Probes of layer silicate surfaces

MCBRIDE Paramagnetic Probes of Layer Silicate Surfaces... 

McBride, M. B. (1986). Paramagnetic probes of layer silicate surfaces. In Geochemical Processes at Mineral Surfaces, Davis, J. A, and Hayes, K. F., eds., American Chemical Society Washington, D.C., pp. 362-388. 

Electron spin resonance (ESR) is a useful technique for investigating the mobility and orientation of exchange cations at the surface of layer silicate clays in various states of hydration. Using Cu2+ and the charged nitroxide spin probe, TEMPAMINE+... 

In a study of dental silicate cements, Kent, Fletcher Wilson (1970) used electron probe analysis to study the fully set material. Their method of sample preparation varied slightly from the general one described above, in that they embedded their set cement in epoxy resin, polished the surface to flatness, and then coated it with a 2-nm carbon layer to provide electrical conductivity. They analysed the various areas of the cement for calcium, silicon, aluminium and phosphorus, and found that the cement comprised a matrix containing phosphorus, aluminium and calcium, but not silicon. The aluminosilicate glass was assumed to develop into a gel which was relatively depleted in calcium. 

Atomic force microscopy (AFM) is another technique used to characterize nanocomposites.AFM can provide information about the mechanical properties of a surface at a length scale that is limited only by the dimensions of the AFM tip. AFM tips with 10 nm radius of curvature are readily available from commercial suppliers. When probing mechanical properties, the attractive and repulsive force interactions between the tip and sample are monitored. Schematic depicting the intercalation process between a polymer melt and an organic-modified layered silicate is shown on Figure 6.8. 

While the shape of silicate features in emission from protoplanetary disks is often used to argue for general particle growth, it should be kept in mind that they trace only a very limited part of the total particle population. Apart from the fact that the silicate features only probe silicate grains, particles with radii larger than 10-20 xm do not produce silicate features at all, and, in protoplanetary disks, this population may very well dominate by mass. The emission features are only produced in the innermost regions and the uppermost layers of the disk. For a solar-type star, the 10 p.m band is produced in the so-called surface layer of the disks within 1AU, although this radius increases with stellar luminosity (Kessler-Silacci et al. 2007). 

Nonchloride species are probably rapidly converted to oxides (Na20, K2O) on leaving the flame front. The volatile alkalies may condense on the surfaces of fly-ash particles carried by the flue gas or on cooler boiler surfaces. Wibberly and Wall [j ] performed drop-tube experiments in which silica particles were exposed to synthetic combustion gases containing sodium at temperatures of 1200 to 1600°C. Sodium silicate layers ranging in thickness from 0.03 to 0.3 pm were observed on the particle surfaces, and sintered deposits formed rapidly on stainless steel probes inserted into the lower part of the furnace. Such alkali-silicate layers are molten at the temperatures of interest. The thickness of the sodium silicate layers was decreased by a factor of three when the sodium was introduced in the form of NaCl, rather than in chlorine-free forms. 

To this end, loop-based injection is preferred and the sample aspiration tube (Fig. 6.9) behaves as a sampling probe, as demonstrated in the landmark work of Thomsen et al. who determined reactive silicate in coastal waters during a cruise from Monterey Bay to San Francisco Bay in the USA [3]. The sample was continuously aspirated from a water layer about 2 m below the sea surface and a situation of "infinite sample volume" was attained in the main channel of a shipboard reagent injection (reversed flow) flow injection system. A reagent aliquot was injected into the flowing sample every 45 s and the resulting coloured zone was quantified by spectrophotometry. The height of the recorded peak was directly related to the reactive silicate concentration at the specified location and depth. 

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