends to become inverted, glycerin facilitates the phase change and keeps the resulting film even, homogeneous and plastic. Finally, it is perfectly stable in these conditions of use, as well as being non-sticky, emollient and pleasant to the touch. What other cosmetic material can boast of such remarkable versatility ? I cannot conclude this brief review without referring to the special utility of glycerin in aerosol formulations. As long ago as 1939, glycerin had been acclaimed as the ideal agent in shaving creams "for keeping the cream mobile and the lather in a moist state when applied. "•9 More recently, a noted worker at the Illinois Institute of Technology'-8 has pointed out that glycerin is often put to work in studies on fluid foams and anti-fogging agents, and is especially valuable in the preparation of dynamically stable