Aqueous Polyelectrolyte Electrodeposition: The Effects of Alkyl Substitution and Varying Supporting Electrolyte Concentrations on the Deposition Efficiency

Salt ('n'Pepper): the background electrolyte of hexacyanoferrate/polymer mixtures has a distinct effect on the shape of hydrodynamic voltammograms obtained by a rotating ring disk electrode (RRDE). The voltammetry indicates electrodeposition, providing access to the deposition efficiency. Salt conditions leading to maximum deposition at minimum current are identified, being dependent on the chemical structure. The water content in the obtained gel films can be adjusted.

Abstract

The controlled growth of surface-modifying polymer films by electrodeposition often fails because of the lack of redox activity of these compounds. Here, electroactive complexants help to electrodeposit non-electroactive polymers. Hence, we investigate the counterion-induced electrodeposition of polyelectrolytes: three quaternized poly(N,N-dialkylaminoethyl methacrylate)s (qPDAAEMA), in particular their methyl, ethyl, and isopropyl derivatives (i. e. qPDMAEMA, qPDEAEMA, and qPDPAEMA), provide transparent solutions in the presence of hexacyanoferrate(II) (ferrocyanide) at specific concentration windows of the KCl supporting electrolyte. Below a certain KCl concentration, insolubility dominates irrespective of the hexacyanoferrate valency, whilst above an upper threshold, full solubility is observed. Between these limits, oxidation reversibly electrodeposits polymer/hexacyanoferrate(III) (ferricyanide) complexes. Hydrodynamic voltammetry (and data analysis using in-house software) provides access to the deposition efficiency (DE). qPDEAEMA with ethyl substituents shows highest DEs; larger or smaller substituents fall short because of a balance between “hydrophobicity” and charge separation, shifting the window toward smaller salt concentrations with increasing alkyl size. We always observe a DE maximum close to the minimum salt concentration, whilst electrochemical quartz crystal microbalance (EQCM) measurements indicate a change in film water content close to the maximum. These effects, being also discussed in terms of polymer conformation, can direct the future engineering of electroassisted coatings.