Thermodynamic properties of pentanol in dodecyltrimethylammonium bromide aqueous solutions

Abstract

Pentanol in dodecyltrimethylammonium bromide (DTAB) aqueous solutions was investigated extensively in the course of our studies of the thermodynamic properties of micellar solutions. Volumes, heat capacities and enthalpies of solution, and dilution and mixing as a function of the surfactant concentration were then rationalized in terms of the distribution constant ( K) of the additive between the aqueous and the micellar phase, and the corresponding property of the additive in the micellar phase. In this paper we summarize the linear relationships obtained for a given property as a function of the mole fraction of alcohol in the aqueous phase. These equations were solved by least-squares analysis by adjusting K in order to minimize the standard deviation of the plot. Within the experimental uncertainties, the minimizing procedure applied to all the thermodynamic properties investigated allows us to obtain the same K value, indicating the successful application of the model to the experimental data. The thermodynamic properties of pentanol in DTAB micellar phase determined up to now are similar to those in pure liquid pentanol, whereas they differ from those in water and in octane. From this experimental evidence the schizophrenic nature of micelles as solvents is evidenced. In addition, heat capacity and pair interaction parameters for volume, heat capacity and enthalpy of dilution indicate the existence of a structural transition at about 0.35 m DTAB. Solubility data do not show peculiarities around this concentration, but they do at about 0.08 m DTAB. However, since at about 0.25 m DTAB, when pentanol concentration is greater than 1.2 m liquid crystals start forming, it seems that the micellar structural transition can be related to this mesophase formation.