Hysteresis coexistence curve

It is very difficult to measure the coexistence curves of confined fluid experimentally, as this requires estimation of the densities of the coexisting phases at various temperatures. Therefore, only a few experimental liquid-vapor coexistence curves of fluids in pores were constructed [279, 284,292,294-297]. In some experimental studies, the shift of the liquid-vapor critical temperature was estimated without reconstruction of the coexistence curve [281-283, 289]. The measurement of adsorption in pores is usually accompanied by a pronounced adsorption-desorption hysteresis. The hysteresis loop shrinks with increasing temperature and disappears at the so-called hysteresis critical temperature Teh. Hysteresis indicates nonequilibrium phase behavior due to the occurrence of metastable states, which should disappear in equilibrium state, but the time of equilibration may be very long. The microscopic origin of this phenomenon and its relation to the pore structure is still an area of discussion. In disordered porous systems, hysteresis may be observed even without phase transition up to hysteresis critical temperature Teh > 7c, if the latter exists [299]. In single uniform pores, Teh is expected to be equal to [300] or below [281-283] the critical temperature. Although a number of experimentally determined values of Teh and a few the so-called hysteresis coexistence curves are available in the literature, hysteresis... 

As indicated before the locus of the phase diagram of a confined fluid compared to the coexistence curve of the bulk fluid is of importance for the occurence of pore condensation and hysteresis, i.e. for the shape of the sorption isotherm. It is expected that the critical temperature and the triple point temperature will be shifted to lower values for a confined fluid compared to a bulk fluid, i.e. the smaller the pore width the lower the critical temperature and triple point temperature of the pore fluid [8,9,20]. Pore condensation occurs whenever the pore... 

The disappearance of the adsorption-desorption hysteresis with temperature may be used as a very rough estimation of the liquid-vapor coexistence curve in confinement. In Vycor glass, the difference between the bulk critical temperature and the pore hysteresis critical temperature... 

In the plateau region, which indicates the coexistence of two solid phases, hysteresis of the curve can be observed. 

It is by now well documented that the spin transition behaviour, i.e. the molar fractions of the different spin states at a given temperature, the amount of residual paramagnetism at low temperature, the transition temperature Tc (= temperature of equal amounts of the coexisting spin states), the shape of the conversion curve (more or less gradual), eventual hysteresis, may be influenced by various factors, most important among these being... 

It has been found that in the region of concentrations where two hydrogen-palladium alloys coexist (the perpendicular sections of the curves of Fig. 40) it is not possible to trace out the same isobaric curve on sorption and desorption. There is a hysteresis effect illustrated by Fig. 40. It is interesting to find in the study by Lombard, Eichner and Albert (96) that the permeability-temperature curve follows in an inverse manner the absorption isobar, there being a great increase in the permeability in the region 180-200° C. It may be that this rapid alteration in permeability marks the change from )ff-phase to a-phase alloy. 

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