The surface tension of water is about 72 mN/m at room temperature which is one of the highest surface tension for liquid. There is only one liquid having higher surface tension and that’s mercury which is a liquid metal with a surface tension of almost 500 mN/m. Why the surface tension of mercury is so high will also be evident after reading this short blog post.
The surface tension arises due to cohesive interactions between the molecules in the liquid. At the bulk of the liquid, the molecules have neighboring molecules on each side. Molecules are pulling each other equally in all directions causing a net force of zero. However, at the interface, the liquid molecules have only half of the neighboring liquid molecules compared to the bulk of the liquid. This makes the molecule associate more strongly with the molecules at its sides and causes a net inward force towards the liquid. This force resists the breakage of the surface and is called surface tension.
With this explanation in mind, it’s then clear that all the liquids will have the same property but why the surface tension of water is much higher than ethanol for example.
To understand this we need to think of the bonds between the molecules. As explained, the cohesive force between the molecules causes surface tension. The stronger the cohesive force, the stronger the surface tension. The water molecule has two hydrogen atoms bond to an oxygen atom through covalent bonding. Due to the high electronegativity of oxygen, it will have a large portion of the negative charge on its side whereas hydrogen will be more positively charged. This causes an electrostatic attraction between the hydrogen atom in one molecule and the oxygen atom in another. Formed bonds are called hydrogen bonds which lead to strong cohesive forces between the water molecules and high surface tension of water.
As mentioned at the beginning of the blog, this also explains why mercury has so high surface tension. As mercury is a metal, the bonds between the molecules are metal bonds that are much stronger than the hydrogen bonds leading to very high cohesive forces and high surface tension.
To learn more about the importance of surface tension through the link below
Surface tension can be measured with force tensiometer using Du Noüy ring or Wilhelmy plate. Or with optical tensiometer using the pendant drop method.
Du Noüy method utilizes the interaction of a platinum ring with the surface of the liquid.
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