Special effect of beta cyclodextrin on the aggregation behavior of mixed cationic/anionic surfactant systems.
Controllable aggregate transitions are achieved in this work by adding due amounts of beta cyclodextrin (beta CD) to mixed cationic/anionic surfactant aqueous solutions. In contrast to its "aggregate breaking" effect in single surfactant systems, aggregate growth is observed in nonstoichiometrical mixed cationic/anionic surfactant systems upon addition of beta CD. The aggregate growth typically undergoes a micellar elongation and a following micelle to vesicle transition, which in turn greatly influences the viscosity and absorbance of the solutions. A possible mechanism of this beta CD induced aggregate growth is proposed. In mixed cationic/anionic surfactant systems, the surfactants strongly tend to reach electroneutral equilibrium in aggregates. In the present case, added beta CD is found to greatly facilitate the equilibrium by transferring the "major" component (whose molar fraction>0.5) of a cationic/anionic surfactant mixture from the aggregates to beta CD cavities. Consequently, the surfactants in the aggregates approach electroneutral mixing, in favor of low curved aggregates such as vesicles. This work shows that beta CD provides an additional degree of freedom to control microstructures and macroproperties for the whole class of mixed cationic/anionic surfactant systems.
The journal of physical chemistry. B