Vapour adsorption

When an adsorbing surface is exposed to a gas or vapour adsorption will take place, being accompanied by the absorption or evolution of heat. Such thermal changes have already been noted in the extension and contraction of surface films of liquids. Although the direct determination of the surface energy of solid surfaces presents many experimental difficulties yet of its existence there is no doubt. On the adsorption of a gas or vapour a diminution in the free surface energy of the system likewise occurs. From the Gibbs-Helmholtz relationship dcr... 

Activated charcoal or carbon is widely used for vapour adsorption and in the removal of organic solutes from water. These materials are used in industrial processes to purify drinking water and swimming pool water, to de-colorize sugar solutions as well as other foods, and to... 

Laszlo K, Czakkel O, Dobos G, Lodewyckx P, Rochas C, Geissler E (2010) Water vapour adsorption in highly porous carbons as seen by small and wide angle X-ray scattering. Carbon 48 1038-1048... 

Ldszld K, Rochas C, Geissler E (2008) Water vapour adsorption and contrast-modified saxs in microporous polymer-based carbons of different surface chemistry Adsorption 14 447-455... 

Fig. 12.2. An increase of mass leads to an increase of the resonance frequency of elastic mechanic oscillator (left). Similar principle is used for detection of mercury vapour adsorption onto gold electrode of a quartz resonator (right).

Gawrys, M., Fastyn, P., Gawlowski, J., Gierczak, T. and Niedzielski, J. (2001) Prevention of water vapour adsorption by carbon molecular sieves in sampling humid gases. Journal of Chromatography A, 933, 107-16. 

Abstract. Activated carbon Norit R 08 Extra, and molecular sieve type 4A, were investigated using dynamic (tert-butylbenzene (TBB), cyclohexane (CHX) and water vapour) adsorption methods. The TBB, CHX and water breakthrough plots for fixed activated carbon - molecular sieve beds were analyzed. It was found that the type of bed composition with mechanically mixed activated carbon with molecular sieve, or separated activated carbon and molecular sieve layers, affects the dynamic adsorption characteristics. 

Two different adsorbents, activated carbon Norit R 0.8 Extra (Norit N.V., The Netherlands) and molecular sieve (type 4A, Merck), were used to study tert-butylbenzene, cyclohexane, and water vapour breakthrough dynamics. Structural parameters of the carbon adsorbent were calculated from benzene vapour adsorption-desorption isotherms measured gravimetrically at 293 K using a McBain-Bakr quartz microbalance, and nitrogen adsorption-desorption isotherms recorded at 77.4 K using a Micromeritics ASAP 2405N analyzer described in detail elsewhere.22,24 Activated carbon Norit has a cylindrical... 

A more general equation, derived by Everett (1979), is appropriate if the vapour pressure is high or the liquid-vapour adsorptive is close to its critical state. 

Immersion of samples partially pre-covered by vapour adsorption... 

It is evident that there is ample evidence to support the view that the initial stage of water vapour adsorption is dependent on the presence of a number of polar sites on the carbon surface. In the theory of Dubinin and Seipinski (1981), these sites are pictured as primary adsorption centres of uniformly high energy. Water molecules are first adsorbed on the primary centres in a 1 1 ratio and these molecules then act as secondary adsorption centres for the adsorption (by H-bonding) of other molecules. 

Immersion of various dry samples in the same liquid 227 Immersion of dry samples in liquids of different molecular size 228 Immersion of samples partially pre-covered by vapour adsorption 229... 

Matsunaga, M., Homma, T. and Tanaka, A., Investigation of Vapour Adsorption on Molybdenum Disulphide Surfaces by Auger Electron Spectroscopy, ASLE Trans., 25, 323, (1982). 

Over the years, vapour adsorption and condensation in porous materials continue to attract a great deal of attention because of (i) the fundamental physics of low-dimension systems due to confinement and (ii) the practical applications in the field of porous solids characterisation. Particularly, the specific surface area, as in the well-known BET model [I], is obtained from an adsorbed amount of fluid that is assumed to cover uniformly the pore wall of the porous material. From a more fundamental viewpoint, the interest in studying the thickness of the adsorbed film as a function of the pressure (i.e. t = f (P/Po) the so-called t-plot) is linked to the effort in describing the capillary condensation phenomenon i.e. the gas-Fadsorbed film to liquid transition of the confined fluid. Indeed, microscopic and mesoscopic approaches underline the importance of the stability of such a film on the thermodynamical equilibrium of the confined fluid [2-3], In simple pore geometry (slit or cylinder), numerous simulation works and theoretical studies (mainly Density Functional Theory) have shown that the (equilibrium) pressure for the gas/liquid phase transition in pores greater than 8 nm is correctly predicted by the Kelvin equation provided the pore radius Ro is replaced by the core radius of the gas phase i.e. (Ro -1) [4]. Thirty year ago, Saam and Cole [5] proposed that the capillary condensation transition is driven by the instability of the adsorbed film at the surface of an infinite... 

Fig. 4. Water vapour adsorption-desorption Fig. 5. a) N2 adsorption-desorption isotherms...

Table 3. Results of analysis of water vapour isotherms and of characterisation, before and after water vapour adsorption, by nitrogen adsorption at 77K and XRD. Vo.95(l), Vo.95(n)- volume adsorbed from uptake of water vapour at 0.95p° on first and last runs.

Fig. 6. Water vapour adsorption-desorption isotherms determined at 298K on modified MCM-41 sample (HX16-2T) O/ 1 isotherm,...

Because of their narrow microporosity, the textural characterisation of CMS is not easy. Gas and vapours adsorption, especially nitrogen adsorption at 77 K, may be kinetically hindered, and large equilibrium times are needed to obtain the adsorption isotherms. In this way, immersion calorimetry emerges as a powerful technique for the characterisation of pore size distributions in CMS. By choosing hquids with different molecular sizes, the micropore volumes can be obtained as a function of their size by applying the relationship obtained by Stoeckli and Kraehenbuehl [5] ... 

Direct pressure measurements may be made with conventional manometers using mercury, silicone oils, hydrocarbon oils, butyl phthalate and other fluids. Apart from the danger of their affecting the reaction studied directly, by chemical reaction, vapour adsorption or modifying the surface of the RV, they produce a varying dead space . It is important to keep the dead space between 1 and 4 %... 

J. Hagymassy, Jr., I. Odler, M. Yudenfreund, J. Skalny and S. Brunauer, Pore structure analysis by water vapour adsorption 111. Analysis of hydrated calcium silicates and Portland cement. /. Colloid Interface Sci., 38 (1972) 20. 

Table 3.4. Analysis of water vapour adsorption by wood (Acer saccharum and Picea sitchensis) using the BET equation (Stamm, 1964).

Figure 3. Isotherms of CCI4 vapour adsorption by the samples of the composition (% mass) ...

Empirical adsorption equilibrium models like Langmuir and Freundlich can generally be used to describe type I pure water vapour adsorption isotherms on aluminas [6] ... 

Figure 1 also shows the pure water vapour adsorption isotherms on UOP 13X zeolite and a sample of Davison silica gel at 30°C. These data were measured at Air Products laboratories. They are, respectively, type I and IV in shape. The zeolite adsorbs water very strongly (very high capacity at low x). The water adsorption capacities of the silica gel is comparable to those of the aluminas at low x but it exhibits higher water adsorption... 

Figure 4. Polanyi potential plot for water vapour adsorption at 30 and 70°C on Alcan AA-300 alumina.

Figure 7 shows an example of type IV isotherms for adsorption of trace water from toluene and p-xylene mixtures on Alcoa H-152 alumina at 22° C [22], The abcissa of the plot represents relative saturation of water (xi/x ), where x is the molar fraction of water at the solubility limit in the hydrocarbon liquid. Equilibrium isotherm models analogous to those used for pure water vapour adsorption can be derived for describing trace water adsorption from liquid mixtures [20-22],... 

Such form of composite isotherm of components adsorption may be qualitatively explained by the heats of vapour adsorption of these components on silica. The heats of dioxane, benzene and n-hexane adsorption are 66.9, 42.7 and 36.8 kj/ mol respectively at about half monolayer coverage [9]. In accordance with heats of adsorption dioxane strong adsorbs and n-hexane is displaced from the adsorbent surface. The heat of benzene adsorption is smaller than those of dioxane and is larger than those of n-hexane owing to these the... 

Denoyel, R., Beurroies, I., and Vincent, D. (2000). Microcalorimetric methods for studying vapour adsorption and wetting of powders. J. Thermal Anal. Calorimetry, 70, 483-92. 

Hubinger, M., Menegalli, EC., Aguerre, R.J., and Suarez, C. Water vapour adsorption isotherms of guava, mango and pineapple, /. Food ScL, 57, 1405, 1992. 

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