Lesson 6 Surface Tension and Surfactant

6.1 Surface Tension

It is well known that short-range Van der Waals forces of attraction exist between molecules 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 the average，subjected to equal forces of attraction in all directions，whereas those located，for example，at a liquid-air interface experience unbalanced attractive forces resulting in a net inward pull. As many molecules as possible will leave the liquid surface for the interior of the liquid;therefore，the surface will tend to contract spontaneously. For this reason droplets of liquid and bubbles of gas tend to attain a spherical shape.

The molecules at a surface have higher potential energies than those in the interior. This is because they interact more strongly with the molecules in the interior of the substance than they do with the widely spaced gas molecules above it. Work is therefore required to bring a molecule from the interior to the surface.

Surface tension and the more fundamental quantity，surface free energy，fulfil an outstanding role in the physical chemistry of surfaces. The surface tension of a liquid is often defined as the force acting at right-angles to any line of unit length on the liquid surface. However，this definition （although appropriate in the case of liquid films，such as in foams） is somewhat misleading，since there is no elastic skin or tangential force as such at the surface of a pure liquid. It is more satisfactory to define surface tension and surface free energy as the workrequired to increase the area of a surface isothermally and reversibly by unit amount.Attractive force between molecules at surface and in the interior of a liquid is illustrated in the following Figure.

Attractive Forces between Molecules at Surface and in the Interior of a Liquid

The same considerations apply to the interface between two immiscible liquids. Again there is an imbalance of intermolecular forces but of a lesser magnitude. Interfacial tensions usually lie between the individual surface tensions of the two liquids in question.

The above picture implies a static state of affairs. However，it must be appreciated that an apparently quiescent liquid surface is actually in a state of great turbulence on the molecular scale as a result of two-way traffic between the bulk of the liquid and the surface，and between the surface and vapour phase.^① The average lifetime of a molecule at the surface of a liquid is ca. 10^-6 seconds.