A Structural Investigation of CaAOT/Water/Oil Microemulsions

Abstract

The microstructural features of ternary microemulsions in the CaAOT/water/isooctane system are investigated by conductivity and NMR self-diffusion measurements. The results are compared with the corresponding NaAOT system. Experimental data are collected along water and oil dilution lines with the aim of investigating the interactions among the surfactant aggregates. Previous studies on the microemulsion regions of CaAOT/water/decane are also considered in order to evaluate the influence of the oil (branched or linear) on the microstructural transitions. Water-in-oil spherical droplets with a hard-sphere behavior are likely to occur in a very limited region of the L2 phase, namely, at low volume fractions of the disperse phase, φd. Both conductivity and water self-diffusion demonstrate the occurrence of important modifications of the water-in-oil droplet organization and suggest the occurrence of transient fusion-fission processes among the droplets. These processes become more and more important with increasing φd. The microstructure of the system is discussed in view of different approaches based on percolation theory, attractive interactions among discrete particles, and a multiconnected water network.

Effect of counterion substitution on the viscosity anomaly in AOT microemulsions

“…The values of X cited are the stoichiometric ones, neglecting the small amount of water of hydration ( X < 0.5) that normally cannot be removed from the surfactant. For KAOT, systems with 0 X 20 were investigated, while for Ca(AOT) 2 , systems with 5 X 22 were investigated, since Ca(AOT) 2 is not soluble in the waterless system [18]. As described below, the Ca(AOT) 2 systems displayed a viscosity maximum, so measurements were taken at additional intermediate values of X to confirm the viscosity behavior for this system.…”

“…They found that the hydrated radius of counterions strongly impacts the shape of aggregates, with larger counterions favoring rodlike aggregates over spherical aggregates [14]. Other investigators have systematically investigated the viscosity, phase diagram and microstructure of Ca(AOT) 2 /water/ndecane systems versus NaAOT/water/n-decane systems [16][17][18].…”

“…Although other groups have performed detailed studies of the impact of counterion substitution on the self-assembly, structure, and droplet interactions in AOT microemulsions [14][15][16][17][18], to our knowledge there have been no studies that have connected the maximum in viscosity to interdroplet interactions for any counterions other than Na + . Finally, because the charge fluctuation model was developed for spherical droplets, it is important to note here that we are restricted in this study to systems that form spherical droplets over a wide range of values of X and volume fraction Φ.…”

Microemulsions: Thermodynamic and Dynamic Properties

“…The critical exponent t generally ranges between 1.5 and 2, whereas the exponent s allows the assignment of the time dependent percolation regime. Thus, s > 1 (generally around 1.3) identifies a dynamic percolation (Cametti et al, 1992, Pitzalis et al, 2000Mehta et al, 2005). The static percolation is related to the appearance of bicontinuous microemulsions, where a sharp increase in conductivity, due to both counter -ions and to lesser extent, surfactant ions, can be justified by a connected water path in the system.…”

Effect of cations on the structure of sodium bis(2-ethylhexyl)sulfosuccinate water-in-oil microemulsion

“…Recently, theoretical studies on the structure of reversed micelles (w/o microemulsion at low water content) occluding small amounts of water and ions have been carried out (Faeder & Ladanyi, 2000;Bulavchenko et al, 2002;Abel et al, 2004;Pal et al, 2005). Experimentally, the effect of salts on the AOT w/o microemulsion have been studied intensively by using various spectroscopic methods such as infrared, dielectric, conductivity, luminescence, electron paramagnetic resonance and nuclear magnetic resonance (Eastoe et al, 1992;Fioretto et al, 1999;Alvarez et al, 1999;Pitzalis et al, 2000;Burrows & Tapia, 2002). In spite of these previous studies, there is little direct evidence about the effect of different salts on the microemulsion structures (Capuzzi, 1997).…”