Influence of Lennard-Jones Parameters in the Temperature Dependence of Real Gases Diffusion through Nanochannels.

Umbrella Sampling Molecular Dynamics has been used to determine transition energies for different guest molecules through hydroquinone ß-clathrate nanochannels, as well as their temperature trend. This clathrate has been shown to successfully enclathrate different types of small gases with remarkable selectivity, and thus it has been proposed as a potential gas separation and storage medium. Most of these potential guest gases can be successfully modeled as single Lennard-Jones spheres. Then, to obtain a general view of diffusion probabilities for different potential guest molecules, a comparative study for different virtual guest molecules described by different Lennard-Jones parameters has been performed. A regular temperature trend has been obtained for the transition energies for the molecular model characteristic parameter range explored. Finally, to locate the transition energy values of real gases within the space of phases explored, calculations have been repeated for molecular models of different noble gases and H2. The correlation results presented allow a wide interpolation ability for determining the transition energies of potential guest molecules stored or diffusing through the nanochannels of the studied clathrate structure.

Diffusion and dynamics of noble gases in hydroquinone clathrate channels.

In the present work, we study the behavior of the noble gases He, Ne, Ar, and Kr inside a hydroquinone clathrate (HQC) by using all-atom molecular dynamics. Larger elements of the same group were not considered due to their inability to fit inside the HQC cavities. By using the umbrella sampling technique, we have obtained the following inter-cage transition barriers-which are arguably the main factor determining the type of diffusion of the gases-at 310 K and 0.1 MPa: 1192; 2204; 6450; 10 730 kJ mol-1 for the guests He, Ne, Ar, and Kr, respectively. These energy barriers were found to have a linear relation with atomic radii (σ). We have tested this tendency with CH4, due to its intermediate size between Ar and Kr, obtaining a barrier of 8926 kJ mol-1, in excellent agreement with the results for noble gases.