Water vapor, adsorption

Favor adsorption for processes that require essentially complete removal of water vapor (adsorptive dehydration is capable of achieving dew point depres >45° C (80°F) molecular sieves are favored adsorbents. 

The effect of physical aging on the crystallization state and water vapor sorption behavior of amorphous non-solvated trehalose was studied [91]. It was found that annealing the amorphous substance at temperatures below the glass transition temperature caused nucleation in the sample that served to decrease the onset temperature of crystallization upon subsequent heating. Physical aging caused a decrease in the rate and extent of water vapor adsorption at low relative humidities, but water sorption could serve to remove the effects of physical aging due to a volume expansion that took place in conjunction with the adsorption process. 

Figure 2. FTIR spectra of water vapor adsorption under increased pressures (a) 2.25 kPa, (b) 4.20 kPa and (c) 7.55 kPa at 100°C on H-ZSM-5 zeolite and spectrum of the activated zeolite at 450°C (d).

Fig. 1 Water vapor adsorption and deliquescence of a water-soluble solid (a) Atmospheric relative humidity, RHj < RHq (b) RH = RH0 and (c) RHj > RH0.

Dogadkin, Skorodumova, and Kovaleva (127) studied the reaction of carbon black with sulfur at low temperatures. A solution in toluene was used at 145° in the presence of an accelerator. The sulfur sorption was negatively influenced by surface oxides. The oxygen-containing groups were not affected by the reaction, since no change in the water vapor adsorption was detected. No hydrogen sulfide was evolved under the reaction conditions. 

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. ... 

The surface chemistry of coesite and stishovite was studied by Stiiber (296). The packing density of hydroxyl groups was estimated from the water vapor adsorption. More adsorption sites per unit surface area were found with silica of higher density. Stishovite is especially interesting since it is not attacked by hydrofluoric acid. Coesite is dissolved slowly. The resistance of stishovite is ascribed to the fact that silicon already has a coordination number of six. Dissolution of silica to HaSiFg by hydrogen fluoride is a nucleophilic attack. It is not possible when the coordination sphere of silicon is filled completely. In contrast, stishovite dissolves with an appreciable rate in water buffered to pH 8.2. The surface chemistry of. stishovite should be similar to that of its analog, rutile. 

A packing density of 6.6 Ti + ions per 100 A was estimated on a theoretical basis by Hollabaugh and Chessick (301). From the values of irreversible and reversible water vapor adsorption, a surface density of 3.7 OH/100 A was calculated for the substance activated at 450° and of 11.4 OH/100 A for a fully hydroxylated rutile surface. 

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). 

Effect of RAMEB on Water Vapor Adsorption on Soils. Experimental adsorption isotherms for the RAMEB-treated soils are presented in Figure 1 [see p. 127]. As pure RAMEB sorbs a very high amount of water (ca. 1 g g" at p/po = 0.99), an increase in water sorption was expected after RAMEB addition to all soils. However, the isotherms for RAMEB-treated clay-rich S6 and S7 soils showed lower adsorption than the original soils, which is illustrated for S7 soil with 49% clay. This potentially indicates that RAMEB decreases the amount of water-available surfaces in clay-rich soils, similar to what was observed for pure clay minerals (20). In sandy soils (S1-S4), the water sorption markedly increased, particularly at higher RAMEB doses, as is illustrated for S2 soil. 

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 (%). 

Dubinin MM (1980) Water vapor adsorption and microporous structures of carbonaceous adsorbents. Carbon 18 355-364... 

Another special application of adsorption in space is presented by Grover et al. (1998). The University of Washington has designed an in situ resource utilization system to provide water to the life-support system in the laboratory module of the NASA Mars Reference Mission, a piloted mission to Mars. In this system, the Water Vapor Adsorption Reactor (WAVAR) extracts water vapor from the Martian atmosphere by adsorption in a bed of type 3A zeolite molecular1 sieve. Using ambient winds and fan power to move atmosphere, the WAVAR adsorbs the water vapor until the zeolite 3A bed is nearly saturated, and then heats the bed within a sealed chamber by microwave radiation to drive off water for collection. Tire water vapor flows to a condenser where it freezes and is later liquefied for use in tire life-support system. 

Teng, C.D., Zarrintan, M.H., and Groves, M.J. (1991). Water vapor adsorption and desorption isotherms of biologically active proteins. Pharm. Res., 8(2), 191-195. 

We have studied the effect of chemical modification on adsorption properties of natural clinoptilolite (18). Studies of water vapor adsorption show a decrease in adsorption for Dzegvi clinoptilolite, decationized and dealuminized on the water bath, with increased acid concentration, compared with the adsorption of the natural clinoptilolite. The main contribution to adsorption is from primary porosity. 

Pierce and Smith (56) have observed a Type III isotherm for water vapor on a highly graphitized carbon black (graphon, 80 square meters per gram) at 28.9° C. They suggest that water vapor adsorption occurs only on the most active sites and that the entire surface is probably never covered. 

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.

BX in the glass Geometric area (mJ/g) Kr specific area (m /g) Water-vapor adsorption ... 

Figure 1. Water vapor adsorption kinetics of ammonia-treated ramie cellulose ( first exposure to water vapor after drying from ammonia O repeated exposures thereafter to water vapor after drying from 6% moisture content) (36)...

FIGURE 2.21 Changes in weight, volume, BET surface area, and water vapor adsorption during the carbonization of poly(furfuryl alcohol) to form glass-like carbon. (Courtesy of Dr. S. Yamada, Tokai Carbon Co. Ltd., Shizuoka, Japan. With permission.)... 

Type V isotherms are related to the type III isotherms and are also characteristic of weak adsorbate-adsorbent interactions (i.e., water vapor adsorption on charcoal). 

IV sites. A protonated pyridine has never been observed, and the Lewis acid sites on titanium oxides cannot be converted into Br0nsted sites by water vapor adsorption (217). Although Jones and Hockey (216) suggest that the chemistry of surface hydroxyl on rutile corresponds more closely to that of the OH" ion rather than that of the hydroxyl group, no surface reactions similar to that observed with alumina [Eq. (14)] have since been reported. 

Rates of water vapor adsorption from air by silica gel." Ind. Eng. Chem., 31 988-992. 

The effect of temperature on the activity of the fresh catalysts was first studied. For the Hopcalite catalyst with feed A there was complete conversion above 300 °C. However, significant loss of activity was observed at lower temperatures, though activity could be restored by heating the catalyst above 300 °C. This is probably due to water vapor adsorption, given the known sensitivity of other Hopcalite formulations to water. There appears to be almost an on off switch for this material at 300 °C, but the long-term... 

I believe that this is a fine confirmation of the correctness of the Vm values obtained by water vapor adsorption. On the other hand, nitrogen adsorption gave surface areas that ranged from 20 to 100% of the surface areas obtained by water vapor adsorption. I said that nitrogen did not measure the total surface, and I give one evidence for it now and one later. Dr. Copeland determined the specific surface of a hydrated portland cement by small-angle x-ray scattering. The value he obtained checked the water adsorption value within 2 or 3%. On the other... 

Early experiments were concerned with the protection against water-vapor adsorption afforded a pair of soft steel electrodes by coating each with FEP Teflon resin. The two coated electrodes were cleaned as before, mounted in the gas cell, and then discharged with gaseous ions while the metal pins were grounded. The resulting contact potential difference was zero. The same result was obtained using a pair of resin-coated aluminum electrodes. 

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