title: Influence of calcium ions on rhamnolipid and rhamnolipid/anionic surfactant adsorption and self-assembly.
abstract: The impact of Ca(2+) counterions on the adsorption at the air-water interface and self-assembly in aqueous solution of the rhamnolipid biosurfactant and its mixture with the anionic surfactant sodium dodecylbenzenesulfonate, LAS, has been studied using neutron reflectometry and small-angle neutron scattering. The results illustrate how rhamnolipids are calcium tolerant and how their blending with conventional anionic surfactants improves the calcium tolerance of the anionic surfactant. Ca(2+) has relatively little effect upon the adsorption and self-assembly of the monorhamnose, R1, and dirhamnose, R2, rhamnolipids, even at high pH, due to their predominantly nonionic nature. For R1/R2 mixtures the addition of Ca(2+) has little impact upon the adsorbed amount or the surface composition. For R2/LAS mixtures the addition of Ca(2+) results in an increased adsorption and a surface slightly richer in R2. The weak binding of Ca(2+) to R1 and R2 does result in a change to the degree of ionization of the micelles and especially for mixed R1/R2 micelles at R1-rich solution compositions. The stronger binding of Ca(2+) to LAS results in the addition of Ca(2+) having a much greater impact on the self-assembly of R1/LAS and R2/LAS mixtures. For R1/LAS mixtures the addition of Ca(2+) promotes the formation of more planar structures, even at low surfactant concentrations where in the absence of Ca(2+) mixed globular micelle formation dominates. In R2/LAS mixtures, where there is a greater contrast between the high and low preferred curvatures associated with R2 and LAS, the addition of Ca(2+) results in a more complex evolution in micellar aggregation and the degree of ionization of the micelles. This results in variations in Ca(2+) binding that promotes micellar structures in which a spatial segregation of the two surfactant components within the micelle occurs.
authors: Physical and Theoretical Chemistry Laboratory, Oxford University, Oxford, UK.
authors: Minglei
authors: M
authors: Chen
authors: Chuchuan
authors: C
authors: Dong
authors: Jeff
authors: J
authors: Penfold
authors: Robert K
authors: RK
authors: Thomas
authors: Thomas J P
authors: TJ
authors: Smyth
authors: Amedea
authors: A
authors: Perfumo
authors: Roger
authors: R
authors: Marchant
authors: Ibrahim M
authors: IM
authors: Banat
authors: Paul
authors: P
authors: Stevenson
authors: Alyn
authors: A
authors: Parry
authors: Ian
authors: I
authors: Tucker
authors: I
authors: I
authors: Grillo
mesh_terms: D000327:Adsorption; D000388:Air; D001557:Benzenesulfonates / Q000737:chemistry*; D002118:Calcium / Q000737:chemistry*; D002413:Cations, Divalent; D006017:Glycolipids / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D008823:Micelles; D008968:Molecular Conformation; D012210:Rhamnose / Q000737:chemistry*; D012996:Solutions; D013500:Surface Tension; D013501:Surface-Active Agents / Q000737:chemistry*; D014867:Water
chemical_list: D001557:Benzenesulfonates; D002413:Cations, Divalent; D006017:Glycolipids; D008823:Micelles; D012996:Solutions; D013501:Surface-Active Agents; C418382:rhamnolipid; D014867:Water; C001114:dodecylbenzenesulfonic acid; D012210:Rhamnose; D002118:Calcium