Effect of pressure on the freezing of pure fluids and mixtures confined in nanopores.
Monte Carlo simulations combined with the parallel tempering technique are used to study the freezing of Ar, CH(4), and their mixtures in a slit graphite nanopore. For all systems, the solid/liquid coexistence line is located at higher temperature than that for the bulk phase, as expected for fluids for which the wall/fluid interaction is stronger than the fluid/fluid interaction. In the case of the mixtures, the phase diagram for the confined system is of the same type as that for the bulk (azeotropic). It is also found that the freezing temperatures for the confined fluids and mixture are much more affected by pressure than those for the bulk phase. By calculating the isothermal compressibility of the confined fluids and determining the slope of the solid/liquid coexistence line (T,P) from the Clapeyron equation, we show that such a strong effect of pressure is not related to reduced compressibility within the pores. On the other hand, the pressure dependence of the in-pore freezing temperature is correctly described in the frame of the model proposed by Miyahara et al. [ Miyahara , M. ; Kanda , H. ; Shibao , M. ; Higashitani , K. J. Chem. Phys. 2000 , 112 , 9909. ], which is based on the pressure difference between the bulk and confined phases (capillary effect). In this model, a change in the in-pore freezing temperature with pressure is explained by a drastic change in the in-pore pressure, which varies very sharply with the bulk external pressure. We present an extended version of this model to confined systems for which an increase in the freezing temperature is observed.
Coasne
Benoit
B
Institut Charles Gerhardt Montpellier, CNRS (UMR5253), Université Montpellier 2, ENSCM, Place Eugène Bataillon, 34095 Montpellier Cedex 05, France.
Czwartos
J
J
Sliwinska-Bartkowiak
M
M
Gubbins
Keith E
KE