Amphiphilic activities of anionic sodium cholate (NaC), zwitterionic 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS) and their mixtures: a comparative study.

Anionic biological surfactant sodium cholate (NaC) and the zwitterionic nontoxic surfactant CHAPS (3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate) having common steroidal skeleton have not been studied in a comparative level to explore their amphiphilic behaviors separately and in mixed states. Their adsorption at the interface in relation to self-association in solution has been examined in both pure and mixed states with reference to detailed thermodynamic analysis of their surface chemical and solution behaviors. Their head group differences are responsible for the differences in their properties viz. critical micelle concentration (CMC), aggregation number (N), hydrodynamic diameter, Gibbs surface excess (Γmax), minimum head group area (Amin), mixed micelle composition and their mixing ideality/nonideality. Although the CMC values found from tensiometry and isothermal titration calorimetry (ITC) are in good agreement, the standard enthalpy of micellization (ΔHm(0)) found from ITC at different temperatures are different from that evaluated by the van't Hoff method. This is because van't Hoff rationale is a differential method which estimates the enthalpy of the monomer ↔ micelle equilibrium process whereas calorimetry is an integral process that evaluates all probable/possible processes in addition to monomer to micelle conversion. Results are analyzed, compared and discussed in the light of physicochemical parameters of pure NaC and CHAPS and their mixtures. These fundamental findings are considered prospective in pharmaceutical, biochemical and biomedical applications of the studied bio-friendly amphiphiles.