Thermodynamic Properties for Applications in Chemical Industry via Classical Force Fields - Multiscale Molecular Methods in Applied Chemistry - page 235

This model was extensively tested with respect to its ability to yield transport
properties. E.g., the self-diffusion coefficient was predicted in the temperature range
from 203 to 473 K for pressures between 10 and 200 MPa with a mean deviation of
15% over the whole range of studied conditions. As an example, Fig. 6 shows the
temperature dependence of the self-diffusion coefficient at 10 and 200 MPa in
comparison to experimental data [ 250 ].
The thermal conductivity and the shear viscosity of ammonia were also predicted
with a good accuracy on the basis of the force field by Eckl et al. [ 97 ] in the same
temperature and pressure range. The predictions of the thermal conductivity and the
shear viscosity deviate on average by 3 and 14%, respectively, from the experimen-
tal data.
7 Case Study: Binary Mixtures Containing CO 2
CO 2 is an important substance which is present in many processes in the chemical
industry. In the following, a case study on the prediction of the Henry's law constant
for CO 2 in ethanol and the vapor-liquid equilibrium of the binary mixture CO 2 þ
C 2 H 6 is discussed. The aim is to explore the capabilities of force fields to predict the
temperature dependence of gas solubility and to predict azeotropic behavior.
7.1 Force Fields
The Van der Waals interactions of the force fields for CO 2 and C 2 H 6 were described
by two LJ 12-6 sites and one point quadrupole (16). Both force fields were
empirically parameterized to experimental critical temperatures, saturated liquid
densities, and vapor pressures by means of a nonlinear optimization algorithm. For
both pure substances, the vapor-liquid equilibrium properties from simulation
deviate by less than 1% from experimental saturated liquid density data and less
than 3% from experimental vapor pressure and enthalpy of vaporization data.
The force field for ethanol [ 252 ] consists of three LJ 12-6 sites plus three point
charges and was parameterized to ab initio and experimental data. The nucleus
positions of all ethanol atoms were computed by QM at the HF level of theory with
a 6-31G basis set. This force field is also based on the anisotropic approach of
Ungerer et al. [ 130 ]. The LJ parameters and the anisotropic offset were fitted to
experimental saturated liquid density, vapor pressure, and enthalpy of vaporization.
The simulation results from this ethanol force field deviate on average from
experimental values of vapor pressure, saturated liquid density, and heat of vapori-
zation by 3.7, 0.3, and 0.9%, respectively.