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Reversed Anionic Hofmeister Series: The Interplay of Surface Charge and Surface Polarity
N. Schwierz et al.
Langmuir 2010, 26(10), 7370–7379 (doi)
We describe a two-scale modeling approach towards anion specificity at surfaces of varying charge and polarity. Explicit-solvent atomistic molecular dynamics simulations at neutral hydrophobic and neutral hydrophilic self-assembled monolayers furnish potentials of mean force for Na+ and the halide anions F-, Cl-, and I- which are then used within Poisson-Boltzmann theory to calculate ionic distributions at surfaces of arbitrary charge for finite ion concentration. Based on calculated long-ranged electrostatic forces and coagulation properties, 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. This is in full accordance with a recent experimental classification of colloidal coagulation kinetics and also reflects the trends of the ion specific solubility properties of proteins. A schematic Hofmeister phase diagram is proposed. Partial series reversal is understood as a transient phenomenon for surfaces of intermediate polarity or charge.
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 recent research highlight:
Electrohydraulic Power Conversion in Planar Nanochannels
D. Bonthuis et al.
PRL 2009, 103, 144503 (doi)
The electrophoretic mobility of neutral solutes like oil droplets and air bubbles in water has been a highly debated issue for over a century. Conventional theory states that a non-zero zeta-potential cannot exist without mobile charged species present, but the polarity of water molecules is generally not included in the analysis. Using the Navier-Stokes equation, generalized to include the effect of the polar nature of water, we show that the dipolar water ordering does not contribute to the zeta potential of neutral solutes. Nevertheless, rotating electric fields can be used to drive pure water through a neutral nanochannel based on the coupling between molecular spin and vorticity. This electrohydraulic power conversion offers a new alternative for pumping mechanisms in nanodevices.
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Soft matter easily deforms when excited by thermal motion or by external fields and is
abundant in biological and synthetic systems. Typical questions concern electrostatic effects,
water structure, complexes of polymers and colloids, or non-equilibrium hydrodynamics at structured
surfaces (for more info see the research page).
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 recent research highlight:
Peptide adsorption on a hydrophobic surface results from an interplay of
solvation, surface and intrapeptide forces
D. Horinek et al.
Proc. Nat. Acad. Sci. USA 2008, 105, 2842
(doi)
see also a recent
TUM press release
The hydrophobic effect in protein folding involves structural changes on all length scales,
and the various energy contribution cannot be disentangled. We address this issue by looking at the force
necessary to peel a single spider silk molecule from a flat hydrophobic diamond surface in the presence of
water. Using atomic-force spectroscopy, we determine the mean desorption force as F = 58 ± 8 pN.
All-atomistic MD simulations including explicit water yield F = 54 ± 15 pN. An energy analysis shows
that solvation effects, dispersion, and intrapeptide forces give contributions have to be taken into account.
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We use a wide arsenal of analytical and simulation techniques, ranging from hydrodynamic simulations
over Green’s-functional approach to quantum-chemistry ab-initio calculations, for describing soft
matter systems. Our group is located at the Technical University in Munich. We collaborate
with many of the excellent experimental groups in the region. There are many exciting research topics, so
applications for Diploma and Ph.D. positions are always welcome!
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Conference info:
Ion Specific Phenomena in Physics, Chemistry and Biology (15. - 17.09.08)
 New Journal: Biointerphases
an Open Access Journal for the Biomaterials Interface Community
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Visitors, 10.06.08 |
last update 30.03.2009 by n. schwierz |
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