Thermodynamic and structural properties of the two isomers of solid propanol

Abstract

We have measured the specific heat, C p, of the two chemical isomers of propanol, both at low temperatures (1.5–30 K) and around the glass-transition region (≈100 K) . Although the two substances present only the stereoisomeric difference, their glass-transition temperatures differ ( T g ≃98 K for 1-propanol and T g ≃115 K for 2-propanol), though with comparable changes in specific heat. At temperatures below 10 K, both isomers in the glassy state have the typical maximum in C p/ T 3. Nevertheless, the magnitude of the low-temperature specific heat is much larger in 2-propanol than in 1-propanol, a difference which is also found in the Debye contributions in the corresponding crystalline states. To investigate the structural differences between the two isomers of propanol, and their possible influence on the thermodynamic properties, X-ray scattering experiments have been performed on samples of the glassy and crystalline states. In the glassy state, both isomers present the expected amorphous pattern, with characteristic distances of 0.40±0.06 and 0.42±0.07 nm for 1- and 2-propanol, respectively. Preliminary structural analysis of the crystalline states would assign a triclinic structure for 1-propanol and a monoclinic structure for 2-propanol.