KINETICS OF DEGRADATION OF PARABENS 501 0.8 0.6 0.4 0.2 ' O G - 0 '-ø"---- soø c 70 ø C 400 C o 2b do 6b ,oo TIME IN HOURS Figure 2. Log of concentration of ethyl paraben against time at various temperatin'es in 0.1M phosphate buffer, pH 9.16, and ionic strength 0.3 Table IV Energies of Activation of the Parabens as Determined from the Slopes of the Arrhenius Plots at pH 9.16 in 0.1M Phosphate Buffer and Ionic Strength 0.3 Paraben Energy of Activation (kcal/mole) Methyl 20.8 Ethyl 18.7 Propyl 19.3 n-Bury1 21.0 Table IV. It is difficult to rationalize a change in the mechanism of hydrolysis for a simple ester of p-hydroxybenzoic acid. The effect of temperature on the reaction rate can be expressed using the Arrhenius equation (10). Plots of log k versus 1/T yielded a straight-line relationship with a negative slope for each of the parabens studied, as shown in Fig. 3 for ethyl paraben. This indicates that the mechanism for the degra- dation of the parabens does not change with temperature at pH 9.16. Energies of activation for each paraben are shown in Table IV. These values are in good agreement with literature values for the hydrolysis of simples esters such as methyl acetate and ethyl benzoate as reported by Newling and Hinshel- wood (11). By extrapolating the Arrhenius plots of each paraben to 23øC, rate constants and half-lives were predicted, as shown in Table V. It is valid to fol-

502 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 2.oe 1.2 0.4 I/Tx IO $ Figure 3. Arrhenius plot showing temperature dependence of the ethyl paraben hydrolysis at pH 9.16 in 0.1M phosphate buffer and ionic strength 0.3 Table V Specific Rate Constants and Half-life Periods for the Hydrolysis of Parabens at 25øC in 0.1M Phosphate Buffer, pH 9.16 and Ionic Strength 0.3, as Calcu',ated from Arrhenius Plots Paraben k x 10 • t 1/• (hours-•) (hours) Methyl 4.015 1726 Ethyl 1.514 4577 Propyl 1.122 6176 n-Butyl 1.096 6323 low the extrapolated value for the rate constant if the experimental activation energy falls within a range of 10-30 keal/mole ( 12, 13). Although data already obtained showed that the reaction of the parabens at constant hydrogen ion concentration was first order with respect to paraben, this first-order dependency was further verified by a study of the effect of initial paraben concentration on the hydrolysis rate. The results shown in Table VI indicate that for each paraben studied between concentrations of 4-7 rag/l, the rate constants were essentially independent of the initial con centration of paraben.