Identification and Molecular Understanding of the Odd–Even Effect of Dicarboxylic Acids Aqueous Solubility

The solubilities of the homologous series of dicarboxylic acids, HOOC–(CH2) n−2–COOH (n = 2–10), in water have been measured at temperatures ranging from 288.15 to 323.15 K by a static analytic method at atmospheric pressure. Dicarboxylic acids with even numbers of carbon atoms exhibit lower solubilities than acids with adjacent odd carbon numbers. The odd–even effect of solubility is most likely associated with the twist of the molecules, which influences the molecular packing in the solid state: the molecules stack with some offset in the cases of even (n = even) series, but without offset in the cases of odd (n = odd) series, whereas the carboxyl groups are twisted in even members. The interlayer packing is looser in odd members than that in even ones. The energies of intramolecular torsion were calculated using Materials studio 6.0 (Accelrys Software Inc.). Finally, the molar Gibbs energies were predicted, which also showed odd–even alternation.