True colloids

Figure 18-82 illustrates the relationship between solids concentration, iuterparticle cohesiveuess, and the type of sedimentation that may exist. Totally discrete particles include many mineral particles (usually greater in diameter than 20 Im), salt crystals, and similar substances that have httle tendency to cohere. Floccnleut particles generally will include those smaller than 20 [Lm (unless present in a dispersed state owing to surface charges), metal hydroxides, many chemical precipitates, and most organic substances other than true colloids. 

It has been established that plutonium hydrolysis products exhibit colloidal behaviour (147-151) and may adsorb onto minerals and other surfaces to form radiocolloids. However, it is difficult to determine whether a radiocolloid is a true colloid or a pseudocolloid formed by adsorption of the plutonium species onto other colloidal impurities in the solution (152). In some cases both forms may be present... 

Occasionally, the phosphate slime is difficult to settle in the lagoons because of its true colloidal nature, and the use of calcium sulfate or other electrolytes can promote coagulation, agglomeration, and settling of the particles. Usually an addition of calcium sulfate is unnecessary, because it is present in the wastewater from the sand-flotation process. Generally, it has been shown [33] that the clear effluent from the phosphate mining and beneficiation operation is not deleterious to fish life, but the occurrence of a dam break may result in adverse effects [19]. 

Table III shows the result of SANS analysis on fully polymerized PB/PS IPN s, seml-IPN s, and chemical blends by Fernandez et al. [ n.] The specific interfacial surface area was shown to increase with Increasing crosslink density, S decreasing in the order full-IPN s, semi-I IPN s, seml-II IPN s afid chemical blends, as expected from many earlier studies. Its value ranges from 20 to 200 m /gm, in the range of true colloids. This result is particularly important because interfacial surface area is closely related to toughness and impact strength.

The so-called bi-liquid foams described by Sebba and Vincent [17-20] will not be discussed since they are not true colloidal emulsions, but resemble conventional foams more closely, possessing polyhedral cells of centimetre dimensions. 

Mack (58, 59) points out that asphaltenes from different sources in the same petro-lenes give mixtures of approximately the same rheological type, but sols of the same asphaltenes in different petrolenes differ in flow behavior. Those in aromatic petrolenes show viscous behavior and presumably approach true solution. Those in paraffinic media show complex flow and are considered to be true colloidal systems. Pfeiffer and associates (91) consider that degree of peptization of asphaltene micelles determines the flow behavior. Thus, a low concentration of asphaltenes well peptized by aromatic petrolenes leads to purely viscous flow. High concentrations of asphaltenes and petrolenes of low aromatic content result in gel-type asphalts. All shades of flow behavior between these extremes are observed. 

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). 

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... 

The earliest routes for forming ceramics from sol-gel solutions involved the precipitation of metal oxide particles from solutions. These form a true colloidal suspension a sol. Upon destabilization of this sol, aggregation takes place and a rigid network is formed a gel. A gel is intermediate between a solid and a liquid. The term sol—gel has since been used by the materials science community to describe, albeit erroneously, virtually all chemical processing of ceramics from solutions (e.g., metal oxide particle precipitation or metalorganic decomposition). This discussion focuses on the gel aspects of sol—gel synthesis and not on the sol aspects, which are treated separately in this book. 

Two types of colloids are recognized in the literature. Intrinsic colloids (also called true colloids, type I colloids, precipitation colloids, or Eigencolloids ) consist of radioelements with very low solubility limits. Carrier colloids (also known as pseudocolloids, type II colloids or EremdkoIIoides ) consist of mineral or organic phases (in natural waters primarily organic complexes, silicates and oxides) to which radionuclides are sorbed. Both sparingly soluble and very soluble radionuclides can be associated with this type of colloid. In addition, radionuclides can be associated with microbial cells and be transported as biocolloids. 

As for ternary systems, a study of the effect of the order of adding constituents has shown that when the carrier colloid is added after the colloid 1, there is no effect, whatever the systems. This is related to the irreversible character of the sorption of colloid 1. When a true colloid is added after interacting the surface with the carrier colloid, if the conditions are in favour of a presorbed layer (opposite charges), an increase of sorption is observed. When the pseudocolloid interacts with the mineral surface, the charge effect is important and can lead to a strong decrease in heavy element sorption (Figures 6-7). 

Scanning electron microscopy, using an instrument with a resolution of about 350 nm, failed to resolve the individual particles, which thus appear to form a true colloid. Higher resolution using electron transmission microscopy with very thin, dipped layers showed many particles of dimensions of about 100 nm. " The colloidal interaction between mucopolysaccharides and aminoglycosides was studied by Deguchi who observed that the stability of the complexes depended on the ionic strength. 

A second class of colloids of particular importance in the general context of surface chemistry, but less familiar to most people, is that of the so-called association colloids. Association colloids consist of aggregates or units of a number (sometimes hundreds or thousands) of molecules that associate in a dynamic and thermodynamically driven process leading to a system that may be simultaneously a molecular solution and a true colloidal system. As we shall see, the formation of association colloids involving specific substances will often depend on various factors such as concentration, temperature, solvent... 

Study of the dynamics of fluid flow is concerned with the forces acting on the bodies in the fluid. In the earher chapters on soUd dispersions, emulsions, and foams, fluid dynamics was largely ignored in favor of the true colloidal interactions. In aerosols, the nature of the continuous medium makes the subject of fluid dynamics much more important to the understanding of the system, so that the following discussion will introduce a few basic relationships that can be important in the study of aerosols. 

Schwartz and co-workers [52, 63] applied a silica layer on the inner surfaces of polymeric, copper, and steel capillaries of 4-400 m length and =0.5 mm inside diameter. The authors [62] ran into difficulties in using a suspension containing micron-size particles and therefore they decided in favor of true colloidal silica solutions. They used [52] 22% silica sol in [water-2-propanol] system commercially available under the trade mark Nal-coag 1092. Such colloidal silica sols are produced for preparing adsorption and antistatic impregnated systems. They have a low viscosity and therefore can readily pass through a capillary to form a thin... 

The surfactant in grease is a truly multipurpose component. It has to function as a viscosity modifier, an emulsifier/dispersant, and a lubricant adjuvant, all in an interconnected manner. First, the surfactant, or soap, has to thicken the lubricating fluid in order to form a grease with the proper viscosity and hardness characteristics. However, a grease differs from viscous oil by having a true colloidal nature. 

The flow properties of a colloidal system are very much dependent on its microstructure, as determined by the molecular arrangement and interaction of its components. ME systems show flow typical of a Newtonian liquids, for which the shear stress is directly proportional to the shear rate. Since viscosity measurements are dynamic experiments, they will give information on dynamic properties of the ME. These will depend on the miCTOStructure, type of aggregates, or interactions within the ME, which in turn are determined by the concentration of the various components and the temperature. The dispersion of one component in another, e.g., water in oil, will generally increase the bulk viscosity in comparison to the individual components (oil and water) [58]. For at true colloidal dispersion, viscosity will increase with increasing volume fraction of dispersed phase according to the formula generated by Einstein ... 

It is assumed by many investigators that during adsorption only that part of the substance present as crystalloid is taken up. This is probably true in many cases as, for instance, in coloring with dyestuffs where the portion present as crystalloid is large and the remainder is in the form of relatively large particles. This point of view is untenable, however, in the case of true colloids where the part dissolved as crystalloid is very small. Here it must be assumed that the colloidal particles are themselves adsorbed. In specific instances this can be verified experimentally as in the case of gold and aluminimn hydroxide gels. 

For those colloidal particles in which the molecule is identical with the primary particle and in which the individual atoms of the colloidal particle are linked together by normal valences, we propose the term macromolecules. Such colloidal particles form true colloidal materials, which, in accordance to the bonding power of carbon, occur particularly in organic chemistry and in organic natural substances. Here the colloidal properties are determined by the structure and size of the molecule [15] (Fig. 2.4). 

In [158,159] the structures of semi- and full polybutadiene-PS sequential IPNs were studied and the results of determining the equivalent sphere diameter were compared to TEM studies. It was found that the dimensional characteristics obtained by the two methods are in approximate agreement. Surface areas in the range of 150-200 m g indicated true colloidal sizes for the phase domains. Correlation lengths of 35-60 A were found for IPNs, 50-100 and 160-80 A for various semi-IPNs. It was also established that these IPNs are characterized by the dual-phase continuity. 

Total internal reflection microscopy enables the measurement of colloidal forces down to weak forces of 10 fNf under conditions of free Brownian motion that may better resemble true colloidal systems compared to other methods where force distance curves are recorded via enforced movement of surfaces. However, its application is limited to transparent surfaces and repulsive interaction potentials. Magnetic tweezers allow the measurement of forces down to 10 pN. Therefore, magnetic tweezers have mainly been applied to the measurement of molecular interactions. One advantage is the possibility to apply a defined torque, which has made them the most prominent tool to study twisting of molecules such as DNA. 

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