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

Abstract

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.

Introduction

Soil organic matter (SOM) represents a complex, heterogeneous and polydisperse mixture of numerous organic compounds. The major constituent of SOM is represented by humic substances (HSs),(Woodwell et al., 1978) which are heterogeneous high-molecular-weight organic materials mixtures formed by biogeochemical reactions throughout the decay and transformation of plant and microbial remains (humification). Due to their structural complexity and flexibility, a large number of interactions have been observed for HSs, such as hydrogen bonding, anion and cation exchange, ligand exchange, cation bridges, van der Waals and hydrophobic interactions. Each of those interactions can contribute to overall properties of soil organic matter with mineral surfaces (Arnarson and Keil, 2000, Xiaojuan et al., 2005). In current approaches, HSs are described as supramolecular associations (Piccolo, 2001, Piccolo, 2002, Piccolo and Conte, 1999, Wershaw, 1999) leading to a model in which relatively small amphiphilic compounds assemble in supramolecular structures mainly stabilized by non-bonded interactions such as van der Waals, π–π, and CH–π bonds and hydrogen bonds.

In accordance with this picture recent theoretical and experimental investigations showed that individual SOM molecules are linked via intermolecular interactions by bridges of water molecules (Aquino et al., 2009, Schaumann and Bertmer, 2008, Schaumann and Leboeuf, 2005) or by cation bridging sites (Aquino et al., 2008, Aquino et al., 2011, Lu and Pignatello, 2004, Rudolph and Schaumann, 2006, Schaumann, 2006a, Schaumann, 2006b, Scheel et al., 2007, Scheel et al., 2008, Schwesig et al., 2003). It is important to point out that because of the intrinsic heterogeneity of HSs there are no definite structures of HSs available. Therefore, theoretical approaches range from application of minimal models restricted to basic functionality (Aquino et al., 2007) via larger molecular clusters to extended aggregates (Ahn et al., 2008, Aquino et al., 2004, Aquino et al., 2000, Aquino et al., 2001, Aristilde and Sposito, 2008, Kalinichev and Kirkpatrick, 2007, Kubicki and Apitz, 1999, Schulten et al., 2001, Sutton and Sposito, 2006). Hydrogen bonded interactions constituted the major mechanism for attractive forces and the carboxyl group (neutral or deprotonated) had been chosen as a characteristic representative of polar functional groups in many of the referenced investigations.

Soils pose lastingly questions concerning wetness conditions in field environment. Swelling and wetting strongly influence the sorption properties of soils organic material, which has an impact on the accessibility of pollutants and nutrients in soils (Schaumann et al., 2005). It was observed that wetting is not a homogeneous process due to the existence of water repellent spots, which have its origin in amphiphilic substances existing in SOM (Schaumann, 2006b). Soil's wettability strongly influences the structure of SOM. Hence, in dry conditions hydrophobic groups are expected to orient themselves to the outside of clusters of organic molecules, whereas polar functional groups tend to be directed to the inside due the formation of mutual hydrogen bonds. Nevertheless, none of the models cited above considered so far the existence of water molecule bridges having an antiplasticizing effect on the organic matrix, and potentially preventing solvent molecules to access or leave certain microregions in SOM. In this context, it is of great interest to understand the interplay between water, organic solvents and SOM with respect to the rigidity of molecular segments and to estimate the antagonistic effects of these antiplasticizing intermolecular cross-links by water molecules on one hand and swelling and solvent-induced plasticization on the other hand.

In our previous work oligomeric fragments of polyacrylic acid (PAA) representing a polymer model of HSs were used in density functional theory (DFT) investigations of interactions of the herbicide 2-methyl-4-chlorophenoxyacetic acid (MCPA) including calcium bridges (Aquino et al., 2007, Aquino et al., 2008). These calculations showed strong hydrogen bonding of the MCPA with the carboxyl groups of PAA. In a recent work (Aquino et al., 2009) the capacity of water molecules to form stable associations of hydrogen bonds restrained in the nanopores of a HS matrix were investigated. The main focus of this work was laid on the role of the carboxylic group and the hydrogen-bonded interactions including especially the network of the water molecules. A pronounced stabilizing effect of the water network formed with carboxyl groups was observed.

In the present work the environmental effects on the structure and the energetic stability of the water bridges connecting aliphatic chains terminated with carboxyl groups are investigated. The objective is to analyze interactions of these carboxylic groups with a variable content of water molecules representing micro-hydration domains of polar centers in HAs and to study especially the effect of environment characterized by different polarity on these micro-domains.