Abstract
Gas adsorption in metal-organic-frameworks (MOFs) can dramatically affect the size of the crystals (expansion and/or shrinkage) or lead to distortion of their porous structure/framework. This can strongly affect mechanical properties of MOFs potentially leading to loss or improvement of the performance of applications such as membranes, filters, or sensors. Here, we utilize white light multiple-beam-interferometry (MBI) in a surface forces apparatus (SFA) to measure in real-time the deformations taking place in HKUST-1 crystals during humidity adsorption/desorption cycles. MBI provides a real-time measurement of crystal deformations during guest molecule uptake with msecs time and angstrom distance resolution. We find unusual and unexpected dynamic and nonmonotonic deformation behavior upon humidity loading in HKUST-1 crystals, which we attributed to the gradual filling of the different adsorption sites in the MOF crystal framework. Also, the effect of the external mechanical pressure applied to hydrated/dehydrated crystals strongly affects the flexibility of HKUST-1 crystals and hence their deformation characteristics during guest molecule loading. Our results may have important implications for better understanding/modeling of the effect of gas adsorption on the mechanical properties of MOFs. MBI may be extended not only to other MOFs but also to any other guest-molecule systems in which adsorbate/adsorbent interactions affect structure-property relationships.