Experiments have shown that negatively charged colloids experience a long-range attraction and spontaneously order in a solution of counterions, but positively charged colloids do not order under similar circumstances. This asymmetry can be understood in terms of the bent geometry of water molecules as shown in the diagram on the right. There is frustration in the arrangement of water molecules around neighboring like-charged particles in (A) and (B). Due to the bent-core geometry of the water molecules, they arrange themselves in a more favorable configuration for larger negative particles with smaller positively charged particles (C) than in the reverse scenario (D). A mean-field analytical model and numerical simulations were applied to a colloidal system of negatively charged disks and counterions that are absorbed on an interface in a compressible binary system. The system results in a dilute isotropic ionic phase and a condensed hexagonal lattice phase.
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