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

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, alpha-pinene, fluoranthene, p,p'-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 acid-contaminant interactions. The resulting values for the enthalpies of transfer from water to dissolved organic carbon were -2.2, +0.9, -6.4, -4.0, and -4.3 kcal/mol for limonene, alpha-pinene, fluoranthene, p,p'-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 55 degrees C. The values of fluoranthene and p,p'-DDT compared favorably with their experimental values of -4.4 and -5.0 kcal/mol. Prior to this, a general scaling factor for electrostatic interactions in both the pure liquid (sliq = 2.89) and the hydrated solute (shyd = 3.70) was derived from the experimental values of enthalpies of vaporization and aqueous solution. Resulting predicted enthalpies of vaporization deviated by -3.1 (water), 0.4 (limonene), 0.2 (alpha-pinene), 3.9 (fluoranthene), -3.0 (p,p'-DDT), and 0.9 (2,4-D) kcal/mol from experimental data, whereas enthalpies of aqueous solution deviated by -1.3 (limonene), -5.7 (alpha-pinene), 4.2 (fluoranthene), and 0.3 (p,p'-DDT) kcal/mol. Results are discussed in terms of molecular structures and their interactions.