Computer simulation of the phase diagram for a fluid confined in a fractal and disordered porous material

Abstract

We present a grand canonical Monte Carlo simulation study of the phase diagram of a Lennard-Jones fluid adsorbed in a fractal and highly porous aerogel. The gel environment is generated from an off-lattice diffusion limited cluster-cluster aggregation process. Simulations have been performed with the multicanonical ensemble sampling technique. The biased sampling function has been obtained by histogram reweighting calculations. Comparing the confined and the bulk system liquid-vapor coexistence curves we observe a decrease of both the critical temperature and density in qualitative agreement with experiments and other Monte Carlo studies on Lennard-Jones fluids confined in random matrices of spheres. At variance with these numerical studies we do not observe upon confinement a peak on the liquid side of the coexistence curve associated with a liquid-liquid phase coexistence. In our case only a shouldering of the coexistence curve appears upon confinement. This shoulder can be associated with high density fluctuations in the liquid phase. The coexisting vapor and liquid phases in our system show a high degree of spatial disorder and inhomogeneity.

Confinement effects on phase behavior of soft matter systems

“…However, in this case the random irregularity of the confining geometry is a serious obstacle for a detailed understanding. There is ample evidence (both from experiment [126,127] and simulations [128,129,130]) that the liquid-vapor type phase separation or demixing of binary fluid mixtures under such confinement is seriously modified, but the character of this modification has been under discussion since a long time [126,127,128,129,130,131,132]. De Gennes [131] argued that due to the random arrangement of the pore walls (which prefer one of the coexisting phases over the other) the problem can be mapped to the random field Ising model [133,134].…”

Finite-size scaling study of the vapor-liquid critical properties of confined fluids: Crossover from three dimensions to two dimensions

“…Examples of the former include the occurrence of surface transitions which are absent in the bulk, such as layering and wetting. 2,6,7 Examples of the latter include the shift in the common phase transitions, such as the vapor-liquid transition [8][9][10][11][12][13] and the liquid-liquid transition for binary mixtures. [14][15][16] The effects of confinement on the phase behavior of fluids, if properly understood, can be used as a powerful tool to probe the thermodynamic properties of the confined fluid.…”

Colloid–polymer mixtures in the presence of quenched disorder: a theoretical and computer simulation study

“…However, there is also a link to fundamental physics, since quenched disorder in fluids gives rise to 'randomfields' [2]. Consequently, these systems have received considerable attention in experiments [3,4], and sparked numerous theoretical and simulational investigations [5][6][7][8][9][10][11][12][13][14][15]. On the theoretical side, the study of fluids with quenched disorder has had a boost, thanks to the concept of the 'quenchedannealed' mixture, proposed by Madden and Glandt [5].…”