Equilibrium molecular dynamics simulations are used to investigate the effect
of phase transitions on the transport properties of highly-confined water
between parallel graphene sheets. An abrupt reduction by several orders of
magnitude in the mobility of water is observed in strong confinement, as
indicated by reduced diffusivity and increased shear viscosity values. The bulk
viscosity, which is related to the resistance to expansion and compression of a
substance, is also calculated, showing an enhancement compared to the bulk
value for all levels of confinement. An investigation into the phase behaviour
of confined water reveals a transition from a liquid monolayer to a rhombic
frozen monolayer at nanochannel heights between 6.8-7.8 \r{A}; for larger
separations, multilayer liquid water is recovered. It is shown how this phase
transition is at the root of the impeded transport.