Influence of calcium ions on rhamnolipid and rhamnolipid/anionic surfactant adsorption and self-assembly.

Journal article

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: Chen, M; Dong, C; Penfold, J; Thomas, R; Smyth, T

• Publication status: Published
• Journal: Langmuir : the ACS journal of surfaces and colloids
• Volume: 29
• Issue: 12
• Pages: 3912-3923
• Publication date: 2013-03-01
• DOI: 10.1021/la400432v
• EISSN: 1520-5827
• ISSN: 0743-7463
• Language: English
• Keywords: Adsorption; Solutions; Cations, Divalent; Glycolipids; Calcium; Rhamnose; Benzenesulfonates; Surface Tension; Air; Surface-Active Agents; Water; Micelles; Molecular Conformation; Hydrogen-Ion Concentration