IL-based surfactants

A number of ILs containing long alkyl chain substituents have been reported to be surface active and are currently termed as IL-based siufactants [7]. In fact, the term IL-based surfactants is more precise than IL-siufactants, especially considering that the majority of the IL-based surfactants have melting points above 100°C. IL-based surfactants, which possess characteristics of cationic surfactants, undoubtedly constitute a new class of surfactant system. It must be considered that there is only a limited number of conventional cationic surfactants that have wide applicability in separation science. 

Obviously, once an IL is dissolved in water, the anion/cation pair no longer constitutes a neat IL. Indeed, there will be only cations and anions in solution. However, the ability to modify the micellar properties of IL-based surfactant solutions by changing easily the structural makeup of the IL-based surfactant is clearly correlated with the tunability of neat ILs. In other words, simple modifications in the structure of the anion/cation belonging to the IL-based surfactant are accompanied by significant changes in the micellar properties. This is hardly observed with classical surfactant solutions. It is well known that simple modifications in ILs structures are accompanied by huge changes in their properties. 

Table 23.1 [8-40] includes a list of the most successful IL-based surfactants employed in analytical separations and applications. Their structures and abbreviations are accompanied with their reported CMCs and available aggregation number (N ). 

At present, a number of analytical applications have emerged in separation science regarding the use of IL-based surfactant, not only as solubilization media for proteins [41], but in analytical techniques (such as chromatographic or electrophoretic methods), and also in sample preparation schemes. Given their different performances, it is important to study each application field in an independent way. 

TABLE 23.1 IL-Based Surfactants Most Widely Used in Analytical Separation Science Applications... 

Analytical Applications of IL-Based Surfactants in Analytical Techniques... 

Lately, IL-based surfactants have been used in separation techniques of wide apph-cability, such as high-performance hqnid ehromatography (HPLC) and capillary electrophoresis (CE). This section will only focus on those applications that take advantages of the micellar properties of IL-based sinfactants, that is, works that use micellar solutions of IL-based surfactants in HPLC or in CE. In this consideration, applications that employ IL-based surfactant as mere background electrolytes have not been taken into account. 

Utilization of IL-Based Surfactants in HPLC MLC is a mode of HPLC in which micelles are used as pseudophases in the mobile phase. Surfactants vahd for MLC applications should have a low CMC value, which is a common trend for many IL-based surfactants (Table 23.1). 

First application of IL-based surfactants as pseudophases in MLC was reported in 2009 [42], It was a preliminary study regarding the retention behavior of benzene using three different LC stationary phases (Eehpse XDB C8,Zorbax C8,and Gemini C18) and Cj C Im-Br. A comparison was also made with the conventional cationie surfactant cetyltrimethylammonium bromide (CTAB) showing similar performanee. 

Utilization of IL-Based Surfactants in CE IL-based surfactants have been widely employed as baekgroimd eleetrolytes in eapOlary zone electrophoresis (CZE). However, these appheations do not take advantage of the micellar properties of IL-based smfactants, and so their performanee is not related to any micellar property. 

IL-Based Surfactant (Concentration, mmol Analytes (Number) Separation Time (min) Detection Comments Comparison with Conventional Surfactants Reference... 

It is interesting to note that the first application involved the use of chiral IL-based surfactants and their polymeric analogues to separate chiral acidic analytes [38] with successful results in this difficult task. 

IL-based surfactants have also fonnd a large application field in sample preparation, apart from their intense uses in the previously mentioned analytical techniques. In sample preparation, IL-based snrfactants are mainly used as extracting solvents, acting as adeqnate substitutes for toxic organic solvents. 

When introducing a SPME fiber in an aqueous solution containing analytes and IL-based surfactant micelles, the following equation is applicable ... 

If an analyte does not suffer partitioning to an IL-based surfactant micelle, the plot 1/C versus (Eq. 23.2) is a constant, as it can be observed in Figure 23.3a. However, if an analyte suffers partitioning, the plot reflects such interaction (Fig. 23.3b), and calculations can be performed to obtain (Fig. 23.3c).Therefore,... 

GC) [53] and HPLC [54], Using this approach, values varied from 30 for methyl caproate and dimethyl phthalate with the dicationic IL-based surfactant C Ts ) C -2Br, and being also of 30 for o-cresol with the monocationic IL-based surfactant C C Im-Br [53], to 6140 for acenaphthene with C MIm-Br [54]. Clearly, hydrophobic analytes presented higher partition coefficient values over polar... 

Figure 23.3 Peak area obtained in solid-phase microextraction at increasing IL-based surfactant concentrations keeping constant the analyte concentration in solntion for (a) analytes that do not suffer partition to the IL-based surfactant miceUes and (b) analytes that suffer partitioning to the IL-based surfactant micehes. (c) Plots to calcnlate partition coefficients (from the slope) for analytes that suffer partitioning.

Furthermore, differences in the obtained K., values were also attributable to the micellar structure of IL-based surfactants. These results aimed to employ IL-based surfactant in efficient and selective extraction methods by adequate selection of the IL-based surfactant nature and/or the group of compounds to be determined. 

It is worthy of mentioning that Yao et al. [54] also proved that the partition coefficient values obtained for a group of analytes and IL-based surfactants were higher than those obtained for the conventional cationic surfactant CTAB, which points out to the superior performance in terms of extraction abilities for these IL-based surfactants. 

More recently, an in situ preconcentration strategy to improve the sensitivity of the overall extraction method for solid samples using IL-based surfactants as extracting solvents has been proposed [56]. The method is based on transforming a water-soluble IL-based surfactant into a water-insoluble IL-based surfactant by means of... 

Figure 23.4 (a) Overall extraction scheme for analytes contained in solid samples when using aqueous solutions of IL-based surfactants (at concentrations above the CMC) as extracting solvents. No chlorinated organic solvent are needed in the extraction step and the matrix can be finally removed by filtration or centrifugation, (b) In situ preconcentration step with ILBSs. R and R are long alkyl chains. 

---