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

Abstract

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][C(2)SO(4)], 1-ethyl-3-methylimidazolium n-butyl sulfate [C(2)mim][C(4)SO(4)], and 1-ethyl-3-methylimidazolium n-hexyl sulfate [C(2)mim][C(6)SO(4)]. 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][C(6)SO(4)] compared to that for [C(2)mim][C(4)SO(4)] and [C(2)mim][C(2)SO(4)]. 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][C(2)SO(4)], the average rotational relaxation time remains the same (1.12 ns), whereas with the addition of 800 mM [C(2)mim][C(6)SO(4)] 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][C(2)SO(4)] and [C(2)mim][C(6)SO(4)] (Rao, V. G.; Ghatak, C.; Ghosh, S.; Mandal, S.; Sarkar, N. Chem. Phys. Chem. DOI: 10.1002/cphc.201100866).

Study of Fluorescence Resonance Energy Transfer in Zwitterionic Micelle: Ionic-Liquid-Induced Changes in FRET Parameters

“…The emission maximum is red-shifted from 536 to 541 nm with the addition of 800 mM [C 2 mim][C 6 SO 4 ] to 28 mM aqueous SB-16 micelles. The increased micellar hydration is responsible for the observed red shift with the addition of RTILs 60,61. We have also observed the absorption and emission behavior of R6G in the 28 mM aqueous SB-16 micelles.…”

“…The average lifetime of the donor molecule (τ 0 D ) in 28 mM aqueous SB-16 was found to be 4.10 ns (Table 2 SO 4 ] the microfluidity increases drastically. 60,61 This increased microfluidity around the C-153 molecule opens up an additional pathway for nonradiative decay, which causes reduction in the average lifetime.…”

“…60,61 We have already shown there that with the addition of [C 2 mim][C 6 SO 4 ] the penetration of water molecule increases to a great extent, 60 so the probe in the palisade layer undergoes a relative migration toward the micellar surface. 61 This migration causes reduction in distance between donor and acceptor, which results in the observed increase in rate of fluorescence energy transfer (k FRET ) or decrease of rise component. In order to lend support to this reasoning, the distances between donor and acceptor (R DA ) were determined.…”

“…61 In addition, the same study also revealed the movement of C-153 toward the outer surface of SB-16 micelles with the addition of ILs (particularly in case of [C 2 mim][C 6 SO 4 ]). 61 In this paper we have investigated FRET using C-153 as the donor and R6G as the acceptor because R6G, being cationic in nature, preferentially resides at the extreme outer surface of the micellar aggregate, whereas C-153, being hydrophobic in nature, preferentially resides in the inner nonpolar regions of the micellar aggregates. So, if the conclusion of the earlier paper (movement of C-153 toward the outer surface of SB-16 micelle) is correct, then we should observe a decrease in donor−acceptor distance with the addition of ILs, particularly [C 2 mim][C 6 SO 4 ].…”

“…Due to its unusual properties, an IL may demonstrate a unique role in altering the properties of aqueous surfactant solutions. − In our earlier work, we have shown a comparative study of changes in physicochemical properties of an aqueous solution of N -hexadecyl- N , N -dimethylammonio-1-propanesulfonate (SB-16) micelles upon addition of the three room temperature ionic liquids (RTILs), 1-ethyl-3-methylimidazolium ethylsulfate [C 2 mim][C 2 SO 4 ], 1-ethyl-3-methylimidazolium n -butylsulfate [C 2 mim][C 4 SO 4 ], and 1-ethyl-3-methylimidazolium n -hexylsulfate [C 2 mim][C 6 SO 4 ] . We have also observed the effect of added RTILs on the nature of water molecules in the palisade layer of a zwitterionic micelle using solvation and rotational relaxation studies of C-153 dye . In addition, the same study also revealed the movement of C-153 toward the outer surface of SB-16 micelles with the addition of ILs (particularly in case of [C 2 mim][C 6 SO 4 ]) .…”