Electrostatic stabilization in non-aqueous media

This paper discusses electrostatic stabilization of dispersions in non-aqueous media. It begins with the theory of repulsion, with particular attention to the roles of the dielectric constant and ionic strength of the liquid medium. Results for flat plates and spheres are compared. Methods are reviewed to measure the dielectric and electric parameters in non-aqueous media, in conjunction with procedures to determine the mechanical properties of electrostatically stabilized, concentrated suspensions. From theoretical considerations it appears that the extent of the electrostatic stabilization in non-aqueous media is extremely sensitive to the dielectric constant ϵ of the liquid, affecting stability in particular through the degree of dissociation of the stabilizing electrolyte. It is essential that, besides the presence of charge on the particles, there also are certain levels of ions in the solution to ensure a sufficient force of repulsion. It is expedient to distinguish three regimes of ϵ: (a) ϵ ⩾ 11, the (semi-)polar range, where systems can be charge-stabilized more or less as in aqueous systems, (b) the low-polar regime (5 ⩽ ϵ ⩽ 11), where electrostatic stabilization is possible provided some dissociated electrolyte is present and (c) the apolar range (ϵ⩽ 5), where screening is exclusively determined by the polarization of the solvent, and where electrostatic stabilization may be more problematic. Concentrated dispersions of solids in liquid nonionic carriers with dodecyl-benzene sulphonic acid (HDBS) as the stabilizer arc good models for the ‘low polar’ category, as detailed experimental data illustrate. In such media HDBS creates a ζ-potential, and enhances the dielectric constant and the ionic strength of the continuous phase. Especially when attraction between suspended salt particles in liquid nonionics is weak, electrostatic stabilization is easily achieved. This is particularly noticed in the rheology of the concentrated suspensions of some salts.