Diffuse Double‐Layer Models, Long‐Range Forces, and Ordering in Clay Colloids

Abstract

The modern view of colloidal particle behavior in water is based on the DLVO (Derjaguin‐Landau‐Verwey‐Overbeek) theory, which hypothesizes strictly repulsive long‐range interactions among charged particles. However, numerous phenomena of colloidal behavior, some recently discovered, cannot be explained quantitatively or qualitatively by this theory. An alternative description of the fundamental forces involved in the formation of dispersions and gels was outlined in 1938 by Langmuir, and further developed theoretically by Sogami, Ise, and Smalley. This model, unlike DLVO, hypothesizes a long‐range Coulombic attractive force countering osmotic repulsion, which may explain experimentally confirmed multiparticle phenomena such as long‐range attraction and transitions among ordered (liquid crystalline) and disordered phases of colloidal suspensions. The key concepts of Sogami–Ise theory and its experimental support are reviewed in the present article. This review begins with an analysis of the extent to which dynamic (Brownian) motion and interparticle steric hindrance influence the macroscopic properties of clay dispersions and gels. It is shown that the covolume theory of Onsager in some cases could explain the existence of apparent long‐range interparticle interactions, and phase transitions between ordered and disordered states in colloidal dispersions, without invoking long‐range forces. Nevertheless, in situations where the composition of the electrolyte solution influences the observed interparticle interactions, the experimental evidence for long‐range attraction among charged colloidal spheres points to a force that is fundamentally electrostatic in nature, of the type envisioned by Langmuir. After consideration of all experimental and theoretical results to date, it is concluded that the DLVO theory adequately describes the repulsive interaction between isolated like‐charged particles, whereas a long‐range attractive force is needed to explain multiparticle interactions in suspensions under conditions of low electrolyte concentration and high particle charge.