1. Introduction
Surfactants are amphiphilic compounds that reduce solution interfacial tension, playing a key role in detergents, cosmetics, and pharmaceuticals. They are classified by structure (ionic, non-ionic, amphoteric) and origin (natural, synthetic).
Synthetic surfactants, though widely used, carry environmental risks—components like sulfonates and phosphates lower water dissolved oxygen and cause eutrophication. Some also trigger allergies or endocrine disruption. Natural surfactants, by contrast, are biodegradable and safe, making them ideal alternatives.
Tea saponin, a natural non-ionic surfactant from tea seed meal (a by-product of tea seed oil extraction), has surface activity but suffers from weak detergency, insufficient foam, and low purity, limiting commercial use. Most tea seed meal is currently underutilized, leading to resource waste.
Rhamnolipid, an anionic biosurfactant produced by microorganisms like Pseudomonas, is biodegradable, non-toxic, and extreme-environment resistant. However, its high production cost hinders commercialization.
Combining surfactants often creates synergies, outperforming single surfactants in detergency, surface tension reduction, and pollutant elution. This article focuses on the tea saponin-rhamnolipid combined system, exploring each component’s advantages, synergistic benefits, and application results to support the development of eco-friendly, commercial surfactants.
2. Advantages of Individual Surfactants
2.1 Tea Saponin
Tea saponin offers dual benefits: using tea seed meal for extraction cuts waste and daily chemical production costs, while its biodegradability avoids aquatic pollution from synthetic surfactants.
In terms of surface activity, at 25°C, its critical micelle concentration (cmc) is 1.33 g/L, and minimum surface tension is 40.07 mN/m—reducing water’s surface tension (73 mN/m) by 45.11%. It also has strong stability: cmc remains unchanged at 25–65°C; surface tension and cmc fluctuate minimally under varying salinity (via NH₄Cl), pH (4–9), or water hardness (0–500 mg/L), ensuring reliability across scenarios.
2.2 Rhamnolipid
Rhamnolipid stands out for safety and environmental performance: it is non-irritating to skin, non-endocrine-disrupting (suitable for cosmetics and food-related fields), and fully biodegradable, avoiding environmental accumulation.
Its surface activity is exceptional: at 25°C, cmc is 0.08 g/L, and minimum surface tension is 34.67 mN/m—reducing water’s surface tension by 52.51% (higher than tea saponin). The low cmc enables efficient surface activity at low doses. It also resists extreme conditions (high temperature, salt, pH) to expand application scope. Additionally, its microbial fermentation uses renewable raw materials, aligning with green manufacturing.
3. Benefits of the Combined System
3.1 Enhanced Surface Activity via Synergy
Calculated by cmc, the optimal tea saponin-rhamnolipid mass ratio is 16.63:1. At this ratio, the combined system’s theoretical cmc is 1.26 g/L, but actual cmc is 1.01 g/L (19.84% lower); theoretical minimum surface tension is 39.76 mN/m, actual is 31.33 mN/m (21.20% lower). Lower cmc reduces dosage for micelle formation (cutting costs and environmental impact), while reduced surface tension improves interface modification for better wetting and emulsification.
3.2 Improved Functional Properties
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- Foam Performance: Tea saponin’s maximum 0-min foam height is 124.67 mm (at 1.00 g/L). The combined system reaches 143.33 mm (14.97% higher) and maintains 128.67 mm after 5 min—rhamnolipid enriches at the liquid-air interface to strengthen charge repulsion and stabilize foam.
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- Detergency: Tea saponin’s detergency balances at 29.67% (at 10 g/L). The combined system achieves 33.81% (13.95% higher) by enhancing wetting, breaking down dirt, and emulsifying oil.
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- Wetting Power: Tea saponin’s minimum wetting time is 4.12 min (at 4.0 g/L). The combined system shortens it to 1.26 min (69.42% reduction) via lower surface tension and smaller contact angles.
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- Emulsifying Power: Tea saponin’s emulsifying power peaks at 1.6 g/L. The combined system extends emulsifying time to 23.41 min (14.36% improvement) by forming stable interfacial films.
3.3 Better Stability in Adverse Conditions
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- Temperature Resistance: Pure tea saponin’s surface tension drops sharply when heated to 25–85°C; the combined system’s surface tension changes slightly, suiting high-temperature processes.
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- Salt Resistance: Both resist salt, but the combined system stabilizes at lower NH₄Cl concentrations, fitting high-salt environments like offshore oil recovery.
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- pH Adaptability: Both have minimum surface tension at pH 6.5, but the combined system’s surface tension varies less with pH, ideal for variable-pH scenarios.
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- Hard Water Resistance: With 200–800 mg/L water hardness, both show slight surface tension increases, but the combined system remains stable—usable in hard water without softening.
4. Conclusion
The combined system has broad prospects: in daily chemicals, it can make detergents and cosmetics with good foam, detergency, and mildness; in environmental protection, it remediates oil-contaminated sites without secondary pollution; in agriculture, it enhances pesticide/fertilizer efficacy; in energy, it boosts oil recovery.
Challenges remain: rhamnolipid’s high production cost needs optimization via strain selection and fermentation improvements; synergy mechanisms at the molecular level require further study; compatibility with other ingredients and application optimization in specific scenarios also need exploration.
Overall, the tea saponin-rhamnolipid system is a promising natural, high-performance surfactant, set to contribute to greener industries amid growing environmental awareness.