The Physics of Water

The self-assembly of living systems is widely thought to necessarily involve the thermodynamics of primordial biomolecules interacting with water. The fact that oil and water do not mix has profound implications for the formation of biological structures, such as the lipid bilayer membrane that surrounds each and every living cell. Water has unique properties that place it at the top of the list of abiogenic ingredients. We begin with the often-neglected nuclear physics of water, how water’s essential hydrogen was produced in the Big Bang, and how oxygen was formed from nucleosynthesis in stars. The quantum mechanics of the water molecule is explored through its symmetry with the use of discrete point group theory. The water molecule’s wave functions are used to construct an energy-level diagram that informs X-ray spectroscopic studies. Intermolecular hydrogen bonding of water molecules is the key to understanding water’s many anomalous thermodynamic properties; its expansion on freezing is one of the reasons that Earth harbored early life. Studies of ice and liquid water are consistent with a two-state model in which there exist both low- and high-density components. Essentially, electrostatic models of water include the intermolecular TIP5P potential. Theoretical investigations are informed by experiments that measure water’s velocity and spatial autocorrelation functions and its radial distribution function. NMR spectroscopy is used to determine the temperature dependence of water’s hydrogen bonds. The hydrophobic effect drives the segregation of lipids, the structures of proteins, and the hydration of ions.