AFM measurements of Hofmeister effects on clay mineral particle interaction forces

Interaction forces between clay minerals determine particle aggregation and dispersion of clay suspension. Strong Hofmeister effects have been found in surface reactions of clay mineral particles, clay aggregation kinetics, and clay aggregate stability. However, Hofmeister effects observed so far lack experimental evidence from direct measurements of clay mineral particle interaction forces. Through introducing a parameter of "effective radius" for clay mineral particle with a rough surface, AFM was successfully employed to determine the interaction forces between montmorillonite (with roughly surface) and mica (with smooth surface), and strong Hofmeister effects on particle interaction forces were directly observed. The directly measured repulsive forces between the two particles followed the sequence of Li+ > Na+ > K+ > Cs+, and the force strength in solution with 10(-4) mol L-1 Li+ was about 3.6 times that in solution with 10-4 mol L-1 Cs+. Correspondingly, the determined average Stern potential of montmorillonite and mica decreased in the sequence of Li+ > Na+ > K+ > Cs+, and the Stern potential in solution with 10(-4) mol L-1 Li+ was about 2.58 times that in solution with 10-4 mol L-1 Cs+. Analyses of AFM measurements showed that the cation surface adsorption energy determined the ability of cations to shield the electric field around clay mineral particles, which led to the Hofmeister sequence of the Stern potentials of clay mineral particle and the electrostatic repulsive forces between the two clay mineral particles.