Static and Transport Properties of Alkyltrimethylammonium Cation

Article pubs.acs.org/JPCB

Static and Transport Properties of Alkyltrimethylammonium CationBased Room-Temperature Ionic Liquids Shiro Seki,*,† Seiji Tsuzuki,‡ Kikuko Hayamizu,‡ Nobuyuki Serizawa,† Shimpei Ono,† Katsuhito Takei,† Hiroyuki Doi,§ and Yasuhiro Umebayashi§ †

Materials Science Research Laboratory, Central Research Institute of Electric Power Industry (CRIEPI), 2-11-1, Iwado-kita, Komae, Tokyo 201-8511, Japan ‡ National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1, Umezono, Tsukuba, Ibaraki 305-8568, Japan § Graduate School of Science and Technology, Niigata University, 8050, Ikarashi, 2-no-cho, Nishi-ku, Niigata, 950-2181, Japan S Supporting Information *

ABSTRACT: We have measured physicochemical properties of five alkyltrimethylammonium cation-based room-temperature ionic liquids and compared them with those obtained from computational methods. We have found that static properties (density and refractive index) and transport properties (ionic conductivity, self-diffusion coefficient, and viscosity) of these ionic liquids show close relations with the length of the alkyl chain. In particular, static properties obtained by experimental methods exhibit a trend complementary to that by computational methods (refractive index ∝ [polarizability/molar volume]). Moreover, the self-diffusion coefficient obtained by molecular dynamics (MD) simulation was consistent with the data obtained by the pulsed-gradient spin−echo nuclear magnetic resonance technique, which suggests that computational methods can be supplemental tools to predict physicochemical properties of room-temperature ionic liquids.

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INTRODUCTION Room-temperature ionic liquids (RTILs)1,2 are liquid salts that consist of cations and anions without any organic solvent. RTILs are composed of various cations and anions, thus having varieties of chemical structures, and exhibit wide ranges of physicochemical and functional properties. In particular, RTILs have been developed for many applications that require negligible flammability and volatility.3 For example, they can be effectively used in electrochemical applications4 (e.g., lithium-ion secondary batteries,5−12 dye-sensitized solar cells,13−15 electric double-layer capacitors,16−18 fuel cells,19−22 and organic field-effect transistors23−25) owing to their reliable and safe performance with high ionic concentration. To choose RTILs suitable for such applications, it is of importance to systematically understand the basic physicochemical properties of RTILs in detail. Up to now, we have reported a general rule for predicting the molecular volume ratio26 and refractive index27 for many well-known RTILs. On the other hand, Petrowsky and Frech et al. reported mass and charge transport (conductivity, fluidity, and self-duffusion) models of RTILs.28,29 Moreover, we have also reported that a unique phase transition and the ionic conduction of the RTIL composed of slightly asymmetric phosphonium cation.30 To further improve the performance of RTILs, it will be necessary to clarify their bulk characteristics as well as their liquid properties from several viewpoints. To systematically determine physicochemical properties of RTILs, it is necessary to identify each RTIL in terms of its static physical properties measured by static © 2014 American Chemical Society

methods, its transport physical properties based on motional methods, and the different diffusive properties of cations and anions.31−34 In this paper, bulk properties of RTILs obtained both by experiment and by computational techniques are summarized and the alkyl chain length dependence of static properties (density, refractive index, and polarizability) and transport properties (viscosity, ionic conductivity, and selfdiffusion coefficients of cation and anion) are discussed. Specifically, analysis of the results with changing the alkyl chain length for five quaternary ammonium cations (alkyltrimethylammonium, alkyl: propyl, butyl, pentyl, hexyl, and octyl) is reported. We expect that the prediction obtained by computational study will eventually serve as a bridge explain experimental results and can be used to design novel RTILs.

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EXPERIMENTAL SECTION Materials. Bis(trifluoromethanesulfonyl)amide (N(SO2CF3)2, TFSA; Fw 280.15) anion-based RTILs were used to examine five cationic species of RTILs. The cation species were trimethylpropylammonium (TMPA; Fw 102.20), butyltrimethylammonium (TMBA; Fw 116.22), trimethylpentylammonium (TMPeA; Fw 130.25), hexyltrimethylammonium (TMHA; Fw: 144.28), and trimethyloctylammonium (TMOA; Fw 172.33) (chemical structures, Figure 1). All samples were Received: January 6, 2014 Revised: April 3, 2014 Published: April 4, 2014 4590

dx.doi.org/10.1021/jp500123q | J. Phys. Chem. B 2014, 118, 4590−4599

The Journal of Physical Chemistry B

Article

temperatures between 353.15 and 283.15 at 10 K intervals while the samples were cooled. The samples were thermally equilibrated at each temperature for at least 90 min prior to the measurement. The self-diffusion coefficient (D) was measured by the pulsed-gradient spin−echo nuclear magnetic resonance (PGSENMR) method using a Tecmag Apollo-NTNMR and a widebore 6.4 T SCM equipped with a JEOL pulsed-field gradient (PFG) multiprobe. The samples were inserted into a 5 mm (outer diameter) NMR microtube (BMS-005J, Shigemi). The spectra of the cations (alkyltrimethylammonium) and anion (TFSA) were measured using 1H and 19F atoms at frequencies of 270.2 and 254.2 MHz, respectively. The attenuation of the echo signal E was obtained by varying the duration δ of the PFG at a fixed amplitude g. D was determined by the regression of the Stejskal−Tanner equation36 ⎡ ⎛ δ ⎞⎤ E = exp⎢ −γ 2δ 2g 2D⎜Δ − ⎟⎥ ⎝ ⎣ 3 ⎠⎦

(1)

onto the attenuation data, where γ is the gyromagnetic ratio of the observed nuclei and Δ is the interval between diffusion measurements. D is independent of Δ for the homogeneous samples, as implied by eq 1. The measurements were performed at temperatures between 353.15 and 283.15 K while cooling with thermal equilibration at each temperature for 30 min prior to the measurement. High energy X-ray diffraction technique (HEXRD) measurements were carried out at 298 K using the BL04B2 beamline of SPring-8 at the Japan Synchrotron Radiation Research Institute (JASRI).38,37 Sample RTILs were set in a cell consisting of 2 mm thick polyetheretherketone plates as a body with Kapton films as X-ray window hermetically sealed with Kalrez O rings, and stainless steel cover plates. Monochrome 61.6 keV X-rays were obtained using a Si(220) monochromator. For the HEXRD measurement of ionic liquid samples, 6−8 h of Xray irradiation was needed to achieve enough S/N. In our own experience, there is no sample damage by such high-energy Xray irradiation. The observed X-ray intensity was corrected for absorption39 and polarization. Incoherent scatterings40−42 were subtracted to obtain coherent scatterings, Icoh(Q). The X-ray structure factor SHEXRD(Q) was obtained from

Figure 1. Chemical structures of TMPA, TMBA, TMPeA, TMHA, and TMOA cations.

purchased from Kishida Chemical Co., Ltd.,35 dried in a vacuum chamber at 323 K for more than 48 h, and stored in a dry argon-filled glovebox ([O2] < 0.4 ppm, [H2O] < 0.1 ppm, Miwa Mfg. Co., Ltd.) before measurements were carried out. No significant amounts of impurities or residual moisture were detected by 1H NMR. Measurements. Density (ρ/gcm−3) and viscosity [η/(mPa s)] were measured using a thermoregulated Stabinger-type viscosity and density/specific gravity meter (Anton Paar, SVM3000G2). The measurements were performed during cooling from 353.15 to 283.15 at 5 K intervals with an airtight stopper to avoid moisture and air contamination. The samples were thermally equilibrated at each temperature for at least 15 min prior to the measurement. The refractive index was measured using a thermoregulated refractive index measurement system (Anton Paar, Abbemat WR/HT; measured wavelength 589.3 nm) in air. The refractive index was also measured during heating from 283.15 to 353.15 at 5 K intervals. The samples were thermally equilibrated at each temperature for at least 10 min prior to the measurement. The ionic conductivity (σ) was measured on [stainless steel (SUS)/RTIL sample/SUS] hermetically sealed cells and determined by the complex impedance method using an ac impedance analyzer (Princeton Applied Research, PARSTAT2263, 200 kHz to 50 mHz; applied voltage 10 mV) at

S HEXRD(Q ) =

Icoh(Q ) − ∑ nifi (Q )2 (∑ nifi (Q ))2

+1 (2)

where ni and f i(Q) denote the number and atomic scattering factor of atom i,43 respectively. All data treatment was carried out using the program KURVLR.44 Computational Methods. The Gaussian 03 program45 was used for ab initio molecular orbital calculations. The basis sets implemented in the Gaussian program were used. The geometries of isolated ions were optimized at the MP2/6311G** level. The molecular polarizabilities of isolated ions were calculated at the MP2/aug-cc-pVDZ level using the optimized geometries. The molecular polarizabilities of the five alkyltrimethylammonium cations and N(SO2CF3)2 anion are summarized in Table 1 as the averages of the calculated αxx, αyy, and αzz. The polarizabilities of ion pairs were obtained as the sum of the calculated molecular polarizabilities of the isolated cations and anion in Table 1. The MPDyn program46 was used for molecular dynamics (MD) simulations. The MD simulations of 125 ion pairs were 4591



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all samples in the temperature range examined in this study. Generally, in a narrow temperature range, ρ (g cm−3) is expressed as

Table 1. Polarizabilities Calculated for Cations and Anions (in au) at the MP2/aug-cc-pVDZ Level cation

polarizability

anion

polarizability

TMPA TMBA TMPeA TMHA TMOA

77 89 102 115 140

N(SO2CF3)2

98

ρ = b = aT

(3)

where a, b, and T are the coefficient of volume expansion (g cm−3 K−1), the density at 0 K (g cm−3), and the temperature (K), respectively. The best-fit parameters of eq 3 are summarized in Table 2. The observed ρ decreased with Table 2. Density Equation (ρ = b − aT) Parameters and Molar Concentration at 303.15 K = 30 °C (M30) for RoomTemperature Ionic Liquids

carried out in the NPT ensemble.47 After a 1 ns equilibration run, a 20 ns production run was carried out at 403 K to calculate the mean square displacements (MSDs) of the ions. To minimize possible artifacts in the initial configurations, the system was equilibrated at 453 K starting from a low-density condition before equilibration at 403 K. The self-diffusion coefficients were determined by calculating the slopes of the MSDs of the ions versus time. Further details of the conditions of the MD simulations are described elsewhere.48,49 The OPLS force field for RTILs reported in previous work48,49 was used for the simulations. The force field was developed on the basis of the OPLS force field for RTILs proposed by Lopes et al. with some modifications.50−53 The partial atomic charges of the ions were determined on the basis of the atomic charges obtained by electrostatic potential fitting using the Merz−Singh−Kollman scheme54,55 from the MP2/6-311G**-level wave functions of the isolated ions.

ionic liquids TMPA-TFSA TMBA-TFSA TMPeATFSA TMHA-TFSA TMOA-TFSA

Mw/ g mol−1

a/10−4 g cm−3 K−1

b/ g cm−3

M30/10−3 mol cm−3

382.35 396.37 410.40

8.93 8.73 8.77

1.693 1.652 1.619

3.721 3.501 3.298

424.43 452.48

8.39 8.18

1.579 1.524

3.121 2.820

RESULTS AND DISCUSSION Static Properties of Alkyltrimethylammonium CationBased RTILs. To investigate the relationships between the basic physicochemical properties and molecular structures of the RTILs (in terms of alkyl chain length), the temperature dependences of various physicochemical parameters were measured. Figure 2 shows the temperature dependence of the densities of the alkyltrimethylammonium cation-based RTILs, and all the experimentally observed densities of the RTILs are given in Table S1 (Supporting Information). Highly linear relationships (R > 0.999) with temperature were obtained for

increasing alkyl chain length of the alkyltrimethylammonium cations, similarly to conventional imidazolium cation-based RTILs.57 On the other hand, the absolute values of ρ of the alkyltrimethylammonium cation-based RTILs were smaller than those of previously reported imidazolium cation-based RTILs.56 This is considered to be due to the tightening effect induced by the packing of aromatic rings (containing a double bond and a planar molecule). In addition, the molar concentrations of the alkyltrimethylammonium cation-based RTILs at 303.15 K = 30 °C (ρ/Mw x 103, M30) decreased with increasing alkyl chain length owing to both the increasing molecular weight and decreasing density. Figure 3 shows the temperature dependences of the refractive index of the alkyltrimethylammonium cation-based RTILs, and all the experimentally observed refractive indices of the RTILs are given in Table S2 (Supporting Information). A highly linear relationship with temperature (R > 0.999) was also

Figure 2. Temperature dependences of densities (ρ) of TMPA-TFSA, TMBA-TFSA, TMPeA-TFSA, TMHA-TFSA, and TMOA-TFSA upon heating (353.15−283.15 K).

Figure 3. Temperature dependences of refractive indices for TMPATFSA, TMBA-TFSA, TMPeA-TFSA, TMHA-TFSA, and TMOATFSA upon heating (353.15−283.15 K).

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the ratio in the entire temperature range, similarly to many conventional RTILs,27 and it was clearly shown that the refractive index of the RTILs is determined by the polarizability of the ion pair normalized by the molar volume of the alkyltrimethylammonium cation-based RTIL at all temperatures (see Figure 4c, which includes results from ref 10). The linear relationship suggests that the refractive index can be estimated sufficiently accurately by ab initio calculation of the molecular polarizability and density measurements. Although the molecular polarizability depends on the ions, the refractive index is proportional to the polarizability normalized by the molar volume in all RTILs. Moreover, even when the temperature was changed, the linearity between polarizability/ molar volume and the refractive index was maintained within the error range. This result suggests that there is no temperature dependence of polarizability. Generally, polarizability expresses the ease of molecular polarization under an external electric field, and three factors [(1) electronic polarization, (2) transformation polarization, and (3) orientational polarization] contribute to molecular polarization. The main factor contributing to orientational polarization is molecular rotation, which has a slow response under an electric field (respond at infrared frequency region). Similarly, transformation polarization also has a slow response. When an electric field of high frequency is applied, such as visible light, orientational and transformation polarization hardly contributes to molecular polarization. On the other hand, the response of electronic polarization is very rapid, and it makes up the largest contribution to polarization by visible light. Because the main objective of this analysis is the determination of the relationship between the refractive index (589.3 nm) and the polarizability, the magnitude of the electronic polarization calculated by ab initio calculation was assumed to correspond to the polarizability. Moreover, the cause of orientational and transformation polarization is the movement of molecules into the liquid, which may be affected by the temperature. On the other hand, it appears that the effect of temperature on electronic polarization (relates to the electron potential in the molecules) is very small. From the above consideration, we conclude that there is negligible temperature dependence of total polarizability. Therefore, it is clear that the refractive index of alkyltrimethylammoniumbased RTILs has a strong relationship with the density and polarizability of ions, similarly to other RTILs. Transport Properties of Alkyltrimethylammonium Cation-Based RTILs. Figure 5 shows the temperature dependences of the viscosity of the alkyltrimethylammonium cation-based RTILs as Arrhenius-type plots, and all the experimentally observed viscosities of the RTILs are given in Table S3 (Supporting Information). The obtained data deviated from linear Arrhenius-type behavior and appeared to closely follow VFT57−59 or WLF60 behavior (curves fitted by the VFT equation are indicated as solid lines). In this study, the glass transition temperatures (Tg) of the RTILs were not measured because understanding the thermal properties was not a major objective. Therefore, a detailed discussion based on the VFT or WLF parameters of each equation is not given in this article. Note that discontinuous behavior was not observed for the temperature range and samples used in this study. The viscosity increased with the alkyl chain length of the alkyltrimethylammonium cations as well as the molecular weight and size. Viscosity is considered to be affected by the inter/intramolecular frictional force between cations and

obtained for all samples in the temperature range examined in this study, similarly to that obtained for the density, and the range of observed refractive indices (1.39−1.43) was narrower than that of the measured densities. The refractive indices of the alkyltrimethylammonium cation-based RTILs increased with increasing alkyl chain length of the cations. We reported that the ratio of the polarizability of an ion pair (obtained by ab initio calculation) to the molar volume (obtained from density measurements) is proportional to the refractive index.27 Therefore, in this study, we also investigated the relationships between the refractive index and the polarizability divided by the molar volume for all samples at all measured temperatures. Figure 4 shows the relationship between the polarizability (cation + anion, shown in Table 1) divided by the molar volume and the refractive index for the alkyltrimethylammonium cation-based RTILs ((a) RTIL dependences, (b) temperature dependences). For all RTILs with various alkyl chain lengths, the refractive index has a linear relationship with

Figure 4. Relationship between polarizability/molar volume and refractive index for TMPA-TFSA, TMBA-TFSA, TMPeA-TFSA, TMHA-TFSA, and TMOA-TFSA: (a) RTIL dependences; (b) temperature dependences with five kinds RTILs used in this study; (c) results in (a) with those of other RTIL samples from ref 10. 4593



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upward curves, following VFT57−59 or WLF60 behavior (curves fitted by the VFT equation are indicated as solid lines), similarly to those for the viscosity. The obtained values of σ decreased with increasing alkyl chain length of the alkyltrimethylammonium cations. Generally, σ in a unit volume is defined as

σ=

∑ njqjμj j

(4)

where n, q, and μ are the number, charge, and mobility of the carrier ions in the specific volume, respectively. The suffix j corresponds to the alkyltrimethylammonium cation and TFSA anion. In this case, i.e., q = 1, σ values should depend on n (related to the carrier density and ionic dissociation) and μ (related to the viscosity). The experimental values of σ for TMPA-TFSA is 4.5 times higher than that for TMOA-TFSA at 303.15 K. On the other hand, the ratios of viscosity (η30) and molar concentration (M30) of TMPA-TFSA to those of TMOA-TFSA are 2.3 and 1.3, respectively. Thus, one of the main reasons for the decrease in σ might be the increase in viscosity attributed to the increasing alkyl chain length; a higher viscosity decreases the mobility of all the carrier ions. If the molar concentration was not sensitive to n, the ionic activity (ionicity; vide infra) should be related to the value of σ. The molar conductivity (Λimp) was calculated from ac impedance measurements as

Figure 5. Temperature dependences of viscosities (η) for TMPATFSA, TMBA-TFSA, TMPeA-TFSA, TMHA-TFSA, and TMOATFSA upon heating (353.15−283.15 K). Curves fitted by the VFT equation are indicated as solid lines.

anions. We previously studied the introduction of a flexible substitution group (e.g., ether oxygen)61−64 and changing the center atom (e.g., to phosphorus)65,66 for quaternary aliphatic cation-based RTILs in an attempt to achieve low-viscosity RTILs. In this study, viscosity monotonically increased with the chain length as well as the molecular weight of the alkyltrimethylammonium cations; the molecular fluidity should decrease with increasing alkyl chain length of the cation, similarly to conventional molecular liquid materials and other RTIL systems. Figure 6 shows the temperature dependences of the ionic conductivity σ of the alkyltrimethylammonium cation-based RTILs as Arrhenius-type plots, and all the experimental ionic conductivities of the RTILs are given in Table S4 (Supporting Information). The Arrhenius-type plots of σ show convex

Λ imp = σM/ρ

(5)

where M is the molecular weight of the RTILs. Λimp is plotted against the inverse viscosity (1/η) in Figure 7 as Walden plots

Figure 7. Relationships between inverse viscosity (1/η) and molar conductivity (Λimp) for TMPA-TFSA, TMBA-TFSA, TMPeA-TFSA, TMHA-TFSA, and TMOA-TFSA.

for the alkyltrimethylammonium cation-based RTILs. From the Walden product (Λimp × η), the rate of increase of Λimp with respect to η−1 is well-known to be correlated with the apparent electrochemical ionicity (i.e., the activity of ions) of RTILs.67 A small amount of variation was observed (particularly for TMPA-TFSA). Also, the gradient for the measured RTILs decreased with increasing alkyl chain length of the alkyltrimethylammonium cations. A detailed analysis will be given later using the self-diffusion coefficient data. To investigate the diffusion of the cations and anion in the RTILs, PGSE-NMR measurements were carried out. Figure 8 shows the temperature dependences of the self-diffusion coefficients of the cations (Dcation, obtained by 1H NMR) and anion (Danion, obtained by 19F-NMR) for the alkyltrimethy-

Figure 6. Temperature dependences of ionic conductivities (σ) for TMPA-TFSA, TMBA-TFSA, TMPeA-TFSA, TMHA-TFSA, and TMOA-TFSA upon cooling (353.15−283.15 K). Curves fitted by the VFT equation are indicated as solid lines. 4594



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Figure 9. Relationships between kT/πη and self-diffusion coefficients (Dcation and Danion) for TMPA-TFSA, TMBA-TFSA, TMPeA-TFSA, TMHA-TFSA, and TMOA-TFSA: (a) cation; (b) anion.

Figure 8. Temperature dependences of self-diffusion coefficients (Dcation and Danion) for TMPA-TFSA, TMBA-TFSA, TMPeA-TFSA, TMHA-TFSA, and TMOA-TFSA upon cooling (353.15−283.15 K).

Table 3. Values Obtained from the Stokes−Einstein Relation for TMPA-TFSA, TMBA-TFSA, TMPeA-TFSA, TMHATFSA, and TMOA-TFSA

lammonium cation-based RTILs, and all the experimentally observed self-diffusion coefficients of the RTILs are given in Tables S5 (Supporting Information) (cations) and S6 (TFSA). Dcation and Danion monotonically decreased with increasing alkyl chain length of the alkyltrimethylammonium cations at all measured temperatures. The opposite dependence on the alkyl chain length was observed for viscosity, as shown in Figure 5. When the alkyl chain length was small (propyl, butyl), the diffusion of the cation was faster than that of the anion. However, the reverse was observed for the long octyl chain. Using the classical Stokes−Einstein (or Einstein−Sutherland) relation, we will describe our experimental results for the individual ion diffusion coefficients and viscosities. The Stokes−Einstein equation is given as D = kT /cπηa

ionic liquids

ion

van der Waals radius36 (nm)

c

TMPA-TFSA

TMPA cation TFSA anion TMBA cation TFSA anion TMPeA cation TFSA anion TMHA cation TFSA anion TMOA cation TFSA anion

0.311 0.329 0.324 0.329 0.336 0.329 0.347 0.329 0.367 0.329

3.62 3.98 3.53 3.77 3.73 3.96 3.67 3.76 4.14 4.22

TMBA-TFSA TMPeA-TFSA TMHA-TFSA TMOA-TFSA

We carried out MD simulations to evaluate the self-diffusion coefficients. The self-diffusion coefficients of the cations (Dcation) and anions (Danion) in the RTILs obtained by the MD simulations are summarized in Table 4. Although the

(6)

where a is the radius of the diffusing species, k is the Boltzmann constant, and the constant c theoretically ranges between 4 and 6 for slip and stick boundary conditions, respectively. We assume that c is an empirical parameter. Figure 9 shows the relationships between kT/πη and the self-diffusion coefficients (Dcation and Danion) for TMPA-TFSA, TMBA-TFSA, TMPeATFSA, TMHA-TFSA, and TMOA-TFSA (a, cation; b, anion). Assuming that the Stokes−Einstein equation is valid, the slope of each line corresponds to the inverse of c × a. The experimental values of c × a obtained from the linear plots in Figure 9 are summarized in Table 3. Here, we assume that the van der Waals radius68−70 calculated from the van der Waals volume can be used as the radius of diffusing ions, as given in Table 3. Although the c value depends on the chemical structure and the states of ions in RTILs, it may be difficult to calculate a radius uniformly from the volume of nonspherical molecules. The values of c for the cations were almost constant of 3.5−3.7, except for TMOA-TFSA. Also, the values of c for the TFSA anion were also almost constant at 3.8−4.0, except for TMOA-TFSA (c > 4.2). We have reported c(TFSA) values for RTILs with various cations, for which we obtained similar values (approximately 3.7, 1-ethyl-3-methylimidazolium and Nmethyl-N-propylpyrrolidinium cation).71−74

Table 4. Self-Diffusion Coefficients of Ions in RoomTemperature Ionic Liquids Obtained by Molecular Dynamics Simulations at 403 Ka Dcation

ionic liquids TMPA-TFSA TMBA-TFSA TMPeA-TFSA TMHA-TFSA TMOA-TFSA a

2.0 1.7 1.5 1.4 1.1

× × × × ×

10−11 10−11 10−11 10−11 10−11

Danion 1.7 1.5 1.6 1.4 1.1

× × × × ×

10−11 10−11 10−11 10−11 10−11

The self-diffusion coefficients are expressed in m2 s−1.

calculated self-diffusion coefficients at 403 K are about 1 order smaller than the experimental values at 403 K obtained by extrapolation, they agree well with the experimental tendency of the alkyl chain length dependence. The calculated Dcation and Danion also decrease with increasing alkyl chain length. Dcation is larger than Danion when the alkyl chain length is small (propyl, butyl), whereas the difference between Dcation and Danion becomes small with increasing alkyl chain length. This result 4595



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indicates that the alkyl chain length dependences of Dcation and Danion can be qualitatively estimated from MD simulations. The molar conductivity (ΛNMR) was calculated by the Nernst−Einstein equation (eq 7) using the self-diffusion coefficients obtained by the PGSE-NMR measurements (ΛNMR),

chains is promoted in a nonpolar domain (charge delocalization) when the alkyl chain in the imidazolium cation ring is butyl or longer. That is, the microphase separation of the hydrophilic and hydrophobic parts might also be promoted in the trimethylammonium cation-based RTILs. The alkyl chain length not only affects the structure of the single-molecule but also changes the liquid structure. Synchrotron X-ray diffraction measurements, which reveal the detailed structure of liquids, and MD simulations, which reveal the intra- and intermolecular motion, would be effective for obtain by an essential understanding of the behavior of molecules in RTILs. As noted above,82,83 it is widely accepted that the structural heterogeneity of RTILs yields a specific distinct peak in the low-Q region of X-ray scattering spectra (Q: scattering vector defined as Q = 4π sin θ/λ; θ and λ represent the scattering angle and the irradiated X-ray wavelength, respectively). Hereafter, we call this the low-Q peak. Figure 11 shows

ΛNMR = NAe 2(Dcation + Danion)/kT = F 2(Dcation + Danion)/RT

(7) −1

where NA is the Avogadro number (6.022 × 10 mol ), e is the electric charge on each ionic carrier of the RTIL (1.602 × 10−19 C), k is the Boltzmann constant (1.38 × 10−23 J K−1), F is the Faraday constant (9.469 × 105 C mol−1), and R is the universal gas constant (8.314 J mol−1 K−1). ΛNMR can be calculated with the experimental parameters, which is different from Λimp. Generally, ΛNMR is always larger than Λimp at all temperatures in all RTIL systems. Note that because the NMR measurements (2 × 108 Hz) cannot distinguish between charged and paired ions, ΛNMR includes the contributions from all diffusing species. Molar conductivities obtained from the electrochemical conductivity (Λimp, from AC impedance) and self-diffusion coefficient (ΛNMR, from PGSE-NMR) measurements were compared to estimate the active ionic ratios (the so-called ionicity or Haven ratio)56,75−79 of the alkyltrimethylammonium cation-based RTILs. Figure 10 shows the molar 23

Figure 11. X-ray structure factors S(Q) for TMPA-TFSA, TMBATFSA, TMHA-TFSA, and TMOA-TFSA. Experimental details and data treatment were similar to those in our previous works.

HEXRD84−88 data for the alkyltrimethylammonium cationbased RTILs for different alkyl chain lengths in the range of Q ≤ 2.5 Å−1, which provides information about intermolecular (interionic) interaction. Low-Q peaks clearly appeared at approximately 0.3−0.5 Å−1, whose peak intensity increased with the peak shift toward the low-Q side with increasing alkyl chain length, similarly to 1-alkyl-3-methylimidazolium cationbased RTILs.86 The appearance of the low-Q peak suggests a heterogeneous liquid structure consisting of polar and nonpolar domains (probably related to the microphase separation and ionicity); a similar structure exists in the alkyltrimethylammonium cation-based RTILs. Some interpretations of the low-Q peak have been proposed.89,90 We have reported the liquid structures of 1-alkyl-3-imidazolium86 and the structures of 1alkylammonium-based87 RTILs on the basis of HEXRD data with the aid of MD simulations. According to our previous works, the low-Q peak can be ascribed to the long-range correlation among polar domains, particularly among anions. The same techniques and analyses can be applied to this class of RTILs to elucidate the molecular origin of the low-Q peak in a future study. In this study, we examined the effect of increasing the simple alkyl chain length of trimethylammonium cation-based RTILs on the experimental and computational physicochemical properties. The fundamental properties of RTILs may be determined by examining the effect on the physicochemical properties for a more complicated substituent group (e.g., electron donating/accepting substituent). In the future, we aim

Figure 10. Molar conductivity ratios (Λimp/ΛNMR) for TMPA-TFSA, TMBA-TFSA, TMPeA-TFSA, TMHA-TFSA, and TMOA-TFSA.

conductivity ratios (Λimp/ΛNMR) of the RTILs. The experimental ratio Λimp/ΛNMR decreased with increasing alkyl chain length of the alkyltrimethylammonium cations (clear boundary was seen between TMPeA and TMHA), even though some temperature dependence was observed (particularly for TMPATFSA). Similar alkyl-chain dependences were reported for five imidazolium cation-based RTILs (Cnmim: n = methyl to octyl, Λ was more than 2 times larger than that of the alkyltrimethylammonium cation system), and the variation in the range of Λimp/ΛNMR ratios for the alkyltrimethylammonium cation-based RTILs was smaller than that for the imidazolium cation-based RTILs.56 Λimp/ΛNMR should be affected by the ion-size (van der Waals volume), cationic geometry (shape, rotational energy, and frequency factor), inter/intramolecular interactions, physical properties (thermal and static properties), and other factors. For example, Lopes et al. reported, on the basis of MD simulations, 80,81 that the occurrence of intermolecular microphase separation increases with the length of the alkyl chain. They reported that the aggregation of alkyl 4596



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to advance the understanding of RTILs not only though various methods of measurement and analysis (including computation) but also though the molecular design of RTILs.

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CONCLUSIONS In this study, the static, transport, and diffusive properties of alkyltrimethylammonium bis(trifluoromethanesulfonyl)amides were investigated by both experimental and computational approaches. We used both approaches to elucidate the relationships among the physicochemical properties of RTILs. The static properties (density and refractive index) were affected by the alkyl chain length of the trimethylammonium cation-based RTILs; a linear relationship was confirmed between the polarizability normalized by the molar volume and the refractive index in the entire temperature range for all samples. The transport (viscosity and ionic conductivity) and diffusive parameters decreased with increasing alkyl chain length of the alkyltrimethylammonium cations. The ionicity obtained by analysis (Λimp/ΛNMR) also decreased with increasing alkyl chain length of the alkyltrimethylammonium cations and might have affected the polar (generally ionic) and nonpolar (alkyl chain) parts of the cations, and the intra/ intermolecular interactions. The self-diffusion coefficients obtained by MD simulations were generally in qualitative agreement with experimental results, which suggests that computational methods can be supplemental tools to predict physicochemical properties of room-temperature ionic liquids.

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ASSOCIATED CONTENT

S Supporting Information *

Table S1 (experimental density values), Table S2 (experimental refractive index values), Table S3 (experimental viscosity values), Table S4 (experimental ionic conductivity values), Table S5 (experimental self-diffusion coefficients of cations), and Table S6 (experimental self-diffusion coefficients of anions). These materials are available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*S. Seki: e-mail, [email protected]; fax, +81-3-34803401; tel, +81-3-3480-2111. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by KAKENHI (#24750192) from Japan Society for the Promotion of Science (JSPS).

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REFERENCES

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