UV-Visible and AFM Studies of Nonionic Microemulsions

The systems investigated by UV-visible spectroscopy and atomic force microscopy were water/nonionic surfactant/oil+ethanol. The surfactants were the single sucrose laurate and ethoxylated mono-di-glyceride and a mixture of the two. The mixing ratios (w/w) of sucrose laurate and ethoxylated mono-di-glyceride and that of ethanol/oil equal unity. The oils were R(+)-limonene and isopropylmyristate. The absorption spectra of the oils as function of water volume fraction in the microemulsions along the N60 dilution line where the surfactant/oil/ethanol weight ratio is 6/2/2 were recorded. The wavelengths corresponding to the maximum absorbance of the oil in microemulsions () were compared to the wavelengths of the pure oil (o). It was found that the relative wavelength (/o) decrease with the increase in the water volume fraction in the microemulsions. The decrease in /o was correlated to structural transitions. The onset of these structural transitions is dependent on the oil and surfactant structure. In the systems based on sucrose laurate the transition from water in oil to bicontinuous microemulsions occur at higher water volume fractions compared to those observed in the ethoxylated mono-di-glyceride based one. This behavior was attributed to the difference in the surfactant head group. Similar behavior of /o as function of water volume fraction was also observed in the mixed surfactants based systems. In the R(+)-limonene based microemulsions, the values of /o were lower compared to those observed on the isopropylmyristate based systems. This behavior is attributed to the extent of oil penetration in the surfactant layer. The UV-visible results are supported and consistent with the atomic force microscopy measurements. In the regions of water-in-oil and oil-in-water microemulsions uniform singly separated droplets were observed, while continuously connected microemulsion systems were observed in the bicontinuous region as expected.