Volumetric Properties of Dilute Aqueous Alcohol Solutions at Different Temperatures

The densities of dilute aqueous solutions of normal alcohols (from methanol to 1-hexanol) and branched-chain alcohols (2-butanol, 2-methyl-1-propanol, and 2,2-dimethyl-1-propanol) were measured at 5, 15, 25, 35, and 45 °C by means of an oscillating-tube densimeter. The apparent molar volumes of the alcohols in aqueous solutions were determined. The temperature dependences of the limiting partial molar volume, excess partial molar volume, concentration dependence, and methylene group contribution to volume were discussed. An alternative analysis based on the use of specific units instead of molar units was also attempted. The characteristic volumetric behavior of aqueous alcohol solutions is more pronounced for shorter- or branched-chain alcohols rather than hydrophobic alcohols. There is no definitive evidence that so-called hydrophobic hydration around the alkyl groups of alcohol in water is accompanied by a prominent volume change. The unique properties, such as negative partial molar expansion, may originate from the hydrogen-bonding interactions between water and alcohol OH groups.