| Popis: |
In the past few years, more and more attention has been focused on processes occurring in small volumes such as microdroplets and thin films due to the increases in reaction rates and other phenomena recently reported in the literature. Our group has primarily investigated the thermodynamics of physical and chemical changes taking place in acoustically-levitated droplets ranging in volume from five to 10 microliters in size. Levitating a droplet using a standing acoustic wave allows for convenient study of reactions occurring within the droplet or any process reliant on the droplet’s surface using visualization techniques such as microphotography and infrared imaging. We have used this technique in the past to study the evaporation process of levitated solvent droplets, including those of ionic liquids, as well as the neutralization thermodynamics of strong acids and bases within a levitated droplet. In this work we present the initial characterization of droplet “shells” formed on the surface of levitated solvent droplets which contain a noble metal salt and a small piece of transition metal wire. In these experiments silver nitrate or a gold (III) salt was dissolved in a levitated solvent droplet (either in deionized water, silver bis(trifluoromethanesulfonyl)imide ionic liquid, or a DMF/water solution) after which a small (~0.5cm) section of zinc or copper wire was dropped into the levitated drop. The resulting reduction of the noble metal salt to the noble metal solid was observed for each solvent/noble/transition metal combination using a DSLR camera equipped with a close-up lens. In many cases a dark precipitate formed on the surface of the wire, indicating the formation of metal solid much in the same fashion as the silver “tree” general chemistry experiment. However, in the case of the silver nitrate/DI water/copper and gold(III)/DI water/zinc combinations, a metallic solid ( shiny, silver-colored for silver nitrate and black for gold(III)) formed a shell at the droplet/air interface. More study of the shell structures is planned including examination using SEM. While specific applications of these structures is not immediately clear, we envision they may be of interest as high surface area catalysts. Figure 1 |
