Population Inversion of a NAHS Mixture Adsorbed into a Cylindrical Pore

Abstract

A cylindrical nanopore immersed in a non-additive hard sphere binary fluid is studied by means of integral equation theories and Monte Carlo simulations. It is found that at low and intermediate values of the bulk total number density the more concentrated bulk species is preferentially absorbed by the pore, as expected. However, further increments of the bulk number density lead to an abrupt population inversion in the confined fluid and an entropy driven prewetting transition at the outside wall of the pore. These phenomena are a function of the pore size, the non-additivity parameter, the bulk number density, and particles relative number fraction. We discuss our results in relation to the phase separation in the bulk.

Controlling the composition of a confined fluid by an electric field

“…The simplest one is the mixture of nonadditive hard spheres [24] in which the cross diameter is σ ij = 1/2(σ i + σ j )(1 + δ). Previous studies [9,13,14] have shown that when the pore is in equilibrium with a mixture in which one species is in minority (say x 2 = 0.02 for a non additivity parameter δ = 0.2 ) a population inversion occurs in the pore when the total bulk density is varied. This occurs here for ρ b between 0.55 and 0.56 for a pure non-additive HS mixture and between 0.54 and 0.55 for the hard-sphere dipole mixture.…”

“…Previous studies considered the role of the pressure in one-component fluids (eg. [8]), or the total density and the mole fractions [9,13,14] in bulk mixtures but without field. As shown below, the combination of both will play here a crucial role.…”

“…small variation of the applied field produces the inversion: the dipolar particles are selectively absorbed by a weak change about E tr , the converse being possible, perhaps with some hysteresis[13]. Near the adorption jump (ρ b = 0.53, E * = 0.5 for µ * = 1) a slight change in temperature (δT = 25 o C infigure 4) produces a detectable change in adsorption.…”

Structure of penetrable sphere fluids and mixtures near a slit hard wall: A modified bridge density functional approximation

“…This modified bridge density functional approach can be extended to the nonadditive hard-sphere mixtures adsorbed into pores in our previous work. 26,27 It would also be interesting to apply the present approximation for studying the exotic phase behavior occurring in model penetrable spheres such as the clustering transitions and the re-entrant melting phenomena. In addition to structural properties observed in this work, we are currently developing molecular dynamics simulation codes for studying the time-dependent transport properties of penetrable spheres with ultrasoft repulsive interactions.…”

Wall–particle interactions and depletion forces in narrow slits

“…Hence, σ p s0 corresponds to the AO limit (ideal polymers) and σ p N 0 to a particular non-additive hard sphere binary mixture. The phase separation of the NASH model was also studied for symmetric binary mixtures with different composition (σ a = σ b and Δ N 0) under confinement [53] leading to the so called population inversion effect [54] (clearly related to the capillarity condensation/evaporation phenomenon). It consists on that, under certain conditions, the more concentrated bulk species turns the less concentrated species inside the pore [55,56].…”