Surface Energy

in thermodynamics, the excess energy in a thin layer of a substance at the surface of contact with other bodies or phases relative to the energy of the substance within the body.

The total surface energy is composed of (1) the energy of surface formation, that is, the energy required to overcome the forces of intermolecular or interatomic interaction in displacing the molecules or atoms from the bulk of the phase to the surface layer, and (2) the thermal effect related to this process. In accordance with thermodynamic functions, the specific total surface energy is

u = σ + q = σ –T(∂σ/∂T)

where σ is the specific free surface energy, identical for mobile liquids to the surface tension; q is the latent heat, or binding energy, of a unit of surface area; T is the absolute temperature; and ∂σ/∂ T the specific surface entropy, which usually has a negative value.

The free surface energy decreases with increasing temperature, whereas the total surface energy of nonpolar, or nonas-sociated, liquids remains constant and the total surface energy of polar liquids increases slightly. Thus, for water at 0°, 20°, and 100°C, the values for u are equal to 117, 120 and 129 mJ/m² or erg/cm², respectively.

As the critical temperature is approached, the differences in the composition and properties of the contacting phases diminish, the phase boundary surface disappears, and the surface energy becomes zero.

The surface energy effects many physicochemical properties of solids and liquids. Its role is becoming especially prominent in highly dispersed colloidal systems in which the phase boundary surface is exceedingly large.]