Transport of ions through the oil phase of W 1/O/W 2 double emulsions

Abstract

Using a capillary video microscopy technique, the ion transport at liquid–liquid interfaces and through a surfactant-containing emulsion liquid membrane was visually studied by preparing a double emulsion globule within the confined space of a thin-walled, transparent, cylindrical microtube. NaCl and AgNO 3 were selected as the model reactants and were prepared to form a NaCl/AgNO 3 pair across the oil film. By observing and measuring the formed AgCl deposition, it was found that both Cl − and Ag + could transport through a thick oil film and Ag + was transported faster than Cl −. Interestingly, the ion transport was significantly retarded when the oil film became extremely thin ( < 1 μm ). The results suggested that the transport of ions mainly depends on the “reverse micelle transport” mechanism, in which reverse micelles with entrapped ions and water molecules can be formed in a thick oil film and their construction will get impeded if the oil film becomes extremely thin, leading to different ion transport rates in these two cases. The direction of ion transport depends on the direction of the osmotic pressure gradient across the oil film and the ion transport is independent of the oil film thickness in the investigated thick range. Ions with smaller Pauling radii are more easily entrapped into the formed reverse micelles and therefore will be transported faster through the oil film than bigger ions. Oil-soluble surfactants facilitate ion transport; however, too much surfactant in the oil film will slow down the ion migration. In addition, this study showed no support for the “molecular diffusion” mechanism of ion transport through oils.