Argonadsorption

Argon adsorption refers to the physical adsorption of argon gas onto solid surfaces, typically used to characterize the surface area and porosity of porous materials. Argon is a noble, monatomic gas that interacts with surfaces primarily through van der Waals forces, and adsorption is generally reversible at the low temperatures used in these measurements.

Experiments are performed at cryogenic temperatures, often near the boiling point of liquid argon (about 87 K) or at 77 K using liquid nitrogen for calibration. The sample is placed in a sample cell and thermally degassed to remove adsorbed species before cooling. A controlled amount of argon is introduced, and the amount adsorbed is measured as a function of relative pressure to produce an adsorption isotherm.

Data are analyzed with standard adsorption models. The Langmuir model describes monolayer adsorption on a homogeneous surface, while the Brunauer–Emmett–Teller (BET) model extends to multilayer adsorption and is used to determine the BET surface area. Pore-size distributions are commonly derived from argon isotherms using non-local density functional theory (NLDFT) or density-functional theory kernels, alongside methods such as the t-plot for micropore assessment.

Applications include characterization of activated carbons, silica, zeolites, metal–organic frameworks, and other porous materials. Argon is often used as an adsorption probe gas when nitrogen's quadrupole moment or specific interactions with the surface are undesirable, or when a more uniform adsorption is desired on certain materials.

Limitations include the need for cryogenic operation and the model-dependent interpretation of microporous data; results can vary with the chosen analysis kernel. Despite these considerations, argon adsorption remains a standard method for evaluating surface area and porosity.