Separations Based on Solubility

Selective transfer of material in microgram to gram quantities between two immiscible liquid phases separations based on solubility differences selectivity achieved by pH control and complexation. 

Partition—Mixture components are separated based on solubility differences in the liquid stationary phase. 

Asphalt Solubility in Normal Alkanes. The separation of an asphalt into two fractions—asphaltenes and maltenes—by precipitation with low molecular weight alkanes is a physical method of separation based on solubility. Figure 1 shows that pentane precipitates more asphalt components (17.0 wt %) than does heptane (10.6 wt %). It would be expected that when pentane asphaltenes are treated with heptane, the amount of material equal to 10.6 wt % (based on asphalt) would be precipitated. However, more—14.8 wt %—is precipitated. A similar, but even more pronounced, effect can be seen when heptane and decane treatments are compared (see Figure 1). 

Figure 16.9 shows graphically how ions can be separated based on solubility. The relation between the concentration of Pb and Cl ions in contact with solid PbCF... 

The following example illustrates how solubility-product calculations are used to determine the feasibility of separations based on solubility differences. 

The use of chiral, nomacemic solvents in methods of separation of enantiomers has been reviewed by Eliel et al. (1994). They conclude that separation based on solubility difference between enantiomers in a chiral solvent is unlikely to lead... 

Other modes of LC operation include liquid-liquid partition chromatography (LLC) and bonded phase chromatography. In the former, a stationary liquid phase which is immiscible with the mobile phase is coated on a porous support, with separation based on partition equilibrium differences of components between the two liquid phases. This mode offers an alternative to ion exchange in the fractionation of polar, water soluble substances. While quite useful, the danger exists in LLC that the stationary phase can be stripped from the column, if proper precautions are not taken. Hence, it is typical to pre-equil-ibrate carefully the mobile and stationary phases and to use a forecolimn, heavily loaded with stationary phase 9). 

This chapter focuses on gas-liquid chromatography, in which compounds in a sample are separated based on vapor pressures and differences in affinity for the stationary phase (a high boiling point liquid) versus the gaseous mobile phase. The time between sample injection and detection of the individual compound eluting from the column is called the retention time. Compounds that have limited solubility in the stationary phase will exit the column quickly as a large proportion will remain in the mobile phase. Compounds with polarity similar to that of the stationary phase will have longer retention times and potentially broader peaks, due to increased interaction with the stationary phase. 

Lastly, we consider Boesten et al. (2001) (excerpt 5E), which describes an asymmetric Strecker synthesis. Recall that the synthesis results in two diastereomers, which can be separated based on their different solubilities in water. In exercise 2.14 (chapter 2), you were asked to decide where the Discussion section began in this article. That exercise was more challenging than you may have realized. The article presents numerous results, with only a few sections of integrated discussion. 

Chromatography and electrophoresis are used to separate dissolved constituents in seawater. Chromatography is based on partition of the individual components between gas or liquid passed through a column and the liquid or solid stationary phase. This partition is based on solubilities of dissolved material in the different phases and specific chemical interactions with column components. Electrophoresis separates materials on the basis of electrical charge and size as solvents flow through the plate. 

Petroleum can be fractionated into four generic types of materials representing general chemical properties. These include saturated hydrocarbons, aromatic hydrocarbons, resins, and asphaltenes. The standard ASTM separation procedure (D2007) for isolating the asphaltenes and the other components in petroleum is based on solubility behavior and chromatography, as shown in Fig. 5. Commerically, many refineries utilize solvent separations to produce a solvent deasphalted oil which has lower impurity levels. 

Boduszynski et al. (1980) also employed the more conventional separation procedure based on solubility properties (Corbett, 1969) to provide asphaltene and maltene samples from the 675°C+ residuum. Asphaltenes are isolated by precipitation in an alkane solvent, with further separation of maltenes by chromatography in solvents of increasing elution strength. The FIMS results in Fig. 9 illustrate, significantly, that asphaltenes are not necessarily the highest-molecular-weight components in residuum. Asphaltenes have, rather, a relatively low but broad distribution of molecular weights. 

Solvation in supercritical fluids depends on the interactions between the solute molecules and die supercritical fluid medium. For example, in pure supercritical fluids, solute solubility depends upon density (1-3). Moreover, because the density of supercritical fluids may be increased significantly by small pressure increases, one may employ pressure to control solubility. Thus, this density-dependent solubility enhancement may be used to effect separations based on differences in solute volatilities (4,5). Enhancements in both solute solubility and separation selectivity have also been realized by addition of cosolvents (sometimes called entrainers or modifiers) (6-9). From these studies, it is thought that the solubility enhancements are due to the increased local density of the solvent mixtures, as well as specific interactions (e.g., hydrogen bonding) between the solute and the cosolvent (10). 

Various separation methods have been used to isolate, fractionate, and characterize humic materials. Originally it was fractionation, based on solubility differences of humic components in diluted alkalis and acids, which laid the ground work for the first classifications of humic substances (HS) in the 19th century (Mulder, 1861 Sprengel, 1837) and provided for operational definition of HS (Kononova, 1966). And now, alkali extraction is the method of choice for isolating HS from solid humus-containing substrates like soil, peat, coal, and so on (Swift, 1996), while hydrophobic resins (e.g., Amberlite XAD resins) are typically used to extract HS dissolved in natural waters (Aiken, 1985). Initial research on HS began with the used simple separation methods to prove, examine, and define characteristics of components of humic matter (Oden, 1919).Today, however, advances in HS research require ever more sophisticated techniques of separation combined with structural analysis (Orlov, 1990 Stevenson, 1994). 

The separation methods described in Section IV for pairs of optically active diastereoisomers have also been applied to mixtures of diaste-reoisomeric pairs of enantiomers. By one or other of the following methods, it was possible to separate diastereoisomeric pairs of enantiomers completely or, at least, to obtain enriched fractions. In chromatography under achiral conditions, enantiomers are eluted at identical rates. Therefore, the.pairs of enantiomers (RR )l(SS ) and (RS )I(SR ) can be separated chromatographically in the same way as a pair of optically active diastereoisomers (RS1) and (SS ). By application of the methods based on solubility differences for many examples, fractions containing the less soluble pair of enantiomers could be separated from fractions containing the more soluble pair of enantiomers. However, with respect to the separation.by solubility differences, the situation with two diastereoisomeric pairs of enantiomers is more complicated than that of a pair of... 

Maize starch may be separated after irradiation into several fractions, based on solubility in alcohol and aqueous alcohol. The size of the fractions and their composition depends on the radiation dose, as shown in Table X which also shows the distribution of organic products of destruction (aldehydes and carboxylic acids) in particular fractions.118 The relations presented in this Table are S-shaped. Under irradiation with increasing doses, the destruction of starch obviously increases. The nature of the increase of acidity in com starch has also been studied by Athanassiades and Berger.119 Thollier and Guilbot120 have conducted similar studies on potato starch, and Raffi et al99 have extended their studies to more varieties of starch. The results expressed as free and total acidities, as well as the quantity of formic acid at equilibrium water content, are given in Table XI. These data vary rather nonlinearly with increase of the irradiation dose and water content. 

Compounds in solution are often separated based on their solubility differences. During liquid-liquid extraction (also called solvent extraction), a second solvent immiscible to the first is added to the solution in a separatory funnel (shown at right). Usually one solvent is nonpolar and the other is a polar solvent like water. The two solvents are immiscible and separate from each other after the mixture is shaken to allow solute exchange. One layer contains the compound of interest, and the other contains impurities to be discarded. The solutions in the two layers are separated from each other by draining liquid through the stopcock. 

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