title: Role of ion hydration for the differential capacitance of an electric double layer.
abstract: The influence of soft, hydration-mediated ion-ion and ion-surface interactions on the differential capacitance of an electric double layer is investigated using Monte Carlo simulations and compared to various mean-field models. We focus on a planar electrode surface at physiological concentration of monovalent ions in a uniform dielectric background. Hydration-mediated interactions are modeled on the basis of Yukawa potentials that add to the Coulomb and excluded volume interactions between ions. We present a mean-field model that includes hydration-mediated anion-anion, anion-cation, and cation-cation interactions of arbitrary strengths. In addition, finite ion sizes are accounted for through excluded volume interactions, described either on the basis of the Carnahan-Starling equation of state or using a lattice gas model. Both our Monte Carlo simulations and mean-field approaches predict a characteristic double-peak (the so-called camel shape) of the differential capacitance; its decrease reflects the packing of the counterions near the electrode surface. The presence of hydration-mediated ion-surface repulsion causes a thin charge-depleted region close to the surface, which is reminiscent of a Stern layer. We analyze the interplay between excluded volume and hydration-mediated interactions on the differential capacitance and demonstrate that for small surface charge density our mean-field model based on the Carnahan-Starling equation is able to capture the Monte Carlo simulation results. In contrast, for large surface charge density the mean-field approach based on the lattice gas model is preferable.
authors.affiliation: Department of Physics, Institute of Biosciences, Letters and Exact Sciences, Sao Paulo State University, Sao Jose do Rio Preto, Sao Paulo 15084-080, Brazil and Department of Physics, North Dakota State University, Fargo, ND 58108-6050, USA. sylvio.may@ndsu.edu.
authors.forename: Daniel L Z
authors.initials: DL
authors.lastname: Caetano
authors.affiliation: Department of Physics, North Dakota State University, Fargo, ND 58108-6050, USA. sylvio.may@ndsu.edu.
authors.forename: Guilherme V
authors.initials: GV
authors.lastname: Bossa
authors.affiliation: Department of Physics, Institute of Biosciences, Letters and Exact Sciences, Sao Paulo State University, Sao Jose do Rio Preto, Sao Paulo 15084-080, Brazil.
authors.forename: Vinicius M
authors.initials: VM
authors.lastname: de Oliveira
authors.affiliation: Laboratory for Surface Science and Technology, Department of Materials, ETH Zürich, CH-8093 Zurich, Switzerland.
authors.forename: Matthew A
authors.initials: MA
authors.lastname: Brown
authors.affiliation: Department of Physics, Institute of Biosciences, Letters and Exact Sciences, Sao Paulo State University, Sao Jose do Rio Preto, Sao Paulo 15084-080, Brazil.
authors.forename: Sidney J
authors.initials: SJ
authors.lastname: de Carvalho
authors.affiliation: Department of Physics, North Dakota State University, Fargo, ND 58108-6050, USA. sylvio.may@ndsu.edu.
authors.forename: Sylvio
authors.initials: S
authors.lastname: May