Water binding and hygroscopicity in π-conjugated polyelectrolytes

Cindy Guanyu Tang, Mazlan Nur Syafiqah, Qi-Mian Koh, Mervin Chun-Yi Ang, Kim-Kian Choo, Ming-Ming Sun, Martin Callsen, Yuan-Ping Feng, Lay-Lay Chua (), Rui-Qi Png () and Peter K. H. Ho ()
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Cindy Guanyu Tang: National University of Singapore
Mazlan Nur Syafiqah: National University of Singapore
Qi-Mian Koh: National University of Singapore
Mervin Chun-Yi Ang: National University of Singapore
Kim-Kian Choo: National University of Singapore
Ming-Ming Sun: National University of Singapore
Martin Callsen: National University of Singapore
Yuan-Ping Feng: National University of Singapore
Lay-Lay Chua: National University of Singapore
Rui-Qi Png: National University of Singapore
Peter K. H. Ho: National University of Singapore

Nature Communications, 2023, vol. 14, issue 1, 1-14

Abstract: Abstract The presence of water strongly influences structure, dynamics and properties of ion-containing soft matter. Yet, the hydration of such matter is not well understood. Here, we show through a large study of monovalent π-conjugated polyelectrolytes that their reversible hydration, up to several water molecules per ion pair, occurs chiefly at the interface between the ion clusters and the hydrophobic matrix without disrupting ion packing. This establishes the appropriate model to be surface hydration, not the often-assumed internal hydration of the ion clusters. Through detailed analysis of desorption energies and O–H vibrational frequencies, together with OPLS4 and DFT calculations, we have elucidated key binding motifs of the sorbed water. Type-I water, which desorbs below 50 °C, corresponds to hydrogen-bonded water clusters constituting secondary hydration. Type-II water, which typically desorbs over 50–150 °C, corresponds to water bound to the anion under the influence of a proximal cation, or to a cation‒anion pair, at the cluster surface. This constitutes primary hydration. Type-III water, which irreversibly desorbs beyond 150 °C, corresponds to water kinetically trapped between ions. Its amount varies strongly with processing and heat treatment. As a consequence, hygroscopicity—which is the water sorption capacity per ion pair—depends not only on the ions, but also their cluster morphology.

Date: 2023
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DOI: 10.1038/s41467-023-39215-9

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