HEAT-CAPACITIES OF SOME PRIMARY ALCOHOLS IN DODECYLTRIMETHYLAMMONIUM BROMIDE AQUEOUS-SOLUTIONS

Heat capacities of the ternary systems water-dodecyltrimethylammonium bromide (DTAB)-alcohols (propanol, butanol, pentanol, hexanol, and heptanol) were measured at 25-degrees-C as a function of the surfactant and the additive concentration. At fixed low surfactant concentrations m(S), the apparent molar heat capacity of alcohols C-PHI-,R increases linearly with alcohol concentration m(R), whereas at higher m(S), the sign of the slope of C-PHI,R vs m(R) changes from positive to negative at a m(R) value that, regardless of the surfactant concentration, depends on the alcohol alkyl chain length. The longer the alcohol chain, the smaller the m(R) value at which this occurs. Microheterogeneity formation by the alcohol can account for this behavior. From the least-squares analysis at low alcohol concentrations the standard partial molar heat capacities C(p)-degrees-R of propanol and hexanol in micellar solutions have been obtained as a function of m(S). The plot of C(p)-degrees-R vs m(S) for hexanol displays a maximum of about 30 J K-1 mol-1 at ca. 0.3 mol kg-1 DTAB; according to the literature, it can be ascribed to the DTAB postmicellar transition. In the case of propanol the maximum was not detected. The standard partial molar heat capacities of propanol and hexanol in micellar solutions in the region above the cmc and below the structural transition were rationalized in terms of the alcohol properties in the aqueous and micellar phases and the alcohol distribution between the two phases by using a previously reported equation, expanded to account for the temperature effect on the change of the mole fraction of alcohol solubilized in both phases. Previously, this effect was considered to be negligible. The same equation was also used to analyze the heat capacity data of butanol and pentanol already published. An important result obtained is that the standard heat capacity of alcohol in the micellar phase increases with the length of the alcohol alkyl chain. This effect was not previously observed when the temperature effect on the alcohol distribution was neglected.