Surface Tension: Definition, Explanation and Methods

In this article we will discuss about Surface Tension:- 1. Definition of Surface Tension 2. Explanation for Surface Tension 3. Method of Determination 4. Factors Affecting 5. Gibbs-Thomson Principle 6. Physiological Importance.

Contents:

1. Definition of Surface Tension
2. Explanation for Surface Tension
3. Method of Determination
4. Factors Affecting
5. Gibbs-Thomson Principle
6. Physiological Importance

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1. Definition of Surface Tension:

The force with which the surface molecules are held together is called surface tension.

2. Explanation for Surface Tension:

The interior molecules of a homogeneous liq­uid are equally attracted in all directions by surrounding molecules.

They are free to move in all directions. But the molecules in the surface of the liquid are attracted downward and sideways but not upward (except for the little attraction of air molecules).

As a result, the molecules of the sur­face are not so free to move. They are held together and form a membrane over the surface of the liquid.

Therefore, when finely powdered sulphur or other non-wetting powders are sprinkled upon water, they do not sink but are suspended on the surface.

A great part of the energy required to convert a liquid into a gas is essential to overcome surface tension and drag the molecules free from the sur­face of the liquid.

There is also an interfacial tension which is biochemically very important, especially in the process of adsorption.

This tension lies at the boundary between immiscible liquids, e.g., oil drops emulsified in water.

The tension is due to unequal attraction of the film molecules as compared with the molecules in the interior of the liquid.

Surface tension x Surface area = Surface en­ergy. A falling drop of liquid assumes a spherical form because the ratio of surface area and total free surface energy is the least.

3. Method of Determination of Surface Tension:

γ = 1/2 hdgr

where, h = height of the liquid.

d = density of the liquid,

g = acceleration due to gravity,

r = radius of the capillary tube.

4. Factors Affecting Surface Tension:

1. Temperature:

Surface tension decreases with the increase in temperature.

2. Dissolved substances:

(a) Most inorganic salts slightly raise surface tension of water although po­tassium permanganate lowers it.

(b) Organic substances usually lower sur­face tension. Soaps and bile salts are most effective in this respect.

(c) Alkalis increase surface tension but ammonia lowers it. Strong mineral acids also decrease surface tension.

(d) In liquid-liquid and solid-liquid sys­tems, dissolved substances generally lower interfacial tension.

5. Gibbs-Thomson Principle in Relation to Surface Tension:

a. Substances that lower the surface tension become concentrated at the interface.

b. Substances that increase surface tension tend to move away from the interface.

c. Lipids and proteins which are both effec­tive in lowering surface tension are found concentrated in the cell wall.

6. Physiological Importance of Surface Tension:

Surface tension is involved in the process of diges­tion; because bile salts reduce the surface tension of lipids and thus assist emulsification.

As a result, the surface area is increased which favours lipase activity on lipids.