Ionic Liquid-Based Surfactants: Recent Advances in Their Syntheses, Solution Properties, and Applications.

Omar A El Seoud,Paula D Galgano,Naved I Malek,Nicolas Keppeler

doi:10.3390/polym13071100

Omar A El Seoud, Paula D Galgano + Show 2 more

Open Access

https://doi.org/10.3390/polym13071100

Copy DOI

Abstract

The impetus for the expanding interest in ionic liquids (ILs) is their favorable properties and important applications. Ionic liquid-based surfactants (ILBSs) carry long-chain hydrophobic tails. Two or more molecules of ILBSs can be joined by covalent bonds leading, e.g., to gemini compounds (GILBSs). This review article focuses on aspects of the chemistry and applications of ILBSs and GILBSs, especially in the last ten years. Data on their adsorption at the interface and micelle formation are relevant for the applications of these surfactants. Therefore, we collected data for 152 ILBSs and 11 biamphiphilic compounds. The head ions of ILBSs are usually heterocyclic (imidazolium, pyridinium, pyrrolidinium, etc.). Most of these head-ions are also present in the reported 53 GILBSs. Where possible, we correlate the adsorption/micellar properties of the surfactants with their molecular structures, in particular, the number of carbon atoms present in the hydrocarbon “tail”. The use of ILBSs as templates for the fabrication of mesoporous nanoparticles enables better control of particle porosity and size, hence increasing their usefulness. ILs and ILBSs form thermodynamically stable water/oil and oil/water microemulsions. These were employed as templates for (radical) polymerization reactions, where the monomer is the “oil” component. The formed polymer nanoparticles can be further stabilized against aggregation by using a functionalized ILBS that is co-polymerized with the monomers. In addition to updating the literature on the subject, we hope that this review highlights the versatility and hence the potential applications of these classes of surfactants in several fields, including synthesis, catalysis, polymers, decontamination, and drug delivery.

Highlights

Ionic liquids (ILs) are electrolytes whose melting points are, by operational definition, ≤100 ◦C
The effect of the spacer length on cmc is complex because it is a sum of several factors, including rigidity of the molecule, hydrogen bonding, hydration of head group (HG), Coulombic repulsion between HGs. This complex behavior was shown by Pal et al [157] for a series of GILBSs containing two imidazolium rings in the HG and spacer from 2 to 12 methylene groups
They observed a lower cmc value for the (CH2)3 spacer, after which the cmc values increased and reached a plateau (Figure 7). This was explaTinheedefifnectteormf tsheofspriagciedrilteynagnthdopnlacnmacr insactoumreploefxtbheecaimusiedaitzioslaiusmumHoGf,swevheircahl fianctteorrfse,re inwcluitdhinthgerisgpiadciteyr opfatchkeinmgo, lecaudlien,ghytodriongdeenpebnodnednint gbe(whahveiroeraopfpeliaccahblsei)n,ghlyedcrhaationnboefyHonGd, a Cosuplaocmerboicfs3.ion between HGs

Summary

Introduction

Ionic liquids (ILs) are electrolytes whose melting points are, by operational definition, ≤100 ◦C. Demonstrating this purity is important because uncertainty in the value of the critical micelle concentration (cmc) bears on the calculated adsorption and micellar parameters [5,6]; removing surface-active impurities from the surfactant solution is, at best, time-consuming and laborious [7] Another aspect that should be considered when discussing ILBSs is their stability in aqueous media. We stress that this instability problem should not be overlooked; its potential effect on micellar parameters and other applications should be assessed Based on these considerations, we feel justified in our decision to exclude from the parts of surfactant adsorption at the water/air interface and micellization of ILBSs with hydrolytically unstable ions, in particular BF4- and PF6-. 86 [140] 111 [140] 173.4 [143] 86.76 [144] 84.7 [133], 66 [137] 68.5 [133], 80 [139] 122.64 [144]

Abbreviations

Findings

Nanotechnology