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

It is currently thought that electric fields at air−water interfaces can accelerate organic reactions in microdroplets. Most studies have attributed these strong fields to charge separation near the air–water interface. Herein, we propose an alternate origin for these strong electric fields based on the dipole alignment of organic molecules perpendicular to the air–water interface. Full-spectral confocal fluorescence microscopy images show that the voltage-sensitive fluorophore, 4-Di-1-ASP, exhibits an 18 nm shift in emission at the air–water interface. Solvatochromic measurements 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 region of enhanced field extends ~1 µm into the droplet and is largely insensitive to the addition of up to 2 M NaCl. Thus, these fields cannot be explained solely by charge separation near the air–water interface. Measurements in a range of organic 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 orienting the dyes. These results set the stage for molecular-level control over electric fields at gas–liquid interfaces.