Formation of Supraparticle Monolayers Using Liquid Interfaces

Colloidal supraparticles are spherical, micrometer-scale assemblies of primary colloidal particles that exhibit emergent properties arising from the defined internal structural arrangement. A strategy to exploit their functionality on the macroscale is to use supraparticles themselves as building blocks to prepare materials with hierarchical structural order. In this study, we investigate the formation of macroscopic, two-dimensional supraparticle monolayers by the self-assembly of polystyrene and silica supraparticles at the air/water and oil/water interface. At the air/water interface, hydrophilic silica supraparticles self-assemble into densely packed monolayers, while hydrophobic polystyrene supraparticles form loosely packed, disordered agglomerates due to dominant capillary interactions arising from differences in contact angle of both supraparticle types. The decane/water interface promotes the formation of nonclose-packed hexagonal arrangements of polystyrene supraparticles, which collapse into agglomerates at increasing packing densities. The addition of Span 80 as a surfactant to the decane phase lowers the interfacial tension and increases the contact angle, thereby reducing capillary attraction and enabling the formation of close-packed supraparticle monolayers. Monolayers transferred to solid substrates retain their structural order and exhibit structural coloration. This study establishes general design principles for the interfacial self-assembly of supraparticles as a key requirement to form defined macroscopic materials from supraparticle building blocks.