Deborah Wakeham

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.

Structure of the Ethylammonium Nitrate Surface: An X-ray Reflectivity and Vibrational Sum Frequency Spectroscopy Study

X-ray reflectivity and vibrational sum frequency spectroscopy are used to probe the structure of the ethylammonium nitrate (EAN)-air interface. X-ray reflectivity reveals that the EAN-air interface is structured and consists of alternating nonpolar and charged layers that extend 31 A into the bulk. Vibrational sum frequency spectroscopy reveals interfacial cations have their ethyl moieties oriented toward air, with the CH(3) C(3) axis positioned approximately 36.5 degrees from interface normal. This structure is invariant between 15 and 51 degrees C. On account of its molecular symmetry, the orientation of the nitrate anion cannot be determined with certainty.

Probing the protic ionic liquid surface using X-ray reflectivity

The structure of the free liquid surface of three protic ionic liquids, ethylammonium nitrate (EAN), propylammonium nitrate (PAN), and ethylammonium formate (EAF), has been elucidated using X-ray reflectivity. The results show all three liquids have an extended interfacial region, spanning at least five ion pairs, which can be divided into two parts. Adjacent to the gas phase are aggregates consisting of multiple cations and anions. Below this are layers oriented parallel to the macroscopic surface that are alternately enriched and depleted in cation alkyl chains and polar domains of cation ammonium groups and their anions, gradually decaying to the isotropic sponge-like bulk structure. The most pronounced layering is observed for PAN, driven by strong solvophobic interactions, while reduced hydrogen bonding in EAF results in the least structured and least extensive interfacial region.

Nonionic surfactant adsorption at the ethylammonium nitrate surface: a neutron reflectivity and vibrational sum frequency spectroscopy study

The adsorbed layers of polyoxyethylene n-alkyl ether surfactants C(12)E(4), C(14)E(4), and C(16)E(4) at the EAN surface have a headgroup layer that is thin and compact (only approximately 30 vol % EAN). The headgroups do not adopt a preferred orientation and are disordered within the ethylene oxide layer. Alkyl tails contain a significant number of gauche defects indicating a high degree of conformational disorder. The thickness of the tail layer increases with increasing alkyl chain length, while the headgroup layer shows little change. Lowering the C(12)E(4) concentration from 1 to 0.1 wt % decreases the adsorbed amount, and the headgroup layer becomes thinner and less solvated, whereas C(14)E(4) and C(16)E(4) adsorbed layers are unaffected by dilution over the same concentration range. The C(16)E(4) layer thickness increases and area per molecule decreases on warming to 60 degrees C, but the adsorbed layer structures of C(12)E(4) and C(14)E(4) are unchanged. Both effects are attributed to surfactant solubility.

Surfactant adsorption at the surface of mixed ionic liquids and ionic liquid water mixtures

Surface tensiometry and neutron reflectivity have been used to elucidate the structure of the adsorbed layer of nonionic surfactant tetraethylene glycol tetradecyl ether (C(14)E(4)) at the free surface of the ionic liquids ethylammonium nitrate (EAN) and ethanolammonium nitrate (EtAN) and their binary mixtures with each other and with water. Surface tensions reveal that the critical micelle concentration (cmc) depends strongly on solvent composition. The adsorbed surfactant structure elucidated by neutron reflectivity shows that the level of solvation of the ethylene oxide groups varies for both the pure and mixed solvents. This is attributed to solvent-solvent interactions dominating solvent-surfactant interactions.

Surface structure of a “non-amphiphilic” protic ionic liquid

The nanostructure of the ethanolammonium nitrate (EtAN)-air surface has been investigated using X-ray reflectometry (XRR), vibrational sum frequency spectroscopy (VSFS) and neutral impact collision ion scattering spectroscopy (NICISS). The XRR data decays more rapidly than expected for a perfectly sharp interface, indicating a diffuse electron (scattering length) density profile. Modelling of the XRR data using three different fitting routines produced consistent interfacial profiles that suggest the formation of interfacial EtAN clusters. Consistent with this, VSFS reveals that the EtAN surface is predominantly covered by -CH(2)- moieties, with the -NH(3)(+) and -OH groups of the cation buried slightly deeper in the interface. The elemental profiles determined using NICISS also show enrichment of carbon relative to nitrogen and oxygen in the outermost surface layer, which is consistent with the surface cation orientation deduced from VSFS, and with the presence of EtAN aggregates at the liquid surface.

Surface Composition of Mixtures of Ethylammonium Nitrate, Ethanolammonium Nitrate, and Water

Surface tensiometry of binary mixtures of ethylammonium nitrate (EAN), ethanolammonium nitrate (EtAN), and water reveals distinctive amphiphilic character for the ethylammonium cation, but not for ethanolammonium. Results also show that the surface film incorporates nitrate counterions, and that electrostatic and H-bonding interactions, rather than alkyl chain packing, determines the saturated adsorbed film structure and limiting molecular area.

In-situ study of substrate – catalyst interactions in a Juliá–Colonna epoxidation using quartz crystal microbalance with dissipation

Quartz crystal microbalance with dissipation (QCM-D) analysis of the hexa-l-Leucine (PLL)-catalyzed epoxidation of chalcone gives in-situ experimental evidences which demonstrate that the reaction proceeds mainly via the formation of a PLL-bound hydroperoxide complex followed by the reversible addition of chalcone. The observations offer an alternative rationalization for the viability of the preferred catalytic pathway.

Mixed monolayers of alkane thiols with polar terminal group on gold: Investigation of structure dependent surface properties

Adsorption of thiols with cationic or anionic terminal group on gold has been studied from mixed solutions of 11-Amino-1-undecanethiol (AUT) and 3-Mercaptopropionic acid (MPA) using Quartz Crystal Microbalance with Dissipation (QCM-D), X-ray Photoelectron Spectroscopy (XPS), atomic force microscopy (AFM) and contact angles. The goal is to probe the nature of such layers, and the additivity or otherwise of the pH responsiveness, with a view to evaluate their suitability as smart materials. For each of the two pure (unmixed) cases, ordered molecular monolayers are formed with sulfur binding to gold and the alkane chain pointing out from the surface as expected. Adsorption from the thiol mixtures, however, leads to a more complex behaviour. The surface concentration of thiols from the mixtures, as determined by QCM-D, is considerably lower than for the pure cases and it reaches a minimum at a 3:1 MPA/AUT relative concentration in the solution. The XPS results confirm a reduction in adsorbed amount in mixtures with the lowest overall intensity for the 3:1 ratio. Monolayers formed from mixtures display a wettability which is much lower and less pH sensitive. Collectively these results confirm that for adsorption from mixed systems, the configuration is completely different. Complex formation in the mixed solutions leads to the adsorption of molecules parallel to the surface in an axially in-plane configuration. This parallel layer of thiols is mechanically relatively robust to nano-shaving based on AFM measurements. These results will have a significant impact on the design of biomimetic surface coatings particularly when mixtures of oppositely charged molecules are present on the surface, as is commonly the case in biological, proteinaceous surfaces (e.g. hair and skin).

Se–C Cleavage of Hexane Selenol at Steps on Au(111)

Selenols are considered as an alternative to thiols in self-assembled monolayers, but the Se-C bond is one limiting factor for their usefulness. In this study, we address the stability of the Se-C bond by a combined experimental and theoretical investigation of gas-phase-deposited hexane selenol (CH3(CH2)5SeH) on Au(111) using photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory (DFT). Experimentally, we find that initial adsorption leaves atomic Se on the surface without any carbon left on the surface, whereas further adsorption generates a saturated selenolate layer. The Se 3d component from atomic Se appears at 0.85 eV lower binding energy than the selenolate-related component. DFT calculations show that the most stable structure of selenols on Au(111) is in the form of RSe-Au-SeR complexes adsorbed on the unreconstructed Au(111) surface. This is similar to thiols on Au(111). Calculated Se 3d core-level shifts between elemental Se and selenolate in this structure nicely reproduce the experimentally recorded shifts. Dissociation of RSeH and subsequent formation of RH are found to proceed with high barriers on defect-free Au(111) terraces, with the highest barrier for scissoring R-Se. However, at steps, these barriers are considerably lower, allowing for Se-C bond breaking and hexane desorption, leaving elemental Se at the surface. Hexane is formed by replacing the Se-C bond with a H-C bond by using the hydrogen liberated from the selenol to selenolate transformation.