The Micromeritics Accelerated Surface Area and Porosemitry System is used to measure the pore size and surface area of aerogels and xerogels. The system utilizes the physical adsorption principle for gas. Gas molecules strike solid surfaces repeatedly and linger for some time – this is known as adsorption. Adsorption occurs at all gas-solid interfaces and is a function of temperature and pressure. When the sample is exposed to the analysis gas (usually nitrogen), the number of molecules adsorbed on the surface increases. Dynamic equilibrium is achieved when the rate of adsorption is equal to the rate of molecules leaving the surface. The pressure at which the equilibrium occurs is measured and the universal gas law is applied to determine the quantity of gas adsorbed. Knowing the mean surface area that each gas molecule occupies, an analysis can be done using the BET theory (Brunnauer, Emmett and Teller theory) to find the total surface area of the aerogel. As more and more gas molecules are adsorbed with increasing pressure, there comes a point when condensation occurs. At this point, the BJH theory (Barrett, Joyner, and Halenda theory) is applied to investigate the porosity. For these experiments, the aerogel and xerogel samples must be crushed so that the entire surface area is exposed to the nitrogen gas.
