Is fast relaxation water really a free water?

The origin of fast relaxation water in the terahertz frequency range have been elusive for years. In this contribution we have experimentally investigated the change in dynamical structure of water in salt solution using broadband dielectric relaxation spectroscopy (DRS) in the GHz to THz region to study the fast water. Numerical simulations have been conducted to quantify the dynamical properties in the hydration shell of the ions, including rotational and translational residence times. As the amount of the fast water increases with salt solution concentration, the fraction of water molecules with zero or one hydrogen-bond (HB) is also increased. We can conclude that the fast water is composed of the free water and one hydrogen-bonded water.