PHYSICAL CHEMISTRY IN BRIEF

CHAP. 13: PHYSICAL CHEMISTRY OF SURFACES [CONTENTS] 439

13.1.4 Surface tension and surface energy

When we are concerned with a liquid-gas (vapour) interface, we use the term surface tension
rather than interfacial tension.
S Symbols: When denoting the surface tension, the subscripts indicating the phases in contact
are often left out, i.e.γ`g≡γ.
In a one-component system the surface tension depends only on temperature (pressure is
equal to the vapour equilibrium pressure), and the surface energy is

σ=γ−T

(

dγ

dT

)

. (13.10)

Example

Explain why a liquid in a zero-gravity space occupies spontaneously the shape of a sphere.

Solution

Since the sphere is a body that has a minimum outer surface area for a given volume, its surface

energy is also the lowest. Any other shape of a body has a higher energy and consequently would

be less advantageous as far as energy is concerned.

13.1.5 Work of cohesion, work of adhesion, and spreading

coefficient

In addition to interfacial tension and energy, some other quantities are also employed for in-

terfaces description. The work needed for pulling apart a continuous liquid column of a unit

cross-section in a way that allows for the formation of vapour in the space between them is

called thework of cohesion,wc. It is related to surface tension by the relation

wc= 2γ`g= 2γ. (13.11)

The work needed for pulling apart a liquid and a solid phase liquid-vapour and solid-vapour

interfaces is called thework of adhesion,wa. It is related to unit area,

wa=γ`g+γsg−γs`. (13.12)