Andrew Nelson

Co‐refinement of multiple‐contrast neutron/X‐ray reflectivity data using MOTOFIT

The contrast-variation technique is employed in multiple-contrast neutron/X-ray reflectometry experiments to highlight scattering from different structural components that are present at a surface or interface. The advantage of this technique is that the structural model used to describe the interfacial scattering length density profile must apply to all the contrasts measured. A new reflectivity analysis package, MOTOFIT, which runs in the IGOR Pro environment (http://www.wavemetrics.com), has been created to aid the simultaneous fitting (with the same structural model) of these multiple-contrast data, using an intuitive graphical user interface, which most co-refinement packages do not possess. MOTOFIT uses a slab-model approach with the Abeles matrix method, and extensions for surface roughness to perform non-linear least-squares regression on the experimental reflectivity curves. Other features, such as the ability to create complicated interparameter constraints or analyse reflectivity from multilayers, simulated annealing, etc., make MOTOFIT a powerful reflectometry analysis package

Electrical Double-Layer Capacitance in Room Temperature Ionic Liquids: Ion-Size and Specific Adsorption Effects

The electrical double-layer structure and capacitance in room temperature ionic liquids at electrified interfaces were systematically studied with use of the self-consistent mean-field theory. The capacitance curve departs from symmetry with respect to the point of zero charge when unequal ion-size is implemented or when specific adsorption of ions is introduced. For the case of unequal ion-size, the shape of the capacitance curve is strongly determined by the size of the counterion and only weakly influenced by the co-ion size. When present, specifically adsorbed ions would change the capacitance within a limited range of applied potential from the point of zero charge, which itself varies with the strength of specific adsorption.

Structure of [C4mpyr][NTf2] Room-Temperature Ionic Liquid at Charged Gold Interfaces

The structure of 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([C(4)mpyr][NTf(2)]) room-temperature ionic liquid at an electrified gold interface was studied using neutron reflectometry, cyclic voltammetry, and differential capacitance measurements. Subtle differences were observed between the reflectivity data collected on a gold electrode at three different applied potentials. Detailed analysis of the fitted reflectivity data reveals an excess of [C(4)mpyr](+) at the interface, with the amount decreasing at increasingly positive potentials. A cation rich interface was found even at a positively charged electrode, which indicates a nonelectrostatic (specific) adsorption of [C(4)mpyr](+) onto the gold electrode.

Motofit – integrating neutron reflectometry acquisition, reduction and analysis into one, easy to use, package

The efficient use of complex neutron scattering instruments is often hindered by the complex nature of their operating software. This complexity exists at each experimental step: data acquisition, reduction and analysis, with each step being as important as the previous. For example, whilst command line interfaces are powerful at automated acquisition they often reduce accessibility by novice users and sometimes reduce the efficiency for advanced users. One solution to this is the development of a graphical user interface which allows the user to operate the instrument by a simple and intuitive push button approach. This approach was taken by the Motofit software package for analysis of multiple contrast reflectometry data. Here we describe the extension of this package to cover the data acquisition and reduction steps for the Platypus time-of-flight neutron reflectometer. Consequently, the complete operation of an instrument is integrated into a single, easy to use, program, leading to efficient instrument usage.

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.

Invited Article: Polarization “Down Under”: The polarized time-of-flight neutron reflectometer PLATYPUS

This review presents the implementation and full characterization of the polarization equipment of the time-of-flight neutron reflectometer PLATYPUS at the Australian Nuclear Science and Technology Organisation (ANSTO). The functionality and efficiency of individual components are evaluated and found to maintain a high neutron beam polarization with a maximum of 99.3% through polarizing Fe/Si supermirrors. Neutron spin-flippers with efficiencies of 99.7% give full control over the incident and scattered neutron spin direction over the whole wavelength spectrum available in the instrument. The first scientific experiments illustrate data correction mechanisms for finite polarizations and reveal an extraordinarily high reproducibility for measuring magnetic thin film samples. The setup is now fully commissioned and available for users through the neutron beam proposal system of the Bragg Institute at ANSTO.

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.

The effects of acid hydrolysis on protein biosurfactant molecular, interfacial, and foam properties: pH responsive protein hydrolysates

The success of hydrolysis in improving the functional foaming properties of surface-active proteins is usually attributed to three factors: decreased molecular size; increased hydrophobicity; and microchemical changes, specifically deamidation of glutamine and asparagine. Studying these individual factors is difficult using naturally-occurring proteins, as hydrolysate products are complex mixed systems, and the mechanisms of foam stabilization are likewise complex. To address this complexity we report studies of a recombinant protein (DAMP4) which comprises four peptide surfactant (DAMP1) molecules connected by acid-labile amino acid (Asp-Pro) linkers. Hydrolysis of DAMP4 under conditions of low pH and high temperature produced h-DAMP1, a mixture of deamidated variants of the chemically-synthesized DAMP1 peptide surfactant. By examining foaming performance of these molecules, we are able to isolate the effects of molecule size (DAMP1 vs. DAMP4) and deamidation (h-DAMP1 vs. DAMP1). Molecule size had little effect on foaming for the conditions studied. However, deamidation completely changed foaming behaviour, most likely due to alteration of interfacial charge structure (through deamidation of glutamine to glutamic acid) and consequent effects on thin-film stability. Good foaming was observed only at pH values away from the isoelectric points (pI) of the biomolecules where an electrostatic barrier to film rupture can occur. The addition of Zn2+ to DAMP4, h-DAMP1 and DAMP1 caused visible aggregation under all conditions, which assisted in stabilising foams only in situations where a net charge would be expected.

Enhanced specific ion effects in ethylene glycol-based thermoresponsive polymer brushes

The thermoresponse of poly(di(ethyleneglycol) methyl ether methacrylate) (PMEO2MA) brushes has been investigated in the presence of monovalent anions at either end of the Hofmeister series using ellipsometry, neutron reflectometry (NR) and colloid probe atomic force microscopy (AFM). NR measurements in deuterium oxide showed no evidence of vertical phase separation perpendicular to the grafting substrate with a gradual transition between a block-like, dense structure at 45°C and an extended, dilute conformation at lower temperatures. All three techniques revealed a shift to a more collapsed state for a given temperature in kosmotropic potassium acetate solutions, while more swollen structures were observed in chaotropic potassium thiocyanate solutions. No difference was observed between 250mM and 500mM thiocyanate for a 540Å brush studied by ellipsometry, while the lower molecular weight ∼200Å brushes used for NR and AFM measurements continued to respond with increasing salt concentration. The effect of thiocyanate on the temperature response was greatly enhanced relative to PNIPAM with the shift in temperature response at 250mM being five times greater than a PNIPAM brush of similar thickness and grafting density.

Calcium mediated interaction of calf-thymus DNA with monolayers of distearoylphosphatidylcholine: a neutron and X-ray reflectivity study

X-ray and neutron reflection studies, the latter in conjunction with contrast variation, have been combined to study the interaction of calf thymus DNA (ctDNA) with monolayers of distearoylphosphatidylcholine (DSPC) in the presence of 20 mM Ca2+ ions, at the air–liquid interface as a function of surface pressure (10, 20, 30 and 40 mN m−1). Analysis of the X-ray and neutron reflection data showed that, regardless of the surface pressure of the monolayer, a layer of ctDNA was present below the DSPC lipid head groups and that this ctDNA-containing layer (thickness ∼12.5 to 15 A) was separated from the DSPC head groups by a layer of water of ∼9 A thickness. The thickness of the ctDNA-containing layer was thinner than that reported for monolayers of cationic lipid at the air–water interface (18–25 A) although in these monolayers no water layer separating the lipid head groups from the layer containing ctDNA has been reported. At all surface pressures the amount of ctDNA present in the layer was in the range 30–40% by volume. As no significant re-arrangement of the DSPC film was required to accommodate the presence of the ctDNA, this suggests that the distribution of charges in the lipid film matches well the charge spacing of ctDNA. Brewster angle microscopy measurements of DSPC on water in the absence of Ca2+ showed the presence of a continuous film containing small, regular shaped domains at all four surface pressures examined. When Ca2+ ions were present in the sub-phase, although the film was still continuous, the domains comprising the film were more irregular in appearance while the presence of Ca2+ ions and ctDNA resulted in the domains becoming smaller and more regularly packed on the surface.