Conductometric Studies of Sodium Tetraphenylborate

Article pubs.acs.org/jced

Conductometric Studies of Sodium Tetraphenylborate, Tetrabutylammonium Bromide, and Sodium Tetrafluoroborate in N,N-Dimethylformamide at Temperatures from (283.15 to 318.15) K Agnieszka Boruń and Adam Bald* Department of Physical Chemistry of Solutions, University of Łódź, 90-236 Łódź, Pomorska 163, Poland ABSTRACT: The electrical conductances of dilute solutions of sodium tetraphenylborate (NaBPh4), tetrabutylammonium bromide (Bu4NBr), and sodium tetrafluoroborate (NaBF4) in N,N-dimethylformamide have been measured over the temperature range from (283.15 to 318.15) K. The ionic association constant, KA, limiting molar conductances, Λo, and distance parameters, R, were obtained using the Fuoss−Justice equation for the investigated systems. The examined electrolytes are weakly associated in DMF at all experimental temperatures. From the temperature dependence of the limiting molar conductances the Eyring's activation enthalpy of charge transport was estimated. The thermodynamic functions such as Gibbs free energy, entropy, and enthalpy for the process of ion pair formation were calculated from the temperature dependence of the ion association constants. The limiting ionic conductivities and the activation enthalpy of charge transfer for these ions were determined.

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earlier work.6 Furthermore, the mixtures containing electrolyte solutions in DMF apply in the technology, in organic electrolyte batteries, wet electrolyte capacitors, electrodeposition, or photoelectrochemical cells.12 From the experimental data, the values of the limiting molar conductivities, Λo, and the association constants, KA, have been obtained for the investigated mixtures. The Gibbs free energy, ΔGoA, enthalpy, ΔHoA, and entropy, ΔSoA, of ion pair formation as well as the Eyring activation enthalpy of charge transport, ΔH‡λ , for the electrolytes have been evaluated. A more accurate description of conductivity properties of the electrolyte and the interactions of ions with the dipoles of solvent molecule will be possible, when the analysis of the conductivity data for individual ions is made. To determine ionic conductivities, we used the assumption on equality of ionic mobilities for Bu4N+ and BPh4−. On the basis of the limiting ionic conductivities, the activation enthalpy of charge transport for these ions was obtained.

INTRODUCTION The conductivity properties of electrolytes in the mixtures of water with N,N-dimethylformamide (DMF) have been a subject of our interest for many years.1−5 Recently, we decided to study the ionic association and solvation phenomenon in DMF as a function of the temperature.6 This type of study allows us to understand the behavior of electrolytes in solution. In the literature, we can find papers on the conductivity properties of NaBPh4 and Bu4NBr in DMF, usually at a temperature of 298 K.7−9 A survey of the literature indicates that the electrical conductances of electrolytes in DMF as a function of the temperature have not been studied in a systematic way so far. Sharma et al.10 have reported conductance data of tetrabutylammonium bromide, sodium tetraphenylborate, and sodium bromide in DMF at (308.15, 313.15, 318.15, and 323.15) K. They have estimated the limiting ionic conductivities on the basis of the limiting molar conductance of tetrabutylammonium tetraphenylborate as a “reference electrolyte”. Conductance data of sodium bromide and sodium tetraphenylborate in DMF at temperatures from (233.15 to 318.15) K have been also reported by Shmukler et al.11 Safonova et al.12 have published the limiting ionic conductivities for different ions in DMF at various temperatures. They have calculated these values using the Walden product of Bu4N+. In this paper, we present precise conductometric studies of dilute solutions of NaBPh4, Bu4NBr, and NaBF4 in DMF at T = (283.15 to 318.15) K. The choice of these electrolytes was mainly due to the fact that NaBPh4 and Bu4NBr are used to split the limiting molar conductances into their ionic components. For NaBF4, the values of conductances are necessary to split the Λo for the ionic liquids studied in our © XXXX American Chemical Society

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EXPERIMENTAL SECTION The specifications of used chemicals are summarized in Table 1. Sodium tetraphenylborate was dried in vacuo at 353.15 K and tetrabutylammonium bromide and sodium tetrafluoroborate at (333.15 and 373.15) K, respectively. All of the solutions were prepared by mass using an analytical balance (Sartorius RC 210D) with a precision of ± 1·10−5 g. Received: March 15, 2012 Accepted: May 29, 2012

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dx.doi.org/10.1021/je300252d | J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

where M is the molar mass of electrolyte. The plot of molar conductances, Λ, versus the square root of the molar concentration, c1/2, for the investigated systems monotonically decreases as shown in Figures 1, 2, and 3, respectively. The conductivity data were analyzed in the framework of the low concentration chemical model (lcCM).13 This approach uses the set of equations

Table 1. Specification of Chemical Samples chemical name

source

initial mole fraction purity

purification method

final mole fraction purity

DMFa NaBPh4b Bu4NBrc NaBF4d

Aldrich Aldrich Fluka Aldrich

0.998 0.995 ≥ 0.990 ≥ 0.980

none none recrystallization recrystallization

≥ 0.990 ≥ 0.980

a

DMF = N,N-dimethylmethanamide (N,N-dimethylformamide). NaBPh4 = sodium tetraphenylborate. cBu4NBr = tetrabutylammonium bromide. dNaBF4 = sodium tetrafluoroborate.

b

Λ = α[Λo − S(αc)1/2 + E(αc)ln(αc) + J(αc)

Conductivity measurements were performed with a threeelectrode cell with the use of a precise component analyzer type 6430B (Wayne-Kerr, UK) under argon atmosphere and at different frequencies, ν, of (0.2, 0.5, 1, 2, 3, 5, 10, and 20) kHz. The temperature was kept constant within 0.003 K (Calibration Thermostat Ultra UB 20F with through-flow cooler DLK 25, Lauda, Germany). The details of the experimental procedure for conductometric measurements were described in our previous paper.6 Densities were measured with an Anton Paar DMA 5000 oscillating U-tube densimeter equipped with a thermostat with a temperature stability within ± 0.001 K. The densimeter was calibrated with extra pure water, previously degassed ultrasonically.

εr

283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15

958.096 953.346 948.584 943.817 939.042 934.255 929.458 924.650

1.0158 0.9545 0.8985 0.8455 0.7990 0.7553 0.7172 0.6826

39.61 38.68 37.75 36.81 35.88 34.95 34.01 33.07

a Standard uncertainties u are u(T) = 0.01 K, u(p) = 0.05p, and the combined expanded uncertainties Uc are Uc (ρο) = 2·10−2 kg·m−3, and Uc(v) = 0.0030·10−6 m2·s−1 (level of confidence = 0.95).

To convert molonity, m̃ , into molarity, c, the values of density gradients, b, were determined from the equation c /m̃ = ρ = ρο + bm̃

ln Λo + 2/3 ln ρo = −

(1a)

where ρο is the density of the solvent. Molar concentrations, c, were necessary to use the conductivity equation. The density gradients and the molar conductances of the investigated salts in solution, Λ, as a function of the electrolyte molality, m, and temperature are presented in Table 3. The relationship among m, m̃ , and c is the following m̃ = c /ρ = 1/(1 + mM )

(3)

(4)

In these equations, Λo is the limiting molar conductance; α is the dissociation degree of an electrolyte; KA is the ionic association constant; R is the distance parameter of ions; y± is the activity coefficient of ions on the molar scale; A and B are the Debye−Hückel equation coefficients. The analytical form of the parameters S, E, J, and J3/2 was presented previously.13 The values of Λo, KA, and R were obtained using the well-known procedure given by Fuoss14 and are collected in Table 4. As seen from Table 4, the values of limiting molar conductances are the highest in the case of NaBF4, somewhat smaller for Bu4NBr, and definitely the smallest for NaBPh4. The large difference between the values of Λo for NaBF4 and NaBPh4 results from the large differences in the mobility of BPh4− and BF4− anions. Such differences might be expected considering the size of these anions. The association constants are very small, and one can assume that these electrolytes exist essentially as free ions in the DMF. The small values of association constants, especially in the case of electrolytes containing sodium cation, may be due to the fact that this cation can be well-solvated by DMF molecules. This, in turn, effectively reduces the possibility of formation of contact ion pairs, which significantly affect the resultant value of the association constant. The determined values of Λo and KA we decided to compare with the values designated by other authors (see Table 4). As can be seen from Table 4, the limiting molar conductance values obtained in this paper are in good agreement with the data determined by Shmukler et al.11 A significantly less compatibility shows the data presented by Sharma et al.10 These conclusions confirm especially the comparison of KA values. From the temperature dependence of Λo, the Eyring activation enthalpy of charge transport, ΔH‡λ , was obtained

Table 2. Densities, ρο, Viscosities, v, and Relative Permittivities, εr, of N,N-Dimethylformamide as a Function of Temperature, T, at Pressure p = 0.1 MPaa v·106/m2·s−1

KA = (1 − α)/(α 2cy±2 )

ln y± = −(Aα1/2c1/2)/(1 + BRα1/2c1/2)

RESULTS AND DISCUSSION The physical properties (densities, kinematic viscosities, and relative permittivities) of DMF as a function of temperature are summarized in Table 2. The values of viscosities, densities, and relative permittivities of solvent were taken from our earlier work.6

ρο/kg·m−3

(2)

and

■

T/K

+ J3/2 (αc)3/2 ]

ΔHλ‡ +D RT

(5)

where D is an empirical constant. From the slope of the linear function of ln Λo + 2/3 ln ρo versus the inverse of the temperature (1/T), which is shown in Figure 4, we obtained ΔH‡λ values. ΔH‡λ values are (9366, 8980, and 8884) J·mol−1 for NaBPh4, Bu4NBr, and NaBF4, respectively. For Bu4NBr and NaBF4, the values of ΔH‡λ are comparable, and for NaBPh4, the value is clearly higher. It is the result of the presence of the large size of the BPh4− anion and strongly solvated cation Na+.

(1b) B



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Table 3. Molar Conductances, Λ, Corresponding Molalities, m, and Density Gradients, b, of Electrolytes in DMF over the Temperature Range from (283.15 to 318.15) K at the Pressure p = 0.1 MPaa m·104 −1

mol·kg

Λ·104 S·m ·mol 2

m·104 −1

mol·kg

−1

Λ·104

m·104 −1

S·m ·mol 2

mol·kg

−1

Λ·104

m·104 −1

S·m ·mol 2

mol·kg

−1

Λ·104 S·m2·mol−1

NaBPh4 T = 283.15 K b = 71.0 kg2·m−3·mol−1 1.1210 43.111 3.0060 42.460 5.3051 41.942 7.5384 41.545 11.842 40.963 19.588 40.172 29.638 39.424 40.140 38.787 59.051 37.902 79.081 37.146 99.530 36.529 118.91 36.010 T = 303.15 K b = 73.7 kg2·m−3·mol−1 1.0234 56.938 3.2618 55.956 5.5653 55.300 7.6450 54.821 12.289 53.997 20.169 52.952 30.179 51.984 40.007 51.217 58.920 50.075 78.407 49.118 98.738 48.292 118.83 47.579

T = 288.15 K b = 71.7 kg2·m−3·mol−1 0.8405 46.533 3.3754 45.596 5.5319 45.097 8.2616 44.598 11.735 44.103 19.355 43.259 30.885 42.350 39.466 41.800 59.261 40.813 78.879 40.010 98.931 39.356 118.58 38.782 T = 308.15 K b = 74.3 kg2·m−3·mol−1 1.3499 60.462 3.2643 59.608 5.2295 58.999 7.5393 58.422 12.204 57.534 20.024 56.421 29.695 55.410 39.497 54.581 59.258 53.300 79.231 52.265 98.851 51.430 118.72 50.687




T = 298.15 K b = 73.0 kg2·m−3·mol−1 1.1596 53.250 3.1569 52.442 5.5037 51.809 7.6179 51.351 12.173 50.592 20.070 49.609 29.644 48.739 39.394 48.020 59.173 46.899 78.895 45.990 98.075 45.256 118.86 44.558 T = 318.15 K b = 75.6 kg2·m−3·mol−1 1.1152 68.379 2.6182 67.508 4.8702 66.648 7.4573 65.904 12.340 64.856 20.377 63.572 30.000 62.427 39.852 61.473 59.308 60.029 78.776 58.897 99.139 57.949



Bu4NBr

C



Article

Table 3. continued m·104

Λ·104

m·104

Λ·104

m·104

Λ·104

m·104

Λ·104

mol·kg−1

S·m2·mol−1

mol·kg−1

S·m2·mol−1

mol·kg−1

S·m2·mol−1

mol·kg−1

S·m2·mol−1

NaBF4 T = 283.15 K b = 70.4 kg2·m−3·mol−1 1.9236 66.930 3.9524 66.184 6.4508 65.533 8.2646 65.141 12.951 64.340 20.932 63.285 30.901 62.291 40.936 61.456 59.822 60.221 79.832 59.166 99.644 58.320 119.92 57.567 T = 303.15 K b = 71.1 kg2·m−3·mol−1 1.6628 87.670 3.9176 86.544 5.8607 85.855 7.7456 85.290 12.787 84.093 20.884 82.612 30.441 81.277 41.050 80.061 59.638 78.384 79.484 76.906 100.09 75.615 119.81 74.596




Standard uncertainties u are u(T) = 0.01 K, u(p) = 0.05p, u(m) = 10−5 m, and the combined expanded uncertainty Uc is Uc(Λ) = 0.00035 Λ (level of confidence = 0.95). a

Figure 2. Molar conductance, Λ, of Bu4NBr solutions in DMF versus c1/2 at experimental temperatures: ○, 283.15 K; □, 288.15 K; ■, 293.15 K; ×, 298.15 K; ▲, 303.15 K; ●, 308.15 K; ⧫, 313.15 K; △, 318.15 K. The lines represent the calculations according to eqs 2 through 4.

Figure 1. Molar conductance, Λ, of NaBPh4 solutions in DMF versus c1/2 at experimental temperatures: ○, 283.15 K; □, 288.15 K; ■, 293.15 K; ×, 298.15 K; ▲, 303.15 K; ●, 308.15 K; ⧫, 313.15 K; △, 318.15 K. The lines represent the calculations according to eqs 2 through 4.

The temperature dependence of the association constant was used to calculate Gibbs free energy of ion formation, ΔGoA ΔGAo (T ) = −RT ln KA(T )

ΔGoA(T)

The values of parameters A0, A1, and A2 of eq 7 and correlation coefficients, r2, are summarized in Table 5. The entropy and enthalpy of ion association are defined as

(6)

⎛ ∂ΔGAo ⎞ ΔSAo = −⎜ ⎟ = −A1 − 2A 2 T ⎝ ∂T ⎠ p

can also be expressed by the polynomial

ΔGAo (T ) = Ao + A1T + A 2 T 2

(7) D

(8)



Article

Figure 3. Molar conductance, Λ, of NaBF4 solutions in DMF c1/2 at experimental temperatures: ○, 283.15 K; □, 288.15 293.15 K; ×, 298.15 K; ▲, 303.15 K; ●, 308.15 K; ⧫, 313.15 318.15 K. The lines represent the calculations according to through 4.

versus K; ■, K; △, eqs 2

ΔHAo = ΔGAo + T ΔSAo = Ao − A 2 T 2

Figure 4. Plot of ln Λo + 2/3 ln ρο as a function of 1/T for ■, NaBPh4; ●, Bu4NBr; and △, NaBF4 in DMF.

Table 5. Coefficients of Equation 7 and Correlation Coefficients, r2, for NaBPh4, Bu4NBr, and NaBF4 in DMF

(9)

(ΔGoA,

The thermodynamic functions of the ion pair formation ΔSoA, and ΔHoA) at different temperatures are presented in Table 6. The values of ΔGoA presented in Table 6 indicate that, in the case of studied electrolytes, the phenomenon of ionic association is a spontaneous process. In the case of Bu4NBr and NaBF4, the increase of temperature leads to more negative

Ao

A1

A2

salt

J·mol−1

J·mol−1·K−1

J·mol−1·K−2

r2

NaBPh4 Bu4NBr NaBF4

−110679 4190 98538

705.50 −36.56 −644.75

−1.159 −0.008 0.992

0.97386 0.99989 0.99718

ΔGoA values, which means shifting the equilibrium toward the ion pair formation. In the case of sodium tetraphenylborate, the

Table 4. Limiting Molar Conductances, Λo, Association Constants, KA, Distance Parameters, R, and Standard Deviations, σ(Λ), for the Investigated Salts in DMF at Different Temperaturesa Λo·104/S·m2·mol−1 T/K

this work

KA·103/m3·mol−1 lit.

this work

R/nm

σ(Λ)

3.1,b 32e 30e 4.9,b 29e

0.78 0.80 0.79 0.80 0.79 0.78 0.76 0.76

0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.02

49e 45e 49e

0.91 0.87 0.85 0.84 0.84 0.85 0.86 0.88

0.03 0.01 0.01 0.01 0.01 0.01 0.01 0.02

0.74 0.78 0.81 0.82 0.81 0.81 0.79 0.77

0.01 0.01 0.01 0.02 0.02 0.02 0.03 0.03

lit.

NaBPh4 283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15

44.20 47.55 50.96 54.61 58.31 62.13 66.05 70.10

± ± ± ± ± ± ± ±

0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15

63.58 68.50 73.46 78.48 83.55 88.69 93.88 99.12

± ± ± ± ± ± ± ±

0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15

68.62 73.79 79.04 84.37 89.78 95.27 100.84 106.48

± ± ± ± ± ± ± ±

0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

54.92,b 53.6,c 54.6d 62.27,b 61e 65e 68,e 69.51b

5.05 4.62 4.17 3.89 3.76 3.65 3.66 3.87

± ± ± ± ± ± ± ±

0.04 0.03 0.03 0.02 0.03 0.02 0.03 0.05

17.76 18.47 19.16 19.74 20.38 21.00 21.68 22.41

± ± ± ± ± ± ± ±

0.07 0.03 0.02 0.03 0.02 0.02 0.02 0.03

6.54 7.71 8.52 9.12 9.57 10.03 10.10 10.32

± ± ± ± ± ± ± ±

0.03 0.03 0.03 0.03 0.03 0.04 0.04 0.05

2.7b

Bu4NBr

76.4c 92e 94e 100e NaBF4

a

In all cases, ΔR = 0.05 nm. bReference 11. cReference 9. dReference 7. eReference 10. E



Article

decreases as the temperature increases. For sodium tetraphenylborate, the entropy of association at lower temperatures is negative and from the temperature of 303.15 K changes the sign on the positive. It means that in the case of this electrolyte, the temperature increase eliminates the effects of arrangement in the structure of solvation shells occurring at 283.15 K. From the temperature of 303.15 K the effects of disrupting the structure of shells during the ion pair formation occur. They intensify with increasing temperature. A more detailed discussion on the mobility of ions in terms of ion−solvent interactions can be conducted on the basis of limiting ionic conductivities, λo±. Therefore, we decided to split the limiting molar electrolyte conductances into their ionic components. For this purpose, we used the values of limiting molar conductances for NaBPh4 and Bu4NBr determined in this paper and for NaBr determined by Shmukler et al.11 The choice of the values obtained by these authors is justified by the analysis of the results shown in Table 4. The values of limiting ionic conductivities for BPh4−, Bu4N+, Na+, Br−, and BF4− ions and cations of the ionic liquids, that is, 1-ethyl-3-methylimidazolium [emim] + and 1-butyl-3-methylimidazolium [bmim]+, calculated on the basis of the Fuoss−Hirsch assumption (λo(BPh4−) = λo(Bu4N+)),15 are presented in Table 7. For the calculation we used also the values of limiting molar conductances for [emim][BF4] and [bmim][BF4], determined in our earlier paper.6 As seen from Table 7, the lowest values of conductivity have BPh4− and Bu4N+ ions. This would be expected taking into account their large size. The small values of conductivity of the Na+ ion may result from the strong interactions of this ion with DMF dipoles, caused by the high surface charge density of this ion. As a result of these interactions, the effective radius of sodium ion is comparable with the radius of large organic ions. The effective size of the sodium ion is probably larger than cations such as: [emim]+ and [bmim]+. In these cations, the positive charge localized on the nitrogen atom is covered by a −CH3 group, which hinders the cation solvation. One should also pay attention to the fact that the ionic conductivity of [emim]+ is greater than [bmim]+, due to the differences in the sizes of ethyl and butyl group occurring in these cations. From Table 7 it follows that the Br− and BF4− anions have an apparently anomalously high conductivity. The reason for this can be weak solvation of such anions by DMF molecules, postulated in the literature.16,17 A slightly higher conductivity of the BF4− anion than the Br− anion may be due to the fact that the surface charge density in the case of tetraphenyloborate ion is less, and thus the ion−dipole interactions are weaker. This may be because the so-called “dielectric friction” is less, and the mobility of BF4− ion is greater than Br−.

Table 6. Thermodynamic Functions of Association of NaBPh4, Bu4NBr, and NaBF4 Solutions in DMF at Different Temperatures ΔGoA

ΔSoA

ΔHoA

T/K

J·mol−1

J·mol−1·K−1

J·mol−1

283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15

−3810 −3670 −3480 −3370 −3340 −3320 −3380 −3580

283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15

−6770 −6990 −7200 −7390 −7600 −7800 −8010 −9230

283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15

−4420 −4890 −5220 −5480 −5690 −5910 −6020 −6170

NaBPh4 −49.1 −37.5 −25.9 −14.3 −2.7 8.9 20.4 32.0

−17720 −14480 −11080 −7640 −4170 −590 3020 6610

40.9 41.0 41.1 41.2 41.2 41.3 41.4 41.5

4810 4830 4850 4880 4900 4930 4950 4970

82.8 72.8 62.9 53.0 43.1 33.1 23.2 13.3

19010 16090 13220 10320 7360 4300 1250 −1950

Bu4NBr

NaBF4

dependence of ΔGoA = f(T) is more complex. To a temperature of about 303.15 K the spontaneity of the process slightly decreases but, at higher temperatures, slightly increases. The increase of spontaneity of the ion pair formation with increasing temperature means an increase of the interactions energy between the ions. The temperature increase may reduce the interaction between the ions and solvent dipoles, which leads to an increase of the ion−ion interaction and the tendency to formation of the contact ion pairs. The values of entropy and enthalpy of association collected in Table 6 indicate that the spontaneity of association process is mainly due to the entropic effects. In the case of Bu4NBr and NaBF4, the entropy values are positive, which means an increase of disarrangement in the solvation shells of ions during the ion-pairing process. In the case of NaBF4 this effect Table 7. Limiting Ionic Conductivities, λo±, in DMF

λo±·104/S·m2·mol−1 T/K

BPh4− = Bu4N+

Na+

Br−

BF4−

[emim]+

[bmim]+

283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15

19.91 21.52 23.15 24.91 26.70 28.55 30.46 32.43

24.29 26.03 27.81 29.70 31.61 33.58 35.59 37.66

43.67 46.98 50.31 53.57 56.85 60.13 63.41 66.69

44.33 47.76 51.23 54.67 58.16 61.68 65.25 68.82

32.38 34.45 36.57 38.81 41.09 43.46 45.83 48.31

28.79 30.94 33.03 35.17 37.29 39.42 41.47 43.53

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(2) Szejgis, A.; Bald, A.; Gregorowicz, J. Conductivity properties of some tetraalkylammonium iodides in the water + N,N-dimethylformamide mixtures at 298.15 K. Phys. Chem. Liq. 1997, 35, 165−173. (3) Szejgis, A.; Bald, A.; Gregorowicz, J.; Kinart, C. M. Conductivity studies on LiBr, NaBr, KBr and CsBr solutions in binary mixtures of N,N-dimethylformamide with water at 298.15 K. Phys. Chem. Liq. 1997, 34, 189−199. (4) Szejgis, A.; Bald, A.; Gregorowicz, J. Conductance studies of iAm3BuNI and NaBPh4 and the limiting ionic conductance in water + DMF mixtures at 298.15 K. J. Mol. Liq. 1998, 75, 237−252. (5) Szejgis, A.; Bald, A.; Gregorowicz, J.; Ż urada, M. Conductance studies in mixtures of water with DMF at 298.15 K. Part VI. Lithium and sodium nitrates, sodium perchlorate and propionate, potassium picrate and thiocyanate, and limiting ionic conductance. J. Mol. Liq. 1999, 79, 123−136. (6) Boruń, A.; Bald, A. Conductometric studies of 1-ethyl-3methylimidazolium tetrafluoroborate and 1-butyl-3-methylimidazolium tetrafluoroborate in N,N-dimethylformamide at temperatures from (283.15 to 318.15) K. J. Chem. Eng. Data 2012, 57, 475−481. (7) Gill, D. S.; Cheema, J. S. Conductance measurements of some electrolytes in N,N-dimethylformamide and in binary mixtures of N,Ndimethylformamide containing some organic bases and acetonitrile. Electrochim. Acta 1982, 27, 755−758. (8) Gill, D. S.; Cheema, J. S. Conductance measurements of some electrolytes in N,N-dimethylformamide and its binary mixtures with other organic solvents-II. Electrochim. Acta 1982, 27, 1267−1271. (9) Jauhar, S. P.; Sandhu, S. Conductance and viscosity measurements of some 1:1 electrolytes in dimethylformamide + methanol mixtures at 25, 30 and 40 °C. Indian J. Chem. A 2000, 39, 392−399. (10) Sharma, R.; Pradhan, B.; Subba, D.; Das, C. Electrical conductances of tetrabutylammonium bromide, sodium tetraphenylborate, and sodium bromide in N,N-dimethylformamide at (308.15, 313.15, 318.15, and 323.15) K. J. Chem. Eng. Data 2009, 54, 2902− 2905. (11) Shmukler, L. E.; Safonova, L. P.; Patsatsiya, B. K.; Sakharov, D. V. Electric conduction of NaBr and NaBPh4 solutions in N,Ndimethylformamide at 233−318 K. Zh. Fiz. Khim. 1997, 71, 1795− 1798. (12) Safonova, L. P.; Sakharov, D. V.; Shmukler, L. E.; Kolker, A. M. Conductance studies of 1−1 electrolytes in N N-dimethylformamide at various temperatures. Phys. Chem. Chem. Phys. 2001, 3, 819−823. (13) Barthel, J. M. G.; Krienke, H.; Kunz, W. Physical Chemistry of Electrolyte SolutionsModern Aspects; Steinkopff, Springer: Darmstadt, NY, 1998. (14) Fuoss, R. M. Conductance-concentration function for the paired ion model. J. Phys. Chem. 1978, 82, 2427−2440. (15) Fuoss, R. M.; Hirsch, E. Single ionic conductances in nonaqueous solvents. J. Am. Chem. Soc. 1960, 82, 1013−1017. (16) Parker, A. J. The effects of solvation on the properties of anions in dipolar aprotic solvents. Q. Rev. Chem. Soc. 1962, 16, 163−187. (17) Cox, B. G.; Hedwig, G. R.; Parker, A. J.; Watts, D. W. Solvation of ions. XIX. Thermodynamic properties for transfer of single ions between protic and dipolar aprotic solvents. Aust. J. Chem. 1974, 27, 461−475.

On the basis of the limiting ionic conductivities, the activation enthalpy of charge transport, ΔH‡λ , for these ions was obtained (see Figure 5). ΔH‡λ values are (8551, 8884, 8884,

Figure 5. Plot of ln λo± + 2/3 ln ρο as a function of 1/T for ⧫, NBPh4−, Bu4N+; ■, Na+; ○, Br−; ▲, BF4−; ◊, [emim]+; and ●, [bmim]+ in DMF.

8063, 8333, 9936, and 9936) J·mol−1 for Br−, BF4−, Na+, [emim]+, [bmim]+, Bu4N+, and BPh4−, respectively. The highest value of ΔH‡λ for Bu4N+ and BPh4− ions results from their large sizes. The relatively high value of ΔH‡λ for the Na+ ion is the result of the earlier mentioned strong solvation of this cation by the DMF molecules, as a consequence of its large effective radius. It is much more difficult to explain the activation enthalpy values for the other, weakly solvated anions (Br−, BF4−), and also weakly solvated, but much larger cations of the ionic liquids. For a more detailed analysis of these values, the measurements of electrical conductances of other electrolytes should be performed.

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CONCLUSIONS Molar conductances of solutions of sodium tetraphenylborate, tetrabutylammonium bromide, and sodium tetrafluoroborate in DMF have been reported at T = (283.15 to 318.15) K. The conductivity data have been analyzed using the Fuoss−Justice equation. A slight ionic association was observed for the examined salts in DMF at all experimental temperatures. The evaluated values of thermodynamic functions of association suggest the spontaneity of the association process, which results mainly from the entropic effects. The large ions, that is, BPh4− and Bu4N+, have low conductivity and high activation enthalpy values of charge transport. The low conductivity and high activation enthalpy of the Na+ ion result from the strong interaction of this ion with DMF dipoles because of high the surface charge density of this ion. Br−, BF4−, [emim]+, and [bmim]+ ions are poorly solvated by DMF molecules.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +48 426355846. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Szejgis, A.; Bald, A.; Gregorowicz, J. Conductivity studies on some alkali metal iodides in aqueous N,N-dimethylformamide solutions at 298.15 K. Monatsh. Chem. 1997, 128, 1093−1100. G


Conductometric Studies of Sodium Tetraphenylborate

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