AbstractsBiology & Animal Science

Wetting phenomena in electrolyte solutions

by Ingrid Ibagon




Institution: University of Stuttgart
Department: Fakultät Mathematik und Physik
Degree: PhD
Year: 2014
Record ID: 1105476
Full text PDF: http://elib.uni-stuttgart.de/opus/volltexte/2014/9659/


Abstract

The present study analyzes wetting phenomena in electrolyte solutions. They are investigated by means of classical density functional theory. First, the wetting of a charged substrate by an electrolyte solution is studied with emphasis on the influence of the substrate charge density and of the ionic strength on the wetting transition temperature and on the order of the wetting transition. The corresponding models consist of solvent particles, anions, and cations. Two mean field approaches are used: (1) A lattice model (Chap. 3) within which particles occupy the sites of a semi-infinite simple cubic lattice. Each site is either empty or occupied by a single particle and the particles interact among each other via an attractive nearest-neighbor interaction which is taken to be the same for all pairs of particles. In addition, ion pairs interact via the Coulomb potential. The substrate can carry a homogeneous surface charge density and additionally attracts particles in the first layer adjacent to it. (2) A continuum model (Chap. 4) with short- and long-ranged solvent-solvent and substrate-solvent interactions and with ions interacting among each other and with the wall only via the electrostatic field. For the lattice model, the pure, i.e., salt-free, solvent exhibits a second-order wetting transition for all strengths of the substrate-particle and the particle-particle interactions for which the wetting transition temperature is nonzero. If the substrate is neutral, the addition of salt to the solvent changes neither the order nor the transition temperature of the wetting transition of the system. On the other hand, if the surface charge is nonzero, upon adding salt this continuous wetting transition changes to first-order within the range of substrate surface charge densities and ionic strengths considered here. As the substrate surface charge density is increased, for fixed ionic strength, the wetting transition temperature decreases. Moreover, the wetting transition temperature decreases when the ionic strength is decreased for fixed surface charge density. For the continuum model, expressions for the effective interface potential are derived analytically. The analysis of these expressions renders the conditions under which corresponding wetting transitions can be first- or second-order. The analytic results reveal in a transparent way that wetting transitions in electrolyte solutions, which occur far away from their critical point (i.e., the bulk correlation length is less than half of the Debye length), are always first-order if the solvent-solvent and solvent-wall interactions are short-ranged. In contrast, wetting transitions close to the bulk critical point of the solvent (i.e., the bulk correlation length is larger than the Debye length) exhibit the same wetting behavior as the pure, i.e., salt-free, solvent. If the salt-free solvent is governed by long-ranged solvent-solvent as well as long-ranged substrate-solvent interactions and exhibits critical wetting, adding salt can cause the occurrence of an…