Measurement of pore size distribution of low-surface-area materials by krypton gas adsorption method

Pore size estimation of materials, whose surface areas and volumes are so small that nitrogen adsorption cannot be measured, is investigated by applying krypton adsorption at 77 K. The krypton adsorption-desorption isotherms at 77 K on the porous silicas whose pore radii are in the range of 1.36 to 5.20 nm showed capillary condensation and evaporation phenomena, and exhibited definite hysteresis loops. The measurement of the pore sizes using the Kelvin equation revealed that the pore sizes estimated from the krypton desorption isotherms did not coincide with the values estimated from nitrogen desorption isotherms. These differences in pore size estimation will be attributed to the changes in the physical properties (surface tension and density) of the adsorbed krypton in the narrow pores. Considering these changes, the modified Kelvin equation and t-curve for calculating the correct pore size from krypton desorption isotherms are proposed. In the range of pore radii from 1.36 to 5.20 nm, the measurement of krypton adsorption-desorption at 77 K is useful for estimating the pore sizes of porous materials which have small surface areas and pore volumes, such as membranes, coating alms and sintered bodies.