Surface Tension and capillary action, a characteristic property of Liquids

Surface Chemistry, Surface Tension and Capillary Action

Surface Tension:

→ Surface tension is a characteristic of surface of liquids due to which it try to decrease its area. For this purpose, a force of attraction is applied betweem the molecules of liquids on the surface. For this reason, surface of a liquid behave like a stretched membrane.

→ Consider a molecule P some where in the body of the liquid. This is attracted equally in all dieactions by auther molecules which surround it as shown in fig and therefore cancel the effect of one another.

→ Consider, next, a olecule R at the surface of the liquid. THe downward attractive force are greater than the upwardr forces because there are more molecules of liquid below than that in air above the surface. These inbalanced atttractive forces acting downward tend to draw the surface mlecules into the body of the liquild and therefore, tend to reduce the surface to a minimum.

→ It is well know that force of attraction tend to decrease the energy of a system. The molecules at the surface possess greateenergy than those in the bilk. The molecules tends to move from a state of higher energy to a state of lower enegy. As a result, the number of molecules at the surface becomes less than that in bulk. 

→ The surface molecules tend to move closer to one another in order to acquire a normal distance between them. It is the reason that drops of a liquid or bubbles of a gas are spherical in shape. A sphere has minimum surface for a gives volume.

→ A a result of the tendency to contract, surface of a liquid behave as if it were in a state of tension. the force that tends to contract the surface of a liqid is called surface tension.

→ Mathematically surface tension may br defined as the force acting at right angle to the surface along unit length of the surface.

It is generally represented by the symbol r and 

  unit dyne cm-1

S.I unit is Newtons per meter 

Nm-1      1N= 105 dyne

i dyne cm-1 = 10-3 N-m-1

Surface tension of water nitrobenzene, benzens, acetic acid, ethyl alcohol and ethyl ether are 72.8 dyne cm-1 (0.0728 Nm-1), 41.8 dyne cm-1 28.9 dyne cm-1, 27.6 dyne cm-1, 22.3 dyne cm-1 and 17.0 dyne cm-1 respectively.

For most liquids. surface tension at room temperature varies between 27 and 42 dyne cm-1. For water, r is 72.8 dyne cm-1. This high value is due to strong initermolecular forces which exist in water as a result of extension hyfrogen bonding.

Capillary Action:

when a capilary tube is dipped in a liquid, there occures either a rise or a fall of liquids in tube. This phenomenon is called capillary action and is basically due to surface tension of the liquid. If the forces of attraction between the molecules of a liquid and those of the solid surface of the tube (adhesion) are greater than those existing amongs the molecules of the liquid, then the liquid has a tendency to spread on the solid surface and its meniscus in the tube is concave upwords such type of liquid are known as wetting liquids and they rise in the capillary tube

→The angle of contact, which is measured within the liquid from the side of the tube to the tangent drawn at the meniscus touchiing the surface of the tube, is less than 90˚

→ If the cohesive force in the liquid are greater than solid-liquid attraction forces (adhesive) or if there occures repulsion between the molecules of the liquid and those of the solid surface, the liquid deraches from the surface of the solid. THe meniscus of such a liquid in the tube is convex upwards and its level falls withini the tube. The angle of contact is greater than 90˚. 

→ The rise and fall of a liquid in a capillary tube is due to surface tension. Eq. Take the case of wetting liquid. The surface tension forces act all arround the capillary tube in the direction shown in the fig.

Thde liquid rises in the tube because of these upward forces. It continues to rise till the vertical component of the lifting force becomes equal to the weight of the liquid in the capillary tube

Lifting Force = (r cosθ) (2𝝿rc)

rc→radius of capillary tube

2𝝿rc→ circumference of capillary tube

weight of the liquid in the capillary

tube = {( 𝝿rc2)h} ρ.g

At equilibrium, lifting force is equal to the downward force due to the weight of the liquid in the capillary tube. Therefore-

(γ cosθ) (2𝝿rc) = 𝝿rc2h.ρ.g

where α is now new angle of contact

The rise and fall of a liquid in a capilliary tube id due to surface tension.

eg. Take the case of wetting liquid. The surface tension forces act all around the capillary tube in the direction shown in the fig.

The liquid rises in the tube because of these upward forces. It continues to rise till the vertical component of the lifting force becomes equal to the weight of the liquid in the capillary tube

Lifting force = (γ cosθ)(2𝝿rc)                        rc → radius of capillary tube

weight of the liquid in the capillary-                 2𝝿rc → circumference of capillary tube

tube= {( 𝝿rc2)h}ρ.g

At equilibrium, lifting force is equal to the downward force due to the weight of the liquid in the capillary tube. Therefore

(γ cosθ)(2𝝿rc) =  𝝿rc2h.ρ.g

The above mentioned topic is essential for CSIR Coaching in Chemistry. It is also a hot topic for IIT JAM Coaching in Chemistry