title: Ionization at a solid-water interface in an applied electric field: Charge regulation.
abstract: We investigate ionization at a solid-water interface in an applied electric field. We attach an electrode to a dielectric film bearing silanol or carboxyl groups with an areal density Γ0, where the degree of dissociation α is determined by the proton density in water close to the film. We show how α depends on the density n0of NaOH in water and the surface charge density σmon the electrode. For σm> 0, the protons are expelled away from the film, leading to an increase in α. In particular, in the range 0 < σm< eΓ0, self-regulation occurs to realize α ≅ σm/eΓ0for n0≪ nc, where ncis 0.01 mol/L for silica surfaces and is 2 × 10-5mol/L for carboxyl-bearing surfaces. We also examine the charge regulation with decreasing the cell thickness H below the Debye length κ-1, where a crossover occurs at the Gouy-Chapman length. In particular, when σm∼ eΓ0and H ≪ κ-1, the surface charges remain only partially screened by ions, leading to a nonvanishing electric field in the interior.
authors: Department of Chemistry, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan.
authors: Ryuichi
authors: R
authors: Okamoto
authors: Department of Physics, Kyoto University, Kyoto 606-8502, Japan.
authors: Akira
authors: A
authors: Onuki