Surface Tension of Liquid in Chemistry

Surface tension of a liquid origin in surface and tension can be related to the intermolecular force present in the liquids. In other words, the amount of energy required to increase the area of the water or liquid by one unit is the measurement of surface tension of wetting or non-wetting liquids in chemistry.

Origin of Surface Tension

Let us consider a molecule A in the bulk of the liquid. Therefore the molecules uniformly surrounded by other molecules and attracted from all directions. The resultant force on the bulk molecule becomes nil. But the molecule B on the interface partiality surrounded by other molecules and experience a resultant inward pull.

As a result, the molecules on the surface try to leave and enter into the bulk of the liquid. Therefore the liquid under tension and tries to get the minimum area. This unbalanced force of attraction on these molecules in the liquid is the origin of the property surface tension.

Cohesive Force of Liquid

Since γ is under tension any attempt to make a penetration along any line on the interface will require an application of force to holds the separate position of the surface together. This force is called the cohesive force of the liquid.

Higher the Inter-molecular attraction force (cohesive force) of liquid greater will be the magnitude of γ.

Unit of Surface Tension

γ expressed as force per length.

∴ Unit of surface tension = unit of force/unit of length

CGS unit of γ = dyne/cm
=dyne cm-1

SI unit of γ = newton /metre
=newton meter-1

Surface Tension of Water

Water assumes a shape that has minimum interfacial energy or area because that enables the maximum number of molecules to remain on the bulk rather than the surface.

For this reason, when water droplets fall freely it takes the spherical shape because the surface tension or energy of the spheres is minimum.

Wetting and Non-Wetting Liquids

In chemistry liquids are classified into two types, wetting, and non-wetting, depending on the ability to wet the solid interface. But the measurement of the extent of wetting provides the angle of contact on the solid interface.

Therefore the properties of wetting and non-wetting of depending on the angle of constant on the solid interface.

  • Water on glass is an example of wetting liquid with the angle of contact = 180.
  • But mercury on the glass is an example of non-wetting liquid with the angle of contact = 1400.

Properties of Wetting Liquids in Chemistry

When the forces of attraction between the molecules of the liquid and solid are greater, then the liquids have the tendency to spared on the solid. These types of liquids are known as wetting liquids chemistry.

Thus the properties of wetting of liquids

  1. Wetting liquids have a tendency to spared on the solid interface.
  2. The liquids in the capillary tube are concave upwards and so there rise of liquids in the tube.
  3. The intermolecular attraction measurement of liquid molecules is less than the attraction between solid and liquid molecules.

Properties of Non-Wetting

  • The liquids that origin the properties to detach the solid interface are known non-wetting liquids.
  • The intermolecular attraction between the liquid molecules (cohesion) is greater than between the liquid molecule and solid molecule (adhesion).
  • Thus the liquid in the capillary tube given convex upward because there is a fall of liquid in the tube.

Surface Energy of Liquid

If the area of the liquid increases than more molecules from the balk going to its interface of the liquid.  This requires some energy to bringing molecules from the bulk against the inward attractive forces.

Therefore, the amount of work done, which increases the surface area by unity is the origin of the property surface energy. Hence any increases in the area of the liquids against its natural tendency to contract will require performance to work.

Measurement of Surface Tension and Energy

Let us consider a film of liquid stretched on the ware frame having a moveable membrane.

Origin, measurement and unit of Surface tension and surface energy of liquid

If the liquid film stretched by an area = l × x.

Thus the opposing force against the attaching due to γ.

∴ Opposing force = γ × 2l

Since the length of the film in contact with the ware l on each side. Thus the total length = 2l.

So the work required to increase the interfacial area

W = opposing force × displacement
= (γ × 2l) × x = γ × 2(l × x)
= γ × ΔA
where ΔA = increase of the surface area of the film on both sides.

Thus W = measurement of the surface energy or tension of liquids or water associated with the area ΔA.

∴ γ = W/ΔA

The amount of work or energy required to increase the surface area of the liquids by unity is the measurement surface tension of the liquid.

Unit of Surface Energy and Tension

γ = W/ΔA

Therefore the energy per unit area, numerically equal to the surface energy and tension of the pure liquid.

SI unit of γ CGS unit of γ
J m-2 erg cm-2
newton m-1 dyne cm-1

∴ Unit of γ = (unit of F × unit of l)/unit of ΔA
Thus SI unit of γ = N m-1

These are the SI units of surface energy and the tension of the liquid.