Enlargement of Structure-H Clathrate Hydrates by New Large Molecular Guest Substances and Lattice Size Limitation

Abstract

The size of structure-H clathrate hydrates (sH hydrates) depends on the size of large molecule guest substances (LMGSs) that are en-captured in icosahedral (51268, E) cages. Especially, the c-axis in an sH hydrate structure is sensitive toward the longest molecular length of the LMGSs. In this study, we report four new LMGSs for structure-H hydrates containing CH4, namely, iodocyclopentane (ICP), chlorocycloheptane (ClCHP), bromocycloheptane (BrCHP), and trans-1,4-dichlorocyclohexane (t14DClCH). The longest molecular length of t14DClCH is 0.975 nm, considering its van der Waals radius. Moreover, t14DClCH is assumed to be the longest possible LMGS for sH hydrates containing CH4 because of the relationship between its lattice parameter and its longest molecular length. The phase equilibrium pressure–temperature conditions in the four CH4–LMGS–water systems were also measured by an isochoric method. The temperature shifts of the four LMGS systems from the pure CH4–water system increased in the following order: ICP ∼ BrCHP (3.7 K) < ClCHP (4.7 K) < t14DClCH (5.1 K).