Strong Electric Field Observed at the Interface of Aqueous Microdroplets

Abstract

Chemical reactions in aqueous microdroplets often exhibit unusual kinetic and thermodynamic properties not observed in bulk solution. While an electric field has been implicated at the water interface, there has been no direct measurement in aqueous microdroplets, largely due to the lack of proper measurement tools. Herein, we employ newly developed stimulated Raman excited fluorescence microscopy to measure the electric field at the water−oil interface of microdroplets. As determined by the vibrational Stark effect of a nitrile-bearing fluorescent probe, the strength of the electric field is found to be on the order of 10 7 V/cm. This strong electric field aligns probe dipoles with respect to the interface. The formation of the electric field likely arises from charge separation caused by the adsorption of negative ions at the water−oil interface of microdroplets. We suggest that this strong electric field might account in part for the unique properties of chemical reactions reported in microdroplets.

Can Electric Fields Drive Chemistry for an Aqueous Microdroplet?

“…Kathmann et al using ab initio molecular dynamics found that the vapor-water interface has an average interfacial electric eld strength of − 1.5x10 2 MV/cm 27,28 , which is the order of magnitude expected for electric elds needed to drive most chemical reactions if well-aligned with relevant chemical bonds. This is in disagreement with recent stimulated Raman excited uorescence (SREF) microscopy that determined an averaged electric eld magnitude of only ~ 8 MV/cm at the surface for a similar oilwater microdroplet interface 29 , elds that are on the lower end for generating any signi cant surfacereactivity.…”

Absolute ion hydration free energy scale and the surface potential of water via quantum simulation

“…30 that the anion in the tetraphenyl-arsonium/tetraphenyl-borate (TATB) hydrophobic ion pair prefers the interface while spectroscopy indicates the anion is more strongly hydrated in bulk water (72) gives a clear indication of an electrostatic driving force at work near the interface. It is also interesting to note that, in recent spectroscopic experiments (35), electric fields of magnitude 10 7 V/cm were estimated from Stark shifts of fluorescent probes near droplet surfaces, very close to the effective field observed in the present simulations. Ref.…”

“…We have suggested previously that there may be physical and chemical consequences to solvent-induced potential gradients through an interface even though the shifts are not directly measurable (29). The consequences include nonuniform ion density profiles (8,30), altered acid-base chemistry (31)(32)(33)(34), and electric fields probed by spectroscopic measurements at droplet surfaces (35). Experimental evidence is accumulating that suggests the effective surface potential of water (passing from the gas phase or a hydrophobic medium into water) is approximately −0.4 V, in agreement with earlier estimates (29,36,37).…”

A critical analysis of electrospray techniques for the determination of accelerated rates and mechanisms of chemical reactions in droplets

“…73 Measured electric eld strengths were in the range of 0.2-1 V nm À1 for charged nanodroplets 104 and in the order of 10 7 V nm À1 at the oil-water interface of rhodamine microemulsions. 105 It was reported that strong electric elds accelerate single molecule reactions by stabilizing charge separation in the transition state. 74,75 By analogy, strong elds may have similar effects within nano-and micro-droplets.…”