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Surface Tension

Surface Tension of Liquid in Chemistry

Surface tension or surface energy of liquid molecules origin at the surface can be related to intermolecular forces or net inward pull of the larger number of molecules toward the liquid side than the vapour side. In general, liquids are obtained by colling gas molecules below their critical temperature by the treatment of high pressure and from the solid by the specific heat to overcome translational kinetic energy. The effect of colling decreases the thermal energy and high pressure increases the density, thereby increasing the forces of attraction amongst the molecules. Therefore, the surface of wetting or non-wetting liquids is some sort of tension and tends to contract to the smallest possible area in order to have the minimum number of molecules at the surface. 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 physical and chemical science.

Origin of Property 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.

Surface Tension and Formation of Bubbles

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.

The formation of bubbles basically due to surface tension. The total pressure acting on the concave side must be larger than the pressure acting on the convex side. Therefore, the pressure inside the bubbles must be larger than the external pressure. If the external pressure not balanced by these two forces, then the bubbles are not stable and immediately collapsed. Therefore, for getting stable bubbles, these external forces balanced by other forces namely cementing forces.

Cohesive Force  and Unit of Surface Tension

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.

Surface tension expressed as force per length. Therefore, the unit of surface tension equal to the unit of force by unit of length. Hence the unit of surface tension in CGS and SI-system = dyne cm-1 and newton m-1 respectively.

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 = 18°.
  • But element mercury on the glass is an example of non-wetting liquid with the angle of contact = 140°.

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 in learning chemistry or physical science. Therefore, the properties of wetting of liquids are,

  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 equilibrium the forces from the bulk against the inward attractive forces. 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. With the increasing concentration of liquids, surface energy or tension also decrease. Therefore, the values of γ for water, benzene, methyl alcohol at 20°C = 72.8, 28.87, 22.55 dyne cm-1 respectively.

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

The opposing force against the attaching due to surface tension = 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 of the liquids.

∴ W = opposing force × displacement
= (γ × 2l) × x = γ × 2(l × x)
= γ × ΔA

ΔA = increase of the surface area of the film on both sides. Therefore, W = measurement of the surface energy or tension of liquids or water associated with the area ΔA. Hence, 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.

Dimensions of Surface Energy and Tension

From the definition of γ = W/ΔA. Therefore, the energy per unit area, numerically equal to the surface energy and tension of the pure liquid. Hence the CGS and SI unit of surface energy = Joule m-2 and erg cm-2 respectively.

∴ Unit of γ = (unit of F × unit of l)/unit of ΔA
Therefore, the SI unit of γ = N m-1
Again, Joule m-2 = N × m × m-2 = N m-1

Therefore, the dimensions of surface energy and the tension are equal quantity.