July, 1956
BINARY FREEZING POINT DIAGRAM FOR 2-AMlNOPYRIDINE
963
and indicate that a monolayer of adsorbed titanium (12 X 1014 atoms/cm.2) is found at about 0.01 weight yo concentrations. A monolayer of adsorbed chromium (13 X loL4 atoms/cm.2) is formed a t about 1.0 weight ~o'chromium. Since there are 15.3 x l O I 4 atoms/cm.2 in the plane of densest packing of oxygen atoms in Al2O3,I5and the surfaces used were randomly oriented and small grained, this corresponds to about one excess metal atom for each oxygen ion a t the surface. The solid curves in Fig. 2 have been drawn in conformance with the Langmuir adsorption equat , i ~ n i.e. ,~,~
r = cu+ C co y=yo-ukTln
(4)
(1 + -3
(5)
I
where u and co are experimentally determined constants. These curves fit the experimental data reasonably well. Without information as to activities, speculation regarding deviations seems pointless. These results and other measurements of interface energies of metals on oxide s y s t e m ~ ~indi-~ cate that the oxide surface is dominated by large oxygen anions, and that there is little attraction between an oxide surface and an inert metal. It is only highly electropositive constituents (such as chromium and titanium) which tend to concentrate a t the interface. This transfer to the interface layer is a chemisorptioii process and may be represented as (15) W. L. Bragg, "Atomic Structure of Minerals," Cornel1 Univ. Press, Ithaca, N. Y . , 1937, p. 93.
1
IO00
0001
001 WEIGHT
Fig. 2.-Effect Ml(solute)
I
01
10
10
PERCENT ADDITION
of Sn, Ti, In and Cr on the Ni(l)-AlzOl(s) interfacial energy a t 1475'.
+ Mz-O(surface)
=
+
MI-O(surface) Mz(solvent) (6)
with the free energy change for this process directly related to the standard free energies of formation of the oxides. Materials forming more stable oxides (Cr, Ti, Si6)than the solvent (Ni, Fe6) are adsorbed a t the oxide surface while solutes forming less stable oxides (In, Sn) show little interfacial adsorption. The concentration a t which essentially complete surface coverage is obtained (1, 0.1, and 0.01 for Cr, Si and Ti) decreases with increasing oxide stability ( A F O T i O = - 117 kcal./mole; A F 0 ~ / 2 ~ i 0= 2 -110; A F " l j a c r 2 0 3 = -96).
BINARY FREEZING POINT DIAGRAMS FOR 2-AMINOPYRIDINE WITH SATURATED AND UNSATURATED LONG CHAIN FATTY ACIDS BY ROBERT R. MODAND EVALD L. SKAU Southern Regional Research Laboratoru,l New Orleans, Louisiana Received J a n u a r y 81 1.966 ~
Binary freezing point data have been obtained for 2-aminopyridine with lauric, myristic, palmitic, stearic, oleic, elaidic and a- and p-eleostearic acids, The binary diagrams prove the existence of two congruently-melting, crystalline, molecular compounds in each case. For the saturated acid systems, the compounds had the compositions RCOOH.NCrH4NH2 and 4RCOOH.NC6H4NH2,and for the unsaturated acid systems, R'COOH.NC6H4NHn and 2R'COOH.NC6HnNH2. If the deviation from ideal freezing point depression can be attributed to molecular compound formation alone the data show that the degree of dissociation of the acid-amine compound in the molten state varies with the chain length and the degree of unsaturation. For the saturated acids dissociation is greater the longer the chain length of the acid. For the C18acids the dissociation is greater the less the degree of unsaturation.
Though amine salts of mixtures of long chain fatty acids have long been used as soaps, emulsifiers and detergents very little effort has been made toward the isolation and characterization of the pure individual compounds. Ramsay and Patterson2 isolated an equimolecular compound of 2-aminopyridine with stearic acid in connection with the use of this amine in the separation of homologous fatty acids by partition chromatography. In previous (1) One of the laboratories of the Southern Utilizatlon Research Branch, Agricultural Research Service, U. S. Department of .4griculture. ( 8 ) L. L. Rainsay and W . I . Patterwn, J. Anaoc. Of. A g r . Chemists, 31, 139 (1948).
publications3-6 from this Laboratory it was shown by binary freezing point measurements that acetamide forms a similar 1 : 1 compound with stearic acid but that the corresponding CIS, mono-unsaturated oleic and elaidic compounds have incongruent melting points and the more highly unsaturated eleostearic acids form no crystalline molecular (3) F. C. Magne and E. L. Skau, J. Am. Chem. Sue.. 74, 2628 (t952). (4) R. R. Mod,and E. L. Skau, T H I SJOURNAL, 66, l O l G (1952). ( 8 ) R . R. Mod, E. L. Skau and R. W. Planck, J. A m . Oil Chem. Soc.. 3 0 , 368 (1983). (G) R. T. O'Connor, R. R. Mod, A l . n. Murray and E. L. Skau, J. Am. Chem. Ror., 7 1 , 892 (195.5).
,
ROBERT R. MODAND EVALD L. S u o
964
compounds. The binary freezing point diagrams for 2-aminopyridine with a number of saturated and unsaturated fatty acids reported below show that two crystalline compounds form in each casean equimolecular compound and a compound consisting of 2 or 4 molecules of acid per molecule of amine depending upon whether the acid is unsaturated or saturated. Experimental The fatty acids were purified by the procedures previously described.3-6 The pure 2-aminopyridine, f .p. 58.0", was obtained by repeated recrystallization from benzene. The freezing point determinations were made with a probable accuracy of f 0 . 2 ' by the thermostatic synthetic procedure,8 which involves finding two temperatures a few tenths of a degree apart a t which, in one case, liquefaction is complete and, in the other, a few crystals persist after a long period with agitation at constant temperature.
Results and Discussion The data for the complete binary freezing point diagrams are given in Table I and are represented graphically for the saturated acids in Fig. 1 and for the various unsaturated acids in Fig. 2. For the oleic acid system only two points were obtained in equilibrium with the stable modification of the acid ; the metastable equilibria are represented by the broken lines in Fig. 2. I n all of the eight systems two inolecular compounds form, each having a congruent melting point. TABLE I BINARYFREEZING P O I N T DATAFOR 2-AMISOPYRIDINE WITH VARIOUS ACIDS" Mole % 2-aminopyridine
:.p,,
C.
Mole % 2-aminopyridine
F.P., 'C.
Lauric acid Myristic acid 0.00 43.9 0.00 53.9 6.23 41.8 12.27 50 2 12.41 39.2 16.68 47.6 (17.7)* (46.8)) 16.00 36.5 li.Gl 35.0 (20.00)' (47.2)C (18.5)b (33.8)* 21.06 47.2 19.23 33.9 30.68 44.5 (20.00)" (33.9)' (33.20)b (43.2)b 21.29 33.9 35.88 46.1 26.15 32.9 (50.00)" (51.3)" 30.10 32.1 51.33 51.3 (32.4)b (31.6)) 68.62 46.8 35.08 34.6 (73.2)b (14.4)b 39.07 38.2 79.43 48.9 45.5G 11,3 89.71 54.2 (50.00)' (41.6)' 100.00 58.0 50.15 41.6 54.51 41.4 59.78 40.1 64.84 3 8 . 1 (67.1)b (36.8)b 70.21 40.0 74 88 41.3 79.51 17.7 84.67 50.8 94.77 55.G 100.00 58.0
Mole ?' & 2-aminopyridine
F.
08:p
Palmit,ic acid 0.00 62.5 (3.56 61.3 12.50 59.4 17.28 57.2 (18.0)b (56.6)* 18.47 56.7 18.93 56.9 (20.00)' (57.0)" 20.58 57.0 23.15 56.9 26.11 56.5 29.17 55.6 31.47 54.1 (33.4)) (52. 6)b 34.64 53.5 37.16 55.2 41.71 57.3 44.90 58.2 (50.00)" (58.8)' 50.67 58.8 54.64 58.4 59.86 57.3 69.30 54.5 73.69 53.0 75.27 52.4 (80.00)* (50.5)* 80.00 50.5 82.11 51.4 89.71 54.4 100.00 58.0
Mole % 2-aminopyridine
:.p., C.
Vol. 60 Mole % 2-aminopyridine
:.e:,
Mole. %
2-aminopyridine
:8:'
Stearic acid a-Eleostearic acid 0-Eleostearic acid 0.00 48.4 0.00 69.3 0.00 70.5 14.58 43.4 12.40 66.8 12.28 67.2 16.11 42.3 15.77 65.5 17.83 64.7 17.82 41.4 (18.0)b (64.5)) 19.87 63.6 21.61 (20.00)' (65.0)' 38.1 24.36 60.7 22.07 65.0 29.81 28.9 28.51 57.4 31.95 62.8 31.42 26.9 (28.5)b (57. 4)* (35.2)b (60.3)b (31. 6)b (26,4)* (33.33)' (58.7)" 37.44 61.3 (33.33)' (26.5)" 33.59 58.7 (50.00)' (64.7)' 34.68 26.5 36.31 58.4 50.69 64.7 35.26 26.4 (4O.O)* (57.0)b 65.34 62.1 37.72 26.5 40.19 57.1 79.90 57.3 42.19 24.6 43.42 58.4 85.88 54.9 (42.2)b (24.6)b 48.31 59.3 27.3 (87. 40)b (54. 2)b 46.76 (50.00)' (59.4)" 90.26 55.2 47.91 27.6 51.05 59.4 94.45 56.5 49.96 27.9 58.54 58.1 100.00 58.0 (50.00)' (27.9)c 63.56 56.3 54.89 27.2 71.93 53.1 58.35 26.3 75.88 50.8 60.56 25.6 79.76 48.9 (62. 4)b (24.6)) (80. (48.4) 63.93 29.0 85.03 51.0 67.11 35.3 87.99 52.5 88.15 52.3 100.00 58.0 100.00 58.0 Elaidic acid Oleic acid 0.00 43.8 0.00 16.3, 15.5d 13.58 40.6 4.44 15.8, 12.8 19.71 37.6 8.20 11.6 21.89 36.3 14.27 8.7 22.34 36.0 (19.1)b (5.9)b (24.0)) (34.8)b 22.17 8.6 11.8 27.05 35.G 26.68 32.72 13.7 30.50 36.4 33.17 36.7 (33.33)' (13.8)' (33.33)' (36.7)" 36.40 13.4 11.1 34.24 36.6 41.61 85.05 36.5 (45.5)) (8.9)b (36.6)b (36.0)b 47.56 9.4 36.85 36.1 49.66 9.8 37.12 36.3 (50.00)" (9.8)' 9.8 43.50 39.3 51.17 (50.00)' (40.2)' 51.85 9.9 50.77 40.1 (53.6)b (9.5)) 12.9 62.06 38.2 54.65 (65.9)b (36.7)) 56.80 19.4 29.9 65.93 36.7 61.41 43.4 G7.85 39.9 71.38 74.58 46.0 80.50 49.9 58.0 85.85 52.8 100.00 100.00 88.0 The values in parentheses were obtained by graphical interpolation. b Eutectic. Freezing point of compound. Values in italics are for metastable equilibria.
For the saturated fatty acids these compounds have the compositions RCOOH.NC5HdNH2 and 4RCOOH.NC5H4NH2. The freezing points of the 1: 1 and 1:1 compounds for stearic acid are nearly identical, about 4.3 and 4.6", respectively, below that of the acid. As the number of carbon atoms in the acid decreases from 18 t o 12 the difference
.
July, 1956
BINARYFREEZING P O I N T DIAGRAM FOR 2-AMINDPYRIDINE
between the freezing points of the acid and the 4: 1 compound increases from 4.3 to 10.0" while the corresponding difference for the 1:1 compound decreases from 4.6 to 2.3'. As a result, the freezing point of the 4: 1 compound in the stearic acid system is 0.3" higher than that of the 1:1 compound, but for palmitic, myristic and lauric acids it is 1.8, 4.1 and 7.7" lower, respectively. Since all of the molecular compounds melt congruently there are three eutectics, viz., between the acid and the 4: 1 compound, between the 4: 1 and 1: 1 compounds, and between the 1: 1 compound and the amine. For stearic acid these eutectic compositions are 18.0, 35.2 and 87.4 mole yo of amine, respectively. As the number of carbon atoms in the acid molecule decreases the first of these eutectic compositions remains virtually unchanged and the others change gradually so that the eutectic compositions for the lauric acid system are 18.5, 32.4 and 67.4 mole yoof amine, respectively. In contrast to the behavior of the saturated acids, the molecular compounds formed by the CIB unsaturated fatty acids have the compositions RCOOH.NCsH4NH2 and 2RCOOH.NCsH4NH2. The 1 : l compound of elaidic (trans) acid has a higher freezing point than the 2: 1 compound, and for the oleic (cis) acid the reverse is true. For the (cis-trans-trans) a- and (trans-trans-trans) p-eleostearic acids the 1: 1 compound has a slightly higher freezing point than the 2 :1in both cases, It is apparent from the amine side of Fig. 1 that the lowering of the freezing point of 2-aminopyridine by a given mole yo of saturated fatty acid is greater the shorter the chain length of the acid. This can be construed as meaning that the degree of dissociation of the molten molecular compounds into free acid and amine decreases witjh a decrease in the length of the fatty acid molecule, as was previouslya found to be true for the molecular compounds between acetamide and these acids. According to the same reasoning the degrees of dissociation of the acid-amine compounds for the oleic, elaidic and myristic acids are practically identical; that for the p-eleostearic acid is about the same as lauric acid; and that for the a-eleostearic acid is the lowest of all. This would indicate that for the CIS acids the degree of dissociation of the acid-amine compound decreases as t,he degree of unsaturation increases. For the saturated acids the relative degrees of dissociation of the acid-amine compounds can also be deduced from the data on the acid side of Fig. 1; i.e., from the freezing point depression of the acid by the amine. Since the heats of fusion of the individual saturated fatty acids are known, the ideal freezing point lowering can be calculated. In the range of concentrations where the acid is the solid phase the experimental freezing points fall below the calculated values and the deviation increases as the chain length of the acid decreases. If the deviation is attributed entirely to compound formation the data show that, at the temperature in question, the proportion of free acid present in the molten
965
70
60 .c' WK
3
F
2w 5 0 0
e l-
40
30
0
20
40
60
80
100
MOLE PERCENT 2-AMINOPYRIDINE.
Fig. 1.-Binary freezing point diagrams for 2-aminopyridine with: A, stearic acid; B, palmitic acid; C, myristic acid; D, lauric acid.
0
40 60 80 MOLE PERCENT 2-AMINOPYRIDINE.
20
100
Fig. 2.--Binary freezing-point diagrams for 2-aminop ridine with: A, 6-eleostearic acid; B, a-eleostearic acid; Zelaidic acid; D, oleic acid. Broken lines represent metastable equilibria.
acid-amine compound is lowest for the short chain acids. This is consistent with the above conclusion that the degree of dissociation of the saturated acidamine compounds increases as the length of the acid molecule increases. Acknowledgment.-The authors are indebted to Ralph W. Planck for supplying the purified a- and @-eleostearicacids.
