80 research outputs found
The many origins of charge inversion in electrolyte solutions: effects of discrete interfacial charges
We show that charge inversion, i.e. interfacial charges attracting
counterions in excess of their own nominal charge, is a general effect that
takes place in most charged systems next to aqueous solutions with multivalent
ions and identify three different electrostatic origins for this effect 1)
counterion-counterion correlations, 2) correlations between counterions and
interfacial charges and 3) complexation. We briefly describe the first two
regimes and provide a detailed characterization of the complexation regime from
united atom molecular dynamics simulation of a phospholipid domain in contact
with an aqueous solution. We examine the expected conditions where each regime
should apply and describe a representative experimental example to illustrate
each case. We point out that our results provide a characterization of ionic
distributions irrespectively of whether charge inversion takes place and show
that processes such as proton release and transfer are also linked to ionic
correlations. We conclude with a discussion of further experimental and
theoretical implications.Comment: 22 pages, 7 figure
Micellar Crystals in Solution from Molecular Dynamics Simulations
Polymers with both soluble and insoluble blocks typically self-assemble into
micelles, aggregates of a finite number of polymers where the soluble blocks
shield the insoluble ones from contact with the solvent. Upon increasing
concentration, these micelles often form gels that exhibit crystalline order in
many systems. In this paper, we present a study of both the dynamics and the
equilibrium properties of micellar crystals of triblock polymers using
molecular dynamics simulations. Our results show that equilibration of single
micelle degrees of freedom and crystal formation occurs by polymer transfer
between micelles, a process that is described by transition state theory. Near
the disorder (or melting) transition, bcc lattices are favored for all
triblocks studied. Lattices with fcc ordering are also found, but only at lower
kinetic temperatures and for triblocks with short hydrophilic blocks. Our
results lead to a number of theoretical considerations and suggest a range of
implications to experimental systems with a particular emphasis on Pluronic
polymers.Comment: 12 pages, 11 figures. Note that some figures are extremely low
quality to meet arXiv's file size limit
Ground state structure and interactions between dimeric 2D Wigner crystals
We study the ground state ordering and interactions between two
two-dimensional Wigner crystals on neutralizing charged plates by means of
computer simulation. We consider crystals formed by (i) point-like charges and
(ii) charged dimers, which mimic the screening of charged surfaces by elongated
multivalent ions such as aspherical globular proteins, charged dendrimers or
short stiff polyelectrolytes. Both systems, with point-like and dimeric ions,
display five distinct crystalline phases on increasing the interlayer distance.
In addition to alteration of translational ordering within the bilayer, the
phase transitions in the dimeric system are characterized by alteration of
orientational ordering of the ions.Comment: Revised versio
Properties of Quantum Hall Skyrmions from Anomalies
It is well known that the Fractional Quantum Hall Effect (FQHE) may be
effectively represented by a Chern-Simons theory. In order to incorporate QH
Skyrmions, we couple this theory to the topological spin current, and include
the Hopf term. The cancellation of anomalies for chiral edge states, and the
proviso that Skyrmions may be created and destroyed at the edge, fixes the
coefficients of these new terms. Consequently, the charge and the spin of the
Skyrmion are uniquely determined. For those two quantities we find the values
and , respectively, where is electron charge,
is the filling fraction and is the Skyrmion winding number. We
also add terms to the action so that the classical spin fluctuations in the
bulk satisfy the standard equations of a ferromagnet, with spin waves that
propagate with the classical drift velocity of the electron.Comment: 8 pages, LaTeX file; Some remarks are included to clarify the
physical results obtained, and the role of the Landau-Lifshitz equation is
emphasized. Some references adde
The Phase Diagram of the Sigma Model
We study the phase diagram of the scalar model in
dimensions. We find that the phase transition is of first order in most of the
parameter space. The theory can still be relevant to continuum physics (as an
effective theory) provided the transition is sufficiently weakly first order.
This places restrictions on the allowed coupling constants.Comment: 3 pages (Latex), 2 eps figures, uses espcrc2.sty, epsf, talk given at
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