Silanol density, measurement

From silanol densities measured between 4.2 and 5.7 OH per square nanometer, Zhuravlev (39) found a mean value of 4.9 OH per square nanometer at 180-200 °C under vacuum for 100 samples of different, fully... 

From silanol densities measured between 4.2 and 5.7 OH per nm, Zhuravlev [47] found a mean value of 4.9 OH per nm at 180-200°C under vacuum for 100 samples of different, fully hydroxylated, amorphous silicas. Thermogravimetry of silica A showed that the silanol density that must be compared to these values, in the same conditions, is about 3.3 OH per square nanometer. As received, silica A is thus not completely hydroxylated. Other results corroborate this conclusion the D2 Raman band decreases under a further exposure to water vapor (Figure 26.8, curve a, 40), and the meth-oxyl density of 4.9 per nm observed after reaction with methanol at 460°C is larger than the initial OH density (Table 26.1). 

Analogous to the electrical conditioning, the characteristics can be improved by a thermal treatment prior to the deposition of the source/drain metallization. On the basis of XPS measurements it could be shown that a heat treatment of the SiOa/Ca double layer results in a promoted interface reaction reducing the interfacial silanol density and the metallic fraction in the Ca overlayer. The former results in a reduction of the electron trap density and the latter in a reduction of screening effects by the metallic Ca. Since a cyclic electrical stress of an already annealed transistor does not yield a significant additional improvement in device performance, it was concluded that the observed OFET improvement for both device treatments, have the same origin. This implies that the oxidation of the metallic fraction in the Ca overlayer is promoted by the electrical cyclic stress as well as by the heat treatment. 

Very few direct measurements of the reaction of surface silanol groups on quartz have been reported. This is apparently caused by the small effects due to the limited surface areas available. Adsorption of sodium ions on quartz was measured by radioactive tracer techniques by Gaudin et al. (293). Saturation was achieved at high pH (>10) and sodium ion concentrations above 0.07 Jlf. The calculated packing density of silanol groups was 4.25/100 A. Goates and Anderson (294) titrated quartz with aqueous sodium hydroxide and alcoholic sodium ethylate. The occurrence of two types of acidic groups was reported. 

Many attempts have been made to measure silanol surface density (aon)-Her [7] estimated Ooh to be equal to 8 groups/nm on the basis of the [100] face of P-cristobalite. However, most porous amorphous silicas show surface silanol concentration on the level of 4.6 to 5 groups/nm [6]. 

Pioneers in measuring polymer bound fractions by IR were Fontana and Thomas 1), who studied poly(alkyl methacrylates) in n-dodecane on silica. Upon adsorption, part of the carbonyl stretching bemd (originally at 1736 cm ) shifted to a lower frequency (1714 cm ) due to formation of bound carbonyls. Fontana and Thomas also observed shifts in the vibration of silanol groups from the substrate and used these to obtain the train density. The IR technique has been exploited by several authors using different poljrmer systems, mostly with non-aqueous media and silica or tltanla as the substrates ll. The ATR approach has been used in several more recent studies ). 

A better solution is to couple the method with mass spectrometry. Here, the signal for water molecules is clearly separated from the combustion products, leading to a better quantification. Measurement of the silanol group density by deuterium exchange with CF3COOD followed by H NMR spectroscopy is a chemical method combined with a spectroscopic measurement and gives reliable values. 

One hundred fully hydroxylated silica samples prepared by different processes and having different structural characteristics such as specific surface area, type or size distribution of the pores, particle packing density, and structure of the underlying silica skeleton were analyzed two or more times each. Eighty-five percent of the 231 measurements were from samples with a specific surface area less than 400 m2/g. The OH groups in ultramicropores, into which only water molecules can penetrate, were not classified as surface silanol groups but as structurally bound water. 

From models of medium-pore and wide-pore zeolite structures, we estimate that the density of hydroxyl groups on the external surfaces of zeolite crystals is one hydroxyl group per 35( 2) A. The number of hydroxyl groups measured by IRS for the ZSM-5 sample used in this study was 2.20 X lO" mol/g (sum of silanol groups plus twice the number of acidic OH groups). Using these values, the external surface of the studied sample was estimated as follows ... 

This latter observation is clearly highlighted when plotting the TMS surface densities, calculated using both the specific surface areas calculated from nitrogen and CTAB adsorption measurements, versus the surface density of residual silanol groups as depicted on Fig. 1. 

The density of silanol groups was measured by means of an acid-base titration using a solution of NaOH in a water/methanol mixture (50 50 % w/w), following a procedure described by Sears [20]. It should be noted that only silanol groups on the silica surface are measured with this method. 

Specific surface area Owing to the constant density of the silanol groups, the specific area is a direct indicator of the adsorption capacity of a silica gel in chromatogaphy (Section II.A.2). The specific surface area Sbet of silica gel in thin-layer chromatography ranges from 200 to 800 m /g. A possible method of determination of Sbet >s based on the measurement of nitrogen adsorption isotherms (9). 

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