Adsorption of vapors

In the second picture, an interfacial layer or region persists over several molecular diameters due to a more slowly decaying interaction potential with the solid (note Section X-7C). This situation would then be more like the physical adsorption of vapors (see Chapter XVII), which become multilayer near the saturation vapor pressure (e.g.. Fig. X-15). Adsorption from solution, from this point of view, corresponds to a partition between bulk and interfacial phases here the Polanyi potential concept may be used (see Sections X-7C, XI-1 A, and XVII-7). 

The concluding chapters, Chapters XVI through XVIII, take up the important subjects of physical and chemical adsorption of vapors and gases, and heterogeneous catalysis. As with the earlier chapters, the approach is relatively quantitative and problem assignments regain importance. 

Also, many of the stnictural and spectroscopic techniques are now being applied to new types of systems, such as those involving the physical adsorption of vapors. Such bridging of methodologies will surely help to keep surface chemistry a single, broad field with good intercommunication between the various subareas. 

The surface film or sprea ding pressure, FI, is used to account for the change in gas—soHd interaction caused by adsorption of vapor evaporated from the Hquid. A Hquid is called wetting if the contact angle from soHd to Hquid through gas, 65 0 nonwetting if > 90°. Because it is easier to... 

The actual adsorption of vapor molecules takes place mainly on the surface of internal passages within the adsorbent particles, since that is where most of the available surface exists. The adsorption process may be either physical or chemical in nature. Physical adsorption is a readily reversible process that occurs as a result of the physical attraction between the gas molecules and the molecules of the solid surface. If the gas-solid intermolecular attraction is greater than the intermolecular attractions in the gas phase, the gas will condense on the solid surface, even though its pressure is lower than its vapor pressure at the prevailing temperature. For example, the equilibrium adsorption pressure of acetone on activated carbon may, under some conditions, be as little as 150 to 1,100 of the equilibrium vapor pressure at... 

An additional comphcating factor with respect to air samphng rate calibration is the uncertainty associated with determinations of vapor-phase and particle sorbed concentrations of analytes by HiVol sampling. These systems suffer from artifacts such as the volatilization of particle-bound contaminants, insufficient retention of small particles, and adsorption of vapor-phase contaminants on the GFFs (Ockenden et al., 1998 Lohman et al., 2001). These artifacts may cause concentrations in the vapor-phase to be overestimated or underestimated, which results in sampling rates that are too low or too high. 

Incomplete methylation of silica gel with diazomethane in etheric solution was also observed by Shcherbakova (234) and Shcherbakova and Slovetskaya (235). The reaction product showed decreased adsorption capacity for methanol vapor. The influence of esterification of silica surfaces on the adsorption of vapors was also studied by Slinyakova and Neimark (236), Ganichenko et al. (237), and Aristov et al. (238). 

It should be noted that, in the above equations, the effects of adsorption of vapor or gas on the solid surfaces are completely neglected. The amount of adsorption can be quite large, and may approach or exceed the point of monolayer formation at saturation. The spreading pressure, n, which is the amount of the reduction in surface energy on the solid surface due to the adsorption of vapor in equilibrium, is given by (Adamson, 1982)... 

Adsorption of Vapors on Mercury (Standard State ic = 0.0608 dynes/cm.)... 

Spreading Pressure Values, v>,, for the Adsorption of Vapors on Analaae and Graphite ... 

Utilization of coal and oil shale to produce liquid and gaseous synfuels results in the generation of many hazardous sub-tances. Workers in these synfuel plants are likely to be exposed to potentially carcinogenic materials present in coal tars and oils. Among the various pathways of exposure, skin contamination by direct contact transfer or by adsorption of vapors and particulates into the skin presents a serious occupational health hazard. The skin irritant and potential carcinogenic properties of raw syncrudes and their distillate fractions have been reported (1. 2, 3). 

Expls prepd by treating porous AN grains with vapors of volatile organic substances. Immediately after the adsorption of vapors, the vapors are sealed by a coating with small amts of waxes)... 

Several hundred different vapor sources have been considered, with almost 200 having been investigated in the laboratory. Avenues of approach have included the use of dispropor-tionating salts, the direct adsorption of vapor taggants into the elastomeric plug material of detonators, and the microencapsulation of taggant materials ... 

In recent years the understanding of the adsorption of vapors has significantly progressed. This is mainly due to the fact that more materials with defined pore sizes can be made [395,... 

Fig. 30 CTL glow curves from the catalyst heated at a rate of 0.5 °C/s in synthetic air after adsorption of vapor, a Adsorption of 100 ppm ethanol in air for 1, 2, 5, and 10 min. b Adsorption of 40, 60, and 80 ppm acetone for 2 min...

As pressure increases, new islands form, and some of them flow together until the whole surface becomes covered with adsorbed liquid when the vapor pressure is reached. Using this model, Polanyi and Goldmann concluded that adsorption of vapors on charcoal was not unimolecular, although the data obeys the Langmuir equation. Goldmann and Polanyi (1928), in ref. 23. See Brunauer, The Adsorption of Gases and Vapours, 116-119. 

The adsorption of vapors in complex porous systems takes place approximately as follows [1-3] at first, micropore filling occurs, where the adsorption behavior is dominated nearly completely by the interactions of the adsorbate and the pore wall after this, at higher pressures, external surface coverage occurs, consisting of monolayer and multilayer adsorption on the walls of mesopores and open macropores, and, at last, capillary condensation occurs in the mesopores. 

As pointed out above, Affi for the process studied here is the net heat of adsorption. AH2 is then a net differential heat of adsorption. AHJ is the same as it would be for the adsorption process involving adsorption of vapor instead of liquid. 

M. Dubinin (Academy of Sciences of the USSR, Moscow, USSR) The survey paper considers the fundamental problems of adsorption of vapors on zeolites within the framework of a general theory of molecular adsorption on nonporous and porous adsorbents. Among other things, it quotes examples of description of adsorption equilibria on zeolites over temperature ranges not exceeding 50° for initial and intermediate regions of isotherms by an exponential series with virial coefficients taking into account both adsorbent—adsorbate and adsorbate—adsorbate interactions. It is assumed that these equations can be used for practical calculations of adsorption equilibria. 

Equation 5 at n = 2 describes, to a good approximation, the adsorption of vapors on active carbons, for instance, the adsorption of benzene with a variation of the characteristic energy 2 from 3000 to 6000 cal/ mole. However, for active carbons with the finest micropores, when 2 of benzene substantially exceeds 6000 cal/mole—for example, for active carbon obtained from polyvinylidene chloride ( 2 = 7240 cal/mole)— Equation 5 with n = 2 is applicable only to 0.5 with the effective... 

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