Formation of 2,4-D complexes on montmorillonites - An ab initio molecular dynamics study

Sorption of the anionic form of the pesticide 2,4-D (2,4- dichlorophenoxyacetic acid) on the surface of the clay mineral montmorillonite was investigated using a short-time ab initio molecular dynamics (MD) simulation at room temperature. Three different situations were modelled: sorption on a dry surface, on a hydrated surface and an intercalation between montmorillonite layers. In all three cases, the calcium cation compensates the excess negative charge of the montmorillonite layer and the negative charge of the 2,4-D anion. It was found that in all models with direct contact of the Ca²⁺ cation with the montmorillonite layer, the most stable position of Ca ²⁺ is above the ditrigonal hole of the mineral layer. While in the case of a dry surface very stable bidentate binding is created between the 2,4-D anion and the Ca²⁺ cation, the formation of the monodentate complexes is preferred in all models that include water molecules. Hydrogen bonds formed between water molecules and the 2,4-D anion make a considerable contribution to the formation of the monodentate complexes. Tetrahedral substitutions in the montmorillonite layer have a significant effect on the formation of the complexes of any type. However, the MD simulations did not support the role of Ca²⁺ as a cation bridge in the adsorption mechanism. Calculations showed that hydrated 2,4-D⋯Ca²⁺ complexes are thermodynamically more stable than complexes in which the Ca ²⁺ cation acts as a bridge to the surface. On the other hand, it is possible that phyllosilicates with a greater concentration of isomorphic substitutions (e.g. mica) will be able to form stable surface complexes with a cation bridge mechanism.