Application to the Solid-Solution Interface

In practice, a convenient range of solid crystalline materials, which can be polished to optical flatness over manageable areas, exists and includes silicon, quartz, and sapphire. The equivalent transmission through a block of amorphous quartz or silicon is 10—15%. This has so far precluded them from use, and limited the application to crystalline substrates. In aqueous solution, the most commonly use contrasts are D20, H20, and water (H20/D20 mixture) index matched to the solid phase. In D20, the refractive index (or scattering length density) difference between that and the solid phase is significantly different for silicon, quartz, and sapphire (see Table 1). 

From silicon to sapphire, the difference in scattering length density or refractive index compared with D20 is progressively smaller, which makes the region of total reflection more difficult to access, but enhances the sensitivity of 

Scattering length density of commonly used solid substrates 

This is an important step as the surfaces are not always as well defined as the example shown in Fig. 2. Quartz is particularly troublesome, and thick gel layers often exist. For example, McDermott et al. [24] obtained a 85-A thick layer of inhomogeneous material, with a scattering length density intermediate between crystalline quartz and amorphous silicon, on the surface of crystalline quartz. On silicon 111) a more reproducible oxide layer, in thickness and density, is generally achieved. However, in the past some variability in the hydrophilic nature and hence reproducibility of adsorption has been experienced. This was discussed in detail by Penfold et al. [25], who found that the mild piranha treatment produced a reproducible hydrophilic surface. Other treatments have also been found to be reliable and are discussed in more detail elsewhere [26]. Another important aspect that was discussed by Penfold et al. [25] and that is evident from a number of related studies is that the history of the surface and hence the experimental route can be of vital importance. 

The nature of the adsorption of surfactants at both the hydrophilic and hydrophobic solid surfaces has been subject to extensive studies, and a number of excellent recent reviews exist [27-29]. The structure of the adsorbed layer, at both the hydrophilic and hydrophobic surfaces, has been the subject of much conjecture. From the form of the adsorption isotherm at the hydrophilic surface, the cooperative nature of the adsorption was established, and the evolution of the structure with concentration was inferred [30] (see Fig 3). 

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