The role of confined water in the emergence of electrostatic strong coupling as revealed by nanoseparated charged lipid layers

TL;DR

This paper reveals that hydration water can induce electrostatic strong coupling between charged lipid layers, even with monovalent ions, challenging previous assumptions about multivalent ion necessity.

Contribution

It demonstrates that hydration water enhances electrostatic interactions leading to strong coupling, supported by combined experimental and numerical analysis, expanding understanding of lipid layer interactions.

Findings

Hydration water induces strong coupling attraction at moderate surface charges.

Hydration repulsion is stronger in DPPS than DPPC.

Charge screening by hydration water explains the emergence of strong coupling.

Abstract

This study investigates the interplay between Strong Coupling (SC) attraction and hydration repulsion in nanoconfined water between like-charged phospholipid layers. It challenges the assumption that SC attraction requires multivalent counterions by showing that hydration water can enhance electrostatic interactions. 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 precise determination of charged surface positions by numerical simulations, enable us to compare our experiments with a theoretical model. It shows 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 to DPPC. These findings offer insights into the forces that control interactions between phospholipid layers and have important implications for biological and colloidal systems.