Effect of Pore Size Distribution on Capillary Condensation in Nanoporous Media

The occurrence of capillary condensation is often ignored in many naturally occurring nanoporous media, such as shale rock, simply because their isotherms do not adhere to the prescribed shapes presented in the literature. In particular, it is apparent from the literature that most shale isotherms do not display a clear capillary condensation step, which is commonly observed for much simpler adsorbents, such as MCM-41. We contend that the absence of this step from the isotherms for natural adsorbents is not due to the absence of nanoconfinement-induced phase behavior. Rather, it is due to the broad pore size distribution characteristic of such materials. By mechanically mixing different sizes of MCM-41 together and measuring isotherms for propane and n-butane in them at a variety of temperatures, we show that phase behavior in different pore sizes is additive and suppresses the commonly observed appearance of capillary condensation. By comparing the isotherms in the mixtures of MCM-41 to those measured in single pore sizes of MCM-41, we develop correlations, using the Lorentzian function, that make the determinations of porosity and fluid density from the mixture isotherms straightforward.