Pseudo-colloids

The entire picture is still more confusing because of the fact that several different types of colloids are distinguished—i.e., radiocolloids, pseudo-colloids (7, 8, 28, 33), and true colloids. Radio-colloids refer to systems of radiotracers which appear to be in colloidal form although they are in concentrations well below their ionic solubility (25, 26). The term pseudo-colloid is used to describe the formation of a colloid system... 

All the previous theoretical considerations have been established assuming an ideal system without any boundary conditions. It should be pointed out however that in practice, all the studied systems, especially in SHE chemistry, have finite dimensions (time and volume). As only ideal system were considered, edge effects, pseudo-colloid formation, sorption phenomena, redox processes with impurities or surfaces, medium effects have not been taken into account. All these effects, representing the most important part from the deviation to ideality, cannot be predicted with formal thermodynamics and/or kinetics. Thus, radiochemists who intend to perform experiments at the scale of one atom must be aware that the presence of any solid phase (walls of capillary tubes, vessels, etc.) can perturb the experimental system. It is important to check that these edge effects are negligible at tracer level before performing experiments at the scale of the atom [11]. The following section describes experimental techniques used in SHE chemistry. 

Kelly J. W., Aguilar R., and Papenguth H. W. (1999) Contribution of mineral-fragment type pseudo-colloids to the mobile actinide source term of the Waste Isolation Pilot Plant (WIPP). In Actinide Speciation in High Ionic Strength Media (eds. D. T. Reed, S. B. Clark, and L. Rao). Kluwer/ Plenum, New York, pp. 227-237. 

Sol-Gel Techniques. Sol-gel powders (2,13,15,17) are produced as a suspension or sol of coUoidal particles or polymer molecules mixed with a Hquid that polymerizes to form a gel (see Colloids SoL-GELtechnology). Typically, formation of a sol is foUowed by hydrolysis, polymerization, nucleation, and growth. Drying, low temperature calciaation, and light milling are subsequently required to produce a powder. Sol-gel synthesis yields fine, reactive, pseudo-crystalline powders that can be siatered at temperatures hundreds of degrees below conventionally prepared, crystalline powders. 

Lest I leave the erroneous impression here that colloid science, in spite of the impossibility of defining it, is not a vigorous branch of research, I shall conclude by explaining that in the last few years, an entire subspeciality has sprung up around the topic of colloidal (pseudo-) crystals. These are regular arrays that are formed when a suspension (sol) of polymeric (e.g., latex) spheres around half a micrometre in diameter is allowed to settle out under gravity. The suspension can include spheres of one size only, or there may be two populations of different sizes, and the radius ratio as well as the quantity proportions of the two sizes are both controllable variables. Crystals such as AB2, AB4 and AB13 can form (Bartlett et al. 1992, Bartlett and van... 

One major question of interest is how much asphaltene will flocculate out under certain conditions. Since the system under study consist generally of a mixture of oil, aromatics, resins, and asphaltenes it may be possible to consider each of the constituents of this system as a continuous or discrete mixture (depending on the number of its components) interacting with each other as pseudo-pure-components. The theory of continuous mixtures (24), and the statistical mechanical theory of monomer/polymer solutions, and the theory of colloidal aggregations and solutions are utilized in our laboratories to analyze and predict the phase behavior and other properties of this system. 

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 ... 

A number of reagents containing oxide components are used in zeolite manufacture [19]. Silica is provided by addihon of sodium or other alkali silicate solutions, precipitated, colloidal, or fumed silica, or tetraalkylorthosihcate (alkyl = methyl, ethyl) and certain mineral silicates such as clays and kaolin. Alumina is provided as sodium aluminate, aluminum sulfate soluhon, hydrous aluminum oxides such as pseudo boehmite, aluminum nitrate, or aluminum alkoxides. Additional alkali is added as hydroxide or as halide salts, while organic amines and/or... 

Colloidal Dispersions or Solutions (Sols) and Colloids. Colloidal solutions (or rather "pseudo solutions ), also called sols (or in case of liquids hydrosols) are heterogeneous systems consisting of a "dispersion medium (mostly a liquid) and a "dispersed or "suspended medium known as a "colloid . Colloidal particles are invisible under ordinary microscope but detectable by the ultramicroscope. Their size ranges from ca 1 x 10 7 to 1 x 5 smm. If the dispersion is a viscous, sticky, transparent liquid, it is what is generally known as a "colloidal solution . As examples of this may be cited a soln of gum-arabic in water and sol ns of NC in acetone, ethyl acetate or ether alcohol. When "solns are dialized, most of the colloidal particles do not pass thru the membrane. This is their principal distinction from "crystalloids , which are substances like Na chloride, etc. If part of the volatile liquid (dispersing medium), is evaporated the resulting tacky, jellylike substance is known as a gel. 

For colloidal semiconductor systems, Albery et al. observed good agreement between the value of the radial dispersion obtained from dynamic light scattering and the value found from application of the above kinetic analysis to flash photolysis experiments [144], It should be remembered that this disperse kinetics model can only be applied to the decay of heterogeneous species under unimolecular or pseudo-first order conditions and that for colloidal semiconductors it may only be applied to dispersions whose particle radii conform to equation (37), i.e., a log normal distribution. However, other authors [145] have recently refined the model so that assumptions about the particle size distribution may be avoided in the kinetic data analysis. 

A similar model has been advanced by Brown and Rajapakse [186], again based on the work done by Albery et al. [84], They also suggest that electron-hole recombination can be effectively slowed by the presence of interband hole traps, some of which can be attributed to sulphur vacancies, in agreement with conclusions made by Ramsden and Gratzel [169]. Work must be done to refine this model and to identify the nature of the interband hole traps. Nonetheless, the pseudo-first order rate constant for electron loss to them is 0.168 0.052 s 1 a value which has far-reaching implications for the design of photocatalytic systems employing colloidal CdS in photoreduction reactions. 

Hydrazone cyclization and hydroalkylation [138-140] are rare examples of reactions conducted on a preparative scale, since the products were isolated in milligram amounts and not just identified in solution. As already mentioned in Section 6.2.5, photocorrosion of the semiconductor photocatalyst often prevents its use in preparative chemistry. This is very true also for colloidal semiconductors although the pseudo-homogeneous nature of their solutions allows one to conduct classical mechanistic investigations, until now they were too labile to be used in preparative chemistry [107, 141, 142]. In contrast to the above-mentioned reactions, in recent years we have isolated novel compounds on a gram-scale employing photostable zinc and cadmium sulfide powders as photocatalysts [97, 107, 143-145]. During this work we found also a new reaction type which was classified as semiconductor photocatalysis type B [45]. In contrast to type A reactions, where at least one oxidized and one reduced product is formed, type B reactions afford only one unique product, i.e., the semiconductor catalyzes a photoaddition reaction (see below). 

Cross, M.M. Rheology of non-Newtonian fluids a new flow equation for pseudo-plastic systems. J. Colloid Sci. 1965, 20, 417 37. 

Figure 1 Four examples of cluster structures (a) Na2[Pt3(CO)6] ( = 2,3,5) inset TEM picture of Pt nanoparticles from [Pt3(CO)e]fo precursor (b) unit cell of Pt2Ru4(CO)is cluster. Coordination distances in a cluster are i pt Ru = 2.70 A, = 2.66 A, and Rru.ru = 2.16 A (c) unit cell of Ru4Se2(CO)n cluster. Coordination distances in a cluster are Rru-ru = 2.79 2.77 A, i Ru-se = 2.56 2.60 A. Inset TEM picture of Ru Se, colloidal nanoparticles from Ru3(CO)i2 and Se, via the Ru4Se2(CO)n chemical precursor (the stabilizer was octade-canthiol) (d) Chevrel cluster phase MoeScg. Coordination distances in a cluster are Rmo-mo = 2.68 2.83 a, and RMo-se = 2.598 A. For a pseudo-ternary compound Mo4Ru2Ses. Coordination distances in a cluster are 7 Me-Me = 2.659, 2.71 A i Me-se = 2.623 A.

With metal ions, some organic reagents form coloured compounds that are sparingly soluble both in water and in organic solvents. They are either polynuclear complexes, such as formed by phenylfluoron with Sn(IV) or with Ge(IV), or adsorption-type compounds such as those formed by titanium yellow with Mg. In such cases, the absorbance is measured for suspensions of coloured pseudo-solutions stabilized with protective colloids [e.g., gum arabic, gelatine, poly(vinyl alcohol)]. 

Aluminon (ammonium aurintricarboxylate) (formula 6.1) was formerly an important reagent for aluminium (e 2-10 ) [4,31,43]. It forms a sparingly soluble red chelate with A1 ions in acetate buffer. Protective colloids (e.g., gelatine) are necessary to stabilize the pseudo-solution. 

The addition of an aqueous solution of sodium diethyidithiocarbamate (Na-DDTC, cupral) (formula, 4.40) to a solution (at pH 4-11) containing small amounts of copper(II) ions produces a yellow-brown colour owing to a colloidal suspension of the sparingly-soluble copper 1 2 chelate with DDTC. The reagent co-ordinates with copper through the two sulphur atoms to form a chelate with four-membered rings, which is a rather rare configuration. Protective colloids (e.g., gum arabic) stabilize the pseudo-solution, and permit the spectrophotometric determination of copper. Cu (11) has been determined in aqueous solutions in the presence of surfactants [17],... 

The molar absorptivity of the pseudo-solution of the magnesium compound with Titan Yellow is 3.6-10 at 545 (a = 1.5). The intensity and reproducibility of the colour obtained are affected by the method of pH adjustment, the excess of Titan Yellow, the protective colloid used, the temperature of the solution, and the time of standing. Immediately after the start of the colour reaction, an increase in absorbance is noticed, but after 10-30 min the colour of the solution remains almost constant. After this it weakens progressively. Hydroxylamine is reported to stabilize the colour [12]. 

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