IONIC POLYMERIZATION UNDER A N ELECTRIC FIELD Now K --t means energy is given by Q)
2 :
+
1 and thus the excess free
4543
+
where c = r / ( r - 1) and'q = 1 [(r - l)/rc]. Equations A3 and A10 allow us to find the solvent activity coeffoient as a function of the chain length. This relation is
At the maximum value of GE the activity coefficients of A and B are equal, so we have This gives US for (GE/RT)m,x the equation Equations A3 and A4 may be combined with eq A9 to give the composition at the maximum. This calculation gives Table I gives (GE/RT)max and XAO for the various values of r.
Ionic Polymerization under an Electric Field. XII.
Living Anionic
Polymerization of Styrene in the Binary Mixtures of Benzene and Tetrahydrofuran by Norio Ise, Hideo Hirohara, Tetsuo Makino, and Ichiro Sakurada Department of Polyner Chemistry, Kyoto University, Kyoto, Japan (Received June 11, 1068)
The anionic polymerization of the lithium salt of living polystyrene was investigated in the binary mixtures of benzene and tetrahydrofuran (THF) a t 25' in the presence and absence of an electric field. The apparent propagation rate constant increased with increasing field strength and dielectric constant. The electric field did not affect the ion-pair propagation rate constant] whereas it increased the k,"K'IP term; k," is the free-ion propagation rate constant and K is the dissociation constant of the ion pair. The observed field effect was larger than that calculated by the Onsager theory on the second Wien effect. This discrepancy is discussed in terms of k,", which was found to increase with increasing field intensity. At a T H F content of 60 vol %, the k," reached a limiting value of -lo5 M-lsec-l when the field was intensified over 3 kV/cm. Evidence is presented which shows that the observed acceleration effect was not caused by the electroinitiation mechanism,
In previous papers, the influence of an electric field on polymerization reactions has been studied. The results have shown that cationic polymerizations can generally be accelerated, and the degree of polymerization of the polymers produced can be increased in the presence of a high-intensity electric field, --B whereas free-radical polymerizations are n o t influenced. Most recently it has been demonstrated that the monomer reactivity ratio in copolymerizationscan also be affected by the field.? These field influences have been shown to coincide with the changes caused by an increase in the dielectric constant of the solvent. This agreement
supports our interpretation that the field effects observed are due to the field-facilitated dissociation8of the (1) I. Sakurada, N. Ise, and T. Ashida, Makromol. Chem., 8 2 , 284 (1964); 95, l ( l 9 6 6 ) . (2) I. Sakurada, N. Ise, Y. Tanaka, and Y. Hayashi, J . Polyn. Sci., Part A-1, 4, 2801 (1966). (3) I. Sakurada, N. Ise, and S. Hori, Kobunshi Kaoaku, 24, 145 (1967). (4) I. Sakurada, N. Ise, and Y. Hayashi, J. Macromol. Sci. Chem., A I , 1039 (1967). (6) I. Sakurada, N. Ise, and Y. Tanaka, Polymer, 8 , 626 (1967). (6) I. Sakurada, N. Ise, Y. Hayashi, and M. Nakao, Kobunshi Kagaku, 25,41 (1968).
Volume 72, Number 13 December 1968
4544 growing chain ends. Though this interpretation appears reasonable, it lacks clear verification because of the intricate mechanism of the cationic polymerizations. Therefore, it is very interesting to study the field effect on much simpler systems. As one such system, we have chosen a living anionic polymerization of styrene. This choice enables us to observe the field effect on the propagation step directly. In the present paper, the binary mixtures of benzene and tetrahydrofuran (THF) are used as solvents, and Li+, which has been found to be strongly affected by ~ o l v e n t sis , ~chosen as the gegenion. This particular combination of living polymer-monomerinitiator-solvent has been studied by Bywater and Worsfold, lo and the living anionic polymerizations in ethereal solvents have been investigated by the Syracuse and 1Vlainz The propagation has been shown to proceed by two states of the growing chain ends, namely free ion and ion pair. The rate constant of free ions has been found to be much larger than that of ion pairs. A brief description of the present work has been published earlier. l 3
Experimental Section I . Puri$cation of Reagents. A . Benzene. Commercial benzene was washed by the usual method, refluxed over CaH2 for 1 week after drying, and distilled under nitrogen atmosphere onto a Na-K alloy. Using a high-vacuum line, the benzene was dried under violent stirring at room temperature, cooled down near to the melting point, and deaerated instantaneously. Several cycles of drying and deaeration were repeated and the benzene was distilled onto a Na mirror, After confirming the brightness of the Na mirror, the benzene was distilled into ampoules with break-seals which were adequately flamed, and the ampoules were sealed off from the vacuum line. B. Tetrahydrofuran. A raw sample of T H F was dried and distilled at atmospheric pressure in the same manner as the benzene. Then, on the vacuum line, the T H F was brought into contact with the Na-K alloy. The deaeration was effected by cooling the THF to -78” and then opening a cock to the vacuum line for about 10 min. Distillation into a trap at liquid nitrogen temperature did not occur. The adequately dried and deaerated T H F showed a characteristic blue color. C. Styrene. Commercially available styrene was washed and dried by the usual method and distilled onto CaHz under reduced pressure. The styrene was then dried using a vacuum line and CaH2 was cooled to the melting point and deaerated. The styrene was then distilled onto the Na mirror. The distillation was repeated until the brightness of the mirror did not disappear. I I . Preparation of Living Polymer. Following the The Journal of Physical Chemistry
N. ISE,H. HIROHARA, T. MAKINO,AND I. SAKURADA method described by Fujimoto, et aZ.,l4 n-butyl bromide was allowed to react with metal lithium in ether and then the solvent was replaced by benzene. Using the butyllithium thus prepared, poly(styryl1ithium) with an approximate degree of polymerization of 15 was prepared and used as the seed of the polymerization reaction. III. Polymerization Procedures and Apparatus. As is well known, traces of oxygen, water, and carbon dioxide can inhibit living polymerization. In the preliminary experiments carried out under a nitrogen atmosphere in an open system, no reproducible results were obtained, and, in addition, the rate of polymerization was occasionally decreased by an application of an electric field.l5 Thereafter, a high-vacuum-line technique was always employed for purification of reagents and for polymerization. Figure 1 shows our polymerization vessel. Ampoules A and B contained dilute and concentrated benzene solutions of the seed polymer, respectively, and ampoules C and D contained T H F and a benzene solution of the monomer. The polymerization vessel was connected to the high-vacuum line, evacuated, flamed, and sealed off. Then the break-seal on ampoule B was crushed, and the whole vessel was rinsed with the concentrated solution of the living polymer. All the contents were then collected in container E and the vessel was washed by cooling the walls with a pad at Dry Icemethanol temperature and condensing the benzene. From E the solvent was distilled into F, and E was (7) I. Sakurada, N. Ise, Y . Hayashi, and M.Nakao, Macromolecules, 1, 265 (1968). (8) (a) M. Wien, Ann. Phys., 83, 327, 795 (1927); (b) M. Wien, Phys. Z., 28,751,834 (1927). (9) D. N. Bhattacharyya, J. Smid, and M. Szwarc, J . Phys. Chem., 69, 624 (1965). (10) 8. Bywater and D. J. Worsfold, ibid., 70,162 (1966). (11) (a) D. N. Bhattacharyya, C. L. Lee, J. Smid, and M.Szwarc, Polymer, 5, 54 (1964) ; (b) D. N. Bhattacharyya, C. L. Lee, J. Smid, and M. Szwarc, J . Phys. Chem., 69, 612 (1965); (c) M. van Beylen, D. N. Bhattacharyya, J. Smid, and M. Szwarc, ibid., 70,157 (1966); (d) T. Shimomura, K. J. Tolle, J. Smid, and M. Szwarc, J . Amer. Chem. Soc., 89, 796 (1967); (e) T. Shimomura, J. Smid, and M. Szwarc, ibid.,89, 5743 (1967). (12) (a) H. Hostalka, R. V. Figini, and G. V. Schulz, Makromol. Chem., 77, 240 (1964); (b) H. Hostalka and G. V. Schulz. J . Phys. Chem. (Frankfurt am Main), 45, 286 (1965); (c) W. K. R. Barnikol and G. V. Schulz. ibid., 47, 89 (1965). (13) I. Sakurada, N. Ise, H. Hirohara, and T. Makino, J . Phys. Chem., 71, 3711 (1967). (14) T. Fujimoto, N. Ozaki, and M. Nagasawa, J . Polyn. Sci., Part A , 3, 2259 (1965). (15) It appears highly plausible that an unremoved trace of water can be electrolyzed by the application of the field to produce oxygen which can inhibit the polymerization. Thus the presence of the electric field gives rise to a “negative” field effect as mentioned in the text. The negative field effect has also been observed for f r e e radical polymerization of methylmethacrylate,’ when the monomer had been allowed to absorb water. I n this connection, Khalestskii, et al., have reportedi6 the “retarding” influence of an electric field on radical polymerization. It appears to us that this influence is due to the electrolysis of foreign substances, since no detailed description of reagent purification was given. (16) M. M. Khalestskii and B. I. Sukhoruker, Vysokomol. Soedin., 3 , 1347 (1961).
IONIC POLYMERIZATION UNDER AN ELECTRIC FIELD VACUUM
OPTICAL CELL
Figure 1. The apparatus used for kinetic studies of living polymerization under an electric field.
sealed off. The break-seals on ampoules A, C, and D were crushed, and, by turning the whole vessel upside down, the dilute solution of living polymer was admitted into G, and T H F and the monomer solution into H. The apparatus was again turned upside down, the contents were vigorously shaken, and the mixture was allowed to collect in F. About 9 ml of the polymerizing solution was transferred into the attached optical cell, the cell was placed in a spectrophotometer, the recorder was switched on, and a high (dc) field was applied. The quartz optical cell (optical path, 1 cm) was furnished with a pair of parallel platinum electrodes (cell constant, 0.15 cm-l). The spectrophotometer was anEPS-3T of HitachiManufacturing Co., Hitachi, Japan. The progress of polymerization was followed by monitoring the adsorption of styrene a t 291.5 mp, The concentration of the living end was determined by measuring adsorption a t 335 mp a t regular intervals during the polymerization. The temperature of the optical cell was believed to be maintained a t 25 i 0.02" by using a specially designed thermostated cell holder. The spectrophotometric method was more appropriate for our purpose than the capillary flow technique, since a uniform electric field can be much more easily produced in the optical cell than in the capillary. By using spectrophotometry, the polymerization a t concentrations of living ends lower than M could be investigated, as was noted ear1ier.l'" At these concentrations, the polymerization proceeded fairly slowly. It can be claimed, therefore, that the field was applied during the whole course of polymerizations, though a stationary high field was set up about 15 sec later after the onset of the polymerization. The initial monomer concentration of styrene was usually about 25 times that of the living end. The dielectric constants of the solvent mixtures were deter-
4545 mined by using a capacitance bridge and a three-terminal guarded cell. The viscosity was determined with a Cannon-Ubbelohde viscometer. The electric current caused by the application of the high field gives rise to two undesired phenomena, even though the current intensity was small. One is a temperature rise in the polymerizing solution. Under the present experimental conditions, calculations based on the assumption that the solution is adiabatic show that themaximum rise amounted to 1.0" in 30 sec, during which time most of the optical measurements were taken. According to control experiments performed without the cell holder mentioned above, a slow distillation of the solvent from the cell to F took place during a prolonged period. This was prevented by using a cell holder and by keeping the room temperature a t about 25". A second perplexing effect of the electric current is the electrolytical production of active propagating species or polymerization-initiating species. Since the elimination of this effect is most essential to our experiments, this matter will be discussed at length in the Discussion section.
Results The dielectric constant and viscosity of a series of mixtures of benzene and T H F a t 25" are given in Figure 2. The observed values of the pure liquids are in excellent agreement with the literature values. All of the polymerizations showed an internal first-order disappearance of styrene, as was observed previously. l7 The apparent propagation constant (k,) determined from our measurements is given as a function of the inverse half-power of the polystyryllithium concentration in Figure 3. According to Szwarc, et uZ.,l1b
'h
{o.60
a
I /0.45
0
20
A
40 60 80 Volume % THF
100
Figure 2. Dielectric constant (O), and viscosity(0) of THFbenzene mixtures a t 25'. (17) C. Geaointov, J. Smid, and M. Szwaro, J. Amer. Chem. Soc., 84, 2508 (1982).
Volume 78, Number IS December 1968
N. ISE,H. HIROHARA, T. MAKINO, AND I. SAKURADA
4546 the intercept of this plot gives the ion-pair propagation constant (k,’) and the slope is equal to k,”K‘/’, where k,” is the propagation constant of the free ion, and K is the dissociation constant of the ion pair. The filled symbols and the blank ones in Figure 3 give the k, values obtained in the presence of an electric field of 5 kV/cm and in its absence, respectively. Thus it is clear that the electric field increases the kp”K‘/z values for T H F contents of 30,40, 50, and 60 ~ 0 1 % . On the other hand, the electric field has no influence on the k,’ values, which were 10.8, 19.0, 31, and 48 M-l sec-l at T H F contents of 30, 40, 50, and SO%, respectively.
5
0
r
-
~~~
Table I: Field Effects at Various Field Strengths on the Living Anionic Polymerization of Styrene
1
THF vol fraction,
Field strength,
%
kV/crn
kp”K”*, M-”’ 888 -1
40.0
0 1.8 4.0 5.0
0.05 0.06 0.09 0.11
1 1.20 1.80 2.20
1
50.0
0 1.0 3.0 5.0 7.6
0.11 0.12 0.16 0.20 0.33
1 1.09 1.47 1.82 3.00
1 1.012 1.037 1.061 1.093
60.0
0 0.5 1.0 2.0 3.0 5.0
0.42 0.46
1
1 1.006 1.011 1.022 1.033 1.055
0.57 0.77 0.88 0.91
kp,E”KE”’/ (KE/Ko)‘/* kP,o~’K~’/’ (calcd)
1.09 1.46 1.83 2.10 2.16
1.027 1.056 1.070
30
Discussion It would be useful to mention a t first the temperature effect on the apparent propagation rate constant ( k p ) ,
in 0 ’ - - - 1 - _ _ 1 0 50 I00 150 I
/m
200
250
w!
‘0 L50U 100 _ l 150- - 200 i _250i 1
Irn
Figure 3. Dependence of the appararent propagation rate constant on poly(styryl1ithium) concentration at 25”. THF content (vol %): 0 , 0, 60; ,. 0,50; A, A, 40; e, 0, 30. The filled symbols are for 5 kV/cm, and the blank ones are for 0.
According to Bywater and Worsfold,’O the activation energy (Emt) of the propagation process of the present system decreases with increasing T H F content in the solvent. I n the solvent mixtures, in which the fieldaccelerating effect was observed, the Eaotvalue is believed to be smaller than 4 kcal. Thus the temperature rise due to the Joule effect, which amounts a t the most to 1.0” under our experimental conditions, should increase the k, value by 2%. The apparent increase in k, resulting from the application of the field, on the other hand, is 55%, which greatly exceeds the temperature effect. Thus it was concluded that the observed field-accelerating effect is not due to the Joule effect.’* Next we turn to the possibility of the so-called electroinitiation me~hanism.’~Control experiments were carried out using monomer solutions containing no polystyryllithium. Application of an electric field (5 kV/cm) resulted in no change in the styrene absorption at 291.5 mp and no living-end absorption a t 335 mp, even when approximately the same quantity of electricity was allowed to pass through the monomer solutions as in the case of polymerization. This fact rules out the possibility of the direct electron addition to the monomer by electrode reaction. Furthermore, it should be remarked that the time independence of the living-end concentration also excludes the possible role
By combining the kp“K”’ values observed in the absence of the field and the K values (at 20”) reported by Bywater and Worsfold,lo approximate values of k,” can be estimated to be 2 X lo4,3 X lo4,3.5 X lo4, and 5 X lo4M-I sec-’ for T H F contents of 30, 40, 50, and 60 vol %, respectively. These values are in good agreement with those reported earlier,IO increasing with rising dielectric constant as was found earlier.’O At T H F fractions of 10 and 20%, the k, values were independent of the concentration of the living end both in the presence and absence of an electric field of 5 kV/cm. The k,’ values were 2.07 and 5.7 M-l sec-l at 10 and 20%, respectively. No field effects on k, and k,’ were observed. Table I shows the field effects at various field strengths. The k,”K’/’ values in the third column were determined based on the fact that k,‘ is inde/’/~~,~~ pendent of the field strength. I C I ) , E “ K ~ ~‘KO1/’ (18) It is interesting to recall that in this laboratory field-accelerating is the ratio of the slopes of the k,-l/Z/W] plot where effects were also observed for some cationic polymerizations which [LE] is the concentration of thelivingend, with andwithhad negative apparent activation energies for their polymerization rates. out an electric field, which denotes the field-accelerating (l9j See, for example: B. L. Funt, S. N. Bhadani, and D. Richardeffect. It should be noted that the ratio increases son, J. Polym. Sci., Part A-1, 2871 (1966); N. Yamazaki, S. Nakawith increasing field strength. hama, and €3. Kambara, PoZym. Lett., B3, 67 (1966). The Journal of Physical Chemistry
1 . i IONIC POLYMERIZATION UNDER AN ELECTRIC FIELD
R
z
-w.
(n
0
d
6
-
OO
100
sec.
200
300
Figure 4. Time dependence of t h e concentration of living ends under a n electric field a t 25": a, d, e, polystyryllithium monomer solvent; b, c, polystyryllithium solvent.
+
+
+
of the electroinitiation mechanism as follows. As curve a of Figure 4 shows, the living-end concentration in most cases stayed practically constant within the polymerization period. This implies that the living ends can be neither produced nor consumed by electrode reactions. The possibility that an equal amount of living ends can be produced at one electrode and simultaneously be consumed at the other can also be eliminated by curves b and c in Figure 4, which were obtained for polystyryllithium solution without monomers in the presence of the electric field (5 kV/cm). The results show that consumption, if it occurred a t all, is only to a very small extent. Furthermore, attention should be given to the fact that the field effect on the apparent propagation rate constant increases with decreasing living-end concentration (Figure 3)) whereas the quantity of electricity being passed through the polymerizing solution, which is calculable from the field strength and current intensity, increases with increasing living-end concentration. By this experimental fact, we can assert that the observed field acceleration is not due to electrolytically initiated polymerization. 2o It is to be noted further that the trend of decreasing of [LE] in the early stage of polymerization, as demonstrated by curves b and e, could be observed only when the high-vacuum line was clumsily handled so that the decrease was due to the introduction of retarding agents by electrolysis. We note that the increasing tendency demonstrated by curve d is associated with the decreasing trend of the specific conductivityzz in the early stage of polymerization. The increase in [LE] was frequently observed when white precipitates were produced in n-BuLi solutions. Probably, an impurity (presumably LiBr) was reduced to metal lithium, from which a living anionic polymerization with two living ends started. When all of the LiBr was electrolyzed, [LE] reached a constant value. It should be noted that the k , value was obtained from this constant value.
4547
From the foregoing discussion, we conclude that the observed field-acceleration effect is not caused by the electroinitiation mechanism. 23 In previous papers'-' we ascribed the observed increase in the rate of cationic polymerizations to the second Wien effect, and we anticipated, as mentioned at the beginning of this paper, that the present anionic polymerization can be enhanced under an electric field as a result of this effect. As a matter of fact, the results show that the propagation rate constant was increased by the field. However, it should be noted that the field influence was observed on the kprrK'/z term. This situation invites some critical remarks on the mechanism of the field acceleration. We confine ouselves to the field effect on K , namely, the second Wien effect. If the observed field effect is wholly attributed to the second Wien effect, the kp,ErrKE1/z/kp,OrrKoL/z given in the fourth column of Table I should be equal to KE~"/KO~'~.According to the Onsager theory,24the field effect on the dissociation constant can be written
(KE/K~)'/' 1
1 + -b21 + -b2 +. 24
,
.
(1)
where b = (9.636 X 10-3)E/eT2for 1: l-type electrolytes, E is the field intensity in kilovolts per centimeter, and r is the dielectric constant. The fifth column of Table I gives the (KE/KO)l/' values calculated by eq 1. Evidently, the theoretical values are much smaller than the experimental ones given in the fourth column. This disagreement suggests first that the models and assumptions involved in the Onsager theory do not apply for the polymer systems. I n this theory, the ions were assumed to be small metallic spheres. In our system, on the other hand, the living polymers having a degree of polymerization of about 15 at the onset of the polymerization exist in solvent containing benzene, which is a good solvent for polystyrene. Thus it is reasonable to assume a randomly coiled and partially free-draining model for the living polymers. This con(20) We recall here that the conductivity increase of weak-electrolyte solutions under high-intensity electric fields is independent of the pulse duration, if the latter is larger than the threshold value characteristic of electrolytes.2' This independence supports the conclusion that the observed conductivity increase does not have its origin in the electrolysis but is due to the field-facilitated dissociation of the weak-electrolyte molecules, or the second Wien effect, (21) H. C. Eckstrom and C. Schmelzer, Chem. Rev., 24, 367 (1939). (22) The specific conductivity was obtained from the field strength and current intensity. Being determined by such an inaccurate method, it is not intended to develop a quantitative discussion of conductivity. We note, however, that a linear relation was obtained between the logarithm of the equivalent conductivity and that of the living-end concentration with a slope of -I/z, indicating the coexistence of free ions and ion pairs. (23) Various evidence against the electroinitiation mechanism was discussed for cationic polymerizations.'-' Among them, the field influence on the monomer reactivity ratio in copolymerizations most definitely excluded the role of the electrolytical initiation,' since the reactivity ratio is determined solely by the propagation process. (24) L. Onsager, J. Chem. Phys., 2, 599 (1934).
Volume 72, Number 13 December 1068
N. ISE,H. HIROHARA, T. MAKINO,AND I. SAKURADA
4548
2t 0
I
I
25 2
4 E KV/cm
6
8
Figure 5. Field-intensity dependence of the free-ion rate constant a t 25”. THF content (vol %): 0, 60; 0,50; A, 40.
tradicts the metallic sphere model. Accordingly, the failure of the theory does not exclude the second Wien effect as a potential factor causing the observed field effect. 25 The disagreement between theory and experiment may then be interpreted to imply that the free-ion rate constant (k,”) becomes larger with increasing field intensity, if the Onsager theory correctly accounts for the field-facilitated dissociation phenomenon taking place in the present system. By using the Onsager theory and the observed values of k, ,B“KE’/ykp ,o”Kol/a, k p , ~ was ” calculated and is shown in Figure 5 as a function of the field strength. I n this calculation, the l ~ , , ~ ‘ l value was taken as 5 X lo4,3.5 X lo4,and 3 X lo4M-’ sec-l for T H F contents of 60, 50, and 40 vel%, respectively. It is seen from Figure 5 that k,,~” increases with increasing field intensity. At a T H F content of 60 vol %, the k,,~” reaches a constant value of -lo5 M-’ sec-l above 3 kV/cm. It has been suggested that the anionic growing chain ends of a free-ion type can be solvated by THF molecules only weakly.llbvd If this is true, it would be tempting to suggest that THF molecules can be removed by the applied electric field so that the desolvated growing ends become more accessible to monomers. Furthermore, the limiting value of JZ,,E” (-lo5 M-l sec-l) might be ascribed to the “naked” growing ends. This interpretation appears to be supported by the experimental fact that k,,~” values at lower T H F contents (and hence at lower dielectric constants) increase more slowly with increasing field strength, as shown in Figure 5 ; 2 6 in other words, the lower the dielectric constant, the higher the field strength necessary to strip the T H F molecules from the growing ends. This would be understandable if the solvation is primarily determined by the coulombic forces. The desolvation theory seems to be acceptable at first sight. However, it is accompanied by the following defects. It should be pointed out that the intensity of the applied electric field (-lo3 V/cm) is considerably lower than that due to a univalent ion, which may The Journal o j Physical Chemistry
30 35 IO(€- 11/(2€+1)
1
40
Figure 6. Dielectric constant dependence of the ion-pair rate constant at 25“: 0, present work; 0, previous work;l*b - - -,previous work at 2Oo.l0
amount to as much as lo7 V/cm at 1-B separation. The calculation by the Born2’ and Moelwyn-Hughes theories2*shows that it is impossible to remove the solvent molecules from the growing chain ends by an external field, although the application of these theories to the present case is fairly questionable. Thus if desolvation is responsible for the observed increase in k,”, forces other than coulombic ones must be predominant. This is inconceivable under the usual conditions. From the foregoing discussion, it is clear that further studies are necessary in order to find out the true case for the observed field acceleration. High-precision conductivity measurements are urgently needed to judge the pros and cons of the interpretation in terms of the second Wien effect. Simultaneously, studies on the solvation state of the growing chain ends are also indispensable. Efforts along this line are currently being made in this laboratory. These experiments show that the ion-pair propagation rate constant (k,’) is not affected by the electric field. The field intensity used in the present work was not high enough to vary the distance between charge centers of the ion pairs. The Laidler-Eyring theory29 on reaction rates between dipole molecules can be written
where I.C and r are the dipole moment and radius, respectively, and A and B and indicate the reactants and
*
(26) A most direct evidence for the Wien effect should be obtainable from the electric conductivity increase by the electric field. The conductivity determined as noted above was not, however, reliable enough t o show the increase with increasing field. (26) Experiments at field intensity higher than 8 kV/cm were impossible because of electric discharge in the optical cell. (27) M. Born, 2. Phys., 1, 46 (1920). (28) E. A. Moelwyn-Hughes, Proc. Cambridge Phil. Soc., 45, 477
(1 948). (29) K. J. Laidler and H. Eyring, Ann. N . Y . Acad. Sci., 39, 303
(1940).
THEGIBBSFREEENERGY OF SOMEELECTROLYTES critical complex, respectively. Figure 6 shows that eq 2 holds for the present systems. The filled circle, which denotes the result reported for 100% THF, lib
4549 falls on the linear relation. The broken line in Figure 6 gives the results reported previously for 2O0.l0 The agreement is fairly satisfactory.
The Gibbs Free Energy of Transfer of the Alkali Perrhenates and Perchlorates between Pure Water and Pure Nitromethane by G. R. Haugen Department of Thermochemistry and Chemical Kinetics, Stanford Research Institute, Menlo Park, California 94026
and H. L. Friedman Department of Chemistry, State U n i v e r s ~ of y New York at Stony Brook, Stony Brook, New York 11790 (Received June 18, 1968)
Previously reported partition and salting-out experiments in the ternary system nitromethane, water, and electrolyte were combined with two series of new experiments to furnish the Gibbs free energy of transfer of each of the electrolytes between the various solvents. The determination of the partition of CsC104 between the equilibrium water and nitromethane phases and the determination of the solubility of LiReOd in nitromethane as a function of the concentration of added water are reported herein. The standard Gibbs free energies of formation at 25" of the ions Li, Na, K, Rb, Cs, C104, and Re04 in each of the solvents, of water saturated with nitromethane, of nitromethane saturated with water, and of pure nitromethane are reported. In addition, the standard Gibbs free energies of formation of the ions H, C1, I, Cr(NH&(SCN)d, and PFs in Dome of these solvents are reported. Both the electrostatic and nonelectrostatic contributions to the free energy of transfer of an ion from pure water to pure nitromethane tend to make this free energy more negative for larger ions. The Gibbs free energy of transfer of an electrolyte between pure water and pure nitromethane is accurately given by the Born charging processes. The specific solvation effects are apparently more noticeable in enthalpies and entropies of transfer than the free energies.
Introduction The changes in free energy, enthalpy, and entropy accompanying the solvation of a series of ions of similar structure are quantities that are fundamental to a molecular-level understanding of electrolyte solutions in the solvent. The changes in the same thermodynamic quantities accompanying the transfer of a series of ions from one solvent to another are also important. While more complicated than the solvation energies, the energetics of transfer have the advantage of being experimentally accessible for complex as well as for monatomic ions. A recent study of enthalpies of transfer from water to several aprotic solvents' illustrates what one may learn from such data, even with only a qualitative treatment. The free energies of transfer are perhaps most directly determined by either emf cell measurements, including polarography, or by measurement of the ratio of the solubility of a salt in the two solvents, but it often happens that these methods are not applicable in a case of interest. The problem of obtaining accurate free
energy data in nonaqueous solvents is complicated by the increase in ion-ion interaction in the saturated salt solutions in these solvents and the difficulty of finding reversible electrodes that operate in these solvents. Furthermore, the systematic study of & group of similar ions is handicapped by the difficulty encountered in finding a series of salts with the right properties. I n a series of earlier publications2-4 we have worked out an indirect, but rather general, method /for determining free energies of transfer and applied it to the transfer of the alkali metal ions from water to nitromethane. This method comprises the following steps for the
(1) H. L. Friedman, J . Phys. Chem., 71, 1723 (1967). (2) G.R. Haugen and H. L. Friedman, ibid., 60, 1303 (1956). (3) H.L. Friedman and G. R. Haugen, J . Amer. Chem. SOC.,76, 2060 (1964). (4) G. R. Haugen and H. L. Friedman, J . Phys. Chem., 67, 1757 (1963).
Volume 7@,Numbm IS December 1968
