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Thursday, 29 November, 2007

Specific Ion Adsorption at Hydrophobic Solid Surfaces

D. Horinek and R. R. Netz -
Phys. Rev. Lett. 99, 226104 (2007)

Periodic mesoporous materials formed through the cooperative self-assembly of surfactants and framework building blocks can assume a variety of structures and their widely tuneable properties make them attractive hosts for numerous applications. Because the molecular movement in the pore system is the most important and defining characteristic of porous materials, it is of interest to learn about this behaviour as a function of local structure. Generally, individual fluorescent dye molecules can be used as molecular beacons with which to explore the structure of—and the dynamics within—these porous hosts, and single-molecule fluorescence techniques provide detailed insights into the dynamics of various processes, ranging from biology to heterogeneous catalysis. However, optical microscopy methods cannot directly image the mesoporous structure of the host system accommodating the diffusing molecules, whereas transmission electron microscopy provides detailed images of the porous structure, but no dynamic information. It has therefore not been possible to 'see' how molecules diffuse in a real nanoscale pore structure. Here we present a combination of electron microscopic mapping and optical single-molecule tracking experiments to reveal how a single luminescent dye molecule travels through linear or strongly curved sections of a mesoporous channel system. In our approach we directly correlate porous structures detected by transmission electron microscopy with the diffusion dynamics of single molecules detected by optical microscopy. This opens up new ways of understanding the interactions of host and guest.Molecular dynamics simulations of ions at a hydrophobic self-assembled monolayer with polarizable force fields for water and ions are used to extract potentials of mean force for Na+ and the halide ions Cl-, Br-, and I-. Similar to the air-water interface, the large halide ions are attracted to the surface, which is traced back to surface-modified ion hydration. The total effective interaction is parametrized and used within Poisson-Boltzmann theory to calculate surface potentials and interfacial tensions at finite ion concentration in qualitative agreement with experiments.

 

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