In the present report, I introduce a technique for focusing electrochemically generated molecules in solution using an external magnetic field. The acceleration of charge-carrying ions in an applied field is due to the generation of a magnetic body force. This force per volume (N/m~3) is given by Eq. 1, where J (C m~(-2) s~(-1)) is the flux of F_(vol) = JXB (1) ions, and B (T) is the applied field. ~1 The transfer of momentum from the accelerated ions to the solvent gives rise to magnetohydrodynamic (MHD) flow of electrolytic solution. ~(2,3)For single-electrode systems, the magnetic force can significantly alter the rate of mass transport to the electrode surface. ~(4-6) When a dual-electrode configuration is employed, induced flow patterns create a unique molecular transport system. The dual-electrode configuration, used to transport focused ions, is shown in Fig. 1. Two Pt inlaid microdisk electrodes (radius = 250 #mu#pm) are at facing angles of 0 deg and 180 deg, with respect to the applied field. Both microelectrodes are poised at a potential of -2.0 V (vs. Ag/Ag_xO), in an acetonitrile solution containing 2 M nitrobenzene (NB). NB radical anion generated at the electrodes is dark red and easily imaged by video microscopy. When a uniform magnetic field of 1.0 T is applied, the magnetic force induces a rotational flow of solution at the circumference of one microelectrode. Mass conservation requires an inward flow to balance the outward flow at the inlaid disk edge. The inward flow is a tightly focused beam and can transport the NB anion over distances up to 1.0 cm at velocities of approx 0.25 cm/s. The direction of the beam can be varied by changing the positions of them microelectrodes. Molecules can also be focused into other geometrical shapes, e.g., 1 cm diameter, 50 #mu#m wide circular sheets.
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