Vishal Govind Rao

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.

A Step toward the Development of High-Temperature Stable Ionic Liquid-in-Oil Microemulsions Containing Double-Chain Anionic Surface Active Ionic Liquid

Owing to their fascinating properties and wide range of potential applications, interest in nonaqueous microemulsions has escalated in the past decade. In the recent past, nonaqueous microemulsions containing ionic liquids (ILs) have been utilized in performing chemical reactions, preparation of nanomaterials, synthesis of nanostructured polymers, and drug delivery systems. The most promising fact about IL-in-oil microemulsions is their high thermal stability compared to that of aqueous microemulsions. Recently, surfactant-like properties of surface active ionic liquids (SAILs) have been used for preparation of microemulsions with high-temperature stability and temperature insensitivity. However, previously described methods present a limited possibility of developing IL-in-oil microemulsions with a wide range of thermal stability. With our previous work, we introduced a novel method of creating a huge number of IL-in-oil microemulsions (Rao, V. G.; Ghosh, S.; Ghatak, C.; Mandal, S.; Brahmachari, U.; Sarkar, N. J. Phys. Chem. B2012, 116, 2850-2855), composed of a SAIL as a surfactant, room-temperature ionic liquids as a polar phase, and benzene as a nonpolar phase. The use of benzene as a nonpolar solvent limits the application of the microemulsions to temperatures below 353 K. To overcome this limitation, we have synthesized N,N-dimethylethanolammonium 1,4-bis(2-ethylhexyl) sulfosuccinate (DAAOT), which was used as a surfactant. DAAOT in combination with isopropyl myristate (IPM, as an oil phase) and ILs (as a polar phase) produces a huge number of high-temperature stable IL-in-oil microemulsions. By far, this is the first report of a huge number of high-temperature stable IL-in-oil microemulsions. In particular, we demonstrate the wide range of thermal stability of [C6mim][TF2N]/DAAOT/IPM microemulsions by performing a phase behavior study, dynamic light scattering measurements, and (1)H NMR measurements and by using coumarin-480 (C-480) as a fluorescent probe molecule.

Phase Boundaries, Structural Characteristics, and NMR Spectra of Ionic Liquid-in-Oil Microemulsions Containing Double Chain Surface Active Ionic Liquid: A Comparative Study

A method developed for the first time, to create a huge number of ionic liquid (IL)-in-oil microemulsions has been discussed in our earlier publication (Rao, V. G.; Ghosh, S.; Ghatak, C.; Mandal, S.; Brahmachari, U.; Sarkar, N. J. Phys. Chem. B 2012, 116, 2850-2855). Here, we present facile methods to adjust the structural parameters of microemulsions using different ionic liquids (ILs) as additives (polar phase). We have characterized ILs/[C(4)mim][AOT]/benzene ternary system by performing a phase behavior study, dynamic light scattering (DLS) measurements, and (1)H NMR measurements. The IL loading capacity of microemulsions (area of single phase region) (i) increases with increase in alkyl chain length of cation of ILs and follows the trend [C(6)mim][TF(2)N] > [C(4)mim][TF(2)N] > [C(2)mim][TF(2)N], (ii) increases with decrease in cation anion interaction strength of added ILs and follows the trend [C(4)mim][TF(2)N] > [C(4)mim][PF(6)] > [C(4)mim][BF(4)]. So depending on the IL used, the amount of IL within the core of microemulsions can be easily manipulated to directly affect the size of aggregates in microemulsions. The size increase with increasing R value (R value is defined as the molar ratio of RTILs to [C(4)mim][AOT]) was found to be maximum in the case of [C(2)mim][TF(2)N]/[C(4)mim][AOT]/benzene microemulsions and follows the trend [C(2)mim][TF(2)N] > [C(4)mim][TF(2)N] > [C(6)mim][TF(2)N]. However, the size increase was almost the same with increase in R value in the case of ILs with different anions. The most promising fact about IL-in-oil microemulsions is their high thermal stability compared to that of aqueous microemulsions, so we investigated the effect of temperature on size of aggregates in microemulsions at R = 1.0. It is evident from dynamic light scattering measurements that the aggregates in microemulsions remain monodisperse in nature with increasing temperature, and in all the cases, the size of aggregates in microemulsions decreases with increasing temperature. The effect of water addition on IL-in-oil (IL/O) microemulsions was also studied in detail. By far, this is the first report where the effect of water addition on microemulsions containing hydrophobic ILs is being reported and compared with microemulsions containing hydrophilic ILs. We observed that the added water has a prominent effect on the microstructure of the microemulsions. In all the cases, (1)H NMR spectra provide more detailed information about intra/intermolecular interactions thus affording a clear picture of locations of (i) the RTILs in RTILs/[C(4)mim][AOT]/benzene microemulsions and (ii) the added water molecules in microemulsions.

Ionic Liquid-in-Oil Microemulsions Composed of Double Chain Surface Active Ionic Liquid as a Surfactant: Temperature Dependent Solvent and Rotational Relaxation Dynamics of Coumarin-153 in [Py][TF2N]/[C4mim][AOT]/Benzene Microemulsions

In the recent past, nonaqueous microemulsions containing ionic liquids (ILs) have been utilized for performing chemical reactions, preparation of nanomaterials, and synthesis of nanostructured polymers and in drug delivery systems. The most promising fact about IL-in-oil microemulsions is their high thermal stability compared to that of aqueous microemulsions. In our earlier publication (Rao, V. G.; Ghosh, S.; Ghatak, C.; Mandal, S.; Brahmachari, U.; Sarkar, N. J. Phys. Chem. B 2012, 116, 2850-2855), we presented for the first time the possibility of creating huge number of IL-in-oil microemulsions, just by replacing the inorganic cation, Na(+), of NaAOT by any organic cation and using different ionic liquids as the polar core. In this manuscript we are interested in exploring the effect of temperature on such systems. We have characterized the phase diagram of the [Py][TF2N]/[C4mim][AOT]/benzene ternary system at 298 K. We have shown that in the experimental temperature range employed in this study, the microemulsions remain stable and a slight decrease in the size of the microemulsions is observed with increasing temperature. We have reported the detailed study of solvent and rotational relaxation of coumarin 153 (C-153) in neat IL, N-methyl-N-propylpyrrolidinium bis((trifluoromethyl)sulfonyl)imide ([Py][TF2N]), and in [Py][TF2N]/[C4mim][AOT]/benzene microemulsions using steady state and picosecond time-resolved spectroscopy. We have monitored the effect of (i) varying the [Py][TF2N]/[C4mim][AOT] molar ratio (R value) and (ii) temperature on solvent and rotational relaxation of C-153. The features observed in absorption and emission spectra clearly indicate that (i) the probe molecules reside at the polar interfacial region of the [Py][TF2N]/[C4mim][AOT]/benzene microemulsions and (ii) with increasing R value the probe molecules move toward the polar IL-pool of the microemulsion. We have shown that the increase in solvation time on going from neat [Py][TF2N] to [Py][TF2N]-containing microemulsions is very small compared to the increase in solvation time on going from pure water to water-containing microemulsions. The average solvation time decreases with increasing R values at 298 K, but it shows only a small R dependence compared to microemulsions containing solvents capable of forming hydrogen bonds. We have also shown that the temperature has substantial effect on the solvent and rotational relaxation of C-153 in neat [Py][TF2N] compared to that of [Py][TF2N]/[C4mim][AOT]/benzene microemulsions at R = 0.69.

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.