Generalized Interface Polarity Scale Based on Second Harmonic Spectroscopy

Quantitative polarity scales have been of great value in advancing our understanding of chemical and physical processes in bulk solvents. However, an understanding of the polarity at liquid interfaces has been more elusive and no scale of interface polarity currently exists. Following our demonstration that second harmonic spectroscopy can be used to measure interface polarity, we have determined the polarity of several liquid/ liquid and vapor/liquid interfaces. The polarities of the water/1,2-dichloroethane and water/chlorobenzene interfaces have been investigated using the polarity indicator molecule N,N-diethyl-p-nitroaniline (DEPNA). The betaine dye 4-(2,4,6-triphenylpyridinium)-2,6-diphenylphenoxide, (ET(30)), was used to probe the polarity of the air/water interface. The intramolecular charge transfer (CT) absorption band positions of both DEPNA and ET(30) are measured and used to define the interface solvent polarity. An important finding is that the polarity of the liquid interfaces is simply related to the polarity of the bulk phases. The interface polarity is found to be equal to the arithmetic average of the polarity of the adjoining bulk phases. This surprisingly simple result suggests the possible dominance of the long-range solute-solvent interactions, not the local interface interactions, in determining the difference in the excitedand ground-state solvation energies of the interface adsorbed molecules.