Dependence on External Electrical Field

Recently, Toney et al. [140] published the results of X-ray reflectivity measurements on dilute aqueous sodium fluoride solutions near positively and negatively charged silver electrodes with (111) surface geometry. The analysis shows an oscillatory density profile with three or four density maxima near the interface, qualitatively similar to the results in Fig. 7. They also found a very large shift of the distance of the first 

9 molecules per surface atoms would be expected from MD simulations. In spite of the simple, or even simplistic, nature of the interaction models, it seems very unlikely from the simulations and geometric considerations that water coverages as high as the ones quoted in Ref. 140 can occur. A simple calculation based on the jellium model by Schmickler et al. [68] appears to be able to reproduce qualitatively the observed change in metal oxygen distance. This work, however, does not address the problem of the extremely high densities. 

The experiments by Toney and coworkers are very important since they provide at the present time the only direct experimental evidence for the layering of water molecules in the vicinity of metal surfaces. However, they also have been highly controversial because of the extreme increase in density. Simulations show an increase in correlation, not in total density. The discrepancies are probably an indication for specific adsorption or another yet unknown electrode process in the experiments. In the near future another series of experiments will be performed by Ocko and coworkers which will hopefully resolve the controversy. 

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Moreover, for the observables depending on external electric field, its specific effect has to be investigated the electric field induces new terms in the nuclear Hamiltonian, due to the change of equilibrium geometry and the nuclear motion perturbation. Pandey and Santry (14) has brought to the fore this effect and calculated the correction which only concerns the parallel component. It is represented by the following expression ... 

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