THE ROLE OF GLYCEROL IN SKIN HYDRATION By K. LADEN, Pi. D.* THE UTILITY of glycerol in cosmetic formulations is well established. I Thus, glycerol has been used as a humectant, lubricant, plasticizer, binder, • spreading agent, etc. For many years, one of the functions regularly attributed to glycerol •was'a "softening" action on skin. That glycerol per se does not have a •softening effect on keratin has been shown clearly by investigators during I the past ten years (1-3). Indeed in aqueous products containing glycerol, I the glycerol may actually serve to slow down the hydration of keratin when I these products are used on the skin. This may be illustrated by soaking •strips of desiccated callus in various glycerol-water solutions and studying I their hydradon by observing the rate at which they swell, soften, and become I opaque (hydrated callus is opaque, desiccated callus is translucent). Re- • sults indicate that callus hydrates most rapidly in pure water, and the addi- I tion of 5 per cent glycerol has an appreciable effect of slowing down the • rate of hydration. Higher concentrations of glycerol slow down the hydra- I tion even more markedly. Flesch (4) recently reported the results of experiments in which the rate of moisture loss from skin was studied. This was accomplished by mount- •ing a piece of excised skin over the mouth of a vessel filled with water and , then measuring moisture loss through this piece of skin by daily weighings. I When a film of glycerol is placed on this skin membrane, the rate of mois- ture loss from that system is decreased. From these results, the author I concluded that glycerol decreased the diffusion of water through the skin membrane probably via occlusion of the skin surface. An alternate expla- •nation of these results can be postulated, however, that does not involve , any alteration of the diffusion rate of water through the skin membrane. I Thus some of the moisture lost through the skin membrane could readily 'have been trapped by the glycerol film, thereby lowering the rate at which •moisture was being lost from the complete system. Indeed this would I be expected, since glycerol is more hygroscopic than skin. This would •mean that the skin would lose moisture at the same rate with and without , a surface film of glycerol, but, in the case of glycerol coating, some of this • moisture would not escape from the system. * The Toni Co., Div. of The Gillette Co., Chicago 54, Ill. 455

456 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Powers and Fox (5) studied the effect of various cosmetic ingredients' (including glycerol) on the rate of moisture loss from the skin in an in vivol situation. In their experiments, small desiccator cups containing silica• gel were strapped on the skin, and the rate of water loss measured from an l untreated site in comparison to a site rubbed with anhydrous glycerol. I The authors found that glycerol increased the rate at which water was lostl from the skin. They conclude that glycerol accelerates the diffusion ofl water from moist dermal tissue to the surface, and that this accounts forl the beneficial effect of humectants on skin during dry weather (6). It has been shown that the rate at which moisture diffuses to the surfacel of the skin is governed by a rate regulating barrier membrane (1). Ill Powers and Fox's interpretation of their data is correct, one must concludel that glycerol increases the rate of water diffusion through this barrierl membrane. This does not appear to be a likely action of glycerol. Another possible explanation for these results is that during the processl of rubbing the anhydrous glycerol on the skin, it picked up water from thel air and the weight changes really measured water lost by the skin plus l water lost by the glycerol. An experiment was set up to determine how much water could be taken up from the air by 4.9 cm. 2 surface area of glycerol (equivalent to the sur- face area of the desiccator cups used by Powers and Fox) in short periods of time. Exposing 4.9 cm. 2 of anhydrous glycerol to 65 per cent R.H. for one minute resulted in a moisture uptake of 3.8 mg., 3.0 mg. and 4.5 rag. on successive trials. In a second experiment, about 30 rag. of anhydrous glycerol was spread over an area of 4.9 cm. 2 using a glass stirring rod in a manner analogous to that described by the authors. (This amount ofglycerol corresponds to the amount of glycerol Powers and Fox had covering their one-inch circular area.) After spreading the glycerol for one minute at 65 per cent R.H., the moisture uptake was determined. The results were 3.9 mg. and 3.5 mg. on successive trials. In both experiments, short, wide-mouthed, glass weighing bottles were used in which a circular area of 4.9 cm. 2 was inscribed on the bottom. In the first experiment, glycerol was dropped into the vessel until it formed a circular drop equal to the inscribed area. In the second experiment, 30 rag. of glycerol was placed in the center of the inscribed area and spread with a micro glass stirring rod. These values are of the right magnitude to explain the increased moisture loss observed by Powers and Fox from the glycerol treated site. Thus, they report increased moisture loss on one subject of 3.6 rag., 4.4 rag. and 3.6 rag. during three successive hourly measurements, and on a second subject of 1.7 rag., 1.5 mg., 1.4 rag. and 1.9 rag. during four successive hourly measurements. Since the desiccant cups were removed each hour