Monte Carlo studies of aqueous solution of nitrogen using different potential energy surfaces

Abstract

A Monte Carlo simulation of a dilute solution of N2 in H2O was performed under canonical (T, V, N) ensemble conditions at 25°C. Two pairwise analytical potential functions (APF), fitted to ab initio 4–31 G** and 7/3 basis set SCF calculations, and a simple Lennard-Jones potential, fitted to experimental data, were used in three runs of 8000 K steps each. Analysis of the solution from the microscopic point of view was performed through the calculation of a number of radial and frequency distribution functions, subdividing also the solution in shell and bulk regions. Evidences of water structuring promotion by N2 are provided. According to the used potentials, the first hydration shell is predicted to include 17 ∼ 20 water molecules, values very close to that characteristic of the corresponding chlatrate. The partial molar energies calculated by the three APFs show that (7/3) potential gives the value (-21·7 ∓ 14·6 kJ mol-1), close to the experimental solvation enthalpy (-10·4 kJ mol-1). On the other hand the statistical analysis shows that (4–31 G**) potential promotes a structuring of the solvent correctly decreasing in the region far from N2.