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

In this study, we systematically investigate the interactions between mobile ions generated from added salts and immobile charges within a sulfobetaine-based polyzwitterionic film in the presence of five salts (KCl, KBr, KSCN, LiCl, and CsCl). The sulfobetaine groups contain quaternary alkylammonium and sulfonate groups, giving the positive and negative charges. The swelling of the zwitterionic film in the presence of different salts is compared with the swelling behavior of a polycationic or polyanionic film containing the same charged groups. For such a comparative study, we design cross-linked terpolymer films with similar thicknesses, cross-link densities, and charge fractions, but with varying charged moieties. While the addition of salt in general leads to a collapse of both cationic and anionic films, the presence of specific types of mobile anions (Cl-, Br-, and SCN-) considerably influences the swelling behavior of polycationic films. We attribute this observation to a different degree of ion-pair formations between the different types of anionic counterions and the immobile cationic quaternary alkylammonium groups in the films where highly polarizable counterions such as SCN- lead to a high degree of ion pairing and less polarizable counterions, such as Cl-, cause a low degree of ion pairing. Conversely, we do not observe any substantial effect of varying the type of cationic counterions (K+, Li+, and Cs+), which we assign to the lack of ion pairing between the weakly polarizable cations and the immobile anionic sulfonate groups in the films. In addition, we observe that the zwitterionic films swell with increasing ionic strength and the degree of swelling is anion dependent, which is in agreement with previous reports on the “antipolyelectrolyte effect”. Herein, we explain this ion-specific swelling behavior with the different cation and anion abilities to form ion pairs with quaternary alkylammonium and sulfonate in the sulfobetaine groups.</p

Specific Counterion Effects on the Swelling Behavior of Strong Polyelectrolyte Brushes

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

Antimicrobial PDMS Surfaces Prepared through Fastand Oxygen-Tolerant SI-SARA-ATRP, Using Na₂SO₃ as a Reducing Agent

[Image: see text] Poly(dimethylsiloxane) (PDMS) is an attractive, versatile, and convenient material for use in biomedical devices that are in direct contact with the user. A crucial component in such a device is its surface in terms of antimicrobial properties preventing infection. Moreover, due to its inherent hydrophobicity, PDMS is rather prone to microbial colonization. Thus, developing an antimicrobial PDMS surface in a simple, large-scale, and applicable manner is an essential step in fully exploiting PDMS in the biomedical device industry. Current chemical modification methods for PDMS surfaces are limited; therefore, we present herein a new method for introducing an atom transfer radical polymerization (ATRP) initiator onto the PDMS surface via the base-catalyzed grafting of [(chloromethyl)phenylethyl]trimethoxysilane to the PDMS. The initiator surface was grafted with poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) brushes via a surface-initiated supplemental activator and reducing agent ATRP (SI-SARA-ATRP). The use of sodium sulfite as a novel reducing agent in SI-SARA-ATRP allowed for polymerization during complete exposure to air. Moreover, a fast and linear growth was observed for the polymer over time, leading to a 400 nm thick polymer layer in a 120 min reaction time. Furthermore, the grafted PDMAEMA was quaternized, using various alkylhalides, in order to study the effect on surface antimicrobial properties. It was shown that antimicrobial activity not only depended highly on the charge density but also on the amphiphilicity of the surface. The fast reaction rate, high oxygen tolerance, increased antimicrobial activity, and the overall robustness and simplicity of the presented method collectively move PDMS closer to its full-scale exploitation in biomedical devices