Rotational Diffusion of Nonpolar and Ionic Solutes in 1-Alkyl-3

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Rotational Diffusion of Nonpolar and Ionic Solutes in 1-Alkyl-3methylimidazolium Tetrafluoroborate–LiBF4 Mixtures: Does the Electrolyte Induce Structure-Making or Structure-Breaking Effect? Sugosh R Prabhu, and G. Bhaskar Dutt J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b10047 • Publication Date (Web): 09 Nov 2015 Downloaded from http://pubs.acs.org on November 15, 2015

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The Journal of Physical Chemistry

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

Sugosh R. Prabhu and G. B. Dutt* Radiation & Photochemistry Division Bhabha Atomic Research Centre Trombay, Mumbai 400 085, INDIA

———————————————————— *To whom correspondence should be addressed. E-mail: [email protected] Phone: (91) 22-2559-0302

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Abstract Rotational diffusion of three structurally similar solutes, 9-phenylanthracene (9-PA), fluorescein (FL) and rhodamine 110 (R110) has been investigated in 1-butyl-3methylimidazolium 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 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]. 9PA 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 structure-making effect in the ionic liquid.

Keywords: Fluorescence Anisotropy, Hydrodynamics, Ionic Liquid, Electrolyte, Reorientation Time, Specific Interactions

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1. Introduction The ability of ionic liquids to form organized structure and its subsequent influence on dynamical processes has been well-established.1-10 The existence of cooperative network of hydrogen bonds between the cations and the anions leads to the nanostructural organization of the ionic liquids.1 Even though the organized structure of the ionic liquids has been extensively investigated, little information is available concerning the influence of electrolytes on this nanostructure. During the past two years, efforts have been made to elucidate the structure of protic ionic liquids in the presence of inorganic salts.11-14 A majority of these studies deal with LiNO3 in protic ionic liquids such as ethylammonium nitrate (EAN) and propylammonium nitrate (PAN). It has been inferred that the organized structure of the ionic liquids is not appreciably affected even at high concentration of the salt (0.2 mole fraction of LiNO3). Presence of Li+ and extra NO3− ions in the polar domain disrupts packing in the nonpolar domain and produces a weak structure-breaking effect.12 Moreover, fluorescence anisotropy measurements carried out with a pair of structurally similar nondipolar solutes in EAN–LiNO3 system indicate that addition of LiNO3 to EAN induces only viscosity related effects on the rotational diffusion of the two solute molecules.15 Thus, the results of the structural and dynamical studies carried out in these systems suggest that addition of the electrolyte neither affects the organized structure of the protic ionic liquid nor the mobility of the dissolved solute molecules.

Unlike protic ionic liquid–electrolyte systems, virtually no information is available pertinent to the influence of inorganic salts on the organized structure of imidazolium-based ionic liquids. The sole exception being the study by Lawler and Fayer,16 wherein fluorescence anisotropy measurements of a planar organic solute, perylene have been carried out in 1-butyl-3-

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methylimidazolium bis(trifluoromethyl)sulfonylimide with varying amounts of lithium bis(trifluoromethyl)sulfonylimide. It has been observed that the in-plane and out-of-plane friction coefficients of perylene decrease by over a factor of 6 and 3, respectively, as the electrolyte concentration increases from 0.0 to 0.4 mole fraction. The observed results have been assimilated by considering the changes associated with the organized structure of the medium. However, it is unclear from the above-mentioned study whether similar trends will be noticed in case of other imidazolium-based ionic liquid/electrolyte combinations. Furthermore, it is not apparent how the rotational diffusion of charged solutes that are likely to be located in the ionic region of the ionic liquids, will be affected. The present study attempts to find answers to these issues by investigating rotational diffusion of three structurally similar solutes, 9-phenylanthracene (9-PA), fluorescein (FL) and rhodamine 110 (R110) in 1-butyl-3-methylimidazolium tetrafluoroborate– lithium tetrafluoroborate ([BMIM][BF4]–LiBF4) mixtures at 0.00, 0.15 and 0.30 mole fraction of LiBF4 (xLiBF4). Figure 1 gives molecular structures of the solutes used in this study and it can be noticed that 9-PA, FL and R110 are, respectively, nonpolar, anionic and cationic in nature. Due to its nonpolar nature, 9-PA will be solubilized in the nonpolar domains, whereas FL and R110 are likely to be located in the ionic region of the ionic liquid. Thus, by carrying out the rotational diffusion of the three solutes in this manner we hope to shed light on whether the added electrolyte induces structure-making or structure-breaking effect in [BMIM][BF4]. In other words, this work focuses on how the organized structure of the ionic liquid will be affected in the presence of an electrolyte. It may be noted that rotational diffusion of various types of organic solutes in ionic liquids17-37 and ionic liquid–organic solvent mixtures38-44 has been extensively investigated in recent times. The information gleaned from these studies enabled us to

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understand solute-ionic liquid interactions, the local environment offered by these systems and the organized structure of the ionic liquids in a general sense.

2. Experimental Section [BMIM][BF4] and LiBF4 were purchased from io-li-tec, Germany and Aldrich, USA, respectively. The fluorophore 9-PA was obtained from Aldrich, whereas FL and R110 were from Exciton. The stated purity of the ionic liquid is >99% with

Rotational Diffusion of Nonpolar and Ionic Solutes in 1-Alkyl-3

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