Characterisation of aerosol OT-stabilised oil-in-water microemulsions using a time-resolved fluorescence method

Abstract

Oil-in-water microemulsion phases stabilished by sodium bis(2-ethylhexyl)sulphosuccinate (AOT) form in equilibrium with conjugate phases containing virtually pure oil when the concentration of added aqueous electrolyte (NaCl) is low. Using turbidity titrations, the molar ratio of alkane: AOT (=R_alkane) in the equilibrium microemulsion phase has been determined as a function of the NaCl and AOT concentrations, the chain length of the alkane component and the temperature. Increasing [NaCl], increasing [AOT], decreasing alkane chain length and decreasing temperature all cause the R_alkane value to increase. The aggregation numbers (N_agg) of the microemulsion particles have been determined using time-resolved fluorescence measurements of the intensity decays of solubilised pyrene. N_agg is found to depend on R_alkane, but is independent of the particle concentration. The variation of N_agg is consistent with the microemulsion droplets being spherical and monodisperse. The AOT monolayer is not penetrated by the solubilised alkane. The rate measurements indicate that the local viscosity of the dispersed alkane is tenfold higher in small droplets than its bulk value, but decreases to its bulk value as the droplet size increases.