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ABSTRACT: We apply recently developed version of a density functional theory [Z. Wang, L. Liu, and I. Neretnieks, J. Phys.: Condens. Matter 23, 175002 (2011)] to study adsorption of a restricted primitive model for an ionic fluid in slit-like pores in the absence of interactions induced by electrostatic images. At present this approach is one of the most accurate theories for such model electric double layers. The dependencies of the differential double layer capacitance on the pore width, on the electrostatic potential at the wall, bulk fluid density, and temperature are obtained. We show that the differential capacitance can oscillate as a function of the pore width dependent on the values of the above parameters. The number of oscillations and their magnitude decrease for high values of the electrostatic potential. For very narrow pores, close to the ion diameter, the differential capacitance tends to a minimum. The dependence of differential capacitance on temperature exhibits maximum at different values of bulk fluid density and applied electrostatic potential.
The Journal of chemical physics 12/2012; 137(23):234705. · 3.09 Impact Factor
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ABSTRACT: We use a version of the density functional theory to study the solvation
force between two plates modified with a tethered layer of chains. The chains
are built of tangentially jointed charged spherical segments. The plates are
immersed in an electrolyte solution that involves cations, anions and solvent
molecules. The latter molecules are modelled as hard spheres. We study the
dependence of the solvation force and the structure of chains and of solute
molecules on the grafting density, length of chains, architecture of the chains
and on concentration of the solute.
10/2012;
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ABSTRACT: We apply density functional theory to study adsorption of ions, treated in the framework of the restricted primitive model (RPM), on surfaces modified by tethered polyampholytes. The residual electrostatic contribution to the free energy functional is approximated by using the approach proposed by Wang et al. [J. Phys.: Condens. Matter 23, 175002 (2011)] for simple nonuniform RPMs systems. Our research concentrates on the problems how the distribution of the charges within chains of polyampholytes changes the selectivity of adsorption of ions species, the structure of the surface layer, and its electric properties.
The Journal of chemical physics 08/2012; 137(7):074707. · 3.09 Impact Factor
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ABSTRACT: A recently proposed local second contact value theorem [Henderson D., Boda
D., J. Electroanal. Chem., 2005, 582, 16] for the charge profile of an electric
double layer is used in conjunction with the existing Monte Carlo data from the
literature to assess the contact behavior of the electrode-ion distributions
predicted by the density functional theory. The results for the contact values
of the co- and counterion distributions and their product are obtained for the
symmetric valency, restricted primitive model planar double layer for a range
of electrolyte concentrations and temperatures. Overall, the theoretical
results satisfy the second contact value theorem reasonably well, the agreement
with the simulations being semi-quantitative or better. The product of the co-
and counterion contact values as a function of the electrode surface charge
density is qualitative with the simulations with increasing deviations at
higher concentrations.
07/2012;
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ABSTRACT: The paper discusses the ground state properties and dynamics of monolayer films formed by atomic adsorbates on a square lattice,
being the (100) plane of a face centered cubic crystal. The vibrations of films ordered into the commensurate c(2×2) as well as into the recently discovered ordered phase exhibiting a distorted Archimedean tiling of the type (32.4.3.4) are considered. The dispersion relations and the densities of states are determined and discussed.
Adsorption 04/2012; 15(3):254-263. · 2.00 Impact Factor
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ABSTRACT: Using the results of extensive Monte Carlo simulations in the canonical and grand canonical ensembles, we discuss the phase behavior of mixed submonolayer films of krypton and xenon adsorbed on the graphite basal plane. The calculations have been performed using two- and three-dimensional models of the systems studied. It has been demonstrated that out-of-plane motion does not affect the properties of the films as long as the total density is well below the monolayer completion and at moderate temperatures. For the total densities close to the monolayer completion, the promotion of particles to the second layer considerably affects the film properties. Our results are in a reasonable agreement with the available experimental data. The melting point of submonolayer films has been shown to exhibit non-monotonous changes with the film composition, and reaches minimum for the xenon concentration of about 50%. At the temperatures below the melting point, the structure of solid phases depends upon the film composition and the temperature; one can also distinguish commensurate and incommensurate phases. Two-dimensional calculations have demonstrated that for the xenon concentration between about 15% and 65% the adsorbed film exhibits the formation of a superstructure, in which each Xe atom is surrounded by six Kr atoms. This superstructure is stable only at very low temperatures and transforms into the mixed commensurate (√3×√3)R30° phase upon the increase of temperature. Such a superstructure does not appear when a three-dimensional model is used. Grand canonical ensemble calculations allowed us to show that for the xenon concentration of about 3% the phase diagram topology of monolayer films changes from the krypton-like (with incipient triple point) to the xenon-like (with ordinary triple point).
The Journal of chemical physics 04/2012; 136(14):144702. · 3.09 Impact Factor
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ABSTRACT: A density functional study of adsorption from binary solutions on surfaces modified with tethered chains is presented. The tethered chains are modeled as freely jointed tangent spheres. The fluid molecules are spherical. All species interact via the Lennard-Jones (12-6) potential. The substrate is neutral with respect to all chain segments but the surface-binding segment, and it interacts with the fluid molecules via a Lennard-Jones (9-3) potential. Depending on the parameters of the model, different shapes of the relative excess adsorption isotherms are found. The density profiles of all components are analyzed. It is shown that the surface region is highly inhomogeneous. An influence of the grafting density, the length of grafted chains, the nature of the solution, and its composition on the adsorption mechanism and the structure of the bonded-phase is investigated. The theoretical predictions are consistent with the results of computer simulations and experimental data.
The Journal of Physical Chemistry B 03/2012; 116(10):3115-24. · 3.70 Impact Factor
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ABSTRACT: We use a version of the density functional theory to study the changes in the
height of the tethered layer of chains built of jointed spherical segments with
the change of the length and surface density of chains. For the model in which
the interactions between segments and solvent molecules are the same as between
solvent molecules we have discovered two effects that have not been observed in
previous studies. Under certain conditions and for low surface concentrations
of the chains, the height of the pinned layer may attain a minimum. Moreover,
for some systems we observe that when the temperature increases, the height of
the layer of chains may decrease.
02/2012;
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ABSTRACT: Density functional approach is applied to study the phase behavior of Lennard-Jones(12,6) fluid in pillared slit-like pores. Our focus is in the evaluation of phase transitions in fluid adsorbed in the pore of a fixed width. If the length of pillars is sufficiently large, we observe additional phase transitions of the first and second order due to the symmetry breaking of the distribution of chain segments and fluid species with respect to the slit-like pore center. Re-entrant symmetry changes and additional critical, critical end points and tricritical points then are observed. The scenario of phase changes is sensitive to the energy of fluid-solid interaction, the amount, and the length of the pillars. Quantitative trends and qualitative changes of the phase diagrams topology are examined depending on the values of these parameters.
The Journal of chemical physics 08/2011; 135(5):054703. · 3.09 Impact Factor
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ABSTRACT: Using Monte Carlo simulation methods in the grand canonical and semigrand canonical ensembles, we study the phase behavior of two-dimensional symmetrical binary mixtures of Lennard-Jones particles subjected to a weakly corrugated external field of a square symmetry. It is shown that the both vapor-liquid condensation and demixing transition in the liquid phase are not appreciably affected by a weakly corrugated external field. On the other hand, even a weakly corrugated external field considerably influences the structure of solid phases and the liquid-solid transition. In particular, the solid phases are found to exhibit uniaxially ordered distorted hexagonal structure. The triple point temperature increases with the corrugation of the external field, while the triple point density becomes lower when the surface corrugation increases. The changes in the location of the triple point are shown to lead to the changes of the phase diagram topology. It is also demonstrated that the solid phase undergoes a demixing transition, which is also very slightly affected by the weakly corrugated external potential. The demixing transition in the solid phase is shown to belong to the universality class of the Ising model.
The Journal of chemical physics 06/2011; 134(21):214705. · 3.09 Impact Factor
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ABSTRACT: Density functional approach is applied to study the phase behavior of symmetric binary Lennard-Jones(12,6) mixtures in pillared slit-like pores. Our focus is in the evaluation of the first-order phase transitions in adsorbed phases and lines delimiting mixed and demixed adsorbed phases. The scenario of phase changes is sensitive to the pore width, to the energy of fluid-solid interaction, the amount, and the length of the pillars. Quantitative trends and qualitative changes of the phase diagrams topology are examined depending on the values of these parameters. The presence of pillars provides additional excluded volume effects, besides the confinement due to the pore walls. The effects of attraction between fluid species and pillars counteract this additional confinement. We have observed that both the increasing surface pillar density and the augmenting strength of fluid-solid interactions can qualitatively change the phase diagrams topology for the model with sufficiently strong trends for demixing. If the length of pillars is sufficiently large comparing to the pore width at low temperatures, we observe additional phase transitions of the first and second order due to the symmetry breaking of the distribution of chain segments and fluid species with respect to the slit-like pore center. Re-entrant symmetry changes and additional critical points then are observed.
The Journal of chemical physics 06/2011; 134(21):214702. · 3.09 Impact Factor
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ABSTRACT: A density functional study of adsorption of oligomers on weakly attractive surfaces modified with grafted chains is presented. The chain molecules are modeled as freely jointed tangent spheres. The segments interact via Lennard-Jones (12-6) potentials. Two types of substrates are considered - a neutral surface and the Lennard-Jones (9,3) surface. The mechanism of adsorption is discussed. Depending on the system characteristics, the primary, secondary and ternary adsorption is found. The effects of selected factors on adsorption and separation of mixtures are analyzed.
Journal of Colloid and Interface Science 04/2011; 356(1):267-76. · 3.07 Impact Factor
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ABSTRACT: The direct correlation function of the complex discrete potential model fluids is obtained as a linear combination of the first-order mean spherical approximation (FMSA) solution for the simple square well model that has been reported recently [Hlushak et al., J. Chem. Phys. 130, 234511 (2009)]. The theory is employed to evaluate the structure and thermodynamics of complex fluids based on the square well-barrier and square well-barrier-well discrete potential models. Obtained results are compared with theoretical predictions of the hybrid mean spherical approximation, already reported in the literature [Guillen-Escamilla et al., J. Phys.: Condens. Matter 19, 086224 (2007)], and with computer simulation data of this study. The compressibility route to thermodynamics is then used to check whether the FMSA theory is able to predict multiple fluid-fluid transitions for the square barrier-well model fluids.
The Journal of chemical physics 03/2011; 134(11):114101. · 3.09 Impact Factor
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ABSTRACT: A density functional approach to the retention in a chromatography with chemically bonded phases is developed. The bonded phase is treated as brush built of grafted polymers. The chain molecules are modelled as freely jointed spheres. Segments of all components interact with the surface via the hard wall potential whereas interactions between the segments are described by Lennard-Jones (12-6) potential. The structure of the bonded phase is investigated. The distribution of different solutes in the stationary phases is determined. An influence of the following parameters on the retention is analyzed: the grafting density, the grafted chains length, the strength of molecular interactions, the solute sizes, temperature. The theoretical predictions are consisted with numerous experimental results.
Journal of chromatography. A 02/2011; 1218(5):711-20. · 4.19 Impact Factor
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ABSTRACT: We study the phase behavior of a two-component fluid in a pore with the walls modified by tethered chains. The walls are completely permeable for one component of the fluid and completely impenetrable for the second component. The fluid is perfectly mixed in a bulk phase. We have found that depending on the details of the model the fluid undergoes capillary condensation inside the pore and wetting and layering transitions at the outer walls. Moreover, we have found transitions connected with the change of symmetry of the distribution of chains and fluid inside the pore.
The Journal of chemical physics 01/2011; 134(4):044705. · 3.09 Impact Factor
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ABSTRACT: Using Monte Carlo simulation methods in the grand canonical and semigrand canonical ensembles, we study the phase behavior of two-dimensional symmetrical binary mixtures of Lennard-Jones particles. We discuss the interplay between the demixing transition in a liquid and the freezing in detail. Phase diagrams for several systems characterized by different parameters describing interactions in the system are presented. It is explicitly demonstrated that different scenarios involving demixing and freezing transitions, described in our earlier paper [A. Patrykiejew and S. Sokołowski, Phys. Rev. E, 81, 012501 (2010)], are possible. In one class of systems, the λ-line representing a continuous demixing transition in a liquid phase starts at the liquid side of either the vapor-liquid or liquid-solid coexistence. The second class involves the systems in which the λ-line begins at the liquid side of the vapor-liquid coexistence, in the lower critical end point, and then terminates at the liquid side of the liquid-solid coexistence, in the upper critical end point. It is also shown that in such systems the solid phase may undergo a demixing transition at the temperature above the upper critical end point.
The Journal of chemical physics 12/2010; 133(24):244501. · 3.09 Impact Factor
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ABSTRACT: We discuss an application of a density functional approach to describe a bilayer, or a simplified model of a membrane, that is built of tethered chain molecules. The bilayer integrity is maintained by tethering chain particles to two common sheets. We study the structure of a two-component mixture in contact with the bilayer, as well as the solvation force acting between two bilayers, immersed in a fluid. The fluid is a binary mixture involving the component that can cross freely the bilayer and the second impenetrable component. All the calculations are carried out for athermal system, in which only hard-core interactions are present.
The Journal of chemical physics 06/2010; 132(24):244704. · 3.09 Impact Factor
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ABSTRACT: We report the phase diagrams for a square-shoulder, square-well fluids in two dimensions (2D), as well as confined in slitlike pores. The diagrams are obtained by histogram reweighting Monte Carlo simulations. The critical points are located by using finite size scaling analysis. Our calculations indicate that the phase behavior of the systems investigated is distinct from that observed in three dimensions. In particular, we have not found the liquid-liquid transition that occurs in the bulk.
The Journal of chemical physics 04/2010; 132(16):164702. · 3.09 Impact Factor
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ABSTRACT: The interplay between demixing and freezing in two-dimensional symmetrical binary mixtures of Lennard-Jones particles is studied using Monte Carlo simulation. It is demonstrated that different scenarios are possible. For example, the line of continuous liquid demixing transition can start at the liquid side of the vapor-liquid coexistence at the lower critical end point and then it can terminate at the liquid side of the liquid-demixed solid coexistence at the upper critical end point. Other situations are also possible. We distinguish four different scenarios depending on the interactions between unlike particles.
Physical Review E 01/2010; 81(1 Pt 1):012501. · 2.26 Impact Factor
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ABSTRACT: Using the Monte Carlo simulation method in the grand canonical ensemble, we study two-dimensional symmetrical binary mixtures subjected to an external potential of square symmetry and finite corrugation. The results reveal a rich variety of mixed liquid-like and solid-like structures. It is demonstrated that even very weakly corrugated external potential leads to the development of axially ordered striped solid-like phases. The results of finite temperature simulation confirm quite well the predictions stemming from the ground state considerations and show the formation of commensurate, high-order commensurate, and incommensurate mixed phases. It is also shown that the corrugation potential affects the demixing transition.
The Journal of Physical Chemistry B 12/2009; 114(1):396-406. · 3.70 Impact Factor
