Understanding the Microscopic Structure of Counterion Condensation Near a Charged Polypeptoid Surface

The molecular-level study of the electrical double layer is key to understanding ion solvation, adsorption, and transport in many electrochemical and biological systems. Here, we combine the modified Gaussian renormalized fluctuation theory and molecular dynamics simulations to investigate the microscopic structure of counterion condensation near a charged nanofiber created by self-assembly of amphiphilic polypeptoids in water. Our theory shows a peak shift of the counterion distribution, indicating the existence of a water monolayer between condensed counterions and surface charges. Without any fitting parameters, the peak position and the shape of the counterion distribution predicted by our theory are in quantitative agreement with recent experimental results using cryogenic electron microscopy. We provide a possible explanation for the formation of such solvent-separated ion pairs in terms of the image charge repulsion at an interface with a dielectric mismatch. This important feature arising from electrostatic fluctuations cannot be captured by either classical Poisson-Boltzmann theory or simulations using nonpolarizable force fields.