Stabilizing Capacity of Water Bridges in Nanopore Segments of Humic Substances: A Theoretical Investigation: A theoretical investigation

Molecular simulations using density functional theory (DFT/PBE and DFT/tight-binding (DFTB)) have been performed to study wetting processes of model nanopore segments in humic substances (HS). A complex of two poly(acrylic acid) trimers (trimer complex, TC) arranged in parallel alignment was used to provide the structural example for supramolecular contact of two HS chains by means of hydrogen bonds. Their interaction with a local network of water molecules represented the influence of wet spots. Displaced TC structures were constructed by horizontal motion of the chains relative to each other in order to study the capacity of the water cluster to hold the two chains together even though their distance is too far for direct hydrogen bonding between the carboxyl groups. Geometry optimizations and molecular dynamics simulations were used to investigate the hydrogen-bonded structures formed and to compute their energetic stabilities. At shorter distances between the two oligomer chains an outer solvation was most stable. However, with increasing distance of the two polyacrylic trimers the water molecules penetrated into the inside of the created free space, keeping the two chains together by means of a hydrogen-bonded network. Significant stabilization effects of 10-20 kcal/mol were observed by this intrusion of water molecules at trimer distances of ̃13 Å. The present model, therefore, strongly supports the hypothesized bridging function of water molecules in humic substances provided a local distribution of appropriate functional groups is available in the HS matrix.