Microstructure of Ca-AOT/water/decane w/o microemulsions

The microstructural features of the ternary microemulsion CaAOT/water/n-decane are investigated by viscosity, conductivity, dynamic light scattering, and NMR self-diffusion measurements. The phase diagram shows a significant shrinkage of the microemulsion region, compared with the corresponding NaAOT system, which implies a decrease of the capability of water uptake. This is also observed for the L-alpha, phase of the binary CaAOT/water system. A preliminary analysis of viscosity data suggests the occurrence of nonspherical particles at high volume fraction of the disperse phase, phi(d), whereas the conductivity measurements show a percolative behavior at a critical phi(d)(c) = 0.142, along an oil dilution line at constant molar ratio w/s = 26.4. The percolation threshold is not temperature dependent in the range 15-30 degrees C, thus suggesting the possibility of a static percolation. The comparison between the diffusion coefficients calculated from composition and those obtained by DLS and NMR measurements reveals that spherical droplets with a hard-sphere behavior are likely to occur in a very limited region of the L-2 phase, namely, at low phi(d). The conductivity measurements and the NMR self-diffusion coefficients of water, measured along several water dilution lines, display significant maxima, appearing at w degrees ([H2O]/2[CaAOT]) between 6 and 13 (conductivity, diffusion, and w degrees values, at the maximum, increase with increasing the s/o ratio), in evident conflict with a hard-sphere model. All experimental data demonstrate the occurrence of important modifications of the water-in-oil droplet organization. The microstructure of the system is discussed in view of different approaches based on percolation theory, attractive interactions among discrete particles, and multiconnected water network. Microstructural evolutions can be justified in terms of transient fusion-fission processes among the droplets occurring over time scales that are comparable with the experimental (conductivity and NMR observation times. Similar results have been obtained by considering the geometrical DOC model set up to interpret the transition from a bicontinuous to a water-in-oil droplet microstructure in DDAB microemulsions.