1,367 research outputs found
Effective Electrostatic Interactions in Suspensions of Polyelectrolyte Brush-Coated Colloids
Effective electrostatic interactions between colloidal particles, coated with
polyelectrolyte brushes and suspended in an electrolyte solvent, are described
via linear response theory. The inner cores of the macroions are modeled as
hard spheres, the outer brushes as spherical shells of continuously distributed
charge, the microions (counterions and salt ions) as point charges, and the
solvent as a dielectric continuum. The multi-component mixture of macroions and
microions is formally mapped onto an equivalent one-component suspension by
integrating out from the partition function the microion degrees of freedom.
Applying second-order perturbation theory and a random phase approximation,
analytical expressions are derived for the effective pair interaction and a
one-body volume energy, which is a natural by-product of the one-component
reduction. The combination of an inner core and an outer shell, respectively
impenetrable and penetrable to microions, allows the interactions between
macroions to be tuned by varying the core diameter and brush thickness. In the
limiting cases of vanishing core diameter and vanishing shell thickness, the
interactions reduce to those derived previously for star polyelectrolytes and
charged colloids, respectively.Comment: 20 pages, 5 figures, Phys. Rev. E (in press
Electrostatic Disorder-Induced Interactions in Inhomogeneous Dielectrics
We investigate the effect of quenched surface charge disorder on
electrostatic interactions between two charged surfaces in the presence of
dielectric inhomogeneities and added salt. We show that in the linear
weak-coupling regime (i.e., by including mean-field and Gaussian-fluctuations
contributions), the image-charge effects lead to a non-zero disorder-induced
interaction free energy between two surfaces of equal mean charge that can be
repulsive or attractive depending on the dielectric mismatch across the
bounding surfaces and the exact location of the disordered charge distribution.Comment: 7 pages, 2 figure
Ground state of two unlike charged colloids: An analogy with ionic bonding
In this letter, we study the ground state of two spherical macroions of
identical radius, but asymmetric bare charge ((Q_{A}>Q_{B})). Electroneutrality
of the system is insured by the presence of the surrounding divalent
counterions. Using Molecular Dynamics simulations within the framework of the
primitive model, we show that the ground state of such a system consists of an
overcharged and an undercharged colloid. For a given macroion separation the
stability of these ionized-like states is a function of the difference
((\sqrt{N_{A}}-\sqrt{N_{B}})) of neutralizing counterions (N_{A}) and (N_{B}).
Furthermore the degree of ionization, or equivalently, the degree of
overcharging, is also governed by the distance separation of the macroions. The
natural analogy with ionic bonding is briefly discussed.Comment: published versio
Bacteria isolated from the airways of paediatric patients with bronchiectasis according to HIV status
Background. Knowledge of which bacteria are found in the airways of paediatric patients with bronchiectasis unrelated to cystic fibrosis (CF) is important in defining empirical antibiotic guidelines for the treatment of acute infective exacerbations.Objective. To describe the bacteria isolated from the airways of children with non-CF bronchiectasis according to their HIV status.Methods. Records of children with non-CF bronchiectasis who attended the paediatric pulmonology clinic at Chris Hani Baragwanath Academic Hospital, Johannesburg, South Africa, from April 2011 to March 2013, or were admitted to the hospital during that period, were reviewed. Data collected included patient demographics, HIV status, and characteristics of the airway samples and types of bacteria isolated.Results. There were 66 patients with non-CF bronchiectasis over the 2-year study period. The median age was 9.1 years (interquartile range 7.2 - 12.1). The majority of patients (78.8%) were HIV-infected. A total of 134 samples was collected (median 1.5 per patient, range 1 - 7), of which 81.3% were expectorated or induced sputum samples. Most bacteria were Gram negatives (72.1%). Haemophilus influenzae was the most common bacterium identified (36.0%), followed by Streptococcus pneumoniae (12.6%), Moraxella catarrhalis (11.1%) and Staphylococcus aureus (10.6%). There were no differences between HIV-infected and uninfected patients in prevalence or type of pathogens isolated.Conclusion. Bacterial isolates from the airways of children with non-CF bronchiectasis were similar to those in other paediatric populations and were not affected by HIV status
Many-Body Electrostatic Forces Between Colloidal Particles at Vanishing Ionic Strength
Electrostatic forces between small groups of colloidal particles are measured
using blinking optical tweezers. When the electrostatic screening length is
significantly larger than the particle radius, forces are found to be
non-pairwise additive. Both pair and multi-particle forces are well described
by the linearized Poisson-Boltzmann equation with constant potential boundary
conditions. These findings may play an important role in understanding the
structure and stability of a wide variety of systems, from micron-sized
particles in oil to aqueous nanocolloids.Comment: 5 pages 2 figure
The Poisson-Boltzmann Theory for Two Parallel Uniformly Charged Plates
We solve the nonlinear Poisson-Boltzmann equation for two parallel and likely
charged plates both inside a symmetric elecrolyte, and inside a 2 : 1
asymmetric electrolyte, in terms of Weierstrass elliptic functions. From these
solutions we derive the functional relation between the surface charge density,
the plate separation, and the pressure between plates. For the one plate
problem, we obtain exact expressions for the electrostatic potential and for
the renormalized surface charge density, both in symmetric and in asymmetric
electrolytes. For the two plate problems, we obtain new exact asymptotic
results in various regimes.Comment: 17 pages, 9 eps figure
Casimir-Polder interatomic potential between two atoms at finite temperature and in the presence of boundary conditions
We evaluate the Casimir-Polder potential between two atoms in the presence of
an infinite perfectly conducting plate and at nonzero temperature. In order to
calculate the potential, we use a method based on equal-time spatial
correlations of the electric field, already used to evaluate the effect of
boundary conditions on interatomic potentials. This method gives also a
transparent physical picture of the role of a finite temperature and boundary
conditions on the Casimir-Polder potential. We obtain an analytical expression
of the potential both in the near and far zones, and consider several limiting
cases of interest, according to the values of the parameters involved, such as
atom-atom distance, atoms-wall distance and temperature.Comment: 11 page
"Blue energy" from ion adsorption and electrode charging in sea- and river water
A huge amount of entropy is produced at places where fresh water and seawater
mix, for example at river mouths. This mixing process is a potentially enormous
source of sustainable energy, provided it is harnessed properly, for instance
by a cyclic charging and discharging process of porous electrodes immersed in
salt and fresh water, respectively [D. Brogioli, Phys. Rev. Lett. 103, 058501
(2009)]. Here we employ a modified Poisson-Boltzmann free-energy density
functional to calculate the ionic adsorption and desorption onto and from the
charged electrodes, from which the electric work of a cycle is deduced. We
propose optimal (most efficient) cycles for two given salt baths involving two
canonical and two grand-canonical (dis)charging paths, in analogy to the
well-known Carnot cycle for heat-to-work conversion from two heat baths
involving two isothermal and two adiabatic paths. We also suggest a slightly
modified cycle which can be applied in cases that the stream of fresh water is
limited.Comment: 7 Figure
Mixtures of Charged Colloid and Neutral Polymer: Influence of Electrostatic Interactions on Demixing and Interfacial Tension
The equilibrium phase behavior of a binary mixture of charged colloids and
neutral, non-adsorbing polymers is studied within free-volume theory. A model
mixture of charged hard-sphere macroions and ideal, coarse-grained,
effective-sphere polymers is mapped first onto a binary hard-sphere mixture
with non-additive diameters and then onto an effective Asakura-Oosawa model [S.
Asakura and F. Oosawa, J. Chem. Phys. 22, 1255 (1954)]. The effective model is
defined by a single dimensionless parameter -- the ratio of the polymer
diameter to the effective colloid diameter. For high salt-to-counterion
concentration ratios, a free-volume approximation for the free energy is used
to compute the fluid phase diagram, which describes demixing into colloid-rich
(liquid) and colloid-poor (vapor) phases. Increasing the range of electrostatic
interactions shifts the demixing binodal toward higher polymer concentration,
stabilizing the mixture. The enhanced stability is attributed to a weakening of
polymer depletion-induced attraction between electrostatically repelling
macroions. Comparison with predictions of density-functional theory reveals a
corresponding increase in the liquid-vapor interfacial tension. The predicted
trends in phase stability are consistent with observed behavior of
protein-polysaccharide mixtures in food colloids.Comment: 16 pages, 5 figure
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