Micellization and characterization of micelle microenvironment of N-alkylimidazolium ionic liquids in aqueous solution

G. G. FORTUNATO, M. V. BRAVO AND C.G. ADAM

Foz iguazu

Conferencia; 12th Latin Conference on Physical Organic Chemistry. April 07 - 12, 2012, Foz do Iguaçu, Brasil; 2013

The replacement of harmful and volatile organic solvents traditionally used in industry has become a mayor interest as part of green chemistry. Therefore, novel compounds like ILs are under intensive examination. The physicochemical and solvation properties of ILs can be effectively tuned by simply tailoring the cation and/or the anion. Besides, ILs possessing long aliphatic chains can undergo micellization in aqueous solutions. Thus, due to their structural feature, ILs may represent a unique class of “designer surfactants” in which the substituent on the headgroup and the counterion can be widely varied. Micelle formation is undoubtedly important in relation to the future uses, applications and environmental fate of ILs. ILs composed of 1-alkyl-3-methylimidazolium cation are being extensively studied in the field of colloid and interface science. In contrast, ILs based on 1-alkylimidazolium cation have been less investigated. The main aim of this work is to investigate the self-aggregation behavior of a series of ILs with N-alkylimidazolium cations CnH2n+1Im (n= 8, 10, 12, 14) having trifluoracetate (CF3CO2), methanesulfonate (CH3SO3) and tetrafluoroborate (BF4) anions as counterions. All ILs and the related N-alkylimidazoles were properly synthesized and purified. The aggregation behavior of these ILs in aqueous solution has been investigated employing electrical conductivity and absorption and fluorescence Uv-Vis spectroscopy. The latter was accomplished by employing hydrophobic probe molecules, such as nile red (NR), hidroxi-nile red (HONR) and pyrene (Py). This last probe was also employed to determine the mean aggregation number (Nag) of the micellar systems through fluorescence quenching. As expected, the micellization process was highly dependent on the anion nature and on the length of the alkyl chain, being [BF4] the less favorable anion to promote self-aggregation for alkyl substituent with 8 and 10 carbon atoms. The NR and HONR probes turned out to be highly sensitive molecules in order to analyze the micellization process allowing also evaluating the pre-micellar region. The micellar phase polarity was evaluated by means of I1/ I3 parameter (from pyrene) and ENR or EHONR molar transition energies. The last ones were correlated with the ET (30) polarity. The results suggest that the most likely probe location is the interface. The experimental results confirmed that N-alkylimidazolium ILs can undergo micellization in aqueous solution. Additionally, from the critical aggregation concentrations values obtained it can be inferred that, for a same counterion, the N-alkylimidazolium cation is more effective to promote self-assembling than a N,N-dialkyl one