(56u) Pi Electron Cloud Mediated Separation of Aromatics Using Supported Ionic Liquid Membrane (SILM) Having Antibacterial Activity
- Conference: AIChE Spring Meeting and Global Congress on Process Safety
- Year: 2019
- Proceeding: 2019 Spring Meeting and 15th Global Congress on Process Safety
- Group: Spring Meeting Poster Session and Networking Reception
- Time: Monday, April 1, 2019 - 5:00pm-7:00pm
Mohanad Kamaza, Mahmood Jebura, Arijit Senguptaa, Xianhong Qianb, Ranil Wickramasinghea
aRalph E Martin Department of Chemical Engineering, University of Arkansas, Fayetteville, AR 72701, United States
cDepartment of Biomedical Engineering, University of Arkansas, Fayetteville, AR 72701, USA
Supported ionic liquid membranes (SILM) using imidazolium based ionic liquids and hydrophobic PTFE base membrane were used for the Ï electron mediated separation of aromatic compounds including dyes in aqueous and non-aqueous medium. The FTIR of SILMs showed the signature of the functionalities for both base membrane and the imidazolium moiety. A significant reduction in contact angle was observed for SILMs compared to the base membrane indicating the enhancement in surface hydrophilicity of the membrane, while considerable changes zeta potential was measured at different pH indicating the modification in surface charge. The SEM image confirmed the pore filling of base membrane with ionic liquids. The stability of the SILMs was studied using hexane, water and DMF solvents. Tuning the anionic moieties, ionic liquids were utilized for aqueous and non-aqueous application through SILMs. For non-aqueous applications, the trend in separation efficiency was found to be divinylbenzene > styrene > toluene, which was attributed to the more Ï electron cloud density on extended conjugation leading to favorable interaction with ionic liquid influencing its transfer through SILMs. Preferential separation of Congo red (CR) and Ramazol Brilliant Blue R (RBBR) was achieved in water based application of the SILMs.
The ionic liquids were having affinity for the Ï electron cloud. This Ï electron cloud density for toluene is lower than styrene followed by divinyl benzene, due to the presence of additional double bonds in conjugations with the aromatic Ï electron cloud. Due to this extended conjugation, divinyl benzene interacted strongly with the ionic liquid sitting inside pores. This might be responsible for bringing the selectivity in the separation for the SILMs. The lower transport in case of allyl ionic liquid might be attributed to the presence of electron withdrawing sp2 hybridized carbon of the ally group resulting reduction in the electron density over imidazolium ring. On the other hand, for choloro and bromo ionic liquids, butyl and hexyl groups were present in the side chain of the imidazolium ring. The (+) inductive effect of the alkyl group containing sp3 hybridized carbon atom pushed the electron density towards imidazolium ring. More electron cloud on the imidazolium ring, it will be more favorable for the Ï electron interaction resulting enhancement in the separation efficiency.
Similar investigation was also carried out to understand the selectivity in separation of different dyes using SILMs with water insoluble ionic liquids. In this case three different dyes CR, EBT and RBBR were used. The SILMs were found to be more selective for CR. In case of hexyl imidazolium ionic liquid, almost 60 % CR was transferred to the receiver side, while for allyl imidazolium ionic liquid % transport of CR reached around 50 % after two hours. For the other dyes RBBR and EBT, the % transport through both the SILMs were found to be less than 5 %. Though all these dyes have aromatic moieties, their complicated structures with different functionalities and stereo-chemical arrangement might lead to different overall interaction beside electron cloud resulting in different selectivity through SILMs.
The stability of the supported liquid membranes is of major concerns. Therefore, it is interesting to investigate the stability of the SILMs. In view of this SILMs were allowed to be equilibrated in presence of water, DMF and hexane and at different time interval the conductivity of the medium were measured. For SILMs with water soluble ionic liquid, the conductivity of the medium was found to be modified in presence of water and DMF. In presence of water, the water soluble ionic liquids came out of the membrane pores resulting in modifying the supernatant conductivity. DMF being polar in nature, certainly interacts with the ionic liquid moieties in the membrane due to electrostatic interaction. As result, ionic liquid might get dissolved in DMF. This also led to changes in conductivity in DMF, while in case of hexane, up to 60 days, there is no change in the conductivity of the medium indicating the stability of the SILMs. Using these equilibrated membranes (30 daysâ equilibration), the separation of toluene, styrene and divinyl benzenes were carried out. There modification in separation efficiency were found to be within 10â15 %. These investigations revealed the high stability of SILMs with water soluble ionic liquid in hexane medium.
The antimicrobial activity of these SILMs were investigated using gram-positive bacteria (staphylococcus aureus and pseudomonas aeruginosa). More the number of carbon atom on the side chain of the imidazolium ring, more antimicrobial activity was observed. For staphylococcus aureus, the L/D ratio for the virgin membrane was kept 52. Over the time frame of 24 hours, the virgin membrane did not show any appreciable modification in the ratio, though some fluctuation is noticed. In case of SILM, a drastic decrease in L/D values were observed for all the ionic liquids upto a definite time (3 hours for bromohexane and chlorobutane ionic liquids; 5 hours for allylbromide ionic liquid), followed by almost constant values for L/D upto 24 hours. This investigating clearly demonstrates the antibacterial activity of SILMs compared to the virgin membrane. The antibacterial activity of allylbromide SILM was found to be slightly lower than that of chlorobutane or bromohexane SILMs. The end point was taken as 24 hours, because beyond that, there might be some appreciable modification in the L/D ratio for the original bacteria due to their life cycle.
Similar study was also carried out with another gram positive bacteria, pseudomonas aeruginosa starting with L/D ratio ~ 56 by monitoring the L/D ratio using luminescent spectra for 24 hours with a regular time interval. The trends were found to be similar to staphylococcus aureus. Though, this method is very convincing for evaluating the antibacterial activity, still it has some limitations. The bacteria having compromised membranes may be able to recover and reproduce but this method identify them as âdeadâ bacteria. Similarly, some bacteria may have intact cell membrane but unable to reproduce in nutrient medium, are assigned as âliveâ bacteria. Moreover, since imidazolium moiety is also able to give fluorescence, it may interfere in the determination of fluorescent intensity of the dyes indicating live and dead bacteria. Therefore, additional non fluorescent based technique should also be used to confirm the antimicrobial activity of SILMs.
The quantitative antibacterial analysis was carried out in terms of colony forming units per mL for bacteria. Large CFU/mL values, 2.82 E+09 and 3.5 E+09 were taken as the starting point for staphylococcus aureus andpseudomonas aeruginosa, respectively. Within 24 hours, there was almost no change in the CFU/mL values for both the bacteria for virgin membrane, while significant reduction in CFU values were observed for SILMs. After 5 hours of contact with the membranes the CFU/mL values become 2.82E+09, 4.4E+06, 1.10E+03 and 1.2E+04 for virgin membrane, SILMs with allylbromide, bromohexane and chlorobutane ionic liquids, respectively; whereas after 24 hours CFU/mL became 2.80 E+09, 9.01E+04, 2.10E+0 and 2.04E+01. The investigation clearly demonstrates, not only the antibacterial activity of SILMs, but also the trend of allylbromide < cholorobutane < bromohexane. This trend was found to be similar than that of earlier investigation. More the number of the carbon atom on the side chain of the imidazolium ring more pronounced antibacterial activity was observed. The quantitative antibacterial analysis was carried out in terms of colony forming units per mL for bacteria. Large CFU/mL values, 2.82 E+09 and 3.5 E+09 were taken as the starting point for staphylococcus aureus and pseudomonas aeruginosa, respectively. Within 24 hours, there was almost no change in the CFU/mL values for both the bacteria for virgin membrane, while significant reduction in CFU values were observed for SIL membranes. After 5 hours of contact with the membranes the CFU/mL values become 2.82E+09, 4.4E+06, 1.10E+03 and 1.2E+04 for virgin membrane, SILMs with allylbromide, bromohexane and chlorobutane ionic liquids, respectively; whereas after 24 hours CFU/mL became 2.80 E+09, 9.01E+04, 2.10E+0 and 2.04E+01. The investigation clearly demonstrates, not only the antibacterial activity of SILMs, but also the favorable transport trend of allylbromide < cholorobutane < bromohexane.