Interfacial Phenomena

Abstract

A molecule in a bulk of material is pulled from all the sides whereas a molecule at the surface experiences no pull from the top and is pulled only from the sides and below. Bonding between molecules takes place with lowering of free energy. The bond energies between particles are negative because energy is evolved on bond formation. Therefore, atoms, molecules, or ions on the surface of a liquid or solid possess higher energies than those located in the bulk of the substance. The energy of the surface increases with the increase of the surface area. Hence, the surface free energy can be defined as the energy required to create 1 m2 of new surface. This energy can be expressed in Joule per square meter (or ergs per square cm). As the molecules in solids are rigid, no such effect is realized. Surface free energy is a characteristic property of the liquid phase making a free surface. Since reduction in the surface area reduces the free energy, the surface of the liquid will tend to contract. Liquids tend to form spherical drops because spheres have the least surface area to volume ratio among all geometrical shapes. The tensional force causing the contraction of the free liquid surface is known as surface tension. The surface tension is defined as the force in newtons acting at right angles to a line 1 m long on the surface. The work done in producing 1 m2 of new surface is equivalent to the work done in stretching the surface by 1 m along a 1 m line against the surface tension and this is the surface free energy, σ. Thus, the surface tension σ, newton per meter (Nm−1), is numerically equal to the surface free energy Jm−2. Surface tension is generally used with reference to liquid, whereas surface free energy and interfacial energy are used with reference to liquids and solids, respectively. Solids definitely have surface free energy, but due to their rigid surfaces, they are neither smooth nor free to allow the solid to fill the shape of a container, and therefore, the concept of surface tension is not applicable in solids. Table 6.1 lists the values of surface tension of a few liquids.