1-Propanol hydrate by IR spectroscopy

The mid-IR attenuated total reflectance (mid-IR-ATR) spectra of a series of 1-propanol and water mixtures were obtained. Factor analysis (FA) applied to spectra gave the spectra of three principal species and their abundance: pure water, pure propanol, and a 1-propanol hydrate (1:1). When compared to the pure solvents, the hydrate propanol bands were modified which indicated that the valence bonds were perturbed. The assignment of the hydrate IR bands was made by comparing them with those of pure 1-propanol and pure water. No monomer was observed which indicated that all species were associated. The simplest representation of the different associations is by groups of two on which an intermolecular bond (H-bond) can be placed. The exchange between the molecules can be represented by the following equation: (H2O...H2O) + (CH3CH2CH2OH...OHCH2CH2CH3) reversible arrow 2(CH3CH2CH2OH...OH2). The hydrate's formation constant (K-f) is 1.2 (+/-0.2). This value is less than the one expected from the association of freely moving molecules, indicating that the hydrophobic interactions of the 1-propanol aliphatic chains decrease the strength of the hydrogen bond between water and alcohol in the hydrate.