Effect of Mixing Entropy on the Static Yield Stress of a Liquid Dispersion of Solid Particles: Comparison between Si3N4and Ca3(PO4)2Aqueous Suspensions

Abstract

Experimental measurements of the static yield stresses τ of silicon nitride (SN, Si3N4) and α-tricalcium phosphate (TCP, α-Ca3(PO4)2) aqueous dispersions have been performed for different pH values of the liquid medium and, as a result, the two τ vs pH behaviors are quite different. In agreement with the DLVO theory, the static yield stress value for silicon nitride is maximal at the isoelectric point of the slurry (pHiep(SN)= 8.0 ± 0.1) and, for higher and/or lower suspension pH values, it decreases progressively. On the other hand, in tricalcium phosphate dispersed systems, the maximum value of τ is not observed at the isoelectric point (pHiep(TCP)= 6.7 ± 0.1) but two relative maximum values, τaand τb, are observed for two suspension pH values in the acid/basic environments, namely, for pH ≶ pHiep(TCP). First, displacements of the pH from the isoelectric point in both environments are accompanied by an increase in τ; second, after the maximum τ values have been reached, the static yield stress decreases with the increase in the [H+]/[OH+] ions in the solution. It is shown that this phenomenon can be interpreted as an effect of the mixing entropy relative to the solid TCP aggregates, which is very sensitive to the suspension pH. Phenomenological and theoretical explanations are developed, respectively, by a heuristic recasting of the Hamaker expression for the London–van der Waals forces and by a relationship between the static yield stress and the number of solid aggregates; this relation is based on recently proposed methods for investigating the agglomeration/adsorption phenomena in a dispersed system.