joURNAL OF TliE SOCIETY OF cosMETIC CliEMISTS -4.0 -4.5 -5.0 -6.0 -6.5 -7.0 800 1200 1600 0 400 Time Ihours) Apparent first-order plots of the decomposition of methyl paraben in a t0% aqueou glycoliacid Figure 4. and 40øC ([])' 80øC (&)' and 120øC ([])' solution at pH 1.87 EFFECT OF LACTIC ACID coNCENTRATION ON TIdE STABILITY OF ExCIPIENTStheexcipiinincreaseofmaintathe(roflinearlyorinatincreaseanbufferedAnToacidacid.increasedemuls*onacid120øC'lacticdecompoandAlIAtheparaben The effect of lactic acid concentration (0.06-0-22 M) on the was studied at a pH of 3.8 and temperatures of 40, 80, solutions at a constant pli, sodium lactate was added to most lactic acid concentration did not increase the degradation of the lactic ß . . .. entration of lactic and However, the decomposition rate for methyl excipients- n•c .... ¬• 'n an . with an increase m m•t)al 0.9962) ,•:•,•o % The use ot metny• P• .... n * temperature kr,• ...... ß 3.8 and containing a high concentration of lactic acid is not recomme' DiscUSSION From the DSC compatibility eYaluation interactions actually predicted kinetic consequencea'marebeinacidofpossiblethematerilacticwhichWhenbecausewould(4,5,8).concludedmostlybenotapplicationcouldit was solution- Users of thermal analysis in compatibility testing agree that there number of other difficulties in its general

STABILTY OF LACTIC AND GLYCOLIC ACIDS 173 4.0 2.0- 1.0- =0 •'• ! ! 0.00 0.05 0.10 0.15 0.20 0.25 Concentration (M) Figure 5. Effect of lactic acid concentration on the apparent first-order rate constant of methyl paraben decomposition at pH 3.83 and 40øC (I), 80øC ('), and 120øC (O). tested have almost identical melting points, it is difficult to distinquish the thermal events attributed to each component. Interpretations of DSC curves are also impossible when one component dissolves in another. In these studies, dissolution occurred either in the melt of the lower melting glycolic acid (melting point equal to 79øC) or in the lactic acid solution. All this indicated that false positives were likely when DSC data are used to predict possible incompatibilities. Therefore, to substantiate interactions seen by DSC, the thermal stability of aqueous solutions was studied to determine the kinetics of decomposition of various combina- tions of the compounds. Information on the stability of AHAs is scarce, but lactic acid and glycolic acid are reported to be very stable in aqueous solution. This was found true because both lactic acid and glycolic acid remained stable (Figure 3), regardless of exposure to high temperature, for six months. Furthermore, combination with excipi- ents, and exposing these mixtures to high temperature, did not decrease the AHA stability. Information about the stability of the other excipients at low pH was also not readily available. Results obtained from accelerated thermal stability studies of AHAs and excipient mixtures revealed that only methyl paraben and sorbic acid were unstable at