Aggregate-based sub-CMC Solubilization of n-Alkanes by Monorhamnolipid Biosurfactant

Microbe cultivation guidelines to optimize rhamnolipid applications

In the growing landscape of interest in natural surfactants, selecting the appropriate one for specific applications remains challenging. The extensive, yet often unsystematized, knowledge of microbial surfactants, predominantly represented by rhamnolipids (RLs), typically does not translate beyond the conditions presented in scientific publications. This limitation stems from the numerous variables and their interdependencies that characterize microbial surfactant production. We hypothesized that a computational recipe for biosynthesizing RLs with targeted applicational properties could be developed from existing literature and experimental data. We amassed literature data on RL biosynthesis and micellar solubilization and augmented it with our experimental results on the solubilization of triglycerides (TGs), a topic underrepresented in current literature. Utilizing this data, we constructed mathematical models that can predict RL characteristics and solubilization efficiency, represented as logPRL = f(carbon and nitrogen source, parameters of biosynthesis) and logMSR = f(solubilizate, rhamnolipid (e.g. logPRL), parameters of solubilization), respectively. The models, characterized by robust R2 values of respectively 0.581-0.997 and 0.804, enabled the ranking of descriptors based on their significance and impact-positive or negative-on the predicted values. These models have been translated into ready-to-use calculators, tools designed to streamline the selection process for identifying a biosurfactant optimally suited for intended applications.

Enhanced remediation of oil-contaminated intertidal sediment by bacterial consortium of petroleum degraders and biosurfactant producers

Oil pollution in intertidal zones is an important environmental issue that has serious adverse effects on coastal ecosystems. This study investigated the efficacy of a bacterial consortium constructed from petroleum degraders and biosurfactant producers in the bioremediation of oil-polluted sediment. Inoculation of the constructed consortium significantly enhanced the removal of C₈-C₄₀n-alkanes (80.2 ± 2.8% removal efficiency) and aromatic compounds (34.4 ± 10.8% removal efficiency) within 10 weeks. The consortium played dual functions of petroleum degradation and biosurfactant production, greatly improving microbial growth and metabolic activities. Real-time quantitative polymerase chain reaction (PCR) showed that the consortium markedly increased the proportions of indigenous alkane-degrading populations (up to 3.88-times higher than that of the control treatment). Microbial community analysis demonstrated that the exogenous consortium activated the degradation functions of indigenous microflora and promoted synergistic cooperation among microorganisms. Our findings indicated that supplementation of a bacterial consortium of petroleum degraders and biosurfactant producers is a promising bioremediation strategy for oil-polluted sediments.

Enhanced bioremediation of diesel oil-contaminated seawater by a biochar-immobilized biosurfactant-producing bacteria Vibrio sp. LQ2 isolated from cold seep sediment

This study investigated the effect of immobilized biosurfactant-producing bacteria on the bioremediation of diesel oil-contaminated seawater. Initially, a biosurfactant-producing bacterium, LQ2, was isolated from a marine cold-seep region, and identified as Vibrio sp. The biosurfactant produced by LQ2 was characterized as a phospholipid, exhibiting high surface activity with strong stability. Meanwhile, the inoculation of biochar-immobilized LQ2 demonstrated superior efficiency in removing diesel oil (94.7%, reduction from 169.2 mg to 8.91 mg) over a seven-day period compared to free-cell culture (54.4%), through both biodegradation and adsorption. In addition, the microbial growth and activity were greatly enhanced with the addition of immobilized LQ2. Further experiment showed that degradation-related genes, alkB and CYP450-1, were 3.8 and 15.2 times higher in the immobilized LQ2 treatment, respectively, than those in the free cell treatment. The findings obtained in this study suggest the feasibility of applying immobilized biosurfactant-producing bacteria, namely LQ2, in treating diesel oil-contaminated seawater.

Cooperativity in micellar solubilization

Sudden onset of solubilization is observed widely around or below the critical micelle concentration (CMC) of surfactants. It has also been reported that micellization is induced by the solutes even below CMC and the solubilized solute increases the aggregation number of the surfactant. These observations suggest enhanced cooperativity in micellization upon solubilization. Recently, we have developed a rigorous statistical thermodynamic theory of cooperative solubilization. Its application to hydrotropy revealed the mechanism of cooperative hydrotropy: hydrotrope self-association enhanced by solutes. Here we generalize our previous cooperative solubilization theory to surfactants. We have shown that the well-known experimental observations, such as the reduction of CMC in the presence of the solutes and the increase of aggregation number, are the manifestations of cooperative solubilization. Thus, the surfactant self-association enhanced by a solute is the driving force of cooperativity and a part of a universal cooperative solubilization mechanism common to hydrotropes and surfactants at low concentrations.

Contribution of Nonaqueous-Phase Liquids to the Retention and Transport of Per and Polyfluoroalkyl Substances (PFAS) in Porous Media

Per and polyfluoroalkyl substances (PFAS) cocontamination with nonaqueous-phase organic liquids (NAPLs) has been observed or suspected at various sites, particularly at fire-training areas at which aqueous film-forming foams (AFFFs) were applied. The objectives of this study are to (1) delineate the relative significance of specific PFAS-NAPL processes on PFAS retention, including partitioning into the bulk NAPL phase and adsorption to the NAPL-water interface; (2) investigate the influence of NAPL properties, saturation, and mass-transfer constraints on PFAS retention; and (3) determine whether PFAS may impact NAPL distribution through mobilization or dissolution. Perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) are used as representative PFAS, and trichloroethene (TCE) and decane are used as representative NAPLs. NAPL-water interfacial adsorption was quantified with NAPL-water interfacial-tension measurements; partitioning into NAPL was quantified with batch experiments, and retardation factors (R) in the absence and presence of residual NAPL were determined with miscible-displacement transport experiments. R values increased in the presence of residual NAPL, with adsorption to the NAPL-water interface accounting for as much as ∼77% of retention and solid-phase adsorption also significantly contributing to retention. Additionally, this study provides the first QSPR analysis focused on NAPL-water interfacial adsorption coefficients, with results consistent with those from previous air-water studies. Lastly, this initial investigation into PFAS impacts on NAPL behavior determined that PFOS/PFOA are unlikely to enhance solubilization or mobilization of NAPL under the conditions present at many AFFF legacy sites.

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.

Surfactant-enhanced aquifer remediation: Mechanisms, influences, limitations and the countermeasures

In recent years, surfactant-enhanced aquifer remediation (SEAR) has attracted increasing interest duo to the high efficiency of removing non-aqueous phase liquids (NAPLs) from aquifer. A thorough understanding of SEAR is necessary for its successful implementation in field remediation. This paper reviewed the SEAR technology in a comprehensive way based on the recent research advances. Firstly, an overview of the basic processes and mechanisms underlying the technology was presented. Secondly, applications of SEAR and the factors that influence the performance were summarized. Thirdly, the key limitations of SEAR, which are downward migration of dense-NAPLs, secondary pollution of surfactants, adsorptive, precipitative and partitioning loss of surfactants, and heterogeneity of the aquifer, were reviewed. Finally, the recent advances in modifying SEAR to overcome the limitations were discussed in detail. The review will promote our understanding of SEAR technology and provide some useful information to improve the performance of SEAR in applications.

Biosurfactants: Production and application prospects in the food industry

There has been considerable interest in the use of biosurfactants due to the diversity of structures and the possibility of production from a variety of substrates. The potential for industrial applications has been growing, as these natural compounds are tolerant to common processing methods and can compete with synthetic surfactants with regards to the capacity to reduce surface and interfacial tensions as well as stabilise emulsions while offering the advantages of biodegradability and low toxicity. Among biosurfactant-producing microorganisms, some yeasts present no risks of toxicity or pathogenicity, making them ideal for use in food formulations. Indeed, the use of these biomolecules in foods has attracted industrial interest due to their properties as emulsifiers and stabilizers of emulsions. Studies have also demonstrated other valuable properties, such as antioxidant and antimicrobial activity, enabling the aggregation of greater value to products and the avoidance of contamination both during and after processing. All these characteristics allow biosurfactants to be used as additives and versatile ingredients for the processing of foods. The present review discusses the potential application of biosurfactants as emulsifying agents in food formulations, such as salad dressing, bread, cakes, cookies, and ice cream. The antioxidant, antimicrobial and anti-adhesive properties of these biomolecules are also discussed, demonstrating the need for further studies to make the use of the natural compounds viable in this expanding sector.

Different submicellar solubilization mechanisms revealed by 1H NMR and 2D diffusion ordered spectroscopy (DOSY)

Different submicellar solubilization mechanisms of two systems, Triton X-100 (TX-100)/tetradecane and sodium dodecyl sulfate (SDS)/butyl methacrylate, are revealed on the molecular scale by ¹H NMR spectroscopy and 2D diffusion ordered spectroscopy (DOSY). It is evident that the apparent solubilities of both tetradecane and butyl methacrylate are enhanced, even at much lower surfactant concentrations than the CMCs. Solubilized solutes also contribute to the early formation of surfactant micelles. In general, the molar solubilization ratios (MSRs) of both solutes linearly increase as the surfactant concentrations increase. However, variations in MSRs of the two systems are different below and above the CMC, which is probably related to the different solubilization mechanisms. For TX-100/tetradecane, as the TX-100 concentration increases, the tetradecane resonance in the independent state transforms into that of the aggregated state and the corresponding evolution of diffusions is shown in the 2D DOSY spectra. These results demonstrate that below the CMC, tetradecane is first solubilized in TX-100 solutions, and then solubilized in TX-100 micelles above the CMC. For SDS/butyl methacrylate, the appearance of oligomeric SDS resonances below the CMC indicates that butyl methacrylate is partially solubilized in SDS oligomers. Then, when the CMC is reached, the dominant, monomeric SDS molecules aggregate into oligomers, and the similar diffusivity trend of butyl methacrylate with that of SDS indicates that a proportion of butyl methacrylate molecules are solubilized in it. Finally, the fusion of SDS resonances in the two states and the tendency of co-diffusion of SDS and butyl methacrylate indicate that all the SDS molecules gradually aggregate into micelles, and almost all the butyl methacrylate molecules are solubilized in them. In conclusion, above the CMCs, the solubilization manners of these two systems are similar. However, they are different below CMCs. The solubilization of tetradecane by TX-100 is driven by the intermolecular hydrophobic interaction, i.e., molecular-pair formation. However, the polar interaction between functional groups of butyl methacrylate and the polar head of SDS contributes to the solubilization of butyl methacrylate. The different submicellar solubilization mechanisms are mainly caused by the different properties of solutes and surfactants, which also results in different MSRs and solubilization sites in the micelles.

Sub-CMC solubilization of n-alkanes by rhamnolipid biosurfactant: the Influence of rhamnolipid molecular structure

Solubilization of n-alkanes by dirhamnolipid (diRL) biosurfactant at sub-critical micelle concentrations (sub-CMC) was studied and the results were compared to that for monorhamnolipid (monoRL) obtained in our prior study. The results show that the apparent solubility of the four alkanes (decane, dodecane, tetradecane and hexadecane) increases linearly with the increase of diRL concentration at diRL concentrations below CMC. The solubilization potential of diRL indicated by molar solubilization ratio (MSR) is higher at sub-CMC than at hyper-CMC concentrations (0.43 vs 0.03, 7.91 vs 0.02, 5.16 vs 1.98 and 3.71 vs 1.26 for decane, dodecane, tetradecane and hexadecane, respectively). The maximum access at the surface of the alkane-rhamnolipid aggregates (Гmax) is smaller for diRL than for monoRL (2.47 vs 3.09, 2.35 vs 2.86, 2.26 vs 3.60, and 2.01 vs 4.09 mol/m2 for decane, dodecane, tetradecane and hexadecane, respectively). In addition, more significant impact of surface access change on the size of the aggregates were observed for diRL, indicating that the difference in the structure of polar group between diRL and monoRL (one more rhamnose ring in diRL polar group) has influence on alkane-solubilization behavior. Due to the smaller access at the surface of the aggregates, diRL can produce larger number of aggregates than monoRL in the solution and thus show higher solubilization efficiency of dodecane, tetradecane and hexadecane. The findings of the study add to the knowledge of solubilization of hydrophobic organic compounds by surfactants and contribute to application of rhamnolipid biosurfactant in the area of remediation.

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.

Microbial Surfactants: The Next Generation Multifunctional Biomolecules for Applications in the Petroleum Industry and Its Associated Environmental Remediation

Surfactants are a broad category of tensio-active biomolecules with multifunctional properties applications in diverse industrial sectors and processes. Surfactants are produced synthetically and biologically. The biologically derived surfactants (biosurfactants) are produced from microorganisms, with Pseudomonas aeruginosa, Bacillus subtilis Candida albicans, and Acinetobacter calcoaceticus as dominant species. Rhamnolipids, sophorolipids, mannosylerithritol lipids, surfactin, and emulsan are well known in terms of their biotechnological applications. Biosurfactants can compete with synthetic surfactants in terms of performance, with established advantages over synthetic ones, including eco-friendliness, biodegradability, low toxicity, and stability over a wide variability of environmental factors. However, at present, synthetic surfactants are a preferred option in different industrial applications because of their availability in commercial quantities, unlike biosurfactants. The usage of synthetic surfactants introduces new species of recalcitrant pollutants into the environment and leads to undesired results when a wrong selection of surfactants is made. Substituting synthetic surfactants with biosurfactants resolves these drawbacks, thus interest has been intensified in biosurfactant applications in a wide range of industries hitherto considered as experimental fields. This review, therefore, intends to offer an overview of diverse applications in which biosurfactants have been found to be useful, with emphases on petroleum biotechnology, environmental remediation, and the agriculture sector. The application of biosurfactants in these settings would lead to industrial growth and environmental sustainability.

Ion-selective molecular inclusion of organic dyes into pH-responsive gel assemblies of zwitterionic surfactants

The pH-Responsive sol–gel transition of a surfactant gel took place along with ion-selective capture and release of dye molecules.

Mechanisms for rhamnolipids-mediated biodegradation of hydrophobic organic compounds

The widespread existence of hydrophobic organic compounds (HOCs) in soil and water poses a potential health hazard to human, such as skin diseases, heart diseases, carcinogenesis, etc. Surfactant-enhanced bioremediation has been regarded as one of the most viable technologies to treat HOCs contaminated soil and groundwater. As a biosurfactant that has been intensively studied, rhamnolipids have shown to enhance biodegradation of HOCs in the environment, however, the underlying mechanisms are not fully disclosed. In this paper, properties and production of rhamnolipids are summarized. Then effects of rhamnolipids on the biodegradation of HOCs, including solubilization, altering cell affinity to HOCs, and facilitating microbial uptake are reviewed in detail. Special attention is paid to how rhamnolipids change the bioavailability of HOCs, which are crucial for understanding the mechanism of rhamnolipids-mediated biodegradation. The biodegradation and toxicity of rhamnolipids are also discussed. Finally, perspectives and future research directions are proposed. This review adds insight to rhamnolipids-enhanced biodegradation process, and helps in application of rhamnolipids in bioremediation.

Self-aggregation of bio-surfactants within ionic liquid 1-ethyl-3-methylimidazolium bromide: A comparative study and potential application in antidepressants drug aggregation

Aggregation behavior of bio-surfactants (BS) sodium cholate (NaC) and sodium deoxycholate (NaDC) within aqueous solution of ionic liquid (IL) 1-ethyl-3-methylimidazolium bromide [Emim][Br] has been investigated using surface tension, conductivity, steady state fluorescence, FT-IR and dynamic light scattering (DLS) techniques. Various interfacial and thermodynamic parameters are determined in the presence of different wt% of IL [Emim][Br]. Information regarding the local microenvironment and size of the aggregates is obtained from fluorescence and DLS, respectively. FT-IR spectral response is used to reveal the interactions taking place within aqueous NaC/NaDC micellar solutions. It is noteworthy to mention that increasing wt% of [Emim][Br] results in an increase in the spontaneity of micelle formation and the hydrophilic IL shows more affinity for NaC as compared to NaDC. Further, the micellar solutions of BS-[Emim][Br] are utilized for studying the aggregation of antidepressants drug promazine hydrochloride (pH). UV-vis spectroscopic investigation reveals interesting outcomes and the results show changes in spectral absorbance of PH drug on the addition of micellar solution (BS-[Emim][Br]). Highest binding affinity and most promising activity are shown for NaC as compared to NaDC.

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.

Effect of rhamnolipid solubilization on hexadecane bioavailability: enhancement or reduction?

In this study, liquid culture systems containing rhamnolipid-solubilized, separate-phase, and multi-state hexadecane as the carbon source were employed for examining the effect of rhamnolipid solubilization on the bioavailability of hexadecane. Experimental results showed that the uptake of rhamnolipid-solubilized hexadecane by Pseudomonas aeruginosa ATCC 9027, a rhamnolipid producing strain, was enhanced compared to the uptake of mass hexadecane as a separate phase, indicating rhamnolipid solubilization increased the bioavailability of hexadecane for this bacterium. For Pseudomonas putida CICC 20575 which does not produce but degrade rhamnolipid, the uptake of either rhamnolipid-solubilized hexadecane or multi-state hexadecane was inhibited. The reduction of bioavailability was assumed to be the consequence of the blocking effect caused by the partition of rhamnolipid molecules at the hexadecane-water interface. The results show that how rhamnolipid solubilization changes the bioavailability of hexadecane depends on the bacterial compatibility to rhamnolipid. The study adds insight into the knowledge of biosurfactant-associated bioavailability of hydrophobic organic compounds (HOCs), and is of importance for application of biosurfactants in bioremediation of HOCs.

Sub-CMC solubilization of dodecane by rhamnolipid in saturated porous media

Experiments were conducted with a two-dimensional flow cell to examine the effect of monorhamnolipid surfactant at sub-CMC concentrations on solubilization of dodecane in porous media under dynamic flow conditions. Quartz sand was used as the porous medium and artificial groundwater was used as the background solution. The effectiveness of the monorhamnolipid was compared to that of SDBS, Triton X-100, and ethanol. The results demonstrated the enhancement of dodecane solubility by monorhamnolipid surfactant at concentrations lower than CMC. The concentrations (50–210 μM) are sufficiently low that they do not cause mobilization of the dodecane. Retention of rhamnolipid in the porous medium and detection of nano-size aggregates in the effluent show that the solubilization is based on a sub-CMC aggregate-formation mechanism, which is significantly stronger than the solubilization caused by the co-solvent effect. The rhamnolipid biosurfactant is more efficient for the solubilization compared to the synthetic surfactants. These results indicate a strategy of employing low concentrations of rhamnolipid for surfactant-enhanced aquifer remediation (SEAR), which may overcome the drawbacks of using surfactants at hyper-CMC concentrations.