Investigations into the formation and characterization of microemulsions. II. Light scattering conductivity and viscosity studies of microemulsions |
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Authors: | R.C. Baker A.T. Florence R.H. Ottewill Th.F. Tadros |
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Affiliation: | 2. Department of Pharmacy, University of Strathclyde, Royal College, 204 George Street, Glasgow G1 1XW, Scotland;3. Department of Physical Chemistry, School of Chemistry, University of Bristol, Bristol BS8 1TS, England |
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Abstract: | 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, φH2O, the Rayleigh ratio, R90 increased slowly with an increase in φH2O but above a critical volume fraction of water, φH2OS, R90 increased almost linearly with increase of φH2O, reached a maximum at another critical water volume fraction, φH2OC, above which R90 decreased with further increase in φH2O. 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 φH2O for the four systems investigated. The results showed a linear increase of R with increase in φH2O. 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-φH2O was observed at a critical volume fraction of water φH2OS above which κ decreased and then remained almost constant over a range of φH2O values. The conductance then sharply increased at another φH2O value, namely φH2OC. This φH2OC 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 φH2O 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 ηr increasing exponentially with φH2O. 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. |
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Keywords: | Author to whom correspondence should be addressed. |
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