Chemistry Review

Hydrophobic Interactions

Nonpolar molecules cannot form H-bonds with H₂O, and are therefore insoluble in H₂O. These molecules are known as hydrophobic (water hating), as opposed to water loving hydrophilic molecules which can form H-bonds with H₂O. Hydrophobic molecules tend to aggregate together in avoidance of H₂O molecules; hydrophobic interactions are clearly demonstrated with Vinegar and Oil Salad Dressing. The oil floats on the vinegar (an aqueous solution). The oil can be broken up into small droplets suspended in the vinegar, but they always fuse together to form larger and larger droplets until there are two layers again. This attraction/repulsion is known as the hydrophobic (fear of water) force.

Surface : Volume Ratios

One of the most important mathematical concepts in Biology is the relationship between Surface Area and Internal Volume. Briefly stated this relationship says that for any 3-dimensional form, the ratio of Surface Area to Internal volume is NOT constant - it is inversely related to size. In other words, as size gets larger, the Surface Area becomes smaller relative to the Internal Volume.

In this numerical example a sphere is used, but the concept is the same for any 3 dimensional object.

ASIDE: One of the best examples of how this mathematics applies to Biology is in Ecology. Bergmann's Rule and Allen's rule were first discovered by Ecologists, but their explanation is pure Geometry!! See Dr. Lott for an explanation and Dr. Dalby for actual examples!

ASIDE: Another excellent example of how this mathematics applies to Biology is the question of cell size - why are cells so small? Why is it impossible to have a giant amoeba which eats Manhattan? See Dr. Smith or Dr. Eggleton for an explanation!

Insolubility of Hydrophilic and Hydrophobic Molecules

Suppose we consider the 9 small droplets of hydrophobic molecules - is there a difference in their surface areas if they are separated as 9 small droplets, or if they all fuse into a single large one? YES!

A single large droplet would have the same internal volume as 9 small ones, but considerably less surface area than the total surface area of the nine small ones.

The amount of oil (volume) is the same whether it is dispersed in the 9 smaller droplets or a single large droplet. However the single large droplet has far less surface area exposed to the water molecules.

Decrease in Surface Area 452.4 - 217.5 = 234.92 mm² less surface area with a single large droplet instead of 9 small ones.

Per cent Decrease in Surface Area 234.92 / 452.4 = 51.9%. The decrease in surface area is 51.9% of the total for the 9 small droplets.

This means that 51.9% more water molecules can H-bond with each other if the hydrophobic molecules are present in a single large droplet rather than 9 smaller ones.

Therefore the situation is this:

lowest energy state is when the number of H bonds is maximized.
decreasing surface area presented by hydrophobic molecules increases number of H-bonds which can be formed.
surface area presented by hydrophobic molecules decreases as the size of the droplets increases.
Therefore attractions of the water molecules for each other will tend to "squeeze" the fat droplets together into larger (and fewer) droplets.

The end result is that the hydrophobic and hydrophilic molecules separate from each other completely (like the layers in vinegar/oil salad dressing)! They are INSOLUBLE in each other.

RETURN to "Chemistry Review" Page