A. Patrykiejew

Phase transitions in adsorbed layers formed on crystals of square and rectangular surface lattice

Abstract This article gives a survey of phase transitions in adsorbed films on well defined surfaces of square and rectangular symmetry of the lattice. The discussion concentrates on the effects of periodic changes of the adsorbate–substrate potential on the structure and thermodynamic properties of adsorbed films. Different theoretical approaches are briefly reviewed, with an emphasis on those which explicitly take into account final corrugation of the surface potential. Several aspects of statistical mechanical description of phase transitions in surface layers, such as order–disorder, melting, commensurate–incommensurate transitions in monolayer films as well as transitions connected with the formation of multilayer films (layering and wetting) are presented. Theoretical discussion is followed by the presentation of numerous experimental and computer simulation studies for various systems. Then the properties of monolayer films of molecular adsorbates on different substrates of a square and rectangular symmetry is discussed. It is pointed out that computer simulation methods provide a very powerful tool which allows to probe the inner structure of such systems and provides direct information concerning both orientational and positional ordering.

Absence of superheating for ice Ih with a free surface: a new method of determining the melting point of different water models

Molecular dynamic simulations were performed for ice Ih with a free surface. The simulations were carried out at several temperatures and each run lasted more than 7 ns. At high temperatures the ice melts. It is demonstrated that the melting process starts at the surface and propagates to the bulk of the ice block. Already at the temperatures below the melting point, we observe a thin liquid layer at the ice surface, but the block of ice remains stable along the run. As soon as the temperature reaches the melting point the entire ice block melts. Our results demonstrate that, unlike in the case of conventional simulations in the NpT ensemble, overheating of the ice Ih with a free surface does not occur. This allows one to estimate the melting point of ice at zero pressure. We applied the method to the following models of water: SPC/E, TIP4P, TIP4P/Ew, TIP4P/Ice and TIP4P/2005, and found good agreement between the melting temperatures obtained by this procedure and the values obtained either from free energy calculations or from direct simulations of the ice–water interface.

Lattice gas models for multilayer adsorption: variation of phase diagrams with the strength of the substrate potential

Abstract The simple cubic lattice gas model with nearest-neighbor attractive interaction is considered for the case where the potential V ( z ), that an adatom at a distance z from the surface experiences due to the substrate, is V ( z ) = − A / z 3 . Exact ground state phase diagrams are obtained for different A , while the behavior at nonzero temperatures is studied both by Monte Carlo simulations and the molecular field approximation. We show that the detailed sequence of the layering transitions in the first few layers depends very strongly on the strength of the substrate potential: for strong potentials individual first-order layering transitions in layers 1, 2, 3, …, while for intermediate potential strength the transition of layers 1 and 2 coincide whereas the remaining transitions are still distinct. For yet weaker substrate potentials the separate layering of the third layer occurs only for temperatures exceeding a triple point which disappears for still weaker substrate potentials where the layering of the first three layers coincide. These different patterns of behavior lead to remarkedly different adsorption isotherms. We discuss these findings in the context of recent theoretical predictions.

On the structure of Lennard-Jones fluids confined in crystalline slitlike pores

The structure of Lennard-Jones fluid in slitlike pores with crystalline walls is studied by canonical ensemble Monte Carlo simulation method. It is assumed that each pore wall is a perfect (100) plane of the face centered cubic crystal. The effects of the wall–wall separation on the ordering and freezing of a confined fluid are investigated for a series of systems with the density corresponding to the two completely filled layers of commensurate c(2×2) phase. It is demonstrated that the structure of a solid phase very strongly depends on the pore diameter. Thus, the formation of partially filled film that spans the space between the both pore walls occurs only when the pore width is smaller than a certain value. For wider pores only two monolayers at each pore wall occur. Then, the formation of commensurate structures of square symmetry in each layer occurs only over certain ranges of the pore width, while for other pore widths, the incommensurate phases of hexagonal symmetry in each layer are formed. The...

Simulation and density functional study of a simple membrane. II. Solvent effects using the solvent primitive model

The simple membrane, supporting charge densities σ1 and σ2=−σ1 on its inner and outer surfaces that we studied previously is extended to include solvent molecules, modeled as hard spheres. In addition to the electrostatic potential, the membrane can interact with the surrounding electrolyte by a short-range van der Waals-type potential that can be attractive or repulsive. The fluid beyond the outer surface is four component electrolyte consisting of the hard sphere solvent, two species of cations, and one species of anions. The membrane is impermeable to one of the cation species so that the fluid in the membrane and beyond the inner surface is a three component electrolyte. Monte Carlo simulations and density functional theory are used to study the density profiles of the electrolyte and the charge-electrostatic potential relationship for the membrane surfaces. The presence of the solvent molecules leads to oscillatory profiles. The potential versus charge relationship is strongly influenced by the solve...

Monte Carlo study of melting and disordering in monolayer films formed on the (100) face of face centered cubic crystals

Monte Carlo simulation studies have been carried out for monolayer films formed on the (100) plane of model fcc crystals with different corrugation of the gas‐solid potential. It has been demonstrated that the low temperature structure of monolayer films depends strongly on the gas‐surface potential corrugation as well as on the size of adsorbed atoms. The effects of the gas‐surface potential corrugation on the melting transition have been determined for a series of systems, exhibiting different structure of the solid phase.

Multilayer adsorption of binary mixtures: mean-field theory and Monte Carlo simulation

Abstract The formation of two-component multilayer films on homogenous solid surfaces is discussed using the mean-field theory and Monte Carlo simulation. Adsorption systems exhibiting complete and incomplete mixing of components are discussed. It is shown that the structure of adsorbed films depends strongly on the relative strength of the adsorbate-adsorbate and the adsorbate-substrate interactions for both components. In particular, it is demonstrated that the enhanced adsorption of one component due to adsorption of another gas, which is observed in monolayer films, occurs also in multilayer films. It is shown that in the systems considered in this work the displacement of preadsorbed film due to adsorption of the second component may occur via the desorption or via the transfer of molecules to the higher layers. The mechanism of the displacement is shown to depend strongly on the details of gas-gas and gas-solid interactions of both components.

Dynamics of multilayer adsorption: a Monte Carlo simulation

Abstract The growth of an adsorbed film at an initially empty surface which is exposed at time t = 0 to a gas is studied within the framework of a kinetic lattice gas model by Monte Carlo simulation. The model includes an attractive potential V ( z ) between adsorbed particles at distance z from the surface, V(z) = −A z 3 and a nearest-neighbor attractive interaction between the gas atoms. Several choices of the surface potential depth A , corresponding to different sequence of layering transitions, are considered. The Monte Carlo process assumes random evaporation/condensation events of gas atoms in adsorbed layers close to the surface, while surface diffusion is disregarded. For temperatures T somewhat less than the interfacial roughening temperature T R of the considered lattice gas model, the time-dependent coverage θ( t ) exhibits distinct steps, representing the adsorption of the first, second, third, etc. layer. This layerwise adsorption kinetics also shows up in the surface excess energy due to the adsorbed layer. At temperatures far below T R , pronounced metastability of the empty surface as well as hysteresis is observed. On the other hand, for T > T R a smooth increase of coverage with time is observed, corresponding to wetting behavior rather than layering. We relate this behavior to the corresponding phase diagrams of the model and briefly discuss pertinent experimental applications.

The structure of fluids confined in crystalline slitlike nanoscopic pores: Bilayers

Grand canonical and canonical ensemble Monte Carlo simulation methods are used to study the structure and phase behavior of Lennard-Jones fluids confined between the parallel (100) planes of the face centered cubic crystal. Ultra thin slit pores of the width allowing for the formation of only two adsorbate layers are considered. It is demonstrated that the structure of adsorbed phases is very sensitive to the wall-wall separation and to the strength of the fluid-wall potential. It is also shown that the structure of low temperature (solid) phases strongly depends on the fluid density. In particular, when the surface field is sufficiently strong, then the high density phases may exhibit a domain wall structure, quite the same as found in monolayer films adsorbed at a single substrate wall. On the other hand, the weakening of the surface potential leads to the regime in which only the hexagonally ordered bilayer structure is stable. The phase diagrams for a series of systems are estimated. It is shown that, depending on the pore width and the temperature, the condensation leads to the formation of the commensurate or incommensurate phases. The incommensurate phases may have the domain-wall or the hexagonal structure depending on the pore width and the strength of the fluid-wall potential.

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

An extension of the Kierlik-Rosinberg version of density functional theory to the case of a polydisperse fluid of hard spheres in contact with a single wall and also adsorbed in slit-like pores is presented. A comparison with Monte Carlo results show that the theory is capable of precisely reproducing density profiles of a polydisperse fluid near a hard wall. Several model calculations of adsorption in slit like pores are presented. The influence of the confinement on the polydispersity, as compared with bulk fluid, is discussed.