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

Silk fibroin, the structural protein extracted from the cocoons of the silkworm Bombyx mori, has long been prized in biomaterials science for its biocompatibility, controllable biodegradability, and remarkably low immunogenicity. Surgeons and tissue engineers have used it in sutures, films, sponges, and hydrogels, and a growing body of literature documents its usefulness in wound dressings and bone regeneration scaffolds. Yet silk fibroin hydrogels on their own suffer from two persistent weaknesses: they lack intrinsic antibacterial activity, leaving implanted or dressings-based scaffolds vulnerable to infection, and their mechanical strength can be insufficient for demanding structural applications. The Kashmir group set out to address both shortcomings simultaneously without compromising the protein’s natural compatibility with human tissue.

The first stage of the fabrication process focused on the nanofibers themselves. Using sol-gel electrospinning, the researchers produced titanium dioxide nanofibers with silver nanoparticles embedded within them. Electrospinning draws a precursor solution through an electrified needle, whipping it into fibers with diameters in the nanometer range as the solvent evaporates; a subsequent sol-gel conversion and calcination step transforms the polymer-ceramic composite into pure titanium dioxide fibers. Silver was incorporated into the precursor so that the antimicrobial metal became an integral part of the fiber structure rather than a surface decoration that might wash away. This combination matters because titanium dioxide contributes its own antibacterial and photocatalytic behavior, while silver nanoparticles release silver ions that disrupt bacterial membranes, proteins, and DNA—a dual mechanism that has been extensively documented against both Gram-positive and Gram-negative organisms.

Turning liquid silk fibroin into a solid hydrogel quickly and controllably is a challenge in its own right, and here the team exploited a surfactant-based trick. Sodium dodecyl sulfate, or SDS, was added to the fibroin solution to trigger the transition from soluble random coils to the beta-sheet-rich network that gives silk its gel structure. The researchers found a strikingly concentration-dependent effect: at 0.1 molar SDS, gelation was fastest, completing in just twenty to thirty minutes. Above that threshold, however, the trend reversed. At concentrations between 0.12 and 0.5 molar, gelation times increased progressively, which the authors attribute to micellar repulsion—above a critical concentration, SDS molecules assemble into micelles whose charged surfaces interfere with the protein aggregation needed for gel formation. The finding gives future scaffold designers a precise dial for tuning how quickly a silk hydrogel sets, which is critical when the material must conform to a wound bed or be cast into a mold before solidifying.

With the hydrogel formed, the silver-loaded titanium dioxide nanofibers were deposited onto its surface using ultrasonication. The high-frequency vibrations drive the fibers into intimate contact with the soft, porous gel surface, anchoring them without harsh chemical adhesives that could compromise biocompatibility. Scanning electron microscopy revealed that the hydrogel retained its desired porous morphology both before and after the coating step, an essential feature because interconnected pores allow nutrient diffusion, waste removal, and cell infiltration in tissue engineering applications. The porosity of a scaffold is one of the most important determinants of how well cells populate it, and the team’s microscopy confirmed that the decoration process did not clog or collapse this architecture.

Structural confirmation came from X-ray diffraction and Fourier-transform infrared spectroscopy. The XRD patterns verified the crystalline phases of both the titanium dioxide and the embedded silver, while FT-IR confirmed the characteristic beta-sheet signatures of the silk fibroin matrix and showed no adverse chemical interactions between the components. Mechanical testing added a welcome bonus: incorporating the Ag-TiO2 nanofibers significantly improved the compressive strength of the hydrogel, indicating enhanced structural stability. For a material intended to line or fill a wound, resisting deformation under load can mean the difference between a dressing that protects tissue and one that disintegrates during handling or movement.

Durability of the coating was assessed by immersing the decorated hydrogels in phosphate-buffered saline, a standard physiological mimic. Post-immersion electron microscopy showed that the ultrasonically deposited nanofibers remained firmly attached to the hydrogel surface, demonstrating that the coating can withstand the aqueous, ion-rich conditions it would encounter in the body. This adhesion is not a trivial detail; many nanoparticle-laden biomaterials lose their active layers through leaching, which both diminishes antibacterial protection over time and raises concerns about where the released particles ultimately travel.

The antibacterial performance of the finished scaffolds was quantified using zone-of-inhibition assays, in which the material is placed on a lawn of bacteria and the surrounding cleared area is measured. The nanofiber-coated hydrogels produced killing zones of 15.491 plus or minus 0.46 millimeters against E. coli and 12.706 plus or minus 0.47 millimeters against S. aureus. Both organisms are clinically significant: E. coli is a common Gram-negative cause of wound and urinary infections, while S. aureus, including its antibiotic-resistant strains, is a leading culprit in surgical site and chronic wound infections. The somewhat larger zone against the Gram-negative organism suggests effective silver ion diffusion from the fiber surface, and the results position the scaffold as a candidate for infection-prone wound environments where conventional antibiotics struggle against biofilms.

Crucially, antimicrobial potency did not come at the cost of cytotoxicity. The team evaluated biocompatibility using the MTT assay with mouse embryonic fibroblast cells, a colorimetric test in which metabolically active cells convert a yellow tetrazolium compound into a purple formazan product, providing a readout of viability and proliferation. The nanocomposite hydrogels proved biocompatible, and the coating nanofibers showed no counterproductive effect on cell growth. This balance—lethal to bacteria, hospitable to mammalian cells—is the central design goal of infection-resistant biomaterials, and it is one that many silver-based systems fail to strike, since free silver at high concentrations can damage healthy tissue and provoke inflammatory responses.

The authors suggest that the multifunctional silk-based hydrogel holds particular promise for tissue engineering, and the broader context supports that optimism. Chronic wounds afflict millions of patients worldwide, and infected or poorly vascularized wound beds are a major barrier to healing, driving demand for dressings that combine moisture retention, mechanical protection, infection control, and cellular support in one material. By tuning SDS concentration to control gelation speed, embedding silver within electrospun titanium dioxide fibers for durable antimicrobial action, and using ultrasonication to bond those fibers to a porous silk scaffold, the Kashmir team has assembled a modular recipe in which each step can be independently optimized. The work, supported by the University of Kashmir and published as volume 83, article 649 of Polymer Bulletin, adds to a rapidly expanding toolkit of silk fibroin nanocomposites and points toward preclinical testing in wound models as the logical next step for a material that kills bacteria, holds its shape, and lets cells thrive.

Subject of Research: Silk fibroin hydrogels decorated with silver nanoparticle-encapsulating titanium dioxide nanofibers for antimicrobial and cell-supportive tissue engineering applications

Article Title: Silk fibroin hydrogels decorated with titanium dioxide nanofibers encasing silver nanoparticles for antimicrobial action and cell-supportive properties: Using gelation, electrospinning, and ultrasonication

Article References: Hamid, I., Khan, R. S., Kabli, S. A., Rather, A. H., Khanday, F. A., Abdal-Hay, A., & Sheikh, F. A. (2026). Silk fibroin hydrogels decorated with titanium dioxide nanofibers encasing silver nanoparticles for antimicrobial action and cell-supportive properties: Using gelation, electrospinning, and ultrasonication. Polymer Bulletin, 83(12), Article 649. https://doi.org/10.1007/s00289-026-06703-z

Image Credits: AI Generated

DOI: 10.1007/s00289-026-06703-z

Keywords: silk fibroin, hydrogels, titanium dioxide nanofibers, silver nanoparticles, antimicrobial biomaterials, electrospinning, ultrasonication, wound healing, tissue engineering, biocompatibility, sodium dodecyl sulfate, gelation