38 research outputs found
Understanding depletion forces beyond entropy
The effective interaction energy of a colloidal sphere in a suspension
containing small amounts of non-ionic polymers and a flat glass surface has
been measured and calculated using total internal reflection microscopy (TIRM)
and a novel approach within density functional theory (DFT), respectively.
Quantitative agreement between experiment and theory demonstrates that the
resulting repulsive part of the depletion forces cannot be interpreted entirely
in terms of entropic arguments but that particularly at small distances
( 100 nm) attractive dispersion forces have to be taken into account
Depletion forces near curved surfaces
Based on density functional theory the influence of curvature on the
depletion potential of a single big hard sphere immersed in a fluid of small
hard spheres with packing fraction \eta_s either inside or outside of a hard
spherical cavity of radius R_c is calculated. The relevant features of this
potential are analyzed as function of \eta_s and R_c. There is a very slow
convergence towards the flat wall limit R_c \to \infty. Our results allow us to
discuss the strength of depletion forces acting near membranes both in normal
and lateral directions and to make contact with recent experimental results
Glasses in hard spheres with short-range attraction
We report a detailed experimental study of the structure and dynamics of
glassy states in hard spheres with short-range attraction. The system is a
suspension of nearly-hard-sphere colloidal particles and non-adsorbing linear
polymer which induces a depletion attraction between the particles. Observation
of crystallization reveals a re-entrant glass transition. Static light
scattering shows a continuous change in the static structure factors upon
increasing attraction. Dynamic light scattering results, which cover 11 orders
of magnitude in time, are consistent with the existence of two distinct kinds
of glasses, those dominated by inter-particle repulsion and caging, and those
dominated by attraction. Samples close to the `A3 point' predicted by mode
coupling theory for such systems show very slow, logarithmic dynamics.Comment: 22 pages, 18 figure
Depletion potential in hard-sphere mixtures: theory and applications
We present a versatile density functional approach (DFT) for calculating the
depletion potential in general fluid mixtures. In contrast to brute force DFT,
our approach requires only the equilibrium density profile of the small
particles {\em before} the big (test) particle is inserted. For a big particle
near a planar wall or a cylinder or another fixed big particle the relevant
density profiles are functions of a single variable, which avoids the numerical
complications inherent in brute force DFT. We implement our approach for
additive hard-sphere mixtures. By investigating the depletion potential for
high size asymmetries we assess the regime of validity of the well-known
Derjaguin approximation for hard-sphere mixtures and argue that this fails. We
provide an accurate parametrization of the depletion potential in hard-sphere
fluids which should be useful for effective Hamiltonian studies of phase
behavior and colloid structure
Polymer depletion interaction between two parallel repulsive walls
The depletion interaction between two parallel repulsive walls confining a
dilute solution of long and flexible polymer chains is studied by
field-theoretic methods. Special attention is paid to self-avoidance between
chain monomers relevant for polymers in a good solvent. Our direct approach
avoids the mapping of the actual polymer chains on effective hard or soft
spheres. We compare our results with recent Monte Carlo simulations [A. Milchev
and K. Binder, Eur. Phys. J. B 3, 477 (1998)] and with experimental results for
the depletion interaction between a spherical colloidal particle and a planar
wall in a dilute solution of nonionic polymers [D. Rudhardt, C. Bechinger, and
P. Leiderer, Phys. Rev. Lett. 81, 1330 (1998)].Comment: 17 pages, 3 figures. Final version as publishe
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
Effective forces in colloidal mixtures: from depletion attraction to accumulation repulsion
Computer simulations and theory are used to systematically investigate how
the effective force between two big colloidal spheres in a sea of small spheres
depends on the basic (big-small and small-small) interactions. The latter are
modeled as hard-core pair potentials with a Yukawa tail which can be both
repulsive or attractive. For a repulsive small-small interaction, the effective
force follows the trends as predicted by a mapping onto an effective
non-additive hard-core mixture: both a depletion attraction and an accumulation
repulsion caused by small spheres adsorbing onto the big ones can be obtained
depending on the sign of the big-small interaction. For repulsive big-small
interactions, the effect of adding a small-small attraction also follows the
trends predicted by the mapping. But a more subtle ``repulsion through
attraction'' effect arises when both big-small and small-small attractions
occur: upon increasing the strength of the small-small interaction, the
effective potential becomes more repulsive. We have further tested several
theoretical methods against our computer simulations: The superposition
approximation works best for an added big-small repulsion, and breaks down for
a strong big-small attraction, while density functional theory is very accurate
for any big-small interaction when the small particles are pure hard-spheres.
The theoretical methods perform most poorly for small-small attractions.Comment: submitted to PRE; New version includes an important quantitative
correction to several of the simulations. The main conclusions remain
unchanged thoug
Macromolecular theory of solvation and structure in mixtures of colloids and polymers
The structural and thermodynamic properties of mixtures of colloidal spheres
and non-adsorbing polymer chains are studied within a novel general
two-component macromolecular liquid state approach applicable for all size
asymmetry ratios. The dilute limits, when one of the components is at infinite
dilution but the other concentrated, are presented and compared to field theory
and models which replace polymer coils with spheres. Whereas the derived
analytical results compare well, qualitatively and quantitatively, with
mean-field scaling laws where available, important differences from ``effective
sphere'' approaches are found for large polymer sizes or semi-dilute
concentrations.Comment: 23 pages, 10 figure
Le comportement de la gentamicine dans le parenchyme renal. [Behavior of gentamicin in the kidney parenchyma. Studies on rats during and after treatment with therapeutic doses]
The injection in rats of 4 mg/kg gentamicin results in accumulation and continued presence of the antibiotic in the renal parenchyma compared with its rapid elimination from other tissues. With repeated injections at 24-hour intervals over a period of 3 weeks, gentamicin concentrations in the cortex reach 356 +/- 139 microgram/g after 1 week of treatment, i.e. levels 50 times higher than the maximum serum levels. They then stay at a saturation plateau before decreasing very slowly to levels of 50 microgram/g 2 weeks and 15 microgram/g 15 weeks after the last injection. The accumulation takes place mainly in the cells of the proximal convoluted tubule. In the medulla, gentamicin behaves similarly but the levels are lower. Weeks after the last injection, urine levels of gentamicin are still appreciable. In spite of this enormous accumulation, the treatment does not result in disturbance of renal function or striking morphological changes. These observations are based on dosages of gentamicin corresponding to those used clinically. Compared to earlier observations based on the supratherapeutic administration of the antibiotic, the present study provides a better understanding of the mechanisms involved in gentamicin nephrotoxicity