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Surface Tension

Definition, Examples, Measurement

Surface Tension of Liquid in Chemistry

Surface tension or surface energy is the most important characteristic property of liquids origin at the surfaces and displayed when a liquid contact with its vapour or stretched the liquid by an elastic membrane. In chemistry or physics, surface tension simply define the amount of energy causes to increase the unit area of the water or any other liquid ( wetting or non-wetting) and measure by the units dyne cm-1 (CGS) or newton meter-1 (SI). In general, liquids are obtained by cooling gas molecules below their critical temperature with high pressure or from the solid by the specific heat to overcome translational kinetic energy.

Origin of Property Surface Tension of liquid

The effect of cooling or gas to liquid phase causes to decreases the thermal energy and high pressure causes to increases the density, thereby increasing the forces of attraction amongst the molecules. In the liquid phase, the surface is some sort of tension and tends to contract the smallest possible area in order to contain the minimum number of molecules at the outside. The phenomenon of surface tension in science clearly observed in the spherical shape of small liquid drops (water or rain drop) or soap bubbles in the air.

Origin of Property Surface Tension

Consider a molecule in the bulk of the liquid, the molecules are uniformly surrounded by other molecules and attached from all directions. Therefore, the resultant force on the bulk molecule becomes nil. But a molecule on the interfacial molecule partially surrounded by other molecules and experience a resultant inward pull. As a result, the molecules on the surface try to leave and enter into the bulk of the liquid or the liquid under tension and tries to get the minimum area. This unbalanced force of attraction on these molecules in the liquid is the origin of the properties surface tension.

Surface Tension and Formation of Bubbles

Water drops in nature displayed spherical shape with minimum interfacial energy or area, due to surface tension because the maximum number of water molecules to remain on the bulk rather than the interface. For this reason, when water droplets fall freely it takes a spherical shape.

Another example of bubble formation methods in nature also caused by the fact surface tension or energy. The total pressure acting on the concave side must be larger than the pressure acting on the convex side. Therefore, the pressure inside the bubbles must be larger than the external pressure. If the external pressure is not balanced by these two forces, then the bubbles are not stable and immediately collapsed. Therefore, for getting stable bubbles, these external forces are balanced by other forces namely cementing forces.

Unit of Surface Tension

From the definition in chemistry, surface tension defined as the force acting along the surface of a liquid at the right angle to any line of unit length. This force is called the cohesive force of the liquid. The higher the Inter-molecular attraction force (cohesive force) of liquid greater will be the magnitude of γ. Therefore, surface tension is expressed by force per length having the unit dyne cm-1 (CGS system) and newton meter-1 (SI system).

Wetting and Non-Wetting Liquids

In learning chemistry, liquids are classified into two types, wetting, and non-wetting, depending on the ability to wet the solid surface or interfacial tension. The properties of wetting and non-wetting of depending on the angle of contact on the solid interface. For example, water on glass (angle of contact = 18°) and chemical element like mercury on the glass (angle of contact = 140°).

From common experience, some liquids spontaneously wet the surface of some solids and speed over them while other liquids do not wet or spread on certain surfaces. For examples of wetting liquids like water on the glass, alcohol on the glass or cement, oil on the cloth will easily wet the surface and speed over them but non-wetting liquids like mercury on glass, water on solid paraffin will not speared or wet.

Surface Energy of Liquid

If the area of the liquid increases, the more number of molecules going to its interface from the balk. For equilibrium, some energy is required for balancing upward and inward attractive forces. Therefore, the work (in joule) necessary to create or extend one square meter of the area defines the surface energy of liquid per square meter. With the increasing concentration of liquids, surface energy also decreases. For example, the values of surface tension or interfacial energy for water, benzene, methyl alcohol at 20°C = 72.8, 28.87, 22.55 dyne cm-1 respectively.

Measurement of Surface Tension and Energy

For the measurement and explanation of surface tension or energy of the liquid, we use a film contains a rectangle ware frame and a moveable membrane. Suppose the film stretched by moving membrane x meter and length of membrane = l meter.

Surface tension or surface energy, property of liquids or water acting if stretched unit area of liquid by elastic membrane

If γ symbol represented the surface tension acting per meter, the forces acting towards the film = γ × 2l. Since the length of the film in contact with the ware l on each side. Therefore, the total length = 2l. The work required to increase the interfacial area of the liquids (W) = opposing force × displacement = (γ × 2l) × x = γ × 2(l × x) = γ × ΔA, where ΔA = increase of the area of the film on both sides. Therefore, this mathematical formula uses for the measurement or calculation of the surface energy or tension of any liquids or water in terms of work or energy required to increase the unit area.

Unit and Dimensions of Surface Energy

From the above derivation, surface tension (γ) = W/ΔA or the energy per unit area, numerically equal to the surface energy pure liquid. Therefore, the CGS and SI unit of surface energy = Joule meter-2 and erg cm-2 respectively.

The dimensions of the quantity joule meter-1 or erg cm-1 and newton meter-1 or dyne cm-1  are the same. Therefore, the dimensions of surface energy and surface tension are the same, and different types of instruments or apparatus like the capillary tube, stalagmometer used for measuring it.