Conductance studies of Et4NIO4, Et4NClO4, Bu4NI, Et4NI and the limiting ionic conductance in water + acetonitrile mixtures at 298.15 K

Journal of Molecular Liquids 190 (2014) 54–58

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Conductance studies of Et4NIO4, Et4NClO4, Bu4NI, Et4NI and the limiting ionic conductance in water + acetonitrile mixtures at 298.15 K Anna Wypych-Stasiewicz a, Jăn Benko b, Ol'ga Vollărovă b, Adam Bald a,⁎ a b

Department of Physical Chemistry of Solutions, University of Łódź, 90-236 Łódź, Pomorska 163, Poland Department of Physical and Theoretical Chemistry Comenius University, SK 842 15 Bratislava, Slovakia

a r t i c l e

i n f o

Article history: Received 27 November 2012 Received in revised form 22 October 2013 Accepted 24 October 2013 Available online 5 November 2013 Keywords: Conductance Mixtures of water and acetonitrile Walden products

a b s t r a c t The electric conductivities of Bu4NI, Et4 NI, Et4NIO4 and Et4NClO4 were measured in the mixed solvent of water and acetonitrile (AN) within the whole range of contents at a temperature of 298.15 K. The electrical conductivities of NaBPh4, NaI, and NaClO4 were measured for a few selected compositions of the mixed solvent. The experimental data of conductance were analyzed on the basis of low concentration Chemical Model (lcCM). The − − + + − + limiting ionic conductivities of BPh− 4 , Bu4N , Na , I , Et4N , IO4 , ClO4 were also determined. The dependencies of the limiting conductance and Walden products versus mixed solvent composition have been analyzed. © 2013 Elsevier B.V. All rights reserved.

1. Introduction

2. Experimental

This work is a continuation of our studies of electrolyte solutions in the mixtures of water and polar aprotic solvents such as N,Ndimethylformamide, tetrahydrofuran and acetonitrile [1–14]. Some of these studies concerned the electrical conductivity of electrolytes in the mixtures of water and N,N-dimethylformamide [1–7]. Review of the literature indicates that only a few works are devoted to research of conductometric properties of electrolyte solutions in the mixtures of water and acetonitrile [15–20]. Until now, the values of molar limiting ionic conductance have been determined only for several ions. Aim of this work is to increase knowledge of the conductometric properties of the ions in these mixtures. The subject of interest of this work is the conductometric properties of Et4NIO4, Et4NClO4, Et4NI and Bu4NI solutions in the mixtures of water and acetonitrile at 298.15 K. To calculate the values of ionic limiting conductance, the values of limiting molar conductance determined by us for Et4NIO4, Et4NClO4, Et4NI, Bu4NI and the data contained in the work of other authors [15,16] for NaBPh4 and NaI were used. For mixtures containing 0.01 (for NaBPh4 and NaI) and 0.20 (for NaBPh4) mole fractions of acetonitrile in the mixtures our own data were used. Data concerning NaClO4 in water + acetonitrile mixtures that were included in the work [17] were also analyzed. For the ClO− 4 ion, the limiting molar conductance values determined by us were different from those obtained by Niazi [17]. Therefore, the limiting conductance of NaClO4 in the mixtures of water and AN was tested for acetonitrile mol fractions x2 = 0.01, 0.02 and 0.05. The conductivity of salt having an anion IO− 4 , in acetonitrile and its mixtures with water, has been investigated for the first time.

2.1. Materials

⁎ Corresponding author. Tel.: +48 426355846. E-mail address: [email protected] (A. Bald). 0167-7322/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molliq.2013.10.023

Sodium iodide (99.99%, Aldrich) was used without purification. Sodium tetraphenylborate (99.5%, Sigma–Aldrich) was dried in vacuum at 80 °C for 12 h. Sodium perchlorate (99.0%, Fluka) was recrystallized from water and dried at 80 °C. Tetraethylammonium iodide (98.0% Aldrich) and tetrabutylammonium iodide (99.0%, Aldrich) used in this work were further purified by recrystallization and dried at temperature of 60 °C under lowered pressure to ensure the maximum purity. Tetraethylammonium periodate (Et4NIO4) and tetraethylammonium perchlorate (Et4NClO4) were prepared at the Comenius University, Bratislava, by the neutralization of the 20% water solution of Et4NOH (Merck p.a.) with HIO4 (Merck p.a.) and HClO4 (Merck p.a.) respectively. Both salts were two times recrystallized from water – AN mixtures (1:1). In order to prepare solutions, acetonitrile (99.9%) from Sigma – Aldrich and double distilled, deionized and degassed water with a specific conductance better than 0.2 · 10− 6 S cm− 1 were used. 2.2. Measurements Solutions and mixed solvent compositions were prepared by weight. The compositions were accurate to ±0.01 wt.%. The measurements of the conductivity were performed in the range of 0.0002 – 0.0060 [mol dm−3], usually for 11 or more concentrations, with the use of Precise Component Analyser type 6425 (Wayne – Kerr, UK). All conductance values were the results of extrapolation to infinite frequency. All data were corrected with specific conductance of the solvent. All measurements were made at 298.15 K. The temperature was kept constant within 0.005 K. (Calibration Thermostat Ultra UB 20 F, Lauda,

A. Wypych-Stasiewicz et al. / Journal of Molecular Liquids 190 (2014) 54–58

ions on the molar scale (the activity coefficient of the ion pairs is assumed to be equal to unity, as usual for dilute solutions) A and B are the Debye – Hückel equation coefficient. The analytical form of the parameters S, E, J and J3/2 is presented in papers [21]. The values of Λo, KA and R, were obtained using the well known procedure given by Fuoss [22]. The values of Λo, KA and R thus obtained together with their standard deviations are collected in Table 1. Additionally the limiting ionic conductivities of NaI, NaBPh4 and NaClO4 were taken from papers [15–17]. The values of the Walden products Λoη for the electrolytes under investigation are also presented in Table 1. The dependencies of limiting molar conductance Λo and viscosities η versus the mixed solvent composition are presented in Fig. 1. As can be seen in Fig. 1, slight additions of acetonitrile to water cause a decrease in the limiting molar conductivity of electrolytes. After reaching a minimum at 0.1 mole fraction content of acetonitrile, Λo values gradually increase with increasing the content of non-aqueous component. The dependence of dynamic viscosity shown in Fig. 1 can well approximately explain the character of Λo value changes. This results from the simple hydrodynamic models according to which the ion mobility is inversely proportional to the viscosity of the solvent. From these models it also follows that the product of molar conductivity and viscosity (Walden product, Λoη) depends on the effective size of ions. Therefore, in the aspect of electrolyte–solvent interactions, considerably more information can be supplied by the dependence of Walden

Germany). Taking into account the measurement procedure, equipment and reagents purity, we estimated the accuracy of the conductivity values measured as better than 0.05%. The values of relative permittivity, density and viscosity necessary for the calculation were taken from literature [15–20]. 3. Results and discussion The conductivity data were analyzed within the framework of the low concentration chemical model (lcCM) [21]. This approach uses the set of equations: h i 1=2 3=2 Λ ¼ α Λ o –SðαcÞ þ EðαcÞlnðαcÞ þ J ðαcÞ þ J 3=2 ðαcÞ

ð1Þ

  2 2 K A ¼ ð1−α Þ= α c y

ð2Þ

and     1=2 1=2 1=2 1=2 = 1 þ BRα c lny ¼ − Aα c

55

ð3Þ

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 and y± is the activity coefficient of

Table 1 Limiting molar conductances Λo, association constant KA, their standard deviations (σ Λo, σKA), parameters R and Walden products Λoη for Et4NIO4, Et4NClO4, Bu4NI, Et4NI, NaI, NaBPh4 and NaClO4 in the water + acetonitrile mixtures. x2

Λo

KA 2

Et4NIO4 0.010 0.020 0.050 0.100 0.150 0.200 0.300 0.400 0.500 0.600 0.700 0.800 0.900 1.000 Bu4NI 0.010 0.020 0.050 0.100 0.150 0.200 0.300 0.400 0.500 0.600 0.700 0.800 0.900 1.000 NaI 0.010 0.200

−1

R 3

−1

Λo η · 10−2 2

−1

S cm mol

dm mol

nm

S cm mol

85.91 85.01 82.77 82.43 ⁎85.27 89.07 99.68 113.65 ⁎126.16 141.17 155.37 167.91 ⁎175.82 182.19[21]

2.5 ± 0,3 3.1 ± 0.4 2.8 ± 0.3 4.6 ± 0.4 – 4.7 ± 0.4 5.0 ± 0.5 6.3 ± 0.6 – 9.8 ± 0.6 11.3 ± 0.8 13.2 ± 0.8 – –

0.99 0.99 1.00 1.01 – 1.02 1.04 1.05 – 1.08 1.08 1.10 – –

0.789 0.800 0.809 0.801 0.790 0.775 0.756 0.735 0.700 0.674 0.649 0.630 0.617 0.624

93.45 91.59 87.99 86.50 88.31 91.26 98.00 105.47 ⁎114.20 123.12 ⁎134.00 144.92 155.80 163.89[21]

6.5 6.3 6.8 6.7 6.5 6.2 5.8 5.6 – 5.4 – 7.8 8.4 –

1.02 1.03 1.03 1.04 1.05 1.06 1.06 1.08 – 1.11 – 1.14 1.16 –

0.859 0.862 0.860 0.840 0.818 0.794 0.743 0.682 0.633 0.588 0.559 0.544 0.547 0.562

122.39 –

– –

0.62 –

1.126 –

± ± ± ± ± ± ± ±

0.3 0.2 0.3 0.3 0.4 0.3 0.5 0.5

± 0.6 ± 0.7 ± 0.9

Λo mPa s

R

Λo η · 10−2

dm mol

nm

S cm2mol−1 mPa s

2.0 ± 0.3 2.4 ± 0.2 3.0 ± 0.4 4.8 ± 0.3 4.6 ± 0.5 4.8 ± 0.5 – 7.1 ± 0.7 8.0 ± 0.8 9.8 ± 0.6 – 11.0 ± 0.6 12.5 ± 0.8 –

0.95 0.95 0.96 0.97 0.97 0.98 – 1.01 1.02 1.04 – 1.06 1.08 –

0.898 0.905 0.906 0.884 0.867 0.844 0.816 0.795 0.755 0.726 0.703 0.685 0.662 0.651

2.6 ± 0.3 3.1 ± 0.4 3.2 ± 0.4 2.5 ± 0.4 2.5 ± 0.4 2.6 ± 0.5 2.4 ± 0.6 2.5 ± 0.6 3.4 ± 0.5 4.1 ± 0.5 6.6 ± 0.6 9.8 ± 0.8 10.1 ± 0.9 –

0.93 0.93 0.94 0.95 0.95 0.96 0.98 0.99 1.01 1.02 1.03 1.04 1.05 –

0.975 0.974 0.964 0.936 0.913 0.879 0.829 0.779 0.728 0.679 0.645 0.623 0.624 0.642

– –

0.91 0.93

0.629 0.567

KA 2

−1

S cm mol

Et4NClO4 97.70 96.17 92.69 91.02 93.52 97.02 ⁎107.59 122.91 136.19 152.13 ⁎168.32 182.64 188.54 190.03[21] Et4NI 106.07 103.48 98.63 96.40 98.52 100.95 109.31 120.51 131.26 142.19 154.60 166.11 177.74 187.46 NaBPh4 68.32 67.18

3

−1

NaClO4 0.010 0.020 0.050

114.21(116.7)⁎⁎ 111.68(115.3)⁎⁎ 106.45(111.4)⁎⁎

In all cases, ΔR = 0.05 nm, σΛo/Λo b 0.0005. ⁎ values interpolated from own data, ⁎⁎ values interpolated from the data presented in [17].

– – –

0.65 0.65 0.66

1.0494(1.069)⁎⁎ 1.0514(1.076)⁎⁎ 1.0404(1.772)⁎⁎

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A. Wypych-Stasiewicz et al. / Journal of Molecular Liquids 190 (2014) 54–58

200.00

0.0120

Table 2 Limiting ionic conductance and ionic Walden products. X2

160.00

λo±/S cm2 0.000 0.010 0.020 0.050 0.100 0.150 0.200 0.300 0.400 0.500 0.600 0.700 0.800 0.900 1.000

0.0080 120.00

80.00 0.0040 40.00

0.00 0.000

0.200

0.400

0.600

0.800

0.0000 1.000

Fig. 1. Viscosity η water + acetonitrile mixtures and limiting molar conductances Λo in water + acetonitrile mixtures for the NaBPh4 (♦) [15,16], Bu4NI (•), NaI (Δ) - [15,16], Et4 NIO4 (▲), Et4NClO4 (■), Et4NI (×), NaClO4 (o), viscosity (□).

product value changes versus composition of the mixtures. Particularly it takes place, when the values of ionic Walden products are analyzed. Individual ionic limiting conductances were obtained from the division proposed by Fuoss – Hirsch [23].   þ − ½Λ o ðNaBPh4 Þ þ Λ o ðBu4 NIÞ – Λ o ðNaIÞ=2 ¼ Bu4 N ¼ ðBPh4 Þ

− NBu+ 4 = BPh4

mol−1 19.54 19.67 19.92 20.37 20.96 21.78 23.60 28.34 33.67 39.24 44.15 49.15 53.42 57.26 59.91

Na+

I−

NEt+ 4

IO4−

ClO4−

50.41 48.77 47.40 44.92 42.78 41.88 41.55 43.00 46.29 50.11 54.36 57.91 61.89 66.87 75.20

76.78 73.78 71.67 67.62 65.54 66.53 67.66 69.66 71.80 74.96 78.97 84.85 91.50 98.54 103.98

32.35 32.29 31.81 31.01 30.86 31.99 33.29 39.65 48.71 56.30 63.22 69.75 74.61 79.20 83.48

54.70 53.62 53.20 51.76 51.57 53.28 55.78 60.03 64.94 69.86 77.95 85.62 93.30 96.62 98.71

67.00 65.42 64.36 61.68 60.16 61.53 63.73 67.94 74.20 79.89 88.91 98.57 108.03 109.34 106.55

0.684 0.678 0.675 0.661 0.637 0.616 0.589 0.528 0.464 0.416 0.377 0.354 0.343 0.346 0.356

0.288 0.297 0.299 0.303 0.300 0.296 0.290 0.301 0.315 0.312 0.302 0.291 0.280 0.278 0.286

0.487 0.493 0.501 0.506 0.501 0.494 0.485 0.455 0.420 0.387 0.372 0.357 0.350 0.339 0.338

0.596 0.601 0.606 0.603 0.584 0.570 0.555 0.515 0.480 0.443 0.425 0.412 0.405 0.384 0.365

λo± η.10−2/S cm2 mol−1 mPa s 0.000 0.174 0.449 0.010 0.181 0.448 0.020 0.188 0.446 0.050 0.199 0.439 0.100 0.204 0.416 0.150 0.202 0.388 0.020 0.205 0.362 0.300 0.215 0.326 0.400 0.218 0.299 0.500 0.218 0.278 0.600 0.211 0.260 0.700 0.205 0.242 0.800 0.200 0.232 0.900 0.201 0.235 1.000 0.205 0.258

ð4Þ The limiting conductance values of the other ions were obtained in the following way: o λþ

  þ o − ¼ Λo ðNaBPh4 Þ−λ− ðBPh4 Þ Na

o − λ− ðI Þ

¼

o Λ o ðNaIÞ−λþ



þ

Na



¼

o − λ− ðI Þ

0

ð5aÞ ð5bÞ

  o þ o − λþ Et4 N ¼ Λ o ðEt4 NIÞ−λ− ðI Þ

ð5cÞ

  o o þ − λ− ðIO4 Þ ¼ Λo ðEt4 NIO4 Þ−λþ Et4 N

ð5dÞ

  o o þ − λ− ðClO4 Þ ¼ Λo ðEt4 NClO4 Þ−λþ Et4 N

ð5eÞ

The values of Λo(NaClO4) were used as follows: o − λ− ðClO4 Þ

¼

o Λo ðNaClO4 Þ−λþ

  þ Na

radii, rS, calculated using Eq. (7), are listed in Table 3. The values of ionic radii, ri, are also presented in Table 3.

ð6Þ

Λo (NaI) and Λo (NaBPh4) values used for calculations in the following Eqs. (4), (5a) and (5b) were obtained by the interpolation of the data contained in papers [15,16]. In order to determine λo− value (ClO− 4 ), the Eq. (5e) and Λo values (Et4NClO4) obtained in this study were used. We decided not to use the Λo values (NaClO4) given in paper [17] because data presented there concerned mixtures containing 0.01, 0.02 and 0.05 mole fraction of acetonitrile and differed significantly from our data (Table 1). Whereas λo− values (ClO− 4 ) calculated with the use of our data and Eqs. (5e) and (6) showed very good agreement. The limiting ionic conductance and Walden products are collected in Table 2. The limiting molar conductances of electrolytes given in literature [18,21] were used for the calculation of the limiting molar conductances of ions and ionic Walden products in water. The values of Stokes

λ η ¼ 0:820=r S

ð7Þ

The dependencies of ionic Walden products versus the mixed solvent composition are illustrated in Fig. 2. As shown in Fig. 2, λo± η = f (x2) relationship for large ions NBu+ 4 , BPh4 − and also for cation NEt+ 4 is different than for other ions. The values of Walden product for these ions almost do not depend on the Table 3 Stokes radii (rS) and crystallographic radii (rcryst) for investigated ions. Na+

I−

NEt+ 4

IO4−

ClO4−

r cryst/nm [21] 0.494

0.098

0.224

0.400

0.280⁎

0.240

r S/nm 0.000 0.010 0.020 0.050 0.100 0.150 0.200 0.300 0.400 0.500 0.600 0.700 0.800 0.900 1.000

0.183 0.183 0.184 0.187 0.197 0.211 0.227 0.252 0.274 0.295 0.315 0.339 0.353 0.349 0.318

0.120 0.121 0.121 0.124 0.129 0.133 0.139 0.155 0.177 0.197 0.218 0.232 0.239 0.237 0.230

0.285 0.276 0.274 0.271 0.273 0.277 0.283 0.272 0.260 0.263 0.272 0.282 0.293 0.295 0.287

0.168 0.166 0.164 0.162 0.164 0.166 0.169 0.180 0.195 0.212 0.220 0.230 0.234 0.242 0.243

0.138 0.136 0.135 0.136 0.140 0.144 0148 0.159 0.171 0.185 0.193 0.199 0.202 0.214 0.225

X2

− NBu+ 4 = BPh4

0.471 0.453 0.436 0.412 0.402 0.406 0.400 0.381 0.376 0.376 0.389 0.400 0.410 0.408 0.400

a − * r S (NBu+ 4 ) = r S (BPh4 ) according to the assumption of Fuoss–Hirsch, crystallographic radii, b calculated by the method of Robinson and Stokes, c determined by Krumgalz, d values estimated on the basis of the difference between the crystallographic radii of ions Cl− and I−.

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composition of the mixed solvent. Simple hydrodynamic models of ions assume that the Walden product value is inversely proportional to the effective radius of the ions (Eq. (7)). On this basis, it can be assumed − + that the effective radii of large ions — NBu+ 4 , BPh4 (and also NEt4 cation) are virtually unchanged in the whole range of mixture compositions. This may be due to the low surface charge density of these ions resulting from their large size. This makes such ions practically unsolvated (or very weakly solvated) and causes the mobility of these ions to be affected only by their own size and independent of the mixed solvent composition. It may be somewhat surprising that the Stokes radii of these ions are smaller than the radii given by Stokes and Robinson [24] (Table 3). This applies in particular to the ion NEt+ 4 . However, this is a common phenomenon in many solvents. The method, – in which the Stokes radius rS was corrected to give effective radius (reff) of ion in solution, – was proposed by Robinson and Stokes [24]. This method has been extended by Nightingale [25]. Unfortunately, these methods were applied only to the pure solvents. In the case of mixed solvents, the use of these methods requires one to know the values of ionic conductivity for several ions NR+ 4 , for each of the mixture + tested. For ions BPh− 4 and NBu4 , the values of rs are consistent with the values given by Robinson and Stokes only in the case of water. The addition of acetonitrile to water causes a decrease in the rS for these ions. In mixtures with higher contents of acetonitrile, the values of − + Stokes radii for ions NBu+ 4 , BPh4 , NEt4 , become very similar to those given by Krumgalz [26]. According to Krumgalz, solvents forming three-dimensional net-works surround hydrophobic organic ions in clathrate-like structures [26]. As a result, in water and mixtures with a high water content, the values of rS may differ from those in organic solvents or aqueous-organic solvents of low water content. This applies, to a greater extent, to more hydrophobic ion NBu+ 4 , and to a lesser extent + to ion NEt+ 4 . Differences in the hydrophobic hydration of ions NEt4 and can be clearly seen by analyzing the character changes in the NBu+ 4 enthalpy and entropy of transfer of these ions [27]. A small addition of acetonitrile to water causes a strong endothermic effect of changes in the enthalpy of solvation of ions NBu+ 4 . Further increase in the acetonitrile content practically does not change the enthalpy of solvation. In the case of ion NEt+ 4 , a change in the content of acetonitrile does not produce significant changes in the enthalpy of solvation. − In the case of other ions (Na+, I−, ClO− 4 and IO4 ), the values of the Walden products in water – acetonitrile mixtures differ significantly.

0.800

0.700

0.600

0.500

0.400

0.300

0.200

0.100

0.000 0.000

0.200

0.400

0.600

0.800

1.000

Fig. 2. Ionic Walden product λo± η in water + acetonitrile mixtures for the Bu4N+ = + + − − − BPh− 4 (♦), Et4N (■), Na (Δ), I (▲), ClO4 (•), IO4 (+).

57

Widely available literature data show that for the studied anions − (I−, ClO− 4 and IO4 ) the value of r S in the water is much smaller than the radii ri. In the case of acetonitrile, values of rS are very close to the values of ri (Table 3). This may be due to differences in the structure of these two solvents and the nature of the ion–solvent interactions. The use of Robinson and Stokes' method for these solvents is difficult because of the small values of rS. Nevertheless, larger values of the Walden products in water as compared to acetonitrile for the − ions I−, ClO− 4 and IO4 , may suggest that the effective sizes of these ions in acetonitrile are much larger (lower values of λo± η) than in water. This, in turn, may mean that the solvation shells of these ions in acetonitrile are larger than those in water. However, the nature of variations of the ionic Walden product as a function of mixture composition is different. In the case of I− ion, the addition of small amount of acetonitrile to water causes a more significant reduction in λoη value which may be due to the exchange of water molecules for acetonitrile molecules in the solvation shells of these ions. Perhaps, this results from high polarizability of I− ion as well as from a large dipole moment of acetonitrile molecule which is larger than the dipole moment of water molecules. To sum up, it can be assumed that in the case of I− ions, the process of exchange of water molecules for acetonitrile molecules is preferential in the mixtures of water – acetonitrile, and starting from acetonitrile content x2 = 0.6 solvation shells of iodide ion are composed probably mainly of acetonitrile molecules (the stabilization of ionic Walden product value occurs). This can be confirmed by the negative values of enthalpy of transfer of ion I− in mixtures rich in acetonitrile [27,28]. In the case of ion Cl− (smaller polarizability), the − values of enthalpy of transfer are positive. In the case of ClO− 4 and IO4 oxyanions, the values of Walden product are lower than that for iodide ion, which is obviously related to the fact that these ions have a larger radius than the I− ion. The fact that λ0η value for ClO− 4 ion is higher than that for IO− 4 ion seems to confirm this suggestion. Also, the course − of λo± η = f(x2) dependence for ions ClO− 4 and IO4 is a bit different in − comparison to that for I ion. This applies mainly to the mixture compositions with low acetonitrile content. As shown in Fig. 2, the addition of a small amount of acetonitrile does not result in the simultaneous − decrease in the Walden product of ClO− 4 and IO4 , but leads to its slight increase. This effect cannot be explained by changes in the hydration shells of these anions, which would result from the exchange of water molecules for the acetonitrile in the hydration shells. Such a phenomenon would lead to an increase in effective radii of these ions and thus, to lower values of the ionic Walden product. Experimental data do not confirm these assumptions. This may be explained by the hypothesis that the addition of small amounts of acetonitrile to water does not − lead to changes in the hydration shells of ClO− 4 and IO4 ions and thus their nearest surroundings has a different composition than the mixed solvent. Therefore, ion mobility is not reduced to the same extent as macroscopic viscosity increases. This leads to the increase in the value of Walden products. Starting from acetonitrile content ca. x2 = 0.05, the stabilization, and then decline in value of Walden products are observed and this must be the result of progressing exchange of water molecules by acetonitrile molecules in the solvation shells of these ions. Partially this may be related to an increase in the exothermic enthalpy of transfer of ClO− 4 ion in the transition from water-rich mixtures to pure acetonitrile [29]. It should also be noted that the value of the − − is much less varied Walden products of the ions ClO− 4 , IO4 and I than in the case of water. In the case of sodium ion, a slight stabilization of the ionic Walden products occurs only when a very small amount of acetonitrile is added to water. This may be related to the fact that small additions of acetonitrile to water (x2 b0.05) did not produce larger changes in the enthalpy of transfer [27]. Further addition of nonaqueous component results in the reduction of λo± η value for this ion. It may be associated with rapidly escalating exothermic nature of the enthalpy of transfer, during the further increase in the content of acetonitrile [27]. It is worth mentioning that the effective size of sodium ion in pure acetonitrile is visibly larger than that of the iodide ion and only

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slightly smaller than that of tetrabutylammonium ion. This confirms the strong solvation of this ion as evidenced by its extensive solvation shell. This may be related to a negative value of the enthalpy of transfer of Na+ ion from water to acetonitrile [27–30]. However, it should be clearly stated that the discussed conclusions are only an attempt to explain the observed experimental facts. Interactions with the solvent dipoles substantially affect the conduction of ion. Boyd and Zwanzig tried to describe these phenomenon in detail [31–33]. A moving ion orients the solvent dipoles around it. Therefore, in addition to the forces generated by the viscous drag, the dielectric retarding forces should be taken into account during the movement of ions [31,32]. The fact that some of the solvent molecules near the ion are dragged by viscous force and does not need to be reorientated should also be taken into account [33]. Unfortunately, in the case of pure solvents, works of Boyd and Zwanzig provide only qualitative, partially satisfactory description of the ion mobility. In the case of mixed solvents, a big hurdle in using Zwanzig model is the need of knowledge of relaxation times. It should also be noted that the use of the macroscopic viscosity in the analysis of Walden product, may arouse a number of doubts. Changes in the macroscopic viscosity of mixed solvents do not necessarily correlate with changes in the viscosity in the immediate surroundings of ions (the so-called “microscopic viscosity”). This is particularly important in the case of selective solvation of ions. The significance of these phenomena has been repeatedly highlighted in our previous works [1–5,13,34–39]. An important obstacle to the study of ion conductivity in mixed solvents is also the low accuracy of many of literature data. We pointed this out in our previous works [35,36,38]. Thus, full explanation of the experimental facts associated with the electrical conductivity of ions in mixed solvents requires further intense investigations. 4. Conclusions The conductivities of Bu4NI, Et4NI, Et4NIO4 and Et4NClO4 solutions in mixtures of water and acetonitrile in full composition range of the studied mixtures were measured. On the basis of the low concentration Chemical Model (lcCM) the relationship between molar conductivity and electrolyte concentration was analyzed and limiting molar conductivity, association constants and ion approximation parameters were determined. The values of limiting ionic conductivity and ionic Walden products were calculated on the basis of the limiting molar conductivity values and literature data. The analysis of the nature of the relationship of ionic Walden products as a function of the composition of the studied mixtures was carried out mainly in terms of ion – solvent interactions. The role of ion size, its polarizability and acetonitrile dipole moment

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