Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2021 / Paper 1 / 10 / ii / c

DMSO is a strong hydrogen-bond acceptor. It competes for the NH donors of the UPy units and solvates the association sites, weakening their mutual hydrogen bonding.

For A, the intermolecular chains should shorten or dissociate, reducing the association-driven viscosity. For B, the intramolecular ring should become less populated, increasing the fraction of open conformations. Sufficient DMSO also weakens rings of A and any intermolecular associations of B. The covalent molecular skeletons remain intact.

The extent depends on DMSO fraction and the association free energies in the mixed solvent. solvent viscosity itself also changes, so the prediction concerns the supramolecular contribution rather than an unconditional numerical value for total solution viscosity.