Charged colloidal particles and small mobile ions near the oil-water interface: destruction of colloidal double layer and ionic charge separation.

We study suspensions of hydrophobic charged colloids in a demixed oil-water solvent with salt by means of a modified Poisson-Boltzmann theory, taking into account image-charge effects and partitioning of the monovalent ions. We find that the ion's aversion for oil can deform the double layers of the oil-dispersed colloids, which qualitatively affects the colloidal density profiles. The same theory also predicts crystallization of colloid-free micron-sized water-in-oil droplets at water volume fractions as small as approximately 10(-3) in a narrow range of the oil-dielectric constant. These findings explain recent observations by M. E. Leunissen et al. [Proc. Natl. Acad. Sci. U.S.A. 104, 2585 (2007)10.1073/pnas.0610589104].