THF (tetrahydrofuran) hydrates form at 101.325 kPa and at temperatures at or below 277.15 K. This system is of interest because unlike conventional gas hydrates where the guest is a small hydrophobic molecule, THF is a polar molecule which is capable of forming hydrogen bonds with the water molecules in the cages and is miscible in water. The study of THF hydrates is therefore of interest from a science perspective and also storage of hydrogen is envisioned in THF hydrates at relatively milder conditions. THF hydrates form sII hydrate structure and our initial configuration consists of unit cells with the THF molecules occupying the large cages. The force fields employed in our simulations are TraPPE-UA for THF and TIP4P/Ice for water with the Lorentz-Berthelot combining rules employed for the cross-terms. There are 1088 water molecules and 64 THF molecules. We have performed one-box Metropolis Monte Carlo simulations in the isothermal-isobaric ensemble in the entire temperature range from 200.15 K to 310.15 K at atmospheric pressure using the open source software package, MCCCS Towhee. For the melting point determination, the specific density is one order parameter. For host water molecules, we have also employed other order parameters which measure the deviation from a tetrahedral structure of water such as F3, Sg and z. We have also used two 4-body orientational order parameters, F4f and F4t which measure the planarity of two oxygen atoms within a spherical cut-off of 3.5 Å. These order parameters have specific values for clathrate-like structures and liquid water enabling the determination of the melting point. We have further explored the structure of the THF hydrates using site-site intermolecular radial distribution functions.
