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CPP: Fachverband Chemische Physik und Polymerphysik
CPP 34: Emerging Topics in Chemical and Polymer Physics, New Instruments and Methods I
CPP 34.10: Talk
Thursday, March 21, 2024, 12:15–12:30, H 0106
Thermal expansion on molecular scale: heterogeneity and packing — •Martin Treß1, Jan Gabriel2, and Friedrich Kremer1 — 1Universität Leipzig — 2Roskilde University, Denmark
While typically used for chemical characterization, infrared spectroscopy also reveals physical properties on molecular scale which are difficult to access otherwise. We study the thermal expansion of molecular bonds in amorphous systems, i.e. a series of polyalcohols as well as liquid water. By analyzing specific molecular vibrations and correlating them with interatomic bond lengths, the thermal expansion of covalent bonds and intermolecular hydrogen(H)-bonds is quantified. Pronounced differences between intra- and intermolecular expansion verify the dominance of the latter. Surprisingly, in polyalcohols the macroscopic thermal expansion (i.e. the cube root of inverse density) is even bigger than that of the strong H-bonds. This suggests that additional weak H-bonds or van-der-Waals contacts dominate the thermal expansion. Since strong H-bonds are the largest barriers to molecular rotation, i.e. they control the structural relaxation time, a clear explanation for the failure of density scaling - an attempt to link structural relaxation time with density - in H-bonding liquids is revealed. Liquid water exhibits an even more complex connection of intermolecular bond expansion and macroscopic expansivity due to the anomalous temperature dependence of its density. Our results demonstrate that heterogeneities in intermolecular contacts play distinct roles in densification and structural relaxation which requires explicit consideration in comprehensive theoretical descriptions of liquids and glass formers.
Keywords: Infrared spectroscopy; hydrogen bonding; molecular expansivity; glycerol; water