Vesicle phases from N-methyl-N-alkanoylglucamin and various co-surfactants

Abstract

Abstract We have studied the phase behavior of N -methyl- N -alkanoylglucamin (GA) in water and the influence of various additives on the phases of GA. We find that GA forms a large L 1 -phase that extends up to 60 wt.% of surfactant. The viscosity in this phase increases with increasing concentration and decreasing temperature. When solutions are cooled down below the Krafft-temperature (21–28 °C) T k the samples transform into clear gels that are stable for several months. The transformation of the gel to the L 1 -phase proceeds in two separate steps. Acid–base titration experiments show that the commercially produced GA is not a pure well defined compound but contains about 10% of an anionic surfactant, most likely dodecanoic acid. Solutions of GA can be continuously mixed with the anionic surfactant sodiumdodecylsulfate (SDS) or sodiumdodecylethoxysulfate (SDES) to clear and low viscous phases. Solutions of GA mixed with increasing amount of cationic surfactant tetradecyltrimethylamoniumbromide (TTABr) transform first into two phase systems and then again into low viscous single phases. The influence of several chemically different co-surfactants like n -alcohols, octanoic acid, oleic acid, 2-ethylhexylmonoglyceride (EHMG) and oleylmonoglyceride on the phase behavior of phases with 5% GA has been studied. With increasing mole fraction of the co-surfactants the well-known sequence of phases is observed, namely a L 1 -phase, a two phase region L 1 /L α - and a L α -phase. However, the properties of the L α -phase for the different systems are very different. For samples with octanol the L α -phase is an optically isotropic, transparent, highly viscoelastic gel. Without shear the gel like phase shows no birefringence. FF-TEM micrographs show that it consists of rather monodisperse, unilamellar vesicles with a diameter of about 500 A. The L α -phases for the other co-surfactants are turbid and have a rather low viscosity. They also contain vesicles but with a rather broad size distribution ranging from 200 to 1000 A. For the same co-surfactant/surfactant ratio the various systems differ also in their conductivity. For some systems the conductivity is only about 20% lower than in the corresponding L 1 -phase while in other systems the difference is more than a factor two. These results are an indication that small uni-lamellar vesicles contain more ionic groups at the outside than on the inside of the bilayer and that some systems are composed of uni-lamellar vesicles while others are composed of multilamellar vesicles (onions).