Lennard-Jones fluids in cylindrical pores: Nonlocal theory and computer simulation

Abstract

We present adsorption isotherms, phase diagrams, and density profiles for a Lennard-Jones fluid confined to a cylindrical pore. In particular, we concentrate on the gas–liquid transition in the pore (capillary condensation). We compare simulations for a series of radii and different temperatures with mean field density functional theory (MFT). Two forms of MFT are considered, the simple local density approximation (LDA) and Tarazona’s nonlocal or smoothed density approximation (SDA). We find that the SDA provides a quite accurate description of fluid structure in the pore and that it produces phase diagrams in good agreement with the simulation data. For larger radii and temperatures T/Tc≳0.6 the SDA shows steep rises in adsorption close to the transition. This strongly affects the shape of the coexistence curve in the T, ρ̄ plane. Here ρ̄ is defined as the average density inside the pore. This behavior is confirmed by the simulation. In contrast, LDA gives a poor representation of the fluid structure and this underlies the failure to reproduce the phase diagrams and adsorption isotherms found with SDA or simulation. For extremely small radii (R*≈1) the simulation adsorption isotherms are smooth, and for not too low a temperature they are accurately described by an approach which starts from the potential distribution theorem and uses perturbation theory for the true one-dimensional fluid.

Liquid–vapor oscillations of water in hydrophobic nanopores

“…Near R ϭ 0.55 nm both states are almost equally probable although they do not coexist spatially because the pore is finite and small. In infinite pores spatially alternating domains of equal length would be expected (35) and were actually observed in MD simulations (36). The oscillating states in short pores, on the other hand, alternate temporally, thus displaying a kind of ''time-averaged'' coexistence.…”

Structure and solvation forces in confined films of alkanes

“…Theoretical work and computer simulations of Lennard-Jones fluids and hard spheres show that the oscillatory solvation force originates from the stratification or ordering of the molecules in layers when the fluid is confined by the surfaces [5][6][7][8][9][10][11][12][13][14][15][16][17][18].…”

Molecular modelling of ionic liquids in the ordered mesoporous carbon CMK-5

“…For example, molecular simulations [69] have shown that methane inside neighbouring cylindrical pores in the zeolite AlPO 4-5 can undergo a gas-liquid phase transition; however, no phase transitions are observed when methane is inside an isolated cylindrical pore of the same material and size because the pore diameter is smaller than twice the molecular diameter of methane and thus, the system is very close to the one-dimensional limit. [70] Very recently, an order-disorder transition was also observed in molecular simulations of water inside a membrane with neighbouring, narrow cylindrical pores arranged in a square lattice. [71] Kondrat et al [72] and Péan et al [73] indicate that ILs inside sub-nm pores can experience very fast dynamics during charging processes due to a complex interplay between factors such as confinement in very narrow pores, ion crowding, screened interactions and changes in the structure of the double layer caused by the charging process.…”

“…Interactions between molecules located in neighbouring pores can act in a cooperative way and are known to induce important changes in the behaviour of several nonpolar and polar fluids inside nanopores. [70,71] Correlation effects might play an important role in determining the macroscopic properties of the electrical double-layer near charged surfaces [25,26]; these effects deserve further investigation in follow-up studies.…”