A research team from Kyoto University, including Xiangmei Xiang, Dr. Zaoming Wang, Professor Kenji Urayama, and Professor Shuhei Furukawa from the Institute for Integrated Cell-Material Sciences (WPI-iCeMS), along with collaborators in Japan and Taiwan, has developed a new method to adjust the thickness of type I porous liquids — materials that have tiny, empty spaces inside them — by over eight orders of magnitude, without changing their internal structure. This breakthrough was published in the Journal of the American Chemical Society. The team used special cage-like molecules called cuboctahedral rhodium-based metal-organic polyhedra (MOPs), which naturally have porous structures. They attached twelve flexible chains of polyethylene glycol (PEG) to each MOP core, forming star-shaped molecules that then self-assemble into solvent-free type I porous liquids. By altering four different MOP surface groups and using two different PEG chain lengths, the researchers created eight unique porous liquids. In each set of liquids with the same PEG chain length, the MOP cavity size and the number and length of PEG chains remained the same, with only the MOP surface chemistry varying. Using advanced techniques like synchrotron X-ray scattering and molecular dynamics simulations, the team discovered that the chemistry of the MOP surface influences the shape of the PEG chains. When the surface had dodecyloxy groups — which are hydrophobic — the chain ends were drawn to these groups, causing the chains to fold tightly around the MOP surface. This tight wrapping made the polymer shells less likely to interpenetrate, allowing the liquid to flow more easily, resulting in lower viscosity. On the other hand, when the MOP surface had hydroxy groups — which are hydrophilic — the chains remained extended, forming a network that resisted flow and increased viscosity. At 60°C (140°F), the viscosity of these liquids ranged from 18 Pa·s to 3.0 billion Pa·s, a difference of about 170 million times. The most viscous materials behaved like solids for short periods but eventually flowed over longer times, showing that they are viscoelastic — meaning they have both liquid and solid properties. Carbon dioxide (CO₂) adsorption tests confirmed that all these porous liquids retained their internal cavities at 30°C (86°F). In one series of liquids with long PEG chains, the polymer shell acted like a temperature-sensitive gate. When the surface was dodecyloxy-functionalized, CO₂ uptake increased significantly with temperature, unlike typical porous solids. This is because at lower temperatures, the PEG chains blocked the cavity entrances, while at higher temperatures, the chains moved more freely, opening the cavities. When the surface was hydroxy-functionalized, the chains remained extended, keeping the cavities accessible even at lower temperatures. This research separates two properties — pore volume and liquid viscosity — that were previously difficult to adjust independently. Low-viscosity liquids could be useful in systems that need to circulate gases, while high-viscosity liquids might be better for separating gases using membranes. The polymer layer can also act as a gate, controlling access to the pores. Future studies will examine how these materials handle mixed gases, their performance in membranes, durability over time, and their behavior at low temperatures. Overall, this modular approach allows scientists to design porous liquids tailored for specific chemical applications.