In-situ-SANS Investigations of C5F12 Condensation in Mesoporous Silicas with a Hierarchical Pore Structure

1. Introduction The condensation of liquids from the gas phase and the pore emptying mechanism in mesoporous frameworks (pore size 2 – 50 nm) represents an important scientific issue owing both to the significant industrial relevance (catalysis, separation process, etc.), but also to its importance for the fundamental understanding of the behavior of fluids in confined space. In general, the field of physisorption suffers from the lack of suitable independent techniques to verify theoretical models, which themselves are the basis for the determination of exact pore sizes. Furthermore, the basic phenomenon of filling and emptying large mesopores (> ca. 5 nm) from the filled state through smaller pores is still a matter of intense discussion and has to be regarded as one of the most fundamental topics in the field of physisorption [1]. In our previous work successful results were obtained performing in-situ small-angle neutron scattering (SANS) study during N 2 sorption at 77 K taking advantage of contrast matching between amorphous SiO 2 and liquid N 2. In essence, liquid N 2 has practically the same scattering length density as amorphous silica regarding neutrons. Since the small-angle scattering intensity I(s) is related to the difference in scattering length densities (SLD) of void and silica/ N 2 , I(s) ~ (δ void-δ silica/nitrogen) 2 , only unfilled pores contribute to SANS. If performed at various relative pressures p/p 0 during adsorption/desorption, the SANS curves therefore comprise information of the sorption mechanism [2].