Mechanistic Studies of Microdroplet Chemistry
Abstract
Water is so common that one might think that all aspects of its behavior had already been discovered and understood, but such is far from the case. Recent work has shown that seemingly simple water droplets give rise to unexpected and accelerated reactivity compared to the bulk liquid. This proposal is focused on understanding and exploiting such reactivity, which has the potential for efficient synthesis of materials and molecules, high throughput chemical analysis and relevant DOD applications such as decontamination. We have assembled a team of six experts with complementary expertise required to address the physical and chemical complexities of droplets. Because this is a necessarily a multiscale topic, our team has the expertise to engage this problem at a range of length scales, spanning from sub-nm (molecular cluster models) to several cm (chemical imaging of electrospray plumes). The team will employ a diverse and complementary set of techniques including nonlinear microscopy of droplets and their interfaces, fluorescence and IR imaging of plumes, molecular-scale cluster models, spectroscopy of levitated droplets, mass spectrometry of accelerated reactions, and molecular-scale and coarse-grained theoretical models. With these techniques, we aim to answer the following types of questions under three interrelated research thrusts- (1) Microdroplet Environments- One of the primary hypotheses about the air-water interface of a microdroplet is the influence of strong electric fields that can make or break chemical bonds or induce redox reactions. We will measure such fields using vibrational and electronic Stark shift spectroscopy in conjunction with computational models of the droplet interfaces. This problem is closely tied to ion concentration gradients, surface pKa, molecular orientation and concentration gradients of reactants, products, and intermediates. (2) Dissecting Reaction Mechanisms in Microdroplets- A large body of evidence has accumulated over the past decade that shows drastically different reactivity in droplet surfaces compared to bulk. Differences in solvation of reactants between bulk and interface have been suggested to explain reaction acceleration in organic solvents. We will explore the relative contributions of droplet charge, strong fields, and partial solvation in aqueous reactions. To understand the influence of these factors on reaction mechanism, we will study a target set of archetypal reactions, with both ionic and neutral intermediates. We will explore whether the mechanisms and reaction intermediates generated at the interface are the same as those present in the bulk. (3) Synthetic and Analytical Applications- We will exploit the special properties of droplets in applications where warranted. How can droplets be used to synthesize new compounds? How can one scale up synthetic processes to make microdroplet chemistry preparative? We propose to address these questions as well as to explore opportunities to create new methodologies ranging from high throughput chemical analysis to decontamination? The culmination of our work will be an experimentally verified and theoretically justified understanding reaction mechanisms in droplets. It will open avenues for driving chemistry using droplets, in particular for problems of interest to DoD such as decontamination, screening-scaling up of accelerated reactions and making new materials.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Mar 07, 2023
- Source ID
- FA95502110170
Entities
People
- Richard Zare
Organizations
- Air Force Office of Scientific Research
- Stanford University
- United States Air Force