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Making devices based on ion-ion correlations in nanofluidics, biology, and beyond
=Dirk Gillespie, Ph.D.= ===February 2, 2012=== Making devices based on ion-ion correlations in nanofluidics, biology, and beyond Electrolyte solutions are used in nanofluidics and in biology to do work. For example, ions can be moved through nanofluidic channels with pressure to produce a voltage. In biology, ion channels conduct ions across membrane to produce action potentials and initiate muscle contraction. Different kinds of ion channels can select which kinds of ions to conduct (e.g., Ca2+ in preference to Na+). Other applications include electrochemical (super) capacitors. Designing, optimizing, and understanding these systems requires modeling. Theories like Poisson-Nernst-Planck that model ions as point charge are very useful in many applications. However, when ions are near highly-charged device walls or inside ion channels, the size of the ions produces first-order effects because the ions’ concentration is very large and/or because the ions are in a crevice or pore that is not much wider than the ions themselves. Density Functional Theory (DFT) of electrolytes (not electron orbitals) is a thermodynamically-derived theory that includes the effect of ion-ion correlations due to ion size. Applications of DFT to be discussed are as varied as modeling of ions at dielectric interfaces and ion currents through calcium channels and through nanofluidic devices. Dirk Gillespie is a member of the Department of Molecular Biophysics and Physiology, Rush University Medical Center, Chicago IL
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