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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 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.