Molecular Structures at Free Surfaces of Liquid n-Heptadecane Investigated by Infrared External Reflection Spectroscopy

Molecular structures at the free surface of liquid n-heptadecane (n-C 17 H 36 ; freezing point (T f ) 22°C, surface freezing point (T s ) 24 °C) were studied by infrared external reflection spectroscopy. Upon successively cooling the liquid sample from 28 °C, at which the sample assumes an isotropic state, to 26, 25, and 24 °C, the infrared external reflection spectra exhibited drastic changes, indicating changes in the orientation and conformation of surface species. At 26 °C, an irregular structure of n-C 17 H 36 with a gauche conformation changes into a regular one with a mainly trans conformation, and simultaneously the symmetry axis of the terminal methyl group tilts away from the surface normal. At 25 °C, the hydrocarbon chain raises its axis to the direction of the surface normal, and the carbon skeleton plane inclines to the direction of the surface plane. At 24 °C (T s ), the molecular axis is raised further to the direction of the surface normal, and the surface species form a rotator phase at the surface. Thus, upon cooling from a temperature a few degrees above T s , surface freezing of n-C 17 H 36 proceeds through discrete structural and orientation changes of the surface species, contrasting with the explanations of previous studies in terms of an abrupt structural change at the free surface occurring at T s .