The self-assembly in solution and adsorption at the air-water interface, measured by small-angle neutron scattering, SANS, and neutron reflectivity, NR, of the monorhamnose and dirhamnose rhamnolipids (R1, R2) and their mixtures, are discussed. The production of the deuterium-labeled rhamnolipids (required for the NR studies) from a Pseudomonas aeruginosa culture and their separation into the pure R1 and R2 components is described. At the air-water interface, R1 and R2 exhibit Langmuir-like adsorption isotherms, with saturated area/molecule values of about 60 and 75 Å(2), respectively. In R1/R2 mixtures, there is a strong partitioning of R1 to the surface and R2 competes less favorably because of the steric or packing constraints of the larger R2 dirhamnose headgroup. In dilute solution (<20 mM), R1 and R2 form small globular micelles, L(1), with aggregation numbers of about 50 and 30, respectively. At higher solution concentrations, R1 has a predominantly planar structure, L(α) (unilamellar, ULV, or bilamellar, BLV, vesicles) whereas R2 remains globular, with an aggregation number that increases with increasing surfactant concentration. For R1/R2 mixtures, solutions rich in R2 are predominantly micellar whereas solutions rich in R1 have a more planar structure. At an intermediate composition (60 to 80 mol % R1), there are mixed L(α)/L(1) and L(1)/L(α) regions. However, the higher preferred curvature associated with R2 tends to dominate the mixed R1/R2 microstructure and its associated phase behavior.

The use of small angle neutron scattering, SANS, neutron reflectivity, NR, and surface tension to study the mixing properties of the biosurfactant rhamnolipid with a conventional anionic surfactant, sodium dodecyl 6-benzene sulfonate, LAS, is reported. The monorhamnose rhamnolipid, R1, mixes close to ideally with LAS at the air-water interface, whereas for mixtures of LAS with the dirhamnose rhamnolipid, R2, the LAS strongly partitions to the air-water interface relative to R2, probably because of the steric hindrance of the larger R2 headgroup. These trends in the binary mixtures are also reflected in the ternary R1/R2/LAS mixtures. However, for these ternary mixtures, there is also a pronounced synergy in the total adsorption, which reaches a maximum for a LAS/rhamnolipid mole ratio of about 0.6 and a R1/R2 mol ratio of about 0.5, an effect which is not observed in the binary mixtures. In solution, the R1/LAS mixtures form relatively small globular micelles, L(1), at low surfactant concentrations (<20 mM), more planar structures (lamellar, L(α), unilamellar/multilamellar vesicles, ulv/mlv) are formed at higher surfactant concentrations for R1 and LAS rich compositions, and a large mixed phase (L(α)/L(1) and L(1)/L(α)) region forms at intermediate surfactant compositions. In contrast, for the R2/LAS mixtures, the higher preferred curvature of R2 dominates the phase behavior. The predominant microstructure is in the form of small globular micelles, except for solution compositions rich in LAS (>80 mol % LAS) where more planar structures are formed. For the ternary mixtures, there is an evolution in the resulting phase behavior from one dominated by L(1) (R2 rich) to one dominated by planar structures, L(α), (R1, LAS rich), and which strongly depends upon the LAS/rhamnolipid and R1/R2 mole ratio.

Biosurfactants are one of the numerous adaptations of microorganisms metabolizing hydrocarbons and broadly represent a physiological response to specific requirements encountered by the cells depending on their environment. Some bacteria have developed the ''pseudosolubilization'' strategy to gain access to poorly soluble substrates and therefore produce highly dynamic low-molecular mass biosurfactants characterized by the capability to self-assembly in micelles, hemi-micelles or aggregates. Other bacteria interact with hydrocarbons directly by means of wall-bound biosurfactants that confer on the cell surface the appropriate hydrophobicity. High molecular mass bioemulsifiers in general adsorb tightly and cover the hydrocarbons thus dramatically increasing their apparent solubility. Although a variety of mechanisms and specializations distinguish biosurfactants, they all share a few essential traits. They are part of the process of interaction of microbial cells with surfaces as well as substrates. In addition to working on hydrocarbon solubilization they also act at the level of the cellular outer membrane allowing temporary and reversible modifications that control substrate access by the cells. As a consequence, a ''substrate effect'' can often be observed. Although the biosynthetic mechanisms for most remain unclear, substrates seem able to influence structural variations that make the biosurfactants particularly active towards the same substrate. Finally, many biosurfactants and their producing strains stimulate the growth of differently specialized bacteria which suggests that these molecules play a vital role in the interaction between the oil-degrading microbial communities and their environment.

Approximately 80% of the Earth's biosphere is cold, at an average temperature of 5°C, and is populated by a diversity of microorganisms that are a precious source of molecules with high biotechnological potential. Biosurfactants from cold-adapted organisms can interact with multiple physical phases - water, ice, hydrophobic compounds, and gases - at low and freezing temperatures and be used in sustainable (green) and low-energy-impact (cold) products and processes. We review the biodiversity of microbial biosurfactants produced in cold habitats and provide a perspective on the most promising future applications in environmental and industrial technologies. Finally, we encourage exploring the cryosphere for novel types of biosurfactants via both culture screening and functional metagenomics.

Thermophilic bacterial cultures were isolated from a hot spring environment on hydrocarbon containing mineral salts media. One strain identified as Pseudomonas aeruginosa AP02-1 was tested for the ability to utilize a range of hydrocarbons both n-alkanes and polycyclic aromatic hydrocarbons as sole carbon source. Strain AP02-1 had an optimum growth temperature of 45 degrees C and degraded 99% of crude oil 1% (v/v) and diesel oil 2% (v/v) when added to a basal mineral medium within 7 days of incubation. Surface activity measurements indicated that biosurfactants, mainly glycolipid in nature, were produced during the microbial growth on hydrocarbons as well as on both water-soluble and insoluble substrates. Mass spectrometry analysis showed different types of rhamnolipid production depending on the carbon substrate and culture conditions. Grown on glycerol, P. aeruginosa AP02-1 produced a mixture of ten rhamnolipid homologues, of which Rha-Rha-C10-C10 and Rha-C10-C10 were predominant. Rhamnolipid-containing culture broths reduced the surface tension to approximately 28 mN and gave stable emulsions with a number of hydrocarbons and remained effective after sterilization. Microscopic observations of the emulsions suggested that hydrophobic cells acted as emulsion-stabilizing agents.