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 sc attering, and NMR self-diffusion measurements. The phase diagram shows a significant shrinkage of the microemulsion region, compared with th e corresponding NaAOT system, which implies a decrease of the capabili ty of water uptake. This is also observed for the L-alpha, phase of th e binary CaAOT/water system. A preliminary analysis of viscosity data suggests the occurrence of nonspherical particles at high volume fract ion of the disperse phase, phi(d), whereas the conductivity measuremen ts 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 percolati on threshold is not temperature dependent in the range 15-30 degrees C , thus suggesting the possibility of a static percolation. The compari son between the diffusion coefficients calculated from composition and those obtained by DLS and NMR measurements reveals that spherical dro plets with a hard-sphere behavior are likely to occur in a very limite d region of the L-2 phase, namely, at low phi(d). The conductivity mea surements and the NMR self-diffusion coefficients of water, measured a long several water dilution lines, display significant maxima, appeari ng at w degrees ([H2O]/2[CaAOT]) between 6 and 13 (conductivity, diffu sion, and w degrees values, at the maximum, increase with increasing t he s/o ratio), in evident conflict with a hard-sphere model. All exper imental data demonstrate the occurrence of important modifications of the water-in-oil droplet organization. The microstructure of the syste m is discussed in view of different approaches based on percolation th eory, attractive interactions among discrete particles, and multiconne cted water network. Microstructural evolutions can be justified in ter ms of transient fusion-fission processes among the droplets occurring over time scales that are comparable with the experimental (conductivi ty and NMR observation times. Similar results have been obtained by co nsidering the geometrical DOC model set up to interpret the transition from a bicontinuous to a water-in-oil droplet microstructure in DDAB microemulsions.