CHAP. 13: PHYSICAL CHEMISTRY OF SURFACES [CONTENTS] 436
Chapter 13
Physical chemistry of surfaces
13.1 Phase interface
When a system contains more phases (see1.1.4), the area where two phases come in contact is
referred to as theinterface. It is characterized by its area, and for curved interfaces also by the
surface curvature. If the interface area is relatively small, its influence on the overall behaviour
of the system can be usually neglected. If, however, the interface is larger, its properties have
to be considered in the overall description of the system.13.1.1 Interfacial tension
The surface work needed for the creation of an interface of an infinitesimally small area dAis̄dw=γffdA , (13.1)where the proportionality constantγffis theinterfacialtension.
S Symbols:The subscriptffdenotes the type of phases meeting at the interface, e.g.γs`would be
used for a solid-liquid interface.
U Main unit: J m−^2 = N m−^1.
Note:The term interfacial tension is used because its numerical value is equal to the force
acting perpendicularly on 1 m of length in the direction of the surface (tangential force)
against its expansion.