Surface tension

Surface tension

A molecule inside the liquid mass (a) is pulled uniformly on all the sides by intermolecular forces. But a surface molecule (b) suffers a much greater intermolecular attraction towards the interior of the liquid than towards the vapour phase, because fewer molecules are present in the vapour phase. The excess of inward force on the surface layer accounts for the surface tension. Surface tension (¡) is defined as the force acting perpendicularly inwards on the surface layer of a liquid to pull its surface molecules towards the interior of the liquid mass.

1. Factors affecting surface tension

·              Density - Macloed’s equation relates surface tension to the density of the liquid (r) and that of its vapour (r’).

∝(r-r')2.

·              Temperature- Temperature and surface tension are inversely related to each other. As the temperature of the liquid increases, the surface tension decreases and becomes zero at the critical temperature.

·              Solutes - Solutes that enter the liquid raise the surface tension of the solvent, while solutes that concentrate on the surface lower the surface tension.

2. Biological Importance

·              Emulsification of fats by bile salts - Bile salts lower the surface tension of the fat droplets in the duodenum, which aids in digestion and absorption of lipids.

·              Surface tension of plasma: The surface tension of plasma is 70 dynes/cm, which is slightly lower than that of water.

·              Hay’s test for bile salts - The principle of surface tension is used to check the presence of bile salts in urine. When fine sulphur powder is sprinkled on urine containing bile salts ( as in jaundice), it sinks due to the surface tension lowering effect of bile salts. If there are no bile salts in urine as in normal individuals, it floats.

·              Dipalmitoyl lecithin is a surfactant that is secreted by the lung alveoli, which reduces the surface tension and prevents the collapse of lung alveoli during expiration. Certain pre-mature infants have low levels of this surfactant leading to acute respiratory distress.