Surface structure of amorphous and

A survey of the physical and chemical techniques to characterize the surface structure of amorphous and crystalline silica is presented by Unger in this book (Chapter 8). Methods to measure particle size and particle size distribution and surface area are discussed by Kirkland (Chapter 18) and by Allen and Davies (69). The use of some of these techniques by Morrow et al. (Chapter 9), Burneau et al. (Chapter 10), Vidal and Papier (Chapter 12), Kohler et al. (42), and Legrand et al. (17) to provide new insights into the silica surface structure was already mentioned in the section Silica Surface in this chapter. 

Surface Structure of Amorphous and Crystalline Porous Silicas... 

Substantial progress in the elucidation of the surface structure of crystalline and amorphous silicas has been achieved by means of high-resolution spectroscopic techniques, for example, Si cross-polarization magic-angle spinning NMR spectroscopy and Fourier transform IR spectroscopy. The results lead to a better understanding of the acidity, dehydration properties, and adsorption behavior of the surface. These properties are key features in the design of novel advanced silica materials. The current methods of characterization are briefly reviewed and summarized. 

Porous silicas employed as packings in HPLC are amorphous i.e., they do not possess a long-range order of their bulk structure. Most information on the bulk and surface structure of silica and its bonded derivatives can be drawn from Si and C solid-state NMR spectroscopy [7],... 

Many studies have shown that the accessibility and reactivity depend on the structural parameters of cellulose-specific surface, degrees of amorphicity and crystallinity, as well as paracrystallinity parameters. So, the improvement of acetylation process of cellulose under effect of acetic acid is explained by the increase of specific surface of the porous system (Papkovetal., 1976]. As was established, non-crystalline domains of cellulose are accessible for water, while crystallites and their paracrystalline layers are inaccessible for this polar liquid (loelovich et al., 2009 loelovich et al., 2010]. 

More recently, simulation studies focused on surface melting [198] and on the molecular-scale growth kinetics and its anisotropy at ice-water interfaces [199-204]. Essmann and Geiger [202] compared the simulated structure of vapor-deposited amorphous ice with neutron scattering data and found that the simulated structure is between the structures of high and low density amorphous ice. Nada and Furukawa [204] observed different growth mechanisms for different surfaces, namely layer-by-layer growth kinetics for the basal face and what the authors call a collected-molecule process for the prismatic system. 

Figure 8 illustrates the transformations induced on the surface structure of HT materials after 10 days of corrosion. The amorphous gel layer that appears during corrosion is observed on most HT samples and reveals pits, holes, and corrosion paths from case to case. For many samples, crystalline secondary phases in the 1-10 pm range cover the altered surface, and spallation/cxfoliation of the gel layer is observed or at least suggested in several instances, indicating that corrosion is a discontinuous process, even under static conditions. 

Surface characterization includes also the study of the modification of a surface under cathodic load or after some pretreatments. The presence of residual surface oxides can explain some observations otherwise inexplicable. Activation in situ usually results in composite structures which are difficult to identify by X-ray, and may contain metallic and non-metallic components. Particularly crucial is the case of the surface structure of glassy metals or amorphous alloys. 

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