Taras Patsahan

Fluids in random porous media: Scaled particle theory*

The scaled particle theory (SPT) is applied to describe thermodynamic properties of a hard sphere (HS) fluid in random porous media. To this purpose, we extended the SPT2 approach, which has been developed previously. The analytical expressions for the chemical potential of an HS fluid in HS and overlapping hard sphere (OPH) matrices, sponge matrix, and hard convex body (HCB) matrix are obtained and analyzed. A series of new approximations for SPT2 are proposed. The grand canonical Monte Carlo (GGMC) simulations are performed to verify an accuracy of the SPT2 approach combined with the new approximations. A possibility of mapping between thermodynamic properties of an HS fluid in random porous media of different types is discussed. It is shown that thermodynamic properties of a fluid in the different matrices tend to be equal if the probe particle porosities and the specific surface pore areas of considered matrices are identical. The obtained results for an HS fluid in random porous media as reference systems are used to extend the van der Waals equation of state to the case of a simple fluid in random porous media. It is observed that a decrease of matrix porosity leads to lowering of the critical temperature and the critical density of a confined fluid, while an increase of a size of matrix particles causes an increase of these critical parameters.

Nanostructures in a binary mixture confined in slit-like pores with walls decorated with tethered polymer brushes in the form of stripes: Dissipative particle dynamics study

Using dissipative particle dynamics, we investigate the behavior of a binary mixture, exhibiting demixing in a bulk phase, confined in slit-like pores with walls modified by the stripes of tethered brush of chains. Our main interest is to determine possible morphologies that can be formed inside the pore, depending on the geometrical parameters characterizing the system (the size of the pore and the width of the stripes). In order to describe the observed morphologies we calculate several characteristics, as the density and local temperature profiles, the radii of gyration for the attached polymers, and the minimum polymer-polymer distances in the direction parallel and perpendicular to the pore walls. The summary of our findings is presented as a sketch of the diagram of morphologies.

Isotropic–Nematic Transition and Demixing Behavior in Binary Mixtures of Hard Spheres and Hard Spherocylinders Confined in a Disordered Porous Medium: Scaled Particle Theory

We develop the scaled particle theory to describe the thermodynamic properties and orientation ordering of a binary mixture of hard spheres (HS) and hard spherocylinders (HSC) confined in a disordered porous medium. Using this theory, the analytical expressions of the free energy, the pressure, and the chemical potentials of HS and HSC have been derived. The improvement of obtained results is considered by introducing the Carnahan-Starling-like and Parsons-Lee-like corrections. Phase diagrams for the isotropic-nematic transition are calculated from the bifurcation analysis of the integral equation for the orientation singlet distribution function and from the conditions of thermodynamic equilibrium. Both the approaches correctly predict the isotropic-nematic transition at low concentrations of hard spheres. However, the thermodynamic approach provides more accurate results and is able to describe the demixing phenomena in the isotropic and nematic phases. The effects of porous medium on the isotropic-nematic phase transition and demixing behavior in a binary HS/HSC mixture are discussed.

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

Abstract We study the vapour-liquid critical parameters of an ionic fluid confined in a disordered porous medium by using the theory which combines the collective variables approach with an extension of the scaled particle theory. The ionic fluid is described as a two-component charge- and size-asymmetric primitive model, and a porous medium is modelled as a disordered matrix formed by hard-spheres obstacles. In the particular case of the fixed valencies 2:1, the coexistence curves and the corresponding critical parameters are calculated for different matrix porosities as well as for different diameters of matrix and fluid particles. The obtained results show that the general trends of the reduced critical temperature and the reduced critical density with the microscopic characteristics are similar to the trends obtained in the monovalent case. At the same time, it is noticed that an ion charge asymmetry significantly weakens the effect of the matrix presence.

Thermodynamics of Molecular Liquids in Random Porous Media: Scaled Particle Theory and the Generalized Van der Waals Equation

A new approach to the theoretical description of molecular liquids confined in random porous media is proposed in order to study their thermodynamic properties. The models applied in our study are characterized by the intermolecular interactions consisting of repulsive and attractive parts, both of which are of the anisotropic nature. To take into account an anisotropy of the repulsion the scaled particle theory (SPT) is extended for the system of a hard convex body (HCB) fluid in a quenched matrix of hard particles forming a random porous medium. A contribution of the anisotropic attractive interaction is considered on the level of the mean-field or Van der Waals approximation. Therefore, combining the obtained analytical results within the framework of the perturbation theory the equation of state for confined liquids is derived. On the basis of the developed approach we can consider a fluid in a random matrix using various models. A reliability of the SPT theory is proved on the examples of hard sphere and hard spherocylinder fluids in different matrices. For a spherocylinder fluid with attractive intermolecular interaction the phase transition diagrams are constructed to study a vapour-liquid-nematic equilibrium and the effect of confinement on it. It is shown that a matrix porosity decrease leads to decreasing of the critical temperature and the critical density of vapour-liquid phase transition. In the case of long spherocylinders (\(L_{1} /D_{1} = 10\)) the vapour-liquid transition of a fluid in a matrix can disappear completely being suppressed by the isotropic-nematic phase transition. On the other hand the coexistence between vapour and nematic phases is observed for a spherocylinder fluid at the conditions comparable to the Onsager limit (\(L_{1} /D_{1} = 80\)). The anisotropy of attractive potential causes the broadening of the liquid-nematic coexistence region and in the case of essentially high rates of anisotropy the vapour-liquid transition vanishes. It is noticed that the presence of porous medium enhances this effect. The presented review is aimed to illustrate an application of the SPT approach which developed recently for fluids of non-spherical molecules confined in random porous media.