Confinement effects on glass-forming mixtures: Insights from a combined experimental approach to aqueous ethylene glycol solutions in silica pores.

We perform nuclear magnetic resonance, broadband dielectric spectroscopy, and differential scanning calorimetry studies to ascertain the dynamical behaviors of aqueous ethylene glycol (EG) solutions in silica pores over broad temperature ranges. Both translational and rotational motions are analyzed, and the pore diameter (2.4-9.2 nm) and the EG concentration (12-57 mol. %) are varied, leading to fully liquid or partially crystalline systems. It is found that the translational diffusion coefficient strongly decreases when the diameter is reduced, resulting in a slowdown of nearly three orders of magnitude in the narrowest pores, while the confinement effects on the rotational correlation times are moderate. For the fully liquid solutions, we attribute bulk-like and slowed down reorientation processes to the central and interfacial pore regions, respectively. This coexistence is found in all the studied pores, and, hence, the range of the wall effects on the solution dynamics does not exceed ∼1 nm. Compared to the situation in the bulk, the concentration dependence is reduced in confinements, implying that the specific interactions of the molecular species with the silica walls lead to preferential adsorption. On the other hand, bulk-like structural relaxation is not observed in the partially frozen samples, where the liquid is sandwiched between the silica walls and the ice crystallites. Under such circumstances, there is another relaxation process with a weaker temperature dependence, which is observed in various kinds of partially frozen aqueous systems and denoted as the x process.