Aggregation behavior of non-cytotoxic ester functionalized morpholinium based ionic liquids in aqueous media

Self-assembly of choline-based surface-active ionic liquids and concentration-dependent enhancement in the enzymatic activity of cellulase in aqueous medium

The micellization of choline-based anionic surface-active ionic liquids (SAILs) having lauroyl sarcosinate [Sar]-, dodecylsulfate [DS]-, and deoxycholate [Doc]- as counter-ions was investigated in an aqueous medium. Density functional theory (DFT) was employed to investigate the net interactional energy (Enet), extent of non-covalent interactions, and band gap of the choline-based SAILs. The critical micelle concentration (cmc) along with various parameters related to the surface adsorption, counter-ion binding (β), and polarity of the cores of the micelles were deduced employing surface tension measurements, conductometric titrations and fluorescence spectroscopy, respectively. A dynamic light scattering (DLS) system equipped with zeta-potential measurement set-up and small-angle neutron scattering (SANS) were used to predict the size, zeta-potential, and morphology, respectively, of the formed micelles. Thermodynamic parameters such as standard Gibb's free energy and standard enthalpy change of micellization were calculated using isothermal titration calorimetry (ITC). Upon comparing with sodium salt analogues, it was established that the micellization was predominantly governed by the extent of hydration of [Cho]+, the head groups of the respective anions, and the degree of counter-ion binding (β). Considering the concentration dependence of the enzyme-SAIL interactions, aqueous solutions of the synthesized SAILs at two different concentrations (below and above the cmc) were utilized as the medium for testing the enzymatic activity of cellulase. The activity of cellulase was found to be ∼7- to ∼13-fold higher compared to that observed in buffers in monomeric solutions of the SAILs and followed the order: [Cho][Sar] > [Cho][DS] > [Cho][Doc]. In the micellar solution, a ∼4- to 5-fold increase in enzymatic activity was observed.

Purposefully Designed Surfactants for Facile and Controllable Gold Colloidal Nanocrystal Synthesis

Three new cationic surfactants-N-cetyl-bis(2-dimethylaminoethyl)ether bromide (CBDEB), N-dodecyl-bis(2-dimethylaminoethyl)ether bromide (DBDEB), and N-hexyl-bis(2-dimethylaminoethyl)ether bromide (HBDEB)-have been designed herein using a simple and tailorable synthesis route. CBDEB and DBDEB, the 16- and 12-carbon chain surfactants, demonstrate facile, rapid, and controllable aqueous syntheses of gold nanoparticles (AuNPs) as dual-action reducing and capping agents. The synthesis strategy, using only surfactant and HAuCl4 salt, and 4 min of heating at 80 °C, results in spherical AuNPs (average diameters of 13.4 ± 3.8 nm for CBDEB and 12.0 ± 3.8 nm for DBDEB). Microwave irradiation was also investigated as a heating method and produces AuNPs in as little as 30 s. Control over the size and shape of AuNPs was proven to be feasible (toward populations of Euclidean shapes) by appropriately tuning reaction parameters, such as the molar ratio of surfactant to Au3+, temperature, incorporation of a time delay before heating, or shape control agents, such as Cu2+. Frustratingly, the cytotoxicity of CBDEB is similar to that of cetyltrimethylammonium bromide (CTAB), a popular 16-carbon chain cationic surfactant. Notably, while the shorter HBDEB (6-carbon chain) does not produce AuNPs under the applied conditions, it does appear to improve cell viability upon cytotoxicity evaluation and may be favorable as a new biological surfactant.

Water Induced Alterations in Self-Assembly of a Bio-Surfactant in Deep Eutectic Solvent for Enhanced Enzyme Activity

Deep eutectic solvents (DESs) meet important requirements for green solvent technology, including non-toxicity, biodegradability, sustainability, and affordability. Despite possessing low cohesive energy density than water, DESs have been found to support the self-assembly of amphiphiles. It is very much pertinent to examine the effect of water on self-assembly of surfactants in DESs as the presence of water alters the inherent structure of DES, which is expected to affect the characteristic properties of self-assembly. Following this, we have investigated the self-assembly of amino-acid based surfactant, Sodium N-lauroyl sarcosinate (SLS), in DES-water mixtures (10, 30 and 50 w/w% of water) and explored the catalytic activity of Cytochrome-c (Cyt-c) in the formed colloidal systems. Investigations using surface tension, fluorescence, dynamic light scattering (DLS), and isothermal titration calorimetry (ITC) have shown that DES-water mixtures promote the aggregation of SLS, resulting in the lower critical aggregation concentration (cac ∼1.5-6-fold) of the surfactant as compared to water. The nanoclustering of DES at low water content and it's complete de-structuring at high water content affects the self-assembly in a contrasting manner governed by different set of interactions. Further, Cyt-c dispersed in DES-water colloidal solutions demonstrated 5-fold higher peroxidase activity than that observed in phosphate buffer.

Investigating the Influence of Aromatic Counterions on the Micellar Structure and Aggregation Behavior of Morpholinium-Based Surface-Active Ionic Liquids in an Aqueous Solution

Two morpholinium-based surface-active ionic liquids (SAILs) with aromatic counterions were synthesized, namely, n-dodecyl-n-methylmorpholinium salicylate [C12mmor][Sal] and n-dodecyl-n-methylmorpholinium 3-hydroxy-2-naphthoate [C12mmor][3-h-2-n], and explored their aggregation behavior in aqueous solutions systematically. Electrical conductivity, small-angle neutron scattering (SANS), surface tension (ST), and UV-vis spectroscopy measurements were employed to determine various thermodynamic, micellar, and interfacial parameters, like the degree of counterion binding (β), critical micelle concentration (CMC), minimum area per molecule (Amin), surface excess concentration (Γmax), standard Gibbs free energy of adsorption (ΔGad0), aggregation number (Nagg), standard Gibbs free energy of micellization (ΔGm0), standard enthalpy of micelle formation (ΔHm0), and the standard entropy of micellization (ΔSm0) in an aqueous solution. Incorporating the aromatic counterions favors significantly excellent micellization properties over conventional halogenated SAILs such as [C12mmor][Br]. SANS analysis revealed that upon changing the counterion from salicylate to 3-hydroxy-2-naphthoate, the structure changed from prolate ellipsoidal micelles to large unilamellar vesicles. Also, increasing the concentration in the case of [C12mmor][Sal] resulted in a lower aggregation number.

Mixed Aggregates of Surface-Active Ionic Liquids and 14-2-14 Gemini Surfactants in an Aqueous Medium as Fluid Scaffolds for Enzymology of Cytochrome-c

The aggregation behavior of the surface-active ionic liquid (SAIL), 3-(2-(hexadecyloxy)-2-oxoethyl)-1-methyl-1H-imidazol-3-ium chloride, [C16Emim][Cl], and a gemini surfactant (GS) (14-2-14) in the whole mole fraction range has been investigated in an aqueous medium employing various techniques. Experimentally obtained values of critical aggregation concentration (cac) are in good agreement with the theoretical cac values obtained using Clint's equation. Rubingh's model has been employed to evaluate the extent of synergistic interactions between two components, which has been found to be dependent upon the composition of a mixture of surfactants. The polarity index, hydrodynamic diameter (Dh), zeta potential (ζ-Pot.), and morphology of the aggregates have been found to be dependent upon the extent of hydrophobic as well as dipolar interactions and the degree of counterion binding governed by the content of the GS in mixed aggregates. Thermodynamic parameters evaluated employing isothermal titration calorimetry have revealed the aggregation as an entropy-driven process. Density functional theory calculations provide a detailed account of the SAIL-GS interactions at the molecular level. The reduced density gradient (RDG) along with the calculated isosurfaces asserts that the dominant interactions are noncovalent interactions. Furthermore, the enzymology of cytochrome-c in the aqueous SAIL-GS aggregated systems has been investigated and a two-fold increase in the enzyme activity has been observed in the aggregates formed by the GS as compared to that in buffer.

Synthesis and Surface Properties of Piperidinium-Based Herbicidal Ionic Liquids as a Potential Tool for Weed Control

A series of piperidinium-based herbicidal ionic liquids (HILs) were synthesized and investigated. The designed HILs, obtained with high yields, consisted of cation 1-alkyl-1-methylpiperidinium with surface activity and a commercially available herbicidal anion: (3,6-dichloro-2-methoxy)benzoates (dicamba). The above-mentioned compounds were characterized in terms of surface activity and phytotoxicity. Preliminary results were obtained at higher wettability for all HILs when compared to the wettability of commercial Dicash, with HIL having 18 atoms in the carbon chain being the best effectiveness in wetting surfaces (weeds and crop leaves), whereby a drop of HILs with short alkyl chains (C₈-C₁₀) could not slide down a leaf. Our findings present that wettability or mobility of HILs drops varied depending on the plant species. Moreover, in this study, by zeta potential and atomic force microscopy measurements, we provide conclusive evidence to demonstrate that alkyl chain elongation plays a significant role in the evolution of surface properties of HILs.

Synthesis, Aggregation Behavior and Drug-binding Interactions of Fatty acid-imidazolium-based Surface-active Ionic Liquids

The renewable fatty acid-based surface-active ionic liquids (SAILs) containing ethyl-substituted imidazolium head groups were prepared and structurally analyzed by Fourier transform infrared spectroscopy (FTIR), ¹HNMR and ¹³CNMR spectroscopy. The products were named as; 3-ethyl-1-(2-dodecanoyl oxy) ethylimidazolium bromide [C₁₂Eeim]Br, 3-ethyl-1-(2-tetradecanoyl oxy) ethylimidazolium bromide [C₁₄Eeim]Br and 3-ethyl-1-(2-hexadecanoyl oxy) ethylimidazolium bromide [C₁₆Eeim]Br. The critical micelle concentration (cmc) values of the three SAILs have been evaluated using conductivity measurements, probe-less UV-visible spectroscopy and fluorescence spectroscopy. The obtained cmc values were compared with the earlier reported non-functionalized SAILs such as [C_nmim]Br and [C_neim]Br where n= 12, 14, 16. The values were found to be 3 to 9 times lower mainly due to the presence of ester chain and also ethyl substituted imidazole ring. Thermodynamic parameters were evaluated by conductivity data at three different temperatures. Further, the aggregation behavior of SAILs with anesthetic drug, lidocaine hydrochloride (LC) has been studied using fluorescence. The fluorescence and UV-visible studies showed strong synergistic interactions operating between SAILs and drug molecules involving H bonding and cation-π interactions. The interactions grew stronger with the elongation of SAIL-chain length (12C-16C). Dynamic light scattering (DLS) and transmission electron microscopy (TEM) measurements suggested the formation of vesicles in SAIL-LC mixtures. These studies may thus offer an effective candidate which would serve as vectors for drug molecules in terms of their enhanced solubilization, permeability and target-specific delivery.

Emerging impacts of ionic liquids on eco-environmental safety and human health

Owing to their unique physicochemical properties, ionic liquids (ILs) have been rapidly applied in diverse areas, such as organic synthesis, electrochemistry, analytical chemistry, functional materials, pharmaceutics, and biomedicine. The increase in the production and application of ILs has resulted in their release into aquatic and terrestrial environments. Because of their low vapor pressure, ILs cause very little pollution in the atmosphere compared to organic solvents. However, ILs are highly persistent in aquatic and terrestrial environments due to their stability, and therefore, potentially threaten the safety of eco-environments and human health. Specifically, the environmental translocation and retention of ILs, or their accumulation in organisms, are all related to their physiochemical properties, such as hydrophobicity. Based on results of ecotoxicity, cytotoxicity, and toxicity in mammalian models, the mechanisms involved in IL-induced toxicity include damage of cell membranes and induction of oxidative stress. Recently, artificial intelligence and machine learning techniques have been used in mining and modeling toxicity data to make meaningful predictions. Major future challenges are also discussed. This review will accelerate our understanding of the safety issues of ILs and serve as a guideline for the design of the next generation of ILs.

Stimuli Responsive, Self-Sustainable, and Self-Healable Functionalized Hydrogel with Dual Gelation, Load-Bearing, and Dye-Absorbing Properties

The motivation for designing low-molecular-weight gelators with self-healing characteristics originates from elegant examples in biology such as vines of the genus Aristolochia whose internal secondary growth exhibits rapid self-healing in their stems. In the present work, we had explored the stimuli-responsive dual gelation characteristics for the ester-functionalized surfactant (4-(2-(hexadecyloxy)-2-oxoethyl)-4-methylmorpholin-4-ium bromide, C₁₆EMorphBr) in aqueous medium at 7.20% (w/v) critical gel concentration and pH 7.4. The hydrogel provides an excellent platform to study dynamic phase behavior within a supramolecular network as it exhibits transformation from a fibrillar opaque hydrogel to a transparent hydrogel upon heating. Molecular interactions, arrangement within the supramolecular framework, and mechanical properties of the hydrogels were characterized using Fourier transform infrared, small-angle neutron scattering, rheological analysis, and tensile strength and cyclic loading-unloading tests. The fibrillar opaque gel has been characterized for its morphology using scanning electron microscopy, field emission scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. The self-sustained, self-healable porous fibrillar opaque xerogel was further explored for selectively absorbing anionic dyes and for its load-bearing characteristics. We conclude a perspective on designing a new-age gelator that can open entirely new avenues in environmental protection and wearable "smart" devices.

Hydrophobically Driven Morphologically Diverse Self-Assembled Architectures of Deoxycholate and Imidazolium-Based Biamphiphilic Ionic Liquids in Aqueous Medium

Biamphiphilic ionic liquids (BAILs) having amphiphilic cation and anion are thought to exhibit improved surface activity and colloidal stability to be utilized in different applications. For their effective use, a control over synergetic hydrophobic and electrostatic interactions between oppositely charged ions along with the possibility of tuning of hydrophobicity of the core of aggregates is required. Focusing on this, new BAILs comprising a bile salt anion, deoxycholate, [DC]^-, and 1-alkyl-3-methylimidazolium cations, [C _nmim]⁺ ( n = 2, 4, 6, 8, and 12), were synthesized and characterized for their behavior at air-solution interface as well as in bulk. The synthesized BAILs exhibit high surface activity and self-assemble in the form of different architectures ranging from nanosheets (NSs), nanorods, and vesicles with varying hydrophobicities of the formed core of aggregates, depending on the length of alkyl chain of [C _nmim]⁺. Analysis of various parameters obtained from investigated techniques suggested the changing role of [C _nmim]⁺ from a counterion ( n = 2 and 4) to a cosurfactant ( n = 8 and 12) via a borderline case of [C₆mim]⁺. This changeover in the nature of counterion controlled by hydrophobicity of alkyl chain resulted in morphological diversification in self-assembled architectures via varying set of interactions. It is believed that the present work would offer new perspectives in the self-assembly phenomenon of surfactants in general and surface active ionic liquids in particular to devise new strategies for inducing morphology-dependent functionality in self-assembled structures of BAILs.

Thermo-Switchable de Novo Ionic Liquid-Based Gelators with Dye-Absorbing and Drug-Encapsulating Characteristics

An ionic liquid-based surfactant with ester functionality self-aggregates in an aqueous medium and forms ionogels at 8.80% (w/v) concentration at physiological pH. The ionogel exhibited a remarkable change in its appearance with temperature from fibrillar opaque to transparent because of the dynamic changes within its supramolecular structure. This gel-to-gel phase transition occurs below the melting point of the solid ionic liquid. The ionogels were investigated using turbidity, differential scanning calorimetry, scanning electron microscopy (SEM), field emission SEM (FE-SEM), inverted microscopy, transmission electron microscopy imaging, Fourier transform infrared spectroscopy, and rheological measurements. The fibrillar opaque ionogel and transparent ionogel were studied for their ability to absorb dyes (methyl orange and crystal violet) and to encapsulate drugs (diclofenac sodium and imatinib mesylate).

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.

Effect of Imidazolium-Based Surface-Active Ionic Liquids on the Orientation of Liquid Crystals at Various Fluid/Liquid Crystal Interfaces

A series of imidazolium-based surface-active ionic liquids (IM-SAILs), viz., single-chained IM-SAILs, 1-alkyl-3-methylimidazolium bromide ([C_nmim]Br, n = 12, 14, 16), 1-dodecyl-3-methylimidazolium salicylate ([C₁₂mim]Sal), 1-dodecyl-3-methylimidazolium 3-hydroxy-2-naphthoate ([C₁₂mim]HNC), 1-dodecyl-3-methylimidazolium cinnamate ([C₁₂mim]CA), 1-dodecyl-3-methylimidazolium para-hydroxy-cinnamate ([C₁₂mim]PCA), gemini IM-SAIL, and 1,2-bis(3-dodecylimidazolium-1-yl)ethane bromide ([C₁₂-2-C₁₂im]Br₂), along with three short-chained ionic liquids (ILs) [ethylammonium nitrate (EAN), propylammonium nitrate (PAN), and butylammonium nitrate (BAN)] were synthesized and applied to nematic liquid crystal (LC)/fluid interfaces. First, we evaluated the influence of the length and number of aliphatic chains as well as the counterion in the IM-SAIL structures on the anchoring of LCs at the aqueous/LC interface. It was observed that the threshold concentration of [C_nmim]Br (n = 12, 14, 16) decreased with the increase in aliphatic chain length. And double-chained [C₁₂-2-C₁₂im]Br₂ has a far lower threshold concentration than single-chained [C₁₂mim]Br. But the alteration of counterions (e.g., Br^- and aromatic counterions) scarcely affected the anchoring of LCs at the interface. Second, we investigated the alignment of LCs at the diverse IL/LC interfaces in the presence of IM-SAILs. It is found that the variations in both aliphatic chain length and number can remarkably change the trigger points of the orientational transition of LCs at the EAN/LC interface. Specifically, with a slight increase in the alkyl chain length of short-chained ILs, as the fluid medium, the orientation of LCs varied tremendously at the IL/LC interface. Therefore, the higher threshold concentration of IM-SAILs and the corresponding greater stability in the optical appearance of LCs at the EAN/LC interface compared to that of the aqueous/LC interface can be ascribed to the discrepancy in the microstructure of water and IL. Finally, we verified that the volume ratio of H₂O to EAN could more dramatically affect the alignment of LCs than the change in IM-SAIL concentration in aqueous solution. This work first illustrated the impact of SAIL structure on the LCs orientation at the aqueous/LC, IL/LC, and H₂O-IL mixture/LC interfaces, which will inspire us to obtain a stabilized molecular alignment of LCs at the IL/LC interfaces and to further design novel functionalized SAIL molecules for various chemical and biological applications.

Effect of alkyl chain functionalization of ionic liquid surfactants on the complexation and self-assembling behavior of polyampholyte gelatin in aqueous medium

The interactional behavior of ILSs towards gelatin forming structurally different ILS mediated self-assemblies depending on the nature of the ILS and counterion binding is shown.

Modulating the mixed micellization of CTAB and an ionic liquid 1-hexadecyl-3-methylimidazollium bromide via varying physical states of ionic liquid

The physical nature of [C₁₆mim][Br] as monomers/micelles led to different IL–CTAB mixed self-assembled structures.

A new strategy to prepare giant vesicles from surface active ionic liquids (SAILs): a study of protein dynamics in a crowded environment using a fluorescence correlation spectroscopic technique

A simple procedure for the preparation of giant vesicles using surface active ionic liquids (SAILs) has been provided in this paper.

Aggregation Behavior of Imidazolium-Based Surface-Active Ionic Liquids with Photoresponsive Cinnamate Counterions in the Aqueous Solution

Two imidazolium-based surface active ionic liquids (SAILs) with photoresponsive cinnamate aromatic counterions, viz. 1-dodecyl-3-methylimidazolium cinnamate ([C12mim][CA]) and 1-dodecyl-3-methylimidazolium para-hydroxy-cinnamate ([C12mim][PCA]), were newly synthesized, and their self-assembly behaviors in aqueous solutions were systematically explored. Results of surface tension and conductivity measurements show that both [C12mim][CA] and [C12mim][PCA] display a superior surface activity in aqueous solutions compared to the common imidazolium-based SAIL, 1-dodecyl-3-methylimidazolium bromide (C12mimBr), which implies the incorporation of cinnamate aromatic counterions can promote the micellar formation. Furthermore, [C12mim][CA] shows higher surface activity due to the higher hydrophobicity of its counterion in comparison to [C12mim][PCA] that has a hydroxyl group. Both hexagonal liquid-crystalline phase (H1) and cubic liquid-crystalline phase (V2) were constructed in the [C12mim][CA] aqueous solutions. In contrast, the [C12mim][PCA]/H2O system only exhibits a single hexagonal liquid-crystalline phase (H1) in a broad concentration region. These lyotropic liquid crystal (LLC) phases were comprehensively characterized by polarized optical microscopy (POM), small-angle X-ray scattering (SAXS), and rheometer. Investigation on the temperature-dependent self-assembly nanostructures demonstrates that the higher temperature leads to a looser arrangement. Under UV irradiation, trans-cis photoisomerization of the phenylalkene group results in inferior surface activity of the prepared SAILs in aqueous solution with higher cmc values. Moreover, UV light irradiation induces obvious change of the structural parameters without altering the LLC phases. This work is expected to enrich the investigations of phase behaviors formed in SAILs systems and receive particular attention due to their unique properties and potential applications in drug delivery, biochemistry, materials science, etc.

Aggregation behaviours and bactericidal activities of novel cationic surfactants functionalized with amides and ether groups

Novel cationic surfactants exhibit high surface activity, interesting aggregation behaviors in aqueous solution and excellent bacterial activity.

Ionic liquid assisted nanofabrication of ferromagnetic Co-doped La–Ce ferrites

The effect of Co(ii) doping on the various physico-chemical properties of La–Ce ferrites synthesized via a hydrothermal route using an ILS as a structure directing template is investigated. The doped NPs are found to be magnetically improved with doping.