Morphology Modulation of Ionic Surfactant Micelles in Ternary Deep Eutectic Solvents

Cationic micelles in deep eutectic solvents: effects of solvent composition

Deep eutectic solvents (DES) are mixtures of hydrogen bond donors and acceptors that form strongly hydrogen-bonded room temperature liquids. Changing the H-bonding components and their ratios can alter the physicochemical properties of deep eutectic solvents. Recent studies have shown p-toluenesulfonic acid (pTSA) forms room temperature liquids with choline chloride (ChCl) at different molar ratios: 1 : 1, 1 : 2 and 2 : 1 [Rodriguez Rodriguez et al., ACS Sustain. Chem. Eng., 2019, 7(4), 3940]. They also showed that the composition affects the physical properties of these liquids and their ability to dissolve metal oxides. In this work we evaluate the solubility and self-assembly of cationic surfactants alkyltrimethyl ammonium bromides (CnTAB) in these pTSA/ChCl based liquids. CnTABs are insoluble in 1pTSA : 2ChCl, whereas in 1pTSA : 1ChCl and 2pTSA : 1ChCl they form micelles. We characterise CnTAB (n = 12, 14, 16) micelles using small angle neutron scattering and also look at interaction of water with the micelles. These studies help determine the interaction of DES components with the surfactant and the influence of varying pTSA and water ratios on these interactions. This provides potential for controlled surfactant templating and for tuning rheology modification in such systems.

Stabilization of Non-Native Folds and Programmable Protein Gelation in Compositionally Designed Deep Eutectic Solvents

Proteins are adjustable units from which biomaterials with designed properties can be developed. However, non-native folded states with controlled topologies are hardly accessible in aqueous environments, limiting their prospects as building blocks. Here, we demonstrate the ability of a series of anhydrous deep eutectic solvents (DESs) to precisely control the conformational landscape of proteins. We reveal that systematic variations in the chemical composition of binary and ternary DESs dictate the stabilization of a wide range of conformations, that is, compact globular folds, intermediate folding states, or unfolded chains, as well as controlling their collective behavior. Besides, different conformational states can be visited by simply adjusting the composition of ternary DESs, allowing for the refolding of unfolded states and vice versa. Notably, we show that these intermediates can trigger the formation of supramolecular gels, also known as eutectogels, where their mechanical properties correlate to the folding state of the protein. Given the inherent vulnerability of proteins outside the native fold in aqueous environments, our findings highlight DESs as tailorable solvents capable of stabilizing various non-native conformations on demand through solvent design.

Evidence for an L3 phase in ternary deep eutectics: composition-induced L3-to-Lα transition of AOT

Pure and hydrated deep eutectic solvents (DES) are proposed to form self-assembled nanostructures within the fluid bulk, similar to the bicontinuous L3 phase common for ionic liquids (ILs). Labelled choline chloride : urea : water DES were measured using small-angle neutron scattering (SANS), showing no long-range nanostructure. However, solutions of the surfactant AOT in this DES yielded scattering consistent with the L3 "sponge" phase, which was fitted using the Teubner-Strey model. A disclike model gave local structural information, namely, a linear increase in radius versus solvent water content (w = molar ratio of DES : water), eventually forming large, turbid lamellar phases at 10w; an L3-to-Lα transition was observed. Simultaneous multi-contrast SANS fits show the surfactant headgroup region is dominated by interactions with poorly-soluble Na+ at low water contents, and numerically-superior [cholinium]+ as water content increases. The modified interfacial Gaussian curvature from cation : anion volume matching stabilizes the lamellar morphology, allowing the bilayer aggregation number to increase.

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.

Molecular Architecture Effects on Bulk Nanostructure in Bis(Orthoborate) Ionic Liquids

A series of 19 ionic liquids (ILs) based on phosphonium and imidazolium cations of varying alkyl-chain lengths with the orthoborate anions bis(oxalato)borate [BOB]- , bis(mandelato)borate, [BMB]- and bis(salicylato)borate, [BScB]- , are synthesized and studied using small-angle neutron scattering (SANS). All measured systems display nanostructuring, with 1-methyl-3-n-alkyl imidazolium-orthoborates forming clearly bicontinuous L3 spongelike phases when the alkyl chains are longer than C6 (hexyl). L3 phases are fitted using the Teubner and Strey model, and diffusely-nanostructured systems are primarily fitted using the Ornstein-Zernicke correlation length model. Strongly-nanostructured systems have a strong dependence on the cation, with molecular architecture variation explored to determine the driving forces for self-assembly. The ability to form well-defined complex phases is effectively extinguished in several ways: methylation of the most acidic imidazolium ring proton, replacing the imidazolium 3-methyl group with a longer hydrocarbon chain, substitution of [BOB]- by [BMB]- , or exchanging the imidazolium for phosphonium systems, irrespective of phosphonium architecture. The results suggest there is only a small window of opportunity, in terms of molecular amphiphilicity and cation:anion volume matching, for the formation of stable extensive bicontinuous domains in pure bulk orthoborate-based ILs. Particularly important for self-assembly processes appear to be the ability to form H-bonding networks, which offer additional versatility in imidazolium systems.

Critical Properties of Ternary Deep Eutectic Solvents Using Group Contribution with Extended Lee-Kesler Mixing Rules

One of the most commonly used molecular inputs for ionic liquids and deep eutectic solvents (DESs) in the literature are the critical properties and acentric factors, which can be easily determined using the modified Lydersen-Joback-Reid (LJR) method with Lee-Kesler mixing rules. However, the method used in the literature is generally applicable only to binary mixtures of DESs. Nevertheless, ternary DESs are considered to be more interesting and may provide further tailorability for developing task-specific DESs for particular applications. Therefore, in this work, a new framework for estimating the critical properties and the acentric factor of ternary DESs based on their molecular structures is presented by adjusting the framework reported in the literature with an extended version of the Lee-Kesler mixing rules. The presented framework was applied to a data set consisting of 87 ternary DESs with 334 distinct compositions. For validation, the estimated critical properties and acentric factors were used to predict the densities of the ternary DESs. The results showed excellent agreement between the experimental and calculated data, with an average absolute relative deviation (AARD) of 5.203% for ternary DESs and 5.712% for 260 binary DESs (573 compositions). The developed methodology was incorporated into a user-friendly Excel worksheet for computing the critical properties and acentric factors of any ternary or binary DES, which is provided in the Supporting Information. This work promotes the creation of robust, accessible, and user-friendly models capable of predicting the properties of new ternary DESs based on critical properties, thus saving time and resources.

Constituent- and Composition-Dependent Surfactant Aggregation in (Lanthanide Salt + Urea) Deep Eutectic Solvents

Due to the ease of tailoring the physicochemical properties by simply changing a constituent or composition, deep eutectic solvents (DESs) possess widely varying capabilities for surfactant self-assembly that could depend on the surfactant headgroup charge. The self-aggregation process of three surfactants, sodium dodecylsulfate (SDS), cetyltrimethylammonium bromide (CTAB), and Triton X-100 (TX-100), dissolved in DESs composed of a lanthanide salt (Ln) and urea (U) is investigated. The role of the identity of the metal salt is assessed by using [La(NO₃)₃·6H₂O] (La) and [Ce(NO₃)₃·6H₂O] (Ce) and that of the composition is deciphered by systematically changing the mole ratio of the metal salt and urea in (La/U) DESs. The response to a fluorescence probe pyrene-1-carboxaldehyde along with electrical conductance and surface tension measurements is used to obtain the critical aggregation concentration (CAC). While the CACs in 1:3.5 (Ln/U) for SDS are significantly lower than that in water, the values are marginally higher for CTAB and TX-100. The CACs for all three surfactants are similar in 1:3.5 (La/U) and (Ce/U) DESs, implying that the identity of the metal in the salt is not so important. Increasing the urea composition in (La/U) DESs results in increased CAC for SDS and CTAB; however, a minimal decrease in CAC is observed for TX-100. From the temperature dependence of CAC, thermodynamic parameters, ΔG_agg⁰, ΔH_agg⁰, and ΔS_agg⁰, of the surfactant self-aggregation process are estimated. These parameters reveal that while at a lower urea content, the SDS/CTAB self-assembly process is enthalpically driven, it becomes entropically favored at higher urea concentrations. The TX-100 self-aggregation in these DESs is found to be strongly enthalpically favored and entropically un-favored. These parameters are explained as a combination of passage of the solvophobic surfactant chain from the bulk DES to the aggregate pseudo-phase and differential orientation/organization of DES constituents around surfactant monomers and/or aggregates.

How Does Nanostructure in Ionic Liquids and Hybrid Solvents Affect Surfactant Self-Assembly?

Ionic liquids (ILs) have recently emerged as novel classes of solvents that support surfactant self-assembly into micelles, liquid crystals, and microemulsions. Their low volatility and wide liquid stability ranges make them attractive for many diverse applications, especially in extreme environments. However, the number of possible ion combinations makes systematic investigations both challenging and rare; this is further amplified when mixtures are considered, whether with water or other H-bonding components such as those found in deep eutectics. In this Perspective we examine what factors determine amphiphilicity, solvophobicity and solvophilicity, in ILs and related exotic environments, in what ways these differ from water, and how the underlying nanostructure of the liquid itself affects the formation and structure of micelles and other self-assembled materials.

Deep Eutectic Systems as Novel Vehicles for Assisting Drug Transdermal Delivery

In recent years, deep eutectic systems (DES) emerged as novel vehicles for facilitating the transdermal delivery of various drugs, including polysaccharides, proteins, insulin, vaccine, nanoparticles, and herb extracts. The objective of this study is to conduct a comprehensive review of the application of DES to transdermal drug delivery, based on previous work and the reported references. Following a brief overview, the roles of DES in TDDS, the modes of action, as well as the structure-activity relationship of DES are discussed. Particularly, the skin permeation of active macromolecules and rigid nanoparticles, which are the defining characteristics of DES, are extensively discussed. The objective is to provide a comprehensive understanding of the current investigation and development of DES-based transdermal delivery systems, as well as a framework for the construction of novel DES-TDDS in the future.

Amphiphilic Deep Eutectic Solvent Based on Lidocaine and Lauric Acid: Formation of Microemulsion and Gel

Deep eutectic solvent (DES), as a new type of promising green solvent, showed great advantages of easy preparation and no need for purification after synthesis and displayed great potential applications in various fields. Herein, we have constructed a new type of therapeutic DES based on lidocaine and lauric acid. The DES displayed good surface activity in constructing a nonaqueous microemulsion with 1,2-propanediol (PG) and isopropyl myristate (IPM) being the polar phase and nonpolar phase, respectively. The obtained nonaqueous microemulsion displayed a structural transition from W/O type to O/W type via a bicontinuous structure with an increase of the PG content. The size, morphology, and microstructure of the microemulsion were explored using dynamic light scattering (DLS), transmission electron microscopy (TEM), and UV-vis absorption spectra measurements. Furthermore, this novel DES can act as a gelator to form a gel in a certain water content range. The rheological measurements suggested the presence of a strong colloidal force. Therefore, the results presented herein were expected to provide a new perspective in the applications of deep eutectic solvent as a surfactant.

Deep eutectic systems: An overview of fundamental aspects, current understanding and drug delivery applications

The deep eutectic system (DES) is a relatively new concept in the field of drug delivery science. DES is a class of eutectic mixtures comprised of two or more components, with a eutectic point far below than the melting temperature of the pure components. The strong hydrogen bonding interactions between DES constituents are responsible for significant lowering of melting point in DES. A significant number of molecules cannot reach from drug discovery phase to drug development phase because of poor biopharmaceutical attributes, such as solubility and permeability. DES can be a novel alternative to overcome these issues. In last few years DESs have been widely used in different pharmaceutical and chemical processes. However, comprehensive information regarding their drug delivery potential is not available. This review deals with fundamental aspects such as types, preparation, thermodynamics, toxicity, biodegradability and their applications in the field of drug delivery. Current challenges, future prospects and translational aspects of DES as drug delivery system have also been discussed.

Long-Range Electrostatic Colloidal Interactions and Specific Ion Effects in Deep Eutectic Solvents

While the traditional consensus dictates that high ion concentrations lead to negligible long-range electrostatic interactions, we demonstrate that electrostatic correlations prevail in deep eutectic solvents where intrinsic ion concentrations often surpass 2.5 M. Here we present an investigation of intermicellar interactions in 1:2 choline chloride:glycerol and 1:2 choline bromide:glycerol using small-angle neutron scattering. Our results show that long-range electrostatic repulsions between charged colloidal particles occur in these solvents. Interestingly, micelle morphology and electrostatic interactions are modulated by specific counterion condensation at the micelle interface despite the exceedingly high concentration of the native halide from the solvent. This modulation follows the trends described by the Hofmeister series for specific ion effects. The results are rationalized in terms of predominant ion-ion correlations, which explain the reduction in the effective ionic strength of the continuum and the observed specific ion effects.

Self-assembly of ionic and non-ionic surfactants in type IV cerium nitrate and urea based deep eutectic solvent

Understanding and manipulating micelle morphology are key to exploiting surfactants in various applications. Recent studies have shown surfactant self-assembly in a variety of Deep Eutectic Solvents (DESs) where both the nature of surfactants and the interaction of the surfactant molecule with the solvent components influence the size, shape, and morphology of the micelles formed. So far, micelle formation has only been reported in type III DESs, consisting solely of organic species. In this work, we have explored the self-assembly of cationic surfactant dodecyl trimethylammonium nitrate/bromide (C₁₂TANO₃/C₁₂TAB), anionic surfactant sodium dodecyl sulfate (SDS), and non-ionic surfactants hexaethylene glycol monododecyl ether (C₁₂EO₆) and octaethylene glycol monohexadecyl ether (C₁₆EO₈) in a type IV DES comprising metal salt, cerium (III) nitrate hexahydrate, and a hydrogen bond donor, urea, in the molar ratio 1:3.5. C₁₂TANO₃, C₁₂TAB, C₁₂EO₆, and C₁₆EO₈ form spherical micelles in the DES with the micelle size dependent on both the surfactant alkyl chain length and the head group, whereas SDS forms cylindrical micelles. We hypothesize that the difference in the micelle shape can be explained by counterion stabilization of the SDS headgroup by polycations in the DES compared to the nitrate/bromide anion interaction in the case of cationic surfactants or molecular interaction of the urea and the salting out effect of (CeNO₃)₃ in the DES on the alkyl chains/polyethoxy headgroup for non-ionic surfactants. These studies deepen our understanding of amphiphile self-assembly in this novel, ionic, and hydrogen-bonding solvent, raising the opportunity to use these structures as liquid crystalline templates to generate porosity in metal oxides (ceria) that can be synthesized using these DESs.

Electrodeposition of Silver in a Ternary Deep Eutectic Solvent and the Electrochemical Sensing Ability of the Ag-Modified Electrode for Nitrofurazone

The determination of nitrofurazone (NFZ) has received substantial attention because it is a kind of antibiotic drug. Herein, a rapid and low-cost electrochemical sensor for the analysis of NFZ is reported. The method uses Ag-modified electrodes in which different surfactants, hexadecyltrimethylammonium bromide and sodium dodecyl sulfate, in a ternary choline chloride-urea-glycerol deep eutectic solvent were deposited. The physical properties of the solutions with various surfactants are investigated by a conductivity meter, viscometer, and tensiometer. The morphologies and crystallinity of the Ag-modified electrodes were characterized by using scanning electron microscopy, transmission electron microscopy, and powder X-ray diffraction. Electrochemical impedance spectroscopy and CV analyses indicate that the as-prepared Ag-SDS electrode exhibited better performance as a NFZ sensor. The dynamic linear range of NFZ is 0.66-930 μM with a corresponding detection limit of 0.37 μM. The proposed electrochemical sensor was applied to detect NFZ in the aquaculture water sample, and the results showed good recovery in the range from 100.28 to 102.65%.