The Chameleon-Like Nature of Zwitterionic Micelles: The Effect of Ionic Liquid Addition on the Properties of Aqueous Sulfobetaine Micelles

We used fluorescence probing, ζ potentials, and dynamic light scattering measurements to study the interactions between the zwitterionic surfactant N‐hexadecyl‐N,N‐dimethylammonio‐1‐propanesulfonate (SB‐16) and three ionic liquids (ILs), 1‐ethyl‐3‐methylimidazolium ethylsulfate ([C2mim][C2SO4]), 1‐ethyl‐3‐methylimidazolium n‐butylsulfate ([C2mim][C4SO4]), and 1‐ethyl‐3‐methylimidazolium n‐hexylsulfate ([C2mim][C6SO4]). The three ILs have the same cationic part and their anionic parts differ only in the length of the alkyl chain. The aim of our work is to offer a comparative study and establish the role of the alkyl chain length of the anion of ILs on 1) the incorporation of these anions in the zwitterionic micelles of SB‐16 (selectivity of anions) and 2) the physicochemical properties of aqueous solutions of SB‐16. Results show that, at lower concentrations (i.e. ≤20 mM), the different ILs modify the properties of the aqueous SB‐16 solution in similar manner. All of them bring about a decrease in the critical micelle concentration (CMC) and also in size, and increase the aggregation number of the SB‐16 micelles; these effects are more dramatic when [C2mim][C6SO4] is used as the additive rather than [C2mim][C4SO4] and [C2mim][C2SO4]. It is proposed that, in case of [C2mim][C6SO4], the presence of a hexyl chain on the hexylsulfate ion allows the ion to align itself with the tail part of SB‐16, whereas, in the case of [C2mim][C2SO4], the presence of ethyl chain in the ethylsulfate ion is not sufficient to bring about a similar alignment of the ethylsulfate anion with the tail part of SB‐16. This difference in the location of the anions of the ILs is responsible for the different behavior of the ILs. Now you see me…︁ Fluorescence probing, ζ potentials, and dynamic light scattering measurements were used to study the interactions between the zwitterionic surfactant N‐hexadecyl‐N,N‐dimethylammonio‐1‐propanesulfonate (SB‐16) and three methylimidazolium ionic liquids with varying sulfate chain lengths (see picture).