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Twoisomers of C2H7N, dimethylamine (DMA)and ethylamine (EA), are known to be clathrate hydrate formers bythemselves. Here we introduced methane gas as a secondary guest intoboth dimethylamine and ethylamine clathrate hydrates and identifiedtheir structural transitions using powder X-ray diffraction (PXRD)and solid-state NMR spectroscopy. We observed the structural transitionsof amine clathrate hydrates from expanded structure I (cubic Pm3n) to structure II (cubic Fd3m). In addition, from experimental results obtainedthrough neutron powder diffraction (NPD) and PXRD, we found that neithertemperature nor pressure affected the hydrate structural transition.Raman spectroscopy was used to identify the structural transitionoccurring in these amine clathrate hydrate systems. In addition, wemeasured the hydrate equilibrium conditions for amine–water–methanehydrates. The DMA and EA act as hydrate inhibitors in DMA/EA + H2O + CH4hydrate systems compared with pure methanehydrate over our experimental pressure and temperature ranges. [ABSTRACT FROM AUTHOR]

Anomalous chloride concentration enrichment has been detected in marine sediments comprising methane hydrates (MHs). In this study, we designed an electric circuit system linked to the high-pressure resistance cell in which the chloride ion concentration can be directly measured within reliable accuracy under in situ conditions of the deep-sea floor pressure and temperature. Chloride concentration increased under a fast MH formation rate, but no noticeable concentration change was detected under a relatively low-rate. Furthermore, we suggested that the MH formation rate must be maintained at least ∼102 mol m−2 yr−1 so as to efficiently enrich chlorides and retain the acquired chlorinity. The present experimental system dose not fully reflect the relatively minor effective variables such as vertical advections in real system, but the results seem to be sufficient for revealing chloride enrichment phenomena induced by fast MH formation rate with free methane gas. © 2011 American Institute of Chemical Engineers AIChE J, 2012 [ABSTRACT FROM AUTHOR]

This study focuses on the cage occupancy of guest molecules in the infinitely dilute state. At the extreme conditions of highly diluted guest concentrations the direct measurements of the cage occupancy ratio representing the competitive inclusion of multiguest species appear to be so difficult because of spectroscopic intensity limitation, but its thermodynamic significance might be considerable due to the fact that the infinite-dilution value of the cage occupancy ratio can provide the valuable thermodynamic information as a very unique and guest-specific parameter. To experimentally identify gaseous guest populations in structure I (sI) and structure II (sII) cages, we used the solid-state nuclear magnetic resonance (NMR), gas chromatography, and direct gas measurements. Furthermore, we derived the simple and generalized thermodynamic equation related to cage occupancies at infinite dilution from the van der Waals-Platteeuw model. Both experimental and predicted values agree well within the experimental error range.