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

The size of structure-H clathrate hydrates (sH hydrates)dependson the size of large molecule guest substances (LMGSs) that are en-capturedin icosahedral (5(12)6(8), E) cages. Especially,the c-axis in an sH hydrate structure is sensitivetoward the longest molecular length of the LMGSs. In this study, wereport 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, consideringits van der Waals radius. Moreover, t14DClCH is assumed to be thelongest possible LMGS for sH hydrates containing CH4 becauseof the relationship between its lattice parameter and its longestmolecular length. The phase equilibrium pressure-temperatureconditions in the four CH4-LMGS-water systemswere also measured by an isochoric method. The temperature shiftsof the four LMGS systems from the pure CH4-watersystem increased in the following order: ICP & SIM; BrCHP (3.7 K)< ClCHP (4.7 K) < t14DClCH (5.1 K).