Interface Driven Dynamics at Nanoscales:Polymer thin films and Electrical Double Layer
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The electrical double layer (EDL) is formed due to the accumulation of charge at the interface of a metal surface in contact with an electrolyte. The total charge in the EDL compensates the charge on the metal surface. As EDL is the layer that "connects" the electrode to the "bulk", all electrode mediated transport and redox reaction depends on the structure and dynamics of the ions in the EDL. Thus the ion dynamics in the EDL are critical to a wide range of physical and biological phenomena such as electrochemical reaction, flow in channels of nanofluidic devices, wetting of fluids; to biology, for example, folding and function of proteins, conformation change of DNA and ionic flow through cell membranes. EDL polarization is the ion accumulation or depletion in the EDL due to the potential of the metal surface. The conventional method of measuring the EDL polarization is by monitoring the current flowing through the electrochemical system. Thus, the electrical characteristics of the EDL are inferred indirectly from the total current that is implicitly related to effects such as the impedance of the bulk solution. We have developed a sensitive optical interferometric technique to directly measure the polarization of the metal-electrolyte interface. The key advantage of our method is high sensitivity, and the measurement is specific only to the changes at the metal-electrolyte interface. The ion accumulation in the EDL of a simple salt like NaCl is studied as a function of the frequency and the amplitude of the applied potential on the metal electrode. The amplitude of modulation of the ions is linearly proportional to the amplitude of the applied AC potential. The linearity is observed up to high amplitude (up to 2V) and salt concentration as high as 0.5M. Furthermore, the local segmental dynamics of polyelectrolytes such as polystyrene sulfonate have been measured. Next we extend this novel technique to study electrochemical redox reactions. The oxidation of the widely used redox ion [Fe(CN)6]4- is followed by measuring the response to an AC potential (amplitude ~100mV) as a function of a superimposed saw-tooth potential ramp, at a time period 106 fold slower and amplitude 5-10 fold larger than the AC potential. The sensitivity of the optical method is significantly better than the measurement of the AC current. For a redox process on the electrode, the change in the optical signal is over two orders of magnitude larger than the electrical signal. Using the optical technique, we can separate the kinetic events in redox processes: transport of charged species to the electrode surface and charge transfer across the electrode-electrolyte interface. Because we measure the local electrochemical process, the method can be used to probe redox reaction at multiple spots on the same electrode (i.e., combinatorial electrochemistry).
- Doctoral Dissertations