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Research / Main fundamental results / Surface physics and heterogeneous catalysis

Surface physics and heterogeneous catalysis

The investigations started in the mid 1970ies and were aimed at studying the electrolyte solutions at the surfaces. Original results dealing with the expressions for distribution functions have been obtained (M.F.Holovko, O.O.Pizio, I.Yo.Kurylyak, Ye.M.Sovyak). The peculiarities of structural ordering at the surface were described which made it possible to introduce the membrane filtration mechanisms based on the charge separation in the near-surface region. The corresponding transport equations have been derived, and evaluations for several model systems have been performed, corroborating these methods to be very promising (V.V.Tokarchuk, I.P.Omelyan, R.I.Zhelem).

Starting from the mid 1980ies the research was carried out in several directions, among which one has to mention the study of the behavoiur of dense fluids behaviour in a porous medium (M.F.Holovko, A.D.Trokhymchuk, Ye.M.Sovyak, T.M.Patsahan), investigation of structural and adsorptive properties of fluids at the interfaces (M.F.Holovko, E.V.Vakarin, A.Ya.Duda, A.F.Kovalenko, T.G.Sokolovska), the development of various quantum statistical methods of describing the reaction-diffusion processes and surface reconstruction (M.V.Tokarchuk, V.V.Ignatyuk, T.M.Bryk), computer simulation of the surface layers (I.V.Stasyuk, N.I.Pavlenko, I.M.Mryglod, I.P.Omelyan, R.Ya.Stetsiv, T.S.Mysakovych).

In the theory of porous media, the dependence of the solvent density on its dielectric properties, porosity, and ionization degree was found. The averaged dielectric properties of porous media were studied from the condition of phase coexistence. It was shown that for porous matrices with a small dielectric constant, the ion concentration in the pore can exceed its value in the bulk phase.

The multidensity formalism was developed to describe the fluid properties in the porous media, which in conjunction with the replica method allowed one to investigate the phase diagrams of methane in silica xerogel. It was shown that the porous medium extends the interphase region, decreases the critical temperature, and narrows the “pressure–density” phase diagrams. It has been proven that the states with a negative compressibility can emerge in the pores at certain conditions, which are accompanied by a sharp change of the local structure. The properties of methane adsorbed in silicagel were studied by the methods of molecular dynamics. It was shown that the formation of the contact layers at the pore surface leads to a decrease of the mobility of molecules. The obtained values of the diffusion coefficient are in a good agreement with experiment. The effect of a charged and dielectric porous medium on screening in the ionic-molecular fluids was studied. It was shown that the screened potentials can change their signs and asymptotics due to correlation between the particles from different pores.

The conditions of cooperative adsorption of molecules on the crystal surface were studied. The cooperative adsorption of polymeric and network-forming systems on the adhesive surfaces was shown to be observed at strong dilution due to the competition between the intra- and intermolecular correlations of the adsorbed molecules. This phenomenon was observed experimentally. A possibility of the wetting-type phase transition along with the adsorbed layers generation was observed during adsorption of the network-forming liquids.

It was shown that adsorption of a liquid on a crystal surface can induce a distorting phase transition on the substrate, whose threshold is determined by competition between the rigidity and the strain of the substrate. The structure of the surface originating from this process is a result of the compromise between the proper symmetry of the crystal and the hexagonal symmetry caused by the increase of the adsorbate density. The conditions for the local lattice destruction and the adsorbate condensation on originating inhomogeneities were studied.

A modification of the integral equation method was proposed for the investigation of the near-surface nematic ordering. Based on the current notion of the ionic dimerization mechanism there was suggested an explanation of anomalous behaviour of the double electric layer capacity at low temperatures and/or small values of dielectric susceptibility of the solution. A contact theorem for the charge density profile has been proven by integrating the Bogolyubov equation for the unary distribution function of the ions at the charged surface. The obtained expression is of a non-local character and relates the surface structure to its electric properties. The effect of surface charge on the water structure in the near-surface layer was studied by the method of integral equations. The possibility of water hydrolysis at the charged surface was revealed. The investigation of structural and thermodynamic properties of the “liquid-vapour” and “liquid-liquid” interfaces was performed for the Ising-like fluid. The effect of magnetic field on the surface properties was studied.

In the theory of layers and heterostructures, simple analytic expressions were obtained for the interaction energy and wedging pressure caused by the structural forces. The stability conditions for thin films formed by mono- and bidispersive colloidal suspensions are studied. Large particles are shown to localize at the surface when the volume fraction of small particles increases.

The theoretical investigation of optic properties of photonic crystals and thin nanolayers is carried out using the time- and frequency-domain approaches. The dispersion relations and transmission spectra for systems with various geometry of the structure (spiral, polygonal, helical etc.) are evaluated with taking into account the periodicity defects. The optimal parameters of the nanosystems with defects are determined which is important for the construction of optical interference filters.

The investigation of the “conductor-insulator” transition in heterostructures based on films formed by oxides of transition metals is carried out by the density functional method. It was found that structural deformations and ionic distortions in such materials are accompanied by the appearance of static dipole moments and the Janh-Teller electron level splitting of the d-orbitals at the interface. It is a key mechanism that induces the charge redistribution and changes the basic properties of the above mentioned systems.

The processes of the impurity charge separation at the “ice-water” interface are investigated by methods of computer simulations. The basic characteristics of the interfaces for their various orientations are obtained, and the free energy profiles for the impurity ions are evaluated allowing one to explain the selectivity of the ion behaviour at the “ice-water” interface. The properties of inhomogeneous fluids in external fields are studied by the density functional method.

The investigation of the effect of chemical association on structural and thermodynamic properties of the “liquid-vapour” interface is performed. The surface energy and density profiles as functions of temperature and the strength of associative interaction are calculated.

The interaction of water and hydroxyl groups (formed as a result of dissociation of the water molecules) with the silica surface was studied by means of quantum chemistry methods. The adiabatic potentials are evaluated; both the ion localization sites and the values of their effective charges are found; the adsorption energy is calculated, and the redistribution of the electronic density is investigated in detail.

The corrosion processes of solids were studied in the framework of the cellular automata model. The parameters describing the surface roughness and the dynamics of the corrosion front are evaluated.