Presence Of Colloidal Particles Research Articles

Modeling of simultaneous exchange of colloids and sorbing contaminants between streams and streambeds.

Contaminant transport in streams can be significantly modified by both stream-subsurface exchange and the presence of colloidal particles, but the interaction of these effects is notwell understood. Exchange with the hyporheic zone exposes contaminants to surface-chemical reactions with streambed sediments, while colloidal particles have a large reactive surface area that allows them to carry pollutants that would otherwise be transported primarily as dissolved species. A new theoretical model is developed to predict the role of colloids in mediating advective contaminant exchange between streams and streambeds. Bedform-induced pumping theory is applied to model physical transport, and colloid filtration and reversible contaminant sorption are used to calculate the local distributions of colloids and contaminants within the streambed. Residence time functions of both colloids and contaminants in the bed are then used to link contaminant concentrations in the pore water and streamwater. Model simulations indicate that, under conditions of low colloid filtration and strong contaminant sorption to colloids, contaminants are mobilized by colloids and there is less retention of contaminants in the streambed. This is the case of "colloid-facilitated contaminant transport" commonly considered in groundwater transport. On the other hand, when colloid filtration is high and contaminants still sorb strongly to colloids, contaminant mobility decreases and there is greater contaminant retention in the streambed. We term this case "colloid-impeded contaminant transport". Thus, we find that a variety of contaminanttransport behavior can occur depending on the concentration and mobility of suspended particles in the system and the relative affinity of contaminants for colloids and other solid phases.

Foaming mechanisms in surfactant free particle suspensions

AbstractPhysical mechanisms for foam formation in the presence of colloidal particles are poorly understood. This study is aimed at enhancing the basic understanding of the mechanisms that produce foaming in a three‐phase system, containing solid, liquid, and gas, and identifying key parameters that aggravate foaming in the absence of surface‐active agents. The foaming ability of a system (“foaminess”) was investigated by aerating a suspension of nanosized hydrophilic silica particles. We observed that the foaminess was directly proportional to particle concentration and inversely proportional to particle size. However, even a small degree of bidispersity in particle size drastically reduced foaminess. To explain these observations, the drainage of a single foam lamella containing nanosized particles was studied using a novel capillary force balance apparatus, which incorporates the microinterferometric technique. The single foam lamella thins in a stepwise manner, as the colloidal particles self‐organize into a layered structure between the gas bubbles. This layer provides a barrier against coalescence of bubbles, thereby stabilizing the foam lamella. The number of stepwise film thickness transitions increased with particle concentration and decreased with the increase in the particle size, consistent with the results from foaming experiments. Particle bimodal distribution greatly decreases the foam‐lamella stability, consistent with the theoretical prediction.

Film Stratification in the Presence of Colloidal Particles

This is probably the first study about the drainage of curved liquid films in the presence of colloidal particles. The systems did not contain any surfactant. In the presence of monodispersed colloidal particles, thinning occurs in a stepwise manner (stratification). It has been shown that the size of the film is an important parameter in the stepwise thinning process. This investigation found a critical film size below which at least one layer of particles stays in the film at equilibrium; a “spot” (a thinner section of the film), even if formed, does not expand and is in equilibrium with the film. The area of the spot expands linearly with time. The rate of spot-area expansion increases linearly with the film perimeter and can be increased or decreased merely by changing the film size. The stepwise film thinning and the effects of film size and particle concentration on film stability are discussed on the basis of the diffusive-osmotic mechanism.

Computer simulation of multiblock-copolymers in presence of colloidal particles

Computer simulation of multiblock-copolymers in presence of colloidal particles

Colloidal and polymeric nature of fossil amber

Fossilized ambers from several geographical origins, copal from the Kauri pine tree of New Zealand and modern white spruce tree resin have been analyzed by the following analytical techniques: dynamic light scattering, optical and scanning electron microscopy, gel permeation chromatography, IR spectroscopy, rheology and viscometry. The results dispute the prevailing view that amber is largely an insoluble, continuously crosslinked, integral polymer network. Rather, the data indicate that a large portion of amber consists of insoluble though solvent-swellable colloidal particles, ranging from submicron to multimicron in size, which can be dispersed in organic solvents such as N, N-dimethyl-formamide. The presence of colloidal particles in fossil amber, copal and even fresh resin suggests a mechanism for resin exudation involving the presence or formation of discrete ‘packets’ of resin, where the surface of each packet has been modified by the interaction with oxygen and water.

Comparison of Kinetic and Hybrid-Equilibrium Models Simulating Colloid-Facilitated Contaminant Transport in Porous Media

The presence of colloidal particles in groundwater can enhance contaminant transport by reducing retardation effects and carrying them to distances further than predicted by a conventional advective/dispersive equation with normal retardation values. When colloids exist in porous media and affect contaminant migration, the system can best be simulated as a three-phase medium. Mechanisms of mass transfer from one phase to another by colloids and contaminants can be kinetic or equilibrium-based, depending on the sorption–desorption reaction rate between the aqueous and solid phases. When the rate of sorption between the water phase and the solid phase(s) is not much greater than the rate of change in contaminant concentration in the water phase, kinetic sorption models may better describe the phenomenon. In some cases of modeling one or more mass transfer processes, a useful simplification may be to introduce the local equilibrium assumption. In this study, the local equilibrium assumption for sorption processes on colloidal surfaces (hybrid equilibrium model) was compared with kinetic-based models. Sensitivity analyses were conducted to deduce the effect of major parameters on contaminant transport. The results obtained from the hybrid equilibrium model in predicting the transport of colloid-facilitated groundwater contaminant are very similar to those of the kinetic model, when the point of interest is not at contaminant and colloid source vicinities and the time of interest is sufficiently long for imposed sources.

Interference of colloidal particles in the determination of orthophosphate concentrations in soil water extracts

Accurate determination of orthophosphate (P) in soil solutions is a prerequisite for rigorously assessing soil P availability for plants as well as the potential risk of P transfer to water bodies. Although P is usually measured on extracts filtered at 0.45 μm, little is known on the potential interfences due to the presence of colloidal particles. This work was conducted to study the influence of colloidal particles on P concentration by measuring it both by the phosphoantimonylmolybdenum blue colorimetry and ion chromatography in soil water extracts filtered at 0.45, 0.2, or 0.025 μm. Results strongly suggest that direct measurements with colorimetry might lead to an overestimation of the P concentration due to the solubilization of P‐containing particles in the acid medium prevailing in the solution during colorimetric essay. In the present study, the largest overestimations were observed in soil extracts rich in iron (Fe) and aluminum (Al) that had been filtered at 0.45 μm, on the opposite this overestimation remained lower but constant for the three levels of filtration in the studied calcareous soil. Ion chromatography constantly underestimated P concentration especially when the solutions were rich in Al and Fe. This underestimation could not be related to the presence of dissolved polyvalent cations in the solution and, therefore, might be accounted for by the sorption of P at the surfaces of colloids. In order to limit these interferences, it is proposed to measure P concentration by colorimetry after a filtration of the solution through a 0.2‐ or 0.025‐um membrane, instead of a 0.45‐μm filter. In some cases for instance for soil solutions rich in calcium phosphate particles, it might be advisable to determine P concentration with ion chromatography.

Inactivation of a subtilisin in colloidal systems

The aim of the present study is to establish the relation between the inactivation of the proteolytic enzyme Savinase TM and its adsorption at different types of solid-liquid interfaces. The loss of activity has been determined both in solution and in the presence of colloidal particles, which provide a surface area for adsorption of 25% of the enzyme population. Analysis of the remaining solution at different periods of incubation of the various systems shows that the intact protein is converted into autolysic degradation products at the expense of biological activity. The different particles, however, deactivate the enzymes to a different extent. Under the experimental conditions the half-life of the enzymatic activity in solution is 3.5 hours. In the presence of particles that have hydrophobic surface properties (teflon- or polystyrene latex) the half-life is reduced to 0.7 hours. On the contrary, hydrophilic silica particles stabilize the enzyme against autolysis as compared to the inactivation in solution. Polystyrene latex particles which are chemically grafted with short poly(ethylene oxide) chains ([EO] 8) are, for steric reasons, also mild with respect to the reduction of enzymatic stability. It is thus concluded that the type of surface determines the mode in which the enzyme is adsorbed on a particle which, in turn, affects the autocatalytic rate.

X-ray diffraction investigations on the structure of some thoria gels

The structure of some samples of thoria gels has been investigated by X-ray small-angle diffraction and by radial electronic distribution analysis, performed through elaboration of wide-angle spectra. The presence of colloidal particles has been revealed, tending to spherical shape, which have nearly equal dimensions in samples which were not treated at high temperatures. Such particles are formed by a core of oxide, coated by a hydroxyl groups shell, located in the three (0,0 1 2 interstitial positions of the cubic face-centered lattice (fluoryte type), and by water molecules on a more external layer, which contributes to the formation of bonds between the different particles. The average particles dimensions, their polydispersity and the order degree in the oxide core may largely change with preparation procedures of gels and with annealing temperature.

Study of a coagulation bath as a dispersed system

1. The authors have experimentally confirmed the presence of colloidal particles in the coagulation bath.

Limitations of the Spectropolarimeter in reference to Optical Activity in Meteorites

CONFLICTING claims have arisen concerning the significance of the discovery of a small laevo-rotation in the saponified material of a carbonaceous chondrite1–6. The discovery has been questioned on two grounds—that the optical activity may be caused by terrestrial contamination and that the reported optical activity is an instrumental artefact caused by the high optical density and by the presence of colloidal particles in the extract of the meteorite. Unless the second objection, is resolved, it is doubtful whether projected tests for optical activity on extra-terrestrial bodies can have much significance.