Microscopic density functional theory of wetting and drying of a solid substrate by an explicit solvent model of ionic solutions

Abstract

Classical density functional theory (DFT) of inhomogeneous fluids is applied to an explicit solvent "semi-primitive" model (SPM) of ionic solutions to investigate the influence of ionic solutes on the wetting behaviour of a solvent in contact with a neutral or charged planar substrate. The SPM is made up of 3 species of hard sphere particles with different diameters, interacting via an attractive Yukawa potential to model excluded volume and cohesion. The solvent particles are neutral, while the monovalent anions and cations are oppositely charged. The polar nature of the solvent is modeled by a continuum dielectric permittivity linked to the local solvent density. All 3 species interact with the impenetrable substrate via an attractive external potential. While excluded volume effects are accurately described by a Rosenfeld "fundamental measure" free energy functional, the short range Yukawa attraction and Coulombic interactions are treated within the mean-field approximation. The ionic solutes are found to have a significant impact on the wetting behaviour of the solvent, in particular on the wetting temperature. Strong electric fields, or long-ranged (weakly screened) Coulombic forces are shown to have the propensity to change the wetting transition from second to first order. The cation-anion size asymmetry leads to charge separation on the liquid-vapour interface of the solution, which in turn can induce a drying transition on the liquid side of liquid-vapour coexistence.

Kinetic Density Functional Theory: A Microscopic Approach to Fluid Mechanics

“…In the present section we briefly discuss recent extensions and applications of the self-consistent dynamical method, to binary neutral or to ternary charged mixtures comprised of positively and negatively charged hard spheres carrying point like charges (the ions) plus neutral spheres representing the solvent [38]. Each species, denoted by α, has mass m α , diameter σ αα .…”

Wetting and drying scenarios of ionic solutions

“…1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 The "semi-primitive" model (SPM) briefly introduced in Section 3 is a step in that direction. Solvent molecules are taken to be neutral hard spheres of diameter d 0 , while cations and anions are charged hard spheres of diameters d + and d − ; the investigations in reference [16] are restricted to monovalent ions (q + = +e, q − = −e), and the diameters taken equal to their Pauling values. This is the basic model examined in ref.…”

“…Since the model ignores the polar nature of the solvent molecules, it must be supplemented by the introduction of a local dielectric permittivity ε(z) depending on the local solvent density to allow for solute dissociation [15,16]. This model will be considered in Section 5.…”

“…In the following Sections the phenomenological and microscopic DFT approaches are generalised to the case of simple models of ionic solutions [13][14][15][16][17]. It is important to stress at the outset that for any theory of wetting or drying to be consistent, the interfacial and bulk thermodynamic properties must be calculated throughout from the SAME free energy functional, which reduces to a function of the constant densities in the bulk.…”

“…In the present paper we review this recent work [13][14][15][16][17], and present some new results on the variation of the substrate/fluid and liquid/vapour surface tensions with ion concentration. After a brief reminder of mean field theories of wetting and drying transitions in simple fluids (Section 2), we introduce the models and approximations used to describe inhomogeneous ionic solutions (Section 3).…”