Dynamics in microemulsions with nonionic surfactant. A combined NMR relaxation and self-diffusion study

Abstract

Data on molecular self-diffusion and 2H-NMR relaxation, on deuterons specifically attached to the surfactant molecule, are presented for two ternary microemulsion systems with nonionic surfactant, covering the whole range of water to oil ratios. In one of the systems, the hydrophobic solvent was made up by a 1:1 by weight mixture of cyclohexane and hexadecane. By comparing the diffusion coefficients of the two hydrocarbons, conclusions are drawn concerning the dominating diffusion process of the hydrocarbon molecules. A marked temperature dependence of the self-diffusion properties, as well as in the relaxation parameters, is observed. Of particular interest are the dramatic changes occuring at low oil content within a narrow temperature interval. There, at lower temperatures, swollen micelles are identified that grow in size with increasing temperature. At higher temperatures, data suggest that rapid coalescence and breakup of aggregates provide a mechanism for the macroscopic transport of oil molecules, resulting in a marked increase of the hydrocarbon diffusion coefficient.