Influence of Anion Hydrophilicity on the Conformation of a Hydrophobic Weak Polyelectrolyte Brush

Abstract

The conformation of a hydrophobic, weak cationic poly(2-diisopropylamino)ethyl methacrylate (PDPA) brush was studied using neutron reflectometry as a function of aqueous solution pH, ionic strength, and anion identity. In pH 4, 10 mM potassium nitrate the brush is highly charged, resulting in an extended, dilute conformation; at pH 9 the uncharged brush collapses to a single, dense layer. The brush response to added salt at constant pH (4.5) for varying concentrations of the potassium salts of acetate, nitrate, and thiocyanate revealed ion-specific conformations of the brush. At low ionic strength (0.1 mM) the brush was collapsed, independent of salt identity, while at higher ionic strengths (up to 500 mM) the conformation was dependent on counterion identity. The brush exhibited extended conformations in the presence of kosmotropic acetate counterions, while collapsed conformations were retained in the presence of strongly chaotropic thiocyanate counterions. The brush showed a richer set of behaviors in ...