Critical solubility point

Although aniline does possess a dipole moment the cohesion energy is predominantly of the London type. The critical solubility point—technically actually the i i demixing point—with hydrocarbon mixtures (aniline point) is an important characteristic quantity. The aniline point rises with the molecular weight in a homologous series. For aromatic hydrocarbons the critical solubility point is the lowest, for paraffins the highest, olefins and cyclic hydrocarbons are in between, in agreement with the variation of the specific cohesion (Table 33). 

The condition for the critical solubility point Tc can easily be derived from the thermodynamic conditions for a critical point ... 

Fig. 10. The mole fraction of carbon dioxide in saturated solutions in air at — 110°C (above the lower critical end point). The full line is the experimental curve of Webster and the dashed curves are 1, an ideal gas mixture 2, an ideal gas mixture with Poynting s correction and 3, the solubility calculated from Eq. 8 and the principle of corresponding states.

Holder and Maass34 found that the lower critical end point was at 44.85°C, that is, 12.5°C above the critical temperature of pure ethane. They did not measure the pressure and their claim of having detected different solubilities in different parts of the fluid at temperatures above this point probably does not apply to a system at equilibrium in the absence of a gravitational field.66... 

Detailed measurements of the solubility between the lower and upper critical end points have been made only for the solutions in ethylene of naphthalene,14 hexachlorethane,30 and />-iodochloro-benzene.21 Atack and Schneider2 have used dilute solutions of the last-named substance to study the formation of clusters near the gas-liquid critical point of ethane. 

Sediment may be added by bulk mixing via imbricate thnisting (Bebout and Barton 2002), dehydration (Class et al. 2000), or melting (Johnson and Plank 1999). The latter two may differ in their P-T conditions and, therefore, residual mineralogy as well as relevant partition coefficients. In general, fluids are less effective transport agents than melts (i.e., trace elements are more soluble in melt than in pure water or even brine), but fluid/solid partitioning can fractionate some elements, notably Ba-Th and U-Th, more than melt/solid. However, as pressure increases, the distinction between fluid and melt decreases as their mutual solubility increases and they approach a critical end-point. 

The system FLO - C02> H20 - ELS and H20 - (C HjOoO can be described by assuming cross-associationf The particular temperature dependence of the solubility for diethyl ether was reproduced by the calculation without making it the object of a fitting process. This suggests that the method might be able to describe systems with both an upper and lower critical solution point. 

Before the reaction, at temperatures below the critical solution point, the catalyst is insoluble in the organic solvent. When heated to temperatures above the critical solution point, the catalyst is soluble in the organic solvent and a homogeneous system is formed in which the catalyzed reaction takes place. After reaction, on cooling to temperatures below the critical solution point, the catalyst precipitates from the organic phase which contains the product. Thus, the catalyst can be easily separated from the product by decantation or filtration and reused. 

For designing a RESS-experiment one is recommended to study the solubility of the substance to be crystallized in the supercritical gas phase near the upper critical end-point... 

The S-L-V curve intersects the gas-liquid critical curve in two points the lower critical end point (LCEP) and the upper critical end point (UCEP). At these two points, the liquid and gas phases merge into a single fluid-phase in the presence of excess solid. At temperatures between Tlcep and Tucep a S-V equilibrium is observed. The solubility of the heavy component in the gas phase increases very rapidly with pressure near the LCEP and the UCEP. Near the LCEP the solubility of heavy component in the light one is limited by the low temperatures. In contrast, near the UCEP the solubility of heavy component in the light one is high, owing to the much higher temperatures [6],... 

Studies on non-ionic surfactants as effective drag-reducing additives have been submitted by Zakin (1972). He studied various effects on three non-ionic surfactants formed from straight-chain alcohols and ethyleneoxide. These surfactants have an upper and a lower temperature limit for solubility in water and prove effective drag reducers near their upper critical solubility temperature or clouding point. The clouding point is the point at which a solution of a non-ionic agent in water becomes turbid as the temperature is raised. 

Lamb, D. M., Barbara, T. M. Jonas, J. NMR Study of Solid Naphthalene Solubilities in Supercritical Carbon Dioxide Near the Upper Critical End Point. J. Phys. Chem. 1986, 90, 4210M215. 

Dlepen and Scheffer ( 6) were the first to show that near either the lower or upper critical end point the solubility of a solid in a supercritical fluid is enhanced and also very sensitive to changes in temperature and pressure our solubility isotherms show this effect for both end points. First, the isotherms cross at about 140 bar so that the solubility at the lowest temperature (50.0°C) is largest at 120 bar. This is a result of approaching the lower critical end point region (which should be close to the critical point of pure C02 as previously mentioned). At temperatures and pressures near this LCEP the solubility enhancement results in lower temperature isotherms having the greater solubilities. The effect of the upper critical end point is also well shown by our data. The 58.5°C isotherm shows a large increase in solubility at about 235 bar the slope of the isotherm is near zero. As Van Welie and Diepen... 

The solubility of a solid in a supercritical fluid has been described by Gitterman and Procaccia.(lO) The region of interest chromatographically will be for infinitely dilute solutions whose concentration is far removed from the lower critical end point (LCEP) of the solution. Therefore the solubility of the solute in a supercritical fluid at infinite dilution far from criticality can be approximated as,... 

At the point C the two liquid layers become identical, and this is called the critical solution point or con-solute point. If the total applied pressure is varied, both the critical temperature and composition of the critical mixture alter and we obtain a critical solution line. As an example of this we give in table 16. If the dependence of the critical solution temperature on pressure for the system cyclohexane -f aniline. An increase of pressure raises the critical solution temperature, and the mutual solubility of the two substances is decreased. We saw earlier that the applied pressure had only a small effect on the thermodynamic properties of condensed phases, and we notice in this case that an increase of pressure of 250 atm. alters the critical temperature by only 1.6 °C. 

Type II (Solid-Fluid) System. In type II systems (when the solid and the SCF component are very dissimilar in molecular size, structure, and polarity), the S-L-V line is no longer continuous, and the critical (L = V) mixture curve also is not continuous. The branch of the three-phase S-L-V line starting with the triple point of the solid solute does not bend as much toward lower temperature with increasing pressure as it does in the case of type I system. This is because the SCF component is not very soluble in the heavy molten solute. The S-L-V line rises sharply with pressure and intersects the upper branch of the critical mixture (L = V) curve at the upper critical end point (LfCEP), and the lower temperature branch of the S-L-V line intersects the critical mixture curve at the lower critical end point (LCEP). Between the two branches of the S-L-V line there exists S-V equilibrium only (13). 

Let us consider two liquids A and B that are not very soluble in each other. Addition of liquid C increases the miscibility of B in A and of A in B. The addition of C has the same effect as increasing the temperature in the binary phase diagram. The major difference is that the tie lines are no longer necessarily parallel to the baseline, and the critical end point is no longer at the maximum of the miscibility gap (Figure 3.4). This is because C does not partition evenly between the two coexisting phases. In the present case, C goes preferably into B. The critical end point is located near the A corner. An isothermal critical end point is usually referred to as a plait point. 

If an experiment is performed at an overall composition equal to x in figure 3.2d, the vapor-liquid envelope is first intersected along the dew point curve at low pressures. The vapor-liquid envelope is again intersected at its highest pressure, which corresponds to the mixture critical point at T2 and x. This mixture critical point is identified with the intersection of the dashed curve in figure 3.2b and the vertical isotherm at T2. At the critical mixture point, the dew point and bubble point curves coincide and all the properties of each of the phases become identical. Rowlinson and Swinton (1982) show that P-x loops must have rounded tops at the mixture critical point, i.e., (dPldx)T = 0. This means that if the dew point curve is being experimentally determined, a rapid increase in the solubility of the heavy component will be observed at pressures close to the mixture critical point. The maximum pressure of the P-x loop will depend on the difference in the molecular sizes and interaction energies of the two components. 

Schmitt (1984) verified the entrainer behavior reported by Kurnik and Reid. Schmitt and Reid (1984) show that very small amounts of an entrainer in the SCF-rich phase have very little effect on the solubility of a second component in that phase. This observation is consistent with the work of Kohn and Luks for ternary mixtures at cryogenic temperatures. The data of Kurnik and Reid have been corroborated for the naphthalene-phenanthrene-carbon dioxide system (Gopal et al., 1983). Lemert and Johnston (1989, 1990) also studied the solubility behavior of solids in pure and mixed solvents at conditions close to the upper critical end points. Johnston finds that adding a cosolvent can reduce the temperature and pressure of the UCEP while simultaneously increasing the selectivity of the solid in the SCF-rich phase. In these studies Johnston found the largest effects with a cosolvent capable of hydrogen bonding to the solute. 

T room temperature T reaction temperature Tcp cloud point or critical solubility temperature... 

In the case of Type III, the involalile solute has only limited solubility in the near-critical solvent. The critical line breaks off at a critical end point (CEP). Additional solute added collects in a second liquid phase. A three-phase line LLV extends downward from the critical endpoint. At the solute critical point, a critical line starts that can either move almost straight up to higher pressures, or may fust appear to move to lower temperatures, but ultimately turns over and moves to high pressures. On this critical line, it is possible for both temperature and pressure to both be higher than those of the solute. In that case, the critical line is said to be gas-gas, even though the two coexisting supercritical phases may be quite dense due to the high pressures. 

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