Effect of β-Cyclodextrin on the Micellization of Cetyltrimethylammonium in Aqueous Solution at High Temperature

Abstract

Molar volume and conductivity measurements have been carried out at 338.2 K for cetyltrimethylammonium chloride (CTAC) + H₂O and CTAC + β-cyclodextrin (β-CD) + H₂O systems. The apparent critical micelle concentrations, the dissociation degree of the micelle, the transfer free energy for the hydrocarbon chain of CTAC, the standard partial molar volumes of CTAC in aqueous β-CD solution and the stoichoimetry 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 are analyzed under this temperature. 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. 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, and the stoichoimetry is found to be 2:1. This supports our conductivity results.

Keywords:

• β-Cyclodextrin
• cetyltrimethylammonium
• conductivity
• micellization
• partial molar volume
• stoichoimetry

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (Grant No. 20703023), the Natural Science Foundation of Jiangxi Province (Grant No. 2007GQH2100), and the Science Research Program of the Education Department in Jiangxi Province (Grant No. [2007]14).