(193bf) Single-Step Synthesis of Novel Polyionic Liquids Having Antibacterial Activity and Showing ?-Electron Mediated Selectivity in Separation of Aromatics

Kamaz, M., University of Arkansas
Sengupta, A., University of Arkansas
Jebur, M., University of Arkansas
Qian, X., University of Arkansas
Wickramasinghe, S. R., University of Arkansas
One pot synthesis of polyionic liquids (PILs) from pyridinium and phosphonium family was reported in the present investigation. The size exclusion chromatographic (SEC) method was employed for the determination of the molecular weight of the polymer and the dispersity during synthesis. The aggregation behavior of these PILs was investigated in polar- protic, polar- aprotic and apolar-aprotic solvents. The FTIR spectroscopic analysis was utilized for the characterization of the functional groups in PILs, whereas 1H NMR analysis was employed for structural characterization of the PILs in terms of the existence of magnetically non-equivalent protons. UV-Vis spectroscopic characterization was also employed for these PILs. The anti-bacterial activity of these PILs were investigated using two gram positive bacteria staphylococcus aureus and pseudomonas aeruginosa. Plate reader method was used to determine the ratio of live to dead bacterial cell in presence of PILs as a function of time by monitoring the fluorescence wavelength of the corresponding cells after tagging them suitably with the appropriate dye. The plate counting method was also employed for the direct counting of the number of colony of the bacteria as a function of time in presence of the synthesized PILs. A π electron mediated separation was found to bring the selectivity in sorption of aromatics on these PILs.

The size exclusion chromatographic (SEC) analysis or gel permeation chromatographic (GPC) analysis was carried out for the determination of molecular weight of the synthesized PILs. The calibration curve was established based on the different retention time for the dextran of different molecular weight ranging from 6 kDa to 500 kDa. The response i.e. retention time (Rt) was found to vary with the molecular weight by the following equation with a regression coefficient 0.9973. For the PILs from 2-vinyl pyridine and triphenyl allyl phosphonium bromide, the chromatogram showed single peak with retention time 11.4 and 11.8 min, respectively. This indicated that the presence mono-disperse polymers with molecular weight 20 kDa and 6 kDa, respectively. For PIL from 4 vinyl pyridine showed the major peak at 11.0 min with a very small peak in higher retention time region. This revealed that the PIL from 4-vinyl pyridine predominantly exists as a polymer with 60 kDa molecular weight, though a very small amount of low molecular weight polymer (less than 6 kDa) is generated during the fabrication. The relative intensity ratio calculated from the chromatogram revealed that 60 kDa PIL is almost 5 times more than the lower molecular weight PIL (less than 6 kDa) obtained from 4-vinyl pyridine.

Though the polymers are characterized by their definite molecular weight, yet in different medium, they may exist in different aggregated forms. The interaction between the polymers and the solvent molecules basically decides the size of aggregation of the polymer in different media. The aggregation behavior is very important as it decides the surface area as well as surface charge of the polymer. By changing the media, the aggregation behavior of the PIL can be fine-tuned and hence its particular application can also be fine-tuned. For the present case laser diffraction technique was employed for the determination of the size of aggregation. Different solvents were chosen such as: water and methanol (polar, protic); toluene (apolar, aprotic) and acetone (polar, aprotic). In most of the cases, the polymers were found to form aggregates of a definite size (except for toluene solution of PILs of 4-vinyl pyridine and triphenul allyl phosphonium bromide. A general trend was also observed for all the PILs, i.e. the size of the aggregation followed the trend water < methanol < acetone < toluene. In polar and protic solvents, the solvent molecules are expected to have stronger electrostatic interaction with the PIL molecules resulting smaller size of aggregates (as seen in case of water and methanol). On the other hand, toluene being apolar aprotic solvent might lead to less electrostatic interaction and more π stacking interaction leading to large size of aggregates. The molecular weight of the PILs were evaluated using SEC, and the size of the aggregates of these PILs in different media was evaluated using laser diffraction technique.

In case of water, the size indicated the presence of single PIL molecules, while in toluene maximum molecules were involved in forming aggregates. The above calculation is based on the assumption that the size of the aggregates is only filled up by the PIL molecules. There is almost negligible void space and the SEC analysis strictly gave the molecular weight of PIL molecules without any association/dissociation. The self-assembly of poly(4vinyl pyridine) derivatives have been extensively investigated in solvents having different polarity and at different temperature [54]. Spectroscopic (1H-NMR, Fluorescence and UV-Vis) and imaging (Scanning electron microscopy and Transmission Electron Microscopy) techniques were employed in understanding the aggregation behavior revealing the significant contribution of H-bonding and π-π stacking interaction.

These PILs were used for the uptake of toluene, divinyl benzene and styrene. These aromatics have different π electron density due to the difference in conjugation. For all these three ionic liquids, the trend in uptake of the aromatics was found to follow the order: toluene < styrene < divinyl benzene. More extended the conjugation of double bond, more will be the π electron density on the aromatic molecules. The trend in % sorption of these aromatic can be explained on the basis of π stacking interaction between the PIL polymers and the aromatics. More the electron density on the aromatics, more will be the π electron cloud mediated interaction, more will be the % of sorption. The selectivity of separation was found to more for the PIL from 2vinylpyridine followed by 4 vinyl pyridine, while for the PIL from phosphonium ionic liquid, the selectivity of sorption is not really impressive.