Investigating the Influence of Aromatic Counterions on the Micellar Structure and Aggregation Behavior of Morpholinium-Based Surface-Active Ionic Liquids in an Aqueous Solution.

Two morpholinium-based surface-active ionic liquids (SAILs) with aromatic counterions were synthesized, namely, n -dodecyl- n -methylmorpholinium salicylate [C 12 mmor][Sal] and n -dodecyl- n -methylmorpholinium 3-hydroxy-2-naphthoate [C 12 mmor][3- h -2- n ], and explored their aggregation behavior in aqueous solutions systematically. Electrical conductivity, small-angle neutron scattering (SANS), surface tension (ST), and UV-vis spectroscopy measurements were employed to determine various thermodynamic, micellar, and interfacial parameters, like the degree of counterion binding (β), critical micelle concentration (CMC), minimum area per molecule ( A min ), surface excess concentration (Γ max ), standard Gibbs free energy of adsorption (Δ G ad 0 ), aggregation number ( N agg ), standard Gibbs free energy of micellization (Δ G m 0 ), standard enthalpy of micelle formation (Δ H m 0 ), and the standard entropy of micellization (Δ S m 0 ) in an aqueous solution. Incorporating the aromatic counterions favors significantly excellent micellization properties over conventional halogenated SAILs such as [C 12 mmor][Br]. SANS analysis revealed that upon changing the counterion from salicylate to 3-hydroxy-2-naphthoate, the structure changed from prolate ellipsoidal micelles to large unilamellar vesicles. Also, increasing the concentration in the case of [C 12 mmor][Sal] resulted in a lower aggregation number.