Impact of Aromatic Counter-Ions Charge Delocalization on the Micellization Behavior of Surface-Active Ionic Liquids

The nature of counter-ions governs the micellar and structural characteristics of surface-active ionic liquids (SAILs). Especially, the introduction of aromatic counter-ions significantly increases their surface adsorption and induces the formation of various types of aggregates like prolate ellipsoidal micelles, rodlike micelles, vesicles, lamellars, etc. The present study reports the role of charge delocalization of two different aromatic counter-ions in the micellization behavior of their respective SAILs in aqueous medium. For this purpose, we have synthesized two SAILs, namely, 1-tetradecyl-3-methylimidzolium phenolate [C₁₄mim][PO] and 1-tetradecyl-3-methylimidzolium benzoate [C₁₄mim][BZ]. The O-atom of phenolate (PO^-) possesses negative charge, which is delocalized on its phenyl ring. Conversely, the negative charge of benzoate (BZ^-) is not delocalized on its phenyl ring. The more hydrophobic BZ^- counter-ion increases the hydrophobic interactions and reduces the electrostatic repulsions more efficiently as compared to PO^-, which results in a lower critical micelle concentration (cmc) of [C₁₄mim][BZ] than that of [C₁₄mim][PO]. Interfacial properties obtained by tensiometry reveal better surface activity and absorption efficiency of [C₁₄mim][BZ] as compared to [C₁₄mim][PO]. The increase of cmc and degree of counter-ion binding (β) with the rise of temperature for both SAILs has been observed by conductometry. The decrease in the polarity of pyrene microenvironment explains the higher compactness of [C₁₄mim][BZ] aggregates than that of [C₁₄mim][PO], observed by fluorimetry. The position of PO^- and BZ^- is in the stern and palisade layers of C₁₄mim⁺ aggregates, respectively, located by ¹H NMR. The existence of prolate ellipsoidal micelles for both SAILs has been established by small-angle neutron scattering measurements. Thus, the interfacial and bulk properties of [C₁₄mim][PO] lie somewhere in between those of the SAILs having perfect aromatic counter-ions, [C₁₄mim][BZ], and the SAILs having regular inorganic counter-ions like Cl^-, Br^-, etc.