Understanding the Microscopic Behavior of Binary Mixtures of Ionic

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Understanding the Microscopic Behavior of Binary Mixtures of Ionic Liquids Through Various Spectroscopic Techniques Manjari Chakraborty, Tasnim Ahmed, Ranu Satish Dhale, Debashis Majhi, and Moloy Sarkar J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b09699 • Publication Date (Web): 29 Nov 2018 Downloaded from http://pubs.acs.org on November 29, 2018

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The Journal of Physical Chemistry

Understanding the Microscopic Behavior of Binary Mixtures of Ionic Liquids Through Various Spectroscopic Techniques

Manjari Chakraborty,[a] Tasnim Ahmed,[b]Ranu Satish Dhale,[a]Debashis Majhi[a]and Moloy Sarkar[a]

[a]Manjari Chakraborty, Ranu Satish Dhale, Debashis Majhi, Dr. Moloy Sarkar School of Chemical Sciences, National Institute of Science Education and Research, Bhubaneswar 751005, India, Email: [email protected] [b]Tasnim Ahmed Department of chemistry, University of Hyderabad, Hyderabad – 500046, India.

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Abstract In recent times it has been shown that certain binary mixture of pure ionic liquids having appropriate chemical composition can behave like a new chemical entity. However, current knowledge about the microscopic behavior of these interesting systems is rather limited. The present study is undertaken with an objective to understand the microscopic behavior in terms of intermolecular interaction, structure and dynamics of these systems. In the present study, few (IL+IL) mixtures are chosen with a common cation and a variation of anion. The investigations are also carried out by taking individual pure ILs so that the difference in behavior between pure IL and (IL+IL) mixtures are understood. Initially the systems have been investigated by studying the thermophysical properties of the concerned mixtures. Synergistic effect between combining pure ILs through photochromism has also been studied. These mixtures have been investigated further through steady state and time resolved fluorescence spectroscopy, electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR) and fluorescence correlation spectroscopy (FCS). Interestingly, time-resolved fluorescence data also pointed out that (IL+IL) mixtures are not only spatially heterogeneous but they are dynamically heterogeneous too. EPR measurements have suggested that the micro-polarity (ET(30)) of (IL+IL) mixture is close to aliphatic polyalcohol. Measurements of translational diffusion coefficients of the diffusing species through NMR and FCS studies have provided idea about the nano-structural organization within (IL+IL) binary mixtures. The analysis of data essentially reveals that the mixtures of ILs that are used in the current study do not behave like a non-ideal solution. The behavior of the IL mixtures is observed to be more like quasi-ideal type.


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1. Introduction Ionic Liquids (ILs) are now considered as one of the most important classes of material in material sciences. The emergence of ionic liquids from a simple laboratory reagent to specialized material has been possible owing to its unique physiochemical properties such as large electrochemical window, negligible vapor pressure, high viscosity, high thermal stability etc.16

By virtue of this, ILs are used in wide range of applications such as in catalysis,7-9in batteries10-

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and even in rocket sciences13. Another interesting aspect of ILs is that the physiochemical

properties of ILs can be tuned by appropriately selecting the cation and anion that they are composed of. Because of this reason, they are also known as “designer solvents.”14 Despite of the several interesting properties of ILs one inherent problem in ionic liquid is its low conductivity.15 Tuning of ionic liquid properties is therefore highly desirable as low conductivity of ILs can restricts its uses in several electrochemical applications.5,15,16 One of the ways through which properties of ILs can be tuned is to combine two or more ILs through simple mixing. In fact, it has been shown that a library of compounds with a range of properties can be generated by simply mixing two or more ILs.17 Tuning of ILs by simple mixing is economical and less cumbersome than altering the molecular structure of the concerned ILs by chemical synthesis. 18 It is interesting to note that an increase conductivity for the mixture of ILs have been found in comparison to individual ILs.19-21 Essentially, the variability of ionic liquid properties is expected to increase by mixing two or more ionic liquids.22 The (IL+IL) mixtures are interesting solvent systems but the usefulness of these system as a fluid material for several applications can only be realized when the solute-solvent interactions as well as the solvent-solvent interactions within the constitute of this material is completely understood. However, very limited numbers of studies have been carried out on these systems. 3 ACS Paragon Plus Environment


Welton’s group have discussed the ideal and non-ideal behavior of ionic liquid mixtures mainly focusing on their properties and applications.22 Recent studies have also shown that it is possible to generate a library of liquids with range of properties by simply mixing two or more ILs.17A major finding consistent with the range of IL mixtures predicts the structure of IL mixtures dominated by the random distribution of ions driven by Columbic interaction.23-28 In the context of ideal and non-ideal behavior of (IL+IL) mixture, the work by Marrucho and coworkers25 is noteworthy. They have estimated the densities, viscosities and the molar volume of many IL mixtures with a common cation. Small deviation in the molar volumes for the IL mixtures has been observed from their study. With the help of these data they have concluded that mixtures of ILs that are used in that study do not deviate in a significant extent from the ideal behavior.25A few studies on IL mixtures have also looked at the effect of H-bonding interactions with the IL mixtures. Brussel et al.29-30have investigated [C2C1im]Clx[SCN]1-x using ab-nito molecular dynamics and NMR experiments. They have observed that the [SCN]– anion in the mixture is displaced from interacting with the C(2) hydrogen of the imidazolium cation by strong coordination of Cl– anion illustrating the preferential H-Bond formation by the cation with a particular anion over the other. Similar H-bonding effects have also been studied by Rebelo and coworkers31-32 for a mixtures of [NH4][SCN] with ILs. In another interesting work, Brenecke et.al33 have studied a library of (IL+IL) mixtures and have observed the viscosities of the combinations of phosphonium ILs with imidazolium ILs showed a positive deviation from the excepted viscosities of pure ILs whereas the combination of imidazolium ILs with phosphonium ILs showed a negative deviation. Through this observation, they have shown that Arehinius model is not adequate to explain the observed data. However, additional IL-IL interactions are needed for explaining the complete viscosity data. Recently, while working on the hydrogen


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bonding interactions in mixture of two protic ionic liquids Ludwig and coworkers34 through molecular dynamics (MD) simulation studies have demonstrated that the behaviors of the ionic mixtures yield to a simple statistical analysis based upon the number of hydrogen bonding sites of the constituent ions. From the above discussions it is evident that though some studies on (IL+ IL) mixtures are carried out but studies which aim to understand kinship among the interactions, structure and dynamics of the (IL+IL) mixtures are rather limited. Additionally, the nanostructural organization of (IL+IL) mixtures which is expected to depend on the combining ILs is also not well understood for such systems. In this context we would like to note that time resolved fluorescence anisotropy, NMR and FCS techniques has proven to be quite effective in providing molecular level understanding on the microscopic behavior of the ILs,35-36 and therefore, these techniques are also expected to be quite helpful in understanding the intermolecular interactions, dynamics and nano structural organization of the (IL+IL) mixtures. Keeping the above facts in mind, in the present work several mixtures of ILs (Figure 1) have been investigated through Time resolved Fluorescence, NMR spectroscopy and Fluorescence Correlation Spectroscopy (FCS). The (IL+IL) mixtures are constituted by mixing the concerned ILs in a definite mole fraction. For example, Mix I (A) is composed of 0.4 mole fraction of [BMIM][BF4] and 0.6 mole fraction of [BMIM][PF6], Mix I (B) is composed of 0.6 mole fraction of [BMIM][BF4] and 0.4 mole fraction of [BMIM][PF6], Mix I (C) is composed of 0.8 mole fraction of [BMIM][BF4] and 0.2 mole fraction of [BMIM][PF6],Mix II (A) comprises of 0.4 mole fraction of [BMIM][PF6] and 0.6 mole fraction of [BMIM][NTF2], Mix II (B) comprises of 0.6 mole fraction of [BMIM][PF6] and 0.4 mole fraction of [BMIM][NTF2], Mix II (C) comprises of 0.8 mole fraction of [BMIM][PF6] and 0.2 mole fraction of [BMIM][NTF2], Mix III (A) constitutes of 0.4 mole fraction of [BMIM][BF4] and 0.6 mole fraction of 5 ACS Paragon Plus Environment


[BMIM][NTF2], Mix III (B) constitutes of 0.6 mole fraction of [BMIM][BF4] and 0.4 mole fraction of [BMIM][NTF2] and Mix III (C) constitutes of 0.6 mole fraction of [BMIM][BF4] and 0.4 mole fraction of [BMIM][NTF2]. Prior to spectroscopic studies, thermophysical properties of (IL+IL) mixtures have also been investigated. Synergistic behavior between the combining ILs have also been investigated through the photochromism studies. Solute-solvent and solventsolvent interactions in the (IL+IL) mixtures are carried out using the time-resolved fluorescence anisotropy studies. The data on (IL+IL) mixtures have also been compared with the constituent pure IL so that the synergism that exists between the individual ILs are also understood. Additionally, all the relevant systems have been investigated through NMR, EPR and FCS so that a comprehensive understanding on the microscopic behavior in terms of intermolecular interaction, structure and dynamics of (IL+IL) mixtures comes out. The present study depicts that the nano-structural organization of (IL+IL) mixtures are different from the constituent ILs. The outcome of the present study also reveals that (IL+IL) mixtures that are used in the current study do not behave like a non-ideal solution rather they behave more like a quasi-ideal mixture. Since the (IL+IL) mixtures used in the present study do not provide distinct thermodynamic behavior, it is not appropriate to term them as Double Salt Ionic Liquids (DSIL).The outcome of the present study is expected to entail a significant step forward in understanding the microscopic behavior of these (IL+IL) mixtures.


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(IL+IL) mixtures with their compositions as mole fraction

[BMIM][BF4]x[PF6]y

[BMIM][PF6]x[NTF2]y

[BMIM][BF4]x[NTF2]y

x = 0.4; 0.6; 0.8 y = 0.6; 0.4;0.2

x = 0.4; 0.6; 0.8 y = 0.6; 0.4; 0.2

x = 0.4; 0.6; 0.8 y = 0.6; 0.4; 0.2

Figure 1. Structures and abbreviations of ILs and (IL+IL) mixtures used in this study.

2. Experimental Section 2.1. Materials The ionic liquids [BMIM] [BF4] and [BMIM] [NTF2] were purchased from TCI Chemicals (>98% purity) and [BMIM][PF6] was obtained from Sigma Aldrich (>99% purity). The halide ions and water content in these ILs are

Understanding the Microscopic Behavior of Binary Mixtures of Ionic

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