Influence of polyethylene glycol and surfactant mixed system on the rate of alkaline hydrolysis of benzocaine

Abstract The rate of alkaline hydrolysis of benzocaine has been studied spectrophotometrically in the presence of polyethylene glycols (PEGs), surfactants (cetyltrimethylammonium bromide; CTABr and sodium dodecyl sulphate; SDS) and in the mixed media of surfactants-polysurfactants (PEGs-CTABr and PEGs-SDS). The observed values of rate constant increased linearly with the increase in [NaOH] at fixed [benzocaine] obeying the equation: k obs  = k 2 [NaOH]. The rate constant – [CTABr] plot showed peaked behaviour whereas in case of SDS, the rate constant decreased with increase in [SDS]. The increase in the molecular weight of PEGs increased the binding tendency of PEG-CTABr system for benzocaine, in turn, an increase in inhibitory effect on the rate of hydrolysis was observed. The obtained values of the binding constant decreased with the increase in the molecular weight of PEGs in PEG-SDS mixed system. The binding constant and the values of rate constant in the surfactant, PEG-surfactant media were determined by using Pseudophase Ion Exchange (PIE) and pseudophase models.