Hydroxyl group surface concentration

Surface adhesion of polyester was controlled by a combination of parameters such as molecular weight of polyester, concentration of hydroxyl groups, and concentration of fluorocarbon additive. To eliminate surface defects, the amount of additive should be increased, but its increase contributes to the loss of adhesion between basecoat and primer. 

Fig. 5.16 Surface concentration of hydroxyl groups of silica, as a function of the temperature of dehydration. Data are +, from Fripiat and Uytterhoeven A, from Kiselev and Zhuralev O, from Taylor (cf.

The relationship between the BET monolayer capacity of physically adsorbed water and the hydroxyl content of the surface of silica has been examined by Naono and his co-workers in a systematic study, following the earlier work by Morimoto. Samples of the starting material—a silica gel—were heated for 4 hours in vacuum at a succession of temperatures ranging from 25 to 1000°C, and the surface concentration of hydroxyl groups of each sample was obtained from the further loss on ignition at 1100°C combined with the BET-nitrogen area. Two complete water isotherms were determined at 20°C on each sample, and to ensure complete... 

In Table 5.3, is compared with the total hydroxyl concentration (Ni, + N ) of the corresponding fully hydroxylated, sample. The results clearly demonstrate that the physical adsorption is determined by the total hydroxyl content of the surface, showing the adsorption to be localized. It is useful to note that the BET monolayer capacity n JH2O) (= N ) of the water calculated from the water isotherm by the BET procedure corresponds to approximately 1 molecule of water per hydroxyl group, and so provides a convenient means of estimating the hydroxyl concentration on the surface. Since the adsorption is localized, n.(H20) does not, of course, denote a close-packed layer of water molecules. Indeed, the area occupied per molecule of water is determined by the structure of the silica, and is uJH2O) 20A ... 

The BET monolayer capacity N, calculated from the first water isotherm included both physisorbed and chemisorbed water, whereas that from the second isotherm iV, included only the physisorbed water. Thus the difference (iV, - N,) gave the amount of chemisorbed water taken up as hydroxyl groups during the isotherm determination. N, + iV ) was therefore the total concentration of hydroxyl groups on the surface when the second water isotherm was being measured. 

It should be noted that, due to the strong polarity of the hydroxyl groups on the silica, the initial adsorption of the ethyl acetate on the silica surface is extremely rapid. The individual isotherms for the two adsorbed layers of ethyl acetate are shown in Figure 8. The two curves, although similar in form, are quite different in magnitude. The first layer, which is very strongly held, is complete when the concentration of ethyl acetate is only about l%w/w. At concentrations in excess of l%w/w, the second layer is only just being formed. The formation of the second layer is much slower and the interactions between the solvent molecules with those already adsorbed on the surface are much weaker. 

Of all the aluminas, y-alumina is the most used alumina, due to its high surface area and the high concentration of surface hydroxyl groups. 1 1-alumina is u.sed in i.somerization becau.se it has a higher density of acid sites at the surface. 

The thromboresistance of heparinized films, as shown in Table VI, was enhanced by higher hydrogel content since the concentration of hydroxylic groups on modified PET film surface, able to react with glutaralde-hyde in order to bind heparin, was higher. 

The number of leaving alkyl groups seems to depend on the degree of hydroxylation of silica. When the silica is heated, water is driven off and the surface concentration of OH groups decreases. At 450°C siloxane bonds are also formed (Scheme 7.8). 

The stoichiometry of these reactions can be controlled by modulating the concentration of hydroxyl groups on the surface of silica. When starting with the tetra-alkyl complex, subsequent reaction with an alcohol R OH (Equation(4)) is necessary this generally occurs under conditions mild enough to maintain the anchoring bond SiO—M. 

An appreciable amount of Ti + may also exist in such samples. Indeed, materials treated at this temperature are gray, suggesting the presence of reduced forms of titania. Surface Type III Hydrogen reduction at 720 K probably produces a surface in which there is no molecular water and only a small number of hydroxyl groups. Furthermore, the surface following this treatment may have a high concentration of Ti + species. Materials treated in this manner were found to be pastel blue in color. The surface "type" of each sample is given in Table I and II. 

Zhuravlev, L.T. Concentration of hydroxyl groups on the surface of amorphous silicas. Langmuir 1987, 3, 316-318. 

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