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.