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

Abstract

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.

Keywords:

• Aggregate transitions
• DDAB
• NaDHC
• Regular Solution Theory
• Vesicles

Acknowledgments

The authors acknowledge the financial support from the Universidad Nacional del Sur, Agencia Nacional de Promoción Científica y Tecnológica (ANPCyT) and Concejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina (CONICET). PM is an adjunct researcher of (CONICET). M.D. Fernández-Leyes and C. Luengo have a fellowship from the CONICET. The authors want to especially thank Dr. Marcelo Avena (Departamento de Química, Universidad Nacional del Sur, INQUISUR-CONICET. 8000 Bahía Blanca, Argentina) for his collaboration.