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

Reverse micellar solutions made from the surfactant bis(2-ethylhexyl)sulfosuccinate sodium salt (AOT), water, and decane have been studied using NMR inversion-recovery and pulsed-field gradient experiments. Spin-lattice relaxation times, T-1, and diffusion coefficients, D, were measured for the individual components of the reverse micellar phase of the water-AOT-decane microemulsion as a function of temperature, 277 < T < 313 K, and reverse micelle volume fraction, 0 < phi < 1. Activation energies for the decane rotational 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. Both 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 onset of percolation. This percolation characterization, together with the measured diffusion coefficients and relaxation times, is consistent with previous descriptions of the structure and dynamics of water-AOT-decane and similar microemulsions reported in the literature. This study also continues the evaluation of the water-AOT-decane microemulsion as the signal-bearing solution in a magnetic resonance imaging phantom. The T-1 of the water phase matches that of nonfatty tissues in the body; however, the hydrocarbon component 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 needed to minimize a standing wave artifact in a magnetic resonance image of the phantom.