Journal of SoftMaterials, 7(4): 258–276, (2009)
We report on the properties of water/mixed nonionic surfactants/R (+)-limonene/ isopropylmyristate microemulsions. The surfactants were sucrose monolaurate and ethoxylated mono-di-glyceride. The total area of the one phase microemulsion region in the pseudoternary phase diagrams is dependent on the mixing ratios (w/w) of the oils and that of the mixed surfactants. The largest microemulsion area formed with a surfactants mixing ratio (w/w) equals unity. The one phase microemulsion area increases with the increase in the R (+)-limonene/ isopropylmyristate-mixing ratio (w/w). Electrical conductivity was measured at different temperatures and the activation energy of conduction flow was evaluated. At the percolation threshold, the activation energy of conduction flow reaches a minimum value. Beyond the percolation threshold, a small increase is observed in the activation energy of conduction flow, then it decreases with the increase in the water volume fraction indicating structural transitions. Dynamic viscosity was measured as function of temperature at different water volume fractions. The measured viscosities for the samples in all of the systems decrease as the temperature increases. The thermodynamic parameters of viscous flow that include enthalpy, entropy, and Gibbs free energy were estimated. In all of the systems studied, the enthalpy of viscous flow remains constant as function of temperature and varies as the water content in the microemulsions vary. The entropy and the Gibbs free energy vary with both the temperature and the composition of the microemulsions. Linear relationships were observed between the enthalpy and entropy of viscous flow. The enthalpy-entropy compensation temperature was determined and found to be 247 K. Small angle X-ray scattering results indicate that the periodicity increases linearly with the increase in the water volume fraction. The correlation length increases with the increase in the water volume fraction to a certain value, then stabilizes.