Water Dynamics in the Hydration Shell of Amphiphilic Macromolecules

In the present work, the dynamics of water in the hydration shell of amphiphilic hyperbranched polyglycerol (HPG) molecules was investigated using THz and GHz dielectric relaxation spectroscopies. Different from the typical small amphiphilic molecules, the HPG macromolecule provides a rather dispersionless background and allows the direct observation of four types of water molecules in the hydration shell, including bulk-like water, undercoordinated water, slow water (water molecule hydrating hydrophobic groups and water molecule accepting hydrogen bond from OH hydroxyl groups), and super slow water that are strongly hydrogen-bonded to ether groups. For bulk-like water and undercoordinated water, the time constant remains invariant with HPG concentration, while the time constant of slow water molecules increases from 15 to 24 ps with increase of HPG concentration. Differently, the time constant of super slow water molecules decreases dramatically with HPG concentration.