Intermolecular Forces:
It is defined as the force between the two molecules of a substance (force of attraction & repulsion). Intermolecular forces are weak relative to intramolecular forces ( force that binds together the atoms making up a molecule or compound). Information on intermolecular forces is obtained by macroscopic measurements of properties like viscosity, pressure, volume, temperature (PVT) data.
a. Force of Cohesion: It is defined as the force of attraction between two similar molecules of a substance. This is totally different from the gravitational forces which holds the inverse square law.
Examples: Force between two water molecules, force between two glass molecules,
b. Force of Adhesion: It is defined as the force of attraction between molecules of different substances.
Examples: Force between glass & water molecule.
SURFACE TENSION:
Surface tension is also defined as the force acting on the liquid surface per unit length of the liquid surface.
T=F/l
where T=Surface Tension, F=Total force, l=Length of the liquid surface
The dimensional formula of surface tension is MT-2.
Examples of surface tension(ST):
1. Raindrops are spherical due to ST.
2. Oil floats on water surface because ST of oil is less than that of water.
3. Small insects such as the water strider can walk on water because their weight is not enough to penetrate the surface.
4. A carefully placed small needle can be made to float on the surface of water even though it is several times as dense as water.
5. Aluminum coin supported by the surface tension of water.
6. Cleaning of clothes by soaps and detergents which lowers the surface tension of the water.
7. when a hair brush is dipped inside liquid, the hairs get separated from each other but when it is taken out of the liquid, the hairs cling together due to ST.
SURFACE ENERGY:
The surface tension always try to decrease the area of the liquid surface, so to increase the area of the liquid surface, some energy is required called as potential energy. This potential energy per unit area is called surface energy.
T=ST, l=length of the liquid surface
Hence force F=2Tl (soap solution has two surfaces)
So workdone dw=Fdx=2Tldx
dw=TdA (since 2ldx=dA= Increase in area)----------------(1)
a. Isothermal Process:
b. Adiabatic Process:
Hence surface energy is numerically equal to surface tension.