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
B. Dorrego et al., 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, J PHYS CH B, 105(21), 2001, pp. 4912-4920
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 exist
ence of a significant concentration of uncomplexed CD was observable in equ
ilibrium with the micellar system. Contrary to expectations, the concentrat
ion of uncomplexed CD increased together with the hydrocarbon chain of the
surfactant. This result was interpreted as a consequence of the balance bet
ween the complexation processes of the surfactant monomers by the CD and it
s 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 un
complexed 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 st
oichiometry of 2:1. One of the main consequences of this study is that the
critical micellar concentration of the mixed CD-surfactant systems cannot b
e 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.