Electric-field induced entropic effects in liquid water

Externally applied electric fields in liquid water can induce a plethora of effects with wide implications in electrochemistry and hydrogenbasedtechnologies. Although some effort has been made to elucidate the thermodynamics associated with the application of electric fields inaqueous systems, to the best of our knowledge, field-induced effects on the total and local entropy of bulk water have never been presentedso far. Here, we report on classical TIP4P/2005 and ab initio molecular dynamics simulations measuring entropic contributions carried bydiverse field intensities in liquid water at room temperature. We find that strong fields are capable of aligning large fractions of moleculardipoles. Nevertheless, the order-maker action of the field leads to quite modest entropy reductions in classical simulations. Albeit moresignificant variations are recorded during first-principles simulations, the associated entropy modifications are small compared to the entropychange involved in the freezing phenomenon, even at intense fields slightly beneath the molecular dissociation threshold. This finding furthercorroborates the idea that electrofreezing (i.e., the electric-field-induced crystallization) cannot take place in bulk water at room temperature.In addition, here, we propose a molecular-dynamics-based analysis (3D-2PT) that spatially resolves the local entropy and the number densityof bulk water under an electric field, which enables us to map their field-induced changes in the environment of reference H2O molecules.By returning detailed spatial maps of the local order, the proposed approach is capable of establishing a link between entropic and structuralmodifications with atomistic resolution.