14 Externe wissenschaftliche Einrichtungen
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Item Open Access Wetting phenomena in electrolyte solutions(2014) Ibagon, Ingrid; Dietrich, Siegfried (Prof. Dr.)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 ion-induced first-order thin-thick transition which precedes the subsequent continuous wetting as for the salt-free solvent. The phenomenon of electrowetting, i.e., the dependence of the macroscopic contact angle of a fluid on the electrostatic potential of the substrate, is studied in Chap. 5 for a vertical parallel plate capacitor in contact with two immiscible fluids, where at least one of the two fluids is an electrolyte solution. Here, the possibility of the formation of films of microscopic thickness on the substrates, widely ignored in the context of electrowetting, is taken into account. This approach allows one to transparently derive the electrowetting equation. The derivation shows that electrowetting is a consequence of the voltage-dependence of the depth of the effective interface potential. Finally, the line tension and the three-phase contact line structure of a drop of an electrolyte solution on a charged substrate are investigated within the lattice model in Chap. 6. For the pure solvent, the equilibrium liquid-gas interface profile approaches its asymptote from above, as expected for second-order wetting transitions and the line tension depends linearly on the contact angle the drop makes with the substrate. For the electrolyte solution, the equilibrium liquid-gas interface profile approaches its asymptote from below as expected for first-order wetting transitions. When the contact angle is changed by varying the temperature while keeping the surface charge fixed, the line tension increases as the temperature is increased, i.e, as the contact angle is decreased. When the contact angle is changed by varying the surface charge density at fixed temperature, the line tension increases as the surface charge is increased. The equilibrium structure of the three-phase contact line for different charge densities has been calculated.Item Open Access Phase behavior of colloidal suspensions with critical solvents(2013) Mohry, Thomas F.; Dietrich, Siegfried (Prof. Dr.)Colloidal suspensions, i.e., nano- to micrometer sized particles immersed in a solvent, are interesting, both for applications which are present in various kinds in our daily life as well as for exploring fundamental physics. In this work colloidal suspensions are studied the solvents of which are binary liquid mixtures which exhibit a miscibility gap. The focus is set on the thermodynamic region close to the critical point of demixing of this solvent. In that region the fluctuations of the concentration of the two components forming the binary liquid mixture are correlated on meso- to macroscopic length scales. The spectrum of these correlations and the concentration profile of the solvent particles are affected by the presence of the colloidal solute particles. Among others, these alterations result in an effective force acting between the solute particles. Close to the critical point of the solvent this effective force acquires an universal contribution which is known as the critical Casimir force. This effective force acting between surfaces confining a critical medium is studied, in particular its dependence on the bulk ordering field. In the case of a binary liquid mixture the bulk ordering field is related to the deviation of the concentration of the solvent from its critical value. A small value of the bulk ordering field can enhance the strength of the critical Casimir force up to ten times as compared with its value for zero bulk field. The critical Casimir force which acts between solute particles with the same adsorption preferences for one of the two components forming the solvent is attractive. This attraction, which can be tuned by minute temperature changes, can lead to reversible aggregation of the solute particles. These aggregation phenomena are studied theoretically, e.g., in terms of the radial distribution function or the second virial coefficient. The effective attraction due to the critical Casimir forces may also lead to a phase separation into a colloidal rich and a colloidal poor phase. Colloidal suspensions are often treated as effective one-component systems of colloidal particles between which solvent mediated effective interactions act. This effective (and often successful) description is shown to fail in general in the presence of phase-separating solvents. The thermodynamics and the phase diagram of such colloidal suspensions are discussed thoroughly and approaches for the description of the full ternary mixture are presented. The theoretical results are compared in detail with in the literature available experimental data.