Enthalpy Measurements in Aqueous SDS/DTAB Solutions Using Isothermal Titration Microcalorimetry

Abstract

Isothermal titration microcalorimetry has been used to measure enthalpies of aggregate formation and binding, at 25 °C, for aqueous solutions of the single surfactants sodium dodecyl sulfate (SDS) and dodecyltrimethylammoniumbromide (DTAB) and mixtures of these at 1 wt % total surfactant concentration, over accessible composition ranges. The enthalpy of formation for SDS micelles is 0.6 kJ g-1 mol-1, and that for DTAB micelles is −2.1 kJ g-1 mol-1. This calorimetric experiment also produces a direct measurement of the critical micelle concentration (cmc) for each surfactant. The enthalpy of formation for 5:95 SDS/DTAB mixed micelles is −5.5 kJ g-1 mol-1 total surfactant and −6.7 kJ g-1 mol-1 total surfactant for 85:15 SDS/DTAB mixed micelles. Vesicles at an SDS/DTAB ratio of 62:38 have an enthalpy of formation of −13.4 kJ g-1 mol-1 total surfactant, indicating a strong enthalpic contribution to their free energy of formation. The enthalpies of aggregate formation are in good agreement with predictions fro...