title: Impact of Electrolyte on Adsorption at the Air-Water Interface for Ternary Surfactant Mixtures above the Critical Micelle Concentration.
abstract: The composition of the air-water adsorbed layer of the ternary surfactant mixture, octaethylene monododecyl ether, C12E8, sodium dodecyl 6-benzenesulfonate, LAS, and sodium dioxyethylene glycol monododecyl sulfate, SLES, and of each of the binary mixtures, with varying amounts of electrolyte, has been studied by neutron reflectivity. The measurements were made above the mixed critical micelle concentration. In the absence of electrolyte adsorption is dominated by the nonionic component C12E8but addition of electrolyte gradually changes this so that SLES and LAS dominate at higher electrolyte concentrations. The composition of the adsorbed layer in both binary and ternary mixtures can be quantitatively described using the pseudo-phase approximation with quadratic and cubic interactions in the excess free energy of mixing (GE) at both the surface and in the micelles. A single set of parameters fits all the experimental data. A similar analysis is effective for a mixture in which SDS replaces SLES. Addition of electrolyte weakens the synergistic SLES-C12E8and LAS-C12E8interactions, consistent with them being dominated by electrostatic interactions. The SLES-LAS (and SDS-LAS) interaction is moderately strong at the surface and is little affected by addition of electrolyte, suggesting that it is controlled by structural or packing factors. Most of the significant interactions in the mixtures are unsymmetrical with respect to composition, with the minimum in GEat the 1:2 or 2:1 composition. There is a small structural contribution to the LAS-C12E8interaction that leads to a minimum intermediate in composition between 1:2 and 1:1 (LAS:C12E8) and to a significant residual GEin strong electrolyte.
authors.affiliation: Physical and Theoretical Chemistry Laboratory, Oxford University , South Parks Road, Oxford OX1 3QZ, United Kingdom.
authors.forename: Jessica R
authors.initials: JR
authors.lastname: Liley
authors.affiliation: Physical and Theoretical Chemistry Laboratory, Oxford University , South Parks Road, Oxford OX1 3QZ, United Kingdom.
authors.forename: Robert K
authors.identifier: 0000-0002-0539-0859
authors.initials: RK
authors.lastname: Thomas
authors.affiliation: Physical and Theoretical Chemistry Laboratory, Oxford University , South Parks Road, Oxford OX1 3QZ, United Kingdom.|Rutherford-Appleton Laboratory , Chilton, Didcot, Oxon OX11 0RA, United Kingdom.
authors.forename: Jeffrey
authors.initials: J
authors.lastname: Penfold
authors.affiliation: Unilever Research and Development Laboratory , Port Sunlight, Quarry Road East, Bebington, Wirral CH63 3JW, United Kingdom.
authors.forename: Ian M
authors.initials: IM
authors.lastname: Tucker
authors.affiliation: Unilever Research and Development Laboratory , Port Sunlight, Quarry Road East, Bebington, Wirral CH63 3JW, United Kingdom.
authors.forename: Jordan T
authors.initials: JT
authors.lastname: Petkov
authors.affiliation: Unilever Research and Development Laboratory , Port Sunlight, Quarry Road East, Bebington, Wirral CH63 3JW, United Kingdom.
authors.forename: Paul
authors.initials: P
authors.lastname: Stevenson
authors.affiliation: STFC, Rutherford-Appleton Laboratory , Chilton, Didcot, Oxfordshire OX11 0RA, United Kingdom.
authors.forename: John R P
authors.initials: JRP
authors.lastname: Webster