1,283 research outputs found
Electrostatic interactions between discrete helices of charge
We analytically examine the pair interaction for parallel, discrete helices
of charge. Symmetry arguments allow for the energy to be decomposed into a sum
of terms, each of which has an intuitive geometric interpretation. Truncated
Fourier expansions for these terms allow for accurate modeling of both the
axial and azimuthal terms in the interaction energy and these expressions are
shown to be insensitive to the form of the interaction. The energy is evaluated
numerically through application of an Ewald-like summation technique for the
particular case of unscreened Coulomb interactions between the charges of the
two helices. The mode structures and electrostatic energies of flexible helices
are also studied. Consequences of the resulting energy expressions are
considered for both F-actin and A-DNA aggregates
Interfacial Tension of Electrolyte Solutions
A theory is presented to account for the increase in surface tension of water
in the presence of electrolyte. Unlike the original ``grand-canonical''
calculation of Onsager and Samaras, which relied on the Gibbs adsorption
isotherm and lead to a result which could only be expressed as an infinite
series, our approach is ``canonical'' and produces an analytic formula for the
excess surface tension. For small concentrations of electrolyte, our result
reduces to the Onsager-Samaras limiting law.Comment: contains two figures. Journal of Chemical Physics, in pres
All-Electron Path Integral Monte Carlo Simulations of Warm Dense Matter: Application to Water and Carbon Plasmas
We develop an all-electron path integral Monte Carlo (PIMC) method with
free-particle nodes for warm dense matter and apply it to water and carbon
plasmas. We thereby extend PIMC studies beyond hydrogen and helium to elements
with core electrons. PIMC pressures, internal energies, and pair-correlation
functions compare well with density functional theory molecular dynamics
(DFT-MD) at temperatures of (2.5-7.5) K and both methods together
form a coherent equation of state (EOS) over a density-temperature range of
3--12 g/cm and 10--10 K
Nonlinear dielectric effect of dipolar fluids
The nonlinear dielectric effect for dipolar fluids is studied within the
framework of the mean spherical approximation (MSA) of hard core dipolar Yukawa
fluids. Based on earlier results for the electric field dependence of the
polarization our analytical results show so-called normal saturation effects
which are in good agreement with corresponding NVT ensemble Monte Carlo
simulation data. The linear and the nonlinear dielectric permittivities
obtained from MC simulations are determined from the fluctuations of the total
dipole moment of the system in the absence of an applied electric field. We
compare the MSA based theoretical results with the corresponding Langevin and
Debye-Weiss behaviors.Comment: 10 pages including 4 figure
Ionic profiles close to dielectric discontinuities: Specific ion-surface interactions
We study, by incorporating short-range ion-surface interactions, ionic
profiles of electrolyte solutions close to a non-charged interface between two
dielectric media. In order to account for important correlation effects close
to the interface, the ionic profiles are calculated beyond mean-field theory,
using the loop expansion of the free energy. We show how it is possible to
overcome the well-known deficiency of the regular loop expansion close to the
dielectric jump, and treat the non-linear boundary conditions within the
framework of field theory. The ionic profiles are obtained analytically to
one-loop order in the free energy, and their dependence on different
ion-surface interactions is investigated. The Gibbs adsorption isotherm, as
well as the ionic profiles are used to calculate the surface tension, in
agreement with the reverse Hofmeister series. Consequently, from the
experimentally-measured surface tension, one can extract a single adhesivity
parameter, which can be used within our model to quantitatively predict hard to
measure ionic profiles.Comment: 14 pages, 6 figure
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
Lattice Models of Ionic Systems
A theoretical analysis of Coulomb systems on lattices in general dimensions
is presented. The thermodynamics is developed using Debye-Huckel theory with
ion-pairing and dipole-ion solvation, specific calculations being performed for
3D lattices. As for continuum electrolytes, low-density results for sc, bcc and
fcc lattices indicate the existence of gas-liquid phase separation. The
predicted critical densities have values comparable to those of continuum ionic
systems, while the critical temperatures are 60-70% higher. However, when the
possibility of sublattice ordering as well as Debye screening is taken into
account systematically, order-disorder transitions and a tricritical point are
found on sc and bcc lattices, and gas-liquid coexistence is suppressed. Our
results agree with recent Monte Carlo simulations of lattice electrolytes.Comment: 25 pages, 3 figures, ReVTeX 4, Submitted to J. Chem. Phy
Nonlinear effects in charge stabilized colloidal suspensions
Molecular Dynamics simulations are used to study the effective interactions
in charged stabilized colloidal suspensions. For not too high macroion charges
and sufficiently large screening, the concept of the potential of mean force is
known to work well. In the present work, we focus on highly charged macroions
in the limit of low salt concentrations. Within this regime, nonlinear
corrections to the celebrated DLVO theory [B. Derjaguin and L. Landau, Acta
Physicochem. USSR {\bf 14}, 633 (1941); E.J.W. Verwey and J.T.G. Overbeck, {\em
Theory of the Stability of Lyotropic Colloids} (Elsevier, Amsterdam, 1948)]
have to be considered. For non--bulklike systems, such as isolated pairs or
triples of macroions, we show, that nonlinear effects can become relevant,
which cannot be described by the charge renormalization concept [S. Alexander
et al., J. Chem. Phys. {\bf 80}, 5776 (1984)]. For an isolated pair of
macroions, we find an almost perfect qualitative agreement between our
simulation data and the primitive model. However, on a quantitative level,
neither Debye-H\"uckel theory nor the charge renormalization concept can be
confirmed in detail. This seems mainly to be related to the fact, that for
small ion concentrations, microionic layers can strongly overlap, whereas,
simultaneously, excluded volume effects are less important. In the case of
isolated triples, where we compare between coaxial and triangular geometries,
we find attractive corrections to pairwise additivity in the limit of small
macroion separations and salt concentrations. These triplet interactions arise
if all three microionic layers around the macroions exhibit a significant
overlap. In contrast to the case of two isolated colloids, the charge
distribution around a macroion in a triple is found to be anisotropic.Comment: 10 pages, 9 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
Crossover Scales at the Critical Points of Fluids with Electrostatic Interactions
Criticality in a fluid of dielectric constant D that exhibits Ising-type
behavior is studied as additional electrostatic (i.e., ionic) interactions are
turned on. An exploratory perturbative calculation is performed for small
ionicity as measured by the ratio of the electrostatic energy to the strength
of the short-range nonionic (i.e., van der Waals) interactions in the uncharged
fluid.
With the aid of distinct transformations for the short-range and for the
Coulombic interactions, an effective Hamiltonian with coefficients depending on
the ionicity is derived at the Debye-Hueckel limiting-law level for a fully
symmetric model. The crossover between classical (mean-field) and Ising
behavior is then estimated using a Ginzburg criterion. This indicates that the
reduced crossover temperature depends only weakly on the ionicity (and on the
range of the nonionic potentials); however, the trends do correlate with the,
much stronger, dependence observed experimentally.Comment: 25 pages, 4 figure; submitted to J. Chem. Phy
- âŠ