2520
J. Phys. Chem. 1981, 85, 2520-2524
Molecular Interactions in Mixtures of Carboxylic Acids with Amines. 1. Melting Curves and Viscosities Frledrlch Kohler,” H. Atrops, H. Kalall, Lehrstuhl fiir Thermodynamlk, Ruhr-Universitiit Bochum, Postfach 10 2 1 48, D-4630 Bochum, German Federal Republic
E. Llebermann, Emmerlch Wllhelm, Institut fur Physikalische Chemle, Unlversit8t Wlen, A- 1090 Wlen, Austrh
F. Ralkovlcs, and T. Salamon Vegyipari Egyetem, Flzlkai K6miai Tansz6ke. H-820 1 Veszpr6m, ff. 28 Hungary (Received: January 27, 198 1)
In a previous paper, various properties of the mixture acetic acid + triethylamine were investigated and explained in terms of a very strong attractive interaction between a 1:l complex acid + amine and the cyclic dimer of acetic acid, leading to a 3:l aggregate of extreme thermodynamic stability. This gives rise to phase separation at negative values of the excess Gibbs energy. In the present series of articles, an attempt is made to present a unified treatment of systems of carboxylic acid + amine, with emphasis on triethylamine, but with inclusion of some recent results involving the tri-n-butylamine as well as secondary and primary n-butylamine. In this paper we present experimental results on solid-liquid equilibria of formic, propionic, and trimethylacetic (pivalic) acids + triethylamine and viscosities of formic, trifluoroacetic, propionic, and trimethylaceticacids triethylamine, propionic and trimethylacetic acids + tri-n-butylamine,propionic acid + di-n-butylamine,and propionic acid + n-butylamine.
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Introduction In several papers on carboxylic a~ids,l-~ properties of the pure acids and properties of their mixtures with other substances have been discussed in terms of formation of cyclic dimers, and of strong attractive interaction between the monomer and the cyclic acid dimer. This interaction is conceived as a special inductive interaction between the polar monomer and the highly polarizable, distortable ring of bond moments, as illustrated by the model, shown in Figure 1. It is thought not to be strongly dependent on the relative orientation of monomer and dimer; it would suffice to assume that a partial charge of a monomer is near the ring of the hydrogen-bonded dimer. In addition to the association equilibrium and the attractive interaction between the monomer and the cyclic dimer, the possibility has to be considereds that the monomer can flip from the cis configuration of the OH group relative to the CO group into the extremely polar trans configuration and that open dimers and consequently chain associates can be formed. Apart from formic acid, where chain associates with units probably in the trans configuration are im(1) Affsprung, H. E.; Findenegg, G . H.; Kohler, F. J. Chem. SOC. A 1968, 1364. (2) Kohler, F. Monatsh. Chem. 1969,100, 1151. (3) Becker, G.;Kohler, F. Monatsh. Chem. 1972,103, 556. (4) Kohler, F.;Liebermann, E.; Miksch, G.; Kainz, C. J.Phys. Chem. 1972, 76,2-64. (5) Kohler, F.; Liebermann, E.; Wilhelm, E. “Proceedings”, Symposium on Thermodynamic, Spectral and Structural Features of Interactions between Molecules, Excited Molecules or Ions and Liquids, Wepion/Louvain, Belgium, Sept 16-20,1974, p 108. Kohler, F.; Huyskens, P. Adv. Mol. Relaxation Processes 1976,8, 125. (6) Kohler, F.; Findenegg, G. H.; Bobik, M. J.Phys. Chem. 1974, 78, 1709. (7) Kohler, F. Ber. Bumenges. Phys. Chem. 1978,82, 582. (8)Bobik, M.; Kohler, F.; Heger, G.; Lischka, H.; Schuster, P. Chem. Phys. Lett. 1976,40,66. Hocking, W. H. Z . Naturforsch A 1976,31,1113. Bjarnov, E.; Hocking, W. H. Ibid. 1978,33, 610. According to the experimental determinationof Hocking, the trans conformer of formic acid has an energy -17 kJ/mol higher than the cis conformer (trans and cis relative to the C=O group), much higher than the value given by the theoretical calculations in the paper of Bobik et al. Accordingly, still higher energy differences can be expected for the other carboxylic acids. 0022-385418112085-2520$01.2510
portant and account for the high dielectric constant, these additional complications add only comparatively minor new aspects to the behavior which is dominated by the cyclic dimerization and the strong attraction between polar species and the cyclic dimer. This interaction between a polar group and a cyclic hydrogen-bonded ring-which might be an essential feature in cyclic hydrogen-bonded structures, occurring also in systems of biochemical interest-determines to a large extent the properties of carboxylic acid + amine systems. It was the purpose of this investigation to get an overview on these systems, where salt formation is less important than one would expect at first sight. Complexes of acid + amine with a molecular ratio of 1:l (or 2:l) can be found in the solid state. However, in the case of formic acid triethylamine and acetic acid triethylamine, the 1:l complex melts at relatively low temperature forming two liquid phases. In the case of trifluoroacetic acid triethylamine, the complex melts congruently, but at slightly lower acid concentrations phase separation prevails. As can be inferred from dielectric measurements of mixtures of higher alkanoic acids with triethylamine: and also from our own NMR data (presented in part 2 of this series), the 1:l complex is highly polar. Thus, when the complex replaces the monomer in Figure 1, the extremely attractive interaction with the cyclic dimer results in the formation of an orientationally ill-defined but thermodynamically significant 3:l (acid amine) aggregate. Therefore, equimolar or nearly equimolar mixtures separate4 into two liquid phases with predominance of either 3:l aggregates or of free amine, respectively, while at the same time exhibiting large negative values of the excess Gibbs energy
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+
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+
GE.
There is one competitive reaction to the formation of the 3:l aggregate: the addition of an acid monomer to the polar 1:l complex, resulting in the formation of a 2:l complex (conjugate salts in the sense of Kolthoff*o). (9) Borowikow, Yu. Ya. Zh. Obshch. Khim. 1968,38,1215.
0 1981 American Chemical Society
Mixtures of Carboxylic Acids with Amines
The Journal of Physlcal Chemistry, Vol. 85, No. 17, 1981 2521
TABLE I: Some Propertiesa o f the Pure Components
T/K
property p l ( g c m - 31
293.15 313.15 323.15 303.15 318.15 293.15 313.15
10-34~4 r)
/cP
TrJK
AH,/(kJ m o l - ' )
HCOOH CF,COOH C,H,COOH (CH,),COOH (C,H,),N (n-C,H,),N (n-C,H,),NH n-C,H,NH, 1.21959 1.19446
1.48916 1.44186
0.99348 0.97187
1.041
1.614
1.100
0.90401 0.89434
0.72727 0.70862
0.7712 0.7612
0.75929 0.74284
0.73687 0.71744
1.299
1.040
1.095
1.336
0.3628 0.2954
1.450 0.990
0.85 0.64
0.523 0.401
1.076 1.7890 1.2222 281.47 12.fi6
0.9572 0.7365 258.02 9.41b
1.1005 0.8401 252.31 7.63
T*/K AH,/(kJ mol-')
2.3684 308.82 2.72 278.7 8.3
a Density, p ; coefficient of thermal expansion, c( ; viscosity, 7)bmelting temperature, T , ; enthalpy of melting, AH,; From ref 6. transition temperature, Ttr; and enthalpy of transition, AH^.
310 T/K
C'H I
Figure 1. Schematic model for the
1
special inductive interaction be-
tween a polar molecule (here the monomer) and the cyclic dlmer of acetic acid. Whereas such a complex could be found in acetic acid + triethylamine only by measuring ultrasonic relaxation," its influence is clearly seen in the enthalpy of mixing12 of propionic acid triethylamine and results in the formation of a solid compound in the system trimethylacetic acid + triethylamine. It seems that bulky groups on the C atom in the a position to the carboxylic group hinder somewhat the close approach of a partial charge to the hydrogenbonded ring of the cyclic dimer, thus favoring the competitive 2:l complex formation. Though our results still point to the overwhelming formation of a 3:l aggregate, the partial formation of the 2:l complex apparently suffices to maintain homogeneity in the liquid phase for mixtures with propionic acid or higher homologues. In order to show that our results on carboxylic acids + triethylamine can be generalized to include other tertiary amines, we carried out some measurements with tri-nbutylamine. The results are very similar, if the smaller mobility of the bulkier tri-n-butylamine is taken into account properly. The situation changes drastically when the tertiary amine is replaced by a secondary or primary amine. Then the 1:l complex becomes increasingly important. In particular, this is the case for primary amines, where 1:l complexes seem to form big clusters.
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Experimental Section Source, treatment, and purification of the acids and of triethylamine have been given el~ewhere.'.~~*~~ Tri-n-butylamine, di-n-butylamine, and n-butylamine (all from Fluka, "zur Synthese" quality) were purified by fractional (10) Kolthoff, I. M.; Chantmni, M. K., Jr. J. Am. Chen. SOC.1963,85, 426. (11) Bobik, M. Adv. Mol. Relaxation Interact. Processes 1977,11,191. (12) Munn, R. J.; Svejda, P. Ber. Bunsenges. Phys. Chem. 1979,83, 920.
Figure 2.
Melting curves for formic (H), propionic (CH3CH,), and trl+ triethylamine vs. mole fraction of amine,
methylacetic ((CH3)3C)acids x2.
distillation in a column of -20 theoretical plates, stored over sodium wire, and distilled again before each experiment. A summary of experimental values for selected properties of the pure components is presented in Table I. The technique for measuring melting curves has been described b e f ~ r e . Viscosities ~ were measured with an Ubbelohde viscosimeter, a Hoppler Rheo-Viscosimeter, or a rotational viscosimeter (Rheotest 2-50 HzRV2). For the Ubbelohde viscosimeter, mixtures were prepared by weight and transferred into the visosimeter by pressure; for the other viscosimeters, mixtures were prepared volumetrically by using microburets. Results The direct experimental values were tabulated and are available as supplementary material (see paragraph at end of text regarding supplementary material). Discussion Melting Curves. Melting curves of the systems formic acid + triethylamine, propionic acid triethylamine, and trimethylacetic acid triethylamine are shown (down to -40 "C) in Figure 2. Figures comparing the melting curves of acetic acid + triethylamine and trifluoroacetic acid + triethylamine with those of formic acid + triethylamine may be fo nd in ref 5, 7, and 13. We recall briefly that these me1 ing curves look very similar to that of formic acid + triethylamine. But in acetic acid + triethylamine the 1:I. complex melts already at -21.5 "C with formation of
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t"
(13) Kohler, F. Org. Liq.: Struct. Dyn., Chem. Prop. 1978, 245.
The Journal of Physical Chemistry, Vol. 85, No. 17, 1981
2522
1 */kJ-'
P
/
I 005
010
015
x2
Figure 3. Extrapolation to the inverse value of the heat of melting (see text). The notations (CH3)3CI and (CH3)3CII refer to the two solid modifications of trimethylacetic acid.
two liquid phases, whereas the 1:l complex of trifluoroacetic acid + triethylamine melts congruently at 20 "C, the region of phase separation extending from x E ~ ~=N 0.515 to xw = 0.985 with the crystallization line at 15 "C. Thus it appears that the region of stability of the 1:l complex with triethylamine decreases in the sequence trifluoroacetic, formic, and acetic acids. In mixtures of triethylamine with propionic acid and with trimethylacetic acid, no solid phase corresponding to the 1:l complex could be observed within the temperature interval considered. However, for trimethylacetic acid + triethylamine, the solid 2 1 complex coexists over a considerable composition range, a behavior not observed in mixtures with other acids. From the melting curves with primary crystallization of acid, the heat of melting of pure acid can be deduced and the activity coefficient fi of the acid can be derived. The usual appro~imationl~ for determining the enthalpy of melting, AH,, yields (with the approximation In fl = ax;, 1y the interaction parameter, for the concentration interval considered) AH, = -(RTT,/ATj(ln x1 + In fi) (RTTm/AT)(x2+ x Z 2 / 2 - a~2') (1)
where AT = T, - T, T, is the melting temperature of the pure substance, and x 2 denotes the mole fraction of amine. Since 1 / 2 - a is here a large quantity, we could not use the last expression of eq 2 but present in Figure 3 the quantity AT/(RTT,x2) vs. x2, where the intercept determines 1/ AH,. Figure 4 shows the quantity a = In f l / x ; for various acid triethylamine systems for large mole fractions of acid. We note that a becomes increasingly negative with addition of triethylamine and that the order of the limiting values lim,,a corresponds to propionic acid > trimethylacetic acid > trifluoroacetic acid > acetic acid > formic acid. Starting with x2 N 0.1, however, trifluoroacetic acid interacts more negatively than acetic acid. Viscosities. The dynamic viscosities at 313.15 K of formic, trifluoroacetic, propionic, and trimethylacetic acids + triethylamine are given in Figure 5. Previous meas u r e m e n t ~on~ acetic acid triethylamine have been included for comparison. Recently, Huyskens et al.15 have
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(14)Munn, R.J.; Kohler, F. Monatsh. Chem. 1960,91,381. (15)Huyskens, P.;Felix, N.; Janssens, A.; Van der Broeck, F.; Kapuku, F. J. Phys. Chem. 1980,84,1387.
Figure 4. Quantity (Y = In f l l x 2 vs. mole fraction of amlne (x,) for mixtures wRh formic (H), acetic (CH,), trifluoroacetlc (CF,), propionic (CH3CH,), and trimethylacetic ((CH&C) acids for small mole fractions x2, as calculated from the melting curves without correctlon for the temperature dependence of in f l .
Flgure 5. Dynamic viscosities q of formlc (H), acetic (CH,), trifluoroacetic (CF,), propionic (CH,CH,), and trimethylacetic acids trC ethylamine vs. mole fraction of amine (x,) at 313.15 K.
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also measured viscosities of mixtures triethylamine + formic through pentanoic acids, in general agreement with our results. We note that, with all alkanoic acids except formic acid, a strong maximum of the viscosity is observed at X,id = 0.75,indicating the 3:l aggregate postulated in the introduction. For formic acid + triethylamine, the viscosity still increases toward lower acid mole fractions, and, for trifluoroacetic acid + triethylamine, it shows the dominance of the 1:l complex. Figure 6 presents the viscosities at 313.15 K of propionic acid tri-n-butylamine and of trimethylacetic acid tri-n-butylamine (the respective results for mixtures with triethylamine are shown for comparison). As expeded, the viscosity of the mixtures increases when the tertiary amine becomes bulkier. Finally, the influence upon viscosity when the tertiary amine is replaced by a secondary or primary amine has been investigated. Figure 7 shows the results for propionic
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The Journal of Physical Chemistty, Vol. 85, No. 17, 198 1 2523
Mixtures of Carboxylic Acids with Amines
t
TABLE 11: Relative Molecular Masses (M) of Acid- Amine Complexes in Comparison t o Values Obtained from Viscosities at the Appropriate Mole Fractions'
i
f+\
M
complex (HCOOH),-Et,N (HCOOH),-Et,N (CF,COOH),-Et,N CF,COOH-Et,N (CH,COOH),-Et,N ( C,H,COOH),-Et,N (C,H,COOH),-Et,N [C(CH,),COOH 1,-Et,N [ C( CH,),COOH],-Et,N [ C( CH,),COOH],-BU,N (C, H,COOH),-Bu,N (C;H;COOH);-Bu;NH C,H.COOH-Bu,NH ( C,H,COOH),-BUNH, C,H,COOH-BUNH,
M*
239.3 193.2 443.3 215.2 281.3 323.4 245.3 407.6 305.6 491.8 407.6 351.5 203.3 295.4 147.2
289 316 266 383 275 339 234 465 299 472 372 507 460 400 614
a The abbreviation Et signifies the group C,H,, and Bu denotes CH,CH,CH,CH,.
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Flgure 8. Dynamic viscosities q of propionic acid trlethylamlne (0), proplonlc acld trl-n-butylamine (X), trimethylacetic acid 4- triethylamine (A), and trimethylacetic acid trl-n-butylamine (+) vs. mole fraction of amlne (x,) at 313.15 K.
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35 I
,
I
,
100
200
300
I
I
500
600
k l mol-'
'"
LOO
Figure 8. Calibration curve for the Gibbs energy of activation for viscous flow (AG*/(kJ mol-')) vs. the relative molecular mass M(see text). Points (0)are normal alkanes (thelast polnt Is squalane), crosses (X) are tertlary amlnes, and triangles (0)are carboxylic adds ( M calculated for the dimer).
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Figure 7. Dynamic viscositles of propionic acid trl-n-butylamlne (A), proplonlc acld di-n-butylamlne (X), and proplonlc acid n-butylamlne (0)vs.,moie fraction of amlne (x,) at 313.15 K. For the last system at x , = 0.5, q = 176.5 cP.
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acid di-n-butylamineand propionic acid + n-butylamine at 313.15 K (the curve for tri-n-butylamine is also shown for comparison). The mixture with di-n-butylamineshows a peak a t &id = 0.75 and a rather broad shoulder extending to ?&,,id = -0.5, whereas the mixture with n-butylamine shows only one giant peak at equimolar composition. We do not completely agree with the interpretation of Huyskens et a1.16 that the dramatic increases in viscosity are strictly related to the formation of dissociated ions. An example that formation of ions and viscosity do not always go parallel is provided by formic acid + triethylamine in the range 0.6 < &id < 0.7, where the viscosity q still in-
creases, but the conductivity K and the product K q decrease. However, we agree that the formation of large, orientationally ill-defined aggregates is closely connected with the polarization of hydrogen bonds, and this can be influenced, among others, by ions. Thus we prefer to discuss the observed viscosities rather by estimating the size of the aggregates formed. To this end, an empirical method presented beforel6 is modified. Briefly, we assume that for a flexible molecule each group has to move more or less separately out of the quasi-lattice of a liqgid if the molecule moves as a whole. Therefore, the activation enthalpy for viscous flow should be proportional to the number of groups of the moving entity, and hence for saturated compounds consisting of groups CH2,OH, N, etc. it should be roughly proportional to the relative molecular mass M. Previouslyle the empirical relation was given as
M
AH*
16.7- 34.3 (3) kJ mol-l Here, instead of utilizing the temperature dependence of viscosity q (specificallythe enthalpy of activation L\HI), we correlate data on viscosities themselves. AG*,the Gibbs (16) Ratkovics, F.; Salmon, T.;Domonkos, L. Acta Chim. Acad. Sei. Hung. 1974, 83,71.
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J. Phys. Chem. 1901, 85, 2524-2529
energy of activation as defined by Eyring's equation q = (hNA/v)eAC'/RT
(4)
where u denotes the molar volume, is plotted against M. Such a graph, for several paraffins, tertiary amines, and carboxylic acids, is shown in Figure 8. Table I1 shows the relative molecular masses for various complexes M in comparison with values MARC obtained empirically via Figure 8. The following conclusions can be drawn. For alkanoic acids and tertiary amines, the value M- obtained from viscosities at ?&id = 0.75 corresponds rather well to that expected for a 3:l aggregate. The enhancement M ~ J Mobserved for aggregates with di-n-butyl- and nbutylamine appears to be too large to be an artifact of our empirical method and is perhaps caused by additional attachment of ionized groups. For trifluoroacetic acid, the 3 1 aggregate seems to be rather weak. On the other hand, trifluoroacetic acid forms a very strong 1:l complex with triethylamine, in the form of a zwitterion,as we will discuss in the following paper,l' and these zwitterions seem to be (17) Kohler, F.; Gopal, R.; GWze, G.; Atropa, H.; Demiriz, M. A,; Liebermann, E.; Wilhelm, E.; Rakovics, F.; Palagyi, B. J.Phys. Chem., following paper in this issue. See also ref 7 rind 5.
partly aggregated. Still higher aggregation can be found, at equimolar composition, with di-n-butylamine and especially n-butylamine, where the cluster corresponds to ca. four 1:l complexes. Acknowledgment. The cooperation between the Ruhr-Universitat Bochum and Veszprgmi Vegyipari Egyetem has been supported by the Deutsche Forschungsgemeinschaft and the Office for Cultural Relations of the Hungarian People's Republic, for which we are thankful. Supplementary Material Available: Tables containing the following: (i) melting points for the systems formic acid (1)+ triethylamine (2) (for 0 < x 2 C 0.98), propionic acid (1) + triethylamine (2) (for 0 C x2 C 0.75); (ii) dynamic viscosities of the systems formic acid + triethylamine, trifluoroacetic acid + triethylamine, propionic acid triethylamine, and trimethylacetic acid + triethylamine for various mole fractions a t 293 and 313 K,and for the systems propionic acid + tri-n-butylamine, propionic acid + di-n-butylamine, propionic acid + n-butylamine, and trimethylacetic acid + tri-n-butylamine for various mole fractions and from 293 to 323 K (10 pages). Ordering information is available on any current masthead page.
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Molecular Interactions in Mixtures of Carboxylic Acids wlth Amines. 2. Volumetric, Conductimetric, and NMR Properties Frledrlch Kohler,' Ram Gopal,+ G. Gotze, H. Atrops, M. A. Demlrlr, Lehrstuhi fur Thermodynamik, Ruhr-Universitat Bochum, Postfach 10 2 I 48, 0-4630 Bochum, German Federal Republic
E. Llebermann, Emmerlch Wllhelm, Institut fur Physikaiische Chemie, Universitat Wien, A- 1090 Wien, Austria
F. Ratkovics, and B. Palagyl Vegylparl Egyetem, Fizikai K6miai Tanszgke, H-820 1 Veszprgm, Pf. 28, Hungary (Received: January 27, 198 1)
In this series of articles, we attempt to present a unified treatment of systems of carboxylic acid + amine. In part 1,experimental results on melting curves and viscosities were reported. Here, we present excess volumes for formic, trifluoroacetic, propionic, and trimethylacetic (pivalic) acids + triethylamine, as well as their temperature dependence, and excess volumes for propionic acid + n-butylamine and propionic acid + di-nbutylamine. Electrolytic conductivitieshave been measured for formic, trifluoroacetic, acetic, and propionic acids + triethylamine,propionic acid + tri-n-butylamine,propionic acid + di-n-butylamine,and propionic acid + n-butylamine. In addition, chemical shifts for trifluoroacetic, formic,acetic, and propionic acids + triethylamine, propionic acid + tri-n-propylamine,propionic acid + tri-n-butylamine, and propionic acid + n-propylamine are represented.
Introduction In the first paper of this series,l we have summarized the peculiar properties of mixtures of carboxylic acids with amines in terms of molecular interaction and complex formation. Without going into details, we recapitulate briefly the basis of our interpretation. For most mixtures we assume (1)predominance of cyclic dimerization of the carboxylic acid, (2) formation of a strongly polar 1:l complex between carboxylic acid and amine, and (3) a very attractive interaction between this polar complex and the t C-1087 Sector A, Mahangar, Lucknow-226006, U.P., India. 0022-3654/81/2085-2524$01.25/0
easily distortable ring (containing two hydrogen bonds) of the cyclic dimer. This gives rise to the formation of orientationally ill-defined 3:l aggregates with extreme influence on thermodynamic and structural properties. The only acid where these 3:l aggregates seem to be less important relative to the 1:l complex is trifluoroacetic acid, where the 1:l complex is a zwitterion. With formic acid, where open dimers and higher chain associates play a dominant role in the pure acid, the 3:l aggregate seems (1) Kohler, F.;Atrops, H.; Kalali, H.; Liebermann, E.; Wilhelm, E.; Ratkomcs, F.; Salamon, T. J.Phys. Chern., preceding paper in this issue.
0 1981 American Chemical Society
