Effect of Potential on the Nanostructure Dynamics of Ethylammonium Nitrate at a Graphite Electrode.

Video-rate atomic force microscopy (AFM) is used to study the near-surface nanostructure dynamics of the ionic liquid ethylammonium nitrate (EAN) at a highly oriented pyrolytic graphite (HOPG) electrode as a function of potential in real-time for the first time. The effects of varying the surface potential and adding 10 wt% water on the nanostructure diffusion coefficient are probed. For both EAN and the 90 wt% EAN-water mixture, disk-like features ≈9 nm in diameter and 1 nm in height form above the Stern layer at all potentials. The nanostructure diffusion coefficient increases with potential (from OCP -0.5 V to OCP +0.5 V) and with added water. Nanostructure dynamics depends on both the magnitude and direction of the potential change. Upon switching the potential from OCP -0.5 V to OCP +0.5 V, a substantial increase in the diffusion coefficients is observed, likely due to the absence of solvophobic interactions between the nitrate (NO3 - ) anions and the ethylammonium (EA+ ) cations in the near-surface region. When the potential is reversed, EA+ is attracted to the Stern layer to replace NO3 - , but its movement is hindered by solvophobic attractions. The outcomes will aid applications, including electrochemical devices, catalysts, and lubricants.

Nanostructure and Dynamics of Aprotic Ionic Liquids at Graphite Electrodes as a Function of Potential.

Atomic force microscope (AFM) videos reveal the near-surface nanostructure and dynamics of the ionic liquids (ILs) 1-butyl-3-methylimidazolium dicyanamide (BMIM DCA) and 1-hexyl-3-methylimidazolium dicyanamide (HMIM DCA) above highly oriented pyrolytic graphite (HOPG) electrodes as a function of surface potential. Molecular dynamics (MD) simulations reveal the molecular-level composition of the nanostructures. In combination, AFM and MD show that the near-surface aggregates form via solvophobic association of the cation alkyl chains at the electrode interface. The diffusion coefficients of interfacial nanostructures are ≈0.01 nm2 s-1 and vary with the cation alkyl chain length and the surface potential. For each IL, the nanostructure diffusion coefficients are similar at open-circuit potential (OCP) and OCP + 1V, but BMIM DCA moves about twice as fast as HMIM DCA. At negative potentials, the diffusion coefficient decreases for BMIM DCA and increases for HMIM DCA. When the surface potential is switched from negative to positive, a sudden change in the direction of the nanostructure motion is observed for both BMIM DCA and HMIM DCA. No transient dynamics are noted following other potential jumps. This study provides a new fundamental understanding regarding the dynamics of electrochemically stable ILs at electrodes vital for the rational development of IL-based electrochemical devices.

Molecular Resolution Nanostructure and Dynamics of the Deep Eutectic Solvent-Graphite Interface as a Function of Potential.

Interest in deep eutectic solvents (DESs), particularly for electrochemical applications, has boomed in the past decade because they are more versatile than conventional electrolyte solutions and are low cost, renewable, and non-toxic. The molecular scale lateral nanostructures as a function of potential at the solid-liquid interface-critical design parameters for the use of DESs as electrochemical solvents-are yet to be revealed. In this work, in situ amplitude modulated atomic force microscopy complemented by molecular dynamics simulations is used to probe the Stern and near-surface layers of the archetypal and by far most studied DES, 1:2 choline chloride:urea (reline), at the highly orientated pyrolytic graphite surface as a function of potential, to reveal highly ordered lateral nanostructures with unprecedented molecular resolution. This detail allows identification of choline, chloride, and urea in the Stern layer on graphite, and in some cases their orientations. Images obtained after the potential is switched from negative to positive show the dynamics of the Stern layer response, revealing that several minutes are required to reach equilibrium. These results provide valuable insight into the nanostructure and dynamics of DESs at the solid-liquid interface, with implications for the rational design of DESs for interfacial applications.

Bulk nanostructure of a deep eutectic solvent with an amphiphilic hydrogen bond donor.

Neutron diffraction with empirical potential structure refinement (EPSR) show the deep eutectic solvent (DES) 1 : 4 choline chloride : butyric acid is amphiphilically nanostructured. Nanostructure results from solvophobic interactions between the alkyl chains of the butyric acid hydrogen bond donor (HBD) and is retained with addition of 10 wt% water. EPSR fits to the diffraction data is used to produce a three-dimensional model of the liquid which is interrogated to reveal the interactions leading to the solvophobic effect, and therefore nanostructure, in this DES at atomic resolution. The model shows electrostatic and hydrogen bond interactions cause the cation, anion and HBD acid group to cluster into a polar domain, from which the acid alkyl chains are solvophobically excluded into theapolar domain. The polar and apolar domains percolate through the liquid in a bicontinuous sponge-like structure. The effect of adding 10 wt% water is probed, revealing that water molecules are sequestered around the cation and anion within the polar domain, while the neat bulk structure is retained. Alkyl chain packing in the apolar domain becomes slightly better-defined indicating water marginally strengthens solvophobic segregation. These findings reveal bulk self-assembled nanostructure can be produced in DESs via an amphiphilic HBD.

Intracellular Communication between Synthetic Macromolecules.

Non-viral delivery is an important strategy for selective and efficient gene therapy, immunization, and RNA interference, which overcomes problems of genotoxicity and inherent immunogenicity associated with viral vectors. Liposomes and polymers are compelling candidates as carriers for intracellular, non-viral delivery, but maximal efficiencies of around 1% have been reported for the most advanced non-viral carriers. Here, we develop a library of dendronized bottlebrush polymers with controlled defects, displaying a level of precision surpassed only by biological molecules like DNA, RNA, and proteins. We test concurrent and competitive delivery of DNA and show for the first time that, while intracellular communication is thought to be an exclusively biomolecular phenomenon, such communication between synthetic macromolecular complexes can also take place. Our findings challenge the assumption that delivery agents behave as bystanders that enable transfection by passive intracellular release of genetic cargo and improve upon coarse strategies in intracellular carrier design lacking control over polymer sequence, architecture, and composition, leading to a hit-or-miss outcome. Understanding the communication that takes place between macromolecules will help improve the design of non-viral delivery agents and facilitate translation of genome engineering, vaccines, and nucleic acid-based therapies.

Polycation radius of gyration in a polymeric ionic liquid (PIL): the PIL melt is not a theta solvent.

The conformation of the polycation in the prototypical polymeric ionic liquid (PIL) poly(3-methyl-1-aminopropylimidazolylacrylamide) bis(trifluoromethylsulfonyl)imide (poly(3MAPIm)TFSI) was probed using small-angle neutron scattering (SANS) and ultra-small-angle neutron scattering (USANS) at 25 °C and 80 °C. Poly(3MAPIm)TFSI contains microvoids which lead to intense low q scattering that can be mitigated using mixtures of hydrogen- and deuterium-rich materials, allowing determination of the polycation conformation and radius of gyration (Rg). In the pure PIL, the polycation adopts a random coil conformation with Rg = 52 ± 0.5 Å. In contrast to conventional polymer melts, the pure PIL is not a theta solvent for the polycation. The TFSI- anions, which comprise 48% v/v of the PIL, are strongly attracted to the polycation and act like small solvent molecules which leads to chain swelling analogous to an entangled, semi-dilute, or concentrated polymer solution in a good solvent.

Selective ion transport across a lipid bilayer in a protic ionic liquid.

Ionic liquids (ILs) have exhibited enormous potential as electrolytes, designer solvents and reaction media, as well as being surprisingly effective platforms for amphiphile self-assembly and for preserving structure of complex biomolecules. This has led to their exploration as media for long-term biopreservation and in biosensors, for which their viability depends on their ability to sustain both structure and function within complex, multicomponent nanoscale compartments and assemblies. Here we show that a tethered lipid bilayer can be assembled directly in a purely IL environment that retains its structure upon exchange between IL and aqueous buffer, and that the membrane transporter valinomycin can be incorporated so as to retain its functionality and cation selectivity. This paves the way for the development of long-lived, non-aqueous microreactors and sensor assemblies, and demonstrates the potential for complex proteins to retain functionality in non-aqueous, ionic liquid solvents.

Stiffness-Dependent Intracellular Location of Cylindrical Polymer Brushes.

Cylindrical polymer brushes (CPBs) are macromolecules with nanoparticle proportions. Their modular synthesis enables tailoring of their chemical composition as well as the dialing-up of overall dimensions and physicochemical properties. In this study, two rod-like poly[(ethylene glycol) methyl ether methacrylate] (PEGMA)-based CPBs with varying stiffness but otherwise comparable features and functionality, are synthesized. Differences in particle stiffness are assessed using small angle neutron scattering (SANS). It is observed that the fate of the two CPBs within cells is distinctly different. Stiffer CPBs seem to gravitate toward the mitochondria, whereas CPBs with reduced stiffness are present in different intracellular vesicles.

Amphiphilic nanostructure in choline carboxylate and amino acid ionic liquids and solutions.

The liquid structures of six choline carboxylate/amino acid ionic liquids (bio-ILs) and their mixtures with water and various n-alkanols have been investigated by small-angle X-ray scattering (SAXS). The ILs exhibit long-range amphiphilic nanostructure comprised of polar and apolar domains that can be controlled by choice of anion, and which is tolerant to water dilution. Mixtures with n-alkanols can lead to marked changes in domain size and ordering. Utilising the Teubner-Strey model, we find amphiphilicity factors in many of these mixtures are comparable to those observed in conventional microemulsions, and that cooperative assembly in bio-IL/alkanol mixtures can enhance amphiphilicity, with potential to improve performance in a range of applications.

The Double-Faced Nature of Hydrogen Bonding in Hydroxy-Functionalized Ionic Liquids Shown by Neutron Diffraction and Molecular Dynamics Simulations.

We characterize the double-faced nature of hydrogen bonding in hydroxy-functionalized ionic liquids by means of neutron diffraction with isotopic substitution (NDIS), molecular dynamics (MD) simulations, and quantum chemical calculations. NDIS data are fit using the empirical potential structure refinement technique (EPSR) to elucidate the nearest neighbor H⋅⋅⋅O and O⋅⋅⋅O pair distribution functions for hydrogen bonds between ions of opposite charge and the same charge. Despite the presence of repulsive Coulomb forces, the cation-cation interaction is stronger than the cation-anion interaction. We compare the hydrogen-bond geometries of both "doubly charged hydrogen bonds" with those reported for molecular liquids, such as water and alcohols. In combination, the NDIS measurements and MD simulations reveal the subtle balance between the two types of hydrogen bonds: The small transition enthalpy suggests that the elusive like-charge attraction is almost competitive with conventional ion-pair formation.

Aqueous Polymeric Hollow Particles as an Opacifier by Emulsion Polymerization Using Macro-RAFT Amphiphiles.

A robust polymerization technique that enables the surfactant-free aqueous synthesis of a high solid content latex containing polymeric hollow particles is presented. Uniquely designed amphiphilic macro-reversible addition fragmentation chain transfer (RAFT) copolymers were used as sole stabilizers for monomer emulsification as well as for free-radical emulsion polymerization. The polymerization was found to be under RAFT control, generating various morphologies from spherical particles, wormlike structures to polymer vesicles. The final particles were dominantly polymeric vesicles which had a substantially uniform and continuous polymer layer around a single aqueous filled void. They produced hollow particles once dried and were successfully used as opacifiers to impart opacity into polymer paint films. This method is simple, can be performed in a controllable and reproducible manner, and may be performed using diverse procedures.

Effect of Deep Eutectic Solvent Nanostructure on Phospholipid Bilayer Phases.

Phospholipids are shown by solvent penetration experiments to form lamellar phases and spontaneously spawn vesicles in a wide range of deep eutectic solvents (DESs) composed of alkylammonium halide salts and glycerol or ethylene glycol, which are shown to be nanostructured by X-ray scattering. In contrast with molecular solvents, the chain melting temperature of each phospholipid, which determines the stability of the swellable bilayer phase, depends on the structure of the cation, anion, and molecular H-bond donor that constitute the DES. Chain melting is most sensitive to the length of the alkyl chain of the cation, which is partitioned between apolar domains in the bulk, nanostructured DES and those within the lipid bilayer. This is moderated by the structures of the anion and the molecular hydrogen bond donor, which determine the extent of polar/apolar segregation in the bulk liquid.

Hydrophobic Monomer Type and Hydrophilic Monomer Ionization Modulate the Lyotropic Phase Stability of Diblock Co-oligomer Amphiphiles.

The phase behavior and self-assembly structures of a series of amphiphilic diblock co-oligomers comprising an ionizable hydrophilic block (5 to 10 units of acrylic acid) and a hydrophobic block (5 to 20 units of n-butyl acrylate, t-butyl acrylate, or ethyl acrylate), synthesized by RAFT polymerization, have been examined by polarizing optical microscopy and small-angle X-ray scattering (SAXS). Self-assembled structure and lyotropic phase stability in these systems is highly responsive to the degree of ionization of the acrylic acid hydrophilic block (i.e., pH), concentration, and nature of the hydrophobic block. Increasing headgroup ionization switched the amphiphiles from behaving like soluble to insoluble surfactants. Liquid isotropic (micellar), hexagonal, lamellar, and discrete cubic phases were found under different solution conditions. The surfactant packing parameter was adapted to understand the self-assembly structures in these diblock co-oligomers. The hydrophobic chain structure and length were shown to strongly affect the relative stabilities of these phases, allowing the self-assembled structure to be varied at will.

Effect of protic ionic liquid nanostructure on phospholipid vesicle formation.

The formation of bilayer-based lyotropic liquid crystals and vesicle dispersions by phospholipids in a range of protic ionic liquids has been investigated by polarizing optical microscopy using isothermal penetration scans, differential scanning calorimetry, and small angle X-ray and neutron scattering. The stability and structure of both lamellar phases and vesicle dispersions is found to depend primarily on the underlying amphiphilic nanostructure of the ionic liquid itself. This finding has significant implications for the use of ionic liquids in soft and biological materials and for biopreservation, and demonstrates how vesicle structure and properties can be controlled through selection of cation and anion. For a given ionic liquid, systematic trends in bilayer thickness, chain-melting temperature and enthalpy increase with phospholipid acyl chain length, paralleling behaviour in aqueous systems.

Study of (Cyclic Peptide)-Polymer Conjugate Assemblies by Small-Angle Neutron Scattering.

We present a fundamental study into the self-assembly of (cyclic peptide)-polymer conjugates as a versatile supramolecular motif to engineer nanotubes with defined structure and dimensions, as characterised in solution using small-angle neutron scattering (SANS). This work demonstrates the ability of the grafted polymer to stabilise and/or promote the formation of unaggregated nanotubes by the direct comparison to the unconjugated cyclic peptide precursor. This ideal case permitted a further study into the growth mechanism of self-assembling cyclic peptides, allowing an estimation of the cooperativity. Furthermore, we show the dependency of the nanostructure on the polymer and peptide chemical functionality in solvent mixtures that vary in the ability to compete with the intermolecular associations between cyclic peptides and ability to solvate the polymer shell.

Kamlet-Taft Solvation Parameters of Solvate Ionic Liquids.

The Kamlet-Taft solvent parameters of solvate ionic liquids (SILs) prepared from lithium salts with glyme and glycol ligands are determined. The dipolarity/polarisibilities (π*) are high, similar to those found in conventional ionic liquids. The H-bond basicities (ß) depend strongly on the anion. The H-bond acidities (α) are high in both glyme and glycol SILs, indicating that the lithium is acting as a H-bond donor site. "Poor" SILs have glyme-rich and salt-rich regions. In these liquids the π* and ß values are almost identical to the parent glyme or glycol, and the α values are determined by the salt alone.

Spontaneous vesicle formation in a deep eutectic solvent.

Solvent penetration experiments and small-angle X-ray scattering reveal that phospholipids dissolved in a deep eutectic solvent (DES) spontaneously self-assemble into vesicles above the lipid chain melting temperature. This means DESs are one of the few nonaqueous solvents that mediate amphiphile self-assembly, joining a select set of H-bonding molecular solvents and ionic liquids.

Dissolved chloride markedly changes the nanostructure of the protic ionic liquids propylammonium and ethanolammonium nitrate.

The bulk nanostructure of 15 mol% propylammonium chloride (PACl) dissolved in propylammonium nitrate (PAN) and 15 mol% ethanolammonium chloride (EtACl) in ethanolammonium nitrate (EtAN) has been determined using neutron diffraction with empirical potential structure refinement fits. For both the PAN:PACl and EtAN:EtACl mixtures, data for three different scattering contrasts were simultaneously fit, and the structures determined and compared to that of the pure ionic liquids. Strong electrostatic interactions between chloride and cation charged groups, as well as the alcohol moiety of EtAN, lead to marked changes in local ion packing that alter the liquid structure. In PAN, the addition of chloride modifies but does not significantly disrupt the bicontinuous amphiphilic nanostructure of the IL. Tight packing of ammonium groups around chloride favours a gauche conformer for the cation which shrinks the apolar domains and brings the terminal methyls nearer the polar domains. The weakly-clustered nanostructure of EtAN, a consequence of the terminal hydroxyl, is overwhelmed by strong chloride-cation interactions. Ethanolammonium binds tightly to chloride in a monodentate fashion via either its alcohol or ammonium charge centre, or through both in a bidentate arrangement by adopting a gauche or eclipsed conformer.

Structural effect of glyme-Li(+) salt solvate ionic liquids on the conformation of poly(ethylene oxide).

The conformation of 36 kDa polyethylene oxide (PEO) dissolved in three glyme-Li(+) solvate ionic liquids (SILs) has been investigated by small angle neutron scattering (SANS) and rheology as a function of concentration and compared to a previously studied SIL. The solvent quality of a SIL for PEO can be tuned by changing the glyme length and anion type. Thermogravimetric analysis (TGA) reveals that PEO is dissolved in the SILs through Li(+)-PEO coordinate bonds. All SILs (lithium triglyme bis(trifluoromethanesulfonyl)imide ([Li(G3)]TFSI), lithium tetraglyme bis(pentafluoroethanesulfonyl)imide ([Li(G4)]BETI), lithium tetraglyme perchlorate ([Li(G4)]ClO4) and the recently published [Li(G4)]TFSI) are found to be moderately good solvents for PEO but solvent quality decreases in the order [Li(G4)]TFSI ∼ [Li(G4)]BETI > [Li(G4)]ClO4 > [Li(G3)]TFSI due to decreased availability of Li(+) for PEO coordination. For the same glyme length, the solvent qualities of SILs with TFSI(-) and BETI(-) anions ([Li(G4)]TFSI and [Li(G4)]BETI) are very similar because they weakly coordinate with Li(+), which facilitates Li(+)-PEO interactions. [Li(G4)]ClO4 presents a poorer solvent environment for PEO than [Li(G4)]BETI because ClO4(-) binds more strongly to Li(+) and thereby hinders interactions with PEO. [Li(G3)]TFSI is the poorest PEO solvent of these SILs because G3 binds more strongly to Li(+) than G4. Rheological and radius of gyration (Rg) data as a function of PEO concentration show that the PEO overlap concentrations, c* and c**, are similar in the three SILs.

Ionic liquid nanostructure enables alcohol self assembly.

Weakly structured solutions are formed from mixtures of one or more amphiphiles and a polar solvent (usually water), and often contain additional organic components. They contain solvophobic aggregates or association structures with incomplete segregation of components, which leads to a poorly defined interfacial region and significant contact between the solvent and aggregated hydrocarbon groups. The length scales, polydispersity, complexity and ill-defined structures in weakly structured solutions makes them difficult to probe experimentally, and obscures understanding of their formation and stability. In this work we probe the nanostructure of homogenous binary mixtures of the ionic liquid (IL) propylammonium nitrate (PAN) and octanol as a function of composition using neutron diffraction and atomistic empirical potential structure refinement (EPSR) fits. These experiments reveal why octanol forms weakly structured aggregates in PAN but not in water, the mechanism by which PAN stabilises the octanol assemblies, and how the aggregate morphologies evolve with octanol concentration. This new understanding provides insight into the general stabilisation mechanisms and structural features of weakly structured mixtures, and reveals new pathways for identifying molecular or ionic liquids that are likely to facilitate aggregation of non-traditional amphiphiles.