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Reversible structure of carbon dioxide response ionic liquid microemulsion

2020-04-06 3892105

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Label: cutting edge of discipline
Keywords: ionic liquid microemulsion, carbon dioxide reversible structure
Microemulsion is an isotropic mixed system composed of oil, water and surfactant. Because of its excellent stability and dispersibility, microemulsions are widely used in extraction, catalysis and ore flotation. Recently, compared with conventional microemulsions, responsive microemulsions have more advantages in the field of sustainable development. The research of microemulsion with unique pH, light, redox reaction, carbon dioxide (CO2) and so on has attracted wide attention in recent years.
Because of the non-volatile and low toxicity of ionic liquids, the microemulsion based on ionic liquids has greater significance for the development of green chemistry and technology. In particular, ionic liquids with good CO2 adsorption capacity are ideal candidates for preparing CO2 responsive green microemulsions.
CO2 responsive ionic liquid microemulsion with reversible structure
(image source: Langmuir)
Based on this, Professor Huang Zhiyu and Professor Lu Hongsheng from the school of chemistry and chemical engineering of Southwest Petroleum University combined 1,1,3,3-tetramethylguanidine (TMG) with HOA to obtain the ionic liquid 1,1,3,3-tetramethylguanidine oleate (tmg-oa). Subsequently, the ionic liquid microemulsion with CO2 response was prepared by HOA, isopropanol (IPA), water and TMG-OA, and the microstructure change induced by CO2 was also studied.
Figure 1. Preparation process and FTIR and NMR characterization of ionic liquid tmg-oa
(image source: Langmuir)
Figure 1 shows the preparation process of ionic liquid tmg-oa. The materials and products before and after preparation were characterized and analyzed by IR and NMR, which proved that the preparation of ionic liquid tmg-oa was successful.
Figure 2. TGA curves of TMG, HOA and ionic liquid tmg-oa
(image source: Langmuir)
TMG, as a convertible solvent with excellent CO2 absorption capacity, has strong volatility and toxicity. Therefore, TGA was used to evaluate the thermal stability of TMG, HOA and ionic liquid tmg-oa. As shown in Figure 2, the decomposition of TMG mainly occurs at 64 ° C and 116 ° C, while the decomposition temperature of tmg-oa is about 298 ° C, indicating that the formation of tmg-oa can greatly reduce the loss of TMG, and tmg-oa is also more green than TMG.
Figure 3. PH change of tmg-oa solution
(image source: Langmuir)
Figure 4. Morphology of tmg-oa at different pH
(image source: Langmuir)
In addition, tmg-oa also shows reversible structure conversion performance. As shown in Figure 3, when CO2 is introduced, the pH value of tmg-oa solution decreases from 9.65 to 6.59, while when N2 / 65 ℃ is added, the pH value increases to 8.56. The corresponding micrograph of tmg-oa solution at different pH values is shown in Figure 4. In addition, with the decrease of pH value, spherical droplets gradually form and the size of droplets increases, which indicates that microemulsion gradually forms when CO2 gas is continuously introduced.
Figure 5. Reversibility of tmg-oa
(image source: Langmuir)
The above experiments prove that TMG-OA has CO2 response characteristics, so the researchers have further discussed the CO2 response behavior of TMG-OA based microemulsion. As shown in Fig. 5, for the sample a (O/W microemulsion), the BC structure will be formed when CO2 is continuously injected into the CO2, and then N2 will return to O/W structure at N2 C. DLS analysis shows that the diameter of sample a increases from 3.1 nm to 6.7 nm when CO2 is introduced. On the contrary, when N2 is introduced, the diameter will decrease to 3.6 nm. It shows that tmg-oa has structural reversibility.
In conclusion, in this work, the researchers have prepared CO2 reactive microemulsion without volatile amine by ionic liquid TMG-OA. The reversible microstructure of the microemulsion can be regulated by CO2. The change of microstructure is attributed to the expansion behavior and the decrease of single phase area. The CO2 responsive microemulsion with reversible microstructure is different from the traditional microemulsion. It may be able to achieve reversible oil / water separation after adding CO2 or N2.
Original link:
https://pubs.acs.org/doi/full/10.1021/acs.langmuir.9b03327
Original author:
Xiaojiang Li, Baogang Wang, Shanshan Dai Hongsheng Lu and Zhiyu Huang
DOI: 10.1021/acs.langmuir.9b03327
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