J. TSAUAND D. F. R. GILSON
4082
Polymorphism in n-Alkylammonium Halides.
A Differential
Scanning Calorimetric Study by J. Tsau' and D. F. R. Gilson Department of Chemistry, McGill University, Montreal, Quebec, Canada,
(Received April 18, 1868)
A differential scanning calorimetric study of the series of monosubstituted n-alkylammonium chlorides and bromides, from C1 to CX, showed that several solid-phase transformations occur for these compounds in the temperature range from - 150' to their melting points. Transition temperaturesand enthalpies and enthalpies and entropies of fusion have been determined.
Introduction The existence of various solid phases of the ammonium salts has long been established, but of the n-alkylsubstituted compounds only the methyl- and n-pentylammonium chlorides appear to have been studied in any detaiL2t3 Heat capacity studies on these compounds revealed transitions at 220 and 264.5'K and a t 210 and 242°K) respectively. Stammler4has examined the transitions in methylammonium halides using differential thermalanalysis. An X-ray structural investigation of n-dodecylammonium chloride and bromide indicated that the chloride undergoes a transition at 348°K and the powder pattern of the bromide shows changes a t 330, 334, and 345°K.6 A study of the infrared spectra of some n-alkylammonium halides as a function of temperature by Sandorfy and Chevaliera showed that several phases exist for these compounds. To obtain more complete information on the transition temperatures and enthalpies, we have studied the series of chloride and bromide salts of monosubstituted n-alkylamines from C1 to c16,using differential scanning calorimetry (dsc). This method affords a rapid and convenient technique for examining thermal transformations and has the advantage over dta that the enthalpy changes can also be ~ b t a i n e d . ~ Experimental Section Methylammonium halides were prepared by neutralization of 40% aqueous solutions of methylamine with the appropriate acid and were purified by sublimation (chloride) or recrystallization from ethanol (bromide). Higher alkylammonium salts were obtained by passing hydrohalide gas through a solution of the amine in chloroform or ether, followed by recrystallization (3 x) from mixed solvents ethanol-ether (Ct-C,) or chloroform-ether (C,-Cl,) or from chloroform (C15-cl6). Thermograms were obtained using a Perkin-Elmer DSC-1 differential scanning calorimeter at a scanning rate of 6"/min. Helium was used as the transfer gas at low temperatures (down to -150'), and nitrogen was used at high temperatures. The temperature The Journal of Physical Chemistrg
scale was calibrated using the melting temperatures of standard compounds. The transition in succinonitrile at 233'K, 1482 cal/mol,* and the fusion of high-purity indium, 6.8 cal/g, were used as calibration standards for obtaining enthalpies of transitions. Peak areas were measured with a planimeter. Based on the enthalpy of fusion of indium as the calibrant, the transition in succinonitrile was obtained as 1420 f 40 cal/mol. Using both indium and succinonitrile as standards, a measurement of the transition in ammonium nitrate at 399°K gave an enthalpy of 1042 f 40 cal/mol (lit.g 1060 f: 7 cal/mol). Results The measured transition temperatures and enthalpies for the chlorides and bromides are given in Tables I and 11,respectively, for both increasing and decreasing directions of temperature scan. Two transitions were observed for the chloride salts of C1, C3-Cs, and CIOamines and for all bromide salts except Cz and Ce-CQ, although some transitions were composite transitions in which two or more closely spaced peaks were observed in the thermograms. The chloride salts of CQand C11-C16 amines and CTand C Qammoniumbromides showed three thermal anomalies. When the substituent chain contained four or more carbon atoms, the high-temperature transitions increased more or less regularly in temperature with (1) National Research Council Studentship, 1966-1968. (2) J . C. Aston and C. W. Ziemer, J . Amer. Chem. SOC.,68, 1405 (1946). (3) C. Southard, R . T . Milner, and S. B. Hendricks, J . Chem. Phys., 1, 95 (1932). (4) M . Stammler, J . Inorg. Nucl. Chem., 2 9 , 2203 (1967). ( 5 ) M. Gordon, E. Stenhagen, and V. Vand, Acta Crgstallogr., 6, 739 (1953). (6) P. Chevalier, Ph.D. Thesis, Universitb de Montreal; C. Sandorfy, private communication. (7) E. S. Watson, 11. J. O'Neill, J. Justin, and N. Brenner, Anal. Chem., 36, 1233 (1964). (8) C. A. W-ulff and E. F. Westrum, Jr., J . Phys. Chem., 67, 2376 (1963). (9) M . Nagatani, T . Seiyama, M. Sakiyama, H. Suga, and S. Seki, Bull. Chem. floc. Jap., 40, 1833 (1967).
POLYMORPHISM IN ~-ALKYLAMMONIUM HALIDES
4083
Table I : Dsc Transitions of n-Alkylammonium Chlorides’”
?t
1
--Heating-Transition temp, OK
264* 223* 264 223
170* 650* 850 140
-Cooling-Transition temp, OK
205
188 408
276 992
187 396
280 909
4
237 253
188 588
235
889
259
1490
217 239
461 620
160
209
220
214 265 272
202 810
264
994
260 303
753 446
193 302
450
276 305 308 313
348
275 300 3041
6
7 8
9
10
11
11
304 331 339
774 1260
12
333* 324 334 347
7390* 1200 623 658
320*
2460* 551
700
3
5
Heating----Enthalpy, Transition temp, OK cal mol-1
1110
Cooline-
r
Enthalpy, cal mol-1
No transitions observed
2
a
Enthalpy, cal mol-1
13
,
14
1100 310
259 270 325
312 38 736
258 267 324
407
305* 323* 303 321
300* 3000* 587 983
305 323
668 893
Enthalpy,
303 327 335
778 1300
321 338 346
1180 715 617
466 1040 960
cal mol-I
318 343 359
833 922
313 318 337 357
343* 334 343 365
7210* 1520 579 942
330 343 365
1030 764 1080
6860* 1180
320
1120
1030 1330
342 372
1340 1380
332 344 371
1180 1050 1400
15 327 348 374
350
Transition temp, OK
5120* 2270
16 343 372
1360
691
Asterisks denote transitions observed in the first, but not subsequent, scans.
increasing chain length, but the low-temperature transitions showed alternation with odd and even members of the series. Supercooling of transitions was observed for methyl-, n-pentyl-, and n-heptylammonium chlorides and flor nearly all bromides, and for some salts the dsc thermograms differed, both in transition temperature and enthalpy, if the same sample was scanned more than once. I n certain of these cases the sample returned t o its original state after storage at room temperature for 1 month, e.g., CIC1, CaBr, CI.IEIr, and CleBr, but for C12C1, CleC1, and C9Br the thermograms were still different after nearly 2 months at room temperature.
Discussion For homologous series such as n-alkanes, 1-alkenes, or n-alcohols, the alternation of transition temperatures, temperatures of fusion, enthalpies, and entropies of transition and fusion, etc., as a function of the number of atoms in the chain is well known.lO The alternation in melting points is attributed to melting
from different crystal structures of the odd and even members of the series, even though, in many cases, the structures are not known. Structural data on the n-alkylammonium halides are limited and, with very few exceptions, are based on room temperature studies. I n general, the room temperature forms are tetragonal up to (210, except for ethylammonium chloride and bromide which are monoclinic.” Above Clz the structures are either monoclinic, Clz, (214, and (216, or orthorhombic, C13 and C I ~ . ~ The , ~ low-temperature ~ , ~ ~ form of n-propylammonium chloride is also monoclinic.l 4 For (10) (a) A . R. Ubbelohde, “Melting and Crystal Structure,” Clarendon Press, London, 1965; (b) 111. R. Cines in “Physical Chemistry of the Hydrocarbons,” Vol. 1, A. Farkas, Ed., Academic Press, New York, N. Y., 1950, Chapter 8; (c) E. F. Westrum, Jr., and J. P. McCullough in “Physics and Chemistry of the Organic Solid State,” Vol. 1, D. Fox, M.M. Labes, and A. Weissberger, Ed., Interscience Publishers, New York, N. Y., 1963, Chapter 1. (11) F. Jellinek, Acta Crystallogr., 11, 626 (1958). (12) G . L. Clark and C. R. Hudgens, Science, 112, 309 (1950). (13) J. D. Bernal, Nature, 129, 870 (1932). (14) M. V. King and W. N. Lipscomb, Acta Crystallogr., 3 , 227 (1950).
Volume 72, Number 19 November 1968
J. TSAUAND D. F. R. GILSON
4084 Table I1 : Dsc Transitions of n-Alkylammonium Bromides" Enthalpy, ea1 mol-'
1
285 389
1040 306
281
1290
2
363
2700
357
2270
3
138
450
136
450
4
198 257
223 1240
191 244
244 856
5
275
1400
261
780
6
254 282 316
794 183
227 283
608 189
7
260 30 1 326
2800 370 480
214 312 329
2080 370 480
8
312* 326
3010* 502
321
507
306* 255. 324 3271
1850* 1230
223
n
9
527 333 10
11
'
-CoolingTransitionI temp, OK
-Heating-Transition temp, OK
12
13
324* 295 337
3970* 1670 513
320* 284 341
5970* 1700 1070
334 * 310 347
4690* 2900 1460
331 * 295 353
3820 2340 1450
16
E) 334
608
269 336
830 510
249 278 339
1150 366 801
277 343
1680 1550
273 295 350
1390 252 1490
297 359
1720 2030
290 305 360
2070
296 362
1320 1950
6390*
14
15
Enthalpy, ea1 mol-1
355* 319 360
3060 1940
333* 313 362
5410* 2480 1900
322 * 353* 319 360
3230* 2880" 2310 2070
" Asterisks denote transitions observed in the first, but not subsequent, scans.
substituent chain lengths of about ten carbon atoms, the dsc anomalies commenced at about room temperaThe Journal of Physical Chemistry
ture, so it is reasonable to generalize and assume that the low-temperature phases of these salts are monoclinic and that transitions occur to a high-temperature tetragonal structure. Long chain molecules commonly have three polymorphic forms: the a! phase, which has the chains perpendicular to the plane of the end groups and rotating about their long axes; the /3 form, with nonrotating vertical chains; and the y form, which has nonrotating chains tilted at an angle to the plane of the terminal groups. This elementary description is obviously inappropriate in cases where three transitions were observed. Supercooling behavior has been reported previously for methyl- and n-pentylammonium ~ h l o r i d e s . ~The , ~ room temperature phase (CY.phase) of methylammonium chloride supercooled to about 220°K when it changed to the /3 phase, but on heating, the p phase transformed first to the CY phase at 220°K and then to the CY. form at 264°K. Thus there was only one transition on cooling but two on heating. This result was confirmed in the present investigation. Rapid quenching of n-pentylammonium chloride to below 165°K led to a metastable phase, which reverted to the stable form on heating to about 175"K.a From an X-ray diffraction study of n-dodecylammonium s that the crystal rechloride, Gordon, el d J reported turned to a form with a slightly different long spacing after heating above a transition at 75". Supercooling of transitions has been attributed to the "freezing in" of rotational disordersa If the high-temperature transition is assigned to the rotational transition, then this, rather than the low-temperature transition, would be expected to be susceptible to supercooling. Freezing in of disorder could also explain the differences between the first and subsequent dsc thermograms. Annealing at room temperature allowed some of the compounds to relax to the ordered form. The melting temperatures of the chloride and bromide salts showed an initial increase to a maximum for the n-pentylammonium salts, followed by a gradual decrease. There was no alternation of melting temperature with odd and even members of the series, which might be further evidence that these compounds melt from the same crystal structures. The entropies of fusion, Table 111, were, with the exceptions of ethylammonium chloride and n-propylammonium bromide , less than 5 cal mol-' deg-'. (No enthalpy or entropy of fusion is given in Table I11 for compounds for which sublimation was observed.) No transition was observed for ethylammonium chloride, and the occurrence of rotation in n-propylammonium halides has been questioned.6 Entropies of fusion of long-chain compounds consist of contributions from positional, orientational (rotational) , and configurational disorder. lo& I n the case of "plastic crystals," which comprise rigid pseudospherical molecules exhibiting a rotator phase in the solid, the entropy change arises essentially from positional disorder alone and is less than 5 cal mol-'
POLYMORPHISM IN n-ALKYLAMMONIUM HALIDES
4085
Table I11 : Enthalpies and Entropies of Fusion’
n
MP, OK
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
(506) 381 439 (4871 (502) (493) 480 477 469 465 462 458 453 447 443 436
-
Bromides
Chlorides AH (fusion), cal mol-1
Ad(fueion), oal mol-1 deg -1
2340 1410
6.1 3.2
1580 1520 1800 1730 1760 1840 2180 2010 2150 2130
3.3 3.2 3.8 3.7 3.8 4.0 4.8 4.5 4.9 4.9
AS(fusion), MP, OK
(537) 434 456 (479) (501) (502) 490 483 477 471 467 465 463 453 451 447
AH(fusion), cal mol-’
cal mol-’
2000 2990
4.6 6.6
1390 1440 1531) 1550 1720 1660 1830 1780 1720 1870
2.9 3.0 3.2 3.3 3.7 3.6 4.0 3.9 3.8 4.2
deg -1
Values in parentheses hidioate that sublimation occurred.
deg-l. l6 Thus, since positional disorder alone could account for the entropy of fusion of the n-alkylammonium salts, it must be concluded that configurational and/or positional disorder occur in the solid or tha; there exists some degree of ordering in the liquid phase. The latter possibility is reasonable, since the polar ends of the molecules will cluster together and restrict translational and configuration motions.. As the sole mechanism, however, the extent of such ordering is not sufficient to account for the observed small entropies of fusion. Evidence for more extensive disorder in the solid phase is available from preliminary wide-line nmr studies in this laboratory, which indicate that the experimental second moments above the hightemperature transitions are lower than the values calculated for long-axis rotation alone. I n their studies of long-chain carboxylic acids and their salts, Dunnell and
coworkersle concluded that a motion of ribbonlike chains of molecules occurred in the solid phase several degrees below the melting points. Sandorfy and Chevaliere describe an “a-oid” phase in the solid n-alkylammonium halides which have infrared spectra characteristic of the liquid. We are at present studying the wide-line nmr spectra and X-ray powder diffraction diagrams of these series of compounds to determine the nature of the transitions and the extent of molecular motion in the solid phase. Acknowledgments. We are grateful to the National Research Council of Canada for financial assistance. (15) J. Timmermans, J. Phys. Chem. Sotids, 18, 1 (1961). (16) T.Cyr, W.R. Janzen, and B. A. Dunnell, Advances in Chemistry Series, No. 63, American Chemical Society, Washington, D.C., 1967,p 13.
Volume 78,Number 18 November 1968
