This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Porous liquids combine permanent nanoscale cavities with fluidity, making them promising for applications like carbon dioxide capture, gas separation and other chemical processes. Their viscosity must be tailored to the intended application: Lower-viscosity liquids are easier to pump and circulate while facilitating faster heat and mass transfer, whereas higher-viscosity liquids provide greater structural stability in membrane formation.
Until now, viscosity and pore volume have been notoriously difficult to tune independently. While diluting solvent-containing porous liquids reduces viscosity, it simultaneously lowers the concentration of the porous components. In solvent-free "type I" porous liquids, where the porous molecules themselves form the fluid, changing the length of the attached polymer chains alters the ratio of nonporous material and can inadvertently block access to the pores altogether.
A Kyoto University research team—including Xiangmei Xiang, Dr. Zaoming Wang, Professor Kenji Urayama and Professor Shuhei Furukawa from the Institute for Integrated Cell-Material Sciences (WPI-iCeMS), working with collaborators in Japan and Taiwan—has now developed a molecular strategy that changes the viscosity of a type I porous liquid by more than eight orders of magnitude while retaining its intrinsic porosity. The study was published in the Journal of the American Chemical Society.
The team used cuboctahedral rhodium-based metal-organic polyhedra (MOPs), cage-shaped molecules with intrinsic porosity, as porous hosts. Twelve flexible polyethylene glycol (PEG) chains were attached to each MOP core, forming star-shaped molecules that assemble into solvent-free type I porous liquids. Four MOP surface groups and two PEG chain lengths yielded eight distinct porous liquids.
Within each chain-length series, the MOP cavity and the number and length of the polymer chains stayed the same, with only the surface functional group of the MOPs differing. Synchrotron X-ray scattering and molecular dynamics simulations showed that surface chemistry dictates polymer conformation. Dodecyloxy groups attract the hydrophobic chain ends, promoting chain folding around the MOP surface.
These compact polymer shells exhibit less interpenetration and slide past one another more easily, resulting in lower viscosity. In contrast, hydroxy groups allow the chains to extend into neighboring MOPs, forming a transient interpenetrated network that strongly resists flow and increases viscosity. At 60°C (140°F), the zero-shear viscosity—a measure of a liquid's resistance to flow under near-rest conditions—ranged from 18 Pa·s to 3.0 billion Pa·s, spanning a factor of about 170 million.
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