Ab Initio Molecular Dynamics Study of Dissociation of Water under an Electric Field

The behavior of liquid water under an electric field is a crucial phenomenon in science and engineering. However, its detailed description at a microscopic level is difficult to achieve experimentally. Here we report on the first ab initio molecular-dynamics study on water under an electric field. We observe that the hydrogen-bond length and the molecular orientation are significantly modified at low-to-moderate field intensities. Fields beyond a threshold of about $0.35\mathrm{V}/\AA$ are able to dissociate molecules and sustain an ionic current via a series of correlated proton jumps. Upon applying even more intense fields ($\sim 1.0\mathrm{V}/\AA$), a 15%–20% fraction of molecules are instantaneously dissociated and the resulting ionic flow yields a conductance of about $7.8{\Omega }^{-1}{\mathrm{cm}}^{-1}$, in good agreement with experimental values. This result paves the way to quantum-accurate microscopic studies of the effect of electric fields on aqueous solutions and, thus, to massive applications of ab initio molecular dynamics in neurobiology, electrochemistry, and hydrogen economy.

Figure 1

Distributions of instantaneous O-O and O-H distances for electric fields in the range between zero (purple curves) and $1\mathrm{V}/\AA$ (red curves). The insets show the evolution of most probable distances with the field intensity. Top panel: intermolecular distance between an oxygen atom and its third hydrogen neighbor; central panel: intramolecular distance between an oxygen atom and its second hydrogen neighbor; bottom panel: oxygen-oxygen closest-neighbor distance. The experimental data were smoothed with the Savitzky-Golay filter method, as typically shown for the zero-field curve in the bottom panel.

Figure 2

Counts of ${\mathrm{OH}}^{-}$ and ${\mathrm{H}}_3{\mathrm{O}}^{+}$ ionic species with lifetimes as abscissas, for field intensities $0.25\mathrm{V}/\AA$, $0.35\mathrm{V}/\AA$, and $1\mathrm{V}/\AA$.

Figure 3

Snapshot of a typical dissociation-diffusion mechanism; the molecules involved in the process are rendered with red coloring, while the surrounding molecules have gray-colored oxygen atoms. In both cases, hydrogens are depicted in white. In particular, a ${\mathrm{H}}_3{\mathrm{O}}^{+}-{\mathrm{H}}_2\mathrm{O}-{\mathrm{OH}}^{-}$ entity is clearly visible and its relevant hydrogen bonds are indicated with a dashed white line. The electric field is oriented along the positive $z$ direction.

Figure 4

Ionic current-voltage characteristic for a cubic cell of 12.42 Å side. The dots represent the calculated values, while the solid line is a guide for the eye.