On effective radii of dodecahedral cages in semiclathrate hydrates for van der Waals and Platteeuw model

Abstract

In this communication, effective cage radii of dodecahedral (D) cages in semiclathrate hydrates are investigated based on van der Waals and Platteeuw model. According to recent findings in semiclathrate hydrate structures, D cages in semiclathrate hydrates have anisotropic shapes which provide unique gas capacity and selectivity under mild temperature and pressure conditions. Since applications of these materials for cool energy storage and gas capture and storage technologies are expected, thermodynamic modeling of the semiclathrate hydrates is an emerging issue. So far, the van der Waals and Platteeuw model which is based on Langmuir adsorption theory has been used for modeling of canonical gas hydrates and semiclathrate hydrates. The model applies spherical cell potential to predict guest gas inclusion in the cages, and each cage is characterized by its radius with sphere approximation. While sizes of D cages in semiclathrate hydrates are quite similar to those in gas hydrates, their shapes are found to be irregularly anisotropic dodecahedra that can provide unique gas capture and storage properties. Therefore, when the van der Waals and Platteeuw model is applied to semiclathrate hydrates, it is necessary to discriminate the D cages in the model from those in canonical gas hydrates. Adjusting D cage radius in spherical cell potential model is a simple and convenient way to describe differences in cage shape between semiclathrate hydrates and canonical gas hydrates. Here, we investigated effective cage radii of D cages in the semiclathrate hydrates based on the conventional cell potential model with sphere approximation. Effective radii of D cages in the semiclathrate hydrates were found to be different from those in canonical gas hydrates. With the presently suggested radii, the conventional cell potential model can predict experimental data for cage occupancy in the semiclathrate hydrates more precisely.