Nonadditive ion effects on the coil-globule equilibrium of PNIPAM: a computer simulation study

Area of Science:

• Physical Chemistry
• Polymer Science
• Computational Chemistry

Background:

• The lower critical solution temperature (LCST) of poly(N-isopropylacrylamide) (PNIPAM) is influenced by ions in solution.
• Previous studies indicated a nonadditive effect of mixed weakly and strongly hydrated anions on PNIPAM's LCST.

Purpose of the Study:

• To investigate the molecular origins of nonadditive ion effects on PNIPAM's coil-to-globule transitions.
• To elucidate the interplay between different ion types and their impact on polymer behavior in solution.

Main Methods:

• Atomistic molecular dynamics simulations were employed.
• Simulations focused on a 40mer PNIPAM chain in solutions containing sodium sulfate (Na2SO4) and sodium iodide (NaI) at varying concentrations.

Main Results:

• Nonadditive ion effects stem from the depletion of strongly hydrated sulfate ions and preferential accumulation of weakly hydrated iodide ions on the PNIPAM chain.
• Favorable PNIPAM-iodide interactions and cation partitioning between ion clouds were identified as key mechanisms.
• A mutual enhancement of ion depletion and accumulation effects was observed, driven by cation redistribution and altered water affinity.

Conclusions:

• The study reveals a coupled mechanism involving ion depletion, ion accumulation, and cation redistribution that explains the nonadditive ion effects on PNIPAM.
• Understanding these molecular interactions is crucial for controlling polymer solution properties and designing advanced materials.