J. Phys. Chem. 1985, 89, 1528-1531
1528
4
c+++--%
7 t
I
6
;5 v
4 -
3 -
2 -
It 00
i -I
-2
-3
-4
-5 4-6 log [Cu2+],
-7
-8
-9
4
M
Figure 5. The rate of decomposition of V03+.02H,kd,in 0.4 N H2S04 at 22 "Cas a function of added Cu2+. The calculated curve was obtained by numerical integration of eq 14, 16,-16, and 17 (see text).
of more reactive reductants (e.g., Fez+, hydroquinone) was prohibited by the large excess of V03+ in this system ([V03+. OzH]/[V03+] I 0.05); V03+ is itielf a strong oxidant. Effect of Cuz+on Decay of V03+-02H.The addition of Cu2+ in the second mixer increased the rate of disappearance of V03+.02H from kd = 3.5 s-' to kd = 6.4 s-' in 0.4 N H2S04. This to 3 X M. effect was irldependent of [Cu2+] from 1.3 X The dependence of kdon [Cu2+] is shown in Figure 5. Cu2+reacts with free HOz according to Cu2+
+ HOZ-% Cu+ + H+ + 0,
-
(22)
and proceeds by an inner-sphere process (kZ2 IO* M-' s-I ) .29 The reoxidation of Cu+by HOz is very fast resulting in an effective catalytic rate of 2kz2for the disproportionation of HOz by Cuz+ ions. The absence of a direct reaction between Cu2+ and V03+-OzHis consistent with the inner-sphere requirement of reaction 22. Thus in the presence of Cuz+, kd is dependent on
eq 13,-13, 14, and 22. At the limiting value of kd (6.4 [Cu2+] scavenges all the free HOz resulting from the dissociation of + k I 4 .Therefore k-13 = (6.4 V03+-02Hand hence kd = kWL3 3.5) s-l = 2.9 s-' and K , , = k13/lf..13 = 1 X los M-I in 0.4 N H2S04at 22 "C. The fate of Cu(1) in a solution containing several strong oxidizing species is uncertain. One of the referees has pointed out that if Cu(1) reacts with V03+.02Hin preference to free H 0 2 (or excess V03+)the reported value of k-13 is high by a factor of 2. In the event that Cu(1) reacts with V03+,the value of k F I 3is unchanged but kz2 would seem to exceed lo9 M-I s-l. Equations 13, 14, and 22 were numerically integrated to obtain kd as a function of [CIA'+]using the appropriate rate constants and initial conditions. The solid curve in Figure 5 requires a value of 2kZ2= 6 X lo8 M-I s-I, which is in reasonable agreement with the estimate of Rabani et al.29
Conclusions We have shown that HOz forms transient complexes with the peroxovanadium(V) species but not with V 0 2 + . This is in agreement with the results of S a m u d who detected ESR-active transients only in V(V) solutions containing peroxide. The presence of a faster decaying complex, which we have designated VOS-.02H, was not detected by Samuni and Czapski6 in a perchloric acid medium. The faster rate of decomposition of VOs--O2H relative to V03+.0zH reflects the respective oxidizing strengths of V05- and V03+. Also, high proton concentration increases the rate of peroxide decomposition by V(V) complexes. This may explain the [H+] dependence of k,, (Figure 3), if, at pH < I , protonation of a water molecule in the solvation sphere of V03+.0,H occurs, thus decreasing the stability of the complex. Acknowledgment. This research was carried out at Brookhaven National Laboratory under contract DE-AC02-76CH00016 with the U S . Department of Energy and supported by its Division of Chemical Sciences, Office of Basic Energy Sciences. Registry No. Q. 106-51-4; V02+, 20644-97-7; V 0 2 + , 18252-79-4; VOj*, 12179-36-1; H02, 3170-83-0; OH, 3352-57-6; H202, 7722-84-1; H20, 7732-18-5; 12, 7553-56-2; Cu2*, 15158-11-9; H, 12385-13-6. (29) Rabani, J.; Klug-Roth, D.; Lilie, J. J . Phys. Chem. 1973, 77, 1169.
Thermoelectric Power of the High Ionic Conductivity Glasses AgI:Ag,OB,O, Albert0 Schikldi,* Elisabetta Pezzati: and Prim0 Baldini Dipartimento di Chimica Fisica, Universitd di Pavia, 271 00 Pavia, Italy (Received: February 7 , 1984; In Final Form. November 21, 1984)
Thermoelectric power determinations allow one to obtain the thermodynamic properties of the ionic species exchangeable at the electrode of the thermocell employed. This may be of some relevance in the case of the high ionic conductivity glasses containing AgI, such as those formed in the pseudobinary AgI:Ag20.B203. The thermoelectric power of the glasses investigated becomes almost T independent in the vicinity of the corresponding TB. When these T independent values are plotted vs. the composition, expressed as AgI equivalent fraction, the resulting trend is comparable with those observed for binaries of molten salts. The system is treated as a mixture of two kinds of silver ions, Le., those from AgI and those from silver borate, respectively. By employing the well-known relationship between the thermoelectric power and the entropy of the ionic species exchangeable at the electrode, one may state the suitable ideal behavior of the thermoelectric power vs. the composition and describe the deviations from this as due to the excess entropy of mixing. The trend in the latter is indeed very close to that predicted by the model of regular mixtures. Finally, as the plots of the logarithm of the room temperature conductivity, activation energy, and Tgmay be fairly fitted vs. the AgI equivalent fraction by straight lines, it seems likely to suggest that the Ag+ coming from AgI plays a major role in determining the properties of these glasses.
Introduction A large family of glasses showing high ionic conductivity at rOOmtemperature has been recently recognized in the systems Ccntro di studio per la Tennodinamica ed Elcttrochimica dci sistemi salini fusi e solidi del CNR, Dipartimento di Chimica Fisica, Universitl di Pavia.
0022-3654/85/2089-1528$01 S O / O
AgI-Ag20-M,O,, where M,O, is a Lewis acid which, in some cases, can form a glass by itself. Studies concerning such glasses are spread throughout a number of experimental and theoretical works,'-10 mainly devoted to the interpretation of the mechanism (1) Schiraldi, A. Electrochim. Acta 1978, 23, 1039.
0 1985 American Chemical Society
The Journal of Physical Chemistry, Vol. 89, No. 8, 1985 1529
Thermoelectric Power of AgI:Ag20.B203 of ionic transport and, more generally, of the role played by AgI. It must be mentioned that, when the Lewis acid can form a stable glass by itself, AgI may be replaced by AgCl or AgBr6 in the ternary system with no relevant modifications of the extention of the glass formation region. The corresponding glasses, however, show an ionic conductivity which is significantly lower than for the equivalent AgI based ones. Conversely, when the Lewis acid is not able to form glasses either by itself or when added with Ag20, e.g., AszOs, W 0 3 , etc., the glass formation region in the ternary AgI-Ag,O-Lewis acid is substantially restricted to the composition line with the fixed stoichiometric Ag20/Lewis acid ratio corresponding to the stable Ag oxy salt.' In such cases no formation of stable glasses has ever been observed when AgI is replaced by the other silver halides. Accordingly, it seems that the presence of AgI always improves the ionic conductivity and sometimes allows the glass formation itself (e.g., when added to nonglass-forming Lewis acids). It is also of some relevance that, in no system of this family studied so far, the composition of the AgI-rich border of the glass formation region exceeds the value N = 0.65 (where N is the AgI equivalent fraction). As a rule, for N 0.6, one obtains glasses with approximately the same room temperature conductivity (i.e., 2 X loT2+ X lo-, ohmw1cm-', within the errors connected either with the evaluation of the cell constant or with the difference between actual dc and frequency-independent ac conductivities), whatever the Lewis acid concerned. This might mean that the migration of silver ions actually occurs in the middle of the iodide ions, with no significant effect due to the chemical nature of the oxyanions. This seems also to be the case for the system AgI:Ag20.B203, which shows a very wide glass formation region and, for this reason, has been thoroughly investigated.s-10 This hypothesis has been the subject of s t u d i e ~ ' -devoted ~ , ~ to the quantitative interpretation of the role played by AgI. A common point in these works is that the silver ions of these glasses belong to two main groups, those moving rapidly and those almost permanently bound. However, this kind of interpretation has never been confirmed by a thermodynamic property approach, which should allow us to verify whether the silver ions of these glasses behave as a mixture of two populations. A suitable experimental tool for this scope may be found in thermoelectric power determinations." This procedure has been employed in the present work for glasses corresponding to the pseudobinary AgI:Ag,O. B203. Other compositions with different Ag20/B203molar ratios will be the subject of further work in progress at the present time.
-
Experimental Section The preparation of the samples has been the same reported in ref 8; the only difference concerns the dimensions of the cylindrical glass ingots, viz., 1 cm length and 0.8 cm diameter, directly obtained by pouring the melt into a stainless steel mold. Silver electrodes have been applied by vacuum sputtering onto the base faces of the glass cylinder and the final cell has been set into a horizontal furnace between hollow silver bars, containing the insulated thermocouples and acting as terminal leads. Details of ~
(2) Malugani, J. P.; Waniewski, A.; Doreau, M.; Robert G.; AI Rikabi, A. Mater. Res. Bull. 1978, 13,427. Malugani, J. P.; Waniewski, A.; Doreau, M.; Robert, G.; Mercier, R. Ibid. 1978, 13, 1009. (3) Souquet, J. L. Annu. Reu. Muter. Sci., 1981, 1 1 , 211. (4) Ingram, M. D.; Vincent, C. A. Annu. Rep. Chem. Soc., Sect. A: Phys. Inorg. Chem. 1977, 23. ( 5 ) Martin, S. W.; Schiraldi, A., to be submitted for publication. (6) Minami, T.; Tanaka, M. Rev. Chirn. Min. 1979, 16, 283. (7) Minami, T.; Nambu, H.; Tanaka, M. J. A m . Ceram. SOC.1977, 60, 283. (8) Magistris, A.; Chiodelli, G.; Schiraldi, A. Electrochim. Acta 1979, 24, 203. (9) Chiodelli, G.; Magistris, A.; Villa, M.; Bjorkstam, J. L. J . Non-Cryst. Solids 1982, 51, 143. (10) Chiodelli, G.; Campari Vigane G.; Flor, G.; Magistris, A.; Villa, M. Solid State Ionics 1983, 8, 31 1. (1 1) Schiraldi, A.; Baldini, P.; Pezzati, E. Solid Stare Ionics, in press.
7004
P %
I
500400-
\
9
300100-
d6
03
0 :7
1
Tr Figure 1. Thermoelectric power of some of the compositions investigated. Each point corresponds to the slope of the straight line fitting the experimental AE vs. AT data. Abscissa represents the reduced temperature, viz., T, = T/T,. Each curve is quoted with the corresponding composition expressed as AgI equivalent fraction, N. TABLE I: Thermoelectric Power, t, in R Units, Fitted vs. the Reduced Temperature, T,= T/T, for the Compositions Investigated" X N clR = AT.) T.IK 0.05 0.03 6.28 - 5.227;' + 2.54T;' 623 0.10 0.05 6.03 - 5.23T;' + 2.52T;' 618 0.20 0.11 10.40 - 13.84T;' 6.31T;' 603 0.30 0.18 10.15 - 13.84T;' + 6.32R;' 588 0.40 0.25 7.74 - 9.01T;' + 4.30T;' 573 0.50 0.33 5.32 - 4.23T;' + 2.3OTF2 548 0.60 0.43 4.56 - 2.60T;' + 1.85 T;' 523 0.70 0.54 13.76 - 18.16T;' + 9.12T;' 503 0.79 0.65 12.93 - 11.867';' + 5.50T;' 488 ~
+
" X and N are the AgI molar and equivalent fractions, respectively.
Tgvalues are those of ref 8. the apparatus are given elsewhere.12 The measurements have been carried out in air, viz., without any special control of the atmosphere (these glasses are indeed extremely stable to the moisture). A Leeds & Northrup K5 potentiometer has been employed for checking either the temperature (chromelalumel thermocouples) or the electric potential drop, AE,across the sample, the maximum V T imposed being 14 K cm-I. The thermoelectric power, e, at every mean temperature is the slope of the straight line fitting the corresponding AE vs. AT experimental data, which have been obtained under the conditions of a stationary temperature gradient across the sample.
Results and Discussion For the sake of convenience, it has been expedient to use as temperature and composition units the following quantities: reduced temperature T, = T / T , where Tgis the glass transition temperature observed for each composition, and equivalent AgI fraction
N =X/[X
+ 2(1 - X)]
where X is the AgI molar fraction in the pseudobinary. Figure 1 shows some examples of the experimental results, the whole body of which may be represented by the fitting expressions reported in Table I, where the thermoelectric power is given in R units, R being the gas constant. (12) Schiraldi, A.; Pezzati, E. Z . Naturfirsch. A 1976, 31, 1077.
Schiraldi et al.
1530 The Journal of Physical Chemistry, Vol. 89, No. 8, 1985
5 4
3 2 1
0 -1
-2
-3 I
0
I
I
.2
,
1
~
f
I
~1
I
.6 .% 1 N Figure 2. Thermoelectric power vs. the AgI equivalent fraction, N,at T, = 1. In order to attain the values at N = 0 and N = 1, the data have been fitted with a polynomial regression function, viz., -c/R = 3.54 7.02N + 17.31N2, where R is the gas constant.
.4
According to the thermodynamics of irreversible processes,13 the thermoelectric power of a thermocell such as AglA electrolyte A 7g 4+%T is given by
where S(Ag) and S(Ag+) are the molar entropies of the silver electrode and of the silver ions in the glass, respectively, and S*(i) is the transport entropy of the ith mobile species within the electrolyte. Equation 1 holds whatever the nature of the electrolyte, viz., solid, liquid, or glass; however, when the electrolyte is a pure ionic conductor and its transport properties are due to a single ionic species, e.g., Ag', eq 1 reduces14 to e = S(Ag) - S(Ag+) - S*(Ag+) (2)
0
0.6 0.8 1 N Figure 3. Mixing excess entropy, S, and Gibbs free energy, G , of the silver ions, calculated at T, = 1. For comments see text. 0.2
0.4
common temperature. The present case concerns glasses with fairly different electric conductivity and activation energy (e.g., the quantity employed to approach the transport entropy"), both ranging from values typical of solid electrolytes, for N > 0.5, to values typical of insulating materials, for N < 0.3. Accordingly, an overall view, allowing one to recognize the actual role played by AgI in each glass, may not be gained unless the different glasses are considered in a special condition where they become comparable to one another. Such a condition may be easily recognized a t the so-called glass transition, viz., when any glass actually looks like an undercooled liquid (see photographs in ref 13), trapping some excess energy and entropy. Hence, the discussion of the results obtained in this work has focused upon an elaboration of the t values at the respective glass transition, Le., at T, = 1. Figure 2 shows the plot of t vs. N at T , = 1. Although like expected, the similarity with analogous plots obtained for mixtures of molten salts is actually noteworthing, so that these data have been analyzed with the same procedure proposed so far.12 The system has been treated as a mixture of two kinds of silver ions, viz., those introduced as silver borate, subscript 1, and those coming from AgI, subscript 2, in order to represent the ideal behavior as
Although eq 2 remains a phenomenological expression, it has been repeatedly employed in a more or less explicit form to check the value and the sign of the transport entropy in ionic solids containing points d e f e c t ~ ' ~ and - l ~ in ionic melt^,^^,^^ in order to gain some €id= (1-N)el+Nt2+R[NlnN+(1-N)ln(1-N)] information about the transport mechanism. Such an approach is indeed possible when one is able to previously evaluate terms where both el and e2 values are given by the polynomial fitting like S(Ag+), e.g., through the Pitzer-Wagner e x p r e s ~ i o n ; l ~ > ~ ~represented in the caption of Figure 2. The difference [(tid - €)IT,=] however, this is not the case when the electrolyte considered has may be fitted as an "open structure", such as AgI, RbAg41,, etc., or it is a dis[(tid - €)]~,=1 = (19.82N2 - 19.85N - 0.06)R ordered material, such as a glass. In these cases, eq 2 may still be relevant if used in the opposite sense, viz., to obtain the value which is almost perfectly symmetric with respect to N = 0.5. By of terms like S(Ag+) through a previous estimation of the transport definition, (tid - t) accounts for the excess entropic terms conentropy.]' By doing so, one is able to obtain information that may cerning both the ionic entropy and the transport entropy, viz. be joined with those from conductivity, self-diffusion, electrochemical, and spectroscopic determinations in order to produce S*(Ag+,exc)] ( t i d - t) = [S(Ag+,exc) a reliable model of the structure and of the behavior of the comIn 'id the transport term is pound considered . I I For the molten salts, use of eq 2 may be easily extended to the S*(Ag+) = (1 - N)S*(Ag+,l) S*(Agf,2) study of the thermodynamic properties of mixtures of various compo~ition'~ by determining the corresponding e's at a given where both S*(Ag+,l) and S*(Ag+,2) correspond to the ionic entropy of transport of Ag+ in the liquidlike state attained at TB. This means that S*(Ag+,l) and S*(Ag+,2) are expected to be very (13) Schonert, H.; Sinistri, C.; Z . Elektrochem. 1962, 66, 413. (14) Pitzer, K. S. J . Phys. Chem. 1961, 65, 145. close to each other, just as for the transport entropy of Ag+ in (15) Howard, R. E.; Lidiard, A. B. Discuss. Faraday SOC.1957, 23, 113. various silver molten salts considered under comparable condi(16) Haga, E. J . Phys. SOC.Jpn. 1958, 13, 1090. 1959, Z4, 992, 1176. tions,20 viz. (17) Christy, R. W. J. Chem . Phys. 1961, 34, 1148. (18) Magistris, A.; Schiraldi, A.; Chiodelli, G . Z . Naturforsch. A 1974, S*(Ag+,l) S*(Agf,2) I (3.5 f 0.2)R 29, 1330.
+
-
(19) Pezzati, E.; Schiraldi, A.; Magistris, A. 2.Naturforsch. A 1973, 28, 1334. (20) Schiraldi, A.; Pezzati, E. Z . Naturforsrh. A 1978, 33, 42. (21) Wagner, C. Prog. Solid Stare Chem. 1972, 7, 1.
at the temperature considered here. This also means that any deviation from S*(Ag+), viz., S*(Ag+,exc), is expected to be at most of the same order of magnitude as the difference between
J. Phys. Chem. 1985,89, 1531-1537 S*(Ag+) in molten salts, viz., IS*(Ag+,exc)l I0.2R, which is about 5% of [(ea - t)lTrElfor N = 0.5 (about which the excess quantities generally assume their highest value). Hence, one may safely neglect S*(Ag+,exc) and put
-
[ S ( A g + , e ~ c ) ] ~ , , ~ 19.8RN(l - N) [C(Ag+,exc)lT,=,
-
-19.8RTgN(1 - N)
-
[H(Ag+,exc)l~,=~ 0 where G and H are the Gibbs free energy and the enthalpy, respectively. Figure 3 shows the trend of the excess thermodynamic quantities, at T, = 1, vs. N. It is necessary to emphasize that any comparison with analogous representations of excess thermodynamic quantities in regular binary mixtures is conceptually limited by the nonisothermal character of the thermodynamic functions plotted in this figure. As for the exam entropy, one might indeed assume as physically reliable the symmetric trend shown in Figure 3, since, for temperatures close to T, = 1, the thermoelectric power becomes, within the experimental error, practically T independent for each composition (see Figure 1). Nonetheless, the corresponding excess
1531
free energy trend, which is directly affected by the variation of Tg with N, is not symmetric with respect to N = 0.5. Finally, also the third assumption about the excess enthalpy should be restricted to the peculiar low viscosity state attained at Tg, Accordingly, at T, = 1, i.e. under conditions of maximum energy content compatible with the existence of the glass phase, the silver ions may be actually described as a mixture of two main species, those introduced as AgI and those coming from the silver borate. The former kind of silver ions should play a major role in determining some properties of these glasses, such as the conductivity, u, and the glass transition. This statement, which was speculatively reported by one of us1some years ago, can be supported here by the fair straight lines fits, obtainable for room temperature log u, activation energy, E,,, and Tgdata,8 plotted vs. N instead of vs.
x
log u (ohm-' cm-') = -5.34
+ 6.08N
(std dev = 0.1 1)
E,,, (kcal mol-') = 10.395 - 8.567N (std dev = 0.17) Tg (K) = 627.88 - 227.22N (std dev = 4.36) where std dev is the standard deviation. Registry No. AgI, 7783-96-2;silver borate, 13465-88-8.
Solute/Solvent and Solvent/Soivent Interactions in Methanol Solutions: Quantitative Separation by Raman Difference Spectroscopy Keiji Kamogawa and Teizo Kitagawa* Institute for Molecular Science, Okazaki National Research Institutes, Myodaiji, Okazaki, 444 Japan (Received: June 13, 1984; In Final Form: November 26, 1984)
Solute effects in methanol solutions were investigated with Raman difference spectroscopy. Small frequency shifts of the CH3 symmetric stretching vibration of CH30H upon mixing with polar liquids such as H 2 0 and CF,COOH, nonpolar liquids such as CC14 and C6D6,and methanol isotopes such as CH30D and CD30D were observed with the optical multichannel detection system described previously. We propose here a practical method for dividing the total solute effect into contributions from the solvent/solvent interaction and from the solute/solvent interaction. In practice, two quantities, namely, homogeneous and heterogeneous interaction factors, which are indicative of the strength of methanol/methanol and methanol/solute interactions, respectively, are defined and evaluated from the concentration dependence of the frequency shifts. These quantities appeared to reflect specific interactions of CH30H with each liquid.
The effect of the solvent on the spectroscopic properties of the solute has been extensively studied.' When a solute molecule is influenced through its interaction with solvent molecules, conversely the solvent molecules receive a reaction effect from the solute, which may be tentatively called a solute effect. The solute effect has been little investigated hitherto for nonelectrolyte solutions.* Since the replacement of a solvent molecule with a solute implies not only the occurrence of a solute/solvent interaction but also some modification of solvent/solvent interactions, which otherwise are the same as those in neat liquid, quantitative separation of the total solute effect into solute/solvent and solvent/solvent interactions is desired for understanding the solution. A macroscopic approach to this problem was carried out through thermodynamic measurement^.^ Microscopic understanding of the solute effect requires a spectroscopic investigation. Molecular vibrations are sensitive to the intermolecular as well as to the intramolecular potential function. Accordingly, infrared spectroscopy has been applied successfully to study the solvent effect of molecules having particular groups such as the O-H, (1) C. H. Wang, Mol. Phys., 33, 207 (1979). (2) M. L. Josien in "Molecular Spectroscopy for Dense Phases" S. G.
Elkomoss and J. Ringeissen, Eds., Elsevier, New York, 1975,p 583. (3) M.C. A. Donkersloot, J . Solution Chem., 8, 293 (1979).
0022-3654/85/2089- 1531$01.50/0
N-H, S-H, and C=O groups: although molecules without such functional groups were difficult to treat. Generally, the vibrations which give rise to strong infrared absorption are different from those which give intense Raman lines. The recent development of Raman difference spectroscopy5allowed us to detect very small frequency shifts in the Raman lines and thus to discuss weaker intermolecular interactions between nonpolar groups. Previously we constructed a system to measure Raman difference spectra with high sensitivity.6 Application of this technique to extremely dilute solutions of alkyl sulfates revealed that the Raman line frequencies for C-H stretching modes were affected upon collapse of micelles. To get further insight into the effect of intermolecular interactions on the C-H stretching modes, we chose, in this study, a simple compound (CH,OH) and its binary mixture with polar liquids such as water and trifluoroacetic acid, with nonpolar liquids such as carbon tetrachloride and benzene, and with isotopically substituted methanol. We have (4) C. N. R. Rao and A. S. N. Murthy in "Developments in Applied Spectroscopy", Vol. 7B,E. L. Grove and A. J. Perkins, Eds., Plenum, New York, 1970,p 54. ( 5 ) J. Laane in "Vibrational Spectra and Structure", Vol. 11, J. R. Durig, Ed., Elsevier, Amsterdam, 1983,Chapter 6. (6)K. Kamogawa, K. Tajima, K. Hayakawa, and T. Kitagawa, J . Phys. Chem., 88,2494 (1984).
0 1985 American Chemical Society
