Diffusion in cationic vesicle and micellar solutions- a NMR study

Doctoral thesis, 2006

Double tailed cationic surfactants are the active component in softeners. In the beginning of the 90’s the active component in softener formulations was changed from a hydrolytically stable dialkyl dimethyl ammonium chloride (dialkyl quat) to a cleavable and biodegradable product containing ester groups in the alkyl chains (dialkyl ester quat). During the formulation process the softener surfactants will form vesicles with a large variety in size and number of bilayers in the membrane. The major part of this thesis deals with investigation, by NMR diffusometry, of the permeability of water across the vesicle membranes as a function of the molecular structure of the surfactant. It was shown that the permeability through the membranes could be controlled by changes in the molecular structure and the temperature of the system. Below the transition temperature it was found that a hydrogenated dialkyl ester quat did not allow water to permeate the membranes during the time span for NMR diffusometry experiments. The concentration of water in the bilayers was calculated for the hydrogenated dialkyl ester quat as a function of the temperature indicating that the mechanism of the water transport is changed at the transition temperature from the solubility-diffusion mechanism to a combination of the solubility-diffusion and transport through pores and dislocations in the membrane. Interactions between benzyl benzoate and dialkyl and dialkyl ester quats have also been studied by NOESY-NMR showing that benzyl benzoate is both incorporated into the hydrophobic layer of the aggregates and interacting with the quaternary head group by a π-cation interaction. A minor part of this thesis concerns the micellar kinetics of gemini surfactants. It has been shown that the life time of the micelles increases as the length of the spacer decreases. It has also been shown that the micelles grow in one direction as prolates with the largest growth for the gemini with the shortest spacer.