Solution self-assembly of the sophorolipid biosurfactant and its mixture with anionic surfactant sodium dodecyl benzene sulfonate

Biosurfactant/surfactant mixing properties at the air-water interface: comparing rhamnolipids and sophorolipids mixed with the anionic surfactant sodium dodecyl benzene sulfonate

There is an increasing interest in the use of biosurfactants in the development of more biocompatible and biosustainable surfactant-based products. To optimise performance and mitigate production costs, biosurfactants are commonly mixed with different synthetic surfactants. Understanding in detail their mixing properties at interfaces and in solution is key to the development of optimal formulations. Reported here is a detailed thermodynamic analysis, using the latest developments in the pseudo phase approximation, PPA, of the mixing behaviour at the air-water interface of two glycolipid biosurfactants, rhamnolipids, RL, containing the mono and di-rhamnose isomers R1 and R2, and the sophorolipids, SL, containing the lactonic and acidic isomers LS and AS, with the anionic surfactant sodium dodecyl benzene sulfonate, LAS. The analysis uses the previously reported adsorption data, from neutron reflectivity measurements, NR, for the associated binary and ternary mixtures. The different rhamnolipid and sophorolipid biosurfactant structures and their relative surface activities have a profound effect on their mixing properties at the air-water interface with the anionic surfactant LAS, due predominantly to the steric constraints of the different molecular structures. This results in different synergistic excess free energies of mixing and different optimal compositions.

Liquid Crystal Cubic Phases Constructed from Sophorolipids Micelles

Biosurfactants are considered to be desirable alternatives to synthetic surfactants. Sophorolipids produced by nonpathogenic yeast strains are one of the main types of glycolipid biosurfactants and have various applications. In this work, the aqueous phase behavior of the glycolipid-based biosurfactant sophorolipids (SL) was investigated using polarized microscopy, small-angle X-ray diffraction (SAXS), nuclear magnetic deuterium spectroscopy (2H NMR), transmission electron microscopy (TEM), freeze-etched transmission electron microscopy (FF-TEM), and dynamic light scattering (DLS). The binary phase diagram of the SL/H2O system was constructed, and a liquid crystalline cubic phase constructed by sophorolipids micelles was observed. A micellar phase at low concentrations (<50 wt %) was found. As the concentration increases, after a transition phase, it is a strictly micellar cubic phase at concentrations up to 70 wt %. The micellar cubic phase is an isotropic, highly viscous liquid crystal composed of three-dimensionally ordered arrangements of spherical micelles, which are arranged in simple cubic (CubI/Pm3m) or body-centered cubic (CubI/Im3m). The rheological properties at different concentrations and temperatures were studied. The micellar cubic phase is highly viscoelastic, and the viscosity tends to decrease uniformly with increasing temperatures (15-90 °C) and then returns to its original state after cooling, indicating that the micellar cubic phase possesses satisfactory reversibility at high temperature. The results are expected to be instructive for the application of the sophorolipids.

Investigating the interactions between an industrial lipase and anionic (bio)surfactants

In laundry formulations, synergies between amphiphiles and other additives such as enzymes increase sustainability through a large decrease in energy consumption. However, traditional surfactants are derived from petroleum, requiring chemical modifications (sulfonation, ethoxylation, or esterification) and generating environmental pollution through toxicity and low degradability. Use of biosurfactants removes these issues. To provide a firmer basis for the use of biosurfactants, we report on the interactions between the industrial lipase LIPEX® and three common biosurfactants, rhamnolipids, sophorolipids, and surfactin. The model surfactant sodium dodecyl sulfate (SDS) is included in the study for comparison. A thorough characterization by Small-angle X-ray scattering (SAXS) provides valuable information on the enzyme's oligomerization and the surfactant micelles' ellipsoidal morphology. Additionally, the enzymatic activity and complex formation in different surfactant mixtures are studied using isothermal titration calorimetry, activity assays, and SAXS. SDS activates the enzyme while promoting a controlled association of monomers while the biosurfactants inhibit the enzyme, independent of their effects on its quaternary structure. Rhamnolipids and surfactin promote lipase dimerization while sophorolipids have no significant effect on lipase quaternary structure. Based on these data, we propose a partial replacement that allows the enzyme to retain enzymatic activity while improving the environmental footprint of the formulation.

A Novel Biosurfactant-Based Oil Spill Response Dispersant for Efficient Application under Temperate and Arctic Conditions

Synthetic oil spill dispersants have become essential in offshore oil spill response strategies. However, their use raises significant concerns regarding toxicity to phyto- and zooplankton and other marine organisms, especially in isolated and vulnerable areas such as the Arctic and shorelines. Sustainable alternatives may be developed by replacing the major active components of commercial dispersants with their natural counterparts. During this study, interfacial properties of different types of glycolipid-based biosurfactants (rhamnolipids, mannosylerythritol lipids, and trehalose lipids) were explored in a crude oil-seawater system. The best-performing biosurfactant was further mixed with different nontoxic components of Corexit 9500A, and the interfacial properties of the most promising dispersant blend were further explored with various types of crude oils, weathered oil, bunker, and diesel fuel in natural seawater. Our findings indicate that the most efficient dispersant formulation was achieved when mannosylerythritol lipids (MELs) were mixed with Tween 80 (T). The MELs-T dispersant blend significantly reduced the interfacial tension (IFT) of various crude oils in seawater with results comparable to those obtained with Corexit 9500A. Importantly, no leaching or desorption of MELs-T components from the crude oil-water interface was observed. Furthermore, for weathered and more viscous asphaltenic bunker fuel oil, IFT results with the MELs-T dispersant blend surpassed those obtained with Corexit 9500A. This dispersant blend also demonstrated effectiveness at different dosages (dispersant-to-oil ratio (DOR)) and under various temperature conditions. The efficacy of the MELs-T dispersant was further confirmed by standard baffled flask tests (BFTs) and Mackay-Nadeau-Steelman (MNS) tests. Overall, our study provides promising data for the development of effective biobased dispersants, particularly in the context of petroleum exploitation in subsea resources and transportation in the Arctic.

Self-assembly of sophorolipid and eugenol into stable nanoemulsions for synergetic antibacterial properties through alerting membrane integrity

Exploring the natural, safe, and effective antimicrobial is one of the preferable ways to control foodborne bacteria. In this work, novel oil-in-water nanoemulsions were formulated with sophorolipids and eugenol without any co-surfactant using a self-assembling strategy. These nanoemulsions showed high stability with sizes less than 200 nm when exposure to low concentrations of salt ions, various pH values (5.0, 7.0, 10.0), storage temperature and time. The synergistic antibacterial effects against both Gram-negative Escherichia coli and Gram-positive Bacillus cereus were determined with a minimum inhibitory concentration (MIC) value of 0.5 mg/mL and 0.125 mg/mL, respectively. Further microscopy (SEM, TEM, LCSM) examination and ATP/Na+-K+-ATPase assay results showed that the morphological changes, intensive cell membrane permeability, leakage of ATP, and decreased Na+-K+-ATPase contributed to the antibacterial effects. Moreover, the bonding mechanism between nanoemulsions and cell membranes were further evaluated by FTIR and ITC using a DPPC vesicle model, which demonstrated that the nanoemulsions adsorbed on the surface of bilayer, interacted with the hydrophobic chains of DPPC membrane mainly through the hydrophobic interaction, and altered the structural integrity of the lipid bilayer. These results not only provide a facile green strategy for fabricating stable nanoemulsions, but also highlight a new perspective for stabilizing essential oils for their widely application in food industry.

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.

Palmitic acid sophorolipid biosurfactant: from self-assembled fibrillar network (SAFiN) to hydrogels with fast recovery

Nanofibres are an interesting phase into which amphiphilic molecules can self-assemble. Described for a large number of synthetic lipids, they were seldom reported for natural lipids like microbial amphiphiles, known as biosurfactants. In this work, we show that the palmitic acid congener of sophorolipids (SLC16:0), one of the most studied families of biosurfactants, spontaneously forms a self-assembled fibre network (SAFiN) at pH below 6 through a pH jump process. pH-resolved in situ small-angle X-ray scattering (SAXS) shows a continuous micelle-to-fibre transition, characterized by an enhanced core-shell contrast between pH 9 and pH 7 and micellar fusion into a flat membrane between pH 7 and pH 6, approximately. Below pH 6, homogeneous, infinitely long nanofibres form by peeling off the membranes. Eventually, the nanofibre network spontaneously forms a thixotropic hydrogel with fast recovery rates after applying an oscillatory strain amplitude out of the linear viscoelastic regime: after being submitted to strain amplitudes during 5 min, the hydrogel recovers about 80% and 100% of its initial elastic modulus after, respectively, 20 s and 10 min. Finally, the strength of the hydrogel depends on the medium's final pH, with an elastic modulus fivefold higher at pH 3 than at pH 6. This article is part of the theme issue 'Bio-derived and bioinspired sustainable advanced materials for emerging technologies (part 1)'.

From bumblebee to bioeconomy: Recent developments and perspectives for sophorolipid biosynthesis

Sophorolipids are biobased compounds produced by the genera Starmerella and Pseudohyphozyma that gain exponential interest from academic and industrial stakeholders due to their mild and environmental friendly characteristics. Currently, industrially relevant sophorolipid volumetric productivities are reached up to 3.7 g∙L^-1∙h^-1 and sophorolipids are used in the personal care and cleaning industry at small scale. Moreover, applications in crop protection, food, biohydrometallurgy and medical fields are being extensively researched. The research and development of sophorolipids is at a crucial stage. Therefore, this work presents an overview of the state-of-the-art on sophorolipid research and their applications, while providing a critical assessment of scientific techniques and standardisation in reporting. In this review, the genuine sophorolipid producing organisms and the natural role of sophorolipids are discussed. Subsequently, an evaluation is made of innovations in production processes and the relevance of in-situ product recovery for process performance is discussed. Furthermore, a critical assessment of application research and its future perspectives are portrayed with a focus on the self-assembly of sophorolipid molecules. Following, genetic engineering strategies that affect the sophorolipid physiochemical properties are summarised. Finally, the impact of sophorolipids on the bioeconomy are uncovered, along with relevant future perspectives.

Synergy between Sophorolipid Biosurfactant and SDS Increases the Efficiency of P. aeruginosa Biofilm Disruption

Biofilms are communities of bacteria encased in self-secreted extracellular polymeric substances (EPS) that adhere stubbornly to submerged surfaces. Once established, these communities can cause serious chronic illnesses in medical settings, while they can promote corrosion and biofouling in industrial settings. Due to the difficulty of their removal, strongly oxidizing chemicals and detergents can be used to degrade and remove biofilms by killing the cells and degrading the matrix; however, the choice of compounds is limited in delicate environments due to the potential damage they may cause. In the case of detergents, most are synthesized from nonrenewable petrochemicals that have a degree of aquatic toxicity. There is a growing need to identify and characterize alternatives to synthetic surfactants. Biosurfactants, which are surfactants produced by microorganisms, are a promising alternative since they can be synthesized from renewable resources, have low environmental toxicity, and have been shown to have higher degrees of specificity in the mechanism of action. Sophorolipids are a class of glycolipid surfactants produced by yeast that have demonstrated great promise due to large yields from renewable feedstocks and for antimicrobial properties; however, the effect of the application of sophorolipids to Gram-negative bacterial biofilms has not been well studied. We investigate the antibiofilm properties of sophorolipids by demonstrating its ability to cause the catastrophic disruption of Pseudomonas aeruginosa PAO1 biofilms in microfluidic channels. We show that while sophorolipids inflict little damage to the bacteria, they weaken the EPS biofilm matrix, leading to surface-detachment and breakup of the biofilm. Furthermore, we find that sophorolipids act cooperatively with the widely used surfactant, sodium dodecyl sulfate. When combined, concentrations ∼100-fold lower than the minimum effective concentration, when used independently, recover potency. Biosurfactants are typically expensive to produce, thus our work demonstrates a means to improve efficacy while simultaneously reducing both cost and the amount of environmentally harmful substances used.

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.

Bio-based glyco-bolaamphiphile forms a temperature-responsive hydrogel with tunable elastic properties

A bio-based glycolipid bolaamphiphile (glyco-bolaamphiphile) has recently been produced (Van Renterghem et al., Biotechnol. Bioeng., 2018, 115, 1195-1206) on a gram scale by using the genetically-engineered S. bombicola strain Δat Δsble Δfao1. The glyco-bolaamphiphile bears two symmetrical sophorose headgroups at the extremities of a C16:0 (ω-1 hydroxylated palmitic alcohol) spacer. Its atypical structure has been obtained by redesigning the S. bombicola strain Δat Δsble, producing non-symmetrical glyco-bolaamphiphile, with an additional knock out (Δfao1) and feeding this new strain with fatty alcohols. The molecular structure of the glyco-bolaamphiphile is obtained by feeding the new strain a saturated C16 substrate (palmitic alcohol), which enables the biosynthesis of bolaform glycolipids. In this work, we show that the bio-based glyco-bolaamphiphile readily forms a hydrogel in water at room temperature, and that the hydrogel formation depends on the formation of self-assembled fibers. Above 28 °C, the molecules undergo a gel-to-sol transition, which is due to a fiber-to-micelle phase change. We provide a quantitative description of the Self-Assembled Fibrillar Network (SAFiN) hydrogel formed by the glyco-bolaampiphile. We identify the sol-gel transition temperature, the gelling time, and the minimal gel concentration; additionally, we explore the fibrillation mechanism as a function of time and temperature and determine the activation energy of the micelle-to-fiber phase transition. These parameters allow control of the elastic properties of the glyco-bolaamphiphile hydrogel: at 3 wt% and 25 °C, the elastic modulus G' is above the kPa range, while at 5 °C, G' can be tuned between 100 Pa and 20 kPa, by controlling the undercooling protocol.

Antibacterial properties of biosurfactants against selected Gram-positive and -negative bacteria

The antibacterial properties and ability to disrupt biofilms of biosurfactants (rhamnolipids, sophorolipids) and sodium dodecyl sulphate (SDS) in the presence and absence of selected organic acids were investigated. Pseudomonas aeruginosa PAO1 was inhibited by sophorolipids and SDS at concentrations >5% v/v, and the growth of Escherichia coli NCTC 10418 was also inhibited by sophorolipids and SDS at concentrations >5% and 0.1% v/v, respectively. Bacillus subtilis NCTC 10400 was inhibited by rhamnolipids, sophorolipids and SDS at concentrations >0.5% v/v of all three; the same effect was observed with Staphylococcus aureus ATCC 9144. The ability to attach to surfaces and biofilm formation of P. aeruginosa PAO1, E. coli NCTC 10418 and B. subtilis NCTC 10400 was inhibited by sophorolipids (1% v/v) in the presence of caprylic acid (0.8% v/v). In the case of S. aureus ATCC 9144, the best results were obtained using caprylic acid on its own. It was concluded that sophorolipids are promising compounds for the inhibition/disruption of biofilms formed by Gram-positive and Gram-negative microorganisms and this activity can be enhanced by the presence of booster compounds such as caprylic acid.

Sophorolipid biosurfactants: Possible uses as antibacterial and antibiofilm agent

Biosurfactants are amphipathic, surface-active molecules of microbial origin which accumulate at interfaces reducing interfacial tension and leading to the formation of aggregated micellular structures in solution. Some biosurfactants have been reported to have antimicrobial properties, the ability to prevent adhesion and to disrupt biofilm formation. We investigated antimicrobial properties and biofilm disruption using sophorolipids at different concentrations. Growth of Gram negative Cupriavidus necator ATCC 17699 and Gram positive Bacillus subtilis BBK006 were inhibited by sophorolipids at concentrations of 5% v/v with a bactericidal effect. Sophorolipids (5% v/v) were also able to disrupt biofilms formed by single and mixed cultures of B. subtilis BBK006 and Staphylococcus aureus ATCC 9144 under static and flow conditions, as was observed by scanning electron microscopy. The results indicated that sophorolipids may be promising compounds for use in biomedical application as adjuvants to other antimicrobial against some pathogens through inhibition of growth and/or biofilm disruption.

Environmental fate, toxicity, characteristics and potential applications of novel bioemulsifiers produced by Variovorax paradoxus 7bCT5

The aims of this work were the characterisation and the evaluation of potential environmental applications of the bioemulsifiers produced by Variovorax paradoxus 7bCT5. V. paradoxus 7bCT5 produces a mixture of high molecular weight polysaccharides. The extracellular bioemulsifiers were able to produce a thick stable oil/water emulsion and maintained the emulsification activity after boiling and at low temperatures. Environmental behavior and impact of bioemulsifiers release were assessed by evaluating biodegradability, toxicity and soil sorption. Respirometric tests showed that moderate biodegradability occurred by soil bacterial inoculum. Furthermore, the produced compounds did not show any toxic properties through different ecotoxicological tests. The K(d) values ranged from 1.3 to 7.3 L/kg indicating a high sorption affinity of the bioemulsifier molecules to soil particles. The soil sorption affinity likely affected the bioemulsifier ability to remove hydrocarbons from contaminated soils. In fact, V. paradoxus 7bCT5 bioemulsifiers significantly increased the removal of crude-oil from sandy soil compared to water.