Surface area, creating adsorption isotherms

Weidler, P.G. (1997) BET sample pretreatment of synthetic ferrihydrite and its influence on the determination of surface area and porosity. J. Porous Materials 4 165-169 Weidler, P.G. Degovics, G. Laggner, P. (1998) Surface roughness created by acidic dissolution of synthetic goethite monitored with SAXS and N2 adsorption isotherms. J. Colloid Interface Sd. 197 1-8... 

Research in this area has been directed toward (a) finding or preparing activated carbons with high micropore volume (or BET surface area) in order to increase the CH4 adsorption capacity, and (b) to increase the bulk density of the carbon (reduce void volume of the storage vessel) to minimize the amount of unadsorbed CH4 in the tank. Chemical activation of carbon precursors by heating in alkali solutions and phosphoric acid to create micropores as well as surfece activation of microporous carbons by heating in COj and steam are common techniques used for this purpose [38, 39]. Table 22.10 summarizes the published values of isothermal deliverable CH4 capacities for several activated carbon samples[18, 40—42]. 

Adsorption isotherms are used not only to examine the amount of binding on a surface, or in a sorbent, but also to estimate the number of layers bound. When the protein structure or dimensions are known, and the surface is non-porous, plateaus in the isotherm (such as those observed in Figure Ic) could help elucidate the molecular orientation (assuring close-packed monolayer, bilayers, and etc. are formed). Figure 1 compares the amount of protein bound on alumina and a porous DEAE support. Surface area is usually a known variable used to create an isotherm. For this study, the mass of the beads was known, but not the surface area. Therefore, mass was used in place of surface area. When the size of the adsorbed molecule is much smaller than the dimensions of the pores, the surfaces of the pores are available for adsorption. However, proteins can be larger than some pores, which restrict the available total area. The area, including that from the pores. 

In 1938, Brunauer, Emmett and Teller(12) and Emmett and de Witt(13) developed what is now known as the BET theory. As in the case in Langmuir s isotherm, the theory is based on the concept of an adsorbed molecule which is not free to move over the surface, and which exerts no lateral forces on adjacent molecules of adsorbate. The BET theory does, however, allow different numbers of adsorbed layers to build up on different parts of the surface, although it assumes that the net amount of surface which is empty or which is associated with a monolayer, bilayer and so on is constant for any particular equilibrium condition. Monolayers are created by adsorption on to empty surface and by desorption from bilayers. Monolayers are lost both through desorption and through the adsorption of additional layers. The rate of adsorption is proportional to the frequency with which molecules strike the surface and the area of that surface. From the kinetic theory of gases, the frequency is proportional to the pressure of the molecules and hence ... 

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