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L. Moreno Ostertag, X. Ling, K. Domke, S. Parekh, M. Valtiner:
"Characterizing the hydrophobic-to-hydrophilic transition of electrolyte structuring in proton exchange membrane mimicking surfaces";
Physical Chemistry Chemical Physics, 20 (2018), 11722 - 11729.

The surface density of charged sulfonic acid head groups in a perfluorosulfonic acid (PFSA) proton
exchange membrane determines the hydrophilicity of the ionic channels and is thus critical for the
structuring and transport of water and protons. The mechanism through which the head group density
affects the structuring of water and ions is unknown, largely due to experimental challenges in
systematically varying the density in an appropriate model system resembling the ionic channels. Here,
we present a model system for PFSA membrane ionic channels using self-assembled monolayers with a
tunable surface density of sulfonic acid and methyl groups to tune surface hydrophilicity. Atomic force
microscopy force-distance measurements were used to quantify the hydration forces and deduce the
interfacial electrolyte structure. The measured force profiles indicate a pronounced change of the electrolyte
layering density at the surface with an unexpectedly sharp hydrophobic-to-hydrophilic transition
when the surface shows a contact angle of B371. Using an extended Derjaguin-Landau-Verwey-Overbeek
model including the Hydra force, we quantify diffuse double layer charges and characteristic
hydration lengths as a function of sulfonic acid group density on the surface. Translating our results to
PFSA membranes, these findings have two implications: (1) the density of sulfonic acid head groups can
have a dramatic effect on the local solvent structuring of water inside the ionic channels and (2) they
support a view where two types of water (solution) exist in PFSA ionic channels - a structured (shell/
surface) and a non-structured (bulk) water. This offers an interesting perspective on how different head
group densities lead to changes in water and proton transport and macroscopic membrane conductivity
properties based on hydration layer characteristics.