501 research outputs found
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
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
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
Roughness-Induced Wetting
We investigate theoretically the possibility of a wetting transition induced
by geometric roughness of a solid substrate for the case where the flat
substrate does not show a wetting layer. Our approach makes use of a novel
closed-form expression which relates the interaction between two sinusoidally
modulated interfaces to the interaction between two flat interfaces. Within the
harmonic approximation, we find that roughness-induced wetting is indeed
possible if the substrate roughness, quantified by the substrate surface area,
exceeds a certain threshold. In addition, the molecular interactions between
the substrate and the wetting substance have to satisfy several conditions.
These results are expressed in terms of a lower bound on the wetting potential
for a flat substrate in order for roughness-induced wetting to occur. This
lower bound has the following properties: A minimum is present at zero or very
small separation between the two interfaces, as characteristic for the
non-wetting situation in the flat case. Most importantly, the wetting potential
needs to have a pronounced maximum at a separation comparable to the amplitude
of the substrate roughness. These findings are in agreement with the
experimental observation of roughness-induced surface premelting at a glass-ice
interface as well as the calculation of the dispersion interaction for the
corresponding glass-water-ice system.Comment: 17 pages, 8 figure
Diblock copolymer ordering induced by patterned surfaces
We use a Ginzburg-Landau free energy functional to investigate diblock
copolymer morphologies when the copolymer melt interacts with one surface or is
confined between two chemically patterned surfaces. For temperatures above the
order-disorder transition a complete linear response description of the
copolymer melt is given, in terms of an arbitrary two-dimensional surface
pattern. The appearance of order in the direction parallel to the surface is
found as a result of the order in the perpendicular direction. Below the
order-disorder point and in a thin-film geometry, our procedure enables the
analytic calculation of distorted perpendicular and tilted lamellar phases in
the presence of uniform or modulated surface fields.Comment: 8 pages, 3 figures, to be published in Europhys. Let
Kinetics of Surfactant Adsorption at Fluid-Fluid Interfaces: Surfactant Mixtures
The adsorption at the interface between an aqueous solution of several
surface-active agents and another fluid (air or oil) phase is addressed
theoretically. We derive the kinetic equations from a variation of the
interfacial free energy, solve them numerically and provide an analytic
solution for the simple case of a linear adsorption isotherm. Calculating
asymptotic solutions analytically, we find the characteristic time scales of
the adsorption process and observe the behavior of the system at various
temporal stages. In particular, we relate the kinetic behavior of the mixture
to the properties of its individual constituents and find good agreement with
experiments. In the case of kinetically limited adsorption, the mixture
kinetics is found to be considerably different from that of the
single-surfactant solutions because of strong coupling between the species.Comment: 19 pages, 7 figures, to be published in Langmui
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