Formation and thermodynamic behavior of THF-water hydrates in confined mesoporous media

Abstract

Tetrahydrofuran (THF) is a benchmark guest for probing clathrate hydrate thermodynamics because a stoichiometric aqueous solution (THF·17H2O) forms structure-II (sII) hydrate at ambient pressure with a well-defined dissociation temperature. Here, we combine differential scanning calorimetry (DSC) and wide-angle x-ray scattering (WAXS) in bulk and confined media to resolve how composition, pore filling, and cooling rate govern hydrate formation in SBA-15 mesoporous silica. Bulk DSC establishes mass-balanced enthalpies for ice and sII hydrate and confirms reversible dissociation/melting temperatures. In confinement, the heating traces separate into a Gibbs–Thomson depressed ice melt (=−14.7 ± 0.2 °C), an in-pore hydrate dissociation (=−13.2 ± 0.2 °C). Confined hydrate appears only when two criteria are met: near-percolating filling (φ = 1.0–1.1 cm3/g) and sufficient THF (≥1:16 mol:mol). Cooling-rate experiments (1.0 vs 0.5 °C/min) demonstrate that slower precooling increases the confined-hydrate fraction and reduces confined ice without shifting equilibrium temperatures: at φ = 1.1, the hydrate enthalpy rises by ∼60% at 1:11 and ∼54% at 1:14, but by ≤17% at 1:16. Temperature-cycling tests show invariant reheating peak positions, indicating that capillarity and composition, rather than kinetic history, fix the liquidus and dissociation temperatures. WAXS indicates that the phase formed in pores is crystallographically identical to bulk sII. Finally, the variation of melting points (ΔTm) plotted against inverse pore radius follows the Gibbs–Thomson law for both ice melting and hydrate dissociation, quantitatively linking the observed shifts to crystalline size and clarifying how confinement, cooling rate, and composition govern the competition between hydrate formation and water crystallization.