Michael T. L. Casford

Hexadecylamine Adsorption at the Iron Oxide–Oil Interface

The adsorption behavior of a model additive, hexadecylamine, onto an iron surface from hexadecane oil has been characterized using polarized neutron reflectometry, sum-frequency generation spectroscopy, solution depletion isotherm, and X-ray photoelectron spectroscopy (XPS). The amine showed a strong affinity for the metal surface, forming a dense monolayer at relatively low concentrations; a layer thickness of 16 (±3) Å at low concentrations, increasing to 20 (±3) Å at greater amine concentrations, was determined from the neutron data. These thicknesses suggest that the molecules in the layer are tilted. Adsorption was also indicated by sum-frequency generation spectroscopy and XPS, the latter indicating that the most dominant amine–surface interaction was via electron donation from the nitrogen lone pair to the positively charged iron ions. Sum-frequency generation spectroscopy was used to determine the alkyl chain conformation order and orientation on the surface.

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.

Structure of Mixed Phosphatidylethanolamine and Cholesterol Monolayers in a Supported Hybrid Bilayer Membrane Studied by Sum Frequency Generation Vibrational Spectroscopy

The structure of hybrid bilayer membranes (HBMs) containing either a pure cholesterol or mixed cholesterol/dipalmitoylphosphatidylethanolamine (DPPE) proximal layer adsorbed onto an octadecanethiol (ODT) self-assembled monolayer (SAM) on a gold substrate have been investigated by sum frequency generation (SFG) spectroscopy. The HBMs were formed by the adsorption of either a pure cholesterol or mixed DPPE/cholesterol monolayer from the air/water interface of a Langmuir-Blodgett trough at surface pressures of 1, 20, or 40 mN·m(-1). SFG spectra were also recorded of HBMs where cholesterol was replaced by cholesterol-d(7), in which the terminal isopropyl group of the alkyl chain of cholesterol was isotopically labeled. In order to isolate the contribution to the SFG spectra from the cholesterol in the mixed cholesterol/phospholipid films, DPPE-d was used, in which the alkyl chains of the phospholipid were deuterated. The infrared spectra of solvent-cast cholesterol and cholesterol-d(7) films were recorded to aid with assignment of the SFG spectra of the HBMs. Features corresponding to methyl, methylene, and methine stretches of cholesterol were identified in the SFG spectra. Information on the polar orientation of SFG-active groups was obtained from the phases of the spectral features. The structure of the HBMs showed little dependence on the surface pressure at which they were formed. SFG spectra of HBMs with a mixed cholesterol/DPPE proximal layer were very similar to the spectra of HBMs with a pure cholesterol proximal layer, although the features in the spectra were more intense than anticipated for a film with half the number of cholesterol molecules, indicating that DPPE did have some effect on the orientation of cholesterol molecules in the film.

Cation Bridging Studied by Specular Neutron Reflection

The binding of an anionic surfactant onto an anionic surface by addition of divalent ions is reported based on experimental data from specular neutron reflection (NR) and attenuated total internal reflection IR spectroscopy (ATR-IR). Similar measurements using monovalent ions (sodium) do not show any evidence of such adsorption, even though the amount of surfactant can be much higher. This data is interpreted in terms of the so-called bridging mechanism of ion binding.

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.

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 β-face is inclined toward the gold substrate.

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.

Effect of multiple group orientations on sum frequency generation spectra

Sum Frequency Generation (SFG) spectra of monolayer films on metal surfaces have been simulated where the monolayer contains methyl groups in two different orientations. The two methyl group tilt angles were taken into account by including a second resonant term in the expression for the second-order non-linear susceptibility of the film, χR, and allowing the two resonant terms to vary independently with tilt angle. The metal surface gives rise to a constant non-resonant susceptibility, χNR. The phases of the SFG signals are determined by the interference between the two resonant susceptibilities and the non-resonant susceptibility of the metal surface. Whether the features corresponding to the methyl symmetric (r+) and asymmetric (r−) stretching modes appeared as peaks or dips in the simulated spectra showed a strong dependence on the two tilt angles, and a weaker dependence on R, the ratio of the two hyperpolarisabilities β aca and β ccc. From the simulated spectra it was determined that there are combinations of the two methyl group tilt angles that result in the symmetric and asymmetric methyl resonances having different spectral profiles with one a peak and the other a dip. A general method has been developed from which boundary conditions for the two methyl group tilt angles can be formulated based on the experimentally determined resonant phases of the methyl symmetric and asymmetric stretching modes.

Orientation of cholesterol in hybrid bilayer membranes calculated from the phases of methyl resonances in sum frequency generation spectra

The phases of Sum Frequency Generation (SFG) vibrational resonances recorded from thin films on metal surfaces provide information on the orientation and tilt angles of the functional groups of molecules in the film. SFG spectra have been simulated for monolayer films in which the adsorbed molecule has an unequal number of methyl groups oriented in two different directions. The phases, on resonance, of the methyl symmetric (r(+)) and asymmetric (r(-)) resonances are determined as a function of the two methyl group tilt angles and the fraction of groups pointing in that particular direction. The results are first presented as two-dimensional projection plots for the r(+) and r(-) resonances and then combined to show the tilt angles of the methyl groups, and fraction of methyl groups in that orientation, for which both resonances are in phase or out of phase with one another. Mathematical expressions have been calculated to identify the precise boundary conditions for when the phases of the r(+) and r(-) resonances change. The results of these simulations are compared to the phases of the methyl resonances in experimental SFG spectra of d7-cholesterol in a hybrid bilayer membrane.