Sommaire:

Although conceptually simple, the air-water interface displays complex static and dynamics properties, which become relevant when describing interfacial ion effects.

1) Different definitions of the electrostatic potential, each relevant for distinct experimental scenarios, lead to widely varying surface potential magnitudes and even different signs. Based on quantum-chemical density-functional molecular dynamics simulations, a few different surface potentials are evaluated and compared. The spatially averaged surface potential, accessible to electron holography, is dominated by the trace of the water molecular quadrupole moment and amounts to more than + 4 Volt inside the water phase, very different from results obtained with force-field water models. The surface potential inside a cavity is much smaller, less than 200 mVolt in magnitude. This is the electrochemical surface potential relevant for ion transfer reactions and ion surface adsorption. Charge transfer between water molecules leads to pronounced surface potentials as well. However, when probing electrophoresis by explicitly applying a lateral electric field, the zeta potential turns out to be zero. Thus, charge transfer between water molecules does not translate to a non-zero electrophoretic mobility at the pristine vapor-liquid water interface. [1]

2) The zeta-potential of mineral and biological surfaces can be used to characterize the specific surface affinity of ions. The non-vanishing zeta-potential of zwitterionic net-neutral phospholipid vesicles in neat water is consistently observed in experiments but not fully understood theoretically. Using atomistic molecular dynamics simulations in combination with the modified Poisson-Boltzmann equation, we study the effects of the dielectric and viscous properties of various aqueous interfaces and specific ion-surface interactions. We quantitatively reproduce and explain experimental results for the zeta-potential by using that in a thin interfacial water layer the dielectric and viscous properties are different from bulk water. For lipid membrane surfaces we need to postulate minute amounts of surface-active anionic impurities in lab water and also in the added salt. [2,3]

[1] Electrokinetic, electrochemical, and electrostatic surface potentials of the pristine water liquid–vapor interface, Maximilian R. Becker, Philip Loche and Roland R. Netz, J. Chem. Phys. 157, 240902 (2022)

[2] Interplay of Interfacial Viscosity, Specific-Ion, and Impurity Adsorption Determines Zeta Potentials of Phospholipid Membranes, Amanuel Wolde-Kidan and Roland R. Netz, Langmuir 2021, 37, 8463−8473

[3] Impurity effects at hydrophobic surfaces, Yuki Uematsu, Douwe Jan Bonthuis and Roland R. Netz, Current Opinion in Electrochemistry 2019, 13:166–173

Pour plus d'informations, merci de contacter Palmeri J.