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.