92168.pdf

4 Liquid—gas and liquid-

liquid interfaces

Surface and interfacial tensions

It is well known that short-range forces of attraction exist between

molecules (see page 215), and are responsible for the existence of the

liquid state. The phenomena of surface and interfacial tension are

readily explained in terms of these forces. The molecules which are

located within the bulk of a liquid are, on average, subjected to equal

forces of attraction in all directions, whereas those located at, for

example, a liquid-air interface experience unbalanced attractive

forces resulting in a net inward pull (Figure 4.1). As many molecules

as possible will leave the liquid surface for the interior of the liquid;

the surface will therefore tend to contract spontaneously. For this

reason, droplets of liquid and bubbles of gas tend to attain a spherical

shape.

Surface tension (and the more fundamental quantity, surface free

energy) fulfil an outstanding role in the physical chemistry of surfaces.

Air

Liquid

Figure 4.1 Attractive forces between molecules at the surface and in the interior of a
liquid