  Condensation, melting and freezing of nitrogen in a powder of mesoporous
silica grains (SBA-15) has been studied by combined volumetric sorption
isotherm and scanning calorimetry measurements. Within the mean field model of
Saam and Cole for vapor condensation in cylindrical pores a liquid nitrogen
sorption isotherm is well described by a bimodal pore radius distribution. It
encompasses a narrow peak centered at 3.3 nm, typical of tubular mesopores, and
a significantly broader peak characteristic of micropores, located at 1 nm. The
material condensed in the micropores as well as the first two adsorbed
monolayers in the mesopores do not exhibit any caloric anomaly. The
solidification and melting transformation affects only the pore condensate
beyond approx. the second monolayer of the mesopores. Here, interfacial melting
leads to a single peak in the specific heat measurements. Homogeneous and
heterogeneous freezing along with a delayering transition for partial fillings
of the mesopores result in a caloric freezing anomaly similarly complex and
dependent on the thermal history as has been observed for argon in SBA-15. The
axial propagation of the crystallization in pore space is more effective in the
case of nitrogen than previously observed for argon, which we attribute to
differences in the crystalline textures of the pore solids.
