SKIN CREAM FOR DRY AND FLAKY SKIN 525 Table I. Effect of 5• humectant solutions on water holding capacity and extensibility at 81% r.h. of solvent-damaged guinea pig footpad corneum Water held Extensibility Humectant (mg 100 mg -x dry corneum) (% per 100 g load) Glycerol 19.9 (9) 1.24 (10) Water 17.4' (9) 0.60* (10) Sorbitol 18.6 (8) 0.82 (8) Water 17.5 (8) 0.53* (8) Sodium lactate 21.6 (9) 1.71 (20) Water 17.8' (9) 0.43* (20) NaPCA 23.3 (10) 2.07 (10) Water 17.5' (10) 0.54* (10) Figures in brackets represent number of replicates. * Significant difference (P 0.05) between humectant and water. Treatment times 30 min. Table II. Effect of rinsing on water holding capacity and extensibility at 81 • r.h. of solvent-damaged guinea pig footpad corneum with 5 % humectant solutions Treatment Water held Extensibility (mg 100 mg -• dry corneum) (% per 100 g load) Glycerol: Water Not done 0.76 (20) Water: Water 0.76 (20) Sorbitol: Water 16.5 (10) 0.44 (10) Water: Water 17.1 (10) 0.42 (10) Sodium lactate: Water 17.2 (18) 0.48 (30) Water: Water 17.5 (18) 0.41 (30) NaPCA: Water 15.6 (10) 0.33 (10) Water: Water 15.5 (10) 0.41 (10) Figures in brackets are numbers of replicates. All treatment times were 30 min. In no case was there a significant difference between the water holding capacity or extensibility of corneum treated with humectant followed by water and corneum treated with water alone. The supposition is that water removes the added humectant and this results in a loss of the increased water holding capacity and extensibility.

526 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Adsorption studies The results given above indicate that any beneficial effect of most humectants will only be temporary and that the effect will be lost when the skin is immersed in water. A more effective humectant would be one which is adsorbed by the corneum so that it is not easily rinsed out. The adsorption of glycerol, sorbitol and sodium lactate on to solvent- damaged corneum was studied by determining the reduction in concentra- tion of the humectant in an aqueous solution in contact with corneum. No reduction in the concentration of these three humectants could be detected, indicating that there was no adsorption to the corneum. This observation was consistent with the loss of effect on water holding and extensibility after rinsing corneum treated with the three humectants. In the next series of experiments the adsorption of aromatic carboxylic acids was investigated in the same manner. Carboxylic acids have been shown to adsorb to hair keratin (15) and aromatic compounds were selected as examples of carb- oxylic acids because of the ease of estimating their concentration in aqueous solution by ultra-violet absorption. Table III shows the adsorption of three aromatic carboxylic acids after immersing the corneum in an aqueous solution of the acid for 4 h at a concentration which gave a convenient reading on the spectrophotometer. Table III. Adsorption of aromatic carboxylic acids by solvent-damaged corneum Concentration Wavelength Adsorption Acid (mmol) (nm) (mg acid mg -• dry corneum) Phthalic 0.5 278.5 0.0041 Salicylic 0.2 295 0.0089 Mandelic 3.0 256 0.0021 The results showed that there was some adsorption of carboxylic acids. The acids investigated were not hygroscopic. Further experiments were therefore carried out with a hygroscopic acid, lactic acid. In preliminary experiments using Barker and Summerson's method (11) for determining lactic acid, an average adsorption of 0.056 mg lactic acid mg -• corneum was obtained after immersing corneum in 0.5• lactic acid solution for 2 h. The adsorption was investigated in more detail using C •4 labelled lactate which allowed a simpler and more accurate analytical deter- mination of lactate and lactic acid.