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The State Dependency of the Effective Pair Potential Parameters

Keshavarzi E., Hashemi F., Ghazvini Z.

Polish Journal of Chemistry

2004

Vol. 78 / nr 10

1893-1906

The state dependency of the effective pair potential parameters [epsilon eff (T,ro ), sigma eff (T)] has been studied for gas, liquid and supercritical regions for different types of fluids.We have assumed that the configuration of potential energy between Nmolecules can be divided into independent pair clusters with an effective pair potential, in which the parameters of the potential are state dependent.We have obtained the values of epsilon eff (T, ro),and sigma eff (T) using p-V-T data. The results show, that epsilon eff (T, ro) increases with density for those thermodynamic states, at which pair interactions are dominant, while it decreases with density for those systems, where triplet and higher clusters are dominant. Both epsilon eff (T, ro) and sigma eff (T) decrease with temperature, which coincides with the literature data. Aremarkable result of the present work is the determination of density, at which triplet clusters come into account. We have also shown new corresponding states for the effective well depth parameter, which is held for all examined fluids, including Ar, Xe, CH4, N2, CO, H2O, CO2, CH3OH, C2H6 and C6H6 for different isotherms. Alinear dependence of epsilon eff (T, ro) versus density and temperature on the zeno-line is predicted.

Freezing Behavior in Porous Materials : Theory and Experiment

Śliwińska-Bartkowiak M., Dudziak G., Sikorski R., Gras R., Gubbins K. E., Radhakrishnan R., Kaneko K.

Polish Journal of Chemistry

2001

Vol. 75, nr 4

547-555

We report both experimental measurements and molecular simulations of the melting and freezing behavior of simple fluids in porous media. Activated carbon fibers, having a mean pore width of 1.7 nm, were chosen as the porous medium. Differential scanning calorimetry (DSC) and dielectric relaxation spectroscopy (DS) were used to determine the melting point in these materials. The melting point was found to be very sensitive to the relative strength of the fluid-wall interaction compared to the fluid-fluid interaction. Monte Carlo simulations and Landau free energy formalism were used to determine the shift in the melting point, Tm, for simple fluids in pores having repulsive, weakly attractive and strongly attractive walls. The strength of the interaction of the fluid with the pore wall is shown to have a large effect on the shift in Tm, with Tm being reduced for weakly attracting walls. The theory of corresponding states is used to compare the experimental results for several systems to the simulation results. This approach also provides a unified approach in understanding the diverse freezing behavior in porous media.