Isomeric effects in structure formation and dielectric dynamics of different octanols
Résumé
The understanding of the microstructure of associated liquids promoted by hydrogen-bonding and
constrained by steric hindrance is highly relevant in chemistry physics, biology and for many aspects of
daily life. In this study we use a combination of X-ray diffraction, dielectric spectroscopy and molecular
dynamics simulations to reveal temperature induced changes in the microstructure of different octanol
isomers, i.e., linear 1-octanol and branched 2-, 3- and 4-octanol. In all octanols, the hydroxyl groups
form the bases of chain-, cyclic- or loop-like bonded structures that are separated by outwardly
directed alkyl chains. This clustering is analyzed through the scattering pre-peaks observed from X-ray
scattering and simulations. The charge ordering which pilots OH aggregation can be linked to the
strength of the Debye process observed in dielectric spectroscopy. Interestingly, all methods used here
converge to the same interpretation: as one moves from 1-octanol to the branched octanols, the
cluster structure evolves from loose large aggregates to a larger number of smaller, tighter aggregates.
All alcohols exhibit a peculiar temperature dependence of both the pre-peak and Debye process, which
can be understood as a change in microstructure promoted by chain association with increased chain
length possibly assisted by ring-opening effects. All these results tend to support the intuitive picture of
the entropic constraint provided by branching through the alkyl tails and highlight its capital entropic
role in supramolecular assembly.
Domaines
Matériaux
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