Mixing Behavior of the Biosurfactant, Rhamnolipid, with a Conventional Anionic Surfactant, Sodium Dodecyl Benzene Sulfonate

Abstract

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.

Microbiotechnology Based Surfactants and Their Applications

“…In buffer the I 3 /I 1 ratio remains stable at 0.6 until 0.1 mM RL, after which it increases to ∼1.05 at 1 mM RL ( Figure 1B). This concentration range is in good agreement with other studies which report cmc values of rhamnolipid mixtures between 0.1 and 1 mM (Sanchez et al, 2007;Chen et al, 2010). Neither CZ nor LT affected the pyrene fluorescence pattern, indicating that the enzymes do not aid RL micelle formation below the cmc.…”

Evolution of Aggregate Structure in Solutions of Anionic Monorhamnolipids: Experimental and Computational Results

“…9,14 Thus, mRLs were studied with DLS across a range of concentrations between 0.1 and 100 mM to further explore this behavior. The size and population abundance of aggregates observed as a function of mRL concentration are shown in Figure S3.…”

“…9–12 Given the potential of rhamnolipids as green surfactants, a thorough understanding of both their interfacial and bulk solution aggregation properties is needed to advance their utilization.…”

“…10,13–15 Although multiple researchers have reported critical micelle concentration (CMC) values in the micromolar regime from surface tensiometry, 14,16,17 and dynamic light scattering (DLS) has provided insight into solution aggregate size, with aggregates of hydrodynamic radii ranging from several nanometers to a few hundred nanometers reported, 10,14,17 detailed information about the early stages of aggregation and the growth of aggregate size as solution concentration increases is missing. Although small-angle neutron scattering (SANS) studies have provided some definitive information about aggregate size and shape, results have only been reported for solutions 20 mM and higher in concentration, 9,10,15,18,19 with aggregates of ~47 monomers reported for 20 mM monorhamnolipids in pH 9 aqueous borax buffer. 10 The results at 20 mM are consistent with the small micelles observed using cryo-TEM in solutions of basic pH; 20 at higher concentrations, SANS results indicate the presence of lamellar structures.…”

Stimulation of rhamnolipid biosurfactants production in Pseudomonas aeruginosa AK6U by organosulfur compounds provided as sulfur sources

“…It is also tempting to propose that the type of rhamnolipids (congener profile) produced in the 4,6-DM-DBT cultures facilitated better access to the substrate and consequently, enhanced the growth rate in the 4,6-DM-DBT cultures. The congeners fingerprint has been reported to influence the physicochemical properties of rhamnolipid biosurfactants [12], [14]. Biosurfactants can also be detrimental to growth depending on the mode of action.…”