The Relation of Interface Properties and Bulk Phase Stability: Molecular Dynamics Simulations of Carbon Dioxide

Abstract

The limit of metastability, the so-called spinodal, is calculated for pure carbon dioxide by molecular dynamics simulation. The determination of the spinodal is based on properties of the liquid vapor interface using a recently developed method. This method relates the tangential pressure component through the vapor-liquid interface to the van der Waals loop in the two-phase region of the phase diagram. By application of the thermodynamic stability criteria, the location of the spinodal can be determined. The spinodal determined in this way is called interface spinodal here. Furthermore, the simulation provides equation of state properties in the complete metastable region of the phase diagram. The performance of different correlation equations for the density and the pressure tensor profiles with respect to the estimation of the spinodal is compared. It has been found that the interface spinodal coincides with the thermodynamic mean field spinodal within some reasonable deviation. Finally the influence of the size of the simulation box on the spinodal properties is investigated showing that the temperature-density spinodal data are independent of the interface thickness. Additional simulations using a Lennard-Jones fluid confirm these results over a range of 1.5 orders of magnitude for the systems size. A further result is that interface systems require a very long simulation time in order to obtain reliable results.

Stability limits of n-nonane calculated from molecular dynamics interface simulations

“…As a consequence the normal pressure is the vapor pressure of the system. As discussed earlier [15] Bakker also related the deviation between the normal a tangential pressure, to the surface tension. He called the difference between the pressure components the total deviation to Pascal's law.…”

“…As discussed earlier [22], it is more suitable that for example the Harasima method [23]. waves, however, the tanh functions are as good for the analysis required here [9,15].…”

“…It should be mentioned that the periodic boundary conditions in connection with the finite size limit the wave length of the capillary waves at the interface [16]. However, this effect is not big here as discussed in detail earlier [15]. To avoid the collapse of the film to a sphere it has to have a minimum thickness [17].…”

“…For a more detailed discussion see an earlier paper [15]. The z-coordinate at the extreme value of pT are used to calculate the corresponding density of the spinodal using the fit function of the density profile ρ(z) .…”

Thermo-molecular orientation effects in fluids of dipolar dumbbells

“…Our methodology neglects some important factors, and improved methods certainly exist for making a more accurate determination of the critical point in similar systems [13,17,30].…”