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 on August 4, 2026.
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, 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. The most viscous materials showed solid-like behavior over short times but eventually flowed over longer times, confirming their viscoelastic nature.
"We were able to modulate the macroscopic flow behavior without changing the polymer length or the porous volume," says Xiang, a doctoral student at Kyoto University's Graduate School of Engineering. "A change confined to the MOP surface reorganized the conformation of surrounding chains, resulting in a 170-million-fold change in viscosity."
Carbon dioxide (CO2) adsorption measurements showed that all the liquids retained accessible MOP cavities at 30 °C. In the long-chain porous liquid series, the polymer shell acted as a temperature-responsive molecular gate. For the dodecyloxy-functionalized porous liquid, CO2 uptake increased from 0.29 mol per mol of porous liquid at 0 °C to 1.25 mol per mol at 30 °C, opposite to the typical temperature dependence observed in porous solids. At low temperatures, reduced chain motion blocks the cavity entrance, whereas heating increases chain mobility and opens access. In contrast, replacing the surface functional group with a hydroxy group held the chains in a more extended form, allowing gas accessibility to remain open even at low temperatures.
"This work decouples two properties that have previously been difficult to tune independently: pore volume and liquid viscosity," says Furukawa of WPI-iCeMS. "Relatively low-viscosity liquids could be circulated through gas-processing systems, while high-viscosity liquids may be advantageous for membrane-based separations. The surrounding polymer layer can also act as a gate that regulates molecular access to the pores."
Further work will evaluate mixed-gas selectivity, membrane permeation, long-term durability under repeated use, low-temperature flow behavior, production cost, and scalability. More broadly, independently designing the MOP cavity, surface chemistry, and polymer shell provides a modular strategy for tailoring porous liquids to different chemical processes.
