Equilibrium Surface Adsorption Behavior in Complex Anionic/Nonionic Surfactant Mixtures

Abstract

Neutron reflectivity (NR) and small angle neutron scattering (SANS) have been used to investigate the equilibrium surface adsorption behavior and the solution microstructure of mixtures of the anionic surfactant sodium 6-dodecyl benzene-4 sulfonate (SDBS) with the nonionic surfactants monododecyl octaethylene glycol (C12EO8) and monododecyl triiscosaethylene glycol (C12EO23). In the SDBS/C12EO8 and SDBS/C12EO23 solutions, small globular mixed micelles are formed. However, the addition of Ca2+ ions to SDBS/C12EO8 results in a transition to a vesicle phase or a mixed vesicle/micellar phase for SDBS rich compositions. In contrast, this transition hardly exists for the SDBS/C12EO23 mixture, and occurs only in a narrow composition region which is rich in SDBS. The adsorption of the SDBS/C12EO8 mixture at the air-solution interface is in the form of a mixed monolayer, with a composition variation that is not consistent with ideal mixing. In water and in the presence of NaCl, the nonideality can be broadly accounted for by regular solution theory (RST). At solution compositions rich in SDBS, the addition of Ca2+ ions results in the formation of multilayer structures at the interface. The composition range over which multilayer formation exists depends upon the Ca2+ concentration added. In comparison, the addition of a simple monovalent electrolyte, NaCl, at the same ionic strength does not have the same impact upon the adsorption, and the surface structure remains as a monolayer. Correspondingly, in solution, the mixed surfactant aggregates remain as relatively small globular micelles. In the presence of Ca2+ counterions, the variation in surface composition with solution composition is not well described by RST over the entire composition range. Furthermore, the mixing behavior is not strongly correlated with variations in the solution microstructure, as observed in other related systems.

The role of electrolyte and polyelectrolyte on the adsorption of the anionic surfactant, sodium dodecylbenzenesulfonate, at the air–water interface

“…In the absence of PEI and at higher LAS concentrations (2 mM) the addition of 1-2 mM CaCl 2 resulted in surface multilayer formation [22]. However, there is no evidence for such extreme forms of surface layering in the data presented here.…”

“…However, when comparing directly in terms of ionic strength and where there is overlap in the data, it would seem that NaCl and CaCl 2 have a similar impact upon the surface adsorption and surface structure. This was not the case for the formation of surface multilayer structures with LAS in the absence of PEI [22], where in that case surface multilayer formation was induced by CaCl 2 , but not by NaCl at the same ionic strength. In that context it is clear that the original arguments about the effect of electrolyte on SDS/PEI adsorption [17] hold here.…”

“…1, surfactant was synthesised in two forms, with and without the dodecyl alkyl chains and phenyl ring deuterium labelled (referred to as d-LAS and h-LAS respectively), using a synthetic route and characterisation described in detail elsewhere [22]. The chemical purity of the surfactants was checked using the surface tension measurement, and no minimum in the surface tension was observed at the cmc of 1.5 mM.…”

Enhanced perfume surface delivery to interfaces using surfactant surface multilayer structures

“…The anionic surfactant LAS, sodium dodecyl-6-benzenesulphonate, was custom synthesized at Oxford and Unilever Research and Development [27] as the near symmetrical isomer with the phenyl ring joined at the middle of the C 12 alkyl chain (C 6 position) and is referred to here as LAS-6. This applies to both the hydrogeneous and deuterated surfactant, and for the deuterated surfactant the alkyl chain and phenyl ring were both deuterium labeled.…”

“…Of particular relevance to this work is the surface multilayer formation for sodium dodecyl-6-benzenesulfonate, LAS-6, with CaCl 2 [27] and for sodium dodecyldioxyethylenesulfate, SLES, with AlCl 3 [28]. Depending on the surfactant and counterion concentrations surface multilayer structures with numbers of bilayers ranging from 1 to >30 can be formed at the surface.…”