London Dispersion Forces and Surface Area

The surface area of non-polar particles (atoms or molecules) has a huge effect on the strength of the London Dispersion Forces between them. Specifically, the more surface area two particles have, the stronger the force between them will be.

To understand this, let's picture two molecules made only of carbon and hydrogen -- CH₄ and C₆H₁₄. In the Lewis Dot Structures below the colored oval represents the electrons distributed around the molecules.

If the electrons shift to the left in each of these molecules we will get partial charges just like before.

Now if we had two of the CH₄ molecules next to each other, we would see a small attraction between them.

If, on the other hand, we had 2 of the C₆H₁₄ molecules next to each other, we would have a much larger attraction (really just LOTS of little attractions that add up to a bigger attraction).

This increase in the attraction can be seen looking at simple hydrocarbons (compounds made of carbon and hydrogen).

• CH₄ is a gas at room temperature (weak attraction). 
• C₃H₈ (called propane) can be a liquid at room temperature at high pressure (think of the tank of propane on a gas grill). 
• Butane, C₄H₁₀, has strong enough attractions to be a liquid at room temperature, with only slightly elevated pressure -- it is the fuel in cigarette lighters. 
• Octane (C₈H₁₈) is one of the major components of gasoline, and clearly a liquid (although it evaporates pretty quickly. 
• By the time the chain of carbons has grown to the low teens (for example C₁₃H₂₈) the liquid has become thick due to the extra attractions (this is the type of molecule found in motor oil). 
• Airline fuel (containing, among other things, C₁₈H₃₈) is almost jelly-like in consistency. 
• Add a few more carbons to the chain and you get solid paraffin wax (molecules of C₂₂H₄₆ and others).