Amphiphilically Nanostructured Deep Eutectic Solvents.

Deep eutectic solvents (DESs) are neoteric liquids produced by mixing a high-melting-point salt and a molecular hydrogen-bond donor. Amphiphilic (self-assembled) liquid nanostructure, which is key for many of the useful properties of the related ionic liquid class, has not previously been experimentally demonstrated in DESs. Here we show how amphiphilically nanostructured DESs can be prepared using primary ammonium cations. The bulk structure of alkylammonium bromide (alkyl = ethyl-, propyl-, and butyl) and glycerol DESs at a 1:2 mol ratio is examined using neutron diffraction and empirical potential structure refinement fitting. Analysis reveals cation alkyl chain association, which is the signature of amphiphilic liquid nanostructure, in all systems, which becomes better defined with increasing chain length. The ability to form amphiphilically nanostructured DESs will enable the translation of ionic liquid properties associated with liquid nanostructure to DESs.

Small angle neutron scattering study of the conformation of poly(ethylene oxide) dissolved in deep eutectic solvents.

HYPOTHESIS: The conformation of poly(ethylene oxide) (PEO) in deep eutectic solvents (DESs) is determined by the polymer-solvent interactions, especially hydrogen bonding interactions. The hypothesis for this work is that the hydrogen bonding environment of a DES can be varied via changing the cation or hydrogen bond donor (HBD), and therefore the solvent quality for PEO; the anion species will also effect hydrogen bonding, but this is not examined here. EXPERIMENTS: Small angle neutron scattering (SANS) is used to probe the concentration dependent conformation of 36kDa PEO dissolved in DESs formed by mixing ethyl or butyl ammonium bromide with a molecular HBD (glycerol or ethylene glycol) in a 1:2 molar ratio. FINDINGS: The radius of gyration (Rg), Flory exponent and crossover concentration (c*) from the dilute to the semi-dilute regime of PEO in the DESs revealed by SANS and Zimm plot analysis show that these DESs are moderately good solvents for PEO. When the ammonium alkyl chain length is increased, the hydrogen bond density per unit volume decreases, and with it the solvent quality for PEO. The solvent quality is improved when the HBD is changed from glycerol to ethylene glycol due to differences in the hydrogen bonding environment for PEO.

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.

Metal ion adsorption at the ionic liquid-mica interface.

Mica has been employed in many studies of ionic liquid (IL) interfaces on account of its atomic smoothness and well defined surface properties. However, until now it has been unclear whether ions dissolved in ILs can compete with the IL cation and adsorb to mica charge sites. In this work amplitude modulated atomic force microscopy (AM-AFM) has been used to probe metal ion adsorption at the interface of mica with propylammonium nitrate (PAN), a room temperature IL. Lithium, sodium, potassium, magnesium and calcium nitrate salts were added to PAN at a concentration of ∼60 mM. Aluminum nitrate was also investigated, but only at 5 mM because its solubility in PAN is much lower. The AM-AFM images obtained when the metal ions were present are strikingly different to that of pure PAN, indicating that the ions compete effectively with the propylammonium cation and adsorb to negatively charged sites on the mica surface despite their much lower concentration. This is a consequence of electrostatic attractions between the mica charge sites and the metal ions being significantly stronger than for the propylammonium cation; compared to the metal ions the propylammonium charged group is relatively constrained sterically. A distinct honeycomb pattern is noted for the PAN + Al(3+) system, less obviously for the divalent ions and not at all for monovalent ions. This difference is attributed to the strength of electrostatic interactions between metal ions and mica charge sites increasing with the ion charge, which means that divalent and (particularly) trivalent ions are located more precisely above the charged sites of the mica lattice. The images obtained allow important distinctions between metal ion adsorption at mica-water and mica-PAN interfaces to be made.

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.

Interfacial and bulk nanostructure of liquid polymer nanocomposites.

Liquid polymer nanocomposites (l-PNCs) have been prepared using silica nanoparticles with diameters of 15 nm (l-PNC-15) and 24 nm (l-PNC-24), and Jeffamine M-2070, an amine-terminated ethylene oxide/propylene oxide (PEO/PPO, ratio 31/10) copolymer. Jeffamine M-2070 was used as the host liquid in which the particles were suspended and was also grafted onto the particle surface to prevent aggregation. The grafting density of Jeffamine M-2070 on the particle surfaces was ∼0.75 chains nm(-2). When the total polymer content (surface layer + host) was greater than ∼30 wt %, the PNC was a liquid, while at lower polymer volume fractions the PNC was solid. In this work, the bulk and surface structures of l-PNCs with ∼70 wt % polymer and 30% silica are characterized and compared. Small-angle neutron scattering (SANS) was used to probe the bulk structure of the l-PNCs and revealed that the particles are well-dispersed with minor clustering in l-PNC-15 and substantial clustering in l-PNC-24. This is attributed to stronger van der Waals attractions between particles due to the larger particle size in l-PNC-24. Corresponding effects were revealed using tapping mode atomic force microscopy (TM-AFM) at the l-PNC-air interface; clustering was minimal on the surface of l-PNC-15 but significant for l-PNC-24 droplets. In regions of the l-PNC where the particles were well-dispersed, the spacing between particles is consistent with their volume fractions. This is the first time that the distribution of polymer and particles within l-PNCs has been imaged in situ.