Influence of the hydration and dynamics behaviour of polyvinylpyrrolidone on the rate of alkaline hydrolysis of benzocaine

Abstract

The aggregation and hydrodynamic behaviour of polyvinylpyrrolidone (PVP) in aqueous medium have been studied through the kinetics involving the alkaline hydrolysis of benzocaine. The absorbance values recorded for benzocaine at different [PVP] demonstrate that the absorbance intensities change with the increase in [PVP]. The kinetics studies on the hydrolysis of benzocaine at different [PVP] and [PVP-surfactant] have also been undertaken to further elaborate the morphological changes in PVP in the absence and presence of surfactants. The values of rate constant decrease with the increase in [PVP] in the lower PVP concentrations range. In the middle concentration range, the rate constant values do not change with change in [PVP]. At higher [PVP], i.e. in the polymer chain entanglement region of [PVP], the values of rate constant decreased again. The obtained results for the absorbance and the rate constant values are the consequences of different morphological arrangement of PVP and the hydration ratio around PVP chains. The values of rate constant gave peak behaviour with [CTABr] in the presence of PVP. The plot obtained for kΨ-[SDS] contains two crests with the increase in [SDS] due to different conformational arrangements formed between the PVP chain and SDS molecules. The resulting trends in the values of rate constant versus [surfactant] are the result of partitioning of the reacting species (OH– and Bz) into different pseudophases due to divergent organisation between PVP and surfactant molecules. Viscosity measurements and TEM images at different [PVP] support the structural changes occurring in the PVP chains in aqueous medium.