7,751 research outputs found
Theoretical study of phase behaviour of DLVO model for lysozyme and -crystalline aqueous electrolyte solutions
Mean spherical approximation (MSA), second-order Barker-Henderson (BH)
perturbation theory and thermodynamic perturbation theory (TPT) for associating
fluids in combination with BH perturbation theory are applied to the study of
the structural properties and phase behaviour of the
Derjaguin-Landau-Verwey-Overbeek (DLVO) model of lysozyme and
-cristalline aqueous electrolyte solutions. Predictions of the MSA for
the structure factors are in good agreement with the corresponding computer
simulation predictions. The agreement between theoretical results for the
liquid-gas phase diagram and the corresponding results of the experiment and
computer simulation is less satisfactory, with predictions of the combined
BH-TPT approach being the most accurate.Comment: 9 pages, 3 figure
Non-additivity of pair interactions in charged colloids
It is general wisdom that the pair potential of charged colloids in a liquid
may be closely approximated by a Yukawa interaction, as predicted by the
classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. We experimentally
determine the effective forces in a binary mixture of like-charged particles,
of species 1 and 2, with blinking optical tweezers. The measured forces are
consistent with a Yukawa pair potential but the (12) cross-interaction is not
equal to the geometric mean of the (11) and (22) like-interactions, as expected
from DLVO. The deviation is a function of the electrostatic screening length
and the size ratio, with the cross-interaction measured being consistently
weaker than DLVO predictions. The corresponding non-additivity parameter is
negative and grows in magnitude with increased size asymmetry.Comment: Final versio
Anisotropic electrostatic screening of charged colloids in nematic solvents
The physical behaviour of anisotropic charged colloids is determined by their
material dielectric anisotropy, affecting colloidal self-assembly, biological
function and even out-of-equilibrium behaviour. However, little is known about
anisotropic electrostatic screening, which underlies all electrostatic
effective interactions in such soft or biological materials. In this work, we
demonstrate anisotropic electrostatic screening for charged colloidal particles
in a nematic electrolyte. We show that material anisotropy behaves markedly
different from particle anisotropy: The electrostatic potential and pair
interactions decay with an anisotropic Debye screening length, contrasting the
constant screening length for isotropic electrolytes. Charged dumpling-shaped
near-spherical colloidal particles in a nematic medium are used as an
experimental model system to explore the effects of anisotropic screening,
demonstrating competing anisotropic elastic and electrostatic effective pair
interactions for colloidal surface charges tunable from neutral to high,
yielding particle-separated metastable states. Generally, our work contributes
to the understanding of electrostatic screening in nematic anisotropic media.Comment: 15 pages, 5 figures, SM under ancillary file
Forces between Colloidal Particles in Aqueous Solutions Containing Monovalent and Multivalent Ions
The present article provides an overview of the recent progress in the direct
force measurements between individual pairs of colloidal particles in aqueous
salt solutions. Results obtained by two different techniques are being
highlighted, namely with the atomic force microscope (AFM) and optical
tweezers. One finds that the classical theory of Derjaguin, Landau, Verwey, and
Overbeek (DLVO) represents an accurate description of the force profiles even
in the presence of multivalent ions, typically down to distances of few
nanometers. However, the corresponding Hamaker constants and diffuse layer
potentials must be extracted from the force profiles. At low salt
concentrations, double layer forces remain repulsive and may become long
ranged. At short distances, additional short range non-DLVO interactions may
become important. Such an interaction is particularly relevant in the presence
of multivalent counterions.Comment: Submitted on 30th of May 2016 as invited article to Curr. Opinion
Colloid Interf. Sci. Edited by W. Ducker and P. Claesson. 15 Pages, 7 Figures
82 reference
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
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