Non‐classical polarization of cations increases the stability of clay aggregates: specific ion effects on the stability of aggregates

Abstract

SummaryThe stability of soil aggregates is closely connected with particle interaction determined by the combination of the van der Waals attractive force and electric repulsive force according to Derjaguin–Landau–Verwey–Overbeek (DVLO) theory. Recently, hydration force and dispersion force were put forward to explain the different behaviours of cations or anions of the same valence at the ion–surface interface, namely the specific ion effect, where the application of classical DLVO theory had failed. Here, we employed two cation species, potassium and sodium (K+ and Na+), to discover how the specific ion effect would influence clay aggregate stability. The stability of K+– and Na+–montmorillonite aggregates was determined under different electrolyte concentrations, indicated by the mass percentages of particles with diameters of < 10, < 5 and < 2 µm released after aggregate breakdown. There were large differences in the stability of the K+‐ and Na+‐ aggregates, and strong specific ion effects were shown. These effects could not be explained by the differences in ionic size, hydration and ion–surface dispersion forces between K+ and Na+. We have proved that the difference in polarization between the K+ and Na+ at the charged clay surface was responsible for the specific ion effects. The difference in polarization observed between the adsorbed K+ and Na+ was hundreds to thousands of times larger than classical values; these results were also verified independently with different methods. The strong non‐classical polarization of the adsorbed cation decreased the electric field and the electrostatic repulsion between adjacent particles in the aggregates, and thus strongly increased the aggregate stability.