The Spontaneous Electron-Mediated Redox Processes on Sprayed Water Microdroplets

Abstract

Water is considered as an inert environment for the dispersion of many chemical systems. However, by simply spraying bulk water into microsized droplets, the water microdroplets have been shown to possess a large plethora of unique properties, including the ability to accelerate chemical reactions by several orders of magnitude compared to the same reactions in bulk water, and/or to trigger spontaneous reactions that cannot occur in bulk water. A high electric field (∼109 V/m) at the air–water interface of microdroplets has been postulated to be the probable cause of the unique chemistries. This high field can even oxidize electrons out of hydroxide ions or other closed-shell molecules dissolved in water, forming radicals and electrons. Subsequently, the electrons can trigger further reduction processes. In this Perspective, by showing a large number of such electron-mediated redox reactions, and by studying the kinetics of these reactions, we opine that the redox reactions on sprayed water microdroplets are essentially processes using electrons as the charge carriers. The potential impacts of the redox capability of microdroplets are also discussed in a larger context of synthetic chemistry and atmospheric chemistry.

Hydrocarbon Degradation by Contact with Anoxic Water Microdroplets

“…This electron transfer at water microdroplet interfaces is more recently shown to give rise to unexpected redox reactions . The potential applications of this chemistry range from ecosystem to industry, where contact electrification at the water–gas and water–solid interfaces of water microdroplets makes possible formation of aqueous reactive oxygen species (ROS) and what are believed to be hydrogen radical (•H). − We extend the contact-electric redox chemistry to the highly deformable water–oil microdroplet interfaces by describing the spontaneous generation of H 2 and ROS from microdroplets of water in contact with the model oil hydrocarbon, hexadecane, which leads to the evolution of CO 2 and the production of short-chain hydrocarbons (mainly C 1 and C 2 ).…”

Size-Dependent Sigmoidal Reaction Kinetics for Pyruvic Acid Condensation at the Air–Water Interface in Aqueous Microdroplets

“…Aqueous microdroplets have been shown to largely enhance reaction rates and/or initiate reactions that do not occur to any great extent compared to bulk solutions. − The high surface-to-volume ratio and the unique environment at the air–water interface have been suggested as the origins of these enhanced rates. − Reactions in microdroplets of organic solvents have also been shown to be accelerated compared to bulk solutions. , …”

P-Band Intermediate States Mediate Electron Transfer at Confined Nanoscale

“…The calculated results can also be understood from the viewpoint of molecular orbital (MO): the σ bond of the O–H bond in SWs could be weakened due to the formation of multiatom π bonds in the water chain where the electrons of fully occupied π* antibonding orbital can fill into the σ* antibonding orbital of the O–H Bonds (Figure d). This probably answers the extremely low dielectric constant of water confined in the 2D cavity in graphite with a nominal height of ∼6.7 Å , and also the origin of spontaneous generation of hydrogen peroxide on the bubble interfaces from aqueous microdroplets. − …”