Abstract: To obtain an O/W microemulsion,blend of sodium dodecyl benzene sulfonate and N,N-dimethyl-N-dodecyl tert-amine with equimolar ratio was used as main emulsifier,butanol as co-emulsifier and n-heptane as oil phase.The reversible conversion of microemulsion-phase separated liquid-microemulsion can be realized by alternate action of CO₂ and N₂.The radii of the original and recovered microemulsion are (10.89±0.21) and (11.50±0.47) nm respectively.The oil removal rate for mineral oil on porous solid,textiles and industrial crude oil sand are 99.13%±0.32%,98.30%±0.28% and 54.52%±0.25% respectively.On an equal mass fraction basis,the removal rate of soil made by mark pen of the diluted microemulsion to that of laundry powder from market achieves 1.90 while that of the microemulsion without dilution achieves 2.02.The phase separation of oil containing waste liquid could be quickly and completely realized by bubbling of CO₂.After separating the oil phase,the water phase left was treated with anion and cation exchange resins and activated carbon.The values of chemical oxygen demand (COD) and total organic carbon (TOC) of the waste water drop to 40.62 and 17.51 mg/L respectively,which are in conformity with the first class standard of discharged water (COD≤100 mg/L,TOC≤20 mg/L) according to GB 18918-2002.

Key words: microemulsion, CO₂/N₂ switchable response, oil dirt washing, separation