Electrochemical and Spectroscopic Investigation of Counterions Exchange in Polyelectrolyte Brushes

Abstract

Scanning electrochemical microscopy (SECM) is employed to characterize the transport of redox-active probe ions through quenched polyelectrolyte brushes. The counterion exchange through polyelectrolyte brushes is also investigated by infrared spectroscopy in attenuated total reflection (FTIR-ATR), X-ray photolectron spectroscopy (XPS), and cyclic voltammetry (CV). The synthesis of poly(methacryloyloxy)ethyl trimethylammonium chloride (PMETAC) brushes is performed using surface-initiated atom transfer radical polymerization followed by in situ quaternization reaction. The chloride (Cl⁻) counterions of the positively charged polymer brush are exchanged by ferrocyanide (Fe(CN)₆⁴⁻) and ferricyanide (Fe(CN)₆³⁻) ions that are both detectable by spectroscopy and electrochemically active. A good agreement is found when comparing the results obtained by spectroscopic (FTIR-ATR and XPS) and electrochemical (SECM and CV) methods. The counterions exchange is completely reversible and reproducible. We show that (Fe(CN)₆⁴⁻) and (Fe(CN)₆³⁻) species form stable ion pairs with the quaternary ammonium groups of the polymer brush. The transport of iodide (I⁻) redox-active ions is also investigated. In all cases (ferrocyanide, ferricyanide, or iodide), we find that chloride counterions are partially replaced by electroactive ions. This partial exchange may be attributed to an osmotic effect, since the external salt concentration for the exchange is much lower than the counterion concentration inside the brush.