Structure formation and dielectric dynamics of n-octanols

Hydrogen bonds are essential for understanding the microscopic structure of water and aqueous solutions. They stabilize the structure of peptides [1] and proteins [2] and are the driving force for association in molecular fluids [3]. In this study, we use a combination of X-ray diffraction, dielectric spectroscopy, and molecular dynamics simulations to reveal changes in microstructure as a function of temperature for different octanol isomers, i.e., linear 1-octanol and branched 2-, 3-, and 4-octanol. In all octanols, the hydroxyl groups form chain-, cycle-, or loop-like bonded structures separated by outwardly directed alkyl chains. This clustering is analyzed by the scattering peaks observed in X-ray scattering and simulations. The charge order controlling OH aggregation can be related to the strength of the Debye process observed in dielectric spectroscopy. The cluster structure of octanol isomers evolves from loose large aggregates to a larger number of smaller, denser aggregates, with increasing branching. The temperature-dependent results of XRD experiments and dielectric spectroscopy show a change in the distance between meta-objects and an increase in the supramolecular dipole moment with decreasing temperature, indicating a structural change in all octanols. This change in microstructure is promoted by chain association with increasing chain length, possibly aided by ring opening effects. [1] Ludwig, R., Reis, O., Winter, R., Weinhold, F.& Farrar, T. Quantum cluster equilibrium theory of liquids: temperature dependence of hydrogenbonding in liquid N-methylacetamide studied by IR spectra. The Journal of PhysicalChemistry B 102, 9312–9318 (1998). [2] Jeffrey, G. A. & Saenger, W. Hydrogen bonding in biological structures (Springer Science & Business Media, 2012). [3] Böhmer, R., Gainaru, C. & Richert, R. Structure and dynamics of monohydroxy alcohols—Milestones towards their microscopic understanding, 100 years after Debye. Physics Reports 545, 125–195 (2014).