The Functionality of Cation Bridges for Binding Polar Groups in Soil Aggregates

Structural and energetic properties of cation bridges in soil supramolecular aggregates were studied by means of a model system in which two carboxylate groups connected to aliphatic oligomer chains were bound by Ca ²⁺ and Na⁺, respectively. Wetting/drying conditions in soils because of environmental changes were modeled by different content of water molecules forming the solvation shell of the cations. Density functional theory calculations were performed for unpolar (isolated structures) and polar (water as continuum polarization model) environment. Direct and indirect complexes of the cations with the carboxylate groups were found to have different stability. Formation of indirect complexes was computed by means of potential energy curves for the approach of bare (reference set) and solvated cations to the dimer chain containing the carboxylate groups. Characteristic difference on drying (stepwise reduction of water molecules) between Ca ²⁺ and Na⁺ complexes were observed affecting the predicted mobility of monovalent and divalent cations in soils.