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Colloidal particles can form unexpected two-dimensional ordered colloidal crystals when they interact with surfactants of the opposite charge. Coulomb interactions lead to self-limited adsorption of the particles on the surface of vesicles formed by the surfactants. The adsorbed particles form ordered but fluid rafts on the vesicle surfaces, and these ultimately form robust twodimensional crystals. This use of attractive Coulomb interaction between colloidal particles and surfactant structures offers a potential new route to selfassembly of ordered colloidal structures.
The self-assembly of colloidal particles is a promising route to materials synthesis that combines considerable flexibility in the choice of materials with the opportunity to create highly ordered structures on length scales from nanometers to micrometers. Self-assembled structures can form resilient coatings, templates for growth of ordered micro- and nanoporous materials (1), and even photonic devices (2-4). Self-assembly depends critically on thermal energy; the resultant motion allows particles to attain the optimum positions to create desired highly ordered structures.
To exploit thermal motion, great care must be taken in controlling the interaction energies between the particles themselves and between the particles and their surroundings. Thermally induced self-assembly typically requires repulsive or only very weakly attractive interactions. Strong attraction usually leads to the formation of highly disordered aggregates, rather than ordered colloidal crystal structures. Because of this, attractive Coulomb interactions, which are strong on thermal scales, are usually not effective in producing self-assembled, ordered structures. Thus, for example, when charge-stabilized colloidal particles are mixed with surfactants of the opposite charge, they typically destabilize and form disordered fractal aggregates (5) because the surfactant binds Coulombically to the particles. This process makes their surfaces hydrophobic and leads to strongly attractive interparticle interactions.
Because of this expectation of destabilization, few studies of mixtures of particles and oppositely charged surfactants have been reported. However, surfactants can self-assemble into a rich array of structures, and the possibility of the interaction of these structures with the oppositely charged colloidal particles is ignored in the disordered-aggregate scenario. Indeed, recent studies of mixtures of positively charged lipids with negatively charged DNA have shown that highly ordered structures can ensue (6, 7).
We report the unexpected formation of large colloidal crystals when aqueous charge-- stabilized polystyrene latex particles are combined with a mixture of an oppositely...