ENERGY CONVERSION IN NANOCHANNELS GRAFTED WITH POLYELECTROLYTE AND POLYZWITTERION BRUSHES
Day: August 1st
Location: 2162 Glenn L. Martin Hall (DeWalt Conference Room)
Dr. Siddhartha Das, Chair
Dr. Amir Riaz
Dr. Taylor Woehl
Nanochannels, functionalized by grafting with a layer of charged polyelectrolyte (PE) brushes, have been employed for a large number of applications such as flow control, ion sensing, ion manipulation, current rectification and nanoionic diode fabrication. Recently, we established that such PE-grafted nanochannels, often denoted as “soft” nanochannels, can be employed for highly efficient, streaming-current-induced electrochemomechanical energy conversion in the presence of a background pressure-driven transport. In this thesis, we first decouple the electrostatic effects from the excluded volume and entropic effects in the free energy of a nanoconfined PE brush layer. We extend our calculation for the practically realizable situation when the PE brush layer, grafted on the inner walls of the nanochannel, demonstrates a pH-dependent charge density. Consideration of such pH dependence necessitates consideration of hydrogen and hydroxyl ions in the electric double layer charge distribution, cubic distribution of the monomer profile, and a PE layer-induced drag force that accounts for this given distribution of the monomer profile. Subsequently, we extend our analysis to polyzwitterion-grafted nanochannels. Our results express a hitherto unknown dependence of the streaming electric field (or the streaming potential) and the efficiency of the resultant energy conversion on parameters such as the pH of the surrounding electrolyte and the pKa and the pKb of the ionizable group that ionizes to produce the PE charge—we demonstrate using an integro-differential equation that the energy conversion efficiency substantially increases with an increase in the pH and the PE layer thickness. Similarly, this energy conversion is also concurrent during a decrease in the pKa and ion concentration of the nanochannel fluid. We anticipate that our calculations will provide the design basis for a new form of nanochannel based electrical energy generator by utilizing the mechanical energy of the fluid flow and the chemical energy of the electric double layer.