Adsorption vapor

Thus D(r) is given by the slope of the V versus P plot. The same distribution function can be calculated from an analysis of vapor adsorption data showing hysteresis due to capillary condensation (see Section XVII-16). Joyner and co-woikers [38] found that the two methods gave very similar results in the case of charcoal, as illustrated in Fig. XVI-2. See Refs. 36 and 39 for more recent such comparisons. There can be some question as to what the local contact angle is [31,40] an error here would shift the distribution curve. 

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. 

Characterization. When siHca gel is used as an adsorbent, the pore stmcture determines the gel adsorption capacity. Pores are characterized by specific surface area, specific pore volume (total volume of pores per gram of solid), average pore diameter, pore size distribution, and the degree to which entrance to larger pores is restricted by smaller pores. These parameters are derived from measuring vapor adsorption isotherms, mercury intmsion, low angle x-ray scattering, electron microscopy, gas permeabiHty, ion or molecule exclusion, or the volume of imbibed Hquid (1). 

Langmuir equations The mathematical expressions that describe vapor adsorption equilibria. 

Loading was accomplished by exposing the activated zeolites to benzene-de vapors. The extent of vapor adsorption was determined by the increase in weight of the zeolite. The samples were found to be extremely hydroscopic and thus kept in a vacuum desiccator until their use. After two or three temperature runs the samples began to adsorb water vapor. Therefore, only the data obtained for the first two variable temperature cycles are presented here. The samples studied are listed in Table I. 

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).

III. MODELS DESCRIBING VAPOR ADSORPTION A. Brunauer, Emmett, and Teller Equation... 

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.

As an example of composite core/shell submicron particles, we made colloidal spheres with a polystyrene core and a silica shell. The polar vapors preferentially affect the silica shell of the composite nanospheres by sorbing into the mesoscale pores of the shell surface. This vapor sorption follows two mechanisms physical adsorption and capillary condensation of condensable vapors17. Similar vapor adsorption mechanisms have been observed in porous silicon20 and colloidal crystal films fabricated from silica submicron particles32, however, with lack of selectivity in vapor response. The nonpolar vapors preferentially affect the properties of the polystyrene core. Sorption of vapors of good solvents for a glassy polymer leads to the increase in polymer free volume and polymer plasticization32. 

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. 

The vapor adsorption of Mo(CO),5 in an EMT zeolite followed by nitration with ammonia under a thermal treatment proved to be an appropriate method to introduce molybdenum oxynitrides into the zeolite, preserving the zeolite crystallinity and giving a homogeneous distribution of the molybdenum species in the zeolite [35]. 

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 (%). 

Equation 10.27 is generally known as Freundlich equation. Equation 10.27 with concentration replaced by pressure was also used to describe the adsorption isotherms of gases on solids, suggesting the incorrect idea that adsorption from solution by a solid could be paralleled with gas or vapor adsorption on the same adsorbents. Whereas in some cases the restriction to dilute solutions was imposed by the solubility of solids (e.g., benzoic acid in water or stearic acid in benzene) it was not imposed on the investigation of mixtures of completely miscible liquids, e.g., acetic acid in water. 

The data which are plotted as isotherms in the case of adsorption from liquid solutions on solid adsorbents are different in nature from those of gas (or vapor) adsorption on the same adsorbents. In fact, while the isotherm for adsorption of a single gas by a solid represents directly the quantity (weight or volume under standard conditions) of gas adsorbed per unit weight of the solid, the experimental measurement in adsorption from solution is the change in concentration of the solution which results from adsorption. The fact that a change in concentration is measured emphasizes that there are at least two components in the solution [13]. 

McDow, S. R., and J. J. Huntzicker, Vapor Adsorption Artifact in the Sampling of Organic Aerosol—Face Velocity Effects, Atmos. Environ., 24A, 2563-2571 (1990). 

An advantage of isotherms constructed from mercury intrusion-extrusion curves is the capability of extending the isotherm well beyond the limits of vapor adsorption-desorption isotherms. Intruded and extruded volumes can be measured for pores of several hundred micrometers in diameter at pressures below 1 psia. 

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