Vesicle-to-micelle transition in dioctadecyldimethylammonium bromide and dioctadecyldimethylammonium chloride dispersions induced by octaethylene glycol n-dodecyl monoether. An isothermal titration calorimetry study

Abstract

We have used isothermal titration calorimetry to investigate the vesicle-to-micelle transition in dioctadecyldimethylammonium bromide (DODAB) and chloride (DODAC) vesicle dispersions induced by the nonionic surfactant octaethylene glycol n-dodecyl monoether (C₁₂E₈) at room temperature. Small and giant unilamellar vesicles were prepared by sonication and without sonication, respectively, of the pure cationic surfactants at low concentrations in water. The titration of 1.0 mM DODAX (X=Cl⁻ and Br⁻) by a concentrated micellar solution of C₁₂E₈ shows that the enthalpy of interaction (ΔH _obs) of C₁₂E₈ in micellar form with DODAX is always endothermic. The titration curves are understood on the basis of superposition of the enthalpies of partitioning of C₁₂E₈ into the bilayer, of micelle formation and of vesicle-to-micelle transformation. The enthalpy, ΔH _ods, initially in creases owing to the incorporation of C₁₂E₈ into the vesicle bilayer until the C₁₂E₈/DODAX saturation ratio (R _sat) is reached, then ΔH _obs decreases, in different ways for DODAB and DODAC, owing to degradation of vesicles and formation of mixed micelles and intermediary structures up to the C₁₂E₈/DODAX solubilization ratio, R_sol. Above R _sol only mixed micelles exist. The surfactant solubilization takes place in three stages. All the critical ratios are lower for DODAB than for DODAC, meaning that C₁₂E₈ solubilizes more strongly in DODAB; for example, R _sat is 0.8 for DODAB and 1.2 for DODAC. Sonication has no significant effect on the transition.

This work is part of the results presented at the First Nordic-Baltic Meeting on Surface and Colloid Science, in Vilnius, Lithuania, from 21-25 August 1999