Design of Janus Nanoparticles with pH-Triggered Switchable Amphiphilicity for Interfacial Applications

The ability of amphiphilic Janus nanoparticles (JNPs) to partition at the oil–water and air–water interfaces can be especially attractive for a plethora of new applications. Unlike molecular surfactants, the JNPs could act as "bulk-to-surface" carriers of different small molecules/actives or bulklike properties—magnetic, optic, or electric—without a dramatic effect on their ability to partition at interfaces. Here we report surfactant-free JNPs with pH-triggered switchable amphiphilicity that could be used in such interfacial applications. The polarity balance of the JNPs can be tuned by the pH such that the amphiphilicity of JNPs is switched on or off at low (pH <6.0) or high (pH >7.0) pH values, respectively. When the amphiphilicity of JNPs is switched off, the interfacial activity of JNPs is comparable to that of homogeneous nanoparticles (HNPs), and when switched on, the interfacial activity is enhanced, proving that the amphiphilicity of JNPs plays a role in enhancing the interfacial activity even at scales above 100 nm. In addition, we show that the ability to lower the interfacial tension (ΔIFT) is likely the most appropriate indicator to compare the interfacial activities of different particles because it does not scale with the size of the particles like the calculated activation energy Ea, or attachment energy ΔE. By comparing the mentioned parameters, we conclude that the enhancement in the interfacial activity is slightly larger than the earlier theoretical predictions that only compare the ΔE values. These pH-responsive JNPs were further applied in the generation of pH-responsive Pickering emulsions. Upon adsorption of these JNPs at the oil–water interfaces, the particle-laden interface acquires a pH-responsiveness functionality. For example, Pickering emulsions stabilized by the above pH-responsive JNPs undergo reversible water-in-oil (w/o) to oil-in-water (o/w) phase inversion when the aqueous phase changes from basic to acidic.