Molecules liquid state experience strong intermolecular attractive
forces. When those forces are between like molecules, they are referred
to as cohesive forces. For example, the molecules of a water droplet are
held together by cohesive forces, and the especially strong cohesive
forces at the surface constitute surface tension.
When the attractive forces are between unlike molecules, they
are said to be adhesive forces. The adhesive forces between water
molecules and the walls of a glass tube are stronger than the cohesive
forces lead to an upward turning meniscus at the walls of the vessel and
contribute to capillary action.
The attractive forces between molecules in a liquid can be viewed as residual electrostatic forces and are sometimes called van der Waals forces or van der Waals bonds.
Cohesion is the property of like molecules (of the same substance) to stick to each other due to mutual attraction. Adhesion
is the property of different molecules or surfaces to cling to each
other. For example, solids have high cohesive properties so they do not
stick to the surfaces they come in contact. On the other hand, gases
have weak cohesion. Water has both cohesive and adhesive properties.
Water molecules stick to each other to form a sphere. This is the result
of cohesive forces. When contained in a tube, the water molecules
touching the surface of the container are at a higher level (see Meniscus). This is due to the adhesive force between the water molecules and the molecules of the container.
The cohesive forces between liquid molecules are responsible for the phenomenon known as surface tension. The molecules at the surface
do not have other like molecules on all sides of them and consequently
they cohere more strongly to those directly associated with them on the
surface. This forms a surface "film" which makes it more difficult to
move an object through the surface than to move it when it is completely
submersed.
Surface tension is typically measured in dynes/cm, the force in
dynes required to break a film of length 1 cm. Equivalently, it can be
stated as surface energy in ergs per square centimeter. Water at 20°C
has a surface tension of 72.8 dynes/cm compared to 22.3 for ethyl
alcohol and 465 for mercury.
Surface tension is a contractive tendency of the surface of a liquid
that allows it to resist an external force. It is revealed, for
example, in the floating of some objects on the surface of water, even
though they are denser than water, and in the ability of some insects
(e.g. water striders) to run on the water surface. This property is caused by cohesion of similar molecules, and is responsible for many of the behaviors of liquids.
Surface tension has the dimension of force per unit length, or of energy per unit area. The two are equivalent—but when referring to energy per unit of area, people use the term surface energy—which is a more general term in the sense that it applies also to solids and not just liquids.
In materials science, surface tension is used for either surface stress or surface free energy.
Cohesion and Surface Tension
The cohesive forces
between molecules down into a liquid are shared with all neighboring
atoms. Those on the surface have no neighboring atoms above, and exhibit
stronger attractive forces upon their nearest neighbors on the surface.
This enhancement of the intermolecular attractive forces at the surface
is called surface tension.
Surface Tension Examples
Walking on water
Small insects such as the water strider can walk on water because their weight is not enough to penetrate the surface.
Floating a needle
If carefully placed on the surface, a small needle can be made to float
on the surface of water even though it is several times as dense as
water. If the surface is agitated to break up the surface tension, then
needle will quickly sink.
Don't touch the tent!
Common tent materials are somewhat rainproof in that the surface tension
of water will bridge the pores in the finely woven material. But if you
touch the tent material with your finger, you break the surface tension
and the rain will drip through.
Soaps and detergents
help the cleaning of clothes by lowering the surface tension of the
water so that it more readily soaks into pores and soiled areas.
Clinical test for jaundice
Normal urine has a surface tension of about 66 dynes/cm but if bile is
present (a test for jaundice), it drops to about 55. In the Hay test,
powdered sulfur is sprinkled on the urine surface. It will float on
normal urine, but sink if the S.T. is lowered by the bile.
Washing with cold water
The major reason for using hot water for washing is that its surface
tension is lower and it is a better wetting agent. But if the detergent
lowers the surface tension, the heating may be unneccessary.
Surface tension disinfectants
Disinfectants are usually solutions of low surface tension. This allow
them to spread out on the cell walls of bacteria and disrupt them. One
such disinfectant, S.T.37, has a name which points to its low surface
tension compared to the 72 dynes/cm for water.
