Do Internal and External Surfaces of Metal-Organic Frameworks Have the Same Hydrophobicity? Insights from Molecular Simulations.

Reliable information on the hydrophobicity of porous materials is important in the design of many catalytic and separation processes. In general, hydrophobicity is assessed by measuring the contact angle of water (external surface) or the adsorption isotherm of water (internal surface). However, it is not clear how these different assessments are related. In this paper, molecular dynamics simulations of microscopic water droplets on the external surfaces of metal-organic frameworks are used to investigate the influence of the surface nature and hydrophobicity on the contact angle. The metal-organic frameworks MOF-5 and CAU-10 were modeled with external surfaces of different hydrophobicities, while the internal surface was maintained. It was observed that microscopic droplets orientate their spreading to the nature of the external surfaces. Comparing the simulated contact angles and adsorption isotherms confirms the necessity to distinguish between internal and external hydrophobicity.