ILBSs

We dwell on certain aspects of ILBSs that are important to their applications.This includes their synthesis and aggregation in water and nonaqueous solvents and the properties of the aggregates formed. This is followed by showing representative applications that exploit the surface-active nature of the ILBSs. 

We advance several reasons for discussing the deceptively simple synthesis of ILBSs and their parent ILs a search in the catalog of a major chemical reagents supplier has shown that only a very few ILBSs are commercialized. If the purities in these surfactants are similar to those of the structurally related ILs, commercialized by the same supplier, then this purity is not satisfactory, because 81% have purity less than or equal to 98% (Fig. 4.2). Note that surface-active purity ... 

Figure 4.1 Evolution of publications on ILs and ILBSs during the period 1989-2013. Basic research papers and patents on ILs are shown in (a), whereas the corresponding plots for ILBSs are depicted in (b).

Figure 4.3 Molecular structure of the ILBSs that we cover in this review.

The most employed synthetic routes for ILs or ILBSs are summarized in Scheme 4.1. Their synthesis involves one or two distinct steps an S reaction between a tertiary amine, for example, 1-methylimidazole and an alkyl halide or alkyl methane sulfonate, to yield halides or methane sulfonates. The second step consists, in most cases, of metathesis (from the Greek words meta for change and tithemi for place) of the halide or methane sulfonate counterion, leading to ILBSs containing complex... 

Scheme 4.1 Summary of synthetic paths to obtain ILBSs.

The most frequently employed procedures for the reaction include reflux in an appropriate molecular solvent, for example, acetonitrile, usually for many hours [8-14]. Alternatively, ILBSs have been synthesized in the absence of solvents, in a MW reactor [15], or by using MW and ultrasound irradiation [7].The metathesis step is usually carried out in one- or two-phase systems [16,17]. One convenient route to convert first- to second-generation IL is to exchange, for example, the halide into hydroxide on macro porous ion exchange column, followed by careful neutralization of the IL-OH by an appropriate acid. 

Alternatively, gemini ILBSs have been synthesized by reacting imidazole with a di-haloalkane, followed by alkylation of the two outer nitrogen atoms (see Scheme 4.3). 

A new type of gemini ILBSs that carries no imidazolium rings has been synthesized by Li and coworkers. These novel compounds were found to be ILs at room temperature —apart from (Cj -E02-Cj )Br2 and (C -EOj-CJEr n = 14 or 16), which were yellow waxy solids at room temperature but melt at low temperatures. 

ILBSs that carry ester, amide, or ether groups are interesting especially because these groups render the ILBSs more biodegradable [20]. ILBSs with functionalized side chain have been synthesized by the S reaction of substituted imidazole with a-bromoester or a-bromoamide (Scheme 4.6). 

Zwitterionic ILBSs have been synthesized by reacting bromoacetic acid with A-alkylimidazole and then neutralizing with NaOH (Scheme 4.7). Alternatively, the... 

We set to compile literature data that show the effects of structure of ILBSs on the properties of their solutions, hence on their applications. Because the imidazole ring carries two nitrogen atoms that can be substituted independently, most of these... 

Scheme 4.6 Synthetic route to ILBSs with side ester or amide group side chain. Source Kanjilal et al. [13] and reproduced with permission from Wiley-VCH.

Scheme 4.7 Synthesis of zwitterionic ILBSs, from Af-alkyUmidazole and bromoacetic add. Source Wang et aL [21, 22] and reproduced with permission from Elsevier and ACS, respectively.

The progress of application of ILBSs in diverse fields, for example, organic and polymer synthesis and nanotechnology [5, 6,24-27] rests on a clear understanding of the molecular structure/property/performance relationships of both bulk compounds and their solutions in water and organic solvents. 

TABLE 4.1 Published Data on the Micellar Properties ol ILBSs ... 

Cationic ILBSs with hydrocarbon side chains 1.1. 1,3-Dialkylimidazolium halides 1.1.1. Bromide counterion... 

Cationic ILBSs with functionalized side chains Amides and esters ... 

TABLE 4.2 Thermodynamic Parameters o( MiceDe Formation o( ILBSs ... 

As shown earlier, the structural versatility of ILs and the derived ILBSs results in concomitant variation in the properties of the solvent proper or the micellar solution of ILBS these are important for applications. Examples that are worth mentioning include solvents for organic, organometallic, and inorganic compounds [71] and biopolymers [25] nonaqueous, polar solvents whose miscibility with water and organic solvents can be employed for carrying out reactions in two-phase systems [72] and templates for specific nanostructure [24,26,27]. 

Many conventional cationic surfactants have antibacterial properties, in particular the 1-alkylpyridinium halides and the benzaUconium chlorides (Af-benzyl-A Af-dimethyl-A-R chloride, R=Cg to C ). It is, therefore, only natural that ILBSs were tested for the same property, especially because this bears on their impact in the environment, for example, their toxicological effects on aqnatic organisms [73-76]. The emphasis in these stndies has been on the types of microorganism sensitive to ILBSs and the structme/activity relationships (SAR), in particnlar with regard to the length of the alkyl gronp, the heterocycle employed, and the counterion. 

Figure 4.7 Dependence of the antimicrobial activity (MIC) of the ILBSs RMelmBr and RPyBr on the corresponding cmc. Source Cornelias et al. [40] with copyright permission from Elsevier.

Figure O Molecular structures of the synthesized functionalized ILBSs.

The beneficial effect of the hydroxyethyl side chain on the antimicrobial properties of ILBSs was investigated further by synthesizing silver nanoparticles stabilized by two surfactant series l-Cj -3-hydroxyethylimidazolium bromide,... 

Figure 4.10 Correlation between the efSdency of ILBSs as antimicrobial agents (log [MIC]) with their surface activity (pC20). Source Luczak et al. [80] and reproduced with permission...

The volume of work on the antitumor activity and cytotoxicity of ILs and ILBSs is limited, with more publications on the effects ILs than their surface-active counterparts. Although discussion on the former class of compounds is outside the scope of the present article, we note that imidazolium-, phosphonium-, and ammonium-based ILs have been employed against 60 human tumor cell lines, including brain tumor cells, [83], human hepatocellular liver carcinoma [84], HeLa cancer cells [85], and human melanoma cell lines [86],... 

The effects of the molecular structure of phosphonium- and ammonium-based ILs and ILBSs side chain counterion BF , PF ,Tf2N") on the antitumor activity and cytotoxicity were evaluated in vitro for the first time for the NCI-60 human tumor cell lines. All surface-active compounds (see Rg. 4.12) were found to be active against leukemia, melanoma, lung, colon, kidney, ovary, breast, prostate, and central nervous system cancer cells the tetra(l-butyl)ammonium TfjN" was not [87]. 

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