Direct Determination of Hydration Shell Thickness of Molecular Clusters by Analytical Ultracentrifugation

The hydration shell of charged solutes is important for their solution behaviors, influencing interactions with other species and subsequent self-assembly, phase transitions, and potential applications. While modern spectroscopy techniques and simulations have provided insights into the hydration shells of simple ions and biomacromolecules, accurately determining the hydration shell thickness of hydrophilic macroions (size ranging from 1 to 6 nm) remains challenging.

In this study, we employed analytical ultracentrifugation (AUC) to measure the hydration shell thickness of a series of macroions, using some structurally well-defined metal oxide and metal peroxide clusters as model systems. The accurate hydrated radius of macroions is determined via sedimentation velocity (SV) experiments. Their hydration shell thicknesses are obtained by subtracting the bare radius of the clusters. We found that the hydration shell thickness increases linearly with the charge density of macroions and shows almost no temperature dependence in the range of measurements.