Janus particles with varying configurations for emulsion stabilization

Analogous to molecular surfactants, amphiphilic Janus particles can function as surface active agents to stabilize emulsions. For molecular surfactants, the shape and amphiphilicity, described by packing parameters and hydrophilic-lipophilic balance (HLB), respectively, determine the type of emulsions that are produced. However, it remains unclear which features of Janus particles determine the type of emulsion that can be stabilized by these particles. In this study, a homologous series of amphiphilic Janus particles with shifting Janus boundaries and tunable chemical properties are synthesized via seeded emulsion polymerization. We systematically study the effect of the relative size of the hydrophilic domain to the total particle (configuration) and chemical nature of Janus particles on their emulsification properties. The concept of the Janus structure parameter (JSP) is introduced to describe the configuration of Janus particles. The emulsification results show that the configuration of Janus particles has a limited effect on the emulsion type when the contrast in the water affinity of the two domains is weak. In contrast, the configurations of Janus particles determines the emulsion type when there is a significant difference in water affinity between the two regions of Janus particles. In this case, the threshold of JSP value for emulsion inversion is found to be 0.48, which indicates that Janus particles with larger hydrophobic domains prefer to stabilize water-in-oil (w/o) emulsion and those with larger hydrophilic domains stabilize oil-in-water o/w emulsion. Our findings on the correlations between the configurations of Janus particles and the type of emulsion that are stabilized are crucial for the design of Janus particles for the stabilization of multiphasic fluid mixtures and their applications.