Separable cooperative and localized translational motions of water confined by a chemically heterogeneous environment

Abstract

We report quasi-elastic neutron scattering experiments at two resolutions that probe timescales of picoseconds to nanoseconds for the hydration dynamics of water, confined in a concentrated solution of N-acetyl-leucine-methylamide (NALMA) peptides in water over a temperature range of 248 K to 288 K. The two QENS resolutions used allow for a clean separation of two observable translational components, and ultimately two very different relaxation processes, that become evident when analyzed under a combination of the jump diffusion model and the relaxation cage model. The first translational motion is a localized β-relaxation process of the bound surface water, and exhibits an Arrhenius temperature dependence and a large activation energy of ∼8 kcal mol⁻¹. The second non-Arrhenius translational component is a dynamical signature of the α-relaxation of more fluid water, exhibiting a glass transition temperature of ∼116 K when fit to the Volger Fulcher Tamman functional form. These peptide solutions provide a novel experimental system for examining confinement in order to understand the dynamical transition in bulk supercooled water by removing the unwanted interface of the confining material on water dynamics.