Ali Zarbakhsh

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.

Structural studies of amphiphiles adsorbed at liquid–liquid interfaces using neutron reflectometry

We report the application and refinement of a recently developed method for structural studies at a liquid liquid interface using neutron reflectometry. The technique involves the entrapment of a thin oil layer between a silicon substrate and an aqueous subphase. The thin oil film is prepared by spin-coating an oil film on to an oleophilically treated silicon substrate. During the reflectivity measurement the sample is maintained in a horizontal position, and the angle of incidence of the neutron beam is varied using a supermirror. Attenuation of neutron reflectivity at the lowest angle of incidence is used to determine the oil-layer thickness. We report information regarding the structure at the interface between hexadecane and a 0.1% w/v aqueous solution of the triblock copolymer Pluronic L64 with EO13PO30EO13 (EO = ethylene oxide; PO = propylene oxide) and the interface between hexadecane and a 3.7 mmol dm(-3) (approximately critical micelle concentration) aqueous solution of the cationic surfactant tetradecyltrimethylammonium bromide (C14TAB). For the C14TAB system, the reflectivity data unambiguously reveal the presence of a region highly concentrated in C14TAB on the oil side of the interface. For the Pluronic L64 system, the data suggest that the polymer adsorbs at the interface occupying both oil and water sides of the interface. Model scattering length density profiles that capture these features are presented and further models that better fit the data are discussed.

Study of Off-Specular Neutron Reflectivity Using a Model System

The specular reflection of neutrons gives information on the neutron refractive-index profile normal to an interface. The refractive index is simply related to the scattering-length density and hence the composition. However, in-plane inhomogeneities or corrugated surfaces can give rise to strong off-specular scattering. This has been observed in several systems. The difficulty arises in the interpretation of off-specular data from systems having poorly defined or unknown structure. We have measured off-specular reflection from a gold replica optical diffraction grating with 1 μm line spacing. A series of data was obtained using the CRISP neutron reflectometer at the Rutherford Appleton Laboratory in Oxfordshire, England. The analysis of specular and off-specular data, and the determination of the periodicity of the grating and the thickness of the metal film will be discussed. The maximum periodicity that could be observed was 42 μm. This is in agreement with the off-specular resolution which is determined by the horizontal divergence of the ribbon-shaped beam, but very much larger than the longitudinal coherence length of the neutron wave packet.

Comparative Adsorption of Saturated and Unsaturated Fatty Acids at the Iron Oxide/Oil Interface

A detailed comparison of the adsorption behavior of long straight chain saturated and unsaturated fatty acids at the iron oxide/oil interface has been considered using a combination of surface study techniques. Both depletion isotherms and polarized neutron reflectometry (PNR) show that the extent of adsorption decreases as the number of double bonds in the alkyl chains increases. Sum frequency generation spectroscopic measurements demonstrate that there is also an increase in chain disorder within the adsorbed layer as the unsaturation increases. However, for the unsaturated analogues, a decrease in peak intensity is seen for the double bond peak upon heating, which is thought to arise from isomerization in the surface-bound layer. The PNR study of oleic acid adsorption indicates chemisorbed monolayer adsorption, with a further diffuse reversible adsorbed layer formed at higher concentrations.

Small-angle neutron scattering studies of sodium dodecyl sulfate interactions with gelatin. Part 2.—Effect of temperature and pH

Structural perturbations and interactions between the anionic surfactant sodium dodecyl sulfate (SDS) and the biopolymer and polyampholyte gelatin, have been studied using small-angle neutron scattering. The effects of temperature and changes in pH have been investigated. Although the physical properties of the systems change dramatically with temperature, from a gel at 25 °C to a fluid at 65 °C, the effects on the structure over the dimensions probed by small-angle neutron scattering are rather weak. Changes in pH, which alter the net charge of the gelatin, lead to dramatic changes in the scattering for pure gelatin solutions and for surfactant–gelatin mixtures.

Surfactant Adsorption at the Metal-Oil Interface

The structure of the adsorbed palmitic acid at the iron oxide/oil interface has been investigated using polarized neutron reflectometry. The palmitic acid was found to be strongly adsorbed at the oxide/oil interface resulting in a monolayer of thickness 16 ± 4 Å for 150 and 500 ppm palmitic acid concentrations (16 ± 5 Å for the 1000 ppm solution). These layer thicknesses suggest tilt for the palmitic acid molecules with respect to the interface. The model also requires a second diffuse layer extending in the bulk oil. The thickness of this diffuse layer was 35 ± 17 Å for the 150 ppm solution and 45 ± 22 Å for 500 and 1000 ppm solution. The composition profiles at the interface suggest a depletion of the oil in the vicinity of the interface as the concentration of palmitic acid increases.

Neutron reflectivity study of alkylated azacrown ether at the air-liquid and the liquid-liquid interfaces.

We report the neutron reflectometry study of partially deuterated di-hexadecyl-diaza-18-crown-6 ether (d-ACE-16) at the air-water and the oil-water interfaces. At the air-water interface, the thickness of the monolayer is smaller than that for a fully stretched d-ACE-16 molecule, suggesting a tilt of the alkyl chains with respect to the normal. At the oil-water interface, the same molecules were found to form a more diffuse layer distribution stretching across both sides of the interface. On the oil side, the molecules are densely packed within a thickness of 17 A, the hydrophilic part of the molecule with the azacrown ether ring being immersed in the adjacent aqueous side of the interface. The latter consists of a thick 38 A layer comprising staggered, loosely adsorbed d-ACE-16 molecules. With increasing spread amount, the adsorbed layer density increases at the oil side until saturation at ca. 2.25 x 10(-6) mol m(-2), above which the layer collapses.

Polarized Neutron Reflectometry of Nickel Corrosion Inhibitors

Polarized neutron reflectometry has been used to investigate the detailed adsorption behavior and corrosion inhibition mechanism of two surfactants on a nickel surface under acidic conditions. Both the corrosion of the nickel surface and the structure of the adsorbed surfactant layer could be monitored in situ by the use of different solvent contrasts. Layer thicknesses and roughnesses were evaluated over a range of pH values, showing distinctly the superior corrosion inhibition of one negatively charged surfactant (sodium dodecyl sulfate) compared to a positively charged example (dodecyl trimethylammonium bromide) due to its stronger binding interaction with the surface. It was found that adequate corrosion inhibition occurs at significantly less than full surface coverage.

Adsorption of polystyrene–poly(ethylene oxide) block copolymers on quartz using a parallel-plate surface-force apparatus and simultaneous neutron reflection

Block copolymers of polystyrene and poly(ethylene oxide) adsorbed on quartz plates under compression have been studied using neutron reflectometry. Adsorption from a good solvent (toluene) leads to the preferential adsorption of the ethylene oxide chains, but because of the very high asymmetry of the polymers and the favourable adsorption energy, the polystyrene is also adsorbed. The extent of the polystyrene adsorbed layer is less than the radius of gyration for both samples studied. On the approach of a second coated plate, the polystyrene volume fraction increases near the interface, indicative of a strong interlayer repulsion. On changing the solvent to octane the profiles indicate collapse of the polymer layer and there is a dramatic decrease in the pressure required to hold the quartz plates at the same separation.

Adsorption of Bovine Serum Albumin (BSA) at the Oil/Water Interface: A Neutron Reflection Study.

The structure of the adsorbed protein layer at the oil/water interface is essential to the understanding of the role of proteins in emulsion stabilization, and it is important to glean the mechanistic events of protein adsorption at such buried interfaces. This article reports on a novel experimental methodology for probing protein adsorption at the buried oil/water interface. Neutron reflectivity was used with a carefully selected set of isotopic contrasts to study the adsorption of bovine serum albumin (BSA) at the hexadecane/water interface, and the results were compared to those for the air/water interface. The adsorption isotherm was determined at the isoelectric point, and the results showed that a higher degree of adsorption could be achieved at the more hydrophobic interface. The adsorbed BSA molecules formed a monolayer on the aqueous side of the interface. The molecules in this layer were partially denatured by the presence of oil, and once released from the spatial constraint by the globular framework they were free to establish more favorable interactions with the hydrophobic medium. Thus, a loose layer extending toward the oil phase was clearly observed, resulting in an overall broader interface. By analogy to the air/water interface, as the concentration of BSA increased to 1.0 mg mL(-1) a secondary layer extending toward the aqueous phase was observed, possibly resulting from the steric repulsion upon the saturation of the primary monolayer. Results clearly indicate a more compact arrangement of molecules at the oil/water interface: this must be caused by the loss of the globular structure as a consequence of the denaturing action of the hexadecane.