Alkaloid extract of seed Citrullus colocynthis as novel green inhibitor for mild steel corrosion in one molar HCl acid solution: DFT and MC/MD approaches

The corrosion rate of a steel alloy, submerged in a 1 M HCl solution, is assessed using the potentiostatic method. Corrosion inhibitor, in the form of Citrullus Colocynthis seed alkaloid extracts, is added to a battery of tests. The objective is to pinpoint the most effective concentration of this extracted inhibitor. This investigation is conducted under-regulated parameters, when the inhibitor is present and while it is absent. The concentration of HCl and the temperature of the reaction media may be modified. To ascertain the most favorable concentration of inhibitors given the specified parameters, we do comparative analysis of outcomes derived from concentrations spanning from 0.5 to 2 g/L.Chemical partChemical test of alkaloidsA qualitative chemical test was conducted to confirm the extraction of alkaloids from Citrullus Colocynthis seeds. The results of these tests, which involved different reagents, are presented in Table 1.Table 1 Chemical tests of the CSEA.GC–MS analysis resultsThe results of the Alkaloid Extract GC–MS Analysis are depicted in Fig. SI2 and detailed in Table 2. The seeds of Citrullus Colocynthis were determined to possess 85.68% of chemicals that can be directly analyzed, with roughly 62.38% of these compounds being classified as alkaloids. A comprehensive identification of 18 ingredients was conducted, including 7 distinct alkaloids and 11 other chemical compounds that have potential for extraction in an acidic environment. 5-Quinolinol is a member of the quinoline family, which consists of heterocyclic aromatic chemical compounds. In several applications, this molecule has been used as an internal standard for the quantification of morphine content in serum or plasma. The presence of this compound as a significant component in the alkaloid extract derived from the seeds of Citrullus Colocynthis is really intriguing.Table 2 The chemical makeup of the extracted acetylated alkaloids from Citrullus Colocynthis seed.Electrochemical studiesOpen circuit potentialThe variation in the open-circuit electrical potential of mild steel in 1 M HCl in the absence and presence of CSEA at different concentrations, after 30 min immersion is shown in Fig. SI3. For the blank alone, the free potential becomes less and less noble (loss of electrons), anodic dissolution of the metal is easy. The introduction of CSEA inhibitor in the corrosive medium leads the shift of the EOCP towards more negative values, probably due to the adsorption of CSEA on the MS surface , and therefore, the improvement of its corrosion resistance in hydrochloric acid pickling solution. It is, moreover, noteworthy there was no a specific relation between the potential shift of EOCP and inhibitor concentration.Polarization studiesThe same conditions were used to carry out the experiments thrice. The ZiveZman 2.3 software program was utilized to determine the corrosion parameters by analyzing potentiodynamic curves. Figure 3 shows the Tafel graphs of the MS-substrate electrode at various inhibitor doses. Extrapolation characteristics such as anodic slope (a), cathodic slope (c), corrosion potential (Ecorr), and inhibiting performance (%) are listed in detail in Table 3.Figure 3Tafel plots of MS-substrate in 1 M HCl, both in the absence and presence of varying concentrations of CSEA.Table 3 The effects of CSEA on MS polarization in 1 M HCl at 298 K.The cathodic polarization curve of the MS-substrate electrode in a 1 M hydrochloric acid solution exhibits no significant change in slope subsequent to the introduction of CSEA, as seen in the accompanying image. The absence of any impact of the CSEA inhibitor on the corrosion process is shown by the almost parallel cathodic branch seen in the polarization map. Furthermore, in the realm of the anode. There has been a consistent increase in the polarization potential over a period of time, accompanied by the convergence of the curves. The desorption of inhibitors from the electrode surface is likely attributed to the rapid breakdown of the MS-substrate at elevated polarization potential.Figure 3 demonstrates that the addition of CSEA to the experimental solution results in a deceleration of hydrogen release in the cathodic zone and a deceleration of metal dissolution in the anodic domain. The reduction in corrosion potential leads to a slight shift towards cathodic values. As anticipated, the introduction of inhibitors into the solution results in a deceleration of MS-substrate breakdown40.The following equations elucidate the steps involved in the cathodic reduction of hydrogen:$$Fe \, + \, H^{ + } \to \left( {FeH^{ + } } \right)_{ads} ,$$
(3)
$$\left( {FeH^{ + } } \right)_{ads} + \, e^{ – } \to \left( {FeH} \right)_{ads} ,$$
(4)
$$\left( {FeH} \right)_{ads} + \, H^{ + } + e^{ – } \to Fe \, + \, H_{2} .$$
(5)
The following equations detail the steps for the anodic dissolution reaction:$$Fe + H_{2} O \leftrightarrow FeOH_{ads} + \, H^{ + } + e^{ – } ,$$
(6)
$$FeOH_{ads} \to FeOH^{ + } + e^{ – } ,$$
(7)
$$FeOH^{ + } + \, H^{ + } \to Fe^{2 + } + H_{2} O.$$
(8)
The data shown in Table 3 suggests that the reduced corrosion current density seen in the MS substrate may be ascribed to the inhibitor solution with the greatest concentration, namely 2 g/L. This phenomenon may be well explained by the probability that, at a significant concentration, the surface of steel is enriched with chemicals that serve as corrosion inhibitors41. However, in a 1 M HCl solution without an inhibitor, the corrosion potential (Ecorr) of the MS-substrate electrode is − 498 mV/ECS, but it reaches − 526 mV/SCE when 2 g/L of CSEA is added. The consensus among scholars is that when the change in the magnitude of the corrosion potential exceeds 85 mV, it may serve as a reliable indicator for distinguishing between anodic and cathodic corrosion inhibitors. Otherwise, it is categorized as a mixed type15. In this particular instance, the introduction of the inhibitor CSEA into a 1 M hydrochloric acid (HCl) solution resulted in a displacement of the corrosion potential towards greater negative values in comparison to the absence of the inhibitor.The achieved displacement range was 85 mV. As a result, CSEA may be classified as a mixed-type inhibitor that controls the cathodic reaction predominately. The study also found that as the concentration of the investigated CSEA compound rose, slight modifications occurred in the anode and cathode Tafel slopes (βc and βa). Furthermore, the corrosion current density transitioned from 983 µA cm–2 (blank) to 56 µA cm-2 for the optimal concentration of CSEA. These findings are associated with inhibitor adsorption on metal surfaces, where a barrier film might lessen the interaction of the active site with corrosive conditions. As demonstrated in Table 3, the inhibiting efficiencies increase with the concentration of the CSEA, achieving a peak performance of 94.3% at 2 g/L.EIS studiesTo comprehend the protection mechanism (diffusion, adsorption, and charge transfer) of MS by the CSEA extract in a 1 M HCl solution, we carried out electrochemical impedance measurements. The aforementioned measures possess the capacity to provide insights into the underlying mechanisms involved in corrosion and other aspects of protective procedures42. The validity of this technique in defining the mode of activity of the inhibitor is often confirmed by electrochemical impedance investigations of the inhibitory mechanism. It allows for monitoring their advancement using different factors and assists in evaluating the dielectric characteristics of the resulting film, as shown in Fig. 4. The EIS (Nyquist) diagrams for MS are shown, excluding the CSEA. According to the findings shown in Fig. 5, it can be noticed that all plots display a single time constant, and the inclusion of CSEA does not have any impact on the semicircular form43.Figure 4Nyquist plots for the MS at 298 K and 1 M HCl in water with different CSEA concentrations.Figure 5Bode graphs of MS at 298 K in the presence and absence of CSEA at concentrations ranging from 0 to 10 mol l1 show that the acidity of the solution is mildly decreasing.This indicates that the process of corrosion remains consistent and the reaction of corrosion is controlled by the transfer of charge. The Nyquist plot also reveals how the widths of the semicircles grow larger with higher inhibitor concentrations.The Bode graphs shown in Fig. 4 demonstrate the existence of a stable single-phase component at the interface between the metal and solution inside the analogous circuit. According to the Bode plots, the observed increase in absolute impedance at low frequencies suggests that higher doses of inhibitors result in a greater degree of protection44. Furthermore, an alteration in the phase angle displacement and the emergence of a novel phase angle in the upper and medium frequency ranges were seen when the quantity of inhibitors increased. The phenomenon under consideration arises when the steel surface undergoes the formation of an inhibitor layer, hence altering the interface structure of the electrode. Significantly, it is possible to demonstrate a linear relationship between frequency (f) and logarithm (Z) with a slope that is almost equal to − 1. The aforementioned exhibits the customary attributes of a capacitor, therefore confirming the establishment of a meticulously organized CSEA adsorption layer16.The impedance parameters have been computed, with Rs representing the solution’s resistance, Rp denoting the polarization resistance, and CPE representing the constant phase element. A capacitor has been substituted with a CPE in order to enhance the accuracy of the impedance semicircle data. The formation of a double layer on the metal surface via the inhibitory of adsorption may occasionally exhibit features that are more indicative of a CPE rather than a pure capacitor. The formula supplied45 represents the impedance function of the CPE.$$ZCPE \, = \, \left[ {Q\left( {j\omega } \right)^{ndl} } \right]^{ – 1} ,$$
(9)
where Q represents the magnitude of the CPE. J Is an imaginary number with the value (j2 =  − 1). ω: the angular frequency.The different parameter deviation, termed as ndl (where − 1 ≤ ndl ≤  + 1), embodies the concept of a phase shift.When ndl = 0, it signifies that the CPE represents a pure resistor. For ndl =  − 1, it acts as an inductor and for ndl =  + 1, it behaves like a pure capacitor46.Simultaneously, the double layer capacitance, Cdl, for a circuit with a CPE was calculated using the following equation provided47.$$Z_{CPE} = Q^{ – 1} (i\omega )^{{ – n_{dl} }} .$$
(10)
In this case, the largest value of the imaginary component of the impedance spectrum occurs at a frequency fmax, where ωmax = 2fmax. Table 4 displays the calculated impedance parameters. The following conclusions can be drawn from the data.Table 4 EIS outcomes for the MS/1 M HCl system before and after adding CSEA.Table 4 demonstrates that the n values are higher in the presence of CSEA compared to the blank, indicating that the presence of a green inhibitor reduces surface inhomogeneity. This phenomenon is shown by the formation of a protective coating on the surface of the steel.However, due to the adsorption of CSEA molecules, n values change as inhibitor concentration increases. This suggests that CSEA molecules interact with steel morphology, gradually replacing H2O molecules at the steel/solution interface with CSEA compounds.Furthermore, as inhibitor concentration increases, Rp values increase while Cdl values decrease. This is attributed to enhanced coverage of the MS surface by inhibitor molecules, resulting in a decrease in corrosion rate. The increase in the thickness of the double layer with the rise in CSEA concentration is also associated with the decrease in the value of Cdl. This relationship can be adequately explained by the given equation48.$$C_{dl} = \left( {\frac{{\varepsilon_{0} \varepsilon }}{d}} \right) \times S.$$
(11)
In this context, ε0 is the permittivity of the area, ε is the local dielectric constant, d is the film thickness, and S is the area morphology. According to the equation, Cdl is directly proportional to ε and inversely proportional to d. As various water molecules are replaced, the local dielectric constant decreases, leading to a reduction in the Cdl value. This is due to the replacement of water molecules (which have a higher ε value) by inhibitor molecules (which have a lower ε value). Furthermore, an increase in CSEA concentration leads to an observed increase in the thickness (d) of the electric double layer. This also significantly contributes to the decrease in the Cdl value49. As the concentration of the inhibitor increases CSEA, the inhibition efficiency (ƞimp%) also increases, reaching a maximum value of 93.6% at a concentration of 2 g/L.Adsorption isotherms and thermodynamicsIsotherm plots reveal details regarding the inhibitor’s adsorption onto the metal surface. Partial surface coverage data (θ) and inhibitor concentration need to be plotted on a line in order to determine the type of isotherm. Using EIS, the values of can be determined by dividing the ηEIS% by 100 (as displayed in Table 4).In order to empirically find the most suitable isotherm for the adsorption of inhibitors on the surface of the MS-substrate, several adsorption isotherm models, namely Langmuir, Frumkin, Freundlich, and Temkin, have been investigated. The Langmuir isotherm was found to be the most suitable model for describing the adsorption behavior of the molecules under investigation, as shown by Eq. (12).$$\frac{\theta }{1 – \theta } = K_{ads} \times C_{inh} .$$
(12)
It can be approximated as:$$\frac{{C_{inh} }}{\theta } = \frac{1}{{K_{ads} }} + C_{inh} .$$
(13)
In this context, Kads refers to the equilibrium constant adsorption and Cinhib represents the molar concentration of the molecule inhibiting. The plot of Cinhib/θ versus Cinhib exhibits a linear shape, as depicted in Fig. 6.Figure 6CSEA adsorption on an MS-substrate in 1 M HCl: an adsorption isotherm.The free energy of adsorption (ΔGads) may be determined with the use of Eq. (14) and the adsorption constant (Kads). In this equation, the value 1000 represents the concentration of water in solution in g/L50:$$\frac{1}{{K_{ads} }} = \frac{1}{1000}\exp \,\left( {\frac{{ – \Delta G_{ads}^{{}} }}{RT}} \right).$$
(14)
Or:$$\Delta {G}_{ads}^{0}=-RTLn\left(1000\times {K}_{ads}\right).$$
(15)
Figure 9 demonstrates the straight line that results from the relationship between \({\raise0.7ex\hbox{${C_{inh} }$} \!\mathord{\left/ {\vphantom {{C_{inh} } \theta }}\right.\kern-0pt} \!\lower0.7ex\hbox{$\theta $}}\) and Cinh at 298 K, with a correlation coefficient R2 that is nearly equivalent to one \(\approx\) 0.9999 and a slope that is equal to unity. Furthermore, Strong relationship between the adsorbate and the surface is indicated by a high Kads value (Table 5). The observed phenomenon indicates that the adsorption of CSEA on a surface composed of mild metals adheres to the Langmuir adsorption isotherm. Moreover, it is crucial to emphasize that due to the lack of knowledge regarding the molecular masses of the constituents in the extract, it is not feasible to characterize the behavior of the adsorption isotherm when employing natural product extracts as inhibitors based on the conventional free energy of adsorption quantity50Table 5 In molar hydrochloric acid with and without CSEA at 298 K, the values of the MS-substrate’s adsorption Langmuir parameters were measured.Corrosion activation parameters and temperature effectsThe reaction of the steel electrode in the acid medium can be altered by temperature, in the presence and absence of the inhibitors being used49. Investigating the impact of temperature on the rate of corrosion is beneficial for determining the kinetic and thermodynamic parameters of the adsorption process. These parameters are extremely useful in understanding the nature of the adsorption that is taking place51. The impact of temperature on the inhibitor’s performance was investigated by the analysis of potentiodynamic curves of Mild Steel (MS) in molar hydrochloric acid. This investigation was carried out under two conditions: with and without the ideal inhibitor concentration of 2 g/L (Fig. 7). The curves shown in Fig. 8 were graphed throughout a temperature range spanning from 298 ± 2 to 328 ± 2 K. The related data is presented in Table 6.Figure 7The addition of 2 g/L of CSEA at 298–328 K altered the Tafel plots of MS-substrate in molar hydrochloric acid.Figure 8Comparison of the Arrhenius curves (a) and transition state (b) for mild steel in molar hydrochloric acid at 298–328 K with and without 2 g L–1 CSEA.Table 6 The optimum concentration of CSEA (2 g/L) in solution and its effect on the MS-substrate potentiodynamic polarization parameters in molar hydrochloric acid.Based on the Tafel plots acquired in an acidic environment, it was noted that an elevation in temperature led to a concomitant augmentation in the corrosion current (icorr), irrespective of the presence or absence of inhibitory molecules. The rise was most evident at a temperature of 328 K.Furthermore, slight differences were observed in the anodic and cathodic branches of all Tafel plots. It was inferred from these observations that the rate of corrosion increased with rising temperature in a 1 M HCl solution, irrespective of whether inhibitors were present or not. Consequently, the performance of the inhibitor decreased with increasing temperature. This can be attributed to a decline in the adsorption capacity of the inhibiting compound at elevated temperatures.In this segment of our research, we observed an Arrhenius-type relationship between the corrosion rate and temperature. This allowed us to calculate the activation energy values for the corrosion process (Ea) at various temperatures. We did this both in the absence and presence of CSEA, according to the given relationship40:$$i_{corr} = \, Ae^{( – Ea/RT)} ,$$
(16)
Using the transition state equation, we calculated the enthalpy (ΔHa) and entropy (ΔSa) of the corrosion process17:$$ln\left( {i_{corr} /T} \right) \, = \, \left[ { \, ln\left( {R/hN_{a} } \right) \, + \, \left( {\Delta S_{a} /R} \right) \, } \right] \, {-} \, \Delta Ha/RT,$$
(17)
Corrosion rate (icorr), gas constant (R), Avogadro number (N), pre-exponential factor (h), and Planck constant (h).By plotting ln (icorr) against 1/T, as shown in Fig. 8a, we can calculate the activation energy for metal dissolution both with and without 1 mM of CSEA. This is done by determining the slope of the line, which is equal to -Ea/RT. Furthermore, Fig. 8b shows a plot of ln (icorr/T) versus 1/T. The slope and intercept of this plot allow us to calculate the values of ΔHa and ΔSa52.The activation data, calculated according to the Arrhenius and transition state plots, are displayed in Table 7. The activation energy values obtained for the solutions containing inhibitors were seen to be greater than those for the solutions without inhibitors, as shown in Table 7. The blank solution exhibited an activation energy value of 21.0 kJ/mol, but the solution containing inhibitors had an activation energy value of 46.7 kJ/mol at a concentration of 2 g/L.Table 7 Different factors CSEA 2.0 g L–1 activates corrosion of mild steel in 1 M HCl acid media, both alone and in combination with no CSEA.Upon closer examination of these parameters, it’s clear that the activation energy increases in the presence of CSEA. According to reference53, it may be inferred that the CSEA inhibitor creates a slender barrier layer that effectively hinders the corrosion process. Undoubtedly, it is well acknowledged that the rise in activation energy seen when an inhibitor is present, in contrast to a blank medium, may be ascribed to the electrostatic attraction existing between the metal and the inhibitor. The phenomenon of attraction results in the formation of a protective coating on the surface of the metal, therefore impeding the rate of corrosion54. The dissolution of the MS-substrate has an endothermic nature, as shown by the positive enthalpy value. Conversely, all activation energy (Ea) values are greater than their corresponding enthalpy analogs (ΔHa).The reduction of proton (H+) is the result of a gaseous reaction involved in the corrosion process, which is indicated by this. When ΔSa takes on negative numbers, it means that the disorder is decreasing. The rate of a reaction is determined by the time it takes for the reagents to transfer to the activated complex and for the activated complex to form an association.SEM observationScanning electron microscopy (SEM) images were captured to examine the interaction between CSEA and the surface of steel. Figure 9a depicts the bare steel surface without CSEA, while Fig. 9b shows the steel surface in the presence of 2 g/L of CSEA.Figure 9SEM and EDX micrographs of MS (a) after immersion in 1 M HCl without CSEA, (b) after immersion in 1 M HCl solution containing 2 g/L of CSEA.In Fig. 9a, the steel specimen exhibits a rough texture and a significant degree of porosity, suggesting a quick occurrence of corrosion in the absence of CSEA. Nevertheless, the presence of CSEA, as seen in Fig. 9b, results in a smooth appearance of the specimen. This is attributed to the creation of a protective layer by the components of CSEA. These findings indicate that CSEA hinders the process of iron dissolving, thereby decreasing the rate of corrosion of steel when exposed to a 1 M HCl solution.Energy-Dispersive X-ray (EDX) spectra were used to identify the elements present on the metal surface before and after exposure to the inhibitor solution. Figure 9a presents the EDX spectrum for a specimen in a 1 M HCl solution, showing characteristic peaks for elements found in a MS-sample. Notably, peaks for oxygen and an additional line indicating the presence of Cl on the surface were detected.However, when CSEA is present, the EDX spectrum reveals additional peaks for nitrogen and carbon, which are indicative of adsorbed extract species, as shown in Fig. 9b. Furthermore, the signals for O and Cl are significantly reduced. These data show the adsorption of alkaloid molecules on the surface of the steel.DFT resultsIn this section, the analysis of the DFT study will be discussed according to frontier molecular orbitals (FMO), including the electron density distribution of the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO). Features such as electrophilicity, hardness, softness, the difference between the HOMO and LUMO energies (EHOMO and ELUMO), the proportion of electron transfer, and the exchange energy of donation and return donation are all factors to consider (Fig. SI4).Frontier molecular orbitals (FMO) and electrostatic potential mapsThe optimized molecular structures, the electron distribution of the HOMO and LUMO surfaces, and the electrostatic potential maps, as well as the energy gap diagrams of the investigated neutral inhibitors and their protonated form are shown in Fig. 10. In both forms, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) include a significant portion of the inhibitors under investigation. This observation indicates the inhibitors’ propensity to give and take electrons to and from the metal surface via several active centers. The plotting of the ESP maps, shown in Fig. 10, illustrates the presence of electron-rich areas (red color) and electron-poor regions (blue color), corresponding to electrophilic and nucleophilic assaults, respectively.Figure 103D-isosurfaces (isovalue = 0.02) of the electron distribution of the HOMO and LUMO and the energy gap diagrams for inhibitors 2 and 3 in their forms in left: neutral forms and in right: protonated forms. Blue loops correspond to the positive electron density distribution and the yellow loops correspond to the negative electron density distribution.Furthermore, these results can be well illustrated by plotting the total density at both HOMO and LUMO surfaces (TD-HOMO and TD-LUMO), see Fig. SI1 (supplementary provided).The energies of HOMO, LUMO and energy gapsIn order to establish a very useful correlation between the molecular structure of the investigated inhibitors and their corrosion inhibition efficacy, the quantum chemical calculations were calculated and discussed. In reliable with Fukui’s theory, transition of electron is due to the interaction between HOMO and LUMO. The higher values of EH and the lower values of EL tend the molecule to denote and accept electrons to and from the surface, respectively35,36,37,38,39,40,41. According to our results it can be deduced that the corrosion inhibition efficiency of the neutral and protonated forms of Q1 inhibitor molecule is higher that of Q2 inhibitor molecule. The results show also that the energy gaps of the neural forms are less than those of the protonated ones, illustrating that the probed inhibitors in their neutral forms have higher efficiencies to protect the metal surface than the protonated forms (Figs. SI5 and SI6).One of the most useful global quantum chemical descriptors as a function of reactivity of the inhibitor molecule towards the adsorption on the metal surface is the energy gap, which measures the difference between EH and EL of the inhibitor (ΔE = EL–EH)42,43,44,46. The lower the ΔE value the stronger interaction between HOMO and LUMO, the higher chemical reactivity and the lower kinetic stability. According to our results, the corrosion inhibition efficacy of Q1 inhibitor molecule is higher than that of Q2 one. It is also found that the neutral forms are chemical more reactive the protonated form (see Table SD1).The metal-inhibitor energy interactions can be investigated in terms of the energy gaps between the HOMO and LUMO values of the inhibitors and the metal surface (\(\Delta {{\varvec{E}}}_{1}={E}_{L}-{E}_{H}^{Fe}\) and \(\Delta {{\varvec{E}}}_{2}={E}_{L}^{Fe}-{E}_{H}\))47,48,49. The \({E}_{H}^{Fe}\text{and }{E}_{L}^{Fe}\) are the HOMO and LUMO energies of the Fe, which are taken to be − 7.9024 and − 0.151 eV, respectively. The results of \(\Delta {{\varvec{E}}}_{1}\) and \(\Delta {{\varvec{E}}}_{2}\), together with the energy gap (ΔE) are graphically shown in Fig. SI5. General inspection of Fig. SI6 and Table 8 indicates that ΔE1 and ΔE2 of Q1N (6.111 and 6.062 eV) are less than those of Q2N (6.458 and 7.093 eV), signifying that the electron flow of electrons from HOMO of the metal to the LUMO of the inhibitor is energetically favored compared to reverse process. In contrast, electrons flow from the protonated inhibitor molecule to the unoccupied 3d orbitals of the metal is energetically favored39,40.Table 8 Global quantum reactivity descriptors.The EH and EL were used to calculated the most representative global chemical reactivity such as ionization potential (I = − EH), electron affinity (A = − EL), energy gap (ΔE = EL–EH), absolute electronegativity (χ = (I + A)/2), global hardness (η = (I–A)/2), softness (σ = 1/η), electrophilicity (ω = χ2/2η), nucleophilicity (ε = 1/ω)and energy change of donation and back-donation (ΔEb–d)41,42,43,46. Inspection of Table 8 shows that the investigated neutral inhibitor Q1N and its protonated form Q1P are characterized by low electronegativity, hardness and nucleophilicity values and high softness and electrophilicity values compared to the Q2N inhibitor and its protonated form Q2P. These results confirm that the inhibition corrosion efficiency of the Q1N inhibitor and its protonated form is higher than that of the Q2N and its protonated form.In case the inhibitor and the metal surface come closer to each other, electrons flow from the lower χ (inhibitor) to higher χ (metal) until reaching equilibrium (χFe = χinh). Therefore, the number of electron transfer to or from the Fe (110) plan is given by ΔN110 as follow:$$\Delta {N}_{110}=\frac{{\phi }_{Fe}-{\chi }_{inh}}{2({\eta }_{Fe}+{\eta }_{inh})}.$$
(19)
In the above equation, χinh denotes the electronegativities of inhibitor, ηFe and ηinh are the chemical hardness of Fe metal and inhibitor, respectively, and \({\phi }_{Fe}\) is the work function of Fe110 plan and is given as 4.82 eV. Using of work function is specific for each F plan, where each plan has its own work function. On the other hand, according to Hard and Soft Acids and Bases (HSAB), the fraction of electron transferred (ΔN) was calculated for iron metal, without considering the Fe plans. For this purpose, the χFe was taken to be 7.0 eV. In both cases, the global hardness of ηFe = 0 eV was taken, by assuming that or a metallic bulk I = A because they are softer than the neutral metallic atoms. Previous studies showed that the positive value of \(\Delta {N}_{110}\) indicates that the inhibitors act as an electron acceptor (Lewis acid), while a negative number indicates that the inhibitors act as electron donors (Lewis base). Therefore, the inhibition efficiency of the inhibitors increases with increasing the negative number of \(\Delta {N}_{110}\). Lukovits et al.49 proposed that if ΔN < 3.6, the molecule acts as a good inhibitor and its inhibition efficiency increases with increasing electron donating ability at the metal surface. In this study, it is found that the \(\Delta {N}_{110}\) < 3.6 for all species, indicating the tendency to act as good inhibitors.Following up the global chemical reactivity descriptors strongly suggests that both inhibitors can act as good inhibitors, and the inhibition efficiency of Q1 inhibitor in its two forms is higher than that of Q2.Local reactivity indicesThe natural atomic charges (NAC) as computed by natural population analysis (NPA) of the neutral form of the investigated inhibitors are summarized in Tables 9 and SD1. For Q1N inhibitor, the most negative atoms are O2, N9, and C4, which are able to donate electrons to the metallic surface. Whereas, the most positive atoms are C1, C5 and C11, respectively, which are responsible to accept electrons from the metal. For Q2N inhibitor, the most positive atoms are, C7, C2 and C5, respectively, and the most negative atoms are C1, O3 and O9, respectively. These results are consistent with obtained by visualizing the HOMO and LUMO surface and ESP maps.Table 9 Natural atomic charges in elementary charges, NC, Hirshfeld charges for N, N + 1 and N–1 electron systems (\({Q}_{A}^{N}\), \({Q}_{A}^{N+1}\), and \({Q}_{A}^{N-1}\)) in elementary charge, “e”, condensed Fukui functions and condensed dual descriptors (\({f}_{A}^{-}\), \({f}_{A}^{+}\) and \({f}_{A}^{2}\)) in elementary charge, “e”), of Q1 and Q2 inhibitors in their neutral forms (Q1N and Q2N).Fukui functionsThe Fukui functions were used to ascertain the local reactivity descriptors linked to electron density. These functions provided an estimate of the locations responsible for nucleophilic and electrophilic attack centers on the inhibitors under investigation in their neutral state33. Figure SI7 displays the three-dimensional is surfaces of Fukui functions pertaining to electrophilic, nucleophilic assaults, and dual Fukui functions.The results of Fukui functions confirm those represented by visualizing the FMOs. In this case, it is observed the electrophilic attacks centers simulate the results obtained by electron density distribution of HOMO (\({f}^{-}\left(r\right)\approx {\rho }_{HOMO})\), while the electrophilic attacks centers simulate the results obtained by electron density distribution of LUMO (\({f}^{+}\left(r\right)\approx {\rho }_{LUMO})\). The results of the dual Fukui function accumulate the results obtained by \({f}^{+}\left(r\right)\) and \({f}^{-}\left(r\right)\) in one map.The condensed Fukui indices for nucleophilic (\({f}_{A}^{+}\)) and electrophilic (\({f}_{A}^{-}\)) attacks and the condensed dual Fukui index (\({f}_{A}^{2}\)) on a atom are defined using the following equations49,50,51:$${f}_{A}^{+}={q}_{N}^{A}-{q}_{N+1}^{A},{f}_{A}^{-}={q}_{N-1}^{A}-{q}_{N}^{A},\text{ and}{ \Delta f}_{A}= {f}_{A}^{2}={f}_{A}^{+}-{f}_{A}^{-}.$$
(20)
In the above equations, \({q}_{N}^{A}, {q}_{N+1}^{A} and {q}_{N-1}^{A}\) are the Hirshfeld charges over the A atom in the N (neutral), N + 1 (anion) and N-1 (cation) electron systems, respectively. The \(\Delta f\left(r\right)\) is considered as the most accurate representation for, simultaneously, both nucleophilic and electrophilic attack centers. The results of the local reactivity indices of the neutral inhibitors (Q1 and Q2N) are summarized in Table 9, while those of the protonated forms Q1P and Q2P are listed in Table SD1 (supplementary provided). For the Q1N inhibitor, it is found that the most sites that are responsible for electrophilic attacks (\({f}_{A}^{-}\)) are C8, O2 and C1, respectively. Whereas, the sites that most favorable for the nucleophilic attacks are C6, N9 and C11, respectively. In addition, the condensed dual Fukui indices indicate that the most responsible sites for electrophilic attacks (\({f}_{A}^{2}\)<0) are C8, O2 and C4, while those that are responsible for nucleophilic attacks with \({f}_{A}^{2}\)>0 are C6, C11 and N9. Similarly, for Q2N inhibitor, our results show that the most active sites that are responsible for electrophilic attacks (\({f}_{A}^{-}\)) are O3, N4 and O9, respectively. Whereas, the favorite sites for the nucleophilic attacks (\({f}_{A}^{+}\)) O9, C7 and O3. Furthermore, the condensed dual Fukui indices indicates that the most favorite sites for electrophilic attacks (\({f}_{A}^{2}\)<0) are N4, O3 and C5, while the most favorite sites for the nucleophilic attacks with \({f}_{A}^{2}\)>0 are C7, O9 and C2. The same results can be also followed for the protonated forms of the investigated inhibitors as shown in Table SD1 (supplementary provided).Monte Carlo and Molecular dynamic simulations resultsThe determination of the adsorption energy for a molecule interacting with the Fe(110) surface involves the application of the following equation, denoted by the symbol “Eads.”35,40. This equation serves as a tool for quantifying the energy associated with the adsorption process, providing valuable insights into the strength and stability of the molecular interaction on the Fe(110) surface28. By utilizing this equation, one can assess the degree to which a molecule is bound to the surface, offering a quantitative measure of the affinity between the adsorbate and the Fe(110) substrate:$${E}_{adsorption}={E}_{Fe{\left(110\right)}_{||}inhibitor }-\left({Fe}_{\left(110\right)}+{E}_{inhibitor}\right),$$
(21)
where \({E}_{Fe{\left(110\right)}_{||}inhibitor}\) is the total energy of the simulated system, EFe, and \({E}_{inhibitor}\) is the total energy of the Fe(110) surface and the corresponding free inhibitor molecules.Following the completion of the Monte Carlo (MC) calculations, an exhaustive analysis was undertaken to scrutinize the adsorption geometry of the inhibitor, aiming to corroborate the outcomes obtained through experimental methods41,42,43,46. In the MC simulation, the equilibrium state was determined by comparing steady-state energy values with initial energy values. The system eventually stabilized in a state of minimal energy, achieved at equidistant intervals throughout the simulation period41,42,43. The components utilized to replicate the corrosive environment are depicted in Fig. 11, showcasing the actual configuration of the adsorbed inhibitor on the simulated Fe(110) plane. The Fe(110) surface featured an array of inhibitors that maintained a nearly parallel orientation during the course of molecular dynamics (MD) simulations, as illustrated46. This meticulous examination not only verified the robustness of the experimental findings but also provided valuable insights into the dynamic behavior and stability of the adsorbed inhibitor under varying conditions.Figure 11Lowest energy geometries obtained from MC and MD calculations for the inhibitors in the study.Figure SI7a illustrates that the presence of heteroatoms (O and N) significantly affects the adsorption pattern on the Fe(110) plane. This leads to interactions between the backbone of the inhibitor molecule and the surface atoms. The complex adsorption behavior seen may be attributed to the molecule’s intrinsic inclination to leave its electron-rich rings and heteroatoms exposed on the surface. This leads to a dynamic interaction between the inhibitor and the Fe(110) substrate41,42,43,46. The involvement of these electron-filled structural components is crucial in promoting a stable and advantageous adsorption arrangement, demonstrating the complex molecular-level interactions at the interface between the inhibitor and the metal surface41,42.The outcomes of employing Monte Carlo (MC) computations to elucidate the distribution of adsorption energies (Eads) for the inhibitors utilized in the simulated corrosion environment are visually presented in Fig. SI7a. The inhibitor exhibits a notably high degree of adsorption to the metal surface, signified by its elevated Eads values46. This intensive interaction results in the formation of a protective layer on the metal surface, effectively acting as a barrier to impede further corrosion14. For a more nuanced understanding of the adsorption dynamics, molecular dynamics (MD) simulations, acknowledged as a superior method, provide a comprehensive representation. As illustrated in Fig. SI7b, following several hundred picoseconds of NVT simulation, the inhibitors undergo a structural transformation, adopting a more planar configuration with the molecular ring securely adhering to the Fe surface. To delve deeper into the adsorption mechanisms of corrosion inhibitors on metal surfaces, a critical analysis of the radial distribution function (RDF) derived from the MD trajectory generated during corrosion simulations becomes imperative. This analytical approach ensures a thorough exploration of the spatial arrangement and intermolecular interactions governing the adsorption process, offering valuable insights into the protective capabilities of the inhibitors on the metal surface46.The radial distribution function (RDF) characterizing the distribution of oxygen within the inhibitors on the Fe(110) surface was derived through a meticulous analysis of the molecular dynamics (MD) trajectory, as depicted in Fig. SI7b. This analytical approach provides a comprehensive depiction of the spatial arrangement and intermolecular interactions governing the adsorption process42. An insightful interpretation of the RDF graph allows for the accurate prediction of the adsorption process by identifying peaks at specific distances from the metal surface. Peaks emerging beyond 3.5 units from the surface suggest physical adsorption processes47, while peaks within the 1 to 3.5 Angstrom range are indicative of chemisorption processes. The RDF analysis revealed prominent peaks at distances below 3.5 Angstrom for the oxygen and nitrogen atoms present on the Fe surface, as illustrated in Fig. SI7b.The presence of these observed RDF peaks, together with the simultaneous occurrence of significant negative energy values, provides strong evidence for a strong chemisorption-type interaction between the metal surface and the inhibitor. This finding provides evidence for the concept of a significant and enduring connection between the oxygen-containing constituents of the inhibitors and the Fe(110) surface, highlighting the efficacy of the inhibitors in creating a chemically resilient protective coating on the metal surface.Inhibition mechanismThe bibliographic data confirms that the corrosion inhibition mechanism in acidic conditions is thoroughly established. The text explains that the inhibitor molecule can be adsorbed onto the metal surface through four different types of adsorptions: (1) electrostatic attraction between charged molecules and the charged metal; (2) interaction between unshared electron pairs in the molecule and the metal; (3) interaction between π-electrons and the metal; and (4) a combination of the aforementioned types. In order for the physical adsorption process to occur in an acidic environment, two conditions must be met: there must be a metal surface with empty electron orbitals, and there must be charged species in the solution, such as molecules with loosely attached electrons or heteroatoms with a lone pair of electrons (Fig. SI8).The 5-Quinolinol molecules that has been adsorbed may build a stronger connection with the metal surface by creating coordinate covalent bonds with the electron pairs of the N atoms in the aromatic rings. The charged oxygen atoms of the 5-Quinolinol moieties further enhance the adsorption capacity. These interactions result in the creation of a protective layer (with a high value of the adsorption energy) on the surface of metal. This layer decreases the rate of corrosion, as shown by the experimental findings. The organic coating that is created serves as a physical barrier, effectively blocking corrosive substances from accessing the metal surface.