Long-Range Dipole Alignment of Molecules Generates Strong Local Electric Fields at Air–Water Interfaces

It is currently thought that chemical and physical properties of air−water interfaces can accelerate organic reactions in microdroplets. Recent studies have focused on strong electric fields due to charge separation near the air–water interface as a potential source of this enhanced reactivity. Herein, we propose that strong electric fields can also arise from the dipole alignment of organic molecules perpendicular to the air–water interface. Full-spectral confocal fluorescence microsco-py images show that the voltage-sensitive fluorophore, 4-Di-1-ASP, aligns its dipole perpendicu-lar to the air–water interface and exhibits an 18 nm shift in emission. Solvatochromic measure-ments and density functional theory calculations estimate the strength of this electric field at 7.2-13 MV/cm and 4.6 MV/cm, respectively. The interfacial region extends ~1 µm into the droplet and is largely insensitive to the addition of up to 2 M NaCl. Thus, these fields cannot be ex-plained solely by charge separation near the air–water interface. Measurements in a range of or-ganic solvents demonstrate that water must be present to observe a shift in the emission maxima of voltage-sensitive dyes at gas–liquid interfaces, indicating that water plays a key role in orient-ing the dyes. These results set the stage for molecular-level control at gas–liquid interfaces.