Argonne National Laboratory

Research Highlight | Materials Science

Multivalent counterions impede lubrication of polyelectrolyte brushes
In a study published in Science, researchers results suggest that the lubricating properties of polyelectrolyte brushes in multivalent solution are hindered relative to those in monovalent solution.
Multivalent ion–induced friction mechanisms
Multivalent ion–induced friction mechanisms. MD simulation showing multivalent ion–induced bridging between apposing polyelectrolyte chains at low and high salt concentrations. (A) Percentage of multivalent cations bridging apposing polyelectrolyte chains plotted against the scaled separation distance (ζ), which is normalized by the polymer contour length (50σ). Error bars represent ±SD, averaged over five replicas. (B to D) Schematics of apposing PSS brushes sliding against each other in (B) 6 mM NaNO3, (C) 0.01 mM Y(NO3)3, and (D) 0.1 mM Y(NO3)3 solution.

Scientific Achievement

Molecular mechanism for and demonstration that multivalent ions diminish the lubricity of polyelectrolyte brushes.

Significance and Impact

Polyelectrolyte brushes arise in varied biological and industrial contexts, including the surfaces of articular cartilage, interfaces between mineralized collagen, anti-fouling surfaces, and medical prostheses. The impact of trace concentrations of multivalent ions on brush structure is critical to designing their robust function in diverse environments.

Research Details

  • Surface Forces Apparatus measurements of the friction of opposing polyelectrolyte brushes in the presence of monovalent, divalent, and trivalent ions.
  • Brushes in monovalent salt show high lubricity, whereas those in multivalent ions show dramatically decreased lubricity.
  • Molecular dynamics simulations identify two molecular mechanisms for decreased lubricity: (I) multivalent ion chain-bridging events and (II) heterogeneous brush collapse leading to increased surface roughness.

DOI: 10.1126/science.aar5877

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