Application of low field NMR T2 measurements to clathrate hydrates

Low field (2 MHz) Nuclear Magnetic Resonance (NMR) proton spin-spin relaxation time (T(2)) distribution measurements were employed to investigate tetrahydrofuran (THF)-deuterium oxide (D(2)O) clathrate hydrate formation and dissociation processes. In particular, T(2) distributions were obtained at the point of hydrate phase transition as a function of the co-existing solid/liquid ratios. Because T(2) of the target molecules reflect the structural arrangements of the molecules surrounding them, T(2) changes of THF in D(2)O during hydrate formation and dissociation should yield insights into the hydrate mechanisms on a molecular level. This work demonstrated that such T(2) measurements could easily distinguish THF in the solid hydrate phase from THF in the coexisting liquid phase. To our knowledge, this is the first time that T(2) of guest molecules in hydrate cages has been measured using this low frequency NMR T(2) distribution technique. At this low frequency, results also proved that the technique can accurately capture the percentages of THF molecules residing in the solid and liquid phases and quantify the hydrate conversion progress. Therefore, an extension of this technique can be applied to measure hydrate kinetics. It was found that T(2) of THF in the liquid phase changed as hydrate formation/dissociation progressed, implying that the presence of solid hydrate influenced the coexisting fluid structure. The rotational activation measured from the proton response of THF in the hydrate phase was 31 KJ/mole, which agreed with values reported in the literature.