Ionic separation methods

Tables 3.10e and 3-1 Of suggest that the ionic separation methods IEC and IPC are the most complicated ones. Both methods involve several mutually dependent primary parameters and a series of additional secondary optimization parameters. Therefore, these techniques are bound to be the subject of many optimization studies in the future.

Separation methods based on size include size exclusion chromatography, ultra-filtration, and ultracentrifugation (see Chapter Appendix). The ionic properties of peptides and proteins are determined principally by their complement of amino acid side chains. Furthermore, the ionization of these groups is pH-dependent. 

Recently, Hurwitz and Kustin have reinvestigated this exchange reaction using the same isotopic procedure and the 2-butanone separation method, in conjunction with a stopped flow apparatus. A rate coefficient of 2.3 x 10 l.mole . sec was obtained for the conditions, temperature 25 °C and ionic strength 0.1 M. Application of the Marcus theory to results obtained for the reaction... 

High polarity is one of the reasons why both the ionic and amphoteric surfactants, and especially their metabolites, are difficult to detect. This property, however, is important for the application tasks of surface-active compounds, but is also the reason for their high water solubility. Due to this fact, their extraction and concentration from the water phase, which can be carried out in a number of very different ways, is not always straightforward. Furthermore, they are often not volatile without decomposition, which thus prevents application of gas chromatographic (GC) separation techniques combined with appropriate detection. This very effective separation method in environmental analysis is thus applicable only for short-chain surfactants and their metabolites following derivatisation of the various polar groups in order to improve their volatility. 

Gas chromatography (GC) has developed into the most powerful and versatile analytical separation method for organic compounds nowadays. A large number of applications for the analysis of surfactants have emerged since the early 1960s when the first GC papers on separation of non-ionics were published. The only major drawback for application of GC to surfactants is their lack of volatility. This can be easily overcome by chemical modification (derivatisation), examples of which will be discussed extensively in the following paragraphs. This chapter focuses on surfactant types, and in addition discusses some structural aspects of alkylphenol ethoxylates (APEOs) that are important for, as well as illustrative of, aspects of separation and identification that are linked to the complexity of the mixtures of surfactants that are involved. 

Figure 3.21 Separation methods in which the ionic nature of the test molecule plays an important part.

Berthod, A., Ruiz-Angel, M. J., and Huguet S., Nonmolecular solvents in separation methods Dual nature of room temperature ionic liquids. Anal. Chem., 77,4071-4080,2005. 

The acid-base properties, and hence ionic character, of peptides and proteins also can be used to achieve separations. Ion-exchange chromatography, similar to that described for amino acids (Section 25-4C), is an important separation method. Another method based on acid-base character and molecular size depends on differential rates of migration of the ionized forms of a protein in an electric field (electrophoresis). Proteins, like amino acids, have isoelectric points, which are the pH values at which the molecules have no net charge. At all other pH values there will be some degree of net ionic charge. Because different proteins have different ionic properties, they frequently can be separated by electrophoresis in buffered solutions. Another method, which is used for the separation and purification of enzymes, is affinity chromatography, which was described briefly in Section 9-2B. 

Hydrotalcite is often too fine grained to produce treatment columns with suitable permeability. As an alternative, the sorbent may be mixed with contaminated water in a tank (Lazaridis et al, 2002). The spent sorbent is then separated from the treated water by flocculation, flotation, or other separation methods (see Section 7.2.4). Lazaridis et al. (2002) investigated the use of surfactants with dispersed-air flotation to separate spent hydrotalcites from treated water. At ionic strengths of 0.1 M using KNO3, effective flotation and separation could be obtained by using a mixture of dodecylpyridinium chloride, sodium dodecylhydrogen sulfate, and a cetyltrimethyl ammonium bromide frother (Lazaridis et al., 2002,322,323). 

Actually, as can be seen in the following section, several of the separation methods mentioned above have already been tested in the purification of at least fresh ionic liquids. However, there is still some development necessary to come up with sustainable regeneration emits. 

The separation of the auxiliary agent can be easily handled on a technical scale if it forms a pure phase. Otherwise more sophisticated separation methods are needed. In the case of ionic liquids a process termed organic solvent nanofiltration has been tested successfully [120,128]. 

Purity is indicated by a clean, single, sharp peak on analytical RP-HPLC. The presence minor peaks of shoulders or a distorted peak shape, are indicators that the material needs further purification. To further confirm purity an orthogonal separation method should be used. For example, analytical ion-exchange HPLC, Fig 2E, for which the separation is based on ionic rather than nonpolar interactions. Analytical isoelectric focusing can also be used. 

Amines have been used as the extractants for the actinide-bearing anionic species. This is usually from high-acidic or high-ionic strength medium, which generally lead to very high decontamination factors. The precondition of their use in SLM-based separation methods is the inertness of the membranes toward high acidic or salt medium. As membranes made from polycarbonate or polyamide may not be very stable toward these medium, most of the reported work in this area is on PTFE or PP membranes. 

One advantage of the ion interaction theory is that it can be applied to solutions of different salinities, that is, to brines, seawaters with different salinities, and estuarine waters. While the ionic medium method provides a very simple solution to many problems, especially for the speciation of constituents in the open ocean, it cannot readily be applied to solutions of different salinity that is, brines, seawater, and estuarine waters must be treated as separate solvents (Pabalan and Pitzer, 1988). 

The most desirable separation method in most cases is phase separation, which can be achieved via tuning of the hydrophobicity or hydrophilicity of the ionic liquid to effect immiscibility with the species for extraction. Phase separation may also be... 

In addition to the three classical separation methods mentioned above, reversed-phase liquid chromatography (RPLC) is becoming increasingly popular for the separation of highly polar and ionic species, respectively. Long-chain fatty acids, for example, are separated on a chemically bonded octadecyl phase after protonation in the mobile phase with a suitable aqueous buffer solution. This separation mode is known as ion suppression [18]. 

Selection of the mobile phase is critical in the characterization of silica sols by SEC. As with the other separation methods, pH should be slightly basic, and low ionic strength must be used to prevent particle aggregation. In addition, the mobile phase must interact with the surface of the packing-particle pores to neutralize undesirable charge effects. Negatively charged surfaces within the pore can result in ion-exclusion effects whereby... 

Kl. Kendall, J., Separation by the ionic migration method. Science 67, 163 (1928). 

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