A Grand Canonical Monte Carlo Study of the N-2, CO, and Mixed N-2 - CO Clathrate Hydrates

In this paper we report the use of Grand Canonical Monte Carlo (GCMC) simulations to characterize the competitive trapping of CO and N-2 molecules into clathrates, for various gas compositions in the temperature range from 50 to 150 K. The simulations evidence a preferential trapping of CO with respect to N-2. This leads to the formation of clathrates that are preferentially filled with CO at equilibrium, irrespective of the composition of the gas phase, the fugacity, and the temperature. Moreover, the results of the simulations show that the small cages of the clathrate structure are always filled first, independent of either the guest structure or the temperature. This issue has been associated with the rather significant differences in the calculated heats of encapsulation (similar to 2-3 kJ/mol) between the smallest and the largest cages. In addition, calculations with the simplified ideal adsorbed solution theory (LAST) are developed to allow a comparison with the results arising from the GCMC simulations. Interestingly, this shows that the occupancy isotherms of the mixed N-2-CO clathrates can be perfectly represented when the occupancy isotherms of the corresponding single-guest clathrates are known. This suggests that experiments performed with the single-guest CO and N-2 clathrates might be sufficient to get information concerning the corresponding mixed clathrates by using the LAST approach.