Study the effect of substituent position in aromatic counterion to self-aggregation of cationic surface active ionic liquid in aqueous medium

Specific effects of m- and p-hydroxybenzoate anions (m-HB and p-HB) as counterion on the aggregation process of surface active ionic liquids based on 1-dodecyl-3-methylimidazolium were investigated by surface tension, conductivity and steady state fluorescence measurements. Compared to that for 1-dodecyl-3-methylimidazolium bromide (C12mimBr), lower critical micelle concentration (cmc) and larger Πcmc and pC20 values for these SAILs indicate that the introduction of aromatous counterions favors micellization and reduction of surface tension. The most incorporated salicylate anion can effectively screen electrostatic repulsion, promoting a closer arrangement of C12mimSal at air-solution interface. Thus m- and p-C12mimHB have smaller Γmax and larger Amin values than C12mimSal. From the results of conductivity measurements, the cmc values of these SAILs increase slightly with increasing temperature, which has similar trend with cationic SAIL, 1-alkyl-3-methylimidazolium bromide, [Cnmim]Br. The negative ΔGmθ values, mainly result from more negative-T ΔSmθ, reveal that the micellization process is spontaneous and entropy-driven in the whole studied temperature range. The data also show that ΔHmθ values of all systems are negative, which implies that the micellization process is exothermic. This work suggests that small changes, such as position of the substitute on aromatic ring of counterion, can considerably influence the micellization behavior of SAILs.