In our work, we employed molecular dynamics and Monte Carlo (MC)
simulations to investigate the supercritical phase of carbon dioxide near its critical
point. Three systems have been studied. The pure carbon dioxide, mixture methane +
carbon dioxide at infinite dilution of supercritical carbon dioxide and water + carbon
dioxide at infinite dilution of supercritical carbon dioxide. The usage of molecular
simulation methods in supercritical region gave us a distinct advantage of knowing
the microstructure of the systems in a qualitative and quantitative way. The
Kirkwood-Buff theory, which predicts the influence of the solvent on the solute,
enabled us to predict thermodynamic properties of supercritical phase and compare
them with experimental values.
We have examined the density effect on structure of the pure carbon dioxide
and its solutions along its critical isotherm 4 K above its critical point. We focused
our research and we present results for two basic sections,
A. Equilibrium and transport properties, namely
Volumetric properties;
Average configurational energy;
Isothermal compressibility;
Diffusivity; and the
Isochoric heat capacity
B. Solution structures at infinite solutions, namely
Radial distribution function; and
Coordination number
We discuss the outcomes based on the density inhomogeneities of the solvent and
critical fluctuations, which are maximised at the critical point. We found that the addition of methane to supercritical carbon dioxide increases the volume of the
solution and a cavitation is formed around it. On the hand, the addition of water gives
a cluster around it in local structure and decrease the volume of solution. We report
results also of the diffusion coefficients for the pure carbon dioxide and the mixtures
in this study, which it shows an anomalous decrease close to the critical point of the
pure carbon dioxide. It is a general conclusion for all the properties we have studied
that the density dependence along the isotherm is maximised at densities close to the
critical one. Further, the usage of both molecular dynamics and Monte Carlo in
supercritical regions validates the extension of the techniques in the supercritical
region and reveals their limitations