Synthesis of pH-Responsive Inorganic Janus Nanoparticles and Experimental Investigation of the Stability of Their Pickering Emulsions.

Pickering emulsions exhibit outstanding stability, especially those prepared with Janus particles, whose desorption energy is expected to be up to 3-fold greater than emulsions of homogeneous particles from theoretical calculations. To the best of our knowledge, however, there remains no experimental proof of this behavior in practice. In this study, inorganic Janus nanoparticles were fabricated by regioselective modification of the separate side of SiO2 nanoparticles with a judiciously selected mixture of trimethoxysilylpropyldiethylenetriamine and n-octyltrimethoxysilane. Janus nanoparticles demonstrated excellent interfacial activity, forming Pickering emulsions with oil phases at oil-water interfacial tensions ranging from 6.6-52.8 mN m-1. Furthermore, as the interface of the Janus nanoparticles was regionally functionalized with -NH2 groups, phase inversion could be realized by tuning pH. This is the first example for the Pickering emulsions stabilized with inorganic Janus particles. Importantly, based on the results of centrifugation experiment, the desorption energy of Janus nanoparticles at the interface was 3.2 times larger than that of homogeneous nanoparticles, which is in accordance with the result from theoretical calculations. These experimental results will substantially enrich our understanding of Janus nanoparticle Pickering emulsions and their interfacial assembly behavior.