The Adsorption and Self-Assembly of Mixtures of Alkylbenzene Sulfonate Isomers and the Role of Divalent Electrolyte

Abstract

In this paper, the role of the different structural isomers of the anionic surfactant sodium para-dodecyl benzene sulfonate, LAS, on surface adsorption and solution self-assembly has been studied. Using a combination of neutron reflectivity, NR, and small angle neutron scattering, SANS, the effect of mixing an isomer with a short symmetric hydrocarbon chain with one which has an asymmetric hydrocarbon chain on both the equilibrium surface adsorption behavior and the solution microstructure of the mixtures, both in the presence and absence of a divalent cation (Ca(2+)), has been investigated. In the absence of electrolyte, the LAS isomer mixtures form small charged globular micelles throughout the composition range studied. The micelle aggregation number increases with the increase in the asymmetric isomer content, reflecting an increase in the packing efficiency within the micelle. The addition of calcium ions promotes the formation of planar aggregates, as multilamellar vesicles, but only when the symmetric LAS isomer is the major component of the mixture. At a surfactant concentration just above the critical micelle concentration, CMC, and in the absence of electrolyte, the variation in the surface composition is close to the solution composition. Regular solution theory, RST, calculations show that this variation is also close to what is expected for ideal mixing. The addition of Ca(2+) ions induces a different surface behavior, resulting in the formation of multilayer structures at the interface throughout the entire composition range.

Mechanism of biosurfactant adsorption to oil/water interfaces from millisecond scale tensiometry measurements

“…Therefore, this model does not fit the data (electronic supplementary material, figure S3). Furthermore, earlier works [28,29] suggest that surfactants' micelle diameter are usually twice the thickness of the adsorption layer, which indicates that it has to be the individual molecules rather than their aggregates that adsorb to the interface. A previous study [28] found that at pH 7.5 surfactin adopts a ball-like structure with a thickness of 14 Å at the air/ water interface and the overall structure of the adsorbed surfactin layer appears to be identical to a thickness of 15 Å at a hydrophobic octadecyltrichlorosilane-coated silicon surface.…”

“…The predicted curves for small micelles (n ¼ 3) are shown in electronic supplementary material, figure S4 and for bigger micelles (n ¼ 20) in electronic supplementary material, figure S5. According to the work of Shen et al [28] and Tucker et al [29], we apply the different values of n for the surfactants as in table 2. For the SCBS surfactants, which are the homologue of sodium dodecyl benzene sulfonate with four more carbon atoms, the size of aggregates in the bulk solution is around 100 nm, which indicates a large aggregation number [27].…”

“…They further showed that the micellar structure of surfactin was a sphere with an overall diameter of 50 Å , which is similar to our DLS results with a diameter of 4.45 nm. Earlier work [29] showed that the thickness of the adsorption layer of symmetric and asymmetric sodium para-dodecyl benzene sulfonate, LAS6 ( phenyl ring is joined to the middle of the alkyl chain) and LAS4 (phenyl ring is functionalized at the 'C4' position), ranged from 20 to 31 Å at the air/water interface by a single layer. Small angle neutron scattering data on this system [29] showed that LAS4 and LAS6 are elliptical with an inner core radius of around 15 Å , an outer shell radius around 17 Å and an elliptical ratio of approximately 1.5.…”

Mathematical Modelling of Surfactant Self-assembly at Interfaces

“…Typically, mixed surfactant systems exhibit more pronounced interfacial adsorption than single surfactant systems [7]. Furthermore, the addition of an electrolyte to a mixed surfactant solution significantly may increase the concentration of surfactant adsorbed at the interface compared with the solution in the absence of electrolyte [9,10,14]. In such cases, surfactant is observed to adsorb not only as a monolayer at an interface but as a multilayer structure that extends into the bulk beneath the interface [9].…”

“…In this paper we present a mathematical model for the self-assembly of surfactant in a multilayer structure such as those observed in [9,10,14], neglecting the effect of any counterions that may be present and any bulk solution effects such as aggregation. We consider two situations: (i) the adsorption of a surfactant multilayer at an interface and (ii) the adsorption of a surfactant solution onto two surfaces that are in close proximity to one another.…”