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...
