chapter  18

Phase Behavior of Atomic and Molecular Nanosystems

WithR. Stephen Berry

The physical and chemical properties of aggregates of matter, typically at a scale we can see and handle but extending to micron scales, even sometimes to nanometer scales, are the substance of thermodynamics and classical mechanics. We recognize that classical Newtonian mechanics must be replaced by quantum mechanics when we want to describe very small systems and the phenomena they exhibit. We can often explain how the passage occurs between the scale that requires a quantum description and the larger scale where classical Newtonian mechanics is completely adequate. We have learned to recognize many ways that very small systems can exhibit different chemical behavior from their macroscopic counterparts, largely through quantum-based behavior. However there has been a general (and tacit) assumption that classical thermodynamics would be valid for all systems, whatever their size. As we shall see, this is not strictly correct, although the explanation for the apparent violations becomes clear when we turn to statistical mechanics, specifically statistical thermodynamics, for the needed insight.