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Item type:Publication, Access status: without files , Static dielectric permittivity profiles and coarse-graining approaches for water in graphene slit pores(AIP Publishing, 2026-03-27); ; ; ; The dielectric response of nano-confined fluids is important for a wide range of technologies and biological systems, ranging from the fundamentals of energy storage to the stability of lipid membrane bilayers. Its calculation from molecular simulations has become a viable tool since the 2010s and is often employed to rationalize experimental observations. However, confusion has emerged on the underlying boundary conditions, and correspondingly, the concept of dielectric profiles is often misunderstood in the literature. Here, we re-derive the Green–Kubo relation for the linear dielectric response profile of fluids in planar confinement, while carefully considering the different underlying boundary conditions; if the latter are correctly accounted for, profiles from equilibrium simulations are perfectly in line with results using externally applied fields. We discuss possible pitfalls and address misunderstandings, regarding both the detailed calculation and the general concept of dielectric profiles, as well as procedures to coarse-grain the microscopic dielectric behavior in order to connect to experimental observables. Simulations consistently reveal that the dielectric response of water is bulk-like down to confining scales as small as ∼1 nm. However, the effective dielectric response of confined systems, which determines their capacitance, depends on the precise location of the dielectric interface. Using concepts from effective medium theory, we show that the long-range reduction of the effective dielectric response, reported in both experimental and theoretical studies, is in line with a dielectric response that reaches the bulk value at about 1 nm from the surface. The effective dielectric response can be interpreted in terms of the commonly employed concept of an interfacial capacitance. We find that the dielectric properties of simulated water are independent of the simulation details and the water model employed, thus revealing the universal properties of water polarization correlations.Publicationtype: Journal Article4 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Water structuring induces nonuniversal hydration repulsion between polar surfaces: quantitative comparison between molecular simulations, theory, and experiments(American Chemical Society (ACS), 2024-04-05); ; ; ; Polar surfaces in water typically repel each other at close separations, even if they are charge-neutral. This so-called hydration repulsion balances the van der Waals attraction and gives rise to a stable nanometric water layer between the polar surfaces. The resulting hydration water layer is crucial for the properties of concentrated suspensions of lipid membranes and hydrophilic particles in biology and technology, but its origin is unclear. It has been suggested that surface-induced molecular water structuring is responsible for the hydration repulsion, but a quantitative proof of this water-structuring hypothesis is missing. To gain an understanding of the mechanism causing hydration repulsion, we perform molecular simulations of different planar polar surfaces in water. Our simulated hydration forces between phospholipid bilayers agree perfectly with experiments, validating the simulation model and methods. For the comparison with theory, it is important to split the simulated total surface interaction force into a direct contribution from surface–surface molecular interactions and an indirect water-mediated contribution. We find the indirect hydration force and the structural water-ordering profiles from the simulations to be in perfect agreement with the predictions from theoretical models that account for the surface-induced water ordering, which strongly supports the water-structuring hypothesis for the hydration force. However, the comparison between the simulations for polar surfaces with different headgroup architectures reveals significantly different decay lengths of the indirect water-mediated hydration-force, which for laterally homogeneous water structuring would imply different bulk-water properties. We conclude that laterally inhomogeneous water ordering, induced by laterally inhomogeneous surface structures, shapes the hydration repulsion between polar surfaces in a decisive manner. Thus, the indirect water-mediated part of the hydration repulsion is caused by surface-induced water structuring but is surface-specific and thus nonuniversal.Publicationtype: Journal Article15 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Dielectric constant of aqueous solutions of proteins and organic polymers from molecular dynamics simulations(American Institute of Physics (AIP), 2022-06-14); The dielectric constant of water/oligomer mixtures, spanning the range from pure water to pure oligomeric melts, is investigated using molecular dynamics (MD) simulations. As prototypical water-soluble organic substances, we consider neutral poly-glycine, poly-ethylene glycol, and charged monomeric propionic acid. As the water content is reduced, the dielectric constant decreases but does not follow an ideal mixing behavior. The deviations from ideal mixing originate primarily in the non-linear relation between the oligomer mass fraction and collective polarization effects. We find that the dielectric constant is dominated by water polarization, even if the oligomer mass fraction exceeds 50%. By a double extrapolation of the MD simulation results to the limit of vanishing water fraction and to the limit of infinite oligomeric chain length, we estimate the orientational contribution to the dielectric constant of the pure polymeric melts. By this procedure, we obtain ϵ = 17 ± 2 for polyglycine and ϵ = 1 ± 0.3 for polyethylene glycol. The large difference is rationalized by polarization correlations of glycine units. Interestingly, we find constant temperature simulations to outperform replica exchange simulations in terms of equilibration speed.Publicationtype: Journal Article11 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Water at charged interfaces(2021-07-01)11 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Molecular interpretation of the non-Newtonian viscoelastic behavior of liquid water at high frequencies(American Physical Society (APS), 2020-10-14); ; ; ; Using classical as well as ab initio molecular dynamics simulations, we calculate the frequency-dependent shear viscosity of pure water and water–glycerol mixtures. In agreement with recent experiments, we find deviations from Newtonian-fluid behavior in the THz regime. Based on an extension of the Maxwell model, we introduce a viscoelastic model to describe the observed viscosity spectrum of pure water. We find four relaxation modes in the spectrum which we attribute to (i) hydrogen–bond network topology changes, (ii) hydrogen–bond stretch vibrations of water pairs, (iii) collective vibrations of water molecule triplets, and (iv) librational excitations of individual water molecules. Our model quantitatively describes the viscoelastic response of liquid water on short timescales, where the hydrodynamic description via a Newtonian-fluid model breaks down.Publicationtype: Journal Article11 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Universal and nonuniversal aspects of electrostatics in aqueous nanoconfinement(2020-05-04); ; ; ; Dielectric water properties, which significantly change in confinement, determine electrostatic interactions and thereby influence all molecular forces and chemical reactions. We present comparative simulations of water between graphene sheets, decanol monolayers, and phospholipid and glycolipid bilayers. Generally, dielectric profiles strongly differ in perpendicular and parallel surface directions and for large surface separation decay to the bulk value 1-2 nm away from the surface. Polar surface groups enhance the local interfacial dielectric response and for phospholipid bilayers induce a giant parallel contribution. A mapping on a box model with asymptotically determined effective water layer widths demonstrates that the perpendicular effective dielectric constant for all systems decreases for confinement below a nanometer, while the parallel one stays rather constant. The confinement-dependent perpendicular effective dielectric constant for graphene is in agreement with experimental data only if the effective water layer width is suitably adjusted. The interactions between two charges at small separation depend on the product of parallel and perpendicular effective water dielectric components; for large separation the interactions depend on the confining medium. For metallic confining media the interactions at large separation decay exponentially with a decay length that depends on the ratio of the effective parallel and perpendicular water dielectric components.Publicationtype: Journal Article4 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Giant axial dielectric response in water-fille nanotubes an effective electrostatic ion-ion interactions from a tensorial dielectric moel(American Chemical Society (ACS), 2019-11-25); ; ; ; Molecular ynamics simulations in conjunction with effective meium theory are use to investigate ielectric effects in water-fille nanotubes. The resulting effective axial ielectric constant shows a ivergent increase for small nanotube raii that epens on the nanotube length, while the effective raial ielectric constant ecreases significantly for thin nanotubes. By solving Poisson's equation for an anisotropic ielectric meium in cylinrical geometry, we show that the axial ion-ion interaction epens for small separations primarily on the raial ielectric constant, not on the axial one. This means that electrostatic ion-ion interactions in thin water-fille nanotubes are on the linear ielectric level significantly enhance ue to water confinement effects at small separations, while at large separations the outsie meium ominates. If the outsie meium is metallic, then the ion-ion interaction ecays exponentially for large ion separation.Publicationtype: Journal Article8 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Comment on “Hydrophobic surface enhances electrostatic interaction in water”(American Physical Society (APS), 2019-07-24); ; ; ; Publicationtype: Journal Article5 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Memory-kernel extraction for different molecular solutes in solvents of varying viscosity in confinement(American Physical Society (APS), 2019-07-19); ; ; ; Publicationtype: Journal Article8 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Simulations of Nanoseparated Charged Surfaces Reveal Charge-Induced Water Reorientation and Nonadditivity of Hydration and Mean-Field Electrostatic Repulsion(American Chemical Society (ACS), 2019-01-15); 5