Effect of a nearby charge-ordered phase on correlation functions in ionic systems

The charge-charge and number-number correlation functions are investigated using a mean-field density functional for the restricted primitive model (hard-sphere and Coulomb interactions) supplemented with short-ranged attractive forces. The system exhibits phase separation into ion-dilute and ion-dense phases, and the latter phase becomes unstable with respect to charge-ordering along the lambda-line. In mean field approximation both the range and the amplitude of the charge-charge correlation function increase as (S-S-lambda)(-1/2) on the approach to the lambda-line, where S=T-*/rho(0)(*) and S-lambda is the value of S at the lambda-line (T-* is the reduced temperature and rho(0)(*) is the dimensionless density). The line dividing the phase diagram into regions where the range of charge-charge correlations is longer (shorter) than the range of the number-number correlations is also determined. We argue that the large range and large amplitude of the charge-charge correlation function is consistent with the formation of aggregates (living polymers) observed in simulations of ionic systems. (C) 2003 American Institute of Physics.