Other Materials for Anion Separations

4 Other Materials for Anion Separations Crown Ether Phases 

Separations of inorganic and organic anions are not only possible by using strongly basic anion exchangers based on organic polymers or silica as substrates. In the mid seventies, Blasius et al. [35] described the separation of ionic species on crosslinked 

The possibility of complexation of inorganic salts, as well as of ionic and non-ionic organic compounds, can be explained by the dipole effect of the C-O bond in the polyether ring. Other hetero atoms such as nitrogen or sulfur may also act as donor atoms. The selective complexation of a specific cation, which is always associated with an anion because of electroneutrality, depends on the number and position of the hetero atoms. 

Molecular structure of a RbNCS ion-pair that is coordinated by 18-crown-6 

Complexation with organic molecules occurs very often via hydrogen bonding. 

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Other Materials for Anion Separations 3.4.3.1 Crown Ether and Cryptand Phases... 

A wide variety of physical properties are important in the evaluation of ionic liquids (ILs) for potential use in industrial processes. These include pure component properties such as density, isothermal compressibility, volume expansivity, viscosity, heat capacity, and thermal conductivity. However, a wide variety of mixture properties are also important, the most vital of these being the phase behavior of ionic liquids with other compounds. Knowledge of the phase behavior of ionic liquids with gases, liquids, and solids is necessary to assess the feasibility of their use for reactions, separations, and materials processing. Even from the limited data currently available, it is clear that the cation, the substituents on the cation, and the anion can be chosen to enhance or suppress the solubility of ionic liquids in other compounds and the solubility of other compounds in the ionic liquids. For instance, an increase in allcyl chain length decreases the mutual solubility with water, but some anions ([BFJ , for example) can increase mutual solubility with water (compared to [PFg] , for instance) [1-3]. While many mixture properties and many types of phase behavior are important, we focus here on the solubility of gases in room temperature IFs. 

Silica-base stationary phases have also been employed for enantiomeric separations in CEC [6,72-81]. In the initial work on chiral CEC, commercially available HPLC materials were utilized, including cyclodextrins [6,74,81] and protein-type selectors [73,75,80] such as human serum albumin [75] and ai-acid glycoprotein [73]. Fig. 4.9, for example, depicts the structure of a cyclodextrin-base stationary phase used in CEC and the separation of mephobarbital enantiomers by capillary LC and CEC in a capillary column packed with such a phase. The column operated in the CEC mode affords higher separation efficiency than in the capillary LC mode. Other enantiomeric selectors are also use in CEC, including the silica-linked or silica-coated macrocyclic antibiotics vancomycin [82,83] and teicoplanin [84], cyclodextrin-base polymer coated silicas [72,78], and weak anion-exchage type chiral phases [85]. Relatively high separation efficiency and excellent resolution for a variety of compounds have also been achieved using columns packed with naproxen-derived and Whelk-0 chiral stationary phases linked to 3 pm silica particles [79]. Fig. 4.10 shows the... 

For polar organic substances more soluble in water than in organic liquids, there will be little movement if an anhydrous mobile phase is used adding water to the solvent will cause those substances to migrate. Thus, butan-1-ol is not a suitable solvent for amino acids unless it is saturated with water addition of acetic acid allows more water to be incorporated, and hence increases the solubility of amino acids, particularly basic ones the addition of ammonia increases the solubility of acidic materials. tert-Butanol and water mixtures are the primary solvent for the separation of many polar anionic species, and many other polar substances with solubility characteristics similar to those of amino acids, such as indoles, guanidines and phenols, can be separated with this mixture. For hydrophobic stationary phases, various mixtures of benzene, cyclohexane and chloroform have been used to good effect as eluants. 

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