170 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 11o lOO '• 90 "- 8o = 70 o • 60 5o -•- Lactic acid -•- Glycolic acid 0 50 100 150 200 Time (days) Figure 3. Mean percentage decrease in lactic acid (I) and glycolic acid (0) concentration with time in aqueous solutions kept at 80øC. Results are the mean plus standard deviation of solutions containing the AHAs and AHA excipient mixtures. STABILITY OF LACTIC ACID AND GLYCOLIC ACID Both lactic acid and glycolic acid were very stable in aqueous solution under the conditions studied (Figure 3), and degradation was not different when combined with the various excipients tested (level of significance, p 0.05). The shelf lives determined for lactic acid (Table II) ranged from a low of 79 years, when combined with isopropyl palmitate, to a high of 98 years when combined with sorbic acid (Table II). Results showed no appreciable influence of the excipients on the decomposition of the acids. These results are not consistent with the DSC results. It was found that DSC frequently gives false positive, seldom-false negative, results (8). STABILITY OF EXCIPIENTS COMBINED WITH LACTIC ACID OR GLYCOLIC ACID Kinetic studies showed that except for methyl paraben and sorbic acid (Tables II, III) all the excipients were stable when combined with AHAs under the study conditions. Methyl paraben and sorbic acid were unstable when combined with lactic acid or glycolic acid. The activation energies calculated from Arrhenius plots were 3.45 and 3.84 kJ/mol -• for the decomposition of methyl paraben in 10% glycolic acid and 10% lactic acid solutions, respectively. The respective shelf lives were 88 and 164 days. The

STABILTY OF LACTIC AND GLYCOLIC ACIDS 171 Table II Stability Results for Lactic Acid and Excipients Combined With Lactic Acid Combination Stability of* pH Concentration left after 6 months at 25øC (%) Shelf life at 25øC (to. 9 = years) Glycolic acid LA 1.91 98.2 83 Methyl paraben LA 1.84 98.6 85 Methyl paraben MP 1.84 46.7 0.5 Sorbic acid LA 1.87 99.2 98 Sorbic acid SA 1.87 53.5 0.5 Sodium lactate LA 3.83 98.4 96 Glyceryl monostearate LA 1.92 97.6 85 Cetyl alcohol LA 1.90 99.2 82 Stearic acid LA 1.89 98.5 84 Glycerol LA 1.87 97.4 84 Lanolin LA 1.94 96.3 82 Sorbitol LA 1.95 97.5 81 PEG 1000 monostearate LA 1.92 97.3 82 Isopropyl myristate LA 1.91 97.6 82 Isopropyl palmitate LA 1.90 99.3 79 * LA = lactic acid MP = methyl paraben GA = glycolic acid SA = sorbic acid. Table III Stability Results for Glycolic Acid and Excipients Combined With Glycolic Acid Combination Stability of* pH Concentration left Shelf life after 6 months at 25øC at 25øC (%) (to. 9 = years) Lactic acid GA 1.88 97.7 70 Methyl paraben GA 1.87 98.2 67 Methyl paraben MP 1.87 32.1 0.2 Sorbic acid GA 1.83 96.3 89 Sorbic acid SA 1.83 44.2 0.5 Glyceryl monostearate GA 1.92 99.6 89 Cetyl alcohol GA 1.94 95.2 82 Stearic acid GA 1.87 98.3 81 Glycerol GA 1.88 97.3 79 Lanolin GA 1.87 98.6 86 Sorbitol GA 1.84 95.4 87 PEG 1000 monostearate GA 1.92 96.3 84 Isopropyl myristate GA 1.90 98.7 90 Isopropyl palmitate GA 1.86 99.2 86 * LA = lactic acid MP = methyl paraben GA = glycolic acid SA = sorbic acid. equivalent acitvation energies were 13.2 and 14.9 kJ/mol -• for the decomposition of sorbic acid. In the presence of glycolic acid, sorbic acid decomposes rapidly with a predicted shelf life at 25øC of only 185 days, and when sorbic acid was combined with lactic acid, the shelf life was 197 days. This is not of practical importance since these preservatives are not commonly used at such low pH, and at very low pH most emulsions are self-preserving.