Ions and dipoles in electric field: Nonlinear polarization and field-dependent chemical reaction

Abstract: We investigate electric-field effects in dilute electrolytes with nonlinear polarization. As a first example of such systems, we add a dipolar component with a relatively large dipole moment $\mu_0$ to an aqueous electrolyte. As a second example, the solvent itself exhibits nonlinear polarization near charged objects. For such systems, we present a Ginzburg-Landau free energy and introduce field-dependent chemical potentials, entropy density, and stress tensor, which satisfy general thermodynamic relations. In the first example, the dipoles accumulate in high-field regions, as predicted by Abrashikin {\it et al}.$[$Phys.Rev.Lett. {\bf 99}, 077801 (2007)$]$. Finally, we consider the case, where Bjerrum ion pairs form a dipolar component with nonlinear polarization. The Bjerrum dipoles accumulate in high-field regions, while field-induced dissociation was predicted by Onsager $[$J. Chem. Phys.{\bf 2}, 599 (1934)$]$. We present an expression for the field-dependent association constant $K(E)$, which depends on the field strength nonmonotonically.