Hydrophobic Exclusion

∞ generated and posted on 2016.12.21 ∞

Minimization of the contact area between water and substances, especially liquids, that do not readily dissolve in water.

Hydrophobic exclusion describes the tendency, for example, of multiple oil droplets in water to coalesce into fewer, larger droplets. This is driven by the tendency for water molecules to "want" to hydrogen bond in combination with an inability to do so with these hydrophobic substances.

The energy contained within these systems is minimized when the fewest number of hydrogen bonds have been disrupted which, in turn, occurs when contact area between water and the hydrophobic substance is minimized. Since surface-to-volume ratios decrease as objects become larger, the coalescing of droplets has the effect of reducing surface area – and therefore water-oil contact area – as water droplets become larger, even as the total volume of the hydrophobic substance remains constant.

Biologically, hydrophobic exclusion plays key roles in maintaining the integrity of lipid bilayers as well as globular proteins, both of which literally are hydrophobic on the inside and hydrophilic on the outside. This is a state that is maintained by the tendency of water molecules to maximize the number of hydrogen bonds within systems, in this case by bonding to hydrophilic interiors and reducing contact with hydrophobic interiors.

Figure legend: Oil droplets are found in yellow versus water molecules in black and white. Water molecules that are surrounding, that is, are in contact with oil molecules are able to participate in fewer hydrogen bonds. The number of water molecules surrounding the three separated oil drops and therefore participating in fewer hydrogen bonds somewhat exceeds (by about two-fold) the number of water molecules surrounding the three droplets that have nestled together (as circled with the dashed line).

Note that hydrophobic substances additionally display weak interactions among themselves, via van der Waals interactions. These work in addition to hydrophobic exclusion to maintain the integrity of lipid bilayers along with globular proteins within aqueous solutions.