Fluctuations of local electric field and dipole moments in water between metal walls

We examine the thermal fluctuations of the local electric field $E_k^{\rm loc}$ and the dipole moment $\mu_k$ in liquid water at $T=298$ K between metal walls in electric field applied in the perpendicular direction. We use analytic theory and molecular dynamics simulation. In this situation, there is a global electrostatic coupling between the surface charges on the walls and the polarization in the bulk. Then, the correlation function of the polarization density $p_z(r)$ along the applied field contains a homogeneous part inversely proportional to the cell volume $V$. Accounting for the long-range dipolar interaction, we derive the Kirkwood-Fr$\ddot{\rm{o}}$hlich formula for the polarization fluctuations when the specimen volume $v$ is much smaller than $V$. However, for not small $v/V$, the homogeneous part comes into play in dielectric relations. We also calculate the distribution of $E_k^{\rm loc}$ in applied field. As a unique feature of water, its magnitude $|E_k^{\rm loc}|$ obeys a Gaussian distribution with a large mean value $E_0 \cong 17~$V$/$nm, which arises mainly from the surrounding hydrogen-bonded molecules. Since $|\mu_k|E_0\sim 30 k_{\rm B}T$, $\mu_k$ becomes mostly parallel to $E_k^{\rm loc}$. As a result, the orientation distributions of these two vectors nearly coincide, assuming the classical exponential form. In dynamics, the component of $\mu_k(t)$ parallel to $E_k^{\rm loc}(t)$ changes on the timescale of the hydrogen bonds $\sim 5$ ps, while its smaller perpendicular component undergoes librational motions on timescales of 0.01 ps.Comment: 17 pages, 15 figures. Accepted in J. Chem. Phy

Solvation and Dissociation in Weakly Ionized Polyelectrolytes

We present a Ginzburg-Landau theory of inhomogeneous polyelectrolytes with a polar solvent. First, we take into account the molecular (solvation) interaction among the ions, the charged monomers, the uncharged monomers, and the solvent molecules, together with the electrostatic interaction with a composition-dependent dielectric constant. Second, we treat the degree of ionization as a fluctuating variable dependent on the local electric potential. With these two ingredients included, our results are as follows. (i) We derive a mass reaction law and a general expression for the surface tension. (ii) We calculate the structure factor of the composition fluctuations as a function of various parameters of the molecular interactions, which provides a general criterion of the formation of mesophases. (iii) We numerically examine some typical examples of interfaces and mesophase structures, which strongly depend on the molecular interaction parameters.Comment: 10 pages, 3 figures. to be published in Journal of Physical Chemistry

Electric double layer composed of an antagonistic salt in an aqueous mixture: Local charge separation and surface phase transition

We examine an electric double layer containing an antagonistic salt in an aqueous mixture, where the cations are small and hydrophilic but the anions are large and hydrophobic. In this situation, a strong coupling arises between the charge density and the solvent composition. As a result, the anions are trapped in an oil-rich adsorption layer on a hydrophobic wall. % while the cations are expelled from it. We then vary the surface charge density $\sigma$ on the wall. For $\sigma>0$ the anions remain accumulated, but for $\sigma<0$ the cations are attracted to the wall with increasing $|\sigma|$. Furthermore, the electric potential drop $\Psi(\sigma)$ is nonmonotonic when the solvent interaction parameter $\chi(T)$ exceeds a critical value $\chi_c$ determined by the composition and the ion density in the bulk. This leads to a first order phase transition between two kinds of electric double layers with different $\sigma$ and common $\Psi$. In equilibrium such two layer regions can coexist. The steric effect due to finite ion sizes is crucial in these phenomena.Comment: 8 Pages, 8 Figs, accepted for Phys. Rev. Let

Selective solvation in aqueous mixtures: Interface deformations and instability

We briefly review the effects of selective solvation of ions in aqueous mixtures, where the ion densities and the composition fluctuations are strongly coupled. We then examine the surface tension \gamma of a liquid-liquid interface in the presence of ions. We show that \gamma can be decreased drastically due to the electrostatic and solvation interactions near the interface. We calculate how the free energy is changed due to small surface undulations in the presence of an electric double layer. A surface instability occurs for negative \gamma, which can easily be realized for antagonistic ion pairs near the solvent criticality. Three-dimensional simulation shows how the surface instability is induced.Comment: 16 pages, 4 figure