Ganster, F.TUM*

2025

Abstract: The effect of ion-specific hydration on the structure and charge transport of zinc-salt dopedpolyzwitterionic poly(2-(N-3-sulfopropyl-N,N-dimethyl ammonium)ethyl methacrylate) (PSBMA)has been investigated using electrochemical impedance spectroscopy (EIS), Fourier-transforminfrared (FTIR) spectroscopy, and quasi-elastic neutron scattering (QENS). Accordingto the Hofmeister series, the zinc salts ZnSO4, Zn(CH3COO)2, and ZnI2 serve as stronglykosmotropic, moderately kosmotropic, and chaotropic representatives, respectively. A fourterminalsetup was designed, which enables humidity-dependent EIS measurements in combinationwith a custom-designed humidity chamber. At 25 ◦C, ionic conductivities of (22.4 ±0.9) mScm−1 and (150±40) mScm−1 were measured for the fully humidified ZnSO4-containingand ZnI2-containing PSBMA-hydrogel, respectively. While the former value is in the range of what has been reported for a similar system, the latter significantly exceeds ionicconductivities from previous studies. FTIR spectroscopy allowed for linking these macroscopicobservations to salt-dependent restructuring of water and the hydration of functionalgroups in the hydrogel. It could be validated that the chaotropic I – -ions weaken the inner saltstructure of the polyzwitterionic hydrogel, thus leading to larger ion channels for ion transportation.Kosmotropic SO42 – -ions, on the other hand, tighten the water network and retrievewater molecules from the free volume between the polymer chains. The deconvolutionof the O– H-stretching vibration band further revealed different water subpopulations that arepresent (quasi-crystalline, solid-like, and liquid-like water), whose areas also fall in line withthe Hofmeister series. QENS provided complimentary water dynamics data, permitting detailedinsights into water-PSBMA interactions. Fits with the isotropic jump model revealedthat the hydration water in the ZnSO4-containing sample has the largest diffusion coefficient,whereas the ZnI2-containing sample exhibits the smallest, indicating more confined interfacialwater despite the higher overall water uptake.The combination of EIS, FTIR spectroscopy, and QENS led to the following conclusions:Kosmotropic anions organize water but constrict ion pathways. In contrast, chaotropic anionsloosen polymer packing and thus form larger ion channels. Consequently, the Zn2+-iontransport in the ZnI2-containing hydrogel is facilitated despite the slowly diffusing hydrationwater.

Keyword(s): Energy (1st) ; Materials Science (2nd) ; Soft Condensed Matter (2nd)

1. E13 (E13)
2. MLZ (MLZ)
3. RESEDA (RESEDA)

1. Measurement at external facility