Molecular Selectivity of CO–N₂ Mixed Hydrates: Raman Spectroscopy and GCMC Studies

Abstract

This paper reports a novel quantitative investigation concerning the COselectivity properties for mixed CO-N2 hydrates. The study was developed by combining Raman Scattering experiments and Grand Canonical Monte Carlo simulations. Both approaches advocate in favor to say that mixed CON 2 hydrates are CO-selective and quantitatively compare with regard to the measured and calculated molecular selectivities. Intriguingly, this preferential CO encapsulation is enhanced by decreasing the CO molar fraction of the gas mixture. Here, an original explanation to this peculiar behavior is provided in terms of a structural change of the clathrates from SI to SII and in terms of the thermodynamics of mixtures. This last issue was clearly evidenced by using the Ideal Adsorption Solution Theory, which gives results that compare favorably with those obtained from Raman Scattering and Grand Canonical Monte Carlo simulations.

Adsorption of CO and N2 molecules at the surface of solid water. A grand canonical Monte Carlo study

“…Preliminary tests of the interaction models for CO First, the accuracy of the two models selected to represent the interactions between CO and H 2 O molecules has been checked, in the context of bulk clathrates, for which recent experimental results are available in the literature. 44,48 Two series of GCMC calculations have thus been performed, both of them considering the competitive trapping of CO with respect to N 2 when the gas phase in contact with clathrates contains mixtures of CO and N 2 molecules.…”

“…Rather, the real composition of the gas phase is determined directly from GCMC simulations of the CO-N 2 mixture at conditions imposed to the clathrate. 44 The corresponding variations of α CO/N 2 values as a function of the CO molar fraction in the gas phase (y CO ) are given on Figure 2, together with the experimental data for comparison. As it can be seen on this figure, the selectivity calculated by using model A seems more accurate than the one calculated with model B, when y CO < 0.2, i.e., in the range of CO molar fraction where the clathrate is supposed to form a sII structure.…”

“…On the basis of previous works devoted to trapping of CO and N 2 in clathrates, 44,45 , the water molecules have been modeled by the TIP4P-Ew model, 40 whereas the model proposed by Potoff and Siepmann on the basis of the TraPPE force field, 41 has been used for N 2 . When considering the CO molecules, two different force fields have been considered, on the basis of their ability for describing CO-water interactions in various contexts.…”

“…It has been recently used in combination to the Potoff's model for N 2 , 41 to investigate the trapping of CO and N 2 molecules inside the small and large cages of sI ans sII clathrates at 150 K, and it has been proven to represent qualitatively well the experimental observations. 44 The second force field (hereafter named model B) has been developped by Straub and Karplus, 43 in their molecular dynamics simulations of biomolecular systems, in which the interaction between CO and water molecules needs to be explicitely taken into account. The parameters of the aforementioned models are given in Table I.…”

“…The Raman spectroscopy data as well as the selectivities for model A, were reproduced from our previous work. 44…”