234 research outputs found
Phase behavior and structure of model colloid-polymer mixtures confined between two parallel planar walls
Using Gibbs ensemble Monte Carlo simulations and density functional theory we
investigate the fluid-fluid demixing transition in inhomogeneous
colloid-polymer mixtures confined between two parallel plates with separation
distances between one and ten colloid diameters covering the complete range
from quasi two-dimensional to bulk-like behavior. We use the
Asakura-Oosawa-Vrij model in which colloid-colloid and colloid-polymer
interactions are hard-sphere like, whilst the pair potential between polymers
vanishes. Two different types of confinement induced by a pair of parallel
walls are considered, namely either through two hard walls or through two
semi-permeable walls that repel colloids but allow polymers to freely
penetrate. For hard (semi-permeable) walls we find that the capillary binodal
is shifted towards higher (lower) polymer fugacities and lower (higher) colloid
fugacities as compared to the bulk binodal; this implies capillary condensation
(evaporation) of the colloidal liquid phase in the slit. A macroscopic
treatment is provided by a novel symmetric Kelvin equation for general binary
mixtures, based on the proximity in chemical potentials of statepoints at
capillary coexistence and the reference bulk coexistence. Results for capillary
binodals compare well with those obtained from the classic version of the
Kelvin equation due to Evans and Marini Bettolo Marconi [J. Chem. Phys. 86,
7138 (1987)], and are quantitatively accurate away from the fluid-fluid
critical point, even at small wall separations. For hard walls the density
profiles of polymers and colloids inside the slit display oscillations due to
packing effects for all statepoints. For semi-permeable walls either similar
structuring or flat profiles are found, depending on the statepoint considered.Comment: 15 pages, 13 figure
Application of automated electron microscopy imaging and machine learning to characterise and quantify nanoparticle dispersion in aqueous media
For many nanoparticle applications it is important to understand dispersion in liquids. For nanomedicinal and nanotoxicological research this is complicated by the often complex nature of the biological dispersant and ultimately this leads to severe limitations in the analysis of the nanoparticle dispersion by light scattering techniques. Here we present an alternative analysis and associated workflow which utilises electron microscopy. The need to collect large, statistically relevant datasets by imaging vacuum dried, plunge frozen aliquots of suspension was accomplished by developing an automated STEM imaging protocol implemented in an SEM fitted with a transmission detector. Automated analysis of images of agglomerates was achieved by machine learning using two free open‐source software tools: CellProfiler and ilastik. The specific results and overall workflow described enable accurate nanoparticle agglomerate analysis of particles suspended in aqueous media containing other potential confounding components such as salts, vitamins and proteins
Modelling colloids with Baxter's adhesive hard sphere model
The structure of the Baxter adhesive hard sphere fluid is examined using
computer simulation. The radial distribution function (which exhibits unusual
discontinuities due to the particle adhesion) and static structure factor are
calculated with high accuracy over a range of conditions and compared with the
predictions of Percus--Yevick theory. We comment on rigidity in percolating
clusters and discuss the role of the model in the context of experiments on
colloidal systems with short-range attractive forces.Comment: 14 pages, 7 figures. (For proceedings of "Structural arrest in
colloidal systems with short-range attractive forces", Messina, December
2003
Nanoparticle corona artefacts derived from specimen preparation of particle suspensions
Progress in the implementation of nanoparticles for therapeutic applications will accelerate with an improved understanding of the interface between nanoparticle surfaces and the media they are dispersed in. We examine this interface by analytical scanning transmission electron microscopy and show that incorrect specimen preparation or analysis can induce an artefactual, nanoscale, calcium phosphate-rich, amorphous coating on nanoparticles dispersed in cell culture media. We report that this ionic coating can be induced on five different types of nanoparticles (Au, BaTiO3, ZnO, TiO2 and Fe2O3) when specimen preparation causes a significant rise in pH above physiological levels. Such a pH change reduces ionic solubility in the suspending media to permit precipitation of calcium phosphate. Finally, we demonstrate that there is no indication of a calcium-phosphorus-rich coating on BaTiO3 nanoparticles suspended in culture media when prepared without alteration of the pH of the suspending media and imaged by cryo-STEM. Therefore we recommend that future reports utilising nanoparticles dispersed in cell culture media monitor and report the pH of suspensions during sample preparation
Effects of polymer polydispersity on the phase behaviour of colloid-polymer mixtures
We study the equilibrium behaviour of a mixture of monodisperse hard sphere
colloids and polydisperse non-adsorbing polymers at their -point, using
the Asakura-Oosawa model treated within the free-volume approximation. Our
focus is the experimentally relevant scenario where the distribution of polymer
chain lengths across the system is fixed. Phase diagrams are calculated using
the moment free energy method, and we show that the mean polymer size at which gas-liquid phase separation first occurs decreases with increasing
polymer polydispersity . Correspondingly, at fixed mean polymer size,
polydispersity favours gas-liquid coexistence but delays the onset of
fluid-solid separation. On the other hand, we find that systems with different
but the same {\em mass-averaged} polymer chain length have nearly
polydispersity-independent phase diagrams. We conclude with a comparison to
previous calculations for a semi-grandcanonical scenario, where the polymer
chemical potentials are imposed, which predicted that fluid-solid coexistence
was over gas-liquid in some areas of the phase diagram. Our results show that
this somewhat counter-intuitive result arose because the actual polymer size
distribution in the system is shifted to smaller sizes relative to the polymer
reservoir distribution.Comment: Changes in v2: sketch in Figure 1 corrected, other figures improved;
added references to experimental work and discussion of mapping from polymer
chain length to effective radiu
Entropic Interactions in Suspensions of Semi-Flexible Rods: Short-Range Effects of Flexibility
We compute the entropic interactions between two colloidal spheres immersed
in a dilute suspension of semi-flexible rods. Our model treats the
semi-flexible rod as a bent rod at fixed angle, set by the rod contour and
persistence lengths. The entropic forces arising from this additional
rotational degree of freedom are captured quantitatively by the model, and
account for observations at short range in a recent experiment. Global fits to
the interaction potential data suggest the persistence length of fd-virus is
about two to three times smaller than the commonly used value of .Comment: 4 pages, 5 figures, submitted to PRE rapid communication
Universal law of fractionation for slightly polydisperse systems
By perturbing about a general monodisperse system, we provide a complete description of two-phase equilibria in any system which is slightly polydisperse in some property (e.g., particle size, charge, etc.). We derive a universal law of fractionation which is corroborated by comprehensive experiments on a model colloid-polymer mixture. We furthermore predict that phase separation is an effective method of reducing polydispersity only for systems with a skewed distribution of the polydisperse property
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