Surface Polarization Effects on Ion-Containing Emulsions

Surface polarization in ion-containing heterogeneous dielectric media such as cell media and emulsions is determined by and determines the positions of the ions. We compute the surface polarization self-consistently as the ions move and analyze their effects on the interactions between electro-neutral, ion-containing droplets using coarse-grained molecular dynamics simulations based on the true energy functional. For water droplets immersed in oil, the interdroplet interaction is attractive, and the surface polarization makes the major contribution. By contrast, for oil droplets in water, the ion-surface induced charge interaction is repulsive and counteracts the attraction between the ions, leading to a small attractive interaction between the droplets. This research improves our understanding of self-assembly in mixed phases such as metal extraction for recovering rare earth elements and nuclear waste as well as water purification.

Figure 1

Ion-ion (blue circles, fit in blue dotted lines), ion-induced charge (red triangles, fit in red dashed lines) and total (black squares, fit in solid black lines) interdroplet interactions as a function of interdroplet distance (a) between water droplets immersed in oil and (b) between oil droplets immersed in water. Interdroplet interaction with uniform water permittivity [green crosses and fit in green solid lines in (a)] and with uniform oil permittivity [green pluses and fit in solid lines in (b)] is shown. Insets are snapshots of the simulations. Red means positive charges, and blue means negative charges. Videos are available in Supplemental Material [13].

Figure 2

(a) The average magnitude of dipole moment, (b) the orientation factor between dipoles, and (c) the distance between the center of dipoles formed by ions in each droplet for water droplets in oil (black squares), oil droplets in water (red circles), uniform water permittivity (blue triangles), and uniform oil permittivity (green diamonds).