6533b7d8fe1ef96bd126b5f2

RESEARCH PRODUCT

Thermodynamic properties of water-β-cyclodextrin-dodecylsurfactant ternary systems

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