Effect of Mixing Entropy on the Static Yield Stress of a Liquid Dispersion of Solid Particles: Comparison between Si3N4and Ca3(PO4)2Aqueous Suspensions
✍ Scribed by Stefano A. Mezzasalma
- Publisher
- Elsevier Science
- Year
- 1997
- Tongue
- English
- Weight
- 158 KB
- Volume
- 190
- Category
- Article
- ISSN
- 0021-9797
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✦ Synopsis
Although the picture of all forces acting in a dispersed Experimental measurements of the static yield stresses t of silisystem is very complex (34)(35)(36)(37)(43) so that the DLVO con nitride (SN, Si 3 N 4 ) and a-tricalcium phosphate (TCP, atheory is not always valid, a maximum value of the static Ca 3 (PO 4 ) 2 ) aqueous dispersions have been performed for different yield stress at the isoelectric point and t vs pH behavior pH values of the liquid medium and, as a result, the two t vs pH decreasing with the adsorption progress onto the solid partibehaviors are quite different. In agreement with the DLVO theory, cle surface are expected (24)(29)(30). the static yield stress value for silicon nitride is maximal at the Results of several works (49-51), concerning colloidal isoelectric point of the slurry (pH (SN) iep Å 8.0 { 0.1) and, for higher dispersions of various solid substances and reporting experiand/or lower suspension pH values, it decreases progressively. On mental measurements of t vs the suspension pH, agree with the other hand, in tricalcium phosphate dispersed systems, the maximum value of t is not observed at the isoelectric point the aforesaid DLVO-based interpretation. In this paper, too, (pH ( TCP ) iep Å 6.7 { 0.1) but two relative maximum values, t a and it is shown that the static yield stresses of silicon nitride t b , are observed for two suspension pH values in the acid/basic (SN) dispersed systems are maximum at the isoelectric environments, namely, for pH ≠ pH ( TCP ) iep
. First, displacements of point. When the t vs pH behavior does not follow a ''bell'' the pH from the isoelectric point in both environments are accomcurve, it is usually believed that additional forces occur panied by an increase in t; second, after the maximum t values among solid particles (24). have been reached, the static yield stress decreases with the in-In this paper, it will be shown that a mixing entropy crease in the [H / ]/[OH / ] ions in the solution. It is shown that contribution, related to solid agglomerates, can play a sigthis phenomenon can be interpreted as an effect of the mixing nificant role in the rheology of suspensions together the usual entropy relative to the solid TCP aggregates, which is very sensi-DLVO interaction. This effect can be observed in tricalcium tive to the suspension pH. Phenomenological and theoretical exphosphate (TCP) dispersed systems, where the occurrence planations are developed, respectively, by a heuristic recasting of the Hamaker expression for the London-van der Waals forces and of many solubility products involving H / /OH 0 ions proby a relationship between the static yield stress and the number of duces an additional contribution that favors the agglomerasolid aggregates; this relation is based on recently proposed methtion of TCP solid particles. Therefore, the maximum static ods for investigating the agglomeration/adsorption phenomena in yield stress value no longer corresponds to the isoelectric a dispersed system. ᭧ 1997 Academic Press point of a dispersed system.