Lower consolute boundary

A plot of the temperatures required for clouding versus surfactant concentration typically exhibits a minimum in the case of nonionic surfactants (or a maximum in the case of zwitterionics) in its coexistence curve, with the temperature and surfactant concentration at which the minimum (or maximum) occurs being referred to as the critical temperature and concentration, respectively. This type of behavior is also exhibited by other nonionic surfactants, that is, nonionic polymers, // - a I k y I s u I Any lalcoh o I s, hydroxymethyl or ethyl celluloses, dimethylalkylphosphine oxides, or, most commonly, alkyl (or aryl) polyoxyethylene ethers. Likewise, certain zwitterionic surfactant solutions can also exhibit critical behavior in which an upper rather than a lower consolute boundary is present. Previously, metal ions (in the form of metal chelate complexes) were extracted and enriched from aqueous media using such a cloud point extraction approach with nonionic surfactants. Extraction efficiencies in excess of 98% for such metal ion extraction techniques were achieved with enrichment factors in the range of 45-200. In addition to metal ion enrichments, this type of micellar cloud point extraction approach has been reported to be useful for the separation of hydrophobic from hydrophilic proteins, both originally present in an aqueous solution, and also for the preconcentration of the former type of proteins. 

It was also found with non-ionic surfactants that flocculation of the system occurred either at or very close to the cloud point (the lower consolute boundary) of the surfactant. Moreover, this was found to be a reversible process in that on cooling below the cloud point the particles redispersed provided that the temperature was not taken too far above the cloud point (c. 5-10°C) [109]. 

As an example of the different phases of surfactants. Figure 3.27 shows the phase diagram of a pure nonionic surfactant of the alkyl polyglycol ether type (20). In particular, the phase behaviour of nonionic surfactants with a low degree of ethoxylation is very complex. As the lower consolute boundary is shifted to lower temperatures with a decreasing EO (ethylene oxide) number of the molecule, an overlapping of this boundary... 

Lower consolute boundary (or cloud point) The temperature above which micelles separate from solution and form a cloudy suspension at a particular nonionic surfactant concentration. 

It is common to encounter a liquid-liquid miscibility gap having a lower critical temperature in nonionic surfactants. The phase boundary responsible for such behavior is termed a lower consolute boundary [62] Fig. 6 displays an example. A related kind of phase behavior that is inverted with respect to temperature has also been found, mostly among zwitterionic surfactants (Fig. 7). This boundary has an upper critical point and is termed an upper consolute boundary. Phase separation occurs on heating with the lower con-... 

Critical opalescence is observed within the liquid phase below a lower consolute boundary, but it vanishes (to the naked eye) 5-10 degrees below the critical temperature [64]. Scattering intensities along an isothermal mixing path lying just below the critical temperature show a maximum at the critical concentration [65]. 

In the system methanol/carbon disulfide, the lower part of the solubility curve is terminated at the freezing boundary (not shown). Many systems exhibit similar behavior with an upper consolute temperature. Other behaviors are encountered as well, as shown in Figure 1 -7. Some systems exhibit a lower consolute temperature, with full miscibility at low temperatures and partial miscibility above. It is... 

Near the critical temperatures of both lower and upper consolute boundaries, the spatial correlations of densities within the liquid-phase sfructure extend over longer-than-usual distances. Near-critical liquids display, for this reason, unusually intense scattering of visible light ( critical opalescence ), but this phenomenon is in no sense unique to surfactant solutions. Critical opalescence is universally observed near critical points, even in one-component systems such as carbon dioxide, water, and so forth [68]. 

There are a few mixtures, such as water and nicotine, that have both an upper and a lower consolute point, so that the boundary of the tie-line region is a closed curve. Below the lower consolute point at 61.5°C water and nicotine mix in all proportions. Above the upper consolute temperature at 233.0 C they also mix in all proportions. Between these temperatures there is a tie-line region in the diagram and the liquids are only partially miscible. Another mixture with both a lower and an upper consolute temperature is butoxyethanol and water, with a lower consolute temperature of48.01°C and an upper consolute temperature of 130.7" C. ... 

In some cases, diffusion occurs much more slowly than expected. This most commonly occurs near a phase boundary where the solution is supersaturated. An example is diffusion in supersaturated solutions of sugar in water, shown in Fig. 6.3-1. Slow diffusion also occurs in solutions near to a consolute point where two liquids first become miscible. Examples of diffusion near consolute points are shown in Fig. 6.3-2. Other related cases occur when an initially homogeneous solution is suddenly quenched to cause a phase separation. This quenching is commonly effected by abruptly lowering the temperature. The phase separation then occurs very rapidly, at a rate proportional to the diffusion. 

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