1,296 research outputs found
Phase Behavior of Polyelectrolyte-Surfactant Complexes at Planar Surfaces
We investigate theoretically the phase diagram of an insoluble charged
surfactant monolayer in contact with a semi-dilute polyelectrolyte solution (of
opposite charge). The polyelectrolytes are assumed to have long-range and
attractive (electrostatic) interaction with the surfactant molecules. In
addition, we introduce a short-range (chemical) interaction which is either
attractive or repulsive. The surfactant monolayer can have a lateral phase
separation between dilute and condensed phases. Three different regimes of the
coupled system are investigated depending on system parameters. A regime where
the polyelectrolyte is depleted due to short range repulsion from the surface,
and two adsorption regimes, one being dominated by electrostatics, whereas the
other by short range chemical attraction (similar to neutral polymers). When
the polyelectrolyte is more attracted (or at least less repelled) by the
surfactant molecules as compared with the bare water/air interface, it will
shift upwards the surfactant critical temperature. For repulsive short-range
interactions the effect is opposite. Finally, the addition of salt to the
solution is found to increase the critical temperature for attractive surfaces,
but does not show any significant effect for repulsive surfaces.Comment: 23 pages, 10 figure
Adhesion-Induced Lateral Phase Separation in Membranes
Adhesion between membranes is studied using a phenomenological model, where
the inter-membrane distance is coupled to the concentration of sticker
molecules on the membranes. The model applies to both for adhesion of two
flexible membranes and to adhesion of one flexible membrane onto a second
membrane supported on a solid substrate. We mainly consider the case where the
sticker molecules form bridges and adhere directly to both membranes. The
calculated mean-field phase diagrams show an upward shift of the transition
temperature indicating that the lateral phase separation in the membrane is
enhanced due to the coupling effect. Hence the possibility of adhesion-induced
lateral phase separation is predicted. For a particular choice of the
parameters, the model exhibits a tricritical behavior. We also discuss the
non-monotonous shape of the inter-membrane distance occurring when the lateral
phase separation takes place. The inter-membrane distance relaxes to the bulk
values with two symmetric overshoots. Adhesion mediated by other types of
stickers is also considered.Comment: 13 pages, 9 PostScript figures included. To be published in Euro.
Phys. J - E. Minor revision
Kinetics of Surfactant Adsorption at Fluid-Fluid Interfaces
We present a theory for the kinetics of surfactant adsorption at the
interface between an aqueous solution and another fluid (air, oil) phase. The
model relies on a free-energy formulation. It describes both the diffusive
transport of surfactant molecules from the bulk solution to the interface, and
the kinetics taking place at the interface itself. When applied to non-ionic
surfactant systems, the theory recovers results of previous models, justify
their assumptions and predicts a diffusion-limited adsorption, in accord with
experiments. For salt-free ionic surfactant solutions, electrostatic
interactions are shown to drastically affect the kinetics. The adsorption in
this case is predicted to be kinetically limited, and the theory accounts for
unusual experimental results obtained recently for the dynamic surface tension
of such systems. Addition of salt to an ionic surfactant solution leads to
screening of the electrostatic interactions and to a diffusion-limited
adsorption. In addition, the free-energy formulation offers a general method
for relating the dynamic surface tension to surface coverage without relying on
equilibrium relations.Comment: 36 pages, latex, 10 figure
Diblock Copolymer Ordering Induced by Patterned Surfaces Above the Order-Disorder Transition
We investigate the morphology of diblock copolymers in the vicinity of flat,
chemically patterned surfaces. Using a Ginzburg-Landau free energy, spatial
variations of the order parameter are given in terms of a general
two-dimensional surface pattern above the order-disorder transition. The
propagation of several surface patterns into the bulk is investigated. The
oscillation period and decay length of the surface -modes are calculated in
terms of system parameters. We observe lateral order parallel to the surface as
a result of order perpendicular to the surface. Surfaces which has a finite
size chemical pattern (e.g., a stripe of finite width) induces lamellar
ordering extending into the bulk. Close to the surface pattern the lamellae are
strongly perturbed adjusting to the pattern.Comment: 9 pages, 14 figures, to be published in Macromolecule
Self-Assembly in Mixtures of Polymers and Small Associating Molecules
The interaction between a flexible polymer in good solvent and smaller
associating solute molecules such as amphiphiles (surfactants) is considered
theoretically. Attractive correlations, induced in the polymer because of the
interaction, compete with intra-chain repulsion and eventually drive a joint
self-assembly of the two species, accompanied by partial collapse of the chain.
Results of the analysis are found to be in good agreement with experiments on
the onset of self-assembly in diverse polymer-surfactant systems. The threshold
concentration for self-assembly in the mixed system (critical aggregation
concentration, cac) is always lower than the one in the polymer-free solution
(critical micelle concentration, cmc). Several self-assembly regimes are
distinguished, depending on the effective interaction between the two species.
For strong interaction, corresponding experimentally to oppositely charged
species, the cac is much lower than the cmc. It increases with ionic strength
and depends only weakly on polymer charge. For weak interaction, the cac is
lower but comparable to the cmc, and the two are roughly proportional over a
wide range of cmc values. Association of small molecules with amphiphilic
polymers exhibiting intra-chain aggregation (polysoaps) is gradual, having no
sharp onset.Comment: 21 pages, 5 figures, RevTex, the published version, see also
cond-mat/990305
Diblock copolymer thin films: Parallel and perpendicular lamellar phases in the weak segregation limit
We study morphologies of thin-film diblock copolymers between two flat and
parallel walls. The study is restricted to the weak segregation regime below
the order-disorder transition temperature. The deviation from perfect lamellar
shape is calculated for phases which are perpendicular and parallel to the
walls. We examine the undulations of the inter material dividing surface and
its angle with the walls, and find that the deviation from its unperturbed
position can be much larger than in the strong segregation case. Evaluating the
weak segregation stability of the lamellar phases, it is shown that a surface
interaction, which is quadratic in the monomer concentration, favors the
perpendicular lamellar phase. In particular, the degeneracy between
perpendicular and unfrustrated parallel lamellar phases for walls without a
preferential adsorption is removed.Comment: 10 pages, 9 figures, submitted to European Physical Journal
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