Effect of electrolytes on the surface behavior of rhamnolipids R1 and R2

Biocompatible Rhamnolipid Self-Assemblies with pH-Responsive Antimicrobial Activity

There is an urgent need for alternative antimicrobial materials due to the growing challenge of bacteria becoming resistant to conventional antibiotics. This study demonstrates the creation of a biocompatible pH-switchable antimicrobial material by combining bacteria-derived rhamnolipids (RL) and food-grade glycerol monooleate (GMO). The integration of RL into dispersed GMO particles, with an inverse-type liquid crystalline cubic structure in the core, leads to colloidally stable supramolecular materials. The composition and pH-triggered structural transformations are studied with small-angle X-ray scattering, cryogenic transmission electron microscopy, and dynamic light scattering. The composition-structure-activity relationship is analyzed and optimized to target bacteria at acidic pH values of acute wounds. The new RL/GMO dispersions reduce Staphylococcus aureus (S. aureus) populations by 7-log after 24 h of treatment with 64 µg mL-1 of RL and prevent biofilm formation at pH = 5.0, but have no activity at pH = 7.0. Additionally, the system is active against methicillin-resistant S. aureus (MRSA) with minimum inhibitory concentration of 128 µg mL-1 at pH 5.0. No activity is found against several Gram-negative bacteria at pH 5.0 and 7.0. The results provide a fundamental understanding of lipid self-assembly and the design of lipid-based biomaterials, which can further guide the development of alternative bio-based solutions to combat bacteria.

Salting out, non-ideality and synergism enhance surfactant efficiency in atmospheric aerosols

In Earth's atmosphere, the surface tension of sub-micron aerosol particles is suspected to affect their efficiency in becoming cloud droplets. But this quantity cannot be measured directly and is inferred from the chemical compounds present in aerosols. Amphiphilic surfactants have been evidenced in aerosols but experimental information on the surface properties of their mixtures with other aerosol components is lacking. This work explores experimentally the surface properties of aqueous mixtures of amphiphilic surfactants (SDS, Brij35, TritonX100, TritonX114, and CTAC) with inorganic salts (NaCl, (NH4)2SO4) and soluble organic acids (oxalic and glutaric acid) using pendant droplet tensiometry. Contrary to what could be expected, inorganic salts and organic acids systematically enhanced the efficiency of the surfactants rather than reduced it, by further lowering the surface tension and, in some cases, the CMC. Furthermore, all the mixtures studied were strongly non-ideal, some even displaying some synergism, thus demonstrating that the common assumption of ideality for aerosol mixtures is not valid. The molecular interactions between the mixture components were either in the bulk (salting out), in the mixed surface monolayer (synergy on the surface tension) or in the micelles (synergy on the CMC) and need to be included when describing such aerosol mixtures.

In Situ Surface Tension Measurement of Deliquesced Aerosol Particles

The surface tension of aerosol particles can potentially affect cloud droplet activation. Hence, direct measurement of the surface tensions of deliquesced aerosol particles is essential but is challenging. Here, we report in situ surface tension measurements based on a novel method that couples a linear quadrupole electrodynamic balance (EDB) with quasi-elastic light scattering (QELS). The EDB-QELS is validated using surface tension measurements of atmospherically relevant inorganic and organic droplets. The surface tension results reasonably agree with the reference values in the range of ∼50-90 mN m^-1. We find a significant size dependence for sodium chloride droplets containing surface-active species (sodium dodecyl sulfate) in the size range of ∼5-18 μm. The surface tension increases from ∼55 to 80 mN m^-1 with decreased size. Relative humidity (RH)-dependent surface tensions of mixed ammonium sulfate (AS) and polyethylene glycol droplets reveal the onset of liquid-liquid phase separation. Droplets containing water-soluble matter extracted from ambient aerosol samples and 2.3-2.9 M AS exhibit a ∼30% reduction in surface tension in the presence of ∼50 mmol-C L^-1 water-soluble organic carbon, compared to pure water (∼72 mN m^-1). The approach can offer size-resolved and RH-dependent surface tension measurements of deliquesced aerosol particles.

Self-Assembly of Rhamnolipid Bioamphiphiles: Understanding the Structure-Property Relationship Using Small-Angle X-ray Scattering

The structure-property relationship of rhamnolipids, RLs, well-known microbial bioamphiphiles (biosurfactants), is explored in detail by coupling cryogenic transmission electron microscopy (cryo-TEM) and both ex situ and in situ small-angle X-ray scattering (SAXS). The self-assembly of three RLs with reasoned variation of their molecular structure (RhaC10, RhaC10C10, and RhaRhaC10C10) and a rhamnose-free C10C10 fatty acid is studied in water as a function of pH. It is found that RhaC10 and RhaRhaC10C10 form micelles in a broad pH range and RhaC10C10 undergoes a micelle-to-vesicle transition from basic to acid pH occurring at pH 6.5. Modeling coupled to fitting SAXS data allows a good estimation of the hydrophobic core radius (or length), the hydrophilic shell thickness, the aggregation number, and the surface area per RL. The essentially micellar morphology found for RhaC10 and RhaRhaC10C10 and the micelle-to-vesicle transition found for RhaC10C10 are reasonably well explained by employing the packing parameter (PP) model, provided a good estimation of the surface area per RL. On the contrary, the PP model fails to explain the lamellar phase found for the protonated RhaRhaC10C10 at acidic pH. The lamellar phase can only be explained by values of the surface area per RL being counterintuitively small for a di-rhamnose group and folding of the C10C10 chain. These structural features are only possible for a change in the conformation of the di-rhamnose group between the alkaline and acidic pH.

Micellar and solubilizing properties of rhamnolipids

We studied the micellar and solubilizing properties of aqueous solutions of unfractionated rhamnolipids produced by Pseudomonas aeruginosa. We used nuclear magnetic resonance (NMR) diffusometry, dynamic light scattering, and conductometry to measure the critical micelle concentration (CMC) of rhamnolipid solutions and determined the effective hydrodynamic radii of rhamnolipid monomers and micelles. Based on selective measurements of the self-diffusion coefficients of molecules, performed by NMR diffusometry, the solubilizing properties of rhamnolipids were studied depending on their concentration in solution; aromatic hydrocarbons, benzene, toluene, ethylbenzene, and para-xylene were taken as solubilizates. On the basis of the measurement results, we estimated the distribution coefficient of the solubilizate between the micellar (solubilized) and free (in the aqueous phase) states and the solubilizing capacity of rhamnolipid micelles.

Rhamnolipid Self-Aggregation in Aqueous Media: A Long Journey toward the Definition of Structure–Property Relationships

The need to protect human and environmental health and avoid the widespread use of substances obtained from nonrenewable sources is steering research toward the discovery and development of new molecules characterized by high biocompatibility and biodegradability. Due to their very widespread use, a class of substances for which this need is particularly urgent is that of surfactants. In this respect, an attractive and promising alternative to commonly used synthetic surfactants is represented by so-called biosurfactants, amphiphiles naturally derived from microorganisms. One of the best-known families of biosurfactants is that of rhamnolipids, which are glycolipids with a headgroup formed by one or two rhamnose units. Great scientific and technological effort has been devoted to optimization of their production processes, as well as their physicochemical characterization. However, a conclusive structure–function relationship is far from being defined. In this review, we aim to move a step forward in this direction, by presenting a comprehensive and unified discussion of physicochemical properties of rhamnolipids as a function of solution conditions and rhamnolipid structure. We also discuss still unresolved issues that deserve further investigation in the future, to allow the replacement of conventional surfactants with rhamnolipids.

Hurdle Technology Approach to Control Listeria monocytogenes Using Rhamnolipid Biosurfactant

This study evaluates the combination of mild heat with a natural surfactant for the inactivation of L. monocytogenes Scott A in low-water-activity (a_w) model systems. Glycerol or NaCl was used to reduce the a_w to 0.92, and different concentrations of rhamnolipid (RL) biosurfactant were added before heat treatment (60 °C, 5 min). Using glycerol, RL treatment (50-250 µg/mL) reduced bacterial population by less than 0.2 log and heat treatment up to 1.5 log, while the combination of both hurdles reached around 5.0 log reduction. In the NaCl medium, RL treatment displayed higher inactivation than in the glycerol medium at the same a_w level and a larger synergistic lethal effect when combined with heat, achieving ≥ 6.0 log reduction at 10-250 µg/mL RL concentrations. The growth inhibition activity of RL was enhanced by the presence of the monovalent salts NaCl and KCl, reducing MIC values from >2500 µg/mL (without salt) to 39 µg/mL (with 7.5% salt). The enhanced antimicrobial activity of RL promoted by the presence of salts was shown to be pH-dependent and more effective under neutral conditions. Overall, results demonstrate that RL can be exploited to design novel strategies based on hurdle approaches aiming to control L. monocytogenes.

Impacts of hesperidin on whey protein functionality: Interacting mechanism, antioxidant capacity, and emulsion stabilizing effects

The objective of this work was to explore the possibility of improving the antioxidant capacity and application of whey protein (WP) through non-covalent interactions with hesperidin (HES), a citrus polyphenol with nutraceutical activity. The interaction mechanism was elucidated using several spectroscopic methods and molecular docking analysis. The antioxidant capacity of the WP-HES complexes was analyzed and compared to that of the proteins alone. Moreover, the resistance of oil-in-water emulsions formulated using the WP-HES complexes as antioxidant emulsifiers to changes in environmental conditions (pH, ion strength, and oxidant) was evaluated. Our results showed that HES was incorporated into a single hydrophobic cavity in the WP molecule, where it was mainly held by hydrophobic attractive forces. As a result, the microenvironments of the non-polar tyrosine and tryptophan residues in the protein molecules were altered after complexation. Moreover, the α-helix and β-sheet regions in the protein decreased after complexation, while the β-turn and random regions increased. The antioxidant capacity of the WP-HES complexes was greater than that of the proteins alone. Non-radiative energy transfer from WP to HES was detected during complex formation. Compared to WP alone, the WP-HES complexes produced emulsions with smaller mean droplet diameters, exhibited higher pH and salt stability, and had better oxidative stability. The magnitude of these effects increased as the HES concentration was increased. This research would supply valuable information on the nature of the interactions between WP and HES. Moreover, it may lead to the creation of dual-function antioxidant emulsifiers for application in emulsified food products.

Limited Role of Rhamnolipids on Cadmium Resistance for an Endogenous-Secretion Bacterium

Rhamnolipids, a type of biosurfactant, represent a potential strategy for both enhancing organismic resistance and in situ remediation of heavy metals contaminations. In-depth study of the mechanism of rhamnolipids synthesis in response to heavy metals stress, is indispensable for a wide use of biosurfactant-secreting microbes in bioremediation. In this study, we employed the wild-type and the rhlAB deficient strain (ΔrhlAB) of Pseudomonas aeruginosa, a prototypal rhamnolipids-producing soil microorganism, to investigate its responses to cadmium resistance based on its physicochemical, and physiological properties. Compared with the wild-type strain, the ΔrhlAB were more sensitive to Cd-stress at low Cd concentration (<50 mg/L), whereas there was little difference in sensitivity at higher Cd concentrations, as shown by spot titers and cell viability assays. Secreted rhamnolipids reduced intracellular Cd²⁺ accumulation to alleviate Cd²⁺ stress, whereas endogenous rhamnolipids played a limited role in alleviating Cd²⁺ stress. Synthesized rhamnolipids exhibited a higher critical micelle concentration (CMC) (674.1 mg/L) and lower emulsification index (4.7%) under high Cd-stress, while these parameters showed no obvious changes. High Cd-stress resulted in high hydrophilic wild-type bacterial surface and lower bioremediation ability. This study could advance a deeper understanding of the mechanism of cadmium resistance and provide a theoretical foundation for the application of biosurfactant and biosurfactant-secreted bacterium in contaminant bioremediation.

Rhamnolipid Micellization and Adsorption Properties

Biosurfactants are naturally occurring amphiphiles that are being actively pursued as alternatives to synthetic surfactants in cleaning, personal care, and cosmetic products. On the basis of their ability to mobilize and disperse hydrocarbons, biosurfactants are also involved in the bioremediation of oil spills. Rhamnolipids are low molecular weight glycolipid biosurfactants that consist of a mono- or di-rhamnose head group and a hydrocarbon fatty acid chain. We examine here the micellization of purified mono-rhamnolipids and di-rhamnolipids in aqueous solutions and their adsorption on model solid surfaces. Rhamnolipid micellization in water is endothermic; the CMC (critical micellization concentration) of di-rhamnolipid is lower than that of mono-rhamnolipid, and both CMCs decrease upon NaCl addition. Rhamnolipid adsorption on gold surface is mostly reversible and the adsorbed layer is rigid. A better understanding of biosurfactant self-assembly and adsorption properties is important for their utilization in consumer products and environmental applications.

Achieving “Non-Foaming” Rhamnolipid Production and Productivity Rebounds of Pseudomonas aeruginosa under Weakly Acidic Fermentation

The rhamnolipid production of Pseudomonas aeruginosa has been impeded by its severe foaming; overcoming the bottleneck of foaming has become the most urgent requirement for rhamnolipid production in recent decades. In this study, we performed rhamnolipid fermentation under weakly acidic conditions to address this bottleneck. The results showed that the foaming behavior of rhamnolipid fermentation broths was pH-dependent with the foaming ability decreasing from 162.8% to 28.6% from pH 8 to 4. The “non-foaming” rhamnolipid fermentation can be realized at pH 5.5, but the biosynthesis of rhamnolipids was significantly inhibited. Further, rhamnolipid yield rebounded from 8.1 g/L to 15.4 g/L after ultraviolet and ethyl methanesulfonate compound mutagenesis. The mechanism study showed that the species changes of rhamnolipid homologs did not affect the foaming behavior of the fermentation but had a slight effect on the bioactivity of rhamnolipids. At pH 8.0 to 5.0, increased surface tension, decreased viscosity and zeta potential, and aggregation of rhamnolipid molecules contributed to the “non-foaming” rhamnolipid fermentation. This study provides a promising avenue for the “non-foaming” rhamnolipid fermentation and elucidates the mechanisms involved, facilitating the understanding of pH-associated foaming behavior and developing a more efficient strategy for achieving rhamnolipid production.

Phase Behaviour, Functionality, and Physicochemical Characteristics of Glycolipid Surfactants of Microbial Origin

Growing demand for biosurfactants as environmentally friendly counterparts of chemically derived surfactants enhances the extensive search for surface-active compounds of biological (microbial) origin. The understanding of the physicochemical properties of biosurfactants such as surface tension reduction, dispersion, emulsifying, foaming or micelle formation is essential for the successful application of biosurfactants in many branches of industry. Glycolipids, which belong to the class of low molecular weight surfactants are currently gaining a lot of interest for industrial applications. For this reason, we focus mainly on this class of biosurfactants with particular emphasis on rhamnolipids and sophorolipids, the most studied of the glycolipids.

Isolation and Characterization of a Biosurfactant Producing Strain Planococcus sp. XW-1 from the Cold Marine Environment

One cold-adapted strain, named Planococcus sp. XW-1, was isolated from the Yellow Sea. The strain can produce biosurfactant with petroleum as sole source of carbon at low temperature (4 °C). The biosurfactant was identified as glycolipid-type biosurfactant species by thin-layer chromatography (TLC) and Fourier transform infrared spectroscopy (FTIR). It reduced the surface tension of water to 26.8 mN/m with a critical micelle concentration measurement of 60 mg/L. The produced biosurfactant possesses high surface activity at wide ranges of temperature (-18-105 °C), pH values (2-12), and salt concentrations (1-18%). The biosurfactant exhibited higher surface activity and higher growth rate of cells with hexadecane and diesel as carbon source. The strain Planococcus sp. XW-1 was also effective in degrading crude oil, after 21 days of growth at 4 °C in medium with 1% crude oil and 1% (v/v) bacteria broth, 54% of crude oil was degraded. The results suggest that Planococcus sp. XW-1 is a promising candidate for use in the bioremediation of petroleum-contaminated seawater in the Yellow Sea during winter. This study reported for the first time that Planococcus isolated from the Yellow Sea can produce biosurfactant using petroleum as the sole carbon source at low temperature (4 °C), showing its ecological role in the remediation of marine petroleum pollution.

Remediation of Smelter Contaminated Soil by Sequential Washing Using Biosurfactants

This paper presents experimental results from the use of biosurfactants in the remediation of a soil from a smelter in Poland. In the soil, concentrations of Cu (1659.1 mg/kg) and Pb (290.8 mg/kg) exceeded the limit values. Triple batch washing was tested as a soil treatment. Three main variants were used, each starting with a different plant-derived (saponin, S; tannic acid, T) or microbial (rhamnolipids, R) biosurfactant solution in the first washing, followed by 9 different sequences using combinations of the tested biosurfactants (27 in total). The efficiency of the washing was determined based on the concentration of metal removed after each washing (C_R), the cumulative removal efficiency (E_cumulative) and metal stability (calculated as the reduced partition index, I_r, based on the metal fractions from BCR sequential extraction). The type of biosurfactant sequence influenced the C_R values. The variants that began with S and R had the highest average E_cumulative for Cu and Pb, respectively. The E_cumulative value correlated very strongly (r > 0.8) with the stability of the residual metals in the soil. The average E_cumulative and stability of Cu were the highest, 87.4% and 0.40, respectively, with the S-S-S, S-S-T, S-S-R and S-R-T sequences. Lead removal and stability were the highest, 64-73% and 0.36-0.41, respectively, with the R-R-R, R-R-S, R-S-R and R-S-S sequences. Although the loss of biosurfactants was below 10% after each washing, sequential washing with biosurfactants enriched the soil with external organic carbon by an average of 27-fold (S-first variant), 24-fold (R first) or 19-fold (T first). With regard to environmental limit values, metal stability and organic carbon resources, sequential washing with different biosurfactants is a beneficial strategy for the remediation of smelter-contaminated soil with given properties.

Rhamnolipids as Effective Green Agents in the Destabilisation of Dolomite Suspension

In this paper, we describe an application of mono- and dirhamnolipid homologue mixtures of a biosurfactant as a green agent for destabilisation of a dolomite suspension. Properties of the biosurfactant solution were characterised using surface tension and aggregate measurements to prove aggregation of rhamnolipids at concentrations much lower than the critical micelle concentration. Based on this information, the adsorption process of biosurfactant molecules on the surface of the carbonate mineral dolomite was investigated, and the adsorption mechanism was proposed. The stability of the dolomite suspension after rhamnolipid adsorption was investigated by turbidimetry. The critical concentration of rhamnolipid at which destabilisation of the suspension occurred most effectively was found to be 50 mg·dm-3. By analysing backscattering profiles, solid-phase migration velocities were calculated. With different amounts of biomolecules, this parameter can be modified from 6.66 to 20.29 mm·h-1. Our study indicates that the dolomite suspension is destabilised by hydrophobic coagulation, which was proved by examining the wetting angle of the mineral surface using the captive bubble technique. The relatively low amount of biosurfactant used to destabilise the system indicates the potential application of this technology for water treatment or modification of the hydrophobicity of mineral surfaces in mineral engineering.

Advanced Simulation of Removing Chromium from a Synthetic Wastewater by Rhamnolipidic Bioflotation Using Hybrid Neural Networks with Metaheuristic Algorithms

This work aims at presenting an advanced simulation approach for a novel rhamnolipidic-based bioflotation process to remove chromium from wastewater. For this purpose, the significance of key influential operating variables including initial solution pH (2, 4, 6, 8, 10 and 12), rhamnolipid to chromium ratio (RL:Cr = 0.010, 0.025, 0.050, 0.075 and 0.100), reductant (Fe) to chromium ratio (Fe:Cr of 0.5, 1.0, 1.5, 2.0, 2.5, 3.0), and air flowrate (50, 100, 150, 200 and 250 mL/min) were investigated and evaluated using Analysis of Variance (ANOVA) method. The RL as both collector and frother was produced using a pure strain of Pseudomonas aeruginosa MA01 under specific conditions. The bioflotation tests were carried out within a bubbly regimed column cell with the dimensions of 60 × 5.70 × 0.1 cm. Four optimization techniques based on Artificial Neural Network (ANN) including Cuckoo, genetic, firefly and biogeography-based optimization algorithms were applied to 113 experiments to identify the optimum values of studied factors. The ANOVA results revealed that all four variables influence the bioflotation performance through a non-linear trend. Their influences, except for aeration rate, were found statistically significant (p-value < 0.05), and all parameters followed the normal distribution according to Anderson-Darlin (AD) criterion. Maximum chromium removal of about 98% was achieved at pH of 6, rhamnolipid to chromium ratio of 0.05, air flowrate of 150 mL/min, and Fe to Cr ratio of 1.0. Flotation kinetics study indicated that chromium bioflotation follows the first-order kinetic model with a rate of 0.023 sec-1. According to the statistical assessment of the model accuracy, the firefly algorithm (FFA) with a structure of 4-9-1 yielded the highest level of reliability with the mean squared, root mean squared, percentage errors and correlation coefficient values of test-data of 0.0038, 0.0617, 3.08% and 96.92%, respectively. These values were evidences of the consistency of the well-structured ANN method to simulate the process.

Isolation and characterization of a novel rhamnolipid producer Pseudomonas sp. LGMS7 from a highly contaminated site in Ain El Arbaa region of Ain Temouchent, Algeria

This study aims to isolate and characterize a novel rhamnolipid producer within the recent bioremediation approaches for treating hydrocarbon-contaminated soils in Algeria. In this context, from a hydrocarbon-contaminated soil, a newly bacterium designated LGMS7 was screened and identified, belonged to the Pseudomonas genus, and was closely related to Pseudomonas mucidolens, with a 16S rRNA sequence similarity of 99.05%. This strain was found to use different hydrocarbons and oils as a sole carbon and energy source for growth. It showed a stable emulsification index E₂₄ (%) of 66.66% ± 3.46 when growing in mineral salts medium (MSM) supplemented with 2% (v/v) glycerol after incubation for 6 days at 30 °C. Interestingly, it was also able to reduce the surface tension of the cell-free supernatant to around 30 ± 0.65 mN m^-1 with a critical micelle concentration (CMC) of 800 mg l^-1. It was found to be able to produce around 1260 ± 0.57 mg l^-1 as the yield of rhamnolipid production. Its biosurfactant has demonstrated excellent stability against pH (pH 2.0-12.0), salinity (0-150 g l^-1), and temperature (-20 to 121 °C). Based on various chromatographic and spectroscopic techniques (i.e., TLC, FTIR, ¹H-NMR), it was found to belong to the glycolipid class (i.e., rhamnolipids). Taken altogether, the strain LGMS7 and its biosurfactant display interesting biotechnological capabilities for the bioremediation of hydrocarbon-contaminated sites. To the best of our knowledge, this is the first study that described the production of biosurfactants by Pseudomonas mucidolens species.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02751-6.

Towards Rational Biosurfactant Design-Predicting Solubilization in Rhamnolipid Solutions

The efficiency of micellar solubilization is dictated inter alia by the properties of the solubilizate, the type of surfactant, and environmental conditions of the process. We, therefore, hypothesized that using the descriptors of the aforementioned features we can predict the solubilization efficiency, expressed as molar solubilization ratio (MSR). In other words, we aimed at creating a model to find the optimal surfactant and environmental conditions in order to solubilize the substance of interest (oil, drug, etc.). We focused specifically on the solubilization in biosurfactant solutions. We collected data from literature covering the last 38 years and supplemented them with our experimental data for different biosurfactant preparations. Evolutionary algorithm (EA) and kernel support vector machines (KSVM) were used to create predictive relationships. The descriptors of biosurfactant (logP_BS, measure of purity), solubilizate (logP_sol, molecular volume), and descriptors of conditions of the measurement (T and pH) were used for modelling. We have shown that the MSR can be successfully predicted using EAs, with a mean R² _val of 0.773 ± 0.052. The parameters influencing the solubilization efficiency were ranked upon their significance. This represents the first attempt in literature to predict the MSR with the MSR calculator delivered as a result of our research.

Congener-dependent conformations of isolated rhamnolipids at the vacuum-water interface: A molecular dynamics simulation

HYPOTHESIS

Molecular dynamics simulation can be used to differentiate between the adsorption properties of rhamnolipid congeners at a vacuum-water interface.

EXPERIMENTS

Adsorption of five congeners with differing alkyl chains (two C10 chains, two C14 chains or mixed C14C10 and C10C14), number of rhamnose rings (mono- or di-) and carboxyl group charge (non-ionic or anionic) are simulated at the vacuum-water interface.

FINDINGS

All rhamnolipids adsorb in the interfacial region with rhamnose and carboxyl groups closer to the water phase, and alkyl chains closer to the vacuum phase, but with differing adsorbed conformations. Headgroups of uncharged congeners show two preferred conformations, closed and partially open. Di-rhamnolipid has a low proportion of closed conformation, due to the steric constraints of the second pyranose ring. Charged congeners show strong preference for closed headgroup conformations. For rhamnolipids with equal alkyl chains lengths (C10C10, C14C14) the distribution of alkyl chain tilt angles is similar for both. Where chain lengths are unequal (C14C10, C10C14) one chain has a greater tendency to tilt towards the water phase (>90°). The order parameter of the alkyl chains shows they are disordered at the interface. Together, these results show congener-dependent adsorbed conformation differences suggesting they will have differing surface-active properties at vacuum-water and oil-water interfaces.

Recent progress and trends in the analysis and identification of rhamnolipids

Rhamnolipids have extensive potential applications and are the most promising biosurfactants for commercialization. The efficient and accurate identification and analysis of these are important to their production, application and commercialization. Accordingly, significant efforts have been made to identify and analyse rhamnolipids during screening of producing strains, fermentation and application processes. Cationic cetyltrimethylammonium bromide-methylene blue (CTAB-MB) test combines a series of indirect assays to efficiently assist in the primary screening of rhamnolipids-producing strains, while the secretion of rhamnolipids by these strains can be identified through TLC, FTIR, NMR, electrospray ionization mass spectrometry (ESI-MS) and HPLC-MS analysis. Rhamnolipids can be quantified by colorimetric methods requiring the use of concentrated acid, and this approach has the advantages of reliability, simplicity, low-cost and excellent reproducibility with very low technological requirements. HPLC-MS can also be employed as required as a more accurate quantification method. In addition, HPLC-ELSD has been established as the internationally acceptable measure of rhamnolipids for commercial purposes. The preparation of well-accepted rhamnolipids standards and modifications of analysis operations are essential to further enhance the accuracy and improve the simplicity of rhamnolipid analysis.Key points• Current status of R&D works on determination of rhamnolipids is listed• Advantages and disadvantages of various types analysis are summarized• Limitations of current rhamnolipid quantification are discussed Graphical abstract.

Structural characterization and stability study of green synthesized starch stabilized silver nanoparticles loaded with isoorientin

Isoorientin (Iso) is a natural flavonoid, the effect of metal nanoparticles loaded with it was unknown. In this study, silver nanoparticles (AgNPs) were synthesized by corn starch and sodium citrate with the green synthesis method, and the structural characterization and stability of AgNPs loaded with Iso (AgNPs-Iso) were examined by UV-vis spectroscopy and zetasizer. Results showed that AgNPs (65 ± 0.87 nm, spheres) successfully loaded with Iso (117 ± 2.13 nm, loading efficiency: 76.60%). There are no significant changes of the stability of AgNPs and AgNPs-Iso in pH 5-9 and 0-0.30 M of NaCl solution. AgNPs-Iso was more stable than AgNPs in the simulated gastrointestinal digestion in vitro. Furthermore, AgNPs-Iso showed the lower erythrocytes hemolysis ratio and cytotoxicity, and exhibited a notably inhibitive effect on α-glucosidase and pancreatic lipase. Therefore, this study could provide the basic support for the further development of highly stable and lowly cytotoxic AgNPs-Iso on Type II diabetes and obesity.

Resveratrol-loaded hollow kafirin nanoparticles via gallic acid crosslinking: An evaluation compared with their solid and non-crosslinked counterparts

The possibility of combining the health benefits of kafirin and polyphenols and improving the bioavailability of resveratrol using hollow kafirin nanoparticles via gallic acid crosslinking was investigated. The size, morphology, charge state, loading efficiency, physicochemical stability, and redispersity after lyophilization of hollow resveratrol-loaded kafirin nanoparticles formed via gallic acid crosslinking were characterized and compared with their solid counterparts and those without crosslinking. The nanoparticles formed were anionic spheres with an average diameter of <100 nm when loading amounts of resveratrol were less than 20%. The hollow nanoparticles were homogenous and still achieved stable colloidal dispersion after lyophilization. The hollow nanoparticles crosslinked with gallic acid displayed stability against pancreatin and delayed release in stimulated digestion. The results suggested that hollow kafirin nanoparticles could be a favorable colloidal delivery system for incorporating resveratrol.

Production of biosurfactant using the endemic bacterial community of a PAHs contaminated soil, and its potential use for PAHs remobilization

Biosurfactants are surface-active agents produced by microorganisms whose use in soil remediation processes is increasingly discussed as a more environmentally friendly alternative than chemically produced surfactants. In this work, we report the production of a biosurfactant by a bacterial community extracted from a polluted soil, mainly impacted by PAHs, in order to use it in a soil-washing process coupled with bioremediation. Nutrient balance was a critical parameter to optimize the production. Best conditions for biosurfactant production were found to be 20 g/L of glucose, 2 g/L of NH₄NO₃, and 14.2 g/L of Na₂HPO₄, corresponding to a C/N/P molar ratio equal to 13/1/2. Purification of the produced biosurfactant by acidification and double extraction with dichloromethane as a solvent allowed measuring the Critical Micellar Concentration (CMC) as equal to 42 mg/L. The capacity of the purified biosurfactant to increase the apparent solubility of four reference PAHs (naphthalene, phenanthrene, pyrene and benzo[a]pyrene) was completed. The solubilisation ratios, in mg of PAH/g of biosurfactant for phenanthrene, pyrene and benzo[a]pyrene are 0.214, 0.1204 and 0.0068, respectively. Identification of the bacteria found in the colony producing the biosurfactant showed the presence of bacteria able to produce biosurfactant (Enterobacteriaceae, Pseudomonas), as well as, others able to degrade PAHs (Microbacterium, Pseudomonas, Rhodanobacteraceae).

Biosurfactants, natural alternatives to synthetic surfactants: Physicochemical properties and applications

Biosurfactants comprise a wide array of amphiphilic molecules synthesized by plants, animals, and microbes. The synthesis route dictates their molecular characteristics, leading to broad structural diversity and ensuing functional properties. We focus here on low molecular weight (LMW) and high molecular weight (HMW) biosurfactants of microbial origin. These are environmentally safe and biodegradable, making them attractive candidates for applications spanning cosmetics to oil recovery. Biosurfactants spontaneously adsorb at various interfaces and self-assemble in aqueous solution, resulting in useful physicochemical properties such as decreased surface and interfacial tension, low critical micellization concentrations (CMCs), and ability to solubilize hydrophobic compounds. This review highlights the relationships between biosurfactant molecular composition, structure, and their interfacial behavior. It also describes how environmental factors such as temperature, pH, and ionic strength can impact physicochemical properties and self-assembly behavior of biosurfactant-containing solutions and dispersions. Comparison between biosurfactants and their synthetic counterparts are drawn to illustrate differences in their structure-property relationships and potential benefits. Knowledge of biosurfactant properties organized along these lines is useful for those seeking to formulate so-called green or natural products with novel and useful properties.

Concentrations and Adsorption Isotherms for Amphiphilic Surfactants in PM1 Aerosols from Different Regions of Europe

Predicting the activation of submicrometer particles into cloud droplets in the atmosphere remains a challenge. The importance of surface tension, σ (mN m^-1), in these processes has been evidenced by several works, but information on the "surfactants" lowering σ in actual atmospheric particles remains scarce. In this work, PM₁ aerosols from urban, coastal, and remote regions of Europe (Lyon, France, Rogoznica, Croatia, and Pallas, Finland, respectively) were investigated and found to contain amphiphilic surfactants in concentrations up to 2.8 μg m^-3 in the air and 1.3 M in the particle dry volume. In Pallas, correlations with the PM₁ chemical composition showed that amphiphilic surfactants were present in the entire range of particle sizes, supporting recent works. This implied that they were present in hundreds to thousands of particles cm^-3 and not only in a few large particles, as it has been hypothesized. Their adsorption isotherms and critical micelle concentration (CMC) were also determined. The low CMC obtained (3 × 10^-5-9 × 10^-3 M) implies that surface tension depression should be significant for all the particles containing these compounds, even at activation (growth factor ∼ 10). Amphiphilic surfactants are thus likely to enhance the CCN ability of submicrometer atmospheric particles.

Extraction of PAHS from an aged creosote-polluted soil by cyclodextrins and rhamnolipids. Side effects on removal and availability of potentially toxic elements

This study evaluated the effect of several cyclodextrins (CDs) and a rhamnolipid (RL) on the removal of polycyclic aromatic hydrocarbons (PAHs) from a co-contaminated soil which had received historically creosote and inorganic wood preservatives for almost 100 years, and the effect of such extractions on the potentially toxic elements (PTEs). The influence on such processes of an electrolyte (0.01 M Ca(NO₃)₂) was also studied. Up to 15.4% of the ∑16 PAHs were extracted using RL in the absence of the electrolyte as washing solution, but decreases until reaching 0.60% in the presence of Ca²⁺ due to RL precipitation and partial inactivation. Only up to 2% of the ∑16 PAHs was extracted with CDs (4-ring PAHs in higher concentrations), but the electrolyte had no effect on extraction. In relation to PTEs, CDs proved to be inefficient for their extraction, and even RL in the presence of the background electrolyte. But in the absence of electrolyte PTEs extraction by RL increased. Apart from that, the availability of Ni, Cr, and As, those more associated to Fe and Al soil surfaces, increased after extraction with RLs in the presence of Ca²⁺ (about 100% for Cr and Ni and 200% for As). Under these conditions Fe and Al availability increased two- and ten-fold, respectively, indicating that Fe-Al soil surfaces were altered. Therefore, the ionic strength and the cations present in the soil solution of soils have to be considered when RLs are used as extractants for remediation purposes.

Efficient cadmium removal from aqueous solutions using a sample coal waste activated by rhamnolipid biosurfactant

A sample coal waste activated by rhamnolipid biosurfactants was used as an efficient adsorbent for the adsorption of cadmium from aqueous solution. The effects of three factors, namely, initial solution pH (3-11), absorbent to cadmium ratio (12.5-162.5) and contact time (3-31 h), on cadmium removal were studied and optimized using a central composite type response surface methodology. The two factors that play a key role in the adsorption process are pH and absorbent dosage. Optimum adsorption conditions achieved at pH 9, absorbent to cadmium ratio of 125 and equilibrium contact time of 10 h, resulted in more than 99% cadmium removal. Kinetic studies revealed that a maximum removal can be achieved before 10 min of adsorption process following a pseudo-second order model. The selectivity study in bimetal aqueous systems using copper, lead and zinc metals showed the adsorption order of Cu²⁺ > Cd²⁺ > Zn²⁺ > Pb²⁺. The cadmium adsorption on activated coal waste was also found to follows the Temkin isotherm model with a correlation coefficient of 92.43%.

Utilization of agro-industrial waste for biosurfactant production under submerged fermentation and its application in oil recovery from sand matrix

This study reports biosurfactant production by Pseudomonas azotoformans AJ15 under submerged fermentation via utilizing the agro-industrial wastes (bagasse and potato peels). The extracted biosurfactant was characterized for its classification (nature, group, and class) and stability against environmental stresses. Further, the biosurfactant was employed to explore its oil recovery efficiency from the sand matrix with 2000 ppm salt concentration. Results revealed that substrates developed by mixing both the agro-industrial wastes account for high yield of biosurfactant. The subsequent experimental studies demonstrated that the biosurfactant might belong to glycolipid group and rhamnolipid class. Moreover, the biosurfactant was stable at a high temperature of 90 °C and enable to persist its activity in the high salt concentration of 6% and varying pH. The biosurfactant was found to be effective in recovering up to 36.56% of trapped oil under saline condition.

Stabilization and Rheology of Concentrated Emulsions Using the Natural Emulsifiers Quillaja Saponins and Rhamnolipids

Concentrated emulsions are widely used in the cosmetic, personal-care, and food industries to reduce storage and transportation costs and to provide desirable characteristics. The current study aimed to produce concentrated emulsions (50 wt % oil) using two natural emulsifiers, quillaja saponins and rhamnolipids. The impacts of emulsifier concentrations on the particle sizes, rheological properties, and stabilities of concentrated emulsions were evaluated. Emulsion particle sizes were negatively correlated with the concentrations of both quillaja saponins and rhamnolipids, and rhamnolipids were more effective in producing smaller droplets. Both emulsifiers formed stable concentrated emulsions against a series of environmental stresses, including various temperatures (30-90 °C), salt concentrations (≤200 mM NaCl), and pHs (pH 5-8). The rheology tests suggested that concentrated emulsions stabilized by quillaja saponins or rhamnolipids presented shear-thinning behaviors and had relatively low consistency coefficients.

An Interaction of Rhamnolipids with Cu2+ Ions

This study was focused on the description of interaction between Cu²⁺ ions and the 1:1 mono- and dirhamnolipid mixtures in the premicellar and aggregated state in water and 20 mM KCl solution at pH 5.5 and 6.0. The critical micelle concentration of biosurfactants was determined conductometrically and by the pH measurements. Hydrodynamic diameter and electrophoretic mobility were determined in micellar solutions using dynamic light scattering and laser Doppler electrophoresis, respectively. The copper immobilization by rhamnolipids, methylglycinediacetic acid (MGDA), and ethylenediaminetetraacetic acid (EDTA) was estimated potentiometrically for the Cu²⁺ to chelating agent molar ratio from 16:100 to 200:100. The degree of ion binding and the complex stability constant were calculated at a 1:1 metal to chelant molar ratio. The aggregates of rhamnolipids (diameter of 43-89 nm) were negatively charged. Biosurfactants revealed the best chelating activities in premicellar solutions. For all chelants studied the degree of metal binding decreased with the increasing concentration of the systems. The presence of K⁺ lowered Cu²⁺ binding by rhamnolipids, but did not modify the complex stability significantly. Immobilization of Cu²⁺ by biosurfactants did not cause such an increase of acidification as that observed in MGDA and EDTA solutions. Rhamnolipids, even in the aggregated form, can be an alternative for the classic chelating agents.

Optimization and characterization of biosurfactant from Streptomyces griseoplanus NRRL-ISP5009 (MS1)

AIMS

This work aimed to study, isolate, characterize and stabilize the biosurfactant isolated from actinomycetes found in petroleum contaminated soil.

METHODS AND RESULTS

Optimized production of the biosurfactant from Streptomyces griseoplanus NRRL-ISP5009, SM1 was obtained on day 6 at 30°C, pH 7, 150 rev min^-1 , in glycerol yeast extract broth medium supplemented with cellulose, yeast extract and 1% NaCl. The stability of the biosurfactant produced was studied at different temperatures, pH and different concentrations of NaCl. The produced biosurfactant was extracted and purified.

CONCLUSION

Streptomyces griseoplanus NRRL-ISP5009, SM1 isolated from oil contaminated soil produced a biosurfactant exhibiting emulsification activity. The produced biosurfactant is a mixture of carbohydrate, lipid and protein. It has promising characteristics, including a higher stability at alkaline pH than at acidic pH, a salinity of 1-3% and stable in the temperature range from 0 and 100°C. Also, the potential antimicrobial activity of the purified biosurfactant was recorded.

SIGNIFICANCE AND IMPACT OF THE STUDY

The research was focused on the isolation of a novel source of biosurfactants that have great importance in the manufacture of food, detergent, pharmaceutical and cosmetics.

In situ surface transfer process of Cry1Ac protein on SiO2： The effect of biosurfactants for desorption

Genetically modified Bacillus thuringiensis (Bt) crops, which have been widely used in agricultural transgenic plants, express insecticidal Cry proteins and release the toxin into soils. Taking into consideration the environmental risk of Cry proteins, biosurfactant-rhamnolipids were applied to desorb Cry proteins from soil environment, which has not been elucidated before. Quartz crystal microbalance with dissipation (QCM-D) was used in this article to investigate the adsorption and desorption behaviors of Cry1Ac on SiO₂ surface (model soil). Results showed that patch-controlled electrostatic attraction (PCEA) governed Cry1Ac adsorption to SiO_2, and the solution pH or ionic strength can affect PCEA. The adsorption kinetics could be fitted by the pseudo-second-order model, and the adsorption isotherm was fitted to Langmuir model with correlation coefficients higher than 0.999. The desorption characteristics of Cry1Ac from SiO₂ were assessed in the presence of mono-rhamnolipid, di-rhamnolipid or complex-rhamnolipid. Mono-rhamnolipid exhibited the most significant positive effect on desorption performance. With a complete removal of Cry1Ac reached when mono-rhamnolipid concentration was up to 50mgL^-1. Additionally, the desorption was enhanced at alkaline pH range, and Cry1Ac can be completely and rapidly desorbed by rhamnolipids from SiO₂ at ionic strength of 5×10^-2M.

Advances in applications of rhamnolipids biosurfactant in environmental remediation: A review

The objective of this review is to provide a comprehensive overview of the advances in the applications of rhamnolipids biosurfactants in soil and ground water remediation for removal of petroleum hydrocarbon and heavy metal contaminants. The properties of rhamnolipids associated with the contaminant removal, that is, solubilization, emulsification, dispersion, foaming, wetting, complexation, and the ability to modify bacterial cell surface properties, were reviewed in the first place. Then current remediation technologies with integration of rhamnolipid were summarized, and the effects and mechanisms for rhamnolipid to facilitate contaminant removal for these technologies were discussed. Finally rhamnolipid-based methods for remediation of the sites co-contaminated by petroleum hydrocarbons and heavy metals were presented and discussed. The review is expected to enhance our understanding on environmental aspects of rhamnolipid and provide some important information to guide the extending use of this fascinating chemical in remediation applications.

Isolation of nitrate-reducing bacteria from an offshore reservoir and the associated biosurfactant production

Biosurfactant producing nitrate-reducing bacteria (NRB) in anaerobic reservoir environments are closely associated with souring (H₂S) control in the offshore oil and gas industry. Five NRB strains were screened from offshore produced water samples and all were identified as Pseudomonas stutzeri. Their biosurfactant producing abilities when fed on either glucose or glycerol media were investigated. P. stutzeri CX3 reduced the medium surface tension to 33.5 and 29.6 mN m⁻¹, respectively, while growing on glucose or glycerol media. The CX3 strain was further inoculated to examine its growth performance, resulting in 32.4% and 94.5% of nitrate consumption over 228 hours of monitoring in two media, respectively. The composition analysis of the biosurfactant product generated by P. stutzeri CX3 was conducted through thin-layer chromatography, gas chromatography with a flame ionization detector (FID) and Fourier transform infrared spectroscopy (FT-IR). The biosurfactant product was identified as a mixture of a small part of lipopeptides and a large part of glycolipids while its critical micellar concentration (CMC) was as low as 35 mg L⁻¹. The biosurfactant product demonstrated high stability over a wide range of temperature (4–121 °C), pH (2–10), and salinity (0–20% w/v) concentration. The results provided valuable technical and methodological support for effective offshore reservoir souring control and associated enhanced oil recovery activities.

Biosurfactant producing nitrate-reducing bacteria (NRB) in anaerobic reservoir environments are closely associated with souring (H₂S) control in the offshore oil and gas industry.

pH-Induced evolution of surface patterns in micelles assembled from dirhamnolipids: dissipative particle dynamics simulation

Dissipative particle dynamics (DPD) simulation is used to study the effect of pH on the morphological transition in micelles assembled from dirhamnolipids (diRLs), and analyze the pH-driven mechanism and influence factors of micellar surface patterns.

Studies on Emulsification Properties of Glycolipids Biosurfactants

Microbial biosurfactants consists of hydrophilic and hydrophobic moieties in its structure and are produced by microorganisms. Glycolipid class of biosurfactants has wide range of surface and interfacial properties. The emulsification activity and emulsion stability of the Glycolipids Trehalose lipids (THL), Mannosylerrythritol lipids (MEL), Sophorolipids (SL) and Rhamnolipids (RL) were investigated using liquid paraffin (hydrocarbon source) and sunflower oil (vegetable source) as hydrophobic source by UV spectroscopy. Emulsification activity and stability are in the order THL > MEL > SL > RL. The stability as a function of the temperature in the range of 20 °C – 80 °C is in order THL > SL > MEL > RL. The effect of pH was studied using buffers of acidic and basic pH. It was observed that RL and SL had excellent emulsification activity at pH 8 while the activity of trehalose lipids and mannosylerrythritol lipids was not affected by pH. Similar effect of various concentrations of salt (NaCl) was studied; THL and MEL emulsion were very resistant to concentration of salt but the stability of SL and RL emulsion decreased with increased salt concentration. Average droplet diameter of emulsion and the polydispersity index were determined by dynamic light scattering. The emulsions of THL and SL have smallest droplet diameter of 422 nm and 625 nm, while emulsions of MEL and RL have a droplet size of 1923 nm and 2245 nm respectively. Emulsions of all investigatesd glycolipid surfactants had good polydispersity index and negative zeta potential, suggesting their possible applications in pharmaceutical, cosmetics, industrial and environmental techniques.