Editorial Highlight: Molecules in confined spaces

Abstract

At interfaces, the properties of molecules are different from those in the bulk gas phase or bulk liquid phase. Several observations make this clear: capillary rise and contact angles, as well as wetting. These are macroscopic phenomena, which are described by thermodynamics with the surface tension or the free energy change per unit surface area as the most important thermodynamic quantity.1 Adsorption, or the tendency of a molecule to have a higher concentration at the interface (gas/liquid, gas/solid, liquid/solid and liquid/liquid) than in the adjacent bulk phases is measured by adsorption isotherms. The physical basis for adsorption is two-fold. On one hand, the surface contains coordinatively unsaturated atoms, functional groups (e.g. OH), exchangeable cations and defects. On the other hand, the adsorbate consists of molecules, which are non-polar, polar or ionic. As a consequence, a multitude of interactions may take place. In the case of physisorption, these interactions are typically intermolecular, namely ion–dipole, ion–apolar, dipolar–dipolar, dipolar–apolar and apolar–apolar. In the case of chemisorption, a chemical bond is established between the surface atoms and the adsorbed molecule.