## 3 Results and Discussions
### Computational Methods
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Atoms** & **EEA4 NB** & **Anion** \\ \hline N1 & 0.0502 & 0.1435 \\ \hline O2 & \(-\)0.1393 & 0.2014 \\ \hline O3 & \(-\)0.1396 & 0.2013 \\ \hline O4 & \(-\)0.4833 & \(-\)0.3275 \\ \hline O5 & \(-\)0.3895 & \(-\)0.3013 \\ \hline C6 & \(-\)0.2217 & 0.0051 \\ \hline C7 & \(-\)0.0601 & 0.0384 \\ \hline C9 & \(-\)0.0952 & 0.0051 \\ \hline C11 & \(-\)0.2104 & 0.0551 \\ \hline C12 & \(-\)0.0939 & 0.0057 \\ \hline C14 & \(-\)0.0603 & 0.0386 \\ \hline C16 & \(-\)0.0843 & \(-\)0.1018 \\ \hline  & & cation \\ \hline N17 & \(-\)0.0062 & 0.0563 \\ \hline O20 & \(-\)0.0217 & \(-\)0.4216 \\ \hline C22 & \(-\)0.0031 & \(-\)0.0581 \\ \hline C25 & \(-\)0.0010 & \(-\)0.0322 \\ \hline C28 & \(-\)0.0005 & 0.0215 \\ \hline C31 & \(-\)0.0006 & 0.0083 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The calculated condensed dual descriptors of the studied inhibitors. It is known that atoms with fractional charges, which are typically involved in most hydrogen bonds, tend to generate significant \"local\" potentials. Therefore, an examination of the potential maps indicates that the oxygen atoms of the anion are most likely to participate in the formation of hydrogen bonds, particularly in the case of hydroxide species (FeOH\({}^{+}\)) on the metal surface. Therefore, a theoretical estimation of the inhibition efficiency of EEA4 NB, the anion, and the cation was conducted at the density functional theory calculation level. The anion exhibits the highest inhibition efficiency compared to EEA4 NB and the cation, primarily due to its highest HOMO and lowest energy gap. Quantum chemical parameters, including hardness (\(\eta\)), electronegativity (\(\chi\)), and the fractions of electrons transferred (\(\Delta\)N), suggest the following order of inhibition efficiencies: anion > EEA4 NB > cation. The presented MEP surface, an overlay of the electrostatic potential (indicating the attraction or repulsion of a molecule), is valuable for describing the overall molecular charge distribution. The most negative potential is identified over the anion's oxygen atoms, while the most positive potential is observed over the cation. The condensed Fukui functions predict the electrophilic and nucleophilic attacking sites of the inhibitors, providing information about the reactivity of the molecules. Additionally, our study proposed a mechanism for acid-base multicomponent systems as corrosion inhibitors, illustrated in Figure 5a. The positively charged carbon steel surface facilitates the adsorption of EEA4 NB through negatively charged oxygen, resulting in a mixed-type adsorption (physisorption + chemisorption), as supported by Langmuir isotherms. The high values of condensed dual descriptors for all oxygen atoms in EEA4 NB and the anion suggest effective inhibition. Notably, the anion contains both electrophilic and nucleophilic attacking sites. Consequently, EEA4 NB in solution can be adsorbed on the carbon steel surface in three forms: the anion form, the cation form, and as undissociated molecules. The observed high inhibition effect for this alkanolammonium salt results from the anion's more powerful electron-donating ability (higher E\({}_{\text{HOMO}}\) value) and the lower value of E\({}_{\text{LUMO}}\), suggesting that undissociated molecules can accept electrons from the  
Figure 8: Molecular electrostatic potential maps for study inhibitors, with the values mapped on the isodensity surface ranging from \(-1\times 10^{2}\) (red) to \(1\times 10^{2}\) (blue). \(d\)-orbital of iron to form a feedback bond, thereby increasing the adsorption of EEA4 NB on the carbon steel surface. We assume that the mechanism presented in Figure 5b is less likely and is not supported by the low quantity of Cl\({}^{-}\) in the organic surface-layer inhibitor. This is probably due to the strong repulsive action towards chloride ions, resulting from both steric hindrance and the electronic properties of the phenyl group. A similar observation was reported for aryl aminoalcohol inhibitors [41].