Experimental Study of the Dielectric Properties of Aqueous Suspensions of Latex Particles for Different Ionic Sizes (abstract)

Abstract

It is well known that the main electrokinetic parameter of suspended colloidal particles is the Z potential, which is usually determined by measurements of the electrophoretic mobility and the standard electrokinetic model. This model assumes that the particles are perfectly smooth and impenetrable, and bear a fixed uniform surface charge. It also assumes that the ions in the electrolyte solution are mathematical points. According to this model, the equilibrium concentrations of ions correspond to the Gouy‐Chapman distribution, the Z potential coincides with the surface potential, and the surface conductivity of the particle coincides with the conductivity of the diffuse double layer. This has been used for many years to determine the Z potential from a single measurement: the electrophoretic mobility. Equally valid alternatives are to determine this potential from the conductivity or permittivity increments, because the three phenomena are interpreted using the same standard model. The problem that arises is that values of Z obtained using these three techniques are usually different. The reason for this discrepancy should reside in some flaw in the hypotheses of the model: the surface of a particle and its surroundings are not as simple as was assumed. One of the possible causes is the finite size of the ionic species present in the electrolyte solution. The influence of the ionic size is studied by means of low‐frequency dielectric spectroscopy measurements of colloidal suspensions of strongly charged latex particles, suspended in aqueous solutions at different electrolyte concentrations and using counterions and co‐ions of different sizes. The conductivity and permittivity increment values deduced from the experimental data are interpreted using existing theoretical and numeric formalisms.