B. J. FONTANA
222
VOl. 67
THE RELATION OF CRYSTAL HABIT TO THE POLYMORPHIC BEHAVIOlC OF LONG-CHAIN PARAFFIN HYDROCARBONS BY B. J. FONTANA California Research Corporation, Richmond, Cdifornia Received June 6, 196%
The relation of the crystal habit to the polymorphic behavior of long-chain paraffin hydrocarbons has been studied utilizing both synthetic normal paraffins and narr.ow fractions isolated from petroleum paraffin wax. The solid state transitions were established and found to be in accord with the results of previous investigators. Thus it appears that, in the region from the meltin& point to ordinary temperatures, normal paraffins of 25 to 38 carbon atoms chain length exhibit only one transition which involves an appreciable energy change. Above 38 carbon atoms chain length, no such transition occurs. It is shown that the unique conic “needle” crystal, previously thought characteristic of non-straight chain araffin hydrocarbons only, is also the natural crystal habit of normal paraffins of the 25 to 38 carbon atom range when cr stal8sed at temperatures above the solid state transition temperature. Below the latter temperature the usual polyhe%al plate crystal forms appear. Normal araffins of greater than 38 carbon atoms chain length can exhibit only the latter plate forms. An analogous relationshipPbetween the transition tem erature and the needle-plate crystal habits of iso- and naphthenic paraffins also exists. These views are shown to account i%r the observations and theories of needle-plate crystal formation of previous investigators. It is suggested that the fundamental nature of the uni ue curvilinear characteristics of the wax paraffin needle crystals may be related to the free rotation about the long - axis oy the molecules which occurs in the solid when above the transition temperature.
Introduction Curvilinear characteristics are not unknown amidst the crystal behavior .of chemical compounds’; however, in the case of paraffins of the wax molecular weight range an acicular from with circular cross section (a true cone) appears to be a definite, stable crystalline modification. These so-called “needle” crystals are not of the type ordinarily identified as merely long thin variations of the usual polyhedral forms. Studies of the nature of wax needles have sometimes been confused with this latter phenomenon as in the early studies by Tanaka, et aL12 (as pointed out by Gray3) and in the recent review by Hughes and Hardman. Carpenter5 was the first to recogniee the occurrence of both the needle form and the normal polyhedral (plate) crystal habit from solvent crystallized paraffin wax fractions. Many theories have been proposed in the attempt to explain the occurrence of these two basically different crystal forms. It will be demonstrated in the present study that the viewpoint considered by Carpenter,6 that the needle-plate transformation was probably related to a solid state transition a few degrees below the melting point, was correct. Carpenter’s crystallization experiments were not directly compared with any of the solid transitions which he detected dilatometrically. Katz6 arrived at the concept of a “temperature of change” above which needles and below which plates were obtained, in qualitative agreement with the results of Carpenter. Katz, however, did not relate his concept to that of solid state transitions. More recently, the increasing recognition that appreciably more than just n-paraffins are present in paraffin wax has resulted in the development of a composition theory of needle and plate formation. Ferris, (1) H. E. Buckley, “Crystal Growth,” John Wiley and Sons, Inc., New York, N. Y.,1951,pp. 514-517. (2) Y.Tanaka, R. Kobayashi and R. Ono, J . Pac. Eng. Tokyo Imp. Uniu., 17, 275 (1928). (3) C. C. Gray, Petroleum, 6, 64 (1943). (4) E. C. Hughes and H. F. Hardman, “Advances in Chemistry Series,” No. 5, Amer. Chem. Soo., 1951,pp. 273-274. (5) J. A. Carpenter, J . Inst. Petrol. Techn., 12, 288 (1926). (6) E. K a t z , ibid., 16, 870 (1930): 17, 37 (1931).
et ai.,? thus suggest that plate crystals can be formed only from normal (straight chain) paraffins and needles only from the iso-paraffinic (branched chain and naphthenic) components of paraffin wax. The effect of temperature was not adequately considered and it will be shown that this condition will lead ta B fortuitous effect of composition. Clarke* found that many iso-paraffins show the expected needle crystal formation at ordinary temperatures and furthermore by crystallizing at sufficiently low temperatures true plate crystals are obtained. Clarke’s data then constitute implied evidence in favor of the solid transition theory of the needle-plate transformation, but this point of view was disregarded. Clarke also suggests that cooling rate is a factor affecting needle and plate formation; however, this was hot tested independently of the effect of the crystallization temperature. Grayg and Sachalienlo suggest that the needle and plate transformation in wax paraffins bears some relationship to the solid state transitions in n-paraffins but consider the problem unsolved. The critical experimental observations required consist of, first, establishing the solid state transition temperature of a given paraffin hydrocarbon and, second, crystallizing the same compound from a solvent at two concentrations such that in the two solutions the cloud points (crystallization temperatures) lie just above and just below the measured transition temperature, respectively. The crystal habits observed will be needles and plates, respectively, as is shown below.
Experimental Materials.-Both synthetic n-paraffins and very narrow fractions isolated from refined petroleum paraffin wax were used in the present study. Two synthetic hydrocarbon?, n-octacosane and n-dotriacontane, from Eastman Organic Chemicals were subjected to fractional solvent crystalliza. Chem.,21, 1090 (1928); 28, 681 (7) S. W. Ferris, et al., I I L ~Eng. (1931); 57, 1054 (1945). (8) E. W.Clarke, ibid., 43, 2526 (1951). (9) C. C. Gray, Petroleum, 7, 94 (1944). (IO) A. N. Sachanen, “The Chemical Constituents of Petroleum,’’ Reinhold Publ. Corp., New York, N. Y., 1946,pp. 306-313.
b
‘
L
POLYMORPHISM OF LONG-CHAIN PARAFFIN HYDROCARBONS
Feb., 1953
223
TABLE I Nrsction
SOURCEAND PROPERTIES OF MATERIALS Paraffin np., Transition
Original source
C.
type
temp., OC.
A
160/165 AMP refined wax Normal 83.6 69.3 Synthetic n-dotriacontane Normal 65.6 B 143/150 AMP refined wax Normal Synthetic n-octacosane Normal 61.3 48.3 C 133/135 AMP refined wax Naphthenic a Determined from the melting point behavior of pure n-paraffins. ebullioscopic molecular weight. ~ U M M A R YO F
Sample
Ethyl lactate Ethyl lactate
Wax fraction B
Ethyl acetoacetate Methyl ethyl ketone Ethyl acetoacetate n-Propyl alcohol Ethyl alcohol Perfluorodimethylc yclohexane Methyl ethyl ketone Ethyl lactate Ethyl alcohol
Wax fraction C
Methyl ethyl ketone n-Propyl alcohol Methyl ethyl ketone
Original source, %
None 1.4372b 41.5 0.89 1.4320 32 18.4 63.7 29.9 0.41 57.4 1.4310 1.4290 28 23.6 53.3 1.4381 28.2" 0.16 28.7 b Extrapolated from measurement at 85". e Froin
TABLE I1 SOLVENT CRYSTALLIZATION EXPERIMDNTS
Solvent
Wax fraction A n-Ca?Hes
Number of carbon atoinsa
nab
Needles Concentration, Cloud point, g./lOO cc, OC.
Transition temp., OC.
..
..
1.6
66.7
0.83
65.5
1.0 8.0 3.0
61.7 59.4 63.8
0.7
60.0
1.4 2.0
57.0 55.8
53.3
1.04
"31.4
28.7
tion and percolation treatment with Floridin clay and silica ge!l?he wax fractions were obtained by fractional vacuum distillation from a three-foot spinning band columnI1followed by fractional solvent crystallization. The source and properties of the three typical wax fractions and two synthetic n-paraffins chosen for study are given in Table I. Melting points and transition temperatures were determined as described in the next section. Refractive indexes, measured with a Bausch and Lomb Abbe Refractometer, are corrected values based on the value of 1.3230 at 80" for pure water.I2 Fraction B, in particular, must closely correspond in purity to the pure n-hydrocarbon n-triacontane. Fraction C, however, although about as narrow a naphthenic fraction as can be reasonably isolated from petroleum wax, still consists of a mixture of a considerable number of compounds. Thermal Analysis .-The transition temperatures were determined by thermal analysis. Approximately one gram of wax is contained in a standard 10 by 75 mm. test-tube. A loosely coiled spiral cylinder, 3 / 4 inch high, made from a 8 / r by 18/8.inch piece of 60 mesh silver screen is completely immersed In the molten wax sample. Some device such as this must be utilized lest poorly defined cooling curves be obtained because of the low heat conductivity of solid paraffin wax. The innermost turn of the silver coil serves to center a single junction (30 B.S.gage) iron-constantan thermocou le in a thin-walled glass protection tube. A multirange &-ownrecording potentiometer was used to obtain the "cooling curves" directly. Temperature readings were reproducible to within less than 0.05". The thermocouple was calibrated, under the actual conditions of use, against standard samples checked with an N.B.S. certified pIati(11) J. A. Lockwood, R. L. LeTourneau, R. Matteson and F. Sipos, Anal. CAem., 88, 1398 (1951). (12) Landolt-B6rnstein, Physikatische-Chemisehe Tabellen, 8 , 956 (1923); 1, 686 (1927); Internationat Critical Tables, 7 , 13 (1930).
Plates Clou! point, Concentration, C. &/loo cc.
None 63.7
57.4
80.6 60.6 47.8 29.4
0.86 0.67 .ll .Ol
21 53.6
.03 .33
51.1 48.6
.50 .33
21 49.4 48.3 29.4 21 21 25.5 26.1
.07 .50 .67 .06 .02 .17 .52 2.0
num resistance thermometer, to give an absolute accuracy of about f 0 . 1 ' . The tube containing the molten sample is suspended during mewurement inside a 6 by ll/! inch testtube which is surrounded by ice in a stoppered, silvered dewar flask. The sharp, prolonged temperature halts observed a t the freezing points and at the transition temperatures are apparent in Fig. 1 where the cooling curves of all five specimens are reproduced (without regard to a fixed scale). Wax fraction
A
Synthetic n-CszHes
Wax fraction
B
Synthetic
Wax
n-CZ8H6B fraction
C
t 2
3
G
a
8 E-l
Fig. 1.-Cooling
6 Time. curves of wax fractions and synthetic n- paraffins.
Solvent Crystallization.-The solvent crystallization studies were made by direct microscopic observation of the crystallization process during all stages a t IOOX to 300X magnification. A Kofler Micro Hot State (A. H. Thomas Co.) was used after modification to give a larger field of observation and more flexible control of heating and cooling. Hot stage temperatures were measured to i ~ 0 . 5 "with a
224
B.J. FONTANA
Vol. 57
thermocouple and these checked satisfactorily with accurately known values of the melting points, transition temperatures, or cloud points of the systems under observation. Solutions of the fractions under study were made up in a volume of 5 cc. or more and the cloud points (see Table 11) determined directly on the whole preparation in the standard manner using a thermometer and stirrer. For micro observation the spheiical depression (0.8 mni. deep) of a preheated culture micro slide was completely filled with a portion of the hot wax solution and then sealed with a micro cover glass; loss of solvent was negligible. In every case studied the crystallization cycle was repeated many times by alternate heating and cooling of the hot stage. Variation of the cooling rate was found to have no significant effect,. The choice of solvents used in the present study was dictated solely by practical requirements. The solubility of wax n-hydrocarbons at temperatures in the vicinity of their solid transitions in most common solvents such as benzene, methyl ethyl ketone or ethyl acetate is of the order of 900 g. per 100 cc. of solvent (see solubility data of Ralston, et aE.,la for n-dotriacontane). Such concentrated solutions are unsuitable for proper observation of crystal growth and prohibitive amounts of sample would be required. For these reasons highly polar solvents were utilized in order to reduce the wax solubility. Relatively high boiling points were necessary also, in order to be able to operate at elevated temperatures. The pertinent phenomena were observed without change in six different solvents as noted in Table 11.
(c) calorimetry16.18 and (d) dilatometry. l9 The curves shown on Fig. 2 , whereon melting and transition temperatures are plotted versus the number of carbon atoms in the n-paraffin chain, were drawn to coincide with the majority of these data to within about &lo. It appears, therefore, that only one solid state transition involving appreciable energy change occurs a t ordinary temperatures, in the molecular weight range of about C26H52 to C38H78. The heat of transition is 0 to 7 kcal. per mole, as compared to 21 to 28 ltcal. per mole for the heat of fusion. Furthermore, above C38H78 this t,ransition no longer occurs. The results obtained by thermal analysis of the nparaffinic wax fractions and compounds used in the present study (Table I) agree with the above data and conclusions, as shown on Fig. 2 . By employing solid carbon dioxide cooling, the cooling curves were extended down to about -60" and no additional thermal transitions were detected. X-Ray diffraction, in particular, has apparently detected more than two crystalline states in some n-paraffins of this molecular weight range. The behavior from one paraffin to the next does not Experimental Results and Discussion appear to be simple and straightforward as might Solid State Transitions.-The solid state poly- be expected for a homologous series of hydrocarbons, morphism of long chain n-paraffins has been demon- however. attempt to classify the crysstrated by previous investigators utilizing (a) talline states of the n-paraffins appear to be unX-ray d i f f r a ~ t i o n , ' ~(b) - ~ ~optical observation116~17justified at the present state of knowledge. For the purpose of the present study, the assumption was made that only the one polymorphic change which occurs with considerable energy change is pertinent. As a consequence of the transition behavior depicted in Fig. 2, then, the predicted relationship between solid state transition temperature and needle-plate crystal formation is expected to occur only in the molecular weight range of from 25 to 38 carbon atoms. Above 38 carbon atoms chain length the n-paraffins should not be capable of forming needle crystals under any conditions. The needle crystals mentioned briefly by Carothers, e l U Z . , ~ ~ obtained from n-CazHlzs and n-C64H130 were undoubtedly merely a normal elongated modification of a true polyhedral form. Crystallization Habit Characteristics.-A summary of the solvent crystallization experiments is given in Table 11. The experimental results are in accord with the previously stated viewpoints. Practical experimental considerations required that the crystallization temperatures (cloud points) be fixed a t not closer than about = k 3 O from the transition temperatures, I n the case of fraction A it is desirable, in order t o make sure of the inability to form needle crystals, to have the cloud point as high as possible. The apparently general formation of two-liquid phase systems in binary mixtures of a long chain n-paraffin with a polar Number of carbon atoms. Fig. 2.-Melting and transition points of n-paraffins. solvent was observed, wherein the wax-rich phase Points: 0,wax fractions; 0 , synthetic h drocarbons, used has a slightly lower melting point than the pure in present study. Curves: best fit of J a t a of references paraffin. In the case of fraction A this restricts 14 through 19, inclusive. (13) A. W.Ralston, C. W. Hoerr and L. T. Crews, J . O T ~Chem., . 9, 319 (1944). (14) A. Muller, Proc. Rov. SOC.(London),A l S 8 , 514 (1932). ( 1 6 ) W.M. Mazee, Rec. t m u . chim., 67, 197 (1948). (16) 8. H.Piper, et al.. Biochem. J . , 86, 2072 (1931). (17) E. C. H.Kolvoort, J . Inat. Petrol. Techn., 84, 338 (1938).
(18) W.E. Garner, K. Van Bibber and A. M. King, J . Chem. Soc.. 1533 (1931). (19) W . F. Seyer. R. F. Patterson and J. L. Keays, J . Am. Chsm. Soc., 66, 179 (1944). (20) C. G.Gray, J . Inst. Petrol. Techn., 89, 226 (1943). (21) W. H. Carothers, J. W. Hill, J. E. Kirby and R. A. Jacobaen, J . Am. Chem. Soe., 6s. 5279 (1930).
Feb., 1953
POLYMORPHISM OF LONG-CBAIN PARAFFIN HYDROCARBONS '
the maximum cloud point to about 3' below the melting point. This is as close or closer to the melting point than was the case when needle crystals were observed in the wax fraction B and the two synthetic hydrocarbons. No difference could be detected in the character of the needle crystals obtained from any of the nparaffinic samples or the naphtheiiic wax fraction. Typical needle growth is illustrated in Plate I, from n-octacosane. The needle crystals grow in
225
The plate crystals obtained a t cloud points just below the transition temperatures of the two synthetic n-hydrocarbons and the wax fraction B are all similar in appearance and resemble those observed under comparable conditions for the nonneedle forming wax fraction A. The particular habit taken by n-paraffinic plate crystals is to some extent a function of the crystallization conditions, which is not unusual in crystal growth as illustrated in the many examples discussed by Wells.22 Such variations in plate crystal form are presumably the result of external forces on the crystal growth and do not involve a change of the internal molecular arrangement in the crystal lattice as is the case in the transformation from plate-to-needle crystals. Plate crystals from n-dotriacontane are illustrated in Plate 111. The drastic modification of nparaffin plate crystals by pour depressants, reported by Goldenberg and Z h u ~ eis, ~unrelated ~ to either the above variations or needle formation.
Plate I.-n-Octacosane from ethyl lactate; cloud point = 57.0"; magnification = 300X.
variations of the simple taper, but are always circular in cross-section and always show the characteristic hole down the center of the long axis. The latter holes are usually readily observed in ordinary light but especially with crossed polarizers as shown in Plate 11. Flat growths of irregular, curvilinear shape always accompany needle growth. The needle crystals often grow out from the latter. These flat growths are probably the result of the initiation of metastable oriented crystallization by the glass surfaces. In this latter respect, it can often be observed that such growths suddenly curl or crumple.
Plate 11.-Same
as Plate I observed between crossed polarizer and analyzer.
A significant effect observed many times with all of the needle forming n-paraffinic substances is the very sudden distortion and internal shattering of the needle crystals when allowed to cool just below the solid transition temperature. Also noted is the initiation of plate growth on the needle. These observations serve to identify the needle crystals obtained by solvent Crystallization with the parent fractions under consideration.
Plate III.--rz-Dotriaco~tane from ethyl lactate; point = 47.8 ; magnification = 300X.
cloud
As was expected, the naphthenic wax fraction was observed to yield plate crystals when crystallized just below its transition temperature. This result is i n accord wit8hthe interpretation of the results obtained by Clarke8 with pure, synthetic isoparaffins. The plate crystal growths from nonnormal paraffinic fractions obtained from petroleum paraffin wax are generally poor as the result of the still complex composition of even an exhaustively purified fraction. These complex plate growths from iso- and naphthenic wax fractions account for the malcrystalline habit described by Ferris, et al.? The transition theory of the crystal habit characteristics of paraffin hydrocarbons is consistent with the experimental observations upon which the composition theory mas based. The majority of the non-normal paraffinic constituents of refined petroleum wax studied had melting points below about 49' and as a result weye normally in the needle crystal forming state a t room temperature. This latter condition is further brought about by the fact that the difference between the melting and transition temperatures is considerably larger for iso- or naphthenic paraffins than for n-paraffins. A rough estimate of the minimum value of the (22) A. F. Wells, Phil. Mag., 37, 184 (1946). (23) N. G . Goldenberg and T. P. Zhure, Kolloid Zhur., 13, 175 (1951)
226
CAI~L A. HELLEB, JR., AND H. AUSTINTAYLOR
melting-transition point difference for pure iso- and naphthenic paraffins can be made from the crystallization data of Clarke.* The average for thirteen compounds is about 21’ (the lowest and highest values were about 12 and 31°, respectively). This is to be compared with the maximum value of 7’ for pure n-paraffins (see Fig. 2). The nparaffins, then, with their relatively high melting points, are normally in the plate crystal forming state at ordinary temperatures. Thus crystallization studies made only in the region of about 20 to 25O, as in the work of Ferris, et ala,’ result in fortuitously consistent observations. High melting point iso- or naphthenic wax fractions which would be in the plate-forming state at ordinary temperatures give rise to the so-called malcrystalline form only because of their complex composition. It should be pointed out that the available data for n-paraffins of less than 25 carbon atoms indicate a distinct departure in transition behavior from that of the 25 to 38 carbon atom n-paraffin group. Hence crystallization characteristics differing from those described in the present work are not to be unexpected for n-paraffins of below 51’ melting point. Preliminary observations indicate the absence of needle-forming characteristics. The Nature of the Needle Crystal.-The results of X-ray diffraction studies14 of the solid modification of long chain n-paraffins which appears between the melting point and the transition temperature have been interpreted to mean that the long paraffin chains rotate freely about their long axes in the solid crystalline lattice. Ferris, et u Z . , ~ quote A. R. Thompson (from a private communication) as having crystallized ammonium nitrate from solution in the form of needles with a hole through the center. No further details of their preparation or appearance are available, but presumably they bear some analogy t o wax needle crystals. Evi-
VOl. 57
dence also exists24 showing that the ammonium and nitrate ions rotate freely about their centers in solid ammonium nitrate above 125’. This suggests that the unusual crystallographic aspect of needle crystals is related in some manner to the free rotation of the molecules. Evansz6 has stated that (‘spherical growth would, presumably, be the normal form of solid accretion in a universe in which surface energy was independent of direction” and that such is the normal form of liquid accretion. A liquid achieves the directional independence of surface energy by virtue of free rotation of the molecules about all axes. In solid long chain paraffins free rotation of the molecules occurs only about one fixed axis. Such a condition, then, may restrict the “spherical growth” to one axis only and hence a crystal is obtained with a circular cross-section such as is the distinguishing characteristic of paraffin hydrocarbon nqedles. Nothing definite is known about the molecular arrangement of the paraffin chains with respect to the shape of the needle crystal. Hubbard,26on the basis of the observation that the birefringence toward the lateral margins of the needles is much stronger than that along the central axis (see, for example, Plate I1 of the present study), concludes that the long axes of the carbon chains radiate perpendicularly from the long axis of the needle crystal. More definitive information is needed, however, such as would be supplied by X-ray diffraction studies. It would also be of interest to study the crystal habit of other long chain compounds which exhibit fr,ee rotation in the solid state as discussed, for example, by Hoffman and SmythZ7for alkyl halides and alcohols. (24) N. H. Hartshorne and A. Stuart, “Crystals and the Polarizing Microscope,” Ed. Arnold Co., London, 1950, pp. 12, 150. (25) U. R. Evans, Discussions oJthe Faraday Soc., No. 6, 77 (1949). (26) B. Hubbard, A m . Mineralogist, 80,645 (1945). (27) J. D. Hoffman and C. P. Smyth, J . A m . Chem. Soc., 7P, 171
(1950).
THE PYROLYSIS OF CADMIUM DIMETHYL1 BY CARLA. HELLER,JR., AND H. AUSTINTAYLOR Department of Chemistry, New York University, New York, N . Y . Received June 1 I . 196d
The pyrolysis of cadmium dimethyl has been studied analytically in the temperature range 212337’. The gaseous products are methane and ethane. Hydrogen, nitrogen and helium have been shown to increase the rate of formation of both methane and ethane, more so for methane than for ethane. In small amounts nitric oxide causes an increase in both methane and ethane formation but with larger amounts methane formation continues to increase while ethane formation is inhibited. Increase of surface decreases both rates but more for ethane than for methane. Increase of temperature causes a more rapid increase in ethane than in methane. A mechanism involving a radical split together with a molecular rearrangement yielding methane may be shown to account qualitatively for these observations.
In previous studies of the pyrolyses of various methyl alkyls evidence has been presented for both free radical and intramolecular mechanisms. During a study of the photolysis of mercury dimethyl Phibbs and Darwent2 carried out three pyrolysis runs in order to find an amount of decomposition to subtract from the high temperature photolysis. (1) Abstract from a thesis presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy a t New York University, October, 1951. (2) M. K. Phibbs and B. de B. Darwent, Trans. Faradoy Soc., 5 ,
541 (1949).
Recently Anderson and Taylor3 carried out a pyrolysis corresponding t o each high temperature photolysis of cadmium dimethyl. It was there found that subtracting the pyrolysis from the total decomposition led to a linear Arrhenius plot for the temperature dependence of the photolysis whereas the total decomposition plot showed marked curvature. This is significant and suggests that the two reactions are entirely independent and do not affect each other. The photolysis is adequately (3) R. D. Bnderson and H. A. Taylor, T H r a JOUBNAL,66,498 (lU52).
