## 1 Introduction  
Undoubtedly, biofouling can be considered a great drawback limiting the use of membranes in the treatment of domestic or agricultural wastewater. Biofouling causes numerous problems, ranging from the flow declining, with a consequent loss of membrane performance, to the cleaning of the membranes, with consequences on the costs of the wastewater treatment. For these reasons, membranes with increased resistance to biofouling are required; hence, the knowledge of the relationships between the structure and properties, correlated with the antibacterial activity of the molecules, is crucial for the synthesis of novel membrane functionalization. Because of their well-known antimicrobial activity, since the first half of the 20 th century, quaternary ammonium salts (QAS) have been used in many human activities (industrial, agricultural, household, and hospital) [1; 2]. They are amphiphilic chemicals with a positive charge related to the quaternary ammonium group and have a hydrophobic tail formed by alkyl chains, hence they are also considered surfactants. Several investigations pointed to the crucial role played by the total positive charge of the ammonium salts and their lipophilicity [3; 4; 5; 6]. [PERSON] et al. [7] showed that the antibacterial effect of quaternary pyridinium salts improved by increasing the number of thimethylammonium head-groups [8]. Such a result was due to the strongest interaction of the ammonium positive charges with the hydrophilic gram-negative cell wall, that is, with the thin layer of lipopolysaccharidesaccharides (LPS), peptidoglycan [9]. However, [PERSON] et al. [10] measured the antimicrobial activity of six QAS tethered polydimethylsiloxane salts as a function of aliphatic chains length (two QAS-linked polymer groups were tested), in order to explain the effect of the chain length on the antiseptic property of QAS-linked polymers. The authors demonstrated that the biocidal activity of the QAS-linked polymer is reduced if the quaternary ammonium groups are exposed to the suppressive effect of dissolved polyanions. On the contrary, QAS with alkyl chains capable of protecting the quaternary nitrogen showed better antibacterial activity [10; 11]. Thus, the charge density on the antiseptic surfaces [12; 13] correlated to the concentration of the quaternary nitrogen and partial charge, as well as the length of the alkyl substituents [14; 15; 16; 17], affect the antibacterial activity of QAS-linked polymers. Polymerizable quaternary ammonium salts (PQASs) have found important applications for preventing biofouling through their anchoring on different polymers, resulting in the formation of antiseptic surfaces [18]. A series of polymerizable surfactants with an alkyl chain of different lengths, as can be seen in Figure 1, were used as antibacterial agents in methacrylate dental composites. This study showed that the antibacterial activity against _S. mutans_ strongly depends on the number of carbon atoms (n) in the main alkyl chain attached to the ammonium group [18], in agreement with previous works. The 2-dimethyl-2-hexadecyl-1-methacryloxyethyl ammonium iodide (C16) showed the highest antibacterial activity, whereas the 2-dimethyl-2-pentyl-1-methacryloxyethyl ammonium iodide (C5) and 2-dimethyl-2-octyl-1-methacryloxyethyl ammonium (C8) did not show any inhibition of the bacteria. PQASs have also found innovative applications in the membrane framework, so as to prevent biofouling. Recently, some authors of this paper synthesized polymerizable quaternary ammonium salts, acryloxyalkyltriethylammonium bromides (AATEABs) [19], used as surfactants for the antimicrobial coatings of commercial membranes [20]. A scheme summarizing the synthesis  
Figure 1: \(N\)-Alkyl-\(N\),\(N\)-dimethyl-\(N\)-[(methacryloyloxy)ethyl]ammonium iodides.