Consolute temperature, lower

The specific rates of hydrolysis of five organic halides in three water-based liquid mixtures near their respective equilibrium consolute points have been observed to be suppressed. The systems studied included t-amyl chloride in isobutyric acid water (upper consolute temperature), and 3-chloro-3-methylpentane in 2-butoxyethanol water (lower consolute temperature). The slowing effect occurred within a few tenths of a degree on either side of the consolute temperature. 

It is widely known that poly(N-isopropylacrylamide), poly(IPAAm), in water has a lower critical solution temperature (LCST) at 32 °C. LCST was originally observed in PEG solutions a long time ago. Rowlinson et al. [40] (1957) explained the lower consolute temperature for PEG in water in terms of negative entropies. The first paper on the LCST of poly(IPAAm) at about 31 °C was presented by Heskins and Guillet in 1968 [41]. They reported that aqueous solution of poly(IPAAm) showed phase separation above this temperature, and ascribed it primarily to an entropy effect on the basis of thermodynamical considerations. 

CONSOLUTF TEMPERATURE. The upper convolute temperature for two partially miscible liquids is the critical temperature above which the two liquids are miscible in all proportions. In some systems where the mutual solubility decreases with increasing temperature over a certain temperature range, the lower convolute temperature corresponds to the critical temperature below which the two liquids are miscible in all proportions. Some systems such as mclhylclhyl ketone and water have both upper and lower consolute temperatures. 

The curve AB shows the decreasing solubility of triethylamine in water while curvers CB shows the decreasing solubility of water in triethylamine. The two curves meet at B, which is the lower critical solution temperature or lower consolute temperature of the system. Any point within the area ABC will give two liquid layers, while any point outisde the area ABC will give a homogeneous solution. 

Methyl ethyl ketone and water has a lower consolute temperature of... 

As a third liquid is added to the partially miscible binary liquid system, the ternary (three-component) system is dependent on the relative solubility of the third liquid in the two liquids. If the third substance is soluble only in one liquid of the original binary mixture or if the solubility of the third in the two liquids is considerably different, the solubility of one liquid in the others will be lowered. The upper consolute temperature should be raised or the lower consolute temperature should be lowered in order to obtain a homogeneous solution. On the other hand, if the third substance is soluble to the same extent in both liquids of the binary system, the complementary solubility of the two liquids is increased. This results in the lowering of an upper consolute temperature or the elevation of a lower consolute temperature. 

Another solubility phenomenon that may depend partly on H bonding is consolute temperature or critical solution temperature formation in mixtures that have a composition region in which they are im -miscible. The region narrows as the temperature is changed. Above an upper consolute temperature, or below a lower consolute temperature, the components are miscible over the entire composition range. Figure 2-11 shows such loops for 2,4- and 2,5-dimethylpyridine in water, as reported by Andon and Cox (45a). Other pyridine-water systems are... 

For conditions of constant pressure, or when pressure effects are negligible, binary LEE is conveniently displayed on a solubihty diagram, a plot of T vs. xi. Figure 14.12 shows binary solubility diagrams of three types. The first diagram [Fig. 14.12(a)] shows curves (binodal curves) that define an "island." They represent the compositions of coexisting phases curve UAL for the a phase (rich in species 2), and curve UBL for the P phase (rich in species 1). Equilibrium compositions jc and at a particular T are defined by the intersections of a horizontal tie line with the binodal curves. Temperature Tl is a lower consolute temperature, or... 

SO lar the critical temperature is showing lower consolute temperature the highest at which two phases... 

Fig. 16.11. Phase diagram of the mixture w-toluidine + glycerol at atmospheric pressure, showing upper and lower consolute temperatures.

Fig. 18.4. Phase diagram with lower consolute temperature.

We conclude therefore that an upper consolute temperature can occur only with endothermic mixtures while lower consolute temperatures are limited to exothermic mixtures. These predictions have been verified in all cases which have been studied. ... 

If a solution possesses an ideal entropy of mixing, the second derivative is always negative and the system can only exhibit an upper consolute temperature. For a non-ideal solution to exhibit a lower consolute temperature, the deviations from ideality must be such that both the sign and the curvature of 5(0 0) is changed (cf, chapter XXIV, 6 chapter XXVI, 7). 

Fig. 23.10. Freezing point diagram for partially miscible liquids with a lower consolute temperature.

On the other hand for a lower consolute temperature for which d%jdxl is positive (18.85) we must have... 

For a lower consolute temperature k must be negative, and the absolute... 

Upper consolute temperatures therefore correspond to phase separation resulting from an energy effect, while lower consolute temperatures result from entropy effects. 

For certain pairs of liquids /3 decreases as the temperature is lowered, within a particular temperature range such S3rstcms exhibit a lower consolute temperature, above which the liquids are not completely miscible. 

An exothermic mixture usually leads to mixing in all proportions. This is the case for water and ethanol. If the mixing is endothermic, the number of coexisting phases and their composition depend on temperature. Increasing the temperature usually results in an increase in the mutual solubility of the two compounds, eventually leading to complete miscibility above a critical temperature, the upper consolute temperature (UCT). Note that some abnormal systems can also have a lower consolute temperature (LCT). Both UCT and LCT are thermodynamic critical points. At a critical point, the compositions of the two phases in equilibrium become identical. 

Generally, liquid-liquid phase equilibrium (or phase separation) occurs only over certain temperature ranges, bounded above by the upper consolute or upper critical solution temperature, and bounded below by the lower consolute or lower critical solution temperature. These critical solution temperatures are indicated on the liquid-liquid phase diagrams given here. All partially miscible mixtures should exhibit either one or both consolute temperatures however, the lower consolute temperature may be obscured by the freezing of the mixture, and the upper consolute temperature will not be observed if it is above the bubble point temperature of the mixture, as vaporization will have instead occurred. ... 

Systems are known in which the solubility decreases with increase in temperature. In some of these systems, a lower consolute temperature is observed Fig. 15.3(a) shows schematically the triethylamine-water system. The lower consolute temperature is at 18.5 °C. The curve is so flat that it is difficult to determine the composition of the solution... 

Some substances exhibit both upper and lower consolute temperatures. The diagram for the system nicotine-water is shown schematically in Fig. 15.3(b). The lower consolute temperature is about 61 °C, the upper one about 210 °C. At all points in the closed loop two phases are present, while the points outside the loop represent homogeneous states of the system. 

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... 

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