Filling-Isotherms in Clathrate-Hydrates.

Abstract

Clathrate hydrates of N2 and CO2 were investigated in situ as a function of gas pressure by neutron powder diffraction. Gas pressure cells developed specifically for this work were employed.The pressure dependency of the filling of the different cages in these clathrate hydrates was established. In contrast to some earlier indirect evidence the small cages in CO2 clathrate hydrate are partly filled, however the generally accepted Langmuir-behaviour of the filling is violated in this case. Likewise, instead of the usually assumed filling of the large cages with only one N2 molecule a double occupancy of these cages is observed for N2 clathrate hydrate.[Clathrates, gas pressure, neutron diffraction, guest-host interaction]

Phase equilibria in the system CO2-H2O I: New equilibrium relations at low temperatures

“…Thermodynamic modeling favors the latter. However, in neutron diffraction study of clathrate cited above Kuhs et al(1997) reported an increase in X-site occupancy by CO 2 in type-I clathrate at 273 K between 15 and 25 bars that agreed with theoretical predictions; whereas in a subsequent study Kuhs et al (1998) reported an improbable decrease in X-site occupancy between 25 and 40 bars with the lower occupancy remaining constant to 60 bars. These data may indicate an order/disorder transformation related to filling of the guest sites between 25 and 40 bars at 273 K. Such a transformation would favor I␣3 I␤.…”

In situ structural properties of N2-, O2-, and air-clathrates by neutron diffraction

“…It may be noted that recent experiments on CO 2 clathrate have indicated that a compensation of mechanical compression and filling-induced expansion may lead to a net increase of lattice constants with increased gas pressure. 29 Despite a slightly larger cage filling in the O 2 case and the almost identical molecular size for N 2 and O 2 in the free gas phase, 58,59 the O 2 -clathrate lattice constant is found to be significantly smaller than that of N 2 -clathrate ͑Table III, Fig. 3͒.…”

“…44 -46 All these experiments were performed on recovered samples and did not provide structural details like cage sizes and cage fillings, which are of considerable importance for a complete understanding of natural air hydrates. 35,36 In a preliminary study using a more limited data set 28,29 we reported on deviations from the generally assumed Langmuir filling behavior of N 2clathrates and observed the co-formation of metastable type I N 2 -clathrates formed at 273 K at pressures above 560 bar together with the usual type II clathrate. In contrast to theoretical predictions of the stability field of type I N 2clathrate 47 it was found that the initially obtained type I structure transformed completely into type II over periods of several weeks up to several months and must, therefore, be considered as metastable.…”

“…The cage filling can also be used to check the validity of the vdWP theory. Our previous work 28,29 has demonstrated that the first assumption of the vdWP theory is not justified in the case of N 2 clathrate, which clearly shows a double occupancy of the large cages of the type I and type II hydrate structure. Recent molecular dynamics simulations 30 were found to be in agreement with our preliminary results.…”

“…27 To obtain assumption-free values of the filling neutron powder diffraction experiments proved to be the most promising approach. 28,29 Diffraction experiments directly yield the time-space averaged absolute filling of small and large cages from which of course the hydration numbers can also be calculated. The cage filling can also be used to check the validity of the vdWP theory.…”