Spontaneous formation of vesicles from the selfassembly of two specific surfactants, one zwitterionic (oleyl amidopropyl betaine, OAPB) and the other anionic (Aerosol-OT, AOT), is explored in water using small-angle scattering techniques. Two factors were found to be critical in the formation of vesicles: surfactant ratio, as AOT concentrations less than equimolar with OAPB result in cylindrical micelles or mixtures of micellar structures, and salt concentration, whereby increasing the amount of NaCl promotes vesicle formation by reducing headgroup repulsions. Small-angle neutron scattering measurements reveal that the vesicles are approximately 30−40 nm in diameter, depending on sample composition. Small-angle X-ray scattering measurements suggest preferential partitioning of OAPB molecules on the vesicle inner layer to support vesicular packing. Heating the vesicles to physiological temperature (37 °C) causes them to collapse into smaller ellipsoidal micelles (2−3 nm), with higher salt concentrations (≥10 mM) inhibiting this transition. These aggregates could serve as responsive carriers for loading or unloading of aqueous cargoes such as drugs and pharmaceuticals, with temperature changes serving as a simple release/uptake mechanism.
A novel neutron and X-ray reflectometry sample environment is presented for the study of surface-active molecules at solid–liquid interfaces under shear. Neutron reflectometry was successfully used to characterise the iron oxide–dodecane interface at a shear rate of $$7.0\times {}10^{2}$$ 7.0 × 10 2 $$\hbox {s}^{-1}$$ s - 1 using a combination of conventional reflectometry theory coupled with the summation of reflected intensities to describe reflectivity from thicker films. Additionally, the structure adopted by glycerol monooleate (GMO), an Organic Friction Modifier, when adsorbed at the iron oxide–dodecane interface at a shear rate of $$7.0\times {}10^{2}$$ 7.0 × 10 2 $$\hbox {s}^{-1}$$ s - 1 was studied. It was found that GMO forms a surface layer that appears unaltered by the effect of shear, where the thickness of the GMO layer was found to be $$24.3^{+9.9}_{-10.2}$$ 24 . 3 - 10.2 + 9.9 Å under direct shear at $$7.0\times {}10^{2}$$ 7.0 × 10 2 $$\hbox {s}^{-1}$$ s - 1 and $$25.8^{+4.4}_{-5.2}$$ 25 . 8 - 5.2 + 4.4 Å when not directly under shear. Finally, a model to analyse X-ray reflectometry data collected with the sample environment is also described and applied to data collected at $$3.0\times {}10^{3}$$ 3.0 × 10 3 $$\hbox {s}^{-1}$$ s - 1 .
The spontaneous adsorption of graphene oxide (GO) sheets at the air--water interface is explored using X-ray reflectivity (XRR) measurements. As a pure aqueous dispersion, GO sheets do not spontaneously adsorb...
As a result of the availability of modern software and hardware, Bayesian analysis is becoming more popular in neutron and X-ray reflectometry analysis. The understandability and replicability of these analyses may be harmed by inconsistencies in how the probability distributions central to Bayesian methods are represented in the literature. Herein advice is provided on how to report the results of Bayesian analysis as applied to neutron and X-ray reflectometry. This includes the clear reporting of initial starting conditions, the prior probabilities, the results of any analysis and the posterior probabilities that are the Bayesian equivalent of the error bar, to enable replicability and improve understanding. It is believed that this advice, grounded in the authors' experience working in the field, will enable greater analytical reproducibility in the work of the reflectometry community, and improve the quality and usability of results.
The adsorption behavior of β-nicotinamide adenine dinucleotide (NADH) at the carbon/electrolyte interface has been studied using a combination of neutron reflectometry (NR) and solution depletion isotherms. Coupling the NR technique with an electrochemical cell allowed in situ observation of the reversible adsorption and desorption of the molecule at the electrode surface over a range of applied potentials. The overall surface coverage was low (30–50%), suggesting adsorption only at specific defect sites on the surface. Isotherms conducted over a range of temperatures were used to extract thermodynamic parameters, which implied strong physisorption via electrostatic interactions. In addition, changes in the outermost layer of the carbon electrode were observed as the applied potential was varied, which were confirmed with ex situ X-ray reflectivity measurements (XRR). X-ray photoelectron spectroscopy (XPS) measurements of the carbon surface demonstrated the majority of carbon atoms were in an sp2 state.
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