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structure of water at interfaces Ion adsorption at interfaces is strongly affected by these properties of interfacial water. Traditionally, the vicinity of hydrophobic interfaces is thought to be free of ions, because they are repelled through image forces. During the last decade, it was shown that this view is not correct for the water / vapor interface (P. Jungwirth and D.J. Tobias, Chem. Rev. 106, 1259 (2006)). Some ionic species with a high polarizability compensate for the image repulsion with an attractive interaction arising from induced dipoles in the electric field in the interfacial region.
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biopolymers at the water/solid interface At high pulling rates, equilibrium is no more satisfied, and friction effects become important. We used MD simulations to study the molecular mechanism of friction of a polymer on a surface. |
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The properties of water in the vicinity of an interface are different than the properties of bulk water, giving rise to many surprising and intriguing effects (S.I. Mamatkulov, Langmuir 20, 4756 (2004)). One key property is the hydrophobicity of the interface. Hydrophobic interfaces break the hydrogen bond structure of water, and the interfacial water molecules reorient to achieve their minimum energy. This induces oscillations of the average dipole moment normal to the interface, which vanishes inside the bulk. A narrow layer of low water density is established at the interface, corresponding to a 0.2 nm thick vacuum. A surface potential of ~500 mV exists along the interface.
We are studying the adsorption of ions at different solid interfaces (diamond and self assembled monolayers along with generic models) with a combination of MD simulations and analytic electrostatics. MD simulations are used for the calculation of the potential of mean force of ion adsorption. These simulations require realistic parameters, of special importance is the polarizability of the ions. Recently, we have calculated these polarizabilities using quantum chemistry, and discussed various solvation models.
Non-covalent forces are responsible for the attraction of polymers in solution to surfaces of solid. These forces can be measured by Atomic Force Microscopy (AFM), where a single polymer strand is pulled off a solid substrate. Generally, they are divided into electrostatic and non-electrostatic interactions. In this project we investigate in the second. Atomistic resolution is hereby required, whereas coarse-grained models are sufficient to describe the electrostatics. We perform MD (molecular dynamics) simulations, where short peptides (< 15 amino acids) are peeled off a hydrophobic surface. The required desorption forces are monitored, and the resulting force curves are compared to AFM experiments, which are done in the