'GLYCERIN AS A CONSTITUENT OF COSMETICS AND TOILET PREPARATIONS An Attempt at Reappraisal by F. V. WELLS* The characteristic properties are critically discussed and the manner in which these properties contribute to its employment in cosmetics. GLYCERIN, called by its Swedish discoverer, Carl Wilhelm Scheele, "the sweet principle of fats," is a substance noted above all else for its exceptional physical versatility a property that is enhanced and accentuated by chemi- cal stability, •esthetic appeal, relatively low cost and a wide range of com- patibility with other materials. I have referred elsewhere to many of the properties and cosmetic applications of glycerin, • and an excellent review of the subject has been written by Kalish.' Other papers and articles to which reference may use- fully be made in this connection include those of Pickthall,' Reckless? Bragg? Stetson,, and Chadwick and Pears.' An informative review of glycerin as a humectant is to be found in the revised' first volume of Harry's work,' and much information of a general character is contained in the broad survey, "Glycerin: its Industrial and Commercial Applications," published in 1945,' and Lawrie's book on glycerin and the glycols. •o While cosmetic chemists do not need to be told, at this rather late hour of the day, that glycerin is one of their most valuable and versatile raw materials, it is nevertheless opportune, and may prove profitable, I think, to attempt a contemporary reappraisal of its pi-operties and uses. The present survey represents, however imperfectly, such an attempt to consider one by one the several and diverse properties that are uniquely contained in this familiar material, and the ways in which they are utili3ed in the cosmetic industry. Probably the most important property of glycerin, so far as the cosmetic chemist is concerned, is its hygroscopicity, or power to absorb and retain moisture. W.C. Griffin and co-workers n have further defined two aspects of hygroscopicity, namely, equilibrium hygroscopicity, which is the state or condition that exists when an aqueous solution of the humectant or hygro- scopic agent will neither gain nor lose water at a given humidity (RH), and dynamic hygroscopicity, or the relative speed with which a humectant or its aqueous solution will gain or lose moisture while approaching equilibrium. In addition to these two factors, certain other properties must be taken into account when selecting a humectant (or moisture content-stabilising agent) for cosmetic use. It is, for example, deskable that the humectant should be characterised by low volatility, although I do not agree with Griffin * Honorary Member Cosmetic Consultant, London. 19

20 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS et al. that complete lack of volatility is necessarily to be preferred. A good cosmetic humectant should, first and foremost, exhibit a high degree of hygroscopicity, and particularly dynamic hygroscopicity, under the normal conditions of use. It should be of low volatility, and cause minimum viscosity changes with variations in temperature and water content. Other desirable features are: lack of toxicity, dermatological suitability, a wide spectrum of compatibility, reasonable cost, and good colour, odour and taste. It is the nicely balanced combination of these and other more general properties that has led such authorities as Kalish to state that "the humectant powers of glycerin are outstanding and account in large measure for its predominant position in cosmetic formulation." A great deal of useful information is contained in the now well-known paper by Griffin, Behrens and Cross (loc. cit.), but in view of the admittedly "erratic" character of some ot the recorded weight losses, it would be as well for chemists not to place too much reliance on them. Even ruling out the ever-present possibility of errors in manipulation or measurement, one is faced, in this admirable piece of supplier-sponsored investigation, with (a) data based on cream formulae that are themselves full of variables and (b) deductions that, while unexceptionable in themselves, do not fully present the story as told by the authors' own tables (i.e., of per cent weight loss in relation to the type and concentration of humectant present). I mention this because it has become almost a fashion to quote these authors' paper as the standard reference to the comparative utility as cosmetic humectants of glycerin, sorbitol and propylene glycol. V•lon pointed out in 1051 •' the high rate of loss of diethylene glycol from a 75 per cent aqueous solution when dried in an oven at 105 ø ½. and examined at intervals of 3, 5, 11, 16, 20, 28 and 36 hours. After the lapse of 36 hours all the diethylene glycol had evaporated. The loss of glycerin, exposed to similar conditions, was also high, but it was nevertheless much more gradual --ranging gradually from 25 per cent after 3 hours to a maximum of 42 per cent after 36 hours. It may be argued with good reason, however, as with so much published data concerning humectants, that this type of experiment bears little relationship to humectaat performance in face creams, where the hygroscopic agent is unlikely to be exposed to the air for long periods or to very high temperatures, and is usually present in proportions not exceeding 10 per cent of the total formula. Also, as Harry truly observes, •-' "the majority of cosmetic and toilet products are packed in air-tight containers, and the humidity inside the container is then purely a function of the temperature to which the container is subjected." In the tabulated results of their numerous experiments, Griffin et al. state that a soap-type cream containing 10 per cent of glycerin showed a loss of 0.24 per cent by weight after 1 hour at R.H. 50 per cent as compared with 0.55 per cent loss shown by sorbitol and 0.48 per cent by propylene