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

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 from a molecular−thermodynamic model. Within the context of regular solution theory (RST), the interaction energy parameter is −51.7 kJ g - 1 mol - 1, again in excellent agreement with the prediction from the molecular−thermodynamic model. Monomer solutions of each surfactant show a measurable enthalpy of dilution with a positive deviation from ideality. Using RST, the activity coefficients for SDS and DTAB monomers in water are 1.035 and 1.025, respectively. The enthalpy of dilution of concentrated micellar solutions (∼5 cmc) of each surfactant up to its cmc is zero, implying that these solutions behave “ideally” over this concentration range. The enthalpy of binding of DTAB monomers to SDS micelles is −28.4 kJ g - 1 mol - 1, to 20:80 SDS/DTAB mixed micelles is −36.4 kJ g - 1 mol - 1, and to 62:38 SDS/DTAB vesicles is −28.6 kJ g - 1 mol - 1 . The enthalpy of binding of SDS monomers to DTAB micelles is −24.7 kJ g - 1 mol - 1 .