Dielectric properties and structure of oil-continuous microemulsions.

摘要

Many previous studies of the dielectric behavior of microemulsions have involved sophisticated experimental techniques and very reliable data have been obtained. But while the effects of temperature, salinity, hydrocarbon media, and composition have been studied previously, the studies are not systematic and have generally been done without serious consideration being given to the influences of these variables on microemulsion morphology. Very little agreement can be found in the literature about the proper interpretation of experimental observations, and no model has been advanced which successfully explains both low-frequency dielectric constants and critical frequencies of dielectric relaxation.;It is proposed that the permittivity behavior of microemulsions has been misinterpreted in the past because the models invoked do not correctly describe microemulsion morphology. Results from dielectric experiments on mercury-in-oil suspensions (Guillien, 1941) suggest strongly that the model of water droplets dispersed in an oil continuum which has prevailed must be modified. It is shown that micelle radii might be deduced from dielectric relaxation results, but not from the large dielectric relaxations observed below ;A systematic experimental study is reported which examines the influence of formulation variables on the permittivity behavior of AOT/water/alkane, DDAB/water/octane, and nonionic surfactant/water/alkane microemulsions. Dielectric constants of the microemulsions are reported as functions of composition, alkane carbon number, temperature, and electrolyte concentration. The data illustrate the basic trends in the dielectric properties and demonstrate the influence of the underlying phase behavior on macroscopic observations.;A model is proposed which explains in detail the permittivity behavior of these oil-continuous microemulsions. The model is based on considerations of the thermodynamics of phase behavior and microemulsion morphology. It is proposed that water-continuous domains coexist with oil-continuous domains even at low water concentrations and low electrical conductivities. The dielectric constant is sensitive to the fraction of each type of domain present in a system, and the critical frequencies of dielectric relaxation observed in the 1-100 MHz range characterize the sizes of the water-continuous domains.