Inclusion complexation of surfactant with β-cyclodextrin and its effect on the mixed micellization of cationic/anionic surfactants

Abstract

Inclusion complexation of surfactant with β-cyclodextrin (β-CD) is an active research topic in supramolecular chemistry. For the mixed system of β-CD and cationic/anionic surfactants, the thermodynamic and molecular dynamics (MD) simulation studies can provide valuable information about synergism between cationic and anionic surfactants and competition between inclusion complexation and mixed micellization. Experimental results indicated a synergistic effect between CTAB and SL in surface tension reduction efficiency and effectiveness, and in mixed micelle formation. The surface tension experiments indicated that for single surfactant CTAB or SL, the binding stoichiometry was β-CD:surfactant = 1:1. For the 1:1 β-CD/surfactant inclusion complexes, umbrella sampling simulation results indicated that SL bound preferentially to the wide rim of β-CD, while, CTAB had a similar probability of binding to either rim of β-CD. Both experimental and simulation results indicated that the inclusion affinity of β-CD with CTAB was significantly stronger than that of β-CD with SL. Due to the electrostatic attraction between the oppositely charged headgroups, the distance between adjacent CTA+ and L− ions was significantly shorter than that between two adjacent CTA+ (or L−) ions. It encouraged us to explore whether the 1:1:1 β-CD/CTAB/SL inclusion complex could form. The MD simulation results indicated that all four possible initial 1:1:1 conformations were unstable. The final stable inclusion complexes were in 1:1 β-CD/CTAB and 1:1 β-CD/SL forms consistent with the umbrella sampling simulation results of the single surfactant. The results suggested that the inclusion interaction becomes less competitive with mixed micellization as the composition approaches the equimolar ratio of CTAB and SL.