The value of knowing the surface area and porosity of a material cannot be underestimated. Surface area affects the dissolution rate in powders, including pharmaceuticals, as well as the adsorption rate of filtration and purifying materials such as activated carbon. Surface area and porosity also govern the performance of catalysts and catalytic supports such as zeolites, porous silica, and alumina. The porosity of materials such as tricalcium phosphate granules and bone graft strips is also vital in the biomedical field.
PTL has several different methods available for determining surface area and porosity.
BET Specific Surface Area
The BET (Brunauer, Emmett and Teller) theory is commonly used to evaluate gas adsorption data and generate a specific surface area result expressed in units of area per mass of sample ( m 2 /g). Prior to analysis, the sample must be preconditioned to remove physically bonded impurities from the surface of the material in a process called degassing or outgassing. The specific surface area of a material is then determined by the physical adsorption of a gas (typically nitrogen, krypton, or argon) onto the surface of the sample at cryogenic temperatures. Once the amount of adsorbate gas has been measured, calculations which assume a monomolecular layer of the known gas are applied.
Mesopore & Micropore Measurement
Mesopores are pores of internal width between 2 and 50 nm, while micropores are defined as pores with internal diameters of less than 2 nm. Characterization of both mesopores and micropores involves the use of physisorptive gases that can penetrate into the pores under investigation. Gases used include nitrogen and argon, which are physically bound at the solid surface in a process referred to as physisorption. Micropores are filled at very low relative pressure, while pores of larger sizes are filled at higher relative pressure ranges. The appropriate calculations are then applied in order to obtain the physisorption isotherm.
Mercury Intrusion Porosimetry
Mercury intrusion porosimetry (MIP) is a technique utilized for the evaluation of porosity, pore size distribution, and pore volume, among other properties. The instrument, known as a porosimeter, employs a pressurized chamber that forces mercury to intrude into the voids within a porous substrate. As pressure is applied, mercury fills the larger pores first. As pressure increases, the filling proceeds to smaller and smaller pores. Both the inter-particle pores (between the individual particles) and the intra-particle pores (within the particle itself) can be characterized using this technique.
To find out more about how surface area and porosity analyses can benefit you, contact PTL at 630.403.0127.
Image: Chorthip “Chip” Peeraphatdit and our Micromeritics Tristar II 3020.