39 research outputs found
Surface Tension of Electrolyte Solutions: A Self-consistent Theory
We study the surface tension of electrolyte solutions at the air/water and
oil/water interfaces. Employing field-theoretical methods and considering
short-range interactions of anions with the surface, we expand the Helmholtz
free energy to first-order in a loop expansion and calculate the excess surface
tension. Our approach is self-consistent and yields an analytical prediction
that reunites the Onsager-Samaras pioneering result (which does not agree with
experimental data), with the ionic specificity of the Hofmeister series. We
obtain analytically the surface-tension dependence on the ionic strength, ionic
size and ion-surface interaction, and show consequently that the
Onsager-Samaras result is consistent with the one-loop correction beyond the
mean-field result. Our theory fits well a wide range of concentrations for
different salts using one fit parameter, reproducing the reverse Hofmeister
series for anions at the air/water and oil/water interfaces.10.1029Comment: 5 pages, 2 figure
Charged Membranes: Poisson-Boltzmann theory, DLVO paradigm and beyond
In this chapter we review the electrostatic properties of charged membranes
in aqueous solutions, with or without added salt, employing simple physical
models. The equilibrium ionic profiles close to the membrane are governed by
the well-known Poisson-Boltzmann (PB) equation. We analyze the effect of
different boundary conditions, imposed by the membrane, on the ionic profiles
and the corresponding osmotic pressure. The discussion is separated into the
single membrane case and that of two interacting membranes. For one membrane
setup, we show the different solutions of the PB equation and discuss the
interplay between constant-charge and constant-potential boundary conditions. A
modification of the PB theory is presented to treat the extremely high
counter-ion concentration in the vicinity of a charge membrane. For two
equally-charged membranes, we analyze the different pressure regimes for the
constant-charge boundary condition, and discuss the difference in the osmotic
pressure for various boundary conditions. The non-equal charged membranes is
reviewed as well, and the crossover from repulsion to attraction is calculated
analytically. We then examine the charge-regulation boundary condition and
discuss its effects on the ionic profiles and the osmotic pressure for two
equally-charged membranes. In the last section, we briefly review the van der
Waals (vdW) interactions and their effect on the free energy between two planar
membranes. We explain the simple Hamaker pair-wise summation procedure, and
introduce the more rigorous Lifshitz theory. The latter is a key ingredient in
the DLVO theory, which combines repulsive electrostatic with attractive vdW
interactions, and offers a simple explanation for colloidal or membrane
stability. Finally, the chapter ends by a short account of the limitations of
the approximations inherent in the PB theory.Comment: 57 pages, 19 figures, From the forthcoming Handbook of Lipid
Membranes: Molecular, Functional, and Materials Aspects. Edited by Cyrus
Safinya and Joachim Radler, Taylor & Francis/CRC Press, 201
Charge regulation: a generalized boundary condition?
The three most commonly-used boundary conditions for charged colloidal
systems are constant charge (insulator), constant potential (conducting
electrode) and charge regulation (ionizable groups at the surface). It is
usually believed that the charge regulation is a generalized boundary condition
that reduces in some specific limits to either constant charge or constant
potential boundary conditions. By computing the disjoining pressure between two
symmetric planes for these three boundary conditions, both numerically (for all
inter-plate separations) and analytically (for small inter-plate separations),
we show that this is not, in general, the case. In fact, the limit of charge
regulation is a separate boundary condition, yielding a disjoining pressure
with a different characteristic separation-scaling. Our findings are supported
by several examples demonstrating that the disjoining pressure at small
separations for the charge regulation boundary-condition depends on the details
of the dissociation/association process.Comment: 6 pages, 3 figure
Complex Fluids with Mobile Charge-Regulated Macro-Ions
We generalize the concept of charge regulation of ionic solutions, and apply
it to complex fluids with mobile macro-ions having internal non-electrostatic
degrees of freedom. The suggested framework provides a convenient tool for
investigating systems where mobile macro-ions can self-regulate their charge
(e.g., proteins). We show that even within a simplified charge-regulation
model, the charge dissociation equilibrium results in different and notable
properties. Consequences of the charge regulation include a positional
dependence of the effective charge of the macro-ions, a non-monotonic
dependence of the effective Debye screening length on the concentration of the
monovalent salt, a modification of the electric double-layer structure, and
buffering by the macro-ions of the background electrolyte.Comment: 7 pages, 5 figure
Osmotic properties of polyethyleneglycols: quantitative features of brush and bulk scaling laws
From glycosylated cell surfaces to sterically stabilized liposomes, polymers
attached to membranes attract biological and therapeutic interest. Can the
scaling laws of polymer "brushes" describe the physical properties of these
coats? We delineate conditions where the Alexander - de Gennes theory of
polymer brushes successfully describes the intermembrane distance vs. applied
osmotic stress data of Kenworthy et al. for PEG-grafted multilamellar liposomes
[Biophys. J. (1995) 68:1921]. We establish that the polymer density and size in
the brush must be high enough that, in a bulk solution of equivalent density,
the polymer osmotic pressure is independent of polymer molecular weight (the
des Cloizeaux semi-dilute regime of bulk polymer solutions). The condition that
attached polymers behave as semi-dilute bulk solutions offers a rigorous
criterion for brush scaling-law behavior. There is a deep connection between
the behaviors of polymer solutions in bulk and polymers grafted to a surface at
a density such that neighbors pack to form a uniform brush. In this regime,
two-parameter unconstrained fits of the Alexander - de Gennes brush scaling
laws yield effective monomer lengths of 3.3 to 3.5 AA, which agree with
structural predictions. The fitted distances between grafting sites are larger
than expected from the nominal content of PEG-lipids; the chains apparently
saturate the surface. Osmotic stress measurements can be used to estimate the
actual densities of membrane-grafted polymers.Comment: 26 pages with figure