Direct Evidence for the Energy of Weak Interactions between Surfactant Molecules Using High Sensitivity Isothermal Titration Calorimetry

Calorimetry is a direct experimental method that can be used to study the thermodynamics of weak interactions between surfactant molecules, allowing the energetic parameters of such interactions to be obtained. In this work, a nano-isothermal titration calorimeter with a thermostat (TAM III) was used to evaluate the thermodynamic behavior of molecular self-assemblies of single and mixed surfactants in aqueous solution. Electrical calibration of this instrument showed that its precision is better than +/- 0.09%. The accuracy of the system was tested by measuring the reaction heat of tris-hydroxymethyl-aminomethane (Tris), employed often as a calorimetric standard substance, with hydrochloric acid. The resulting value ((-47.48 +/- 0.12) kJ . mol(-1)) agreed well with that in the literature. We then determined the critical micelle concentration (cmc) and enthalpy of micellization for dodecyltrimethylammonium bromide (DTAB) with a "head-and-tail" structure, which were consistent with reported values, as well as reliable results for sodium cholate (NaC) with a rigid steroid skeleton composed of hydrophilic and hydrophobic surfaces. Furthermore, for the mixed system of oppositely charged surfactants (DTAB/NaC), the mixed cmc and enthalpy of mixed micellization were also obtained in NaC- and DTAB-rich regions. A stronger synergistic effect was observed between the two types of surfactants in the NaC- rich region than in the DTAB-rich one. Conductivity measurements allowed the thermodynamic behavior of the mixed system (DTAB/NaC) to be discussed in detail.