Chiranjib Ghatak

Probing the Interaction of 1-Ethyl-3-methylimidazolium Ethyl Sulfate ([Emim][EtSO4]) with Alcohols and Water by Solvent and Rotational Relaxation

The effect of the addition of cosolvents in the room temperature ionic liquid (RTIL) 1-ethyl-3-methylimidazolium ethyl sulfate ([Emim][EtSO(4)]) was probed by the solvent and rotational relaxation studies of coumarin 153 in neat ionic liquid [Emim][EtSO(4)] and [Emim][EtSO(4)]-cosolvent mixtures by using steady-state and time-resolved fluorescence spectroscopy. With gradual addition of cosolvents in the RTIL, both the average solvation time and rotational relaxation times gradually decrease. Addition of cosolvents in the IL decreases the viscosity of the medium. We have optimized the geometry of [Emim][EtSO(4)] and [Emim][EtSO(4)]-cosolvent mixtures by using quantum chemical calculations using density functional theory methods, which show the formation of hydrogen bond between cosolvents with [Emim][EtSO(4)]. With addition of the same amount of alcohols in neat [Emim][EtSO(4)], the rotational relaxation time decreases more compared to the addition of the same amount of water.

Spontaneous Transition of Micelle–Vesicle–Micelle in a Mixture of Cationic Surfactant and Anionic Surfactant-like Ionic Liquid: A Pure Nonlipid Small Unilamellar Vesicular Template Used for Solvent and Rotational Relaxation Study

The micelle-vesicle-micelle transition in aqueous mixtures of the cationic surfactant cetyl trimethyl ammonium bromide (CTAB) and the anionic surfactant-like ionic liquid 1-butyl-3-methylimidazolium octyl sulfate, [C4mim][C8SO4] has been investigated by using dynamic light scattering (DLS), transmission electron microscopy (TEM), surface tension, conductivity, and fluorescence anisotropy at different volume fractions of surfactant. The surface tension value decreases sharply with increasing CTAB concentration up to ∼0.38 volume fraction and again increases up to ∼0.75 volume fraction of CTAB. Depending upon their relative amount, these surfactants either mixed together to form vesicles and/or micelles, or both of these structures were in equilibrium. Fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene (DPH), incorporated in this system at different composition of surfactant indicates the formation of micelle and vesicle structures. The apparent hydrodynamic diameter of these large multilamellar vesicles is about ∼200 nm-300 nm obtained by DLS measurement and finally confirmed by TEM micrographs. The large multilamellar vesicles are transformed into small unilamellar ones by sonication using a Lab-line instruments probe sonicator with a diameter of ∼90-125 nm. To investigate the heterogeneity, solvent, and rotational relaxation of coumarin-153 (C-153) have been investigated in these unilamellar vesicles by using picosecond time-resolved fluorescence spectroscopic technique. The solvation dynamics of C-153 in these vesicles is found to be biexponential with average time constant ∼580 ps. This indicates the slow relaxation of water molecules in the surfactant bilayer. In accordance with solvation dynamics, fluorescence anisotropy analysis of C-153 in unilamellar vesicles also indicates hindered rotation compared to bulk water.

Designing a New Strategy for the Formation of IL-in-Oil Microemulsions

Due to the increasing applicability of ionic liquids (ILs) as different components of microemulsions (as the polar liquid, the oil phase, and the surfactant), it would be advantageous to devise a strategy by which we can formulate a microemulsion of our own interest. In this paper, we have shown how we can replace water from water-in-oil microemulsions by ILs to produce IL-in-oil microemulsions. We have synthesized AOT-derived surface-active ionic liquids (SAILs) which can be used to produce a large number of IL-in-oil microemulsions. In particular, we have characterized the phase diagram of the [C(4)mim][BF(4)]/[C(4)mim][AOT]/benzene ternary system at 298 K. We have shown the formation of IL-in-oil microemulsions using the dynamic light scattering (DLS) technique and using methyl orange (MO), betaine 30, and coumarin-480 (C-480) as probe molecules.

An Understanding of the Modulation of Photophysical Properties of Curcumin inside a Micelle Formed by an Ionic Liquid: A New Possibility of Tunable Drug Delivery System

The present study reveals the modulation of photophysical properties of curcumin, an important drug for numerous reasons, inside a micellar environment formed by a surfactant-like ionic liquid (IL-micelle) in aqueous solution. Higher stability of the drug inside IL-micelle in the absence and presence of a simple salt (sodium chloride) as well as considerably large partition coefficient (K(p) = 8.59 × 10(3)) to the micellar phase from water make this system a well behaved drug loading vehicle. Remarkable change in fluorescence intensity with a strong blue-shift implies the gradual perturbation of intramolecular hydrogen bond (H-bond) present within the keto-enol group of curcumin along with considerable formation of intermolecular H-bond between curcumin and the headgroup of surfactant-like IL. Very fast nonradiative decay channels in curcumin mainly caused by the excited state intramolecular proton transfer (ESIPT) are thus depleted remarkably in the presence of IL-micelle of reduced polarity and as a result of restricted rotational and vibrational degrees of freedom when bound to the micelle. Moreover, time-resolved results confirm that not only the keto-enol group of curcumin is playing here but also the phenolic hydroxyl groups are also responsible for such modulation in photophysical properties. From a thermodynamic point of view, our system shows good correlation with its stability parameters (higher binding constant with very less hydrolytic degradation rate ~1%) and higher negative value of binding enthalpy of interaction (-ΔH) than total free energy change (-ΔG) implies that the nature of binding interaction is enthalpy driven not entropy alone. Summarizing all the above observations, we have concluded that the modulation of the intramolecular proton transfer is due to the presence of both intermolecular proton transfer as well as strong hydrophobic interaction between curcumin and the IL-micelle.

Ionic Liquid Containing Microemulsions: Probe by Conductance, Dynamic Light Scattering, Diffusion-Ordered Spectroscopy NMR Measurements, and Study of Solvent Relaxation Dynamics

Room-temperature ionic liquid (RTIL), N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([P(13)][Tf(2)N]), was substituted for polar water and formed nonaqueous microemulsions with benzene by the aid of nonionic surfactant TX-100. The phase behavior of the ternary system was investigated, and microregions of [P(13)][Tf(2)N]-in-benzene (IL/O), bicontinuous, and benzene-in-[P(13)][Tf(2)N] (O/IL) were identified by traditional electrical conductivity measurements. Dynamic light scattering (DLS) revealed the formation of these IL microemulsions because with gradual increase of RTIL contents the droplet sizes of the microemulsions are also gradually increasing. Pulsed-field gradient spin-echo NMR have been studied to measure the diffusion coefficients of neat [P(13)][Tf(2)N] and [P(13)][Tf(2)N] in microemulsions which indicate ionic liquid containing microemulsions is formed. Moreover, the dynamics of solvent relaxation have been investigated in [P(13)][Tf(2)N]/TX100/benzene microemulsions using steady-state and time-resolved fluorescence spectroscopy using coumarin 153 (C-153) and coumarin 480 (C-480) fluorescence probe with variation of RTIL contents in microemulsions. For both of the probes with increasing amount of ionic liquids in microemulsions the relative contribution of the fast components increases and the slow components contribution decreases; therefore the average solvation time decreases.

Microemulsions with Surfactant TX100, Cyclohexane, and an Ionic Liquid Investigated by Conductance, DLS, FTIR Measurements, and Study of Solvent and Rotational Relaxation within this Microemulsion

Room-temperature ionic liquids (RTILs), N,N,N-trimethyl-N-propyl ammonium bis(trifluoromethanesulfonyl) imide ([N(3111)][Tf(2)N]), were substituted for polar water and formed nonaqueous microemulsions with cyclohexane by the aid of nonionic surfactant TX-100. The phase behavior of the ternary system was investigated, and microregions of [N(3111)][Tf(2)N]-in-cyclohexane (IL/O), bicontinuous, and cyclohexane-in-[N(3111)][Tf(2)N] (O/IL) were identified by traditional electrical conductivity measurements. Dynamic light scattering (DLS) revealed the formation of the IL microemulsions. The FTIR study of O-H stretching band of TX100 also supports this finding. The dynamics of solvent and rotational relaxation have been investigated in [N(3111)][Tf(2)N]/TX100/cyclohexane microemulsions using steady-state and time-resolved fluorescence spectroscopy as a tool and coumarin 480 (C-480) as a fluorescence probe. The size of the microemulsions increases with gradual addition of [N(3111)][Tf(2)N], which revealed from DLS measurement. This leads to the faster collective motions of cation and anions of [N(3111)][Tf(2)N], which contributes to faster solvent relaxation in microemulsions.

Ionic liquid-induced changes in properties of aqueous cetyltrimethylammonium bromide: a comparative study of two protic ionic liquids with different anions.

In this work we have shown a comparative study of changes in physicochemical properties of an aqueous solution of a common cationic surfactant cetyltrimethylammonium bromide (CTAB) upon addition of two protic ionic liquids N,N-dimethylethanolammonium [corrected] hexanoate (DAH) and N,N-dimethylethanolammonium [corrected] formate (DAF). The aim of this manuscript is to offer a comparative study and establish the role of the alkyl chain length of the anion of the added protic ionic liquids on the physicochemical properties of aqueous solution of CTAB. At lower concentration (i.e., ≤ 30 mM) both ionic liquids show the same trend in modifying the properties of aqueous CTAB solution, but DAH as an additive shows a more dramatic increase in aggregation number and size of the CTAB micelle compared to that of DAF as an additive. At higher concentrations of additives (DAF and DAH), the properties of aqueous CTAB solution change in an entirely different way. The size of the CTAB micelle was found to be 0.9 nm. With the addition of 215 mM DAH, the size of the CTAB micelle increases to 25.0 nm, whereas with the addition of 215 mM DAF it increases to only 5.6 nm. Zeta potential, electrical conductance, microviscosity, and dipolarity measurements were performed to gain insight into this abrupt size change in the case of DAH. It is proposed that the formate and hexanoate anions undergo Coulombic attractive interaction with cationic head groups of the CTAB micelle at all concentrations. In the case of DAH, the presence of a hexyl chain on the hexanoate ion allows it to align with the tail part of CTAB, whereas in the case of DAF the absence of an alkyl chain in the formate ion is apparently unable to align the formate anion with the tail part of CTAB. So this difference in the location of the anions of DAF and DAH is responsible for the different size changes and different behaviors of the two ionic liquids.

Room temperature ionic liquid in confined media: a temperature dependence solvation study in [bmim][BF4]/BHDC/benzene reverse micelles.

In this work, we reported a detailed study of the solvation dynamics of coumarin-480 in [bmim][BF(4)]/BHDC/benzene reverse micelles (RMs) with varying [bmim][BF(4)]/BHDC molar ratio (R) 1.00, 1.25, 1.50, and also study the solvation dynamics at five different temperatures from 15 to 35 °C RMs at [bmim][BF(4)]/BHDC molar ratio 1.25 for the first time. The average solvation time constant becomes slightly faster with the increase in R values at a temperature 25 °C. The solvation dynamics of the RMs with R value 1.25 becomes faster with the increase in temperature. We have also investigated temperature-dependent solvation dynamics in neat [bmim][BF(4)]. The solvation dynamics in neat [bmim][BF(4)] has a substantial temperature effect but for the [bmim][BF(4)]/BHDC/benzene RMs the temperature effect on the solvation dynamics is not that significant. Time-resolved fluorescence anisotropy studies reveal a decrease in the rotational restriction on the probe with increasing temperature. Wobbling-in-cone analysis of the anisotropy data also supports this finding.

Synthesis of silver nanoparticle inside the nonaqueous ethylene glycol reverse micelle and a comparative study to show the effect of the nanoparticle on the reverse micellar aggregates through solvation dynamics and rotational relaxation measurements.

In this work, we have reported the application of less familiar ethylene glycol-AOT reverse micelle for the synthesis of silver nanoparticle using glucose as mild reducing agent and isooctane as the continuous media. We have also studied the pure ethylene glycol-AOT reverse micelle and the perturbed reverse micelle (containing silver nanoparticle in its womb) through solvation dynamics measurement using steady-state and time-resolved fluorescence spectroscopy. Finally, we compared both of the results to get the valuable information about the perturbed reverse micellar system containing silver nanoparticle. Through the work, we found that in the pure reverse micellar system, with increasing ethylene glycol loading, solvation time was decreasing and anisotropy value became slower. In the perturbed reverse micellar system (containing silver nanoparticle) having the same environmental state, that is, at the same ethylene glycol content, solvent and rotational relaxation became slower and faster, respectively.

Effects of 1-butyl-3-methyl imidazolium tetrafluoroborate ionic liquid on Triton X-100 aqueous micelles: solvent and rotational relaxation studies.

The effect of added room-temperature ionic liquids on the nature of water molecules in the palisade layer of a Triton X-100 (TX-100) micelle has been investigated using solvation and rotational relaxation studies of coumarin 153 in the presence of different wt % of [bmim][BF(4)] and thus to understand the changes in micellar palisade layer, especially the entrapped water structures in the palisade layer. It has been observed that in the presence of added [bmim][BF(4)] the solvation dynamics becomes faster. It has previously been demonstrated (Behera et al. J. Chem. Phys.2007, 127, 184501) that in the present micellar systems, in the presence of [bmim][BF(4)] micellar size and aggregation number (N(agg)) decreases giving rise to more water molecules penetrating in to the micellar phase which results in increased microfluidity. In accordance with solvation dynamics results, fluorescence anisotropy studies also indicate an increased microfluidity for the palisade layer of the TX-100 micelle with the added [bmim][BF(4)]. Wobbling-in-cone analysis of the anisotropy data also supports this finding.