Phase transition between liquid-like and crystal-like structures of deionized colloidal suspensions

Abstract

Transmitted-light spectra exhibiting the characteristic profiles of crystal-like and liquid-like distributions have been observed for suspensions of monodisperse polystyrene spheres (85–173 nm in diameter). The intersphere distance (D) in the liquid-like structures decreases continuously as the sphere concentration increases, and D jumps up by several percent at the critical concentration, at which the liquid-like structure transforms to the crystal-like distribution. A spinodal decomposition type transition is observed in the plots of D against sphere concentration. The critical concentrations of the transition are 0.005, 0.002, 0.005 and 0.025 (in volume fraction) for the spheres of 85, 91, 109 and 173 nm in diameter. The crystal-like distribution transforms to the liquid-like structure at a certain concentration of NaCl accompanied with the dropping down of the D values. A phase diagram containing the regions of (1) liquid-like, (2) face-centred cubic (f.c.c.), (3) body-centred cubic (b.c.c.) distributions and (4) the coexistence of f.c.c. and b.c.c. type crystal-like structures is obtained. These results are consistent with the significant role played by the electrical double layers under the influence of purely electrostatic intersphere repulsion in the effective hard-sphere model.