Supramolecular structure of monohydroxy alcohols

Abstract

Hydrogen bonds are essential for the structure and dynamics of alcohols, aqueous solutions, and water. Because of their low tendency of crystallization and large variability in molecular configuration, monohydroxy alcohols have often been studied as model systems for hydrogen-bonded fluids in general. This thesis contains investigations of the structuring in monohydroxy alcohols on molecular level employing x-ray scattering methods such as x-ray diffraction and x-ray Raman scattering. The x-ray diffraction patterns of monohydroxy alcohols yield information about supramolecular structures via a characteristic prepeak on the low momentum-transfer side of the main diffraction peak. These structures are suggested to be ring- or chainlike in the investigated monohydroxy alcohols. Mixtures of the chain building 2-ethyl-1-hexanol with the halogen alkanes 2-ethylhexyl bromide and 2-ethylhexyl chloride were investigated by x-ray diffraction to track the changing of supramolecular structures due to mixing. Moreover, mixtures of ring building monohydroxy alcohols 4-methyl-3-heptanol, 2-butyl-1-octanol and 2-hexyl-1-decanol were studied by means of x-ray diffraction to observe structural rearrangements leading to chainlike structures in the mixtures. Furthermore, the complex pressure and temperature behavior of 2-ethyl-1-hexanol and 4-methyl-3-heptanol has raised questions about the role of hydrogen bonding in these systems at extreme conditions. The x-ray diffraction patterns of these two liquids were measured at pressures up to 4500 bar at temperatures of 273 K, 300 K, and 343 K and at temperatures in the range from 130 K to 380 K at ambient pressure. These results are discussed in terms of formation and destruction of supramolecular arrangements. Additionally, the dynamic structure factors of four different octanol isomers were measured by x-ray Raman scattering at the oxygen K-edge to obtain further information about the hydrogen bond network in alcohols. An ultimate goal is to investigate the dynamics in liquids on a molecular length scale with femtosecond resolution. Therefore, novel instruments are necessary. A machine delivering highly coherent hard x-rays with a short pulse length is the European XFEL, which is being built up in Hamburg at the moment. This light source asks for an extremely fast sample injection system, delivering a sample of just a few microns thickness. A Microliquids FlatJet system is perfectly suited for this purpose. Such a device was purchased, characterized and tested in the frame of this thesis. First test measurements using a von Hamos spectrometer and the liquid jet system simultaneously at a synchrotron light source are presented.