Protonation equilibria of L-phenylalanine and maleic acid in cationic micellar media

Abstract

Solute-solvent interactions of L-phenylalanine and maleic acid have been studied in various concentrations (0.5-2.5% w/v) of cetyltrimethylammonium bromide solution maintaining an ionic strength of 0.16 mol dm-3 at 303 K. The protonation constants have been calculated using the computer program MINIQUAD75 and the best fit models are arrived at based on statistical grounds employing crystallographic R factor, x2, skewness and kurtosis. These protonation constants values have been found to shift in micellar media as compared to those in pure water. The differences in the values have been attributed to the solvent properties of the interfacial and bulk phases involving contribution from the micellar surface potential in the case of charged micelles. The trend of log values of step-wise protonation constants with mole fraction of the medium have been explained based on electrostatic and non-electrostatic forces operating on the protonation equilibria. Distributions of species, protonation equilibria and effect of influential parameters on the protonation constants have also been presented.