Colloids, lyophilic

For convenience, colloids are divided into two main groups, designated as lyophobic and lyophilic colloids. The chief properties of each class are summarised in Table 11.1, although it must be emphasised that the distinction is not an absolute one, since some gelatinous precipitates (e.g. aluminium hydroxide and other metallic hydroxides) have properties intermediate between those of lyophobic and lyophilic colloids. 

Most lyophilic colloids change their charge readily, e.g. they are positively charged in an acid medium and negatively charged in an alkaline medium. 

Cobalt, sepn. of from nickel, (cm) 532 Codeine and morphine, D. of 740 Coefficient of variation 135 Colloidal state 418 See also Lyophilic, Lyophobic Colorimeters light filters for, 661 photoelectric, 645, 666 Colorimetric analysis 645 criteria for, 672 general remarks on, 645, 672 procedure, 675 solvent selection, 674 titration, 652... 

Liquid junction potential 63, 549 Literature of analytical chemistry 6, 122, 156, 251, 253, 498, 499, 640, 641, 813, 815 Lithium, D. of as aluminate, (g) 459 Litmus 265 Litre xxix, 78 Littrow mounting 661 Logarithms four figure, 843 Lovibond comparator 655 Low voltage d.c. arc 763, 771 Lubricants for glass stopcocks 85 Lyophilic colloids 419 Lyophobic colloids 419 stability of, 419... 

In colloid science, colloidal systems are commonly classified as being lyophilic or lyophobic, based on the interaction between the dispersed phase and the dispersion medium. In lyophilic dispersions, there is a considerable affinity between the two constituent phases (e.g., hydrophilic polymers in water, polystyrene in benzene). The more restrictive terms hydrophilic and oleophilic can be used when the external phase is water and a nonpolar liquid, respectively. In contrast, in lyophobic systems there is little attraction between the two phases (e.g., aqueous dispersions of sulfur). If the dispersion medium is water, the term hydrophobic can be used. Resulting from the high affinity between the dispersed phase and the dispersion medium, lyophilic systems often form spontaneously and are considered as being thermodynamically stable. On the other hand, lyophobic systems generally do not form spontaneously and are intrinsically unstable. 

A colloid is defined as a system consisting of discrete particles in the size range of 1 nm to 1 pm, distributed within a continuous phase [153], On the basis of the interaction of particles, molecules, or ions of the disperse phase with molecules of the dispersion medium-, colloidal systems can be classified as being lyophilic or lyophobic. In lyophilic systems, the disperse phase molecules are dissolved within the continuous phase and in the colloidal size range or spontaneously form aggregates in the colloidal size range (association systems). In lyophobic systems, the disperse phase is very poorly soluble or insoluble in the continuous phase. During the last several decades, the use of colloids in... 

Lyocell process, environmental issues related to, 22 279-280. See also Courtaulds lyocell process Lyondell s process, 23 345 Lyophilic colloids, 7 283-284 Lyophilization, 28 716 Lyotropic liquid crystalline polyesters,... 

Lyophilized copolymer was ground down to colloidal dimensions (<1 iJim) using a laboratory planetary mill (Pulverisette, Fritsch GmbH). A 110-mg sample of copolymer powder, 20 mg of bovine insulin, and 20 mg of GOD were mixed, and the mixture was compressed into a disk-shaped matrix of 5-mm thickness and 15-mm diameter. 

The terms used to distinguish colloidal particles on the basis of their affinity to the fluid in which they are dispersed are lyophilic and lyophobic. These terms mean, literally, solvent loving and solvent fearing, respectively. When water is the medium or solvent, the terms hydrophilic or hydrophobic are often used. This terminology is very useful when considering surface activity such as wettability of a surface however, when used to classify colloids, the distinction is not always clear-cut. We consider these two types of colloids separately in the following subsections. 

The classical use of the term lyophilic colloids refers to soluble macromolecular materials in which the individual particles (macromolecules such as synthetic polymer chains or proteins)... 

Above we used the words continuous phase and dispersed phase to refer to the medium and to the particles, respectively, in the colloidal size range. It should be understood that these are solvent and solute in lyophilic systems. In micellar systems, the micelles are dispersed in an aqueous continuous phase. Furthermore, the system as a whole is generally called a dispersion when we wish to emphasize the colloidal nature of the dispersed particles. This terminology is by no means universal. Lyophilic dispersions are true solutions and may be called such, although this term ignores the colloidal size of the solute molecules. 

As in the case of lyophilic colloids, the use of the adjective lyophobic does not necessarily mean that the surfaces of the colloids are uniformly liquid repelling. For example, ceramic sols such as silica and alumina powders in liquids do have surfaces with varying degrees of affinity to the liquid. Despite the ambiguities in the use of the terms lyophobic and lyophilic, such a classification is convenient. 

Next, we consider another difference between lyophobic and lyophilic colloids in addition to the presence or absence of surfaces between the continuous and dispersed species. This difference deals with the stability of the dispersion, and we examine the meaning(s) of this term in more detail below. 

What are lyophilic colloids What are lyophobic colloids Give some examples. 

What are the difficulties with the characterization of colloids as lyophilic or lyophobic ... 

Until now, we have been primarily concerned with the definition and measurement of viscosity without regard to the nature of the system under consideration. Next we turn our attention to systems containing dispersed particles with dimensions in the colloidal size range. Viscosity measurements can be used to characterize both lyophobic and lyophilic systems we discuss both in the order cited. 

In the quantitative sections of this chapter the primary emphasis has been on establishing the relationship between the electrophoretic properties of the system and the zeta potential. We saw in Chapter 11 that potential is a particularly useful quantity for the characterization of lyophobic colloids. In this context, then, the f potential is a valuable property to measure for a lyophobic colloid. For lyophilic colloids such as proteins, on the other hand, the charge of the particle is a more useful way to describe the molecule. In this section we consider briefly what information may be obtained about the charge of a particle from electrophoresis measurements. 

Solutions of cellulose, its esters and ethers are colloidal solutions. They are reversible lyophilic colloids. The most important characteristics of such solutions are as follows. 

With regard to their viscosity, nitrocellulose solutions demonstrate the typical properties of lyophilic colloids. The action of pressure, temperature and concentration causes anomalies to appear indicating that these solutions deviate from the... 

It is convenient to classify sols into three types (I) tvophilii (solvent loving) colloids, for example, are solutions or gelatin or starch in water (2 association colloids, of which a solution of Soap in water at moderate concentration is an example and (3) Iwphohic (solvent repelling) colloids, for example, sulfur in water. Both lyophilic and association colloids can be prepared in thermodynamic equilibrium, so that when solvent is removed and then returned to the system, the original properties of the system are regained. 

In this paragraph the discussion of the preparation of colloidal dispersions will be limited to those in which a liquid is the dispersion medium and a solid is dispersed, consequently to sols. Sols can be divided into two groups lyophobic (Greek for liquid hating) and lyophilic... 

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