Hydrophobic colloid

The colloidal particles can be crystalline or constitnte an amorphons agglomeration of individual molecnles. The definition also includes nonaggregated large macromolecules such as proteins. An arbitrary distinction is made between hydrophobic colloids (sols) and hydrophilic colloids (gels), which depends on the degree and type of interaction with the aqneons solvent. 

Hydrophobic colloidal particles move readily in the liqnid phase under the effect of thermal motion of the solvent molelcnles (in this case the motion is called Brownian) or under the effect of an external electric field. The surfaces of such particles as a rule are charged (for the same reasons for which the snrfaces of larger metal and insnlator particles in contact with a solution are charged). As a result, an EDL is formed and a certain valne of the zeta potential developed. 

Ferric hydroxide coprecipitation techniques are lengthy, two days being needed for a complete precipitation. To speed up this analysis, Tzeng and Zeitlin [595] studied the applicability of an intrinsically rapid technique, namely adsorption colloid flotation. This separation procedure uses a surfactant-collector-inert gas system, in which a charged surface-inactive species is adsorbed on a hydrophobic colloid collector of opposite charge. The colloid with the adsorbed species is floated to the surface with a suitable surfactant and inert gas, and the foam layer is removed manually for analysis by a methylene blue spectrometric procedure. The advantages of the method include a rapid separation, simple equipment, and excellent recoveries. Tzeng and Zeitlin [595] used the floation unit that was devised by Kim and Zeitlin [517]. 

Hydrophilic colloids are colloidal particles that attract water molecules, whereas hydrophobic colloids are colloidal particles that repel water molecules. 

C. Tribet, R. Audebert, J.-L. Popot (1997) Stabilisation of hydrophobic colloidal dispersions in water with amphiphilic polymers application to integral membrane proteins. Langmuir, 13 5570-5576... 

The important forces involved in the adsorption of metals on to particles are attractive electrostatic or van der Waals forces. These concepts explain many of the properties of colloids with respect to the adsorption of contaminants or ion-exchange factors and the aggregation of the colloids into larger particles. These larger particulates may then descend the water column to the sediment. This occurs most notably in estuarine environments, as increases in salinity lead to estuarine silting. Binding of electrolytes to hydrophobic colloids is often used to facilitate their coagulation and precipitation. 

In seawater the thickness" of the double layer as given by k1 (Eq. 3.9) is a few Angstroms, equal approximately to a hydrated ion. In other words, the double layer is compressed and hydrophobic colloids, unless stabilized by specific adsorption or by polymers, should coagulate. Some of this coagulation is observed in the estuaries where river water becomes progressively enriched with electrolytes (Fig. 7.14a). That these colloids exist in seawater for reasonable time periods is caused... 

The laboratory must be informed when the therapeutic regimens include drugs specifically administered to change the blood level of a biochemical constituent. Cholestyramine resin, a nonabsorbable anion exchange resin administered orally to patients with hyperlipoproteinemia produced a 24% decline in serum cholesterol levels in 14 patients with essential hypercholesterolemia. In these patients the mean cholesterol fell from 414 98 mg/100 ml to 176 21 mg/100 ml (FI). Pectin added to the diet caused a 5% decrease in serum cholesterol values (K4), as did an oral hydrophobic colloid (G4). Levels fell in one case from 220 mg/ 100 ml to 160 mg/100 ml (G4). Nicotinic acid, neomycin, and p-chloro-phenoxyisobutyrate have all been used to reduce serum cholesterol (G7). 

Recent studies indicate that the adsorption of metal ions is controlled only in part by the concentration of the free (aquo) metal ion of considerable importance is the ability of hydroxo and other complex ions and molecules to adsorb. There have been two apparently divergent approaches to describe the role played by hydroxo metal complexes in adsorption at solid-aqueous electrolyte interfaces. Matijevic et al. (9) have proposed that specific hydrolysis products—e.g., Al8(OH)2o4+ in the A1(III)-H20 system, are responsible for extensive coagulation and charge reversal of hydrophobic colloids. It has also been demonstrated by Matijevic that the free (aquo) species of transition and other metal ions... 

When aqueous solutions of silver nitrate and sodium bromide are mixed rapidly, the silver bromide may form a hydrophobic colloidal suspension rather than precipitating. The tiny particles are kept from settling out by Brownian motion, the motion of small particles resulting from constant bombardment by solvent molecules. The sol is further stabilized by the adsorption of ions on the surfaces of the particles. The ions attract a layer of water molecules that prevents the particles from adhering to one another. 

The process in which small amounts of added hydrophilic colloidal material make a hydrophobic colloid more sensitive to coagulation by electrolyte. Example the addition of polyelectrolyte to an oil-in-water emulsion to promote demulsification by salting out. Higher additions of the same material usually make the emulsion less sensitive to coagulation, and this is termed protective action or protection . The protected, colloidally stable dispersions that result in the latter case are termed protected lyophobic colloids . 

Historically, ideas of casein micelle structure and stability have evolved in tandem. In the earlier literature, discussions of micellar stability drew on the classical ideas of the stability of hydrophobic colloids. More recently, the hairy micelle model has focused attention more on the hydrophilic nature of the micelle and steric stabilization mechanisms. According to the hairy micelle model, the C-terminal macropeptides of some of the K-casein project from the surface of the micelle to form a hydrophilic and negatively charged diffuse outer layer, which causes the micelles to repel one another on close approach. Aggregation of micelles can only occur when the hairs are removed enzymatically, e.g., by chymosin (EC 3.4.23.4) in the renneting of milk, or when the micelle structure is so disrupted that the hairy layer is destroyed, e.g., by heating or acidification, or when the dispersion medium becomes a poor solvent for the hairs, e.g., by addition of ethanol. 

Substances like metals, metal sulphides cannot be brought into the colloidal state simply by bringing them in contact with water and, therefore, special methods are devised for the purpose. Hence, they are known as hydrophobic colloids (hydro = water phobic = hating). In case of solvent other than water, the general term lyophobic is used. Further, if these colloids are precipitated, then it is not very easy to reconvert the precipitate directly into the colloidal state. Hence, they are termed as irreversible colloids (colloidal state — precipitate), irresoluble or electrocratic colloids. 

Colloids are either hydrophilic (water-loving) or hydrophobic (water-hating). Hydrophilic colloids (e.g., proteins, humic substances, bacteria, viruses, as well as iron and aluminum hydrated colloids) tend to hydrate and thereby swell. This increases the viscosity of the system and favors the stability of the colloid by reducing the interparticle interactions and its tendency to settle. These colloids are stabilized more by their affinity for the solvent than by the equalizing of surface charges. Hydrophilic colloids tend to surround the hydrophobic colloids in what is known as the protective-colloid effect, which makes them both more stable. 

The colloidal state in these systems is poorly defined. It depends on the level of organic solvent, the number of ionizable groups in the resin and the degree of neutralization. In an anodic system, for example, the polymer may be completely soluble at high pH and change gradually to a hydrophobic colloid as the pH is decreased to 7. At lower pH, the system flocculates. 

The hydrophobic colloids do not have affinity for water thus, they do not contain any bound water. In general, inorganic colloids are hydrophobic, while organic colloids are hydrophilic. An example of an inorganic colloid is the clay particles that cause turbidity in natural water, and an example of an organic colloid is the colloidal particles in domestic sewage. 

The electrical forces are produced due to the charges that the particles possess at their surfaces. These charges called primary charges are, in turn, produced from one or both of two phenomena the dissociation of the polar groups and preferential adsorption of ions from the dispersion medium. The primary charges on hydrophobic colloids are due to preferential adsorption of ions from the dispersion medium. 

Though the theory of Derjaguin-Landau-Verwey-Overbeek (DLVO) [17, 18] was essentially designed for hydrophobic colloids, it is often applied to the analysis of the stability of polyelectrolyte solutions. According to this approach an overlap of the electrical double-layers of two charge-like colloidal spheres in an electrolyte solution always yields a repulsive screened Coulomb interaction, and the van der Waals forces are responsible for the attraction. A number of experiments in the recent decades, however, provide evidence that the effective interparticle potential shows a long-range attraction which cannot be ascribed to the van der Waals forces [15, 88-93], In spite of numerous theoretical attempts to explain this phenomena (for a review see [7, 8, 10, 94,... 

Hydrophobic colloids contain molecules with nonpolar groups that are only weakly attracted to water. Examples include fats that form emulsions (milk and mayonnaise). 

The Tyndall Effect 14-17 The Adsorption Phenomenon 14-18 Llydrophilic and Hydrophobic Colloids... 

Recognize and describe colloids the Tyndall effect, the adsorption phenomenon, hydrophilic and hydrophobic colloids... 

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