Beyond the Poisson-Nernst-Planck Model: The Impacts of Ion Specificity and Electrostatic Correlations on Biological Systems

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 DESCRIPTION: The classical Poisson-Nernst-Planck (PNP) model of electrolyte solutions has been widely used in biological systems to not only explain experiments but also go beyond to provide a rational design tool for lab- on-a-chip devices. However, many experiments contradict this picture for high salt concentrations, in particular, with multivalent ions, and high surface potentials: there are a significant number of striking qualitative discrepancies between experiments and the PNP model. For example, at high concentrations and high potentials, in the presence of multivalent ions, a reversal of electrophoretic mobility of DNA molecules, the attraction between similarly charged surfaces, flow disappearance, and salt dependence were widely reported in literature. All these observations cannot be captured by the PNP model. Clearly, key physical ingredients are missed in the classical PNP model for the case of high salts and high surface potentials. Consider that in biologically relevant applications, the electrolyte has a high concentration and contains multivalent cations. A new model capturing the missing key physics is necessary to bridge the knowledge gap. Ion specificity and electrostatic correlations are prominent at high salts and high surface potentials. The PNP model assumes that ions are point charges with no volume and interact electrostatically only. The PNP model cannot account for ion specificity and electrostatic correlations. Hence, the specific aims of this application are (1) Go beyond the PNP model and develop a simple local continuum model by integrating ion specificity and electrostatic correlations with non-equilibrium thermodynamic principles; (2) Employ this new model to understand the dynamics of electrolytes at high salt concentrations and high surface potentials, more specifically, advance the fundamental knowledge of electrostatic interactions, and bridge the striking discrepancies between experiments and the existing theoretical model. The developed continuum model will serve as a tool for molecular biophysicists and physiologists to understand, study, and control electrostatic interactions ubiquitously in biology, therefore aiding relevant clinical and technological applications.