Capillary Condensation, Freezing, and Melting in Silica Nanopores: A Sorption Isotherm and Scanning Calorimetry Study on Nitrogen in Mesoporous SBA-15

摘要

Condensation, melting and freezing of nitrogen in a powder of mesoporoussilica grains (SBA-15) has been studied by combined volumetric sorptionisotherm and scanning calorimetry measurements. Within the mean field model ofSaam and Cole for vapor condensation in cylindrical pores a liquid nitrogensorption isotherm is well described by a bimodal pore radius distribution. Itencompasses a narrow peak centered at 3.3 nm, typical of tubular mesopores, anda significantly broader peak characteristic of micropores, located at 1 nm. Thematerial condensed in the micropores as well as the first two adsorbedmonolayers in the mesopores do not exhibit any caloric anomaly. Thesolidification and melting transformation affects only the pore condensatebeyond approx. the second monolayer of the mesopores. Here, interfacial meltingleads to a single peak in the specific heat measurements. Homogeneous andheterogeneous freezing along with a delayering transition for partial fillingsof the mesopores result in a caloric freezing anomaly similarly complex anddependent on the thermal history as has been observed for argon in SBA-15. Theaxial propagation of the crystallization in pore space is more effective in thecase of nitrogen than previously observed for argon, which we attribute todifferences in the crystalline textures of the pore solids.