Slowing Down of the Molecular Reorientation of Water in Concentrated Alkaline Solutions

It is generally accepted that the hydroxide ion (OH–) is a strong hydrogen bond acceptor and that its anomalously high diffusion constant in water results from a Grotthuss-like structural diffusion mechanism. However, the spatial extent over which OH– ions influence the dynamics of the hydrogen-bond network of water remained largely unclear. Here, we measure the ultrafast dynamics of OH groups of HDO molecules interacting with the deuterated hydroxide ion OD–. For solutions with OD– concentrations up to 4 M, we find that HDO molecules that are not directly interacting with the ions have a reorientation time constant of ∼2.7 ps, similar to that of pure liquid water. When the concentration of OD– ions is increased, the reorientation time constant increases, indicating a strong slowing down of the structural dynamics of the solution.