Aggregate Structural Transitions Noticed for the Didodecyldimetilammonium Bromide-Sodium Dehydrocholate Catanionic Mixed System at Low Concentration

The phase behavior for the very diluted DDAB-NaDHC-water system was investigated at 25°C. Fluorescence, conductivity, and surface tension experiments were carried out to determine the cac values. Comparison between experimental and calculated values shows the suitability of the regular theory to predict the critical aggregation concentrations for the tested systems. The interaction between both amphiphiles inside aggregates is not ideal, showing a large synergism. Such fact is reflected by the activity of coefficients values. The aggregates are mainly comprised by DDAB (18–30 % mol NaDHC) regardless of the NaDHC solution molar fraction; these molecules act as a good solvent of bile salt type molecule. Nevertheless, the gradual inclusion of NaDHC molecules inside DDAB bilayers leads to structural transformations. These facts are supposed to be due the combination of two effects: (i) the reduction of head groups hydration and (ii) the increment of chain repulsion.