GLYCERIN AS A CONSTITUENT OF COSMETICS 21 glycol in comparable experiments. In a non-ionic cream 2 per cent of glycerin showed a loss of 0.40 per cent after 1 hour at R.H. 70 per cent as compared with 0.55 per cent loss with sorbitol and 0.72 with propylene glycol. After 24 hours at R.H. 70 per cent the non-ionic cream containing glycerin had lost only 5-34 per cent of its weight, as compared with 6.04 per cent for the comparable sorbitol-containing cream. I do not quote these results in support of any claims that can be made for glycerin as the cosmetic humectant of choice, but merely by way of comment upon the rather misleading generalisations put forward on pages 17 and 29 of the paper under reference. • • A further important aspect of humectancy that has not so far been touched upon is the humectant action of the film of cosmetic deposited on the skin. Glycerin has long been utilised in pharmacy, medicine and cos- metics for this purpose. The osmotic action of glycerin and dried magnesium sulphate is utilised, for example, in Morison's paste to withdraw water from boils and carbuncles. Glycerin is similarly used in cataplasma kaolini, where it also helps to prevent the poultice from drying out during storage. The traditional softening "emollient" action on the skin of diluted glycerin, as in glycerin and rosewater and glycerin-containing lotions and o/w creams generally, is, of course, largely due to humectancy. Some later published work of Irvin H. Blank • relates to the prevention or relief of dry skin conditions by the application of glycerin solutions, designed to "help attract water to the stratum corneum from the environment." The subject of humectancy cannot be left, however, without at least a passing reference to the fact that this property can sometimes prove unde- sirable in cosmetic practice if utilised to excess. Excessive concentrations can prove too searching to the skin. The concentrated material is rather unpleasantly sticky, though not so markedly so as sorbitol syrup. Excessive use of glycerin in creams and lotions may cause what Florence Wall •5 aptly calls "polka dot" spottiness in face powder or make-up films subsequently applied over it. Finally, it must be conceded that glycerin is not always to be preferred to the other humectants. In certain formulae the use of sorbitol, propylene glycol or even hexylene glycol may, for one reason or another, p•rove more satisfactory. Thus I have used hexylene glycolin in a liquid hand cleaner as a combined humectant and solvent. Bandelin and Tuschhoff have recommended the use of a mixture of glycerin and sorbitol as the most effective means of minimising evaporation losses in ascorbic acid syrups during storage. Many more examples of this kind will come to mind. Another important property of glycerin, from the cosmetic point of view, is its versatile behaviour as a solvent, coupling agent and vehicle. It is almost certainly the most cosmetically useful of water-miscible substances, being miscible in all proportions with both water and ethyl alcohol. It has limited solubility in ketones and esters. The miscibility of aliphatic and

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