Solution Self-Assembly and Adsorption at the Air−Water Interface of the Monorhamnose and Dirhamnose Rhamnolipids and Their Mixtures

Abstract

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.

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

“…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. 10 Work investigating the early stages of aggregation, and a detailed, systematic analysis of the evolution of aggregate structure with concentration is lacking in the literature.…”

“…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. 10 Work investigating the early stages of aggregation, and a detailed, systematic analysis of the evolution of aggregate structure with concentration is lacking in the literature.…”

“…10,14,16,17,20 Thus, a need still exists to fully characterize the aggregation properties of these surfactants in solution as well as to probe the chemical microenvironments within these aggregates, especially at concentrations below ~20 mM. Toward this end, we report the combined use of surface tensiometry, fluorescence spectroscopy, 21,22 fluorescence quenching, 23,24 and DLS combined with all-atom molecular dynamics (MD) simulations to better understand the aggregation properties of anionic monorhamnolipids in aqueous solution at basic pH.…”

pH-Driven Self-Assembly of Acidic Microbial Glycolipids

“…21,41 Nevertheless, the transition between the high pH, low-scattering, to the lowpH, high-scattering, regimes occurs at sensibly different pH values among the compounds. If this fact could seem odd because of the strong resemblance of the fatty acid tails for all compounds, one should not forget that saturated and unsaturated (poly or mono) fatty acids do not necessarily have the same pKa.…”

Chemical characterization and surface properties of a new bioemulsifier produced by Pedobacter sp. strain MCC-Z

“…Many prokaryotic and eukaryotic microorganisms synthesize a wide range of surface active compounds (SACs) which exhibit surface activities at interfaces, including the ability to lower surface and interfacial tension of liquids and to form micelles and microemulsions between different phases [1][2][3]. They exist in a wide variety of structurally different amphiphilic molecules containing both hydrophilic and hydrophobic units.…”