Light scattering and fluorescence studies of o/w microemulsion: The sodium 4-dodecylbenzenesulfonate-butanol-water-NaCl-octane system

Fluorescence polarization, excimer emission, and dynamic light scattering have been used to investigate the effect of system parameters on the physical (translational diffusion coefficient of microemulsion droplets, microfluidity at the oil-water interface) and structural (droplet size) properties of oil-in-water microemulsions. The microfluidity at the oil-water interface was found to be independent of the total oil volume fraction. The uptake of oil resulted in an increase in droplet size (with concomitant increase of aggregation number) and a decrease in droplet concentration so as to keep the total interface area,A_t, and surfactant molecular area, σ, constant. Some results on the effect ofn-butanol on these physical and structural properties are also investigated. This study suggests that the oil-water interface rigidity is mainly governed by the alcohol molecules adsorbed at the interface at low bulk concentration. The interface is saturated when the alcohol to surfactant molecular ratio is approximately equal to 3. The changes in micelle and microemulsion size depend on the balance between the partition of butanol between the oil-water interface (as reflected by a decrease of the droplet size) and the oil core (as reflected by an increase of the droplet size). Partitioning into the oil-water interface is dominant at low alcohol concentration (C_a <0.7M), whereas partitioning in the oil core is dominant at high alcohol concentration (C_a > 1.2M). In the intermediate concentration region, these two effects are balanced and the droplet size is relatively invariant with alcohol concentration.