(462f) Pi Electron Cloud Mediated Separation of Aromatics Using Supported Ionic Liquid (SIL) Membrane Having Antibacterial Activity

Authors: 
Wickramasinghe, R., University of Arkansas
Sengupta, A., University of Arkansas
Qian, X., University of Arkansas
Supported ionic liquid (SIL) membranes using imidazolium based ionic liquids and hydrophobic PTFE base membrane were used for the π electron mediated separation of aromatic compounds including dyes in aqueous and non-aqueous medium. The FTIR of SIL membranes showed the signature of the functionalities for both base membrane and the imidazolium moiety. A significant reduction in contact angle, especially the dynamic contact angle, was observed for SIL membranes compared to the virgin membrane indicating the enhancement in surface hydrophilicity of the membrane, while considerable changes zeta potential was seen 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 SIL membranes were studied using hexane, water and DMF as solvents. Tuning the anionic moieties, ionic liquids were utilized for aqueous and non-aqueous application through SIL membranes. For non-aqueous applications, the trend in separation efficiency was found to be divinylbenzene > styrene > toluene, which was attributed to the more π electron cloud density on extended conjugation leading to favorable interaction with ionic liquid influencing its transfer through SIL membranes. Preferential separation of Congo red (CR) and Ramazol Brilliant Blue R (RBBR) was achieved in water based application of the SIL membranes.

The ionic liquids were having affinity for the π electron cloud. This π 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 π electron cloud. Due to this extended conjugation, divinyl benzene interacted strongly with the ionic liquid sitting inside pore. This might be responsible for bringing the selectivity in the separation for the SIL membranes. 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 π electron interaction resulting enhancement in the separation efficiency.

Similar investigation was also carried out to understand the selectivity in separation of different dyes using SIL membranes with water insoluble ionic liquids. In this case three different dyes CR, EBT and RBBR were used. The SIL membranes were found to be more selective for CR. In case of hexyl imidazolium ionic liquid, almost 60 % CR was transferred to the receiver side within one and half hour, 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 SIL membranes 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 electron cloud density on them resulting different selectivity through SIL membranes.

The stability of the supported liquid membranes is of major concerns. Therefore, it is interesting to investigate the stability of the SIL membranes. In view of this SIL membranes 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 SIL membranes 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 pore resulting changes in water 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, upto 60 days, there is no change in the conductivity of the medium indicating the stability of the SIL membranes. Using these equilibrated membranes (30 days’ equilibration), the separation of toluene, styrene and divinyl benzenes were carried out. There modification in separation efficiency were found to be within 10 – 15 %. These investigations revealed the high stability of SIL membranes with water soluble ionic liquid in hexane medium.

The anti-microbial activity of these SIL membranes 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 SIL membrane 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 SIL membranes compared to the virgin membrane. The antibacterial activity of allylbromide SIL membrane was found to be slightly lower than that of chlorobutane or bromohexane SIL membranes. 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 signaturing 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 ‘dead’ bacteria. Similarly, some bacteria may have intact cell membrane but unable to reproduce in nutrient medium, are assigned as ‘live’ 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 SIL membranes.

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, SIL membranes 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 SIL membranes, 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, SIL membranes 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 SIL membranes, but also the trend of allylbromide < cholorobutane < bromohexane. This trend was found to be similar than that of earlier investigation.