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

Abstract

We develop a theory based on the method of collective variables to study the vapor-liquid equilibrium of asymmetric ionic fluids confined in a disordered porous matrix. The approach allows us to formulate the perturbation theory using an extension of the scaled particle theory for a description of a reference system presented as a two-component hard-sphere fluid confined in a hard-sphere matrix. Treating an ionic fluid as a size-and charge-asymmetric primitive model (PM) we derive an explicit expression for the relevant chemical potential of a confined ionic system which takes into account the third-order correlations between ions. Using this expression, the phase diagrams for a size-asymmetric PM are calculated for different matrix porosities as well as for different sizes of matrix and fluid particles. It is observed that general trends of the coexistence curves with the matrix porosity are similar to those of simple fluids under disordered confinement, i.e., the coexistence region gets narrower with a decrease of porosity, and simultaneously the reduced critical temperature T * c and the critical density ρ * i,c become lower. At the same time, our results suggest that an increase in size asymmetry of oppositely charged ions considerably affects the vapor-liquid diagrams leading to a faster decrease of T * c and ρ * i,c and even to a disappearance of the phase transition, especially for the case of small matrix particles.

Vapour-liquid critical parameters of a 2:1 primitive model of ionic fluids confined in disordered porous media

“…As an example, we present some of them in Fig. 1 to demonstrate that for Z = 2 the general trend remains similar to the case of Z = 1 studied in our previous work [14], i.e., a decrease of the porosity φ 0 leads to a shift of the phase coexistence region toward lower temperatures and densities, and simultaneously this region gets narrower. On the other hand, the phase coexistence region of a 2 : 1 PM fluid can get broader due to a size-asymmetry, as it is observed in Fig.…”

“…Both the critical temperature and the critical density are affected by the matrix porosity. It is shown for Z = 1 [14] that the smaller is the porosity the lower is the critical temperature. temperature and the critical density for Z = 1 and Z = 2.…”

“…Recently, we have started a systematic investigation of the phase behaviour of ionic fluids confined in a disordered porous medium, addressing the problem of the effects of confinement on the vapour-liquid phase diagrams [11][12][13][14]. To this end, we have developed two theoretical approaches: the approach exploiting the concept of ion association [15,16] and the other one which is based on the collective variable (CV) method [17][18][19][20].…”

“…The advantage of this approach is that one can derive an analytical expression for the relevant chemical potential which includes the effects of correlations between ions up to the third order and takes into account both the charge-and size-asymmetry at the same level of approximation. In [14], using this expression we calculate the phase diagrams of a monovalent size-asymmetric PM with σ + /σ − = 1, 2 and 3 confined in hard-sphere matrices of different porosities and different diameters of matrix obstacles.…”

Clustering effects on the diffusion of patchy colloids in disordered porous media

“…The SPT2 approach was generalized for fluids of anisotropic particles [17,18], for a hard sphere mixture [19], for a mixture of hard spheres and anisotropic particles [20] in disordered porous media. The obtained results were also used as a reference system to take into account the attractive [21,22], the associative [2] and Coulomb [23][24][25][26][27] interactions. The SPT2 approach is useful not only for the description of thermodynamic properties, but it can be also applied for the description of the static structure of hard sphere fluids in disordered porous media.…”