Controlling the geometry (Janus balance) of amphiphilic colloidal particles.

A simple, generalizable method is described to produce Janus colloidal particles in large quantity with control over their respective hydrophobic and hydrophilic areas (Janus balance) in large quantity. To this end, charged particles adsorb onto the liquid-liquid interface of emulsions of molten wax and water in the presence of surfactants of opposite charge, whose concentration modifies how deeply particles penetrate the oil-water interface, and subsequent surface chemical modification of the resulting colloidosomes is performed after lowering temperature to solidify the wax. Silica particles modified in this way using different amounts of didodecyldimethylammonium bromide (DDAB) display contact angles that vary controllably between 37 degrees and 75 degrees. Janus balance also varies but over a more limited range with control of pH, salt concentration, or the presence of nonionic surfactant (Tween 20 or ethanol). Purity, Janus balance, and colloidosome structure are evaluated by a combination of fluorescence microscopy, flow cytometry, and scanning electron microscopy (SEM). The three-phase contact angle is obtained by observing SEM images of voids left by particles escaped from the surface. Colloidosomes made in the presence of DDAB are markedly improved with respect to the hexagonal close packing, which helps increase the efficiency of the method. Gram-sized quantities of particles are synthesized.