1,368 research outputs found
Single chain properties of polyelectrolytes in poor solvent
Using molecular dynamics simulations we study the behavior of a dilute
solution of strongly charged polyelectrolytes in poor solvents, where we take
counterions explicitly into account. We focus on the chain conformational
properties under conditions where chain-chain interactions can be neglected,
but the counterion concentration remains finite. We investigate the
conformations with regard to the parameters chain length, Coulomb interaction
strength, and solvent quality, and explore in which regime the competition
between short range hydrophobic interactions and long range Coulomb
interactions leads to pearl-necklace like structures. We observe that large
number and size fluctuations in the pearls and strings lead to only small
direct signatures in experimental observables like the single chain form
factor. Furthermore we do not observe the predicted first order collapse of the
necklace into a globular structure when counterion condensation sets in. We
will also show that the pearl-necklace regime is rather small for strongly
charged polyelectrolytes at finite densities. Even small changes in the charge
fraction of the chain can have a large impact on the conformation due to the
delicate interplay between counterion distribution and chain conformation.Comment: 20 pages, 27 figures, needs jpc.sty (included), to appear in Jour.
Phys. Chem
Structure of Polyelectrolytes in Poor Solvent
We present simulations on charged polymers in poor solvent. First we
investigate in detail the dilute concentration range with and without imposed
extension constraints. The resulting necklace polymer conformations are
analyzed in detail. We find strong fluctuations in the number of pearls and
their sizes leading only to small signatures in the form factor and the
force-extension relation. The scaling of the peak in the structure factor with
the monomer density shows a pertinent different behavior from good solvent
chains.Comment: 7 pages, 5 figures. submitted to EP
Finite Size Polyelectrolyte Bundles at Thermodynamic Equilibrium
We present the results of extensive computer simulations performed on
solutions of monodisperse charged rod-like polyelectrolytes in the presence of
trivalent counterions. To overcome energy barriers we used a combination of
parallel tempering and hybrid Monte Carlo techniques. Our results show that for
small values of the electrostatic interaction the solution mostly consists of
dispersed single rods. The potential of mean force between the polyelectrolyte
monomers yields an attractive interaction at short distances. For a range of
larger values of the Bjerrum length, we find finite size polyelectrolyte
bundles at thermodynamic equilibrium. Further increase of the Bjerrum length
eventually leads to phase separation and precipitation. We discuss the origin
of the observed thermodynamic stability of the finite size aggregates
Correlation length of hydrophobic polyelectrolyte solutions
The combination of two techniques (Small Angle X-ray Scattering and Atomic
Force Microscopy) has allowed us to measure in reciprocal and real space the
correlation length of salt-free aqueous solutions of highly charged
hydrophobic polyelectrolyte as a function of the polymer concentration ,
charge fraction and chain length . Contrary to the classical behaviour
of hydrophilic polyelectrolytes in the strong coupling limit, is strongly
dependent on . In particular a continuous transition has been observed from
to when decreased from 100% to
35%. We interpret this unusual behaviour as the consequence of the two features
characterising the hydrophobic polyelectrolytes: the pearl necklace
conformation of the chains and the anomalously strong reduction of the
effective charge fraction.Comment: 7 pages, 5 figures, submitted to Europhysics Letter
Polyelectrolyte Bundles
Using extensive Molecular Dynamics simulations we study the behavior of
polyelectrolytes with hydrophobic side chains, which are known to form
cylindrical micelles in aqueous solution. We investigate the stability of such
bundles with respect to hydrophobicity, the strength of the electrostatic
interaction, and the bundle size. We show that for the parameter range relevant
for sulfonated poly-para-phenylenes (PPP) one finds a stable finite bundle
size. In a more generic model we also show the influence of the length of the
precursor oligomer on the stability of the bundles. We also point out that our
model has close similarities to DNA solutions with added condensing agents,
hinting to the possibility that the size of DNA aggregates is under certain
circumstances thermodynamically limited.Comment: 10 pages, 8 figure
Conductance of the single-electron transistor: A comparison of experimental data with Monte Carlo calculations
We report on experimental results for the conductance of metallic
single-electron transistors as a function of temperature, gate voltage and
dimensionless conductance. In contrast to previous experiments our transistor
layout allows for a direct measurement of the parallel conductance and no ad
hoc assumptions on the symmetry of the transistors are necessary. Thus we can
make a comparison between our data and theoretical predictions without any
adjustable parameter. Even for rather weakly conducting transistors significant
deviations from the perturbative results are noted. On the other hand, path
integral Monte Carlo calculations show remarkable agreement with experiments
for the whole range of temperatures and conductances.Comment: 8 pages, 7 figures, revtex4, corrected typos, submitted to PR
On the pearl size of hydrophobic polyelectrolytes
Hydrophobic polyelectrolytes have been predicted to adopt an unique
pearl-necklace conformation in aqueous solvents. We present in this Letter an
attempt to characterise quantitatively this conformation with a focus on ,
the pearl size. For this purpose polystyrenesulfonate (PSS) of various
effective charge fractions and chain lengths has been adsorbed
onto oppositely charged surfaces immersed in water in condition where the bulk
structure is expected to persist in the adsorbed state. \emph{In situ}
ellipsometry has provided an apparent thickness of the PSS layer. In
the presence of added salts, we have found:
( is the monomer size) in agreement with the scaling predictions for
in the pearl-necklace model if one interprets as a measure of the
pearl size. At the lowest charge fractions we have found
for the shorter chains, in agreement with a necklace/globule transition.Comment: 7 pages, 4 figures, 1 table. Published in Europhysics Letters, Vol.
62, Number 1, pp. 110-116 (2003
Optimizing end-labeled free-solution electrophoresis by increasing the hydrodynamic friction of the drag-tag
We study the electrophoretic separation of polyelectrolytes of varying
lengths by means of end-labeled free-solution electrophoresis (ELFSE). A
coarse-grained molecular dynamics simulation model, using full electrostatic
interactions and a mesoscopic Lattice Boltzmann fluid to account for
hydrodynamic interactions, is used to characterize the drag coefficients of
different label types: linear and branched polymeric labels, as well as
transiently bound micelles.
It is specifically shown that the label's drag coefficient is determined by
its hydrodynamic size, and that the drag per label monomer is largest for
linear labels. However, the addition of side chains to a linear label offers
the possibility to increase the hydrodynamic size, and therefore the label
efficiency, without having to increase the linear length of the label, thereby
simplifying synthesis. The third class of labels investigated, transiently
bound micelles, seems very promising for the usage in ELFSE, as they provide a
significant higher hydrodynamic drag than the other label types.
The results are compared to theoretical predictions, and we investigate how
the efficiency of the ELFSE method can be improved by using smartly designed
drag-tags.Comment: 32 pages, 11 figures, submitted to Macromolecule
The Tensor-Vector-Scalar theory and its cosmology
Over the last few decades, astronomers and cosmologists have accumulated vast
amounts of data clearly demonstrating that our current theories of fundamental
particles and of gravity are inadequate to explain the observed discrepancy
between the dynamics and the distribution of the visible matter in the
Universe. The Modified Newtonian Dynamics (MOND) proposal aims at solving the
problem by postulating that Newton's second law of motion is modified for
accelerations smaller than ~10^{-10}m/s^2. This simple amendment, has had
tremendous success in explaining galactic rotation curves. However, being
non-relativistic, it cannot make firm predictions for cosmology.
A relativistic theory called Tensor-Vector-Scalar (TeVeS) has been proposed
by Bekenstein building on earlier work of Sanders which has a MOND limit for
non-relativistic systems.
In this article I give a short introduction to TeVeS theory and focus on its
predictions for cosmology as well as some non-cosmological studies.Comment: 44 pages, topical review for Classical and Quantum Gravit
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