Functionalization of Monolithic MOF Thin Films with Hydrocarbon Chains to Achieve Superhydrophobic Surfaces with Tunable Water Adhesion Strength

Abstract

While the accessible pores render an enormous variety of functionalities to the bulk of metal-organic frameworks (MOFs), the outer surfaces exposed by these crystalline materials also offer unique characteristics not available when using conventional substrates. By grafting hydrocarbon chains to well-defined MOF thin films prepared using layer-by-layer methods (SURMOFs), we were able to fabricate superhydrophobic substrates with static water contact angles over 160°. A detailed theoretical modelling of the hydrocarbon chains grafted on the outer SURMOF surface with well-defined spacing between anchoring points reveal that the grafted hydrocarbon chains compare well to polymer brushes during wetting, where conformational entropy is traded with mixing entropy: The chains are coiled and can access many different conformations, as evidenced directly by infrared spectroscopy. The entropic contributions from the coiled state lead to a pronounced reduction of the surface free energy, rendering superhydrophobic properties to the functionalized SURMOFs. On the other side, the water adhesion strength could be decreased by increasing the surface roughness on the nanometer scale.