Saturday, 15 July 2006

Theoretical Calculations of Glyphosate Adsorption in Montmorillonite Interlayers.

George Khoury, Todd Gehris, and Lorena Tribe. Penn State Berks, Tulpehocken Rd, Reading, PA 19610

The interactions of glyphosate molecules with montmorillonite clay were studied with theoretical methods, ranging from molecular mechanics to ab initio quantum mechanical calculations to explore the ways in which this herbicide may bind to the interlayer surfaces of the mineral. Glyphosate is complex to model in a realistic fashion due to the presence of its three pH-dependent functional groups, which lead to the formation of zwitterions in aqueous solutions in a wide range of acidities and provides multiple opportunities for intra and inter-molecular hydrogen bonding. The environment of the glyphosate molecule in the present calculations is the interlayer of a clay and the surrounding water molecules and cations. The energy landscape is therefore rougher than for the isolated molecule and it is safe to assume that a number of local minima are available. Preliminary calculations showed the barriers between these minima to be low, so the molecule is likely to undergo many transitions in the course a simulation. Therefore, we do not stress the search for the global minima in these conditions but rather determine a reasonable structure for the glyphosate molecule in solution. The following specie was considered: PO3OHCH2NH2+CH2COO-. An initial geometry was established by adding sufficient water molecules surrounding the glyphosate molecule and minimizing the system with PM3, a semi-empirical molecular orbital calculation. A model portion of potassium montmorillonite was too large to make calculations tractable to semi empirical molecular orbital calculations, and was minimized within the molecular mechanics approximation with the MMFF94 force field to ensure its stability. The swelling of montmorillonite as a function of the humidity was reproduced. A ring of water molecules was placed around the cluster of atoms representing the portion of clay and the system was allowed to evolve. A number of water molecules entered the interlayer region and d001 had increased accordingly. To further explore the evolution of the system as a function of the presence of water molecules, groups of water molecules were placed in the interlayer and the system was energy minimized. The results for d001 as a function of the number of water molecules indicate a sustained growth of the interlayer distance as a function of the number of water molecules. These two scenarios indicate that the theoretical approach is appropriate to simulate macroscopic behaviors of the montmorillonite clay when interacting with water. A problem that may occur while attempting to simulate the interactions of large molecules is the biasing of the results due to the consideration of only some of the possible scenarios. To avoid this effect, the first approach to model the interaction of the fully protonated glyphosate molecule with the montmorillonite clay consisted of a molecular modeling calculation where the glyphosate molecule was placed in the montmorillonite interlayer and was allowed to move freely, without constraints. The goal of these calculations was to establish if one of the three functional groups (COOH, NH2+ or PO3OH) is consistently closer to the montmorillonite in the final state than the other two. The final distances between atoms of these moieties and atoms of the surface for seven trials supported the hypothesis that the charged amino moiety is attracted to the negative surface of the interlayer. There was only exception where the initial position of the glyphosate may have led the molecule to remain trapped in a local minimum. The effect of pH on glyphosate molecules was modeled by repeating these calculations for PMG-, PMG2- and PMG3-, the successively less protonated species, with similar results. Selected calculations were repeated with 20 water molecules to provide a more realistic representation of the hydrated clay, finding very similar results. Another approach used to study the interaction of the glyphosate molecule with the montmorillonite interlayer was to estimate the adsorption energy of the herbicide with the clay. This issue was addressed in two ways in the current work. In the first place, the energy of the montmorillonite sample and of the glyphosate molecule was subtracted from the total final energies of the seven conformations above. The values were obtained as the single point energy after the minimization process with molecular mechanics. The adsorption energy estimated this way is a lower order result of the difference of two higher order quantities, and therefore, susceptible to large errors. Nevertheless, an overall trend to more negative adsorption energies was found for smaller interatomic distances of the NH2+ moiety to the surface. The adsorption energy was also estimated by comparing the energies when the adsorbate is bound to the surface and when it is free. In this case it was necessary to make an assumption about the nature of the adsorption . Calculations were done for monodentate and bidentate complexes of phosphonate with the silicon atoms of montmorillonite. The positive values obtained suggest that the formation of those complexes is not favored.

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