The O2/N2 Ratio Gas Solubility Mystery

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Oxygen (O2) and nitrogen (N2) are the two most abundant gases in the Earth’s atmosphere and have generally similar physical properties, yet O2 is twice as soluble in water as N2, a feature that may have physiological and other biological consequences. Furthermore, examination of 47 other solvents shows that the mole-fraction solubility of O2 always exceeds that of N2, with O2/N2 solubility ratios ranging from 1.20 in n-dodecane to 2.31 in nitromethane. The greater solubility of O2 is especially puzzling since the molecular polarizability of N2, a feature normally associated with higher solubility, is greater than that of O2. Several theoretical and empirical approaches are explored in an effort to understand this observation: (1) molecular structure–property relationships, (2) thermodynamic analysis, (3) scaled particle theory, (4) ideal solution theory, and (5) quantum-chemical calculations. Speculations on the causes are offered.