5464
J. Phys. Chem. 1003, 87, 5464-5467
hydrogen bond with large proton polarizability with the reaction field of the solvate. The term M I o is always negative and becomes larger with increasing polarity of the solvent, shifting the equilibrium to the right-hand side. With deuterati0n-h the m e s studied-the equilibrium is always shifted completely to the left-hand side. This result suggests that deuteration causes very significant
changes in the terms A"," and AS".
Acknowledgment. We thank the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie for providing the facilities for this work. Registry No. 1, 87586-62-7;2, 87586-63-8; 3, 87586-64-9; 4, 87586-65-0; 5, 87586-66-1; 6,87586-67-2.
Thermal Diffusion In Carbon Tetrachloride-Methanol and Carbon Tetrachloride-Ethanol Mixtures Nlng-Yuan Richard Ma, Davld Stanford, and Adolph L. Beyerleln' Department of Chemistry and Geology, Clemson Universl&, Clemson, South Carollna 2983 1 (Received: January 3, 1983; In Flnal Form: April 28, 1983)
Thermal diffusion factors are reported on carbon tetrachloride-ethanolat 25 "C over the complete concentration range and on carbon tetrachloride-methanol mixtures at 25 "C in both the dilute carbon tetrachloride and dilute alcohol regimes. As the alcohol concentrations decrease to the very dilute regime the thermal diffusion factors of both mixtures change from a negative to positive sign and proceed through a maximum at an alcohol mole fraction of about 0.02. This behavior is in accordance with molecular association theory of Baranowski et al. The dimer dissociation constants estimated from the experimental data in mole fraction units range from 0.005 to 0.03 depending on the distribution of associated alcohol species assumed in the calculation. These values compare very well with dimer dissociation constants of 0.017 and 0.022 obtained from NMR chemical shifts for carbon tetrachloride-methanol and carbon tetrachloride-ethanol mixtures, respectively. These results along with earlier results on benzene as well as benzene derivative-alcohol mixtures provide strong support for the general validity of molecular association theory of thermal diffusion.
Introduction The thermal diffusion theory of Baranowski, de Vries, Haring, and Paul1s2 for binary mixtures with an inert component and a self-associating component predicts a positive "chemical" contribution to the thermal diffusion factor (defined so that it is positive when the inert component diffuses in the direction of the temperature gradient) which results from the self-association processes in the mixture. This chemical contribution increases and gradually proceeds through a maximum as the associated species depolymerize with decreasing concentrations of the associating component. If the inert component has the higher molecular weight, there may be a change in sign of the thermal diffusion factor before it maximizes because its chemical contributions begin to dominate the more normal tendency for the higher molecular weight component to diffuse to the colder regions of the mixture. A change in sign and maximum has been observed for thermal diffusion factors of alcohol-benzene derivative mixtures (methanol-benzene,2 and ethanol-toluene3) but have not been observed for carbon tetrachloride-alcohol mixture^.^-^ Belton and Tyrrel13 indicated that carbon tetrachloride is much less effective in causing depolymerization of alcohol than benzene or benzene derivatives and as a consequence the maximum in the thermal diffusion factor is shifted to lower alcohol concentrations than (1) Baranowski, B.; deVries, A. E.; Haring, A.; Paul, R. Adu. Chem. Phys. 1969,16, 101. (2) Johnson, J. C.; Beyerlein, A. L. J. Phys. Chem. 1978, 82, 1430. (3) Belton, P. S.; Tyrrell, H. J. V. 2.Naturforsch. A 1971, 26, 48. (4) Whitaker, S.; Pigford, R. L. Ind. Eng. Chem. 1958,50, 1026. (5) Story, M. J.; Turner J. C. R. Trans. Faraday SOC.1969,65,1523. (6) Beyerlein, A,; Bearman, R. J. J. Chern. Phys. 1968, 49, 5022, (7) Stanford, D. J. Ph.D. Thesis, Clemson University, Clemson, SC, 1974, University Microfilms Order No. 74-30,062,
were investigated in earlier work."' Therefore, an experimental investigation of the thermal diffusion factor of carbon tetrachloride-methanol and carbon tetrachloride-ethanol mixtures was performed and the results are reported in this paper.
Experimental Methods and Results In order to study the thermal diffusion factor of carbon tetrachloride-alcohol mixtures at lower alcohol concentrations (below 0.02 alcohol mole fraction) than were studied previously, a more sensitive method of measurement must be used. The thermogravitational thermal diffusion method using a cylindrical apparatus whose annulus is closed at both ends is the most suitable for this purpose because its sensitivity can be readily increased by decreasing the annular gap width.8 Thermal diffusion factors calculated by using measured thermogravitational thermal diffusion separations corrected for temperature difference effectsgand Horne and Bearman's equationgare very accurate as judged by their excellent agreement with those obtained by the other independent methods, namely, the pure thermal diffusionlo and flow cellll methods. The apparatus employed in this investigation has a 0.65-mm annular gap which resulted in 5.6 times larger steady-state separations than are obtained with the apparatus employed in our previous investigation^.^^^ The chemicals, methanol and carbon tetrachloride, were spectranalyzed reagent obtained from Fisher Scientific Co. and the ethanol was USP quality reagent that was dried and distilled by using magnesium ethylate according to a (8) Horne, F. H.; Bearman, R. J. J. Chern. Phys. 1968,49,2457. (9) Stanford, D. 3.; Beyerlein, A. L. J. Chern. Phys. 1973, 58, 4338. (10)Anderson, T. G.; Horne, F. H. J. Chern. Phys. 1971, 55, 2831. (11) Turner, J. C. R.; Butler, B. D.; Story M. J. Trans. Faraday SOC. 1967,63, 1906.
OO22-36~4/83/2O87-5464~O1.5OlO 0 1983 American Chemlcal Society
Thermal Dlffusion in CC1,-MeOH and CC14-EtOH
The Journal of Physlcal Chemlstry, Vol. 87, No. 26, 1983 5465
T A B L E I: Thermal Diffusion Factors for Carbon Tetrachloride- Alcohol Mixtures at 25 "C carbon tetrachloride-ethanol
0.800 0.897 0.950 0.960 0.970 0.980 0.990 0.995
2.0
[ccl41,
tCCl41, mole fraction 0.020 0.050 0.100 0.200 0.300 0.400 0.500 0.600 0.700
carbon tetrachloride-methanol
a:
- 2.3, -2.5, -2.7, -3.0, -3.3, -3.6, -3.9,
mole fraction 0.020 0.900 0.950 0.960 0.970 0.980 0.990
I
a:
- 2.9, -2.1, -1.5, 0.4, 0.8,
- 4.0
1.0, 0.1,
-6.01
-4.2, -3.8, -3.1,
-1.6,
,I
I
I
,
,
0.2
0.4
0.6
0.8
1.0
MOLE FRACTION C C L 4
-0.6,
- 0.2, -0.0,
Figure 1. Thermal diffusion factors for carbon tetrachloride-ethanol mixtures at 25 OC obtained In this work (O),the work of Whltaker and Plgford4(M), the work of A.L.Ba6(0),and the work of Stanford7 (A).
0.6, 0.4, 0.1,
2.0
procedure given in Vogel.12 The measured refractive indices of all reagent chemicals at 20 "C (carbon tetrachloride, 1.4603; methanol, 1.3291; and ethanol, 1.3615) agreed very well with the literature values (1.4601,1.3288, and 1.3611, respectively).13 Stanford found that carbon tetrachloride and alcohol react to form HC1 and other products when in contact with a metal apparatus made of iron alloy^.^ Kuppers reported that the reaction is catalyzed to various degrees by most metals but does not occur in the presence of gold.14 Therefore, in this work these reactions have been eliminated by constructing the apparatus parts from brass and then gold plating them before assembly. The thermal diffusion factors, a,were calculated from the measured separations using Horne and Bearman's equations8
a =
1l.J
waw
W(11-F
19 vz- Vl AW F = -AB1430 V B = 1/ ( VvD-)
(3)
The quantity A is a well-defined apparatus constant8 whose value is 3.2195 X lo4 for the apparatus used in this work. The measured separation, AW, is the difference between the weight fraction of carbon tetrachloride in the upper part of the apparatus and that in the lower part of the apparatus. It is measured by differential refractometry using a procedure previously described.'J6 The diffusion coefficients D are obtained from the work of Longsworthls and Hammond and Stokes;17 and the viscosities 7 are obtained from the work of Jones et al.18 The specific volumes of the pure components V1 and V,, the specific (12) Vogel, A. I. "A Textbook of Practical Organic Chemistry", 3rd ed.; Wiley: London, 1956. (13) "Handbook of Chemistry and Physics", 60th ed.; The Chemical Rubber Publishing Co.: Boca Raton, FL, 1980. (14) Kuppers, J. R. J.Electrochem. SOC.1978, 125, 97. (15) Horne, F. H.; Bearman, R. J. J . Chem. Phys. 1962, 37, 2857. (16) Longsworth, L. G. J. Colloid Interface Sci. 1966,22, 3. (17) Hammond, B. R.; Stokes, R. H. Tram. Faraday SOC.1955,51, 1641: 1956.52.781. , -~ (1'8) Jones, W. J.; Bowden, S. T.; Yarnold, W. W.; Jones, W. H. J. Phys. Chem. 1948,52,753. - 7
-
.V
1
+:. .
-2.0
0
-4.0
-8.01
I
p
oloo
I
,{
0.2 0.4 0.6 0.8 1.0 MOLE F R A C T I O N C C L 4 Flgure 2. Thermal diffusion factors for carbon tetrachloride-methanol mixtures at 25 OC obtalned In this work (O),the work of Whtaker and Pigford' (M), and the work of Story and Turner' (0).
m/.2
AB
8
0
0.0
volumes of the mixtures V, and the thermal expansion coefficients p are obtained from density data in Timmermans19and the International Critical Tables.20 A total of 34 experiments were performed on carbon tetrachloride-methanol mixtures and 49 experiments were performed on carbon tetrachloride-ethanol mixtures. At least two thermal diffusion factor measurements were obtained at each carbon tetrachloride weight fraction ( W) investigated. The best average values obtained for a are given in Table I. An error analysis showed that the experimental errors at the very high carbon tetrachloride mole fractions (X), X > 0.96, are 12% and at the remaining concentrations they range from 3% for 0.02 < X < 0.8 to about 8% for 0.8 < X < 0.96. Since earlier measurementsP7 of the thermal diffusion factor for carbon tetrachloride-alcohol mixtures were affected quite unexpectedly by chemical reactions between carbon tetrachloride and alcohol, it is instructive to compare them with the measurements of this work as is done in Figures 1 and 2. In general, Figure 1 shows good agreement between the various thermal diffusion factor (19) Timmermans,J. "Physicochemical Constants of Binary Systems"; Interscience: London, 1959. (20) National Research Council. "International Critical Tables of Numerical Data, Physics, Chemistry, and Technology"; McGraw-Hill: New York, 1928; Vol. 111.
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The Journal of Physical Chemistty, Vol. 87, No. 26, 1983
measurements for carbon tetrachloride-ethanol mixtures. Some significant deviations ranging from 25% to 40% occur between the results of this work and those from early work of one of the authors (A.L.BJ6 at the higher carbon tetrachloride concentrations (above 0.5 mole fraction) where the separations become somewhat smaller and the errors caused by the reaction become proportionately larger. In later work Stanford’ used an apparatus without reservoirs which required less than one-fifth the time to reach steady state as the apparatus of the previous work6 which had reservoirs at each end of the annulus. The effect of the reaction is thereby considerably reduced as is evidenced by the much improved agreement obtained by comparing Stanford’s thermal diffusion factors with those of this work. Whitaker and Pigford‘s4 measured thermal diffusion factors on carbon tetrachloride-ethanol mixtures agree extremely well with those of this work excepting at dilute alcohol concentrations ( X = 0.83 and 0.90) where their thermal diffusion factors suddenly become very small in magnitude. It is believed the thermal diffusion factors of this work are more accurate at these dilute concentrations because of the significantly higher sensitivity of the thermogravitational thermal diffusion method over the pure thermal diffusion method used by Whitaker and Pigford. The brass apparatus used by Whitaker and Pigford also catalyzes the reaction between carbon tetrachloride and alcohol but the catalysis is very slowl4 and these comparions show that errors caused by reaction are small. Story and Turner5 used a flow cell constructed from brass to obtain their thermal diffusion factors on carbon tetrachloridemethanol mixtures and at the extreme concentrations where their results overlap with our results there is good agreement (see Figure 2). Even if one considers the difference in mean temperatures, Figure 2 shows significant differences between the results of Story and Turner5 and Whitaker and Pigford4 on carbon tetrachloridemethanol mixtures at intermediate carbon tetrachloride concentrations. We are unable to obtain accurate thermal diffusion factors at these concentrations because the diffusion coefficient data required to calculate them from thermogravitational thermal diffusion separations using eq 1 are not available. However, because the interest in this investigation is primarily for the dilute alcohol regime, the available data a t intermediate concentrations are sufficiently accurate for purposes of this work, Le., for interpretation in terms of molecular association theory.1,2
Discussion and Conclusions The experimental data are discussed in terms of the Johnson and Beyerlein extension2of the Baranowski et al. molecular association theory1 to the following multipleequilibria model for alcohol self-association: ROH + ROH + (ROH), (ROH),
+ ROH + (ROH)3
(ROH),1
+ ROH + (ROH),,
(4)
Ma et al.
%hem
+
=
RT
2 fjDj
j= 1
where R is the gas constant, Xk is the mole fraction of associated species (ROH)k,[ k is the fraction of alcohol that is in the form of species (ROH)k, and Dk is a polynary diffusion coefficient defined by the following linear relation to the center of mass diffusion flux, J:
-J =
5 DkVXk + 5 xi(1-
k=l
i=l
xJ)aoVIn T
(9)
j=l
The parameter a. which is contained in the equation for the “physical” contribution to a, i.e., (Yphys, is analogous to the thermal diffusion factor of a binary mixture with no association reactions. The chemical contribution is obtained by imposing the criteria of local chemical equilibrium on the mole fraction gradients ox,. The property r is a well-defined quantity (see ref 2) that accounts for the effects of nonideal mixing of the associated species on the equilibrium constants for the association equilibria. The above equations are somewhat simpler than those derived by Johnson and Beyerlein,2because the same enthalpy, AH, is assumed for the dissociation of all associated alcohols, (ROH)k to (ROH)k-l and the monomer. Calculations predict that a p h y s is nearly constant and independent of concentration.2 Johnson and Beyerlein, demonstrated that, for reasonable assumptions regarding the dependence of Dk on k,21(Ychem is a positive definite quadratic form that increases with decreasing alcohol concentrations. It maximizes a t dilute alcohol concentrations where the fraction of alcohol that is monomer, El, begins to become larger than any fractions, f k , for the other associated species. The thermal diffusion factors for carbon tetrachloridemethanol and carbon tetrachloride-ethanol mixtures change from negative to positive values as the alcohol mole fraction is decreased below 0.04 and proceed through a maximum a t about 0.02 f 0.005 alcohol mole fraction. This is in accordance with the predictions of molecular association theory. According to theory the equilibrium constants dominate all other factors that determine the concentration where a maximizes.2 Application of theory to the experimental data for both mixtures predicts dimer dissociation equilibrium constants in mole fraction units ranging from 0.005 to 0.03 depending on assumptions made regarding the concentration distribution of associated alcohol species in the mixture.22 Dimer dissociation constants estimated from PMR chemical shifts are 0.017 and 0.022 for the methanol and ethanol mixtures, re~ p e c t i v e l y . ~It~is gratifying that these literature values
Their equations for a are a=
aphys
+ %hem
i+r
(5)
(21) Johnson and Be erlein (ref 2) showed that, if D kvaried monotonically with k as k(’-m{then cychernis positive definite for d < 0.5. Since
Dk for a real mixture is expected to decrease with the size of the associated species, it is reasonable to expect that d < 0.0. Thus, the condition for a positive definite ache,,,should be valid for a real mixture. (22) The equilibrium constant model for the multiple association equilibria (eq 4)which determines the distribution of associated species is described in ref 2.
J. Phys. Chem. 1983, 87, 5467-5472
are in the middle of the range of values estimated in this work. In principle eq 5-8 can be used to simulate a over the entire concentration range. However, in this work and the earlier work on benzene-methanol mixtures? theory cannot accurately predict the relatively large negative a at intermediate concentrations unless a concentration dependence for the parameter a, is assumed. Since the definition of a. is analogous to that for a of a binary mixture with no molecular association, the assumed concentration independence of a. is based on the near concentration independence of experimental a for nearly ideal mixtures. Thus, a concentration dependence for a. implies that the nonideal behavior of the alcohol mixtures is not entirely due to molecular association, but must result from other types of intermolecular interactions such as van der Waals forces. Such nonideal contributions are partially accounted for by r insofar as they affect the association equilibrium constants. But calculations show that r is much too small (its magnitude is about 0.1 or less) to account for the concentration behavior of a at intermediate alcohol mole fractions. Theoretical and experimental studies of the dependence of a on intermolecular forces and mixture nonidealities have been the subject of both (23) Littlewood, A. B.; Willmott, F. W. Trans. Faraday SOC.1966,62, 3287.
5467
theoretical and experimental p ~ b l i c a t i o n s . ~ - ~HowJ~-~~ ever, at this point more experimental and theoretical work needs to be done before one can formulate a useful estimate for the concentration dependence of cyo. In conclusion, the observed change in sign and maximum for the thermal diffusion factor of carbon tetrachloridealcohol mixtures confirms expectations based on theoretical and experimental investigations of benzene and benzene derivative-alcohol mixture^.^,^^^,^ The agreement obtained with theory for both benzene (or benzene derivative) and carbon tetrachloride as the inert component provides strong verification for the general validity of molecular association theory and illustrates its usefulness for the interpretation of liquid thermal diffusion data. Acknowledgment. We gratefully acknowledge the National Science Foundation for supporting this research under Grant CPE 8019544. Computer time provided by the Clemson University computer center is also acknowledged. Registry No. Carbon tetrachloride,56-23-5; methanol, 67-56-1; ethanol, 64-17-5. (24) Bearman, R. J.; Kirkwood, J. G.; Fixman, M. Adu. Chem. Phys. 1958, 1, 1. (25) Bearman, R. J. J. Chem. Phys. 1959, 30, 835. (26) Bearman, R. J.; Horne, F. H. J. Chem. Phys. 1965, 42, 2015. (27) Story,M. J.; Turner, J. C. R. Trans. Faraday SOC.1969,65,349. (28) Farsang, G.; Tyrrell, H. J. V. J . Chem. SOC.A 1969, 1839.
Silver Complexation by the Ionophorous Antibiotic Monensin in Nonaqueous Solvents J. Garcia-Rosas,’ H. Schneider, Max-Planck-Instltut fur blophysikallsche Chemle, 0-3400 Giittlngen, West Germany
and B. 0. Cox Chemistry Department, University of Stirling, Stirllng FK9 4LA, Scotland, U.K. (Received: January 10, 1983; In Final Form: April 25, 1983)
The stability and kinetics of dissociation and formation of the silver complex of monensin in several solvents have been investigated. In dipolar aprotic solvents, the stability constant of the complex decreases monotonically with increasing Ag+ ion solvation. The formation rates are practically diffusion controlled. In MeOH, the stability constant is much lower than expected from considerationsof Ag+ solvation alone and the formation of the complex takes place at a rate between 3 and 5 times lower than the rate for a diffusion-determinedreaction. It is suggested that the complexation of Ag+ by monensin in protic media is unfavorable (both kinetically and thermodynamically) with respect to that in aprotic environments with similar solvating properties toward Ag+.
Introduction Since the isolation of the monocarboxylic acid antibiotic monensin (Figure 1) from cultures of Streptomyces cinnamonensisl a number of studies on the biological activity: molecular ~tructure,”~ and solution chemistry5 of this antibiotic have been carried out. The ability of monensin (1) A. Agtarap, J. W. Chamberlin, M. Pinkerton, and L. Steinrauf, J. Am. Chem. SOC.,89, 5737 (1967). (2) S. Estrada-0, B. Rightmire, and H. A. Lardy, Antimicrob. Agents Chemother., 279 (1967). (3) M. Pinkerton and L. K. Steinrauf, J. Mol. Biol., 49, 533 (1970). (4) W. K. Lutz, H.-K. Wipf, and W. Simon, Helu. Chim. Acta, 53,1741 (1970). (5) P. Gertenbach and A. I. Popov, J. Am. Chem. SOC.,97,4738 (1975). 0022-365418312087-546780 1.5010
to complex selectively metal ions and transport them across natural and artificial membranes6 has constituted the main reason for the interest in this ionophorous antibiotic. Similar complexing properties are also exhibited by synthetic macrocylic ligands such as crown ethers’ and cryptands? where the complexed metal ion is located inside the cyclic structure of the ligand. X-ray analyses of the silver salt of monensin indicate that the monensin molecule adopts a ringlike structure around the silver ion, which is (6) B. C. Pressman, Fed. Proc., Fed. Am. SOC.Exp. B i d , 27, 1283 (1968). (7) C. J. Pedersen, J. Am. Chem. Soc., 89, 7017 (1967). (8) J. M. Lehn, Struct. Bonding (Berlin), 16, 1 (1973).
0 1983 American Chemical Society
