Micelle reorganisation characterised by differential scanning calorimetry for aqueous solutions containing hexadecyltrimethylammonium bromide and tetradecyltrimethylammonium bromide

Abstract

Over a small concentration range in the 10–2 mol dm–3 region, the differential isobaric heat capacities δCp(sln; T) of aqueous solutions containing hexadecyltrimethylammonium bromide (CTAB) show extrema near 310 K when measured as a function of temperature over the range 209 ⩽T/K⩽ 360. The temperatures corresponding to maxima in δCp(sln; T) depend on the concentration of CTAB(aq). Similar extrema are observed for CTAB solutions in D2O, but in aqueous solutions the extrema become less intense when either hexanol or KBr is added, the maxima being completely lost in the latter case. A pattern in δCp(sln; T) similar to that shown by CTAB(aq) is observed for aqueous solutions containing tetradecyltrimethylammonium bromide (TDTAB). At fixed total concentration, 0.02 mol dm–3, the temperatures at maxima in δCp(sln; T) show a simple dependence on the molar ratio CTAB to TDTAB in mixed solutions of the two surfactants. The dependence on temperature of the relaxational (configurational) contribution of the solute Cpm(sln; config) to the molar heat capacity of the solution, ΔCpm(sln; T) is analysed in terms of both single and coupled equilibria. The results are discussed in terms of changes in the shape of the micelles and in the extent of counterion binding resulting from an increase in temperature.