Electric Double Layers in Soils as Basis for Micro-Aggregation

The behavior of ions at the electric double layer interfacial region between solid and liquid phases is well known in surface and material science. However, this is not fully the case in soil science, where the electric double layer develops due to the inherent complexity and diversity of negatively charged soil organic matter molecules that sorb at the mineral surface. This lack of knowledge opens questions regarding the interaction mechanisms at the molecular scale that lead to the stabilization of such soil building blocks, the so-called organo-mineral aggregates. By using classical molecular dynamics simulations, we studied the interactions between Kaolinite and four standardized soil organic matter samples with different aromatic and carboxylic content and Ca2+ cations clustering OM molecules and stabilizing them to the surface. We assessed the influence of (i) organic content, (ii) water content, and (iii) competing Cl− ions. Cluster aggregates are favored in soils with a high aliphatic fraction at low-water content, whereas aggregation is favored in soils with high aromatic fraction at high-water content and in the presence of Cl− ions. Moreover, samples with a low acid fraction favor clustering due to non-bonded interactions and hydrogen bonds. All of these environments are reflected as unique charge-density “fingerprints” and surface coverage for each OM sample. This work represents a step towards the characterization and study of natural soils tunable to specific local conditions.