219 research outputs found
Molecular diffusion between walls with adsorption and desorption
The time dependency of the diffusion coefficient of particles in porous media
is an efficient probe of their geometry. The analysis of this quantity,
measured e.g. by nuclear magnetic resonance (PGSE-NMR), can provide rich
information pertaining to porosity, pore size distribution, permeability and
surface-to-volume ratio of porous materials. Nevertheless, in numerous if not
all practical situations, transport is confined by walls where adsorption and
desorption processes may occur. In this article, we derive explicitly the
expression of the time-dependent diffusion coefficient between two confining
walls in the presence of adsorption and desorption. We show that they strongly
modify the time-dependency of the diffusion coefficient, even in this simple
geometry. We finally propose several applications, from sorption rates
measurements to the use as a reference for numerical implementations for more
complex geometries.Comment: 4 pages, 2 figures, 1 supplementary material of 3 page
Molecular hydrodynamics from memory kernels
The memory kernel for a tagged particle in a fluid, computed from molecular
dynamics simulations, decays algebraically as . We show how the
hydrodynamic Basset-Boussinesq force naturally emerges from this long-time tail
and generalize the concept of hydrodynamic added mass. This mass term is
negative in the present case of a molecular solute, at odds with incompressible
hydrodynamics predictions. We finally discuss the various contributions to the
friction, the associated time scales and the cross-over between the molecular
and hydrodynamic regimes upon increasing the solute radius.Comment: 5 pages, 4 figure
Taylor Dispersion with Adsorption and Desorption
We use a stochastic approach to show how Taylor dispersion is affected by
kinetic processes of adsorption and desorption onto surfaces. A general theory
is developed, from which we derive explicitly the dispersion coefficients of
canonical examples like Poiseuille flows in planar and cylindrical geometries,
both in constant and sinusoidal velocity fields. These results open the way for
the measurement of adsorption and desorption rate constants using stationary
flows and molecular sorting using the stochastic resonance of the adsorption
and desorption processes with the oscillatory velocity field.Comment: 6 pages, 4 figure
Computer simulations of ionic liquids at electrochemical interfaces
Ionic liquids are widely used as electrolytes in electrochemical devices. In
this context, many experimental and theoretical approaches have been recently
developed for characterizing their interface with electrodes. In this
perspective article, we review the most recent advances in the field of
computer simulations (mainly molecular dynamics). A methodology for simulating
electrodes at constant electrical potential is presented. Several types of
electrode geometries have been investigated by many groups in order to model
planar, corrugated and porous materials and we summarize the results obtained
in terms of the structure of the liquids. This structure governs the quantity
of charge which can be stored at the surface of the electrode for a given
applied potential, which is the relevant quantity for the highly topical use of
ionic liquids in supercapacitors (also known as electrochemical double-layer
capacitors). A key feature, which was also shown by atomic force microscopy and
surface force apparatus experiments, is the formation of a layered structure
for all ionic liquids at the surface of planar electrodes. This organization
cannot take place inside nanoporous electrodes, which results in a much better
performance for the latter in supercapacitors. The agreement between
simulations and electrochemical experiments remains qualitative only though,
and we outline future directions which should enhance the predictive power of
computer simulations. In the longer term, atomistic simulations will also be
applied to the case of electron transfer reactions at the interface, enabling
the application to a broader area of problems in electrochemistry, and the few
recent works in this field are also commented upon.Comment: 12 pages, 10 figures, perspective articl
Frequency-dependent impedance of nanocapacitors from electrode charge fluctuations as a probe of electrolyte dynamics
The frequency-dependent impedance is a fundamental property of electrical
components. We show that it can be determined from the equilibrium dynamical
fluctuations of the electrode charge in constant-potential molecular
simulations, extending in particular a fluctuation-dissipation for the
capacitance recovered in the low-frequency limit and provide an illustration on
water/gold nanocapacitors. This work opens the way to the interpretation of
electrochemical impedance measurements in terms of microscopic mechanisms,
directly from the dynamics of the electrolyte, or indirectly via equivalent
circuit models as in experiments
Hydration of Clays at the Molecular Scale: The Promising Perspective of Classical Density Functional Theory
We report here how the hydration of complex surfaces can be efficiently
studied thanks to recent advances in classical molecular density functional
theory. This is illustrated on the example of the pyrophylite clay. After
presenting the most recent advances, we show that the strength of this implicit
method is that (i) it is in quantitative or semi-quantitative agreement with
reference all-atoms simulations (molecular dynamics here) for both the
solvation structure and energetics, and that (ii) the computational cost is two
to three orders of magnitude less than in explicit methods. The method remains
imperfect, in that it locally overestimates the polarization of water close to
hydrophylic sites of the clay. The high numerical efficiency of the method is
illustrated and exploited to carry a systematic study of the electrostatic and
van der Waals components of the surface-solvant interactions within the most
popular force field for clays, CLAYFF. Hydration structure and energetics are
found to weakly depend upon the electrostatics. We conclude on the consequences
of such findings in future force-field development.Comment: 24 pages, 8 figures. Molecular Physics (2014
Diffusion under confinement: hydrodynamic finite-size effects in simulation
We investigate finite-size effects on diffusion in confined fluids using
molecular dynamics simulations and hydrodynamic calculations. Specifically, we
consider a Lennard-Jones fluid in slit pores without slip at the interface and
show that the use of periodic boundary conditions in the directions along the
surfaces results in dramatic finite-size effects, in addition to that of the
physically relevant confining length. As in the simulation of bulk fluids,
these effects arise from spurious hydrodynamic interactions between periodic
images and from the constraint of total momentum conservation. We derive
analytical expressions for the correction to the diffusion coefficient in the
limits of both elongated and flat systems, which are in excellent agreement
with the molecular simulation results except for the narrowest pores, where the
discreteness of the fluid particles starts to play a role. The present work
implies that the diffusion coefficients for wide nanopores computed using
elongated boxes suffer from finite-size artifacts which had not been previously
appreciated. In addition, our analytical expression provides the correction to
be applied to the simulation results for finite (possibly small) systems. It
applies not only to molecular but also to all mesoscopic hydrodynamic
simulations, including Lattice-Boltzmann, Multi-Particle Collision Dynamics or
Dissipative Particle Dynamics, which are often used to investigate confined
soft matter involving colloidal particles and polymers.Comment: 3 figures and 1 in the supplemental sectio
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