What Is So Special about Aerosol-OT? 2. Microemulsion Systems

Abstract

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.

Effects of small ionic amphiphilic additives on reverse microemulsion morphology

“…This behaviour appears to gradually change with increasing alkanoate tail length. The AOT microemulsions shift stability region with increasing alkanoate chain length to lower temperatures and closer to the temperature range of stability for normal (hydrotrope-free) AOT microemulsions [19]. The longer the tail length the greater the 'alkyl' character of the additive and a stronger positive interaction between the additive and AOT tails is expected.…”

“…Because hydrotropes do not partition strongly into oil (they are only weakly hydrophobic), at low water content the effective hydrotrope concentration in the water pools is high, and hence they might be expected to adsorb strongly at the interface. For a fixed mole fraction x of hydrotrope in the total microemulsion, increasing the water content w decreases the additive concentration in the water droplets, and eventually the micellar geometry change to spherical, as found for the native AOT (hydrotrope-free) systems [19]. However, the extent of axial elongation did not appear to directly correlate to additive architecture, with some less hydrophobic additives inducing longer cylinders as compared to the more hydrophobic homologues and analogues.…”

“…Å and 32.9 Å for w = 10 and w = 20 respectively [19]. The presence of the additives causes a noticeable decrease in the mean radii of the microemulsions for any w value, at the same time the polydispersity increases with water content.…”

“…Reverse AOT/n-heptane/D2O microemulsions are known to be spherical with radius increasing proportionally with water content w [19]. In a previous study alkylhydrotropes induced a varying degree of axial elongation with rods and ellipsoids observed at w values where the corresponding concentration of the additive was above or in close proximity of the corresponding bulk critical aggregation concentration (cac or critical micelle concentration cmc), and below which the microemulsions became spherical [18].…”

“…Linear dependences of fitted radii with w for spherical microemulsions. The slope and intercept of the radii of the equations shown in Table 5 The Porod method for determining radii and head-group areas considers the scattering intensity I(Q) reporting from local interface at high Q values [19].…”