On the Theory of Electrostatic Interactions in Suspensions of Charged Colloids

This study assessed the relative merits of the Derjaguin-Landau-Verwey-Overbeek and Sogami-Ise theories for the description of the electrostatic interaction between like-charged colloid particles in an electrolyte solution, which differ qualitatively in their predictions of the nature of this interaction. The former implies a repulsion at all distances with a screened-Coulomb (or Yukawa-type) dependence on distance, whereas the latter leads to an electrolyte-mediated, long-range attraction. The Sogami-Ise result for the interaction potential depends critically on the notion that the electrostatic parts of the Helmholtz and Gibbs energies of an electrolyte solution are different. By elaborating on the thermodynamic implications of this assumption, it was determined that it leads to a conflict with well-established results, in particular the relation between osmotic pressure and solvent chemical potential, already at the Debye-Huckel level. For this reason alone, the Sogami-Ise theory is untenable. The condensation and ordering phenomena observed with charged colloids in low-ionic-strength solutions, which the Sogami-Ise theory was originally designed to explain, can be interpreted without the need to resort to long-range electrostatic attraction, as has been demonstrated in previously published work.