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

Abstract

Aqueous solutions of β-cyclodextrin (β-CD) or 2,6-di-o-methyl-β-cyclodextrin (DM-β-CD) and dodecylethyldimethylammonium bromide (D12EDMAB) 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 theu measurements: I) as a function of [D12EDMAB] in the presence of several initial cyclodextrin concentrations (β-CD or.DM-β-CD); II) as a function of [cyclodextrin] (β-CD or DM-β-CD), for an initial micellar solution of D12EDMAB and; III) as a function of the [cyclodextrin]/[surfactant] stoichiometric concentrations. Both inclusion complexes formed (β-CD∶D12EDMAB) and (DM-β-CD∶D12EDMAB) have stoichiometries of 1∶1, and their association constantK 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 D12EDMAB is found to increase linearly upon the addition of cyclodextrin (β-CD or DM-β-CD). The free surfactant concentration in the micellar region, [D12EDMAB]f, decreases in the presence of β-CD and slightly increases in the presence of DM-β-CD. The influence of the parcial methylation of the β-cyclodextrin (β-CD⇔DM-β-CD) and of the polar head of the surfactant (D12TAB ⇔D12EDMAB) on the complextion and micellar parameters are also discussed.