34,379 research outputs found
Phase Behavior of Columnar DNA Assemblies
The pair interaction between two stiff parallel linear DNA molecules depends
not only on the distance between their axes but on their azimuthal orientation.
The positional and orientational order in columnar B-DNA assemblies in solution
is investigated, based on the DNA-DNA electrostatic pair potential that takes
into account DNA helical symmetry and the amount and distribution of adsorbed
counterions. A phase diagram obtained by lattice sum calculations predicts a
variety of positionally and azimuthally ordered phases and bundling transitions
strongly depending on the counterion adsorption patterns.Comment: 4 pages, 3 figures, submitted to PR
Simple Fluids with Complex Phase Behavior
We find that a system of particles interacting through a simple isotropic
potential with a softened core is able to exhibit a rich phase behavior
including: a liquid-liquid phase transition in the supercooled phase, as has
been suggested for water; a gas-liquid-liquid triple point; a freezing line
with anomalous reentrant behavior. The essential ingredient leading to these
features resides in that the potential investigated gives origin to two
effective core radii.Comment: 7 pages including 3 eps figures + 1 jpeg figur
Phase behavior of parallel hard cylinders
We test the performance of a recently proposed fundamental measure density
functional of aligned hard cylinders by calculating the phase diagram of a
monodisperse fluid of these particles. We consider all possible liquid
crystalline symmetries, namely nematic, smectic and columnar, as well as the
crystalline phase. For this purpose we introduce a Gaussian parameterization of
the density profile and use it to minimize numerically the functional. We also
determine, from the analytic expression for the structure factor of the uniform
fluid, the bifurcation points from the nematic to the smectic and columnar
phases. The equation of state, as obtained from functional minimization, is
compared to the available Monte Carlo simulation. The agreement is is very
good, nearly perfect in the description of the inhomogeneous phases. The
columnar phase is found to be metastable with respect to the smectic or crystal
phases, its free energy though being very close to that of the stable phases.
This result justifies the observation of a window of stability of the columnar
phase in some simulations, which disappears as the size of the system
increases. The only important deviation between theory and simulations shows up
in the location of the nematic-smectic transition. This is the common drawback
of any fundamental measure functional of describing the uniform phase just with
the accuracy of scaled particle theory.Comment: 17 pages, 5 figure
Phase Behavior of Bent-Core Molecules
Recently, a new class of smectic liquid crystal phases (SmCP phases)
characterized by the spontaneous formation of macroscopic chiral domains from
achiral bent-core molecules has been discovered. We have carried out Monte
Carlo simulations of a minimal hard spherocylinder dimer model to investigate
the role of excluded volume interations in determining the phase behavior of
bent-core materials and to probe the molecular origins of polar and chiral
symmetry breaking. We present the phase diagram as a function of pressure or
density and dimer opening angle . With decreasing , a transition
from a nonpolar to a polar smectic phase is observed near ,
and the nematic phase becomes thermodynamically unstable for . No chiral smectic or biaxial nematic phases were found.Comment: 4 pages Revtex, 3 eps figures (included
Phase Behavior of strongly associating systems
The modeling of associating fluids has been an active area of research for several decades. Attention has gradually shifted from the so called chemical theories, where molecular association is treated as a chemical reaction, to molecular models where association naturally arises from strong attractive intermolecular forces; among the last ones the Statistical Associating Fluid Theory (SAFT) and related approaches are becoming very popular. We will present calculations performed with the soft-SAFT EoS [F.J. Blas and L.F. Vega, Ind. Eng. Chem. Res. 37 (1998) 660-674.] to simulate the equilibrium thermodynamic properties of the acetic acid and the nitriles family (two classes of strongly associating compounds) as well as their mixtures[K. Jackowski and E. Wielogorska, Journal of Molecular Structure355 (1995) 287-290.]. Carboxylic acids form stable double hydrogen bridged dimers which in the gas phase exist in equilibrium with the monomers. Molecular association in liquid phase of the nitriles family is interesting as they are important organic solvents which are soluble in water without any limits. Pure-component molecular parameters are obtained by fitting the equation to available experimental data. The equation enables to search for physical trends, allowing the transferability of the parameters. The complex behavior of these mixtures is also investigated with the same approach
Phase behavior of ionic liquid crystals
Bulk properties of ionic liquid crystals are investigated using density
functional theory. The liquid crystal molecules are represented by ellipsoidal
particles with charges located in their center or at their tails. Attractive
interactions are taken into account in terms of the Gay-Berne pair potential.
Rich phase diagrams involving vapor, isotropic and nematic liquid, as well as
smectic phases are found. The dependence of the phase behavior on various
parameters such as the length of the particles and the location of charges on
the particles is studied
Equilibrium phase behavior of polydisperse hard spheres
We calculate the phase behavior of hard spheres with size polydispersity,
using accurate free energy expressions for the fluid and solid phases. Cloud
and shadow curves, which determine the onset of phase coexistence, are found
exactly by the moment free energy method, but we also compute the complete
phase diagram, taking full account of fractionation effects. In contrast to
earlier, simplified treatments we find no point of equal concentration between
fluid and solid or re-entrant melting at higher densities. Rather, the fluid
cloud curve continues to the largest polydispersity that we study (14%); from
the equilibrium phase behavior a terminal polydispersity can thus only be
defined for the solid, where we find it to be around 7%. At sufficiently large
polydispersity, fractionation into several solid phases can occur, consistent
with previous approximate calculations; we find in addition that coexistence of
several solids with a fluid phase is also possible
Electroneutrality and Phase Behavior of Colloidal Suspensions
Several statistical mechanical theories predict that colloidal suspensions of
highly charged macroions and monovalent microions can exhibit unusual
thermodynamic phase behavior when strongly deionized. Density-functional,
extended Debye-H\"uckel, and response theories, within mean-field and
linearization approximations, predict a spinodal phase instability of charged
colloids below a critical salt concentration. Poisson-Boltzmann cell model
studies of suspensions in Donnan equilibrium with a salt reservoir demonstrate
that effective interactions and osmotic pressures predicted by such theories
can be sensitive to the choice of reference system, e.g., whether the microion
density profiles are expanded about the average potential of the suspension or
about the reservoir potential. By unifying Poisson-Boltzmann and response
theories within a common perturbative framework, it is shown here that the
choice of reference system is dictated by the constraint of global
electroneutrality. On this basis, bulk suspensions are best modeled by
density-dependent effective interactions derived from a closed reference system
in which the counterions are confined to the same volume as the macroions.
Linearized theories then predict bulk phase separation of deionized suspensions
only when expanded about a physically consistent (closed) reference system.
Lower-dimensional systems (e.g., monolayers, small clusters), depending on the
strength of macroion-counterion correlations, may be governed instead by
density-independent effective interactions tied to an open reference system
with counterions dispersed throughout the reservoir, possibly explaining
observed structural crossover in colloidal monolayers and anomalous
metastability of colloidal crystallites.Comment: 12 pages, 5 figures. Discussion clarified, references adde
Phase behavior of three-component ionic fluids
We study the phase behavior of solutions consisting of positive and negative
ions of valence z to which a third ionic species of valence Z>z is added. Using
a discretized Debye-Hueckel theory, we analyze the phase behavior of such
systems for different values of the ratio Z/z. We find, for Z/z>1.934, a
three-phase coexistence region and, for Z/z>2, a closed (reentrant) coexistence
loop at high temperatures. We characterize the behavior of these ternary ionic
mixtures as function of charge asymmetry and temperature, and show the complete
phase diagrams for the experimentally relevant cases of Z/z=2 and Z/z=3,
corresponding to addition of divalent and trivalent ions to monovalent ionic
fluids, respectively.Comment: 6 pages, 4 figures; to appear in the European Physical Journal
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