Properties of microemulsions with sugar surfactants and peppermint oil

In this study, we report on the properties of water + propylene glycol/sugar surfactant/peppermint oil + ethanol. The sugar surfactants were sucrose monolaurate and sucrose dilaurate. The mixing ratios (w/w) of water/propylene glycol and that of ethanol/oil equal 2 and 1, respectively. U-type microemulsions were observed in the sucrose monolaurate while S-type microemulsions were observed in the dilaurate-based systems. Temperature-insensitive microemulsions were formulated using the two surfactants. Water volume fraction percolation thresholds were determined by the study of electrical conductivity and dynamic viscosity. The structural parameters that include the periodicity and the correlation length were estimated using small angle X-ray scattering. The periodicity increases linearly with the increase in the water content whereas the correlation length increases with the increase in the water volume fraction to a certain value then decreases. The diffusion properties investigated by nuclear magnetic resonance confirm a progressive transformation of the microemulsions from water-in-oil to bicontinuous and inversion to oil-in-water upon dilution with water. The hydrodynamic radius of diluted microemulsions measured by dynamic light scattering increases with the increase in temperature. The area per polar head group decreases with the increase in temperature.