TERNARY WATER IN OIL MICROEMULSIONS MADE OF CATIONIC SURFACTANTS, WATER, AND AROMATIC SOLVENTS .2. DROPLET SIZES AND INTERACTIONS AND EXCHANGE OF MATERIAL BETWEEN DROPLETS

Ternary water in oil microemulsions made of cationic surfactants, water, and aromatic solvents have been investigated by means of time-resolved fluorescence quenching, quasi-elastic light scattering, and electrical conductivity in order to determine the surfactant aggregation number N per water droplet, the rate constant ke for the exchange of material between droplets through collisions with temporary merging, the droplet diffusion coefficient D, and the coefficient of interaction between droplets a and to study the occurrence of electrical percolation as a function of the surfactant chain length, head-group size, and water content of system (expressed as the molar concentration ratio ω = [water]/[surfactant]). Most measurements were performed with chlorobenzene as solvent. In one instance, chlorobenzene was substituted by benzene in order to investigate the effect of the nature of the solvent. For a given surfactant, N and ke increased with ω and upon substituting chlorobenzene by benzene. Also, at a given ω, N and ke increased when the surfactant chain length was decreased. The increases of ke were always extremely large. The droplet hydrodynamic radii from quasi-elastic light scattering were found to agree with the droplet sizes calculated with the N values from fluorescence quenching. The droplet interaction coefficient α became more negative as the surfactant chain length decreased, indicating increasingly attractive interdroplet interactions. Finally, electrical percolation was found to occur in all systems were interdroplet interactions were sufficiently attractive. The percolation threshold ω-values increased with surfactant chain length. Our results clearly showed that, under fixed experimental conditions, a decrease of surfactant chain length can result in a moderate increase of N, an increase of the magnitude of attractive interdroplet interactions, a very large increase of ke, and a decrease of the percolation-threshold value. From a more quantitative viewpoint, it was noted that in all instances, including numerous other studies where conductivity data and ke values are available, the percolation threshold corresponds to ke values of about (1-2) × 109 M-1 s-1. This result led us to attribute the electrical conductivity of water in oil microemulsions above the percolation threshold to the motion of surfactant counterions within transient water channels arising in droplet clusters upon opening of surfactant layers separating adjacent water droplets. © 1990 American Chemical Society.