Synergistic influence of inhibition of PEG-surfactant on the rate of alkaline hydrolysis of procaine

The effect of surfactants (sodium dodecyl sulphate; SDS, cetyltrimethyl ammonium bromide; CTABr, and Triton X-100; TX100) and polymeric surfactants (viz. polyethylene glycol; PEG) mixture on the rate of base catalyzed hydrolysis of procaine hydrochloride (2-diethylaminoethyl-4-aminobenzoic acid) has been studied under the pseudo-first-order conditions ([OH¯] ≫ [Procaine]). The rate of hydrolysis of procaine was found to be linearly dependent upon the [NaOH] in the range from 2.0 × 10−2 mol dm−3 to 8.0 × 10−1 mol dm−3 at fixed [procaine] (=5.0 × 10−5 mol dm−3) at 37 °C. The values of rate constant were found to be independent on [procaine] in the concentration range from 1.0 × 10−5 mol dm−3 to 1.0 × 10−4 mol dm−3 at a fixed concentration of NaOH at 5.0 × 10−2 mol dm−3. The values of rate constant decreased with the increase in the concentrations of surfactants (SDS, CTABr and TX100). The rate of hydrolysis further decreased with the addition of PEGs of different molecular weights (1500, 4000, 6000 and 8000) in the reaction media containing surfactant. The increase in the concentration of PEGs from 1% to 5% decreased the rate of hydrolysis, thereby, increasing its stability. The increase in molecular weight of PEG also decreased the rate of hydrolysis. The kinetic parameters and binding constants (e.g. km, Kp for the different reaction media) were calculated for the reactions occurring in the aqueous and surfactant-PEG pseudophases by applying pseudophase ion exchange model. Maximum inhibition in the rate of hydrolysis was observed for PEG-TX100 aggregates.