(376a) Imidazolium Based Poly(ionic liquids), the Tunable Membranes Having Antimicrobial Activity

Due to the favorable properties like low vapour pressure, large electric potential window, high degree of chemical and thermal stability, wide liquidus range, high solubilizing capability of inorganic, organic and polymeric substances; room temperature ionic liquid finds a wide range application in the field of separation, catalysis, synthesis, electrochemistry and even in energy applications and hence they are world-wide considered as potential ‘green’ alternatives to highly volatile organic solvents. The most attractive property of ionic liquid is the high degree of tenability. A very small structural modification can bring significant changes in the physico-chemical properties of the ionic liquid and hence the suitable ionic liquid can be designed based on desired properties and applications, provided the structure-activity relationship is understood properly. Introduction of functional groups onto the ionic liquid has also been reported to be used for task specific applications. The presence of ionic liquid was also demonstrated to show some unusual chemistry, which otherwise is not observed in presence of conventional solvents. For example; the extraordinarily high extraction efficiency of metal ion in presence of ionic liquid was attributed to the predominance of ‘ion exchange’ mechanism, which otherwise proceeds through ‘solvation’ mechanism for conventional molecular diluents. During ‘ion exchange’ mechanism, the separation takes place at the cost of transfer of cationic or anionic part of ionic liquid. Hence, with time we are always going to loose the amount of ionic liquid. Ionic liquid being expensive, loosing of significant amount during large scale industrial applications is not worth. Moreover, usually the high viscosity coefficient of the ionic liquid makes the mass transfer kinetics slower during the biphasic separation.

In view of these, poly(ionic liquid) membranes are the solutions to address such issues. The poly(ionic liquid) (PIL) membranes are the polymeric membranes containing repeated units of ionic liquid and is a nice blend of membrane properties as well as ionic liquid properties. Being membrane, it is a solid polymeric boundary layer used for separation, hence the problems associated with ionic liquid loss and high viscosity become irrelevant. The PIL membranes may be obtained from polymerizable cationic part of ionic liquid or anionic part of ionic liquid or both. The combination of the unique properties of ILs with the macromolecular architecture in PIL membranes is not only fascinating but also provide way to creating new properties and functions in these novel materials in the field of membrane technology and advanced materials for energy applications. The enhanced mechanical stability, durability, spatial controllability over the IL species and improved processability are the major advantages of PIL membranes over ionic liquids.

A simple two step synthetic methodology was optimized for the preparation of three polyionic liquid membranes from imidazolium family. These PIL membranes were found to be hydrophilic, while with increase in polymerization time was found to reduce the hydrophilicity of the membranes. The FTIR spectroscopic analysis indicated the presence of polyvinyl moieties, polystyrene moieties, polyvinyl imidazolium moieties and nitrile group in these PIL membranes. The detailed structural characterization of these membranes were also done by ¹H and ¹³C NMR spectroscopy in terms of magnetically nonequivalent protons and carbons. At acidic pH (below pH 7), the surface charge of these membrane was found to be positive and it appeared zero near pH ~ 7, while alkaline pH showed negative charge on the membrane surface. The AFM and SEM imaging revealed that 15 min of polymerization led to fabrication of microfiltration as well as ultrafiltration membranes. These PIL membranes were found to have appreciable stability in presence of polar-protic; polar-aprotic and apolar-aprotic solvents, indicating its suitability in a wide variation of solvent applications. The plate counting method revealed that, in presence of PIL membranes, the number of colonies of staphylococcus aureus and pseudomonas aeruginosa reduced significantly and also with time. In case of plate reader, the ratio of live to dead bacterial cell was also found to reduce significantly in presence of PIL membranes. This antibacterial activity of PIL membranes were found to depend on the substituents present in the imidazolium ring. Finally, these PIL membranes were used for water flux measurement. After precompaction; membranes from allybromide and bromohexane showed water flux of ~982 LMH and 948 LMH (0.5 Psi), respectively; while that for chlorobutane PIL membrane was 315 LMH (1.5 Psi). Rejection in BSA and dextran revealed the tenability of the membrane’s property by different substituents on imidazolium ring and polymerization time.

The direct plate counting method was employed to count the colony forming unit of bacteria per mL at different time of contact with PIL membranes. In this present investigation, two gram positive bacteria; staphylococcus aureus and pseudomonas aeruginosa were used. The initial CFU/ mL was kept sufficiently high to observe significant reduction in antibacterial activity. The blank did not contain any PIL membranes (but same amount of CFU/mL). In blank, there is no appreciable change in the CFU/mL value upto 24 hours, while for PIL membranes, 5 h of contact led to significant reduction in the number of bacterial colony. This clearly indicated the anti-bacterial activity of the PIL membranes. It was also to be noted, the number of colonies after 5 hours of contact with different PILs are different, indicating difference in the antimicrobial activity of the PIL membranes. Though, all these PIL membranes are from imidazolium family, the nature of substituents on imidazolium ion played significant role in their antibacterial activity. After 24 hours, CFU/mL was found to decrease further. Though the PIL membranes showed similar trend in antibacterial activity for both the gram positive bacteria; staphylococcus aureus and pseudomonas aeruginosa, yet the extent of antibacterial activity was found to be slightly different. The PIL membrane derived from chlorobutane was found to have maximum antibacterial activity compared to that of bromohexane followed by the allylbromide derived PIL membrane.

The direct plate reading method is based on the preferential tagging of two different dyes on dead or live cells of the bacteria. Since these two dyes have their own characteristic photo luminescence emission, monitoring the intensity of their characteristic wavelengths gives a quantitative measurement of the ratio of live bacterial cell to dead bacterial cell. Both the gram positive bacteria staphylococcus aureus and pseudomonas aeruginosa were investigated in present study to confirm the antibacterial activity of these PIL membranes. The live to dead cell ratio of both the bacteria was monitored at each hour interval up to 25 hours. The initial L/D ratio values for staphylococcus aureus and pseudomonas aeruginosa were kept 65 and 58, respectively. For blank, the L/D ratio values for both the bacteria were not found to change drastically, although a minor variation was still seen. For PIL membranes, the L/D ratio values were found to decrease drastically with time. This investigation confirmed the antibacterial activity of these PIL membranes. This investigation also revealed that, the antibacterial activity of the PIL membranes from allylbromide was lesser compared to the other two, while other two PIL membranes showed almost similar antibacterial activity. Though both plate counting and plate reading method almost revealed the similar antibacterial activity, yet some variation was still present.