61 research outputs found
Going beyond standard mixing rules
Using molecular dynamics (MD) simulations in conjunction with the SPC/E water
model, we optimize ionic force-field parameters for seven different halide and
alkali ions, considering a total of eight ion-pairs. Our strategy is based on
simultaneous optimizing single-ion and ion-pair properties, i.e., we first fix
ion water parameters based on single-ion solvation free energies, and in a
second step determine the cation-anion interaction parameters (traditionally
given by mixing or combination rules) based on the Kirkwood-Buff theory
without modification of the ion-water interaction parameters. In doing so, we
have introduced scaling factors for the cation-anion Lennard-Jones (LJ)
interaction that quantify deviations from the standard mixing rules. For the
rather size-symmetric salt solutions involving bromide and chloride ions, the
standard mixing rules work fine. On the other hand, for the iodide and
fluoride solutions, corresponding to the largest and smallest anion considered
in this work, a rescaling of the mixing rules was necessary. For iodide, the
experimental activities suggest more tightly bound ion pairing than given by
the standard mixing rules, which is achieved in simulations by reducing the
scaling factor of the cation-anion LJ energy. For fluoride, the situation is
different and the simulations show too large attraction between fluoride and
cations when compared with experimental data. For NaF, the situation can be
rectified by increasing the cation-anion LJ energy. For KF, it proves
necessary to increase the effective cation-anion Lennard-Jones diameter. The
optimization strategy outlined in this work can be easily adapted to different
kinds of ions
Electrokinetic Lattice Boltzmann solver coupled to Molecular Dynamics: application to polymer translocation
We develop a theoretical and computational approach to deal with systems that
involve a disparate range of spatio-temporal scales, such as those comprised of
colloidal particles or polymers moving in a fluidic molecular environment. Our
approach is based on a multiscale modeling that combines the slow dynamics of
the large particles with the fast dynamics of the solvent into a unique
framework. The former is numerically solved via Molecular Dynamics and the
latter via a multi-component Lattice Boltzmann. The two techniques are coupled
together to allow for a seamless exchange of information between the
descriptions. Being based on a kinetic multi-component description of the fluid
species, the scheme is flexible in modeling charge flow within complex
geometries and ranging from large to vanishing salt concentration. The details
of the scheme are presented and the method is applied to the problem of
translocation of a charged polymer through a nanopores. In the end, we discuss
the advantages and complexities of the approach
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