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Multivalent ions induce lateral structural inhomogeneities in polyelectrolyte brushes
Subtle details about a polyelectrolyte's surrounding environment can dictate its structural features and potential applications. Atomic force microscopy (AFM), surface forces apparatus (SFA) measurements, and coarse-grained molecular dynamics simulations are combined to study the structure of planar polyelectrolyte brushes [poly(styrenesulfonate), PSS] in a variety of solvent conditions. More specifically, AFM images provide a first direct visualization of lateral inhomogeneities on the surface of polyelectrolyte brushes collapsed in solutions containing trivalent counterions. These images are interpreted in the context of a coarse-grained molecular model and are corroborated by accompanying interaction force measurements with the SFA. Our findings indicate that lateral inhomogeneities are absent from PSS brush layers collapsed in a poor solvent without multivalent ions. Together, AFM, SFA, and our molecular model present a detailed picture in which solvophobic and multivalent ion-induced effects work in concert to drive strong phase separation, with electrostatic bridging of polyelectrolyte chains playing an essential role in the collapsed structure formation
Multivalent ions induce lateral structural inhomogeneities in polyelectrolyte brushes
Subtle details about a polyelectrolyte’s surrounding environment can dictate its structural features and potential applications. Atomic force microscopy (AFM), surface forces apparatus (SFA) measurements, and coarse-grained molecular dynamics simulations are combined to study the structure of planar polyelectrolyte brushes [poly(styrenesulfonate), PSS] in a variety of solvent conditions. More specifically, AFM images provide a first direct visualization of lateral inhomogeneities on the surface of polyelectrolyte brushes collapsed in solutions containing trivalent counterions. These images are interpreted in the context of a coarse-grained molecular model and are corroborated by accompanying interaction force measurements with the SFA. Our findings indicate that lateral inhomogeneities are absent from PSS brush layers collapsed in a poor solvent without multivalent ions. Together, AFM, SFA, and our molecular model present a detailed picture in which solvophobic and multivalent ion–induced effects work in concert to drive strong phase separation, with electrostatic bridging of polyelectrolyte chains playing an essential role in the collapsed structure formation.Published versio
Comparing Solvophobic and Multivalent Induced Collapse in Polyelectrolyte Brushes
Coarse-grained
molecular dynamics enhanced by free-energy sampling
methods is used to examine the roles of solvophobicity and multivalent
salts on polyelectrolyte brush collapse. Specifically, we demonstrate
that while ostensibly similar, solvophobic collapsed brushes and multivalent-ion
collapsed brushes exhibit distinct mechanistic and structural features.
Notably, multivalent-induced heterogeneous brush collapse is observed
under good solvent polymer backbone conditions, demonstrating that
the mechanism of multivalent collapse is not contingent upon a solvophobic
backbone. Umbrella sampling of the potential of mean-force (PMF) between
two individual brush strands confirms this analysis, revealing starkly
different PMFs under solvophobic and multivalent conditions, suggesting
the role of multivalent “bridging” as the discriminating
feature in trivalent collapse. Structurally, multivalent ions show
a propensity for nucleating order within collapsed brushes, whereas
poor-solvent collapsed brushes are more disordered; this difference
is traced to the existence of a metastable PMF minimum for poor solvent
conditions, and a global PMF minimum for trivalent systems, under
experimentally relevant conditions