ALESSANDRO D’APRANO AND ROBERTO TRIOLO
3474
Ion Pairs and Solvent-Solute Interaction. 11. Conductance of Lithium Chlorate in Methanol-Dioxane and in Acetonitrile-Dioxane
by Alessandro D’Aprano and Roberto Triolo Institute of Physical Chemistry, University of Palermo, Palermo, Italy
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(Received April 10,1967)
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The conductance of lithium chlorate over the approximate concentration range 5 < lo4 c < 25 has been measured in methanol-dioxane (32.7 2 D 2 10.5) and acetonitrile-dioxane mixtures (36.0 2 D 1 17.0). For mixtures with the same dielectric constant, the association constant K A is over an order of magnitude greater in the acetonitrile mixtures. The slope of the log KA-D-’ plot is less in the methanol mixtures. Both of these effects suggest that solvent-separated ion pairs are formed in the hydrogen bonding systems, while only electrostatic ion-dipole effects appear in the acetonitrile systems.
Recent conductometric work1B2 has shown that the sphere-in-continuum model cannot describe the behavior of lithium perchlorate and chlorate in dioxanewater mixture on account of intense interaction between solute ions and solvent molecules. If, however, shortrange interaction between ions and nearest-neighbor solvent molecules is assumed and the solvent is treated as a continuum beyond this zone, a model results which is consistent with the observations on these systems. The mechanism of interaction was postulated to be different for anion and cation: hydrogen bonding for the former and electrostatic ion-dipole attraction for the latter. From the contact distance found, it was also necessary to assume that the ion pairs are solventseparated in the case of the chlorate. This hypothesis was tested further3 by studying lithium perchlorate in methanol-dioxane and in acetonitrile-dioxane, in order to have the salt in quite different hydrogen bonding environments. The purpose of this paper is to present the results of a study of lithium chlorate in the same solvent mixtures, in order to complete the comparisons.
Experimental Section Lithium chlorate was purified and handled as before.2 Solvents were purified by methods described in the literature.4-6 The electrical equipment,’ conductance cell, and method2 have all been described previously. Solvent properties are summarized in Tables I and 11, where w is weight per cent of dioxane and the other The Journal of Physical Chemistry
Table I: Properties of Methanol-Dioxane Mixtures at 25” No.
W
P
D
1027
1o’K
1 2 3 4 5
0 40.15 50.93 60.07 65.71
0.7867 0.8725 0.8980 0.9202 0.9345
32.66 19.05 15.40 12.32 10.50
0.552 0.584 0.615 0.651 0.681
0.24 1.18 0.60 0.02 0.02
symbols are, respectively, density, dielectric constant, viscosity (centipoise) and solvent conductance. Equivalent conductances a t 25” are given in Table 111, where the solvent systems are identified by the code numbers of Tables I and 11.
Discussion The data for systems 1, 2, 3, 4, and 6 were analyzed on the IBM 1620 computer, using Kay’s programs (1) F. Accascina and S. Schiavo, “Chemical Physics of Ionic Solutions,” B. L. Conway and R. G. Barradas, Ed., John Wiley and Sons, Inc., New York, N. Y., 1966,p 515. (2) F. Accascina, A. D’Aprano, and R. Triolo, J. Phys. Chem., 71, 3469 (1967). (3) F. Accascina, G. Pistoia, and 5. Schiavo, Ric. Sci., 7 , 560 (1966). (4) Acetonitrile: D. S. Berns and R. M. Fuoss, J . Am. Chem. SOC., 83,1321 (1961). (5) Methanol: H.Hartley and H. Raikes, J . Chem. SOC.,127, 524 (1925). (6) Dioxane: J. E.Lind, Jr., and R. M. Fuoss, J . Phys. Chem., 6 5 , 999 (1961). (7) F. Accascina, A. D’Aprano, and R. M. Fuoss, J . Am. Chem. SOC., 81, 1058 (1959).
IONPAIRSAND SOLVENT-SOLUTE INTERACTION
Table I1 : Properties of Acetonitrile-Dioxane Mixtures at 25' No.
W
P
D
lO*q
1O'K
6 7 8 9 10
0 30.31 39.68 49.49 59.92
0.7773 0.8409 0.8622 0.8874 0.9136
36.01 26.85 23.80 20.45 17.03
0.345 0.416 0.449 0.496 0.558
0.34 0.41 0.21 0.09 0.07
3475
these data were therefore analyzed by the simpler Fuoss-Kraus equation lo A
Methanol-Dioxane and in Acetonitrile-Dioxane A
10%
1
4.469 9.695 13.622 18.438 22.483
6 95.81 93.42 92.10 90.77 89.77
5.343 13.328 20.876 26.440 31.742
85.06 80.75 77.49 74.48 72.56
3.964 11.035 16.975 20.821 24.674
74.87 68.10 63.98 60.71 58.64
3.054 6.713 9.965 13.657 17.158
56.25 50.15 45.35 42.59 41.14
4.651 8.504 13.266 16.815 22.437
36.52 29.83 27.04 25.54 24.55
6.089 10.944 14.894 19.853 25.570
2 4.451 8.819 13.617 19.384 24.083
82.79 65.70 57.56 51.48 47.33 9
5 5,384 10.502 14.418 17.317 19.673
98.57 74.86 65.07 60.71 57.19
8
4 4.677 7.999 12.245 15.816 18.186
139.78 120.01 108.24 101.90 96.83 7
3 3.460 7.857 12.071 16.742 20.387
n
104~
56.88 46.09 39.05 35.64 31.82 10 21.880 17.102 14.866 13.098 11.720
for associated electrolytes, in order to obtain the constants for the conductance equationg A = &, f
E c log ~ CY
+ JCY - K A C ~ P A(1)
For systems 5, 7, 8, 9, and 10, the association constants were so large ( K A> 1000) that the E and J terms became negligible compared to the association term;
- SC'/~~'/~)
(2)
The results are summarized in Table IV.
Table IV: Derived Constants No.
Table I11 : Conductance of Lithium Chlorate in
= y(A0
1 2 3 4 5 6 7
8 9 10
A0
3
KA
Methanol-Dioxane 100.97 f 0 . 0 9 1410 =k 134 95.20 =k 0 . 2 7222 =k 273 13134 f 467 87.93 =t0 . 3 83.68 f 0.06 24948 f 65 78.40 ... Acetonitrile-Dioxane 170.00f0.3 1078 f 150 145.20 ... 135.15 ... 125.20 ... 112.00 ...
5*3 46 f 9 219 f 22 1344 =k 5 6625 401 1760 3520 6300 39300
* 60
We note that association is much more pronounced in the aprotic acetonitrile-dioxane mixtures than in the methanol-dioxane systems. In water-dioxane mixtures, association was negligible a t dielectric constants above about 30. These results parallel the behavior of the perchlorate in the same solvents3 and demonstrate strikingly the significant part played by hydrogen bonding in the solvation process, which in turn determines the extent of association by controlling the size of the ionic entities (central ion plus solvating molecules). As shown in Figure 1, the logarithm of the association constant is linear in reciprocal dielectric constant for both solvent systems. From the slope of the lines, contact distances are found to be d K = 5.04 for methanol-dioxane as solvent and 22K = 3.70 for the acetonitrile-dioxane mixtures. The value is, as expected, larger in the hydrogen bonding solvent, and furthermore, is in a consistent sequence with the value of 6.8 found in water-dioxane systems where of course the solvating molecule is HzO instead of HOCHs. The sequence of slopes of log KA-D-' plots clearly shows that solvating molecules remain in the ion pairs in those cases where hydrogen bonds form the solvate cluster. For comparison, the values for lithium perchlorate3 in the same solvents are also shown in Figure 1. Both (8) R. L.Kay, J. Am. Chem. Soe., 82, 2099 (1960). (9) R. M. Fuoss, ibid., 80, 3163 (1958). (10) R. M. Fuoss and C. A. Kraus, ibid., 55, 476 (1933).
Volume 7 1 . Number 11
October 1967
ALESSANDRO D’APRANO AND ROBERTO TRIOLO
3476
sion than the one based only on charge-charge interaction and excluded volume would have the form In K A = In (4nNu3/3000)
+
e2/aDkT - E,/kT 4- AS/L
I
O
0
U2
IOO/D
I
I
I
6
8
IO
Figure 1. Dependence of association constant on dielectric constant. @, LiClOa in acetonitrile-dioxane; 0, LiC104 in acetonitrile-dioxane; 0 , LiClOa in methanol-dioxane; e , LiC104 in methanol-dioxane.
salts in both solvent systems give linear plots, but the acetonitrile plots lie above those for the methanol systems, and the separation between chlorate and perchlorate is greater in the former. These differences show that each salt is associated to a higher extent in acetonitrile systems which are isodielectric with methanol systems, and that different contact distances are required for a given electrolyte in different solvents. These differences show that the simple model of charged spheres in a continuum is inadequate to describe the systems of Figure 1; other effects must be included in the equation for the association constant in addition to electrostatic attraction. First, the energy of solvent molecules with respect to two free ions and to the pair which they can make is different; a factor of the form exp( -E,/kT) calculated by Gilkersonll must be included. Second, the arrangement of solvent molecules will in general be different around free ions and around pairs, so an entropy term exp(AX/k) is needed. Finally, solvent-separated pairs will require different distance parameters to give different slopes on the log &-1/D plots,. Summnrieing, a more general expres-
The Journal of Ph,ysical Chemistr~
(3)
This equation is consistent with the results shown in Figure 1, if appropriate ad hoc assumptions are made. The dipolar structure of the chlorate ion should cause stronger attraction between it and acetonitrile, with the N atom of the solvent molecule adjacent to the (relatively positive) chlorine atom of the ion; in the pair, the lithium ion should be located on the face made up by the three oxygen atoms, and its presence would enhance the attraction for the nitrile molecule; that is, we propose a solvated, but not a solvent-separated pair, for the chlorate. Such an interaction would not be expected for the symmetrical perchlorate ion. Consequently, E , would be larger for the chlorate, and the chlorate curve should lie above the perchlorate curve in the acetonitrile systems. In the methanol systems, if we assume solvent-separated pairs for both salts, the plots in Figure 1 would be less steep than for the acetonitrile systems, as observed. To account for the vertical displacement of the two sets of curves, we recall that methanol is a hydrogen-bonding molecule; therefore, there is more order in a methanol-dioxane system than in an acetonitrile-dioxane system. When a pair forms in the methanol system, its weaker field (compared to that of the free ions which formed it) would permit greater reorientation in methanol than in acetonitrile; that is, the entropy change would be more negative for the former. The exp(AX/k) term in (3) would then lower the met>hanolcurves with respect to those for acetonitrile.
Acknowledgment. Acknowledgment is gratefully made to the Consiglio Naeionale delle Ricerche for partial support of this research. (11) W.R. Gilkerson, J. Chem. Phus., 25, 1199 (1956).
