Heat capacities of butanol and pentanol in aqueous dodecyltrimethylammonium bromide solutions

Heat capacities of the ternary systems water-dodecyltrimethylammonium bromide (DTAB)-butanol and water-DTAB-pentanol were measured at 25°C. The standard partial molar heat capacities of pentanol in micellar solutions show a maximum at about 0.35 mol-kg^–1 DTAB that has been attributed to a micellar structural transition. This maximum tends to vanish by increasing the alcohol concentration and by decreasing the alcohol alkyl chain length; in the case of butanol it was not detected. The behavior of the standard partial molar heat capacities of alcohols in micellar solutions in the region above the cmc and below the structural transition was explained using a previously reported mass-action model for the alcohol distribution between the aqueous and the micellar phase and the pseudophase transition model for micellization. In the resulting equation the contributions due to the temperature effect on the shift of both the micellization equilibrium and the distribution are shown to be negligible so that only the distribution effect and the shift of the micellization equilibrium due to the added alcohol remain. The distribution constant and the partial molar heat capacities of alcohols in the aqueous and micellar phases have been derived by linear regression. The distribution constant for both alcohols agree well with those previously obtained using different techniques. Since the best fit below the structural transition correlates as well with the experimental points above the structural transition, it seems that no difference exists in the standard partial molar heat capacities of alcohols in the two shapes of the micelles. Also, from the present data and those for alkanols in sodium dodecylsulfate reported in the literature it seems that the standard heat capacity of alcohols in the micellar phase does not depend on both the alcohol alkyl chain length and the nature of the hydrophilic moiety of the head group of the micelles.