Zeta potentials and yield stresses of silica suspensions in concentrated monovalent electrolytes: isoelectric point shift and additional attraction.

The zeta potentials and yield stresses of silica suspensions were measured over a wide range of monovalent electrolyte concentrations. The counterions investigated were Li(+), Na(+), K(+), and Cs(+), while the co-ion was always Cl(-). The poorly hydrated ions (Cs(+) and K(+)) adsorb in greater quantity to the silica surface than the well-hydrated ions (Li(+) and Na(+)) and produce lower magnitude negative zeta potentials at high pH. At high electrolyte concentrations and at low pH the poorly hydrated counterions adsorb in great enough quantity to reverse the sign of the zeta potential from negative to positive. This specific adsorption of counterions shifts the iep to higher values. The shift in the iep is directly related to the hydration of the counterion with the least hydrated ions creating the greatest iep shift. The yield stresses of silica suspensions at high pH were found to increase in the order Li(+)<Na(+)<K(+)<Cs(+). The magnitude of the yield stress correlates with the amount of adsorbed ions; i.e., the greatest yield stresses are observed with the least hydrated ions. The yield stresses measured at high pH with high salt concentrations are greater than can be accounted for with just van der Waals attraction. An attractive ion-ion correlation force is presumed responsible for the additional attraction necessary to explain the observed results.