Characterization of the cyclodextrin-surfactant interactions by volume and enthalpy

Volume and enthalpy of transfer of hydroxypropyl-alpha-cyclodextrin (HP-alpha-CD) and hydroxypropyl-gamma-cyclodextrin (HP-gamma-CD) from water to the aqueous solutions of sodium alkanoates (sodium hexanoate, sodium decanoate and sodium dodecanoate) were determined at 298 K. The cyclodextrin concentration was kept constant, and that of the surfactant was varied in order to analyze both the pre- and postmicellar regions. The experimental data in the premicellar region were consistent with the formation of 1: 1 and 1: 2 (1 cyclodextrin: 2 surfactants) inclusion complexes, with the exception of the HP-alpha-CD/sodium dodecanoate system which presented only the 1: 1 complexes. The mechanism of the 1: 2 complexes formation of HP-alpha-CD/surfactant is different from that involving HP-gamma-CD. The quantitative analysis of the experimental data in the post-micellar region supplied parameters indicating that the cyclodextrin-micelles forces are ion-dipole (carboxylate head/hydroxylic group) in nature. The present results combined with the literature ones clarify the effect of the cavity size of the cyclodextrin as well as the hydrophobicity of the surfactant on the cyclodextrin-dispersed surfactant and cyclodextrin-micelle interactions.