Mobilities in cubic liquid crystals (“ringing gels”) determined by NMR self-diffusion measurements

Abstract

Diffusion coefficients in the ternary surfactant system consisting of surfactant (tetradecyldimethylaminoxide, C 14DMAO), hydrocarbon (C 6H 12), and water have been determined by the pulsed field gradient spin-echo NMR method. Concentrations were varied along two sections of the ternary phase diagram. The transition from a micellar phase (L 1) that consists of spherical micelles into a cubic liquid crystalline phase (“ringing gel”) is studied as the volume fraction of the micelles is increased at constant hydrocarbon surfactant ratio. The mobility of the solvent, water, is only moderately decreased for this series of samples. In contrast, the diffusion coefficient of the hydrocarbon is decreased by two orders of magnitude, and the one of the surfactant by three orders of magnitude, when passing from the micellar phase into the cubic phase. In the micellar phase, the observed diffusive motion ( D ∼ 1–3 × 10 −11m 2 s −1) is due to transport of the micelles. This conclusion is reached from the fact that the diffusion coefficients of surfactant and hydrocarbon are equal, and approximately two orders of magnitude lower than that of the neat hydrocarbon liquid. In agreement with expectations, a linear decrease of the diffusion coefficient with the volume fraction of the micelles is observed. In contrast, the mobility of the micelles is strongly reduced in the cubic liquid crystal. The dynamics observed in the latter phase is due to two processes. Hopping of hydrocarbon molecules between neighboring micelles is characterized by a residence time of ∼100 μs, and diffusion coefficients of 3–5 × 10 −13 m 2 s −1. The exchange of surfactant molecules between the micelles occurs on a slower time scale (∼1 ms), with diffusion coefficients on the order of 3 × 10 −14 m 2 s −1.