Conductance studies of i-Am3BuNl and NaBPh4 and the limiting ionic conductance in water+DMF mixtures at 298.15 K

journal of MOLECULAR

LIQUIDS

ELSEVIER

Journal of Molecular Liquids 75 (1998) 237-252

Conductance Studies of i-Am3BuNl and NaBPh4 and the Limiting Ionic Conductance in Water+DMF Mixtures at 298.15 K. Adam Szejgis', Adam Bald'* and Jerzy Gregorowicz" "University of L6dz, Faculty of Physics and Chemistry, PL-90-236 L6dz, Pomorska 163, Poland e-mail: [email protected] ABSTRACT Conductance data of triisoamylbutylammonium iodide, i-Am3BuNI (TABI) and sodium tetraphenylboride (NaBPIu) in the water+N,N-dimethylformamide (DMF), in the whole range of the mixed solvent composition at 298.15 K are reported. The data obtained have been analysed with the Fuoss-Justice equation in terms of the limiting molar conductance (Ao), ionic association constants (KA), and parameter R. Ionic association constant was found negligible for NaBPh4. From the data obtained here and those reported earlier, the individual limiting ionic conductivity of the Li÷, Na +, K ÷, Rb ~, Cs ÷, Me4N÷, Et4N÷, Pr4N+, Bu4N+, I-Iex4N÷, i-Am3BuN +, and of CI, Br, I, Ph4B has been determined using Fuoss-Hirsch assumption. From the values of the individual limiting ionic conductance obtained in this manner, the values of the Walden products for single ions were also calculated. The results obtained here as well as the values of the limiting molar conductance and Walden products for individual ions have been discussed. © 1998Elsevier Science B.V. INTRODUCTION The conductivity properties of unirunivalent electrolytes in the water+DMF have been a subject of our interest for many years. In our previous works of this series [1-4] we reported on the conductivity properties of some alkali metal chlorides [1], bromides [2], iodides [3], and some tetraalkylammonium iodides [4] in the binary mixtures of DMF with water at 298.15 K. In the papers mentioned above [1-4], the discussion of the results obtained were made on the basis of comparison of the differences in the limiting molar conductance and in the Walden products for the salts having the same cation or the same anion, versus the mixed solvent composition, water+DMF. We have also pointed [1-4] to the substantial differences in the interaction of different ions with the solvent dipoles, which is connected with the sign of their charge, their charge density, their size and polarizability, as well as with the character of their interactions (for example hydrophobic solvation). In these papers [1-4] we also suggested, that a more accurate description of conductivity properties of the electrolytes and the interactions of ions with the dipoles of the mixed solvent molecule would be possible, when the analysis of the conductivity data for individual ions was made. Therefore, we have decided to extend our investigations by including the solutions of triisoamybutylammonium iodide, i-Am3BuNI 0167-7322/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved SO167-7322(97) 00107-4

Pll

238 (TABI) and sodium tetraphenylboride (NaBPh4) in water+DMF mixtures at 298.15 K. From the data obtained in this paper and those obtained earlier we were able to calculate the conductivity of the single ions, both cations and anions. EXPERIMENTAL Water and DMF (Apolda, Germany) were purified according to the standard procedures as described earlier [1-4]. The sodium tetraphenylboride (NaBPh4, (Merck)) was purified by recrystallisation and were dried under vacuum at elevated temperatures for 12 hours. The tfiisoamylbutyammonium iodide, i-Am3BuNI (TABI) was prepared and purified as described in paper [5]. All the experimental techniques and procedures for the measurements of conductance of the electrolyte solutions were described in detail previously [1-4]. Solvent properties like relative permittivity and viscosity, necessary for calculation, were derived from the literature [6-8], as well as were numerically interpolated, when necessary. Solutions and mixed solvent compositions were prepared by weight, and were accurate to within _+0.01 wt%. The temperature was kept constant to within +0.005 K, and all the data were corrected at 298.15 K with the specific conductance of the solvent. As previously [1-4], we have estimated the accuracy of the measured values of conductivity as better than 0.05 per cent. RESULTS AND DISCUSSION The conductivity data were analysed with the Fuoss-Justice conductance equation [9-11] using the following set of equations: A= a [A0 -S (~C) 1/2"t"E (otc) In (otc) + J (ctc) + J3/2 (~c) 3/2 ]

(1)

KA = (1 -or) / (o~2 c y2 ±)

(2)

lny± = - (A
(3)

In these equations A0 is the limiting molar conductance, c~ is the dissociation degree of an electrolyte, KA is the ionic association constant, R is the distance parameter of ions, and y± is the mean activity coefficient of ions on the molar scale. The J and J3a coefficients of equation (1) are function of the distance parameter R. The values of J and J3;2 depend considerably on the value of R, but are less significant in equation (3). The other terms have their usual significance (see for example references [9-12]). The values of Ao, KA and R were obtained with the use of procedure given by Fuoss in paper [13]. The error of the R value i.e. AR was estimated as corresponding to 0.1 (O^)ml,. (for more see our papers [1-4] as well as the paper by Fuoss [13] ). The values of Ao, KA and R together with their standard deviation and estimated errors of the R values are given in Tables 1 and 2 for i-Am3BuNI and NaBPI~ solutions, respectively. The dependencies of A0 for TABI and NaBPh4 solutions versus the mixed solvent composition are given in Figure 1. As seen in Figure 1, the values of A0 decrease abruptly, when small amounts of DMF are added to water, reaching minimum at ca. 30-35 tool% content of the amide in the mixture, and increase with the increase of the DMF content

239 100 --o- i-Am3BuNI 90

+

NaBPh~

80 7O o

E

~

60

O3 ~<

50 40 30 20 10

t 0

L

I

I

I

I

10 20 30 40 50 60

I

I

I

70 80 90 100 mol % DMF

F i g u r e 1. The dependencies of the limiting molar conductance (Ao) for i-AmsBuNI and NaBPh4 the w a t e r + D M F mixtures at 298.15 K 1,0

0,~8 0,9

i-AmaBuNI -¢-- NaBPh. + i-Am~BuNBPh.

1,0 0,9

0,8

0,7

' 0,7

0,6

0,6

~ 0,5

0,5

%_

f

0,4

~

'0,4

0,3

" 0,3

0,2 0

I 20

I 40

I 60

I 0,2 80 100 tool % DMF

F i g u r e 2. The dependencies of the Walden products (Aorl) for i-Am~BuNl, NaBPh4, and i-Am3BuNBPha versus the mixed solvent composition at 298 15 K.

240 up to the pure amide. This character of the changes of the limiting molar conductance as the function of the mixed solvent composition is typical for all electrolytes investigated so far, in the water +DMF mixtures [1-4, 14-16]. The sharp minimum on the dependencies A0 = f(x) (where x denotes the mixed solvent composition) responses in most cases to the range of the mixed solvent composition for which macroscopic viscosity reaches its maximum. Therefore, it is better to analyse the dependencies A0r] = f(x), which is independent, from definition (Stokes low), from the values of the macroscopic viscosity. These dependencies for TABI, NaBPh4 and i-Am3BuNBPh4 are presented in Figure 2. As shown in Figure 2, the values of A0rl for salts investigated here changes to a smaller extent than in the case of alkali metal halides, investigated earlier [1-3]. This is probably due to the significantly smaller ionic conductance of i-Am3BuN+ and BPh4 ions in comparison with the alkali metal halides cations and halides anions. The differences in the character of the dependencies of A0rl = f(x) for TABI and NaBPh4 are observed mainly in the mixtures containing small amounts of DMF contents, in the water+DMF mixture. The addition of DMF to the water, in the case of NaBPth leads to the growth of the values of A0rl, but in the case of TABI proves the diminution of the values of A0rl. According to the suggestions given in papers [1-4], it seems, that this fact results from the difference in the conductometric properties of sodium and iodide ions. The observations mentioned above can be confirmed by the relatively small changes of the values of the Walden product for i-AmaBuNBPh4 (Figure 2) calculated from the dependence of A0rl(iAm3BuNBPI~) = A0rl(NaBPh4) + A0rI(TABI) - A0rl(NaI), in which the values of A0rl(NaI), were derived from the data given in paper [3]. The presence of the two large ions (i.e. iAmaBuN+ and BPh4" ), having small charge density, leads to the changes of the values of A0rl(i-Am3BuNBPh4 ) to a relatively smaller extent. In this case the growth of the values of A0rl(i-Am3BuNBPh4) accompanied with the addition of the DMF to the water seems to be slight. It can result from the quite different interaction of ions with the solvent molecule (for example hydrophobic hydration ). In the mixtures containing above ca. 35 tool% of DMF, the values of Aorl(i-Am3BuNBPh4) change insignificantly. However, a more precise conclusion can be obtained on the basis of the analysis of the conductometric data of the individual ions. Therefore, in this paper we have performed a division of the limiting molar conductance of salts investigated here and earlier [1-4], into the ionic contributions. The division of the limiting molar conductance of the salts into the ionic contribution was made according to the three different assumptions, which are well known in the literature [5, 16, 17] (a) - the Fuoss-Hirsch assumption [17], on the basis & t h e dependence of ~o (Bu,N +) = to (BPh,') (b) - the Coplan-Fuoss assumption [5], on the basis of the dependence of (BPh4") = ~0 (i-Am3BuN+) (c) - on the basis of the values of the transference numbers given in paper [16] In the literature an excellent review paper of Krumgalz is also available, which deals with all aspect of the division of the limiting molar conductance into the ionic contributions in several organic solvents, except for DMF and water+DMF mixtures [I 8] In Table 3 are presented the

241

Table 1. Limiting molar conductance (A0), ionic association constants (KA), their standard deviations (oA0) and (OKA), parameters R and their estimated errors AR, and the Walden products (Aorl) for i-Am3BuNI in water+DMF mixtures at 298.15 K DMF/ mol% 0.00 2.50 5.00 10.00 15.00 20.00 25,00 35.00 50.00 70.00 80.00 90.00 100.00

A0

oAo

95.38 74.83 60.96 44.95 36.69 32.96 30.22 30.25 36.56 51.08 59,68 68.41 a 76.70

KA

OKA

R

AR

0,04 0.05 0.05 0.05 0.04 0.03 0.03 0.03 0.04 0.05 0.05

5.5 5.9 5.7 5.2 5.2 4.8 4.2 3.9 4.8 70 9.2

1.5 2.1 2.0 1.8 1.9 1.6 15 1.1 2.1 24 2,4

4.0 4.0 4,0 4.0 4.0 4.5 4.5 4.5 5.0 55 6.0

0.5 0.5 0.5 1.0 1.0 0.5 0.5 0.5 1,0 1.0 0.5

0.07

14.5

3.2

6.0

1.0

Aorl 0.849 0.823 0.808 0.781 0.774 0.763 0.751 0,728 0.682 0.636 0.621 0.616 0.610

(a) - interpolated value calculated from the data obtained in this work

Table 2. Limiting molar conductance (Ao) and standard deviations (~A0), parameters R and their estimated errors AR, and the Walden products (Aorl) for NaBPh4 in water+DMF mixtures at 298.15 K. (The values Of KA were found negligible). DMF/ mol% 0.00 2.50 5.00 10.00 15.00 20.00 25.00 35.00 50.00 70.00 80.00 90.00 100.00

Ao 70.00 58.36 50.42 40.12 33.70 30.14 28.53 27.20 30.92 40.30 45.66 50.83 55.59

oA0 0.01 0,02 0.01 0.01 0.03 0,01 0,01 0.01 0.01 0.04 0.01 0.02 0.04

R 4.5 4.5 4.5 4.5 4,5 5,0 50 5.0 5.5 5.5 6.0 6.0 6.0

AR 0.5 1.0 0.5 0.5 1.0 0.5 0.5 0.5 0.5 1.0 1.0 0.5 0.5

A0rl 0.623 0.642 0.668 0.697 0,710 0,711 0.699 0.654 0.577 0.502 0.475 0.458 0.442

Units in Tables 1 and 2 are: A0 and oA0 / S cm2 tool q • KA and OKA / d m 3 molq • R and AR/10 s cm ; Aorl/S cm2 mol -~ P (where P denotes Poise units)

242 values of the limiting conductance for Na ÷ ion 0.0+) and CI" ion (Lo'), for some solvent mixtures, in order to compare the values obtained from the different assumptions. Table 3. The values of limiting molar conductance for Na ÷ (~0÷) and CI" ( ~ ' ) ions obtained from three different methods in water+DMF mixtures at 298.15 K DMF/moI% 0.00 10.00 25.00 35.00 a 50.33 28.89 19.42 17.78 Na ÷ b 50.79 29.41 19.80 18.16 c 50.22 29.32 1 9 . 9 3 18.03 a 76.21 38.01 23.04 21.40 CI" b 75.76 37.49 22.66 21.02 c 76.33 37.58 22.53 21.15 Units in Table 3 are: ~.0+ and ~,0 / S cm2 molt

50.00 18.85 19.22 19.10 22.85 22.48 22.60

70.00 22.79 23.36 22.90 2987 29.30 29,76

100.00 30.03 30.68 29.93 54.97 54.32 55.07

In Table 3 the symbols a, b and c denote three different methods of the division of the limiting molar conductance into the ionic contribution. As can be seen from the data given in Table 3, the values of the limiting molar conductance for the individual ions ( ~ ) obtained with the use of the three difference methods are almost the same, and it is difficult to choose one of them as the best of all. Furthermore, one can observe, that the differences in the values of ~ did not exceed two per cent (2%). Moreover, the differences in the values of ~± both for sodium and for chloride ions, did not show any influence on the character of the dependencies ~± = f(x) and ~ r l = fix), where x denotes mixed solvent composition, for sodium and chloride ions. A similar observation, that the differences in the values of conductivity of single ions do not depend substantially on the method of the division into the ionic contribution, could be drawn in the case of others cations and anions, which are presented here. For this reason we have decided to calculate the ionic contribution on the basis of the Fuoss-Hirsch assumption [17]. The values of ~.0* and ~0", obtained in this manner, in the range of the mixed solvent compositions up to 40 mol% of DMF in the mixture, are similar to those obtained with the use of the Coplan-Fuoss assumption [5]. Moreover, in the range 0 - 40 mol% content of DMF in the mixture, the values of the conductivity of the single ions change abruptly. For this reason the transference data given in paper [16] are not useful for the reliable numerical interpolation of the values of transference numbers, in the range of the mixed solvent composition mentioned above. The values of the limiting molar conductance for individual cations and anions, calculated on the basis of the Fuoss-Hirsch assumption [17], are presented in Tables 4 and 5, respectively. The dependencies ~0~ = f(x) and ~.0 = f(x) are presented in Figures 3 and 4, respectively. As shown in Figures 3 and 4, the addition of the DMF to the water leads to the diminution in the values of limiting ionic conductance for all ions investigated here. However, the intensity of this diminution caused by the addition of DMF to the water is not the same for all ions. The most strong diminution in the values of limiting ionic conductance is observed in the case of cations of cesium, rubidium and potassium, and for anions of chloride, bromide and iodide.

243 Table 4. The individual limiting ionic conductance 0~o* ) in water+DMF mixtures at 298.15 K. DMF/ tool% 0.00 2.50 5.00 10.00 15.00 20.00 25.00 35.00 50.00 70.00 80.00 90.00 100.00

Li* 38.76 31.47 26.42 19.57

Na ÷

50.33 42.23 36.58 28.89 24.08 13.30 21.25 12.04 19.42 10.80 1 7 . 7 8 12.49 1 8 . 8 5 16.65 22.79 19.34 25.10 22.32 27.55 25.44 30.03

K+

Rb +

Cs*

73.71 60.74 51.30 39.24 32.09 27.47 24.67 22.31 22.48 25.53 27.78 29.85 31.03

78.10 63.80 53.93 40.48 32.62 27.09 24.57 22.71 23.39 26.99 29.81 31.08 32.42

77.68 63.55 53.15 40.12 32.49 27.16 24.02 22.10 23.72 28.07 30.76 33.19 34.69

Me4N+ Et 4N+ 44.80 36.27 31.15 24.15

32.58 27.24 23.02 18.46

17.50 16.47 16.20 19.82

14.06 13.40 14.24 17.72 25.06 29.07 32.76 35.63

Table 4. (continued) DMF Pr4N÷ Bu4N÷ Hex4N* i-Am~BuN+ tool% 0.00 23.65 19.67 18.76 2.50 19.36 16.13 15.39 5.00 1 6 . 2 5 13.84 12.94 10.00 12.92 11.23 10.19 15.00 9.62 8.82 20.00 10.40 8.89 8.23 25.00 10.33 8.70 7.93 35.00 10.81 9.42 8.67 50.00 1 3 . 9 8 12.07 9.48 11.32 70.00 20.17 1 7 . 5 1 13.98 16.38 80.00 23.51 20.56 16.88 19.38 90.00 26.15 23.28 19.31 22.21 100,00 28.69 25.56 21.23 24.26 Units in Table 4: ;Lo*/S cm2 mol l

244

80

~ J_

Lr Na" K°

,

Rb" $+

.2,0

70

•

60

"

E

40'

m

30

/

/ /

• ---o----=.....

\ \

3,0

\11

~le,N" Et, N" Pr,N" Bu,N" Hex N"

2,5

1,s

~

a.

1,0

20

. ~-" oo °

0,5 10

=:~=~:=-

0

0

o"

~

~

i

=

20

40

60

80

0

100 % mol D M F

Figure 3. The dependencies of the limiting ionic conductances of the individual cations (~.0*) versus the mixed solvent composition. T a b l e 5.

The individual limiting ionic (Lo") conductance in water+DMF mixtures at 298.15 K DMF/ c r mol% 0.00 76.21 2.50 61.79 5.00 51,44 10.00 38.01 15.00 30.34 20.00 25.89 25.00 23.04 35.00 21.40 50.00 22.85 70.00 29.87 80.00 36.06 90.00 44.13 100.00 54.97

Br

I"

Ph4B

78,07 62.59 51.35 38,14 30.91 25.98 23.32 2190 24.13 32.38 38.63 45.89 53.98

76.62 59.44 48.02 34.76 27.87 24.13 22.29 21.58 25.24 34.70 40.30 46.20 52.44

19.67 16.13 13,84 11.23 9.62 8.89 8,70 9.42 12.07 17.51 20.56 23.28 25.56

Units in Table 5: ~,o/S cm2 mol"t

245 80

3,0

70

-..i-- Br ~~ rCI" t I Ph,B 2,5

°°1

2,0

40

1,5

50

~o

30

1,0

20 0,5 10 0 0

I

I

t

I

20

40

60

80

0 100

tool % DMF

Figure 4. The dependencies of the limiting ionic conductance of the individual anions (~0) versus the mixed solvent composition. The character of the limiting ionic conductance is not connected with the ion-ion interaction. but only results from the properties (nature) of the mixed solvent investigated, as well as from ion-solvent interaction. Thus, one can assume, that in the case of the ions mentioned above, the relatively small additions of the DMF to the water, influence substantially the interaction of these ions with the mixed solvent molecule. The smallest dimimition in the ionic mobility produced by addition of DMF to the water, is observed in the case of the large tetraalkylammonium cations and tetraphenylborate anion. It can result from the fact, that these ions having considerably large dimension can interact quite differently with the solvent molecule (for exarnple hydrophobic hydration). When the minimum of the values of limiting ionic conductance is attained (at ca 35 mol% of DMF in the mixture in the case of alkali metal cations and halide anions; at ca 30 tool% of DMF in the case of Me.~N* ; at ca. 25 tool% of DMF for others tetraalkylammonium cations and tetraphenylboride anion, BPh4), the values of the limiting ionic conductance increase monotonically with the increasing of the DMF content, up to the pure amide. The position of the minimum of the values of the limiting ionic conductance seems to be the connected with the kind of hydration The supposition mentioned above can be confirmed by conclusion given in papers [19-25], which deal with conductivity properties of ions in the mixtures of water with methanol [19], ethanol [20], tert-butanol [21], acetonitrile (ACN) [22, 23], dilnethylsulfoxide (DMSO) [24], and hexamethylphosphoramide (HMPT) [25]. In all cases [19-25] the position of the minimum of the limiting molar conductance of tetraa[kyammonium ions on the dependencies of )~0* = f(x) is observed in the range of the smaller amounts of organic components in the mixture with water, than it could be

246 Table 6. Walden products for single ions @o+rl) in water+DMF mixtures at 298.15 K. DMF/ mol% 0.00 2.50 5,00 10.00 15.00 20.00 25.00 35.00 50.00 70.00 80.00 90.00 100.00

Li ÷

Na ÷

K+

Rb ÷

Cs +

0.345 0.346 0.350 0.340

0.448 0.465 0.485 0.502 0.508 0.501 0.484 0.428 0.352 0.284 0.261 0.248 0.239

0.656 0.668 0.680 0.682 0.677 0.648 0.613 0.537 0.419 0.318 0.289 0.269 0,247

0.695 0.702 0.715 0.703 0.688 0.639 0.611 0.546 0.436 0.336 0.310 0.280 0.258

0.692 0.699 0.704 0.697 0.686 0.640 0.597 0.532 0.442 0.350 0.320 0.299 0.276

PnN +

Bu4N +

Hex4N ~

i-Am3BuN ~

0.211 0.213 0.215 0.224

0.175 0.]77 0.183 0.195 0.203 0.210 0.216 0.227 0.225 0.218 0.214 0.210 0.203

0.177 0.174 0.176 0.174 0.169

0.167 0.169 0.172 0.177 0.186 0.194 0.197 0.209 0.211 0.204 0.202 0.200 0.193

0.314 0.299 0.260 0.233 0.207 0.201 0.201 0.202

Me4N + E h N + 0.399 0.399 0.413 0.420

0.290 0.300 0.305 0.321

0.413 0.409 0.390 0.370

0.332 0.333 0.342 0.331 0.312 0.302 0.295 0.283

Table 6. (continued) DMF/ tool% 0.00 2.50 5.00 10.00 15.00 20.00 25.00 35.00 50.00 70.00. 80.00 90.00 100.00

0.245 0.256 0.260 0.261 0.250 0.244 0.236 0.228

Units in Table 6: to+q / S cm 2 mol" P,

247 observed in the case of alkali metal cations and halides anions. However the intensity of the growth of the limiting ionic conductance, when the minimum is attained, is comparable for all cations, in the water+DMF mixtures. Only the limiting ionic conductance of tetraethylammonium ion increased more rapidly. Unfortunately, we were not able to analyse or to compare the intensity of the growth of the limiting ionic conductance of tetramethylammonium ion, because of the lack of experimental data, in the investigated range of the mixed solvent composition. From the comparison of the dependencies presented in Figures 3 and 4 one can see, that when the minimum of the values of the limiting ionic conductance (~o~) is attained the values of ~" of anions of chloride, bromide and iodide increase more rapidly, in comparison with cations. Moreover, in the pure amide the values of the limiting ionic conductance of anions are considerably greater in comparison with cations. This can probably result from the fact that the anions are relatively weakly solvated in the pure DMF [26, 27, 28, 29]. It should be pointed here, that the order of the growth of the values of the limiting ionic conductance for different ions, in water and in pure DMF, is not the same (Tables 4 and 5). This means, that the growth of the limiting ionic conductance for different cations in water follows in the order of: Rb ~ ~ Cs + > K + > Na ÷ > Me4N+ > Li÷ > EhN-+ > Pr4N + > Bu4N + > i-Am3BuW, and in DMF: Et4N ÷ > Cs + > Rb + > K* > Na ÷ > Pr4N~ > Bu4N+ -_-Li+ > iAnI3BuN + > Hex4W, while for anions in water follows in the order of: Br" > Y =_.CI" > BPtu, and the pure DMF in order: CI > Br" > I > BPh4". The changes observed in the order of the growth of the values of the limiting ionic conductance (~,ot) surely result from the differences in the interactions of the ions mentioned above, with the molecules of water and DMF. However, one can conclude, that the character of the dependencies of ~,0 ~ = fix) is almost the same. The presence of the minimum on the dependencies of ~0 ~ = f(x) can result from the changes of the macroscopic viscosity, as the function of the mixed solvent composition (Stokes low). Thus, the analysis of the dependencies of the Walden products for ions (;%~rl) should be more interesting. The changes of the values of ~-*rl as the function of the mixed solvent composition are independent from the changes of the macroscopic ?¢iscosity. Thus the difference between the values of (X0±q) for different ions are source of the information on the differences in the ion-solvent interactions. The values of the Walden product for the single cations and anions are presented in Tables 6 and 7 and the dependencies ~+rl = f(x) and X0rl = fix) are presented in Figures 5 and 6, respectively. As shown in Figure 5 the values of ~.0+rl for alkali metal cations increase in the range of the small amounts of DMF in mixture and subsequently decrease with the increasing of DMF content in the mixture up to the pure amide. The above-mentioned growth of the values of X0÷rl of alkali metal cations accompanying to the addition of DMF to the water, was also observed in the case of the earlier-investigated mixtures of water with others organic solvent [19-25, 30-34]. The excess of the cation mobility, accompanying to the addition of the DMF to the water, is difficult to explain in terms of breaking or ordering of the structure of water, caused by an addition of the organic solvent. It is well known in literature [35, 36], that the addition of DMF to the water leads to the breaking of the structure of water. For this reason the small ions, which break the structure of water more weakly, should show greater excess of their mobility. The greater excess mobility of the sodium ions than potassium or cesium ions, connected with the addition of the DMF to water, could confirm the abovementioned conclusion. However, on the other hand, in the case of organic solvent ordering the structure of water, (for example ethanol [20, 21, 31, 37] one can expect, that the ions which break the structure of water to a larges extent should show greater ionic mobility.

248

0,8"

0,7

0,6

.~ Li*

~ • ~

~ ,

K"

8

Rb'

• Cs" ~Me,N"

IK

- -~

0,5 ~ , , ,

0,7

= Et,N"

~ \%, ~

..~

.t

0,8

- Bu,N"

" 0,6

- -o- - H e x , N + - -,~ - i - A n ~ B u N "

"~

. 0,5

0,4

0,4

0,3

0,3

0,2 ~ ~-A

"

" _ ~ ' ~ " ~ : : o .... --o.. -o - -o - .

0,1

0

0,2

0,1

i

I

I

20

40

60

t 80

0 100

tool % D M F

Figure 5. The dependencies of the Walden products for the single cations (~o+q) versus the mixed solvent composition.

Table 7. Walden products for single anions (~,o') in water+DMF mixtures at 298.15 K. DMF/ CI" mol% 0.00 0.679 2.50 0.680 5.00 0.682 10.00 0.660 15.00 0.640 20.00 0.611 25.00 0.573 35.00 0.515 50.00 0.426 70.00 0.372 80.00 0.375 90.00 0.397 100.00 0.437

Br" 0.695 0.689 0.680 0.663 0.652 0.613 0.580 0.527 0.450 0.403 0.402 0.413 0.429

I" 0.682 0.654 0.636 0.604 0.588 0.569 0.554 0.519 0.471' 0.432 0.419 0.416 0.417

Ph4B 0.175 0.177 0.183 0.195 0.203 0.210 0.216 0.227 0.225 0.218 0.214 0.210 0.203

Units in Table 7: ~,orI / S cm2 mol"~P

249 Thus, the maximum of the value of X0+*l should be more distinct for potassium ion than for sodium ion. However, the above conclusion is not confirmed by the experimental results [20, 21, 31]. The assumption, that the small amounts of DMF added to the water, leads to the replacement of dipoles of water to the greater dipoles of DMF, in the solvation shell of cations, seems not justified. It should to lead to the diminution in the ion mobility. Thus, it seems, that the changes of conductivity properties of cations of alkali metals, when the DMF is added to water, can be explained by the assumption of the preferential solvation of these cations by the water dipoles, as was proposed by Kay as so-called ,,sorting effect" [20] (even in the cases when the added organic solvent has more basic properties as for example DMF). Preferential solvation of alkali metal cations by water molecule, in the case when the small amounts of DMF are added to the water, can lead to a change of the microscopic viscosity (around ion) to the smaller extent, than macroscopic viscosity, which is used for the calculation of the values of the Walden products. It can lead to the excess of the mobility of the mentioned above cations and presence of the maximum on the dependencies of M+rl = f(x). Greater amounts of DMF added to the water result in the gradual replacement of the water dipoles to the DMF dipoles in solvation shell of cations, and the values of the ionic Walden products decrease. It should be pointed here that in the case of lithium ion, having great charge density, the abovementioned growth of the ionic mobility is relatively small and comparable with the analogous growth of the mobility of the cesium and rubidium ions. Simultaneously the stabilisation of the values of X0+vl for lithium ion is already reached at ca 50 mol% of DMF in the mixture. Probably in the case of Li+, the replacement of water dipoles to the DMF dipoles take place within the smaller amounts of the DMF in the mixture, and ranging from the 50 tool% of DMF content in the mixture, lithium ions can be considered as solvated only by DMF molecules. The notable disproportion between the excess of the mobility ofLi + and Na + ions was also observed in the water+ethanol mixtures [20]. This disproportion is however smaller than in the case of Li ÷ and Na + ions investigated in the water+DMF mixtures. It is advisable to point here, that the differences in the mobility of alkali metal cations are considerably smaller in DMF than in water. In the case of greater tetraalkylammonium cations and anion of BPh4, the differences between mobility of ions in pure water and in pure amide are relatively small. Moreover, the mobility of the great tetraalkylammonium ions and anion of BPh4" is greater in pure amide. For cation ofEt4N + and probably for cation of Me4N+, the mobility is greater in water than in pure amide. The addition of DMF to the water changes the character of solvation of great tetraalkylammonium and BPh4" ions (i.e. hydrophobic hydration), which leads to the increment of the ion mobility. In the DMF-rich region in the mixture, the ion-dipole interaction takes place, which leads to the decrement of ionic mobility. This seems to be confirmed by the fact, that tetraalkylammonium ions having smaller dimension, and thus greater charge density, lost their mobility more rapidly with the increasing of the DMF contents in the mixture. The ion of Me4N + seems to be similar to the alkali metal cations in its conductivity properties. However, the course of the changes of the values of X0q for chloride, bromide and iodide ions seems to be difficult to interpreted. As shown in Figure 6 (see also Table 7), only in the case of chloride ions, the addition of the DMF to the water leads to an insignificant excess of ionic mobility. In the case of bromide ion the values of ~.0"TIdecrease, and in the case of iodide ion these values decrease considerably, when the small amounts of DMF are added to the water.

250

0,8

0,8 *

CF Br

+ 1

0,71

0,7

0,6

0,6

o- 0,5-

0,5

0,4-

0,4

0,3-

0,3

0,2- ~

0,2

0,1

0

t

f

i

I

20

40

60

80

0,1

100

tool % DMF Figure 6. Tile dependencies of the Walden products for the single anions (X,0q) versus the mixed solvent composition As it results from the literature data [20, 21, 30, 31, 34], the addition of the organic solvent to the water, in the case of mixtures of water with alcohol [20, 21, 31, 34], and water with dioxan [30] leads to excess of anion mobility, explained by the so-called ,,sorting effect" [20], as in the case of cations In the case of the mixtures of water with aprotic solvent, the addition of the organic solvent to the water leads to the drop of values of X0q (in the mixtures of water+ACN [22, 23], water+DMSO [24] ) relative stabilisation (in the mixtures water+sulpholane (TMS) [32, 33]) or increase of these values (in the mixtures water+HMPT [25]), for halide anions. One can observe, that arnong chloride, bromide and iodide anions, the values of Z0rl for chloride ion increase to the greatest extent or decrease to the smallest extent, when the organic solvents are added to the water. In the case of the addition of the small amounts of the DMF to the water, the values of the differences of (X0"rl)" - (X0"q)'V (where m denotes the mixture containing small amounts of DMF and w denotes water) decreases in the order o f C l > B r >I. If one assumes, that by adding of DMF to the water, halides ions are preferentially solvated by a water molecules, then some excess of the anions mobility can be obtained. For this reason the maximum of the function of ~.0-q = f(x) should be observed for chloride, bromide and iodide ions (,,sorting effect" [20]). However the above-mentioned suppositions are not confirmed by experimental data If one assumes, that the addition of DMF to the water causes to the

251 replacement of water molecules by dipoles of amide in soivation shell of anions, then it should lead to the increment of effective radii of ions and to diminution of their mobility. In consequence the values of L0+rl should decrease with the increasing of DMF contents, which in fact, one can observe in the case of bromide and iodide ions. It seems however, that these hypotheses are in contradiction with the opinion presented in the literature [28, 29], on weak solvation of anions by disubstituted amides. On the other hand the conductivity properties of chloride, bromide and iodide anions, in the water+DMF mixtures, can be explained by the assumption, that chloride ions show preferential solvation by water molecules. This preference disappears nevertheless in order of CI', Br and I. In the case of iodide ion, one should even assume lack of this preference. It could be noted, that the addition of DMF to the water leads to the changes in solvation shells of iodide anions. It seems that the above-mentioned opinion can be sufficiently explained by an increment of polarizability of anions in order of CI-, Br" and I'. In the same order should increase the possibility to the interaction of these anions with the DMF dipoles, having considerable great dipole moment (3.86 D [26] ). The above-mentioned opinion can be explained by the fact, that ranging from the DMF-rich region to the pure amide, the values of ~or I for iodide ion change slightly, which means lack of violent changes in the solvation shell of these anions. In the case of the bromide ion, and particularly in the case of chloride ion, one can observe the distinct increment of the mobility, which can suggest desolvation of these ions and decrement their effective radii. G. Petrella and H. Pe~rella [38] have tried to explain the conductometric properties of anions in the mixtures of water with organic solvent in terms of ordering or breaking of the structure of the solvent. However, in our opinion, further studies are necessary, including conductometric investigations of others anions than halides, in other water-organic mixtures, in order to obtain a more complete explanation of the conductometric properties of anions. Appropriate investigations are already under way, and their results will be published as soon as possible.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

Z. Kozlowski, A. Bald, A. Szejgis and J. Gregorowicz, Polish J. Chem., 63 (1989) 547 A. Szejgis, A. Bald, J. Gregorowicz and C. M. Kinart, Phys. Chem. Liq., 34 (1997) 189 A. Szejgis, A. Bald and J. Gregorowicz, Monats. Chem., (1997) accepted, in press A. Szejgis, A. Bald and J. Gregorowicz, Phys. Chem. Liq., 35 (1997) 165 M. A. Coplan and R. M. Fuoss, J.Phys.Chem., 68 (1964) 1177 G. Douheret and M.Morenas, Comptes Rendus Acad. Sci. Paris Serie C, 264 (1967) 729 R. Reynaud, Comptes Rendus Acad. Sci. Paris Serie C, 266 (1967) 489 S. Taniewska-Osinska, A. Piekarska and A. Kacperska, J. Solution Chem., 12 (1983) 717 J. C. Justice, Electrochim. Acta, 16 (1971) 701 E. Renard and J. C. Justice, J. Solution Chem., 3 (1974) 633 J. Barthel, J. C. Justice and R. Wachter, Z.Phys.Chem.N.F., 84 (1973) 100 R. M. Fuoss and L. Accascina, "Electrolytic Conductance", Interscience, New York, (1959), page 195 R. M. Fuoss, J. Phys. Chem., 82 (1978) 2427 D. Singh, L~ Bahadur and M. V. Ramanamurti, J.Solution Chem., 6 (1977) 703 L. Bahadur and M. V. Ramanamurti, Can. J. Chem., .62 (1984) 1051 G. Chittleborough, C. James and B. Stell, J. Solution. Chem., 17 (1988) 1043

252 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38.

R. M. Fuoss and E. Hirsch, J..Am. Chem. Soc., 82 (1960) 1013 B. S. Krumgalz, J. Chem. Soc. Faraday Trans. I, 79 (1983) 571 C. Tissier and G. Douheret, J. Solution Chem., 7 (1978) 87 R. L. Kay and T. L. Broadwater, J. Solution Chem., 5 (1976) 57 S. Taniewska-Osifiska, A. Bald and A. Szejgis, J. Chem. Soc., Faraday Trans. I, 1989, 85, 4147 D. Singh, J. D. Mac Leod and A. J. Parker, J. Solution Chem., 13 (1984) 103 G. Petrella, M Castagnolo, A. Sacco and M. Petrella, J. Solution Chem., 9 (1980) 331 S. Das and D. K. Hazra, Indian J. Chem., 72A (1988) 1073 J. Y. Gal, H. Laville, F. Persin, M. Persin, L C. Bollinger and T. Yvemault, Can. J. Chem., 57 (1979) 1127 M. Spiro, Physical Chemistry of Organic Solvent Systems, (eds.) A.K.Covington and T.Dickinson, Plenum Press, London, New York, 1971 R. Zwanzig, J. Chem. Phys., 52 (1970) 571 A. J. Parker, Quart. Rev. (London), 16 (1962) 163 B. G. Cox, G. R. Hedwig, A. J. Parker and D. W. Wats, Austr. J. Chem., 27 (1974) 477 R. L. Kay and T. L. Broadwater, Electrochim Acta, 16 (1971) 667 T. L. Broadwater and R. L. Kay, J. Phys. Chem., 74 (1970) 3802 G. Petrella, M Castagnolo, A. Sacco and A. De Giglio, J. Solution Chem., 5 (1976) 621 G. Petrella, A. Sacco, M. Castagnolo, M Della Monica and A. De Giglio, J. Solution Chem., 6 (1977) 13 A. Szejgis, J. Gregorowicz and A. Bald, unpublished results A. Fratiello, Molec. Phys., 7 (1963) 565 L. Thakur and R. Prasad, Indian J. Chem., 19A (1980) 520 A. Vivo, C. Moran, M. Sanchez and M. C. Quintana, Ann. Quim., 82 (1986) 161 G. Petrella and H. Petrella, Electrochim Acta, 27 (1982) 1733