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Oil evaporation

In Science Corner on March 9, 2008 at 4:56 pm

Oil evaporation


Unlike water, why do coconut oil and other edible oils not evaporate?


C. S. Tamil Selvan

Pothanur, Tamil Nadu A liquid consists of molecules (or atoms) which perpetually move around. The freedom with which they move depends on the size, weight and shape of the molecules and more importantly on the strength of attraction between the molecules. These characteristic features differ from one liquid to another and decide the rate of evaporation.

At any temperature, the different molecules constituting the liquid perpetually collide against each other and exchange energy so as to possess a distribution of energies.

Most of the molecules have much smaller energy than the intermolecular bond energy and so do not have the ability to leave the liquid surface, but very few molecules at the liquid surface have sufficient energy to break the intermolecular bond and come out of the liquid surface; this is the process of evaporation.

At higher temperatures, the fraction of higher energy molecules is higher and so the evaporation rate is higher at higher temperature. Similarly, if the liquid is spread out to have larger free surface area, the evaporation rate is proportionally higher.

As a general rule, while comparing liquids of similar materials, the ones with higher molecular size and weight evaporate slowly. However, comparison of different classes of liquids needs to be based on careful examination.

Water molecule is made up of one oxygen atom bonded to two hydrogen atoms; but each molecule is not totally free of the neighbouring molecules but they are bonded to each other through a kind of bond called the hydrogen bond.

These bonds are stronger than the dipolar bonds between relatively passive molecules but are weaker than the bonds between atoms inside the molecule itself. This accounts for lower evaporation rate of water as compared to alcohols.

The edible oils, on the other hand, have rather large molecules of long carbonaceous chains consisting of more than about 15 carbon atoms.

The bonding between these molecules is weaker than that in the case of water, but still the evaporation rate is very much lower. This is because the size and anisotropic shape of the oil molecules do not allow free movement past each other.

Another effect is specifically important in the case of such long chain molecules. Often the molecular collision can take place from the lateral side of the chain and the energy transferred in these events gets used up in bending the chain rather than in moving it.

This effect shifts the energy distribution to lower energy side and the energy available for molecular translational motion is reduced while the kinetics of removal of the massive oil molecules require much larger amount of energy to be possessed by the molecules.

Dr. H. K. Sahu

IGCAR, Kalpakkam, Tamil Nadu