Elucidating Anion Effect on Nanostructural Organization of Dicationic

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Elucidating Anion Effect on Nanostructural Organization of Dicationic Imidazolium-Based Ionic Liquids Clarissa P. Frizzo, Caroline R Bender, Izabelle de Mello Gindri, Marcos Antonio Villetti, Giovanna Machado, Otávio Bianchi, and Marcos A. P. Martins J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04262 • Publication Date (Web): 13 Jun 2016 Downloaded from http://pubs.acs.org on June 16, 2016

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The Journal of Physical Chemistry

Elucidating Anion Effect on Nanostructural Organization of Dicationic Imidazolium-Based Ionic Liquids

Clarissa P. Frizzoa*, Caroline R. Bendera, Izabelle de Mello Gindrib, Marcos A. Villettic, Giovanna Machadod, Otavio Bianchie, Marcos A. P. Martinsa

a

Heterocycle Chemistry Group (NUQUIMHE), Department of Chemistry, Federal University of Santa Maria,

(UFSM), CEP 97105-900 Santa Maria, RS, Brazil

b

Biomaterials for Osseointegration and Novel Engineering Lab (BONE), Department of Bioengineering, University

of Texas at Dallas, Richardson,TX, United States 75080

c

Spectroscopy and Polymers Laboratory (LEPOL), Department of Physics, Federal University of Santa Maria,

(UFSM), CEP 97105-900, Santa Maria, RS, Brazil

d

Northeast Strategic Technology Center (CETENE), Electron Microscopy and Nanotechnology, CEP 50740540,

Recife, PE, Brazil.

e

Group for Advanced Composites and Polymers — Post-graduate Engineering and Materials Science program

(PGMAT), University of Caxias do Sul (UCS), CEP 95070560, Caxias do Sul, RS, Brazil.

1 ACS Paragon Plus Environment


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Abstract

This work reports the influence of anion structure (Br-, NO3-, BF4-, and SCN-) in the aggregation process of ionic liquids (ILs), derived from 1,8-bis(3-methylimidazolium-1-yl)octane, in 4.75% ethanol-water solution (v/v). The aggregation behavior was investigated using small angle x-ray scattering (SAXS), nuclear magnetic resonance (NMR), dynamic light scattering (DLS), and transmission electron microscopy (TEM). Spin-lattice relaxation times (T1), obtained by NMR, indicated that the molecular mobility of the ILs changed when aggregates are formed. 1H-NMR showed distinct chemical shifts as a function of the concentration of [BisOct(MIM)2][2X] (in which X = Br, NO3, SCN, and BF4) in solution. This change was associated with different chemical environments experienced by the hydrogen atoms when the aggregation process occurs. This behavior was characterized by the different types of interactions in the aggregates, in accordance with the anion of the IL structure. The SAXS measurements demonstrated that the distance between two molecules, which function as scattering centers, was dependent on the anion hydrophobicity. Less hydrophobic anions resulted in shorter distances between scattering centers due to their better solvating ability. Due to the lower solvating ability of hydrophobic anions, a larger distance between two scattering centers was observed. Furthermore, ILs with more hydrophobic anions (e.g., BF4-) resulted in closely-packed aggregates.


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1. INTRODUCTION

The development of nanostructured materials in liquid state has been advancing rapidly in the recent years due to their wide range of applications in drug delivery systems, energy conversion, lubrication, and the biomedical field.1–4 The emergent physicochemical properties of these materials are associated with the combination of bulk and surface properties of aggregates.2 Moreover, functionalization of nanoparticles using chemical groups that have more affinity with biological molecules, such as viruses, DNA, and RNA, can have a significant impact on the development of virus detections, environmental monitoring, and bio-probe techniques.4 However, the challenge in this field relies on understanding how molecules are packed in the aggregates, and understanding their supramolecular structures. Ionic liquids (ILs) are materials with attractive properties such as high thermal stability, conductivity, and a wide electrochemical window and liquid range.2–4 Their adjustable properties — through structural modification in cationic and anionic moieties — allow for the designing of materials with specific requirements, including functionalization with bioactive molecules.5,6 This adjustable nature of ILs has resulted in constant development of new compounds with various applications, both in the academic and industrial sectors, where they are employed in synthetic procedures, catalysis, batteries, and selective membranes.5,7,8 Due to the amphiphilic characteristics of ILs, aggregation is observed when the critical aggregation concentration (CAC) is achieved in solution.1 This phenomenon is dependent on the IL structure, in which the cationic alkyl chain and anion structure have been shown to play important roles. A series of techniques can be used to evaluate the aggregation behavior of ILs and aggregate features.1 Most commonly, physicochemical properties of IL solutions, such as conductivity, surface tension,



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and fluorescence of probes sensitive to environment polarity, are used to monitor aggregate formation.1 Morphological features of aggregates, such as size and shape, are obtained using dynamic light scattering (DLS) and transmission electron microscopy (TEM).1 Despite these techniques allowing for determination of the CAC and aggregate properties, they are not able to elucidate intermolecular interactions and supramolecular arrangement of molecules in the aggregates. In order to understand the structural organization of ILs at the nanoscale, nuclear magnetic resonance (NMR) and small angle x-ray scattering (SAXS) experiments have been proposed.9,10 Spin-lattice relaxation times (T1), obtained through NMR, enable understanding of the changes in the chemical environment surrounding IL molecules as well as the molecular dynamics in solution.9,10 Variations in T1 may be related to intermolecular cation-anion, cation-solvent, and anion-solvent interactions. SAXS is a valuable technique that enables identification of ordered supramolecular structures of ILs in solution and in the net state.9,10 A comprehensive investigation of intermolecular interactions and the supramolecular structure of imidazoliumbased IL aggregates was performed by Mota et al.9 NMR spin relaxation experiments demonstrated strong interaction between IL molecules and solvent, while SAXS data was used to elucidate the shape and size of aggregates, and how IL moieties interact with water and methanol molecules.9 Nevertheless, structure and aggregate relationship studies are limited to monocationic ILs, and evaluation of anion effect in the aggregation process is even rarer. Dicationic imidazolium-based ILs consist of a doubly-charged cation composed of two singlycharged cationic heads linked by an alkyl chain that is known as the spacer group. Each cationic head has an anionic moiety as counterion. These new structures have demonstrated special properties and high potential for applications in materials science; for example, in the formation


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of liquid crystals,11 metal coatings,12 and high temperature lubricants.13,14 Our research group has been developing important works in the area of ILs.12,15–17 Recently, we showed that longer spacer groups favor the aggregation of dicationic imidazolium-based ILs in water, when the side alkyl chains are shorter (methyl).15 Additionally, we showed that the aggregation properties of dicationic ILs are dependent on anion hydrophobicity.16 In continuing our studies on the investigation of supramolecular self-assembly of dicationic imidazolium-based ionic liquids, we proposed to explore the influence of IL structure on aggregate morphology, using a molecular to supramolecular characterization approach. IL-IL and IL-solvent interactions were studied, and supramolecular self-assembly characterization was done by SAXS, NMR, TEM, and DLS. Dicationic imidazolium-based ILs possessing anions with distinct structural features were evaluated — their structures are illustrated in Figure 1.

Cation

Anions

Figure 1. Structure of the cation and anions investigated.

2. EXPERIMENTAL SECTION

2.1 Materials: 1,8-Dibromooctane, 1-methylimidazole, sodium tetrafluoroborate, silver nitrate, and potassium thiocyanate salts were purchased from Sigma-Aldrich (St. Louis, MO, USA), 5 ACS Paragon Plus Environment


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while acetonitrile and ethyl ether (HPLC) were purchased from Tedia (Rio de Janeiro, RJ, Brazil). All chemicals products were of high-grade purity and were used without further purification.

2.2 Synthesis and characterization: The ILs were synthesized according to methodologies described by Shirota et al.18 and developed in our laboratory.15,16 The structures were confirmed using: 1H and

13

C NMR experiments recorded on a Bruker Avance III (1H at 600.13 MHz and

13

C at 150.32 MHz; BRUKER BioSpin GmBH, Germany) at 298 K; and electrospray ionization

mass spectra (ESI-MS) with Agilent Technologies 6460 Triple quadrupole 6460 (LC/MS-MS) (Agilent Technologies, USA), operating in the positive-ion mode. Thermal characterization was performed using differential scanning calorimetry (MDSC Q2000, T-zeroTM DSC technology, TA Instruments Inc., USA). The data obtained were in accordance with the compounds previously described by our research group.15,16 The water content in the ILs was determined by Karl Fisher titration (Titrando 836, Metrohm, Brazil). Less than 0.5 wt % of water was found for all the studied ILs. The amount of bromide was determined by ion chromatography (IC), using a model 850 Professional IC (Metrohm, Herisau, Switzerland) equipped with an 858 Professional Sample Processor and conductivity detector. The bromide content was 0.008, 0.037, and 0.26 mol kg−1 for the ILs [BisOct(MIM)2][2X], in which X = NO3, SCN, and BF4, respectively.

2.3 Preparation of solution with aqueous ILs: Aqueous IL solutions were prepared by weighing the IL in a volumetric flask, using an analytical balance with a precision of ± 0.001 g (Marte AL 500, Brazil). The volumetric flask was filled with ethanol or a solution of ethanol in double-distilled and deionized Millipore quality water (Elix-03, Barueri, Brazil; and Milli-Q,


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Molsheim, France) (4.75%; v/v). The stock solution of each IL was then further diluted to yield other concentrations.

2.4 Aggregates Characterization

NMR Measurements: NMR 1H and (1H at 600.13 MHz and

13

13

C NMR spectra were recorded on a Bruker Avance III

C at 150.32 MHz) at 303 K and in 5 mm NMR tubes containing a

sealed capillary tube with TMS diluted in CDCl3 as external reference (digital resolution of ± 0.01 ppm). Chemical shifts were expressed in ppm. An inversion recovery pulse sequence [π ̶ τ ̶ π/2]n was used to measure the T1 at 25 °C.19 The proton T1 and 1H experiments were performed in different concentrations of IL in a solution of ethanol-D2O 4.75 % (v/v), in order to evaluate supramolecular assemblies and intermolecular interactions.9,20,21

SAXS Measurements: The behavior of ILs in ethanol-water was evaluated using SAXS measurements. These experiments were performed on the D1B-SAXS1 beamline of the Brazilian Synchrotron Light Laboratory (LNLS), monitored with a photomultiplier, and detected on a Pilatus detector (300k Dectris) positioned at a sample-to-detector distance of 944.2 mm, which generated scattering wave vectors (q) of 0.08 to 2.5 nm-1. The wavelength of the incident x-ray beam ( λ ) was 1.488 Å. Analyses were performed at 25°C. Background and parasitic scattering was determined by separate measurements on an empty holder and subtracted. The exposure time was set at 90 s for all samples. The result of a SAXS experiment is essentially the intensity of the Fourier transform of the electron density, and it must be interpreted in order to determine



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morphology. The x-ray scattering is experimentally determined as a function of the scattering vector, q, whose modulus is given by (1):

= (4 )/

(1)

in which θ is half the scattering angle (2θ). Based on the model described by Porod, the correlation function, γ(r), and interface distribution function, G(r), were generated from SAXS data to obtain nanostructural information about different regions of the system (semi-ordered and disordered phases). The γ(r) is a measure of the electron density in a sample, defined as a Fourier transform of the SAXS data. The G(r) represents the probability distribution of finding two interfaces, and it is obtained by applying the second derivative to the Fourier-transformed SAXS data. In this work, the γ(r) and G(r) of different ILs were obtained using procedures described in the literature.9,22,23

Dynamic light scattering (DLS): The hydrodynamic radius (Rh) of particles was measured via DLS (Zetasizer Nano ZS Malvern Instruments, Malvern, UK), equipped with a He–Ne laser of 5.0 mW. IL solutions of 50, 300, and 500 mM diluted in 4.75% (v-v) ethanol in Mili-Q water; and IL solutions of 50, 300, 500, and 700 mM in ethanol (previously filtered in a filter with 0.45 µm pore size) were analyzed using a quartz cuvette. The intensity of light scattering from solutions at 298.15 K was measured at 173º. Each sample was analyzed three times, with seven runs of 30 s each.


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Transmission Electron Microscopy (TEM): The morphology of aggregates was investigated with an FEI-Morgagni 268D electron microscope operating at an accelerating voltage of 80 kV. TEM samples were prepared by dispersion of IL solution at 298.15 K on a holey carbon-coated copper grid.

3. RESULTS AND DISCUSSION

The effect of anion structure on the aggregation behavior of dicationic imidazolium-based ILs was evaluated. In order to perform a complete investigation of the aggregation process and characterization of aggregate supramolecular structure, a series of techniques was employed. NMR was used to investigate cation-anion, cation-solvent, and anion-solvent intermolecular interactions, while morphological features of aggregates were obtained using SAXS, DLS, and TEM.

3.1. NMR

1

H NMR measurements of T1 were done in order to study the aggregation properties and

molecular dynamics of the ILs [BisOct(MIM)2][2X] (in which X = Br, NO3, SCN, and BF4) in ethanol-D2O solution. In this NMR experiment, the parameters (vdlist and the P30) were optimized for this specific nucleus (H11). The H11nucleus was selected in order to demonstrate the change in the chemical environment, because this nucleus demonstrated the most pronounced chemical shifts. However, the same behavior is expected for other 1H nuclei in the IL molecule. When the aggregation occurs, both the chemical environment and the dynamics of the system



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modify suddenly, promoting a change in the T1 values and thus making it possible to determine the CAC using T1 measurements.9,20 1H NMR T1 measurements of H11 relative to different concentrations of ILs are shown in Figure 2. The T1 of structures in solution clearly decreased with the increase in IL concentration. This behavior occurs because when the aggregates are formed, the molecular motions become slower and the relaxation process is more efficient due to dipole-dipole interactions.24 The influence of anion structure in the dynamic process of ILs in solution was also observed, indicating that intermolecular interactions (water-cation, wateranion, ethanol-cation, and ethanol-anion) are dependent on the anion structure.25,26

Figure 2. T1 values for the H11 of the ILs [BisOct(MIM)2][2X], in which X = Br, NO3, SCN, and BF4, for different concentrations in ethanol-D2O solution at 25 °C. Each point refers to an independent experiment.

The CAC — highlighted by the arrows in Figure 2 — was observed at around 200 mM for ILs with Br- and NO3- anions, and at around 100 mM for ILs with SCN- and BF4- anions. The detection of CAC by T1 1H NMR as a function of IL concentration is in accordance with previous work.16 The CAC obtained for these ILs corroborates with the hydration ability of 10 ACS Paragon Plus Environment

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anions, based on free energy (∆G°hyd, Table 1) [28]. The values decrease in the following order: Br- > NO3- > SCN- > BF4-.27 These values indicate that small and less polarizable anions (e.g., Br) are more hydrated and have greater CAC values in comparison with more hydrophobic anions (e.g., BF4-) which have lower CAC values. Experiments for evaluating the dependence of chemical-shift variation in relation to the concentration of ILs were performed in order to provide information about intermolecular interactions resulting from the aggregation process.28 In more dilute solutions (before aggregation), the chemical shift does not change significantly; however, as the concentration increases and aggregates begin to form, the chemical shift changes become more pronounced. This can be understood by considering that in diluted solutions, the IL is more solvated (the solvent is part of the chemical environment felt by the IL molecules); while in the aggregate state (concentrated solutions) the chemical environment is saturated by the IL (other IL molecules are part of the environment felt by the IL). Thus, as the concentration increases, the interaction between the monomers of the ILs increases for the formation of the aggregate, which causes larger variations in the chemical shift. The chemical shifts of all the hydrogen atoms of the investigated ILs are shown in Figure 3. The ∆δ (δobsd − δmon) represents the variation in the chemical shifts of the analyzed nuclei. Signal changes were observed for both the hydrophobic chain as well as for the hydrophilic head group after aggregation. Chemical shifts of the ILs changed slightly for concentrations below the CAC; whereas above the CAC, all the resonances indicated large shifts. Interestingly, the H2, H4, and H5 of the imidazolium ring, and the H11 and H31 of the carbon atoms directly bound to the imidazolium ring, showed different ∆δ behavior for ILs with Br- and BF4- in relation to NO3- and SCN- anions. While these hydrogen atoms in ILs with Br- and BF4- anions showed an upfield



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shift due to aggregation, the same atoms of ILs with NO3- and SCN- anions showed a downfield shift. The opposite behavior is believed to occur due to the formation of different types of aggregates. The distinct supramolecular arrangements of ILs resulted in different environments experienced by hydrogen atoms as a consequence of intermolecular interactions in the aggregates.


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(a)

(b)

(c)

(d)

Figure 3. Variation of 1H chemical shifts for atom signals of (a) [BisOct(MIM)2][2Br], (b) [BisOct(MIM)2][2NO3], (c) [BisOct(MIM)2][2SCN], and (d) [BisOct(MIM)2][2BF4] versus 1/C of ethanol-D2O solution, at 25 °C.

Studies on monocationic ILs have shown that a decrease in the polarity of the microenvironment due to aggregation leads to the hydrogen atoms in the hydrophilic region shifting downfield.29 A downfield shift may also result from the hydrogen-bonding interactions of aromatic hydrogen with water or anion molecules when the aggregation process occurs.21 Upon aggregation in aqueous solution, an upfield shift of the hydrogens of the hydrophilic region has also been observed, and it is associated with ring stacking between imidazolium rings through π-π interactions.21,28,30,31 13 ACS Paragon Plus Environment


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All hydrogen atoms in the hydrophobic region simultaneously exhibited an upfield shift upon aggregation. Based on the behavior of chemical-shift variation dependence on IL concentration (Figure 3), two forms of aggregates were proposed (Figure 4). ILs with Br- and BF4- anions (Figure 3a and 3d) form aggregates in which an upfield shift of hydrogen in the hydrophilic region suggests that cationic heads interact effectively by CH-π stacking at the aggregate surface (Figure 4a). In this case, the hydrogens are probably located within the shielding cone of the imidazolium rings. ILs with NO3- and SCN- anions form aggregates in which the downfield shift of hydrogens from a hydrophilic region (Figure 3b and 3c) suggests that the imidazolium rings are not oriented to interact with others through CH-π stacking. This indicates that imidazolium rings are probably oriented to interact more effectively with counterions and the hydrogens are possibly located outside of the shielding cone of the imidazolium rings (Figure 4b). When the cationic heads interact through CH-π stacking, the anions are probably located at the aggregate surface. On the other hand, when the imidazolium rings are not orientated to interact through CH-π stacking, the anions are probably located between the cationic heads. For better visualization, the probable positions of the anions in the aggregates were omitted from Figure 4. The indications that the alkyl chains of the IL are folded will be explained in the next topic.


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(b)

(a)

Figure 4. Schematic representation of different possible organizations adopted by IL with anions: (a) Br, BF4; and (b) NO3, SCN.

3.2. SAXS

SAXS is a well-established technique for evaluating the morphology (spheric, cylindric, platelet, cubic, etc.) and size distribution of aggregates in a multiphase sample, as well as for obtaining nanostructural parameters such as orientation, degree of orientation, and the mean distance between aggregates. Moreover, structural information can be obtained based on the electron density distribution in the samples. In this work, the SAXS technique was used to elucidate the molecular organization of cations and anions of ILs, since ILs have complex organization through self-aggregation in polar and non-polar domains. The correlation function, γ(r), and interface distribution function, G(r), of ILs dispersed in ethanol-water (4.75%, v-v) were determined using the two-phase model32–36 — see Figure 5.



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Figure 5. Correlation and interface distribution functions for IL in an ethanol/water solution: effect of the counterion at 600 mM.

The long period (Lp) was taken as the first maximum of the correlation function γ(r). This value corresponds to an enhancement of the most probable distance between the two centers of gravity (IL aggregates). In this case, each dicationic molecule in the aggregate acts as a gravity center; therefore, the electron density is higher at the aggregate surface where the imidazole rings are located. Two observations indicate that the alkyl chains of the ILs are folded in the aggregate: (i) the SAXS data demonstrate that the electron density is higher at the aggregate surface, and the region with the greatest electron density in the IL molecules is the cationic head; and (ii) a previous work showed that dicationic IL with eight carbons in the spacer chain has a tendency to adopt a non-linear conformation at the liquid/air interface.15 Thus, one can presume that this conformation, in which the monomers are folded at the surface, is maintained when the aggregates are formed. As can be seen in Figure 5, ILs with SCN- and BF4- anions showed a shift in the maximum position in the correlation function γ(r) to high distances (r) and, consequently, had greater Lp values. On the other hand, the Lc distance is represented as the position of the first minimum in 16 ACS Paragon Plus Environment

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the correlation function γ(r). The Lc is considered to be the most probable distance between the gravity centers of IL aggregates and their adjacent solvent region. If the aggregate is dispersed in the solvent to form a one-dimensional ideal lattice, the values of Lp and 2*Lc coincide. However, if this lattice is not ideal, the position of the maximum, Lp, and minimum, Lc, in the correlation function γ(r) may be slightly shifted. As can be seen in the values of Lp and 2*Lc, the aggregates formed by dicationic ILs do not form a one-dimensional ideal lattice. This can be attributed to the dynamics of the IL species in solution. Applying the G(r) function in the experimental data, the first maximum — estimated as disorder solvent-phase — is represented by La. The model considers that IL aggregates (semi-ordered phase) are surrounded by solvent molecules (disorder phase). The difference between (Lp)γ(r)(nm) and (La)g(r)(nm) represents the distance between two scattering centers without solvent. The parameters obtained by using γ(r) and G(r) are illustrated in Figure 6 and shown in Table 1. The anions were omitted from Figure 6 for a better representation of the Lp, La, and Lc region. The probable position of the anions in the aggregates of each IL can be seen in Figure 4.

Lp

La Lc

Figure 6. Representation of the Lp, La, and Lc region between two aggregates of dicationic ILs.



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Table 1. SAXS parameters obtained by using γ(r) and G(r).

Counterion

Lpγ(r) (nm)

Lag(r) (nm)

Lcγ(r) (nm)

2*Lcγ(r) (nm)

Sizea (Lp)γ(r) (nm)-(La)g(r) (nm)

Br

3.98

1.69

2.13

4.26

2.29

-75.3

NO3

3.96

1.68

2.08

4.16

2.28

-71.7

SCN

4.36

1.76

2.13

4.26

2.60

-66.9

BF4

4.52

1.78

2.43

4.86

2.74

-45.4

a

∆G°hyd (kcal/mol)

Molecular length between two scattering centers without solvent.

The anions that had less negative ∆G°hyd (more hydrophobic) had a higher Lp, which is related to the distance between two scattering centers in two distinct aggregates (Figure 6). Thus, the lower solvation ability of the anions led to longer distances between the scattering centers.37,38 Consequently, lower or no solvation was detected for ILs with SCN- and BF4- anions (less negative ∆G°hyd and high hydrophobicity), which resulted from the repulsion between anions and solvent (water) and led to the increase in Lp values. This behavior is known as the “solvophobic effect”. ILs with SCN- and BF4- anions are more hydrophobic than ILs with Br- and NO3-, thus the former favor IL-IL interactions and more effectively repel solvent molecules. Therefore, for these compounds, IL molecules are better packed due to more efficient aggregation. Likewise, an increase in the disordered region (larger La), which is represented by solvent molecules, was observed. The La region corresponds to the distance between the interfaces of two aggregates (Figure 6). As expected, the La values increased together with the Lp values, since the distance between two scattering centers increased.39,40 This can be confirmed by the Lp values of 3.98, 3.96, 4.36, and 4.52 nm for the Br-, NO3-, SCN-, and BF4- anions, respectively. An increase in the Lp values indicated a greater repulsion between ILs and solvent due to the solvophobic effect. Thus, for greater Lp values, closely packed aggregates are expected. The larger distance between 18 ACS Paragon Plus Environment

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the molecules is not associated with a higher amount of water involving the structures, but rather the repulsion contribution of each counterion of the system, as seen for SCN- and BF4- anions. In these cases, the hydrophobic characteristics of the counterions become very important.

3.3. DLS and TEM

Morphological characteristics of aggregates, including size and shape, were determined using DLS and TEM. Self-assembled IL structures were examined at three concentrations for [BisOct(MIM)2][2X] (in which X = Br, NO3, SCN, and BF4) in ethanol-water 4.75 % (v/v) solution. From the DLS data, two aggregate populations (Rh1 and Rh2) were found for [BisOct(MIM)2][2Br] — see Figure 7(a). The Rh1 population may be related to light scattering of a very small number of monomers in solution.15,41,42 In this work, the Rh1 is referred to as a small-sized species (~ 1–2 nm) and it was observed at concentrations of 300 and 500 mM. On the other hand, Rh2 was seen to increase with the increase in IL concentration, implying that the formation of larger aggregates was favored at higher concentrations. Furthermore, the scattering intensity of Rh2 decreased for solutions with higher IL concentration, which can be attributed to the occurrence of an Rh1 population at these concentrations (Figure 7(b)). The same behavior was observed for all the other ILs except for [BisOct(MIM)2][NO3], which had only one aggregate population (Rh2) — see supporting information. The average aggregate sizes obtained for dicationic ILs are in accordance with data previously reported in the literature for monocationic ILs43,44 and for [BisOct(MIM)2][2Br] in water.15 The size of the IL aggregates (Rh1 and Rh2) did not change in relation to the anion hydrophobicity, and similar Rh2 values were



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observed for all IL solutions. This result can be attributed to the polydispersity of ILs in solution, which was confirmed by TEM images (Figure 8).

(a)

(b)

Figure 7. DLS results of the [BisOct(MIM)2][2Br]: (a) Size distribution; and (b) Rh2 intensity versus concentration.


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(a)

(b)

(c)

(d)

Figure 8. TEM images at 300 mM (above the CAC) for: (a) [BisOct(MIM)2][2Br], (b) [BisOct(MIM)2][2NO3], (c) [BisOct(MIM)2][2SCN], and (d) [BisOct(MIM)2][2BF4] in an ethanol-water solution.

DLS experiments were also performed for [BisOct(MIM)2][2Br] and [BisOct(MIM)2][2SCN] in ethanol (95%). Two relaxation frequencies related to two different populations (Rh1 and Rh2) were observed — see supporting information. Rh1 represents smaller aggregates (1–2 nm) that can be related to the association of IL monomers in solution, while Rh2 represents larger aggregates (200–600 nm) that were larger than those observed in the ethanol-water solution for these ILs. Although the size of the Rh2 populations was bigger in ethanol (95%), the number of aggregates was higher in the ethanol-water solution. This finding can be explained based on the greater intensity of light scattered by larger aggregates than by the pre-micelar aggregates in ethanol-water. The higher intensity of the scattered light indicates a higher number of aggregates



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in the ethanol-water solution than in ethanol. The Rh1 population did not appear in the dilute ethanol-water solution, but it was notable in the ethanol.

4. CONCLUSION

From the results reported in this work, it was possible to perform an in-depth investigation of the aggregation behavior of the dicationic imidazolium-based ILs [BisOct(MIM)2][2X] (in which X = Br, NO3, SCN, and BF4) in ethanol-water 4.75% (v/v) solution and in ethanol (95%). Spinlattice relaxation times (T1) obtained by 1H NMR measurements demonstrated a clear decrease in the mobility of structures in solution as the IL concentration increased. The 1H NMR provided information about specific interactions that can occur when aggregates are formed. The SAXS data demonstrated that the distance between two scattering centers from two distinct molecules, observed via Lp values, was related to the hydrophobic characteristic of the anionic moiety. The solvophobic effect in [BisOct(MIM)2][2SCN] and [BisOct(MIM)2][2BF4] systems was responsible for more effectively repelling the solvent molecules (larger La), and it also resulted in more effectively packed aggregates. From the DLS data, two populations of aggregates (Rh1 and Rh2) were found for [BisOct(MIM)2][2X] (in which X = Br, NO3, SCN, and BF4) in ethanolwater, in which Rh1 was related to very small-sized species and Rh2 to larger aggregates. The size of the IL aggregates was not correlated with the anion hydrophobicity. TEM images confirmed the polydispersity of IL aggregates in an ethanol-water solution.

ASSOCIATED CONTENT


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Supporting Information The Supporting Information with additional information regarding NMR, SAXS and DLS data is available free of charge on the ACS Publications website at DOI: XXX. Correlation functions of the solutions and aggregates hydrodynamic radius, obtained by DLS data, are available.

AUTHOR INFORMATION

*E-mail: [email protected]. Tel.: +55 5596147313; fax: +55 5532208756.

ACKNOWLEDGMENTS

The authors are grateful for the financial support from: the National Council for Scientific and Technological Development (CNPq) — Universal/Proc. 474895/2013-0 and Universal/ Proc. 554593-2010-6/ Proc. 555231-2010-0/ Proc. 407208/2013-5; the Rio Grande do Sul Foundation for Research Support (FAPERGS) — Proc. 2262-2551/14-1 and 2290-2551/14-1; and the Coordination for Improvement of Higher Education Personnel (CAPES/PROEX). The fellowships from CNPq (M. A. P. M., C. R. B., and G. M.), CAPES (I.M.G.), and FACEPE are also acknowledged.

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