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

24 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS foams. These two uses are considered together by Pickthall' in one of his recent authoritative essays on aerosol formulation. Drawing attention to the general utility of this trihydric alcohol in aerosol-packaged products, he adds that it is in emulsified foam-type products that glycerin really comes into its own. "The most important single factor," he states, in the develop- ment of adequate spreading power, body and foam stability "is glycerin." He goes on to quote a formula for a shaving cream containing 20 per cent of glycerin. Much more could be written on this subject. In tooth pastes and mouth washes glycerin finds application as a sweetening agent, humectant and "throat-conditioner." As Pepper observes, '• it has a subsidiary advantage in vanishing creams intended for sale to very cold countries, in that its freezing point-lowering property prevents emulsion breakdown at low temperatures. Glycerin also reduces the stickiness of gum mucilage and gelatinous films (a property that, with its lubricating action, is used to special advantage in jellies for cosmetic, pharmaceutical and gynaecological use). From the dermatological point of view, little need be said, having regard to this material's known lack of toxicity and freedom from any tendency to provoke harmful skin reactions under normal conditions of use. There is ample evidence in fayour of this view and little but vague and unsubstanti- ated rumours against it. This does not mean to imply that glycerin should be used quite indiscriminately or without intelligent appreciation of all the concomitant factors involved. • Significant, perhaps, among recently published work is that of H. Chino, '• who, in biochemical studies of the Bombyx silkworm, has revealed for the first time the presence in animal tissues of both glycerol and sorbitol as normal products of metabolism. Also noteworthy is the use of glycerin in the preservation of corneal grafts and bull's semen.•' By way of conclusion, I would like once again to second E.G. McDonough's plea for further fundamental work on the physical chemistry of glycerin. This would be of great value, he pointed out to American glycerin producers in 1954, in determining whether many of its interesting, yet varied, properties could be integrated and explained more simply. "For example (he added) is the strong hydrogen bonding property of glycerin a basis and an explanation for such terms now used as the hygroscopic nature of glycerin, the humectant character of glycerin, the strong solvent action of glycerin, the high viscosity of glycerin, and the plasticising action of glycerin on such widely divergent matehals as proteins (e.g., gelatin, casein, zein and gluten), methyl and carboxymethyl celluloses, amylose and starches, polyvinyl alcohols, alginates and pectates ?" The subject is important enough to deserve the lucid and elegant inter- pretation of an A. E. Alexander or tl•e subtle analysis of a Linus Pauling.