Highly and Fully Water Dilutable Sustainable Microemulsions with Dibasic Esters as Oil Phase

In this study, we show a strategy of formulating highly and fully water dilutable sustainable microemulsions with dibasic esters as oil pseudophase. First, dimethyl or diethyl DBEs with different numbers of carbon atoms between the ester functions (succinate C2, glutarate C3, and adipate C4) or DBE blends and a surfactant–cosurfactant melt consisting of sodium dodecylsulfate and 1-pentanol with a constant weight ratio of 1:2 are used. We investigated the extent of the realms of existence of homogeneous single phases and represent them with pseudoternary phase diagrams. It was possible to formulate fully water dilutable oil-rich single-phase solutions with all dimethyl dibasic esters but not with the diethyl ones. By mixing the most hydrophilic dibasic ester with the most hydrophobic one with a mass ratio of 3:2, a fully water dilutable oil-rich single phase could also be obtained. The presence of water-in-oil, bicontinuous, and oil-in-water microemulsions was checked with conductivity and dynamic light scattering measurements. A percolative behavior was observed for all systems. Second, to formulate green and sustainable microemulsions, we replaced successively sodium dodeylsulfate by sodium oleate and 1-pentanol by ethanol or 1,5-pentanediol. With sodium oleate, highly water dilutable microemulsions were obtained. Homogeneous and translucid samples in the very diluted area turned bluish after several days. Longtime pH measurements of systems containing sodium oleate showed no degradation of the dibasic esters at low water content and a rapid hydrolysis at high water content with 1-pentanol as cosurfactant. Always in presence of sodium oleate, the substitution of 1-pentanol by ethanol or 1,5-pentanediol drives the system to microemulsions showing an instability of the ester at any water concentrations. Thermal gravimetric analysis measurements at 32 °C for 24 h confirmed a rapid evaporation of ethanol and water and negligible evaporation of 1,5-pentanediol.