THERMODYNAMIC PROPERTIES OF WATER BETA-CYCLODEXTRIN DODECYLSURFACTANT TERNARY-SYSTEMS

Densities, heat capacities and conductivities of water-surfactant-beta-cyclodextrin (beta-CD) ternary systems were determined at 25-degrees-C. The surfactants studied were sodium dodecylsulfate (NaDS) and dodecyltrimethyl-ammonium bromide (DTAB). From conductivity data, apparent critical micelle concentrations (cmc*) and degree of ionization of micelles were obtained at a fixed beta-CD concentration (m(CD)). From the cmc* value and that in water (cmc) the stoichiometry of the surfactant-beta-CD complex was calculated. At a given m(CD), the apparent molar volume V(phi,CD) and heat capacity C(phi,CD) of beta-CD in the two surfactants were calculated as functions of surfactant concentration m(S). For both NaDS and DTAB, V(phi,CD) increases with m(S) up to about the cmc beyond which it decreases to a constant value at high m(S); the opposite is observed for C(phi,CD). With NaDS, a jump in the C(phi,CD) vs. m(S) trend was detected and ascribed to a structural NaDS micellar transition. The apparent molar volume V(phiS) and heat capacity C(phiS) of NaDS and DTAB in the water-beta-CD mixture 0.017m were also obtained. From these properties and those in pure water, the volume DELTAV(S) and heat capacity DELTAC(S) of transfer of the surfactant from water to water+beta-CD mixture as functions of m(S) were calculated. For both surfactants, the DELTAV(S) vs. m(S) trends increase to the cmc and then decrease in a monotonic manner, whereas DELTAC(S) increases regularly with m(S) in the pre-micellar region and is essentially constant in the post-micellar region. The DELTAV(S) vs. m(S) trends were qualitatively explained in terms of dispersed, complexed and micellized surfactant contributions.