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 degrees 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 kJg(-1) mol(-1), and that for DTAB micelles is -2.1 kJ gl 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 gl mol-l, 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).