Influence of cation species and pH on yield stress, floc formation, and sedimentation behavior in silica particle suspensions

Abstract

This study examined how coagulation conditions—electrolyte concentration, cation species, and pH—influence yield behavior, sedimentation dynamics, and floc formation in amorphous silica suspensions. Zeta potential measurements showed that less hydrated cations adsorbed more strongly onto silica surfaces, reducing double-layer repulsion. Yield stress data indicated stronger interparticle attraction at high pH with poorly hydrated cations due to enhanced surface adsorption. Interestingly, in suspensions containing guanidinium (Gdm⁺)—the least hydrated cation—the yield stress decreased at concentrations above 2.0 M, implying that the large size of Gdm⁺ may cause steric repulsion at high concentrations, reducing the yield stress. Sedimentation tests showed that both sediment layer compression and floc structure were affected by pH and cation identity. Within the pH range of 4–8, sedimentation behavior aligned with yield stress trends, though deviations appeared at pH 10. Floc diameter measurements supported these findings, showing that larger flocs formed under conditions of stronger interparticle attraction, consistent with the yield stress data. Overall, cation hydration and pH strongly influenced interparticle forces and suspension behavior. These findings emphasize the significance of ion-specific effects in colloidal systems and provide insight into the rheological and stability properties of silica suspensions.