Evidence for supramolecular dynamics of non-hydrogen bonding polar van der Waals liquids

TL;DR

This study reveals that non-hydrogen bonding van der Waals liquids can form supramolecular structures, with dielectric and rheological evidence showing coupled slow dynamics driven by dipole-dipole interactions.

Contribution

It provides experimental evidence of supramolecular formation in non-hydrogen-bonding van der Waals liquids using dielectric spectroscopy and rheology, challenging previous assumptions.

Findings

Detection of a Debye-like dielectric process slower than structural relaxation.

Observation of dynamical separation between terminal relaxation and structural rearrangements.

Strong coupling between dielectric relaxation time and rheological terminal time.

Abstract

Non-hydrogen bonding van der Waals liquids with dipole-dipole interactions are typically viewed as non-associative and not considered able to sustain large supramolecular structures. Combining broadband dielectric spectroscopy (BDS) and rheology, we demonstrate the supramolecular formation in a group of non-hydrogen-bonding van der Waals liquids, i.e. 1-bromo-2-ethylhexane, 1-chloro-2-ethylhexane, and 1-bromo-3,7-dimethyloctane. BDS shows an emergence of a Debye-like process slower than their structural relaxation, which follows super-Arrhenius temperature dependence. Meanwhile, rheological measurements reveal a noticeable dynamical separation between the terminal relaxation and the structural rearrangements. Interestingly, the rheological terminal time agrees remarkably well with the dielectric Debye-like relaxation time, pointing to a strong coupling between the terminal flow and the supramolecular dynamics of these van der Waals liquids. These results highlight the role of intermolecular dipole-dipole interactions on the structure and slow dynamics of van der Waals liquids.