Buckling Effect of Sole Zeolitic Imidazolate Framework-8 Nanoparticles Adsorbed at the Water/Oil Interface

The buckling phenomenon of sole zeolitic imidazolate framework-8 (ZIF-8) particles adsorbed at the water/oil interface was systematically studied. The droplet of ZIF-8 water dispersion was pended in oil for a certain time period and manually extracted to decrease the volume. With the reduction of interfacial area, the ZIF-8 particles were jammed together to form a wrinkling solid film at the water/oil interface, which could withstand the extraction of the droplet and be regenerated. The size and concentration of the particles affected the assembly kinetics. The rapidest assembly was observed for the medium-sized ZIF-8 particles (m-ZIF-8) among the three sizes tested (1.81 μm, 258 nm, and 51 nm). The droplet of 0.91 wt % m-ZIF-8 reached a nearly full surface coverage in 13 min, faster than those with the lower concentration of 0.46 or 0.28 wt %. The pH of the solution, ranging between 6 and 10.7, affected both the assembly kinetics and film stability. Cryo-scanning electron microscopy images of frozen m-ZIF-8-stabilized Picking emulsions showed a monolayer of ZIF-8 wetted by both oil and water phases. The observed buckling effect could be attributed to the stable adsorption of ZIF-8 at the water/oil interface and the interparticle interactions, related to the unique surface chemistry and polyhedral shape of the ZIF-8 crystals. This work provided some understanding on the interfacial property of ZIF-8 and the mechanism of sole ZIF-8-stabilized Pickering emulsions.