Experimental Electronic Spectroscopy at Liquid Interfaces

To measure electronic spectra of solute molecules at interfaces, one must overcome the problem that most of the UV and visible signals absorbed or emitted from solute molecules are generated in the bulk. Unless most of the solute molecules are at the interface, which is the case for systems such as adsorbed 

By applying this approach to adsorbed dyes at liquid/liquid interfaces, Eisenthal and coworkers noticed that the polarity of several liquid/liquid interfaces (between water and 1,2-dichloroethane, chlorobenzene, or heptane) is close to the arithmetic average of the polarities of the two bulk phases. Because it is known that most of the contribution to solvation energy comes from the first solvent-solute coordination shell, the simple arithmetic rule indicates that DEPNA and the Ej 30) indicator molecules have a mixed solvation environment at the water/liquid interface. 

The idea of assigning a specific polarity value to a liquid interface region on the basis of a solvatochromic shift of an adsorbed dye molecule involves several assumptions. Although these assumptions have been found to be violated sometimes, they provide valuable new insights into the structure of and molecular interactions at interfaces. The assumptions typically made include the following  

Eor another example at the liquid/liquid interface. Steel and Walker used two different solvatochromic probe molecules, para-nitrophenol (PNP) and 2,6-dimethyl-para-nitrophenol (dmPNP), to study the polarity of the water-cyclohexane interface. These probes give spectral shifts as a function of bulk solvent polarity that are very similar because both solutes are mainly sensitive to the nonspecific solvent dipolar interactions. However, when these two dye molecules are adsorbed at the water/cyclohexane interface, they experience quite different polarities. The more polar solute (PNP) has a maximum SHG peak that is close to that of bulk water, and thus it reports a high-polarity environment. In contrast, the less polar solute (dmPNP) reports a much lower interface polarity, having a maximum SHG peak close to that of bulk cyclohexane. Clearly, the more polar solute is adsorbed on the water side of the interface, keeping most of its hydration shell, and thus reports a higher polarity than does the nonpolar solute. Other examples of the surface polarity dependence on probe molecules are discussed in Ref. 363. 

Dependence on probe location and orientation. Prom our discussion of the neat interface we now know that this region is very narrow, and the SHG peak spectrum will likely depend strongly on the solute location and orientation. This was proved by Steel and Walker who designed a series of surfactant solvatochromic probes they call molecular rulers.Each of these surfactant molecules consists of an anionic hydrophilic sulphate group (which is restricted to the aqueous phase), attached to a hydrophobic solvatochromic probe moiety by a variable length alkyl spacer. The probe is based on para-nitroanisole (PNA), whose bulk solution excitation maximum shifts monotonically with solvent polarity. When these surfactant molecules adsorb at the interface, the anionic end is in the aqueous phase, and the probe moiety resides at variable positions relative to the interface. 

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