Lyophilic colloidal solutions

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... 

Lyophilic colloids can also be desolvated (and precipitated if the electric double layer interaction is sufficiently small) by the addition of non-electrolytes, such as acetone or alcohol to aqueous gelatin solution and petrol ether to a solution of rubber in benzene. 

When a lyophilic colloid loses stability, a separation into two liquid phases may occur. This process is termed coacervation . The phase that is more concentrated in the colloid is the coacervate, and the other phase is the equilibrium solution. See also Cloud Point. 

Colloidal solutions may be divided roughly into two main groups, designated as lyophobic (Greek solvent hating) and lyophilic (Greek solvent loving) when water is the dispersion medium, the terms hydrophobic and hydrophilic are employed. The chief properties of each class are summarized in Table 1.14, but it must be emphasized that the distinction is not an absolute one since some, particularly sols of metallic hydroxides, exhibit intermediate properties. 

Colloidal particles may be attracted (lyophilic) or repelled (lyophobic) by their dispersion medium. (The dispersion medium in a colloid is analagous to the solvent in a solution.) Lyophobic colloids form when amphipathic or charged particles adsorb to the surface of tire colloidal particles stabilizing them in the dispersion medium. Protein in water is an example of a lyophilic colloid emulsyfied fat in water is an example of a lyophobic colloid. 

Some colloidal systems such as polymer solutions and surfactant solutions containing micelles are thermodynamically stable and form spontaneously. These types of colloids are called lyophilic colloids. However, most systems encountered contain lyophobic colloids (particles insoluble in the solvent). In the preparation of such lyophobic colloidal dispersions, the presence of a stabilizing substance is essential. Because van der Waals forces usually tend to lead to agglomeration (flocculation) of the particles, stability of such colloids requires that the particles repel one another, either by carrying a net electrostatic charge or by being coated with an adsorbed layer of large molecules compatible with the solvent. 

Although not so effective as bone char for removal of ash, certain activated carbons do remove appreciable quantities of ash, particularly iron, calcium, alumina, and magnesia.1 This action of carbon appears indirect. Some ash constituents in the untreated liquors are kept in solution by the peptizing action of lyophilic colloids. By adsorbing these protective colloids carbon allows the mineral ash to precipitate and to be mechanically trapped by the carbon. 

In evaluating the relative filtration rate of different carbons, it is important to consider that the filtration rate is a function of mechanical resistance that not only results from the shape and size of the carbon particles, but also depends on the filterability of the liquid. Some carbons are so effective for removing lyophilic colloids, gums, and resins that they give a better over-all filtration performance than does a carbon that has better mechanical structure, but lacks ability to remove colloidal substances. Consequently, comparisons of filtration of different carbons should be based on the solution to be... 

In lyophilic colloidal systems, e.g. in polymer solutions, the disperse phase concentration may be sufficiently high, and the osmotic pressure reaches values that can be reliably measured. In this case osmotic pressure measurements, along with the application of osmosis-related phenomena, such as cryoscopy and ebullioscopy, provide methods for the study of such systems. For example, these methods allow one to determine the molecular weights of polymers. 

Further removal of the dispersion medium results in a conversion of gel into a solid macroscopic phase, i.e. into the soap crystal. Based on the results of the X-ray diffraction analysis, soap crystals were shown to have a lamellar structure. The surfactant - water system can thus undergo transitions into various states, depending on the content of components from a homogeneous system (surfactant molecular solution) to lyophilic colloidal state and further to macroscopic heterogeneous system (soap crystals in water). Different states of the system can be described by a particular thermodynamic equilibrium, i.e. ... 

The formation of lyophilic colloidal systems may also take place via phase separation of polymer solutions. A typical phenomenon that occurs upon phase separation is the formation of the so-called coacervates, which are characteristic nuclei containing higher concentration of polymer, as compared to that in the medium surrounding them. It is speculated that coacervation was a second stage (after the formation of adsorption layers) in the ordered structuring of organic matter on Earth. 

In some cases under the conditions similar to those corresponding to the formation of lyophilic colloidal systems, a spontaneous formation of emulsions, the so-called self-emulsification, may take place. This is possible e.g. when two substances, each of which is soluble in one of the contacting phases, react at the interface to form a highly surface active compound. The adsorption of the formed substance under such highly non-equilibrium conditions may lead to a sharp decrease in the surface tension and spontaneous dispersion (see, Chapter III, 3), as was shown by A.A. Zhukhovitsky [42,43], After the surface active substance has formed, its adsorption decreases as the system reaches equilibrium conditions. The surface tension may then again rise above the critical value, acr. Similar process of emulsification, which is an effective method for preparation of stable emulsions, may take place if a surfactant soluble in both dispersion medium and dispersed liquid is present. If solution of such a surfactant in the dispersion medium is intensively mixed with pure dispersion medium, the transfer of surfactant across the low surface tension interface occurs (Fig. VIII-10). This causes turbulization of interface... 

Lyophilic Colloid An older term used to refer to single-phase colloidal dispersions. Examples polymer and micellar solutions. Other synonyms no longer in use semicolloid or half-colloid. 

In Chapter 1 the importance of the various classes of colloidal systems to modern science and technology was indicated in a general way. Because of the wide variety of colloidal systems one encounters, each having certain unique features that distinguish it from the others, it is convenient to discuss each major classification separately. For that reason, chapters have been devoted to specific systems such as solid dispersions, aerosols, emulsions, foams, lyophilic colloids (i.e., polymer solutions), and association colloids. There is a great deal of overlap in many aspects of the formation, stabilization, and destruction of those systems, and an effort will be made not to repeat more than is necessary. However, for purposes of clarity, some repetition is unavoidable. 

In addition to the colloids composed of insoluble or immiscible components, there are the lyophibc colloids which are in reality solutions, but in which the solute molecules (i.e., polymers) are much larger than those of the solvent. Lyophilic colloids are somewhat unique in that they have been able to cross over into another major area of science—polymer science—and thereby gain a great deal more general attention than more typical colloidal systems. 

Because the lower limit of the colloidal range is just larger than the size of some molecules and solvated species it is difficult to determine exactly where the distinction between surface and bulk ends and a molecularly dispersed system begins. For macromolecular systems, of course, the molecular size is such that even a molecular dispersion or solution easily falls into the size range of colloids. For that reason, primarily, such systems are referred to as lyophilic colloids, even though the properties of such systems are governed for the most part by phenomena distinct from the classic surface interactions considered in lyophobic colloids. It is no trivial matter, therefore, to decide just where surface effects end and the characteristics of the individual free, solvated units begin. 

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