Motions of water, decane, and bis(2-ethylhexyl) sulfosuccinate sodium saltin reverse micelle solutions

Reverse micellar solutions made from the surfactant bis(2-ethylhexyl)sulfos uccinate sodium salt (AOT), water, and decane have been studied using NMR i nversion-recovery and pulsed-field gradient experiments. Spin-lattice relax ation times, T-1, and diffusion coefficients, D, were measured for the indi vidual components of the reverse micellar phase of the water-AOT-decane mic roemulsion as a function of temperature, 277 < T < 313 K, and reverse micel le volume fraction, 0 < phi < 1. Activation energies for the decane rotatio nal motions probed by the T-1 measurements and those for the translational motions probed by the D measurements are compared as a function of phi. Bot h types of activation energies increase by a factor of similar to 2 as phi increases from zero (pure decane) to 0.8 (near the lamellar phase boundary) , indicating the influence of the AOT tails on the decane motions. A T-phi phase diagram has been constructed showing an empirical boundary for the on set of percolation. This percolation characterization, together with the me asured diffusion coefficients and relaxation times, is consistent with prev ious descriptions of the structure and dynamics of water-AOT-decane and sim ilar microemulsions reported in the literature. This study also continues t he evaluation of the water-AOT-decane microemulsion as the signal-bearing s olution in a magnetic resonance imaging phantom. The T-1 of the water phase matches that of nonfatty tissues in the body; however, the hydrocarbon com ponent will need to be adjusted to match its T-1 to that of adipose tissue. The percolation boundary in the T-phi phase diagram helps to define the T and phi values where the solution possesses the high dielectric constant ne eded to minimize a standing wave artifact in a magnetic resonance image of the phantom.