(594j) Phase Behavior of Confined Multiple Sites Associating Lj Fluids in Functionalized Slit Pore: A Monte Carlo Study

Fluids within nonporous materials have very common occurrence in many natural, biological and industrial processes such as shale gas extraction, microfluidics devices, fabrication of nanomaterials, nanotribology etc. The fluid phase behavior at nanoscale confined in different pores is significantly different than those in a bulk phase. The competitive interaction between fluids and surface atoms lead to various kind of phase transitions including layering, prewetting, vapor-liquid, melting/freezing, capillary condensation, along with a shift in critical properties of the system. Molecular insight of those phenomena is very essential for efficient design of micro or nano scale devices such as micro-electromechanical systems (MEMS), nanoelectromechanical systems (NEMS), Lab-on-a-chip (LOC) etc.

Various properties of coexistence phases and critical properties are assessed for multiple sites associating fluids at different slit widths as well as different functionalized surfaces using Grand-Canonical Transition Matrix Monte Carlo (GC-TMMC) method. In this work, one-site, two-sites (opposite to each other) and four-sites(tetrahedral) associating fluid model is considered in which Lenard-Jones potential is used for isotropic interaction between fluid molecules and a short-range directional square-well potential is used for site-site interaction. All interaction parameters are normalized with respect to parameters of fluid particle which is correspond the argon-graphite system. The structural features of coexistence phases are examined through orientation profile, monomer fraction and density profile. The effect of surface site density on the phase behavior under confinement is critically investigated. The presence of functionalized surface significantly alters the orientation of neighboring fluid particles, leading to rich coexistence phase behavior. For example, the orientation of two sites associating fluid molecules is normal to surface, which promote bonding with surface and propagate the bond formation to the bulk of the system.