Self-Aggregation Behavior of Fluorescent Carbazole-Tailed Imidazolium Ionic Liquids in Aqueous Solutions

Synthesis and Properties of Alkyl Bis-Guanidinium Acetates Surfactants

Novel surfactants with double hydrophilic groups (cocopropane and tallowpropane bis-guanidinium acetates), were synthesized and tested to evaluate both the basic surfactant properties and the unique application performance. Surface tension, conductivity and contact angle measurements were used to study the self-aggregation behavior in aqueous solution. Aggregation parameters were calculated such as adsorption efficiency and effectiveness (pC20 and CAC/C20), the maximum surface excess concentration (Гmax) and minimum surface area permolecule (Amin). The thermodynamic parameters of aggregation based conductivity measurements revealed that the aggregation process was spontaneous and entropy-driven. Compared to DTAC and CTAC, the alkyl bis-guanidinium acetates showed a higher emulsification capacity with both liquid kerosene and soybean oil. The evaluation of antimicrobial activity showed that the alkyl bisguanidinium acetates exhibited strong antibacterial activity against the tested strains at a concentration of 50 ppm.

Self-Assembled Vesicles Formed by Positional Isomers of Sodium Dodecyl Benzene Sulfonate-Based Pseudogemini Surfactants

The construction of pseudogemini surfactants based on noncovalent interactions (such as electrostatic interaction and π-π stacking) was a powerful method to assemble well-defined aggregates in aqueous solution. The mixtures of butane-1,4-bis(methylimidazolium bromide) ([mim-C₄-mim]Br₂) and positional isomers of sodium dodecyl benzene sulfonate (SDBS-0,11 or SDBS-3,8) in a molar ratio of 1:2 were studied to characterize the effect of straight and branched alkyl chains on the aggregation behavior of pseudogemini surfactants. Spontaneous phase transition from micelles to vesicles was formed by these two kinds of complexes. Interestingly, a densely stacked onion-like structure (multilamellar vesicles) with more than one dozen layers was fabricated. The micelle and vesicle phases were characterized in detail by cryogenic transmission electron microscopy, polarized optical microscopy, dynamic light scattering, and rheological measurements. It can be clearly demonstrated that the structure of alkyl chain can significantly influence the surface adsorption, solution self-assembly, and aqueous two-phase system of pseudogemini surfactants. Our work provided a convenient technique to achieve controlled self-assembly by introducing positional isomers of surfactants.

The crystal structure of 1,1′-(9-ethyl-9H-carbazole-3,6-diyl)bis(3-ethyl-1H-imidazol-3-ium) bis(hexafluorophosphate(IV)), C24H27N5F12P2

C24H27N5F12P2, monoclinic, P21/n (no. 14), a = 12.395(4) Å, b = 16.630(6) Å, c = 26.899(10) Å, β = 90.395(7)°, V = 5545(4) Å3, Z = 8, Rgt(F) = 0.0650, wRref(F2) = 0.1651, T = 113(2) K.

Molecular packing of surface active ionic liquids in a deep eutectic solvent: a small angle X-ray scattering (SAXS) study

During the past decade, deep eutectic solvents (DESs) have shown promising application in the self-assembly of surfactants. Various aggregates such as micelles, vesicles, lyotropic liquid crystals, microemulsions and gels have been reported. In this research, the phase behaviours of imidazolium surface active ionic liquids (SAILs) CnmimBr (n = 12, 14, 16) were investigated in ChG. With the help of small angle X-ray scattering (SAXS), the types and structure parameters of aggregates were determined. The molecular packing of SAILs was influenced by the solvophobic chain length, surfactant concentration, temperature and solvent, accounting for their different aggregation behaviours. This study would give a good description of the molecular packing of surfactants in DESs.

Synthesis and Characterization of Photosensitive Ionic Liquid Surfactant 4-Butylazobenzene-4′-(Oxyethyl)Methylimidazolium with Br− and BF4− Counterions

The photo-responsive ionic liquid surfactants 4-Butylazobenzene-4′-(oxyethyl)methylimidazolium with Br− (AZMIMBr) and BF4− (AZMIMBF4) counterions were synthesized, and their structures were characterized by means of 1H NMR. Their properties for pre- or post-UV irradiation were investigated by employing tensiometry, electrical conductance, thermal gravimetry-differential scanning calorimetry (TG-DSC) and small-angle X-ray scattering (SAXS). The effect of UV irradiation time on photoisomerization of two surfactants solutions was evaluated, which showed the photoisomerization efficiency decreases with an increase of their concentrations. After UV irradiation, the CMC value of the two surfactants increased, whereas, the surface tensions at CMC (γCMC) and the fraction of counterion binding (β) were approximately the same. SAXS coupled with polarized optical microscopic studies (POM studies) confirmed that the liquid crystal textures of AZMIMBr could be affected by UV-irradiation. The detailed analysis of thermodynamic parameters revealed that the micellizaton of AZMIMBr was entropy-driven, and the micellization of AZMIMBF4 was enthalpy-driven.

Spontaneous wormlike micelles formed in a single-tailed zwitterionic surface-active ionic liquid aqueous solution

Wormlike micelles were successfully fabricated by the self-assembly of a single-tailed zwitterionic surface-active ionic liquid (SAIL), 3-(1-hexadecyl-3-imidazolio) propanesulfonate β-naphthalene sulfonate (C₁₆IPS-Nsa), in aqueous solutions without any additives. With increasing zwitterionic SAIL concentration, spontaneous transition from micelles to wormlike micelles and then a hexagonal phase occurs. Interestingly, the wormlike micelles are closed, stretched and directionally arranged, which was rarely found in previous research studies. This kind of wormlike micelle is useful to serve as novel soft template in the synthesis of functional materials. ¹H NMR illustrated that the introduced π-π stacking and hydrophobic interactions originated from the hydrophobic aromatic counterions are responsible for the structural transformation. Density functional theory (DFT) simulated the optimum configuration of C₁₆IPS-Nsa and calculated the interaction energy of C₁₆IPS-Nsa-H₂O is 39.6 kJ mol^-1. This work paves the way for regulating the self-assembly structures of amphiphiles through changing specific weak interactions.

Nicotine-based surface active ionic liquids: Synthesis, self-assembly and cytotoxicity studies

New ester-functionalized surface active ionic liquids (SAILs) based on nicotine, [C_nENic][Br] (n=8, 10 and 12), with bromide counterions have been synthesized, characterized and investigated for their self-assembly behavior in aqueous medium. Conductivity measurements in aqueous solutions of the investigated SAILs have provided information about their critical micelle concentration (cmc), and degree of counterion binding (β), where cmc was found to be 2-3-fold lower than homologous SAILs or conventional cationic surfactants. The inherent fluorescence of SAILs in the absence of any external fluorescent probe have shed light on cmc as well as interactions prevailing between the monomers in micelle at molecular level. The thermodynamic parameters related to micellization have been deduced from isothermal titration calorimetry (ITC) and conductivity measurements. ¹H NMR, spin-lattice (T₁) relaxation time and 2D ¹H-^IH ROESY measurements have been exploited to get detailed account of internal structure of micelle. The size and shape of the micelles have been explored using dynamic light scattering (DLS) and transmission electron microscopy (TEM) measurements. The synthesized SAILs have been found to be non-cytotoxic towards C6-Glioma cell line, which adds to the possible utility of these SAILs for diverse biological applications.

Drug induced micelle-to-vesicle transition in aqueous solutions of cationic surfactants

Solubilization of the drug diclofenac sodium (DS) in micellar aggregates of ionic liquid-based surfactants and cetyltrimethylammonium bromide at 25 °C induces changes to worm-like micelles and vesicles, with potential use in drug delivery.

Photoresponsive Self-Assembly of Surface Active Ionic Liquid

A novel photoresponsive surface active ionic liquid (SAIL) 1-(4-methyl azobenzene)-3-tetradecylimidazolium bromide ([C14mimAzo]Br) with azobenzene located in the headgroup was designed. Reversible vesicle formation and rupture can be finely controlled by photostimuli without any additives in the aqueous solution of the single-tailed ionic liquid. The photoisomerization of the azobenzene derivative was investigated by (1)H NMR and UV-vis spectroscopy. Density functional theory (DFT) calculations further demonstrate that trans-[C14mimAzo]Br has less negative interaction energy, which is beneficial to aggregate formation in water. The incorporation of trans-azobenzene group increases the hydrophobicity of the headgroup and reduces the electrostatic repulsion by delocalization of charge, which are beneficial to the formation of vesicles. However, the bend of cis-azobenzene makes the cis-isomers have no ability to accumulate tightly, which induces the rupture of vesicles. Our work paves a convenient way to achieve controlled topologies and self-assembly of single SAIL.

On the role of a coumarin derivative for sensing applications: Nucleotide identification using a micellar system

The recognition of nucleotides is of crucial importance because they are the basic constituents of nucleic acids. The present study is focused on the selective interaction between a novel amphiphilic fluorophore containing coumarin and imidazole, CI (1-methyl-3-(12-((2-oxo-2H-chromen-7-yl)oxy)dodecyl)-1H-imidazol-3-ium bromide), and different nucleotide-monophosphates (NMPs). It was supposed that the solubilization of the low water soluble CI in a micelle system of hexadecyltrimethylammonium chloride (CTAC) would make the coumarin moiety of CI available to the interaction with the water-soluble NMPs. Changes in CTAC critical micelle concentration suggested that CI strongly interacted with the host cationic surfactant, thus forming a positively charged interface enriched with coumarin able to interact with the anionic NMPs. Steady-state fluorescence quenching revealed that CI/CTAC system was capable of distinguish between purine- and pyrimidine-based nucleotides. A modified Stern-Volmer equation permitted the use of a quenching model that accounted for the possible interactions between the micelles and the nucleotides. The data analysis allowed calculating selective parameters that differentiated according to the type of nucleotide either at 25 or 50°C. Our results established the utility of the novel coumarin derivative fluorophore, supported by the simple and suitable micellar systems, as a tool for DNA sensing applications.

Aggregation behavior of zwitterionic surface active ionic liquids with different counterions, cations, and alkyl chains

The effects of anionic type, cationic structure and alkyl chain length are illustrated to regulate the self-assembly of zwitterionic SAILs.

Multistimuli Sensitive Behavior of Novel Bodipy-Involved Pillar[5]arene-Based Fluorescent [2]Rotaxane and Its Supramolecular Gel

Fluorescent [2]rotaxane BC12P5 is successfully constructed with 1,4-diethoxypillar[5]arene as wheel over a long alkyl axle with Bodipy chromophore as one stopper for the first time. NMR spectra clearly reveal its molecular shuttle nature triggered by multiple external stimuli including solvent polarity and temperature. In particular, the fluorescence nature introduced into [2]rotaxane BC12P5 renders it a good sensor for the external stimuli. Nevertheless, the supramolecular gel successfully fabricated from this novel rotaxane system via self-assembly in dimethyl sulfoxide (DMSO) also shows reversible gel-sol phase transition upon multiple external stimuli such as heating/cooling, shaking/resting, or the addition of different anions. Interestingly, exposure of the supramolecular gel film to HCl or ammonia vapor induces the change in the film fluorescence intensity, endowing this system with a potential application in gas detecting.

Structural rearrangement in the aqueous solution of surface active ionic liquid 1-butyl-3-methylimidazolium bis(2-ethylhexyl) sulfosuccinate

The aggregation behaviors of surface active ionic liquid 1-butyl-3-methylimidazolium bis(2-ethylhexyl) sulfosuccinate in aqueous solutions were investigated by conductometry, densimetry, fluorimetry, (1)H nuclear magnetic resonance ((1)H NMR) spectroscopy, dynamic light scattering (DLS), and transmission electron microscopy (TEM), which confirmed two distinguished critical concentrations. The first critical concentration was believed to be the critical aggregation concentration (CAC), where two different types of aggregates were formed, namely, micelles with the size of 10 nm and vesicles with the size of 100 nm. The second critical concentration at two-fold CAC was suggested to be resulting from the insertion of the imidazolium cations into aggregates.

Micellization behavior of morpholinium-based amide-functionalized ionic liquids in aqueous media

Morpholinium-based amide-functionalized ionic liquids (ILs) [C(n)AMorph][Br], where n = 8, 12, and 16, have been synthesized and characterized for their micellization behavior in aqueous medium using a variety of state of the art techniques. The adsorption and micellization behavior of [CnAMorph][Br] ILs at the air-solution interface and in the bulk, respectively, has been found to be much better compared to that observed for nonfunctionalized homologous ILs and conventional cationic surfactants, as shown by the comparatively higher adsorption efficiency, lower surface tension at the critical micelle concentraiton (γ(cmc)), and much lower critical micelle concentration (cmc) for [C(n)AMorph][Br] ILs. Conductivity measurements have been performed to obtain the cmc, degree of counterion binding (β), and standard free energy of micellization (ΔG(m)°). Isothermal titration calorimetry has provided information specifically about the thermodynamics of micellization, whereas steady-state fluorescence has been used to obtain the cmc, micropolarity of the cybotactic region, and aggregation number (N(agg)) of the micelles. Both dynamic light scattering and atomic force microscopy have provided insights into the size and shape of the micelles. 2D (1)H-(1)H nuclear Overhauser effect spectroscopy experiments have provided insights into the structure of the micelle, where [C16AMorph][Br] has shown distinct micellization behavior as compared to [C8AMorph][Br] and [C12AMorph][Br] in corroboration with observations made from other techniques.

Water Phase Diagram Is Significantly Altered by Imidazolium Ionic Liquid

We report unusually large changes in the boiling temperature, saturated vapor pressure, and structure of the liquid-vapor interface for a range of 1-butyl-3-methyl tetrafluoroborate, [C4C1IM][BF4]-water mixtures. Even modest molar fractions of [C4C1IM][BF4] significantly affect the phase behavior of water, as represented, for instance, by strong negative deviations from Raoult's law, extending far beyond the standard descriptions. The investigation was carried out using classical molecular dynamics employing a specifically refined force field. The changes in the liquid-vapor interface and saturated vapor pressures are discussed at the atomistic resolution. The reported results guide the search for novel scientific and technological applications of ion-molecular systems.

Aggregation behavior and electrical properties of amphiphilic pyrrole-tailed ionic liquids in water, from the viewpoint of dielectric relaxation spectroscopy

The self-aggregation behavior of amphiphilic pyrrole-tailed imidazolium ionic liquids (Py(CH₂)₁₂mim⁺Br⁻: Py = pyrrole, mim = methylimidazolium) in water is investigated by dielectric spectroscopy from 40 Hz to 110 MHz. Dielectric determination shows that the critical micelle concentration (CMC) is 8.5 mM, which is lower than that for traditional ionic surfactants. The thermodynamic parameter of the micellization, the Gibbs free energy ΔG, was calculated for Py(CH₂)₁₂mim⁺Br⁻ and compared to those of the corresponding C(n)mim⁺Br⁻ (n = 12, 14). It was found that the main driven forces of the Py(CH₂)₁₂mim⁺Br⁻ aggregation were hydrophobic interaction and π-π interactions among the adjacent Py groups. Further, the structure of aggregation was speculated theoretically that Py groups partially insert into the alkyl chains and the staggered arrangement in micelles is formed. When the concentration of Py(CH₂)₁₂mim⁺Br⁻ is higher than CMC, two remarkable relaxations which originated from diffusion of counterions and interfacial polarization between the micelles and solution, were observed at about 1.3 MHz and 55 MHz. The relaxation parameters representing the real properties of the whole system were obtained by fitting the experimental data with Cole-Cole equation. A dielectric model characterizing the structure and electrical properties of spherical micelles was proposed by which the conductivity, permittivity and the volume fraction of micelles as well as electrical properties of solution were calculated from the relaxation parameters. An intriguingly high permittivity of about 150 for the micelle was found to be a direct consequence of the strong orientational order of water molecules inside the core of micelle, and essentially is attributed to the special structure of the micelle. Furthermore, the calculation of the interfacial electrokinetic parameters of the micelles, i.e., the surface conductivity, surface charge density and zeta potential, were also achieved based on the relaxation parameters and phase parameters from higher frequency relaxation. On the basis of the results obtained, the aggregation behaviours and interfacial electrokinetic properties of the special micelles are discussed.

Aggregation of double-tailed ionic liquid 1,3-dioctylimidazolium bromide and the interaction with triblock copolymer F127

The aggregation of ionic liquid-based double-tailed surfactant, 1,3-dioctylimidazolium bromide ([Doim]Br) and its interaction with pluronic copolymer F127 were systematically investigated by nuclear magnetic resonance (NMR), surface tension, dynamic light scattering (DLS), and isothermal titration calorimetry (ITC). It was found that the [Doim]Br aggregates are composed with the alkyl chains embedded in the micellar core and with the imidazolium rings parallel and staggered on the hydrophilic layer of micelles, which was generally different from the single-tailed IL [omim]Br. The hydrogen bonding between protons attached to the imidazolium rings and anion Br(-) was enhanced upon the aggregation of [Doim]Br. The aggregation of F127 was promoted by addition of [Doim]Br, which was more efficient than the single-tailed surfactant. At lower [Doim]Br content, [Doim]Br monomers embedded deeply into F127 micelle core. At higher [Doim]Br concentrations, the F127 micelles were disassociated, and then F127 chains penetrated into [Doim]Br micelle. In addition, the microstructure of F127/[Doim]Br complex can also be tuned by temperature.

Ionic and Molecular Liquids: Working Together for Robust Engineering

Because of their outstanding versatility, room-temperature ionic liquids (RTILs) are utilized in an ever increasing number of novel and fascinating applications, making them the Holy Grail of modern materials science. In this Perspective, we address the fundamental research and prospective applications of RTILs in combination with molecular liquids, concentrating on three significant areas: (1) the use of molecular liquids to decrease the viscosity of RTILs; (2) the role of RTIL micelle formation in water and organic solvents; and (3) the ability of RTILs to adsorb pollutant gases. Current achievements are examined, and future directions for the potential uses of RTILs are outlined.

Physicochemical Properties of Quaternary Ammonium Surfactants with Hydroxyethyl Groups

Two cationic quaternary ammonium surfactants with hydroxyethyl groups, dodecyl-(2-hydroxyethyl)-dimethylammonium chloride (DHDAC), and dodecyl-di(2-hydroxyethyl)-methylammonium chloride (DHHAC) were synthesized. Their physicochemical properties were investigated systematically by surface tension and conductivity measurements. Various physicochemical parameters were calculated to evaluate the surface activities of these surfactants. The results show that the surface activity increases with increasing the number of hydroxyethyl groups on hydrophilic groups of surfactants. The degrees of counterion binding to micelles of these surfactants were also discussed. The thermodynamic parameters obtained from electric conductivity measurements show that the micelle formation of DHHAC is entropy-driven in the temperature range investigated. However, it is enthalpy-driven for DHDAC.