Observation of the Unusual Aggregation Kinetics of Colloidal Minerals in Acidic Solutions

Abstract

Hofmeister (Specific ion) effects have been considered as important to chemical science as Mendel’s work was to genetics. In this work, the aggregation kinetics of montmorillonite colloids in H⁺, Ag⁺, NH$_{4}^{+}$, K⁺ and Na⁺ solutions were measured in situ, detecting the unusual kinetics and ion specificity for H⁺. Activation energies that can quantitatively estimate the underlying specific ion effects change in the order of H⁺ < Ag⁺ < NH$_{4}^{+}<$ K⁺ < Na⁺. Meanwhile, it was found that although the aggregation of montmorillonite colloids with lower particle densities results in higher activation energies, Hofmeister series will not be affected. It was sufficiently testified that interaction energies of colloidal particles with adsorbed ions show fine correlation with activation energies for colloidal aggregation, and larger interaction energy corresponds to lower activation energy. An attempt was made to quantitatively estimate the ion specificity for H⁺, and it was substantialized that the unusual ion specificity of H⁺ is caused by steric effect.