The distribution of the isotope molecules is different in phases that are in thermo­dynamic equilibrium, including the liquid/gas, liquid/solid, and solid/gas phases. Similarly, the solubility of the isotope molecules is also different.

The isotope effects in the liquid/gas phases have been well studied. The effect can be characterized by the partial pressure of the isotope molecules:

where p and p’ are the partial pressure of the lighter and the heavier molecule, respectively, and є is the relative partial pressure. The degree of the isotope effect is usually low: є«1. Of course, the different partial pressures result in different boiling points (Table 3.2).

Usually, the partial pressure of the lighter molecules is greater. If not, an inverse isotope effect exists. Among the molecules in Table 3.2, methane shows an inverse isotope effect.

A well-known isotope effect in the solid/liquid phase is the ice/water system. The boiling point of 2H2O and 1H2O is different. As a result, the deuterium content of the icy seas is greater than the average deuterium content of the oceans.

The adsorption of the isotope molecules can also be different, a fact that is used in adsorption chromatography to separate isotopes. As the pressure and temperature decrease, the isotope effects increase, resulting in increased separation factors.

The different solubility of the isotope molecules have mainly been studied dur­ing the dissolution of light and heavy water in organic solvents. The inorganic salts and some organic compounds dissolve differently in light and heavy water.