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Raman silanols

Several properties of the filler are important to the compounder (279). Properties that are frequently reported by fumed sihca manufacturers include the acidity of the fiUer, nitrogen adsorption, oil absorption, and particle size distribution (280,281). The adsorption techniques provide a measure of the surface area of the filler, whereas oil absorption is an indication of the stmcture of the filler (282). Measurement of the silanol concentration is critical, and some techniques that are commonly used in the industry to estimate this parameter are the methyl red absorption and methanol wettabihty (273,274,277) tests. Other techniques include various spectroscopies, such as diffuse reflectance infrared spectroscopy (drift), inverse gas chromatography (igc), photoacoustic it, nmr, Raman, and surface forces apparatus (277,283—290). [Pg.49]

During the past decade, the main advances in the spectroscopic characterization of silica powders have come from 29Si NMR, Raman diffusion, and Fourier transform infrared (FTTR) studies. NMR studies have given two types of results. First, single and geminal silanols have been quantitatively differentiated as a function of dehydroxylation by thermal treatment and subsequent rehydroxylation by liquid water (7). The fraction of geminal silanols... [Pg.198]

Surface of a Fumed Silica. Several results obtained for silica A, as received and after some contact with air, can be rationalized in the following way. The low silanol surface density (about 3.65 OH per square nanometer, internal silanols excluded), the comparatively high fraction of geminal sites (/g = 0.21), and the presence of a rather strong D2 band in the Raman spectrum indicate an only partial and selective hydrolysis of the surface after the manufacturing of silica A at high temperature. [Pg.214]

Reaction 10 would characterize vicinal silanols Se. It is in agreement both with the Raman spectrum, because band D2 is not enhanced, and with the infrared components simultaneously observed at 908 and 888 cm-1. The resulting two-fold ring, which would be located at edges, is known to be very reactive (3). [Pg.216]

A tentative assignment of the main features of the infrared spectrum of silica A is summarized as follows 3500 cm-1, mainly SP 3620 cm-1, closest Sm in triplet sites i-j-k 3670 cm-1, internal silanols 3680 cm-1, weak proton donors Sm, Se, and Ge in pairs 3715 cm-1, terminal Gp 3736-3742 cm-1, weakly perturbed Gp, Sm, Se, and Ge 3747 cm-1, isolated Sm, Se, and single silanols ex-GP and ex-Ge. This assignment gives a possible explanation of the Raman components appearing at about 3685 and 3615 cm-1 by rehydroxylation of a silica gel pretreated at 600 °C (14). [Pg.216]

Sites Sp-Gp achieve a somewhat concave step, accommodating what has been called inner silanols (22). The present assignment gives an improved picture of the surface and of the hydration mechanism of a fumed silica. However, the infrared spectrum is intricate, and its assignment is still partly speculative. Whereas the comparison of the infrared and Raman spectra supports the conclusion that the formation of three-fold rings is mainly due to the condensation of weakly perturbed silanols (that absorb above about 3600 cm-1), the reciprocity is not warranted. For instance, the contribution of isolated silanols (i/OH = 3750 cm-1), postulated by Brinker et al (16), is excluded. In fact, a further analysis of both spectra will be necessary to know their relation to the various sites of a silica. [Pg.216]

Raman experiments on sllicalite and TS-1 with excitation wavelengths of 1064 nm (non resonant) and 244 nm (resonant) show that (i) the main features associated with Ti insertion in the lattice are vibrations at 1125 and 960 cm" the former being drastically enhanced by UV-resonance, while the latter is not (ii) a mode is observed at 978 cm" on defective silicalites and TS-1, which we attribute to the Si-0 stretching in silanols. The proximity of the 960 and 978 cm" modes has prompted us to re-examine IR spectroscopy in the same region in order to distinguish the 960 cm band from defect modes. [Pg.206]

The vOH absorbance profile of fully dehydrated silica A, G, P is shown in Figure 26.4. A narrow absorption at 3747 cm dominates the spectrum of silica A and characterizes isolated silanols on surfaces that are not completely hydroxylated. This last statement is also based on the Raman spectrum discussed later. About 9% of the silanols are internal, inaccessible to D2O molecules at room temperature (vOH = 3670 cm Figure 26.4). The weakness of the components at 3715 and 3530 cm is also characteristic of fumed silica samples [3]. [Pg.299]


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See also in sourсe #XX -- [ Pg.169 ]




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