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

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.

Keywords: adsorption at water/air interface; catalysis; drug delivery; formation of micelles and microemulsions; gemini ionic liquid-based surfactants; ionic liquid-based surfactants; ionic liquids; mesoporous nanoparticles; molecular structure/properties relationships; polymerization.

Figure 1

Formation of different morphologies as a function of the surfactant molecular structure (1-C₁₆-3-R-imidazolium bromides; R = C₂ to C₁₆) and their concentration in water. S, H, WM, G and P refer to isotropic solution, hexagonal liquid crystals, wormlike micelles, hydrogel and ionic liquid-based surfactants precipitation, respectively [2]. Reprinted with permission from ref. [2]. Copright 2021 Elsevier.

Figure 2

Number of publications on ionic liquids (a) and ionic liquid-based surfactants (b) between years 2000–2020, source SciFinder database.

Scheme 1

Schematic representation of the synthesis of ionic liquids and ionic liquid-based surfactants. MW and US refer to microwave and ultrasound irradiation, respectively.

Scheme 2

Synthetic route to ionic liquid-based surfactants with ester- or amide group side chain. Redrawn from Kanjilal [46].

Scheme 3

Synthetic route to gemini ionic liquid-based surfactants via the use of a protecting group. Redrawn from Baltazar [45].

Scheme 4

Synthetic route to gemini ionic liquid-based surfactants, via the use of dihaloalkanes. Redrawn from Baltazar [45].

Scheme 5

Molecular structures and acronyms of the cationic head-groups of ionic liquid-based surfactants.

Figure 3

Dependence of log cmc on the number of carbons in the hydrophobic chain (C_x) for cationic ionic liquid-based surfactants. Data taken from conductivity measurements. The abbreviations of the surfactant head-ions are those listed in the footnotes of Table 1.

Figure 4

Log critical micelle concentration (cmc) as a function of the size of heterocyclic amine ring structures of cationic ionic liquid-based surfactants. Data taken from Schnee and Palmer [150].

Figure 5

Dependence of log cmc on the number of carbons in the head group side chain (C_y) of (a) cationic ionic liquid-based surfactants and (b) anionic ionic liquid-based surfactants. Data taken from conductivity measurements; see references [80,91,126,151,152].

Scheme 6

Molecular structures of the gemini ionic liquid-based surfactants reported in this review.

Figure 6

Dependence of cmc on the number of carbon atoms in the hydrocarbon chain (from C₈ to C₁₆) for the surfactant series with one ethylene oxide spacer [161]. Reprinted with permission from ref. [161]. Copright 2021 Elsevier.

Figure 7

cmc as a function of spacer length from m = 2–12 in ((C₁₆Im)₂(CH₂)_m)Br₂ series [157].

Figure 8

Dependence of the aggregate morphologies on the structure of the cation and anion in gemini (A) and trimeric (B) biamphiphilic surfactants.

Figure 9

Schematic representation of the electrostatic (a) and steric (b) stabilization mechanisms of nanoparticles. In the former mechanism, the nanoparticles (NPs) are stabilized due to electrostatic repulsion of the positively charged outer layer. Steric repulsion between the surfactant hydrophobic chains contributes to NP stabilization [219]. Reprinted with permission from ref. [219]. Copright 2021 Springer Nature.

Figure 10

Schematic representation for the production of FeO₂H particles coated with an ionic liquid-based surfactant layer [194]. Reprinted with permission from ref. [194]. Copright 2021 Elsevier.

Figure 11

Schematic representation of the effects of C₁₀C₁ImCl and ionic liquid-based surfactant plus the co-template (P123) on the morphology of the formed SiO₂ NPs. HCl is used to hydrolyze tetraethyl orthosilicate silane (TEOS), the silicate precursor [195]. Reprinted with permission from ref. [195]. Copright 2021 Elsevier.

Figure 12

(a) MACn = mesoporous silica nanoparticles prepared by strategy (i) and (b) MBCn = mesoporous silica nanoparticles prepared by strategy (ii) [198]. Reprinted with permission from ref. [198]. Copright 2021 Elsevier.

Figure 13

Schematic representation for the fabrication of functionalized mesoporous silica nanoparticles [200]. Reprinted with permission from ref. [200]. Copright 2021 Elsevier.

Figure 14

Schematic representation for the fabrication of functional molybdenum-containing mesoporous silica nanoparticles [201]. Reprinted with permission from ref. [201]. Copright 2021 Elsevier.

Figure 15

Micrographs showing the dependence of Nd₂O₃ nanoparticles shape on the concentration of N-(3-cocoamidopropyl)-betaine (CAPB), parts (a–d). Part (e) shows the nanoparticles fabricated in the absence of the surfactant [204]. Reprinted with permission from ref. [204]. Copright 2021 Elsevier.

Figure 16

Schematic representation of the effects of surfactant concentrations on the morphologies of Nd₂O₃ nanoparticles. Close to the surfactant critical micelle concentration, small, spherical micelles are formed, leading after calcination, to spherical Nd₂O₃ nanoparticles. Micellar morphology changes at higher [surfactant] lead to the formation of leaf-shaped Nd₂O₃ nanoparticles [204]. Reprinted with permission from ref. [204]. Copright 2021 Elsevier.

Figure 17

Schematic representation of the effect of surfactant anion on the formation of chitosan nanoparticles. Interactions of the chloride ion with the 1-octyl-3-methylimidazolium cations at the aggregate interface lead to the formation of larger aggregates. This is hindered in the case of the voluminous octyl sulfate anion [61]. Reprinted with permission from ref. [61]. Copright 2021 Elsevier.

Figure 18

Schematic representation of the role of the nano-segregated polar and non-polar domains of the ionic liquid-based surfactant in the formation of interconnected network of α-Fe₂O₃ nanoparticles [205]. Reprinted with permission from ref. [205].

Figure 19

Types of microemulsions according to Windsor. Water and oil phases are colored in turquoise and yellow color, respectively. W, O and BC refer to water, oil and bicontinuous phase, respectively [3]. Reprinted with permission from ref. [3]. Copright 2021 Elsevier.

Figure 20

The complete cycle of activators regenerated by electron transfer- atom transfer radical polymerization) (AGET–ATRP) of MMA, polymer precipitation and microemulsion regeneration in the system C₁₂C₁ImBr/C₄C₁ImBF₄/MMA. EBiB and AA refer to the polymerization initiator ethyl-2-bromo-isobutyrate and ascorbic acid, respectively [230]. Reprinted with permission from ref. [230]. Copright 2021 ACS Publications.

Figure 21

Schematic representation of ionic liquid-based surfactant-mediated fabrication of nanoparticles of polystyrene (PS) without and with magnetic properties (MNP) [234]. Reprinted with permission from ref. [234]. Copright 2021 ACS Publications.

Figure 22

SEM images of polymers fabricated by microemulsion polymerization of MMA in the presence of the polymerizable ionic liquid-based surfactant-b. Parts (A–C) refer to polymer gel as produced, the gel after treatment with aqueous solutions of 0.1 mol L⁻¹ of KPF₆ and NaBr, respectively [227].