Macromolecular vs molecular crowding in aqueous solutions: A comparative study of PEG400 and ethylene glycol

Concentrated solutions, also known as crowded solutions, are known to provide a very different environment for dissolved solutes than dilute solutions. Understanding the molecular origin of these differences is important not only for a fundamental understanding of the properties of solutions, but also for the applicative implications. The latter range from making in vitro biochemical assays more similar to in vivo conditions, to the design of aqueous matrices in which the activity of water is reduced (e.g. for batteries). Using a combined computational-experimental approach, we compare the hydration properties of concentrated aqueous solutions of a polymer with an analogue of its monomeric unit in water. The aim of this work is to disentangle the effects related to the chemical nature of a solute from those related to its dimension, and for this purpose we have studied the pair polyethylene glycol, ethylene glycol (PEG versus EG). The analysis of increasingly concentrated solutions of the two systems reveals a completely different microscopic behaviour, in which the different clustering/aggregation of both solutes and water plays the main role. Indeed, water within EG solutions primarily constitutes high-density hydrating water, whereas PEG solutions exhibit confined, bulk-like water patches. These findings indicate that molecular crowding could potentially exhibit greater efficacy than macromolecular crowding in all applications requiring a reduction in water activity.