Heat Capacity, Thermal Expansion Coefficient, and Grüneisen Parameter of CH₄, CO₂, and C₂H₆ Hydrates and Ice I_h via Density Functional Theory and Phonon Calculations

Abstract

Thermal properties of gas hydrates and their underlying fundamental characterization are limited and incomplete but crucial in ongoing basic science research and technological applications. The constant volume heat capacity, the constant pressure heat capacity, the volumetric thermal expansion coefficient, and the Grüneisen parameter of methane, ethane, ethylene oxide, carbon dioxide, and empty structure I hydrates, and of hexagonal ice, as functions of temperature from 0 to 300 K, were calculated using the integration of density functional theory (DFT) simulations at 0 K and phonon calculations at higher temperatures. At low temperatures, DFT predictions replicated experimental values of constant pressure heat capacity for hydrates and ice accurately. Notably, the constant volume heat capacity was lower than when compared with literature values calculated with molecular dynamics (MD) and closer to actual data. Guest molecules were found to contribute slightly more than their ideal gas heat capacities to the overall property of the system. DFT underestimated the thermal expansion coefficient in all cases. The ethane and carbon dioxide hydrates demonstrated behavior that was markedly different when compared to methane and empty hydrates, and hexagonal ice. The Grüneisen parameter was calculated for all systems. DFT overestimated the value of the parameter for filled hydrates and hexagonal ice when compared to experimental hexagonal ice values. Altogether, this systematic atomistic study contributes to the technological applications and basic material science of these crystals whose properties are of significant importance in the fields of energy and the environment and provides a potential input to MD simulations thanks to its performance at low temperatures.

Topics & Concepts