C-steel substrates with an area of 1.0 cm² and subsequent elemental structure silicon, 0.16%; nickel, 0.01%, chromium, 0.02%; sulfur, 0.02%; phosphorous, 0.01%; carbon, 0.16%; manganese, 0.68%; and remaining Fe were used for corrosion measurements.

Soybean oil was hydrolyzed with 20% sodium hydroxide solution for 8 h at a temperature of 85–90°C. This method produced sodium salts of fatty acids with good yield (87%). Then, the sodium salts of fatty acids were reacted with concentrated hydrochloric acid solution (37%) for extracting the fatty acids. The sulfating processes were performed on the extracted fatty acids. The product is a sulfated fatty acid with good yield (81%). The products were characterized by physical–chemical approaches and FT-IR spectroscopy.

The sulfated fatty acids were reacted with the equimolar ratio from 10% mono- and diethanolamine at 25°C to yield mono- and diethanolamine salts by the following structures: [R-CH-(SO₃OX)-COOX] (where X = –NH-C₂H₄-OH; 1-[bis(2-hydroxyethyl)amino]-1-oxooctadecan-9-yl sulfate 2-hydoxyethan-1-aminium (CSM) and –N-(C₂H₄-OH)₂; and 1-[bis(2-hydroxyethyl)amino]-1-oxooctadecan-9-yl sulfate bis[2-hyroxyethyl)aminium (CSD)].

The chemical construction of the extracted natural surfactants was clarified via melting points (Gallen–Kamp); FT-IR spectroscopy was performed in KBr (NICOLET 5700 FT-IR). In addition, the K6 processor-Tensiometer (KRÜSS-Company, Germany) using the ring technique was used to determine the surface tension of synthesized surfactants in 3.5% sodium chloride solution at 25°C.

The corrosive medium, 3.5% sodium chloride, was prepared by dissolving analytical grade sodium chloride in double-distilled H₂O. Before experimentation, the prepared 3.5% sodium chloride was stirred for 40 min in 500 ml cells. Then, the cells were retained for 50 min under a pressure of 0.9 bars in a heater at 323 K. The NaCl medium was saturated with CO₂. The concentration range of the synthesized additives was from 10 to 150 mg L⁻¹ used for corrosion inhibition experiments.

Corrosion experiments were completed in 3.5% brine solution saturated with carbon dioxide with different doses of the prepared natural surfactants. The Gamry Potentiostat/Galvanostat/ZRA device was used for the electrochemical experiments using 3-electrode cell systems, in which Pt-sheet assisted as a counter electrode, saturated silver/silver chloride (Ag/AgCl_(sat)) served as the reference electrode, and C1018-steel was the working electrode (WE). Before all electrochemical measurements, the WE was retained in the corrosive medium for 30 min to reach constant OCP values. The PDP experiments were performed in a potential range of −0.4–1.0 V vs. E _OCP at a sweep rate of 0.2 mV s⁻¹. EIS was accomplished at E _OCP using a 10 mV peak-to-peak voltage excitation in a frequency range of 100 kHz–0.1 Hz. Different parameters were attained from the plot simulations using Gamry Echem Analyst software. All the experimentations were carried out twice, and the value averages were recorded using the best figures. The LPR corrosion rate measurements were completed as discussed previously in Abd El-Lateef (2020).

Full optimization of the two inhibitor molecules CSM and CSD was studied using DFT calculations with B3LYP-functional (Saha et al., 2015; Singh et al., 2016) and basis set 6–31g (d,p) implemented in the Dmol3 module in Materials Studio V. 7.0 program (Haque et al., 2017; Singh et al., 2018a; Singh et al., 2018b). The results obtained from the simulation, for instance, the lowest unoccupied molecular orbital (LUMO), the highest occupied molecular orbital (HOMO), gap energy (ΔE), electronegativity (χ), hardness (η), global softness (σ), dipole moment (µ), electrophilicity index (ω), and number of electrons transferred (ΔN) were investigated. The electronegativity, global softness, hardness, electrophilicity index, and number of electrons transferred were calculated as follows: χ = − E H O M O − E L U M O 2 , (1) η = 1 σ = E L U M O − E H O M O 2 , (2) ω = χ 2 2 η , (3) Δ N = ϕ − χ i n h 2 ( η F e − η i n h ) , (4) where φ is function work of Fe(110), χ_inh represents the inhibitor electronegativity, and η _Fe and η _inh are the chemical hardness of Fe (0 eV) and the inhibitor, respectively.

For MC simulations, the suitable adsorption configurations of the CSM and CSD molecules on the Fe(110) surface were discovered by using the adsorption locator module in the Materials Studio V. 7.0 program (Abd El-Lateef et al., 2020). Initially, the adsorbate molecules had been optimized in the operating COMPASS force field. Then, in a simulation box (37.24 × 37.24 × 59.81 Å), adsorption of the CSM and CSD, Cl⁻ ions, hydronium ions, and water molecules with surface of Fe(110) was accomplished (Tantawy et al., 2020).

In order to inspect the modifications in surface topologies of the C-steel specimens after corrosion experiments, the samples were first immersed in the corrosive solution with and without synthetic surfactants for 20 h, then cleaned with double-distilled H₂O and acetone, and desiccated with cool air. The surface morphology of the investigated specimens was detected by EDX complemented with a FESEM. An XPS study was performed as mentioned in our previous study (Abd El-Lateef et al., 2020). The investigated specimens attained from LPR corrosion were utilized in examinations.

The structural features of purified compounds of the extracted fatty acids were established by FT-IR investigation as presented in Supplementary Table S1; Supplementary Figure S1 (Supporting data). The fatty acid peaks are in agreement with the distinctive peaks described previously (Shapaval et al., 2014). The peak at ≈ 1706 cm⁻¹ is related to the carbonyl group (C=O) of the –COOH group; however, the band at 1,488 cm⁻¹ is attributed to –C=C–. This –C=C– bond was damaged after the practicability of sulfating. The peak at 1,359 cm⁻¹ is owing to the stretching absorption band S–O. It designates the whole abstraction of the –C=C– bond by adding practicability.

The physico-chemical characteristics of the extracted fatty acids before and after the sulfating process were examined. The results exhibited that the acid number augmented was from 134.7 (before sulfating process) to 278.4 (after the sulfating process) and no interaction with iodine was determined after the sulfating process. These outcomes also established the whole elimination of the C=C bond by sulfuric acid addition and production of sulfated fatty acid.

The concentration of critical micelle concentration (CMC) values of the synthesized natural surfactants was estimated by plotting the change in the surface tension (γ) against their concentrations in mg/l in 3.5% NaCl solution at 25°C (Figure 1). The CMC values were found to be 123.1 and 102.3 for CSM and CSD, respectively. Accordingly, the CMC value of the synthesized natural surfactant is declined by increasing the hydrophobic chain length; this could be related to decreasing the solubility and increasing hydrophobicity of the synthesized natural surfactant, that is, by increasing the hydrophobic chain length, the tendency of the surfactant molecule to form a micelle is consequently reduced by CMC (Abd El-Lateef et al., 2020).

The surface-active parameters such as effectiveness (Π _cmc), the surface area (A _min), and efficacy of interfacial adsorption (Γ _max) were calculated based on surface tension measurement. The effectiveness (Π _cmc) values indicated that the most active surfactant is CSD, which provides the largest decline in the surface tension at the CMC. Moreover, the Γ _max and A _min values exhibited that the increase in the length of a hydrophobic chain of the surfactant shifts Γ _max to lesser concentration values and A _min is increased owing to the increasing hydrophobic chain of the area occupied by each surfactant molecule increase.

According to Gibb’s adsorption equations, the thermodynamic parameters of micellization ( Δ G m i c 0 ) and adsorption ( Δ G a d s 0 ) of the prepared natural surfactants were determined. The values of Δ G m i c 0 and Δ G a d s 0 ( Δ G m i c 0 = -23.25 and -23.78 kJ g¹ and Δ G a d s 0 = -25.12 and -25.46 kJ g¹ for CSM and CSD, respectively) are continuously negative, leading to the spontaneity of adsorption and micellization processes, but there are more upsurges in the negativity value of Δ G a d s 0 than Δ G m i c 0 , demonstrating the inclination of the surfactant compounds to be adsorbed at the metal/electrolyte surface (Tantawy et al., 2020). Moreover, Δ G a d s 0 values increase by increasing the hydrophobic chain, signifying that the inclination of the surfactant molecules to be absorbed on the interface follows the order CSD > CSM.

The impact of the natural surfactant (CSM and CSD) dose on the Tafel anodic and cathodic polarization profiles of C-steel in 3.5% brine solution saturated with carbon dioxide at a sweep rate 0.2 mV s⁻¹ and 323 K is presented in Figure 2 A, B. Corrosion parameters were computed based on the anodic and cathodic potential vs. current density features in the potential of Tafel area (Hsissou et al., 2020b). The corrosion current density values (jcor) for the titled C1018-steel lacking and with the surfactant, respectively, were estimated by the anodic and cathodic extrapolation of Tafel lines to the corrosion potential (Ecor). It could be appreciated that the occurrence of surfactant additive results is a noticeable modification in both anodic and cathodic divisions of the polarization plots in the direction of jcor. It is worth noting that the surfactants affect the mutually anodic and cathodic processes. The surfactant additive might decline the corrosion rate by the lessening of C1018-steel reactivity. Based on this phenomenon, a decline of both the cathodic and anodic reactions arises from the surfactant adsorption on the conforming efficient centers (Alnajjar et al., 2022). The protection efficacy (η _T/%) in the occurrence of CSM for CSD was intended from the corrosion current density in the blank ( j c o r 0 ) and inhibited ( j c o r s ) systems by the following equation. (The values are recorded in Table 1): η T / % = [ j c o r 0 − j c o r s j c o r 0 ] × 100 = θ × 100. (5)

The outcomes in Table 1 revealed that jcor declines and η _T/% upsurges as the surfactant dose is augmented. These findings indicated that hindrance of the electrode reactions takes place at mutually anodic and cathodic locations as a consequence of coverage of these places by CSM and CSD species. The maximum value of η _T/% (98.6%) was attained for the maximum dose (150 ppm) CSD in the investigated corrosive media. This might be ascribed to the increase in active position number, molecular size, and electron densities (Hsissou et al., 2021a). The increase in η _T/% with growing the [surfactant] could be inferred based on the adsorption quantity, and part of the covered surface increased with increasing additive concentration (Ansari et al., 2014). The Ecor values of both CSM and CSD additives showed a slight shift toward both anodic and cathodic directions (the differences in Ecor are lower ±85 mV) and did not demonstrate any definite trend in CO₂-saturated NaCl solution. Such findings proposed that the CSM and CSD natural surfactants can be classified as mixed-type corrosion inhibitors, that is, both the hydrogen evolution reaction and the anodic Fe dissolution were blocked (Bentiss et al., 2005).

From Table 1, it is correspondingly detected that the cathodic (βc) and anodic (βa) Tafel slopes of the protected corrosive medium do not follow a definite array with an upsurge in surfactant dose. These outcomes designate that these natural surfactants act by merely hindering the obtainable surface area. The high part of the covered surface θ value close to unity designates nearly a complete coverage of the electrode surface with the adsorbed surfactant additives. The protection capacity of the synthesized natural surfactants was augmented in the subsequent order: CSM < CSD. High electron density and higher surfactant size on the adsorption sites might be accountable for high protection efficacy.

The effect of temperature for the deterioration of C-steel in blank and inhibited test solution was studied. To obtain more knowledge about the adsorption type of the extracted natural surfactants at different temperatures, the Tafel polarization method was performed at 303–333 K. The adsorption mechanism (physically or chemically onto the metal surface) could be proposed from the consideration of these findings. The change in j _cor and η _T/%) with temperature in the uninhibited and inhibited systems is presented in Supplementary Figure S2. It could be seen that j _cor increases with temperature for C-steel in the blank corrosive medium (Supplementary Figure S2A). In the presence of CSM and CSD surfactants, j _cor is smaller than that in blank solution. These results indicate that the CSM and CSD surfactants are effective at the studied temperature range. The effect of temperatures on the inhibition capacity of C-steel in the corrosive medium containing CSM and CSD surfactants is presented in Supplementary Figure S2B. Based on this plot, inhibition efficiency increases by increasing the reaction temperature. This performance is characteristic of a chemisorption mechanism (Tantawy et al., 2020).

To further assess the corrosion inhibition action of the synthesized natural surfactants on C-steel corrosion in blank and inhibited systems, EIS experiments were carried out. The blank and protected C-steel samples display a sole capacitive loop which designates that metal corrosion in the current corrosive medium (3.5% NaCl-saturated with carbon dioxide) includes a charge transfer mechanism (Soltani et al., 2016). Examination of Figure 3A, B exhibited that Nyquist profile diameters increase with increasing concentrations of the two natural surfactants (CSM and CSD), which is ascribed to their augmented adsorption on the interfaces of steel/corrosive solution (Kowsari et al., 2014).

Comparison of experimental Nyquist data (points) and fitted data (line) measured for C-steel in the blank (A) and inhibited systems (B) is presented in Figure 4. The EIS parameters were computed with the assistance of a suitable equivalent circuit (EEC) presented in Figures 4A,B (inset) and are recorded in Table 2. This EEC includes R _e (electrolyte resistance), R _p [resistance of polarization; R _p = R _dl (diffuse layer resistance) + R _ct (charge transfer resistance) + R _a (other collected resistance)], and CPE (constant phase element), which is utilized instead of pure capacitance to recompense the inhomogeneity of the surface (Yilmaz et al., 2016), arising owing to impurities, dislocations, roughness of surface, grain boundaries fractality, and active center distribution (Ortega-Toledo et al., 2011). In the case of the inhibited medium (Figure 4B), CPE is in sequences to the parallel of capacitance as C _ads due to the surfactant adsorption layer and the resistance attributed to inhibitor adsorption (R _ads). The CPE impedance (Z _CPE) is assumed by the subsequent equation (Musa et al., 2011): Z C P E = 1 Y 0 ( j ω ) n , (6) where j represents an imaginary number (j = − 1 ), Y _o characterizes the CPE constant, ω symbolizes the angular frequency, and n is the phase shift factor. The n values = 1, 0.5, 0, and −1 signify pure capacitivity, Warburg impedance resistivity, and inductivity, respectively. The C _dl values (double-layer capacitance) were intended from the frequency at the imaginary part of the maximum impedance (ω _max) by the following equation (Desimone et al., 2011): C d l = Y 0 ( ω max ) n − 1 . (7)

The R _e value recorded in Table 2 is higher for the solution containing CSM and CSD inhibitors than the uninhibited medium. This is ascribed to a decline in the conductivity of electrolytes by the insertion of the instigated surfactants. The outcomes of Table 2 demonstrated that the R _p values improved on augmenting the surfactant dose, which attributed to larger hindering of the efficient part at the steel surface owing to CSM and CSD adsorption onto the C-steel surface and the disarticulation of the adsorbed H₂O molecules from the metal interface. The values of n are generally relative to the metal surface heterogeneity/homogeneity. As possibly detected, the n value is small for the uninhibited medium, which openly clarifies the great coverage of iron hydroxide/oxide on the Fe bar. Nevertheless, after insertion, the quantities of CSM and CSD surfactants in CO₂-saturated 3.5% brine solution and the n value enhanced considerably, demonstrating respectable hindrance in contradiction of sweet corrosion and the products of corrosion process movement from the metal surface.

Generally, when a corrosion additive is inserted into a corrosive solution, the C _dl values will turn lesser, and this infers that the constant of local dielectric declines and/or double-layer thickness increases, supporting the C-steel corrosion protection by the surfactant adsorption at the interface of C-steel/CO₂-saturated 3.5% NaCl (Hsissou et al., 2020c).

The double-layer thickness is the reciprocal of capacitance and could be utilized to follow the inhibitor species adsorption process on the metal interface. For instance, for CSM and CSD surfactants (150 mg L⁻¹), the capacitance (C _dl) value (197.70 μF cm⁻²) attained for the blank system is reduced to 31.71 μF cm⁻² for CSM and 23.42 μF cm⁻² for CSD, indicating that the CSD surfactant performs as a respectable capacitance reducer and consequently a superior corrosion additive. Furthermore, a decline in C _dl with cumulative surfactant dose was detected which might be ascribed to the development of a defensive film by the surfactant additive adsorption at the C-steel/brine solution interface (Shahzad et al., 2020).

Figures 5I,II show the Bode and phase angle modules at different doses of the studied surfactant additives. As observed from Figure 5, Bode profiles denote the presence of a sole constant phase component at the interface of C-steel/CO₂-saturated brine. At low frequencies, the impedance values are augmented owing to the construction of the protection layer by the surfactant molecule adsorption, which shields C-steel from harsh ions. The extreme phase-angle at the middle frequency for a perfect capacitor is −90°. In the lack of CSM and CSD surfactants, the value of the phase angle is lesser than the values detected in the inhibited medium. The increase in maximum phase-angle with an upsurge in the surfactant dose confirms the adsorption of more surfactant molecules on the metal interface (Singh et al., 2019), resulting in a reduction in the dissolution rate of the metal. Additionally, more phase-angle values are detected in the case of CSD, which clarifies its superior inhibition characteristics. In the pre-eminent condition, the protection efficacy value of about 92.6 and 96.1% was obtained in the presence of 150 ppm of CSM and CSD, respectively, after 45 min metal exposure to the corrosive medium.

The LPR corrosion rate was performed to confirm the stability of the extracted natural surfactants in the titled corrosive medium. Figures 6A,B show the change of the rate of (CR) corrosion with dipping time for C-steel in blank brine solution saturated with carbon dioxide and inhibited medium containing different concentrations of (A) CSM and (B) CSD at 50 ± 1 °C. The CSM and CSD surfactant additives were introduced into the 3.5% NaCl saturated with carbon dioxide after 1 hour of dipping (at this period, E _cor becomes stable). In the blank corrosive medium, the CR was augmented with t from 2.354 to 3.969 mm/y after 20 hours of immersion, that is, the CR inclines to upsurge with exposure time in the blank corrosion system. This behavior could be recognized to the galvanic impact among Fe₃C (cementite) and α-Fe (ferrite phase) (Abd El-Lateef et al., 2020). From Figures 6A,B, a sharp decline in the CR was distinguished after the insertion of CSM and CSD surfactants to the 3.5% NaCl saturated with carbon dioxide as a corrosive solution. The CR was decreased from 3.969 mm/y to 0.401 and 0.068 mm/y by the addition of 150 ppm of CSM and CSD, respectively, after 20 h of dipping. These phenomena might be attributed to the adsorption of CSM and CSD molecules and the construction of a nature-protecting layer at the electrode/electrolyte interface; consequently, the metal interface is regularly isolated from the corrosive environments (Tantawy et al., 2020). It is observed from Figures 6A,B that the CR values declined, while the protection capacity enhanced with cumulative (CSM and CSD) and extends to the highest value of 89.9 and 98.2% in the presence of 150 ppm of CSM and CSD, respectively, after 20 h of dipping. The increased protection capacity upon increasing [surfactant] is ascribed to an upsurge in the part of the covered surface of the CSM and CSD molecules. The findings accomplished from the LPR corrosion rate technique were established to have a suitable agreement with the data attained from the EIS and PDP methods.

It is well accepted that the inhibition efficacy of inhibitor additives is motivated by immersion during the time in the aggressive medium. So, to examine the influence of immersion time on the corrosion protection of C-steel under sweet conditions in the presence of different concentrations of (CSM and CSD) surfactants, the LPR corrosion rate method was utilized. It is observed from Figures 6A,B that the values of CR for C-steel in the inhibited systems containing surfactants (at all concentrations) continue from 2 to 20 h when compared with those achieved for the blank system. Nevertheless, from 2 to 20 h of soaking, the values of the corrosion rate remain steady, which may be attributed to the development of a strong surfactant layer on the steel interface which inhibits the corrosive attack of sweet corrosion on the C-steel substrate.

Actually, the H₂O molecules might adsorb at the interface of the electrode/electrolyte. Consequently, the surfactant adsorption from the aqueous medium could be reflected as a quasi-replacement route among the surfactant molecules in the aqueous solution [Sur_(sol)] and H₂O molecules at the metal substrate [H₂O_(ads)] as the following equation (Farelas and Ramirez, 2010). Su r ( s o l ) + n H 2 O ( a d s ) ↔ Su r ( a d s ) + n H 2 O ( a d s ) , (8) where n represents the number of H₂O molecules substituted by one surfactant additive. Basic knowledge of the surfactant adsorption steel interface could be delivered by adsorption isotherm considerations (Singh et al., 2017). The degree of corrosion protection depends on the surface situations and adsorption approach of organic additives. Supposing that the corrosion on the surface-protected parts equalize zero and that the corrosion occurs merely on the exposed parts of the interface, the part of surface coverage is computed from Eq. 3. Some models of adsorption isotherms could be utilized to measure the adsorption mode of the prepared surfactants on the steel interface (Hsissou et al., 2020d; Hsissou et al., 2021b):

Temkin model exp ( f θ ) = K a d s C i n h , (9) Langmuir model θ 1 − θ = K a d s C i n h , (10) Frumkin model θ 1 − θ exp ( − 2 f θ ) = K a d s C i n h , (11) Freundlich model θ = K a d s C i n h , (12) where f represents the active factor in homogeneity, C _inh signifies the surfactant dose, and K _ads denotes the adsorption equilibrium constant (Hsissou et al., 2020d).

In order to obtain the best explanation of the adsorption performance of the prepared surfactants, all of the previous adsorption isotherm models were verified. The C/θ vs. C _inh profile produced a straight line with a correlation coefficient (R ²) of 0.9974 and 0.9997 and a slope of 0.936 and 0.948 for CSM and CSD, respectively, as presented in Figure 7. This indicates that the adsorption of the surfactant on the electrode interface follows the model of Langmuir isotherm. On the other hand, the value of K _ads could be intended from the straight-line intercept of the C/θ vs. C _inh curve, and the associated standard free energy of adsorption ( Δ G a d s 0 ) could be expressed by the subsequent equation (Li et al., 1999): Δ G a d s 0 = − 2.303 R T ⁡ log ( 1 × 10 6 K a d s ) , (13) where the value of 1 ×10⁶ is (H₂O), R is the general gas-constant, and T is Kelvin temperature.

The calculated K _ads values were found to be 62.6 and 91.7 L g⁻¹ and Δ G a d s 0 are −48.2 and −49.3 kJ g⁻¹ for CSM and CSD, respectively. The high K _ads value shows that the surfactant molecule owns strong adsorption capability onto the steel interface, and the negative Δ G a d s 0 values show that the surfactants are instinctively adsorbed on the electrode surface (Ouici et al., 2017). Normally, Δ G a d s 0 values close to –20 kJ g⁻¹ or lesser are reliable with the electrostatic attraction among charged surfactant molecules and charged steel substrates (physical adsorption); while those close to –40 kJ g⁻¹ or greater include transfer from the surfactant additive molecule to the steel substrate to form covalent bonds (chemical adsorption) (Usman et al., 2017). In this study, the Δ G a d s 0 values are more than −40.0 kJ g⁻¹, demonstrating that the prepared natural surfactants are chemically adsorbed on the metal substrate.

Surfactant molecule adsorption takes place due to the attraction energy between the surfactant species and the electrode substrate, which is greater than that between H₂O molecules and the electrode substrate. Subsequently, the mitigation impact by surfactants is related to the robust surfactant adsorption via their active sites onto the electrode substrate. Herein, the findings show that the mechanism of adsorption of the tested surfactants on metal in brine solution saturated with CO₂ is characteristic of chemisorption at the experiment temperature. Chemical adsorption of the surfactant additive could happen owing to the construction of links among the empty d-orbital of Fe atoms, including the H₂O molecule displacement from the steel substrate and the lone pairs of electrons existing on the nitrogen, sulfur, and/or oxygen atoms of the surfactant. The rate of the adsorption process is commonly quick and, therefore, the reactive steel is isolated from the corrosive medium (Shalabi et al., 2019). Moreover, there is a synergism between Cl⁻ ion adsorption and the protonated surfactant molecules. In addition, the adsorbed surfactant species could be combined with the oxide film and react chemically to produce an additional protecting surface system (Cai et al., 2013; Migahed et al., 2018). Consequently, we could conclude that the corrosion inhibition of CSM and CSD in brine solution saturated with CO₂ is mostly owing to electrostatic and chemical attractions.

Supplementary Table S2 (Supporting data) shows the comparison of the inhibition activity of the synthesized natural surfactants based on soybean oil (CSM and CSD) for the corrosion of C-steel alloy under the sweet condition (CO₂-saturated 3.5% NaCl) with some previously designated surfactant additives (Farelas and Ramirez, 2010; Singh et al., 2017; Singh et al., 2019; Shahzad et al., 2020; Hsissou et al., 2021b). The inhibition capacity values, recorded in Supplementary Table S2, were acquired utilizing EIS, LPR corrosion rate, and PDP experiments. By comparing these outcomes, it was found that our natural surfactants (CSM and CSD) are the pre-eminent effective inhibitors in 3.5% NaCl saturated with CO₂. Likewise, the highest inhibition capacity value is attained using a 150 ppm concentration of CSD, that is, 98.6%.

The FESEM complemented with the EDX system was used to confirm the adsorption of the surfactant molecule at the electrode/electrolyte interface. Figure 8 shows the EDX analysis (A, B, and C) and FESEM (A1, B1, and C1) micrograph of (A) pristine C-steel, (B) C-steel immersed in blank 3.5% NaCl saturated with CO₂, and (C) C-steel immersed in 3.5% NaCl saturated with CO₂ containing 150 ppm CSD for 20 h. Figure 8A displays EDX analysis for C-steel substrate. The feature peaks are due to (Fe, Cr, C, P, S, Si, and Mn) elements existing in the alloy structure. Figure 8A1 illustrates the picture of the pristine C-steel surface. The image shows the metal surface brightness lacking any presence. After immersion in the corrosive medium without the inhibitor (Figure 8B), the surface reveals that the feature peaks are due to Mn, P, Fe, Cr, and O elements. This designated that the products of corrosion process on the C-steel substrate creature metal oxides. The corresponding picture (Figure 8B1) exhibited a dense spongy film of corrosion product enclosing all the metal substrates, revealing that in the uninhibited medium, the electrode interface is extremely corroded. However, in the inhibited corrosive medium containing 150 ppm of CSD (Figure 8C), the surfactant displays extra characteristic peaks of nitrogen element. This outcome demonstrated that the surfactant adsorption on the C-steel substrate leads to decline in the corrosion film, and a greater dose of the surfactant is essential to interval the corrosion process, while the SEM picture in the inhibited system (Figure 8C1) shows a smooth surface, which determines a respectable defensive layer existing on the C-steel substrate. These findings support the PDP and EIS measurements.

The XPS investigations were accompanied to verify the CSD surfactant adsorption on the electrode substrate. The XPS exploration attained for C1018-steel in blank brine solution saturated with CO₂ solution (A–D) and the occurrence of 150 mg/L CSD (E-J) are presented in Supplementary Figure S3. The communal bands for Fe-2p, Cl-2p, C-1s, and O-1s were identified for the protected and unprotected sample. Furthermore, the peaks of S-2p and N-1s were detected for the C1018-steel sample dipped in the solutions containing the inhibitor, which approves the CSD surfactant adsorption on a metal interface.

For the blank medium, the profile of C-1s indicates three peaks (Supplementary Figure S3A) at 288.5, 287.2, and 285.3 eV might be labeled to –C=O/COOO⁻, –C–O/COOO^−-,and–-C–C–, respectively, whereas the protected sample at 284.6 might be ascribed to the C–H and–-C–C– bonds, 286.4 could be recognized to –C–S and –C–N bonds, and 288.6 eV can be related to ⁺N–C– bond (Ouici et al., 2017). The occurrence of Cl band (Supplementary Figures S3B,F) on the metal interface in the picked sample lacking and with the CSD surfactant in 3.5% NaCl solutions saturated with carbon dioxide could be ascribed to the attraction between the Cl⁻ ions and the surface positive charge of metal generating iron (III) chloride (Bouanis et al., 2016). The Cl-2p band exposes two peaks for the blank and protected sample which is due to Cl-2p_3/2 at 197.9 and 197.8 and Cl-2p_1/2 at 199.9 and 199.5 eV, respectively (Bouanis et al., 2016).

The XPS profile of iron-2p indicates that four bands (Supplementary Figures S3C,G) for the blank and protected sample at 709.9 and 710.1 eV is given for Fe-2p_3/2 of Fe(II), where 712.9 and 712.8 eV is related to Fe-2p_3/2 of Fe(III), 718.4, 720.2; 724.3, and 723.6 and 729.5 and 731.4 eV are attributed to Fe-2p_3/2, Fe-2p_1/2, and Fe-2p_1/2, respectively (Hashim et al., 2019). In addition, the O-1s profile possesses three bands (Supplementary Figures S3D,I) for the blank and protected C1018-steel sample which might be related to O atoms, CO₃ ⁻, and OH⁻ bonded to Fe(II) and Fe(III) to form the FeO/Fe₂O₃ oxides, FeCO₃, and FeOOH, respectively (Cen et al., 2019).

Furthermore, the metal electrode in the corrosive medium containing the CSD surfactant displays additional peaks for N-1s and S-2p spectra exposing one band (Supplementary Figures S3H,J) at 398.9 and 167.5 eV which might be ascribed to the –N–H and –SO₃ ^- existing in the CSD compound, respectively. According to the XPS outcomes, we could confirm that the CSD surfactant is adsorbed at the interface of steel/solution.

The optimized geometry of the synthesized natural surfactants CSM and CSD at the B3LYP level of theory and their HOMO and LUMO orbital occupation are presented in Figure 9. The binding of surfactant species to the C-steel interface increases with a greater energy value of HOMO, which specifies the superior electron donation of the surfactant molecule to the empty d-orbital of the steel, and a lesser energy level of LUMO denotes the capability of the surfactant additive to gain electrons from the d-orbital of steel (Abdallah et al., 2018). As recorded in Table 3, if we compare among the surfactants, CSD has a greater HOMO and lesser LUMO energy value that demonstrates that CSD displays greater protection capacity than the CSM surfactant additive. The adsorption capability of the surfactant increases with lesser energy gap (ΔE). The lesser energy gap states greater chemical reactivity and protection efficiency (Gece and Bilgiç, 2009). The tendency for ΔE values has the order CSM > CSD or CSMH⁺> CSDH⁺ (Table 3). These outcomes demonstrate that the protonated forms have larger inclination to adsorb on the steel interface than the non-protonated forms.

For the two models, the HOMO and LUMO as seen in Figure 9 are generally localized at the terminal edges of the surfactant molecule, where the cationic part chains are placed. The dipole moment (µ) is a significant theoretical parameter that reveals the global polarity of a surfactant molecule. The µ is associated with protection power. The protection capacity increases with increasing the µ. As revealed in Table 3, the CSD and CSDH⁺ forms have the maximum µ.

Global hardness (η) and softness (σ) for the two CSM and CSD compounds were intended, which estimate the surfactant molecule reactivity. The compound which has greater softness value and lesser hardness value is predicted to have the highest protection efficiency. As a result, as detected in Table 1, the CSD inhibitor displays higher protection power than the CSM surfactant molecule.

The local reactivity of the synthesized surfactants molecules could be assessed by computing the Fukui indices ( f k + and f k − ), local softness descriptor ( σ k ± ), and the local electrophilicity ( ω k ± ) and the dual descriptors ( Δ f k , Δ σ k  and  Δ ω k ) as follows (Hsissou et al., 2021b): σ k ± = σ f k ± . (14) ω k ± = ω f k ± . (15) Δ f k ± = f k + − f k − . (16) Δ σ k = σ k + − σ k − . (17) Δ ω k = ω k + − ω k − . (18) For simplifying, the most important findings are exhibited in Supplementary Table S3. The calculated Fukui indices (Supplementary Table S3) support the electron distribution of the HOMO and LUMO orbitals detected for the neutral and protonated species assigning the sites; thus, the synthesized surfactants will be adsorbed onto the metal surface. f k − represents the reactivity of the electrophilic attack (donation sites), whereas f k + signifies the reactivity of the nucleophilic attack (accepting centers) (Hsissou, 2021). The highest f k − for the neutral form of the synthesized surfactants is found at O11, S12, O13–O15, C22, N23, O24, and O30 and the protonated form at O11, S12, O13–O15, C22, N23, O24, O29, O30, and O80, designating the site for offering electrons, whereas the highest f k + is found at S12, C22, N23, O24, and O30 for the neutral form and at O11, O13–O15, O24, O29, and O30 for the protonated form, exhibiting back-donation capability (Hsissou et al., 2021c). Moreover, the local dual descriptors are more accurate and reliable tools than the Fukui indices ( f k + and f k − ), and local softness ( σ k ± ) and electrophilicity ( ω k ± ) and the graphical representation of the dual local descriptors of the maximum descriptive active sites are demonstrated in Figure 10. The obtained findings disclose that the sites with the Δ f k , Δ σ k , and  Δ ω k < 0 have the affinity to contribute electrons to the metal surface. In contrast, those centers with Δ f k , Δ σ k , and  Δ ω k > 0 have the capability to receive electrons from the metal. It is obvious that in Figure 10, the greatest active sites for electron contribution are at O11, S12, O13–O15, C21, N23, C25–C28, O29, O30, N76, and O79 for the neutral form and O11, S12, O13–O15, C20, C21, N23, O24, C25–C28, O29, O30, C31, N77, and O80 for the protonated form, while the active accepting centers are at C10, C22, and O24 for both neutral and protonated forms.

The electron transferred number (ΔN) is also computed and recorded in Table 3. If the ΔN values ˂ 3.6, the protection capacity upsurges by increasing the capability of surfactant molecules to contribute electrons to the C-steel interface (Obot et al., 2016). As observed in Table 3, the values of ΔN for the prepared natural surfactants are positive, and the CSD displays a greater value than CSM additive, which provides a suggestion that CSD additive has greater protection capacity more than the CSM compound.

From the outcomes in Table 3 the ΔN values are in the order [CSDH⁺ (1.06)> CSMH⁺ (1.05)> CSD (0.82)> CSM (0.81)]. Herein, CSM and CSD surfactants are the electron donors, and the metal interface is the acceptor. This outcome supports the fact that the CSM and CSD adsorption on the metal substrate can happen by donor–acceptor attractions among π-electrons and/or nonbonding lone pair electrons of the N- atom and empty d-orbitals of Fe. Based on the previous computational indices, it could be recognized that the protonated CSMH⁺ and CSDH⁺ forms are more efficient than neutral CSM and CSD forms.

MC simulations were performed to acquire more information about the interaction between the iron surface and surfactant molecule (El-Lateef et al., 2019). We have confirmed that the full system touched equilibrium until both energy and temperature were composed. The equilibrium shape (top, side views, and final equilibrium configurations) of the Fe(110)/Cl⁻/surfactant/H₂O adsorption model is depicted in Figure 11. Figure 11 reveals the greatest suitable adsorption arrangements for the CSM and CSD molecules on the Fe (1 1 0) surface, which are signified in approximately parallel or flat arrangement, and the surfactant compound is adsorbed on the steel interface via nitrogen atom (N), demonstrating an improvement in the magnitude of adsorption and maximum surface coverage (Madkour et al., 2018). Furthermore, the directories assessed from MC simulations are listed in Table 4. As per Table 4, the CSD molecule has high negative adsorption energy (−2836.76 kcal mol⁻¹) than that for the CSM molecule (−2700.14 kcal mol⁻¹), which designates that the adsorption of the CSD molecule on the Fe(110) interface is strong, spontaneous, and stable. This indicated that when the corrosive ions, such as Cl⁻ ions and H₂O molecules, exist at the surface, the inhibitive CSM and CSD molecules could approach the steel interface accordingly, suggesting their tendency for interfacial attachment to the metal adsorbent (Özcan et al., 2004). Moreover, the CSD molecule in the investigated corrosive medium provides higher rigid adsorption energy (−3012.51 kcal mol⁻¹) than the CSM (−2874.08 kcal mol⁻¹) molecule, which indicates that the CSD molecule will adsorb more powerfully on the steel interface and possess excellent protection capacity.

The dE _ads/dN _i is the adsorption energy if one of the adsorbates was eliminated (Zhang et al., 2020). The dE _ads/dN _i values for hydronium ions, water molecules, and Cl⁻ ions are smaller than the values of the CSM and CSD molecules, suggesting that the strong adsorption of the CSM and CSD is higher than that of water molecules, hydronium ions, and Cl⁻ ions. So, this proves the exchange of the other species by the CSM and CSD molecules. Therefore, the CSM and CSD molecules are resolutely adsorbed on the steel surface and produce a robust adsorbed defensive film resulting in the protection for the C-steel substrate in the studied aggressive medium, which is confirmed by experiential and DFT investigations together.