Thermodynamic and Spectroscopic Identification of Guest Gas Enclathration in the Double Tetra-n-butylammonium Fluoride Semiclathrates

Abstract

The precise nature and unique pattern of the double tetra-n-butylammonium fluoride (TBAF) semiclathrates with a guest gas (CH4 or CO2) was closely investigated through thermodynamic and spectroscopic analyses. The three-phase equilibria of semiclathrate (H), liquid water (L-w), and vapor (V) for the ternary CH4 + TBAF + water and CO2 + TBAF + water mixtures with various TBAF concentrations were experimentally measured in order to determine the stability conditions of the double TBAF semiclathrates. The double CH4 (or CO2) + TBAF semiclathrates showed remarkably enhanced thermal stability when compared with pure CH4 (or CO2) hydrate. The highest stabilization effect was observed at the stoichiometric concentration of pure TBAF semiclathrate, which is 3.3 mol %. Gas uptake measurements were undertaken in order to estimate the amount of gas consumed during double semiclathrate formation. CH4 was found to be a relatively more favorable guest for the 5(12) cages of the double TBAF semiclathrate than CO2. From the results of the NMR and Raman spectroscopic analyses it was identified that the guest gas molecules (CH4 or CO2) were enclathrated in the 5(12) cages of the double TBAF semiclathrates. The overall results given in this study are useful for understanding the fundamental guest gas enclathration behavior in the double semiclathrates.