It is well known that the nano-systems exhibit properties different from their bulk analogs. Typically, the phase diagrams are redefined because the position of coexistence lines depends on the size and shape of the nano-objects. This is related to the fact that nano-systems are characterized by high surface-to-volume ratio. The surface atoms are weakly bonded and their contribution to the latent heat is smaller. Consequently, the surface usually transforms at lower temperature and the whole transition may happen smoothly over a finite range of temperatures. This observation suggests that there is no temperature of melting (or any other struc-tural change) in the conventional sense because the structural (phase) changes are gradual and phases are no longer distinguishable.Here we discuss mechanism of methane melting [1-3] and its structural transformations when adsorbed in nanoporous systems. The general analysis of methane melting in slit pores was already discussed by Mi-yahara and Gubbins [1]. In this paper, we emphasize the influence of structural heterogeneity on the mecha-nism of structural transformations. As an example, we discuss the mechanism of melting of methane con-fined in two different structures: first, in 3 and 4 nm slit pores, then, in 2.8 nm square channels of SURMOF porous structure. Mechanism of melting transformation in both cases will be compared and the correlation between the nano-scale and heterogeneity will be emphasized and discussed. Fig. 1 presents an order pa-rameter calculated in different layers of the confined system as a function of temperature.Structural transformations of adsorbate are interesting phenomena from both fundamental and practical point of view. The mechanism of transformations is defined by the characteristic nanosize of the system and the influence of the interactions with the pore framework. This defines the unusual properties of the confined system which can be used for the system characterization.

It is well known that the nano-systems exhibit properties different from their bulk analogs. Typically, the phase diagrams are redefined because the position of coexistence lines depends on the size and shape of the nano-objects. This is related to the fact that nano-systems are characterized by high surface-to-volume ratio. The surface atoms are weakly bonded and their contribution to the latent heat is smaller. Consequently, the surface usually transforms at lower temperature and the whole transition may happen smoothly over a finite range of temperatures. This observation suggests that there is no temperature of melting (or any other structural change) in the conventional sense because the structural (phase) changes are gradual and phases are no longer distinguishable.Here we discuss mechanism of methane melting [1-3] in confined nanoporous systems. The general analysis of methane in slit pores was already discussed by Miyahara and Gubbins [1]. In this paper, we emphasize the influence of structural heterogeneity on the mechanism of structural transformations. As an example, we discuss the differences in mechanism of melting of methane confined in two different structures: first, in 3 and 4 nm slit pores, then, in 2.8 nm square channels of SURMOF porous structure. Mechanism of melting transformation in both cases will be compared and the correlation between the nano-scale and heterogeneity will be emphasized and discussed.

Hydrogen is very likely the ultimate future of the energetic economy of the world: it is ubiquitous in the environment, it can be sustainably produced, and it holds the highest energy per unit mass of any fuel (except nuclear). If combusted, it produces only water. Therefore hydrogen, when used as energy vector, enters the natural water cycle of the Earth, with no impact on the planet’s climate. Today the major issue of the widespread hydrogen use is its low mass and low volume storage, as the energy density of hydrogen gas is low. Physisorption in nanoporous materials seems to be the most promising way of storage, as it does not release much energy during adsorption and in principle do not require the energy input when hydrogen is released. In this paper we will present the Grand Canonical Monte Carlo simulations of hydrogen adsorption in slit-shape carbon nanopores. Our calculations confirm the very controversial experimental results [1-3] showing that under confinement the density of hydrogen film adsorbed on the carbon wall exceeds that of the bulk liquid at low temperature and approaches that of solid hydrogen at extreme pressures. The densification of hydrogen is restricted to the layer in direct contact with the adsorbent, and does not depend significantly on the pore size (at least for the pores of the width from 0.6 to 3.0 nm) nor on the temperature (for 50 K< T < 180 K).The simulations are confronted with the experimental isotherms of hydrogen adsorption in KOH - activated nanoporous carbons [3] obtained from biomass waste [4]. The numerical and experimental results are coherent and prove that the interaction between hydrogen molecules and carbon surface is strong enough to produce adsorbed layer with solid hydrogen density.

It has been widely reported that the jamming transition point phi_J is not uniquely determined but shows protocol dependence.Whereas phi_J is obtained around 65% in three dimensions by compressing a dilute hard sphere fluid,higher value of phi_J is attained when a dense thermally equilibrated fluid is compressed.Recently, we develop a very efficient thermalization setting of a hard sphere system which is composed of continuous polydispersity of the particle diameters and a non-local swap Monte-Carlo algorithm.This method enabled us to thermalize the hard sphere fluid up to nearly 66%, which is beyond phi_J of the system obtained by the compression of dilute states.By compressing these high density equilibrium fluids, we obtain an unprecedentedly wide range of phi_J, so-called J-line, from around 65% to almost 70%.These denser phi_J states also have on average 6 contact number per particle (isostaticity).Also, the power law singularity near contact in the radial distribution function is observed over the entire J-line with a fixed exponent whose value is consistent with the prediction of the mean-field theory. Furthermore, we examine local and global structural properties of high density jammed states.Even though the isostaticity and critical behavior are preserved over the entire J-line, the high density jammed states show qualitatively distinct structural properties compared to the lowest jamming transition point, phi_J = 65%, which has been widely studied in the past.Especially, we demonstrate the following local and global structural properties.Growing local order:Even though our jammed configurations are all isostatic, they do differ at the local structure level.We reveal this by a combination of bond-orientational order analysis and Voronoi tessellation.This analysis detects growing icosahedral order with increasing phi_J and shows no signs of crystallization in our configurations.Disturbing the hyperuniformity: Hyperuniformity, which corresponds to vanishing of the density (or volume fraction) fluctuations at long wavelengths is believed to be one of the characteristic properties of the jamming transition point.We analyze the two body correlation function of the volume fraction to examine the hyperuniformity in our system.We observe excitations at small wavenumbers which disturb the hyperunifromity at the high density jammed states.Also, these excitations increase with increasing phi_J.Suppression of the finite size effect: It has been argued that the jamming transition is a phase transition which occurs at the thermodynamic limit.Also, several length scales diverging at phi_J are well characterized as a consequences of the isostaticity of the system.However, the length scale which causes finite size effects at the jamming transition is not yet understood. We study finite size effects for two protocols, compression of dilute and of dense fluids.Interestingly, we find that finite size effects are significantly suppressed by the protocol using compression of the dense fluid.This observation implies that the length scale causing finite size effects is not related to the isostaticity of the system.Instead we discuss the mechanism of the finite size effect of the jamming transition using an analogy with the potential energy landscape thermal systems.