269 research outputs found
Fluid-Fluid and Fluid-Solid transitions in the Kern-Frenkel model from Barker-Henderson thermodynamic perturbation theory
We study the Kern-Frenkel model for patchy colloids using Barker-Henderson
second-order thermodynamic perturbation theory. The model describes a fluid
where hard sphere particles are decorated with one patch, so that they interact
via a square-well (SW) potential if they are sufficiently close one another,
and if patches on each particle are properly aligned. Both the gas-liquid and
fluid-solid phase coexistences are computed and contrasted against
corresponding Monte-Carlo simulations results. We find that the perturbation
theory describes rather accurately numerical simulations all the way from a
fully covered square-well potential down to the Janus limit (half coverage). In
the region where numerical data are not available (from Janus to hard-spheres),
the method provides estimates of the location of the critical lines that could
serve as a guideline for further efficient numerical work at these low
coverages. A comparison with other techniques, such as integral equation
theory, highlights the important aspect of this methodology in the present
context.Comment: Accepted for publication in The Journal of Chemical Physics (2012
A chloride channel in rat and human axons
Current recordings from single chloride channels were obtained from excised and cell-attached patches of rat and human axons. In rat axons the channels showed an outwardly rectifying current-voltage relationship with a slope conductance of 33 pS at negative membrane potentials and 65 pS at positive potentials (symmetrical 150 mM CsCl). They were measurably for cations (PNa/PCs/PCl=0.1/0.2/1). Channel currents were independent of cytoplasmatic calcium concentration. Inactivation was not observed and gating was weakly voltage dependent. Clâ channels in human axons showed similar gating behavior but had a lower conductance
Properties of asymmetric nuclear matter in different approaches
Properties of asymmetric nuclear matter are derived from various many-body
approaches. This includes phenomenological ones like the Skyrme Hartree-Fock
and relativistic mean field approaches, which are adjusted to fit properties of
nuclei, as well as more microscopic attempts like the Brueckner-Hartree-Fock
approximation, a self-consistent Greens function method and the so-called
approach, which are based on realistic nucleon-nucleon interactions
which reproduce the nucleon-nucleon phase shifts. These microscopic approaches
are supplemented by a density-dependent contact interaction to achieve the
empirical saturation property of symmetric nuclear matter. The predictions of
all these approaches are discussed for nuclear matter at high densities in
-equilibrium. Special attention is paid to behavior of the isovector
component of the effective mass in neutron-rich matter.Comment: 16 pages, 7 figure
A simple patchy colloid model for the phase behavior of lysozyme dispersions
We propose a minimal model for spherical proteins with aeolotopic pair
interactions to describe the equilibrium phase behavior of lysozyme. The
repulsive screened Coulomb interactions between the particles are taken into
account assuming that the net charges are smeared out homogeneously over the
spherical protein surfaces. We incorporate attractive surface patches, with the
interactions between patches on different spheres modeled by an attractive
Yukawa potential. The parameters entering the attractive Yukawa potential part
are determined using information on the experimentally accessed gas-liquid-like
critical point. The Helmholtz free energy of the fluid and solid phases is
calculated using second-order thermodynamic perturbation theory. Our
predictions for the solubility curve are in fair agreement with experimental
data. In addition, we present new experimental data for the gas-liquid
coexistence curves at various salt concentrations and compare these with our
model calculations. In agreement with earlier findings, we observe that the
strength and the range of the attractive potential part only weakly depend on
the salt content
Upper limits on the observational effects of nuclear pasta in neutron stars
The effects of the existence of exotic nuclear shapes at the bottom of the
neutron star inner crust - nuclear `pasta' - on observational phenomena are
estimated by comparing the limiting cases that those phases have a vanishing
shear modulus and that they have the shear modulus of a crystalline solid . We
estimate the effect on torsional crustal vibrations and on the maximum
quadrupole ellipticity sustainable by the crust. The crust composition and
transition densities are calculated consistently with the global properties,
using a liquid drop model with a bulk nuclear equation of state (EoS) which
allows a systematic variation of the nuclear symmetry energy. The symmetry
energy J and its density dependence L at nuclear saturation density are the
dominant nuclear inputs which determine the thickness of the crust, the range
of densities at which pasta might appear, as well as global properties such as
the radius and moment of inertia. We show the importance of calculating the
global neutron star properties on the same footing as the crust EoS, and
demonstrate that in the range of experimentally acceptable values of L, the
pasta phase can alter the crust frequencies by up to a factor of three,
exceeding the effects of superfluidity on the crust modes, and decrease the
maximum quadrupole ellipticity sustainable by the crust by up to an order of
magnitude. The signature of the pasta phases and the density dependence of the
symmetry energy on the potential observables highlights the possibility of
constraining the EoS of dense, neutron-rich matter and the properties of the
pasta phases using astrophysical observations.Comment: 8 pages, 7 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Societ
Effect of glycerol and dimethyl sulfoxide on the phase behavior of lysozyme: Theory and experiments
Salt, glycerol and dimethyl sulfoxide (DMSO) are used to modify the
properties of protein solutions. We experimentally determined the effect of
these additives on the phase behavior of lysozyme solutions. Upon the addition
of glycerol and DMSO, the fluid-solid transition and the gas-liquid coexistence
curve (binodal) shift to lower temperatures and the gap between them increases.
The experimentally observed trends are consistent with our theoretical
predictions based on the thermodynamic perturbation theory (TPT) and the
Derjaguin-Landau-Verwey-Overbeek (DLVO) model for the lysozyme-lysozyme pair
interactions. The values of the parameters describing the interactions, namely
the refractive indices, dielectric constants, Hamaker constant and cut-off
length, are extracted from literature or are experimentally determined by
independent experiments, including static light scattering to determine the
second virial coefficient. We observe that both, glycerol and DMSO, render the
potential more repulsive, while sodium chloride reduces the repulsion.Comment: Manuscript accepted for publication in The Journal of Chemical
Physic
Comparing ion conductance recordings of synthetic lipid bilayers with cell membranes containing TRP channels
In this article we compare electrical conductance events from single channel
recordings of three TRP channel proteins (TRPA1, TRPM2 and TRPM8) expressed in
human embryonic kidney cells with channel events recorded on synthetic lipid
membranes close to melting transitions. Ion channels from the TRP family are
involved in a variety of sensory processes including thermo- and
mechano-reception. Synthetic lipid membranes close to phase transitions display
channel-like events that respond to stimuli related to changes in intensive
thermodynamic variables such as pressure and temperature. TRP channel activity
is characterized by typical patterns of current events dependent on the type of
protein expressed. Synthetic lipid bilayers show a wide spectrum of electrical
phenomena that are considered typical for the activity of protein ion channels.
We find unitary currents, burst behavior, flickering, multistep-conductances,
and spikes behavior in both preparations. Moreover, we report conductances and
lifetimes for lipid channels as described for protein channels. Non-linear and
asymmetric current-voltage relationships are seen in both systems. Without
further knowledge of the recording conditions, no easy decision can be made
whether short current traces originate from a channel protein or from a pure
lipid membraneComment: 13 pages, 9 Figure
A survey of the parameter space of the compressible liquid drop model as applied to the neutron star inner crust
We present a systematic survey the range of predictions of the neutron star
inner crust composition, crust-core transition densities and pressures, and
density range of the nuclear `pasta' phases at the bottom of the crust provided
by the compressible liquid drop model in the light of current experimental and
theoretical constraints on model parameters. Using a Skyrme-like model for
nuclear matter, we construct baseline sequences of crust models by consistently
varying the density dependence of the bulk symmetry energy at nuclear
saturation density, , under two conditions: (i) that the magnitude of the
symmetry energy at saturation density is held constant, and (ii)
correlates with under the constraint that the pure neutron matter (PNM) EoS
satisfies the results of ab-initio calculations at low densities. Such baseline
crust models facilitate consistent exploration of the dependence of crustal
properties. The remaining surface energy and symmetric nuclear matter
parameters are systematically varied around the baseline, and different
functional forms of the PNM EoS at sub-saturation densities implemented, to
estimate theoretical `error bars' for the baseline predictions. Inner crust
composition and transition densities are shown to be most sensitive to the
surface energy at very low proton fractions and to the behavior of the
sub-saturation PNM EoS. Recent calculations of the energies of neutron drops
suggest that the low-proton-fraction surface energy might be higher than
predicted in Skyrme-like models, which our study suggests may result in a
greatly reduced volume of pasta in the crust than conventionally predicted.Comment: 37 Pages, 16 figures, accepted for publication in Astrophysical
Journal Supplement Serie
Self-assembly mechanism in colloids: perspectives from Statistical Physics
Motivated by recent experimental findings in chemical synthesis of colloidal
particles, we draw an analogy between self-assembly processes occurring in
biological systems (e.g. protein folding) and a new exciting possibility in the
field of material science. We consider a self-assembly process whose elementary
building blocks are decorated patchy colloids of various types, that
spontaneously drive the system toward a unique and predetermined targeted
macroscopic structure.
To this aim, we discuss a simple theoretical model -- the Kern-Frenkel model
-- describing a fluid of colloidal spherical particles with a pre-defined
number and distribution of solvophobic and solvophilic regions on their
surface. The solvophobic and solvophilic regions are described via a
short-range square-well and a hard-sphere potentials, respectively.
Integral equation and perturbation theories are presented to discuss
structural and thermodynamical properties, with particular emphasis on the
computation of the fluid-fluid (or gas-liquid) transition in the
temperature-density plane.
The model allows the description of both one and two attractive caps, as a
function of the fraction of covered attractive surface, thus interpolating
between a square-well and a hard-sphere fluid, upon changing the coverage.
By comparison with Monte Carlo simulations, we assess the pros and the cons
of both integral equation and perturbation theories in the present context of
patchy colloids, where the computational effort for numerical simulations is
rather demanding.Comment: 14 pages, 7 figures, Special issue for the SigmaPhi2011 conferenc
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