Screening in Ionic Systems: Simulations for the Lebowitz Length

Simulations of the Lebowitz length, ξ_L(T,ρ), are reported for the restricted primitive model hard-core (diameter a) 1∶1 electrolyte for densities ρ≲4ρ_c and T_c≲T≲40T_c. Finite-size effects are elucidated for the charge fluctuations in various subdomains that serve to evaluate ξ_L. On extrapolation to the bulk limit for T≳10T_c the exact low-density expansions are seen to fail badly when ρ>1/10ρ_c (with ρ_ca³≃0.08). At higher densities ξ_L rises above the Debye length, ξ_D∝√T/ρ, by 10%–30% (up to ρ≃1.3ρ_c); the variation is portrayed fairly well by the generalized Debye-Hückel theory. On approaching criticality at fixed ρ or fixed T, ξ_L(T,ρ) remains finite with ξ_L^c≃0.30a≃1.3ξ_D^c but displays a weak entropylike singularity.