Ionic-Liquid-Induced Changes in the Properties of Aqueous Zwitterionic Surfactant Solution: Solvent and Rotational Relaxation Studies

In the recent past, the chameleon-like nature of zwitterionic micelles has been utilized for performing electrophilic, nucleophilic, base, and acid catalyzed reactions. But the use of simple salts to induce the zwitterionic character limits the variation to inorganic cations and anions only. To overcome this problem, we have used room temperature ionic liquids (RTILs), which can be tailored according to need. More precisely, we have shown the effect of added RTILs on the nature of water molecules in the palisade layer of a zwitterionic (N-hexadecyl-N,N-dimethylammonio-1-propanesulfonate (SB-16)) micelle using solvation and rotational relaxation studies of C-153 dye. We have carried out a comparative study of changes in the solvent and rotational relaxation parameters of C-153 in an aqueous solution of SB-16 upon addition of three different ionic liquids (ILs): 1-ethyl-3-methylimidazolium ethyl sulfate [C(2)mim] [C2SO4], 1-ethyl-3-methylimidazolium n-butyl sulfate [C(2)mim][C4SO4], and 1-ethyl-3-methylimidazolium n-hexyl sulfate [C(2)mim][C6SO4]. It has been observed that in the presence of added RTILs the solvation dynamics become faster and the change in solvation dynamics is more pronounced in the case of [C(2)mim[C6SO4] compared to that for [C(2)mim][C4SO4] and [C(2)mim][C2SO4]. This can be accounted for by considering the increased water penetration (increased microfluidity) with the addition of ILs. In accordance with solvation dynamics results, fluorescence anisotropy studies also indicate an increase in microfluidity of the palisade layer of the SB-16 micelle with the added RTILs. The average rotational relaxation time in 28 mM SB-16 was found to be 1.12 ns. With the addition of 800 mM [C(2)mim][C2SO4], the average rotational relaxation time remains the same (1.12 ns), whereas with the addition of 800 mM [C(2)mim][C6SO4] it decreases to 0.40 ns. This observation is in agreement with our earlier report on the microfluidity of SB-16 solution with the addition of [C(2)mim][C2SO4] and [C(2)mim][C6SO4] (Rao, V. G.; Ghatak, C.; Ghosh, S.; Mandal, S.; Sarkar, N. Chem. Phys. Chem. DOI: 10.1002/cphc.201100866).