Classification of calorimetric titration plots for alkyltrimethylammonium and alkylpyridinium cationic surfactants in aqueous solutions

Abstract

Calorimetric titration plots for deaggregation of micelles formed by alkylpyridinium and alkyltrimethylammonium surfactants are classified into three types, A, B and C, depending on the shape of the plot of the enthalpy of dilution as a function of surfactant concentration. For Type A plots the recorded heat of injection q changes sharply between two parts of the titration curve over which the recorded heats are effectively independent of the composition of the solution in the sample cell. For Type B plots, the change is less sharp and both parts of the plot show dependences of heat q on solution composition, a pattern accounted for in terms of solute–solute interactions. Type C plots are complicated, in that no sharp change in q is recorded, the complexity of the plots being accounted for in terms of micelle–monomer equilibria over a range of surfactant concentrations and related enthalpies of deaggregation.