J.A. Clark, V.M. Prabhu, J.F. Douglas
National Institute of Standards and Technology,
Keywords: polybetaine, zwitterionic polymers, static hydration layer, dynamic hydration layer, water mobility gradients, Debye-Waller parameter, molecular dynamics
Summary:We investigate the hydration of poly(3-[2-(acrylamido) ethyldimethylammonio] propanesulfonate) over a range of temperatures in pure water and with the inclusion of NaCl using atomistic molecular dynamics simulation. Concepts drawn from the field of glass-forming liquids capture a solvent mobility gradient extending 18 Å from the polybetaine backbone. Here, mobility is defined as the mean square displacement at a characteristic time on the order of water’s fast β-relaxation time, i.e., the Debye-Waller parameter (), a fast dynamics property that has been shown to be related to the -structural relaxation time of bulk water. The inner 10.5 Å of this 18 Å interfacial region is explored by pendant conformational motions and the remaining 7.5 Å represents vicinal solvent with hindered mobility. This water mobility gradient defines a dynamic hydration layer that extends far beyond the static hydration layer, defined as the water directly interacting with the polybetaine.