We study the microscopic mechanism of adsorption in nanometric cylindrical pores with strongly heterogeneous walls using grand canonical Monte Carlo simulations. The pore surface structure is modeled by a new lattice-site approach. Each site is characterized by two amplitudes-structural and energetic-that locally modify the structural and energetic properties of the surface. The amplitudes are randomly distributed over the pore wall. We have shown that different structural and energetic distribution functions lead to different mechanism of adsorption. The energetic site distribution plays the most crucial role in the submonolayer region. The structural site distribution modifies the multilayer adsorption. A method to analyze the stability of the adsorbed system using static susceptibility is proposed. Potential applications in multiscale modeling are discussed.

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Molecular Dynamics (MD) simulations of tetracosane (C24H50) monolayer physisorbed on graphite are carried out. C24H50 molecules are simulated with explicit hydrogens and the graphite is represented by six graphene layers. We focus our analysis on the microscopic mechanism of melting, experimentally observed at T = 340 K. We are looking for the pre-melting transformations and analyze if there is a correlation between translational disordering of molecules and their internal degrees of freedom. We analyze several order parameters and their fluctuations along the MD trajectories. We show that the all atom representation of C24H50 is much more sensitive to the model of intramolecular interactions than united atom model. In particular two components: electrostatic forces and scaled 1-4 internal non-bonded interactions can shift melting temperature by several tenths of degree. A correlation between molecular stiffness and the intermolecular forces causes molecules’ footprint reduction during melting which involves a simultaneous lost of intramolecular and translational order. This cooperative mechanism is related to an abrupt increase of gauche defects within the central region of the chain.

Molecular nitrogen (N2) adsorbed on graphite reveals a very rich phase diagram (Fig.1). In particular, at low temperatures a variety of structures appears, depending on surface coverage. The sequence of structural phase transitions at densities close to monolayer is already well understood and described in the literature [1-4]. Therefore, our aim is to investigate the phase transitions in multilayer systems. We want to understand whether the presence of subsequent layers modifies the mechanism and temperature of rotational phase transition and melting within a particular layer. We present here the canonical Monte Carlo simulations of full N2 monolayer absorbed on corrugated surface of graphite and its evolution as a function of the subsequent layers adsorbed on the top of the previous one. The calculations have been performed for a maximum of 4 layers. At low temperature and full monolayer coverage the N2 molecules form so called herringbone structure, commensurate with graphite. It consists of two sublattices of molecules. In solid phase, in both sublattices the molecules are parallel to each other and molecular centers of mass coincide with centers of graphite hexagons. Between the sublattices, the molecules are perpendicular (Fig.2). On the top of the first layer (sites A on Fig.3) a second one is formed, of the same structure but shifted by the 1.25 Å in x direction (sites B). The third layer can be added in two ways: by putting it above the first one (AB A hexagonal-like sequence) or above the hexagons that are still “empty” (sites C, ABC cubic sequence). The four-layer structures are, in consequence, ABAB or ABCA. Both types of molecular stacking has been investigated. Several order parameters and distributions have been analyzed to follow the phase changes as a function of the temperature. The results show that the temperature and mechanism of phase transitions vary with surface coverage and change from layer to layer.