Kinetics and mechanism of biphasic transfer of salted water from an aqueous phase to AOT reverse micelles

The transfer of salted water from an external aqueous phase to AOT reverse micelles is studied in relation with the change of properties from a percolating to a non percolating state of the micelles. A membrane-separated two-compartment cell was used to measure the transfer of salt which was followed by conductivity measurements. We expected a sharp conductivity drop which could have been used as a kind of sensor to detect a threshold of salinity. In fact, this sharp drop was not observed and this is shown to be due to the fact that no excess water is incorporated in the microemulsion phase in these conditions. For this purpose separate analysis of the water and salt incorporated in the microemulsion phase versus time has been performed. Under the conditions used the increase of the salt content is accompanied by a decrease of the water content, even though the initial system was far from being saturated with water. The results are discussed in terms of the different possible mechanisms. Molecular diffusion of water appears to be unlikely and its expulsion can be simply explained by the shrinkage of the microdroplets merging with the interfacial film, this being associated with the electrostatic shielding of the polar head repulsions.