Magnetohydrodynamic transport and confinement of molecules at microelectrodes.

摘要

This dissertation describes magnetic field phenomena associated with electrochemical reactions at microelectrodes. Chapter 1 provides an overview of magnetic field effects on electrochemical reactions. Two magnetic forces are operative whenever an electrochemical reaction is carried out in an external magnetic field. The first force is the Lorentz force (F = q( v × B)) acting on the charge-carrying ions. This force will accelerate the electrogenerated ions, giving rise to magnetohydrodynamic (MHD) flow. The second force, the magnetic gradient force (F _∇B = 2CN_A(m*2/3kT) (B•∇) B), arises when an electrochemical reaction occurs in the presence of a nonuniform field.; Chapter 2 describes the magnetic field driven convective transport at inlaid-disk Pt microelectrodes as a function of the electrode radius. Large enhancements (∼125%) in the voltammetric current for the 1-e− reduction of nitrobenzene (NB) are reported for electrodes with radii >100 μm. Chapter 3 presents a new microfluidic system based on MHD vortex flow between two Pt microelectrodes in an external magnetic field of 1 Tesla. Focused transport of molecules in flow tubes, pulses, and circular-sheets is demonstrated by video-enhanced microscopy.; Chapter 4 describes the focusing of paramagnetic molecules near the electrode surface of magnetized ferromagnetic Fe and Ni microelectrodes. Results show that diminishments in the electrochemical reaction rates as large as 40% are observed for Fe and Ni microelectrodes compared to Pt when immersed in a uniform magnetic field. Chapter 5 demonstrates further how the gradient force can be used to focus and confine paramagnetic molecules into spatial regions surrounding ferromagnetic microelectrodes. The confinement of paramagnetic NB anions in field gradients generated at magnetized cylinder-shaped Fe for tens of seconds is also reported in Chapter 5.; Chapter 6 compares the influence of the magnetic gradient force for paramagnetic 2,26,6-tetramethyl-1-piperidinyloxyl (TEMPO) vs. diamagnetic N,N,N ′,N′,-tetramethyl-p-phenylenediamine (TMPD) solutions. The electrochemical behavior of Fe, Ni, and Pt microelectrodes for the 1-e− oxidation of TEMPO and TMPD is reported. Results indicate that current enhancements as large as 250% can be achieved as a result of magnetophoretic transport of paramagnetic species facilitated by the magnetic gradient force.