Dresden 2011 – scientific programme

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CPP: Fachverband Chemische Physik und Polymerphysik

CPP 6: Charged Soft Matter

CPP 6.11: Talk

Monday, March 14, 2011, 17:00–17:15, ZEU 160

Reversed Hofmeister Series: The Interplay of Surface Charge and Surface Polarity — •Nadine Schwierz, Dominik Horinek und Roland R. Netz — Physik Department, Technische Universitaet Muenchen, James Franck Strasse, 85748 Garching, Germany

More than 120 years ago, Hofmeister discovered that different ions can be ordered reproducibly according to their efficiency in precipitating hen-egg white proteins. That sequence now runs under the name Hofmeister series and can be found everywhere in chemistry and biology. The complex interplay of electrostatics, dispersion forces, hydration, ion size effects and interfacial water structure make it hard to identify a universal law. We describe a two-scale modeling approach toward anion specificity at surfaces of varying charge and polarity. Explicit-solvent atomistic molecular dynamics simulations at neutral hydrophobic (nonpolar) and neutral hydrophilic (polar) self-assembled monolayers furnish potentials of mean force for sodium and the halide anions, which are then used within Poisson-Boltzmann theory to calculate ionic distributions at surfaces of arbitrary charge for finite ion concentration. We obtain the direct anionic Hofmeister series at negatively charged hydrophobic surfaces. Reversal takes place when going to negative polar or to positive nonpolar surfaces, leading to the indirect series, while for positive polar surfaces the direct series is again obtained in full accordance with a recent experimental classification. A schematic Hofmeister phase diagram is proposed. Partial series reversal is understood as a transient phenomenon for surfaces of intermediate polarity or charge.