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Properties are reported for new phenyl-tipped anionic surfactants, which are aromatic chain relatives of the normal aliphatic aerosol-OT (AOT, sodium bis(2-ethyl-1-hexyl)sulfosuccinate). Variations in chain length and branching with these aromatic surfactants have important effects on aqueous and water-in-oil (w/o) microemulsion phase properties. In dilute aqueous systems, chain structure affects the cmc and surface tension behavior:  compared to linear chain analogues, the branched-chain surfactants display lower surface tensions but also reduced packing as measured by molecular area at the cmc acmc. Owing to the phenyl-tipped structure, water-in-oil microemulsions were stabilized with aromatic toluene as an oil but not with aliphatic heptane; the latter is commonly used with normal AOT. Contrast variation small-angle neutron scattering (SANS) was used to characterize the microemulsion aggregates and adsorbed films. These SANS data show that water-in-toluene microemulsions stabilized by aromatic-AOTs contain mildly polydisperse spherical nanodroplets of similar structure to those found in systems containing normal AOT. Molecular areas at the air−water and toluene−water interfaces are found to be of similar magnitude and follow a trend that correlates with variations in surfactant chain structure. The new results with aromatic surfactants build on extensive studies of aliphatic AOT analogues (Nave, S.; Eastoe, J.; Penfold, J. Langmuir 2000, 16, 8733. Nave, S.; Eastoe, J.; Heenan, R. K.; Steytler, D.; Grillo, I. Langmuir 2002, 16, 8741. Nave, S.; Eastoe, J.; Heenan, R. K.; Steytler, D.; Grillo, I. 2002, 18, 1505), suggesting that the versatility of normal AOT originates from an optimized head and chain spacer group rather than from any specific effects of the 2-ethyhexyl chain structure.

Surfactant mixing in model water-in-heptane microemulsion interfaces has been investigated for blends of a cationic didodecyldimethylammonium bromide (DDAB) with poly(ethylene glycol) monododecyl ethers (C12EJ, J = 3, 4, 5, 6, 7, 8, and 23). Phase behavior studies, electrical conductivity, and contrast variation small-angle neutron scattering (SANS) have been employed to delineate the effects of systematic variation of ethylene oxide headgroup size. These mixtures are characterized by an overall surfactant concentration (0.10 mol dm^-3) and mole fraction of nonionic, which was varied up 0.20. The larger ethylene oxide (EO) numbers of 5−7 and 23 lead to significant enhancements in the maximum microemulsion solubilization capacity compared to DDAB only, whereas the shortest surfactant employed, C12E3, caused a decrease in the phase stability. Microemulsion nanostructure and interfacial compositions were studied for the EO3, EO4, EO6, and EO7 systems in partial structure factor type SANS experiments, as described before for the EO5 analogue (Langmuir 1999, 15, 5271). Analysis of contrast variation SANS data showed that C12E7, C12E6, and C12E5 partition strongly into the DDAB layer. Under equivalent conditions the shorter EO chain surfactants C12E4 and C12E3 appear to adsorb much more weakly. Interfacial compositions determined by SANS have been used to rationalize trends in phase behavior and nanostructure, highlighting the importance of partitioning effects with nonionics in multicomponent mixtures of this type.

Syntheses, adsorption, and aggregation properties of two novel double-chain anionic sulfoglutaconate surfactants are described. These compounds are similar to the more common sulfosuccinates, such as Aerosol-OT (see Parts I and II of this study, Nave et al. Langmuir 2000, 16, 8733−8748), but with a modified hydrophilic moiety bearing one extra −CH2− spacer. For comparison purposes, two related linear n-hexyl- and branched 2-ethylhexyl- hydrophobic groups were used to form the surfactants, which are denoted di-C6GLU and AOTGLU. The planar air−aqueous solution surface was characterized by drop volume tensiometry, whereas small-angle neutron scattering and spinning-drop tensiometry were used to study microemulsions and oil−water interfaces. Dilute binary surfactant water and ternary water-in-oil microemulsion phase behavior is also described. In terms of molecular packing both glutaconate and sulfosuccinate systems are found to be very similar, showing the minor role of the headgroup compared to larger effects of the tail structure, as noted before in Parts I and II. Interestingly, the AOTGLU proved to be even more efficient than normal AOT in terms of its capacity to microemulsify water in heptane. The results point to subtle effects of hydrophilic group structure on physicochemical properties of these AOT-type surfactants, especially in microemulsions.

To identify why Aerosol-OT (sodium bis(2-ethylhexyl) sulfosuccinate) is such a versatile surfactant, adsorption and aggregation in aqueous solutions have been investigated with 11 different AOT-related compounds. The first set, denoted di-CnSS with n = 4, 5, 6, 7, and 8, contain linear chains. In addition six different branched chain systems have been studied, including Aerosol-OT itself. These surfactants span extremes of branching, from 3,5,5-trimethyl-1-hexyl, through 2-ethyl-1-hexyl (AOT), to 1-ethyl-2-methyl-1-pentyl (see Figure 1), and therefore it is possible to delineate structure−performance relationships. For one model compound, di-C6SS, adsorption studies were made with two complementary techniques, drop volume tensiometry (DVT) and neutron reflection (NR). The tensiometrically derived area per molecule at the critical micelle concentration Acmc was 60 ± 3 Ų, which is in excellent agreement with that obtained directly by neutrons, 59 ± 2 Ų. The procedures for obtaining a sufficiently high surface chemical purity and the application of the Gibbs equation, to achieve agreement between DVT and NR, are discussed. Partial structure factor experiments with di-C6SS gave quantitative information on the interfacial structure in terms of surfactant and solvent distributions, and the results are compared with those obtained previously for AOT (Prog. Colloid Polym. Sci. 1995, 98, 243 and J. Phys. Chem. B 1997, 101, 1615). For the di-CnSS systems classic effects of chain carbon number on surface tension behavior, critical micelle concentrations (cmc's), and Acmc were observed. With the branched chain compounds a significant increase in Acmc was found, between 10 and 20 Ų, over those for equivalent carbon number straight chain systems. Furthermore, slight variations in Acmc were detected, reflecting changes in packing owing to differing extents of chain branching. The efficiency of packing was correlated with chain structure by introducing an empirical branching factor. In dilute aqueous systems regular Aerosol-OT behaves in a readily understandable fashion, fitting into the general pattern of behavior for all these sulfosuccinates:  no special effects due to the 2-ethyl-1-hexyl chains were noted.

The aim was to identify why Aerosol-OT is such an efficient surfactant for forming microemulsions, and in pursuit of this, 11 Aerosol-OT-related compounds have been investigated. These surfactants were from two separate homologous series, with either linear or branched hydrocarbon tails. Hence, it was possible to examine the effect of chain structure on packing in curved interfacial films at oil−water interfaces. With the linear dichain compounds, water-in-oil microemulsion phases could be formed only in the presence of a short-chain alcohol. On the other hand, all the branched surfactants formed microemulsions without cosurfactant. Within the range of structures studied, it was possible to identify a minimum branching necessary to stabilize a ternary microemulsion. With branched sulfosuccinates the single-phase microemulsion region only differed by its location on the temperature scale, and this was correlated with subtle variations in hydrophobicity, caused by the different chain structures. Small-angle neutron scattering was used to characterize the microemulsion aggregates and adsorbed films. Packing at the oil−water interface, and the water droplet radius, was shown to be related to chain architecture, and the same pattern of behavior was found as at the air−water interface (see the preceding paper in this issue). Therefore, with regard to microemulsion formation Aerosol-OT is no special case, but it possesses a chain structure that gives optimum aqueous-phase solubility around room temperature.

Neutron reflection (NR) and surface tension methods were compared for accessing the equilibrium adsorption isotherm of various ionic surfactants at the air−water interface. Four custom-synthesized anionics were investigated in detail:  sodium dihexyl sulfosuccinate (di-C6SS), bis(1H,1H-perfluoro-n-pentyl)sodium sulfosuccinate (di-CF4), bis(1H,1H,5H-octafluoro-n-pentyl)sodium sulfosuccinate (di-HCF4), and bis(1H,1H,5H-octafluoropentyl)-2-sulfoglutaconate (di-HCF4GLU). Commercial n-alkyltrimethylammonium bromide (Cn-TAB) cationic surfactants with C12, C14, and C16 chain lengths were also studied. The experiments examined the validity of the Gibbs equation, and the prefactor 2, for these seven compounds. Effects of contaminants, trace levels of polyvalent metal ions, and hydrophobic impurities were assessed for the anionics. When added at low levels, tetrasodium ethylenediaminetetraacetate (EDTA) was effective for eliminating effects of metallic impurities, and foam fractionation was used to remove hydrophobic contaminants. The effects of these treatments on the apparent surface excess and procedures for obtaining agreement between the neutronic and tensiometric isotherms are described. Finally, it was confirmed that the Gibbs prefactor of 2 applies for all these 1:1 ionic surfactants.