10,909 research outputs found
Liquid friction on charged surfaces: from hydrodynamic slippage to electrokinetics
Hydrodynamic behavior at the vicinity of a confining wall is closely related
to the friction properties of the liquid/solid interface. Here we consider,
using Molecular Dynamics simulations, the electric contribution to friction for
charged surfaces, and the induced modification of the hydrodynamic boundary
condition at the confining boundary. The consequences of liquid slippage for
electrokinetic phenomena, through the coupling between hydrodynamics and
electrostatics within the electric double layer, are explored. Strong
amplification of electro-osmotic effects is revealed, and the non-trivial
effect of surface charge is discussed. This work allows to reconsider existing
experimental data, concerning Zeta potentials of hydrophobic surfaces and
suggest the possibility to generate ``giant'' electro-osmotic and
electrophoretic effects, with direct applications in microfluidics
Strongly Charged, Flexible Polyelectrolytes in Poor Solvents -- Molecular Dynamics Simulations
We present a set of molecular dynamics (MD) simulations of strongly charged,
flexible polyelectrolyte chains under poor solvent conditions in a salt free
solution. Structural properties of the chains and of the solutions are
reported. By varying the polymer density and the electrostatic interaction
strength we study the crossover from a dominating electrostatic interaction to
the regime of strong screening, where the hydrophobic interactions dominate.
During the crossover a multitude of structures is observed. In the limit of low
polymer density strongly stretched, necklace like conformations are found. In
the opposite limit of high polymer density which is equivalent to strongly
screened electrostatic interactions, we find that the chains are extremely
collapsed, however we observe no agglomeration or phase separation. The
investigations show that the density of free charges is one of the relevant
parameters which rules the behavior of the system and hence should be used as a
parameter to explain experimental results.Comment: 42 pages, including 22 figures and 2 table
Joint Density-Functional Theory of the Electrode-Electrolyte Interface: Application to Fixed Electrode Potentials, Interfacial Capacitances, and Potentials of Zero Charge
This work explores the use of joint density-functional theory, a new form of
density-functional theory for the ab initio description of electronic systems
in thermodynamic equilibrium with a liquid environment, to describe
electrochemical systems. After reviewing the physics of the underlying
fundamental electrochemical concepts, we identify the mapping between commonly
measured electrochemical observables and microscopically computable quantities
within an, in principle, exact theoretical framework. We then introduce a
simple, computationally efficient approximate functional which we find to be
quite successful in capturing a priori basic electrochemical phenomena,
including the capacitive Stern and diffusive Gouy-Chapman regions in the
electrochemical double layer, quantitative values for interfacial capacitance,
and electrochemical potentials of zero charge for a series of metals. We
explore surface charging with applied potential and are able to place our ab
initio results directly on the scale associated with the Standard Hydrogen
Electrode (SHE). Finally, we provide explicit details for implementation within
standard density-functional theory software packages at negligible
computational cost over standard calculations carried out within vacuum
environments.Comment: 18 pages, 5 figures. Initially presented at APS March Meeting 2010.
Accepted for publication in Physical Review B on Jul. 27, 201
Polarizable molecular interactions in condensed phase and their equivalent nonpolarizable models
Earlier, using phenomenological approach, we showed that in some cases
polarizable models of condensed phase systems can be reduced to nonpolarizable
equivalent models with scaled charges. Examples of such systems include ionic
liquids, TIPnP-type models of water, protein force fields, and others, where
interactions and dynamics of inherently polarizable species can be accurately
described by nonpolarizable models. To describe electrostatic interactions, the
effective charges of simple ionic liquids are obtained by scaling the actual
charges of ions by a factor of 1/sqrt(eps_el), which is due to electronic
polarization screening effect; the scaling factor of neutral species is more
complicated. Here, using several theoretical models, we examine how exactly the
scaling factors appear in theory, and how, and under what conditions,
polarizable Hamiltonians are reduced to nonpolarizable ones. These models allow
one to trace the origin of the scaling factors, determine their values, and
obtain important insights on the nature of polarizable interactions in
condensed matter systems.Comment: 43 pages, 3 figure
Interaction of proteins in solution from small angle scattering: a perturbative approach
In this work, an improved methodology for studying interactions of proteins
in solution by small-angle scattering, is presented. Unlike the most common
approach, where the protein-protein correlation functions are
approximated by their zero-density limit (i.e. the Boltzmann factor), we
propose a more accurate representation of which takes into account
terms up to the first order in the density expansion of the mean-force
potential. This improvement is expected to be particulary effective in the case
of strong protein-protein interactions at intermediate concentrations. The
method is applied to analyse small angle X-ray scattering data obtained as a
function of the ionic strength (from 7 to 507 mM) from acidic solutions of
-Lactoglobuline at the fixed concentration of 10 . The
results are compared with those obtained using the zero-density approximation
and show a significant improvement particularly in the more demanding case of
low ionic strength.Comment: 12 pages, 3 figures, to appear in Biophysical Journal (April 2002)
Due to an unfortunate name mismatch, the original submission contained an
incorrect sourc
Shear Viscosity of Clay-like Colloids in Computer Simulations and Experiments
Dense suspensions of small strongly interacting particles are complex
systems, which are rarely understood on the microscopic level. We investigate
properties of dense suspensions and sediments of small spherical Al_2O_3
particles in a shear cell by means of a combined Molecular Dynamics (MD) and
Stochastic Rotation Dynamics (SRD) simulation. We study structuring effects and
the dependence of the suspension's viscosity on the shear rate and shear
thinning for systems of varying salt concentration and pH value. To show the
agreement of our results to experimental data, the relation between bulk pH
value and surface charge of spherical colloidal particles is modeled by
Debye-Hueckel theory in conjunction with a 2pK charge regulation model.Comment: 15 pages, 8 figure
A general study of actinyl hydration by molecular dynamics simulations using ab initio force fields
A set of new ab initio force fields for aqueous [AnO2]
2+/+ (An = Np(VI,V), Pu(VI), Am(VI)) has been developed using the Hydrated Ion
(HI) model methodology previously used for [UO2]
2+. Except for the non-electrostatic contribution of the HI-bulk water interaction, the
interaction potentials are individually parameterized. Translational diffusion coefficients, hydration enthalpies, and vibrational normal mode
frequencies were calculated from the MD simulations. Physico-chemical properties satisfactorily agree with experiments validating the robustness of the force field strategy. The solvation dynamics and structure for all hexavalent actinoids are extremely similar and resemble our
previous analysis of the uranyl cation. This supports the idea of using the uranyl cation as a reference for the study of other minor actinyls. The
comparison between the NpO2+
2 and NpO+
2 hydration only provides significant differences in first and second shell distances and second-shell
mean residence times. We propose a single general view of the [AnO2]
2+/+ hydration structure: aqueous actinyls are amphiphilic anisotropic
solutes which are equatorially conventional spherically symmetric cations capped at the poles by clathrate-like water structures.Junta de AndalucĂa de España, Plan Andaluz de InvestigaciĂłn-FQM-28
Influence of solvent granularity on the effective interaction between charged colloidal suspensions
We study the effect of solvent granularity on the effective force between two
charged colloidal particles by computer simulations of the primitive model of
strongly asymmetric electrolytes with an explicitly added hard sphere solvent.
Apart from molecular oscillating forces for nearly touching colloids which
arise from solvent and counterion layering, the counterions are attracted
towards the colloidal surfaces by solvent depletion providing a simple
statistical description of hydration. This, in turn, has an important influence
on the effective forces for larger distances which are considerably reduced as
compared to the prediction based on the primitive model. When these forces are
repulsive, the long-distance behaviour can be described by an effective Yukawa
pair potential with a solvent-renormalized charge. As a function of colloidal
volume fraction and added salt concentration, this solvent-renormalized charge
behaves qualitatively similar to that obtained via the Poisson-Boltzmann cell
model but there are quantitative differences. For divalent counterions and
nano-sized colloids, on the other hand, the hydration may lead to overscreened
colloids with mutual attraction while the primitive model yields repulsive
forces. All these new effects can be accounted for through a solvent-averaged
primitive model (SPM) which is obtained from the full model by integrating out
the solvent degrees of freedom. The SPM was used to access larger colloidal
particles without simulating the solvent explicitly.Comment: 14 pages, 16 craphic
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