Spatial Structure of Electrical Diffuse Layers in Highly Concentrated Electrolytes: A Modified Poisson-Nernst-Planck Approach

Studies of room-temperature ionic liquids showed that electrical diffuse layers in these highly concentrated electrolytes may exhibit spatially extended nonmonotonic (oscillatory) and monotonic decays. These unconventional properties are fundamentally different from traditional (dilute) electrolytes and demonstrate the limited mechanistic understanding of highly concentrated electrolytes. Moreover, electrolyte behavior placed in close proximity of two charged surfaces becomes even more unclear due to the possible overlap between diffuse layers. The latter is important as many applications require confinement into narrow spaces, e.g., energy and lubrication related applications. To advance the understanding of electrical diffuse layers in highly concentrated electrolytes (and ionic liquids) we use a semiphenomenological modified Poisson-Nernst-Planck equation and regulate weak dilutions. Using spatial dynamics methods and numerical computations, we analyze distinct diffuse layer characteristics (nonmonotonic and monotonic) and provide for each type the analytic conditions and the validity limits in terms of applied voltage, domain size, molecular packing, and short-range electrostatic correlations. We also discuss the qualitative generality of the results and thus believe that these insights will allow us to advance the electrochemical understanding of confined highly concentrated electrolytes and their technological. applications.