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.