THE HYDROLYSIS OF MONOSTEARIN IN AN ACIDIC MEDIUM 709 Figure 1 1.4 1.3 1-2 1.0 0.9 0'8 0 100 200 300 400 Time (min) Concentration-time profile of monostearin in mineral acid (Key: ß 35øC O 40øC, CD 45øC, O 50øC, ß 54.5øC.) Figure o 0.8 0'4 0.2 0 0.30 o.3• 0.32 0.33 44 x •o 3 K-temper&ture profile of the hydrolysis of mo•oste&ri• i• miner&l &cid.

710 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS in Fig. 1, all straight lines having a correlation coefficient greater than 0.95. The rate constants derived from these straight lines are given in Table II. An Arrhenius plot of the rate constants (Fig. 2) gives an activation energy of 76.49kj mole-1 and a collision factor of 9.48 min-1 Table II Hydrolysis of pure monostearin in 25 • v/v mineral acid in acetone Temperature (øC) I 35 40 45 50 54.5 . Rate constant X 103 (min- l) 0.326 0.391 0.688 1.583 2.038 DISCUSSION Ingold et al (4) have determined activation energies and collision factors for the acid hydrolysis of the acetate, propionate, n-butyrate and n-valerate esters of glycerol, and have found that both quantities decrease as the homologous series is ascended. Extrapolation of Ingold's results to the stearate ester gives a calculated activation energy of 81.93 kJ mole-1 and a collision factor of 9.8 min-1, i.e. somewhat higher than those obtained above. Jellinek (9) has pointed out that in hydrolysis reactions, the use of a solvent of low dielectric constant may be expected to lower the energy of activation compared with aqueous systems, which may account for the discrepancy between the calculated values for collision factor and energy of activation, and those obtained experimentally. The rate of hydrolysis of monostearin was determined in a series of buffered solutions of aqueous acetone. The experimental conditions were as described earlier, except that the mineral acid was replaced by quadruple strength Mcllvaine citric acid - phosphate buffer (10). The pH values of the buffer-acetone mixtures were measured on an E.I.L. pH meter standardized against a known aqueous buffer. For this reason, the actual meter readings are quoted as "apparent pH". Rate constants determined at three different temperatures and apparent pH values are given in Table III. Table III Rate constants (X 106) of the hydrolysis of pure monostearin in 25 •o v/v quadruple strength Mcllvaine buffer in acetone Temperature (øC) 38 47 55 "Apparent pH" 2.9 4.03 (min- 7.50 9.72 3.4 3.96 6.11 9.16 4.7 2.92 5.00 6.94