Surface Area, Porosity and Fractal Dimensions

Essential characteristics of fine-grained solids, such as the Fe oxides, include the specific surface area, the porosity and the fractal structure. The standard procedure for measuring these properties is the Brunauer-Em-met-Teller (BET) method (Gregg Sing, 1991) This depends on the fact 

Before adsorption, the solid must be evacuated overnight at a temperature of between 100 and 150°C to remove physically adsorbed water which would otherwise interfere with the N2 adsorption. Usually, this treatment does not lead to phase changes or transformations in most of the iron oxides. Although temperatures of between 90 and 120 °C are necessary to remove surface water from ferrihydrite, the phase transformation is blocked once adsorbed water is removed (Weidler, 1997). 

The better crystalline iron oxides do not have an internal surface area, hence the BET method measures the total surface area of the solid. Ferrihydrite, however, frequently contains aggregates the internal surfaces of which may not be entirely accessible to N2, hence the BET surface area is much lower than that calculated from the average particle size obtained by electron microscope observation. For example, for spherieal particles with a diameter of 3 ran and a density of 3 g/em a 2-line ferrihydrite would have a surface area of ea. 600 m /g whereas its N2-BET surface is hardly ever higher than ea 300 m /g. Indeed, ferrihydrites are usually strongly aggregated irrespeetive of whether dried in air or freeze-dried. 

N2 adsorption is also used to estimate the micro pore volume and the pore size distribution (see e. g. Glasauer et al. 1999) whieh ean be derived from a plot of adsorbed N2 vs. the thiekness of a statistieal monolayer, t, whieh is a function of the relative gas pressure (t-plot method). Mereury porosimetiy serves the same purpose (Celis et al. 1998). N2 adsorption isotherms have also been used to determine the fractal dimensions of Fe oxide particles (c. f.. Celis et al. 1998 Weidler et al. 1998). 

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Weidler, PG and Stanjek, H. (1998). The effect of dry heating of synthetic 2-line and 6-line ferrihydrite II. Surface area, porosity and fractal dimension. Clay Miner, 33, 277-284. 

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