Structural aspects of microemulsions using dielectric relaxation and spin label techniques

The structural characteristics of three similar systems containing sodium stearate, alcohol (n-pentanol, n-hexanol, or n-heptanol), hexadecane, and varying amounts of water were investigated. The systems were identical in composition except for the nature of alcohol used for producing microemulsions. In all the three microemulsion systems, the microemulsion remained isotropic and clear up to the water/oil ratio of 0.48. However, the electrical resistance studies indicated that the three systems were strikingly dissimilar. Upon increasing the water/oil ratio, n-pentanol microemulsion systems exhibited a constant decrease in resistance; hexanol microemulsion systems showed an initial increase and a subsequent sharp decrease whereas n-heptanol microemulsion systems exhibited very high electrical resistance which could not be measured (>l07Ω-cm). The pentanol microemulsion systems exhibited dielectric relaxation at lower water/oil ratios as compared to the hexanol microemulsion systems. However, heptanol microemulsion systems did not exhibit dielectric relaxation. The dielectric relaxation results were interpreted in terms of ionization of the carboxyl group and the concomitant formation of the electrical double layer. This occurred at lower water/oil ratios in pentanol microemulsion systems as compared to hexanol microemulsion systems. However, there was no measurable ionization or electrical double layer formation in heptanol microemulsion systems. The spin label studies using a stearic acid spin probe suggested that the stearate molecule is located at the oil/water interface. Furthermore, the spin label incorporated in microemulsions exhibited anisotropic behavior in contrast to the isotropic behavior of the spin label dissolved in oil or water. The values of order parameter suggested that the order in interfacial film as measured by the spin label decreased in the following order: heptanol > hexanol > pentanol. In other words, the shorter chain alcohol produced a maximum disorderat the oil/water interface. The isotropic hyperfine splitting constant of the label in the three microemulsion systems suggested that the environment experienced by the spin label was intermediate between that of water and oil. In summary, the electric, dielectric, and ESR measurements suggest that the alcohol chain length affects the ionization, interfacial polarization, and disorder at the oil/water interface which in turn influences the properties of microemulsions.