Surface Area Determination
The surface-to-volume ratio is an important powder property since it governs the rate at which a powder interacts with its surroundings, for example, in chemical reactions. The surface area may be determined from size-distribution data or measured directly by flow through a powder bed or the adsorption of gas molecules on the powder surface. Other methods such as gas diffusion, dye adsorption from solution, and heats of adsorption have also been used. The most commonly used methods are as follows:
Mercury porosimetry, were the pores are filled with mercury under pressure [ISO 15901-1:2005 Pore size distribution and porosity of solid materials – Evaluation by Mercury Porosimetry and gas adsorption – Part 1: Mercury Porosimetry]. This method is suitable for many materials with pores in the diameter range of about 3nm to 400µm (especially within 0.1 to 100µm).
Gas adsorption for Micro-, Meso- and Macropores, were the pores are characterized by adsorbing gas, such as nitrogen at liquid nitrogen temperature. This method is used for pores in the range of approximately <2nm (Micro-pores), 2 to 50nm (Meso-pores) and >50nm (Macro-pores) [ISO/FDIS 15901-2: Pore size distribution and porosity of solid materials –Evaluation by Mercury Porosimetry and gas adsorption – Part 2: Analysis of Meso-pores and macro pores by gas adsorption, ISO/FDIS 15901-3: Part 3: Analysis of Micro-pores by gas Adsorption]. An isotherm is generated of the amount of gas adsorbed against gas pressure, and the amount of gas required to form a monolayer is determined.
Many theories of gas adsorption have been advanced. For meso-pores the measurements are usually interpreted by using the BET theory [Brunauer, Emmet, and Teller, J. Am. Chem. Soc., 60, 309 [1938)]. Here the amount of absorbed na >is plotted against the relative pressure p/p0. The monolayer capacity nm is calculated by the BET equation:
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(1) |
The specific surface per unit mass of the sample is then calculated by assessing a value am for the average area occupied by each molecule in the complete monolayer (e.g. am=0.162nm² for N2 at 77K) and the Loschmidt number L:
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(2) |
