Ester aminolysis by morpholine in AOT-based water-in-oil microemulsions

A kinetic study of the aminolysis of p-nitrophenyl acetate (NPA) by morpholine (MOR) in AOT/isooctane/water (w/o) microemulsions was conducted. Based on the solubilities of NPA and MOR in water and isooctane, both compounds partition between the continuous medium, interface and water microdroplets of the microemulsion. Because the rate of the aminolysis reaction decreases with decreasing polarity of the solvent, the reaction must take place to a negligible extent in the continuous medium relative to the interface and the aqueous microdroplets. We used the pseudo-phase model to determine the rate constants at the interface, k(2)(i), and in the water microdroplets, k(2)(w). Both k(2)(i) and k(2)(w) were found to be independent of W in the aminolysis of NPA by MOR. This is a result of the expected increase in k(2)(w) on decreasing W being offset by the decrease in k(2)(i) with increase in the water content of the system. Based on the results, the reaction takes place to an extent of only 16% in the water microdroplets at W=40, the proportion decreasing with decreasing water content.