Differential scanning calorimetric study of n-hexadecyltrimethylammonium chloride in aqueous solutions

Abstract

The isobaric heat capacity of n-hexadecyltrimethylammonium chloride (CTAC) in aqueous solutions containing 0.02 (mol monomer) dm^–3 passes through a maximum at 294 K. The temperature, T_m, of the maximum shifts to higher temperatures with increase in concentration of CTAC. These extrema at T_m in the dependence on temperature of the isobaric heat capacity, C_p, measured using a differential scanning calorimeter, point to structural transitions in aqueous solutions containing CTAC. The processes responsible for the extrema at T_m are sensitive to added alcohol and added KCI. Replacement of water by D₂O produces a shift in T_m to higher temperatures, whereas addition of urea produces a shift in T_m to lower temperatures. T_m shifts gradually to higher temperatures as the proportion of n-hexadecyltrimethylammonium bromide (CTAB) in a mixture of CTAB (aq) and CTAC(aq) increases. The data are quantitatively accounted for in terms of micelle clustering. The stoichiometries required to account for the dependence of C_p on temperature in the region of T_m point to processes involving several hundred monomers, a consequence of micelle clustering in these reasonably concentrated solutions. In the case of CTAC a simple two-state model suffices, whereas a more complex description is required for related CTAB solutions for which the dependence of C_p on temperature is characterised by extrema at 40.6 and 49.8 °C with an aggregation number of 650 where [CTAB]= 0.03 (mol monomer) dm^–3.