Effects of Large Guest Molecular Structure on Thermal Expansion Behaviors in Binary (C₄H₈O + CH₄) Clathrate Hydrates

Investigating the thermal expansion of clathrate hydrates is essential for understanding their complex physicochemical properties. Although there have been various discussions on the thermal expansion, few studies have investigated the structural effect of each guest molecule. To compare the lattice expansion behaviors with cyclic and linear large guest molecules, cyclobutanol and butyraldehyde, having the same formula of C₄H₈O, were used as new sII hydrate formers with a help gas of CH₄. In Raman spectra of the bonding characteristics, the peaks of the C-H stretching mode, O-H stretching mode, and O:H stretching phonon showed unique shift behaviors as the temperature rises. A crystallographic analysis using high-resolution powder diffraction showed that the (cyclobutanol + CH₄) hydrate undergoes smaller lattice expansion than the (butyraldehyde + CH₄) hydrate under thermal stimulations, and the phase equilibria showed that the cyclobutanol case involves milder formation conditions than the butyraldehyde case. These structural effects could be based on the different guest-host interactions, and molecular dynamics simulation results also show that the different thermal expansivity or molecular motion of the large guest in a hydrate cage causes these unique interactions. The results of this study provide insight into distinctive guest-host interactions depending on the guest structure and their effects on the hydrate lattice.