The simplest model for non-congruent fluid-fluid phase transition in Coulomb system

The simplest model for non-congruent phase transition of gas-liquid type was developed in frames of modified model with no associations of a binary ionic mixture (BIM) on a homogeneous compressible ideal background (or non-ideal) electron gas /BIM($\sim$)/. The analytical approximation for equation of state equation of state of Potekhin and Chabrier of fully ionized electron-ionic plasma was used for description of the ion-ion correlations (Coulomb non-ideality) in combination with ``linear mixture'' (LM) approximation. Phase equilibrium for the charged species was calculated according to the Gibbs-Guggenheim conditions. The presently considered BIM($\sim$) model allows to calculate full set of parameters for phase boundaries of non-congruent variant of phase equilibrium and to study all features for this non-congruent phase transition realization in Coulomb system in comparison with the simpler (standard) forced-congruent evaporation mode. In particular, in BIM($\sim$) there were reproduced two-dimensional remarkable (``banana-like'') structure of two-phase region $P-T$ diagram and the characteristic non-monotonic shape of caloric phase enthalpy-temperature diagram, similar to the non-congruent evaporation of reactive plasma products in high-temperature heating with the uranium-oxygen system. The parameters of critical points (CP) line were calculated on the entire range of proportions of ions $0<X<1$, including two reference values, when CP coincides with a point of extreme temperature and extreme pressure, $X_{T}$ and $X_{P}$. Finally, it is clearly demonstrated the low-temperature property of non-congruent gas--liquid transition---``distillation'', which is weak in chemically reactive plasmas.

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