Structural Dynamics (Sep 2022)

Structure and dynamics of supercooled water in the hydration layer of poly(ethylene glycol)

  • Yuqing Li,
  • Zehua Han,
  • Changli Ma,
  • Liang Hong,
  • Yanwei Ding,
  • Ye Chen,
  • Junpeng Zhao,
  • Dong Liu,
  • Guangai Sun,
  • Taisen Zuo,
  • He Cheng,
  • Charles C. Han

DOI
https://doi.org/10.1063/4.0000158
Journal volume & issue
Vol. 9, no. 5
pp. 054901 – 054901-9

Abstract

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The statics and dynamics of supercooled water in the hydration layer of poly(ethylene glycol) (PEG) were studied by a combination of quasi-elastic neutron scattering (QENS) and molecular dynamics (MD) simulations. Two samples, that is, hydrogenated PEG/deuterated water (h-PEG/D2O) and fully deuterated PEG/hydrogenated water (d-PEG/H2O) with the same molar ratio of ethylene glycol (EG) monomer to water, 1:1, are compared. The QENS data of h-PEG/D2O show the dynamics of PEG, and that of d-PEG/H2O reveals the motion of water. The temperature-dependent elastic scattering intensity of both samples has shown transitions at supercooled temperature, and these transition temperatures depend on the energy resolution of the instruments. Therefore, neither one is a phase transition, but undergoes dynamic process. The dynamic of water can be described as an Arrhenius to super-Arrhenius transition, and it reveals the hydrogen bonding network relaxation of hydration water around PEG at supercooled temperature. Since the PEG-water hydrogen bond structural relaxation time from MD is in good agreement with the average relaxation time from QENS (d-PEG/H2O), MD may further reveal the atomic pictures of the supercooled hydration water. It shows that hydration water molecules form a series of pools around the hydrophilic oxygen atom of PEG. At supercooled temperature, they have a more bond ordered structure than bulk water, proceed a trapping sites diffusion on the PEG surface, and facilitate the structural relaxation of PEG backbone.