Cohesion
Nico A.M. Schellart, Dept. of Med. Physics, AMC
Principle
Cohesion or cohesive attraction or cohesive force is the
intermolecular attraction between (nearly) identical molecules. The cohesive
forces between liquid molecules are responsible for phenomena such as surface tension
and capillary force.
Molecules in liquid state experience strong
intermolecular attractive forces. When those forces are between like molecules,
they are called cohesive forces. For example, cohesive forces hold the
molecules of a water droplet together, and the strong cohesive forces
constitute surface tension.
When the attractive forces are between unlike molecules,
they are said to be adhesive forces (see Adhesion). 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.
Mercury is an example of a liquid that has strong cohesive forces, as becomes
clear from the very convex meniscus in the tube of a classical air pressure
meter.
Application
There are many, in medicine, science and daily life.
Often they are based on surface tension.
Clinical tests
Surface tension
disinfectants Disinfectants
are usually solutions of low surface tension. This allows them to spread out on
the cell walls of bacteria and disrupt them.
Walking on water Small insects 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.
Soaps and
detergents They help
the cleaning of clothes by lowering the surface tension of the water so that it
more readily soaks into pores and soiled areas.
Washing With hot water its surface tension is lower
and it is a better wetting agent.
More Info
There are various phenomena, which are based on cohesion.
Surface Tension
Water is a polar molecule due to the high
electronegativity of the oxygen atom, which is an uncommon molecular
configuration whereby the oxygen atom has two lone pairs of electrons. When two
water molecules approach one other they form a hydrogen bond. The negatively
charged oxygen atom of one water molecule forms a hydrogen bond with a
positively charged hydrogen atom in another water molecule. This attractive
force has several manifestations. Firstly, it causes water to be liquid at room
temperature, while other lightweight molecules would be in a gaseous phase.
Secondly, it (along with other inter molecular forces) is one of the principal
factors responsible for the occurrence of surface tension in liquid water.
Water at 20°C has a surface tension of 0.073 N/m compared
to 0.022 N/m for ethyl alcohol and 0.47 N/m for mercury. The latter high value
is the reason why in a capillary filled with mercury the meniscus is very
convex. The surface tension of water decreases significantly with temperature
as shown in the graph. The surface tension arises from the polar nature of the
water molecule. Hot water is a better cleaning agent because the lower surface
tension makes it a better "wetting agent" to get into pores and
fissures rather than bridging them with surface tension. Soaps and detergents
further lower the surface tension.
See for further for an explanation the surface tension the
description of subject itself.
In crystals
In crystals (of molecular-, ionic-, valence- and
metal-type) many types of forces play a role such as van der Waals forces and
forces of chemical bonds. A van der Waals force is the attraction between two
molecules with positively and negatively charged ends. This polarity may be a
permanent property of a molecule (Keesom forces)) or universally occurs in
molecules, as the random movement of electrons within the molecules may result
in a temporary concentration of electrons at one end (