To Distinguish Electrostatic, Coordination Bond, Nonclassical Polarization, and Dispersion Forces on Cation-Clay Interactions

Recent research has suggested that inorganic ions give rise to complex interfacial adsorption effects, but people do not fully understand the mechanisms at present. In this study, the interface adsorption energies of H+ (without extranuclear electron), Li+, and Cs+ (with extranuclear electrons but possessing a large difference in ionic radius) on montmorillonite surface were estimated to elucidate the contribution of electrostatic, coordination bond, nonclassical polarization, and dispersion forces to interface adsorption energies. The results showed that under given cationic concentrations, the equilibrium adsorption energies followed the sequence of Cs+ > H+ > Li+. Moreover, the adsorption energies of H+ (with minimum ion radius) were close to Cs+ (with largest ion radius) but much larger than that of Li+ under relative low cationic concentrations, whereas the adsorption energies of Cs+, H+, and Li+ approached each other under the highest cationic concentration of 0.1 mol L-1, although their ionic sizes are in great difference. With these results, we conclude the following: for Li+, the observed adsorption energy could be fully explained by the classic electrostatic force; for H+, the nonelectrostatic adsorption energy was from the coordinate bond between H+ and O atom at surface, and the coordinate bond adsorption energy of H+ was electric-field-dependent; for Cs+, under relative low electrolyte concentrations, the nonelectrostatic adsorption energy was from the nonclassic polarizability of Cs+, and under the high electrolyte concentration of 0.1 mol L-1, the nonelectrostatic adsorption energy was from the dispersion force of Cs+ and NO3- through ion pair adsorption.