Isoniazid-derived ionic liquids for enhanced C-steels surface finishing via electrochemistry, supported by Monte Carlo, mechanistic study, theoretical, and docking methodology

Abstract

Ecological concerns, stringent environmental laws, and the ongoing need for innovation motivated this investigation into improved metal-finishing techniques. Proper polishing methods can improve qualities such as corrosion and wear resistance by drastically reducing surface roughness and the coefficient of friction. However, traditional polishing methods have certain limitations that cause them to consume the metal. In this study, we introduced a newly aromatic pyridinium salt based isoniazid hybrid compounds (APyHC) as eco-friendly synthesized ionic liquids (ILs) to diminish the electrochemical dissolution rates (Electropolishing EP) of carbon steels (CSs) improving surface protection and demonstrate inhibition efficiencies (%IE) mostly 80% at relatively low concentration 5.93 × 10^− 5 M in 8M H₃PO₄, offering a sustainable alternative to harmful inhibitors. To comprehensively evaluate the inhibitor’s performance, we employed Galvanostatic mass-transfer analysis over a temperature range of 293–308 K. This test revealed a significant improvement in surface finish. The kinetic and thermodynamic coefficient values were calculated. The results show that increasing APyHC concentration enhanced inhibition efficiency, whereas rising temperature reduced it, confirming a temperature-sensitive process. Adsorption followed a mixed physicochemical mechanism obeying the El-Awady model (R² = 0.99 at 298 K). Thermodynamic parameters confirmed endothermic, spontaneous adsorption, with E_a below 80 kJ/mol, positive ΔH°, and ΔG° ranging from − 37.40 to -39.77 kJ/mol. The surface characteristics of CSs, as measured by composition (EDX, XPS), morphology (SEM), and topography (AFM), confirmed the presence of surface shielding. In line with experimental techniques, theoretical insights, such as DFT calculations and quantum-based Monte Carlo simulations (MC), were integrated with experiments to clarify reactivity, adsorption mechanisms at the atomistic scale, deepening understanding of inhibition and supporting a sustainable strategy for designing green ionic-liquid-based protection systems. Additionally, docking simulations examined APyHC’s biological affinity for the sulfate-reducing bacteria (SRB) protein, evaluating its potential as a dual-purpose inhibitor of CSs dissolution in harsh media.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-026-55745-w.

Introduction

The metal finishing industry is adopting cleaner methods due to stricter regulations, recognizing that surface quality dictates metal performance and lifespan. These preventive actions have a positive impact on both the environment and these companies’ ability to compete. This is because fewer raw materials are used, less energy is consumed, and contaminated materials and outflows are reduced, thereby reducing the risks of environmental contamination¹. Using a variety of cleaner production techniques is challenging, though. Furthermore, this might have a detrimental impact, particularly when the whole picture is considered, underscoring the need for efficient decision aids in this industry².

Industrial demand has driven efforts to evaluate surface roughness due to its impact on corrosion resistance. Polishing serves a dual purpose: as a final treatment to reduce roughness and improve durability, and as a pretreatment to enhance coating adhesion, resulting in stronger interfacial bonding and improved product longevity^3,4.

Therefore, Electropolishing (EP) is an electrochemical finishing process that smooths metal surfaces by selectively dissolving microscopic peaks. Still, it requires a controlled current density to prevent oxygen bubble formation, which can cause pitting and reduce corrosion resistance⁵. This is a major problem for carbon steel (used in construction, automotive, and pipelines), as the acidic EP process makes it susceptible to damaging pitting rather than protecting it^6,7.

Specialized corrosion inhibitors are crucial in electro-polishing to prevent pitting and gas evolution⁸. Highly effective organic inhibitors work by adsorbing onto the metal surface using heteroatoms (N, S, O), polar groups (-NH₂, -COOH, -OH), and π-electrons as active adsorption sites^9,10. This creates a stable insulating film that blocks corrosion sites, significantly lowering corrosion rates even at low concentrations and protecting the metal^11,12.

For developing effective organic inhibitors, cyclic imide derivatives have versatile applications, including use as metal inhibitors, resin solvents, and antimicrobial agents¹³. Isonicotinohydrazide derivatives have been the subject of considerable research, which consistently confirms their potent antibacterial efficacy. Notwithstanding their established value in biological contexts, their potential utility as corrosion inhibitors remains largely unexplored^14–17. The corrosion inhibition potential of pyridine-based compounds has been the focus of considerable research. In chemical synthesis, pyridine serves as an organic base, forming pyridinium salts that participate in various acid-base reactions. These salts are commercially valuable and find use in sectors such as pharmaceuticals, cosmetics, and industrial catalysis¹⁸. In their liquid state at ambient temperature, they are classified as pyridinium ionic liquids. Their biological applications include drug and gene delivery, owing to their properties, such as their interactions with biomembranes and their DNA-binding capabilities. A key mechanism of their effectiveness as corrosion inhibitors is anticipated to be intermolecular synergism, in which the large cationic and anionic groups enhance adsorption onto metallic substrates.

Therefore, in line with what was mentioned last, for metal electropolishing, ionic liquids (ILs) offer a sustainable and less hazardous alternative to conventional mechanical surface treatments¹⁹. Critically, these same properties make ionic liquids environmentally benign, with high thermal and chemical stability, and also drive their exceptional corrosion-inhibition efficacy, ensuring resilience in aggressive environments^20,21. Concurrently, non-volatility ensures persistent surface coverage without evaporative loss, a key requirement for long-term protection. While high polarity and tunable design enable durable protective film formation that collectively blocks corrosive agents, minimizing environmental impact and aligning with green chemistry principles^22,23.

Molecular docking is a key computational method for designing antibacterial compounds and anodic dissolution inhibitors. It simulates binding interactions with biological targets to optimize antibacterial activity and assess effects on corrosion (either inhibition or unintentional promotion), aiding development of novel anti-corrosion strategies. Additionally, Monte Carlo (MC) simulations predict metal-inhibitor interactions and adsorption energy at the adsorbate-substrate interface^24,25.

Given the aforementioned reasoning, the current study seeks to improve the CSs’ surface properties to achieve a smooth, shiny microstructure and enhanced corrosion resistance, thereby reducing metal dissolution and surface degradation. So, we introduced this by highlighting the importance of imide, Isonicotinohydrazide, and pyridinium salts in developing two novel hybrid compounds, eco-friendly APyHC, as high-performance inhibitors for the electropolishing of CSs in aggressive 8 M phosphoric acid. Based on the unique physical properties of ILs and on understanding the EP mechanism, our primary objective was to rigorously quantify their inhibition efficacy and elucidate the associated adsorption thermodynamics across a range of concentrations and temperatures. The resulting surface improvements were analyzed using advanced characterization techniques, including scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX) and atomic force microscopy (AFM), to assess morphology and homogeneity. Furthermore, X-ray photoelectron spectroscopy (XPS) was employed to characterize the elemental composition and confirm the successful formation of a protective inhibitory film on the CSs.

Furthermore, this study seeks to connect the experimentally observed inhibitory effects of APyHC on CSs with computational analyses using Density Functional Theory (DFT). These computations elucidated the electronic structures of the inhibitors, identified key reactive sites, and quantified adsorption energetics, thereby establishing a fundamental structure-performance relationship of Fukui indices, Molecular electrostatic potential, and Electron Localization Function (ELF). We also utilize molecular docking simulations to evaluate the compound’s antibacterial activity against Sulfate-Reducing Bacteria (SRB). By integrating these methods, we establish a comprehensive and predictive strategy for designing new multifunctional pyridinium-based compounds APyHC, which hold considerable promise for both industrial and biomedical applications. The theoretical approach analyzes not only the basic quantum parameters but also integrates spatial visualization of inhibitor molecule adsorption onto the Fe(110) surface via Monte Carlo simulation (MC), linking molecular orientation to adsorption energy. The combination of extreme test conditions and in-depth theoretical analysis makes this study an original contribution to developing the best surface protection and demonstrating greenish inhibitors.

Materials and methods

Preparation of ILs

A mixture of N-(1,3-dioxoisoindolin-2-yl)isonicotinamide (3) (0.01 mol) and the appropriate trifluoromethyl benzyl bromide 4a-b (0.01 mol): 1-(bromomethyl)−2-(trifluoromethyl)benzene (4a) or 1-(bromomethyl)−4-(trifluoromethyl)benzene (4b) in 15 ml Acetonitrile. The reaction mixture was heated under reflux for 12 h, and the excess solvent was removed. The obtained solid was filtered off and crystallized from ethanol to afford the desired products. The results obtained from characterization are reported in the following Table 1.. The material, the characterization equipment, and the resulting Figs. S1-S6 are noted in the Supplementary File.

Table 1.

FT-IR, ¹H NMR, and ¹³C NMR data for the synthesized ionic liquids (APyHC).

Compound
Molecular Formula (IL)	C22H15BrF3N3O3(o-APyHC)	C22H15BrF3N3O3 (p-APyHC)
Color	yellowish-white crystals	off-white crystals
Yield	82%	84%
FT-IR νmax (cm−1, KBr)	3389 (NH), 2891 (CH2) sp3-stretch, 1787, 1744 (C=O, acid anhydride), 1699(C=O, secondary amide) were observed as strong bands (S1).	3404 (NH), 2855 (CH2) sp3-stretch, 1790, 1740 (C=O, acid anhydride), 1705 (C=O, secondary amide) were observed as strong bands (S4).
1H NMR; δ (ppm) (500 MHz, DMSO-d6)	12.35 (s, 1H, NH), 9.35 (d, J = 6.0 Hz, 2H, Ar-H), 8.46 (d, J = 6.0 Hz, 2H, Ar-H), 8.20 (t, J = 4.5 Hz, 2H, Ar-H), 7.96 (dd, J = 4.5 Hz, J = 2.0 Hz, 2H, Ar-H), 7.91 (d, J = 7.5 Hz, 1H, Ar-H), 7.72 (t, J = 7.5 Hz, 1H, Ar-H), 7.67 (t, J = 7.5 Hz, 1H, Ar-H), 7.29 (d, J = 7.5 Hz, 1H, Ar-H), 6.23 (s, 2H, CH2) (S2).	12.25 (s, 1H, NH), 9.44 (d, J = 6.0 Hz, 2H, Ar-H), 8.59 (d, J = 6.0 Hz, 2H, Ar-H), 8.01 (dd, J = 5.0 Hz, J = 3.0 Hz, 2H, Ar-H), 7.96 (dd, J = 5.0 Hz, J = 3.0 Hz, 2H, Ar-H), 7.82 (d, J = 8.5 Hz, 1H, Ar-H), 7.74 (d, J = 8.5 Hz, 1H, Ar-H), 6.06 (s, 2H, CH2) (S5).
13C NMR (125 MHz, DMSO-d6)	165.2, 162.0, 147.7, 145.6, 136.2, 134.2, 131.7, 131.1, 130.5, 129.9, 127.5, 127.3, 124.7(Ar-C), 60.9 (CH2) (S3).	165.2, 162.1, 147.3, 145.3, 136.2, 130.3, 129.9, 127.4, 126.6, 126.5, 126.2, 124.6, 123.4(Ar-C), 63.4 (CH2) (S6).
CHN (%) (Found)	C=52.28, H=3.04,N=8.38	C=52.28, H=3.04, N=8.38
CHN (%) (Calculated)	C=52.19, H=2.99, N=8.30	C=52.19, H=2.99, N=8.30

Sample preparation and test solution

All electrochemical measurements were performed using CSs working electrodes with a 10 cm² exposed surface area. A rigorous surface preparation protocol, applied before each experiment, consisted of sequential mirror-polishing with fine-grit emery paper, degreasing with acetone, rinsing with distilled water, and final air-drying at ambient temperature. Elemental analysis using a JEOL JSM-IT200 apparatus confirmed the material composition (wt%) as: Fe (99.15%), C (0.14%), Mn (0.56%), P (0.03%), S (0.02%), Al (0.03%), Si (0.02%), Cr (0.01%), V (0.01%), and Ni (0.01%)^26,27.

The test solution was an 8 M phosphoric acid (H₃PO₄) solution, formulated by diluting analytical-grade reagent (85% w/w, Fisher Chemicals Ltd.) with distilled water. The inhibition performance of o-APyHC and p-APyHC was assessed in a 100 mL electrochemical cell across a concentration range of (0.494–5.928) × 10⁻⁵ M. These test solutions were prepared from a 1.976 × 10⁻³ M stock solution in distilled water. The limiting current (Iₗ_iₘ) was determined for each inhibitor concentration at four different temperatures: 293, 298, 303, and 308 K. This temperature range was deliberately selected to preserve the stability of the ionic liquids, thereby enabling accurate investigation of corrosion processes without interference from solvent decomposition. Moreover, it covers both ambient conditions (20–25 °C) and moderately elevated temperatures (up to 35–40 °C), making it highly representative of practical industrial environments such as pipelines, chemical processing facilities, heat exchangers, and aerospace systems. All measurements were performed in triplicate, with reported values as averages showing excellent reproducibility and a maximum experimental error of ± 0.2%.

Galvanostatic polarization measurements

Galvanostatic polarization was employed to assess inhibitor efficiency, leveraging its well-documented reliability, accuracy, and experimental simplicity in accordance with standardized literature procedures.

The surface coverage (θ) of the electropolished steel and the inhibition efficiency (% IE) were calculated according to Eqs^8,28:

1

2

Where I_limblank is the limiting current in the absence of inhibitors, and I_limAPyHC is the limiting current in the presence of it.

Topography analysis

Surface analysis was conducted on polished CSs specimens (1.0 cm × 1.0 cm × 0.3 cm). These were exposed to 8 M H₃PO₄ under different conditions of concentration and temperatures, testing both in the attendance and nonattendance of the APyHC. After air-drying and storage in a desiccator, the samples were characterized. This included capturing two-dimensional SEM micrographs with a JEOL JSM-IT200, with a coupled EDX detector for quantitative analysis of film chemical composition. All micrographs of the carbon steel coupons were taken at a magnification of 5000. Surface topography and roughness were further examined via AFM using an Auto Probe CP-Research system (ThermoMicroscope), operating in contact mode with a Bruker MLCT Silicon Nitride probe. Topographic images were captured over a 25 × 25 μm² area with a scan rate of 1 Hz and a resolution of 256 × 256 pixels. Data acquisition was managed with ProScan 1.8 software, while image analysis was performed with IP 2.1 software.

X-ray photoelectron spectroscopy (XPS) was carried out using a Thermo Fisher Scientific K-Alpha spectrometer. The analysis utilized monochromatic Al Kα radiation over a binding energy range of −10 to 1350 eV under an ultra-high vacuum of 10⁻⁹ mbar. Survey and high-resolution spectra were acquired with pass energies of 200 eV and 50 eV, respectively, using a 400 μm analysis spot. All binding energies were referenced to the adventitious C 1 s peak.

Quantum chemical calculations

The molecular structures of the investigated APyHC were constructed in GaussView and subsequently optimized. All calculations were performed using Density Functional Theory (DFT) within the Gaussian 09 software package, employing the B3LYP functional and the 6-311G(d, p) basis set. This computational methodology is well-established in corrosion science for its accuracy in predicting molecular properties¹². The molecular dynamics simulations were performed using the Adsorption Locator module with the COMPASS force field. The Fe (110) surface was selected as the adsorption substrate owing to its high thermodynamic stability. The simulation box dimensions were set to 45 × 60 × 45 ų under periodic boundary conditions to realistically simulate the corrosive acidic environment containing inhibitor molecules, water molecules, and H₃PO₄ species. Simulated annealing calculations were performed with 20 annealing cycles of 2500 steps each, under automated temperature control. Geometry optimization was performed before the simulations to minimize the system’s total energy. The adsorption configurations and adsorption energies were subsequently extracted from the equilibrium trajectories to evaluate the interaction strength between the inhibitor molecules and the Fe surface. Key global reactivity descriptors were calculated using equations in the Supplementary File.

Molecular docking simulation

Molecular docking simulations were performed to investigate the binding mode and inhibitory potential of the synthesized ligands o-APyHC and p-APyHC toward the active site of ribosyltransferase from bacterial origin (PDB ID: 3GEY). Docking studies were carried out using AutoDock 4.0 with the Lamarckian Genetic Algorithm (LGA) under default parameters. All ligands were energy-minimized prior to docking, and the grid box was positioned to fully enclose the enzyme’s catalytic pocket. Several conformers were generated for each ligand and ranked according to the AutoDock scoring function. The most stable conformation was selected based on its lowest binding free energy (ΔG), intermolecular interaction energy, internal torsional energy, and predicted inhibition constant (K_i). Protein–ligand interactions were further analyzed using Discovery Studio, with emphasis on hydrogen bonding, halogen interactions, π–cation contacts, π–π stacking, and hydrophobic interactions, providing a comprehensive evaluation of the stability and biological relevance of the docked complexes.

Results and discussion

Synthesis and structural characterization

Herein, our work focused on the preparation of a new ionic liquid based on the isoniazid moiety. As adopted in Scheme 1, the condensation reaction of isonicotinohydrazide (1) and phthalic anhydride (2) in ethanol as solvent and drops of glacial acetic acid to afford N-(1,3-dioxoisoindolin-2-yl)isonicotinamide (3) in a high yield²⁹.

Scheme 1.

Synthesis of new isoniazid-based ionic liquid derivatives.

Two pyridinium bromide ionic liquid derivatives, 5a and were synthesized via reaction of N-(1,3-dioxoisoindolin-2-yl)isonicotinamide (3) with different trifluoromethyl benzyl bromide, namely 1-(bromomethyl)−2-(trifluoromethyl)benzene (4a) or 1-(bromomethyl)−4-(trifluoromethyl)benzene (4b) in acetonitrile as solvent. The structure of the synthesized compounds 5a and 5c was confirmed using spectroscopic data. Their FT-IR spectrum showed a sharp band of the C = O stretching vibration of the secondary amide at 1699 and 1705 cm^− 1, and their 13 C-NMR spectrum showed a peak of CH₂ at 60.9 and 63.4 ppm for compounds 5a and 5b, respectively. Moreover, for compound 5a, its¹H-NMR spectrum showed two triplet peaks in the aromatic range at 7.67 and 7.72 ppm, confirming the presence of a trifluoromethyl group in the ortho position. In contrast, for compound 5b, it appears as two doublet peaks at 7.82 and 7.74 ppm, confirming the presence of a trifluoromethyl group in the para position (Table 1).

Galvanostatic polarization studies

The Galvanostatic polarization (GP) method was employed to evaluate the influence and kinetics of APyHCon the anodic dissolution of CSs during EP. This approach was chosen due to its simplicity, low cost, and numerous advantages over alternative methods^30,28. Additionally, it offers a practical means to determine solid/liquid mass transfer rates, such as measuring electro-recovery kinetics under diffusion-limited conditions. The technique provided important information on the inhibition mechanism, both in the absence and with different concentrations of the APyHC inhibitor, across temperatures ranging from 293 to 308 K in 8 M H₃PO₄.

As shown in Table 2, increasing the APyHC concentration results in a more negative EP rate and a reduction in the limiting current. This occurs because inhibitor molecules bind to active sites on the CS surface, slowing the EP process. Polarization curves exhibit a plateau both with and without the inhibitor (Fig. 1), suggesting the development of a more effective protective oxide layer on the metal. The shape of the plateau changes only marginally at higher inhibitor concentrations. These findings indicate that the inhibitor restricts anodic dissolution, with inhibition efficiency increasing significantly as concentration increases, consistent with an adsorption-based mechanism in which ionic liquid (IL) molecules bind to the steel surface. Maximum inhibition efficiencies reached 80.99% for p-APyHC and 74.67% for o-APyHC (at 5.93 × 10⁻⁵ M), confirming the effectiveness of these ILs as EP inhibitors in acidic media. The percentage %IE (Table 2) and surface coverage were calculated using Eqs. (1) and (2), respectively. Moreover elevating the temperature from 293 to 308, resulting in a shift in limiting current values followed by a decrease for the corresponding %IE as holding the inhibitor concentration constant as in Table 2.

Table 2.

Depicted the variation in EP rates as I_Lim, and (% IE) with different APyHC dosages in 8M H₃PO₄ at different temperatures.

ILs		C x 105 (M)	293 K		298K		303 K		308 K
			ILim(A)	% IE	ILim(A)	% IE	ILim(A)	% IE	ILim(A)	% IE
p-APyHC		Blank	0.75±0.18	0.00	0.80±0.21	0.00	0.86±0.23	0.00	0.95±0.27	0.00
		0.49	0.35±0.02	53.33	0.42±0.03	47.50	0.52±0.04	39.53	0.62±0.16	34.74
		0.99	0.32±0.01	57.33	0.37±0.02	53.75	0.42±0.03	51.16	0.52±0.12	45.26
		1.98	0.24±0.02	68.00	0.30±0.01	62.50	0.36±0.02	58.14	0.45±0.09	52.63
		3.95	0.19±0.01	74.67	0.28±0.01	65.00	0.33±0.02	61.63	0.41±0.05	56.84
		5.93	0.15±0.01	80.00	0.25±0.01	68.75	0.32±0.02	62.79	0.39±0.03	58.95
o-APyHC		Blank	0.75±0.18	0.00	0.80±0.21	0.00	0.86±0.23	0.00	0.95±0.27	0.00
		0.49	0.40±0.05	46.67	0.44±0.03	45.00	0.55±0.04	36.05	0.68±0.18	28.42
		0.99	0.35±0.02	53.33	0.41±0.03	48.75	0.47±0.04	45.35	0.54±0.15	43.16
		1.98	0.27±0.02	64.00	0.32±0.03	60.00	0.41±0.02	52.33	0.49±0.10	48.42
		3.95	0.23±0.01	69.33	0.30±0.02	62.50	0.37±0.02	56.98	0.42±0.08	55.79
		5.93	0.19±0.01	74.67	0.26±0.01	67.50	0.33±0.01	61.63	0.40±0.05	57.89

Fig. 1.

Schematic of the current/voltage profiles during the galvanostatic polarization experiments for p-APyHC at 293 K.

Temperature impact and thermodynamic activation properties

To evaluate the durability of the adsorbed ILs film on the CSs’surface and to calculate the thermodynamic activation parameters for the EP process in the aggressive medium, Galvanostatic measurements were performed over a temperature range of 293–308 K, both with and without the optimal concentrations of APyHC. Based on Table 2, dissolution rates were significantly enhanced at higher temperatures, with a maximum observed at 308 K in the uninhibited solution. This increase in the EP rate at elevated temperatures is caused by faster molecular interactions, which change how the CSs surfaces interact with the acidic solution. Generally, higher temperatures promote anodic reactions, thereby affecting the kinetics of the EP process^27,31.

At a constant APyHC concentration, the %IE decreased with increasing temperature, as illustrated in Fig. 2. This decline is principally due to the increase in thermal motion, which shifts the adsorption-desorption equilibrium toward desorption, explaining the decrease in APyHC’s inhibitory performance, which reduces the integrity (weakening) of the protective layer. This feature is identified as physisorption, relying on weak electrostatic bonds that heat can easily disrupt³². Even with lower efficiency at elevated temperatures, substantial dissolution inhibition was still achieved. This reinforces the potential of these ILs.

Fig. 2.

Figure1 A bar chart illustrating the relative performance of the p-APyHC at different temperatures.

The inhibition mechanism is further elucidated by changes in activation energy (E_a). An observed decline in inhibition efficiency at elevated temperatures, coupled with a rise in E_a. The activation energies were determined through application of the Arrhenius Eq³³:

3

where A is the pre-exponential factor, R is the universal gas constant (8.314 J mol⁻¹ K⁻¹), and T is the thermodynamic temperature (293–308 K). Linear Arrhenius plots (Fig. 3) facilitated E_a calculations. Table 3 summarizes the activation parameters for the EP process of CSs in acid. The rise in activation energy (E_a) with increasing APyHC concentration indicates that a higher energy barrier must be overcome for the EP mechanism to proceed³⁴. All E_a values are significantly below the 80 kJ mol⁻¹ threshold; the adsorption mechanism is primarily physical in nature³⁵. The enthalpy (ΔH ^≠) and entropy (ΔS ^≠) of activation for the metal dissolution reaction were computed using the transition state Eq³⁶.:

Fig. 3.

Figure1 Arrhenius plots depicting the EP of CSs in 8M H₃PO₄, with and without various concentrations of APyHC.

Table 3.

Kinetic activation parameters CSs EP in test acidic solution.

ILs	C x105 M	Ea (kJ/mol)	∆S≠ (J/mol.K)	∆H≠(kJ/mol)	∆G≠(kJ/mol)
p-APyHC	Blank	11.71	−158.37	9.21	56.80
	0.49	28.94	−105.85	26.45	58.25
	0.99	23.72	−124.49	21.22	58.63
	1.98	31.03	−101.82	28.53	59.13
	3.95	37.17	−82.38	34.67	59.43
	5.93	46.87	−50.98	44.37	59.69
o-APyHC	Blank	11.710	−158.37	9.21	56.80
	0.49	27.17	−111.05	24.67	58.05
	0.99	21.58	−130.92	19.08	58.42
	1.98	30.54	−102.62	28.04	58.88
	3.95	30.32	−104.35	27.82	59.18
	5.93	37.14	−82.67	34.65	59.49

4

where N is Avogadro’s number, h is Planck’s constant, and T is the temperature. The activation enthalpy (ΔH ^≠) and entropy (ΔS ^≠) for APyHC inhibitors were calculated from transition state theory plots of ln(Iₗ_iₘ/T) versus 1000/T (Fig. 4). The values of ΔH ^≠ and ΔS ^≠ were obtained from the slope (−ΔH ^≠/R) and intercept (ln(R/Nh) + (ΔS ^≠/R) of the linear regression, respectively, and are presented in Table 3. The increased positive enthalpy of activation (ΔH^≠) confirms that the inhibitor elevates the energy barrier for CSs dissolution, effectively hindering the rate-determining step of the EP process. Also, the positive values of ΔH^≠ for both uninhibited and inhibited systems confirm the endothermic nature of the CSs dissolution process, with values below 80 kJ/mol^37,38. This is consistent with a physical adsorption mechanism, as values below this threshold typically indicate physisorption, while those around 100 kJ/mol suggest chemisorption. As the concentration of APyHC increases, a marked rise in the activation enthalpy (ΔH^≠) is observed. This behavior indicates that the presence of the ionic liquid raises the energy barrier for anodic dissolution of carbon steel, thereby hindering the rate-determining step of the electropolishing process. The inhibitor molecules adsorb onto active surface sites, forming a physical barrier that limits metal dissolution and requires higher energy for the electrochemical reaction to proceed.

Fig. 4.

Transition plots comparison for CSs in 8M H₃PO₄, tested both with and without various concentrations of APyHC.

The negative values of the activation entropy (ΔS^≠), and consequently of TΔS^≠, indicate a decrease in disorder during formation of the activated complex. This reflects an associative mechanism in which the activated state is more ordered than the reactants, consistent with the formation of an adsorbed inhibitor layer on the steel surface. The reduction in entropy is therefore attributed to the organization of APyHC molecules at the metal–solution interface, resulting in a compact, structured protective film³⁹.

The interrelation between the activation enthalpy (ΔH^≠), entropy (ΔS^≠), and Gibbs free energy (ΔG^≠) provides deep insight into the inhibition mechanism of APyHC during the electropolishing of carbon steel. These parameters are related by the transition-state Eq⁴⁰:

5

The activation Gibbs free energy (ΔG^≠) remains positive and increases slightly with inhibitor concentration, demonstrating that the dissolution process becomes thermodynamically less favorable in the presence of APyHC. This confirms the protective role of the ionic liquid film, which suppresses corrosion by increasing the energetic cost of metal dissolution.

Together, the increase in ΔH^≠, the positive and increasing ΔG^≠, and the negative ΔS^≠ confirm that APyHC inhibits electropolishing primarily through a physisorption-controlled mechanism, in which an ordered ionic-liquid film forms on the carbon steel surface, raising the energy barrier for anodic dissolution and effectively suppressing corrosion⁴¹.

Modeling of adsorption data

Adsorption isotherms detail the inhibitor’s behavior on metal surfaces, influenced by the electrolyte, inhibitor chemistry, and the metal’s properties during electropolishing. Adsorption is favored when the inhibitor-metal interaction energy exceeds that of water-metal. In this study, surface coverage (θ) values obtained from Galvanostatic measurements were quantified as a function of inhibitor concentration (C), revealing competitive or synergistic interactions at the interface and identifying the most suitable adsorption isotherm model⁴². The adsorption mechanism of p-APyHC and o-APyHC on CSs was elucidated by applying several adsorption isotherm models (Langmuir, Temkin, Frumkin, and El-Awady).

The Langmuir isotherm posits a monolayer of inhibitor molecules uniformly adsorbing onto a homogeneous metal surface without intermolecular interactions, with one molecule per site³². Conversely, the Temkin model incorporates lateral interactions between adsorbates under assumptions of uniform binding energy, a linear decline in adsorption heat with coverage, and notable interfacial effects⁴³. The Frumkin isotherm accounts for both adsorption energy and adsorbate-adsorbate interactions, making it applicable to systems with significant intermolecular forces, such as corrosion inhibition and catalysis⁴⁴. The El-Awady kinetic-thermodynamic isotherm can be employed to assess the number of water molecules displaced by one inhibitor molecule (y)on a metal surface. The inhibitor adsorbs onto the surface only if its binding affinity exceeds that of water; otherwise, adsorption does not occur.

Applying these models clarified the nature of APyHC adsorption onto the CSs’surface, providing insight into its effectiveness as a dissolution inhibitor, as summarized in Figs. 5 and 6, and Table 4. Ultimately, the El-Awady model demonstrated the best fit to experimental data, indicating its greater accuracy in characterizing the adsorption mechanism⁴⁵. The adsorptive equilibrium constant K_ads is given by: K_ads = K ^,(1/y). The Parameter 1/y provides critical insight into the adsorption behavior of the inhibitor. Values below unity (1/y < 1) indicate multilayer adsorption on the metal surface, whereas values above unity (1/y > 1) suggest that individual inhibitor molecules interact with multiple active sites⁴³. The adsorption equilibrium constant (K_ads) serves as a quantitative measure of the interaction affinity between inhibitor molecules and the metal surface within the electrolyte (Table 5).

Fig. 5.

The series of adsorption isotherms investigated.

Fig. 6.

El-Awady adsorption isotherm of APyHC on CSs surface in 8 M H₃PO₄ at different studied temperatures.

Table 4.

Thermodynamic parameters derived from APyHC adsorption isotherm data.

IL	Isotherms	R2	Kads.	∆Goads.kJ/mol	Parameters
p-APyHC	Langmiur	0.9992	-	-	Slope	1.3981
o-APyHC		0.9979	-	-		1.4129
p-APyHC	Frumkin	0.9399	6.1073E+06	−48.66	a	−3.65
o-APyHC		0.9047	2.5307E+06	−46.48	a	−3.15
p-APyHC	Temkin	0.9734	5.9545E+07	−54.31	a	−5.89
o-APyHC		0.9623	2.4871E+07	−52.14	a	−5.41
p-APyHC	El Awady	0.9769	1.6292E+05	−39.68	y	0.35
					K'	70.24
o-APyHC		0.9637	1.1162E+05	−38.75	y	0.38
					K'	83.40

Table 5.

El-Awady isotherm fitting results.

T(K)	y	K'	Kads	∆Goads(kJ/mol)	R2	∆Hoads(kJ/mol)	∆Soads(J/mol.K)
IL	p-APyHC
293	0.5137	557.70	221892.30	−39.77	0.9780	−74.59	−119.00
298	0.3543	70.24	162594.26	−39.68	0.9700
303	0.3718	69.29	89285.31	−38.84	0.9240
308	0.3903	69.04	51524.81	−38.07	0.9541
IL	o-APyHC
293	0.488	333.12	147659.81	−38.78	0.9915	−69.38	−104.10
298	0.3806	83.40	111624.85	−38.74	0.9637
303	0.4037	82.58	56018.05	−37.66	0.9850
308	0.4767	155.78	39749.23	−37.40	0.9373

Higher K_ads values indicate thermodynamically favorable adsorption, promoting greater surface coverage and, consequently, improved corrosion inhibition efficiency⁴². The inverse correlation between K_ads and temperature suggests that higher thermal energy disrupts the adsorption equilibrium, reducing inhibition efficiency. This behavior indicates that the adsorption of the investigated inhibitors onto CSs becomes less favorable at higher temperatures. Notably, p-APyHC exhibits greater K_ads values than o-APyHC at identical temperatures, reflecting its stronger adsorption affinity for the CSs surface. Accordingly, the inhibitor’s strong adsorption affinity allows it to form a cohesive protective layer on the surface, which acts as a barrier against corrosive agents and consequently slows the EP process⁴⁵.

The Gibbs free energy of adsorption (ΔG^o_ads), a fundamental thermodynamic parameter, was calculated using the Eq. (6)⁴⁶:

6

where R, T, and 55.5 M are the universal gas constant, thermodynamic temperature (293–308 K), and the molar concentration of water in the solution. The ΔG^o_ads serves as a crucial indicator for distinguishing between different adsorption mechanisms. Physisorption, dominated by electrostatic interactions between charged inhibitor molecules and the metal surface, is typically associated with ΔG^o_ads values less negative than − 20 kJ·mol^− 1. In contrast, chemisorption involving the sharing or transfer of electron pairs (or π-electrons) from organic inhibitors to the metal surface to form coordinate covalent bonds is characterized by ΔG^o_ads values more negative than − 40 kJ·mol^− 147 Values in the intermediate range (−40 to −20 kJ·mol⁻¹) suggest a mixed adsorption mechanism combining both physical and chemical adsorption^48,49. The calculated ∆G^o_ads values for both p-APyHC and o-APyHC at each experimental temperature are summarized in Table 5. The ∆G^o_ads values for p-APyHC and o-APyHC range from − 39.77 to −38.07 kJ·mol⁻¹ and − 38.78 to −37.40 kJ·mol⁻¹, respectively, within the temperature range of 293–308 K, confirming a spontaneous adsorption process governed by a combination of chemisorption and physisorption mechanisms (physicochemical). The highly negative ∆G^o_ads values elucidate the spontaneous adsorption of inhibitor molecules and the stability of the adsorbed layer⁴². The adsorption enthalpy (ΔH^o_ads) was determined using the Van’t Hoff equation (Eq. 7)⁵⁰:

7

A linear correlation was observed when plotting ln K_ads versus 1/T, as illustrated in Fig. 7. The slope of this linear relationship corresponds to − ΔH^o_ads/R, enabling determination of the enthalpy of adsorption. The negative ΔH^o_ads values confirmed the exothermic nature of the inhibitor’s adsorption process on mild steel. These results corroborate the observed decrease in inhibition efficiency with rising temperature. The standard entropy of adsorption (∆S^°_ads) can be determined using Eq. (8)⁵¹:

Fig. 7.

Van Hoff equation for APyHC adsorption in 8M H₃PO₄.

8

As obtained slope from Fig. 8, the negative ΔS^o_ads values reflect a decrease in system disorder during adsorption, suggesting the formation of ordered inhibitor-surface complexes. While dissolution typically increases entropy, this observation can be explained by the replacement of disordered water molecules from the metal interface with more structured ionic liquid adsorbates⁵².

Fig. 8.

ΔG^°_ads as a Function of Temperature for variable concentrations of APyHC.

Integrated experimental and computational surface analysis

SEM/EDX

Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectroscopy (EDX) were used in tandem to provide complementary insights into surface morphology and elemental composition during the EP process³⁴. Figure 9 shows the EP SEM image of the uninhibited CSs exposed to 8 M H₃PO₄, which reveals significant surface deterioration. This severe corrosion is driven by the aggressive action of phosphate ions (PO₄³⁻), which facilitate metal dissolution and the buildup of corrosion products. The absence of an inhibitor leaves the surface vulnerable to accelerated damage and material loss.

Fig. 9.

SEM images and EDX spectra of a) Blank, b) p-APyHC (4.94× 10^–6M) (293 K), c) p-APyHC (5.928× 10^–5 M) (308 K), d) p-APyHC (5.928× 10^–5 M) (293 K), and e) p-APyHC (5.928× 10^–5 M) (293 K).

Conversely, the addition of varying concentrations of APyHC at different temperatures significantly improved the condition of the carbon steel (CS). The SEM image of the sample treated with p-APyHC (Fig. 9) reveals a uniform, planar surface devoid of cracks and pits, indicating superior corrosion mitigation compared to o-APyHC (Fig.9). This enhanced surface integrity, attributable to the formation of a protective film, effectively blocks phosphate-ion infiltration and substantially reduces corrosive interactions with the acidic medium, resulting in smoother surfaces with minimal defects⁵³.

Energy-Dispersive X-ray (EDX) spectroscopy (Fig. 9; Table 6) corroborates the SEM findings. Treatment with APyHC led to a significant restoration of iron content, increasing from 37.32% in the corroded state to 56.15%, 62.50%, 69.85%, and 80.00%. Concurrently, the substantially reduced oxygen and phosphorus content indicates a marked decrease in corrosion products, underscoring the effectiveness of these ILs in preventing oxidation and phosphate adsorption onto the carbon steel⁵⁴. The detection of nitrogen and bromine confirms the adhesion of APyHC to the surface. The observed carbon signals are attributed to the organic components of the adsorbed ionic liquids.

Table 6.

Comparative EDX Analysis of steel surface Composition in 8 M H₃PO₄ with and without APyHC.

Mass %	Fe	C	O	P	N	Br	%IE
a)Blank	37.32±0.02	1.21±0.01	54.81±0.93	15.66±0.11	nd	nd	0.00
B) p-APyHC (4.94 × −10⁻6 M) (293 K)	56.16±0.82	1.49±0.07	27.49±0.42	2.711±0.33	0.21±0.88	0.18±0.14	53.33
c) p-APyHC (5.928 × −10⁻⁵ M) (308 K)	62.50±0.76	2.37±0.11	26.09±0.36	8.60±0.22	0.23±0.08	0.21±0.11	58.95
d) p-APyHC (5.928 × −10⁻⁵ M) (293 K)	88.00±1.21	4.85±0.21	4.32±0.21	2.03±0.25	0.31±0.07	0.29±0.13	80.00
e) o-APyHC (5.928 × −10⁻⁵ M) (293 K)	69.85±0.67	3.49±0.07	18.62±0.26	7.48±0.18	0.29±0.06	0.27±0.11	74.67

Collectively, SEM and EDX provide a comprehensive assessment of structural and compositional changes, confirming the formation of a protective layer and reduced corrosive attack. This combined analysis validates the trends from gravimetric and electrochemical studies, offering a holistic view of the inhibition mechanism at microstructural and chemical levels.

AFM-based roughness analysis under harsh conditions

In situ AFM was used as an essential tool to quantitatively measure micro- and nanoscale surface changes on CSs during EP experiments in 8 M H₃PO₄, with and without specific inhibitors APyHC. The 3D AFM data, which revealed that temperature and inhibitor concentration reduce surface roughness (Fig. 10), provided quantitative support for the qualitative morphological and elemental profiles obtained from SEM/EDX³⁴.

Fig. 10.

Topography evolution of the CSs’ surface during AFM measurements.

Quantitative analysis of surface roughness parameters, including the average roughness (Ra), root-mean-square roughness (Rq), and peak-to-valley depth (Rp-v), served as definitive metrics for evaluating the protective efficacy of the inhibitors⁵⁵. Exposure to the uninhibited acid solution resulted in severe corrosion damage, producing a highly irregular surface with prominent bumps and superficial pits. This severe degradation was quantified by a significantly elevated average roughness of 2.711 ± 0.21 μm, indicative of vigorous anodic dissolution and confirming the solution’s inherent aggressiveness (Fig. 10a).

In stark contrast, the addition of the inhibitors p-APyHC and o-APyHC at their optimal concentrations (5.928 × 10⁻⁵ M, 293 K) yielded a dramatic improvement in surface preservation. P-APyHC proved to be the most effective inhibitor, generating the lowest average roughness value of approximately 0.572 ± 0.02 μm. o-APyHC also produced a substantial protective enhancement. The corresponding AFM topographies (Fig. 10) visually corroborate these findings, showing remarkably smooth, uniform surfaces. The pronounced reduction in roughness parameters, especially the maximum peak height of the CSs’ surface, reaches approximately 2.87 ± 0.11 μm in the blank solution (Fig. 10a). In addition to p-APyHC, the maximum peak height is significantly reduced to 630 nm at 5.928 × 10⁻⁵ M, 293 K (Fig. 10c). This provides direct evidence of a coherent protective film formation. These films act as effective barriers, significantly limiting direct interaction between the metal surface and corrosive species and thereby preventing CSs’ deterioration.

As we concluded from the last data, CSs’ samples after electropolishing in H₃PO₄ become microsmooth while retaining nanoroughness, which can be beneficial for many applications.

XPS surface analysis

XPS analysis was employed to characterize the adsorption of APyHC inhibitors on CSs, with a specific focus on elucidating the chemical composition and binding configuration of the resulting protective film. The technique is uniquely capable of probing the specific bonding interactions between heteroatoms in the inhibitor molecules and the steel substrate. A comparative analysis of the surface before and after IL adsorption was pivotal for uncovering the underlying inhibition mechanism. Critically, the XPS survey spectra in (Figs. 11a and 12a, and 13a) provide definitive evidence of successful adsorption, marked by the emergence of a nitrogen (N) peak exclusively on the treated surface, thereby confirming the incorporation of inhibitor molecules onto the steel substrate.

Fig. 11.

XPS analysis for CSs surface (blank) exposed to 8 M H₃PO₄: (a) XPS survey scan and XPS spectrum of (b) Fe 2p, (c) P 2p, (d) C1s and (e) O1s.

Fig. 12.

(a) XPS survey scan and XPS spectrum of (b) Fe 2p, (c) P2p, (d) C1s, (e) O1s and (f) N1s for o-APyHC inhibitor.

Fig. 13.

(a)XPS survey scan and XPS spectrum of(b)Fe 2p, (c) P 2p, (d)C1s, (e) O1s and(f)N1s for p-APyHC inhibitor.

The initial characterization of the phosphoric acid-treated CSs surface (the “blank,” Fig. 11) established a critical baseline of its chemical state before inhibitor introduction. This analysis confirmed the presence of several iron oxides, identified as Fe₃O₄ or FeO (associated with Fe⁺²) and Fe₂O₃ (associated with Fe⁺³). These species were characterized by two primary Fe 2p peaks at binding energies of 711.48 eV (2p₃/₂) and 724.48 eV (2p₁/₂). Further examination revealed additional chemical features: the P 2p spectrum exhibited two deconvoluted peaks at 133.69 eV and 134.66 eV, consistent with P-O and PO₄⁻³ species, respectively. The C 1s spectrum was resolved into three distinct components at 284.99 eV, 286.64 eV, and 288.81 eV, corresponding to C–H/C–C, C–N, and C = O bonds, respectively. Complementing this, the O 1 s spectrum showed two peaks at 531.64 eV and 533.18 eV, attributed to Fe–O/C = O and O–C bonds, respectively. This comprehensive profiling of the blank surface provided a definitive reference for evaluating subsequent chemical changes.

Following inhibitor adsorption, the emergence of new N 1s peaks served as direct evidence of a chemical interaction between the inhibitor and the substrate. As presented in (Fig. 13), the spectrum for the p-APyHC inhibitor reveals two distinct N 1s peaks at 400.15 eV and 402.33 eV, assigned to C-N/N-N and Fe-N/N⁺ species, respectively. This provides conclusive evidence for the formation of a coordinated protective layer. This interaction is further substantiated by systematic shifts in the binding energies of the core Fe, P, and O peaks post-adsorption, confirming their active participation in forming a stable surface coating. Moreover, this inhibition mechanism is consistent across different inhibitors, as a nearly identical N 1s spectral profile is observed for o-APyHC (Fig. 12), with all pertinent peak shift data detailed in Table 7.

Table 7.

XPS-derived binding energies for APyHC inhibited samples.

Element	Binding energy	Assignment	Ref.	Element	Binding energy	Assignment	Ref.
o-APyHC + Fe				p-APyHC + Fe
C	284.96	C-C/C = C/C-H	56	C	285.03	C-N/C-H	57
	286.54	C-N	56,57		286.72	C-N	56,57
	288.06	C = O	58		288.62	C = O	57
N	400.06	N-C/N-N	59	N	400.15	N-C/N-N	58
	402.33	N-Fe/N⁺	60		402.16	N-Fe/N⁺	60,61
O	531.52	Fe-O/C = O	62	O	531.55	Fe-O/C = O	62
	533.31	Fe2O3	63		532.75	C-O	64
P	133.51	P-O	64	P	133.48	P-O	64
	134.71	PO4− 3	65		134.24	PO4− 3	65
Fe	713.92	Fe2p3/2	66	Fe	712.09	Fe2p3/2	67
	725.09	Fe 2p1/2	66		723.74	Fe 2p1/2	67

These XPS findings provide molecular-level confirmation of the adsorption and inhibition mechanisms inferred from electrochemical and microscopic analyses.

Computational insights into the EP mechanism

Alongside the experimental investigations, density functional theory (DFT) calculations were conducted to gain deeper insights into the molecular-level mechanism of corrosion inhibition. The molecular geometries of the two ionic liquids were optimized using the B3LYP functional with the 6-311G(d, p) basis set implemented in Gaussian 09, and the optimized structures were further visualized using Gauss View 5.0 (Fig. 14). Subsequently, a comprehensive set of quantum chemical descriptors was evaluated, including the frontier orbital energies (E_HOMO and E_LUMO), energy gap (ΔE), electron transfer fraction (ΔN), electronegativity (χ), global hardness (η), softness (σ), chemical potential (µ), ionization potential (I), electron affinity (A), dipole moment (µd), electrophilicity (ω), and nucleophilicity in both gas and aqueous phases, as summarized in Table 8⁶⁸. Since the corrosion process occurs in an aqueous acidic medium, solvent effects must be included to obtain a more realistic representation of the inhibitor–metal interactions under practical conditions. Therefore, aqueous-phase calculations were performed using the polarizable continuum model (PCM), enabling a better evaluation of the electronic behavior of the inhibitor molecules in solution. The obtained quantum descriptors in the aqueous phase exhibited trends comparable to those observed in the gas phase, confirming the consistency and reliability of the theoretical calculations. However, slight variations in the electronic parameters were observed due to solvent-induced polarization effects, which significantly influence charge distribution, molecular polarity, and electron transfer capability.

Fig. 14.

HOMO and LUMO orbitals for APyHC.

Table 8.

The calculated quantum chemical indices in eV for the investigated APyHC at DFT/B3LYP/6-311G(d,p) in the gas and aqueous phases.

State	Gas		Aqueous
Cpds	p-APyHC	o-APyHC	p-APyHC	o-APyHC
EHOMO (eV)	−5.71	−5.87	−5.63	−5.70
ELUMO (eV)	−2.66	−2.67	−1.86	−1.96
∆Egap (eV)	3.06	3.20	3.77	3.74
I	5.71	5.87	5.63	5.70
A	2.66	2.67	1.86	1.96
X	4.19	4.27	3.75	3.83
Pi	−4.19	−4.27	−3.75	−3.83
µd (Debye)	10.04	10.56	15.31	17.00
ƞ (eV)	1.53	1.60	1.88	1.87
σ (eV−1)	0.65	0.62	0.53	0.53
ω (eV)	5.73	5.69	3.73	3.92
∆Eb.d (eV)	−0.38	−0.40	−0.47	−0.47
∆N (e)	0.92	0.85	0.86	0.85
T.E (Hartee)	−4111.39	−4111.38	−4001.45	−4000.94
∆Esteel/inh (eV)	1.30	1.16	1.40	1.34
ω- (eV)	8.01	8.02	5.84	6.07
ω+ (eV)	3.83	3.75	2.09	2.24
∆ω± (eV)	3.70	3.63	1.92	2.07

Frontier molecular orbital (FMO) analysis provided valuable insights into the electronic interactions between the inhibitor molecules and the steel surface. A higher E_HOMO value reflects a stronger electron-donating capacity, enhancing the likelihood of charge transfer to the vacant d-orbitals of Fe atoms. In contrast, a lower E_LUMO value indicates a greater propensity to accept back-donation from filled metal orbitals, thereby stabilizing donor–acceptor interactions⁶⁹. Comparative analysis revealed that p-APyHC exhibits both a significantly elevated E_HOMO and a reduced E_LUMO relative to o-APyHC in both gas and aqueous phases (Table 8), highlighting its superior dual electron donor–acceptor character. This favorable electronic configuration rationalizes the experimentally observed enhancement in p-APyHC’s inhibition performance. Furthermore, the presence of solvent molecules in the aqueous phase increased the polarity of the inhibitors and enhanced their interaction with the metal surface, thereby improving adsorption characteristics.

The energy gap (ΔE = E_LUMO – E_HOMO), a fundamental indicator of chemical reactivity, showed an inverse relationship with inhibitor efficiency: smaller ΔE values promote stronger adsorption and enhanced interaction with the metal surface⁷⁰. In this regard, p-APyHC demonstrated a notably reduced ΔE compared to o-APyHC in both phases, confirming its greater chemical reactivity and adsorption potential. The relatively lower ΔE values obtained in aqueous solution further suggest that the inhibitor molecules become more reactive in the presence of solvent effects, which facilitates adsorption onto the steel surface and improves corrosion protection efficiency. Such findings provide a quantum-level explanation for p-APyHC’s superior protective efficiency.

Additionally, the electron transfer fraction (ΔN) was employed to quantitatively assess the capacity of inhibitor molecules to donate electrons to the metallic surface. Positive ΔN values (ΔN > 0) indicate spontaneous electron transfer from the inhibitor to vacant metal orbitals, whereas negative values suggest the reverse⁷¹. Both inhibitors exhibited positive ΔN values in the gas and aqueous phases, consistent with their adsorption capability. Importantly, p-APyHC displayed a considerably higher ΔN value than o-APyHC, underscoring its enhanced charge transfer ability. This stronger electron donation capacity further corroborates the experimentally observed superior inhibition efficiency of p-APyHC.

The concepts of global hardness (η) and global softness (σ) are fundamental descriptors in conceptual DFT, widely employed to rationalize chemical reactivity trends⁷². According to Pearson’s hard and soft acids and bases (HSAB) principle, compounds with smaller HOMO–LUMO gaps exhibit higher polarizability, which in turn enhances their chemical reactivity while reducing kinetic stability, classifying them as soft species. Such softness promotes stronger interactions with metallic surfaces by facilitating charge redistribution. In the present study (Table 8), p-APyHC exhibited a markedly higher softness (σ = 0.654 eV) compared with O-APyHC (σ = 0.624 eV), alongside a correspondingly lower hardness (η = 1.528 eV vs. 1.601 eV) in the gas phase. This electronic profile indicates that p-APyHC is more chemically reactive and possesses a greater tendency to adsorb onto steel surfaces, thereby reinforcing its superior inhibition performance.

Electronegativity (χ) is another crucial quantum descriptor that reflects a molecule’s tendency to attract electrons. In corrosion inhibition studies, lower electronegativity values are generally favorable, as they imply a stronger ability of the inhibitor to donate electrons to the metal’s vacant d-orbitals, facilitating protective film formation through adsorption^73,74. Computed values demonstrate that p-APyHC exhibits lower electronegativity than o-APyHC in both phases, consistent with its enhanced donor characteristics and improved inhibition efficiency. Moreover, the dipole moment (µd), which reflects molecular polarity, increased in the aqueous phase due to solvent effects. The higher dipole moment observed for p-APyHC indicates a stronger interaction with the metallic interface and enhanced adsorption capability in aqueous medium. These findings collectively demonstrate that including solvent effects improves agreement between theoretical calculations and experimental observations, confirming that p-APyHC exhibits superior corrosion inhibition performance owing to its favorable electronic structure and enhanced adsorption properties in both gas and aqueous phases.

• Electrostatic Potential Distribution and Reactive Centers.

Molecular electrostatic potential (MEP) analysis provides a robust computational framework for visualizing the spatial distribution of electron density and identifying the most chemically reactive regions within inhibitor molecules. In these maps, negative potential domains (red) correspond to nucleophilic sites with high electron-donating ability, while positive potential areas (blue) indicate electrophilic centers capable of accepting electrons. The electron density distribution is represented as a continuous gradient, decreasing from red (maximum density) through orange, yellow, and green to blue (minimum density), thereby providing a quantitative visualization of electronic polarization across the molecular structure⁷⁵.

As illustrated in Fig. 15a and b, both p-APyHC and o-APyHC display distinct electron density localization, with the most pronounced negative potential regions (yellow to red) concentrated around oxygen and nitrogen atoms, highlighting them as the primary donor sites. In contrast, the lower-electron-density areas (green to blue) are associated with specific carbon atoms. To further validate these observations, a Mulliken population analysis was performed, which confirmed the presence of multiple active centers, including O13, O14, N15, N16, O19, and N29, in both molecules, as shown in Fig. 16. The presence of multiple electron-rich centers is particularly advantageous for corrosion inhibition, as atoms bearing higher negative partial charges serve as effective donor sites for electron transfer to the vacant d-orbitals of iron. This donor–acceptor interaction enhances the adsorption of inhibitor molecules onto the steel surface, thereby strengthening the protective film. Collectively, the insights from MEP mapping and Mulliken charge distributions provide compelling molecular-level evidence for the superior adsorption behavior and inhibition efficiency of the studied ILs.

Fig. 15.

MEP map of (a) p-APyHC, (b) o-APyHC, ELF color-filled maps in XY-plane, and isosurfaces of valence electrons overview (c) p-APyHC, (d) o-APyHC.

Fig. 16.

Mulliken charge distribution of the most negative centers in APyHC.

The Electron Localization Function (ELF) analysis, visualized through isosurfaces and a color-filled XY-plane map Fig. 15c and d, distinctly identifies the specific electron localization sites governing donor interactions. This mapping reveals a region of high electron density, distinctly marked in red, precisely localized around the coordinating atoms O13, O14, N15, and N16 in both inhibitors, indicating a specialized reactive center optimized for efficient electron transfer. In contrast, the surrounding molecular expanse is characterized by low electron localization, a property consistently evidenced by the characteristic color transition from green to blue across the planar maps^76,77.

• Site-Specific Reactivity: Fukui Indices and Dual Descriptors.

Fukui function analysis was employed to probe the adsorption behavior of the studied inhibitors on the CSs, thereby offering deeper theoretical insights into the experimentally observed inhibition performance. This approach enables the identification of the most reactive atomic sites by distinguishing regions prone to nucleophilic and electrophilic attacks.

The condensed Fukui indices (f_K⁺ for nucleophilic attack and f_K⁻ for electrophilic attack), together with the local electrophilicity indices (ω_K⁺, ω_K⁻) and local softness parameters (σ_k⁺, σ_k⁻), were calculated and summarized in the Supplementary Tables. To allow systematic comparison across different atomic sites, differential parameters Δfₖ, Δσₖ, and Δωₖ were derived, defined as (f_K ⁺ − f_K^⁻), (σ_k⁺ − σ_k⁻), and (ω_K⁺ − ω_K^⁻), respectively⁴¹. Positive values of these descriptors (Δf_K, Δσ_k, Δω_K > 0) denote regions more susceptible to nucleophilic attack, while negative values (Δf_K, Δσ_k, Δω_K < 0) highlight atomic centers favorable for electrophilic attack.

According to these calculations, nucleophilic reactivity was predominantly localized on atoms C2, C3, O13, O14, and N15 in both p-APyHC and o-APyHC, indicating these atoms as key donor sites for interaction with the steel surface. Conversely, electrophilic susceptibility was observed at C22 and C35 in p-APyHC and at C22 and N29 in o-APyHC. The identification of these distinct reactive centers provides a molecular-level explanation of the adsorption mechanism, confirming that the presence of multiple nucleophilic sites in p-APyHC significantly enhances its capacity to donate electrons to the metal surface, thereby rationalizing its superior inhibition efficiency compared to o-APyHC (Fig. 17). Based on Fukui analysis and frontier molecular orbital results, the adsorption process between the investigated inhibitor and the carbon surface is most accurately described as a donor-acceptor interaction.

Fig. 17.

Local dual descriptors of different atoms with respect to Mulliken charges of APyHC.

Molecular docking insights into antibacterial activity

The molecular docking results (Fig. 18; Table 9) demonstrated that both o-APyHC and p-APyHC exhibited strong and stable binding to the ribosyl transferase active site, highlighting their potential as effective antibacterial candidates⁷⁸. The binding energies were − 10.2 kcal/mol for o-APyHC and − 10.4 kcal/mol for p-APyHC, with corresponding inhibition constants of 36.94 nM and 26.41 nM, confirming the efficiency of both ligands in occupying the catalytic pocket and preventing substrate access⁷⁹. o-APyHC formed multiple stabilizing interactions, including two strong hydrogen bonds with LYS518 (2.77 Å) and LEU471 (2.66 Å), a carbon–hydrogen bond (3.18 Å), and three halogen interactions involving F44, F45, and F46. Additional stabilization arose from π-cation interactions with LYS518:NZ (3.80 Å) and HIS550:NE2 (4.65 Å), together with π-alkyl and π–π interactions involving PHE642, LEU471, LYS518, and LYS525. Similarly, p-APYHC displayed an even stronger affinity (–10.4 kcal/mol), forming a key hydrogen bond with LEU471 (3.20 Å) and five significant halogen interactions, particularly with ASN644 (2.91 Å) and ASP643 (3.36–3.59 Å), which played a crucial role in stabilizing the complex. π–cation interactions with HIS550 (4.80 Å), π–π stacking with PHE642 (3.63 Å), and multiple hydrophobic contacts further enhanced the ligand’s stability within the active site. Since ribosyltransferase is essential for RNA/DNA biosynthesis in bacteria, the strong binding of both ligands disrupts nucleic-acid precursor formation, inhibits bacterial growth, and prevents cell damage. Overall, the dense network of hydrogen bonds, halogen contacts, electrostatic interactions, and hydrophobic packing confirms the capability of both ligands to effectively block the enzyme, with p-APyHC showing slightly superior inhibitory characteristics and representing a promising candidate for further antibacterial development.

Fig. 18.

2D and 3D docking interactions of (o-APyHC) and (p-APyHC) in the active pocket of ribosyl transferase (PDB ID: 3GEY), visualized with Discovery Studio software.

Table 9.

Molecular docking scores for o-APyHC and o-APyHC, binding energies, inhibition constant, and Protein–Ligand interaction of the synthesized molecules in the active site of ribosyl transferase (PDB ID: 3GEY).

ILs	Binding Energy (ΔG) Kcal/mol	Inhibition Constant (Ki) nM	Protein–Ligand interaction and bond distance (Å)
o-APYHC	−10.2	36.94	LYS518―O19 (Hydrogen Bond-2.77 Å) LEU471―F44 (Hydrogen Bond-2.66 Å) RES1―F45 (Carbon H-bond-3.18 Å) LYS518―π-system (π–Cation-3.80 Å) HIS550―π-system (π–Cation-4.65 Å)
p-APYHC	−10.4	26.41	LEU471―π-system (Hydrogen Bond-3.20 Å) ASN644―F44 (Halogen-2.91 Å) ASP643―F44 (Halogen-3.36 Å) ASP643―F46 (Halogen-3.59 Å) HIS550―π-system (π–Cation-3.63 Å) PHE642―C43 (π–allkyl-3.59 Å)

Molecular dynamics simulations (MD)

Molecular dynamics (MD) simulations were further performed to provide a deeper molecular-level understanding of the adsorption behavior of the investigated inhibitors on the carbon steel surface in phosphoric acid medium. The simulation procedure was initiated by cleaving pure iron along the Fe (110) crystallographic plane, which is considered one of the most thermodynamically stable surfaces for adsorption studies. A simulation box was subsequently constructed containing the Fe (110) substrate together with inhibitor molecules, H₂O molecules, and H₃PO₄ molecules to realistically mimic the corrosive environment. MD simulations represent an effective theoretical approach for investigating the adsorption mechanism and interaction strength between corrosion inhibitors and metallic surfaces⁸⁰.

The optimized adsorption configurations of p-APyHC and o-APyHC on the Fe (110) surface are illustrated in Fig. 19. The obtained configurations reveal that both inhibitor molecules adsorb nearly parallel to the metal surface, maximizing the contact area between the inhibitor molecules and the Fe substrate. Such adsorption orientation facilitates strong interaction between the electron-rich adsorption centers, particularly nitrogen and oxygen heteroatoms, as well as π-electrons of the aromatic rings, and the vacant 3 d orbitals of iron atoms. Consequently, a compact and stable protective film forms over the metal surface, effectively blocking the penetration of aggressive, corrosive species such as H₃PO₄ molecules and thereby reducing corrosion⁸¹.

Fig. 19.

Adsorption configurations of p-APyHC and o-APyHCinhibitor molecules on the Fe (110) surface in H₃PO₄ medium, showing top and side views obtained from molecular dynamics (MD) simulations.

The adsorption behavior was quantitatively evaluated through the adsorption energy (E_ads), which was calculated according to Eq. (9):

9

where a more negative adsorption energy indicates stronger and more spontaneous adsorption of the inhibitor molecule onto the metal surface. The calculated adsorption energies were found to be − 846.55 kcal/mol for p-APyHC and − 816.88 kcal/mol for o-APyHC. The significantly more negative Eads value obtained for p-APyHC indicates a stronger interaction and higher adsorption affinity toward the Fe (110) surface than for o-APyHC. This result confirms that p-APyHC forms a more stable adsorbed layer on the steel surface, leading to superior corrosion inhibition efficiency. The MD simulation findings therefore agree well with the experimental measurements and DFT calculations, collectively confirming the enhanced inhibition performance of p-APyHC in a phosphoric acid medium.

Proposed dissolution inhibition mechanism

In an acidic medium (Fig. 20), the CSs’surfaces acquires a positive charge²². This charge electrostatically attracts the anionic part However, the presence of CF₃ in the ortho position could proinhibit the close intermolecular interaction due to the steric hindrance. The resulting coordinate complex analog is due to the interaction between the IL cation moity that could promote the physicochemical adsorption of p-APyHC compound. The physical nature of this interaction is corroborated by the negative values of the fractional number of electrons transferred (ΔN), indicating electron donation by the inhibitor.

Fig. 20.

The proposed reaction mechanism that inhibits EP of CSs using APyHC.

Concurrently, the calculated free energy of adsorption (ΔG°_ads) values confirm that chemisorption operates alongside physisorption⁸². This chemical adsorption occurs via dual electron-transfer processes:

1. Electron Donation: Lone pairs from heteroatoms O, N, and π-electrons from aromatic rings are donated into vacant d-orbitals of surface iron atoms⁸³.

2. Back-Donation: Electrons from the 3 d orbitals of iron may be transferred to the vacant antibonding molecular orbitals (π*) of the inhibitor⁴⁵.

The synergy of this donation and back-donation results in a stable, protective film on the metal surface⁵³. Therefore, it is confirmed that the isoniazid-based ionic liquids function via a combined physisorption-chemisorption mechanism, effectively inhibiting both corrosion and wear.

Comparative analysis with pyridinium salt inhibitors

The controlled metal dissolution required for processes like electropolishing relies on the use of acidic media in industry. To achieve the desired surface finish and uniformity, specific inhibitors are needed to modulate the dissolution rate. The pyridinium salt family of organic compounds has proven particularly effective for this role. A comparison of the two aromatic pyridinium salt-based isonized molecules investigated in this work with other reported derivatives is provided in Table 10. These compounds, as numerous studies emphasize, demonstrate excellent performance in regulating metal dissolution, as validated by multiple experimental and computational approaches.

Table 10.

Comparative inhibition efficiencies of pyridinium salt inhibitors reported in literature and the present work under electropolishing conditions. .

Inhibitor	Concentration	Metal/Medium	Technique	Inhibition efficiency (%)	Reference
BPH	(50–100) ppm	C-steel/15% HCl	Potentiodynamic polarization	51.8–84.6	18
[C4Py] Br	(0.25–8) mmol L− 1	MS/1 M HCl	Potentiodynamic Polarization	70.1–90.4	84
Py12	(0.5–15) mmol dm− 3	EN3B mild steel/3.5% NaCl solution	Tafel Polarization	68.92–76.92	85
o-APyHC	(4.94*10− 6−5.928*10− 5) M	C-steel/8M H3PO4	Galvanostatic Polarization	46.67–74.67	Current Work
p-APyHC	(4.94*10− 6−5.928*10− 5) M	C-steel/8M H3PO4	Galvanostatic Polarization	53.33–80	Current Work

The environmental compatibility and safety evaluation of a pyridinium-modified isoniazid compound

This research describes the transformation of isoniazid into pyridinium salt derivatives, aiming to improve its therapeutic properties while actively reducing its environmental impact. The approach aligns with sustainable pharmacy by designing these ionic derivatives to enhance biodegradability and reduce ecotoxicity to non-target organisms, potentially decreasing the drug’s persistence and harm to ecosystems. Furthermore, the increased bioavailability of the salt form could lower the required dosage per patient, thereby reducing the quantity of the active ingredient excreted into the environment. The research also encompasses the application of green chemistry principles in the synthesis process and a detailed study of the compound’s behavior in aquatic systems, collectively embodying a proactive, “benign-by-design” strategy to address pharmaceutical pollution without compromising clinical efficacy^86,87,88.

Conclusion and future outlook

• This study successfully developed and evaluated two novel aromatic pyridinium salt-based isoniazid hybrid compounds (APyHC) as eco-friendly ionic liquid inhibitors for the electropolishing of carbon steel in aggressive 8 M H₃PO₄. The synthesized APyHC inhibitors demonstrated high inhibition efficiencies, reaching 80.99% for p-APyHC and 74.67% for o-APyHC at a low concentration (5.93 × 10⁻⁵ M), showcasing their potential as sustainable alternatives to conventional toxic inhibitors.

• A combined experimental and computational approach was employed to elucidate the inhibition mechanism. Galvanostatic polarization studies revealed that APyHC adsorption follows a mixed physisorption-chemisorption mechanism, obeying the El-Awady isotherm model. Thermodynamic parameters confirmed spontaneous and endothermic adsorption, with ΔG°_ads values ranging from − 37.40 to −39.77 kJ/mol. Surface characterization via SEM-EDX, AFM, and XPS confirmed the formation of a protective film that significantly reduced surface roughness and corrosion product formation.

• DFT calculations provided atomistic insights into the reactivity and adsorption behavior of the inhibitors, highlighting the superior electron-donating ability and reactivity of p-APyHCcompared to o-APyHC.

• The molecular docking results demonstrate that both o-APyHC and pAPyHC exhibit strong and stable binding affinities for ribosyltransferase, indicating a promising inhibitory mechanism against bacterial nucleic acid biosynthesis. The high binding energies and nanomolar inhibition constants (Ki) underscore their potential as effective antibacterial lead compounds.

• Future investigations should focus on in vitro antibacterial activity and enzyme inhibition assays to validate the computational predictions experimentally. Additionally, further structural optimization and comprehensive biological evaluation are recommended to enhance inhibitory efficiency and assess their feasibility for practical antimicrobial application.

• Overall, this work presents a green, efficient, and multifunctional ionic liquid-based inhibitor system that aligns with sustainable industrial practices and offers promising applications in both material protection and biological inhibition.

Supplementary Information

Below is the link to the electronic supplementary material.

Author contributions

Amira Hossam, Ali Atya Harfoush, and Hanaa Hammam finished the study design. Mohamed Hagar and Menna Mamdouh prepared the organic compounds. Amira Hossam and Menna Mamdouh completed experimental studies. Amira Hossam, Hanaa Hammam, and Menna Mamdouh finished the data analysis. Hanaa Hammam, Mohamed Hagar, and Amira Hossam finished the manuscript editing. All authors read and approved the final manuscript.

Funding

Open access funding provided by The Science, Technology & Innovation Funding Authority (STDF) in cooperation with The Egyptian Knowledge Bank (EKB).

Data availability

The data used and analyzed during the current study are available from the corresponding authors upon reasonable request.

Declarations

Competing interests

The authors declare no competing interests.