Surface phase transitions of a Lennard-Jones fluid in contact with a permeable wall of finite thickness: a density functional approach

We study the phase behavior of a Lennard-Jones fluid in contact with a perm eable wall of finite thickness. The wall permeability is modelled by a squa re-well repulsive potential with a finite height of the potential barrier. The external planes of the wall can also provide an attractive field acting on fluid particles. The structure and phase behavior of the fluid are inve stigated by means of a density functional method. For high repulsive potent ial barriers and the walls wide enough, the phase behavior of the system mi mics that characteristic for a fluid in contact with a single adsorbing sur face. In the case of a purely repulsive membrane, exhibiting an intermediat e value of the intramembrane potential barrier, we observe the existence of a surface transition similar to the predrying transition. The dependence o f this transition on the membrane width and on the height of the repulsive barrier is studied. We also consider the case of a membrane exerting an att ractive field on fluid particles located outside the membrane. In such a ca se we find the prewetting type surface phase transitions.