Treating oily solids using a CO2-responsive surfactant-free microemulsion: An enhanced approach for oil removal from solid surfaces

Because switchable water is difficult to apply directly for oil removal, an enhanced approach to improve oil removal is proposed using a surfactant-free microemulsion (SFME) containing switchable water. The SFME is formed using switchable water (aqueous N, N-dimethylethanolamine (DMEA) or N,N-dimethylbutylamine (BDEA)) and a hydrophobic amine (N-butyldiethanolamine (DMBA)). The single-phase region for preparing the SFME is determined using a ternary phase diagram, and the microstructure types are distinguished using the conductivity method. The pH changes and dynamic laser scattering test results demonstrate that the SFME exhibits irreversible transformation behavior regulated by CO₂. Moreover, the recovered SFME containing BDEA has a microstructure with a smaller particle size after removing the CO₂ by bubbling N₂ at 65 °C, which is attributed to the higher proportion of DMEA than that of BDEA. In addition, dodecane and diesel oil are shown to dissolve in the SFME, and the oil solubility of the SFME increases with increasing oil phase (DMBA) content. The interface free energy theory reveals that the oil is more easily detached with the addition of SFME than with switchable water only owing to the reduced interface free energy of oil on the solid surface. Thus, an enhanced approach for removing oil is proposed to wash oil sands, where the oil removal efficiency of switchable water can be improved by using SFMEs containing switchable water.