"half-hydration" at the air/water interface revealed by heterodyne-detected electronic sum frequency generation spectroscopy, polarization second harmonic generation, and molecular dynamics simulation

A solute-solvent interaction at the air/water interface was investigated both experimentally and theoretically, by studying a prototypical surface-active polarity indicator molecule, coumarin 110 (C110), adsorbed at the air/water interface with heterodyne-detected electronic sum frequency generation (HD-ESFG) spectroscopy, polarization second harmonic generation (SHG), and a molecular dynamics (MD) simulation. The second-order nonlinear optical susceptibility ( X⁽²⁾) tensor elements of C110 at the air/water interface were determined experimentally by HD-ESFG and polarization SHG, and information on "intermediate" polarity sensed by C110 at the interface was obtained by HD-ESFG. An MD simulation and a time-dependent density functional theory calculation were used to theoretically evaluate the X⁽²⁾ tensor elements, which were in good agreement with the experimental results of HD-ESFG and polarization SHG. The microscopic "half-hydration" structure around C110 at the water surface was visualized on the basis of the MD simulation data, with which we can intuitively understand the microscopic origin of the surface activity of C110 and the intermediate polarity sensed by C110 at the air/water interface.