Gas-liquid Transition in Charged Fluids

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Abstract
The connexion between the equation of state of a classical fluid of non-polarizable ions, the character of the screening, and the appearance of long range oscillations in the chargetharge radial distribution function is examined. While considerations of stability lead to the usual inequalities for the inverse static dielectric function and the compressibility of the charged fluid, the square of the inverse screening length k 3, does not need to be positive for thermodynamic stability. Through a study of an approximate equation of state for a twocomponent fluid of charged hard spheres, the regions of negative and positive k 2 in the pressuredensity plane are related to a liquid phase and to an ionized-gas phase, respectively. The model fluid displays a gas-liquid critical point, above which the transition between the two types of screening is continuous. This behaviour of the charged-hard-spheres fluid is contrasted with the transition of a real ionic liquid to the molecular gaseous phase.