Anionic surfactants are the major surface active ingredient in most home and personal care products. The ester sulfonate surfactants are a class of anionic surfactants that exhibit increased tolerance to hard water, with a reduced tendency to precipitation in the presence of multivalent counterions. However, the addition of multivalent counterions results in the formation of multilayer structures at the air-solution interface, and is accompanied by advantageous wetting properties.
Changing the surfactant geometry, by altering the headgroup or the structure of the alkyl chain, can have a significant impact upon the nature of the surface adsorption induced by multivalent counter-ions. This study further investigated the impact of the changes in geometry by comparing surfactants with a branched isostearic alkyl chain to a linear saturated alkyl chain, and by modifying the headgroup from methyl- to ethyl-ester, in the MES (methyl ester sulfonate) surfactant.
Using neutron reflectivity on the SURF reflectometer the researchers found that, for the surfactants studied; in contrast to MES, more limited surface multilayer formation is observed with the addition of Al3+ counter-ions. This implies that the original Na+ surfactant counter-ions are more strongly bound, and are therefore less readily displaced by the Al3+ ions.
Their results provide a crucial insight into how these subtle changes in surfactant geometry and structure can be used to manipulate their interaction with cations, and their associated surface properties.
Instrument: SURF, Deuteration Facility
Related publication: “Impact of molecular structure, headgroup and alkyl chain geometry, on the adsorption of the anionic ester sulfonate surfactants at the air-solution interface, in the presence and absence of electrolyte” Journal of Colloid and Interface Science 544 (2019) 293–302
Funding: China Scholarship Council
Authors: Z Wang (China University of Petroleum, ISIS), P Li, K Ma, Y Chen, M Campana (ISIS), J Penfold (ISIS, University of Oxford), RK Thomas (University of Oxford), DW Roberts (Liverpool John Moores University), H Xu (KLK, OLEO), JT Petkov (Lonza UK), Z Yan (China University of Petroleum)