## 3 Results and Discussion  
In addition to the five synthesized AATEABs, surfactants with \(n=10\), \(8\), and \(4\) were also studied. After the QM/MM geometry optimizations, the molecules of water formed a cluster around the surfactant, as shown in Figure 4, with an average distance between the solute atoms, and the nearest water molecules are slightly longer than \(3\) A. As a result, the water molecules do not form strong hydrogen bonds that are able to bend the surfactant or act as thickness between these groups with carbonyl or with ammonium; in fact, water molecules were not found between carbonyl and ammonium, as shown in Figure 4a. Comparing the optimized structures obtained from initial geometries belonging to the \(pcg\) (Figure 4a) and \(sc\) (Figure 4b), respectively, the interaction between the quantum part and the carbonyl of the surfactant is mainly a hydrophobic (dispersion) interaction. It is important to emphasize that this analysis was carried out using the both the QM/MM optimized structures (Figure 4a,b) and those obtained from the pure DFT calculations, in which all of the surfactants were considered as a quantum system, in addition to the dispersion correction and solvent effect. The average charges of the ammonium carbons and hydrogens, as well as the charge of nitrogen are plotted in Figure 5 as a function of the number of C atoms in the aliphatic chain, while, in Figure 6, the ammonium net charge, defined as the algebraic sum of the partial charges of its atoms, is reported. Diamonds refer to the values coming out of the optimizations, where the \(pcg\) structures (i.e., initial geometries from the most populated conformers group) were used, while circles refer to the values obtained from the optimized geometries where the shortest isomer (conformer with the shorter h value) for each surfactant was used as the initial geometry. Figure 4: (**a**) QM/MM optimized structure of the AATEAB with twelve carbons in the aliphatic chain (C12) obtained using one conformer of \(pcg\) as the initial geometry; (**b**) QM/MM optimized structure of C12 obtained using one conformer of the \(sc\) group as initial geometry. Figure 5: Average partial charges calculated with a DFT based approach calculated average partial charges on H ((**a**) Löwdin, (**b**) electro static potential (ESP)) and C ((**c**) Löwdin, (**d**) ESP) of the ammonium group and partial charges on N ((**e**) Löwdin, (**f**) ESP) as a function of the carbons in the AATEABs aliphatic chain. The average partial charges of C and H do not depend on the number of aliphatic carbons, as well as the Lowdin charge of N, in contrast, the ESP charge of the latter varies in an oscillating way. The conformation of the aliphatic chain affects the ESP charges of C and N, as shown in Figure 5d,f, conversely, the Lowdin charges do not depend on the conformer structure chosen as the initial geometry. The ESP net charge of the ammonium group, Figure 6b, changes with the number of C atoms in an oscillating way around 0.88 a.u., while the Lowdin charges remain almost constant, as can be seen in Figure 5a. It is worth noting that the ESP charges depend on the conformer chosen as the initial geometry (Figure 5b). This is physically consistent, because the ESP charges reproduce the quantum electrostatic potential of the entire molecule, thus the conformation affects the potential, and in turn, the ESP charges; conversely, the Lowdin charges depend on the nature of the chemical bonds (i.e., on the electron density). This means that the bonds in the ammonium head do not change with the increase of the aliphatic chain. The surfactant can bend over itself, thus the carbonyl group can interact through an intra-molecular interaction with the ammonium group. The results in Figure 6b show that this happens for the longer surfactants, and markedly for the C4. In Figure 7, the optimized geometries of the C4 surfactant obtained from the DFT and COSMO calculations are shown.