Hydration Contribution to the Solvation Free Energy of Water-Soluble Polymers

Area of Science:

• Physical Chemistry
• Polymer Science
• Computational Chemistry

Background:

• Solvation free energy is crucial for understanding polymer behavior in solution.
• The dynamic hydration layer (DHL) around polymers significantly influences solvation thermodynamics.
• Quantifying entropic contributions from the DHL is essential for accurate solvation models.

Purpose of the Study:

• To investigate the entropic contribution of the dynamic hydration layer (DHL) to the solvation free energy of water-soluble polymers.
• To isolate polymer hydration effects from conformational changes using a rod-like polymer model.
• To analyze specific ion effects on polymer solvation and the DHL structure.

Main Methods:

• Simulated solvation free energy calculations for a rod-like polymer.
• Quantification of the dynamic hydration layer (DHL) using the Debye-Waller parameter (⟨u²⟩).
• Analysis of Kirkwood-Buff integrals to assess ion-specific effects and solvent structure.

Main Results:

• A significant entropic contribution from polymer hydration was identified, correlating with changes in Kirkwood-Buff integrals.
• Water mobility in the DHL depends on polymer-solvent interactions but shows no additional specific ion effect compared to bulk.
• Chaotropic ions (CsCl) showed excess density near the polymer, while kosmotropic ions (NaCl) showed depletion.

Conclusions:

• Polymer hydration, particularly the DHL, plays a substantial role in the overall solvation free energy.
• Specific ion effects influence ion distribution at the polymer interface but not DHL mobility or extent.
• These findings are critical for understanding the thermodynamics of molecular self-assembly and phase separation in polymer solutions.