Investigation of Nanostructure and Interactions in Water-in-Xylene Microemulsions Using Small-Angle X-ray and Neutron Scattering

Abstract

The ability to modulate the size, the nanostructure, and the macroscopic properties of water-in-oil microemulsions is useful for a variety of technological scenarios. To date, diverse structures of water-in-alkane microemulsions stabilized by sodium bis(2-ethylhexyl) sulfosuccinate (AOT) have been extensively studied. Even though the decisive parameter which dictates the phase behavior of micremulsions is the nature of the continuous phase, relatively very few reports are available on the structure and interactions in the microemulsions of aromatic oil. Here, we present a fundamental investigation on water-in-xylene microemulsions using small-angle neutron scattering (SANS) at a fixed molar ratio (ω) of water to AOT. We elucidate the microstructural changes in the water-AOT-xylene ternary system at dilute volume fractions (Φ = 0.005, 0.01, 0.03), where the droplet−droplet interactions are absent, to moderately concentrated systems (Φ = 0.05, 0.10, 0.15, and 0.20), where colloidal interactions become important. We also characterize the reverse microemulsions (RMs) for thermally induced microstructural changes at six different temperatures from 20 to 50 °C. Depending on the magnitude of Φ, the scattering data is found to be well described by considering the RMs as a dispersion of droplets (with a Schulz polydispersity) which interact as sticky hard spheres. We show that while the droplet diameter remains almost constant with increase in the volume fraction, the attractive interactions become prominent, much like the trends observed for water-in-alkane microemulsions. With increase in temperature, the RMs showed a marginal decrease in the droplet size but no pronounced dependence on the interactions was observed with the overall structure remaining intact. The fundamental study on a model system presented in this work is key to understanding the phase behavior of multiple component microemulsions as well as their design for applications at higher temperatures, where the structure of most RMs breaks down.

Modulating shape transition in surfactant stabilized reverse microemulsions

“…86,88 Our previous investigation shows that when dodecane is replaced with p -xylene, the water-in-xylene RMs prepared at similar conditions ( ω = 10 and Φ = 0.005 to 0.20) are found to consist of AOT stabilized water drops that are spherical in shape. 90 Contrary to the case of D 2 O-in-dodecane RMs, the size of spherical D 2 O droplets remains almost constant and the attractive interactions become more pronounced when Φ is increased. Moreover, when RMs containing D 2 O-in-xylene prepared at fixed Φ are subjected to different temperatures ( T ) ranging from 20 °C to 50 °C, only a marginal decrease in the size was observed, whereas the inter-droplet interactions remain almost unchanged.…”

“…6 shows the probability density distribution functions (PDDF) p ( r ) vs. r obtained from SAXS analysis on the D 2 O-in-oil RMs where the continuous (oil) phase is (i) p -xylene (ii) dodecane, and (iii) a mixture of p -xylene and dodecane in the ratio of 1 : 9 by volume when the volume faction of dispersed phase, Φ = 0.005, see Section 2.2 for more details on the procedure. The p ( r ) vs. r for p -xylene/D 2 O/AOT RM at ω = 10 and Φ = 0.005 from our previous investigation 90 is included to draw a comparison of the nanostructure of the RMs when different oils are used. It is observed that the p ( r ) vs. r of p -xylene/D 2 O/AOT RM is a bell shaped curve, depicting spherical droplets whereas for dodecane/D 2 O/AOT system, a peak followed by a long tail is observed, pointing to the presence of elongated droplets (see Fig.…”

What is the origin of conductivity in water-poor reverse micelles?

“…The onset of structuring, as derived from the extrapolation limit of the scattered intensity at zero angle, I(0), corresponds to the transition seen in conductivity. Detailed study of the zero angle in scattering and its relation with chain interdigitation has been made by Rastogi and co-workers using xylene [41]. As we see in the supplementary material, the dismutation regime dominates when no water cores are present and dismutation becomes energetically favourable.…”

“…Since no other study and only five data points are available, it is not possible at the present stage to confirm this observation. A more detailed study linking conductivity measured to carefully established scattering has become available recently [41]. This work was focused on xylene, a polar solvent in which all ternary phase equilibria have been determined: AOT and xylene are fully miscible even in the absence of water and show a large domain of reverse hexagonal phases [12].…”