Electric fields at hydrophobic water interfaces: spectroscopic evidence, physical origin, and implications on...

Water at interfaces exhibits unique properties that differ markedly from those of bulk water. In particular, a myriad of water-interface-related enhanced reactivities including on-water catalysis and microdroplet chemistry have been documented since the 1980s but remain mechanistically unclear. This review focuses on recent advances in optical spectroscopy and imaging techniques-including fluorescence imaging, vibrational Stark spectroscopy, electrochromism, sum-frequency generation, and high-resolution Raman micro-spectroscopy-that have successfully enabled the detection of interfacial electric fields at different hydrophobic water interfaces (air, liquid and solid). We summarize how both probe-based and label-free optical spectroscopic techniques can consistently quantify the on-water electric field strengths to be on the order of tens of MV cm^-1, corroborated by independent non-spectroscopic techniques, such as electrokinetic and surface charge measurements. The surprisingly close agreement among these different measurements and across broad experimental systems strongly hints at the existence of strong electric fields being a general feature of water-hydrophobe interfaces. We further discuss the physical origins of the interfacial electric field with a particular emphasis on the mechanism of preferential hydroxide accumulation at hydrophobic interfaces. Finally, we examine the implications of strong interfacial electric fields for chemical kinetics, radical generation and thermodynamics, thereby making important connections to interfacial water reactivity. These insights not only contribute to our fundamental understanding of water at interfaces but also point toward new strategies for harnessing interfacial water electrostatics in biomedicine, catalysis, green chemistry, and environmental science.