Gregory G. Warr

The Smallest Amphiphiles : Nanostructure in Protic Room-Temperature Ionic Liquids with Short Alkyl Groups

Room-temperature ionic liquids (ILs) are low-melting-point organic salts that, until recently, were thought to have homogeneous microstructure. In this work, we investigate nanoscale segregation of short (<C(4)) alkyl chain ILs using propylammonium nitrate (PAN) and ethylammonium nitrate (EAN). Structure peaks at q = 0.54 A(-1) for PAN and at q = 0.66 A(-1) for EAN, corresponding to Bragg spacings (D* = 2pi/q(max)) of 11.6 and 9.7 A respectively, provide the first experimental evidence of nanoscale heterogeneity for ILs with alkyl chains less than C(4). The observation that these ILs are not optically birefringent and the fits obtained suggest a disordered, locally smectic or sponge-like structure. Solvophobic interaction between alkyl groups is the most important factor for the production of nanoscale heterogeneities, but electrostatic and hydrogen bonding attractions between the amine nitrogen and the nitrate anion will also play a role.

The surface of neat water is basic

Theoretical studies which conclude that the surface of neat water is acidic (with a pH < or = 4.8), due to the preferential adsorption of hydronium ions, are contrary to the available experimental evidence. Air bubbles in water have a negative charge, as do hydrophobic oil drops in water, and streaming potential measurements on inert surfaces such as Teflon in water show a similar negative surface charge. In each case the pH dependence of the zeta potential has an isoelectric point between pH 2-4. An isoelectric point of pH 4 implies a preference for hydroxide over protons of 10(6), the opposite of what was inferred from the theoretical simulations. Water behaves similarly at all inert hydrophobic interfaces with the preferential adsorption of hydroxide ions to give a negatively charged surface at neutral pH. The surface-charge density at the oil/water interface in mM salt solutions is -5 to -7 microC cm(-2), which corresponds to one hydroxide ion on every 3 nm2 of the surface. The homogenisation of an inert oil such as hexadecane in water in the absence of any salt or base still leads to formation of an emulsion. The hydroxide adsorbed on the large surface area of the emulsion greatly exceeds that present at 10(-7) M in neutral water; it is created by the increased autolysis of water, driven by the strong adsorption of hydroxide ions at the oil/water interface. These surfactant-free, salt-free emulsions are stable for some hours, with protons as the only counterions to the negative hydroxide surface.

The Nature of Hydrogen Bonding in Protic Ionic Liquids

The size, direction, strength, and distribution of hydrogen bonds in several protic ionic liquids (PILs) has been elucidated using neutron diffraction and computer simulation. There is significant variation in PIL hydrogen bond interactions ranging from short and linear to long and bi-/trifurcated. The nature of the PILs hydrogen bonds reflects its macroscopic properties.

How Water Dissolves in Protic Ionic Liquids

In recent years, ionic liquids (ILs) have emerged as useful chemical solvents for an enormous number of processes and technologies. 2] Their ions have more complex chemical structures than inorganic salts; by incorporating large, sterically mismatched anions and cations, ILs melt at low temperatures because, compared to typical inorganic salts, Coulombic attractions are weakened and lattice-packing arrangements frustrated. ILs are regarded as “designer solvents”, as molecular control over liquid properties is possible depending on how the ions are functionalized. Hydrogen bonding can play a key role in IL chemistry. Whereas most inorganic salts cannot form hydrogen bonds and are dominated by electrostatic interactions between ions, many ILs have extensive H-bonding capacity. For example, H-bond donor and acceptor sites are created during synthesis of protic ionic liquids (PILs). This enables some PILs to develop dense Hbond networks and thus mirrors a number of remarkable structural and solvent properties of water. Finally, ILs have the capacity to self-assemble, forming well-defined nanostructures in the bulk phase as well as at interfaces. IL nanostructure arises because at least one of the ions (frequently the cation) is amphiphilic, with distinct charged and uncharged moieties. This drives segregation of ionic and nonionic groups in ILs, reminiscent of self-assembly in aqueous surfactant mesophases. 12] Here we elucidate the bulk solvent structure of mixtures of a PIL, ethylammonium nitrate (EAN), and water (Figure 1). EAN is one of the oldest known and most extensively studied PILs. As EAN is completely miscible with water, this raises questions such as: how do EAN and water mix? Are the forces that lead to self-assembly in pure EAN sufficient to maintain a solvophobic nanostructure? What is the nature of ion solvation in such mixtures? If nanostructure persists in aqueous mixtures and key solvent properties are retained, this will increase PIL utility by offering an additional mechanism for tuning liquid behavior and lowering the overall cost of the solvent medium. While primitive (continuum solvent) models of dilute aqueous electrolyte solutions are generally successful, understanding ion–water interactions and concentrated solutions has proved challenging, and is complicated in part by the absence of a satisfactory model for liquid water. 19] The structure in aqueous electrolyte solutions is understood in terms of Hofmeister and hydrophobic effects, which can only be probed using sophisticated experimental and computational techniques. Solvated ions induce a different local structure of water molecules in the first, and even the second or third solvation shells, to accommodate the dissolved species. This leads to ions being classified as either “structure making” or “structure breaking” through the creation of “solute cavities”. Recent, growing interest in IL/water mixtures has been motivated, at least in part, by the desire to understand the dramatic changes in IL solvent properties observed upon water contamination. Water is probably the most common impurity in ILs; even nominally hydrophobic ILs absorb significant quantities of water when exposed to the atmosphere. Many computational models have been developed that examined changes in IL solvent structure by dissolved water, often over the full concentration range. At low water concentration, the models predict that the IL nanostructure is relatively unperturbed, but at high water content the system resembles aqueous solutions of ionic surfactants. However, these studies have overwhelmingly investigated aprotic ILs; largely absent are corresponding studies of PILs and experimental verification of the findings. Only one paper has directly investigated the structure of PIL/water mixtures. Smalland wide-angle X-ray scattering (SWAXS) was used to investigate the effect of water on a range of PILs. For neat PILs like EAN, the SWAXS spectra were consistent with nanoscale structure, and were essentially invariant with increasing water content. A micelle-like model was proposed for the solution structure, with water located in the bulk polar domains and associated with the charge groups on the ions. Figure 2 shows the neutron diffraction data and EPSR fits to the three EAN/water mixtures in a molar ratio of 1:6 for different neutron contrasts ([D3]EAN + D2O, [D8]EAN + Figure 1. Molecular structure and atom types of the ethylammonium (EA) cation, nitrate (NO3 ) anion, and water molecule. Different C, N, O, and H atoms are distinguished using subscripts.

Influence of Temperature and Molecular Structure on Ionic Liquid Solvation Layers

Atomic force microscopy (AFM) force profiling is used to investigate the structure of adsorbed and solvation layers formed on a mica surface by various room temperature ionic liquids (ILs) ethylammonium nitrate (EAN), ethanolammonium nitrate (EtAN), ethylammonium formate (EAF), propylammonium formate (PAF), ethylmethylammonium formate (EMAF), and dimethylethylammonium formate (DMEAF). At least seven layers are observed for EAN at 14 degrees C (melting point 13 degrees C), decreasing as the temperature is increased to 30 degrees C due to thermal energy disrupting solvophobic forces that lead to segregation of cation alkyl tails from the charged ammonium and nitrate moieties. The number and properties of the solvation layers can also be controlled by introducing an alcohol moiety to the cations alkyl tail (EtAN), or by replacing the nitrate anion with formate (EAF and PAF), even leading to the detection of distinct cation and anion sublayers. Substitution of primary by secondary or tertiary ammonium cations reduces the number of solvation layers formed, and also weakens the cation layer adsorbed onto mica. The observed solvation and adsorbed layer structures are discussed in terms of the intermolecular cohesive forces within the ILs.

Propylammonium Nitrate as a Solvent for Amphiphile Self-Assembly into Micelles, Lyotropic Liquid Crystals, and Microemulsions

The phase behavior and self-assembled microstructures of a range of oligo(oxyethylene)-n-alkyl ether (C(i)E(j)) surfactants has been investigated in propylammonium nitrate (PAN), a room temperature ionic liquid. Micelles and single-phase microemulsions were all found to form at alkyl chain lengths from dodecyl to octadecyl, and lyotropic liquid crystals formed with hexadecyl chains or longer. Small-angle neutron scattering (SANS) shows that self-assembly occurs by solvophobic interactions driving the aggregation of the alkyl chains, but several results indicate that these are weaker in PAN than in water or ethylammonium nitrate, due chiefly to the hydrophobicity of PAN. Longer alkyl chains are needed for lyotropic liquid crystals to form, and higher surfactant concentrations are needed to form a single phase microemulsion. Conductivity shows these microemulsions to be weakly structured, and relatively insensitive to oil or surfactant molecular structure, unlike water-based systems. However, SANS contrast variation reveals a nanosegregation of oil from the alkyl tails of surfactants within the microemulsion, and may suggest a cosurfactant-like role for the propylammonium cation. Molecular areas within microemulsions and lamellar phases are larger than corresponding water- or ethylammonium nitrate-based systems due to the large molecular volume of the solvating PANs.

Structure of Nonionic Surfactant Micelles in the Ionic Liquid Ethylammonium Nitrate

The structure of micelles formed by nonionic polyoxyethylene alkyl ether nonionic surfactants, C n E m , in the room-temperature ionic liquid, ethylammonium nitrate (EAN), has been determined by small-angle neutron scattering (SANS) as a function of alkyl and ethoxy chain length, concentration, and temperature. Micelles are found to form for all alkyl chains from dodecyl through to octadecyl. Dodecyl-chained surfactants have high critical micelle concentrations, around 1 wt%, and form weakly structured micelles. Surfactants with longer alkyl chains readily form micelles in EAN. The observed micelle structure changes systematically with alkyl and ethoxy chain length, in parallel with observations in aqueous solutions. Decreasing ethoxy chain length at constant alkyl chain length leads to a sphere to rod transition. These micelles also grow into rods with increasing temperature as their cloud point is approached in EAN.

Structure and self assembly of pluronic amphiphiles in ethylammonium nitrate and at the silica surface.

The self-assembled structures formed by three Pluronic surfactants (P65, L81, L121) in ethylammonium nitrate (EAN, a protic room temperature ionic liquid) in bulk solution and at the silica-EAN interface have been investigated using polarizing optical microscopy, small-angle neutron scattering (SANS), and atomic force microscopy (AFM) to assess how surface active Pluronics are in ionic liquids. Polarizing microscopy revealed optical textures for P65 only, allowing a detailed phase diagram to be determined with features similar to those determined for Pluronics in water and formamide. Small-angle neutron scattering experiments were conducted at 1 and 10 wt % Pluronic at 25 and 63 degrees C to ascertain whether critical micelle temperatures existed in EAN. SANS experiments using 1 wt % solutions at room temperature reveal that the three Pluronics adopt a random flight chain conformation. For 10 wt % at room temperature, P65 and L81 are dissolved as random coils, but L121 forms lamellar vesicles. When the temperature is increased, the solubility of the Pluronics in EAN decreases, mostly on account of the PPO block. At 63 degrees C, P65 forms micelles, 1 wt % L81 forms lamellar stacks, 10 wt % L81 forms unilamellar vesicles, and L121 forms bilayer stacks at both concentrations. AFM images of the P65-silica-EAN at room temperature revealed an amorphous layer of surface aggregates for concentrations above 3 wt %. To our knowledge, this is the first report of aggregates adsorbed to a charged surface in an ionic liquid. For L81 and L121 concentrations between 1 and 10 wt %, AFM images do not reveal structure, but the force profiles recorded are consistent with an adsorbed brush layer. The approach force profiles for the three Pluronics have been modeled using the Alexander de Gennes and Milner-Witten-Cates theories, with the Alexander de Gennes theory generally providing better fits to the data. The L81 retraction force data has been modeled using the wormlike chain theory. The fitted Kuhn lengths are in accordance with those determined for aqueous good solvent polymer systems, but the contour lengths are too long to be due to a single L81 chain, suggesting that L81 aggregates upon confinement between the AFM tip and the surface.

Nanostructure of the ionic liquid-graphite stern layer

Ionic liquids (ILs) are attractive solvents for devices such as lithium ion batteries and capacitors, but their uptake is limited, partially because their Stern layer nanostructure is poorly understood compared to molecular solvents. Here, in situ amplitude-modulated atomic force microscopy has been used to reveal the Stern layer nanostructure of the 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIm TFSI)-HOPG (highly ordered pyrolytic graphite) interface with molecular resolution. The effect of applied surface potential and added 0.1 wt/wt % Li TFSI or EMIm Cl on ion arrangements is probed between ±1 V. For pure EMIm TFSI at open-circuit potential, well-defined rows are present on the surface formed by an anion-cation-cation-anion (A-C-C-A) unit cell adsorbed with like ions adjacent. As the surface potential is changed, the relative concentrations of cations and anions in the Stern layer respond, and markedly different lateral ion arrangements ensue. The changes in Stern layer structure at positive and negative potentials are not symmetrical due to the different surface affinities and packing constraints of cations and anions. For potentials outside ±0.4 V, images are featureless because the compositional variation within the layer is too small for the AFM tip to detect. This suggests that the Stern layer is highly enriched in either cations or anions (depending on the potential) oriented upright to the surface plane. When Li(+) or Cl(-) is present, some Stern layer ionic liquid cations or anions (respectively) are displaced, producing starkly different structures. The Stern layer structures elucidated here significantly enhance our understanding of the ionic liquid electrical double layer.

Optimized Steric Stabilization of Aqueous Ferrofluids and Magnetic Nanoparticles

The preparation and properties of an aqueous ferrofluid consisting of a concentrated (>65 wt %) dispersion of sterically stabilized superparamagnetic, iron oxide (maghemite) nanoparticles stable for several months at high ionic strength and over a broad pH range is described. The 6-8 nm diameter nanoparticles are individually coated with a short poly(acrylic acid)-b-poly(acrylamide) copolymer, designed to form the thinnest possible steric stabilizing layer while remaining strongly attached to the iron oxide surface over a wide range of nanoparticle concentrations. Thermogravimetric analysis yields an iron oxide content of 76 wt % in the dried particles, consistent with a dry polymer coating of approximately 1 nm in thickness, while the poly(acrylamide) chain length indicated by electrospray mass spectrometry is consistent with the 4-5 nm increase in the hydrodynamic radius observed by light scattering when the poly(acrylamide) stabilizing chains are solvated. Saturation magnetization experiments indicate nonmagnetic surface layers resulting from the strong chemical attachment of the poly(acrylic acid) block to the particle surface, also observed by Fourier transform infrared spectroscopy.