Solubilization of 2-propanol and pentanol in sodium dodecyl sulfate micelles: A thermochemical study

Abstract

The solubilization of 2-propanol and pentanol in sodium dodecyl sulfate (SDS) solutions has been studied by titration calorimetry over a wide range of alcohol concentrations in three different SDS solutions. Measurements were made at 25°C and in addition at 35°C for pentanol. The measured enthalpy changes are close approximations of the partial molar enthalpies of solution of the alcohols in surfactant solution. Reported values of the partition coefficient K c derived from self-diffusion measurements using the Fourier transform NMR pulsed gradient spin-echo method were used to interpret the results. In the calorimetric measurements pentanol dissolves predominantly in the micellar phase, while a larger part of added 2-propanol remains in the aqueous phase. To separate the solubilization effects partial molar enthalpies of transfer Δ 1 mic H of alcohol from pure liquid to micellar phase were derived assuming the distribution of alcohol between aqueous and micellar phase to be a simple partition equilibrium. The limiting value of Δ 1 mic H at low alcohol content is −4.4 ± 0.2 kJ mole −1 for pentanol and −6.5 ± 0.5 kJ mole −1 for 2-propanol. The derived values of Δ 1 mic H for pentanol show a pronounced variation with the composition of the micellar phase. The increase is almost linear up to a molar ratio 2:1 of alcohol:SDS in the micellar phase, indicating a smooth change in the milieu for the solubilizate from discrete micelles with low alcohol content to alcohol-rich apolar domains. The heat capacity change for the transfer of liquid pentanol to the micellar phase is zero or close to it over the concentration range studied, which indicates that the alkyl chains are dissolved in a nonaqueous environment.