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

Molar volume and conductivity measurements have been carried out at 298.15 K for cetylpydinium chloride (CPC) + H₂O and CPC + β-cyclodextrin (β-CD) + H₂O systems. The apparent critical micelle concentrations, the dissociation degree of the micelle, the hydrophobic contribution of the transfer free energy for the hydrocarbon chain of CPC, the standard partial molar volumes of CPC in aqueous β-CD solutions and the stoichiometry for the inclusion complex of CPC with β-CD have been determined. The influence of β-CD and its complex on the micellization processes of CPC is analyzed. It is shown that β-CD partly screened the hydrophobic hydrocarbon chain of CPC molecules from contact with the surrounding medium, and retarded the formation of CPC micelles in a certain extent. The thermodynamic activity of CPC is decreased due to the formation of complex. The β-CD and its complexes do not participate the formation of micelles of CPC, and the complex has no effect on the micelle properties once the micelles are formed. The number of CH₂ groups entered the cavity of β-CD was calculated based on a simple model proposed by Gonzalez-Gaitano et al. The result suggests that β-CD forms strong complex with CPC, and the stoichiometry is found to be 1 : 1 at very low concentration of β-CD and 1 : 1 and 2 : 1 complexes coexist at higher concentrations of β-CD. This supports our conductivity results.