Microheterogeneous solvation for aminolysis reactions in AOT-based water-in-oil microemulsions

A kinetic study was carried out on the aminolysis of p-nitrophenyl acetate (NPA) by n-decylamine (DEC), piperazine (PIP) and sarcosine (SAR) in AOT/isooctane/water (w/o) microemulsions. By using the pseudophase model both the rate constants at the interface, k2i, and the water microdroplet, k2w, can be obtained. The obtained results show that k2i increases together with the water content of the microemulsion, whereas k2w increases as the water content of the system decreases. In the aqueous microdroplet the predominant interaction Na+...OH2 causes a decrease in the strength of the hydrogen bonds and therefore facilitates the desolvation of the reagents as W decreases. This desolvation of the reagents causes the increase of k2w as W decreases. In the interface of the microemulsion the predominant interaction SO3-...HOH causes an increase in the electronic density on the water molecules and the consequent decrease in their efficiency in the solvation of the partial negative charge, which develops on the carbonyl oxygen atom in the transition state of the reaction. This decrease in the solvation causes k2i to decrease together with the water content of the system.