Thermodynamic and Structural Investigations of Condensates of Small Molecules in Mesopores

Abstract

Liquids and solids consisting of small, mainly van-der-Waals interacting building blocks, such as Ar, Kr, N2, O2, and CO, are among the most simple systems of condensed matter imaginable. As we shall demonstrate in this microreview on our work sponsored within the Sonderforschungsbereich 277, these cryoliquids condensed in mesoporous hosts with typical mean pore diameters of 7 to 10nm are also particularly suitable for the investigation of fundamental questions regarding the thermodynamics and structure of spatially mesoscale confined systems. An exploration of phase transitions like the vapour–liquid (capillary condensation), the vapour–solid (capillary sublimation), the liquid–solid (freezing and melting) and some solid–solid transformations of such pore condensates reveals a remarkably rich, sometimes perplexing phenomenology. We will show, however, that by experiments combining sorption isotherm, X-ray and neutron diffraction, calorimetric and optical transmission measurements, and by referring to concepts, intermediate between surface and bulk physics, a deeper understanding of the mesoscale mechanisms ultimatively responsible for this complex behaviour can indeed be accomplished, both on a qualitative and a quantitative level.