The Shape and Properties of Mixed Micelles in Surfactant Solutions of Tetradecyldimethylaminoxide and Tetradecyltrimethylammoniumbromide in the L1 Phase: The Influence of Ionic Charge Density and Cosurfactant Concentration

Abstract

Aqueous surfactant solutions of tetradecyldimethylaminoxide (TDMAO) and tetradecyltrimethylammoniumbromide (TTMAB) in the presence of increasing amounts of hexanol were studied in the L 1-phase region. TDMAO forms rod-like and TTMAB globular micelles in the concentration range between 50 and 100 m M. In all mixtures with a mol fraction X c > 0.1 of cationic surfactant globular micelles are found. Hexanol was added to the surfactant mixtures until the phase boundary of the L 1 phase was reached. The micellar structures and the properties of the surfactant solutions were studied with surface and interfacial tension, electric birefringence, viscosity, and conductivity measurements. A sphere-rod transition was observed in all surfactant mixtures with increasing hexanol concentration. In the pure cationic system globular micelles were observed with increasing mol fraction of alcohol X a, up to the phase boundary. Electric birefringence measurements turned out to be most sensitive to the sphere-rod transition. The hexanol concentration which was necessary to reach the sphere-rod transition increased continuously with X c . The concentration of hexanol to reach the phase boundary increased also. The viscosities, the rotational time constants, and the Kerr constants of the mixed surfactant solutions increased exponentially with the hexanol concentration from the sphere-rod transition to the phase boundary. The viscosity which was reached close to the phase boundary decreased with increasing mol fraction of the cationic surfactant. The lengths of the rods at the phase boundary which were determined from the electric birefringence measurements decreased with increasing X c. The persistence length for all charged and unshielded rods seems to be longer than their total length and the rods under these conditions can be visualized as being stiff. The surface and the interfacial tension for all micellar solutions decreased in a similar fashion with increasing X a. Independently of X c the interfacial tension of the micellar solution measured against decane, reached a value of about 0.2 mN/m at the phase boundary. The surface tension was around 25 mN/m and also more or less independent on the surfactant ratio.