Neutron reflectivity measurements

Figure 1 Schematic diagram of the neutron reflectivity measurement with the neutrons incident on the surface and refiected at an angle 6 with respect to the surface. The angie 62 is the angle of refraction. The specimen in this case is a uniform film with thickness d, on a substrate.

Neutron reflectivity measures the variation in concentration normal to the surface of the specimen. This concentration at any depth is averaged over the coherence length of the neutrons (on the order of 1 pm) parallel to the sur ce. Consequendy, no information can be obtained on concentration variadons parallel to the sample surface when measuring reflectivity under specular conditions. More imponantly, however, this mandates that the specimens be as smooth as possible to avoid smearing the concentration profiles. 

Figure 8.6 Comparison of the influence of non-ionic Ci2E6 (hexaoxyethyl-ene ft-dodecyl ether) or anionic SDS (sodium dodecyl sulfate) on adsorbed amount of p-lactoglobulin at the air-water interface (0.1 wt% protein, pH = 6, ionic strength = 0.02 M, 25 °C) as determined by neutron reflectivity measurements. Protein surface concentration is plotted against the aqueous phase surfactant concentration ( ) Ci2E6 ( ) SDS. Reproduced from Dickinson (2001) with permission.

Schwendel, D., Hayashi, T., Dahint, R., Pertsin, A., Grunze, M., Steitz, R., Schreiber, F. (2003). Interaction of water with self-assembled monolayers neutron reflectivity measurements of the water density in the interface region. Langmuir, 19, 2284-2293. 

This reaction can be utilized to determine Fe(lll) at potentials where it is not redox-active at an unmodified glassy carbon electrode. The mediation of this reaction in two different electrolytes, i.e. 1 M perchloric acid and 0.1 M sulfuric acid, will be discussed. EQCM and neutron reflectivity measurements have shown that, while in H2SO4 a swollen structure is obtained, in perchloric acid a much more closed morphology is obtained which inhibits ion movement. 

Fig. 1. Schematic representation of the scattering geometry for the neutron reflectivity measurements at the solid-solution interface.

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. 

In contrast to the measurements by McDermott et al [58], neutron reflectivity measurements for the Ci2E6/Ci6TAB mixture in 0.1 M NaBr at the air-water interface and SANS measurements of the mixed micelles show close to ideal mixing. Penfold et al. [60] has used neutron reflectivity to investigate this mixture at the solid-solution interface. For the hydrophilic silicon surface, the surface composition of the mixed surfactant bilayer adsorbed at the interface depended strongly upon the solution pH. At pH 2.4, the surface composition... 

Penfold et al. [62] have also used neutron reflectivity to study the adsorption (structure and composition) of the mixed anionic/nonionic surfactants of SDS and C12E6 at the hydrophilic silica-solution interface. This is rather different case to the cationic/nonionic mixtures, as the anionic SDS has no affinity for the anionic silica surface in the absence of the Ci2E6. The neutron reflectivity measurements, made by changing the isotopic labelling of the two surfactants and the solvent, show that SDS is coadsorbed at the interface in the presence of the Ci2E6 nonionic surfactant. The variations in the adsorbed amount, composition, and the structure of the adsorbed bilayer reflect the very different affinities of the two surfactants for the surface. This is shown in Fig. 7, where the adsorbed amount and composition is plotted as a function of the solution composition. 

Figure 9 Distribution of the block co-polymer PB-PEO at the (a) air-water and (b) hexadecane-water interfaces, derived from specular neutron reflection measurements...

Fig-i- Theoretical volume fraction vs. distance profile across an interface between PS and PVP polymers. A value of x = 0-12 appropriate to a temperature of 160 °C [ 105] has been used to predict an interface width a7 of 1.6 nm from Eq. (3). This profile is consistent with neutron reflectivity measurements of PS/PVP interface segment density profiles if the apparent broadening of the interface by capillary waves is taken into account [ 106]... 

Lu et al. studied the structure of adsorption layers for three classes of surfactants - cationic, anionic, and non-ionic in a broad concentration range up to the CMC [67], The mean distances between the centre of distribution of the hydrophobic chain of adsorbed molecules and the mean position of the aqueous surface was calculated on the basis of neutron reflection measurement at concentrations close to the CMC. The obtained results show that in this concentration range the hydrocarbon chains are significantly immersed into the water, regardless of their chemical nature. 

Fig. 7.3 H-tunability of the lEC demonstrated by neutron reflectivity measurements (A) and by magnetometric measurements (B) of a Fe/Nb superlattice. Images taken from Klose et ai. [37].

Fig. 5.156. Spectroelectrochemical cell for in situ neutron reflectivity measurements [989]...

This finding is in accordance with earlier observation of Tanaka et al. [11] who reported on nonuniform swelling of PMMA in water indicated by a neutron reflectivity measurement. They found that the thickness of the swelling layer was 10 nm. 

An analysis of the surfactant tilt angles revealed that no significant difference appears between the molecular orientations of the hydrophobic C12 chain at the air/water and the oil/water interface. This result is in discrepancy with experimental investigations of the interaction of C12E5 and dodecane. Neutron reflection measurements suggest an more upright arrangement of the C12 chains if dodecane penetrates into the C12 layer. 

Strutwolf, J., A.L. Barker, M. Gonsalves, D.J. Caruana, P.R. Unwin, D.E. Williams, and J.R.P. Webster (2000). Probing liquid/liquid interfaces using neutron reflection measurements and scanning electrochemical microscopy. J. Electroanal. Chem. 483, 163-173. 

Figure 6.9. Volume fraction profiles of an end-grafted polystyrene brush, of relative molecular mass 105 000, imder various solvent conditions (O, toluene at 21 C and cyclohexane at A, 53.4 °C , 31.5 °C o, 21.4 °C and A, 14.6 "C), deduced from neutron reflectivity measurements (all the solvents are deuterated). Toluene at 21 °C is a good solvent and the solid line is the classical parabolic profile. The theta temperature for d-cyclohexane is 34 °C and the dashed line is the elliptical profile predicted by analytical self-consistent field theory for theta conditions. After Karim et al. (1994).

The self-consistent field theory also allows one to calculate the segment density profiles of each homopol)maer and each block of the copolymer. Forward recoil spectrometry is unable to resolve the details of these concentration profiles — the apparent finite width of the copolymer layer shown in figure 6.6 is entirely due to the instrumental resolution - but from neutron reflectivity measurements on a series of differently labelled samples one is able to extract all four segment density profiles. Figure 6.20 shows an example of this, for a styrene/methyl methacrylate copolymer at an interface between polystyrene and poly(methyl methacrylate). 

The cases in which one or both of the copolymer blocks form a wet brush are somewhat more complicated and are discussed by Dai et al. (1992). Our discussions in section 6.2 should lead us to suspect that this scaling approach is not likely to be completely accurate - we know from neutron reflectivity measurements (for example those shown in figures 6.14 and 6.20) that, even in the dry-brush regime, there is substantial penetration of the brush by the homopolymer. Nonetheless, the theory does succeed in capturing much of the physics and allows at least semi-quantitative predictions of the interfacial excess and interfacial tension. 

Figure 8.7 Neutron reflectivity measurements on deuterated TIPS-pentacene PccMS blends. PaMS had molecular weights of (a) l.SkgmoP and (b)...

The inset of Fig. 3 shows the difference between the model described by Eq. 2 and the measurement. We observe a significant depression of the refractive index at a separation of Drain = 21 1 nm. If interpreted in terms of water density, this is equivalent to a zone of 5-10% reduced density 5-10 nm above the PEG layers. We note that a quantitatively similarly density depression was recently suggested based on neutron reflectivity measurements (Schwendel et al., 2002). 

The structure of adsorbed chains at the air-liquid interface is similar to that at the solid-liquid interface. Adsorption at air-liquid interface has been studied by ellipsometry (54), X-ray and neutron reflectivity (55,56), surface tension measurements (57), X-ray evanescent wave-induced fluorescence (58), and Langmuir trough techniques (55). Neutron reflectivity measurements indicate that in the... 

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