Surface tension is also related to the intermolecular forces in the liquid. The molecules in liquids are held closely and hence attract each other. A molecule in the bulk of the liquid is attracted equally on all sides so that the net attractive pull on the molecule is zero. However, a molecule at the surface is subjected only to the attractive forces of the molecules below it, as there are no molecules above it. Therefore, surface molecules experience a resultant downward attractive force from within the liquid, which tends to make the surface area of the liquid as small as possible. This causes the molecules at the surface to be pulled inwards and so there is always some residual imbalance force acting on the surface of the liquids. This is called surface tension. Whenever we want to stretch the surface, work has to be done against surface tension. Surface tension may be defined as:'Force per unit length acting perpendicular to the tangential line on the surface'. It may also be defined as 'the work done to increase the free surface area of any liquid by one unit at constant temperature and pressure.'
The SI unit of surface tension or force per unit length is Nm-1. Since this unit is too big for any practical purpose we use a smaller unit mN m-1 (millinewton meter-1).
Fig: 2.10 - Forces at the surface and interior of the liquid
Effect of temperature
Surface tension decreases with rise in temperature, almost linearly. The decrease of surface tension with increase in temperature results because the kinetic energy (or speeds) of the molecules increases. Thus, the strength of intermolecular forces decreases resulting in the decrease of surface tension also. For example, clothes are washed more efficiently in hot water than in cold water due to decreased surface tension in hot water.The effect of temperature on surface tension is given by Eotvas equation
where, g = surface tension, k = constant, d = density
M = Molecular mass, Tc = critical temperature; T= temperatureAs 'T' approaches critical temperature, the surface tension becomes zero. At this stage the meniscus between the liquid and vapour disappears.
Nature of liquid
Primarily surface tension of a liquid is governed by the strength of intermolecular attractive forces. Therefore, the magnitude of surface tension is a measure of intermolecular attractive forces. For instance the order of strength between inter molecular forces as well as the surface tensions of water, ethyl alcohol and ether are in their respective orders:Water (72.8)> Ethyl alcohol (22.3) > Ether (17.0).
Impurities present in the liquid
Impurities affect surface tension appreciably. It is observed that impurities, which tend to concentrate on the surface of liquids, compared to its bulk lower the surface tension. Substances like detergents, soaps, alcohol lower the surface tension of water, while inorganic impurities present in the bulk of a liquid such as NaCl tend to increase the surface tension of water.
Increase of pressure on the surface of a liquid increases the surface tension. Such effects are not large.The following two important phenomena are due to surface tension:
Spherical shape of drops
The liquid drops have nearly spherical shapes. Because of surface tension, the free surface of a liquid tends to attain minimum surface area. Since the sphere has minimum surface area for a given volume of liquid, the liquid tries to adopt spherical shape. Examples are water droplets or mercury globules.
When one end of a capillary tube is put into a liquid that wets glass, the liquid rises into the capillary tube to a certain height and then stops. The rise of liquid in a capillary is due to the inward pull of surface tension acting on the surface, which pushes the liquid into the capillary tube. This phenomenon is called capillary action. The rise of liquid in the capillary is very important. For example, water below the surface of the Earth rising to the plants through the roots, oil rising into the wick of an oil lamp, ink rising in a blotting paper, are all examples of capillary action.