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Effect of repulsive and attractive interactions on depletion forces in colloidal suspensions: a density functional theory treatment.

The author employs density functional theory to study colloidal interactions in solution. Hardcore Yukawa potentials with soft tails, either repulsive or attractive, are used to model colloid-solvent and solvent-solvent interactions. We analyze the effect of these interactions on the solvent-mediated potential of mean force between two colloids in solution. Overall, theory is shown to be in good agreement with recent simulation data. We use the theory to study the density dependence of the colloid-colloid second virial coefficient.

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Domains of phase separation in a charged colloidal dispersion driven by electrolytes.

We put forth the idea of treating coexisting phases as a composite system and express its free energy as the average of its constitutent free energies weighted by their respective volume proportions. As a result, the theoretical study of charged colloidal phase separation in the presence of electrolytes reduces to optimizing solely the entities pertaining to colloids and small ions. As concrete illustrations, we demarcated the boundaries of coexisting phases for the simplest colloidal dispersion driven by salts at moderate to high concentrations and compared the results with those obtained in the usual manner to numerically show the robust efficiency of the present theory. Also, for a charged colloidal dispersion at very low ionic strength, we crosshatched both the homogeneous one phase and coexisting phases, and used the domains of coexisting phases to interpret an anomalous "transition" of phase diagrams exhibited in dilute colloidal dispersions induced by salts on dilution.

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Laser trapping of low refractive index colloids in a nematic liquid crystal.

We describe and analyze laser trapping of small colloidal particles in a nematic liquid crystal, where the index of refraction of colloids is smaller compared to the indices of the liquid crystal. Two mechanisms are identified that are responsible for this anomalous trapping: (i) below the optical Fréedericksz transition, the trapping is due to the anisotropic dielectric interaction of the polarized light with the inhomogeneous director field around the colloid, (ii) above the optical Fréedericksz transition, the optical trapping is accompanied by the elasticity-mediated interaction between the optically distorted region of a liquid crystal and the colloid. In the majority of the experiments, the trapping above the Fréedericksz transition is highly anisotropic. Qualitative agreement is found with a numerical analysis, considering the nematic director elastic distortion, dielectric director-light field coupling and optical repulsion due to low refraction index colloid in high index surroundings.

Journal Article↗

Phase diagram and structure of colloid-polymer mixtures confined between walls.

The influence of confinement, due to flat parallel structureless walls, on phase separation in colloid-polymer mixtures, is investigated by means of grand-canonical Monte Carlo simulations. Ultrathin films, with thicknesses between D=3-10 colloid diameters, are studied. The Asakura-Oosawa model [J. Chem. Phys. 22, 1255 (1954)] is used to describe the particle interactions. To simulate efficiently, a "cluster move" [J. Chem. Phys. 121, 3253 (2004)] is used in conjunction with successive umbrella sampling [J. Chem. Phys. 120, 10925 (2004)]. These techniques, when combined with finite size scaling, enable an accurate determination of the unmixing binodal. Our results show that the critical behavior of the confined mixture is described by "effective" critical exponents, which gradually develop from values near those of the two-dimensional Ising model, to those of the three-dimensional Ising model, as D increases. The scaling predictions of and Fisher and Nakanishi [J. Chem. Phys. 75, 5875 (1981)] for the shift of the critical point are compatible with our simulation results. Surprisingly, however, the colloid packing fraction at criticality approaches its bulk (D-->infinity) value nonmonotonically, as D is increased. Far from the critical point, our results are compatible with the simple Kelvin equation, implying a shift of order 1/D in the coexistence colloid chemical potential. We also present density profiles and pair distribution functions for a number of state points on the binodal, and the influence of the colloid-wall interaction is studied.

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Colloidal brushes in complex solutions: existence of a weak midrange attraction due to excluded-volume effects.

Towards the prediction of thermodynamic properties for complex systems and the design of self-assembly techniques for the production of nanocomposite materials, we employ Monte Carlo simulations to calculate the effective interactions between colloidal brushes dissolved in nonadsorbing polymer solutions. For simplicity, all interactions are reduced to excluded-volume potentials. Our results indicate that (1) due to excluded-volume effects and the absence of attractive van der Waals forces in our calculations, the effective interactions between colloidal brushes in a continuum medium are always repulsive; and (2) that the short-ranged repulsion between the colloidal brushes may be coupled to a midrange attraction when nonadsorbing polymers are in solution. We prove that a depletion mechanism is responsible for the midranged attraction and we observe that the strength of the induced attraction is weaker compared to the depletion attraction computed for hard-sphere colloids in nonadsorbing polymer solutions. More interestingly, however, our results indicate that, even for the simple interaction potentials employed here, it is not possible to superimpose depletion attraction and steric repulsion to predict the correct pair potential between colloidal brushes in nonadsorbing polymer solutions.

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Structure and dynamics of colloidal depletion gels: coincidence of transitions and heterogeneity.

Transitions in structural heterogeneity of colloidal depletion gels formed through short-range attractive interactions are correlated with their dynamical arrest. The system is a density and refractive index matched suspension of 0.20 volume fraction poly(methyl methacyrlate) colloids with the nonadsorbing depletant polystyrene added at a size ratio of depletant to colloid of 0.043. As the strength of the short-range attractive interaction is increased, clusters become increasingly structurally heterogeneous, as characterized by number-density fluctuations, and dynamically immobilized, as characterized by the single-particle mean-squared displacement. The number of free colloids in the suspension also progressively declines. As an immobile cluster to gel transition is traversed, structural heterogeneity abruptly decreases. Simultaneously, the mean single-particle dynamics saturates at a localization length on the order of the short-range attractive potential range. Both immobile cluster and gel regimes show dynamical heterogeneity. Non-Gaussian distributions of single particle displacements reveal enhanced populations of dynamical trajectories localized on two different length scales. Similar dependencies of number density fluctuations, free particle number, and dynamical length scales on the order of the range of short-range attraction suggests a collective structural origin of dynamic heterogeneity in colloidal gels.

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Hydroxyethyl starch 120, dextran 70 and acetated Ringer's solution: hemodilution, albumin, colloid osmotic pressure and fluid balance following replacement of blood loss in pigs.

Twenty healthy pigs weighing 12-17 kg were anesthetized and the small intestines were exteriorized into saline-moistened gauze. During a 2-h period 4% of the animals' body weight was bled through an arterial cannula in six increments and replaced immediately by the fluid tested: hydroxyethyl starch 120 (HES, Plasmafusin, Orion Corp., Mw 120,000), dextran 70 (DEX) and Ringer's acetate (RA). The amount of fluid infused for replacement of blood loss was equal to the amount of blood withdrawn in the colloid groups but fourfold in the RA group. Five non-bled pigs served as controls. After the hemodilution the laparotomy was closed and the animals received only 5% dextrose (2 ml/kg/min) during a 5-h follow-up period. The synthetic colloids caused a more effective dilution of hemoglobin and albumin than did RA. The colloid osmotic pressure (COP) was well maintained by the plasma substitutes but decreased in the RA group to 64% of the initial values. A stable urinary output and no edema formation was found in the HES and DEX groups. The RA animals were unable to excrete the excess crystalloid, which resulted in a strikingly positive fluid balance persisting throughout the study. Thus, the synthetic colloids were superior to RA in expansion of the plasma volume, maintenance of the COP and prevention of fluid accumulation. The effect of the two colloids was similar except that COP was slightly better maintained during the follow-up period in animals which received HES 120.

Animals↗

Comparison of outcome in patients with cirrhosis and ascites following treatment with albumin or a synthetic colloid: a randomised controlled pilot trail.

BACKGROUND: The question of which colloid (albumin or synthetic colloids) used for plasma expansion following paracentesis or other complications requiring fluid loading in patients with cirrhosis remains controversial. AIMS: To compare outcome and hospital-related cost in patients with cirrhosis treated with 20% human albumin with those treated with a synthetic colloid (3.5% polygeline). METHODS: The primary end point was occurrence of a first liver-related complication. RESULTS: When the trial was prematurely discontinued because of safety concerns about bovine-derived products, 30 patients were assigned to receive albumin and 38 were assigned to receive a synthetic colloid. Sixty-three patients were included for ascites removal by paracentesis and five patients for ascites removal by paracentesis and renal impairment. The median time to first liver-related complication was not significantly longer in the albumin group (20 vs. 7 days). However, the total number of liver-related complications adjusted to a 100-day period was significantly lower in the albumin group. The median hospital cost for a 30-day period was significantly lower in the albumin group (1915 euros vs. 4612 euros). CONCLUSIONS: In patients with cirrhosis and ascites, human albumin appears to be more effective in preventing liver-related complications than synthetic colloid. This may be associated with decreased hospital costs.

Adult↗

Microgravity effects on thermodynamic and kinetic properties of colloidal dispersions.

Microgravity experiments on the physicochemical properties of colloidal dispersions reported hitherto have been reviewed. In microgravity, reliability and reproducibility of experimental data have been improved significantly by the elimination of convection in the suspension. Vanishment of the excess downward diffusion in microgravity has also produced a significant effect on properties of colloidal suspensions. For example, colloidal crystallization rates decreased in microgravity, whereas colloidal alloy crystallization rates increased. These results demonstrate the important role of the segregation effect in normal gravity. Colloidal silica formation reactions are retarded in microgravity, a phenomenon that is correlated deeply with disappearance of the downward diffusion of heavy products.

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Two-dimensional nematic colloidal crystals self-assembled by topological defects.

The ability to generate regular spatial arrangements of particles is an important technological and fundamental aspect of colloidal science. We showed that colloidal particles confined to a few-micrometer-thick layer of a nematic liquid crystal form two-dimensional crystal structures that are bound by topological defects. Two basic crystalline structures were observed, depending on the ordering of the liquid crystal around the particle. Colloids inducing quadrupolar order crystallize into weakly bound two-dimensional ordered structure, where the particle interaction is mediated by the sharing of localized topological defects. Colloids inducing dipolar order are strongly bound into antiferroelectric-like two-dimensional crystallites of dipolar colloidal chains. Self-assembly by topological defects could be applied to other systems with similar symmetry.

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Effects of the rate of evaporation and film thickness on nonuniform drying of film-forming concentrated colloidal suspensions.

In this paper, we report on nonuniform distribution of film-forming waterborne colloidal suspensions above the critical concentration phi(c) of the colloidal glass transition during drying. We found that colloidal suspension films dry nonuniformly when the initial rate of evaporation E and/or the initial thickness l(0) are high. We found that a Peclet number Pe, defined as Pe = El(0)/D, where D is the diffusion coefficient of the colloids in the diluted suspensions, does not predict uniformity of drying of the concentrated suspensions, contrary to the reported work on drying of diluted suspensions. Since the colloidal particles are crowded and their diffusive motion is restricted in concentrated suspensions, we assumed that above phi(c) water is transported to the drying surface by hydrodynamic flow along the osmotic pressure gradient. The permeability of water through channels between deforming particles is estimated by adapting the theory of foam drainage. We defined a new Peclet number Pe' by substituting the transport coefficient of flow (defined as the permeability divided by the viscosity, multiplied by the osmotic pressure gradient) for the diffusion coefficient. This extended Peclet number predicted the nonuniform drying with a criterion of Pe' > 1. These results indicate that the mechanism of water transport to the drying surface in concentrated suspensions is water permeation by osmotic pressure, which is faster than mutual diffusion between water and particles --that has been observed in diluted suspensions and discussed by Routh and Russel. The theory fits well the experimental drying curves for various thicknesses and rates of evaporation. The particle distribution in the drying films is also estimated and it is indicated that the latex distribution is nonuniform when Pe' > 1.

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Quantitative prediction of crystal-nucleation rates for spherical colloids: a computational approach.

This review discusses the recent progress that has been made in the application of computer simulations to study crystal nucleation in colloidal systems. We discuss the concept and the numerical methods that allow for a quantitative prediction of crystal-nucleation rates. The computed nucleation rates are predicted from first principles and can be directly compared with experiments. These techniques have been applied to study crystal nucleation in hard-sphere colloids, polydisperse hard-sphere colloids, weakly charged or slightly soft colloids, and hard-sphere colloids that are confined between two-plane hard walls.

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Preembedding colloidal gold immunostaining of hypothalamic neurons: light and electron microscopic localization of beta-endorphin-immunoreactive perikarya.

A preembedding immunogold staining (IGS) procedure was developed to identify beta-endorphin/adrenocorticotropic hormone immunoreactive neurons at the light and electron microscopic levels. Colchicine-treated rats were perfused with Nakane's periodate-lysine-paraformaldehyde fixative. Vibratome sections were incubated in primary antisera followed by goat anti-rabbit immunoglobulin G coupled to 16 nm colloidal gold, and, in some cases, rabbit immunoglobulin G coupled to gold. The appearance to pink to light red perikarya, corresponding to colloidal gold deposition at antigenic sites, was monitored under the light microscope. Positive cell bodies in the arcuate region sometimes extended lateral to the nucleus. Only proximal portions of neuronal processes were stained. At the ultrastructural level, colloidal gold labeled the periphery of 90-110 nm dense neurosecretory granules in the perikaryal cytoplasm and a few proximal axons. Clusters of gold particles, appearing free in the neuroplasm, actually labeled secretory granules in adjacent thin sections. Granules associated with the Golgi apparatus were not stained. Colloidal gold labeling of mature beta-endorphin granules, but not progranules, in rat hypothalamic neurons was confirmed using the peroxidase-antiperoxidase technique. The results correlate well with data on the intracellular processing of pro-opiomelanocortin in pituitary cells and prepropressophysin in the paraventricular nucleus. These data demonstrate the first application of the preembedding colloidal gold staining method for the identification of intracellular antigens within the central nervous system. The IGS method provides a definitive marker for single or double labeling of nervous tissue at both the light and electron microscopic levels.

Animals↗

Cationic colloidal gold--a new probe for the detection of anionic cell surface sites by electron microscopy.

Particles of colloidal gold were coated with poly-L-lysine to prepare cationic colloidal gold. Monodispersed colloidal gold with a particle diameter of 5, 8, or 15 nm and poly-L-lysine with a molecular weight of 350,000 or 1500-8000 were used. The resulting complexes were used to label red blood cell membranes. The labeling was sensitive to neuraminidase treatment or acid hydrolysis, demonstrating that cationic colloidal gold binds preferentially to anionic cell surface constituents. Cationic colloidal gold can be used at physiological pH values and ionic strength, as well as at low pH values, making it a flexible probe for detection of anionic cellular components.

Erythrocyte Membrane↗

Colloid cysts: experience with the management of 84 cases since the introduction of computed tomography.

A retrospective review of colloid cysts diagnosed from 1974 to 1986 emphasizes the presenting symptoms of these lesions, their surgical management, and the contribution of modern imaging techniques to their diagnosis and therapy. In this 12-year period, 84 patients (45 men and 39 women) had a colloid cyst diagnosed. The patients' mean age was 46 years (range, 7-82 years). Surgery was performed in 55 patients, 7 of whom had undergone prior surgery elsewhere. The surgical approaches used were transfrontal-transventricular, transcallosal, computer-assisted stereotactic aspiration and resection by stereotactic craniotomy, and shunting of cerebrospinal fluid without removal of the lesion. There was no operative mortality, but complications occurred in 15 patients (27%). Preoperative imaging showed hydrocephalus in 93% of the patients: severe in 43%, moderate in 36%, and mild in 14%. In the surgically treated group, the most common presenting symptoms were headache, change in mental status, ataxia, nausea and vomiting, visual disturbance, emotional lability/inappropriate affect, depersonalization, and hypersomnolence. Twenty-four patients for whom surgery was not recommended are being followed up closely. Most of these patients had normal ventricles. The symptoms in this group included headache, anxiety/nervousness, ataxia, memory impairment, visual disturbance, and seizures. Five autopsy cases of patients with colloid cysts were available during this period and were reviewed. Direct removal of colloid cysts can be accomplished with low morbidity and mortality, avoiding the frequent revisions and complications related to shunt procedures. There is a subgroup of colloid cysts that can be operated upon electively or followed up closely with serial imaging studies.

Adolescent↗

Interobserver variability in thyroid fine-needle aspiration interpretation of lesions showing predominantly colloid and follicular groups.

We studied interobserver variability (IV) in the assessment of thyroid fine-needle aspiration (FNA). We limited our cases to those showing predominantly colloid and follicular cell groups. Twenty cases of thyroid FNA diagnosed by 1 experienced cytopathologist were reviewed by 4 other cytopathologists who made their own diagnoses while unaware of the original diagnoses. Two cytopathologists then assessed the cytologic features of the 20 cases. IV was calculated for noncollapsed and collapsed diagnoses. Diagnoses and observer agreement were compared with cytologic features. There was little correlation among observers regarding the diagnosis of follicular "lesion" vs "neoplasm." IV was somewhat poor before data were collapsed to treatment recommendations (kappa = 0.35) but was relatively good when data were collapsed (kappa = 0.65). Cellularity, cyst change, and amount of colloid correlated with treatment recommendations; no specific features correlated with poor performance. Thyroid FNA shows good interobserver agreement in the diagnoses of lesions showing predominantly colloid or follicular cells (when collapsed). We speculate that IV is poor in some cases owing to difficulty assessing thin colloid, some lack of agreement regarding criteria for adequacy, and a possible "gray zone" that might exist with lesions showing colloid and abundant follicular cells.

Biopsy, Fine-Needle↗

Third ventricle colloid cysts: a consecutive 12-year series.

A continuous follow-up review of colloid cysts including aspects of natural history and evaluation of treatment options is necessary to optimize individual treatment. Thirty-seven consecutive patients with colloid cyst of the third ventricle seen at Karolinska Hospital between 1984 and 1995 were reviewed. Five patients were admitted in a comatose state, and two died despite emergency ventriculostomy. Three had recurrent cysts following previous aspiration procedure. During the study period, patients underwent a total of 10 ventriculostomies, 10 aspirations, 26 microsurgical operations, and two shunt operations. Twenty-four of 26 microsurgical operations were transcallosal and two were transcortical. Twenty-four operations (22 transcallosal and two transfrontal approaches) without permanent morbidity were performed by four surgeons. Transient memory deficit from forniceal traction was noted in 26%. The remaining two transcallosal operations, which led to permanent morbidity or mortality, were performed by two different surgeons. Aspiration of cysts performed by four different surgeons carried a 40% risk of transient memory deficit (10% permanent) and an 80% recurrence rate. One patient was found to be cured on radiological studies obtained at the 5-year follow-up review. Seven cysts were followed by means of radiological studies with no treatment for 6 to 37 months. Five of these cysts grew, indicating that younger patients with colloid cysts will probably need surgical treatment. The main causes of unfavorable results were: 1) failure to investigate symptoms that proved fatal; 2) subtotal resection; and 3) surgical complications. Transcallosal microsurgery produced excellent results when performed by experienced surgeons. A colloid cyst of the foramen of Monro is a disease that should be detected before permanent neurological damage has occurred. Permanent morbidity or mortality should not be accepted in modern series of third ventricle colloid cysts.

Adolescent↗

Uptake of cyclosporine A loaded colloidal drug carriers by mouse peritoneal macrophages in vitro.

AIM: To investigate the uptake of cyclosporine A loaded colloidal drug carriers by mouse peritoneal macrophage (MPM) in vitro. METHODS: The [3H]cyclosporine A loaded colloidal particles: polylactic acid nanospheres, polylactic acid nanocapsules, and microemulsions were prepared. The [3H]cyclosporine A loaded colloidal particles were incubated with MPM for 30 min at 37 degrees C, then the cells were separated from the colloidal particles and the radioactivity was measured by a liquid scintillation counter. RESULTS: In comparison to the cyclosporine A solution, the binding to polylactic acid nanospheres produced a 20-fold increase in the uptake of cyclosporine A by MPM in 30 min incubation, whereas some obvious decrease in the uptake of cyclosporine A by MPM was observed in the binding of cyclosporine A with polylactic acid nanocapsules or microemulsions. The surfactant coating and plasma protein adsorption were found to have marked effects on the uptake of cyclosporine A loaded nanospheres by MPM. CONCLUSION: Our present study indicated that colloidal drug carriers might affect the targeting of cyclosporine A to mononuclear phagocyte system.

Animals↗