## 4 Conclusions  
Recently, some authors of this paper synthesized important polymerizable quaternary ammonium salts with an antibacterial activity. They found a direct correlation between the number of aliphatic carbons and the antibacterial activity of these compounds. Thus, the electrostatic charges, the maximum molecular length, and aspect ratio of these surfactants were evaluated in this work using an accurate _ab initio_-based molecular modeling, with the aim of investigating the correlations between the surfactants' structure and the properties related to their antibacterial activity. In particular, the work aimed to assess whether the folding of the aliphatic chain, correlated to the carbons of the chain, affects the ammonium charges and the maximum molecular length through hydrogen bonds or noncovalent interactions between carbonyl and hydrogens of the ammonium. The water molecules surrounding the surfactants and their dispersion, (i.e., hydrophobic interaction) are necessary to evaluate the folding effect, and they were considered in the modeling. The work showed that the ESP net charge of the ammonium group changes in an oscillating way around 0.88 a.u. with a number of carbons in the main aliphatic chain, whereas the Lowdin charge remains almost constant. However, the net positive charge of the ammonium does not increase with the aliphatic carbons, thus, although the number of carbons influences the antibacterial activity of these surfactants, as previously measured, this influence is not associated with an increase in the positive charge on the ammonium group. Unlike the partial charges, a different trend was obtained for the maximum length of the surfactants. In this case, the average value of \(h\) increases as the number of aliphatic carbons increases, although not linearly. This trend correlates with the experimental antimicrobial activity of these compounds. The analysis of the computed aspect ratios reveals that the AATEABs showing the highest antibacterial activity also showed a correct aspect ratio. Although this _ab initio_ molecular modeling was used to study the structure-property correlations of these surfactants, the computational procedure does not use any adjustable parameters, thus it is quite general and can be applied to predict analogous properties of new hanging structures to improve the biofouling resistance of the membranes.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Number of Carbons in AATEAB** & **Aspect Ratio from** & **Aspect Ratio from** \\
**Aliphatic Chain** & _pcg_ **Conform** & _sc_ **Conform** \\ \hline C6 & 1.99 & 1.53 \\ C9 & 1.34 & 1.57 \\ C11 & 1.54 & 1.50 \\ C12 & 2.35 & 1.36 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Aspect ratios obtained from quantum/molecular mechanics (QM/MM) optimized structures and initial conforms from _pcg_ and _sc_, respectively.  
**Author Contributions:** Conceptualization, G.D.L., L.L., and B.G.; methodology, G.D.L.; formal analysis, G.D.L.; validation, R.M. and B.G.; data curation, R.A.; investigation, R.A.; writing (original draft preparation) G.D.L. and B.G.; supervision and funding acquisition, A.F.  
**Funding:** The work performed has received funding from the European Union's Horizon 2020 Research and Innovation Program under grant agreement no. 689427 for the project **VicInAqua**.  
**Acknowledgments:** The authors are grateful to the support of the Cineca supercomputing center.  
**Conflicts of Interest:** The authors declare no conflict of interest.