261,956 research outputs found
Order of wetting transitions in electrolyte solutions
For wetting films in dilute electrolyte solutions close to charged walls we
present analytic expressions for their effective interface potentials. The
analysis of these expressions renders the conditions under which corresponding
wetting transitions can be first- or second-order. Within mean field theory we
consider two models, one with short- and one with long-ranged solvent-solvent
and solvent-wall interactions. The analytic results reveal in a transparent way
that wetting transitions in electrolyte solutions, which occur far away from
their critical point (i.e., the bulk correlation length is less than half of
the Debye length) are always first-order if the solvent-solvent and
solvent-wall interactions are short-ranged. In contrast, wetting transitions
close to the bulk critical point of the solvent (i.e., the bulk correlation
length is larger than the Debye length) exhibit the same wetting behavior as
the pure, i.e., salt-free, solvent. If the salt-free solvent is governed by
long-ranged solvent-solvent as well as long-ranged solvent-wall interactions
and exhibits critical wetting, adding salt can cause the occurrence of an
ion-induced first-order thin-thick transition which precedes the subsequent
continuous wetting as for the salt-free solvent.Comment: Submitte
The local phase transitions of the solvent in the neighborhood of a solvophobic polymer at high pressures
We investigate local phase transitions of the solvent in the neighborhood of
a solvophobic polymer chain which is induced by a change of the polymer-solvent
repulsion and the solvent pressure in the bulk solution. We describe the
polymer in solution by the Edwards model, where the conditional partition
function of the polymer chain at a fixed radius of gyration is described by a
mean-field theory. The contributions of the polymer-solvent and the
solvent-solvent interactions to the total free energy are described within the
mean-field approximation. We obtain the total free energy of the solution as a
function of the radius of gyration and the average solvent number density
within the gyration volume. The resulting system of coupled equations is solved
varying the polymer-solvent repulsion strength at high solvent pressure in the
bulk. We show that the coil-globule (globule-coil) transition occurs
accompanied by a local solvent evaporation (condensation) within the gyration
volum
Polyelectrolyte chains in poor solvent. A variational description of necklace formation
We study the properties of polyelectrolyte chains under different solvent
conditions, using a variational technique. The free energy and the
conformational properties of a polyelectrolyte chain are studied minimizing the
free energy , depending on trial probabilities that
characterize the conformation of the chain. The Gaussian approximation is
considered for a ring of length and for an open chain of length
in poor and theta solvent conditions, including a Coulomb repulsion
between the monomers. In theta solvent conditions the blob size is measured and
found in agreement with scaling theory, including charge depletion effects,
expected for the case of an open chain. In poor solvent conditions, a globule
instability, driven by electrostatic repulsion, is observed. We notice also
inhomogeneous behavior of the monomer--monomer correlation function,
reminiscence of necklace formation in poor solvent polyelectrolyte solutions. A
global phase diagram in terms of solvent quality and inverse Bjerrum length is
presented.Comment: submitted to EPJE (soft matter
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