Thursday, 3 March 2011

Surface Tension In Brief

I did not really explain what surface tension is. Surface tension is a phenomenon in which the surface of a liquid, where the liquid is in contact with gas, acts like a thin elastic sheet. This term is typically used only when the liquid surface is in contact with gas (such as the air). If the surface is between two liquids (such as water and oil), it is called "interface tension."

What Causes Surface Tension?

Various intermolecular forces, like hydrogen bonds and Van der Waals draw the liquid particles together. In brief, surface tension arises from the strong interactions between water molecules through these interactions. However, you get a better appreciation of the magic of surface tension if you understand the bonding. Two Hydrogens (hydrogen bond donor) are attached to an electronegative oxygen (hydrogen bond acceptor) covalently. However other bonds called hydrogen bonds are between each water molecule. The hydrogen bond (5 to 30 kJ/mole) is stronger than a van der Waals interaction, but weaker than covalent or ionic bonds. The hydrogen bond is described as an electrostatic dipole-dipole interaction but has some features of stronger bonds like covalent bonds. Liquid water is specials since for every water molecule can be H-bonded to four water molecules. It is this strong interaction which also manifests in the other unusual property of water, its high boiling point, melting point, and viscosity compared to otherwise similar liquids.

As mentioned each water molecule can make up to four H-bonds but this only occurs in the bulk of a liquid. (Water basically likes to hang out in a gang). On the surface, however, the interactions with the neighboring molecules are limited and weaker, resulting in a higher free energy and reduced intermolecular hydrogen bonding of the molecules. Surface tension thus finds how strong these hydrogen bonds on the surface are. is. This interaction on the surface can change with temperature or if you put a surfactant like soap into the medium...