Confinement Induced Dilution: Electrostatic Screening Length Anomaly in Concentrated Electrolytes in Confined Space

For electrolytes confined in nanoscale space, a growing body of experimental evidence suggests that the electrostatic screening length (or Debye length) increases with the electrolyte concentration in the concentrated regime (e.g., ionic liquids). This electrostatic screening length anomaly can be understood under the assumption of an effectively dilute electrolyte due to massive ion-pairing, which, however, contradicts with the conventional wisdom that ionic liquids are highly dissociated, given the negative free energy of dissociation. To resolve this puzzle, a phenomenological theory is developed to understand ion-pair dissociation in electrolytes in confined space. Nanoconfinement suppresses lattice expansion upon ion-pair dissociation, shifting the effective free energy of dissociation toward positive values, favoring massive ion-pairing, and rendering a long electrostatic screening length. The dependence of the electrostatic screening length on the temperature and the ionic size is discussed. It is of high priority to investigate the nanoconfinement effect of the differential double-layer capacitance C-dl to further corroborate the confinement induced dilution phenomenon.