Phase separation and structure in a concentrated colloidal dispersion of uniform plates

The structure and phase behaviour of a colloidal dispersion of prate-like p articles are described. The plates are nickel (II) hydroxide and have short -range, repulsive interactions and a low polydispersity. As the concentrati on of the plates is increased, an equilibrium phase separation between a co lumnar phase and a less ordered phase is observed. Complementary measuremen ts using small-angle neutron and small-angle X-ray scattering have been use d to distinguish the columnar phase from other possible ordered structures. Previously isotropic-nematic phase transitions have been observed [1], how ever this dispersion forms the more highly ordered columnar phase, due to t he aspect ratio and the low polydispersity of the plate-like particles. The concentration at which phase separation occurs, increases as the range of the particle interactions is reduced. This system provides an interesting m odel for comparison with theory and calculations of structures in liquid cr ystal and mesophase in which the particle interactions can be altered.