Fluid In Slit-like Pores Research Articles

Density functional approach to adsorption of a polydisperse fluid in slit-like pores

Density functional approach to adsorption of a polydisperse fluid in slit-like pores

Evaluation of liquid–vapor density profiles for associating fluids in pores from density-functional theory

Using density-functional theory we calculate density profiles of an associating fluid in slit like pores as functions of two variables: The distance from the pore wall and the distance along the pore axis. Attention is focused on evaluation of the profiles characterizing the coexistence between two confined phases. We also calculate changes in the grand canonical potential connected with the formation of an interface between two coexisting confined phases. Specific calculations have been carried out for the associating, chain forming Lennard-Jones fluid adsorbed in a slitlike pore.

Application of the density functional method to study phase transitions in an associating Lennard-Jones fluid adsorbed in energetically heterogeneous slit-Like pores

A density functional approach is used to study the adsorption of the four-bonding-site model associating Lennard-Jones fluid in slit-Like pores with energetically heterogeneous walls. The fluid-wall potential is qualitatively similar to that invoked by Röcken, P., Somoza, A., Tarazona, P., and Findenegg, G. H., 1999, J. chem. Phys., 108, 8089, i.e. it consists of a homogeneous part that varies in the direction perpendicular to the wall and a periodic part, varying also in one direction parallel to the wall. Both parts are modelled by Lennard-Jones 9,3-type functions. The structure of the adsorbed film is characterized by the local densities of all particles and the densities of the monomers. The phase diagrams are evaluated for several systems characterized by different corrugation of the adsorbing potential. The adsorbing field is strong enough to allow for the layering transition. As well as the formation of the so-called bridge phase that fills the pore space over the most energetic parts of the wall and of capillary condensation, the layering transition is observed within the first layer adjacent to the pore walls. If the adsorbing potential due to each pore wall is shifted in phase by π/2, the bridge phase is not formed.

Phase transitions in an associating, network-forming, Lennard-Jones fluid in slit-like pores. II. Extension of the density functional method

We study adsorption of hydrogen-bonded fluids in slit-like pores with strongly attractive walls, in the framework of the four-site associating Lennard-Jones model. The density profiles, as well as the phase behavior, are obtained by using a density functional method. We have found that, at temperatures lower than the critical temperature of the bulk fluid, the confined fluid undergoes one or more layering transitions dependent on the pore width, followed by capillary condensation. Each of the transitions is localized by analyzing the grand thermodynamic potential. The density profiles of nonbonded and differently bonded particles demonstrating changes of the structure of the fluid in the pore along the coexistence are discussed briefly. The critical temperature for capillary condensation is lower for confined fluid, compared with that for the bulk liquid–vapor transition, as expected. However, an increase of the energy of association between fluid species increases the critical temperatures for layering transitions and for capillary condensation. The envelope of the capillary condensation is narrower than the bulk liquid–vapor phase diagram. The ratio between the critical temperatures for layering transitions and capillary condensation depends on the pore width. The critical temperature for the second layering is always lower than for the first one. The triple point temperature between either the second layering transition and the capillary condensation (in wider pores) or the first layering transition and the capillary condensation (in narrower pores) increases with decreasing pore width. The triple point temperature between the layering transitions is much lower than the relevant temperature between the second layering transition and the capillary condensation. The triple point temperatures also depend on the association energy. We have shown that highly bonded fluid species prevail at triple point temperatures.

Monte Carlo simulation of hard sphere fluids in planar slits with molecularly rough surfaces

The structural and thermodynamic behaviour of hard-sphere fluids confined in planar slit-like pores with hard rough walls is investigated by canonical Monte Carlo simulations. The surfaces of the pores are modelled by means of parallel impenetrable walls. Additionally a molecular “roughness” of the pore surfaces is introduced using several models of randomly distributed hard spheres located (fixed or restricted in motion) at the walls. The density distribution of the hard spheres in the pore and the pressure normal to the walls are estimated and compared with the behaviour of fluids in slit-like pores with smooth hard walls. The results are discussed in the context of experimental studies of fluids confined in the polar interfaces of bilayers.

Monte Carlo Computer Simulation of Adsorption of Diatomic Fluids in Slitlike Pores

Monte Carlo simulations in the canonical ensemble were carried out to investigate the dependence of the behavior of ethane and a model system (when molecules are longer than ethane) in the adsorbing slitlike pores on the average density, the force of the adsorption field, and the temperature. In the nearest to the walls adsorbed layers the tendency of ethane to orientational transitions was revealed when the temperature was decreased from 180 to 160 K. There are no orientational transitions in the systems of the molecules longer than ethane. At T = 140 K the orientation of ethane molecules perpendicular to the walls is absolutely preferential.

Density profiles of chemically reacting fluids in slit-like pores

A model of a chemically reacting fluid in a slit-like pore is studied. The density profiles of the fluid particles inside the pore, the adsorption isotherms, and the solvation force acting between the walls of the pore are investigated within the integral equation method. The Percus-Yevick and hypernetted chain approximations for the fluid particle-pore correlations are used. The model for the bulk chemically reacting fluid is that proposed by Cummings and Stell for a mixture of overlapping hard spheres. The investigations are performed for pores of different width, at different densities of the bulk fluid which is in an equilibrium with the confined fluid and for different degrees of bulk fluid association. The bulk fluid is described by using the Percus-Yevick approximation. A wide range of thermodynamic states and different bonding lengths between the reactants are investigated. It is shown that the effect of a chemical reaction in the bulk fluid produces qualitative changes in the behaviour of the den...

Perturbation density functional theory and Monte Carlo simulations for the structure of hard triatomic fluids in slitlike pores

A model fluid composed of semiflexible tangent hard sphere trimers is investigated using Monte Carlo simulations and perturbation density functional theory (PDFT), focusing on the density profile and conformational properties in the vicinity of a hard wall. The surface density is reduced (enhanced) at low (high) densities. We also observe preferential end absorption. Both features arise from competition between chain conformational entropy and packing constraints. Molecules adjacent to the wall tend to lie flat against it, particularly at high density and/or stiffness. But the bond angle distribution is only weakly affected by the presence of the wall. For rigid molecules, the density profiles depend strongly on the value of the bond angle. PDFT is in excellent agreement with simulation, and promises to be a successful means of elucidating the interfacial structure of complex fluids.

Integral equation theory for the adsorption of chain fluids in slitlike pores

The adsorption of hard chains in slitlike pores is studied via an integral equation theory. The theory uses the growing adsorbent model in conjunction with the polymer reference interaction site model (polymer-RISM) theory to predict the wall–fluid correlation functions; wall–fluid and fluid–fluid correlations are treated using the Percus–Yevick closure relations. The theory is compared to Monte Carlo simulation data for the density profiles of 4-mers and 8-mers in slitlike pores; the theory is fairly accurate, though it tends to overestimate the density at the wall at low densities and underestimate the density at the wall at high densities. The theory also underestimates the amplitude of the oscillations in the density profile. The theory is very accurate, however, for the adsorption isotherm of 4-mers and 8-mers.