3,012 research outputs found
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 on the surface charge density and
the ionic strength 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
. 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.Comment: accepted for publication in the Journal of Chemical Physic
Smectic phases in ionic liquid crystals
Ionic liquid crystals (ILCs) are anisotropic mesogenic molecules which carry
charges and therefore combine properties of liquid crystals, e.g., the
formation of mesophases, and of ionic liquids, such as low melting temperatures
and tiny triple-point pressures. Previous density functional calculations have
revealed that the phase behavior of ILCs is strongly affected by their
molecular properties, i.e., their aspect ratio, the loci of the charges, and
their interaction strengths. Here, we report new findings concerning the phase
behavior of ILCs as obtained by density functional theory and Monte Carlo
simulations. The most important result is the occurrence of a novel, wide
smectic-A phase , at low temperature, the layer spacing of which is
larger than that of the ordinary high-temperature smectic-A phase .
Unlike the ordinary smectic phase, the structure of the phase
consists of alternating layers of particles oriented parallel to the layer
normal and oriented perpendicular to it
Stability of thin liquid films and sessile droplets under confinement
The stability of nonvolatile thin liquid films and of sessile droplets is
strongly affected by finite size effects. We analyze their stability within the
framework of density functional theory using the sharp kink approximation,
i.e., on the basis of an effective interface Hamiltonian. We show that finite
size effects suppress spinodal dewetting of films because it is driven by a
long-wavelength instability. Therefore nonvolatile films are stable if the
substrate area is too small. Similarly, nonvolatile droplets connected to a
wetting film become unstable if the substrate area is too large. This
instability of a nonvolatile sessile droplet turns out to be equivalent to the
instability of a volatile drop which can attain chemical equilibrium with its
vapor.Comment: 14 pages, 13 figure
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
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.Comment: Submitte
Surface properties of fluids of charged platelike colloids
Surface properties of mixtures of charged platelike colloids and salt in
contact with a charged planar wall are studied within density functional
theory. The particles are modeled by hard cuboids with their edges constrained
to be parallel to the Cartesian axes corresponding to the Zwanzig model and the
charges of the particles are concentrated in their centers. The density
functional applied is an extension of a recently introduced functional for
charged platelike colloids. Analytically and numerically calculated bulk and
surface phase diagrams exhibit first-order wetting for sufficiently small
macroion charges and isotropic bulk order as well as first-order drying for
sufficiently large macroion charges and nematic bulk order. The asymptotic
wetting and drying behavior is investigated by means of effective interface
potentials which turn out to be asymptotically the same as for a suitable
neutral system governed by isotropic nonretarded dispersion forces. Wetting and
drying points as well as predrying lines and the corresponding critical points
have been located numerically. A crossover from monotonic to non-monotonic
electrostatic potential profiles upon varying the surface charge density has
been observed. Due to the presence of both the Coulomb interactions and the
hard-core repulsions, the surface potential and the surface charge do not
vanish simultaneously, i.e., the point of zero charge and the isoelectric point
of the surface do not coincide.Comment: 14 pages, submitte
Effective Landau theory of ferronematics
An effective Landau-like description of ferronematics, i.e., suspensions of
magnetic colloidal particles in a nematic liquid crystal (NLC), is developed in
terms of the corresponding magnetization and nematic director fields. The study
is based on a microscopic model and on classical density functional theory.
Ferronematics are susceptible to weak magnetic fields and they can exhibit a
ferromagnetic phase, which has been predicted several decades ago and which has
recently been found experimentally. Within the proposed effective Landau theory
of ferronematics one has quantitative access, e.g., to the coupling between the
magnetization of the magnetic colloids and the nematic director of the NLC. On
mesoscopic length scales this generates complex response patterns
Free Isotropic-Nematic Interfaces in Fluids of Charged Platelike Colloids
Bulk properties and free interfaces of mixtures of charged platelike colloids
and salt are studied within density-functional theory. The particles are
modeled by hard cuboids with their edges constrained to be parallel to the
artesian axes corresponding to the Zwanzig model. The charges of the particles
are concentrated in their center. The density functional is derived by
functional integration of an extension of the Debye-H\"uckel pair distribution
function with respect to the interaction potential. For sufficiently small
macroion charges, the bulk phase diagrams exhibit one isotropic and one nematic
phase separated by a first-order phase transition. With increasing platelet
charge, the isotropic and nematic binodals are shifted to higher densities. The
Donnan potential between the coexisting isotropic and nematic phases is
inferred from bulk structure calculations. Non-monotonic density and nematic
order parameter profiles are found at a free interface interpolating between
the coexisting isotropic and nematic bulk phases. Moreover, electrically
charged layers form at the free interface leading to monotonically varying
electrostatic potential profiles. Both the widths of the free interfaces and
the bulk correlation lengths are approximately given by the Debye length. For
fixed salt density, the interfacial tension decreases upon increasing the
macroion charge.Comment: 11 pages, submitted to J. Chem. Phy
- …