Peculiar Feature in Pressure of Fluids Confined in Nanospace and their Phase Behavior

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Many porous media such as zeolites, silica and activated carbons posses nanoscale pores, while such a basic information as the phase diagram of fluids confined in nanospace is left unpredictable. This research project pursued phase behavior of the confined fluids using the molecular simulation technique and an atomic force microscopy (AFM) to obtain their characteristics directly, which were utilized to establish a systematic understanding and a predictable model for the phase behavior, based on the 'pressure felt by the molecules in nanospace.' The obtained results are summalized in the followings.1. Molecular SimulationsApplying a molecular dynamics technique developed originally by the head investigator (Miyahara et al., J.Chem. Phys., 1997), phase transitions of fluids confined in nanospace were observed in detail under various conditions of interaction strength of the confining wall and bulk equilibrium pressure.2. Experimental MeasurementsWe developed a unique experimental system … More for observing directly the liquid-solid phase transition of confined liquids as follows. The AFM was equipped with a temperature control unit. A carbon particle with high degree of graphitization was selected and successfully attached to the tip of AFM cantilever to make up a so-called colloidal probe. Force curves between the probe and a cleaved graphite surface immersed in an organic liquid with ambient temperature of the freezing point were measured under various temperatures. The results clarified that the liquid, which was confined in "favorable" walls, exhibits higher freezing temperature than that in bulk. The relation between the freezing point shift and the surface separation was successfully obtained.3. Model Development and VerificationWe developed a new model for predicting phase behavior of confined liquids, quantitatively, based on the concept of the "pressure felt by the confined fluid, " which differs from bulk because of the influence by the attractive interaction from confining walls. The proposed model successfully predicted the phase transition points observed both in simulation systems and experimental ones, without any adjustable parameter, and proved its validity both in the qualitative concept and quantitative aspect, and its usefulness. Less