91 research outputs found
Influence of solvent granularity on the effective interaction between charged colloidal suspensions
We study the effect of solvent granularity on the effective force between two
charged colloidal particles by computer simulations of the primitive model of
strongly asymmetric electrolytes with an explicitly added hard sphere solvent.
Apart from molecular oscillating forces for nearly touching colloids which
arise from solvent and counterion layering, the counterions are attracted
towards the colloidal surfaces by solvent depletion providing a simple
statistical description of hydration. This, in turn, has an important influence
on the effective forces for larger distances which are considerably reduced as
compared to the prediction based on the primitive model. When these forces are
repulsive, the long-distance behaviour can be described by an effective Yukawa
pair potential with a solvent-renormalized charge. As a function of colloidal
volume fraction and added salt concentration, this solvent-renormalized charge
behaves qualitatively similar to that obtained via the Poisson-Boltzmann cell
model but there are quantitative differences. For divalent counterions and
nano-sized colloids, on the other hand, the hydration may lead to overscreened
colloids with mutual attraction while the primitive model yields repulsive
forces. All these new effects can be accounted for through a solvent-averaged
primitive model (SPM) which is obtained from the full model by integrating out
the solvent degrees of freedom. The SPM was used to access larger colloidal
particles without simulating the solvent explicitly.Comment: 14 pages, 16 craphic
Irreducible tensor-form of the relativistic corrections to the M1 transition operator
The relativistic corrections to the magnetic dipole moment operator in the
Pauli approximation were derived originally by Drake (Phys. Rev. A 3(1971)908).
In the present paper, we derive their irreducible tensor-operator form to be
used in atomic structure codes adopting the Fano-Racah-Wigner algebra for
calculating its matrix elements.Comment: 26 page
Temperature and force dependence of nanoscale electron transport via the Cu protein Azurin
The mechanisms of solid-state electron transport (ETp) via a monolayer of
immobilized Azurin (Az) was examined by conducting probe atomic force
microscopy (CP-AFM), both as function of temperature (248 - 373K) and of
applied tip force (6-12 nN). By varying both temperature and force in CP-AFM,
we find that the ETp mechanism can alter with a change in the force applied via
the tip to the proteins. As the applied force increases, ETp via Az changes
from temperature-independent to thermally activated at high temperatures. This
is in contrast to the Cu-depleted form of Az (apo-Az), where increasing the
applied force causes only small quantitative effects, that fit with a decrease
in electrode spacing. At low force ETp via holo-Az is temperature-independent
and thermally activated via apo-Az. This observation agrees with
macroscopic-scale measurements, thus confirming that the difference in ETp
dependence on temperature between holo- and apo-Az is an inherent one that may
reflect a difference in rigidity between the two forms. An important
implication of these results, which depend on CP-AFM measurements over a
significant temperature range, is that for ETp measurements on floppy systems,
such as proteins, the stress applied to the sample should be kept constant or,
at least controlled during measurement.Comment: 24 pages, 6 figures, plus Supporting Information with 4 pages and 2
figure
Nuclear charge radii of ⁶²⁻⁸⁰Zn and their dependence on cross-shell proton excitations
Nuclear charge radii of ⁶²⁻⁸⁰Zn have been determined using collinear laser spectroscopy of bunched ion beams at CERN-ISOLDE. The subtle variations of observed charge radii, both within one isotope and along the full range of neutron numbers, are found to be well described in terms of the proton
excitations across the Z = 28 shell gap, as predicted by large-scale shell model calculations. It comprehensively explains the changes in isomer-to-ground state mean square charge radii of ⁶⁹⁻⁷⁹Zn, the inversion of the
odd-even staggering around N = 40 and the odd-even staggering systematics of the Zn charge radii. With two protons above Z = 28, the observed charge radii of the Zn isotopic chain show a cumulative effect of different aspects of nuclear structure including single particle structure, shell closure, correlations and deformations near the proposed doubly magic nuclei, ⁶⁸Ni and ⁷⁸Ni
Single-molecule measurements of trapped and migrating circular DNA during electrophoresis in agarose gels
The effect of agarose gel concentration and field strength on the electrophoretic trapping of open (relaxed) circular DNA was investigated using microscopic measurements of individual molecules stained with a fluorescent dye. Three open circles with sizes of 52.5, 115, and 220 kbp were trapped by the electric field (6 V/cm) and found to be predominately fixed and stretched at a single point in the gel. The length of the stretched circles did not significantly change with agarose concentration of the gels (mass fractions of 0.0025, 0.01, and 0.02). The relaxation kinetics of the trapped circles was also measured in the gels. The relaxation of the large open circles was found to be a slow process, taking several seconds. The velocity and average length of the 52.5 kbp open circles and 48.5 kbp linear DNA were measured during electrophoresis in the agarose gels. The velocity increased when the agarose concentrations were lowered, but the average length of the open-circle DNA (during electrophoresis) did not significantly change with agarose gel concentrations. The circles move through the gels by cycles of stretching and relaxation during electrophoresis. Linear dichroism was also used to investigate the trapping and alignment of the 52.5 kbp open circles. The results in this study provide information that can be used to improve electrophoretic separations of circular DNA, an important form of genetic material and commonly used to clone DNA
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