STRUCTURE AND DYNAMICS OF CETYLTRIMETHYLAMMONIUM BROMIDE WATER-IN-OIL MICROEMULSIONS

The state of water, the droplet size, and interdroplet exchange of reactants between colliding droplets have been investigated by NMR, time-resolved fluorescence quenching, and electrical conductivity in water-in-oil microemulsions made of cetyltrimethylammonium bromide (CTAB) in chloroform/isooctane (2/1, v/v). The effect of the two additives cetyl bromide (CB) and trimethylamine (TMA) on the properties of the droplets has also been investigated. The droplet size, as expected, increases as the [water]/[CTAB] molar ratio w0 increases. No interdroplet exchange of reactants has been detected in the absence of additives. The addition of CB above 0.6 M has a drastic effect on the droplet properties. A large increase of size is observed which is accompanied by the appearance on the fluorescence time scale (≃1 μs) of interdroplet exchange of reactants. Electrical percolation phenomena are also observed for the systems with CB when the value of the rate constant ke which characterizes the interdroplet exchange of reactants is above (1-2) × 109 M-1 s-1 in agreement with the observation made in previous studies of other microemulsion systems. The addition of TMA slightly decreased the surfactant aggregation number. © 1990 American Chemical Society.