Phase Behavior of Lennard-Jones Fluids in Slit-like Pores with Walls Modified by Preadsorbed Molecules: A Density Functional Approach ² O. Pizio,* ,‡ A. Patrykiejew, § and S. Sokolowski § Instituto de Quı ´mica de la UNAM, Coyoaca ´ n 04510, Me ´ xico, and Department for the Modeling of Physico-Chemical Processes, Maria Curie-Sklodowska UniVersity, 20-031 Lublin, Poland ReceiVed: May 14, 2007; In Final Form: July 9, 2007 We have studied the liquid-vapor coexistence of Lennard-Jones fluids in slit-like pores with the walls modified by preadsorbed molecules in the framework of the density functional approach. During the preadsorption step, the grafting of monomers, trimers, or chains consisting of spherical segments is performed. The model of tangentially bonded spheres is used for chain molecules. It is shown that the presence of grafted species may induce condensation and layering phase transitions. The crossover between evaporation and condensation is also influenced by grafted species. Phase transitions in the adsorbed fluid affect the microscopic structure of grafted chains and can result in the peculiarities of their thermodynamic behavior. The entire calculations are performed by using the semi-grand canonical ensemble. 1. Introduction The study of thermodynamics of adsorption of fluids and mixtures in pores and porous media is one of several important problems in chemical physics and related sciences. Of particular interest is the investigation of phase transitions in such physical systems. 1-4 Academic research in this area is motivated to a great extent by the development of applications concerned with more-efficient and novel technological processes involving adsorbents. Several techniques and areas of applied science, for example, chromatography, heterogeneous catalysis, oil recovery, gas storage, membrane separation, lubricant developments, and some important biological phenomena and processes require the knowledge and control of the thermodynamic behavior of fluids in pores. In narrow pores (with the pore width on the order of a few molecular diameters), fluids exhibit significantly different physical behavior compared to bulk systems. The competition between fluid-pore walls and fluid-fluid interactions leads, under certain thermodynamic conditions, to surface-driven phase transitions, such as layering, wetting, and capillary condensation. Computer simulation techniques, as well as theoretical methods, including density functional theory and integral equations are commonly used to understand these phenomena. 1-6 In a vast majority of theoretical works, adsorption is studied in a single pore, and the pore geometry is assumed to be well- defined. The interaction between pore walls and particles of an adsorbed fluid is usually described by simple potentials that vary only in the direction perpendicular to the surface. However, real porous systems are geometrically and energetically hetero- geneous at various scales. The effects of pore heterogeneity on surface phase transitions have been studied in several works. 7-17 Different models, for example, with spatially varying adsorbing potentials and those employing the concept of quenched- annealed mixtures, have been used to describe surfaces whose roughness varies on the scale of molecular dimensions. The obtained results indicate that adsorption and the surface phase transitions are essentially influenced by the heterogeneity effects. Alternatively, the adsorbing properties of porous solids can be changed by their physical or chemical modification. This possibility is of practical importance and has been explored for several specific purposes. However, modeling and theoretical description of structurally and energetically modified adsorbents is not well-developed so far. Physical modification of the surface of pores may be reached by preadsorption (i.e., at the step preceding adsorption process) of certain molecules. 18-24 Ex- perimentally important and theoretically interesting are the adsorbents in which the pore walls are modified by preadsorp- tion of complex molecules, specifically of chains and colloidal particles. In particular, the understanding and description of the microscopic structure and resulting macroscopic properties of a layer of chain molecules grafted to the solid has attracted much attention. 25-33 Intrinsically, this type of physical system involves not only grafted species under confinement but also solvent particles that adsorb on the solid surface and on the grafted species. Density functional theory is a powerful methodology for the modeling of inhomogeneous systems that include chain mol- ecules. A few years ago, McCoy, Curro, and co-workers proposed a version of the density functional theory to study the density profiles of chains, tethered to a surface. 34-38 The presence of a solvent has been considered at different levels of modeling. Principally, these authors focused on the microscopic structure of tethered chains on a single solid surface in an implicit solvent and analyzed the relation between their density functional approach with the previously developed self- consistent field theories for tethered chains in a continuum solvent. Also, these authors have attempted to account for solvent species explicitly. However, the method intrinsically cannot describe surface-induced phase transitions in tethered polymer-explicit solvent systems. Several alternative density functional formulations exist for nonuniform chain fluids and their mixtures. 39-45 Some of these ² Part of the “Keith E. Gubbins Festschrift”. ‡ Instituto de Quı ´mica de la UNAM. § Maria Curie-Sklodowska University. 15743 J. Phys. Chem. C 2007, 111, 15743-15751 10.1021/jp0736847 CCC: $37.00 © 2007 American Chemical Society Published on Web 08/18/2007