Investigation of the micelle-to-vesicle transition in mixtures of an anionic and a cationic surfactant: the effect of adding salt

Abstract: Catanoinic systems spontaneously form micelles and vesicles, which are self-assembled spherical structures made up by surfactants. In the core of the micelle a drug, or other organic substance, can be kept to stabilize it when placed in an aqueous environment. The micelle-to-vesicle transition corresponds to the moment when the drug is releases, and understanding which factors that trigger this transition is thus of great interest for the pharmaceutical industry. In this study the micelle-to-vesicle transition in water and the effect of salt were studied for the systems 95 mol% SDS/DDAB and 95 mol% SDeS/DDAB with different total concentrations. The static light scattering measurements showed that the micelle-to-vesicle transition for the system 95 mol% SDS/DDAB was shifted to lower total concentrations both when 50 mM NaBr and 100 mM NaBr were added, and that the transition was unaffected by changing the anionic surfactant from SDS to SDeS when no salt had been added. A phase separation was observed when 50 mM NaBr was added to 95 mol% SDeS/DDAB (the Krafft point was probably reached), and when 100 mM NaBr was added to the same system the sample remained opaque one week after mixing the sample and also after heating it to 40°C in a water bath. The curve for sample 95 mol% SDS/DDAB 1/8192 mM + 100 mM NaBr was oscillating implying possible defects in the vesicle membrane. The cryo-TEM images confirmed the light scattering results and additionally showed that at higher total concentrations agglomeration occurred, while whenever salt was added less vesicles seemed to appear, while both discs and broken vesicles were present suggesting that the disc structure is preferred over the spherical structure when salt is present. Also a vesicle inside another vesicle was discovered for the sample 0.95 SDS/DDAB 3.75 mM + 50 mM NaBr. The mole fraction of anionic surfactant in the aggregates (x) was calculated using a MATLAB code based on the Poisson-Boltzmann theory. The results from the calculations showed that a higher amount of SOS was needed for the system 0.95 SOS/CTAB than the amount of SDS and SDeS needed for the systems 0.95 SDS/DDAB and 0.95 SDeS/DDAB when forming aggregates, indicating that a shorter chain of the anion and the higher spontaneous curvature of the cation leads to a higher curvature. Also a larger amount of cation was needed when the tail was single than when it was double in order to form stable spherical structures. Finally, as the total concentration decreased the x value also decreased in all cases, thus the spontaneous curvature was decreased.