Surface Tension, pharmaceutics I year

Chapter 5 Surface Tension The boundary between two phases is generally described as an interface. When one of the phases is a gas or a vapour and the other a liquid or solid the term surface is commonly used. • Surface tension is defined as the force acting over the surface of the liquid per unit length of the surface. Force per unit area (F/A) on surface of liquid. Its unit is newton per meter (N/m2 ) EXPLANATION: The molecule inside the liquid interacts equally with other molecule from all sides, whereas the molecule at the surface is affected only by the molecule below it. The molecules exposed to air behave differently. Surface molecules are compressed more tightly together; forming a sort of skin on the surface, with less distance between them compared to the molecules below. The molecules on the surface of a liquid are attracted only sideways and towards the interior so, surface molecules are being pulled inward the liquid. Liquid surface is therefore under tension and surface area of the surface molecules is reduced. Water in particular has a very high surface tension. This explains the characteristic rounded shape that liquids form when dropping through the air: The molecules are all being pulled toward the center. Factors affecting Surface Tension Intermolecular forces Surface tension increases with the increase in intermolecular forces and decreases with the decrease in intermolecular forces. Hydrogen bonding Surface tension increases with the increase in hydrogen bonding and decreases with the decrease in hydrogen bonding. Temperature Surface tension decreases with the increase in temperature and increases with the decrease in temperature. Applications • Emulsifying agents reduce the surface tension of oil and water phase which results in stabilization of emulsion. • Bile salts reduce the surface tension of fats in duodenum which results in solubilization of lipids which help in the digestion of lipids surface tension with definition and explaination with example of water drop