Solubilization of pentanol in sodium dodecylsulphate micelles. Interpretation of calorimetric results using a theoretical model

Abstract

Microcalorimetric titration experiments have been performed to study the solubilization of pentanol in sodium dodecylsulphate, SDS, micelles. A concentrated SDS solution was titrated into water solutions containing 0.059, 0.118 or 0.176 mol dm^–3 of pentanol. The enthalpic titration curves were determined at each pentanol concentration in the range 0–0.035 mol dm^–3 of SDS at 25 and 35 °C. The observed differential enthalpies of dilution vary in a complex manner with SDS and alcohol concentration and temperature.

A thermodynamic model for the association of ionic amphiphiles proposed by Jönsson and Wennerström and extended to deal with alcohol as a third component has been used to analyse the calorimetric results. The concentrations of SDS and pentanol in monomer and micellar states were calculated and the composition of the mixed micelles was derived using the chosen model. The enthalpy change for the formation of SDS–pentanol micelles is regarded as the sum of the following contributions: the enthalpy of formation of pure SDS micelles, ΔH^ref_mic, the electrostatic and surface enthalpy contributions H_el and H_s, respectively, in mixed micelles and an enthalpy of formation of a hypothetical alcohol micelle ΔH^ref_tr. Values of H_el and H_s were calculated from the model for micelles of varying composition and ΔH^ref_mic was determined from dilution experiments in pure water. This leaves ΔH^ref_tr as the only adjustable parameter in the calculation of theoretical titration curves. A satisfactory agreement is seen between the experimental and calculated titration curves, which means that the model can describe the essential features of the solubilization of pentanol in SDS micelles.