Ultrasonic study of the molecular encapsulation and the micellization processes of dodecylethyldimethylammonium bromide-water solutions in the presence of beta-cyclodextrin or 2,6-di-o-methyl-beta-cyclodextrin

Aqueous solutions of beta-cyclodextrin (beta-CD) or 2,6-di-o-merhyl-beta-cyclodextrin (DM-beta-CD) and dodecylethyldimethylammonium bromide (D(12)EDMAB) have been studied from speed of sound (u) data at 298.15 K, using a pulse-echo-overlap technique. The molecular encapsulation process of the surfactant monomer into the cyclodextrin cavity and its effect in the micellization process of the surfactant have been analyzed from the u measurements: I) as a function of [D(12)EDMAB] in the presence of several initial cyclodextrin concentrations (beta-CD or DM-beta-CD); II) as a function of [cyclodextrin] (beta-CD or DM-beta-CD), for an initial micellar solution of D(12)EDMAB and; III) as a function of the [cyclodextrin]/[surfactant] stoichiometric concentrations. Both inclusion complexes formed (beta-CD:D(12)EDMAB) and (DM-beta-CD:D(12)EDMAB) have stoichiometries of 1:1, and their association constant K have been determined using a model proposed in this work, based on the additivity of the different contributions of the involved species to the speed of sound. The apparent critical micellar concentration, cmc*, of D(12)EDMAB is found to increase linearly upon the addition of cyclodextrin (beta-CD or DM-beta-CD). The free surfactant concentration in the micellar region, [D(12)EDMAB](f) decreases in the presence of beta-CD and slightly increases in the presence of DM-beta-CD. The influence of the parcial methylation of the beta-cyclodextrin (beta-CD double left right arrow DM-beta-CD) and of the polar head of the surfactant (D(12)TAB double left right arrow D(12)EDMAB) on the complextion and micellar parameters are also discussed.