QUATERNARY WATER IN OIL MICROEMULSIONS .1. EFFECT OF ALCOHOL CHAIN-LENGTH AND CONCENTRATION ON DROPLET SIZE AND EXCHANGE OF MATERIAL BETWEEN DROPLETS

Water solubility, electrical conductivity, and time-resolved fluorescence quenching measurements have been performed in water/chlorobenzene/cationic surfactants/1-alcohol water-in-oil (w/o) microemulsions in order to investigate the effect of alcohol chain length and concentration on various properties of these systems: surfactant aggregation number, N, per aggregate; radius, R(w), of the droplet water core; intensity of attractive interdroplet interactions; onset of percolation of electrical conductivity; and rate constant, k(e), for the exchange of material between droplets through collisions with temporary merging. The variations of these properties with the molar concentration ratio omega = [water]/[surfactant] for alcohols of increasing chain length are strikingly similar to those found when investigating the effect of surfactant chain length (Jada, A.; et al. J. Phys. Chem. 1990, 94, 387). In particular, N and R(w) and the intensity of attractive interactions decrease when the alcohol chain length increases as predicted by current theory of the stability of w/o microemulsions. For a series of microemulsions based on alkyltrimethylammonium bromide surfactants, the water solubility results indicate that the stability of the microemulsions containing short chain alcohols (propanol, butanol) is determined by the attractive interdroplet interactions. These systems show a percolation for increasing omega or decreasing z = [alcohol]/[surfactant], and the percolation threshold increases with the alcohol chain length. The value of k(e) at the percolation threshold is of (1-2) x 10(9) M-1 s-1, irrespective of the chain length of the alcohol or surfactant and omega or z values. The abnormally low values of the surface area per surfactant head group at the water core surface calculated for systems showing an electrical percolation and large k(e) values reveal that in these systems the droplets are probably not spherical and that their surface is rough, owing to thermal fluctuations and to the low rigidity of the surfactant + alcohol droplet interfacial layer. The results give further support to the mechanism postulated for electrical conductivity above the percolation threshold, namely, motion of counterions through transient water tubes formed in the droplet clusters present in the systems. Finally, it is shown that simple electrical conductivity and water solubility measurements can yield a number of qualitative and quantitative information about the investigated microemulsions.