arXiv subjects

Markus Bier

Publications and source records attributed to Markus Bier.

At least 37 records · Page 2Linked to original sources

arXiv2017

Salt-induced microheterogeneities in binary liquid mixtures

The salt-induced microheterogeneity (MH) formation in binary liquid mixtures is studied by small-angle X-ray scattering (SAXS) and liquid state theory. Previous experiments have shown that this phenomenon occurs for antagonistic salts, whose cations and anions prefer different components of the solvent mixture. However, so far the precise mechanism leading to the characteristic length scale of MHs remained unclear. Here, it is shown that MHs can be generated by the competition of short-ranged interactions and long-ranged monopole-dipole interactions. The experimental SAXS patterns can be quantitatively reproduced by fitting to the derived correlation functions without assuming any specific model. The dependency of the MH structure with respect to ionic strength and temperature is analyzed. Close to the demixing phase transition, critical-like behavior occurs with respect to the spinodal line in the phase diagram.

cond-mat.soft↗

arXiv2017

Electrolyte solutions at curved electrodes. I. Mesoscopic approach

Within the Poisson-Boltzmann (PB) approach electrolytes in contact with planar, spherical, and cylindrical electrodes are analyzed systematically. The dependences of their capacitance $C$ on the surface charge density $σ$ and the ionic strength $I$ are examined as function of the wall curvature. The surface charge density has a strong effect on the capacitance for small curvatures whereas for large curvatures the behavior becomes independent of $σ$. An expansion for small curvatures gives rise to capacitance coefficients which depend only on a single parameter, allowing for a convenient analysis. The universal behavior at large curvatures can be captured by an analytic expression.

cond-mat.soft↗

arXiv2017

Electrolyte solutions at curved electrodes. II. Microscopic approach

Density functional theory is used to describe electrolyte solutions in contact with electrodes of planar or spherical shape. For the electrolyte solutions we consider the so-called civilized model, in which all species present are treated on equal footing. This allows us to discuss the features of the electric double layer in terms of the differential capacitance. The model provides insight into the microscopic structure of the electric double layer, which goes beyond the mesoscopic approach studied in the accompanying paper. This enables us to judge the relevance of microscopic details, such as the radii of the particles forming the electrolyte solutions or the dipolar character of the solvent particles, and to compare the predictions of various models. Similar to the preceding paper, a general behavior is observed for small radii of the electrode in that in this limit the results become independent of the surface charge density and of the particle radii. However, for large electrode radii non-trivial behaviors are observed. Especially the particle radii and the surface charge density strongly influence the capacitance. From the comparison with the Poisson-Boltzmann approach it becomes apparent that the shape of the electrode determines whether the microscopic details of the full civilized model have to be taken into account or whether already simpler models yield acceptable predictions.

cond-mat.soft↗

arXiv2016

Poisson-Boltzmann study of the effective electrostatic interaction between colloids at an electrolyte interface

The effective electrostatic interaction between a pair of colloids, both of them located close to each other at an electrolyte interface, is studied by employing the full, nonlinear Poisson-Boltzmann (PB) theory within classical density functional theory. Using a simplified yet appropriate model, all contributions to the effective interaction are obtained exactly, albeit numerically. The comparison between our results and those obtained within linearized PB theory reveals that the latter overestimates these contributions significantly at short inter-particle separations. Whereas the surface contributions to the linear and the nonlinear PB results differ only quantitatively, the line contributions show qualitative differences at short separations. Moreover, a dependence of the line contribution on the solvation properties of the two adjacent fluids is found, which is absent within the linear theory. Our results are expected to enrich the understanding of effective interfacial interactions between colloids.

cond-mat.soft↗

arXiv2015

Three-phase contact line and line tension of electrolyte solutions in contact with charged substrates

The three-phase contact line formed by the intersection of a liquid-vapor interface of an electrolyte solution with a charged planar substrate is studied in terms of classical density functional theory applied to a lattice model. The influence of the substrate charge density and of the ionic strength of the solution on the intrinsic structure of the three-phase contact line and on the corresponding line tension is analyzed. We find a negative line tension for all values of the surface charge density and of the ionic strength considered. The strength of the line tension decreases upon decreasing the contact angle via varying either the temperature or the substrate charge density.

cond-mat.soft↗

arXiv2015

Non-equilibrium interfacial tension during relaxation

The concept of a non-equilibrium interfacial tension, defined via the work required to deform the system such that the interfacial area is changed while the volume is conserved, is investigated theoretically in the context of the relaxation of an initial perturbation of a colloidal fluid towards the equilibrium state. The corresponding general formalism is derived for systems with planar symmetry and applied to fluid models of colloidal suspensions and polymer solutions. It is shown that the non-equilibrium interfacial tension is not necessarily positive, that negative non-equilibrium interfacial tensions are consistent with strictly positive equilibrium interfacial tensions, and that the sign of the interfacial tension can influence the morphology of density perturbations during relaxation.

cond-mat.stat-mech↗

arXiv2015

Static dielectric properties of dense ionic fluids

The static dielectric properties of dense ionic fluids, e.g., room temperature ionic liquids (RTILs) and inorganic fused salts, are investigated on different length scales by means of grandcanonical Monte Carlo simulations. A generally applicable scheme is developed which allows one to approximately decompose the electric susceptibility of dense ionic fluids into the orientation and the distortion polarization contribution. It is shown that at long range the well-known plasma-like perfect screening behavior occurs, which corresponds to a diverging distortion susceptibility, whereas at short range orientation polarization dominates, which coincides with that of a dipolar fluid of attached cation-anion pairs. This observation suggests that the recently debated interpretation of RTILs as dilute electrolyte solutions might not be simply a yes-no-question but it might depend on the considered length scale.

cond-mat.soft↗

arXiv2015

Thermal and structural properties of ionic fluids

The electrostatic interaction in ionic fluids is well-known to give rise to a characteristic phase behavior and structure. Sometimes its long range is proposed to single out the electrostatic potential over other interactions with shorter ranges. Here the importance of the range for the phase behavior and the structure of ionic fluids is investigated by means of grandcanonical Monte Carlo simulations of the lattice restricted primitive model (LRPM). The long-ranged electrostatic interaction is compared to various types of short-ranged potentials obtained by sharp and/or smooth cut-off schemes. Sharply cut off electrostatic potentials are found to lead to a strong dependence of the phase behavior and the structure on the cut-off radius. However, when combined with a suitable additional smooth cut-off, the short-ranged LRPM is found to exhibit quantitatively the same phase behavior and structure as the conventional long-ranged LRPM. Moreover, the Stillinger-Lovett perfect screening property, which is well-known to be generated by the long-ranged electrostatic potential, is also fulfilled by short-ranged LRPMs with smooth cut-offs. By showing that the characteristic phase behavior and structure of ionic fluids can also be found in systems with short-ranged potentials, one can conclude that the decisive property of the electrostatic potential in ionic fluids is not the long range but rather the valency dependence.

cond-mat.soft↗

arXiv2015

Implications of interface conventions for morphometric thermodynamics

Several model fluids in contact with planar, spherical, and cylindrical walls are investigated for small number densities within density functional theory. The dependence of the solid-fluid interfacial tension on the curvature of spherical and cylindrical walls is examined and compared with the corresponding expression derived within the framework of morphometric thermodynamics. Particular attention is paid to the implications of the choice of the interface location, which underlies the definition of the interfacial tension. We find that morphometric thermodynamics is never exact for the considered systems and that its quality as an approximation depends sensitively on the choice of the interface location.

cond-mat.stat-mech↗

arXiv2014

Electrostatic interaction between colloidal particles trapped at an electrolyte interface

The electrostatic interaction between colloidal particles trapped at the interface between two immiscible electrolyte solutions is studied in the limit of small inter-particle distances. Within an appropriate model exact analytic expressions for the electrostatic potential as well as for the surface and line interaction energies are obtained. They demonstrate that the widely used superposition approximation, which is commonly applied to large distances between the colloidal particles, fails qualitatively at small distances and is quantitatively unreliable even at large distances. Our results contribute to an improved description of the interaction between colloidal particles trapped at fluid interfaces.

cond-mat.soft↗

arXiv2014

Order of wetting transitions in electrolyte solutions

For wetting films in dilute electrolyte solutions close to charged walls we present analytic expressions for their effective interface potentials. The analysis of these expressions renders the conditions under which corresponding wetting transitions can be first- or second-order. Within mean field theory we consider two models, one with short- and one with long-ranged solvent-solvent and solvent-wall interactions. 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 solvent-wall 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.

cond-mat.soft↗

arXiv2014

Density functional theory of electrowetting

The phenomenon of electrowetting, i.e., the dependence of the macroscopic contact angle of a fluid on the electrostatic potential of the substrate, is analyzed in terms of the density functional theory of wetting. It is shown that electrowetting is not an electrocapillarity effect, i.e., it cannot be consistently understood in terms of the variation of the substrate-fluid interfacial tension with the electrostatic substrate potential, but it is related to the depth of the effective interface potential. The key feature, which has been overlooked so far and which occurs naturally in the density functional approach is the structural change of a fluid if it is brought into contact with another fluid. These structural changes occur in the present context as the formation of finite films of one fluid phase in between the substrate and the bulk of the other fluid phase. The non-vanishing Donnan potentials (Galvani potential differences) across such film-bulk fluid interfaces, which generically occur due to an unequal partitioning of ions as a result of differences of solubility contrasts, lead to correction terms in the electrowetting equation, which become relevant for sufficiently small substrate potentials. Whereas the present density functional approach confirms the commonly used electrocapillarity-based electrowetting equation as a good approximation for the cases of metallic electrodes or electrodes coated with a hydrophobic dielectric in contact with an electrolyte solution and an ion-free oil, a significantly reduced tendency for electrowetting is predicted for electrodes coated with a dielectric which is hydrophilic or which is in contact with two immiscible electrolyte solutions.

cond-mat.soft↗

arXiv2013

Non-equilibrium interfaces in colloidal fluids

The time-dependent structure, interfacial tension, and evaporation of an oversaturated colloid-rich (liquid) phase in contact with an undersaturated colloid-poor (vapor) phase of a colloidal dispersion is investigated theoretically during the early-stage relaxation, where the interface is relaxing towards a local equilibrium state while the bulk phases are still out of equilibrium. Since systems of this type exhibit a clear separation of colloidal and solvent relaxation time scales with typical times of interfacial tension measurements in between, they can be expected to be suitable for analogous experimental studies, too. The major finding is that, irrespective of how much the bulk phases differ from two-phase coexistence, the interfacial structure and the interfacial tension approach those at two-phase coexistence during the early-stage relaxation process. This is a surprising observation since it implies that the relaxation towards global equilibrium of the interface is not following but preceding that of the bulk phases. Scaling forms for the local chemical potential, the flux, and the dissipation rate exhibit qualitatively different leading order contributions depending on whether an equilibrium or a non-equilibrium system is considered. The degree of non-equilibrium between the bulk phases is found to not influence the qualitative relaxation behavior (i.e., the values of power-law exponents), but to determine the quantitative deviation of the observed quantities from their values at two-phase coexistence. Whereas the underlying dynamics differs between colloidal and molecular fluids, the behavior of quantities such as the interfacial tension approaching the equilibrium values during the early-stage relaxation process, during which non-equilibrium conditions of the bulk phases are not changed, can be expected to occur for both types of systems.

cond-mat.soft↗

arXiv2013

Impedance spectroscopy of ions at liquid-liquid interfaces

The possibility to extract properties of an interface between two immiscible liquids, e.g., electrolyte solutions or polyelectrolyte multilayers, by means of impedance spectroscopy is investigated theoretically within a dynamic density functional theory which is equivalent to the Nernst-Planck-Poisson theory. A novel approach based on a two-step fitting procedure of an equivalent circuit to impedance spectra is proposed which allows to uniquely separate bulk and interfacial elements. Moreover, the proposed method avoids overfitting of the bulk properties of the two liquids in contact and underfitting of the interfacial properties, as they might occur for standard one-step procedures. The key idea is to determine the bulk elements of the equivalent circuit in a first step by fitting corresponding sub-circuits to the spectra of uniform electrolyte solutions, and afterwards fitting the full equivalent circuit with fixed bulk elements to the impedance spectrum containing the interface. This approach is exemplified for an equivalent circuit which leads to a physically intuitive qualitative behavior as well as to quantitively realistic values of the interfacial elements. The proposed method is robust such that it can be expected to be applicable to a wide class of systems with liquid-liquid interfaces.

cond-mat.soft↗

arXiv2013

Wetting in electrolyte solutions

Wetting of a charged substrate by an electrolyte solution is investigated by means of classical density functional theory applied to a lattice model. Within the present model the pure, i.e., salt-free solvent, for which all interactions are of the nearest-neighbor type only, 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. The influences of the substrate charge density and of the ionic strength on the wetting transition temperature and on the order of the wetting transition are studied. 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. If the surface charge is nonzero, upon adding salt this continuous wetting transition changes to first-order within the wide range of substrate surface charge densities and ionic strengths studied here. As the substrate surface charge density is increased, at fixed ionic strength, the wetting transition temperature decreases and the prewetting line associated with the first-order wetting transition becomes longer. This decrease of the wetting transition temperature upon increasing the surface charge density becomes more pronounced by decreasing the ionic strength.

cond-mat.soft↗

arXiv2012

Local theory for ions in binary liquid mixtures

The influence of ions on the bulk phase behavior of binary liquid mixtures acting as their solvents and on the corresponding interfacial structures close to a planar wall is investigated by means of density functional theory based on local descriptions of the effective interactions between ions and their solvents. The bilinear coupling approximation (BCA), which has been used in numerous previous related investigations, is compared with a novel local density approximation (LDA) for the ion-solvent interactions. It turns out that within BCA the bulk phase diagrams, the two-point correlation functions, and critical adsorption exhibit qualitative features which are not compatible with the available experimental data. These discrepancies do not occur within the proposed LDA. Further experimental investigations are suggested which assess the reliability of the proposed LDA. This approach allows one to obtain a consistent and rather general understanding of the effects of ions on solvent properties. From our analysis we infer in particular that there can be an experimentally detectable influence of ions on binary liquid mixtures due to steric effects but not due to charge effects.

cond-mat.soft↗

arXiv2011

Electrostatic interactions in critical solvents

The subtle interplay between critical phenomena and electrostatics is investigated by considering the effective force acting on two parallel walls confining a near-critical binary liquid mixture with added salt. The ion-solvent coupling can turn a non-critical repulsive electrostatic force into an attractive one upon approaching the critical point. However, the effective force is eventually dominated by the critical Casimir effect, the universal properties of which are not altered by the presence of salt. This observation allows a consistent interpretation of recent experimental data.

cond-mat.soft↗

arXiv2010

Vapor Pressure of Ionic Liquids

We argue that the extremely low vapor pressures of room temperature ionic liquids near their triple points are due to the combination of strong ionic characters and of low melting temperatures.

cond-mat.mtrl-sci↗