1,413 research outputs found
The Osmotic Coefficient of Rod-like Polyelectrolytes: Computer Simulation, Analytical Theory, and Experiment
The osmotic coefficient of solutions of rod-like polyelectrolytes is
considered by comparing current theoretical treatments and simulations to
recent experimental data. The discussion is restricted to the case of
monovalent counterions and dilute, salt-free solutions. The classical
Poisson-Boltzmann solution of the cell model correctly predicts a strong
decrease in the osmotic coefficient, but upon closer look systematically
overestimates its value. The contribution of ion-ion-correlations are
quantitatively studied by MD simulations and the recently proposed DHHC theory.
However, our comparison with experimental data obtained on synthetic,
stiff-chain polyelectrolytes shows that correlation effects can only partly
explain the discrepancy. A quantitative understanding thus requires theoretical
efforts beyond the restricted primitive model of electrolytes.Comment: 16 pages, 2 figure
Theory and simulations of rigid polyelectrolytes
We present theoretical and numerical studies on stiff, linear
polyelectrolytes within the framework of the cell model. We first review
analytical results obtained on a mean-field Poisson-Boltzmann level, and then
use molecular dynamics simulations to show, under which circumstances these
fail quantitatively and qualitatively. For the hexagonally packed nematic phase
of the polyelectrolytes we compute the osmotic coefficient as a function of
density. In the presence of multivalent counterions it can become negative,
leading to effective attractions. We show that this results from a reduced
contribution of the virial part to the pressure. We compute the osmotic
coefficient and ionic distribution functions from Poisson-Boltzmann theory with
and without a recently proposed correlation correction, and also simulation
results for the case of poly(para-phenylene) and compare it to recently
obtained experimental data on this stiff polyelectrolyte. We also investigate
ion-ion correlations in the strong coupling regime, and compare them to
predictions of the recently advocated Wigner crystal theories.Comment: 32 pages, 15 figures, proceedings of the ASTATPHYS-MEX-2001, to be
published in Mol. Phy
Polyelectrolyte Solutions with Multivalent Salts
We investigate the thermodynamic properties of a polyelectrolyte solution in
a presence of {\it multivalent} salts. The polyions are modeled as rigid
cylinders with the charge distributed uniformly along the major axis. The
solution, besides the polyions, contain monovalent and divalent counterions as
well as monovalent coions. The strong electrostatic attraction existing between
the polyions and the counterions results in formation of clusters consisting of
one polyion and a number of associated monovalent and divalent counterions. The
theory presented in the paper allows us to explicitly construct the Helmholtz
free energy of a polyelectrolyte solution. The characteristic cluster size, as
well as any other thermodynamic property can then be determined by an
appropriate operation on the free energy
Overcharging of DNA in the presence of salt: Theory and Simulation
A study of a model rod-like polyelectrolyte molecule immersed into a
monovalent or divalent electrolyte is presented. Results from the
hypernetted-chain/mean spherical approximation (HNC/MSA) theory, for
inhomogeneous charged fluids, {\ch are} compared with molecular dynamics (MD)
simulations. As a particular case, the parameters of the polyelectrolyte
molecule are mapped to those of a DNA molecule. An excellent qualitative, and
in some cases quantitative, agreement between HNC/MSA and MD is found. Both,
HNC/MSA and MD, predict the occurrence of overcharging, which is not present in
the Poisson-Boltzmann theory. Mean electrostatic potential and local
concentration profiles, -potential and charge distribution functions are
obtained and discussed in terms of the observed overcharging effect.
Particularly interesting results are a very non-monotonic behavior of the
-potential, as a function of the rod charge density, and the
overcharging by {\em monovalent} counterions.Comment: 11 pages, 8 figures, RevTex, published in J. Phys. Chem. B 2001, vol.
105, pags. 1098
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