Investigations into the formation and characterization of microemulsions. II. Light scattering conductivity and viscosity studies of microemulsions

Microemulsions formed with water—xylene—sodium alkyl benzene sulfonate (NaDBS)-hexanol have been investigated using time-average light scattering conductivity and viscosity measurements at 25°C. Four different concentrations of NaDBS were used, namely, 5, 10.9, 15, and 20 wt% and the molar ratio of n-hexanol:NaDBS was kept constant at 3.4, 3.24, 3.4, and 3.4, respectively. The light scattering results showed that at low volume fractions of water, φ_H2O, the Rayleigh ratio, R₉₀ increased slowly with an increase in φ_H2O but above a critical volume fraction of water, φ_H2O^S, R₉₀ increased almost linearly with increase of φ_H2O, reached a maximum at another critical water volume fraction, φ_H2O^C, above which R₉₀ decreased with further increase in φ_H2O. The results were interpreted qualitatively in terms of the possible aggregate units formed. A quantitative analysis of the light scattering data was carried out using a procedure based on the use of a hard sphere model for particle interactions. Using this approach, the water droplet radius R was calculated as a function of φ_H2O for the four systems investigated. The results showed a linear increase of R with increase in φ_H2O. Approximate values of the water radii were also calculated from the interfacial area and these were found to be in full agreement with the radii obtained from light scattering measurements. The conductance κ showed a non-monotonic variation as the water concentration was increased. A maximum in the κ-φ_H2O was observed at a critical volume fraction of water φ_H2O^S above which κ decreased and then remained almost constant over a range of φ_H2O values. The conductance then sharply increased at another φ_H2O value, namely φ_H2O^C. This φ_H2O^C value was reduced with increase in NaDBS concentration. The conductance results indicate structural changes in the system as the water concentration increases and the transition observed correlate with those obtained from light scattering. Moreover, the low κ values found and the non-monotonic variation of κ with φ_H2O are indicative of the presence of definitive water cores with an external surfactant film which acts as a barrier for ion transport. Viscosity results showed the behavior normally encountered with concentrated dispersions, the relative viscosity η_r increasing exponentially with φ_H2O. The viscosity data were fitted to the Mooney equation. The results showed an increase in the Einstein coefficient with increase in NaDBS concentration indicating an increase in the hydrodynamic volume of droplets. This was attributed to the increase of the ratio of the surfactant layer thickness to the droplet core radius as the NaDBS concentration is increased.