DFT and 2D-CA methods unravelling the mechanism of interfacial interaction between amino acids and Ca-montmorillonite

We explored the effect of contact time on the interfacial interaction mechanism of amino acids (AAs) connected to the aluminol group (AlOH) and interlayer Ca ions of Ca-montmorillonite (CaMt) in an aqueous solution using density functional theory (DFT) and two-dimensional correlation analysis (2D-CA) technology. The results showed that these interactions include electrostatic (or van-der-Waals) interaction, cation exchange and hydrophilic interaction. In particular, the electrostatic (or van-der-Waals) interaction between the –COO−(H) (and –NH3+) groups of the AAs and surface negative O atoms of CaMt were found to be the main interaction leading to the adsorption behaviour of AAs onto CaMt. With increasing contact time, the Ca-d0 orbital splitting (dx2+y2 → dx2+y2 + dZ2) not only changes the orbital coupling between the Ca-d0 and O-2p4 orbitals (Ca-dx2+y2-O-2p4 → Ca-dx2+y2 + dZ2-O-2p4) but also enhances the formation of Ca+–COO−(H) p-p σ (neutral: glycine and serine) and Ca+-NH3+ p-p π (charged: glutamate and arginine) hybrid orbitals, as well as the cation exchange (AlOH-Ca + AAs) that mainly contributes to the short-range van-der-Waals interaction. Furthermore, the H-1 s (H2O) orbital is degenerate, which in turn enhances the orbital overlap of H-1 s (H2O) with O-2p4 (-HOCO) and N-2p3 (-NH3), leading to the formation of hydrated clusters: -NH3·(H2O)+ and –HOCO·(H2O)−. The hydrophilic interaction (AlOH-H2O + AAs) mainly contributes to the long-range electrostatic interaction. The results of the study provide a new perspective to understand the adsorption process of AAs onto clay mineral surfaces.