Entropy governs the structure and reactivity of water dissociation under electric fields

Abstract

The response of water to electric fields is critical to the performance and stability of electrochemical devices, and the selectivity of enzymatic, atmospheric, and organic reactions. A key process in this context is the water (auto)dissociation reaction (WD), which governs acid-base aqueous chemistry and shapes reaction rates and mechanisms. Despite its significance, the thermodynamics of the WD reaction in electrified environments remains poorly understood. Here, we investigate the WD reaction under external electric fields using ab initio molecular dynamics simulations within the framework of the modern theory of polarization. Our results reveal that strong electric fields dramatically enhance the WD reaction, increasing the equilibrium constant by several orders of magnitude. Moreover, we show that the applied field transforms the WD reaction from an entropically hindered process to an entropy-driven one. Analysis shows that this is because the electric field alters the tendency of ions to be structure makers or structure breakers. By highlighting how strong electric fields reshape solvent organization and reactivity, this work opens new avenues for designing aqueous electro-catalysts that leverage solvent entropy to enhance their performance.

Figures (4)

• Figure 1: Reactivity and structuring of water under electric fields. a) Schematic representation of the model system in the absence (left) and presence (right) of an external electric field. The external electric field is coupled to the system through the electric enthalpy functional (see main text) and leads to a net polarization. The arrow in the right panel inset depicts the field direction. b) Proton defect concentration as a function of the field strength. The threshold for the detection of WD reaction events in the employed setup is $\textbf{E}=0.3$ V/Å. c) Average orientation of molecular dipoles along the field direction for pure water. A value of 1.00 represents a perfect alignment between the molecular dipoles and the external field.
• Figure 2: Dramatic change of temperature dependence of the WD reaction free energy under electric fields. Helmholtz free energy associated with the WD reaction as a function of temperature and selected values of electric field strengths.
• Figure 3: Water and hydrogen-bond network structure under electric fields. a-c) Oxygen-oxygen and oxygen-hydrogen radial distribution functions, d) vibrational density of states (VDOS) of water stretching mode, e) tetrahedral order parameter, and f) classification of water molecules according to the hydrogen bonding environment. Note that the main peak of the VDOS in panel d appears at approximately 2500 cm$^{-1}$ due to the use of deuterium masses for hydrogen atoms.
• Figure 4: Proton conduction under electric fields disrupts the hydrogen-bond network. a) Number of hydrogen-bonds (HB) per water molecule for pure water and 0.86 M aqueous proton solution. b) Proton transfer free energy barrier along the proton sharing coordinate, $\delta$ (see main text for its definition). c) Average orientation of newly created water molecules after a proton transfer event. d) Schematic representation of proton-induced water reorientation (i) Proton transfer (PT) event leads to a newly formed water molecule, which (ii) reorientates its dipole to align it parallel to the external electric field (dipole relaxation). During this reorientation, existing hydrogen bonds are broken and new ones are formed.