Structure of microemulsions studied by NMR

We discuss the application of NMR to investigate microemulsion structure. There are mainly two important NMR experiments; relaxation and self-diffusion studies. Of these two, self-diffusion studies have been found to be particularly informative in that it is the only technique which can demonstrate bicontinuity in equilibrium microemulsions. Self-diffusion coefficients of water, oil and surfactant can be obtained in the same experiment. For droplet microemulsions, the self-diffusion coefficients of the surfactant and enclosed solvent correspond to the droplet diffusion coefficient, which in turn depends on its hydrodynamic radius and on interactions. The self-diffusion coefficient of the continuous solvent is slightly reduced due to obstruction effects, which depends on the droplet shape. For bicontinuous structures, the solvent diffusion on both sides of the surfactant film as well as the surfactant diffusion within the film is monitored. In particular the diffusion of the two solvents depends on how the surfactant film curves. NMR relaxation is found to be useful in the studies of droplet microemulsions. It is very sensitive to the droplet size while at the same time being insensitive to interactions. The latter allows for reliable size information even at higher droplet concentrations.