Stability of methyl paraben in cosmetics.

Effects of ethanol, polyol and oil in cosmetics on stability of methyl paraben (MP) have been studied. MP was degraded by both hydrolysis and transesterification in the presence of alcohol or polyol. In ethanol or 1, 3-butylene glycol aqueous solution, observed rate constant (k) for degradation of MP increased with water activity (Aw) rising. k was nearly equal to rate constant for hydrolysis. This indicated that transesterification was negligible in these solutions. In glycerin aqueous solution, k increased as Aw rised. At Aw=0.95, k took a maximum value, and above this k decreased. This result suggested that the contribution of transesterification was significant in this solution.In two-phase oil oil-water systems, MP is partitioned into oil and water. MP underwent degradation only in water phase, then transferred from oil phase. Consequently, partition coefficient (P) remained constant. P is determined by kind of oil, and related to pKa of MP and pH. We found that a plot of k versus logP gave a straight line, and that the slope was dependent on the amount of oil. In aqueous solution, k was related to pH and temperature. From these results, we proposed an equation with which residual proportion of MP was estimated. In emulsion system, this equation did not explain the behavior of MP. We obtained another equation leaving the transfer of MP out of consideration.The calculated values agreed with experimental results. This suggested that in this system no MP transferred from oil phase because of some components which interferred with transfer of MP.