Changes in the fraction of uncomplexed cyclodextrin in equilibrium with the micellar system as a result of balance between micellization and cyclodextrin-surfactant complexation.: Cationic alkylammonium surfactants

A study was carried out on the basic hydrolysis of m-nitrophenylacetate in mixed systems made up of trimethylalkylammonium micelles and alpha- or beta -cyclodextrin (CD). The surfactants used enabled us to vary the length of the hydrocarbon chain between 8 and 18 carbon atoms. In all cases the existence of a significant concentration of uncomplexed CD was observable in equilibrium with the micellar system. Contrary to expectations, the concentration of uncomplexed CD increased together with the hydrocarbon chain of the surfactant. This result was interpreted as a consequence of the balance between the complexation processes of the surfactant monomers by the CD and its autoassociation to form micelles. We have found the existence of a linear dependence between the percentage of uncomplexed CD and the inverse of the critical micellar concentration of the surfactant, indicating that, as the tendency toward micellization increases, so too does the percentage of the uncomplexed CD. The results obtained indicate that the concentration of uncomplexed alpha -CD in equilibrium with the micellar system is always lower than that of beta -CD, which has been interpreted as a consequence of the greater tendency of the alpha -CD to form CD-surfactant complexes with a stoichiometry of 2:1. One of the main consequences of this study is that the critical micellar concentration of the mixed CD-surfactant systems cannot be used directly to obtain the stoichiometry of the CD-surfactant complexes, since the micellization occurs without the complexation ability of the CD having been saturated.