Hydrodynamic stability of monolayers at fluid-fluid interfaces. I. Ion interactions

We develop a linear hydrodynamic analysis in order to study the mechanical stability of a charged monolayer at the interface between two immiscible fluids (aqueous and nonaqueous phases). For highly concentrated solutions and high charge densities, the stability depends on the discrete electrical interactions among the ions in the monolayer. The method of images is used to take into account the dielectric discontinuity between the compact layer and the solution. The secular equation shows that the electrical terms give a negative destabilizing contribution to the surface tension and introduce a new coupling between normal and longitudinal displacement modes, due to the image charges. When the solution is considered as a perfect conductor, all the counterions are located at the plane of closest approach (OHP) and the coupling term is always destabilizing. The system remains unstable even for realistic values of viscosities in the bulk phases. When there is only a fraction of counterions at the OHP, the other fraction being distributed in a diffuse layer, the image charges are screened. However, this effect does not stabilize the system, if the OHP remains an equipotential. In this model no exchange of ions between the compact layer and the solution is allowed and the surface charge density at the compact layer is overestimated. This leads to an unstable nonequilibrium state. © 1979.SCOPUS: ar.j