Enthalpies of mixing of dodecyltrimethylammonium bromide and secondary alcohols aqueous solutions

Abstract

The enthalpies of mixing of aqueous solutions of secondary alcohols (from propanol to heptanol) and of dodecyltrimethylammonium bromide were measured, taking as the baseline of the mixing process the enthalpy of dilution of the surfactant solutions. The measurements were made at a given alcohol concentration by changing systematically the surfactant concentration. From the enthalpies of mixing below the CMC the pair and the triplet interaction parameters between alcohol and surfactant molecules were calculated. The trend of these parameters as a function of the number of carbon atoms in the alcohol alkyl chain ( n C ) is quite similar to that obtained for primary alcohols. In fact, h RS increases linearly with n C, h RSS increases in an exponential way, and h RRS shows a minimum for 2-pentanol. Above the CMC the enthalpies of mixing were rationalized using a previously reported approach for the alcohol distribution between the aqueous and the micellar phases. From the resulting equation the distribution constant and the enthalpy of transfer (and consequently the standard free energy and entropy) can be obtained contemporaneously. As predicted, the additivity rule always holds for the standard free energy of transfer but probably only up to 2-pentanol for enthalpy and to 2-hexanol for entropy. It is shown that the free energy of transfer of secondary alcohols from aqueous to DTAB micellar phase practically does not differ from that of their corresponding primary alcohols. Such as for primary alcohols, the enthalpy and entropy of transfer display convex trends with maxima for secondary alcohols localized at alkyl chains immediately longer than those for primary alcohols. The contribution of the branched methyl group to the thermodynamics of alcohol transfer from aqueous to micellar phase was calculated by subtracting from the thermodynamic properties of secondary alcohols those of the corresponding primary alcohols immediately lower. From these calculations it results that the introduction of the branched methyl group allows an increase in the solubilizing power of micelles toward alcohols.