Calculation of the interaction between dissolved organic carbon and organic micropollutants by three dimensional force field methods

Abstract

Abstract-A 3D optimized potential for liquid simulation (OPLS) force field method with 6-31G* atomic charges and a geometrical optimization approach was studied for the calculation of enthalpies of transfer at 300 K of partitioning processes. A 3D fulvic acid model of dissolved organic carbon was built. Limonene, ␣-pinene, fluoranthene, p,pЈ-DDT, and 2,4-D were inserted into a general site of preference, and interaction energies were calculated. Energies of hydration were calculated and subtracted from fulvic acidcontaminant interactions. The resulting values for the enthalpies of transfer from water to dissolved organic carbon were Ϫ2.2, ϩ0.9, Ϫ6.4, Ϫ4.0, and Ϫ4.3 kcal/mol for limonene, ␣-pinene, fluoranthene, p,pЈ-DDT, and 2,4-D, respectively. This led to a change of the partition constant by a factor of 0.17 to 1.27 at a temperature increase from 5 to 55ЊC. The values of fluoranthene and p,pЈ-DDT compared favorably with their experimental values of Ϫ4.4 and Ϫ5.0 kcal/mol. Prior to this, a general scaling factor for electrostatic interactions in both the pure liquid (s liq ϭ 2.89) and the hydrated solute (s hyd ϭ 3.70) was derived from the experimental values of enthalpies of vaporization and aqueous solution. Resulting predicted enthalpies of vaporization deviated by Ϫ3.1 (water), 0.4 (limonene), 0.2 (␣-pinene), 3.9 (fluoranthene), Ϫ3.0 (p,pЈ-DDT), and 0.9 (2,4-D) kcal/mol from experimental data, whereas enthalpies of aqueous solution deviated by Ϫ1.3 (limonene), Ϫ5.7 (␣-pinene), 4.2 (fluoranthene), and 0.3 (p,pЈ-DDT) kcal/mol. Results are discussed in terms of molecular structures and their interactions.