Evaluation of Structural and Compositional Changes of a Model Monoaromatic Hydrocarbon in a Benchtop Hydrocracker Using GC, FTIR, and NMR Spectroscopy.

Hydrogenation is a catalytic process that has the potential to facilitate sustainable chemical production. In this work, a model monoaromatic hydrocarbon, phenyldodecane (PDD), comprising an aromatic ring with a long aliphatic side chain has been chosen as representative of a typical species involved in hydrogenation and hydrocracked at a high pressure and temperature over a platinum catalyst in a bespoke benchtop mini-reactor. Gas chromatography-mass spectrometry (GC-MS), Fourier transform infrared (FTIR) spectroscopy, UV-vis spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy were employed to analyze the changes that took place after hydrocracking for different time periods. By combining the results from these sensitive spectroscopic tools, it was found that along with the saturation of the aromatic ring of PDD by hydrogen addition, new molecules were formed via ring opening and catalytic cracking. For comparison purposes, the spectra of the samples post hydrogenation were compared with those of cyclohexylnonadecane (CHND), which has a saturated six-membered ring and a long aliphatic tail.

Probing the Nanoscale Heterogeneous Mixing in a High-Performance Polymer Blend.

The blend of polyetheretherketone (PEEK) and polybenzimidazole (PBI) produces a high-performance blend (PPB) that is a potential replacement material in several industries due to its high temperature stability and desirable tribological properties. Understanding the nanoscale structure and interface of the two domains of the blend is critical for elucidating the origin of these desirable properties. Whilst achieving the physical characterisation of the domain structures is relatively uncomplicated, the elucidation of structures at the interface presents a significant experimental challenge. In this work, we combine atomic force microscopy (AFM) with an IR laser (AFM-IR) and thermal cantilever probes (nanoTA) to gain insights into the chemical heterogeneity and extent of mixing within the blend structure for the first time. The AFM-IR and nanoTA measurements show that domains in the blend are compositionally different from those of the pure PEEK and PBI polymers, with significant variations observed in a transition region several microns wide in proximity to domain boundary. This strongly points to physical mixing of the two components on a molecular scale at the interface. The versatility intrinsic to the combined methodology employed in this work provides nano- and microscale chemical information that can be used to understand the link between properties of different length scales across a wide range of materials.

A quantitative determination of lipid bilayer deposition efficiency using AFM.

The efficacy of a number of different methods for depositing a dimyristoylphosphatidylcholine (DMPC) lipid bilayer or DMPC-cholesterol (3 : 1) mixed bilayer onto a silicon substrate has been investigated in a quantitative manner using atomic force microscopy (AFM) image analysis to extract surface coverage. Complementary AFM-IR measurements were used to confirm the presence of the lipids. For the Langmuir-Blodgett/Schaefer deposition method at temperatures below the chain-melting transition temperature (T m), a large number of bilayer defects resulted when DMPC was deposited from a water subphase. Addition of calcium ions to the trough led to smaller, more frequent defects, whereas addition of cholesterol to the lipid mixture led to a vast improvement in bilayer coverage. Poor coverage was achieved for deposition at temperatures above T m. Formation of the deposited bilayer from vesicle fusion proved a more reliable method for all systems, with formation of near-complete bilayers within 60 seconds at temperatures above T m, although this method led to a higher probability of multilayer formation and rougher bilayer surfaces.

Adsorption of 4-n-Nonylphenol, Carvacrol, and Ethanol onto Iron Oxide from Nonaqueous Hydrocarbon Solvents.

The adsorption of 4-n-nonylphenol (4NP), carvacrol, and ethanol onto the surface of iron oxide from nonaqueous solutions is presented. It is found that adsorption of 4NP from alkanes is strong and proceeds to monolayer formation, where the molecules are essentially "upright". However, at high relative concentrations, ethanol successfully out-competes 4NP for the iron oxide surface. Estimates of the enthalpy and entropy of binding of 4NP were found to be exothermic and entropically disfavored. Sum frequency generation vibrational spectroscopy data indicate some evidence of binding through a phenolate anion, despite the nonpolar, nonaqueous solvent. Carvacrol is also found to adsorb as a monolayer where the molecules are lying "flat". The adsorption of ethanol onto iron oxide from dodecane was investigated through the use of quantitative NMR, which is a convenient analytical technique for measuring adsorption isotherms. It was concluded that ethanol does not form adsorbed monolayers on the surface. Instead, it partitions onto the surface as a surface-enhanced local phase separation related to its poor solubility in alkane solvents.

The Effect of Water on Quinone Redox Mediators in Nonaqueous Li-O2 Batteries.

The parasitic reactions associated with reduced oxygen species and the difficulty in achieving the high theoretical capacity have been major issues plaguing development of practical nonaqueous Li-O2 batteries. We hereby address the above issues by exploring the synergistic effect of 2,5-di-tert-butyl-1,4-benzoquinone and H2O on the oxygen chemistry in a nonaqueous Li-O2 battery. Water stabilizes the quinone monoanion and dianion, shifting the reduction potentials of the quinone and monoanion to more positive values (vs Li/Li+). When water and the quinone are used together in a (largely) nonaqueous Li-O2 battery, the cell discharge operates via a two-electron oxygen reduction reaction to form Li2O2, with the battery discharge voltage, rate, and capacity all being considerably increased and fewer side reactions being detected. Li2O2 crystals can grow up to 30 µm, more than an order of magnitude larger than cases with the quinone alone or without any additives, suggesting that water is essential to promoting a solution dominated process with the quinone on discharging. The catalytic reduction of O2 by the quinone monoanion is predominantly responsible for the attractive features mentioned above. Water stabilizes the quinone monoanion via hydrogen-bond formation and by coordination of the Li+ ions, and it also helps increase the solvation, concentration, lifetime, and diffusion length of reduced oxygen species that dictate the discharge voltage, rate, and capacity of the battery. When a redox mediator is also used to aid the charging process, a high-power, high energy density, rechargeable Li-O2 battery is obtained.

The structure of lipid bilayers adsorbed on activated carboxy-terminated monolayers investigated by sum frequency generation spectroscopy.

The formation and structure of isotopically asymmetric supported bilayer membranes (SBMs) has been investigated using sum frequency generation (SFG) vibrational spectroscopy supplemented by reflection absorption infrared spectroscopy (RAIRS). The bilayers were composed of a proximal and distal leaflet of the phospholipid dipalmitoyl phosphatidylethanolamine (DPPE) supported on a gold surface. The proximal leaflet was chemically tethered to the gold via an 11-mercapto-undecanoic acid (MUA) self-assembled monolayer (SAM) that had been chemically modified to produce an activated succinimidyl ester headgroup using N-hydroxysuccinimide (NHS) and N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide (EDC). The activation of the MUA and the tethering of the DPPE were monitored and confirmed using SFG and RAIRS. The distal leaflet of the bilayer was added using either vesicle fusion (VF) or Langmuir-Blodgett (LB) deposition. To gain insight into the structure of each layer of the SBM perdeuterated DPPE (d-DPPE) and MUA (d-MUA) were used to create SBMs with a layer that was isotopically distinguishable from the rest. The polar orientation and conformational ordering of the lipids was determined using SFG. It was found that the tethering of the proximal lipid leaflet resulted in an increase in the conformational order of the MUA SAM. Furthermore, by careful analysis and comparison of spectra recorded in both the C-H (2800-3000 cm(-1)) and C-D (2000-2300 cm(-1)) stretching regions it was concluded that a better ordered and more biologically relevant lipid bilayer was formed when the distal leaflet was added using LB deposition. On the other hand the SFG spectra of the SBMs in which the distal leaflet was added by VF showed little evidence of conformational ordering on the time scale of minutes, suggesting the presence of an incomplete monolayer or of multilayer formation.

Sum frequency generation vibrational spectroscopy of cholesterol in hybrid bilayer membranes.

The assignment of the vibrational spectrum of cholesterol is surprisingly incomplete for such a fundamental molecule. To improve our understanding, a new investigation of the spectra of cholesterol in the C-H stretching region has been undertaken using the surface specific technique of Sum Frequency Generation (SFG) vibrational spectroscopy and the complementary technique of Reflection Absorption Infrared Spectroscopy (RAIRS). They were used to record the spectra of monolayers of cholesterol in hybrid bilayer membranes (HBMs). In addition to cholesterol, spectra were recorded of HBMs comprising monolayers of the partially deuterated cholesterol isotopologues d6-cholesterol and d7-cholesterol, and the cholesterol analogues cholestanol and androstanol to aid assignment of the spectra. Monolayers of each of the five molecules were used to form the distal leaflet of HBMs with the proximal leaflet consisting of a monolayer of deuterated mercaptoundecanoic acid (d-MUA) self-assembled on a gold substrate. Although cholesterol has five methyl groups and eleven methylene groups, by using molecules in which certain groups were either deuterated or entirely absent, it was possible to assign vibrational bands to specific sets of methyl or methylene groups either in the alkyl chain or sterol ring system of the molecule. Analysis of the spectra showed that the alkyl chains of cholesterol are orientated away from the substrate, which is opposite to their orientation in HBMs when the proximal leaflet is a hydrophobic self-assembled monolayer of octadecane thiol (ODT) adsorbed on gold. Additionally it was shown that in the d-MUA HBM, the α-face of the cholesterol ring is inclined toward the layer of air above the film, and the ß-face is inclined toward the gold substrate.

Adsorption of 1- and 2-butylimidazoles at the copper/air and steel/air interfaces studied by sum frequency generation vibrational spectroscopy.

The structure of thin films of 1- and 2-butylimidazoles adsorbed on copper and steel surfaces under air was examined using sum frequency generation (SFG) vibrational spectroscopy in the ppp and ssp polarizations. Additionally, the SFG spectra of both isomers were recorded at 55 °C at the liquid imidazole/air interface for reference. Complementary bulk infrared, reflection-absorption infrared spectroscopy (RAIRS), and Raman spectra of both imidazoles were recorded for assignment purposes. The SFG spectra in the C-H stretching region at the liquid/air interface are dominated by resonances from the methyl end group of the butyl side chain of the imidazoles, indicating that they are aligned parallel or closely parallel to the surface normal. These are also the most prominent features in the SFG spectra on copper and steel. In addition, both the ppp and ssp spectra on copper show resonances from the C-H stretching modes of the imidazole ring for both isomers. The ring C-H resonances are completely absent from the spectra on steel and at the liquid/air interface. The relative intensities of the SFG spectra can be interpreted as showing that, on copper, under air, both butylimidazoles are adsorbed with their butyl side chains perpendicular to the interface and with the ring significantly inclined away from the surface plane and toward the surface normal. The SFG spectra of both imidazoles on steel indicate an orientation where the imidazole rings are parallel or nearly parallel to the surface. The weak C-H resonances from the ring at the liquid/air interface suggest that the tilt angle of the ring from the surface normal at this interface is significantly greater than it is on copper.

Sum frequency generation (SFG) vibrational spectroscopy of planar phosphatidylethanolamine hybrid bilayer membranes under water.

Sum frequency generation (SFG) spectroscopy has been used to study the structure of phosphatidylethanolamine hybrid bilayer membranes (HBMs) under water at ambient temperatures. The HBMs were formed using a modified Langmuir-Schaefer technique and consisted of a layer of dipalmitoyl phosphatidylethanolamine (DPPE) physisorbed onto an octadecanethiol (ODT) self-assembled monolayer (SAM) at a series of surface pressures from 1 to 40 mN m(-1). The DPPE and ODT were selectively deuterated so that the contributions to the SFG spectra from the two layers could be determined separately. SFG spectra in both the C-H and C-D stretching regions confirmed that a monolayer of DPPE had been adsorbed to the ODT SAM and that there were gauche defects within the alkyl chains of the phospholipid. On adsorption of a layer of DPPE, methylene modes from the ODT SAM were detected, indicating that the phospholipid had partially disordered the alkanethiol monolayer. SFG spectra recorded in air indicated that removal of water from the surface of the HBM resulted in disruption of the DPPE layer and the formation of phospholipid bilayers.

Structural changes in a polyelectrolyte multilayer assembly investigated by reflection absorption infrared spectroscopy and sum frequency generation spectroscopy.

The structure of polyelectrolyte multilayer films adsorbed onto either a per-protonated or per-deuterated 11-mercaptoundecanoic acid (h-MUA/d-MUA) self assembled monolayer (SAM) on gold was investigated in air using two surface vibrational spectroscopy techniques, namely, reflection absorption infrared spectroscopy (RAIRS) and sum frequency generation (SFG) spectroscopy. Determination of film masses and dissipation values were made using a quartz crystal microbalance with dissipation monitoring (QCM-D). The films, containing alternating layers of the polyanion poly[1-[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1,2-ethanediyl, sodium salt] (PAZO) and the polycation poly(ethylenimine) (PEI) built on the MUA SAM, were formed using the layer-by-layer electrostatic self-assembly method. The SFG spectrum of the SAM itself comprised strong methylene resonances, indicating the presence of gauche defects in the alkyl chains of the acid. The RAIRS spectrum of the SAM also contained strong methylene bands, indicating a degree of orientation of the methylene groups parallel to the surface normal. Changes in the SFG and RAIRS spectra when a PEI layer was adsorbed on the MUA monolayer showed that the expected electrostatic interaction between the polymer and the SAM, probably involving interpenetration of the PEI into the MUA monolayer, caused a straightening of the alkyl chains of the MUA and, consequently, a decrease in the number of gauche defects. When a layer of PAZO was subsequently deposited on the MUA/PEI film, further spectral changes occurred that can be explained by the formation of a complex PEI/PAZO interpenetrated layer. A per-deuterated MUA SAM was used to determine the relative contributions from the adsorbed polyelectrolytes and the MUA monolayer to the RAIRS and SFG spectra. Spectroscopic and adsorbed mass measurements combined showed that as further bilayers were constructed the interpenetration of PAZO into preadsorbed PEI layers was repeated, up to the formation of at least five PEI/PAZO bilayers.

The structure of oleamide films at the aluminum/oil interface and aluminum/air interface studied by Sum Frequency Generation (SFG) vibrational spectroscopy and Reflection Absorption Infrared Spectroscopy (RAIRS).

The structure of oleamide (cis-9-octadecenamide) films on aluminum has been investigated by sum frequency generation vibrational spectroscopy (SFG) and reflection absorption infrared spectroscopy (RAIRS). Three different film deposition strategies were investigated: (i) films formed by equilibrium adsorption from oleamide solutions in oil, (ii) Langmuir-Blodgett films cast at 1 and 25 mN m(-1), (iii) thick spin-cast films. Both L-B and spin-cast films were examined in air and under oil. The adsorbate formed in the 1 mN m(-1) film in air showed little orientational order. For this film, the spectroscopic results and the ellipsometric thickness point to a relatively conformationally disordered monolayer that is oriented principally in the plane of the interface. Direct adsorption to the metal interface from oil results in SFG spectra of oleamide that are comparable to those observed for the 1 mN m(-1) L-B film in air. In contrast, SFG and RAIRS results for the 25 mN m(-1) film in air and SFG spectra of the spin-cast film in air both show strong conformational ordering and orientational alignment normal to the interface. The 25 mN m(-1) film has an ellipsometric thickness almost twice that of the 1 mN m(-1) L-B film. Taken in combination with the spectroscopic results, this is indicative of a well packed monolayer in air in which the hydrocarbon chain is in an essentially defect-free extended conformation with the methyl terminus oriented away from the surface. A similar structure is also deduced for the surface of the spin-cast film in air. Upon immersion of the 25 mN m(-1) L-B film in oil the SFG spectra show that this film rapidly adopts a relatively disordered structure similar to that seen for the 1 mN m(-1) L-B film in air. Immersion of the spin-cast film in oil results in the gradual disordering of the amide film over a period of several days until the observed spectra become essentially identical to those observed for direct adsorption of oleamide from oil.

Adsorption of SDS and PEG on calcium fluoride studied by sum frequency generation vibrational spectroscopy.

The adsorption of sodium dodecyl sulfate (SDS) from aqueous solution onto a calcium fluoride substrate (CaF(2)), in the presence of polyethylene glycol (PEG) of different molecular weights, has been investigated using the interface specific nonlinear optical technique of sum frequency generation (SFG) vibrational spectroscopy. Spectra of adsorbed SDS (in the C-H stretching region) were recorded at the surface of a CaF(2) prism in contact with SDS solutions at concentrations up to the cmc (8 mM) of the pure surfactant and in contact with binary solutions containing SDS and PEG with molecular weights from 400 to 12 000. In contrast with SFG spectra from the same combinations of surfactant and polymer on a hydrophobic surface, there was no evidence of spectra arising from the actual polymer adsorbed on CaF(2) at any polymer molecular weight either in the absence or presence of surfactant. However, there was indirect evidence for the presence of adsorbed polymer from changes in the SDS SFG spectra in the presence of polymer compared with spectra when the polymer was absent. The SFG spectra of SDS at 0.8 mM were closely similar to each other at all polymer molecular weights and different from the spectra in the absence of the polymer. The spectral differences between the polymer present and polymer absent was much smaller when the solution concentration of surfactant was 8 mM.

Adsorption of sodium dodecyl sulfate at the hydrophobic solid/aqueous solution interface in the presence of poly(ethylene glycol): dependence upon polymer molecular weight.

The polar orientation and degree of conformational order of sodium dodecyl sulfate (SDS) adsorbed at the hydrophobic octadecanethiol/aqueous solution interface in the presence of poly(ethylene glycol) (PEG) has been investigated as a function of the surfactant concentration and the molecular weight of the polymer. Sum frequency generation (SFG) vibrational spectroscopy was employed to obtain spectra of interfacial surfactant; weak SFG signals from interfacial polymer were also detected for polymer molecular weights of 900 and above. The phase of the SFG spectra indicated that both the surfactant and polymer had a net orientation of their CH2 and/or CH3 groups toward the hydrophobic surface. Spectra of SDS in the presence of mixed polymer/surfactant solutions showed increasing conformational order as the surfactant concentration was raised. At the lowest surfactant concentrations, the spectra of SDS were weaker in the presence of the polymer than in its absence. All PEG molecular weights investigated, with the exception of PEG 400, gave rise to significant inhibition of ordered surfactant adsorption below the critical micelle concentration. The greatest inhibitory effect was noted for PEG 900. Probing interfacial PEG specifically through the use of perdeuterated SDS revealed that the polymer spectral intensity decreased monotonically as the surfactant concentration was increased for all polymer molecular weights where a PEG spectrum was apparent. These findings are interpreted in terms of the displacement of preadsorbed polymer as the surfactant concentration increases. This result is compatible with observations of adsorption from SDS/PEG solutions at solid/solution and solution/air interfaces made using other techniques.