KINETICS OF ASPIRIN HYDROLYSIS IN AQUEOUS-SOLUTIONS AND GELS OF POLOXAMINES (TETRONIC-1508) - INFLUENCE OF MICROENVIRONMENT

An aqueous solution of Tetronic 1508 (20-25% w/w) undergoes sol-gel transition at about 25-27-degrees-C causing a dramatic increase in bulk viscosity (4 orders of magnitude). A significant increase in viscosity would be expected to reduce the collision frequency of two reacting species in such a gel medium due to a decrease in their aqueous diffusion coefficients. As a result a dramatic increase in the hydrolytic stability of a model ester compound, aspirin by Tetronic 1508 gel was hypothesized. While theoretical calculations predicted almost 7-8 orders of magnitude decrease in the apparent first-order rate constants for aspirin hydrolysis at any given pH, only 2-3-fold reduction was noted in the pH range of 1-10. Moreover, within the pH range of 5-8 there was no apparent reduction in the aspirin hydrolytic rate constant because, within that pH range, the predominant mechanism of aspirin hydrolysis is the intramolecular nucleophilic attack by the neighboring carboxylate anion. The result of only 2-3-fold reduction in the rate constant refutes the original hypothesis and suggests the presence of a low viscosity 'micro' environment within the rigid gel structure. The presence of such a fluid microenvironment within the Tetronic 1508 gel structure was confirmed utilizing an ESR probe technique. While the macroscopic viscosity of the gel may be as high as 10(6) mPa s (cps) the viscosity within the microenvironment was found to be only 1-10 mPa s. Such a low viscosity within the microenvironment indicates that there is probably an abundance of 'free' water in the system. It also helps to explain why the rate of aspirin hydrolysis was not reduced as hypothesized. It is apparent that a complete evaluation of the thermorheologic properties of the Tetronic polymer requires an understanding of both the macroscopic and microscopic viscosities.