Evidence of supercoolable nanoscale water clusters in an amorphous ionic liquid matrix

Molecular Dynamics Study of the Green Solvent Polyethylene Glycol with Water Impurities

Polyethylene glycol (PEG) is one of the environmentally benign solvent options for green chemistry. It readily absorbs water when exposed to the atmosphere. The Molecular Dynamics (MD) simulations of PEG200, a commercial mixture of low molecular weight polyethyelene glycol oligomers, as well as di-, tetra-, and hexaethylene glycol are presented to study the effect of added water impurities up to a weight fraction of 0.020, which covers the typical range of water impurities due to water absorption from the atmosphere. Each system was simulated a total of four times using different combinations of two force fields for the water (SPC/E and TIP4P/2005) and two force fields for the PEG and oligomer (OPLS-AA and modified OPLS-AA). The observed trends in the effects of water addition were qualitatively quite robust with respect to these force field combinations and showed that the water does not aggregate but forms hydrogen bonds at most between two water molecules. In general, the added water causes overall either no or very small and nuanced effects in the simulation results. Specifically, the obtained water RDFs are mostly identical regardless of the water content. The added water reduces oligomer hydrogen bonding interactions overall as it competes and forms hydrogen bonds with the oligomers. The loss of intramolecular oligomer hydrogen bonding is in part compensated by oligomers switching from inter- to intramolecular hydrogen bonding. The interplay of the competing hydrogen bonding interactions leads to the presence of shallow extrema with respect to the water weight fraction dependencies for densities, viscosities, and self-diffusion coefficients, in contrast to experimental measurements, which show monotonous dependencies. However, these trends are very small in magnitude and thus confirm the experimentally observed insensitivity of these physical properties to the presence of water impurities.

Preserving fast ion dynamics while introducing mechanical rigidity in gelatin-based ionogels

Ionogels are gels containing ions, often an ionic liquid (IL), and a gelling agent. They are promising candidates for applications including batteries, photovoltaics or fuel cells due to their chemical stability and high ionic conductivity. In this work we report on a thermo-irreversible ionic gel prepared from a mixture of the ionic liquid 1-butyl-3-methylimidazolium ([BMIM]) dicyanamide ([DCA]), water and gelatin, which combines the advantages of an ionic liquid with the low cost of gelatin. We use (i) dielectric spectroscopy to monitor the ion transport, (ii) dynamic light scattering techniques to access the reorientational motions of the ions, as well as fluctuations of the gel matrix, and (iii) rheology to determine the shear response from above room temperature down to the glass transition. In this way, we are able to connect the microscopic ion dynamics with the meso- and macroscopic behavior of the gelatin matrix. We show, by comparing our results to those for a IL-water mixture from a previous study, that although some weak additional slow relaxation modes are present in the gel, the overall ion dynamics is hardly changed by the presence of gelatin. The macroscopic mechanical response, as probed by rheology, is however dominated by the gel matrix. This behaviour can be highly useful e.g. in battery gel electrolytes which prevent electrolyte leakage and combine mechanical rigidity and flexibility.