Phase transitions in an associating, network-forming, Lennard-Jones fluid in slit-like pores. II. Extension of the density functional method

We study adsorption of hydrogen-bonded fluids in slit-like pores with stron gly attractive walls, in the framework of the four-site associating Lennard -Jones model. The density profiles, as well as the phase behavior, are obta ined by using a density functional method. We have found that, at temperatu res lower than the critical temperature of the bulk fluid, the confined flu id undergoes one or more layering transitions dependent on the pore width, followed by capillary condensation. Each of the transitions is localized by analyzing the grand thermodynamic potential. The density profiles of nonbo nded and differently bonded particles demonstrating changes of the structur e of the fluid in the pore along the coexistence are discussed briefly. The critical temperature for capillary condensation is lower for confined flui d, compared with that for the bulk liquid-vapor transition, as expected. Ho wever, an increase of the energy of association between fluid species incre ases the critical temperatures for layering transitions and for capillary c ondensation. The envelope of the capillary condensation is narrower than th e bulk liquid-vapor phase diagram. The ratio between the critical temperatu res for layering transitions and capillary condensation depends on the pore width. The critical temperature for the second layering is always lower th an for the first one. The triple point temperature between either the secon d layering transition and the capillary condensation (in wider pores) or th e first layering transition and the capillary condensation (in narrower por es) increases with decreasing pore width. The triple point temperature betw een the layering transitions is much lower than the relevant temperature be tween the second layering transition and the capillary condensation. The tr iple point temperatures also depend on the association energy. We have show n that highly bonded fluid species prevail at triple point temperatures. (C ) 2000 American Institute of Physics. [S0021-9606(00)52005-4].