Quantification of Water-Ion Pair Interactions in Polyelectrolyte Multilayers Using a Quartz Crystal Microbalance...

Area of Science:

• Materials Science
• Physical Chemistry
• Polymer Science

Background:

• Water within polyelectrolyte multilayer (PEM) films significantly impacts their properties and applications.
• Existing knowledge on PEM water microenvironments and water-ion pair interactions is limited.
• Factors like ionic strength, salt type, and terminating layers influence PEM swelling.

Purpose of the Study:

• To investigate the influence of assembly and post-assembly conditions on water-ion pair interactions in PSS/PDADMA PEMs.
• To quantify the number of water molecules per ion pair (i) and the hydration coefficient (π_salt ^H).
• To elucidate the role of salt type (NaCl vs. KBr) and terminating layer (PSS) on water-ion interactions.

Main Methods:

• Utilized quartz crystal microbalance with dissipation monitoring (QCM-D) to measure water content and ion interactions.
• Assembled PSS/PDADMA PEMs under varying ionic strengths of NaCl and KBr.
• Exposed assembled PEMs to increasing ionic strengths of matching salt types post-assembly.
• Employed classical and ab initio molecular dynamics for microstructural analysis.

Main Results:

• Established a linear relationship between water per ion pair and post-assembly salt concentration (>0.5 M).
• Observed higher values of 'i' and 'π_salt ^H' for KBr compared to NaCl, attributed to KBr's chaotropic nature.
• Found that a PSS terminating layer decreases 'i' due to its hydrophobic nature.

Conclusions:

• The study provides a quantitative understanding of water microenvironments and water-ion pair interactions in PEMs.
• Assembly and post-assembly conditions, including salt type and terminating layer, critically influence water association with ions.
• Findings offer insights for designing PEMs with tailored properties for energy storage, coatings, and biomedical applications.