Opening image: Methane clathrate (crystalline state). For clarity, only the first hydration shell is shown. spacefill.
Gas hydrates are macroscopic solids termed clathrates in which individual molecules of gas are trapped within a rigid hydrogen-bonded network of water molecules. Gases such as methane are actually hydrophobic, yet the hydrophobic guests are essential in stabilizing the overall structure. They are also of considerable industrial interest, as gas hydrate formation is a major cause of gas and petroleum pipeline blockages; also, there are vast deposits of naturally-occurring methane hydrate, and these could provide a major source of natural gas in the 21st century. Whether or not crystalline clathrates are relevant to the hydrophobic effect, their very existence has strongly influenced our views of how nonpolar solutes behave in water.
The surface representation of the hydration cage.
Even though one liter of the crystalline hydrate contains 164 liters of methane, each methane molecule is completely surrounded by a water cage. Twenty four molecules of water form a rigid cage surrounding one methane molecule. The water molecules are arranged into interlocking pentagons and hexagons.
There are . It is important to note that the both hexagons are planar and not ice-like structurally.
There are . Although pentagons are not found in ice, they are common in liquid water. The hydrogen bonding in a pentagon is closer to ideal geometry than that in the hexagons in the crystalline hydrate.
We return to the spacefilled model. Note that the water molecules avoid pointing their hydrogen-bonding groups toward the methane molecule. To do so would waste hydrogen bonds.
A water molecule that solvates methane is sterically blocked from interacting freely with the surrounding "regular" (bulk) water. Rotation of a water molecule in the bulk phase involves breaking of hydrogen bonds. However, new hydrogen bonds are being formed as old ones are being broken, so the free energy change is essentially zero. The situation is different for a water molecule in the hydration shell of the clathrate. If it rotates inward toward the methane molecule, it loses one or more hydrogen bonds but cannot form new hydrogen bonds. The first-shell water molecules lose entropy to gain hydrogen bonding.
The problem with methane clathrate as a model of the hydrophobic effect is that the clathrate exists only at low temperatures and high pressures. Do hydrated methane molecules stay apart from each other in liquid water, or do they aggregate and eventually escape into the gas phase as the concentration of dissolved methane increases? To answer this question we turn to a landmark paper by Ned Wingreen which appeared in 2007.