Surface of solids. Physisorption - Chemisorption - Segregation

The field of surfaces is of particular importance because every body, whether liquid or solid, interacts with the surrounding environment through the surface that defines its boundary. Atoms on the surface of a solid or liquid are less coordinated than those at the core of the system. It's therefore easy to understand why these atoms confer very specific properties on the surface. The energy required to increase the surface area of a solid is always positive, which means, among other things, that condensed systems tend to minimize this surface energy :

• to reduce the surface area;

• to react with molecules in the ambient atmosphere to form an adsorption layer;

• surface segregation of the solid element with the lowest surface energy;

• or lead to surface relaxations (modification of distances between crystalline planes), or even give rise to deep surface reconstructions.

These particular properties give dispersed systems (with a large specific surface area) an important role in a wide variety of fields, from physics and chemistry to geology and biology. Certain thermodynamically possible chemical reactions are accelerated, or favored when in competition with other possible reactions, thanks to the surface of certain solids. This phenomenon, known as heterogeneous catalysis, is of crucial importance in petrochemistry, for example. Similar observations could be made in the fields of metallurgy, corrosion, adhesion and fracture of solids, lubrication, tribology, crystal growth, electronics, microsystems and so on. Ultimately, in nanosystems, when the number of "surface" atoms becomes equivalent to, or even greater than, the number of "volume" atoms, the very notion of surface, as delimiting a body or a phase, loses its meaning, and physics itself may change in nature.

This article focuses on adsorption and segregation. For a general overview of these phenomena and their applications, please consult the works cited in § 1 .