Vapor-liquid phase behavior of a size-asymmetric model of ionic fluids confined in a disordered matrix: the collective variables-based approach

摘要

We develop a theory based on the method of collective variables to study thevapor-liquid equilibrium of asymmetric ionic fluids confined in a disorderedporous matrix. The approach allows us to formulate the perturbation theoryusing an extension of the scaled particle theory for a description of areference system presented as a two-component hard-sphere fluid confined in ahard-sphere matrix. Treating an ionic fluid as a size- and charge-asymmetricprimitive model (PM) we derive an explicit expression for the relevant chemicalpotential of a confined ionic system which takes into account the third-ordercorrelations between ions. Using this expression, the phase diagrams for asize-asymmetric PM are calculated for different matrix porosities as well asfor different sizes of matrix and fluid particles. It is observed that thecoexistence region gets narrower with a decrease of the matrix porosity, andsimultaneously the reduced critical temperature $T_{c}^{*}$ and the criticaldensity $\rho_{c}^{*}$ become lower. On the other hand, an increase in the sizeof matrix particles at a fixed porosity increases the critical parameters$T_{c}^{*}$ and $\rho_{c}^{*}$, while the size asymmetry of positively andnegatively charged ions essentially strengthens the tendency of $T_{c}^{*}$ and$\rho_{c}^{*}$ towards lower values.