Surface Phase Transition of Associating Fluids on Functionalized Surfaces

Abstract

Surface phase transitions are studied for Lennard-Jones (LJ) based dimer forming associating fluids on modified surfaces with active sites for various association strengths using grand-canonical transition matrix Monte Carlo. We examine adsorption isotherm, density, energy, and monomer profiles to differentiate layering, quasi-2D vapor–liquid and prewetting transitions. Prewetting transition is found for association strengths: εaf = 8 and 10, whereas, for weaker associating fluids, εaf = 4 and 6, we observe quasi-2D vapor–liquid transition. The growth of thick films in the case of quasi-2D vapor liquid transitions is found to suppress with decrease in temperature and eventually splits in layering transitions. For systems exhibiting prewetting transition, wetting temperature and prewetting critical temperature increase with increasing association strength. In addition, we examine boundary tension of quasi-2D and prewetting transitions using finite size scaling formalism of Binder. Our results indicate that the quasi-2D boundary tension is lower than that of the prewetting transition. Surface sites are found to reduce the boundary tension; however, the effect of active sites diminishes with stronger fluid–fluid associating strength.

Surface phase transitions of multiple-site associating fluids

“…Several attempts have been made with multiple-site associating fluids with surface sites in the context of real systems such as water on activated carbon, etc., to study the different surface phase transitions and the results are in good agreement with experimental observations [6,26,27]. However, very few investigations have been performed to study the effect of surface sites and the number of fluid molecule sites on the surface phase transition [24,28]. To address the above, we investigate the influence of surface sites and their density on the surface phase transition of multiple-site associating fluids between the prewetting transition and the quasi-2D vapor-liquid transition.…”

“…We observe a firstorder transition of thick and thin films for different temperature ranges depending on the number of sites and the site strength of the associating fluid. The excess surface densities of thick and thin films are evaluated from the particle probability distribution at the coexistence chemical potential as described in the methodology section, and subsequently subtracted from the bulk phase density, with coexisting thin and thick films as used in our earlier work [28,35]. Figure 2 presents the excess surface density of thick and thin films, on a smooth surface, at various temperatures for different associating fluids.…”

“…In this study, c , r c and r cut are fixed at 27 , 1.00 and 2.5, respectively. In this work, a structureless substrate with active sites is used similar to that used in our earlier work [28]. Sites are distributed uniformly on a rectangular grid on the surface as shown in Figure 1.…”

“…In this study, the associating strength is varied from 4 to 8 for two-site associating fluids and from 4 to 6 for four-site associating fluids. For comparison, we took the results for a one-site associating fluid from our earlier work [28,35]. Surface phase transition simulations are performed with a surface kept at the lower XY plane of the cell.…”

Understanding Wetting Transitions Using Molecular Simulation

“…In our recent works [26,27], we have investigated the prewetting transition of one-site, two-site, and four-site associating fluids on smooth surface for various associating strengths. These sites mimic the strong and short-range directional attraction of real associating fluids such as alcohol, hydrogen fluoride, and water.…”