This thesis is divided into three sections, an Introduction, the Experimental Section and finally the Discussion where the findings from the experiments have been analysed and discussed. The Introduction deals with the mechanisms of ester hydrolysis and the factors which affect its rate. This is then followed by a section dealing with surface active agents, where the basic concepts of micelle physical chemistry, including the structure of micelles, the CMC, the shape, size, and solubilisation are considered together with the factors affecting these parameters. The theories dealing with micelle formation and the effect of micelles on reaction kinetics with special emphasis on reactions involving cationic surfactants, particularly cetyl-trimethylammonium bromide (CTAB), are then reviewed. The experimental work describes the methods by which CMCs under various sets of conditions were determined. This is then followed by details of the kinetic investigations into the effect of CTAB on the base catalysed hydrolysis of p-nitrophenyl acetate. Kinetic studies were carried out in the absence of buffer at pHs between 8.0 and 9.2 using a pH-stat technique as well as in the presence of buffer at pH 9.2 and the results are compared. Below the CMC at pH 9.2 retardation in the rate was noticed which was associated with pre-micellar aggregation; this was not the case at lower pHs. CTAB above its CMC increased the rate of hydrolysis of the ester in all the cases up to a maximum in the region of 8.5 - 9.6 x 10 -3M surfactant and then the effect decreased. Changes in micellar structure in this critical region were evidenced by light scattering and viscosity studies but no change in counter-ion binding as measured by bromide activity was observed. The rate of hydrolysis in the presence of CTAB was found to be independent of the ester concentration giving support to the idea of constant partitioning. Potassium bromide was found to be a much more potent inhibitor of CTAB catalysis than the buffer of various strengths studied. The final section deals with the discussion of the experimental results obtained. The main conclusion drawn is that the mathematical model proposed by Winterborn et at (141) and Shetewi (67) is not suitable for this system where the surfactant accelerated the reactions. The qualitative model proposed by Cordes et at (10,11) based on competitive inhibition and counter-ion concentration also does not explain the maximum in the critical concentration region of CTAB satisfactorily. The change in the micellar structure in the region around 10-2M CTAB is however suggested as the precise reason for the change in surfactant effect at this concentration.
|Date of Award||1977|