Study of solvent effect on the stability of water bridge-linked carboxyl groups in humic acid models

Molecular dynamics simulations have been performed by means of density functional theory with tight binding (DFTB) in order to describe the structure and the energetic stability of water bridges in humic substances (HS) model. This model is constructed from two parallel aliphatic chains geometrically restrained on one end and terminated with a carboxyl group on the other to supply the structural pattern for supramolecular contact of two HS chains through hydrogen bonds. Molecular dynamics simulations were used to analyze the interactions of the carboxylic groups with a variable number of water molecules up to 14 representing domains of micro hydration states of polar centers in humic acids. For the present geometrical arrangements of the model five water molecules form a stable bridge between the two carboxylic groups located at each aliphatic chain. The effect of environment through three solvents of different polarities (n-hexane, acetonitrile and water) was investigated. Distribution profiles of oxygen atoms of carboxyl and chain water molecules show that the environmental effect of the solvent with moderate polarity (acetonitrile) is most pronounced in exerting an ordering effect on the water bridge. Energy profiles for incremental addition of water molecules and hydrogen bond analysis demonstrate the remarkable stability of the five water complex as compared to all other models investigated in both gas phase and in acetonitrile. These findings correlate nicely with experimentally observed antiplasticizing effects of water bridges in organic matrices.

Research Highlights
► Molecular modeling of humic substances at density functional theory level.
► Water bridges and hydrogen bonding as strong stabilizing factors in humic substances.
► Influence of solvent polarity on the structure of the water clusters.
► Dynamics simulations to describe energetic stability and environmental effects.