Molecules at the Air/Water Interface Experience a More Inhomogeneous Solvation Environment than in Bulk Solvents:A Quantitative Band Shape Analysis of Interfacial Electronic Spectra Obtained by HD-ESFG

摘要

To evaluate the inhomogeneity of the solvation environment at the air /water interface, we quantitatively analyzed the shape of the electronic spectra at the air/water interface for the first time. We measured the interface-selective electronic χ~(2) (second-order nonlinear susceptibility) spectra of solvatochromic coumarin molecules at the air/water interface by heterodyne-detected electronic sum frequency generation (HD-ESFG) spectroscopy. The observed imaginary χ~(2) (Im[χ~(2)]) spectra were well reproduced by the convolution of a line shape function with a Gaussian distribution of a frequency shift, which enabled us to quantitatively determine the peak position and bandwidth of the Im[χ~(2)] spectra. The effective polarity of the air/water interface, which was determined by the peak position, was found to be dependent on each coumarin, which agreed with our previous homodyne-detected ESFG study. Interestingly, the spectra at the air/water interface showed substantially broader bandwidths than those in equally polar bulk solvents or even bulk water, indicating that the solvation environment at the air/water interface is more inhomogeneous than that in bulk solvents. At the air/water interface, the stabilization energy of the solvation not only changes with the change of the position and orientation of the surrounding solvents but also varies with the change of the position and orientation of the solute at the interface. We consider that this unique situation arising from the anisotropy along interface normal brings about a broader distribution of the stabilization energy of solvation at the air/water interface. The present work showed that the air/water interface provides a more inhomogeneous solvation environment than equally polar bulk solvents because of this broader distribution of the local solvation structure at the interface.