Ion Binding in Liquid Crystals Studied by NMR.: I. The Cetyltrimethylammonium Bromide/Hexanol/Water System

Abstract

The nuclear magnetic relaxation of ⁸¹Br in the liquid crystalline phases in the ternary system cetyltrimethylammonium bromide (CTAB)/hexanol/water was investigated. The dependence of the nuclear magnetic rasonance line width on the composition of the samples is discussed. In the lamellar mesophase the ⁸¹Br NMR line width decreases when either the hexanolor the water concentration is increased. This is interpreted as a release of counter-ions from the charged surfaces by hexanol, and as a reinforced hydration of the bromide ions, respectively. Below a molar ratio of about one between hexanol and CTAB the binding of the bromide ions is strongly dependent on this ratio. A comparison is made between the line width in the mesomorphous phases with lamellar and hexagonal structure and the line width in the proximate isotropic solution phases. It appears that the binding of the ions is stronger in the hexanol-deficient part of the lamellar mesophase than in the isotropic water-rich solution phase. In the hexanol-rich part of the lamellar mesophase with high water content the line widths are the same as in the isotropic hexanol-rich solution phase with high water concer tration. The binding of the ions in the hexagonal mesophase is similar to that in concentrated aqueous solutions of CTAB.