Capillary condensation, freezing, and melting in silica nanopores: A sorption isotherm and scanning calorimetry study on nitrogen in mesoporous SBA-15

Abstract

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.

Thermotropic orientational order of discotic liquid crystals in nanochannels: an optical polarimetry study and a Landau–de Gennes analysis

“…Also for this first-order bulk transition the movement of the ordering/disordering interface has been intensively discussed and experimentally explored in the past [27,29,30,39]. Interfacial melting with a radial moving solid/liquid boundary has been proposed based on filling fraction dependent investigations [28,29,40,41]. Note however, that here the high temperature (liquid) phase is believed to be nucleated at the pore wall and hence the movement of the front boundary is opposite to the scenario outlined here for the melting of the columnar discotic state.…”

Thermotropic nematic order upon nanocapillary filling

“…In the capillary-condensed state the liquid experiences a tensile pressure, dictated by the concave curvature of the menisci terminating the liquid bridges. It is known that this negative hydrostatic pressure causes not only subtle deformations of the rigid, nanoporous matrix [38][39][40], it also significantly affects density, and thus pressuredependent first-order transitions, most prominently the liquid-solid transition [41][42][43]. Therefore, it is in principal also expected to affect the isotropic-to-nematic transition [44,45].…”

Carbonation Resistance of Alkali-Activated Slag Under Natural and Accelerated Conditions

“…The distinctive isotherm for the sample in the natural outdoor conditions has a steady increase in the adsorbed volume at approximately p/p 0 = 0.4 and a sharp increase in adsorbed volume from 0.4 to around p/p 0 = 0.96. The sharp increase is representative of bulk pore filling such as shown in [40].…”