Water vapor adsorption isotherm

The surface of silica turns hydrophobic on treatment with organo-silicon chlorides. Water vapor adsorption isotherms measured by Stober (219) showed a very marked decrease in reversible adsorption. Less than 0.3 primary adsorption centers per 100 A remained in the surface after covering with the organosiloxane layer. Similar effects were observed in the adsorption of ammonia. About 2.2 silanol groups per 100 A had not reacted with the trimethylsilyl chloride. Nevertheless, the greater part of these had become unaccessible for water vapor. Apparently, they were hidden underneath a trimethylsilyl umbrella. ... 

To test this theory, the authors measured water vapor adsorption isotherms for RAMEB-treated soils. The amount of water adsorbed (kg water/kg soil) was monitored as a function of the partial pressure of water (p/po), the dose of RAMEB in the soil (0, 1, or 9%), and the type of soil. Seven soils were studied and arranged in order of increasing clay content (3, 8, 11, 16, 25, 36, and 49% clay content for SI, S2, S3,... S7, respectively). 

Figure 1. Water vapor adsorption isotherms for RAMEB-treated soils. Soil symbols are as in Table 1. The number in parentheses following the soil symbol is the dose of RAMEB (%). 

Figure 3 shows the water-vapor adsorption isotherms measured on the silane-treated fibers along with those obtained on the untreated fibers. A complete discussion of the water adsorption isotherms of the untreated fibers has already been reported [8]. Here, two new features are immediately evident. First, the presence of the silane overlayer has greatly enhanced the water adsorption capacity of the fibers, and, second, the silane-treated fibers that contain 4% and 6% B,0, adsorb significantly more water than the 0% B20, fibers. It is important to note that these data have been normalized to the specific surface areas of the... 

Figure 3. Water-vapor adsorption isotherms for glass fibers after the silane treatment the isotherms for the untreated fibers with 0% and 6% B,0, are included. APS - 1% solution of y-aminopropyl-silane at pH 10 0%—O,4% Oand6% B.

Figure 5 summarizes the low pressure end of water vapor adsorption isotherms obtained on quartz A. Bound water loss, above the surface condition resulting from 400° C. outgassing in vacuum, is indicated by displacement of the adsorption origin to reflect total water content (bound water plus adsorbed water) as a function of water vapor relative pressure. Vapor desorption curves show that... 

Water vapor adsorption isotherms have been obtained on cotton from room temperature up to 150°C [303,304]. Theoretical models for explaining the water vapor sorption isotherms of cellulose have been reviewed [303]. Only adsorption theories will be discussed here at ambient temperatures. The shape of the isotherm indicates that multilayer adsorption occurs and thus the Brunauer, Emmett and Teller (BET) or the Guggenheim, Anderson and deBoer (GAB) theory can be applied. In fact, the BET equation can only be applied at relative vapor pressures (RVPs) below 0.5 and after modification up to a RVP of 0.8 [305]. The GAB equation, which was not discussed in the chapter in the book Cellulose Chemistry and Its Applications [303], can be applied up to RVPs above 0.9 [306]. Initially as the RVP... 

Figure 3. Comparison of nitrogen, benzene and water vapor adsorption isotherms on PHD and MEA resins.

Figure 10. Water vapor adsorption isotherms as a function of % RH for A, collagen B, RSC C, silk D, nylon. Data from Refs. 18, 51, and 62.

The surface chemical properties of the carbon materials were characterized as follows measurement of pH of carbon slurries (in 0.1 M NaCl solution) [89] neutralization with bases of different strength and dilute HCl according to Boehm s method [63,66] determination of total oxygen/nitrogen content by elemental analysis (with an accuracy of 0.2%) [170] mass loss of carbon samples after heat treatment in a vacuum. Additionally, the number of primary adsorption centers (a,)) was determined from water vapor adsorption isotherms according to the Dubinin-Serpinsky method [171], as was the heat of immersion in water for selected samples [111,172]. The results of these operations are pre.sented in Table 3. For all samples transmission Fourier Transform Infrared (FTIR) spectra and X-ray photoelectron spectra (XPS) were recorded. 

Figure 13.1 Water vapor adsorption isotherms on diamond powder (20m /g). (1) Treated with H2 at 800 °C (measured at 17.8 °C) (2) outgassed in vacuo at 900°C (18.3 °C) (3) oxidized with O2 at 420 °C (19.8 °C). (Reprinted from Ref. [8] with permission from Elsevier.)...

Molecular engineering of an activated carbon surface provides a very interesting opportunity to beneficially alter the carbon s core adsorptive characteristics for gas separation. Polar groups can be introduced to the surface of a weakly polar carbon by judicious surface oxidation. Thus, a hydrophobic carbon can be converted to a hydrophilic adsorbent. Figure 22.9(a) shows the water vapor adsorption isotherms at 297 K (specific amount of water adsorbed tj as a function of relative water vapor pressure x) on the original Ceca carbon (Type V... 

Figure 3. Water vapor adsorption isotherms at room temperature of the basic hydrophilic silica (top) and the hydrophobic grade produced with this material...

Surface fractal dimensions of a number of Cambisols and Luvisols were determined using the FHH equation from data obtained from N2 and water vapor adsorption isotherms. Values were compared with those obtained from the mercury intmsion method and with mass fractal dimensions that were evaluated from particle-size distributions using a modified number-based method [108] (Figure 6.3). This method was proposed by Kozak et al. [116] in order to correct some inconsistencies of previous approaches... 

FIGURE 1.28 Water vapor adsorption isotherm on sugar charcoal. (From Puri, B.R. and Bansal, R.C., Indian J. Chem., 5, 566, 1967. With permission.)... 

Adsorption of water vapors on charcoals, active carbons, graphitized carbon blacks, and several types of dehydrated oxide catalysts is generally Type III or Type V. This is due to the fact that the dispersion component of the interaction energy is usually small compared to the polar contribution. Bansal et al. observed that the water vapor adsorption isotherm on a PVDC charcoal was Type IV, but when the carbon... 

FIGURE 2.21 Water vapor adsorption isotherms on polyvinyledene chloride (PVDC) and urea formaldehyde resin (UF) carbons before and after out gassing. (Source Bansal, R.C., Dhami, T.L., and Prakash, S., Carbon, 16, 389, 1978. Reproduced with permission from Elsevier.)... 

Similar results were also obtained from the analysis of CeHe adsorption isotherms. In a comparative analysis of water vapor adsorption isotherms, the adsorption isotherm referred to the active surface (as suggested in [19]) was used as a standard reference. Comparative linear plots which may be extrapolated to the origin of the coordinates were obtained, their slopes being used to find the Ah 20 values(Table 4) these values may be identified as a surface built by fragments of "prism" faces emd covered by functional oxygen-containing species. 

This idealized situation, of course, does not occur in actual silica gels where the pores consist of the irregular spaces between the randoiftly arranged ultimate spherical particles. As practical examples. Figure 5.10 shows water vapor adsorption isotherms and the pore size distribution curves for a series of typical silica gels, each of a relatively uniform pore size, as discussed by Kiselev (146). Pore volumes were... 

The data from [131, 132] presented in Figs. III.2 and III.3 agree with the results obtained by Luzhnov [133] when studying the force of static friction for Ukhtinsk channel-black powder and the water-vapor adsorption isotherms for the same materials (Fig. ni,4). 

Shape of water vapor adsorption isotherms and pore network... 

The derivation of surface areas from water-vapor adsorption isotherms on the same samples, however, does not lead to constant or nearly constant results for the areas of the two suites of kaolinite in different ion forms. The large variation that is observed is adequately expressed in the data in the last column of Table 1 for the apparent cross-sectional areas of a water molecule at monolayer coverage. This apparent area is derived from the monolayer coverage for water and the nitrogen area. The apparent areas deviate considerably from the accepted value for a monolayer of water on a homogeneous surface, about 10.8 A. Both... 

These data for nonexpanding clays show that the value of surface area determinations by means of water-vapor adsorption isotherms is rather limited because of the considerable effect of the ions on the surfaces on the average area per molecule of water in monolayer coverage. 

Therefore, water-vapor adsorption isotherms only supply a very rough measure of total... 

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