Water−Ethylene Glycol Alkyltrimethylammonium Bromide Micellar Solutions as Reaction Media: Study of Spontaneous Hydrolysis of Phenyl Chloroformate

Abstract

The spontaneous hydrolysis of phenyl chloroformate was studied in water−ethylene glycol (EG) micellar solutions of hexadecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, and dodecyltrimethylammonium bromide, the percentage by weight of EG varying within the 0−50% range. The dependence of the observed rate constant on the surfactant concentration was investigated in all the water−EG mixtures. In addition to the kinetic measurements, conductivity, spectroscopic, and fluorescence measurements were done to obtain information about the micellar solutions used as reaction media. For the three surfactants studied, an increase in the amount of EG present in the mixture resulted in an increase in the critical micelle concentration and in an increase in the micellar ionization degree but in a decrease in the aggregation number of micelles. The polarity of the bulk−micelle interface region seems to increase somewhat by increasing the percentage by weight of EG. A simple pseudophase model was useful in rationalizing the micellar kinetic effects observed. The adjustable parameters obtained from the fittings of the experimental data show that the reaction rate in the micellar pseudophase was similar for all the water−EG cationic micellar solutions investigated. The equilibrium binding constant for the phenyl chloroformate molecules to the micellar aggregates decreases as the amount of EG in the mixture increases, this decrease being responsible for the diminution in the observed rate constant when the amount of EG present in the mixture increases.