Colloidal species

FIG. 1 The mean force potential acting between colloidal species, /3fV (r), in adsorbed colloidal dispersion. In parts (a) and (b) the matrix density is taken as negligibly small, = 10 and = 0.193, respectively. In both parts, the evolution of the mean force potential with solvent density is shown p = 0.2, 0.3 and 0.4 (solid, dashed, and dotted lines, respectively). In part (c) the evolution of the PMF on matrix density is presented. The solvent density is held constant, p =0.3 the matrix density is Pmcr = 0.193, 0.386, and 0.772 (dotted, dashed, and solid lines, respectively). The diameter of the matrix species is = 7.055. The density of colloids is Pcg] = 10 , with Uc = 5, in all the cases in question. 

The presence of colloidal species has been a subject of some debate. Neiman Sarma (1980) found with their material that crystallinity did not develop for at least the first two hours of setting. However, Abdelrazig et al. (1984) and Abdelrazig, Sharp El-Jazairi (1988, 1989) reported that... 

Another more specific characteristic can allowed the differentiation between homogeneous and colloidal catalysts. In Pd-catalysed allylic alkylation rac-3-acetoxy-1,3-diphenyl-l-propene with dimethyl malonate, homogeneous and colloid species reacts at different rates with both substrate enantiomers, obtaining in the case of Pd colloids an excellent kinetic resolution (see Section 4) [44]. 

Removal to sediments. Removal of surface-reactive trace elements from the oceans readily occurs by adsorption onto settling particles, and this process is most pronounced in the typically high-energy, particle-rich estuarine environment. Particles are supplied by rivers, augmented by additions of organic material generated within the estuary. Also, floes are created in estuaries from such components as humic acids and Fe. The interaction between dissolved and colloidal species is enhanced by the continuous resuspension of sediments in... 

Honeyman and Santschi 1989). Therefore, flocculation of colloids to form settling particles in estuaries is an important mechanism for trace element removal (Sholkovitz 1977). This is particularly true of Fe, which is a ubiquitous colloidal species and is removed at low salinities. Additional removal may occur by adsorption onto floes, as demonstrated by mixing of organic-rich waters with seawater in the laboratory (Sholkovitz 1977). 

Copper may exist in particulate, colloidal, and dissolved forms in seawater. In the absence of organic ligands, or particulate and colloidal species, carbonate and hydroxide complexes account for more than 98% of the inorganic copper in seawater [285,286]. The Cu2+ concentration can be calculated if pH, ionic strength, and the necessary stability constants are known [215,265-267]. In most natural systems, the presence of organic materials and sorptive surfaces... 

Trace element samples were acidified to 2% (v/v) using Optima HNO3 prior to melting for analyses. Snow samples were not filtered, so the results represent dissolved and colloidal species as well as acid soluble concentrations dissolved from particulates. Trace elements added through acidification were negligible. 

Colloidal and particulate metal species also contribute to maintaining low free metal ion concentrations. The behaviour of colloidal species of similar size may however be very different, depending on their chemical speciation and on their structure. Metals bound to HA of large size may exchange with the... 

Explain why the occurrence of colloidal species is important in the analysis of soil for other species. 

In order to utilise our colloids as near hard spheres in terms of the thermodynamics we need to account for the presence of the medium and the species it contains. If the ions and molecules intervening between a pair of colloidal particles are small relative to the colloidal species we can treat the medium as a continuum. The role of the molecules and ions can be allowed for by the use of pair potentials between particles. These can be determined so as to include the role of the solution species as an energy of interaction with distance. The limit of the medium forms the boundary of the system and so determines its volume. We can consider the thermodynamic properties of the colloidal system as those in excess of the solvent. The pressure exerted by the colloidal species is now that in excess of the solvent, and is the osmotic pressure II of the colloid. These ideas form the basis of pseudo one-component thermodynamics. This allows us to calculate an elastic rheological property. Let us consider some important thermodynamic quantities for the system. We may apply the first law of thermodynamics to the system. The work done in an osmotic pressure and volume experiment on the colloidal system is related to the excess heat adsorbed d Q and the internal energy change d E ... 

As trivalent americium has a smaller ionic potential than the ions of plutonium it hydrolyses to a much lesser extent than the various plutonium ions. However, like Pu3+, hydrolytic reactions and complex formation are still an important feature of the aqueous chemistry of Am3+. Starik and Ginzberg (25) have shown that Am(III) exists in its ionic form from pH 1.0 to pH 4.5 but above pH 4.5 hydrolysis commences and at pH 7.0 colloidal species are formed. The hydrolytic behaviour of Cm(III) resembles that of Am(III). 

In the presence of colloidal solutions in contact with a liquid junction, anomalous liquid-junction potentials are often measured. This suspension or Palmarm effect [14] has not yet been satisfactorily explained. It is probably a Donnan-type potential with the electrically-charged colloidal species acting as indiffusible ions (cf. section 5.1.3). 

For ultrafiltration, the macromolecular solutes and colloidal species usually have insignificant osmotic pressures. In this case, the concentration at the membrane surface (C ) can rise to the point of incipient gel precipitation, forming a dynamic secondary membrane on top of the primary structure (Figure 7). This secondary membrane can offer the major resistance to flow. 

Ninety years ago, Paneth observed for the first time the occurrence of colloidal species for low-solubility radionuclides and coined the term... 

The potentially important role of colloidal species in the geochemical behaviour of the polyvalent actinides has nevertheless been stated by various authors (e.g., Kim 1991 Kersting et al. 1999). The present paper discusses the role of colloids on the release of radionuclides from a nuclear waste repositoiy with regard to the processes leading to (1) colloid generation and stability (2) radionuclide interaction with aquatic colloids and (3) colloid-borne radionuclide migration. 

Fig. 3, Evolution of Am(HI), Eu(III) and U concentrations with time in spent fuel pellet leaching experiments (leachate 5 mol/kg NaCl solution anaerobic conditions) radionuclides found in ultrafiltered samples (uf filter pore size 1.8 nm) arc considered as truly dissolved radionuclide concentrations found in filtered samples (f filter pore size 450 nm) are attributed to truly dissolved + colloidal species the grey shaded area marks the fraction of colloidal radioelement species in solution the black arrow indicates the pH increase in solution during the leaching experiment (Geckeis et al. 1998).

The epitaxy was maintained for CdS thicknesses up to 100 nm, after which the deposit became polycrystalhne. This transition coincided with the visual formation of CdS in the solution, which resulted in a switch of the mechanism from an ion-by-ion growth, necessary to obtain epitaxy, to one involving colloidal species. Since, in principle, conditions can be chosen so that only an ion-by-ion growth occurs, it can be expected that much thicker epitaxial films are obtainable from CD on suitable substrates. 

Most food products and food preparations are colloids. They are typically multicomponent and multiphase systems consisting of colloidal species of different kinds, shapes, and sizes and different phases. Ice cream, for example, is a combination of emulsions, foams, particles, and gels since it consists of a frozen aqueous phase containing fat droplets, ice crystals, and very small air pockets (microvoids). Salad dressing, special sauce, and the like are complicated emulsions and may contain small surfactant clusters known as micelles (Chapter 8). The dimensions of the particles in these entities usually cover a rather broad spectrum, ranging from nanometers (typical micellar units) to micrometers (emulsion droplets) or millimeters (foams). Food products may also contain macromolecules (such as proteins) and gels formed from other food particles aggregated by adsorbed protein molecules. The texture (how a food feels to touch or in the mouth) depends on the structure of the food. 

When one thinks of electrokinetic phenomena in the context of a first-level course on colloid and surface chemistry, the first thought that probably comes to mind is the use of such phenomena to measure zeta potentials and charges of colloidal species. But, as we have already seen in Chapter 1 and as we see later in this chapter, electrokinetic phenomena play a significant role in many other applications. We take a look at one such application here and see why the topics we consider in this chapter and in others are important in that context. 

Zone electrophoresis is mostly used for biological applications. Peptide separation and the measurement of protein fractions from blood serum (proteinogram of albumin and o-, (3- and 7-globulins) are among the better known applications. This TLC for biochemists is useful for the separation of polysaccharides, nucleic acids (for DNA sequencing), proteins and other colloidal species. 

Although the general effect of the addition of bicarbonate was to increase the size of the colloidal species, Lindenbaum and Westfall obtained the opposite effect with citrate addition over the pH range 4-11, as measured by the percent of plutonium (IV) that was ultrafilterable (22). However, their plutonium concentrations were 2 X 10 5Af, and the solutions probably contained true colloids, rather than pseudocolloids, if one accepts Davydovs analysis. Lindenbaum and Westfall concluded that the mechanism of the citrate action was the complexation of plutonium, thereby preventing the formation of hydrolytic polymers. It should be noted, however, that even with a citrate-plutonium molar ratio of 1800 (3.4 X 10 4Af citrate), about 10% of the plutonium still could not pass through the ultrafilter for solutions aged up to four days (22). 

Colloids are always present in natural waters containing the transuranium elements. (Colloids are defined as particles with sizes ranging from 1 to 450 nm. These particles form stable suspensions in natural waters.) Colloids of the transuranium elements can be formed by hydrolysis of transuranium ions, or by the sorption of transuranium elements on the naturally occurring colloids. The naturally occurring colloids include such species as metal hydroxides, silicate polymers, organics (such as humates), and the like. The mobility of the transuranium elements in an aquifer is determined largely by the mobility of its pseudocolloids, that is, those colloidal species formed by the adsorption of the transuranium ions upon the naturally occurring colloids. 

Such a mechanism would require modifications to some concepts of solubility controlled release. This report concerns a preliminary effort to determine some of the sorption properties of colloidal species representative of those formed during waste form/waste package interaction tests. Sorption of actinides and Tc on those colloids as a function of pH at 25°C was studied. 

To verify whether or not colloid species were present in solution, phase separation was thoroughly examined with different ultrafilters. Tables HI and IV show results from various ultrafiltrations for the Am(III) and Pu(VI) solutions, respectively. The solution at solubility equilibrium was first filtered with the Millex-22 (0.22 u) filter and further passed through different ultrafilters of nearly the same pore size ( 2nm). Table ID demonstrates that the americium concentration in filtrates... 

Simple colloidal dispersions are two-phase systems, comprising a dispersed phase of small particles, droplets or bubbles, and a dispersion medium (or dispersing phase) surrounding them. Although the classical definition of colloidal species (droplets, bubbles, or particles) specifies sizes of between one nanometre and one micrometre, in dealing with practical applications the upper size limit is frequently extended to tens or even hundreds of micrometres. For example, the principles of colloid science can be usefully applied to emulsions whose droplets exceed the 1 tm size limit by several orders of magnitude. At the other extreme, the field of nano-... 

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