Gas-Solid Coexistence in Highly Charged Colloidal Suspensions

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Aqueous suspensions of highly charged polystyrene particles with different volume fractions have been investigated for structural ordering and phase behavior using static light scattering (SLS) and confocal laser scanning microscope (CLSM). Under deionized conditions, suspensions of high charge density colloidal particles remained disordered whereas suspensions of relatively low charge density showed crystallization by exhibiting iridescence for the visible light. Though for unaided eye crystallized suspensions appeared homogeneous, static light scattering measurements and CLSM observations have revealed their inhomogeneous nature in the form of coexistence of voids with dense ordered regions. CLSM investigations on disordered suspensions showed their inhomogeneous nature in the form coexistence of voids with dense disordered (amorphous) regions. Our studies on highly charged colloids confirm the occurrence of gas-solid transition and are in accordance with predictions of Monte Carlo simulations using a pair-potential having a long-range attractive term [Mohanty and Tata, Journal of Colloid and Interface Science 2003, 264, 101]. Based on our experimental and simulation results we argue that the reported reentrant disordered state [Yamanaka et al Phys. Rev. Lett. 1998, 80, 5806 and Toyotama et al Langmuir, 2003, 19, 3236] in charged colloids observed at high charge densities is a gas-solid coexistence state.