Thermodynamics of Finite Size Systems

Abstract

Phase transitions are collective phenomena, meaning that a large number of atoms or molecules react in a coordinated manner. For nanoscale systems the concept of phases and therefore also the Gibbs phase rule break down. The restricted space in narrow pores prevents the limiting coordination number being reached. Melting point and critical point shift to lower temperature, and the phase transitions broaden or disappear completely. In the pores of zeolites there is a continuous transition between immobile (frozen) molecules at low temperature and a gas-like fluid state at high temperature. Capillarity is not only relevant in pores; it also determines contact angles of liquid drops on surfaces. The wetting properties of liquids depend enormously on the micro- and nanostructure of the solid surface. Drops can stick or roll off. These properties can be changed significantly by changing the surface structure via temperature, pH, UV irradiation or by application of an electric field to conducting liquids. This opens fascinating possibilities for the design of smart surfaces. In a similar way to those in semiconductor junctions, ions can transfer across the interface between two different ionic crystals, which leads to local violation of charge neutrality and to space charge zones near the interface, also in chemical equilibrium. When crystals are sufficiently small, the overlap of two space charge zones leads to additional size effects.