Neutron reflectivity, from monolayers

Grundy, M. J Richardson, R. M., Roser, S. J., Penfold, J. and Ward, R. C. (1988). X-ray and neutron reflectivity from spread monolayers. Thin Solid Films 159 43-52. 

The interaction between Humicola Lanuginosa lipase HLL) [57] and sodium dodecyl sulphate (SDS) has been investigated measuring neutron reflectivity from deuterated SDS dissolved in a solution of protonated lipase in D2O. The lipase was found to adsorb readily at the air-water interface but SDS can displace lipase from the interface at higher SDS concentrations [164]. Also, the interaction of dissolved HLL with monolayers of MOG and DPG was studied by GIXD and XR [27]. 

The ability to contrast match the air with a mixture of water and heavy water makes neutron reflectivity an attractive technique [180,181], Under these contrast conditions the scattering arises from the monolayer alone and combining... 

Fig. XV-4. Schematic drawing of four streptavidin molecules bound to biotinylated lipid in a monolayer above heavy water. The scattering length density for neutron reflectivity is shown at the side. (From Ref. 30.)...

Tarek et al. [388] studied a system with some similarities to the work of Bocker et al. described earlier—a monolayer of n-tetradecyltrimethylammonium bromide. They also used explicit representations of the water molecules in a slab orientation, with the mono-layer on either side, in a molecular dynamics simulation. Their goal was to model more disordered, liquid states, so they chose two larger molecular areas, 0.45 and 0.67 nm molecule Density profiles normal to the interface were calculated and compared to neutron reflectivity data, with good agreement reported. The hydrocarbon chains were seen as highly disordered, and the diffusion was seen at both areas, with a factor of about 2.5 increase from the smaller molecular area to the larger area. They report no evidence of a tendency for the chains to aggregate into ordered islands, so perhaps this work can be seen as a realistic computer simulation depiction of a monolayer in an LE state. 

The more recent neutron reflectivity studies have established that flattened surface micelle or fragmented bilayer structure in more detail and with more certainty, using contrast variation in the surfactant and the solvent [24, 31]. However, the extent of the lateral dimension (in the plane of the surface) and the detailed structure in that direction is less certain. From those neutron reflectivity measurements [24, 31] and related SANS data on the adsorption of surfactants onto colloidal particles [5], it is known that the lateral dimension is small compared with the neutron coherence length, such that averaging in the plane is adequate to describe the data. The advent of the AFM technique and its application to surfactant adsorption [15] has provided data that suggest that there is more structure and ordering in the lateral direction than implied from other measurements. This will be discussed in more detail in a later section of the chapter. At the hydrophobic interface, although the thickness of the adsorbed layer is now consistent with a monolayer, the same uncertainties about lateral structure exist. 

Bayerl, T. M., Thomas, R. K., Penfold, J., Rennie, A. and Sackmann, E. (1990). Specular reflection of neutrons at phospholipid monolayers. Changes of mono-layer structure and headgroup hydration at the transition from expanded to the condensed phase. Biophys. J. 57 1095-1098. 

Unique information about the unit cell in quasi-crystaUine monolayers can be obtained from X-ray °, neutron , heUum or low energy electron diffraction (LEED) data. In the grazing incidence X-ray diffraction (GIXD) experiment the beam is directed at the coated surface at a low angle and experiences total internal reflection from the metal support underneath the monolayer. The analysis of reflectivity and diffraction pattern of this reflected beam provides information about the molecular structure of the crystalline films, the thickness and refractive index of the layers and the roughness of the surface s . These experiments, however, require sophisticated and expensive equipment and are not therefore used routinely for monolayer characterization. 

Fig. 6 Schematic structure of the composite layer consisting of a silicon oxide surface with a self-assembled monolayer of octadecyl trichlorosilane and a layer of the surfactant tetraethylene glycol monododecyl ether adsorbed from aqueous solution. The diagram shows the dimensions of the layers as deduced from neutron reflection and represents the proportions of the various components in the layers. The surfactant molecules are strongly tilted with the ethylene glycol head groups pointing towards the aqueous solution...

Table 4 Structural data for monolayers of selected lipid monolayers at the surface of water (neutral / H) extracted from specular X-ray and neutron reflectivity data by modelling in terms of two-slab models/ ... 

For a compressed monolayer of DPPC, contrast variation was employed. Three data sets XR data and neutron reflection data for two different contrasts (deuterated DPPC-d62 on both H2O and D2O) were jointly refined to yield a model with parameters better determined than from any of the data sets separately [148],... 

FIGURE 3.15 Schematic illustration of structure of mixed dodecane-Ci4TAB monolayer showing interdigitation between hydrocarbon and surfactant chains. Structure was deduced from neutron reflection measurements. (Reprinted with permission from Lu, J. et al., J. Phys. Chem., 96, 10971. Copyright 1992 American Chemical Society.)... 

Molecular orientational order in adsorbed monolayers can be inferred indirectly from elastic neutron diffraction experiments if it results in a structural phase transition which alters the translational symmetry of the 2D lattice. In such cases, Bragg reflections appear which are not present in the orientationally disordered state. Experiments of this type have inferred orientational order in monolayers of oxygen (41) and nitrogen (42) adsorbed on graphite. However, these experiments have not observed a sufficient number of Bragg reflections to determine the molecular orientation by comparing relative Bragg peak intensities with a model structure factor. 

Numerous techniques have been employed to examine the monolayer structure of phospholipids at the air/water interface including surface tension, fluorescence, neutron and X-ray reflection, and IR and Raman spectroscopy. In contrast, very few techniques are suitable to examine monolayers at the oil/water interface. Surface tension and fluorescence microscopy [46-48] have shed some light on these buried monolayers, but most other surface techniques are hampered because of effects from the bulk liquids. Since VSFS is insensitive to the bulk, it is an excellent technique for probing these monolayers. 

Neutron reflectometry studies on mixed DPPC/oleic acid monolayers have been conducted using the CRISP reflectometer at RAL. First, the stmcture of DPPC monolayers was determined by measuring reflectivity profiles from three different isotopic forms of the DPPC monolayer system. This was achieved using hydrogenated (h-DPPC) and chain perdeuterated (d-DPPC) phospholipids and two different subphases of D2O and ACMW. The monolayers were studied at three surface coverages of approximately 50, 60, and 70 A /molecule. Examination of the surface pressure-area isotherm reveals that the main LE/LC phase transition for DPPC monolayers occurs over this range of molecular area (Lewis and Hadgraft, 1990). 

Fig. 18 a Neutron reflectometry data for lipid/PEG-lipid monolayers on a pure D2O subphase. The four reflectivity curves correspond to a pure DSPE monolayer and to mixtures of DSPE and DSPE-PEG2000. In this set of data, all of the DSPE and DSPE-PEG2000 lipid hydrocarbon chains were fully deuterated (case 1). Full lines represent free form fits to the individual measurements, b. Corresponding scattering length densities (J3 (z)) obtained from the fits shown in a [47] (reproduced with permission from the American Chemical Society)... 

Figure 62. Neutron diffraction intensity after subtraction of the background from CO on graphite (Papyex) at 1.58 K and at a coverage of 0.78 monolayers (see also Fig. 25 for comparison). Note the presence of the (20) and (21) reflections (at Q = 1.703 A and 2.253 A , respectively) and the absence of the (10) and (11) reflections (at Q = 0.852 A" and 1.475 A", respectively) as marked by the arrows the solid line is a two-dimensional line-shape fit [309]. The diffraction pattern reveals that CO on graphite remains in the commensurate herringbone stmcwre down to very low temperatures. (From Refs. 177 and 381.)...

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