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Résumé:

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.