Molecular Models of Cation and Water Molecule Bridges in Humic Substances

Abstract

The molecular basis for many properties of humic substances (HS) still has to be investigated. Besides advanced analytical methods, molecular modelling offers the opportunity to investigate various features of HS. We investigated the wetting process of model nanopore segments in humic substances as a function of distance between two trimeric chains of polyacrylic acid. At shorter distances an outer solvation was most stable. With increasing distance, the water molecules penetrated into the formed free space connecting both chains by means of a hydrogen-bonded network, which had a significant stabilization effect. Similar stabilization was observed when two aliphatic chains terminated by carboxyl groups were overbridged by a chain of water molecules. Another setting with four undecanoic fatty acids served as a model for spatially fixed aliphatic chains in HS terminated by carboxyl group. The rigidity of the model is significantly enhanced as soon as the water cluster is large enough to comprise all four carboxyl groups. These simulations strongly support the hypothesized bridging function of water molecules in humic substances. The interactions of Na+, Ca2+, and Al3+ with carboxylate and carboxyl groups and the concomitant proton transfer reactions induced by these interactions were investigated. The strongest effect on the local environment was observed for Al3+ cation. The calculations reflect the amphoteric character of the hydrated Al3+ complex showing in most cases its acidic character via proton transfer from the water molecules of the hydration shell to the carboxylate group.