Porosimetry is an analytical technique used to determine various quantifiable aspects of a material's porous nature, such as pore diameter, total pore volume, surface area, and bulk and absolute densities.
Many materials such as cosmetics, pigments, pharmaceuticals and construction materials have varying physical properties and functionalities that depend on a certain porosity or surface area for their performance. Surface area can be affected by cracks, crevices, texture, and pore size of the sample. Gas Adsorption is most commonly used to determine a material’s porous nature and surface area. Gas Adsorption is used to analyze pore diameter for smaller pores such as micropores that are less than 2nm and mesopores that range from 2nm to 50nm in size, while mercury Intrusion is used to analyze much larger pore diameters or macropores (pore diameter 14 – 200 µm)
The BET theory or the Brunauer, Emmett, and Teller theory for gas adsorption or desorption is used to determine specific surface area and is most commonly referenced by ISO, USP and ASTM. With this method sample prep is simple and only requires degassing or out gassing the sample to remove any impurities. This is accomplished by an inert gas flowing over sample at increasing increments of temperature. After sample prep, nitrogen, argon, or krypton gases are absorbed by the sample at cryogenic temperatures (N2) or liquid argon temperatures (Ar). As long as the sample has unrestricted monolayer-multilayer adsorption capabilities (Type II and Type IV isotherms) the surface area of sample can be determined and expressed as the standard volume absorbed as a function of relative pressure. This data is then converted to area per mass of sample (m2/g).
- BET supported surface area analysis utilizes six parallel runs that can be achieved in as little as 30 minutes, which grants a faster turnaround time
- Gas adsorption allows for good bad comparison of porosity for a dry sample vs. moisture contaminated sample unlike mercury intrusion where the sample must be dry
- Gas adsorption/ desorption does not overestimate the volume of the smallest pores like mercury intrusion does
