Recovery and Recycling of CO₂/N₂‐Switchable Anionic Surfactants in Emulsions

Abstract

To develop a mild, effective, and clean strategy for recovery and recycling of anionic surfactants in CO2/N2‐switchable emulsions, a CO2/N2‐switchable anionic surfactant, which is a combination of dodecyl seleninic acid (DSA) and N,N,N′,N′‐tetramethyl‐1,2‐ethylenediamine (TMEDA), here referred to as DSA–TMEDA, was used to stabilize an oil‐in‐water (O/W) emulsion. Upon stimulation with CO2, DSA–TMEDA was switched off to form insoluble DSA and the water‐soluble TMEDA bicarbonate. Upon N2 bubbling and heating, the OFF state of DSA–TMEDA was restored to the surfactant of DSA–TMEDA. In this manner, O/W emulsions stabilized by DSA–TMEDA can be switched reversibly between demulsification (phase separation) and re‐emulsification (recovered emulsion) by triggering with CO2/N2 over ten times. After breakage of the emulsion, nearly all of the OFF state surfactant could be separated conveniently away from the oil phase, thus facilitating recovery and recycling of the surfactant afterward in emulsifying oil. No obvious adverse changes in the dispersed oil particles size and the relative stability of the regenerated emulsions were observed over five cycles, and the surfactant loss can be neglected during the recycling.

Retrieving Oil and Recycling Surfactant in Surfactant-Enhanced Soil Washing

“…Around 2.3% of the DUSNa remained in the aqueous filtrate, while around 97.7% had been converted to DUS, as determined by titration with benzethonium chloride after the samples had been basified with NaOH. Generally, reduction of σ and mechanical, steric, and/or electrical barriers formed by the surfactant at the oil–water interface are thermodynamically and kinetically important stabilizing factors in surfactant-based emulsions. , Thus, an almost complete conversion of DUSNa to DUS, an increase in σ (Figure S5), and loss of the DUSNa-based barrier at the water–oil interface lead to phase separation of the DUSNa-stabilized emulsion after CO 2 treatment (Figure b).…”

“…A mixture of mineral oil D80 and aqueous surfactant solution with a volume ratio of 1:1 was homogenized to produce emulsion. The relative stability of the emulsion was measured as the time needed to separate 1 mL of H 2 O from 6 mL of emulsion ( t 1 mL , min) at 30 °C . The type of emulsion was determined by the dilution method .…”

“…There is increasing awareness of the need for recovery and reusability of the surfactants for SESW, because of the potential negative effects of synthetic surfactants and the cost-effectiveness of the process. ,, Pioneering research revealed that the surfactant could be partially recovered after an appropriate degradation treatment, however, most of the target organic pollutants in the soil washing solution were destroyed. ,, To facilitate demulsification and to retrieve the potentially reusable oil as well as the surfactant simultaneously, a CO 2 -switchable anionic surfactant, which was a combination of dodecyl seleninic acid and N , N , N ′, N ′-tetramethyl-1,2-ethylenediamine (DSA–DMEA) had been reported very recently by us . Unfortunately, DSA–DMEA did not work well in SESW.…”

“…1,9,10 To facilitate demulsification and to retrieve the potentially reusable oil as well as the surfactant simultaneously, a CO 2 -switchable anionic surfactant, which was a combination of dodecyl seleninic acid and N,N,N′,N′tetramethyl-1,2-ethylenediamine (DSA−DMEA) had been reported very recently by us. 17 did not work well in SESW. Therefore, novel CSASs are urgently required for use in SESW, in particular, those for which the inactive form is insoluble in both oil and water.…”

“…The relative stability of the emulsion was measured as the time needed to separate 1 mL of H 2 O from 6 mL of emulsion (t 1 mL , min) at 30 °C. 17 The type of emulsion was determined by the dilution method. 18 Emulsion droplets were visualized using a Nikon-80i fluorescence microscope system (Nikon, Japan) and the emulsions were dyed with trace amount of Nile red in advance.…”