Research

Surface Forces and Electrochemical Measurements of Polyelectrolyte Brushes

Polyelectrolyte Brushes

Schematic representations of polyelectrolyte brushes. (a) When only mono-valent ions are present, the brushes interact with one another in a repulsive and reversible fashion. (b) As multi-valent ions are added to the system, they replace the monovalent ions within the brushes, driving brush collapse. (c) Polyelectrolyte brushes with multi-valent ions exhibit an adhesive attraction when brought into contact due to multi-valent bridging between the brush chains. Image taken from Farina et al, Soft Matter, 9(44), 10458-10472 (2013).

Understanding polyelectrolyte brush behavior has a great deal of practical interest in the realm of novel materials and interactions with physiological systems. In part, this interest is due to growing desire for polymeric materials, which can respond to changes in their environment. The polyelectrolyte brushes studied in this project are composed of the strong polyanion poly(sodium styrene sulfonate) and are highly responsive to changes in the ionic environment of their surrounding solution.

The Surface Forces Apparatus (SFA) measures the forces of interactions between two surfaces coated with polyelectrolyte brushes via compressions and separations at speeds as slow as 1 nm/s. These compression/separation cycles provide both structural (i.e. polyelectrolyte brush height) and behavioral information (i.e. whether the two brushes are adhesive or purely repulsive with each other). When only mono-valent counterions are present, two brushes brought into contact interact in a reversible and repulsive manner (no hysteresis is seen during the SFA measurements). However, with the addition of multi-valent ions to the system, the mono-valent ions which provide charge neutrality within the brush are replaced by multi-valent ions (as measured via electrochemistry). Because of their higher charge valency, these multi-valent ions are able to neutralize a larger number of charges within the polyelectrolyte brush at a lower concentration than mono-valent ions. When multi-valent ions are present in the surrounding solution, brushes readily collapse and adhere strongly to one another. Both the observed collapse and the adhesion are due to the fact that higher valence ions are able to neutralize multiple charges within the polyelectrolyte brush. Consequently, multi-valent ions are able to act as bridging ligands between two brush chains, resulting in adhesion between two brushes that are brought into contact and hysteresis in the forces observed during the SFA measurement. When the multi-valent ions are released from the brush, the chains once again extend themselves and show only repulsive forces when brought into contact. This reversibility depends highly on the amounts of both mono-valent and multi-valent ions present in a given system.

Publications

  • "Brushes of Strong Polyelectrolytes in Mixed Mono- and Tri-valent Ionic Media at Fixed Total Ionic Strengths," R. Farina, N. Laugel, P. Pincus, M. Tirrell, Soft Matter, 9(44), 10458-10472 (2013). [PDF]
  • Featured on the journal cover.

  • “Stability Behavior of Anionic Spherical Polyelectrolyte Brushes in the Presence of La(III) Counterions”, C. Schneider, A. Jusufi, R. Farina, P. Pincus, M. Tirrell and M. Ballauff, Physical Review E, 82, 011401 (2010). [PDF]
  • “Microsurface potential measurements: Repulsive forces between polyelectrolyte brushes in presence of multivalent counterions”, C. Schneider, A. Jasufi, R. Farina, F. Li, P. Pincus, M. Tirrell, and M. Ballauff, Langmuir, 24, 10612-10615 (2008). [PDF]

Collaborators