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

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 a key role of intermolecular dipole-dipole interaction in the supramolecular structure formation and the slow dynamics of van der Waals liquids.