Ionization and pH Stability of Multilayers Formed by Self-Assembly of Weak Polyelectrolytes

Abstract

The formation of polyelectrolyte multilayers using the layer-by-layer sequential self-assembly of a weak polybase and polyacid onto a surface of Si crystal and their stability under acidic conditions were studied. The system was 12%, 20%, 25%, 37%, 90%, or 98% quaternized poly-4-vinylpyridine (Q12, Q20, Q25, Q37, Q90, or Q98) as polybases (generally abbreviated as QPVP) and poly(methacrylic acid) (PMAA) as a polyacid in buffered D2O. The adsorption and ionization of pyridine rings and carboxylic groups were quantified using Fourier transform infrared spectroscopy in attenuated total reflection mode. At pH ) 5, film growth was primarily controlled by electrostatic interactions. When QPVP was included into the top layer, ionization of carboxylic groups was increased compared to its value in solution. The amount of the induced charge increased linearly with the total charge density of the polybase (calculated as a sum of ethylated pyridinium and protonated pyridinium groups), suggesting the formation of both types of electrostatic pairs (COO --NR + and COO --NH + ). The degree of ionization of both pyridine groups and carboxylic groups within the film responded to the charge of the polymer included in the top layer and oscillated as a function of layer number. The average ionization of the carboxylic groups within the multilayer was suppressed when the multilayer was topped with PMAA, and it was enhanced when the top layer was the polybase. The effect was rather long-ranged and persisted through over three polymer layers with a decay length of about 100 Å. When the pH of the buffer solution was lowered below pH 2, multilayers dissolved due to protonation of carboxylic groups and disruption of polymer/polymer ionic contacts. The multilayer stability under acidic conditions was dependent on the alkylation degree of QPVP. With the exception of the largest alkylation degrees, the stability of the film increased with the alkylation degree of QPVP. The multilayer destruction correlated with the protonation of the small (<5%) residual fraction of carboxylic groups that was included into polymer/polymer ionic pairs and with the number of ionic polymer/polymer contacts per QPVP chain, suggesting that the number of polymer/polymer contacts during self-assembly was a critical parameter in multilayer stability. The results show the effect of the QPVP charge density on the ionization of self-assembled PMAA and on the pH stability of polyelectrolyte multilayers.

Interaction of myoglobin with poly(methacrylic acid) at different pH in their layer-by-layer assembly films: An electrochemical study

“…The good stability of the films at lower pH was explained by the onset of hydrogen bonding involving protonated carboxylic acid groups within the films. Moreover, the increased local ionization of weak polycarboxylic acid in the presence of a highly charged polycation in their layer-by-layer films, compared to its ionization in solution, was reported [20,21]. This induced electrostatic interaction might also play an important role in the assembly of multilayer films involving weak polyelectrolytes at relatively low pH.…”

Structural aspects of polyanion and hydrophobically modified polycation multilayers on hydrophilic or hydrophobic surfaces

“…[9][10][11] The driving force for the stable layer association is entropic in nature and results from the release of counter-ions and water. 12,13 Issues concerning the dependence of thickness evolution, [14][15][16][17] morphology 18 and ionization (in the case of weak polyelectrolytes) 19 as a function of alternated deposition are important because they control film stability and, consequently, its final applications.…”

Usage of Layer-by-Layer Method for Obtaining Polyelectrolyte Multilayer Films Containing Transition Metal Cations

“…Our strategy for film construction was first based on previous studies on assembly of weak polyelectrolytes, which showed that the film thickness, molecular organization and the amount of polymer adsorbed can be controlled by simple adjustments of the pH of the dipping solution [14]. Second, we also relied on prior studies on PEM assemblies of polyelectrolytes with different permanent charge densities, which reported thicker films for polymers with lower degrees of permanent charge [15]. Figure 4 (top panel) shows dry thicknesses of (BPEI/PAA) 50 films as a function of deposition pH.…”