Solubilization of a water-insoluble dye in aqueous solutions of dodecyltrimethylammonium halides, and its relation to micelle size and shape

Abstract

The solubilization properties of aqueous sodium halide solutions of dodecyltrimethylammonium halides have been measured for a water-insoluble dye, Sudan Red B, and they are related to the micelle size and shape, by referring to the molecular weight data already reported. The solubilization power of micellar solutions of dodecyltrimethylammonium chloride toward Sudan Red B is 2.8 × 10 −3 mole of dye per mole of surfactant, independent of the NaCl concentration up to 4.0 M, where only spherical micelles are formed. The solubilization power of micellar solutions of dodecyltrimethylammonium bromide slightly increases with increasing NaBr concentration and abruptly increases from 2.9 × 10 −3 to 3.9 × 10 −3 mole of dye per mole of surfactant, at 1.0–1.5 M NaBr, corresponding to the sphere-rod transition. The solubilization power of dodecyltrimethylammonium bromide is higher in rodlike micelles than in spherical micelles. The solubilization capacity of a spherical micelle composed of dodecyltrimethylammonium ions is nearly independent of counterion species, either chloride or bromide ion. The solubilization capacities of both spherical and rodlike micelles increase with their size almost linearly. From the density measurements of aqueous solutions of dodecyltrimethylammonium halides, the partial molar volume of surfactants is derived, and it is found that dodecyltrimethylammonium ions have an equal contribution to the partial molar volume of surfactant. The packing state of dodecyltrimethylammonium ions in micelles is common for both surfactants, and also for spherical and rodlike micelles.