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.

Synthesis and Micellization of Carbosilane Sulfonate Surfactants with Short Alkyl Chains

Carbosilane surfactants, consisting of carbosilane as a hydrophobic group linked to hydrophilic groups, are one kind of silicone surfactants. In this paper, a series of carbosilane sulfonate surfactants with short alkyl chains (Cn-Si2C-SO3Na (n = 1-6)), Me-Si2C-SO3Na, Et-Si2C-SO3Na, Pr-Si2C-SO3Na, Bu-Si2C-SO3Na, Pen-Si2C-SO3Na, and Hex-Si2C-SO3Na, were prepared and characterized by 29 Si NMR, 1H NMR, and FT-IR spectroscopies. The influence of the alkyl chain length on their micellization was studied using surface tension, dynamic light scattering, conductivity, and transmission electron microscopy. The CMC value decreases with increasing length of the short alkyl group. The γCMC value of Cn-Si2C-SO3Na (n = 1-6) increases as the alkyl chain increases from methyl to propyl, while the γCMC value gradually decreases as the alkyl chain increases from propyl to hexyl. The larger and rigid tetramethyldicarbosilane group functioned synergistically with a short alkyl chain, resulting in carbosilane sulfonate surfactants adsorbing at the air/water interface with a rugby ball shape; accordingly, the Amin values of the investigated carbosilane sulfonate surfactants increase with increasing length of the alkyl chain. The micellization process of carbosilane sulfonate surfactants is enthalpy-driven at lower temperatures and entropy-driven at high temperatures. The ΔHm0 values became more negative and ΔSm0 values more positive as the alkyl chain length increased. Aggregates in the range of 10-800 nm were observed for Cn-Si2C-SO3Na (n = 1-6) in an aqueous solution, and the hydrodynamic diameter (Dh) decreased with increasing length of the short alkyl group.

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.

Cholinium-Based Ionic Liquids as Promising Antimicrobial Agents in Pharmaceutical Applications: Surface Activity, Antibacterial Activity and Ecotoxicological Profile

Cholinium-based ionic liquids are compounds increasingly studied in pharmaceutics and biomedicine to enhance bioavailability in drug delivery systems and as bioactive ingredients in pharmaceutical formulations. However, their potential as antimicrobial agents has scarcely been investigated. Herein, we explored the antimicrobial activity of a series of surface-active cholinium-based ionic liquids (Chol-ILs). For this purpose, Chol-ILs with alkyl chains of 10-16 carbon atoms were synthesized and their self-assembly in aqueous medium was investigated. Subsequently, their antimicrobial activity against a panel of clinically relevant bacteria and their ability to eradicate MRSA and P. aeruginosa PAO1 biofilms was evaluated. Finally, we analyzed the ecotoxicological profile of Chol-ILs in terms of susceptibility to aerobic biodegradation and acute aquatic toxicity against D. magna and V. fisheri. Our results reveal that cholinium-based ILs with alkyl chain lengths ≥12 C show a broad spectrum of antibacterial activity. Their antimicrobial efficacy depends on their hydrophobicity, with the C₁₄-C₁₆ homologs being the most effective compounds. These ILs exhibit antimicrobial activity similar to that of imidazolium ILs and quaternary ammonium antiseptics. Moreover, the longer alkyl chain Chol-ILs are able to eradicate established biofilms at concentrations as low as 16-32 µg/mL. The biodegradation rate of cholinium-based ILs decreases with alkyl chain elongation. Our results reinforce the suitability of Chol-ILs as promising multifunctional compounds for application in pharmaceutical and biomedical formulation.

Biocompatible Solvents and Ionic Liquid-Based Surfactants as Sustainable Components to Formulate Environmentally Friendly Organized Systems

In this review, we deal with the formation and application of biocompatible water-in-oil microemulsions commonly known as reverse micelles (RMs). These RMs are extremely important to facilitate the dissolution of hydrophilic and hydrophobic compounds for biocompatibility in applications in drug delivery, food science, and nanomedicine. The combination of two wisely chosen types of compounds such as biocompatible non-polar solvents and ionic liquids (ILs) with amphiphilic character (surface-active ionic liquids, SAILs) can be used to generate organized systems that perfectly align with the Green Chemistry concepts. Thus, we describe the current state of SAILs (protic and aprotic) to prepare RMs using non-polar but safe solvents such as esters derived from fatty acids, among others. Moreover, the use of the biocompatible solvents as the external phase in RMs and microemulsions/nanoemulsions with the other commonly used biocompatible surfactants is detailed showing the diversity of preparations and important applications. As shown by multiple examples, the properties of the RMs can be modified by changes in the type of surfactant and/or external solvents but a key fact to note is that all these modifications generate novel systems with dissimilar properties. These interesting properties cannot be anticipated or extrapolated, and deep analysis is always required. Finally, the works presented provide valuable information about the use of biocompatible RMs, making them a green and promising alternative toward efficient and sustainable chemistry.

Nanoconfinement Raises the Energy Barrier to Hydrogen Atom Exchange between Water and Glucose

In bulk aqueous environments, the exchange of protons between labile hydroxyl groups typically occurs easily and quickly. Nanoconfinement can dramatically change this normally facile process. Through exchange spectroscopy (EXSY) NMR measurements, we observe that nanoconfinement of glucose and water within AOT (sodium bis(2-ethylhexyl) sulfosuccinate) reverse micelles raises the energy barrier to labile hydrogen exchange, which suggests a disruption of the hydrogen bond network. Near room temperature, we measure barriers high enough to slow the process by as much as 2 orders of magnitude. Although exchange rates slow with decreasing temperatures in these nanoconfined environments, the barrier we measure below ∼285 K is 3-5 times lower than the barrier measured at room temperature, indicating a change in mechanism for the process. These findings suggest the possibility of hydrogen tunneling at a surprisingly high-temperature threshold. Furthermore, differences in exchange rates depend on the hydroxyl group position on the glucose pyranose ring and suggest a net orientation of glucose at the reverse micelle interface.

Aggregation Behavior and Intermolecular Interaction of Cationic Trisiloxane Surfactants: Effects of Unsaturation

Imidazolium/pyridinium-based trisiloxane surfactants containing a phenyl or vinyl group in the hydrophobic siloxane chain, bis(vinyldimethylsiloxy)methylsilylpropyl-pyridinium chloride (Vi-Si₃pyrCl), bis(vinyldimethylsiloxy)methylsilylpropyl-imidazolium chloride (Vi-Si₃minCl), and bis(phenyldimethylsiloxy)methylsilylpropylimidazolium chloride (Ph-Si₃minCl), were synthesized and confirmed by nuclear magnetic resonance (NMR) (¹H, ¹³C, and ²⁹Si NMR), mass spectrometry, and Fourier transform infrared spectrometry. The effect of the phenyl/vinyl group on their micellization behavior was studied by surface tension, electric conductivity, dynamic light scattering, 2D nuclear Overhauser effect spectroscopy (NOESY) NMR, and transmission electron microscopy. Owing to the hydrophobicity of the siloxane groups and cationic head groups, the critical micelle concentration (cmc) values follow the order Ph-Si₃minCl < Vi-Si₃pyrCl < Vi-Si₃minCl < Si₃pyrCl. Ph-Si₃minCl has a larger γ_cmc value, resulting from the introduction of the phenyldimethylsiloxy unit (π-π stacking interaction). The β values of Vi-Si₃minCl and Ph-Si₃minCl increase with the increase in temperature, which is attributed to the intermolecular interaction which hinders the association of Cl^- with the imidazolium ring and confirmed by 2D NOESY NMR. In aqueous solutions, the investigated cationic trisiloxane surfactants can self-assemble into spherical aggregates.

Concentration- and Temperature-Responsive Reversible Transition in Amide-Functionalized Surface-Active Ionic Liquids: Micelles to Vesicles to Organogel

A ubiquitous example of DNA and proteins inspires the scientific community to design synthetic systems that can construct various self-assembled complex nano-objects for high-end physiological functions. To gain insight into judiciously designed artificial amphiphilic structures that through self-assembling form various morphological architectures within a single system, herein, we have studied self-aggregation of amide-functionalized surface-active ionic liquids (AFSAILs) with different head groups in the DMSO/water mixed system. The AFSAIL forms stimuli-responsive reversible micelle and vesicle configurations that coexist with three-dimensional (3D) network structures, the organogel in the DMSO/water mixed system. The self-assembly driving forces, self-organization patterns, network morphologies, and mechanical properties of these network structures have been investigated. With the proven biodegradability and biocompatibility, one can envisage these AFSAILs as the molecules with a new dimension of versatility.

Study on the Surface Properties and Aggregation Behavior of Quaternary Ammonium Surfactants with Amide Bonds

A number of techniques, including conductivity, surface tension, dynamic light scattering, transmission electron microscopy, and ¹H nuclear magnetic resonance (¹H NMR), Fourier transform infrared (FT-IR), and ¹H-¹H 2D nuclear Overhauser effect spectroscopy (¹H-¹H 2D NOESY), have been used to investigate the effect of amide bonds on the interfacial and assembly properties of a cationic surfactant, N-anilinoformylmethyl-N-cetyl-N,N-dimethyl ammonium chloride (AMC-C ₁₆ ), in aqueous solutions. The adsorption of AMC-C ₁₆ has been found to be much better than that of the conventional cationic surfactant, benzyl cetyldimethylammonium chloride (BAC-16) at the air/water interface and in solution. The surface tension measurements show the presence of two critical aggregation concentrations (CAC₁ and CAC₂) for AMC-C ₁₆ . The presence of a strong intermolecular hydrogen bond of AMC-C ₁₆ was confirmed by ¹H NMR and FT-TR. The molecular interactions of AMC-C ₁₆ were detected by ¹H-¹H 2D NOESY. The results show that the rigid group (phenyl) of AMC-C ₁₆ was partially overlapped with its alkyl chain in aqueous solution, and the possible aggregation behavior for AMC-C ₁₆ was proposed. The effects of an inorganic salt (NaCl) and an organic salt (C₆H₅COONa) to the aggregates of AMC-C ₁₆ have been discussed.

Aqueous colloidal systems of bovine serum albumin and functionalized surface active ionic liquids for material transport

Physicochemical and computational investigation of complexation between BSA and SAILs with application in material transport.

Aqueous systems of a surface active ionic liquid having an aromatic anion: phase behavior, exfoliation of graphene flakes and its hydrogelation

Surface active ionic liquid (SAIL) induced hydrogelation, in the absence of additives, is important considering the properties of soft-hydrogels that can be utilized in different applications. The present study is concerned with the phase behavior and hydrogelation of a SAIL, 1-hexadecyl-3-methylimidazolium p-toluenesulfonate, [C16mim][PTS]. The obtained information about the phase behavior along with the surfactant like behavior of the SAIL was exploited for effective exfoliation of graphene-flakes from graphite in aqueous medium that remain stable for at least one month. Thus the obtained dispersion of graphene-flakes was subsequently hydrogelated exploiting the observations made from the phase behavior of the SAIL, via entanglement of long worm-like micelles of the SAIL formed at higher concentration. The obtained graphene-flake based hydrogels were found to be equally stable as compared to the blank hydrogel as well as against centrifugation. The low melting point of hydrogel facilitates the extraction of graphene-flakes from the hydrogel matrix by heating and diluting the gel and there is no sign of agglomeration in the extracted graphene-flakes even if the extraction is carried out after a period of three months. The present work is an exemplary study on exfoliation, hydrogelation and extraction of graphene-flakes from a hydrogel, when required, using a SAIL and is expected to provide a new platform for utilization of SAILs for efficient graphene exfoliation and subsequent preparation of functional materials.

Colloidal systems of surface active ionic liquids and sodium carboxymethyl cellulose: physicochemical investigations and preparation of magnetic nano-composites

Carboxymethyl cellulose-surface active ionic liquid colloidal formulations for preparation of magnetic nano-composites.

Surface-Active Ionic Liquid Cholinium Dodecylbenzenesulfonate: Self-Assembling Behavior and Interaction with Cellulase

The conventional sodium dodecylbenzenesulfonate (NaDBS) has been converted into an efficient and nontoxic anionic surface-active ionic liquid, cholinium dodecylbenzenesulfonate (Cho[DBS]). We have investigated its self-assembling behavior, interaction with the enzyme cellulase, and ecotoxicity. The surface-active properties at the air-liquid interface and the aggregation behavior of Cho[DBS] in water have been determined using tensiometry, isothermal titration calorimetry (ITC), conductometry, and dynamic light scattering (DLS). The enzyme activity was observed using dinitro salicylic acid analysis. The enhanced enzyme activity was explained through active-site exfoliation and structural constancy of cellulase in the micellar medium using the results from fluorescence, circular dichroism, DLS, and ITC. The nontoxic nature was confirmed by toxicity analysis on the freshwater microalgae Scenedesmus sp.

Effect of Headgroup Variation on the Self-Assembly of Cationic Surfactants with Sulfonatocalix[6]arene

The effect of headgroup variation on the association of supramolecular amphiphiles composed of 4-sulfonatocalix[6]arene (SCX6) and cationic surfactant possessing tetradecyl substituent was studied in aqueous solutions at pH 7. When the surfactant contained hydrophilic trimethylammonium, pyridinium, or 1-methylimidazolium headgroup, highly reversible temperature-responsive nanoparticle-supramolecular micelle transformation could be attained at appropriately chosen component mixing ratios and NaCl concentrations. In these cases, the substantial negative molar heat capacity change (ΔC_p) rendered nanoparticle formation strongly endothermic at low temperature, whereas the assembly to supramolecular micelle was always accompanied by enthalpy gain. The ΔC_p values became less negative when the charge density and the hydrophilic character of the surfactant headgroup diminished. The association of the more hydrophobic 6-methoxyquinolinium and quinolinium surfactants with SCX6 did not lead to supramolecular micelle formation because the self-assembly into nanoparticles was highly exothermic.

Micellization and Antimicrobial Properties of Surface-Active Ionic Liquids Containing Cleavable Carbonate Linkages

Imidazolium-based ionic liquids (ILs) containing cleavable carbonate linkages, 1-alkyloxycarbonyloxyethyl-3-methylimidazolium chlorides with alkyl chains of 10, 12, and 14 carbon atoms, were synthesized, and their self-assembly behavior and antimicrobial activity were investigated. Differential scanning calorimetry and polarized optical microscopy studies reveal that carbonate-functionalized ILs form stable thermotropic smectic liquid-crystalline phases over a wide range of temperature. The surface activity and aggregation behavior of these new ILs were investigated by tensiometry, conductometry, potentiometry, and spectrofluorimetry. The size of aggregates was examined by dynamic light scattering (DLS). Carbonate-functionalized ILs display a higher adsorption efficiency and a lower critical micelle concentration (cmc) than simple alkyl-chain-substituted ILs. The insertion of a carbonate ester moiety in the alkyl side chain favors adsorption at the air-water interface and micellization in the bulk solution when compared to nonfunctionalized ILs. DLS measurements show that small micellelike aggregates are spontaneously formed above the cmc. Furthermore, carbonate-functionalized ILs were examined for their antimicrobial activity against a panel of clinically relevant microorganisms. Biological activity was found to increase with hydrophobicity. The presence of a carbonate ester moiety significantly enhances the antimicrobial efficiency as compared to nonfunctionalized ILs, with the susceptibility of Staphylococcus sp. toward the action of these compounds being particularly remarkable. It has been demonstrated that the functionalization of the alkyl side chain of the imidazolium salts can not only modify the aggregation behavior but also lead to differences in both efficiency and the spectrum of antimicrobial activity of amphiphilic ILs.

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.

Applications of isothermal titration calorimetry - the research and technical developments from 2011 to 2015

Isothermal titration calorimetry is a widely used biophysical technique for studying the formation or dissociation of molecular complexes. Over the last 5 years, much work has been published on the interpretation of isothermal titration calorimetry (ITC) data for single binding and multiple binding sites. As over 80% of ITC papers are on macromolecules of biological origin, this interpretation is challenging. Some researchers have attempted to link the thermodynamics constants to events at the molecular level. This review highlights work carried out using binding sites characterized using x-ray crystallography techniques that allow speculation about individual bond formation and the displacement of individual water molecules during ligand binding and link these events to the thermodynamic constants for binding. The review also considers research conducted with synthetic binding partners where specific binding events like anion-π and π-π interactions were studied. The revival of assays that enable both thermodynamic and kinetic information to be collected from ITC data is highlighted. Lastly, published criticism of ITC research from a physical chemistry perspective is appraised and practical advice provided for researchers unfamiliar with thermodynamics and its interpretation. Copyright © 2016 John Wiley & Sons, Ltd.

Micelle co-assembly in surfactant/ionic liquid mixtures

HYPOTHESIS

The phase behavior of amphiphiles is known to depend on their solvent environment. The organic character of ionic liquids suggested the possibility to tune surfactant aggregation, even in the absence of water, by selection of appropriate ionic liquid chemistry. To that end the behavior of the surfactant sodium dodecylsulfate in a chemically similar imidazolium ionic liquid, 1-ethyl-3-methyl imidazolium ethylsulfate, was explored.

EXPERIMENTS

The solubility of sodium dodecylsulfate in 1-ethyl-3-methyl imidazolium ethylsulfate was determined, establishing the Krafft temperature. Tensiometry was performed to obtain interfacial properties such as the surface excess and area per molecule. Pulsed-field gradient spin-echo NMR was used to determine the diffusion coefficients of all the major species, including micelles, as a function of surfactant concentration. Importantly, all three methods provided consistent values for the critical micelle concentration.

FINDINGS

Analysis of tensiometry data suggests, and is confirmed by NMR results, that the ionic liquid ions are incorporated along with surfactants into micelles, revealing a complex micellization behavior. In light of these findings past studies with ternary mixtures of surfactants, ionic liquids, and water may merit additional scrutiny. Given the large number of ionic liquids, this work suggests opportunities to further control micelle formation and properties.

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.

New ester based gemini surfactants: the effect of different cationic headgroups on micellization properties and viscosity of aqueous micellar solution

A new series of ester functionalized cationic gemini surfactants having different cationic headgroups (i.e. piperidinium, pyrrolidinium, morpholinium and quaternary ammonium) have been synthesized and characterized using NMR and Mass spectroscopy. These new gemini surfactants were investigated for their micellization and viscosity properties using surface tension, conductivity, fluorescence and rheology thechniques. The physicochemical properties of the aqueous surfactant system were influenced by polarity, size and the nature of cationic headgroups as the surface, thermodynamic and viscosity properties of these gemini surfactants were found to be dependent on the type of cationic headgroup. The current research finding establishes the structure-property relationship of the surfactant molecule specifically taking into account the dominant role displayed by the nature of the cationic headgroup.

Surface Activity and Aggregation Behavior of Siloxane-Based Ionic Liquids in Aqueous Solution

Six novel siloxane-based surface-active ionic liquids (SAILs)--siloxane ammonium carboxylate [Si(n)N(2)-CA(1), (n = 3, 4)]--were designed and synthesized. Their melting points, surface activities, and self-aggregation behavior in aqueous solution were studied. The results showed that because of the bulky hydrophobic siloxane chains at the end of the tail, all six siloxane-based SAILs are room-temperature ionic liquids (RT-SAILs). The introduction of the siloxane group can reduce the melting point of ionic liquids to below room temperature and can promote the micellization and aggregation behavior more efficiently. These siloxane-based SAILs can greatly reduce the surface tension of water, as shown by the critical aggregation concentration (γCAC) values of 20 mN·m(-1); all six siloxane RT-SAILs can form a vesicle spontaneously in aqueous solution, indicating potential uses as model systems for biomembranes and vehicles for drug delivery.

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.