1,366 research outputs found
Properties of alkali-halide salt solutions about polarizable nanoparticle solutes for different ion models
Many-Body Electrostatic Forces Between Colloidal Particles at Vanishing Ionic Strength
Electrostatic forces between small groups of colloidal particles are measured
using blinking optical tweezers. When the electrostatic screening length is
significantly larger than the particle radius, forces are found to be
non-pairwise additive. Both pair and multi-particle forces are well described
by the linearized Poisson-Boltzmann equation with constant potential boundary
conditions. These findings may play an important role in understanding the
structure and stability of a wide variety of systems, from micron-sized
particles in oil to aqueous nanocolloids.Comment: 5 pages 2 figure
Liquid-liquid interfacial tension of electrolyte solutions
It is theoretically shown that the excess liquid-liquid interfacial tension
between two electrolyte solutions as a function of the ionic strength I behaves
asymptotically as O(- I^0.5) for small I and as O(+- I) for large I. The former
regime is dominated by the electrostatic potential due to an unequal
partitioning of ions between the two liquids whereas the latter regime is
related to a finite interfacial thickness. The crossover between the two
asymptotic regimes depends sensitively on material parameters suggesting that,
depending on the actual system under investigation, the experimentally
accessible range of ionic strengths can correspond to either the small or the
large ionic strength regime. In the limiting case of a liquid-gas surface where
ion partitioning is absent, the image chage interaction can dominate the
surface tension for small ionic strength I such that an Onsager-Samaras
limiting law O(- I ln(I)) is expected. The proposed picture is consistent with
more elaborate models and published measurements.Comment: Accepted for publication in Physical Review Letter
Nonlinear screening of charged macromolecules
We present several aspects of the screening of charged macromolecules in an
electrolyte. After a review of the basic mean field approach, based on the
linear Debye-Huckel theory, we consider the case of highly charged
macromolecules, where the linear approximation breaks down and the system is
described by full nonlinear Poisson-Boltzmann equation. Some analytical results
for this nonlinear equation give some interesting insight on physical phenomena
like the charge renormalization and the Manning counterion condensation
Quenched Charge Disorder and Coulomb Interactions
We develop a general formalism to investigate the effect of quenched fixed
charge disorder on effective electrostatic interactions between charged
surfaces in a one-component (counterion-only) Coulomb fluid. Analytical results
are explicitly derived for two asymptotic and complementary cases: i)
mean-field or Poisson-Boltzmann limit (including Gaussian-fluctuations
correction), which is valid for small electrostatic coupling, and ii)
strong-coupling limit, where electrostatic correlations mediated by counterions
become significantly large as, for instance, realized in systems with
high-valency counterions. In the particular case of two apposed and ideally
polarizable planar surfaces with equal mean surface charge, we find that the
effect of the disorder is nil on the mean-field level and thus the plates
repel. In the strong-coupling limit, however, the effect of charge disorder
turns out to be additive in the free energy and leads to an enhanced long-range
attraction between the two surfaces. We show that the equilibrium inter-plate
distance between the surfaces decreases for elevated disorder strength (i.e.
for increasing mean-square deviation around the mean surface charge), and
eventually tends to zero, suggesting a disorder-driven collapse transition.Comment: 13 pages, 2 figure
Non-linear screening of spherical and cylindrical colloids: the case of 1:2 and 2:1 electrolytes
From a multiple scale analysis, we find an analytic solution of spherical and
cylindrical Poisson-Boltzmann theory for both a 1:2 (monovalent co-ions,
divalent counter-ions) and a 2:1 (reversed situation) electrolyte. Our approach
consists in an expansion in powers of rescaled curvature , where
is the colloidal radius and the Debye length of the electrolytic
solution. A systematic comparison with the full numerical solution of the
problem shows that for cylinders and spheres, our results are accurate as soon
as . We also report an unusual overshooting effect where the
colloidal effective charge is larger than the bare one.Comment: 9 pages, 11 figure
Effective interactions in the colloidal suspensions from HNC theory
The HNC Ornstein-Zernike integral equations are used to determine the
properties of simple models of colloidal solutions where the colloids and ions
are immersed in a solvent considered as a dielectric continuum and have a size
ratio equal to 80 and a charge ratio varying between 1 and 4000. At an infinite
dilution of colloids, the effective interactions between colloids and ions are
determined for ionic concentrations ranging from 0.001 to 0.1 mol/l and
compared to those derived from the Poisson-Boltzmann theory. At finite
concentrations, we discuss on the basis of the HNC results the possibility of
an unambiguous definition of the effective interactions between the colloidal
molecules.Comment: 26 pages, 15 figure
Casimir-Polder interatomic potential between two atoms at finite temperature and in the presence of boundary conditions
We evaluate the Casimir-Polder potential between two atoms in the presence of
an infinite perfectly conducting plate and at nonzero temperature. In order to
calculate the potential, we use a method based on equal-time spatial
correlations of the electric field, already used to evaluate the effect of
boundary conditions on interatomic potentials. This method gives also a
transparent physical picture of the role of a finite temperature and boundary
conditions on the Casimir-Polder potential. We obtain an analytical expression
of the potential both in the near and far zones, and consider several limiting
cases of interest, according to the values of the parameters involved, such as
atom-atom distance, atoms-wall distance and temperature.Comment: 11 page
Screening of charged spheroidal colloidal particles
We study the effective screened electrostatic potential created by a
spheroidal colloidal particle immersed in an electrolyte, within the mean field
approximation, using Poisson--Botzmann equation in its linear and nonlinear
forms, and also beyond the mean field by means of Monte Carlo computer
simulation. The anisotropic shape of the particle has a strong effect on the
screened potential, even at large distances (compared to the Debye length) from
it. To quantify this anisotropy effect, we focus our study on the dependence of
the potential on the position of the observation point with respect with the
orientation of the spheroidal particle. For several different boundary
conditions (constant potential, or constant surface charge) we find that, at
large distance, the potential is higher in the direction of the large axis of
the spheroidal particle
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