336 research outputs found
Polyelectrolytes in Solution - Recent Computer Simulations
We present a short overview over recent MD simulations of systems of fully
flexible polyelectrolyte chains with explicitly treated counter ions using the
full Coulomb potential. The main emphasis is given on the conformational
properties of the polymers, with a short discussion on counter ion
condensation.Comment: 10 pages, including 5 figures, to appear in the proceedings of the
50th Yamada Conference on Polyelectrolytes, Inuyama, Japan (1998
Detailed analysis of Rouse mode and dynamic scattering function of highly entangled polymer melts in equilibrium
We present large-scale molecular dynamics simulations for a coarse-grained
model of polymer melts in equilibrium. From detailed Rouse mode analysis we
show that the time-dependent relaxation of the autocorrelation function (ACF)
of modes can be well described by the effective stretched exponential
function due to the crossover from Rouse to reptation regime. The ACF is
independent of chain sizes for ( is the entanglement
length), and there exists a minimum of the stretching exponent as . As increases, we verify the crossover scaling behavior
of the effective relaxation time from the Rouse regime to
the reptation regime. We have also provided evidence that the incoherent
dynamic scattering function follows the same crossover scaling behavior of the
mean square displacement of monomers at the corresponding characteristic time
scales. The decay of the coherent dynamic scattering function is slowed down
and a plateau develops as chain sizes increase at the intermediate time and
wave length scales. The tube diameter extracted from the coherent dynamic
scattering function is equivalent to the previous estimate from the mean square
displacement of monomers.Comment: 8 pages, 7 figures, to be published in EPJST special issue on "Phase
transitions and critical phenomena" (2017
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
Strong electrostatic interactions in spherical colloidal systems
We investigate spherical macroions in the strong Coulomb coupling regime
within the primitive model in salt-free environment. We first show that the
ground state of an isolated colloid is naturally overcharged by simple
electrostatic arguments illustrated by the Gillespie rule. We furthermore
demonstrate that in the strong Coulomb coupling this mechanism leads to ionized
states and thus to long range attractions between like-charged spheres. We use
molecular dynamics simulations to study in detail the counterion distribution
for one and two highly charged colloids for the ground state as well as for
finite temperatures. We compare our results in terms of a simple version of a
Wigner crystal theory and find excellent qualitative and quantitative
agreement.Comment: 30 pages and 17 PS figures. REVTEX. Minor changes. To appear in Phys
Rev
Efficient potential of mean force calculation from multiscale simulations: solute insertion in a lipid membrane
The determination of potentials of mean force for solute insertion in a
membrane by means of all-atom molecular dynamics simulations is often hampered
by sampling issues. A multiscale approach to conformational sampling was
recently proposed by Bereau and Kremer (2016). It aims at accelerating the
sampling of the atomistic conformational space by means of a systematic
backmapping of coarse-grained snapshots. In this work, we first analyze the
efficiency of this method by comparing its predictions for propanol insertion
into a 1,2-Dimyristoyl-sn-glycero-3-phosphocholine membrane (DMPC) against
reference atomistic simulations. The method is found to provide accurate
results with a gain of one order of magnitude in computational time. We then
investigate the role of the coarse-grained representation in affecting the
reliability of the method in the case of a
1,2-Dioleoyl-sn-glycero-3-phosphocholine membrane (DOPC). We find that the
accuracy of the results is tightly connected to the presence a good
configurational overlap between the coarse-grained and atomistic models---a
general requirement when developing multiscale simulation methods.Comment: 6 pages, 5 figure
Like-Charge Colloid-Polyelectrolyte Complexation
We investigate the complexation of a highly charged sphere with a long
flexible polyelectrolyte, \textit{both negatively charged} in salt free
environment. Electroneutrality is insured by the presence of divalent
counterions. Using molecular dynamics (MD) within the framework of the
primitive model, we consider different Coulomb coupling regimes. At strong
Coulomb coupling we find that the adsorbed chain is always confined to the
colloidal surface but forms different conformations that depend on the linear
charge density of the chain. A mechanism involving the polyelectrolyte
\textit{overcharging} is proposed to explain these structures. At intermediate
Coulomb coupling, the chain conformation starts to become three-dimensional,
and we observe multilayering of the highly charged chain while for lower charge
density the chain wraps around the colloid. At weak Coulomb coupling,
corresponding to an aqueous solvent, we still find like-charge complexation. In
this latter case the chain conformation exhibits loops.Comment: 18 pages, 13 (main) eps figures, RevTeX4, submitted to J. Chem. Phy
Effect of colloidal charge discretization in the primitive model
The effect of fixed discrete colloidal charges in the primitive model is
investigated for spherical macroions. Instead of considering a central bare
charge, as it is traditionally done, we distribute \textit{discrete} charges
randomly on the sphere. We use molecular dynamics simulations to study this
effect on various properties such as overcharging, counterion distribution and
diffusion. In the vicinity of the colloid surface the electrostatic potential
may considerably differ from the one obtained with a central charge. In the
strong Coulomb coupling, we showed that the colloidal charge discretization
qualitatively influences the counterion distribution and leads to a strong
colloidal charge-counterion pair association. However, we found that
\textit{charge inversion} still persists even if strong pair association is
observed.Comment: 16 pages, 16 ps figures, REVTEX, accepted for publication in EPJ
Relative Resolution: A Multipole Approximation at Appropriate Distances
Recently, we introduced Relative Resolution as a hybrid formalism for fluid
mixtures [1]. The essence of this approach is that it switches molecular
resolution in terms or relative separation: While nearest neighbors are
characterized by a detailed fine-grained model, other neighbors are
characterized by a simplified coarse-grained model. Once the two models are
analytically connected with each other via energy conservation, Relative
Resolution can capture the structural and thermal behavior of (nonpolar)
multi-component and multi-phase systems across state space. The current work is
a natural continuation of our original communication [1]. Most importantly, we
present the comprehensive mathematics of Relative Resolution, basically casting
it as a multipole approximation at appropriate distances; the current set of
equations importantly applies for all systems (e.g, polar molecules). Besides,
we continue examining the capability of our multiscale approach in molecular
simulations, importantly showing that we can successfully retrieve not just the
statics but also the dynamics of liquid systems. We finally conclude by
discussing how Relative Resolution is the inherent variant of the famous
"cell-multipole" approach for molecular simulations
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