DIFFERENTIAL SCANNING CALORIMETRIC STUDY OF NORMAL-HEXADECYLTRIMETHYLAMMONIUM CHLORIDE IN AQUEOUS-SOLUTIONS

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 KCl. 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 temperature. 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-degrees-C with an aggregation number of 650 where [CTAB] = 0.03 (mol monomer) dm-3.