Hierarchical assembly of mixed 2D surfactants in polyHIPEs: tuning conductive networks through multi-scale structuring

Abstract

Hierarchical assembly of two-dimensional (2D) surfactants within high internal phase emulsions (HIPEs) provides a direct route to multifunctional open-cell foams; however, the relationships between particle distribution, emulsion architecture, and macroscopic properties remain poorly understood. In this study, we investigate polyHIPE composites stabilized by dual 2D surfactants by systematically varying surfactant type, size, and mixing protocols. Specifically, we examine emulsions stabilized either by graphene sheets of distinct lateral sizes (1 µm and 10 µm) or combinations of graphene and boron nitride (BN) sheets. Additionally, we explore how surfactant mixing order, pre-mixed (combining surfactants before emulsification) versus post-mixed (combining separately emulsified components), influences emulsion stability and foam morphology. We observed cell-size variation in pre-mixed, 1 µm vs. 10 µm polyHIPES, with the size varying from 192 µm (for 1 µm control) to ∼1076 µm (for 10 µm control) and 280 µm for the pre-mixed samples. Pre-mixed systems demonstrated that 2D surfactants were well-mixed into the emulsion cell walls, whereas post-mixed emulsions retained distinct phase-separated shapes. The resulting polyHIPEs demonstrated promise in controlling electrical conductivity (0.31 S m⁻¹ for post-mixed G-BN polyHIPEs vs. undetectable for pre-mixed G-BN polyHIPEs) enabling the creation of composites with tunable conductive properties. They also exhibited good viscosity and structural strength, making them suitable for use in 3D-printable material systems. This research provides new insights for designing functional composite materials using 2D surfactant combinations, thereby opening up new possibilities for next-generation emulsion-templated materials.