The molecule, named N1,N4-bis((1H-benzo[d]imidazol-2-yl)methyl)cyclohexane-1,4-dicarboxamide, or BCHDA, was assembled by coupling 2-aminomethylbenzimidazole with cyclohexane-1,4-dicarboxylic acid in pyridine, using triphenyl phosphite as a dehydrating agent. The reaction produced a white solid in 44 percent yield with a melting point of 310 degrees Celsius. Its structure was confirmed through elemental analysis, FT-IR and UV-visible spectroscopy, proton and carbon-13 NMR, high-resolution mass spectrometry and single-crystal X-ray diffraction. The proton NMR revealed a roughly 7:1 mixture of cis and trans isomers around the central cyclohexane ring, a detail the team tracked carefully through the characteristic resonances of the axial and equatorial hydrogens.

What makes BCHDA special is its architecture. Where most previously studied benzimidazole inhibitors carry a single fused aromatic ring, BCHDA carries two, linked through amide bridges to a cyclohexane core. Each benzimidazole unit is rich in electron-donating nitrogen atoms, and the amide groups add further oxygen and nitrogen donor sites. In principle, this gives the molecule multiple anchoring points for bonding to an iron surface, along with an extended conjugated pi-electron system capable of sharing electron density with vacant metal d-orbitals. The researchers set out to test whether this dual-ring design translates into superior protection.

The electrochemical evidence was unambiguous. Using potentiodynamic polarization in 0.5 molar sulfuric acid, the team found that adding BCHDA suppressed both the anodic dissolution of iron and the cathodic evolution of hydrogen gas. The corrosion potential shifted by less than 85 millivolts relative to the blank solution, the accepted signature of a mixed-type inhibitor acting on both halves of the corrosion reaction. At the highest concentration tested, 10 to the minus 3 molar, and at 298 kelvin, the corrosion current density fell so dramatically that the calculated inhibition efficiency reached 99.13 percent. Efficiency rose with concentration but declined with temperature, dropping to 92.80 percent at 328 kelvin at the same dose, a pattern consistent with partial desorption of the inhibitor film as thermal agitation increases.

Electrochemical impedance spectroscopy told a matching story. The Nyquist plots showed depressed capacitive semicircles whose diameters grew steadily with inhibitor concentration, reflecting a rising charge-transfer resistance at the metal-electrolyte interface. The data fitted well to a two-time-constant equivalent circuit, with one relaxation process assigned to the dielectric response of the adsorbed inhibitor film and the other to charge transfer across the double layer. From the impedance measurements the team derived a maximum inhibition efficiency of 94.32 percent at 10 to the minus 3 molar and 298 kelvin. The small discrepancy with the Tafel-derived figure arises because the two techniques probe different aspects of the corrosion process, one under finite overpotential and the other near open circuit, yet both showed the same concentration-dependent trend.

Adsorption analysis showed that the surface coverage data followed the Langmuir isotherm almost perfectly, indicating monolayer adsorption on energetically equivalent sites with negligible interaction between adsorbed molecules. The standard free energy of adsorption came out near minus 40 kilojoules per mole, right at the boundary between physical and chemical adsorption, while the adsorption enthalpy of minus 72.23 kilojoules per mole pointed clearly toward dominant chemisorption. The negative adsorption entropy indicated that the molecules become more ordered as they assemble into a protective layer at the metal-solution interface. Arrhenius analysis added further support, with activation energies for the inhibited systems exceeding those of the blank acid, confirming that the adsorbed film acts as a genuine kinetic barrier.

Direct imaging sealed the case. Scanning electron micrographs of polished steel showed a smooth surface marked only by polishing lines. After 24 hours in uninhibited sulfuric acid the surface was ravaged by cracks, pits and irregular corrosion products. In acid containing BCHDA the damage was dramatically reduced, with the 10 to the minus 3 molar sample retaining the smoothest morphology. Energy-dispersive X-ray analysis quantified the difference: iron content on the surface fell from 98.73 percent on polished steel to 56.35 percent after uninhibited acid exposure, but recovered to 71.52 percent in the presence of the inhibitor, while oxygen from oxide corrosion products dropped from nearly 40 percent to about 13 percent. Crucially, carbon and nitrogen signals appeared on the protected surfaces, direct fingerprints of adsorbed BCHDA molecules.

To understand why the molecule works so well, the team turned to density functional theory at the B3LYP/6-311+G(d,p) level, computing descriptors for both the neutral molecule and its protonated form, which predominates in sulfuric acid. The neutral species showed a HOMO energy of minus 6.09 electron-volts, a small frontier orbital gap of 1.74 electron-volts and a high electrophilicity index, all markers of a molecule well suited to donating and accepting electron density at a metal surface. Protonation widened the gap to 3.06 electron-volts and reduced the electron-donating capacity, yet the protonated form remained strongly surface-active. Natural bond orbital analysis revealed powerful intramolecular charge-transfer interactions, including a sigma-to-lone-pair stabilization of 43.04 kilocalories per mole in the neutral molecule and a pi-to-pi-star delocalization of 45.23 kilocalories per mole in the protonated species, confirming that the conjugated framework stays electronically robust under acidic conditions.

Molecular dynamics simulations on a four-layer iron (110) slab in aqueous sulfuric acid provided the final molecular-scale picture. The neutral BCHDA molecule bound to the surface with an interaction energy of minus 185 kilocalories per mole, lying nearly parallel to the metal to maximize contact between its pi-system and the iron atoms. The protonated form, though slightly weaker at minus 160 kilocalories per mole, still adsorbed spontaneously through a synergistic combination of electrostatic attraction to the sulfate-charged surface and residual donor-acceptor bonding through unprotonated nitrogen sites. Together, the experiments and simulations reveal a dual-mode mechanism: rapid electrostatic coverage by protonated molecules followed by durable chemisorption of neutral ones through nitrogen and oxygen lone pairs and aromatic pi-electrons. The authors argue that this integrated experimental-theoretical strategy offers a blueprint for rationally designing the next generation of corrosion inhibitors, positioning bis-benzimidazole derivatives as promising candidates for protecting steel in some of the harshest acidic environments industry can produce.

Subject of Research: Corrosion inhibition of mild steel in sulfuric acid by a bis-benzimidazole diamide derivative

Article Title: Experimental and computational studies on the corrosion inhibition of Bisbenzimidazole diamide derivative on mild steel in acidic media

Article References: Kumar, A., Jain, R., Kumar, S., Sanyukta, Shabnam, Vashisht, H., Baxi, S., Mahiya, K., Mathias, G. E., Anadebe, V. C., Olasunkanmi, L. O., & Ebenso, E. E. (2026). Experimental and computational studies on the corrosion inhibition of Bisbenzimidazole diamide derivative on mild steel in acidic media. Discover Electrochemistry, 3(1), Article 86. https://doi.org/10.1007/s44373-026-00175-6

Image Credits: AI Generated

DOI: 10.1007/s44373-026-00175-6

Keywords: corrosion inhibition, mild steel, benzimidazole, sulfuric acid, electrochemical impedance spectroscopy, potentiodynamic polarization, density functional theory, molecular dynamics, Langmuir isotherm, chemisorption, SEM-EDX, protective film

In a study published in the journal Aerosol Research, Astrid Nørskov Pedersen, Yosef Knattrup, and Jonas Elm investigated three oxygenated organic molecules, or OOMs, that have all been detected in real atmospheric measurements: 3-methyl-1,2,3-butanecarboxylic acid (MBTCA), carboxyheptanoic acid (CHA), and pinyl diaterpenylic ester (PDPE). Each of these molecules carries three carboxylic acid moieties, a structural motif that previous work had flagged as a promising candidate for nucleation. MBTCA forms from the oxidation of alpha-pinene, the familiar scent of pine forests, and has been observed in chamber experiments and field campaigns in the Sierra Nevada. PDPE is a large accretion product of alpha-pinene ozonolysis, found both in California and in the boreal forests of Hyytiälä, Finland. CHA was identified as a product of d-limonene photooxidation and detected in field studies in Hungary.

The central question the Aarhus team set out to answer is deceptively simple: can these molecules actually help form the initial molecular clusters from which atmospheric particles grow, or do they merely glom onto particles after those particles already exist? Despite decades of study, the exact structure of an organic molecule capable of driving nucleation has never been definitively identified. The sheer diversity of volatile organic compounds emitted into the atmosphere, combined with the labyrinth of possible oxidation pathways, has made this one of the most stubborn puzzles in atmospheric chemistry.

To tackle the problem, the researchers deployed a formidable computational arsenal. They first generated candidate cluster structures using the ABCluster algorithm with the CHARMM force field, then refined them through semi-empirical GFN1-xTB calculations and the CREST sampling code, which uses molecular dynamics and metadynamics to explore how flexible molecules can twist and fold. The most promising structures were then optimized with density functional theory at the ωB97X-D/6-31++G(d,p) level, and the final single-point energies were computed with the gold-standard DLPNO-CCSD(T0)/aug-cc-pVTZ method. This funnel-type workflow, automated with the JKCS toolchain, allowed the team to hunt down the global free energy minimum for clusters containing up to two organic molecules, two sulfuric acid molecules, and two nitrogen bases, with the bases spanning ammonia, methylamine, dimethylamine, and trimethylamine.

The thermochemical results revealed a clear hierarchy. Among the purely organic dimers, PDPE formed the most stable pair, with a binding Gibbs free energy even stronger than that of the well-studied sulfuric acid–dimethylamine dimer, a benchmark system known to nucleate efficiently. The reason, the authors argue, lies in molecular flexibility. PDPE’s long, supple backbone allows all three of its carboxylic acid groups to engage simultaneously in hydrogen bonds with its partner, fully exploiting its bonding potential. MBTCA and CHA, by contrast, have shorter or more branched backbones that restrict how many acid pairs can connect at once. This finding adds nuance to a recent study by Kähärä and colleagues, who had concluded that rigid molecules generally form more stable clusters; here, only PDPE shows a single internal hydrogen bond in its monomer form, and its flexibility turns out to be an asset rather than a liability.

When sulfuric acid and bases were introduced into the clusters, the picture became even more interesting. Adding these inorganic partners lowered the binding free energies by roughly 10 to 25 kilocalories per mole across all three organics. Dimethylamine emerged as the strongest-binding base for nearly all cluster sizes and compositions, consistent with its well-documented prowess in stabilizing sulfuric acid clusters. In the larger mixed clusters, PDPE displayed a remarkable structural strategy: two PDPE molecules wrapped around a core of sulfuric acid and dimethylamine, enclosing the inorganic species in a shell-like arrangement. These clusters approached or exceeded one nanometer in diameter, right at the lower limit of what experimental instruments can detect, making them tantalizing targets for laboratory verification.

Thermodynamics alone, however, does not guarantee that particles will actually form in the turbulent, dilute environment of the real atmosphere. To bridge that gap, the team fed their calculated binding free energies into the Atmospheric Cluster Dynamics Code, or ACDC, which solves the birth–death equations governing how clusters collide, grow, and evaporate. The simulations, run at 278.15 kelvin to mimic springtime boreal forest conditions, used a sulfuric acid concentration of one million molecules per cubic centimeter and varied the base concentrations across realistic lower and upper limits. The organic molecule concentrations ranged from zero to ten parts per trillion, the latter representing a deliberately generous upper bound.

The dynamic simulations delivered the study’s headline result: all three tricarboxylic acids enhanced cluster formation by two to three orders of magnitude in most systems containing one organic molecule, one sulfuric acid, and one base. The largest absolute formation potentials appeared in the sulfuric acid–dimethylamine–organic systems, where the enhancement was strikingly similar across all three organics, rising by two orders of magnitude as the organic concentration increased. The authors interpret this insensitivity to the specific molecule as evidence that the functional groups, rather than the identity of the whole molecule, are what matter most for cluster formation and growth. If that holds up, it could allow atmospheric modelers to lump diverse organic compounds into functional-group categories, dramatically simplifying how organic nucleation is represented in global climate simulations. Flux analysis reinforced the point: for systems with ammonia, methylamine, or dimethylamine, the organic molecule appeared in more than 90 percent of the clusters that grew past the simulation boundary.

The study is candid about its limitations. Because simulating growth all the way to climate-relevant particle sizes is computationally prohibitive, the team counted clusters as outgrowing at a modest size, which means the reported formation potentials should be read as upper limits on true nucleation rates. Sensitivity tests with different cluster boundaries confirmed that the enhancement factors were not artifacts of that choice, though the magnitudes may be somewhat overestimated. The authors also note that extending the calculations to clusters with three or four organic molecules could reveal whether organics can nucleate entirely on their own, without any inorganic help, and that mixed systems of different organic molecules might unlock additional geometric flexibility and expose further carboxyl groups for growth.

What makes this work resonate beyond the computational chemistry community is its implications for how we understand the climate system. Aerosols cool the planet both directly, by scattering sunlight, and indirectly, by brightening clouds, and roughly half of the cloud condensation nuclei in the global atmosphere trace their origin to new particle formation. In rural and forested regions, where organic vapors dominate over sulfuric acid, the tricarboxylic acids studied here may be quietly tipping the balance of when and where new particles are born. By pinpointing molecular flexibility and the tricarboxylic acid motif as key ingredients, the Aarhus team has given field researchers a concrete chemical signature to hunt for, and given climate modelers a principled basis for representing one of the atmosphere’s most elusive processes. The next chapter will belong to the experimentalists, who must now try to catch these nanometer-scale clusters in the act of forming, in laboratories and in the forests where the molecules themselves are made.

Subject of Research: Computational study of how tricarboxylic acids from biogenic volatile organic compound oxidation enhance atmospheric new particle formation with sulfuric acid and nitrogen bases

Article Title: Atmospheric new particle formation enhanced by tricarboxylic acids

Article References: Pedersen, A. N., Knattrup, Y., & Elm, J. (2026). Atmospheric new particle formation enhanced by tricarboxylic acids. Aerosol Research, 4(2), 397-411. https://doi.org/10.5194/ar-4-397-2026

Image Credits: AI Generated

DOI: 10.5194/ar-4-397-2026

Keywords: new particle formation, aerosols, tricarboxylic acids, sulfuric acid, dimethylamine, quantum chemistry, secondary organic aerosol, alpha-pinene oxidation, cluster dynamics, climate, molecular flexibility, atmospheric chemistry

The instruments in question are chemical ionization mass spectrometers, or CIMS, the workhorses of modern atmospheric science. They are the only tools capable of sniffing out the ultra-trace gases that drive new particle formation, chiefly sulfuric acid and highly oxygenated organic molecules known as HOMs. These condensable vapors exist at concentrations ranging from parts per trillion down to parts per quadrillion, they stick tenaciously to any surface they touch, and they must be electrically charged before they can be detected at all. Each of those steps is a potential source of error, and because different research groups have built their instruments differently, results from one study have not always been comparable to another. That fragmentation has long frustrated efforts to build a global picture of how new particles form.

To confront the problem, the European research infrastructure ACTRIS organized its first dedicated CIMS field intercomparison, known as CI-FI1, over two weeks beginning on 26 July 2024 at the SMEAR II station in Hyytiälä, southern Finland. The site, surrounded by managed pine forest with only limited local pollution, is a classic location for studying new particle formation. Five of the six instruments were atmospheric pressure interface time-of-flight mass spectrometers, three fitted with traditional Eisele-type inlets and two with the newer multi-scheme MION inlets capable of rapidly switching between reagent ions. The sixth was a Vocus bipolar time-of-flight instrument coupled to a low-pressure AIM reactor, operated with iodide and benzene cluster cations. All six ran in their routine configurations, and all were calibrated using standard procedures, exactly as they would be during a normal field campaign.

The first week was devoted to side-by-side sampling of ambient air, with every inlet facing the same direction to guarantee that all instruments breathed identical air. The second week focused on calibration: sulfuric acid calibrations using a well-established method, background measurements through HEPA filters, and transmission calibrations that probe how efficiently each instrument detects ions of different masses. The conditions were far from ideal for making easy comparisons. Air temperatures ranged from 13 to 25 degrees Celsius, relative humidity averaged a damp 82 percent, and rain fell on several days, suppressing photochemistry and pushing ambient concentrations of the target vapors close to the instruments’ detection limits. In a sense, that made the exercise more valuable, not less, because it tested the instruments where they are most fragile.

For sulfuric acid measured in nitrate mode, the results were moderately good. The conventional sulfuric acid calibration, which has been the community standard for over a decade, brought the five CI-APi-ToF instruments into reasonable agreement, with a coefficient of determination of about 0.58 across the ensemble. The agreement was strongest during the daytime, when photochemical production pushed sulfuric acid concentrations to their peaks. At night, when concentrations dropped, the instruments diverged, and the scatter plots revealed a fan-like pattern with increasing dispersion at lower concentrations. Intriguingly, the data points clustered not by reagent ion chemistry but by inlet type: the three Eisele-type instruments behaved like one another, and the two MION instruments like each other.

That pattern became even more striking for the heavier molecules. When the team examined monoterpene-derived HOM monomers, spanning mass-to-charge ratios of roughly 240 to 390, and HOM dimers, from 480 to 630, sulfuric acid calibration alone was not enough to make the measurements comparable. The missing ingredient was the mass-dependent transmission of each instrument, the fact that ions of different masses are transmitted through the vacuum interface and detector with unequal efficiency. Correcting for this transmission, measured by depleting the reagent ions with a series of perfluorinated acids, substantially improved agreement for the closed-shell HOM monomers, yielding coefficients of determination of 0.68 for non-nitrate monomers and 0.81 for organonitrate monomers, rising to 0.83 and 0.89 when one problematic instrument was excluded. But the study also delivered a caution: for one instrument whose transmission curve fell to nearly zero at high masses, applying the correction produced absurdly large adjustment factors and actually worsened agreement. The lesson is that transmission corrections must be critically evaluated, not blindly applied.

The dimers told a more stubborn story. Even after all corrections, agreement for these heavy, low-volatility molecules remained poor, with a clear separation between the Eisele-type and MION instruments that persisted across the full range of concentrations. The MION instruments, with their shorter inlet tubes and more efficient delivery of ionized sample to the detector pinhole, showed both lower detection limits and higher relative transmission in the dimer mass range, allowing them to distinguish real signal from background noise more effectively. For the lighter organic and inorganic acids below mass 250, agreement was also weaker than for sulfuric acid, and the team could not pinpoint the cause, noting that applying transmission corrections did not help. These compounds, the authors conclude, remain genuinely challenging to quantify.

One of the most consequential findings concerns bromide ionization, a reagent ion scheme prized for its sensitivity to less-oxidized organic compounds but long suspected of being fragile in humid air. During the campaign, relative humidity ranged from 39 to 100 percent, well above the 20 to 40 percent threshold at which bromide sensitivity is known to decline. Yet the two MION instruments, switching every ten minutes between nitrate, bromide, and a non-ionizing mode, showed good agreement with each other for semi-volatile organic compounds, with a coefficient of determination of 0.853. Comparing bromide and nitrate measurements of the same compounds suggested that humidity did not strongly distort the relative performance of the two modes. Bromide-mode measurements, the results imply, may be more robust under real field conditions than laboratory studies had suggested, at least for compounds that form sufficiently stable clusters with the bromide ion.

Perhaps the most important conclusion of the entire campaign is about what does and does not matter. The team compared how the same compounds were detected across nitrate, bromide, and iodide modes, and found that instruments sharing the same inlet design behaved more alike than instruments using the same reagent ion. The way the three Eisele-type instruments detected a five-oxygen organic compound, for example, was clearly distinguishable from the behavior of the two MION instruments, regardless of which reagent ion was in play. The iodide-mode instrument, with its fundamentally different low-pressure reactor, stood apart from all the rest. Reagent ion chemistry matters, but the physical plumbing of the instrument, its inlet geometry, flows, pressures, and voltages, appears to shape the measurement at least as strongly.

The practical message for the atmospheric science community is twofold. First, harmonized, comparable measurements of condensable vapors across different laboratories and field sites are genuinely achievable, provided that calibration factors, inlet-loss corrections, and mass-dependent transmission corrections are all carefully measured and critically applied. Second, the community needs agreed-upon guidelines, including a consistent definition of the limit of detection, which the study showed can swing conclusions depending on how it is calculated. With new particle formation recognized as a major influence on global aerosol numbers and cloud properties, the ability to trust every instrument’s numbers is not a technical nicety. It is the foundation on which the next generation of climate models, and our understanding of how forests, oceans, and human emissions conspire to seed the sky, will be built.

Subject of Research: Intercomparison of chemical ionization mass spectrometers for measuring condensable vapors involved in atmospheric new particle formation

Article Title: Towards harmonized measurements of condensable vapors: insights from the intercomparison of six chemical ionization mass spectrometers at a boreal forest site

Article References: Righi, C., Ciobanu, M., Deot, N., Jokinen, T., Liu, C., Yuan, Q., Quéléver, L. L. J., Jorga, S., Pospisilova, V., Beck, L. J., Simon, M., Zauner-Wieczorek, M., Kürten, A., Soler, R., Vera, T., Muñoz, A., Ahonen, L. R., Yan, C., Petäjä, T., & Sarnela, N. (2026). Towards harmonized measurements of condensable vapors: insights from the intercomparison of six chemical ionization mass spectrometers at a boreal forest site. Atmospheric Measurement Techniques, 19(19), 6357-6378. https://doi.org/10.5194/amt-19-6357-2026

Image Credits: AI Generated

DOI: 10.5194/amt-19-6357-2026

Keywords: chemical ionization mass spectrometry, condensable vapors, sulfuric acid, highly oxygenated organic molecules, new particle formation, boreal forest, ACTRIS, instrument intercomparison, atmospheric aerosols, SMEAR II, reagent ions, mass spectrometry calibration

The research, published in Discover Green Chemistry by Yog Prakash Yadav, Madhab Gautam, Anju Kumari Das, Sanjay Singh, Chandradip Kumar Yadav, and Ajaya Bhattarai of Tribhuvan University, examines the combined anticorrosive power of saponin and sodium dodecyl sulfate, or SDS, on mild steel immersed in 0.5 molar sulfuric acid. Saponin is a naturally occurring biosurfactant best known as the source of the foamy lather in soap bark, Quillaja saponaria, and soapnut trees. Its molecules carry multiple polar functional groups, including hydroxyl and carbonyl moieties, alongside nonpolar hydrocarbon regions, giving them the amphiphilic character that defines all surfactants. SDS, by contrast, is a synthetic anionic surfactant familiar to anyone who has read the ingredient list of a shampoo bottle. The team reasoned that if the two molecules could cooperate at the steel–acid interface, the resulting protective film might outperform either compound acting alone.

The experimental design was thorough, combining classical gravimetric testing with modern electrochemical and spectroscopic techniques. Mild steel specimens, cut to 3 by 3 centimeter squares containing less than 0.3 percent carbon, were polished with silicon carbide papers of progressively finer grit, from 180 up to 2000, then cleaned in distilled water and sonicated in ethanol for twenty minutes. Pre-weighed samples were immersed in sulfuric acid solutions of varying concentrations, from 0.0125 to 0.5 molar, at temperatures spanning 298.5 to 328 kelvin, for exposure periods ranging from 3 to 96 hours. Some baths contained saponin alone, others contained saponin with SDS, and control baths contained only acid. After immersion, the specimens were reweighed on a precision semi-micro balance, and the weight loss was converted into corrosion rates expressed in millimeters per year.

The gravimetric results told a clear story. Saponin on its own achieved an inhibition efficiency of 64.30 percent at a concentration of 2.92 grams per liter, meaning nearly two-thirds of the corrosion that would have occurred in plain acid was suppressed. When SDS was added to the saponin solution, the efficiency climbed by 13.35 percentage points to 77.66 percent, and the corrosion rate fell to a minimum of 0.37 mils per year in the mixed system. The researchers attributed this improvement to a synergistic effect: the two surfactants interact in solution to form mixed micelles with improved stability, a conclusion supported by conductivity measurements showing that the critical micelle concentration of saponin drops from 0.734 grams per liter to 0.679 grams per liter when SDS is present. A lower critical micelle concentration means the molecules begin assembling into surface-active aggregates at lower doses, which translates into better surface coverage on the metal.

Electrochemical measurements provided even more striking evidence. In potentiodynamic polarization experiments, using mild steel as the working electrode, a saturated calomel electrode as the reference, and graphite as the counter electrode, the team measured corrosion current densities after stabilizing each system at open circuit potential for thirty minutes. Bare steel in sulfuric acid exhibited a corrosion current density of 2.62 times ten to the minus two milliamperes per square centimeter, a signature of severe attack. SDS alone halved the damage, achieving roughly 50 percent inhibition, while saponin alone reduced the current to 8.64 times ten to the minus three milliamperes per square centimeter, corresponding to 67 percent efficiency. But the mixed saponin–SDS system was in a different league entirely: the corrosion current density plummeted to 8.55 times ten to the minus four milliamperes per square centimeter, yielding an inhibition efficiency of 96.70 percent. Crucially, both the anodic reaction, in which iron dissolves into solution, and the cathodic reaction, in which hydrogen gas evolves, were suppressed, identifying the mixture as a mixed-type inhibitor that does not fundamentally alter the corrosion mechanism.

Electrochemical impedance spectroscopy reinforced the picture. Nyquist plots recorded from 100 kilohertz down to 1 hertz showed a very small semicircle for steel in plain acid, indicating low charge transfer resistance and rapid metal dissolution. With inhibitors present, the semicircles grew dramatically, and the largest capacitive loop, the highest charge transfer resistance, and the lowest double-layer capacitance appeared in the saponin–SDS system. Fitting the spectra to a Randles circuit with chi-square values below ten to the minus two confirmed that the mixed formulation delivers inhibition efficiencies of 70 percent or above at the interface. A higher charge transfer resistance means electrons face a greater barrier moving between the metal and the electrolyte, which is exactly what a dense, well-ordered protective film should produce.

Thermodynamic analysis revealed the nature of the protective layer. The experimental data fit the Langmuir adsorption isotherm closely, with correlation coefficients between 0.94 and 0.98 across all temperatures, indicating that the inhibitor molecules arrange themselves as a single monolayer on the steel surface. The standard Gibbs free energy of adsorption remained below minus 20 kilojoules per mole at every temperature tested, a threshold that points to physical adsorption driven by electrostatic attraction and van der Waals forces rather than the formation of new covalent bonds. Notably, both the enthalpy and entropy of adsorption were positive, meaning the process is endothermic and accompanied by increasing disorder, likely as water molecules are displaced from the metal surface. Consistent with this physical mechanism, inhibition efficiency declined as temperature rose, dropping from 93.46 to 80.57 percent for saponin and from 95.55 to 88.33 percent for the saponin–SDS mixture between 298.5 and 328 kelvin, because thermal agitation loosens the grip of adsorbed molecules.

Microscopic and spectroscopic examinations made the protection visible. Optical microscopy revealed extensive pitting and roughening on steel immersed in plain acid, while saponin-treated surfaces were noticeably smoother and the saponin–SDS surfaces were the most intact of all, retaining much of the original polished finish. Field-emission scanning electron microscopy confirmed these observations at higher magnification, showing that the mixed inhibitor preserved the polished surface even in a highly corrosive environment. Energy-dispersive X-ray analysis added elemental proof: carbon and oxygen signals increased on saponin-treated steel, and sulfur appeared when SDS was included, confirming that a carbon-, oxygen-, and sulfur-rich organic film had deposited over the iron. Fourier-transform infrared spectroscopy identified the functional groups involved, including broad hydroxyl stretches near 3400 wavenumbers, carbonyl stretches near 1700, and the characteristic sulfate vibrations of SDS between 1200 and 1300 wavenumbers, with the spectra indicating physical interactions and cooperative adsorption rather than the formation of a new chemical compound.

What makes this work resonate beyond the laboratory is its environmental framing. More than 60 percent of synthetic surfactants used industrially ultimately reach aquatic ecosystems, where petroleum-derived compounds can persist, accumulate in organisms, and harm aquatic life. By anchoring a biodegradable, plant-derived molecule at the heart of the formulation and using the synthetic component only as a synergistic partner, the Nepali team has sketched a template for greener corrosion control: formulations that are effective at low doses, inexpensive, and far less damaging when released. The authors note that efficiency does decline over prolonged immersion as the protective film partially desorbs, and that thermal stability remains a limitation, so engineering challenges remain before industrial deployment. Still, the demonstration that a soap bark extract and a detergent molecule can jointly block more than 96 percent of electrochemical corrosion in sulfuric acid is a vivid reminder that some of the most advanced materials science solutions may be growing quietly in trees, waiting to be tested against humanity’s oldest chemical enemies.

Subject of Research: Synergistic corrosion inhibition of mild steel in sulfuric acid using saponin and sodium dodecyl sulfate

Article Title: Anticorrosive efficacy of saponin and sodium dodecyl sulfate on mild steel specimens immersed in acidic medium

Article References: Yadav, Y. P., Gautam, M., Das, A. K., Singh, S., Yadav, C. K., & Bhattarai, A. (2026). Anticorrosive efficacy of saponin and sodium dodecyl sulfate on mild steel specimens immersed in acidic medium. Discover Green Chemistry, 1(1), Article 10. https://doi.org/10.1007/s44509-026-00008-0

Image Credits: AI Generated

DOI: 10.1007/s44509-026-00008-0

Keywords: corrosion inhibition, mild steel, saponin, sodium dodecyl sulfate, sulfuric acid, surfactants, electrochemical polarization, impedance spectroscopy, Langmuir adsorption isotherm, green chemistry, biosurfactants, physical adsorption

Carbon steel is the workhorse of the petrochemical world. It is inexpensive, strong, and durable, which is why it carries oil and gas through pipelines, lines storage tanks, and forms the backbone of boilers, reactors, and heat exchangers. But wherever steel meets acid, trouble follows. In sulfuric acid, iron atoms at the metal surface lose electrons and dissolve as ferrous ions, while hydrogen ions consume those electrons at cathodic sites to evolve hydrogen gas. The result is gradual thinning, pitting, and eventual failure of equipment, with enormous economic and environmental costs. The standard defense has been organic inhibitor molecules rich in nitrogen, oxygen, or sulfur heteroatoms, which adsorb onto the metal and block the reactive sites. Concerns about the toxicity and environmental persistence of many synthetic inhibitors, however, have pushed corrosion scientists toward greener chemistry.

Chrysanthemum indicum seemed like a promising candidate. Phytochemical analyses of its flowers have revealed volatile compounds such as eucalyptol, alpha-pinene, and germacrene D, along with abundant flavonoids and glycosides including quercitrin, myricetin, and luteolin-7-glucoside. These molecules carry electron-rich oxygen and nitrogen atoms and aromatic pi-systems, exactly the structural features that allow inhibitors to anchor themselves to iron surfaces through donor-acceptor interactions. The Egyptian team collected flowers from the Daqahlia Governorate in June 2023, dried and powdered them, and extracted 200 grams of material in 800 milliliters of methanol for 48 hours before concentrating the crude extract under vacuum.

The researchers then tested the extract at concentrations from 200 to 400 parts per million in 0.5 molar sulfuric acid, using a battery of complementary techniques. Weight-loss measurements on polished steel coupons, performed according to the ASTM G31-72 standard, showed that mass loss fell steadily as extract concentration rose. At the optimum dose of 400 ppm and 25 degrees Celsius, the inhibition efficiency reached approximately 91.53 percent, the highest value recorded in the study. When the temperature was raised to 45 degrees Celsius at the same dose, efficiency dropped to 78.28 percent, a decline that carries important mechanistic information.

That temperature dependence, combined with Arrhenius analysis, told the team that the extract binds to steel largely through physical adsorption. The apparent activation energy of the corrosion process increased with inhibitor concentration, a signature of weak electrostatic interactions between adsorbed molecules and the charged metal surface that weaken as heat disrupts them. Thermodynamic parameters reinforced the picture: the adsorption was exothermic, with negative enthalpy values, while positive entropy changes reflected the displacement of adsorbed water molecules as inhibitor species attached to the surface. The adsorption data fit the Langmuir isotherm almost perfectly, with slopes near one and high correlation coefficients, indicating that the phytochemicals form a monolayer on the steel. Free energies of adsorption fell between the classic thresholds for physisorption and chemisorption, suggesting that both mechanisms contribute, with electrostatic attraction dominating and chemical coordination adding strength.

Electrochemical measurements told a consistent story. Potentiodynamic polarization curves showed that the extract suppresses both the anodic dissolution of iron and the cathodic hydrogen evolution reaction, with corrosion current density falling sharply as concentration increased. Because the corrosion potential shifted by only about 4 millivolts, far below the 85-millivolt threshold, the extract qualifies as a mixed-type inhibitor. The Tafel slopes barely changed with dose, meaning the adsorbed film simply reduces the active surface area rather than altering the fundamental corrosion mechanism. Electrochemical impedance spectroscopy added quantitative depth: charge-transfer resistance jumped from roughly 4.5 ohm-square centimeters in bare acid to about 31.1 ohm-square centimeters at 400 ppm, while double-layer capacitance plunged from about 425 to 108 microfarads per square centimeter, evidence that bulky organic molecules were displacing hydronium ions and thickening the interfacial layer.

Surface imaging provided the most visually striking confirmation. Scanning electron micrographs of steel immersed in uninhibited acid for 24 hours revealed severe roughness and widespread pitting, the classic scars of acidic attack. The surface treated with 400 ppm of the extract, by contrast, appeared smooth and largely free of pits. Energy-dispersive X-ray analysis detected nitrogen and sulfur signals on the protected surface, chemical fingerprints of adsorbed organic constituents, alongside a stronger iron signal indicating suppressed dissolution. Atomic force microscopy quantified the transformation: average roughness fell from 647 nanometers on corroded steel to just 84 nanometers on protected samples, a nearly eightfold smoothing that directly reflects the uniformity of the protective film.

Fourier-transform infrared spectroscopy then identified which functional groups do the anchoring. The free extract shows a broad band at 3237 per centimeter from O-H and N-H stretching; after adsorption this band shifts to 3214 per centimeter, indicating that hydroxyl and amine groups participate in binding through hydrogen bonding and coordination with iron. A carbonyl band at 1635 per centimeter shifted to 1653 per centimeter, consistent with oxygen lone pairs donating electron density into the empty d-orbitals of iron atoms, while C-O and C-N bands near 1032 per centimeter also changed position and intensity. Together, these shifts sketch a mixed-mode mechanism in which electron-rich donor atoms coordinate directly with iron while weaker van der Waals and hydrogen-bonding forces add coverage.

The practical significance is considerable. API 5L X70 steel is a high-strength grade used in long-distance oil and gas transmission pipelines, and acidizing operations, pickling, and industrial cleaning all expose such steels to corrosive acids. An inhibitor derived from an abundant, biodegradable flower, effective at just 400 parts per million, could reduce both the environmental footprint and the health risks associated with conventional formulations. The authors note that this is the first reported use of Chrysanthemum indicum extract for protecting this particular steel grade in sulfuric acid, and they acknowledge that direct comparisons with other plant-based inhibitors remain a task for future work.

The convergence of evidence is what makes the study compelling. Weight loss, polarization, impedance, microscopy, elemental analysis, and spectroscopy all point to the same conclusion: molecules in the chrysanthemum extract spontaneously assemble into an adherent organic film that starves the corrosion reactions of active sites. As industries worldwide face mounting pressure to replace hazardous chemicals with sustainable alternatives, the humble chrysanthemum, already valued for centuries in medicine and ornament, may find a new career guarding the steel arteries of the global energy economy.

Subject of Research: Plant-derived green corrosion inhibitors protecting carbon steel in acidic media

Article Title: Chrysanthemum indicum extract as a sustainable inhibitor for acid-induced corrosion of API 5L X70 steel

Article References: Chrysanthemum indicum extract as a sustainable inhibitor for acid-induced corrosion of API 5L X70 steel. (n.d.). https://doi.org/10.1007/s44373-026-00170-x

Image Credits: AI Generated

DOI: 10.1007/s44373-026-00170-x

Keywords: Chrysanthemum indicum, corrosion inhibition, green inhibitor, API 5L X70 steel, sulfuric acid, Langmuir isotherm, electrochemical impedance spectroscopy, potentiodynamic polarization, adsorption mechanism, plant extract, carbon steel, sustainable chemistry