Doctor of Philosophy
Julio C. Alvarez
Particle Collision Electrochemistry (PCE) has gained considerable attention in heterogeneous catalysis, petroleum chemistry and pharmaceutical fields. The PCE refers to a phenomenon in which a particle strikes on an inert electrode surface as a consequence of its Brownian motion and produces a spike of current for the direct oxidation/reduction of the individual particle. This method allows us characterization of individual particles and in-situ study of electrochemical reactions coupled to the particle.
Herein, emulsion droplets were studied by PCE where toluene droplets contained hydrophobic tetrachloro-1,4-benzoquinone (Q). This was investigated as a model system to study the molecular effects that arise due to hydrogen bonding reagents (oleic acid, acetic acid) inside and outside of the droplets. The emulsions were prepared by sonicating toluene-quinone solution with the water phase containing an ionic liquid to provide conductivity to the droplet. Each droplet produced a current spike while colliding with the electrode surface that was held at a potential to reduce tetrachoro-1,4-benzoquinone. In bulk acetonitrile and toluene, tetrachoro-1,4-benzoquinone undergoes a two electron reduction process to form the tetrachloro phenolate di-anion (Q2-). It was shown that the hydrogen bonding affinity of Q2- for acetic acid (pKa = 4.8) was higher than for oleic acid (pKa = 9.9) for both bulk systems (as acetic acid is stronger hydrogen bonding donor than oleic acid). However, the reversed trend was observed in emulsified toluene droplets when studied by PCE. This was attributed to the preferential partition of the carboxylic acids between the two phases in the emulsion. Oleic acid stays inside the droplets due its hydrophobic nature and hydrogen bonding with Q2- takes place inside the droplet. In contrast, solvation of acetic acid by the surrounding water, causes the hydrogen bonding with Q2- to occur significantly less inside the droplet.
Another redox system studied by PCE was hydrophobic ferrocene (Fc) trapped in the toluene droplet to determine the effect of surfactant on the particle size. The diameter determined electrochemically was compared with Dynamic Light Scattering (DSL) measurements. The presence of nonionic surfactant (triton X-100) was observed to affect the droplet’s size easily monitored by PCE. The mediated oxidation of cysteine by the oxidized Fc inside the droplet was investigated at different concentrations of cysteine.
Dilip Kumar Paul
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VCU Theses and Dissertations
Date of Submission
Available for download on Thursday, May 09, 2019