20 research outputs found
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
The electrical double layer for a fully asymmetric electrolyte around a spherical colloid: an integral equation study
The hypernetted chain/mean spherical approximation (HNC/MSA) integral
equation is obtained and solved numerically for a totally asymmetric primitive
model electrolyte around a spherical macroparticle. The ensuing radial
distribution functions show a very good agreement when compared to our Monte
Carlo and molecular dynamics simulations for spherical geometry and with
respect to previous anisotropic reference HNC calculations in the planar limit.
We report an analysis of the potential vs charge relationship, radial
distribution functions, mean electrostatic potential and cumulative reduced
charge for representative cases of 1:1 and 2:2 salts with a size asymmetry
ratio of 2. Our results are collated with those of the Modified Gouy-Chapman
(MGC) and unequal radius Modified Gouy-Chapman (URMGC) theories and with those
of HNC/MSA in the restricted primitive model (RPM) to assess the importance of
size asymmetry effects. One of the most striking characteristics found is
that,\textit{contrary to the general belief}, away from the point of zero
charge the properties of an asymmetric electrical double layer (EDL) are not
those corresponding to a symmetric electrolyte with the size and charge of the
counterion, i.e. \textit{counterions do not always dominate}. This behavior
suggests the existence of a new phenomenology in the EDL that genuinely belongs
to a more realistic size-asymmetric model where steric correlations are taken
into account consistently. Such novel features can not be described by
traditional mean field theories like MGC, URMGC or even by enhanced formalisms,
like HNC/MSA, if they are based on the RPM.Comment: 29 pages, 13 figure
Ion pairing in model electrolytes: A study via three particle correlation functions
A novel integral equations approach is applied for studying ion pairing in
the restricted primitive model (RPM) electrolyte, i. e., the three point
extension (TPE) to the Ornstein-Zernike integral equations. In the TPE
approach, the three-particle correlation functions are obtained. The TPE results are compared to molecular
dynamics (MD) simulations and other theories. Good agreement between TPE and MD
is observed for a wide range of parameters, particularly where standard
integral equations theories fail, i. e., low salt concentration and high ionic
valence. Our results support the formation of ion pairs and aligned ion
complexes.Comment: 43 pages (including 18 EPS figs) - RevTeX 4 - J. Chem. Phys. (in
press
Violation of the local electroneutrality condition in an inhomogeneous macroions solution
A simple mathematical model for a multivalent macroions solution, next to a charged wall of planar geometry, is solved through a well-established integral equation theory. The macroions structure and the charge induced into the fluid are obtained, as a function to the distance to the electrode. The macroions adsorption to the surface and the induced charge density, both show an atypical structure, not consistent with the predictions of the classical theory of Poisson–Boltzmann. In particular, the induced charge density exhibits an enormous charge overcompensation, localized just next to the electrode, implying a violation of the local electroneutrality condition. A breakdown of the charge neutrality, in confined, charged fluids, has been theoretically predicted in the past, by means of integral equations, density functional approaches, and computer simulations, and recently experimentally reported. However, the results presented here show a charge neutrality breakdown, in unconfined, inhomogeneous fluids. Our results are in qualitative agreement with experimental data for Langmuir films of amphiphilic molecules, in contact with a macroions solution