Conductivity and viscosity properties of associated ionic liquids phosphonium orthoborates

Journal of Molecular Liquids 178 (2013) 57–62

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Conductivity and viscosity properties of associated ionic liquids phosphonium orthoborates Alejandro García a, Luis C. Torres-González a, Karinjilottu P. Padmasree b, Maria G. Benavides-Garcia c, Eduardo M. Sánchez a,⁎ a Laboratorio de Materiales para Almacenamiento y Conversión de Energía, Facultad de Ciencias Químicas, Universidad Autónoma de Nuevo León, Cd. Universitaria, San Nicolás de los Garza N.L. 66450, México b CINVESTAV, Unidad Saltillo, Carr. Saltillo – Monterrey km 13.5, C.P. 25900, México c University of Houston-Downtown, Department of Natural Sciences, Houston, TX 77002, United States

a r t i c l e

i n f o

Article history: Received 14 September 2012 Received in revised form 2 November 2012 Accepted 12 November 2012 Available online 4 December 2012 Keywords: Ionic liquids Phosphoniums Ionic conduction Viscosity Fragility

a b s t r a c t A new group of room temperature molten salts based on asymmetric isobutyl(trihexyl)phosphonium cation with different chelated orthoborate anions are presented in this report. The physicochemical properties of these compounds as glass transition temperature, thermal stability, viscosity and conductivity are determined and discussed on structural basis. The highest room temperature conductivity observed was for the isobutyl(trihexyl)phosphonium bis(malonate)borate. The results indicate that the prepared orthoborates have lower viscosities, better conductivities when compared to their precursor, isobutyl(trihexyl)phosphonium bromide. Our investigation has found that those new compounds have an intermediate-to-fragile behavior. We plotted the conductivity and viscosity data according to Walden's rule and found that they can be classified as an associated ionic liquid (AIL) an intermediate between a true ionic liquid and a molecular species. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Ionic liquids (ILs) are molten salts at room temperature and are composed of a cation and anion having a melting point lower than 373 K. These compounds have a variety of useful properties like low vapor pressure, high thermal and electrochemical stability, non-flammability, good solvent miscibility and high conductivity [1]. These qualities make them useful in a diversity of applications such as catalysis, electrochemistry, photochemistry, organic synthesis, solar cells, batteries and fuel cells [2,3]. Regarding ILs, generally one or both of the ions are large and should preferably be unsymmetrical [4]. These factors tend to reduce binding energy in the crystalline network giving properties to ILs such as existing in the liquid state at temperatures well below 373 K. ILs are known as designer solvents, and one of their most important advantages is their physicochemical properties and their phase behavior of the systems that can be tuned/controlled by tailoring their structures [5]. In particular, ionic liquids used for electrochemical devices include imidazolium, quaternary ammonium, pyridinium, pyrrolidinium and piperidinium derivatives [6]. Recently, phosphonium molten salts have found use in electrochemical applications. For instance, these salts have been used successfully for electrodeposition on CdS [7], alkali metals [8], Titanium [9] and Al [10], among others. Additionally, phosphonium ionic liquids ⁎ Corresponding author. E-mail address: [email protected] (E.M. Sánchez). 0167-7322/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molliq.2012.11.007

have been proposed for electrochemical devices such as capacitors [11], lithium batteries [12] and solar cells [13]. Perfluorinated-based molten salts are the common choice for electrochemical applications but they are relatively expensive and they could be hydrolyzed to generate hydrogen fluoride (HF) in the presence of water traces [14]. To overcome this situation, several alternatives had been investigated. For instance, chelated orthoborate anions of different N-containing cations have been reported by Angell's group [15]. However, they were found to have higher glass transition temperatures and room-temperature viscosities than those with perfluorinated anions. At first glance, those compounds are too viscous for most practical applications but they have many attractive properties as model IL systems for fundamental studies. Nevertheless, tighter safety regulations and cost-wise issues place a potential demand for the use of nonhalogenated electrolytes for electrochemical devices in the near future. Therefore, several orthoborate ionic liquids have been proposed as electrolyte additives to improve stability on lithium batteries [16,17] and electrochemical capacitors [18]. Our research group is seeking alternative routes to decrease glass transition and viscosity and we recently reported [19] the use of asymmetric phosphonium ILs as an attempt to hinder crystallization on an assortment of molten iodide salts. As a continuation of such efforts, we present on this work the conductivity and viscosity of new room temperature ionic liquids made by the combination of asymmetric isobutyl(trihexyl)-phosphonium cation with a variety of orthoborate anions. In particular, we compared their conductivity, viscosity and fragility behavior to similar molten systems and

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attempt to explain these properties using structural considerations. Furthermore, their conductivity and viscosity data are plotted under Walden's rule, where ion association plays a major role.

where X is oxalate, malonate, and salicylate respectively. Intermediate iso-butyl-tri-n-hexyl-phosphonium bromide (iBH3P-Br) preparation [20] was performed according to:

2. Experimental

Pðn  C6 H13 Þ3 þ ðCH3 Þ2 CH2 CH2  Br→½ðCH3 Þ2 CH2 CH2 Pðn  C6 H13 Þ3 ½Br

All reagents used in this work were from Aldrich, except for trihexylphosphine (Cytec) and all of them were used as received. Sodium bis(oxalate)borate (NaBOB), sodium bis(salicylate)borate (NaBSB) and sodium bis(malonate)borate (NaBMB) were prepared by complete evaporation of water from the aqueous solutions of the corresponding organic acid (oxalic acid, salicylic acid and malonic acid respectively) along with boric acid and sodium hydroxide in a 2:1:1 molar ratio. This procedure was previously proposed by [15]. A white solid material was obtained on the three cases and were washed with ethanol, then filtered and dried inside a vacuum oven (Shel Lab 1410) at 353 K for two days. The reaction is as follows

where iBH3P-Br was synthesized by refluxing 1-bromo-2-methylpropane with 10% excess of trihexylphosphine for 5 h at 423 K followed by addition of 3 parts of petroleum ether for separation. The colorless liquid was dried in a vacuum oven at 343 K for 7 days. Phosphonium bromide was reacted with excess of sodium orthoborate salt in anhydrous acetonitrile under refluxing for 6 days at 353 K (see Fig. 1). After cooling, the precipitated sodium bromide was filtered off and the solvent in the filtrate was evaporated. The residue was dried in a vacuum oven at 353 K for one day and then dissolved in a large amount of dichloromethane. The solution was allowed to stand at room temperature in order for remaining sodium orthoborate salt to precipitate. After filtration and evaporation of the solvent, the liquid residue was dried in a vacuum oven at 363 K for 2 days to yield phosphonium orthoborate liquid. Nuclear magnetic resonance (NMR) spectroscopy was used for structure confirmation of the orthoborates in deuterated acetonitrile.

Fig. 1. Synthesis of phosphonium orthoborates. (a) Iso-butyl-tri-n-hexyl)-phosphonium Bis(oxalate)borate (iBH3P-BOB), (b) Iso-butyl-(tri-n-hexyl)-phosphonium Bis(malonate) borate (iBH3P-BMB) and (c) Iso-butyl-(tri-n-hexyl)-phosphonium Bis(salicylate)borate (iBH3P-BSB).

A. García et al. / Journal of Molecular Liquids 178 (2013) 57–62

3. Results and discussion Here we prepared molten salts by substitution of bromide ions by orthoborate anions using an efficient metathesis reaction [21]. The insolubility of sodium chloride in anhydrous acetonitrile, and particularly in dichloromethane, is sufficiently enough to use these solvents in successive steps of the synthesis and refinement. In all cases, an AgNO3 solution test was performed to eliminate bromide anion traces. The prepared molten salts have all the 1H and 31P NMR signals of the corresponding P-containing onium cations as well as the 1H and 11B signals of the B-containing anions. Next we present the NMR signals of the corresponding compounds. iBH3P-Br:viscous colorless liquid. NMR in CD3CN: 1H δ 0.84 (t, 9H), 1.03 (d, 6H), 1.26 (m, 12H), 1.38 (m, 6H), 1.48–1.58 (m, 6H), 2.04 (m, 1H), 2.25 (m, 6H), 2.34 (m, 2H) ppm; 31P δ 38.83 ppm iBH3P-BOB: viscous yellow liquid. NMR in CD3CN: 1H δ 0.87 (t, 9H), 1.05 (d, 6H), 1.28 (m, 12H), 1.40 (m, 6H), 1.50 (m, 6H), 2.03 (m, 1H), 2.17 (m, 6H), 2.27 (m, 2H) ppm; 31P δ 38.98; 11B δ 12.88 ppm. iBH3P-BMB: viscous yellow liquid. NMR in CD3CN: 1H δ 0.87 (t, 9H), 1.05 (d, 6H), 1.29 (m, 12H), 1.40–1.51 (m, 12H), 1.62 (m, 2H), 1.92 (s, 2H), 2.01 (m, 1H), 2.13 (m, 6H), 3.28 (s, 2 H) ppm; 31P δ 39.03 ppm; 11B δ 9.02 ppm. iBH3P-BSB: viscous yellow liquid. NMR in CD3CN: 1H δ 0.85 (m, 9H), 1.01 (d, 6H), 1.24 (m, 12H), 1.35 (m, 6H), 1.48 (m, 6H), 1.81 (m, 1H), 2.02 (m, 6H), 2.13 (m, 2H), 6.85 (d, 2H), 6.89 (m, 2H), 7.43 (m, 2H), 7.84 (m, 2H) ppm; 31P δ 39.03 ppm; 11B δ 9.18 ppm. Thermal properties of prepared molten salts at low temperature were investigated by DSC measurements over the temperature range from 200 to 353 K and the results are presented in Fig. 2 and Table 1. As shown, the synthesized phosphonium compounds have low glass transition temperatures (Tg) ranging from 203 to 224 to and no melting or crystallization peaks were observed on the temperature range studied. None of the phosphonium orthoborates crystallize even on fast cooling in liquid nitrogen. We were not able to find any thermal signal for iBH3P-BMP on the temperature interval under study and here we will assume its Tg is below 200 K. Previously reported asymmetric phosphonium iodides [19] have shown remarkably low Tg values with very small melting points and no crystallization at all. Phosphonium bromide has a relative higher Tg than its counterpart iBH3P-I due to

Heat flow

exo

BSB BOB Br

endo

The spectra of 1H, 31P and 11B NMR were obtained using a Varian Unity Spectrometer at 400 MHz, 161.84 MHz and 128.67 MHz respectively. Thermal properties of prepared compounds were analyzed by a DSC 50 Shimadzu Differential Scanning Calorimeter. Samples were placed inside aluminum cells and sealed after. Then, they were cooled down to 200 K under nitrogen atmosphere and were heated up at 10 K per minute up to 353 K. The endothermic glass transition (Tg) was determined by DSC but the melting temperatures were not detected from the temperature range we used in this study, probably because they are very difficult to crystallize. Decomposition temperatures were obtained by thermal gravimetric analysis (TGA) on T.A. Instruments from 273 to 673 K at platinum cells under air atmosphere with a heating rate of 10 K per minute. Viscosity of the compounds was measured using Viscolab 3000 in the temperature range from 313 to 363 K at 10 K intervals. Bulk conductivities were measured by ac impedance spectroscopy using a Gamry potentiostat/Galvanostat Instrument PC4-750 equipped with a frequency response analyzer. The frequency range under study was from 10,00 Hz down to 0.2 Hz. We use a dip-in parallel platinum conductivity cell. Measurements were made from 303 to 353 K in 10 K intervals. The cell constant was 0.66 and was determined with a standard solution of KCl 27 μM.

59

200

220

240

260

Temperature/K Fig. 2. DSC thermograms of phosphonium orthoborates.

large anion effect. In general, the structure of anions (and cations) has an effect on the Tg of ionic liquids, with smaller anions and alkyl chains leading to lower Tg values. The glass transition temperatures of these compounds increase with ionic size. Also, orthoborate anions preclude the compound to form crystalline phase because of an evident dissymmetry compared to the iBH3P+ cation. The lack of melting points was also observed for the butyl-methyl imidazolium (BMI) orthoborate family [15] corroborating those results. Nevertheless, we note a remarkable decrease on Tg values of phosphonium orthoborates compared to the imidazolium counterparts. This can explained on terms of the flexibility of the alkyl chains compared to the rigidity of the imidazolium ring. For the same iBH3P cation, the Tg of the orthoborates with different anion size decreases in the order BSB− > BOB− > BMB− in accordance with comparable results [15]. Within this group, the iBH3P-BSB has the highest Tg due to the increase of stiffness of the phenyl ring of the BSB− structure. At last, we were not able to find a suitable experimental glass transition for the phosphonium orthomalonate (iBH3P-BMB) under the present experimental conditions probably due to the flexibility provided by the methylene group on BMB− when compared to the orthooxalato anion (BOB−) and its glass transition must be below 200 K. TGA traces for phosphoniums from room temperature to 673 K are shown in Fig. 3. Thermogravimetric analysis shows that phosphonium bromide (iBH3P-Br), iso-butyl-(tri-n-hexyl)phosphonium bis(oxalate)borate (iBH3P-BOB),and iso-butyl-(tri-n-hexyl) phosphonium bis(salicylate)borate (iBH3P-BSB) exhibit similar thermal stability up to 443 K, while iso-butyl-(tri-n-hexyl)phosphonium bis(malonate)borate (iBH3P-BMB) is only stable up to 353 K. Table 1 shows the decomposition temperature (Td) of those compounds. We detect lower decomposition temperatures in similar phosphonium iodides [19] probably because they were measured under air instead of nitrogen atmosphere. We observe that Br−, BOB− and BSB− compounds start to decompose (5% weight loss) at about 473 K. Beginning around 523 K, there is a fast and total weight loss. These results suggest a one-step thermal degradation; however, we are confident about their thermal stability at 373 K, where there is only 1% degradation, on average. However, in the case of the BMB − anion we observe a rather rapid decomposition and it could be unsuitable for above 373 K applications. Upon those results, we could attribute the lower thermal stability of the (iBH3P-BMB) to lower cation–anion interactions favoring higher vapor pressures and it will be discussed later under viscosity and conductivity measurements.

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A. García et al. / Journal of Molecular Liquids 178 (2013) 57–62

Table 1 Molecular weight (g/mol), Glass transition temperature (Tg), decomposition temperature (Td), conductivity (σ) and viscosity (η) of phosphonium orthoborates and related compounds. Compound

Mw(g/mol)

Tg (K)a

Td (K)b

σ (S-m−1/303 K)

η (mPa-s) (313 K)

iBH3P-Ic iBH3P-Br iBH3P-BSB iBH3P-BOB iBH3P-BMB BMIM-BOBd BNM2E-BOBd

470.5 424.5 626.6 530.5 558.5 326.1 317.1

174.7 211.9 ± 0.8 227.2 ± 0.5 206.6 ± 1.0 – 244.0 228.8

612 522.8 ± 1.2 523.3 ± 1.1 535.5 ± 1.0 484.0 ± 1.3 – –

0.38 × 10−4 6.90 × 10−4 ± 3 × 10−5 6.45 × 10−4 ± 2 × 10−5 2.30 × 10−3 ± 4 × 10−4 5.20 × 10−3 ± 2 × 10−4 1.60 × 10−2 2.80 × 10−2

995 829.1 ± 0.6 691.2 ± 5.3 315.7 ± 0.1 149.6 ± 0.1 1259 1000

c d

Glass transition temperature taken as the midpoint transition. Thermal decomposition temperature of 10% weight loss. Ref. [19]. Ref. [15].

Viscosity (η) is an important parameter for understand ILs behavior because of its strong impact on the rate of mass transport. If an IL is to be used as an electrolyte, it is desirable to have low viscositiy because many electrochemical devices are required to operate at room temperatures. The ILs with the lowest viscosities contain small anions that have a diffuse negative charge and are unlikely to take part in any hydrogen bonding. Viscosity of the prepared phosphoniums as a function of temperature is shown on Fig. 4. The synthesized orthoborates have a lower viscosity than their bromide precursor where iBH3P-BMB shows the lower viscosity. This decrease on viscosity is mainly due to the larger size of the anion, where the negative charge is distributed over a larger volume. Polarizable halide anion ions attract positive charges causing the molecules to be closer and then enhancing the viscosity. iBH3P-I has higher viscosity than iBH3P-Br (see Table 1). Orthoborate anions are expected to have a more dispersed anion charge all over their structure resulting in lowering the coulombic interactions with their cation counterparts. Among the orthoborates prepared, iBH3P-BSB has the highest viscosity, which may be due to electronic charge and structural stiffness provided by the aromatic rings in its structure. The iBH3P-BMB possesses the lowest viscosity within this group. These two last results could be explained in terms of the lack of aromatic ring and the increase of flexibility on the aliphatic ring on the BMB− compared to the BOB− structure. Phosphonium aliphatic molten salts generally posses high viscosities [22] within data ranging from 500 to 5500 mPa-s at room temperature due to large van der Waals interactions of long aliphatic chains. An intermediate approach have been proposed by our group [19] using an asymmetrical isobutyl aliphatic substituent with viscosities ranging from 400 to 1100 mPa-s at 313 K. Even lower viscosities had been attained using fluorinated anions using the same asymmetric cation [23]. A recent approach

consisted of the use of smaller chain sizes and the methoxy aliphatic chains attaining 35–40 mPa-s viscosities at room temperature [24]. The 150 mPa-s (at 313 K) viscosity value of the iBH3P-BMB is considerably lower than that of BMIM[BOB] despite strong aliphatic van der Waals interactions. In fact, all three orthoborate phosphonium molten salts have lower viscosity than their imidazolium counterpart. A plausible explanation relies on the asymmetry imposed by the isobutyl group which indeed helps to depart P+ from the orthoborate anion and allowing more fluidity with respect to BMIM+ at room temperature. On the insert of Fig. 4, we display an Arrhenius type plot of the viscosity data. Fitting lines were calculated using the Vogel–Tammann–Fulcher equation: η ¼ η0 expðB=ðT−T 0 ÞÞ

and their corresponding η0, B and T0 values are listed in Table 2. From this data, the strength parameter, D, was calculated as D = B/T0 and could be used as a measure of fragility [25]. In the strength–fragility scheme, originally proposed by Angell [26], a D value lower than 10 represents fragile behavior, and higher values (around 100) are typical for strong liquids, similar to SiO2. The D value of those phosphoniums with different anions decreases in the order Br − > BOB->BSB− >BMB −. The iBH3P[BMB] has a low D value and therefore its glass transitions are difficult to observe in this case. Ionic liquids can be defined as strong or fragile depending on whether their viscosity follows the Arrhenius relationship in the low temperature region where Tg/T is between 0 and 1. Strong liquids have a linear increase in log(η) with Tg/T and follow the Arrhenius relationship, whereas fragile liquids have lower viscosities at a given temperature than those

80

Weight (%)

3.0

800

100

(1)

60

40

(1) BOB (2) BSB (3) BMB (4) Br

20

0 300

ð1Þ

400

(2)

log(η/mPa-s)

b

Viscosity/mPa-s

a

600

2.5

Br BSB BOB BMB

2.0 1.5

400 2.8

3.0

3.2

1000/(T/K) 200

(3) (4) 500

600

Temperature/K Fig. 3. TGA traces of phosphonium orthoborates.

700

0 310

320

330

340

350

360

Temperature/K Fig. 4. Viscosity of phosphonium orthoborates. Insert: viscosity Arrhenius plot.

A. García et al. / Journal of Molecular Liquids 178 (2013) 57–62

given by the Arrhenius relationship. Increased fragility corresponds to a greater decrease in viscosity with increasing temperature. A fragility plot is shown in Fig. 5. Here we present the viscosity data versus the adjusted temperature (Tg/T) to the glass transition. We indicate with a dotted line the extreme behavior of glass forming liquids, whose viscosity behavior does not change abruptly with temperature, as is the case of SiO2. On the other side, we present data for o-terphenyl [27], a model molecular liquid with a high degree of fragility. We have also included data for BMIM[BOB], a proposed [15] glass forming ionic liquid model, which is one of the more fragile ionic liquids reported. We included previously discussed MOMNM2E [BF4] for comparison [28]. At first, there was an indication of lower fragility than previously reported [19] for phosphonium iodides suggesting lesser P+–B− interaction with respect to P+–I−. Despite that MOMNM2E [BF4] has one of the lowest glass transitions reported, a smaller fragile behavior is observed when compared to the orthoborates so we can place them in an “intermediate fragile region”. On the other side, we believe that despite the orthoborate low glass transition (Table 1), the long aliphatic chains of phosphonium cation strongly interact with orthoborate anion to allow slower reduction of viscosity versus temperature, compared to really fragile systems as o-therphenyl. This is true indeed for the iBH3P[Br] and iBH3P[BOB] compounds. However, bigger anions as BMB− and BSB− allow lesser interactions and their fragility is found close to the BMIM[BOB] behavior. This property opens up new possibilities for future molten salt studies where the presence of imidazolium is not desirable allowing phosphonium aliphatic compounds to be selected as the proper solvent choice. The conductivity as a function of temperature for phosphonium bromide and phosphonium orthoborates is shown in Fig. 6. Here, we observe that orthoborates have higher conductivity values compared to their precursor. Among them, iBH3P[BMB] has the highest conductivity in the temperature range 303–373 K. The conductivity measurement was conducted only up to 353 K for iBH3P[BMB], since according to thermo gravimetric analysis, it begins to decompose at this temperature and this would affect the measurements and the conductivity. The temperature dependence of ionic conductivity follows the Arrhenius relation σ ¼ σ 0 e−ðEa =kB T Þ where σ0 is the pre-exponential factor, kB is the Boltzmann constant, T is the absolute temperature and Ea is the activation energy for conduction. The values of Ea and A are listed in Table 3. Ea is regarded as the energy barrier which must be overcome in order for the ions to travel by each other, and the value of Ea can be

14

iBH3P[I] MOENM2E[BF4] iBH3P[Br] iBH3P[BOB] iBH3P[BMB] iBH3P[BSB] BMIMBOB o-terphenyl

12 10

log(η/Pa-s)

8 6

NG RO T S e iat ed rm ility e t In frag

4 2 0

GILE FRA

-2 -4 0.0

0.2

0.4

0.6

0.8

1.0

Tg/T Fig. 5. Fragility plot for viscosity data versus adjusted temperature (Tg/T) to glass transition, along with data from literature and prepared phosphoniums. Iso-butyl-tri-n-hexyl)-phosphonium iodide ■ iBH3P-I [19], (methoxy methyl dimethyl ethyl ammonium tetrafluoroborate, ● MOMNM2E-BF4 [28], butyl-methyl imidazolium bis(oxalato)borate, BMIM-BOB [15] and ▲ o-terphenyl) [27].

61

Table 2 VTF parameters for the viscosities of the prepared phosphoniums using Eq. (1) and the fragility index D.

iBH3P-Br iBH3P-BOB iBH3P-BSB iBH3P-BMB

A

B

To/K

D

0.04 0.07 0.09 0.14

1198 992 909 714

193 196 212 211

6.2 5.1 4.3 3.4

correlated with structural arrangements. From this point of view, the smaller the Ea, the more easily for the ions to move. The minimum activation energy Ea is obtained for iBH3P[BMB]. It is well known that ionic conductivity is proportional to the number of free charge carriers (n), their charge (e), and their mobility (μ). Therefore, dissociation of an IL into cations and anions could be one of the important factors in determining the conductivity of the liquid, as only the dissociated species can function as possible charge carriers in the system. The dissociation process can be thought as dependent on the relative basicity of the anion, the more basic the anion the more likely it is to exist in paired or aggregated forms [29]. The higher conductivity obtained for iBH3P[BMB] could be explained by the weaker basicity of the BMB− structure, which result in more dissociation of the salt into ionic species comparative to the other ionic liquids in this study. This indeed would allow more fluidity among cation and anions species lowering their corresponding viscosity. To clarify the relationship between conductivity and viscosity, we constructed a Walden plot, and the outcome is presented in Fig. 7. According to Angell [28] all compounds that lie below one order of magnitude of the ideal behavior of diluted KCl must be classified as “poor ionic liquids”, indicating that the ionic conductivity is lower than expected due to substantially higher viscosity values. Recently, our group [19] had used a clarifying approach. Molten salts that have strong interactions between ions in ionic liquids are usually located well below the KCl line, due to partial association of neighboring ions. Indeed, this is a broader approach to the ion pair concept originally proposed by Kraus [30] and improved by MacFarlane [31]. Therefore, different cations and anions will have different levels of attractive forces, reducing the degree of ionicity. Consequently, there is a possibility for the formation of ion pairs [32] or aggregates. These associated species do not contribute to the overall conductivity; thus, there must be a permanent effort to reduce their presence in electrolyte applications. BMIM[BOB] ionic liquid model behavior lies on ideal KCl line has an indication of good dissociation between ions. Trihexyl-tetradecyl phosphonium diacetamide (P6,6,6,14[dca]) [32] has long aliphatic chains, yet it lies near the ideal KCl line on the Walden plot and is classified as a “true” ionic liquid as well. On the other hand, methoxymethyl dimethyl ethyl ammonium bis(oxalate)borate (MOMNM2E[BOB]) [15] lies below the ideal KCl line and the degree of association is high and must be classified as an associated ionic liquid (AIL), which should be considered an intermediate between a true ionic liquid and a molecular species. Previously we have reported [19] that iBH3PI should also be considered an AIL. This was explained in terms of its structure, where the large iodide anion straightens out the aliphatic chains, exposing the phosphorous cation to stronger cation– anion interactions, as recently was pointed out by Benavides-Garcia [33]. On this plot we found the prepared compounds to lie within the AIL region but closer to the “true ionic liquid” line which indeed is related to weaker interaction between phosphonium cation and orthoborate anion. This is explained by the fact that single negative charge of these anions is shared by eight oxygen atoms (more accurately four oxygen atoms and four carbonyl groups), hence it is very spread out, and thus the coordinating ability of the anions is consequently very weak. This clarifies the ideal behavior of BMIM[BOB] since there are few interactions among compact and planar imidazolium cations compared to aliphatic chains as MOMNM2E[BOB]. Wide open long alkyl chains on those phoshonium cations [33] are relatively separated from the

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A. García et al. / Journal of Molecular Liquids 178 (2013) 57–62

0.02

" ds ui liq c

"

ni

ds

io

ui liq c ni io te

d

-3

ia

3.2

-4

oc

3.0

1000/(T/K)

ru e

2.8

"t

-2

ss

2.6

BMBBOBBSBBr-

"a

-3

BMIM[BOB] P6,6,6,14[dca] MOMNM2E[BOB] iBH3P[BMB] iBH3P[BOB] iBH3P[BSB] iBH3P[Br] iBH3P[I]

-1

-2

log(Λ/S-m2-mol-1)

0.04

0

-1

log (σ/S-m-1)

Conductivity/S-1-m

0.06

-5 -6

360

380

Fig. 6. Conductivity versus temperature plot of the prepared phosphoniums. Insert: Arrhenius plot of conductivity for prepared phosphoniums.

lK

340

Temperature/K

-7

ea

320

id

300

C ll

in

e

0.00

-8 -3

-2

-1

0

1

2

3

4

5

log(Pa-s/η) corresponding anions so that one may expect weak coulombic interactions as they viscosity values are lower than BMIM[BOB]. However, those same long alkyl chains provide strong van der Waals interactions among themselves hindering fast ion transport as their conductivities are lower than BMIM[BOB] at similar temperatures. In this manner, those phosphonium orthoborates are to be considered as associated ion liquids.

Fig. 7. Walden plot presenting our phosphonium orthoborates, along with similar compounds. Butyl-methyl imidazolium bis(oxalato)borate, ■ BMIM-BOB [15], trihexyl tetradecyl phosphonium dicyanamide, ● P6,6,6,14-dca [32], methoxy methyl dimethyl ethyl ammonium bis(oxalato)borate, ▲ MOMNM2E-BOB [15], iso-butyl-tri-n-hexyl)phosphonium iodide ★ iBH3P-I [19].

Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.molliq.2012.11.007.

4. Conclusions Novel room temperature molten salts based on isobutyl(trihexyl) phosphonium cation with different chelated orthoborate anions have been synthesized and characterized by NMR, DSC, TGA, viscosity and conductivity. The synthesized ionic liquids displayed low glass transition temperatures and thermal stabilities up to 443 K for iBH3P-BSB, iBH3P-BOB and up to 353 K for iBH3P-BMB. Although their room temperature conductivity is somehow low, they could be utilized as potential electrolyte additives. The phosphonium bis(malonate)borate exhibits the best conductivity and viscosity properties. We can classify these salts as having intermediate-to-fragile behavior and being comparable to model ionic liquid BMIM-BOB. Under Walden's rule, we observe data below the ideal KCl line, and this is explained in terms of associated ionic liquid interactions.

Acknowledgements The authors express their gratefulness to projects SEP-CONACyT #151587 and SENER-CONACyT #150111. In addition, the support of the Universidad Autónoma de Nuevo León, Monterrey, México, under PAICyT program is recognized.

Table 3 Activation energies Ea, constant A from the Arrhenius plot of conductivity for the phosphonium bromide and phosphonium orthoborates. Compound

A

Ea (kJ/mol)

iBH3P-Br iBH3P-BSB iBH3P-BOB iBH3P-BMB

21.18 21.18 11.73 15.01

26.08 26.42 17.82 17.62

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