95 research outputs found
Model charged cylindrical nanopore in a colloidal dispersion: charge reversal, overcharging and double overcharging
Using the hypernetted-chain/mean spherical approximation (HNC/MSA) integral
equations we study the electrical double layer inside and outside a model
charged cylindrical vesicle (nanopore) immersed into a primitive model
macroions solution, so that the macroions are only present outside the
nanopore, i.e., the vesicle wall is impermeable only to the external macroions.
We calculate the ionic and local linear charge density profiles inside and
outside the vesicle, and find that the correlation between the inside and
outside ionic distributions causes the phenomena of overcharging (also referred
to as surface charge amplification) and/or charge reversal. This is the first
time overcharging is predicted in an electrical double layer of cylindrical
geometry. We also report the new phenomenon of double overcharging. The present
results can be of consequence for relevant systems in physical-chemistry,
energy storage and biology, e.g., nanofilters, capacitors and cell membranes.Comment: 10 pages, 4 figure
Very long-range attractive and repulsive forces in Model Colloidal Dispersions
Experiments with polymer latex solutions show the coexistence of
order-disorder structures of macroions. Because of the large macroions' sizes,
this order-disorder phase coexistence imply the existence of very long-range
attractive and repulsive forces, which can not be explained in terms of
conventional direct interaction potentials, which are short-range. Here we
apply an integral equations theory to a simple model for colloidal dispersions,
at finite concentrations, calculate the particles distribution functions and
the involved effective forces. We find very long-range attractive and repulsive
forces among the like-charged macroions. The distribution functions are in
qualitative agreement with experimental results. The origin of these forces are
discussed in terms of an energy-entropy balance.Comment: 16 pages, seven figures. ECIS-201
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
Overcharging: The Crucial Role of Excluded Volume
In this Letter we investigate the mechanism for overcharging of a single
spherical colloid in the presence of aqueous salts within the framework of the
primitive model by molecular dynamics (MD) simulations as well as
integral-equation theory. We find that the occurrence and strength of
overcharging strongly depends on the salt-ion size, and the available volume in
the fluid. To understand the role of the excluded volume of the microions, we
first consider an uncharged system. For a fixed bulk concentration we find that
upon increasing the fluid particle size one strongly increases the local
concentration nearby the colloidal surface and that the particles become
laterally ordered. For a charged system the first surface layer is built up
predominantly by strongly correlated counterions. We argue that this a key
mechanism to produce overcharging with a low electrostatic coupling, and as a
more practical consequence, to account for charge inversion with monovalent
aqueous salt ions.Comment: 7 pages, 3 figs (4 EPS files). To appear in Europhysics Letter
- …