The stable state of charge polydisperse colloids: Disordered or charge ordered?

Abstract

Charge polydisperse colloidal crystals have been recently reported to turn amorphous beyond a critical charge polydispersity (CPD). The resulting disordered (amorphous) state, due to its unusual structural properties, has been conjectured to be different from that obtained from quenching of atomic liquids and is probably a stable state. In order to confirm this, Monte Carlo simulations with different initial configurations are performed and properties of equilibrium states investigated. In contrast to a monodisperse system, which always evolves to a crystalline structure irrespective of the initial configuration, the charge polydisperse system with random charge distribution is found to remain disordered. On the other hand, for the same CPD an ordered crystalline state is found to remain in equilibrium when evolved from a charge-ordered initial configuration. Simulated annealing, by gradually decreasing impurity ion concentration ni (which is equivalent to the lowering of temperature) and evolving at each ni, is also carried out to obtain the low-temperature (ground) state. From the present investigations it is concluded that beyond a critical CPD the charge polydisperse system remains in a stable disordered state unlike atomic glasses (obtained by quenching) which are known to be metastable. Based on some common properties an analogy is made between a spin glass and the amorphous state. The stability aspects of this amorphous state and charge-ordered states are also discussed.