Quaternary water-in-oil microemulsion of a cationic surfactant (cetyltrimethylammonium bromide, CTAB), n-hexane, water, and n-pentanol has been investigated using conductivity, quasi-elastic light scattering, nearinfrared absorption spectroscopy, and pulsed field gradient spin-echo NMR measurements. The conductivity behavior shows features characteristic of the migration of charged droplets. Consequently, using the charge fluctuation model, the conductivity data were correlated with the droplet radius obtained from self-diffusion coefficients for different obstruction factors. Conductivity and self-diffusion measurements were found to be self-consistent for spherical droplets with hard-sphere interactions. Comparison between collective diffusion and self-diffusion coefficients fully supports this conclusion. The average head-group area of CTAB, the amount of water free in the organic bulk, and the fraction of alcohol present into the aggregates were evaluated together with the thickness of both the interfacial film and the bound water layer providing a full microscopic picture of the system. IntroductionMicroemulsions are transparent, isotropic, thermodynamically stable dispersions of oil and water, stabilized by surfactant molecules. 1-3 Four-component systems of surfactant, cosurfactant (generally a short chain linear alcohol), oil, and water have many important features and are the most studied microemulsion systems. The reason for the significance of these systems is that the introduction of cosurfactant greatly extends the isotropic solution region, especially with single-chain ionic surfactants. Microemulsions based on the cationic surfactant cetyltrimethylammonium bromide (CTAB) have been extensively used as host for different enzymes. 4 These systems offer the possibility to compare the enzymatic activities performed in a cationic microemulsion with those performed in the wellknown systems AOT/hydrocarbon/water. 5 Furthermore, the system CTAB/n-pentanol/n-hexane/water can be a useful tool to investigate the properties of anionic polyelectrolytes such as nucleic acids, 6 since the system is cationic, is transparent in the UV region (avoiding the limitation imposed by the use of chloroform which is often employed as cosolvent for CTABbased microemulsions 7 ), and can solubilize high quantities of water, up to 80 molecules of water per surfactant molecule. These characteristics are at the basis of two recent papers, where this microemulsion was used as a microreactor to perform the self-replication of oligonucleotides. 8,9 Despite such widespread interest in CTAB water-in-oil (w/ o) microemulsions, little is known about their microstructure. This prevents one from completely understanding the basic mechanism of the phenomena taking place inside them. The problem of the structure of a quaternary microemulsion is not an easy task to afford, as can be deduced by the fact that several studies, making use of a wide range of experimental techniques, gave a small contribution toward a reliable picture of these syst...

The microstructure of the quaternary water-in-oil microemulsion CTAB/water/n-pentanol/n-hexane has been investigated by means of the pulsed gradient spin-echo NMR technique over a wide range of composition. The composition of the continuous organic phase and of the interfacial phase has been determined through the analysis of the n-pentanol self-diffusion coefficient. The size of the reverse aggregates has been evaluated from the CTAB self-diffusion coefficient. The correlation of the reverse micellar size with interfacial composition has therefore been possible. Results coming from both water dilution lines and interface dilution lines have been analyzed according to suitable models. A "master plot", i.e., a graphical representation that allows us to display the data collected at all the possible compositions of the four components system, is also proposed.

The isothermal pseudo-ternary-phase diagram was determined at 25 degrees C for systems composed oflecithin, water, and, as oil, either isooctane or decane. This was accomplished by a combination of polarizing microscopy, small-angle X-ray scattering, and NMR techniques. The lecithin-rich region of the phase diagram is dominated by a lamellar liquid-crystalline phase (Lalpha). For lecithin contents less than 60% and low hydration (mole ratio water/lecithin = W0 < 5.5), the system forms a viscous gel of branched cylindrical reverse micelles. With increase in the water content, the system phase separates into two phases, which is either gel in equilibrium with essentially pure isooctane (for lecithin < 25%) or a gel in equilibrium with Lalpha (for lecithin > 25%). These two-phase regions are very thin with respect to water dilution. For 8 < W0 < 54 very stable water-in-oil emulsions form. It is only after ripening for more than 1 year that the large region occupied by the emulsion reveals a complex pattern of stable phases. Moving along water dilution lines, one finds (i) the coexistence of gel, isooctane and Lalpha, (ii) equilibrium between reverse micelles and spherulites, and, finally, (iii) disconnected reverse micelles that fail to solubilize water for W0 > 54. This results in a Winsor II phase equilibrium at low lecithin content, while for lecithin > 20% the neat water is in equilibrium with a reverse hexagonal phase and an isotropic liquid-crystalline phase. The use of the decane as oil does not change the main features of the phase behavior.