Effect of β-Cyclodextrin on the Micellar Behavior of Cetyltrimethylammonium Chloride in Aqueous Solution

Molar volume and conductivity measurements have been carried out at 298.15 K for cetyltrimethylammonium chloride (CTAC) + H₂O and CTAC + β-cyclodextrin (β-CD) + H₂O systems. The apparent critical micelle concentrations (cmc), the dissociation degree of the micelle (β), the transfer free energy for the hydrocarbon chain of CTAC (ΔG⁰_HP), the standard partial molar volumes of CTAC (V⁰_Φ,S), the stoichoimetry and the association constants for the inclusion complex of CTAC with β-CD have been determined. The influence of β-CD and its complex on the micellization processes of CTAC is analyzed. It is shown that β-CD partly screened the hydrophobic hydrocarbon chain of CTAC molecules from contact with the surrounding medium, and retarded the formation of CTAC micelles in a certain extent. The thermodynamic activity of CTAC is decreased due to the formation of complex. The β-CD and its complexes do not participate the formation of micelles of CTAC, and the complex have no effect on the micelle properties once the micelles are formed. Based on a simple model, the number of CH₂ groups entered the cavity of β-CD was calculated. The result suggests that, β-CD forms strong complex with CTAC; the stoichoimetry is found to be 1:1 complexes coexist at very low concentration of β-CD and 2:1 at higher concentrations of β-CD.