528 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS The adsorption of lactic acid decreases as the pH increases. Above pH 5 there is no detectable adsorption. These observations are consistent with the adsorption of undissociated lactic acid. The pH of lactic acid is 3.88 and above pH 5 there will be very little undissociated lactic acid in solution. Effect of lactic acid solution on water holding capacity and extensibility The adsorption studies showed that lactic acid was adsorbed by solvent- damaged corneum. The effect of lactic acid on corneum water holding and extensibility was then determined. Pieces of guinea pig footpad corneum were immersed in a 105/o W/V solution of lactic acid for 30 min and water holding and extensibility were measured (Table VI). In a separate experi- ment, the effect of a subsequent 30 min immersion in water was investigated. In both experiments the effect of lactic acid was compared with that of water. The results of separate experiments showing the effect of 10• sodium lactate, which is not adsorbed, are included for comparison. Table VI shows the results. Table VI. Effect of 10% lactic acid and sodium lactate solution on water holding capacity and extensibility of solvent-damaged comeurn at 81 •o r.h. Treatment Water held Extensibility (mg 100 mg -• dry corneum) (% per 100 g load) Lactic acid 34.0 (6) 30.6 (6) Water 18.9' (6) 0.5* (6) Lactic acid then water 16.9 (10) 1.1 (10) Water 15.6 (10) 0.6* (10) Sodium lactate 47.3 (10) 29.8 (10) Water 19.1' (10) 0.9* (10) Sodium lactate then water 18.0 (10) 0.85 (10) Water 18.0 (10) 0.61 (10) Figures in parentheses are numbers of replicates. All treatment times were 30 min. * Denotes significant difference (P 0.05). Both lactic acid and sodium lactate caused large increases in water holding and extensibility, provided that the corneum was not rinsed after treatment. After a 30 min rinse the effect of the sodium lactate had been lost, but the corneum treated with lactic acid still had a significantly greater extensibility than corneum treated with water, although much of the effect

SKIN CREAM FOR DRY AND FLAKY SKIN 529 had been lost during the rinsing period. No detectable increase in water holding capacity remained after rinsing corneum treated with lactic acid. These experiments showed that adsorption of lactic acid resulted in the corneum maintaining an increased extensibility after rinsing in water under conditions where the effect of other humectants was lost. Effect of hand lotions containing lactic acid on corneum water holding and extensibility The pH of lactic acid itself is too low for incorporation into hand lotions. Hand lotions containing lactic acid were prepared by partially neutralizing lactic acid with sodium hydroxide to give a pH of 4, and incorporating this into the aqueous phase of a lotion to give a product containing 10• by weight of the lactic acid-sodium lactate mixture, calculated as lactic acid. Similar lotions containing 10•o sodium lactate, and a control lotion con- taining water in place of the lactic acid or lactate, were also prepared. The lotions were rubbed into both sides of pieces of solvent-damaged guinea pig footpad corneum for a total period of 90 s with the fingertips while wearing rubber gloves. Excess lotion was removed by wiping with tissues and the treated corneum was either equilibrated at 81• r.h. or rinsed by immersion in water for 30 min before equilibration. The control pieces of corneum from the same animals were either left untreated or were immersed in water for 30 min as appropriate. Table VII shows the effect of the lotions on corneum water holding capacity and extensibility. Table VII. Effect of hand lotions on water holding and extensibility of solvent- damaged guinea pig footpad corneum at 81 •o r.h. Water held Extensibility Treatment (mg 100 mg -• corneum) (•o per 100 g load) Unrinsed Rinsed Unrinsed Rinsed Control lotion 17.6 (9) 18.6 (12) 0.68 (10) 0.93 (12) Untreated 17.7 (9) 18.1 (12) 0.59 (10) 0.87 (12) Lacticacid lotion 17.4 (16) 18.3 (19) 1.19 (10) 0.98 (ll) Untreated 16.4' (16) 18.3 (19) 0.58* (10) 0.59* (11) Sodium lactate lotion 19.9 (10) 18.1 (11) 0.81 (10) 0.74 (11) Untreated 17.8' (10) 17.4 (11) 0.59* (10) 0.75 (11) Figures in parentheses are numbers of replicates. Treatment time with lotions was 90 s, rinsing time was 30 min. * Denotes significant difference (P 0.05).