Types of Intermolecular Force of Attraction and Molecular Theory of Surface Tension

Types of Intermolecular Force 

There are two types of intermolecular forces. They are

1. Cohesive Force
2. Adhesive Force

Cohesive Force

The inter-molecular force of attraction that exist between molecules of the same substance is known as cohesive force. Its value is maximum for solids and minimum for gases. Due to this reason a solid has definite shape and size and is capable of withstanding external deforming force. Among the liquids, mercury has comparatively high cohesive force. Therefore, mercury doesn't wet yhe glass.

Adhesive Force

The inter-molecular force of attraction that exist between two molecules of different substances is known as adhesive force. Sticking with glue on paper, sticking of water molecules in the gas are the example of adhesive force.

Explanation of Surface Tension based on Molecular Theory

Due to the force of cohesion, the molecules of liquid attract each other. Consider a body O is lying below the free surface of a liquid. Here the molecule attracted from all sides as shown in the figure. So, the net force becomes zero. Let P be another molecule on the surface of the liquid. As there are very few molecules on the surface of liquid. So molecule P is attracted only due to molecules lying in the lower half of the sphere. The result of this force is downward.

If we wish to bring a molecule O to the liquid surface then we have to work against the force. This work done on molecules is stored in the form of potential energy.

For equilibrium system must have minimum potential energy. So, there must be less molecule in the liquid surface and it is possible when liquid surface is minimum. So the liquid surface contract like a stretched elastic membrane.

Surface tension

The surface tension of the liquid is the property by which its free surface behaves as elastic stretch membrane and has a tendency to occupy the minimum surface area.

Measurement of surface tension of a liquid

The surface tension of a liquid is measured as the force per unit length of an imaginary line drawn on the free surface, perpendicular to it at every point and tangentially to the liquid surface.

$$\text {Surface tension} (T) =\frac {\text {Force}(F)}{\text {Length}(l)}$$

It's unit is Nm^-1in SI unit and dyne/cm in CGS units .

Dimension of surface tension = \(\frac{dimrnsion of force}{dimensiin of length}\)

= \(\frac{[MLT^{-2}]}{[L]}\)

= \([MT^{-2}]\)