With the aim of understanding heat transfer during structural changes in breathing porous crystals upon gas loading, we study the effect of pore expansion on the thermal conductivity of a series of idealized materials containing adsorbed gas using molecular dynamics simulations. We calculated the thermal conductivity in three main axes of the crystal lattice starting from a tilt angle of 40° to represent the closed form of the crystal up to a tilt angle of 90° to represent the open form. With no gas present, the thermal conductivity increases in the tilt direction with pore expansion whereas thermal conductivity in other directions remains unchanged. In the presence of adsorbed gas, porous crystals at all states of expansion experience reduced thermal conductivity due to phonon scattering introduced by gas–crystal interactions. Similarly, the thermal conductivity in the tilt direction increases as the pore expands; however, the increase is less pronounced compared to the case with no gas present in the pores. We also show that the diffusivity of gas increases during pore expansion, facilitating mass transport.

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Membrane technologies that enable the efficient purification of impaired water sources are needed to address growing water scarcity. However, state-of-the-art engineered membranes are constrained by a universal, deleterious trade-off where membranes with high water permeability lack selectivity. Current membranes also poorly remove low–molecular weight neutral solutes and are vulnerable to degradation from oxidants used in water treatment. We report a water desalination technology that uses applied pressure to drive vapor transport through membranes with an entrapped air layer. Since separation occurs due to a gas-liquid phase change, near-complete rejection of dissolved solutes including sodium chloride, boron, urea, and N -nitrosodimethylamine is observed. Membranes fabricated with sub-200-nm-thick air layers showed water permeabilities that exceed those of commercial membranes without sacrificing salt rejection. We also find the air-trapping membranes tolerate exposure to chlorine and ozone oxidants. The results advance our understanding of evaporation behavior and facilitate high-throughput ultraselective separations.

Membrane distillation (MD) is an emerging water treatment process that offers near-complete removal of nonvolatile contaminants. However, the transport of volatile compounds in the process is highly varied and poorly understood. In this work, we investigate the transport of a wide range of semivolatile and volatile compounds in membrane distillation (55 compounds total) allowing us to gain broad insights into the compound properties, system designs, and operating conditions that impact transport rates in direct-contact MD. We first use experimentally verified simulations to study the effects of different molecular properties on transport and show that the Henry’s constant and diffusion coefficient are the most important molecular properties in determining solute flux. We then study the transport resistances across distillation membranes and find distinct transport regimes dominated by resistances associated with either diffusion through the membrane or boundary layers on either side of the membrane. Simulations of large-scale MD modules reveal the impact of membrane area, operating temperature, and crossflow velocity on the removal of contaminants. Finally, we comprehensively compare the removal of different classes of contaminants between MD and RO, finding that MD is more effective in removing small neutral compounds with low volatility. Overall, the results of this work demonstrate important compound properties and operating conditions that control volatile mass transport in MD.