Dielectric relaxation spectroscopy of ionic micelles and microemulsions

摘要

Microemulsions represent systems consisting of water, oil and amphiphiles(s). They are single-phase and thermodynamically stable isotropic solutions that find numerous commercial applications with high economic impact. The term microemulsion encompasses diverse structural organizations that are water or oil droplets (spherical or elongated) dispersed in either oil (namely water-in-oil or W/O) or water (oil-in-water or O/W) with sizes lower than 0.1 mm. With equivalent volumes of oil and water, bicontinuous microemulsions may exist. Dielectric relaxation spectroscopy (DRS) is known to be sensitive to all kinds of dipole moments fluctuations in the pico- and nanosecond time range which result from the reorientation of water molecules and ion pairs and from the motions of free and bound counterions surrounding the charged micelles. The present work deals with a DRS study of sodium dodecyl sulfate (SDS) micelles in water, SDS/1-pentanol mixed micelles, SDS/1-pentanol bicontinuous structures, SDS/1-pentanol reverse mixed micelles, and W/O microemulsions produced by a final addition of n-dodecane. During this work, the different structural transitions were evidenced by conductivity measurements. DRS relaxation processes related to free (bulk) and bound water (high frequency relaxation processes), and to free and bound counterions (low-frequency relaxation processes) were found to undergo strong changes during those structural transitions (for example during percolation in microemulsions). However, it could be argued that the DRS relaxation processes of ionic micelles are of the same nature as those found for ionic W/O emulsions. Therefore DRS may be regarded as a valuable technique for microemulsions investigation.