Nanostructure in Amphiphile-Based Deep Eutectic Solvents.

Deep eutectic solvents (DESs) are an emerging class of modern, often "green" solvents with unique properties. Recently, a deep eutectic system based on amphiphilic surfactant N-alkyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (C12 & C14 sulfobetaine) and (1S)-(+)-10-camphor-sulfonic acid in the molar ratio 1:1.5 has been reported. Nanostructuring can be expected in this DES due to the nature of the components. In this work, we have investigated the native nanostructure in the DES comprising C12-C18 alkyl chain sulfobetaines with camphor sulfonic acid and how it interacts with polar and nonpolar species, water and dodecane, respectively, using small angle neutron scattering. By using contrast variation to highlight the relative position of the solvent components and additives, we can resolve the structure of the solvent and how it changes upon interaction with water and dodecane. Scattering from the neat DES shows structures corresponding to the self-assembly of sulfobetaines; the size of the structure increases as the alkyl chain length of the sulfobetaines increases. Water and dodecane interact, respectively, with the hydrophilic and hydrophobic moieties in the DES structure, primarily the sulfobetaine, thereby swelling and solvating the entire structure. The extent of the shift of the peak position, and the swelling, depend on concentration of the additive. The solution phase organization and the interaction of polar and nonpolar species as observed here, have the potential to affect the ordering of inorganic or polymeric materials grown in such solvents, paving new avenues for templating applications.

The effect of polymer end-group on the formation of styrene - maleic acid lipid particles (SMALPs).

A series of block copolymers comprising styrene and maleic acid (SMA) has been prepared using RAFT polymerisation. RAFT often results in a large hydrophobic alkylthiocarbonylthio end group and this work examines its effect on the solution behaviour of the copolymers. SMA variants with, and without, this end group were synthesised and their behaviour compared with a commercially-available random copolymer of similar molecular weight. Dynamic light scattering and surface tension measurements found the RAFT-copolymers preferentially self-assembled into higher-order aggregates in aqueous solution. Small angle neutron scattering using deuterated styrene varients add support to the accepted model that these agreggates comprise a solvent-protected styrenic core with an acid-rich shell. Replacing the hydrophobic RAFT end group with a more hydrophilic nitrile caused differences in the resulting surface activity, attributed to the ability of the adjoining styrene homoblock to drive aggregation. Each of the copolymers formed SMALP nanodiscs with DMPC lipids, which were found to encapsulate a model membrane protein, gramicidin. However, end group variation affected solubilisition of DPPC, a lipid with a higher phase transition temperature. When using RAFT-copolymers terminated with a hydrophobic group, swelling of the bilayer and greater penetration of the homoblock into the nanodisc core occurred with increasing homoblock length. Conversely, commercial and nitrile-terminated RAFT-copolymers produced nanodisc sizes that stayed constant, instead indicating interaction at the edge of the lipid patch. The results highlight how even minor changes to the copolymer can modify the amphiphilic balance between regions, knowledge useful towards optimising copolymer structure to enhance and control nanodisc formation.

Trace Water Changes Metal Ion Speciation in Deep Eutectic Solvents: Ce3+ Solvation and Nanoscale Water Clustering in Choline Chloride-Urea-Water Mixtures.

Eutectic mixtures of choline chloride, urea, and water in deep eutectic solvent (DES)/water molar hydration ratios (w) of 2, 5, and 10, with dissolved cerium salt, were measured using neutron diffraction with isotopic substitution. Structures were modeled using empirical potential structure refinement (EPSR). Ce3+ was found to form highly charged complexes with a mean coordination number between 7 and 8, with the shell containing mostly chloride, followed by water. The shell composition is strongly affected by the molar ratio of dilution, as opposed to the mass or volume fraction, due to the high affinity of Cl- and H2O ligands that displace less favorable interactions with ligands such as urea and choline. The presence of Ce3+ salt disrupted the bulk DES structure slightly, making it more electrolyte-like. The measured coordination shell of choline showed significant discrepancies from the statistical noninteracting distribution, highlighting the nonideality of the blend. Cluster analysis revealed the trace presence of percolating water clusters (25 ≥ n ≥ 2) in solvent compositions of 5 and 10w for the first time.

Surfactant induced gelation of TEMPO-oxidized cellulose nanofibril dispersions probed using small angle neutron scattering.

In this work, we studied TEMPO-oxidized cellulose nanofibril (OCNF) suspensions in the presence of diverse surfactants. Using a combination of small angle neutron scattering (SANS) and rheology, we compared the physical properties of the suspensions with their structural behavior. Four surfactants were studied, all with the same hydrophobic tail length but different headgroups: hexaethylene glycol mono-n-dodecyl ether (C12EO6, nonionic), sodium dodecyl sulfate (SDS, anionic), cocamidopropyl betaine (CapB, zwitterionic), and dodecyltrimethylammonium bromide (DTAB, cationic). Contrast variation SANS studies using deuterated version of C12EO6 or SDS, or by varying the D2O/H2O ratio of the suspensions (with CapB), allowed focusing only on the structural properties of OCNFs or surfactant micelles. We showed that, in the concentration range studied, for C12EO6, although the nanofibrils are concentrated thanks to an excluded volume effect observed in SANS, the rheological properties of the suspensions are not affected. Addition of SDS or CapB induces gelation for surfactant concentrations superior to the critical micellar concentration (CMC). SANS results show that attractive interactions between OCNFs arise in the presence of these anionic or zwitterionic surfactants, hinting at depletion attraction as the main mechanism of gelation. Finally, addition of small amounts of DTAB (below the CMC) allows formation of a tough gel by adsorbing onto the OCNF surface.

Neutron Diffraction Study of Indole Solvation in Deep Eutectic Systems of Choline Chloride, Malic Acid, and Water.

Deep eutectic systems are currently under intense investigation to replace traditional organic solvents in a range of syntheses. Here, indole in choline chloride-malic acid deep eutectic solvent (DES) was studied as a function of water content, to identify solute interactions with the DES which affect heterocycle reactivity and selectivity, and as a proxy for biomolecule solvation. Empirical Potential Structure Refinement models of neutron diffraction data showed [Cholinium]+ cations associate strongly with the indole π-system due to electrostatics, whereas malic acid is only weakly associated. Trace water is sequestered into the DES and does not interact strongly with indole. When water is added to the DES, it does not interact with the indole π-system but is exclusively in-plane with the heterocyclic rings, forming strong H-bonds with the -NH group, and also weak H-bonds and thus prominent hydrophobic hydration of the indole aromatic region, which could direct selectivity in reactions.

Stable Cellulose Nanofibril Microcapsules from Pickering Emulsion Templates.

Electrostatic attractions are essential in any complex formation between the nanofibrils of the opposite charge for a specific application, such as microcapsule production. Here, we used cationized cellulose nanofibril (CCNF)-stabilized Pickering emulsions (PEs) as templates, and the electrostatic interactions were induced by adding oxidized cellulose nanofibrils (OCNFs) at the oil-water interface to form microcapsules (MCs). The oppositely charged cellulose nanofibrils enhanced the solidity of interfaces, allowing the encapsulation of Nile red (NR) in sunflower oil droplets. Microcapsules exhibited a low and controlled release of NR at room temperature. Furthermore, membrane emulsification was employed to scale up the preparation of microcapsules with sunflower oil (SFO) encapsulated by CCNF/OCNF complex networks.

Structural investigation of sulfobetaines and phospholipid monolayers at the air-water interface.

Mixtures of sulfobetaine based lipids with phosphocholine phospholipids are of interest in order to study the interactions between zwitterionic surfactants and the phospholipids present in cell membranes. In this study we have investigated the structure of mixed monolayers of sulfobetaines and phosphocholine phospholipids. The sulfobetaine used has a single 18-carbon tail, and is referred to as SB3-18, and the phospholipid used is DMPC. Surface pressure-area isotherms of the samples were used to determine whether any phase transitions were present during the compression of the monolayers. Neutron and X-ray reflectometry were then used to investigate the structure of these monolayers perpendicular to the interface. We found that the average headgroup and tail layer thickness was reasonably consistent across all mixtures, with a variation of less than 3 Å reported in the total thickness of the monolayers at each surface pressure. However, by selective deuteration of the two components of the monolayers, it was found that the two components have different tail layer thicknesses. For the mixture with equal compositions of DMPC and SB3-18 or with a higher composition of DMPC the tail tilts were found to be constant, resulting in a greater tail layer thickness for SB3-18 due to its longer tail. For the mixture higher in SB3-18 this was not the case, the tail tilt angle for the two components was found to be different and DMPC was found to have a greater tail layer thickness than SB3-18 as a result.

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.

Salt-Responsive Pickering Emulsions Stabilized by Functionalized Cellulose Nanofibrils.

Oil-in-water emulsions have been stabilized by functionalized cellulose nanofibrils bearing either a negative (oxidized cellulose nanofibrils, OCNF) or a positive (cationic cellulose nanofibrils, CCNF) surface charge. The size of the droplets was measured by laser diffraction, while the structure of the shell of the Pickering emulsion droplets was probed using small-angle neutron scattering (SANS), confocal laser scanning microscopy (CLSM), scanning electron microscopy (SEM), and rheology measurements. Both OCNF- and CCNF-stabilized emulsions present a very thick shell (>100 nm) comprised of densely packed CNF. OCNF-stabilized emulsions proved to be salt responsive, influencing the droplet aggregation and ultimately the gel properties of the emulsions, while CCNF emulsions, on the other hand, showed very little salt-dependent behavior.

Synthesis, Properties, and Applications of Bio-Based Cyclic Aliphatic Polyesters.

Cyclic polymers have long been reported in the literature, but their development has often been stunted by synthetic difficulties such as the presence of linear contaminants. Research into the synthesis of these polymers has made great progress in the past decade, and this review covers the synthesis, properties, and applications of cyclic polymers, with an emphasis on bio-based aliphatic polyesters. Synthetic routes to cyclic polymers synthesized from bioderived monomers, alongside mechanistic descriptions for both ring closure and ring expansion polymerization approaches, are reviewed. The review also highlights some of the unique physical properties of cyclic polymers together with potential applications. The findings illustrate the substantial recent developments made in the syntheses of cyclic polymers, as well as the progress which can be made in the commercialization of bio-based polymers through the versatility this topology provides.

Enzyme-Functionalized Cellulose Beads as a Promising Antimicrobial Material.

The extensive use of antibiotics over the last decades is responsible for the emergence of multidrug-resistant (MDR) microorganisms that are challenging health care systems worldwide. The use of alternative antimicrobial materials could mitigate the selection of new MDR strains by reducing antibiotic overuse. This paper describes the design of enzyme-based antimicrobial cellulose beads containing a covalently coupled glucose oxidase from Aspergillus niger (GOx) able to release antimicrobial concentrations of hydrogen peroxide (H2O2) (≈ 1.8 mM). The material preparation was optimized to obtain the best performance in terms of mechanical resistance, shelf life, and H2O2 production. As a proof of concept, agar inhibition halo assays (Kirby-Bauer test) against model pathogens were performed. The two most relevant factors affecting the bead functionalization process were the degree of oxidation and the pH used for the enzyme binding process. Slightly acidic conditions during the functionalization process (pH 6) showed the best results for the GOx/cellulose system. The functionalized beads inhibited the growth of all the microorganisms assayed, confirming the release of sufficient antimicrobial levels of H2O2. The maximum inhibition efficiency was exhibited toward Pseudomonas aeruginosa (P. aeruginosa) and Escherichia coli (E. coli), although significant inhibitory effects toward methicillin-resistant Staphylococcus aureus (MRSA) and S. aureus were also observed. These enzyme-functionalized cellulose beads represent an inexpensive, sustainable, and biocompatible antimicrobial material with potential use in many applications, including the manufacturing of biomedical products and additives for food preservation.

Deep eutectic solvents-The vital link between ionic liquids and ionic solutions.

When selecting a solvent for a given solute, the strongly held idiom "like dissolves like", meaning that polar solvents are used for polar solutes, is often used. This idea has resulted from the concept that most molecular solvents are homogeneous. In a deep eutectic solvent (DES), however, both components can be ionic or non-ionic, polar or non-polar. By tuning the components, DESs can solubilize a wide variety of solutes, often mixing hydrophobic and hydrophilic components, and the mixture can be designed to control phase behavior. The liquids often contain significant short-length order, and preferential solvation of one component often occurs. The addition of small polar molecules such as water or alcohols results in non-homogeneous liquids, which have significantly decreased viscosity and increased ionic conductivity. Accordingly, the areas covered in this special issue focus on structure and dynamics, solvation, the mobility of charged species, and the ability to obtain controllable phase behavior by adding polar diluents or using hydrophobic DESs.

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.

Multienzyme Cellulose Films as Sustainable and Self-Degradable Hydrogen Peroxide-Producing Material.

The use of hydrogen peroxide-releasing enzymes as a component to produce alternative and sustainable antimicrobial materials has aroused interest in the scientific community. However, the preparation of such materials requires an effective enzyme binding method that often involves the use of expensive and toxic chemicals. Here, we describe the development of an enzyme-based hydrogen peroxide-producing regenerated cellulose film (RCF) in which a cellobiohydrolase (TrCBHI) and a cellobiose dehydrogenase (MtCDHA) were efficiently adsorbed, 90.38 ± 2.2 and 82.40 ± 5.7%, respectively, without making use of cross-linkers. The enzyme adsorption kinetics and binding isotherm experiments showed high affinity of the proteins possessing cellulose-binding modules for RCF, suggesting that binding on regenerated cellulose via specific interactions can be an alternative method for enzyme immobilization. Resistance to compression and porosity at a micrometer scale were found to be tunable by changing cellulose concentration prior to film regeneration. The self-degradation process, triggered by stacking TrCBHI and MtCDHA (previously immobilized onto separate RCF), produced 0.15 nmol/min·cm2 of H2O2. Moreover, the production of H2O2 was sustained for at least 24 h reaching a concentration of ∼2 mM. The activity of MtCDHA immobilized on RCF was not affected by reuse for at least 3 days (1 cycle/day), suggesting that no significant enzyme leakage occurred in that timeframe. In the material herein designed, cellulose (regenerated from a 1-ethyl-3-methylimidazolium acetate/dimethyl sulfoxide (DMSO) solution) serves both as support and substrate for the immobilized enzymes. The sequential reaction led to the production of H2O2 at a micromolar-millimolar level revealing the potential use of the material as a self-degradable antimicrobial agent.

Hydrophobization of Cellulose Nanocrystals for Aqueous Colloidal Suspensions and Gels.

Surface hydrophobization of cellulose nanomaterials has been used in the development of nanofiller-reinforced polymer composites and formulations based on Pickering emulsions. Despite the well-known effect of hydrophobic domains on self-assembly or association of water-soluble polymer amphiphiles, very few studies have addressed the behavior of hydrophobized cellulose nanomaterials in aqueous media. In this study, we investigate the properties of hydrophobized cellulose nanocrystals (CNCs) and their self-assembly and amphiphilic properties in suspensions and gels. CNCs of different hydrophobicity were synthesized from sulfated CNCs by coupling primary alkylamines of different alkyl chain lengths (6, 8, and 12 carbon atoms). The synthetic route permitted the retention of surface charge, ensuring good colloidal stability of hydrophobized CNCs in aqueous suspensions. We compare surface properties (surface charge, ζ potential), hydrophobicity (water contact angle, microenvironment probing using pyrene fluorescence emission), and surface activity (tensiometry) of different hydrophobized CNCs and hydrophilic CNCs. Association of hydrophobized CNCs driven by hydrophobic effects is confirmed by X-ray scattering (SAXS) and autofluorescent spectroscopy experiments. As a result of CNC association, CNC suspensions/gels can be produced with a wide range of rheological properties depending on the hydrophobic/hydrophilic balance. In particular, sol-gel transitions for hydrophobized CNCs occur at lower concentrations than hydrophilic CNCs, and more robust gels are formed by hydrophobized CNCs. Our work illustrates that amphiphilic CNCs can complement associative polymers as modifiers of rheological properties of water-based systems.

Filler size effect in an attractive fibrillated network: a structural and rheological perspective.

The effect of the filler size on the structural and mechanical properties of an attractive fibrillated network composed of oxidised cellulose nanofibrils (OCNF) in water was investigated. Silica nanoparticles with a diameter of ca. 5 nm (SiNp5) and and ca. 158 nm (SiNp158) were chosen as non-interacting fillers of the OCNF network. These filler sizes were chosen, respectively, to have a particle size which was either similar to that of the network mesh size or much larger than it. Contrast matched small angle neutron scattering (SANS) experiments revealed that the presence of the fillers (SiNp5 and SiNp158) did not perturb the structural properties of the OCNF network at the nanometer scale. However, the filler size difference strongly affected the mechanical properties of the hydrogel upon large amplitude oscillatory shear. The presence of the smaller filler, SiNp5, preserved the mechanical properties of the hydrogels, while the larger filler, SiNp158, allowed a smoother breakage of the network and low network recoverability after breakage. This study showed that the filler-to-mesh size ratio, for non-interacting fillers, is pivotal for tailoring the non-linear mechanical properties of the gel, such as yielding and flow.

Charge-driven interfacial gelation of cellulose nanofibrils across the water/oil interface.

Interfacial gels, obtained by the interaction of water-dispersible oxidised cellulose nanofibrils (OCNF) and oil-soluble oleylamine (OA), were produced across water/oil (W/O) interfaces. Surface rheology experiments showed that the complexation relies on the charge coupling between the negatively-charged OCNF and OA. Complexation across the W/O interface was found to be dependent on the ζ-potential of the OCNF (modulated by electrolyte addition), leading to different interfacial properties. Spontaneous OCNF adsorption at the W/O interface occurred for particles with ζ-potential more negative than -30 mV, resulting in the formation of interfacial gels; whilst for particles with ζ-potential of ca. -30 mV, spontaneous adsorption occurred, coupled with augmented interfibrillar interactions, yielding stronger and tougher interfacial gels. On the contrary, charge neutralisation of OCNF (ζ-potential values more positive than -30 mV) did not allow spontaneous adsorption of OCNF at the W/O interface. In the case of favourable OCNF adsorption, the interfacial gel was found to embed oil-rich droplets - a spontaneous emulsification process.

Impact of wormlike micelles on nano and macroscopic structure of TEMPO-oxidized cellulose nanofibril hydrogels.

In this work, we investigated the effect of adding surfactant mixtures on the rheological properties of TEMPO-oxidized cellulose nanofibril (OCNF) saline dispersions. Three surfactant mixtures were studied: cocamidopropyl betaine (CAPB)/sodium dodecyl sulfate (SDS), which forms wormlike micelles (WLMs); cocamidopropylamine oxide (CAPOx)/SDS, which forms long rods; and CAPB/sodium lauroyl sarcosinate (SLS), which forms spherical micelles. The presence of micelles in these surfactant mixtures, independent of their morphology, leads to an increase of tan δ, making the gels less solid-like, therefore acting as a plasticizer. WLMs were able to suppress strain stiffening normally observed in OCNF gels at large strains. OCNF/WLM gels have lower G' values than OCNF gels while the other micellar morphologies have a reduced impact on G'. The presence of unconnected micelles leads to increased dissipative deformation in OCNF gels without affecting the connectivity of the fibrils, while the presence of entangled micelles interferes with the OCNF network.

Stability and behaviour in aqueous solutions of the anionic cubic silsesquioxane substituted with tetramethylammonium.

The aqueous behaviour of the anionic octa-tetramethylammonium substituted cubic silsesquioxane, [N(CH3)4]8[Si8O20], was studied with quantitative 29Si-NMR. This molecule partially fragments in aqueous solutions, forming several smaller entities. The most abundant silica species are the monomer, dimer, cyclic trimer, cyclic tetramer and double three-ring. Higher concentrations are required in order to prevent complete fragmentation of the cubic structure. Additives such as alcohols and tetraalkylammonium salts have a stabilising effect on the cubic silsesquioxane, unlike sodium salts that destabilise it. A high concentration solution, containing the non-fragmented molecule as well as entities resulting from fragmentations, was investigated with neutron scattering coupled with modelling, using empirical potential structure refinement (EPSR). The modelling reveals that TMA+ ions coordinates to all different silica species, with approximately three TMA+ per cube. These are located at the faces of the cube.

Bayesian determination of the effect of a deep eutectic solvent on the structure of lipid monolayers.

In this work, we present the first example of the self-assembly of phospholipid monolayers at the interface between air and an ionic solvent. Deep eutectic solvents are a novel class of environmentally friendly, non-aqueous, room temperature liquids with tunable properties, that have wide-ranging potential applications and are capable of promoting the self-assembly of surfactant molecules. We use a chemically-consistent Bayesian modelling of X-ray and neutron reflectometry measurements to show that these monolayers broadly behave as they do on water. This method allows for the monolayer structure to be determined, alongside the molecular volumes of the individual monolayer components, without the need for water-specific constraints to be introduced. Furthermore, using this method we are able to better understand the correlations present between parameters in the analytical model. This example of a non-aqueous phospholipid monolayer has important implications for the potential uses of these solvents and for our understanding of how biomolecules behave in the absence of water.