Redox-switched amphiphilic ionic liquid behavior in aqueous solution

Adsorption and Aggregation Behavior of Mixtures of Quaternary-Ammonium-Salt-Type Amphiphilic Compounds with Fluorinated Counterions and Surfactants

The surface adsorption and aggregation behavior of a mixture of quaternary-ammonium-salt-type amphiphilic monomeric compounds (C₄ FSA, C₈ FSA, and C₄ NTf₂) or gemini compounds (C₁₀-2-C₄ FSA) and various surfactants (nonionic hexaoxyethylene dodecyl ether (C₁₂EO₆), anionic sodium dodecyl sulfate (SDS), cationic dodecyltrimethylammonium bromide (C₁₂TAB), and zwitterionic N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (C₁₂Sb)) was investigated. Both types of compounds contained alkyl chains of nonidentical lengths that used bis(fluorosulfonyl)imide (FSA^-) or bis(trifluoromethanesulfonyl)imide (NTf₂^-) as counterions. The mixtures were analyzed for surface tension, viscosity, electrical conductivity, and pyrene fluorescence, in addition to evaluation by cryogenic transmission electron microscopy, small-angle X-ray scattering, and dynamic light scattering. Our results showed that the surface tension depended on the surfactant structure. For the mixture of C₈ FSA and SDS, as the SDS concentration increased, the surface tension first decreased and became constant at the critical micelle concentration (CMC). In this concentration range, C₈ FSA and SDS were approximately equimolar (2.5 mmol dm^-3), the mixture adsorbed efficiently at the air-water interface, and vesicles and linear-type micelles were formed in the solution owing to the decreased electrostatic repulsion between the hydrophilic groups. As the SDS concentration further increased, the surface tension increased and reached another constant value. The C₈ FSA at the interface was replaced by SDS and the aggregates transformed into spherical micelles. The surface tension plot of the mixture of the amphiphilic compounds and C₁₂Sb showed a minimum at the CMC. The lowest CMC and surface tension were observed for C₁₀-2-C₄ FSA, indicating that the gemini compounds offer excellent adsorption and orientation at the air-water interface. It was revealed that the quaternary-ammonium-salt-type amphiphilic compounds in this study acted as ionic liquids on their own and as surfactants in aqueous solution. Further, they could improve the surface activity of conventional ionic surfactants.

Electrochemically reversible foam enhanced flushing for PAHs-contaminated soil: Stability of surfactant foam, effects of soil factors, and surfactant reversible recovery

Although surfactant foams enhanced-remediation for PAHs-contaminated soil has been proved to be an effective method, lack of simple/economic surfactant recovery methods from the eluent solutions limit its further remediation application for organic contaminated soil. Here, we prepared a electrochemically reversible ferrocene surfactant FcCH₂N⁺(CH₃)C₁₂H₂₅ (Fc12), then investigated the foaming ability and foam stability of Fc12 under its reduced (active state) and oxidation (inactive state) states and explored the flushing efficiency of reduced Fc12 foam for PAHs-contaminated soil and the recovery efficiency of collected eluent solution. The results showed that the foaming ability and foam stability of reduced Fc12 are greatly higher than those of oxidized Fc12, which is indicative of a well reversibly switchable characteristic of Fc12. The contaminated soil flushing efficiencies of reduced Fc12 for phenanthrene and pyrene were 65.28% and 46.45%. The respective desorption efficiency of phenanthrene and pyrene from collected eluent solutions were calculated to be 74.94% and 72.75% by electrochemical oxidation control, which indicates that Fc12 can be well recovered by simple electrochemical control. This study provides a feasible method for the recovery of surfactants from surfactant-enhanced remediation processes by simply electrochemical control.

Surface Adsorption Properties and Layer Structures of Homogeneous Polyoxyethylene-Type Nonionic Surfactants in Quaternary-Ammonium-Salt-Type Amphiphilic Gemini Ionic Liquids with Oxygen- or Nitrogen-Containing Spacers

The amphiphilic ionic liquids containing an alkyl chain in molecules form nano-structure in the bulk, although they also show surface activity and form aggregates in aqueous solutions. Although insights into the layer structures of ionic liquids were obtained using X-ray and neutron scattering techniques, the nanostructures of ionic liquids remain unclear. Herein, the surface adsorption and bulk properties of homogeneous polyoxyethylene (EO)-type nonionic surfactants (C_xEO₆; x = 8, 12, or 16) were elucidated in quaternary-ammonium-salt-type amphiphilic gemini ionic liquids with oxygen or nitrogen-containing spacers [2C_n(Spacer) NTf₂; (Spacer) = (2-O-2), (2-O-2-O-2), (2-N-2), (2/2-N-2), (3), (5), or (6); n = 10, 12, or 14 for (2-O-2) and n = 12 for all other spacers] by surface tension, small- and wide-angle X-ray scattering, cryogenic transmission electron microscopy, and viscosity measurements. The surface tension of C₁₂EO₆ in 2C_n(Spacer) NTf₂ with oxygen-containing spacers increased with increasing concentration of C₁₂EO₆, becoming close to that of C₁₂EO₆ alone, indicating that the amphiphilic ionic liquid adsorbed at the interface was replaced with C_xEO₆. In contrast, both 2C_n(Spacer) NTf₂ with nitrogen-containing spacers and nonionic surfactants remained adsorbed at the interface at high concentrations. In the bulk, it was found that 2C_n(Spacer) NTf₂ formed layer structures, in which the spacing depended on the alkyl chain length of C_xEO₆. These insights are expected to advance the practical applications of amphiphilic ionic liquids such as ion permeation, drug solubilization, and energy delivery systems.

A first-principles investigation of the structural and electrochemical properties of biredox ionic species in acetonitrile

Biredox ionic liquids are a new class of functionalized electrolytes that may play an important role in future capacitive energy storage devices. By allowing additional storage of electrons inside the liquids, they can improve device performance significantly. However current devices employ nanoporous carbons in which the diffusion of the liquid and the adsorption of the ions could be affected by the occurrence of electron-transfer reactions. It is therefore necessary to understand better the thermodynamics and the kinetics of such reactions in biredox ionic liquids. Here we perform ab initio molecular dynamics simulations of both the oxidized and reduced species of several redox-active ionic molecules (used in biredox ionic liquids) dissolved in acetonitrile solvent and compare them with the bare redox molecules. We show that in all the cases, it is necessary to introduce a two Gaussian state model to calculate the reaction free energies accurately. These reaction free energies are only slightly affected by the presence of the IL group on the molecule. We characterize the structure of the solvation shell around the redox active part of the molecules and show that in the case of TEMPO-based molecules strong reorientation effects occur during the oxidation reaction.

Micelle Formation of Morpholinium Bromide Surfactants in Aqueous Solution

The micelle formation of N-alkyl-N-methylmorpholinium bromide (CnMMB, n = 12, 14, 16) surfactants in aqueous solution has been investigated. Their critical micelle concentration (CMC), effectiveness and efficiency of the surface tension reduction, and the maximum surface excess concentration were derived from the surface tension curves. Thermodynamic parameters were then evaluated in the temperature range of 25 °C ∼ 45 °C. The results showed that with increasing the alkyl chain length, the CMC values were decreased gradually, which is consistent with the enhancement of hydrophobic effect. Due to the existence of an oxygen atom in the headgroup, an unusual surface activity of CnMMB surfactants was obtained. The obtained results should help to better understand the effect of the hydrophilic headgroup on the micelle behavior of cationic surfactants containing saturated nitrogen-containing heterocycles.

Physicochemical properties of novel cholinium ionic liquids for the recovery of silver from nitrate media

Linear and ramified cholinium based ionic liquids have been synthesized and their physicochemical properties have been investigated by ¹H NMR, ¹³C NMR, ATR-FTIR and ESI-MS as well as their extraction properties towards Ag(i), Cu(ii) and Fe(iii).

Ionic liquid-induced all-α to α + β conformational transition in cytochrome c with improved peroxidase activity in aqueous medium

Choline dioctylsulfosuccinate [Cho][AOT] (a surface active ionic liquid) has been found to induce all-α to α + β conformational transition in the secondary structure of enzyme cytochrome c (Cyt c) with an enhanced peroxidase activity in its aqueous vesicular phase at pH 7.0.

Self-assembly of new surface active ionic liquids based on Aerosol-OT in aqueous media

New anionic ionic liquid surfactants have been synthesized by replacing the sodium cation of Aerosol-OT (sodium dioctylsulfosuccinate, [Na]AOT) with various biocompatible moieties, such as 1-butyl-3-methyl imidazolium ([C4mim]), proliniumisopropylester ([ProC3]), cholinium ([Cho]), and guanidinium ([Gua]). The Aerosol-OT derived ionic liquids (AOT-ILs) were found fairly soluble in water and formed vesicles above a critical vesicle concentration (CVC) which depended upon the nature of cation, and followed the order: [ProC3]<[C4mim]<[Gua]<[Cho]<Na(+). The self-assembly process was characterized using surface tension (ST), isothermal titration calorimetry (ITC), conductivity, dynamic light scattering (DLS), nuclear magnetic resonance (NMR) and transmission electron microscopy (TEM). Unlike other AOT-ILs, a structural transformation has been observed for [C4mim]AOT above CVC, because of certain amphiphilic character in the cation [C4mim]. Thermodynamic parameters calculated from ITC and conductivity techniques revealed that the vesicle formation process is entropy driven for [C4mim]AOT, whereas the process is both enthalpy and entropy driven for other AOT-ILs. In order to check the versatility of synthesized AOT-ILs we have tested their dissolution behavior in a different class of ionic liquids. All the AOT-ILs were found fairly soluble in the hydrophilic IL, ethanolammonium formate (EOAF), whereas only [C4mim]AOT and [ProC3]AOT were found soluble in hydrophobic IL, [C4mim]Tf2N. Such combinations can have potential for construction of stable colloidal formulations or microemulsions in ionic liquid media.

Photo-induced transformation from wormlike to spherical micelles based on pyrrolidinium ionic liquids

A UV-induced self-assembly based on N-methyl-N-cetylpyrrolidinium bromide (C16MPBr) and sodium (4-phenylazo-phenoxy)-acetate (AzoNa) was fabricated in the present work through facile mixing of C16MPBr and trans-AzoNa in water. Due to the trans-cis photoisomerization functionality of the AzoNa, this self-assembly possesses photo-responsive properties. Under ambient conditions, the morphology of the self-assembly was wormlike micelles with a high viscosity of about 10 Pa s, whereas after applying UV light, the self-assembly transformed into spherical micelles with a dramatically lowered viscosity of about 0.01 Pa s. Different from most reported photo-responsive wormlike micelles consisting of conventional cationic surfactant CTAB, our system was constructed from a novel pyrrolidinium ionic liquid. We hope this study may provide a deeper understanding of photo-responsive wormlike micelles constructed by cationic surfactants and photo-sensitive additives in water.

Effect of cationic head group on micellization behavior of new amide-functionalized surface active ionic liquids

The functionalization of the alkyl chain with an amide group led to marked changes in the characteristic properties of micellization of different SAILs.

Aqueous-biamphiphilic ionic liquid systems: self-assembly and synthesis of gold nanocrystals/microplates

Biamphiphilic ionic liquids (BAILs) based on 1,3-dialkylimidazolium cation and alkyl sulfate anions ([C(n)H(2n+1)mim][C(m)H(2m+1)OSO(3)]; n = 4, 6, or 8; m = 8, 12) have been synthesized and characterized for their self-assembling behavior in the aqueous medium. Effects of alteration of alkyl chain length in cation and anion on surfactant properties of BAILs have been examined from surface tension measurements. The effectiveness of surface tension reduction for BAILs has been found to be exceptionally higher as compared to single chain surface active ILs/conventional surfactants. The thermodynamics of the aggregation process has been studied using isothermal titration calorimetry (ITC) and temperature dependent conductivity experiments. Dynamic light scattering (DLS), nuclear magnetic resonance (NMR), and transmission electron microscopy (TEM) studies showed that BAILs formed distinct aggregated structures depending upon the amphiphilic character present in the cation and anion. BAILs ([C(n)H(2n+1)mim][C(m)H(2m+1)OSO(3)]) form micelles when n = 4, 6; m = 8, intermicellar aggregates when n = 4, 6; m = 12, and vesicles when n = 8; m = 8, 12. Gold nanoparticles and microplates have been synthesized in micellar and vesicle solutions of BAILs using a simple photoreduction method. The studies show the potential of BAILs for constructing micelles and supramolecular assemblies, such as bilayer vesicles, which are effective in preparation of nanomaterials of controlled size and morphology.

Stepwise ordering of imidazolium-based cationic surfactants during cooling-induced crystallization

Surfactants bearing imidazolium cations represent a new class of building blocks in molecular self-assembly. These imidazolium-based cationic surfactants can exhibit various morphologies during phase transformations. In this work, we studied the self-assembly and phase behavior of 1-hexadecyl-3-methylimidazolium chloride (C(16)mimCl) aqueous dispersions (0.5-10 wt %) by using isothermal titration calorimetry, differential scanning calorimetry, synchrotron small- and wide-angle X-ray scattering, freeze-fracture electron microscopy, optical microscopy, electrical conductance, and Fourier transform infrared spectroscopy. It was found that C(16)mimCl in aqueous solutions can form two different crystalline phases. At higher C(16)mimCl concentrations (>6 wt %), the initial spherical micelles convert directly to the stable crystalline phase upon cooling. At lower concentrations (0.5 or 1 wt %), the micelles first convert to a metastable crystalline phase upon cooling and then transform to the stable crystalline phase upon further incubation at low temperature. The electrical conductance measurement reveals that the two crystalline phases have similar surface charge densities and surface curvatures. Besides, the microscopic and spectroscopic investigations of the two crystalline phases suggest that the metastable crystalline phase has preassembled morphology and a preordered submolecular packing state that contribute to the final stable crystalline structure. The formation of a preordered structure prior to the final crystalline state deepens our understanding of the crystallization mechanisms of common surfactants and amphiphilic ionic liquids and should thus be widely recognized and explored.

Aggregation behavior of long-chain N-aryl imidazolium bromide in aqueous solution

The aggregation behavior of three long-chain N-aryl imidazolium ionic liquids (ILs), 1-(2,4,6-trimethylphenyl)-3-alkylimidazolium bromide [C(n)pim]Br (n = 10, 12, and 14), in aqueous solutions was systematically explored by surface tension, electrical conductivity, and (1)H NMR. A lower critical micelle concentration (cmc) for the N-aryl imidazolium ILs is observed compared with that for 1,3-dialkylimidazolium ILs [C(n)mim]Br, indicating that the incorporation of the 2,4,6-trimethylphenyl group into a headgroup favors micellization. The enhanced π-π interactions among the adjacent 2,4,6-trimethylphenyl groups weaken the steric hindrance of headgroups and thus lead to a dense arrangement of [C(n)pim]Br molecules at the air-water interface. An analysis of the (1)H NMR spectra revealed that the introduced 2,4,6-trimethylphenyl group may slightly bend into the hydrophobic regions upon micellization. The micelle formation process for [C(n)pim]Br (n = 10, 12, and 14) was found to be enthalpy-driven in the investigated temperature range, which is attributed to the strong electrostatic self-repulsion of the headgroups and the counterions as well as the π-π interactions among headgroups. Strong, stable fluorescence properties are presented by the new N-aryl imidazolium ILs, indicating their potential application in the field of photochemistry.