  Hydration or interfacial water present in biomolecules and inorganic solids
have been shown to exhibit a dynamical transition upon supercooling. However,
an understanding of the extent of the underlying surface hydrophilicity as well
as the local distribution of hydrophilic/hydrophobic patches on the dynamical
transition is unexplored. Here, we use molecular dynamics simulations with a
TIP4P/2005 water model to study translational and rotational relaxation
dynamics of interfacial water on graphene surfaces. The purpose of this study
is to investigate the influence of both surface chemistry as well as the extent
of hydration on the rotational transitions of interfacial water on graphene
oxide (GO) surfaces in the deeply supercooled region. We have considered three
graphene-based surfaces; a GO surface with equal proportions of oxidized and
pristine graphene regions in a striped topology, a fully oxidized surface and a
pristine graphene surface. The dipole relaxation time of interfacial water
shows a strong-to-strong, strong, and a fragile-to-strong transition on these
surfaces, respectively, in the temperature range of 210-298 K. In contrast,
bulk water shows a fragile-to-strong transition upon supercooling. In all these
cases at high hydration, interfacial water co-exists with a thick water film
with bulk-like properties. To investigate the influence of bulk water on
dynamical transitions, we simulated a low hydration regime where only bound
water (surface water) is present on the GO surfaces and found that the
rotational relaxation of surface water on both the GO and fully oxidized
surfaces show a single Arrhenius behavior. Our results indicate that not only
does the local extent of surface hydrophilicity play a role in determining the
energy landscape explored by the water molecules upon supercooling, but the
presence of bulk water also modulates the dynamic transition.
