The extension of microemulsion regions by combining ethanol with other cosurfactants

• Formulation of green microemulsions.
• Limitations of using of ethanol as cosolvent to extend the microemulsion area.
• Investigation of the film flexibility.

In a previous work, we showed the extension of the microemulsion area towards highly or fully water dilutable systems by adding a short chain alcohol as cosolvent, like ethanol, to the system water/sodium oleate/citronellol/limonene. It was possible to convert an anti-percolative system to a percolative one by making the interfacial film more flexible. The question arises if this is a general concept. For this reason, we investigated pseudo-ternary systems water/surfactant/cosurfactant/ethanol/oil with different cosurfactants (1-pentanol, 1-heptanol, dodecanol, Guerbet alcohols) and fatty acid methyl ester-rapeseed biodiesel as oil phase. Sodium oleate was used as surfactant. Ethanol was added as cosolvent to enhance the film flexibility and so to increase the microemulsion area. With increasing hydrophobicity of the cosurfactants and the oil and without further addition of ethanol, only very restricted single phase areas were obtained. However, with a certain amount of ethanol, sodium oleate, and 1-heptanol a distinct path towards the water-rich corner was found. By replacing 1-heptanol by Guerbet alcohols this extension was lost and monophasic areas only in the surfactant-rich region were preserved. For all systems a limited “optimal” formulation was obtained for a specific percentage of ethanol in the ethanol–cosurfactant blend. With DLS the homogeneous single phase areas were checked to distinguish real solutions from microemulsions. The nano-structure and film flexibility was investigated using electrical conductivity measurements. In the choice of the solvents, we focused on sustainable solvents.

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