Simulations of Electronic Transitions at Interfaces

A limitation of all the experimental methods used to learn about the interface structure from the behavior of adsorbed solute molecules is that the solute perturbs the interface to some degree. Computer simulations that reproduce the experimental data and provide molecular insight help provide a link between the neat and the perturbed structures. For example, simulations that reproduce the spectral shift of an adsorbed solute at a liquid/Uquid interface can reveal the relative contribution of each solvent and provide an explanation of the average polarity rule mentioned earlier. 

The methodology for computing electronic spectra discussed earlier has been used extensively to calculate absorption and emission spectra in bulk liquids. Exactly the same procedures can be used at liquid interfaces. We report here a few results that have provided complementary insight to the experiments. A more detailed review of this topic is provided in Ref. 363. 

The role of many-body polarizable potentials in MD-based predictions of electronic spectra at the water liquid/vapor interface was examined by Benjamin for a model dipolar solute. The peak absorption spectrum was found to be more sensitive to the solvent polarizability than to the solute polarizability. The contribution of many-body polarizabilities was found to be more pronounced as the excited state dipole was increased, and more in the bulk than at the interface. 

The 1-octanol/water interface polarity was studied experimentally by Walker and coworkers using the molecular ruler idea discussed earlier.  

Results suggest that at the water/octanol interface, a region exists whose polarity is less than that of bulk 1-octanol (and of bulk water). This finding was 

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