G. B. Dutt

Rotational Diffusion of Nonpolar and Charged Solutes in Propylammonium Nitrate–Propylene Glycol Mixtures: Does the Organized Structure of the Ionic Liquid Influence Solute Rotation?

Rotational diffusion of two structurally similar nonpolar and charged solutes has been examined in mixtures of an ionic liquid and an organic solvent of comparable size and viscosity with an intent to find out whether the organized structure of the former influences solute rotation. To this effect, temperature-dependent fluorescence anisotropies of 9-phenylanthracene (9-PA) and rhodamine 110 (R110) have been measured in n-propylammonium nitrate (PAN), propylene glycol (PG), and also four different compositions of PAN-PG mixtures. Analysis of the data carried out with the aid of Stokes-Einstein-Debye (SED) hydrodynamic theory indicates that the reorientation times of 9-PA and R110 scale more or less linearly with the ratio of viscosity to temperature and are found to be independent of the mole fraction of PAN. In other words, apart from the viscosity and temperature, rotational diffusion of both the solutes is not affected by the composition of PAN-PG mixtures. It has also been observed that the reorientation times of R110 are significantly longer compared to those of 9-PA due to the specific interactions prevailing between the cationic solute and PAN-PG mixtures. However, the important finding of this work is that, even though PAN forms an organized structure, rotational diffusion of the solute molecules is similar in both the ionic liquid and the organic solvent. The disordered lamellar structure present in PAN probably does not offer compact organized domains unlike ionic liquids with long alkyl chains wherein solute rotation is influenced significantly.

Rotational Diffusion of Nondipolar and Charged Solutes in Alkyl-Substituted Imidazolium Triflimides: Effect of C2 Methylation on Solute Rotation

Rotational diffusion of a nondipolar solute 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and a charged solute rhodamine 110 (R110) has been investigated in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM][Tf2N]) and 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide ([BMMIM][Tf2N]) to understand the influence of the C2 methylation on solute rotation. The measured reorientation times of the nondipolar solute DMDPP are similar in both the ionic liquids and follow Stokes-Einstein-Debye hydrodynamic theory with slip hydrodynamics. In contrast, rotational diffusion of the charged solute R110 in [BMIM][Tf2N] obeys stick hydrodynamics due to specific interactions with the anion of the ionic liquid. Nevertheless, the intriguing result of this study is that the reorientation times of R110 in [BMMIM][Tf2N] deviate significantly from the predictions of stick hydrodynamics, especially at ambient temperatures. The solute-solvent boundary condition parameter Cobs, which is defined as the ratio of the measured reorientation time to the one calculated using the SED theory with stick boundary condition, for R110 is lower by a factor of 2 in [BMMIM][Tf2N] compared to [BMIM][Tf2N] at 298 K. Upon increasing the temperature, Cobs gradually increases and eventually matches with that obtained in [BMIM][Tf2N] at 348 K. It has been well established that methylation of the C2 position in [BMMIM][Tf2N] switches off the main hydrogen-bonding interaction between the anion and the cation, but increases the Coulombic interactions. As a consequence of the enhanced interionic interactions between the cation and anion of the ionic liquid, specific interactions between R110 and [Tf2N] diminish leading to the faster rotation of the solute. However, such an influence is not apparent in case of DMDPP as it does not experience specific interactions with either the cation or the anion of these ionic liquids.

Effect of Low Viscous Nondipolar Solvent on the Rotational Diffusion of Structurally Similar Nondipolar Solutes in an Ionic Liquid

Fluorescence anisotropies of two structurally similar nondipolar solutes, 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP), have been measured in 1-methyl-3-octylimidazolium hexafluorophosphate-dibenzyl ether ([MOIM][PF6]-DBE) mixtures to understand how the addition of a low viscous nondipolar solvent influences solute rotation. The data when analyzed with Stokes-Einstein-Debye hydrodynamic theory reveals that the measured reorientation times of DMDPP are closer to the predictions of slip boundary condition, whereas those of DPP follow stick hydrodynamics. This outcome arises due to specific interactions between DPP and the solvent medium. Nevertheless, the important result of this study is that the rotational diffusion of DMDPP becomes gradually slower with an increase in the mole fraction of DBE (xDBE) for a given viscosity and temperature. In contrast, such a trend is not noticed for the hydrogen-bond donating solute DPP. Instead, two sets of reorientation times have been obtained, one corresponding to xDBE = 0.0-0.2 and the other xDBE = 0.4-1.0. The results for DMDPP have been rationalized on the basis of the organized structure of [MOIM][PF6], which attains homogeneity at the microscopic level with an increase in xDBE. In case of DPP, however, the propensity of the solute to be in the neighborhood of DBE, as a consequence of its stronger hydrogen bond accepting ability compared to the ionic liquid, appears to be the reason for the observed behavior.

Rotational Diffusion of Organic Solutes in 1-Methyl-3-octylimidazolium Tetrafluoroborate–Diethylene Glycol Mixtures: Influence of Organic Solvent on the Organized Structure of the Ionic Liquid

Rotational diffusion of two structurally similar organic solutes, 9-phenylanthracene (9-PA) and rhodamine 110 (R110), has been investigated in 1-methyl-3-octylimidazolium tetrafluoroborate-diethylene glycol ([MOIM][BF4]-DEG) mixtures to understand the influence of organic solvent on the organized structure of the ionic liquid. The reorientation times (τ(r)) of nonpolar and charged solutes have been measured as a function of viscosity (η) by changing the temperature (T) as well as the composition of the ionic liquid-organic solvent mixture. These results when analyzed using the Stokes-Einstein-Debye (SED) hydrodynamic theory follow the relationship τ(r) = A(η/T)(n), where A is the ratio of hydrodynamic volume of the solute to Boltzmann constant. However, in neat [MOIM][BF4] and up to 0.4 mole fraction of DEG (x(DEG)), significant deviations from the SED hydrodynamic theory have been noticed with n being much less than unity. As x(DEG) is increased further, the parameters A and n increase considerably for both solutes, and their rotational diffusion follows the predictions of the SED hydrodynamic theory. It has also been observed that the trends in the variation of τ(r) with η/T for 9-PA and R110 are not similar. These observations have been rationalized by taking into consideration the organized structure of the ionic liquid, which gradually becomes homogeneous at the microscopic level with the addition of the organic solvent.

Does Addition of an Electrolyte Influence the Rotational Diffusion of Nondipolar Solutes in a Protic Ionic Liquid

Rotational diffusion of two structurally similar nondipolar solutes, 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP), has been examined in ethylammonium nitrate-lithium nitrate (EAN-LiNO3) mixtures to understand the influence of added electrolyte on the local environment experienced by the solute molecules. The measured reorientation times of both DMDPP and DPP in EAN-LiNO3 mixtures fall within the broad limits set by the hydrodynamic slip and stick boundary conditions. The hydrogen bond accepting DMDPP and the hydrogen bond donating DPP experience specific interactions with the cation and anion of the ionic liquid, respectively. Addition of LiNO3 (0.1 and 0.2 mole fraction) to EAN induces only viscosity related effects on the rotational diffusion of the two nondipolar solutes. These observations suggest that the local environment experienced by DMDPP and DPP in EAN is not altered upon the addition of LiNO3. Our results are consistent with the structural details available in the literature for EAN-LiNO3 mixtures.

Rotational Dynamics of Imidazolium-Based Ionic Liquids: Do the Nature of the Anion and the Length of the Alkyl Chain Influence the Dynamics?

The rotational dynamics of 1-alkyl-3-methylimidazolium-based ionic liquids has been investigated by monitoring their inherent fluorescence with the intent to unravel the characteristics of the emitting species. For this purpose, temperature-dependent fluorescence anisotropies of 1-alkyl-3-methylimidazolium (alkyl = ethyl and hexyl) ionic liquids with anions such as tris(pentafluoroethyl)trifluorophosphate ([FAP]), bis(trifluoromethylsulfonyl)imide ([Tf2N]), tetrafluoroborate ([BF4]), and hexafluorophosphate ([PF6]) have been measured. It has been observed that the reorientation times (τr) of the ionic liquids with an ethyl chain scale linearly with viscosity and were found to be independent of the nature of the anion. The experimentally measured τr values are a factor of 3 longer than the ones calculated for 1-ethyl-3-methylimidazolium cation using the Stokes-Einstein-Debye (SED) hydrodynamic theory with stick boundary condition, which suggests that the emitting species is not the imidazolium moiety but some kind of associated species. The reorientation times of ionic liquids with a hexyl chain, in contrast, follow the trend τr([FAP]) > τr([Tf2N]) = τr([BF4]) > τr([PF6]) at a given viscosity (η) and temperature (T). The ability of the ionic liquids with longer alkyl chains to form the organized structure appears to be responsible for the observed behavior considering the fact that significant deviations from linearity have been noticed in the τr versus η/T plots for strongly associating anions [BF4] and [PF6], especially at ambient temperatures.

Rotational Diffusion of Charged and Nondipolar Solutes in Ionic Liquid–Organic Solvent Mixtures: Evidence for Stronger Specific Solute–Solvent Interactions in Presence of Organic Solvent

Rotational diffusion of a charged solute, rhodamine 110 (R110), and a nondipolar solute, 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP), has been investigated in ionic liquids, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM][Tf2N]) and 1-butyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([BMIM][FAP]), with 0.8 mole fraction of dibenzyl ether (DBE). This study has been undertaken to find out how specific interactions between the solute and the ionic liquid are affected upon dilution with a nondipolar solvent. It has been observed that at a given viscosity (η) and temperature (T), the reorientation times of R110 increase by 40-60% in the ionic liquid-organic solvent mixtures compared to ones in the corresponding neat ionic liquids. In the case of DMDPP, the influence of DBE is less pronounced, and its reorientation times increase by 25-50% at a given η/T. The addition of DBE weakens the numerous interactions prevailing between the cations and the anions of the ionic liquids, which results in stronger specific interactions between the solutes and the constituent ions, consequently leading to slower rotation of the solutes.

Rotational Diffusion of Nonpolar and Ionic Solutes in 1-Alkyl-3-methylimidazolium Tetrafluoroborate-LiBF4 Mixtures: Does the Electrolyte Induce the Structure-Making or Structure-Breaking Effect?

Rotational diffusion of three structurally similar solutes, 9-phenylanthracene (9-PA), fluorescein (FL), and rhodamine 110 (R110), has been investigated in 1-butyl-3-methylimidazolium tetrafluoroborate-lithium tetrafluoroborate ([BMIM][BF4]-LiBF4) mixtures to understand the influence of the added electrolyte on the mobility of nonpolar, anionic, and cationic solute molecules. It has been observed that the reorientation times of the nonpolar solute 9-PA become progressively shorter with an increase in the concentration of LiBF4 at a given viscosity (η) and temperature (T). In the case of ionic solutes also, a decrease in the reorientation times has been observed upon the addition of the electrolyte compared to those obtained in the neat ionic liquid at a given η/T. However, this decrease is found to be independent of [LiBF4]. 9-PA being a nonpolar solute is located in the nonpolar domains of the ionic liquid. An enhancement in [LiBF4] leads to an increase in the sizes of the nonpolar domains resulting in the faster rotation of the solute. Anionic solute FL and cationic solute R110, which are located in the ionic region experience specific interactions with the cation and anion of the ionic liquid, respectively. In the presence of electrolyte, however, the strengths of these specific interactions diminish as the ions of the ionic liquid are not readily accessible to the solute molecules due to the organized structure, which results in faster rotation. These observations suggest that addition of LiBF4 induces a structure-making effect in the ionic liquid.

Can Critical Packing Parameter Depict Probe Rotation in Block-Copolymer Reverse Micelles?

Rotational diffusion of two ionic probes, cationic rhodamine 110 (R110) and anionic fluorescein (FL), has been examined in reverse micelles formed with the triblock copolymer (EO)13-(PO)30-(EO)13 (L64), where EO and PO represent ethylene oxide and propylene oxide units, respectively, with small amounts of water in p-xylene. This study has essentially been undertaken to explore the influence of mole ratio of water to copolymer (W) as well as copolymer concentration on probe rotation. On the basis of fluorescence lifetimes and reorientation times, it has been established that both R110 and FL are located in the interfacial region of L64/water/p-xylene reverse micellar system. The average reorientation time decreases by 10-35% with an increase in W for both the probes at a given copolymer concentration. However, for a particular W, the average reorientation time increases by 10-30% as the concentration of the copolymer is enhanced. From the micellar structural parameters available in the literature, critical packing parameters have been calculated for the L64/water/p-xylene reverse micellar system, and it has been noticed that the average reorientation times of both the probes scale linearly with the critical packing parameter. In essence, the results of this study indicate that the probe mobility in the interfacial region of block copolymer reverse micelles is governed by the micellar packing.

How Does the Alkyl Chain Length of an Ionic Liquid Influence Solute Rotation in the Presence of an Electrolyte

Fluorescence anisotropies of a nonpolar solute, 9-phenylanthracene (9-PA), have been measured in 1-alkyl-3-methylimidazolium (alkyl = methyl, butyl, octyl, and dodecyl) bis(trifluoromethylsulfonyl)imides ([RMIM][Tf2N]) with varying amounts (0-0.3 mole fraction) of lithium bis(trifluoromethylsulfonyl)imide (LiTf2N). This study has been carried out to understand how the length of the alkyl chain and the concentration of the electrolyte influence the rotational diffusion of a nonpolar solute. It has been observed that the addition of an electrolyte to the ionic liquid increases the bulk viscosity of the system significantly, as the Li+ cations strongly coordinate with the [Tf2N] anions in the polar domains. The reorientation times of 9-PA have been analyzed with the aid of Stokes-Einstein-Debye hydrodynamic (SED) theory, and they fall within the broad limits set by the hydrodynamic slip and stick boundary conditions. However, deviations from the SED theory have been noticed upon addition of LiTf2N, and the influence of the electrolyte is more pronounced in the case of ionic liquids with shorter alkyl chains. The observed trends have been rationalized in terms of electrolyte-induced structural changes in these ionic liquids.