We use the Mayer sampling method, with both direct and overlap sampling, to calculate and compare classical virial coefficients up to B6 for various water models (SPC, SPC/E, MSPC/E, TIP3P, and TIP4P). The precision of the computed values ranges from 0.1% for B2 to an average of 25% for B6. When expressed in a form scaled by the critical properties, the values of the coefficients for SPC water are observed to greatly exceed the magnitude of corresponding coefficients for the simple Lennard-Jones model. We examine the coefficients in the context of the equation of state and the Joule-Thomson coefficient. Comparisons of these properties are made both to established molecular simulation data for each respective model and to real water. For all models, the virial series up to B5 describes the equation of state along the saturated vapor line better than the series that includes B6. At supercritical temperatures, however, the sixth-order series often describes pressure-volume-temperature behavior better than the fifth-order series. For example, the sixth-order virial equation of state for SPC/E water predicts the 673 K isotherm within 8% of published molecular simulation values up to a density of 9 mol/L (roughly half the critical density of SPC/E water).

Higher-order virial coefficients (up to B 6 ) for TIP4P and SPC/E water models are used to characterize molecular clusters (up to hexamers) formed by water at various gas-phase thermodynamic state points between 298 and 773 K. Comparison of cluster statistics with available molecular simulation data for the same models indicates that the virial approach is effective at characterizing the clustering behavior. Significant deviations from experimentally confirmed ab initio results from the literature at 298 K are ascribed to inadequacies in the TIP4P model and to differences in the treatment of "physical" versus "chemical" association in the two approaches. At two conditions where an analysis could be made, the concentration of clusters that are only physically associated was found to be ∼30% of the concentration of those that are chemically associated (hydrogen bonded).

We determine the second and third virial coefficients B 2 and B 3 for the Gaussian charge polarizable model (GCPM) as a function of temperature over the range 210−723 K. The overlap sampling implementation of Mayer sampling molecular simulation is applied to calculate the values, obtaining results to a precision that ranges from 0.1% for B 2 to an average of 1% for B 3. These calculated values compare very well with known values of B 2 and B 3 for real water and outperform both pairwise models and other polarizable models in describing experimental virial-coefficient data. We examine these coefficients in the context of the equation of state and molecular clustering. Comparisons are made to established molecular simulation data, quantum chemical calculations, and experimental data for real water. Under both saturated-vapor and supercritical conditions, the virial series up to B 3 describes the equation of state quite well. The virial coefficients are used to characterize molecular clusters (dimers and trimers) in GCPM water under supercritical, saturated vapor, and atmospheric conditions between 210−673 K. The analysis shows that the extent of clustering in polarizable water is diminished relative to that from a pairwise water model.

Methylamine was oxidized in supercritical water in a Hastelloy tubular flow reactor at 249 atm and temperatures between 390 and 500 °C. The major carbon-containing products were CO2 and CO, with trace amounts of CH3OH. The major nitrogen-containing products were NH3, N2O, and N2. A reaction network consistent with all the results has been constructed. Ammonia appears to be the exclusive nitrogen-containing intermediate between methylamine and the final products, N2O and N2. The disappearance of ammonia during methylamine SCWO is markedly faster than that during oxidation of ammonia alone. Approximately 3−4 times more N2O is produced than N2, whereas the N2O/N2 ratio is essentially zero when ammonia is oxidized alone in SCW. We attribute these differences in the rate of and selectivity from ammonia oxidation in supercritical water in the present experiments to the presence of methylamine in the reaction environment and catalytic chemistry on the reactor walls.