On the relation of structure and dynamics in aromatic ring-tail structured liquids

We present a combined X-ray and depolarized dynamic light scattering study on a series of liquid phenylalkanes, consisting of an aromatic phenyl ring attached to an alkyl chain of varying length. We study the influence of competing interactions of rings and chains on liquid structure and molecular reorientation. The X-ray scattering curves of the investigated liquids show a weak prepeak in a q range below the main scattering peak, indicating a certain degree of structure formation on a larger length scale than commonly found in simple liquids. As a function of temperature and alkyl chain length, we find that the observed prepeak shares some characteristics with that found for ionic liquids, suggesting a similar origin, i.e., domains of ring groups separated by alkyl chains leading to nanometer-scale structuring. Furthermore, with increasing chain length, the scattering curves show a distinct transition in the temperature dependence of the prepeak amplitude, which is mirrored in the activation energy of molecular reorientation, obtained via depolarized dynamic light scattering. As a possible interpretation, we suggest that ring-ring interactions control structure as well as dynamics for short alkyl chains but rapidly lose influence above a certain alkyl chain length. Since phenylalkanes are among the most simple representatives of liquids consisting of aromatic and non-aromatic units, we regard this work as a proof of concept to study the coupling of structure and dynamics in liquids with competing interactions weaker than both Coulombic interactions and hydrogen bonding.