Effects of temperature on the distribution of 1-alcohols in aqueous SDS micellar solutions and ISA (Interaction of Surfactant and Additive) Coefficients

Abstract

The distribution coefficients ( K) of alcohols (1-butanol, 1-pentanol, 1-hexanol, and 1-heptanol) between the aqueous and sodium dodecyl sulfate (SDS) micellar phases at 40 and 55°C were determined in addition to that at 25°C. The standard free energy change ( ΔG° p) of penetration of the alcohol from the aqueous to the micellar phase was obtained using the equation ΔG° p = - RT ln( K). There is a linear relation between Δ G° p and the carbon number of the alcohols, and the increment in Δ G° p per methylene group slightly decreases with increasing temperature. The standard enthalpy change (Δ H° p) and entropy change (Δ S° p) of penetration of the alcohol were estimated at 10, 25, 40, and 55°C by the use of temperature variation of the K and the Gibbs-Helmholtz equation. The fact that both Δ H° p and Δ S° p become less positive with the increase in temperature indicates that at lower temperature the penetration of the alcohol into micelle exhibits a greater entropy dependence. Estimated values of Δ H° p were compared with calorimetrical values. At 40 and 55°C, similar to that at 25°C, linear relations are observed between K and critical micelle concentration (CMC) decrease upon the addition of the alcohols. From the linear relations, ISA (interaction of surfactant and additive) coefficients defined as d ln(CMC)/ dY a = (ISA) K were obtained to be −0.95 and −0.56 at 40 and 55°C, respectively, where Y a is the mole fraction of the alcohol in the aqueous phase, d ln( Q)/ dX a was estimated from ISA, where Q is the degree of ionization of ionic micelles, and X a is the mole fraction of the alcohol in the micellar phase. The theoretical value of d ln( Q)/ dX a from ISA was compared with the experimental one, and a good agreement was obtained between them.