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Criticality in Charge-asymmetric Hard-sphere Ionic Fluids


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      Phase separation and criticality are analyzed in \(z\):1 charge-asymmetric ionic fluids of equisized hard spheres by generalizing the Debye-H\"{u}ckel approach combined with ionic association, cluster solvation by charged ions, and hard-core interactions, following lines developed by Fisher and Levin (1993, 1996) for the 1:1 case (i.e., the restricted primitive model). Explicit analytical calculations for 2:1 and 3:1 systems account for ionic association into dimers, trimers, and tetramers and subsequent multipolar cluster solvation. The reduced critical temperatures, \(T_c^*\) (normalized by \(z\)), \textit{decrease} with charge asymmetry, while the critical densities \textit{increase} rapidly with \(z\). The results compare favorably with simulations and represent a distinct improvement over all current theories such as the MSA, SPB, etc. For \(z\)$\ne\(1, the interphase Galvani (or absolute electrostatic) potential difference, \)\Delta \phi(T)\(, between coexisting liquid and vapor phases is calculated and found to vanish as \)|T-T_c|^\beta\( when \)T\to T_c-\( with, since our approximations are classical, \)\beta={1/2}\(. Above \)T_c\(, the compressibility maxima and so-called \)k\(-inflection loci (which aid the fast and accurate determination of the critical parameters) are found to exhibit a strong \)z$-dependence.

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      27 October 2004
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      25 pages, 14 figures; last update with typos corrected and some added references


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