Phase boundaries, structural characteristics, and NMR spectra of ionic liquid-in-oil microemulsions containing double chain surface active ionic liquid: A comparative study

A method developed for the first time, to create a huge number of ionic liquid (IL)-in-oil microemulsions has been discussed in our earlier publication (Rao, V. G.; Ghosh, S.; Ghatak, C.; Mandal, S.; Brahmachari, U.; Sarkar, N. J. Phys. Chem. B2012, 116, 2850-2855). Here, we present facile methods to adjust the structural parameters of microemulsions using different ionic liquids (ILs) as additives (polar phase). We have characterized ILs/[C₄mim][AOT]/ benzene ternary system by performing a phase behavior study, dynamic light scattering (DLS) measurements, and ¹H NMR measurements. The IL loading capacity of microemulsions (area of single phase region) (i) increases with increase in alkyl chain length of cation of ILs and follows the trend [C₆mim][TF₂N] > [C₄mim][TF₂N] > [C₂mim][TF₂N], (ii) increases with decrease in cation anion interaction strength of added ILs and follows the trend [C ₄mim][TF₂N] > [C₄mim][PF₆] > [C₄mim][BF₄]. So depending on the IL used, the amount of IL within the core of microemulsions can be easily manipulated to directly affect the size of aggregates in microemulsions. The size increase with increasing R value (R value is defined as the molar ratio of RTILs to [C ₄mim][AOT]) was found to be maximum in the case of [C ₂mim][TF₂N]/[C₄mim][AOT]/benzene microemulsions and follows the trend [C₂mim][TF₂N] > [C ₄mim][TF₂N] > [C₆mim][TF₂N]. However, the size increase was almost the same with increase in R value in the case of ILs with different anions. The most promising fact about IL-in-oil microemulsions is their high thermal stability compared to that of aqueous microemulsions, so we investigated the effect of temperature on size of aggregates in microemulsions at R = 1.0. It is evident from dynamic light scattering measurements that the aggregates in microemulsions remain monodisperse in nature with increasing temperature, and in all the cases, the size of aggregates in microemulsions decreases with increasing temperature. The effect of water addition on IL-in-oil (IL/O) microemulsions was also studied in detail. By far, this is the first report where the effect of water addition on microemulsions containing hydrophobic ILs is being reported and compared with microemulsions containing hydrophilic ILs. We observed that the added water has a prominent effect on the microstructure of the microemulsions. In all the cases, ¹H NMR spectra provide more detailed information about intra/intermolecular interactions thus affording a clear picture of locations of (i) the RTILs in RTILs/[C₄mim][AOT]/benzene microemulsions and (ii) the added water molecules in microemulsions.