(Nanowerk News) The “nothing” (empty space) feature of porous soft materials is the basis of their properties and applications, and has been the focus of study for many years. Now, researchers from Japan have solved a long-standing conundrum of these materials that will accelerate research and development, and increase their utility in practical devices.
In a study recently published in Proceedings of the National Academy of Sciences (“Elastic heterogeneity governs asymmetric adsorption-desorption in soft porous crystals”), researchers from the Institute of Industrial Science, The University of Tokyo have revealed the importance of a physical property—elastic heterogeneity—in tuning the molecular adsorption/desorption properties of a general class of soft porous materials: metal-organic frameworks (MOFs).
MOFs, a focus of research since the 1990s, are sponge-like materials. They are highly tunable, meaning that their size, shape, and composition can be changed by changing the metal ions and organic connectors used to make them. This adjustment can eventually change its properties, such as mechanical flexibility. In addition, after adsorbing molecules (known as guests), the MOF crystal structure (known as host) can undergo non-uniform shape changes known as elastic heterogeneity. The resulting stimulus-responsiveness and guest adsorption selectivity of MOFs have led to many applications, such as sensors, supercapacitors, and drug delivery devices.
To improve advanced MOF designs, researchers have long sought to understand the relationship between the macroscopic properties of elasticity and the corresponding microscopic host-guest interactions. Gaining such an understanding by computational simulation is a problem researchers are looking to tackle.
“Our simplified statistical mechanical model reveals the role of guest adsorption and desorption in elastic heterogeneity,” explains Kota Mitsumoto, lead author of the study. “We mathematically relate lattice expansion and contraction to the energetics and thermodynamics of the host-guest interaction.”
The simulation focuses on two domain classes, which are the compositional regions in MOF: adsorbed guests, which are more difficult; and absorbed by the guest, which is softer. The researchers’ main result is that the shape of these domains depends on the differences in elastic stiffness between domains. Guest adsorption is associated with compact domains, while guest desorption is associated with flattened domains.
“We deduce the entropic and energetic contributions to the transition between guest adsorption and desorption,” said Kyohei Takae, senior author. “Thus, we provide physicochemical insight into the origin of elastic heterogeneity in MOFs and analogous materials.”
This work has applications for imparting targeted properties to soft porous materials. For example, compact domains can facilitate robust guest confinement, and thus applications such as gas storage. Alternatively, flattened domains can increase the surface area of the MOF, and thus facilitate chemical reactions. Sensors and many other stimulus-responsive materials will benefit from this insight.