Direct Determination of Hydration Shell Thickness of Molecular Clusters

A general approach is developed by using the analytical ultracentrifugation (AUC) technique through sedimentation velocity (SV) experiments to measure the hydration shell thicknesses of a series of macro-ionic, structurally well-defined metal oxide and metal peroxide molecular clusters in aqueous solution. These clusters with known molar masses and bare sizes serve as valuable models where the accurate hydration shell thickness can be determined by comparing the bare cluster size and its hydrodynamic size from AUC studies. The molecular clusters investigated here have sizes ranging from ∼1 nm to 3 nm. Within the experimental range, the hydration shell thickness increases from less than 0.1 nm for Keggin clusters to ∼0.6 nm for heavily charged clusters (up to 0.79 C/m²). For those relatively large clusters, the relationship between their hydration shell thickness and surface charge density can be described by a linear equation. However, hydration shells of Keggin clusters are found to be negligible. Weak acid-type clusters with pH-dependent charge density (here {Mo₇₂Fe₃₀}) show thicker hydration shells with heavier charge density (at higher pH). Meanwhile, temperature appears to have a minimal effect on the hydration shell within the experimental range, as higher temperatures result in only slightly thinner hydration shells.