Ion Dispositions in Polyelectrolyte Multilayer Films

Polyelectrolyte multilayers (PEMs) fabricated through layer-by-layer (LbL) assembly from sodium chloride-containing solutions of poly(diallyldimethylammonium chloride) (PDDA) and poly(styrene sulfonate) (PSS) were examined by quartz crystal microbalance (QCM), QCM with dissipation (QCM-D), UV–vis spectroscopy, and X-ray photoelectron spectroscopy (XPS) to determine the dispositions of polyelectrolytes and counterions across the PEM thickness. The key experiment was dry film QCM, which by quantifying the incremental mass depositions during LbL assembly uncovered excess polyelectrolyte charge and excess polyelectrolyte charge density as functions of deposition number. Counterion dispositions depended strongly on salt concentration, and trends in the two PEM charge parameters established three salt concentration regimes: zero to near zero salt ([NaCl]  0.1 M), low salt (0.1 M  [NaCl]  0.75 M), and high salt ([NaCl]  1.5 M]). The first two are associated with linear LbL growth while the latter is associated with exponential LbL growth. At zero salt, no counterions are present in the PEM bulk (middle), while at low salt, an excess of PDDA charge across the bulk coincides with an excess of counteranions. Differently, at high salt, deposited PSS permeates the PEM bulk, conveying an excess of countercations. At all salt concentrations, the PEM surface charge alternates according to the capping polyelectrolyte’s identity. Accumulations of small ions in the PEM bulk can be ascribed to property asymmetries between the two deposited polyelectrolytes, but the roles played by different chain properties remain incompletely understood.