Minimal order gases and liquids

Many biochemical processes take place in the aqueous intracellular space, so we need to understand the structure of liquids in general and of water in particular. 

The form of matter with the least order is a gas. In a perfect gas there are no inter-molecular interactions and the distribution of molecules is completely random. In a real gas there are weak attractions and repulsions that have minimal effect on the relative locations of the molecules but that cause deviations from the perfect gas law for the dependence of pressure on the volume, temperature, and amount. Normally there is no need to consider such deviations in biological applications. 

The attractions between molecules are responsible for the condensation of gases into liquids at low temperatures. First, at low enough temperatures the molecules of a gas have insufficient kinetic energy to escape from each other s attraction and they stick together. Second, although molecules attract each other when they are a few diameters apart, as soon as they come into contact, they repel 

The average locations of the molecules in a hquid are described in terms of the radial distribution function, g(r). This frmction is defined so that g(r)dr is the probability that a molecule will be found at a distcmce between r md r + dr from another molecule. It follows that if (r) passes through a maximum at a radius of, for instance, 0.5 nm, then the most probable distance (regardless of direction) at which a second molecule will be found will be at 0.5 nm from the first molecule. 

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