The Role of Confined Water in the Emergence of Electrostatic Strong Coupling As Revealed by Nanoseparated Charged Lipid Layers

This study investigates the interplay between Strong Coupling (SC) attraction and hydration repulsion in nanoconfined water between like-charged phospholipid layers. We combine reflectivities with numerical simulations to analyze supported phospholipid layers under different relative humidity and surface charge densities. X-ray fluorescence demonstrates that we can control the valence of the associated counterions. Experimental measurement of the water thickness, combined with the precise determination of charged surface positions by numerical simulations, clearly demonstrates the existence of electrostatic attraction in the presence of monovalent counterions. It enables us to compare our experiments with a theoretical model showing that charge-screening by hydration water induces SC attraction, even at moderate surface charge densities with monovalent counterions. Furthermore, hydration repulsion is stronger for DPPS compared with DPPC. These findings offer insights into the forces that control interactions between phospholipid layers and have important implications for biological and colloidal systems.