Nonlinearity of Cationic Aromatic Amine Sorption to Aluminosilicates and Soils: Role of Intermolecular Cation-π Interactions

Through the study of substituted anilines and benzylamines, we demonstrated that cooperative cation-pi, pi-pi, and van der Waals interactions can increase aromatic cationic amine sorption to Na/Ca-montmorillonite well beyond the extent expected by cation exchange alone. Cationic amines exhibiting cooperative interactions displayed nonlinear S-shaped isotherms and increased affinity for the sorbent at low surface coverage; parallel cation exchange and cooperative interactions were noted above a sorption threshold of 0.3-2.3% of exchange sites occupied. Our experiments revealed the predominance of intermolecular cation-pi interactions, which occurred between the pi system of a compound retained on the surface via cation exchange and the cationic amine group of an adjacent molecule. Compounds with greater amine charge/area and electron-donating substituents that allowed for greater electron density at the center of the aromatic ring showed a greater potential for cation-pi interactions on montmorillonite surfaces. However, benzylamine sorption to nine soils, at charge loadings comparable to the experiments with montmorillonite, revealed no significant cooperative interactions. It appears that cation-pi interactions may be likely in soils with exceptionally high cation exchange capacities (>0.7 mol charge/kg) and low organic matter contents, abundant in montmorillonite and other expanding clay minerals.