colloids and interfaces Article Adsorption Properties of Soft Hydrophobically Functionalized PSS/MA Polyelectrolytes Ewelina Jarek 1 , Zofia Krasi ´ nska-Krawet 1 , Tomasz Kruk 1 , Lukasz Lamch 2 , Sylwia Ronka 3 , Kazimiera A. Wilk 2, * and Piotr Warszy ´ nski 1, *   Citation: Jarek, E.; Krasi ´ nska-Krawet, Z.; Kruk, T.; Lamch, L.; Ronka, S.; Wilk, K.A.; Warszy´ nski, P. Adsorption Properties of Soft Hydrophobically Functionalized PSS/MA Polyelectrolytes. Colloids Interfaces 2021, 5, 3. https://doi.org/10.3390/ colloids5010003 Received: 13 November 2020 Accepted: 12 January 2021 Published: 14 January 2021 Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional clai- ms in published maps and institutio- nal affiliations. Copyright: © 2021 by the authors. Li- censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con- ditions of the Creative Commons At- tribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Niezapominajek 8, 30-239 Kraków, Poland; ewelina.jarek@ikifp.edu.pl (E.J.); nckrasin@cyf-kr.edu.pl (Z.K.-K.); tomasz.kruk@ikifp.edu.pl (T.K.) 2 Department of Engineering and Technology of Chemical Processes, Faculty of Chemistry, Wroclaw University of Science and Technology, Wybrze˙ ze Wyspia ´ nskiego 27, 50-370 Wroclaw, Poland; lukasz.lamch@pwr.edu.pl 3 Department of Engineering and Technology of Polymers, Faculty of Chemistry, Wroclaw University of Science and Technology, Wybrze˙ ze Wyspia ´ nskiego 27, 50-370 Wroclaw, Poland; sylwia.ronka@pwr.edu.pl * Correspondence: kazimiera.wilk@pwr.edu.pl (K.A.W.); piotr.warszynski@ikifp.edu.pl (P.W.) Abstract: We investigated the adsorption properties of the newly synthesized, hydrophobically functionalized polyelectrolyte (HF-PE), poly(4-styrenesulfonic-co-maleic acid) copolymer (PSS/MA). The hydrophobic alkyl side chains (C 12 or C 16 ) were incorporated into the polyelectrolyte backbone via the labile amid linker to obtain the soft HF-PE product with the assumed amount of 15% and 40% degree of grafting for every length of the alkyl chain, i.e., PSS/MA-g-C 12 NH 2 (15% or 40%) as well as PSS/MA-g-C 16 NH 2 (15% or 40%). In the present contribution, we determined both the effect of grafting density and the length of alkyl chain on adsorption at water/air and water/decane interfaces, as well as on top of the polyelectrolyte multilayer (PEM) deposited on a solid surface. The dependence of the interfacial tension on copolymer concentration was investigated by the pen- dant drop method, while the adsorption at solid surface coated by poly(diallyldimethylammonium chloride)/poly(styrene sulphonate) PEM by the quartz crystal microbalance with dissipation (QCM- D), attenuated total reflection Fourier transform infrared spectroscopy (FTIR-ATR) and contact angle analysis. We found that surface activity of the hydrophobized copolymer was practically independent of the grafting ratio for C 16 side chains, whereas, for C 12 , the copolymer with a lower grafting ratio seemed to be more surface active. The results of QCM-D and FTIR-ATR experiments confirmed the adsorption of hydrophobized copolymer at PEM along with the modification of water structure at the interface. Finally, it can be concluded that the hydrophobically modified PSS/MA can be successfully applied either as the efficacious emulsifier for the formation of (nano)emulsions for further active substances encapsulation using the sequential adsorption method or as one of the convenient building blocks for the surface modification materials. Keywords: hydrophobized polyelectrolyte; adsorption; interfacial tension; wetting; water structure 1. Introduction Nowadays, polyelectrolytes as classical soft-matter materials are of particular interest because they can modify many substrates at any interface, functionalize them, and convert them into various compatible forms with many solvents. Particularly they exert profound effects on colloidal stability for many biomedical purposes and engineered surfaces and can be applied at biointerfaces for many pharmaceutical or cosmetic applications [1–4]. Moreover, they can be designed as responsive polymer materials, playing an increasingly important part in a diverse range of applications, such as drug delivery, diagnostics, tissue engineering, and ‘smart’ optical systems, as well as biosensors, microelectromechanical sys- tems, coatings, textiles and much more [5]. The associative behavior of oppositely charged Colloids Interfaces 2021, 5, 3. https://doi.org/10.3390/colloids5010003 https://www.mdpi.com/journal/colloids