Colloidal properties

Most of the colloidal properties of SOM are due to humus. Humus is highly colloidal and is x-ray amorphous rather than crystalline. The surface area and adsorptive capacities of humus per unit mass are greater than those of the layer silicate minerals. The specific surface of well-developed humus may be as high as 900 x 103 m2 kg-1 its exchange capacity ranges from 1500 to 3000 mmol(+) kg-1. 

FIGURE 6.6. Hypothetical structure of humic acid. (From J. J, Mortvedt, P. M. Giordano, and W. L. Lindsay, eds. 1972. Micronutrients in Agriculture American Society of Agronomy Madison, Wl.) 

FIGURE 6.7. Titration curves of a soil and peat humic acid. The small wavy lines on the curves indicate endpoints for ionization of weak-acid groups having different, but overlapping, ionization constants. (From F. J. Stevenson. 1982. Humus Chemistry. Wiley, New York.) 

The dissociation of carboxyl and phenol groups yields perhaps 85 to 90% of the negative charge of humus. Many carboxylic groups are sufficiently acid to dissociate below pH 6 (zone I, Fig. 6.7) 

No SOM fraction possessing a net positive charge has been found at normal soil pH values (Table 6.2). Protonated groups such as (R—OH2)+ and (R—NH3)+ can yield positive charges, but the overall charge on humus is negative. 

When a solid, powdered material is shaken up in a liquid and left to stand, the solid particles, provided that they are denser than the liquid, sink to the bottom of the container, forming a sediment. Under constant conditions, the rate at which particles settle (or sediment) depends on their size the larger they are, the more rapidly they sink. For spherical particles of radius r m., density dg, in a liquid of density di and viscosity y), the rate of fall v in m.sec-i is given by Stokes s Law, which may be written  

For particles of any other shape, the value of r derived from Stokes Law has no exact geometrical interpretation but is known as the equivalent spherical radius and is used as a convenient measure of the size of the particle. Stokes s Law is only true for particles greater than about l(x in diameter, however for smaller particles their natural thermal motion (called the Brownian Movement) becomes increasingly important. This movement causes small particles to diffuse upwards against the sedimenting force, and if they are sufficiently small they can prevent complete sedimentation. Such a suspension is said to be colloidal, and exhibits 

Atoms and molecules Colloidal particles Emulsions and suspensions 

Clearly, colloidal particles lie on the border-line between coarse particles, which obey the ordinary sedimentation laws, and atoms and molecules, which do not it is therefore not surprising tnat colloidal particles behave as they do. Although we shall be concerned mainly with suspensions of solid particles in water, it should be realized that colloidal suspensions of solid particles in liquids other than water can be produced. 

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Silica sols are often called colloidal silicas, although other amorphous forms also exhibit colloidal properties owing to high surface areas. Sols are stable dispersions of amorphous siUca particles in a Hquid, almost always water. Commercial products contain siUca particles having diameters of about 3—100 nm, specific surface areas of 50—270 m /g, and siUca contents of 15—50 wt %. These contain small (<1 wt%) amounts of stabilizers, most commonly sodium ions. The discrete particles are prevented from aggregating by mutually repulsive negative charges. 

It would be incomplete for any discussion of soap crystal phase properties to ignore the colloidal aspects of soap and its impact. At room temperature, the soap—water phase diagram suggests that the soap crystals should be surrounded by an isotropic Hquid phase. The colloidal properties are defined by the size, geometry, and interconnectiviness of the soap crystals. Correlations between the coUoid stmcture of the soap bar and the performance of the product are somewhat quaUtative, as there is tittle hard data presented in the literature. However, it might be anticipated that smaller crystals would lead to a softer product. Furthermore, these smaller crystals might also be expected to dissolve more readily, leading to more lather. Translucent and transparent products rely on the formation of extremely small crystals to impart optical clarity. 

Staudinger relentlessly championed the molecular, or primary valence, viewpoint in the years which followed. He supported his original contentions with the observation that hydrogenation of rubber, as well as its conversion to other derivatives, does not destroy its colloidal properties. In contrast to association colloids, high polymers (or macromolecules as he chose to call them ) exhibit colloidal properties in all solvents in which they dissolve. Polyoxymethylenes were ex-... 

Doose, S., Tsay, J. M., Pinaud, F. and Weiss, S. (2005) Comparison of photophysical and colloidal properties of biocompatible semiconductor nanocrystals using fluorescence correlation spectroscopy. Anal. Chem., 77, 2235-2242. 

The defined size ranges and limits are somewhat arbitrary since there are no specific boundaries between the categories. The transition of size ranges, either from molecular dispersions to colloids or from colloids to coarse dispersions, is very gradual. For example, an emulsion may exhibit colloidal properties, and yet the average droplet size may be larger than 1 pm. This is due to the fact that most disperse systems are heterogeneous with respect to their particle size [1-2]. 

Steric and Electrostatic Contributions to the Colloidal Properties of Nonaqueous Dispersions... 

FOWKES AND PUGH Colloid Properties of Nonaqueous Dispersions 333... 

Condensed-phase flame retardant mechanisms, 44 484—485 Condensed phosphates, 18 841-852 colloidal properties of, 48 851 complex ion formation in,... 

Schmid, G. Maihack, V. Lantermann, F., and Peschel, S., Ligand-stabilized metal clusters and colloids properties and applications, J. Chem. Soc. Dalton Trans., 589, 1996. 

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