An atom or a molecule located on the surface of a soild is in a different state from an atom or a molecule located inside the solid. Consider, for example, a sodium chloride crystal in a vacuum. A sodium ion inside the crystal is surrounded on all sides by chloride ions, but a sodium ion on the surface has no chloride ion above it. Thus, the force field surrounding a sodium ion on the surface is different from the force field around the sodium ion in the body; consequently, the potential energies of the sodium ions in the two states are different. The same is true of the chloride ions. The potential energy of the sodium chloride molecule in the surface is greater than that of the molecule in the body, and this energy difference is called surface total energy or, briefly, surface energy. A simple way to demonstrate surface energy is to take samples of a substance in a coarse and in a finely divided state, dissolve equal weights of these in equal amounts of the same solvent, and measure the heat of solution; one invariably finds that the finely divided sample evolves more heat than the coarse sample.
Not only is the total energy of a molecule in the surface greater than in the body, the free energy of the molecule in the surface is likewise greater. This excess free energy is responsible for the “activity” of the surface. Any process that results in a decrease of free energy occurs spontaneously. When a gas or vapor is admitted to an evacuated solid, a part of the atoms or molecules of the gas or vapor attach themselves to the surface. This is the phenomenon of adsorption, which is always spontaneous and always results in a decrease of the free energy of the system.
The adsorbed particles are either rigidly held to the surface (localized adsorption) or they can move over the surface in two dimensions (mobile adsorption). Prior to adsorption, the gas molecules were able to move freely in three dimensions; the adsorption process is, therefore, accompanied by a decrease in the entropy of the system. A fundamental equation of thermodynamics is