Colloid-clay systems

Visible light-induced cleavage of water has been reported for a mixed colloidal clay system consisting of a mixture of sepiolite clay-Ru02-Ru(bpy)3 + colloid (for O2 production) and Al,cEui (OH)3-Pt colloid (for H2 production) [165]. A turnover number of 20 with respect to the sensitizer, Ru(bpy)3 +, was observed. It was reported that the gas (H2 + O2) evolution displayed a damped oscillatory behavior. Photo-oxidation of water by tran5-diaqabis-(2,2 -bipyridine)ruthenium(2- -) adsorbed on the surface of hectorite clay has also been reported [166]. 

The cation exchange ability of smectites is isutilized to incorporate [Ru(bpy)3] cations. The luminescence probe studies have been carried out mainly in colloidal clay systems in which an aqueous solution of [Ru(bpy)3] is mixed with an aqueous suspension of smectites. In some cases, a species that quenches the excited state of [Ru(bpy)3] is added (80,82). The concentrations of hosts, [Ru(bpy)3], and quenchers in the suspensions have been varied in order to see the dynamics of the reactions. The photoprocesses of the [Ru(bpy)3] -smectite systems have revealed that the steady-state and time-re-solved luminescence properties of [Ru(bpy)3] + are not simple, and various interpretations of the unusual properties have been proposed (80,83,91). 

In addition to the paving grades of emulsions, a variety of products are available that are suitable for industrial uses, for paper sidings, and for use in the roofing appHcation field. Asphalts emulsified with colloidal clays are especially suitable for outdoor appHcations because of their exceUent weathering properties. Some of these products have been adapted for use with roofing systems, either with bmsh or spray appHcation. 

Polyelectrolytes provide excellent stabilisation of colloidal dispersions when attached to particle surfaces as there is both a steric and electrostatic contribution, i.e. the particles are electrosterically stabilised. In addition the origin of the electrostatic interactions is displaced away from the particle surface and the origin of the van der Waals attraction, reinforcing the stability. Kaolinite stabilised by poly(acrylic acid) is a combination that would be typical of a paper-coating clay system. Acrylic acid or methacrylic acid is often copolymerised into the latex particles used in cement sytems giving particles which swell considerably in water. Figure 3.23 illustrates a viscosity curve for a copoly(styrene-... 

In various kinds of industrial production, materials need to be treated with charged colloidal particles. In such systems, the value of the zeta-potential analyses are needed to control production. For example, in paper, adhesive, and synthetic plastics, colloidal clay can be used as filler. In oil drilling, clay colloidal suspensions are used. The zeta potential is controlled so as to avoid clogging the pumping process in the oil well. It has been found that, for instance, the viscosity of a clay suspension shows a minimum when the zeta potential is changed (with the help of pH from 1 to 7) from 15 to 35 mV. Similar observations have been reported in coal slurry viscosity. The viscosity was controlled by the zeta potential. 

Discussion. The colloidal clay and humus soil fractions are negatively charged and therefore attract and adsorb positive ions (cations) on to exchange sites. These may be the so-called basic cations defined above, or the acidic cations H+ and Al +. These cations are not soluble in water when in the adsorbed state, but can exchange with H+ which is present in the acidic vicinity of the plant root system. They are now in solution and able to be absorbed into the plant. The extent to which the exchange sites are saturated with cations, together with the ratios of the cations to each other, indicates the nutrient supplying power of the soil. 

Simmons, C. F. 1939. The effect of C02 pressure upon the equilibrium of the system hydrology colloidal clay-H20-CaC03. J. Am. Soc. Agron. 31 638-648. 

So far we have been concerned mainly with dispersions of solids in which all three dimensions of the particles of the dispersed phase are in the colloid size range. An important group of colloidal phenomena involve systems having only one dimension in this range (see Figure 1.1). This chapter deals with thin liquid films, present in isolation, as the basic components of liquid foams or as the film between two emulsion droplets in contact. Other examples of such colloidal systems are those in which the solid particles are thin plates, as in many clay systems, and solid foams. These will, however, be omitted from the present discussion. 

Bituminous emulsions are produced by emulsifying an asphalt or coal tar in water using suitable combinations of surfactants along with colloidal clays. After application as coatings, with subsequent evaporation of water, the emulsified bituminous particles coalesce to form a continuous film. Being waterbased, such systems have the potential to comply with VOC regulations. 

Electroultrafiltration (EUF) combines forced-flow electrophoresis (see Electroseparations,electrophoresis) with ultrafiltration to control or eliminate the gel-polarization layer (45—47). Suspended colloidal particles have electrophoretic mobilities measured by a zeta potential (see Colloids Elotation). Most naturally occurring suspensoids (eg, clay, PVC latex, and biological systems), emulsions, and protein solutes are negatively charged. Placing an electric field across an ultrafiltration membrane faciUtates transport of retained species away from the membrane surface. Thus, the retention of partially rejected solutes can be dramatically improved (see Electrodialysis). 

The invasion of particles can be eliminated either by using solids-free systems or by formation of a competent filter cake on the rock surface. If the components forming the filter cake are correctly chosen and blended, they will form a very effective downhole filter element. This ensures that colloidal sized clays or polymeric materials are retained within the filter cake and do not enter the formation. Further protection is provided by ensuring that a thin filter cake is formed due to low dynamic and static filtrate losses. Thus, the cake may be easily removed when the well is brought into production. Additionally, the filter cake can be soluble in acid or oil. 

Natural colloid particles in aqueous systems, such as clay particles, silica, etc. may serve as carriers of ionic species that are being sorbed on the particulates (pseudocolloids). It seems evident that the formation and transport properties of plutonium pseudocolloids can not yet be described in quantitative terms or be well predicted. This is an important area for further studies, since the pseudocolloidal transport might be the dominating plutonium migration mechanism in many environmental waters. 

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