Effects of weak surface fields on the density profiles and adsorption of a confined fluid near bulk criticality

Abstract

Density functional approach to the adsorption of spherical molecules on a surface modified with attached short chains Structure of Lennard-Jones fluids confined in square nanoscale channels from density functional theory A comparison of density functional and integral equation theories vs Monte Carlo simulations for hard sphere associating fluids near a hard wallThe density profile (z) and Gibbs adsorption ⌫ of a near-critical fluid confined between two identical planar walls is studied by means of Monte Carlo simulation and by density functional theory for a Lennard-Jones fluid. By reducing the strength of wall-fluid interactions relative to fluid-fluid interactions we observe a crossover from behavior characteristic of the normal surface universality class, strong critical adsorption, to behavior characteristic of a ''neutral'' wall. The crossover is reminiscent of that which occurs near the ordinary surface transition in Ising films subject to vanishing surface fields. For the ''neutral'' wall (z), away from the walls, is almost constant throughout the slit capillary and gives rise to a ⌫ that is constant along the critical isochore. The same ''neutral'' wall yields a line of capillary coexistence that is almost identical to the bulk coexistence line. In the crossover regime we observe features in (z) similar to those found in the magnetization profile of the critical Ising film subject to weak surface fields, namely two smooth maxima, located away from the walls, which merge into a single maximum at midpoint as the strength of the wall-fluid interaction is reduced or as the distance between walls is decreased. We discuss similarities and differences between the surface critical behavior of fluids and of Ising magnets.

Surface critical behavior of fluids: Lennard-Jones fluid near a weakly attractive substrate

“…In the case of fluids, some strength of the fluid-wall interaction potential could provide equal attraction of the liquid and the coexisting vapor phase to the surface in the vicinity of the critical point. At this unique fluid-wall interaction neither an increase nor a depletion of the density is observed at the critical temperature [30]. At any other fluid-wall interaction a preferential adsorption of one component (molecules or voids) occurs.…”

“…critical bulk LJ fluid [30]. The total number of molecules in the liquid and vapor phases N l + N v in the pore varied from 2700 molecules at low temperature to 2000 at high temperatures.…”

“…At low temperatures this may be understood as a result of adsorption at the weakly attractive substrate. At high temperatures this effect is the result of a higher coexistence pressure due to the confinement in a pore with weakly attractive substrate [30]. The density of the liquid phase in the pore is shifted significantly to lower values due to a depletion of the density near the weakly attractive surface [31] and this effects the liquid density much stronger than the increasing pressure.…”

Critical phenomena at perfect and non-perfect surfaces

“…that without external fields one would be at the ordinary transition, it has been shown [146,147,148] that a small surface field leads to a short-distance increase of m(z). This critical adsorption in systems with weak surface fields has been studied subsequently in a series of papers [149,150,151,152,153]. The increase of the layer magnetisation is due to the fact that a weak surface field gives rise to a macroscopic length scale, yielding a power law behaviour…”

Tunability of critical Casimir interactions by boundary conditions

“…, the surface in this representation. With increasing ~x the scaling field h1 which is often used to theoretically describe functions change systematically from negative to posi-continuously varying BC [29][30][31]. Ellipsometry studies on tive values.…”