Effect of Large Changes in Temperature and Pressure on the Thermodynamic Properties of Micellization and on the Distribution Constant of a Polar Solute in Micellar Solutions

Abstract

Density measurements of pentanol (PentOH)−dodecyltrimethylammonium bromide (DTAB)−water mixtures as functions of both alcohol and surfactant (mS) concentrations were carried out at 0.1 MPa from 45 to 75 °C and at 19 MPa from 25 to 130 °C. The standard (infinite dilution) partial molar volumes and expansibilities of DTAB in water and the corresponding properties in the micellar phase were calculated from the experimental data. As far as PentOH in DTAB micellar solutions is concerned, with the exception of the standard partial molar volume (V°R) data at 130 °C and 19 MPa, all the V°R vs mS trends are monotonic curves with mS. The data of V°R as a function of mS were treated by means of an equation previously used for data obtained at ambient conditions. So it was possible to evaluate simultaneously the distribution constant (K) of PentOH between the aqueous and micellar phases and the standard partial molar volume of alcohol in the aqueous (V°R,f) and the micellar (V°R,m) phases. The standard free energy of transfer (ΔG°t) of PentOH from the aqueous to the micellar phases was calculated from K values. By using experimental literature data for the present system at 25 °C, ΔG°t values as functions of temperature and pressure were calculated. The very good agreement between the simulated and experimental values indicates that the model used also holds at higher temperatures. Equations correlating the standard free energy, enthalpy, and entropy of transfer to temperatures up to 130 °C and pressures up to 19 MPa were proposed.