Ion-induced hydrophobic collapse of surface-confined polyelectrolyte brushes measured by total internal reflection microscopy

Abstract

Cationic poly(2-(methacryloyloxy)ethyl trimethylammonium chloride) (PMETAC) brushes are grown from glass slides via surface-initiated atom transfer radical polymerization (SI-ATRP). The interaction potential energy between the surface-grown polyelectrolyte brushes and a 5 μm diameter polystyrene (PS) particle in different electrolyte environments (NaCl and NaClO4) has been directly measured using total internal reflection microscopy (TIRM). At a NaCl concentration of 0.05 mM, the measured interaction potential energy profiles show a long-range repulsion, arising from an electrical double layer interaction between the positively charged PS particle and the charges from the PMETAC layers. Upon addition of 1 mM NaCl, screening effects reduce the separation distance (h) between the PS particle and the surface, accompanied by the collapse of the PMETAC brushes. However, when the sample solution is replaced with a low concentration NaCl (0.05 mM) solution, h returns to the original values, indicating that the brushes resume the extended conformation. A remarkable difference is observed when similar TIRM experiments are carried out in the presence of NaClO4 solution. At the same concentration or ionic strength, NaClO4 causes a much more abrupt decrease in h between the particle and surface than that with NaCl solution. Conversely, flushing the sample cell with highly diluted NaClO4 or pure water cannot restore the original interaction potentials, thus implying an irreversible collapse of the PMETAC brushes in the presence of ClO4− anions. This apparent change in the PMETAC brush characteristics can be associated with the strong ion-pairing interaction between the ClO4− anions and the quaternary ammonium group in the PMETAC brushes, which experiences not only pure charge screening, but also a sharp hydrophilic-to-hydrophobic change of the brush layer. Using TIRM, we have directly measured such an ion-pairing induced collapse transition of surface-confined polyelectrolyte brushes, even at very low specific anion concentrations.