Specific Counterion Effects on the Swelling Behavior of Strong Polyelectrolyte Brushes

Abstract

It is well known that specific types of counterions affect the hydration of polyelectrolytes both in the bulk and at interfaces, but the mechanisms of this effect have not yet been fully understood. In this work, we have designed a model system, consisting of imidazolium-based cationic polyelectrolyte brushes with controlled grafting densities, to systematically investigate how specific counterion properties affect well-established swelling mechanisms in brushes. With this approach, we show that two swelling mechanisms, namely, counterion influence on the ion osmotic pressure and counterion influence on brush−solvent nonelectrostatic interactions, are simultaneously at play. Here, we demonstrate that the former effect can be related to the polarizability of the counterions, while the latter effect can be correlated to the hydration enthalpy of the counterions. We further demonstrate that the interplay of these two mechanisms depends on the brush grafting density and ionic strength of the medium such that under certain conditions, one effect can dominate over the other. Specifically, at low ionic strength and low grafting density, swelling of the brush is significantly influenced by the polarizability of counterions, while at high grafting density and high ionic strength, the hydration enthalpy of ions is the dominating factor. Moreover, by employing a theoretical model, we rationalize the experimental findings and further quantify the contribution of specific counterion effects as a function of grafting density and ionic strength. We believe such an approach improves the general understanding of the influence of ions on the polyelectrolyte brush swelling and even beyond.

Ion-Specific Antipolyelectrolyte Effect on the Swelling Behavior of Polyzwitterionic Layers

“…This difference in the osmotic regime indicates a lower counterion-induced osmotic pressure in the case of SCN – compared to other ions. Counterion-specific behavior has previously been observed in other cationic films, and here, it has been explained by the effect of specific types of counterions on the counterion-induced osmotic pressure and the polymer–polymer and polymer–solvent nonelectrostatic interactions. ,,− Thus, we have linked the trend of thickness at low ionic strength to the ability of anionic counterions to form ion pairs with the quaternary alkylammonium groups on the polyelectrolyte chains and, thus, become osmotically passive. To this end, on the basis our results, it is expected that the degree of ion-pair formation follows the trend SCN – > Br – > Cl – , which agrees with previous reports. , Beside this overall behavior, the film with SCN – as counterions started to swell with increasing salt concentration, from approximately 800 mM.…”

“…To understand and explain the difference of the counterion effects observed for the polycationic and the polyanionic films, focus should be placed on the mechanism through which ion pairs are formed. Ion-pair formation has been suggested to be correlated to the polarizability of the ions in the pair in such a manner that highly polarizable ions tend to interact stronger with the (also highly polarizable) quaternary ammonium groups and thus form a larger number of ion pairs compared to weakly polarizable ions with sulfonate groups. ,, In our current study, the polarizability of the mobile anions ranged between 3.5 and 7.0 Å 3 , while the polarizability of the mobile cations was much lower (0–2 Å 3 ) . To this end, we suggest that although the cations used here have different polarizabilities, they are all so low that the ion-pair formation in those cases is negligible (i.e., no ion pairing).…”

“…For the polycationic films with different cross-link densities, the swelling ratios (relative to the dry-film thickness) are plotted in Figure as a function of the KCl concentration. The overall swelling behavior of the polycationic film follows the typical behavior of strong polyelectrolyte films with respect to ionic strength. ,, We observed that this film exhibited a high and constant thickness at low salt concentrations, from 0.01 to 10 mM, which corresponds to the osmotic regime, where the counterion-induced osmotic pressure caused considerable swelling of the film. ,, From ∼10–30 mM, the salted regime started, where a decline in swelling was observed because the osmotic pressure difference decreased due to the addition of external salts. In addition to the osmotic and salted regimes, a quasi-neutral regime is sometimes observed for polyelectrolyte films at very high salt concentrations, depending on the films properties. ,, In the current case, this regime is, however, not manifested below 3 M KCl.…”

“…The overall swelling behavior of the polycationic film follows the typical behavior of strong polyelectrolyte films with respect to ionic strength. 10,25,33 We observed that this film exhibited a high and constant thickness at low salt concentrations, from 0.01 to 10 mM, which corresponds to the osmotic regime, where the counterioninduced osmotic pressure caused considerable swelling of the film. 24,29,30 From ∼10−30 mM, the salted regime started, where a decline in swelling was observed because the osmotic pressure difference decreased due to the addition of external salts.…”

“…As implied by the term, the antipolyelectrolyte effect means that the swelling response is opposite to the response of the polyanionic or polycationic films. More specifically, polyzwitterionic films often swell with increasing ionic strength, whereas polyanionic or polycationic films collapse with increasing ionic strength. − …”