I
R. T.
EDWARDS
Technical Service Laboratoiies, Socony Mobil Oil Co., Inc., Brooklyn, N.
Y.
Crystal Habit of Paraffin Wax
Unpredictable malcrystallization often interferes in processing paraffin wax. Re-examination of some old questions is another step toward better production and lower costs
D u N s o A RESEARCH program on composition and its effect on paraffin wax properties, it became apparent that literature references on this aspect of crystallinity showed little agreement (2, 3, 8-70, 76-19, 28, 37, 35, 36). The crystals were usually classified as needles, plates, and malcrystalline forms. nieedles were considered essential for slveating operations and plates essential for filtration or pressing (27). One area of agreement, however, concerns temperature. Carpenter (3) and Katz (76, 77) found that “needles” developed from the melt or concentrated solutions near the melting point, while plates formed from dilute solutions a t lower temperatures. Carpenter (3) and Gray ( 7 7 ) identified the temperature at which the crystal habit changed with the transition point determined thermally or dilatometrically. Fontana (70)) working with pure normal paraffins, gave more definite proof of this point. Clarke ( 5 ) found that hydrocarbons of practically all types could give a variety of crystal forms, depending on temperature and rate of crystallization. One important question has concerned the relationship between needle crystals and a high temperature plate form. Katz (76,77) considered a hex-
750
agonal plate as the normal form, and needles as an abnormal form developed from crystals which first separated as plates. Rhodes, Mason, and Sutton (30) considered needles as rolled plates which then developed further in the needle form. Although Tanaka and others (35, 36) and Hubbard (73) supported this view, it has not been universally accepted. In recent years, modern theories of crystal growth have modified the approach to crystallinity and considerable new information on the crystal structure of normal paraffins has appeared. Therefore, it seems timely to reopen the problem of paraffin wax crystallinity to see if any old questions could be resolved. Normal Crystal Habit of Paraffin Wax Normal paraffin hydrocarbons are by far the major components of paraffin wax and therefore, the most important in determining its crystallinity. Habit and development of most of the crystals formed should be related to the crystal lattice of the normal paraffins as determined by x-ray diffraction (72, 27, 23-26, 29, 32-34). These investigations confirm polymorphism of the normal paraffins indicated by changes in crystal
INDUSTRIAL AND ENGINEERING CHEMISTRY
habit and thermal properties. -41though not all work is in complete agreement, the following summary appears to be consistent with data on the purest hydrocarbons. Above the transition point, when present, normal paraffins in the wax range (C18H38 and higher) crystallize in a close-packed hexagonal lattice with the molecules rotating about their long axes which are perpendicular to a basal plane. ClsH38, C Z O H ~and ~ , hydrocarbons above approximately C40Hg2, when pure, do not show a transition point and crystallize directly from the melt in the low temperature lattice. Below the transition point, the molecules settle to fixed positions in a new lattice and can no longer rotate. For odd-numbered hydrocarbons, the low temperature lattice is orthorhombic with the long axes of the molecules perpendicuIar to a basal plane. The lower even-numbered normal paraffins including C24H60, form a lattice with the long axes of the molecules a t an oblique angle to the basal plane. This has been identified as a triclinic lattice (26, 34). CzgHse and higher even-numbered hydrocarbons have the long axes of the molecules deviating even further from the perpendicular. This lattice is monoclinic but its parameters are yet to be
published. C26H~4 may crystallize in either the triclinic or monoclinic lattice
(34). All investigators have emphasized the importance of purity of compounds in this work. Even small quantities of other members of the series can cause development of the orthorhombic lattice in a compound which when pure, crystallizes in a system with inclined axes; or they can depress the transition. point, increasing the temperature range over which the hexagonal lattice is stable. This effect appears to be due to greater capacity of the hexagonal and orthorhombic lattices to accommodate solid solution formation. Experimental. A North American Philips Co. Type 12021 recording spectrometer operated at a peak voltage of 37 kv. and a c u r r e n t of 6-ma. was used in this work. Copper KCYradiation was isolated by a nickel filter. The waxes were permitted to solidify in a thin layer on a microscope slide which was clamped to the specimen post of the instrument. For work at other than room temperature the slide was replaced by a hollow copper block through which hot or cold water could be circulated. Observations were carried out with a Leitz petrographic microscope with both ordinary illumination and crossed Nicol prisms. Specimens were prepared by placing a drop of molten wax or wax solution on a conventional slide. This was covered with a cover glass and the slide heated on a hot plate until the wax melted or dissolved. The cover glass thus settled on the liquid of its own weight. This method gave a rather thick film in which crystals could develop with limited constraint. Some observations were made on droplets held in the recess of a concave slide where constraint was even less. Temperature was controlled by placing the slide in an insulated chamber similar to the cold stage of Chamot and Mason (4). This could be heated or cooled by circulating a hot or cold liquid through the hollow walls. The slide was heated until the specimen was liquid. Temperature of the circulating liquid was slowly reduced (0.8 to 1.0' F. per minute) until the first crystals separated. I t proved difficult to follow the details of crystallization particularly from the melt with this technique. Motion pictures of crystallization from the melt were taken using a conventional microscope, a n Eastman Cinekodak Special No. 1, and Bausch and Lomb accessories for motion photomicrography. When pictures, taken a t 64 frames per second, were projected a t 16 frames per second the crystallization process could be followed a t leisure. Data reported here were obtained on a low melting paraffin wax, refined by sweating with characteristics as follows:
Low Melting W a x Melting point, ASTM,' F. 127.0 Tensile strength, Tinius-Olsen, Ib./ sa. in. 220_ . _ oil,-% 0.31 Viscosity, kinematic at 210° F., cp. 3.20 Color, Saybolt 4-30
Mass spectrometric analysis (Table I) indicated that this wax was almost free of cyclic compounds and that the content of branched-chain paraffins was low. T o show the effect of temperature on crystal habit, crystallization was observed from the melt and a series of solutions in the petroleum solvent (Table 11) which gave cloud points a t approximately 10' F. intervals from the melting point down to 40' F. The first crystals that develop on cooling a melt or solution of this wax should consist almost entirely of normal paraffins in solid solution. There will be some variation in the composition of these solid solutions depending on the amount of wax crystallized and the range of solid solution possible a t the temperature in question. These crystals should be the most perfect obtainable from paraffin wax and provide a point of reference for studing malcrystallinity. Crystal Habit a n d Temperature. Figure 1A shows the wax crystallizing from its melt in the recess of a hollow slide illuminated with polarized light. Traditional needle crystals and a number of irregular flat plates are apparent. The latter offer much less contrast. The edges of many of the plates are curled and one is developing needlelike projections formed from its curled edges. All of the needles first appeared as plates which curled and grew further in this curled form to yield this crystal type. Figure l B , shows the same field under crossed Nicols. Only curled and tilted crystals are now visible and needles dominate the field. With both types of illumination, but particularly with crossed Nicols, crystals of this type appear characteristic of paraffin wax under these conditions. The true course of their development from the high temperature plate habit was first recognized by Rhodes, Mason, and Sutton (30) and Hubbard (73). The first appearance of these crystals as plates can easily be missed by casual observation, particularly with crossed Nichols. This has led to their continued identification as truly acicular crystals. Figure 2, enlarged frames from a motion picture, shows these crystals in various stages of their development. Most of the needle crystals already formed or in process of formation, developed from individual small crystals. However, much of the crystalline material, perhaps a major part, appears as large, filmlike crystals extending beyond the limits of the microscope field. Surface layers of this film can wrinkle,
Table 1. Refined W a x Mass Spectrometer Analysis Carbon No. 20 21 22 23 24 25 26 27 28 29 30 31 32 33
Normal Paraffins, Mole % 1.5 4.2 7.1 9.3 11.9 11.9 11.8 10.8 8.7 6.3 3.6 1.6
Branched Paraffins, Mole %
... ... ... 0.2
0.3 1.0 1.2 1.8 1.7 1.7 1.1 0.8 0.7 0.4
... ...
Normal paraffins Branched paraffins Monocycloparaffins Condensed cycloparaffins Aromatics
Table II.
Mole % 88.7 10.9 0.2 0.1
0.1
Petroleum Solvent
Gravity, O API Distillation, ASTM, O F. IBP 10% 50% 90% 98%
Residue, yo Loss, % Aniline point, F. Refractive index, n y Specific dispersion
41.6 374 393 410 440 460 1.0
1.0 135.1 1.4553 120.0
peel, curl, and grow further as needles in the same manner as small individual plates. Some hints of such development appear in Figure 2. Because of the lack of contrast, these large filmlike crystals can easily be missed during casual observation. The x-ray diffraction pattern of this wax just below the crystallization point showed a single line in the d=4.5 to 7.5 A. region. This indicates the identity of the 110 and 200 interplanar distances characteristic of'the hexagonal lattice. Since the crystals appear isotropic with crossed Nicols, the optic axis and hence, the c axis of the hexagonal lattice, are perpendicular to the flat surface. The uncurled crystals can be described as thin plates showing extended 001 surfaces, with erratic, curved hk0 boundaries. Essentially this same crystal habit forms from sorutions crystallizing a t temperatures down to about 20' F. below the solidification point of this wax. Figures 3A and B shows crystals forming from a solution which clouds a t 104.5' F. These are obviously similar to those of Figure 1A and B. Differential thermal analysis and dilatometry of the solid wax as a function of temperature indicate that transition sets in just below this temperature. The crystal habit is still that of the hexagonal lattice. VOL. 49, NO. 4
APRIL 1957
751
.
Figure 4A and B, shows the gradual development of more definite form in crystals forming at 99’ F. These crystals are thicker and stiffer with no tendency to curl. Any which appear needlelike with crossed Nicols are definitely tilted crystals. Although the hk0 boundaries are still curved, a n increasing trend toward development of more definite form is apparent. The crystals are showing faint birefringence when seen flatwise with crossed Nicols. The x-ray pat-
terns of a cooling solid wax show a gradual transition from the hexagonal to the orthorhombic lattice in this temperature range as evidenced by the gradual development of separate 110 and 200 reflections. This transition-type of crystal has sometimes been identified as a malcrystalline type. In the present case there can be little question that these crystals are characteristic of solid solutions of normal paraffins crystallizing in the transition range. This is probably the type of plate crystal to which poor sweating properties are attributed. Crystals formed at successively lower temperatures show further development of form. Figure 5A and B, shows the nearly rhombic crystals formed from solution at 66’ F. Crystallization at lower temperatures showed no change in form. hkO Boundaries are now established. This appears to be the stable low temperature habit as Hubbard (73) has indicated. His work and that of McCrone (20) indicated that this crystal is orthorhombic. X-ray diffraction patterns of cooled melts also indicate this. Commercial waxes do not always show definite 001 reflections, although one or two orders are usually obtained. Distillation fractions give sharper lines and more orders. Comparison of d / n values with the molecular weights of the samples used, indicate that long axes of the molecules were always perpendicular to the 001 basal plane. This is characteristic of the orthorhombic lattice. Crystals obtained from solution below the transition point yielded 110 and 200 reflections identical with those of solidified melts. The 001 reflections consisted of a group of broad overlapping lines. This
A.
B.
A.
8.
Figure 1. Refined wax from melt, hollow slide. A, polarized light; 8, crossed Nicols
indicates the simultaneous presence of several solid solutions, each of limited composition range. The higher d,n values obtained from these reflections are compatible only with the orthorhombic lattice. I t is impossible to assign the lower values preferentially to an orthorhombic lattice of shorter molecules or to a lattice with oblique axes of longer molecules. At least, however, the orthorhombic lattice seems to predominate. hlicroscopic observations of other refined waxes show identical changes in crystal habit with temperature. It seems definite that all paraffin waxes are characterized by two ideal plate habitsa soft plastic form with irregular boundaries develops from the hexagonal lattice, while a stiffer rhombic form develops from the orthorhombic lattice. I n the transition range, crystals v ith some of the characteristics of both forms develop. High temperature plates readily curl to form the needles frequently considered characteristic of certain type waxes. Normal and Malcrystalline Forms Investigations cited earlier have shown that malformed crystals seldom occur except in presence of substantial quantities of nonnormal hydrocarbons. The effect of these hydrocarbon types has been interpreted in various ways. However, the usual interpretation has been that the presence of nonnormal hydrocarbons can introduce a new crystal type or an interchange between rypes (2, 8:9, 74, 79). This whole field seemed to require further clarification. Experimental. Paraffin wax crystals, especially those from crude waxes, have always proved troublesome for the micros-
Figure 2. Steps in crystallization from the melt, hollow slide, ordinary illumination
C.
752
D.
INDUSTRIAL AND ENGINEERING CHEMISTRY
E.
P A R A F F I N W A X CRYSTALS
i
A.
A
A.
B.
B.
B.
Figure 3. Refined wax from 61.5% solution, hollow slide; cloud point, 104.5' F. A, polarized light; B, crossed Nicols
Figure 4. Refined wax from 50.5% solution, cloud point, 99' F. A, polarized light; 6,crossed Nicols
Figure 5. Refined wax from 10.7% solution, cloud point, 66' F. A, polarized light; B, crossed Nicols
copist. The crystals are thin and transparent, and show little contrast with normal illumination. Crossed Nicols may help but crystals seen flatwise are only faintly birefringent, if a t all. Edgewise and tilted views always dominate the field. An American Optical Co. phase microscope with bright medium contrast objectives was substituted for the polarizing microscope for the work on crude waxes from solutions. This gave views of the thin crystals that could not be seen with other techniques. Two crude waxes were used in this work. These were produced by distilling a crude wax of broad melting range to give high melting and low melting fractions. Foots from further processing of high melting wax were combined with the low melting distillate. In order to get a picture of all the crystal types that could be obtained from these waxes, they were separated by fractional crystallization from solvent according to the following scheme. With the high-
melting wax, successive filtrations were made a t 85') 75O, 65O, 55', 45', and 35' F. and with the low-melting wax a t 75', 65') 55', 45O, 35O, and 25' F. The solvent (Table 11) was used first and a portion of each filtrate saved for examination. The use of this solvent eliminated difficulty with evaporation from the slide. The procedure was then repeated with iso-octane so that the solvent could be easily removed from the
precipitated wax fractions and their crystallization from the melt examined. The low melting wax gave no appreciable precipitate at 75' F. with thissolvent. Table I11 gives the composition of these fractions by hydrocarbon type as shown by the mass spectrometer. Figures 6 and 7 show distribution of the normal and branched paraffins by carbon number. Distribution of other nonnormal types can be expected to approximate that
25% Solution Cooled to Just Below Crystal Point and Filtered
.1
Composition of Fractions by Hydrocarbon Type High-Melting Crude Wax
Pptn. temp., O F. Compn. mole yo Normal paraffins Branched paraffins Monocyclic para5ns Polycyclic paraffins Monocyclic aromatics Av. C No., normal paraffins Av. C No., brancbed paraffins
85'
75'
65'
55'
45O
35O
Final Filtrate
76.4 15.6 7.8 0.0 0.2
73.3 17.4 8.8 0.2 0.3
73.5 17.1 8.6 0.4 0.4
73.3 16.9 9.2 0.3 0.3
71.6 17.9 9.6 0.6 0.3
67.0 20.7 10.9 0.9 0.5
61.4 24.0 12 0 1.8 0.8
29.8
29.2
28.0
27.2
26.6
25.8
24.9
30.0
29.7
29.3
29.7
29.7
29.4
28.7
Low -Melting Crude Wax
Precipitated Wax
Filtrate Cooled to 10' F. below First Filtration + Precipitated Temperature and Filtered Wax
.1
Table 111.
Pptn. temp., F. Compn., mole % Normal paraffins Branchedparaffins Monocyclic paraffins Polycyclic paraffins Monocyclic aromatics Av. C No., normal paraf5ns Av. C No., branched paraffins
65'
550
450
35"
25'
Final filtrate
74.0 16.6 9.2 0.2
71.1 17.6 10.7 0.3 0.3
67.0 19.9 12.2 0.5 0.4
66.0 20.4 12.7 0.5 0.4
65.1 19.9 13.9 0.6 0.5
58.8 23.1 16.5 1.1 0.5
25.9
24.8
23.8
23.7
23.4
22.1
29.7
29.1
28.5
28.3
28.3
27.1
0.0
Filtrate VOL. 49, NO. 4
APRIL 1957
753
CARBON NO.
C A R B O N NO.
I'
20
25
,
B I.
20
35 I
0
r
1
a
N
5 -
a n
h
a l l B
25
35
30
I
1
.Ill..-
-
10
:
30
Ill,
E
10
&
5 5
..1111111..,
0
:10 L
a
B
0
-.
11111.
5
0
:10 0
A
10
c E
'1
5
15
u- 10
5
b
4 5
N
0
5 5 0
B
0 N-NORMAL
Figure 6.
-
N - NORMAL
Figure
IiBIl,..
8- BRANCHED
7. Low melting
-
B BRANCHED
High Melting
Distribution o f components in crude w a x from mass analysis.
Temperature,
' F.
A.
of the branched paraffins, although it may be shifted somewhat toward higher carbon numbers. Discussion. Mass analyses of the fractions show that as precipitation temperature decreases, average molecular weight of the fractions decreases. Also there are two other trends-the proportion of nonnormals, particularly that of cyclic compounds, increases at lower precipitation temperatures, and the difference between the average carbon number of the nonnormals, as indicated by that of the branched paraffins, and
B.
C.
rhat of the normals increases (Figures 6 and 7). Figure 8 shows that effect of composition on the high temperature crystal habit is not great. With the exception of wax precipitated from the high melting crude wax at 85' F. (Figure 8A) all crystals have the same appearance as those of Figure 1A and B. The difference in appearance of the crystals in Figure 8A is caused by a lesser tendency to curl than those of other specimens. Some curling is evident but it has not proceeded far enough to give the tightly curled needles of other waxes. The crystals of the crudest wax r fractions-those recovered from the final filtrates (Figure 8C and D)-appear to be somewhat smaller and more tightly curled than those of other waxes. Otherwise, there is little evidence that composition affects the form of these crystals. Crystals from filtrates give an entirely different picture (Figure 9). Presence of nonnormal hydrocarbons produces severe deterioration in perfection of form in the low temperature habit. This can be followed by steps. The first step is truncation of the tip of the relatively perfect rhombs of Figures 5A and B. This may proceed far enough to give a pseudohexagonal shape. This is followed by marring of the crystals by indentations and the appearance of what look like fragments of more perfect crystals. Figure 9A and E, shows examples of these degraded forms. In still cruder fractions, the corners of the crystals become rounded (Figure 9B) and this proceeds until practically all semblance of regular form is lost. IYhile the form degenerates, the crystals become thinner (Figure 9D, E, and 8')until they offer so little contrast that they can be seen only with difficulty, even with the advantage of phasecontrast microscopy. Under these conditions little can be seen but the edges of the crystals. I n Figure 9D, F, and G flat or slightly tilted views are only faintly visible in the background. The filtrate from crude low melting wax at 25" F. (Figure 91) showed only faint D.
Figure 8. Crude w a x from melt, hollow slide, polarized light. A, high melting precipitated a t 85" F. and 6 , a t high melting recovered from filtrate a t 35" F. and D, low melting, recovered a t 25" F.
754
INDUSTRIAL AND ENGINEERING CHEMISTRY
75" F.; C,
P A R A F F I N W A X CRYSTALS traces of crystallinity in the form of crystal edges. Method of observation affects considerably what is seen in the microscope. Figure 9B, C, G, and H, compares crystals fr6m two solutions as seen with phase contrast and crossed Nicols. The impression of the nature of the crystals is different with the two methods of observations. Crossed Nicols give a completely erroneous impression since the degraded plates could easily be identified either as needles or as malcrystalline masses. All crystals given by fractions of these waxes from solution appear to be variations of the single plate type shown in Figure 5A and B . The effect of nonnormal hydrocarbons appears to be a degradation of the perfect form through the steps outlined previously. No indication of new crystal-needle, plate, or malcrystalline-has been seen. The degradation of form appears to result from the inhibition of crystal growth by certain types of nonnormal hydrocarbons. Photomicrographs of wax crystals reported earlier in the literature indicate that larger crystals shown can be interpreted on the same basis. I t is not so easy to do this for smaller crystals, particularly when observation has been limited to crossed Nicols. However, small crystals frequently appear to be diminutive examples of the same types reported here. Many malcrystalline masses have the appearance of clusters similar to those of Figure 9C, only of smaller crystals. Under the conditions of crystallization employed, waxes used here gave no examples of the small crystals characteristic of microcrystalline waxes and products intermediate between paraffin and microcrystalline waxes. Preliminary observation of such crystals confirm the opinion that these, too, are of the same type reported here, only smaller. The size of wax crystals appears to depend on factors other than inhibition of growth. A fuller explanation of their development must await further work. Crystal Growth in Paraffin Wax Modern theories of crystal growth emphasize the importance of kinetic factors and indicate that imperfect crystals are the rule rather than the exception. The status of these theories in recent years has been summarized by Verma (37). I t seemed of interest to see if these theories could assist in interpreting the crystallinity of paraffin wax. Two questions were involved. The first is the relationship between normal and malcrystalline forms and the second, further clarification of the nature of the high temperature needle form. I n connec: tion with the second question some ex-
B.
A.
b.
E.
H. G.
Figure 9. Crude wax, phase contrast. A, high melting, first crystals from 25'27, solution; B, high melting, filtrate a t 55" F.; C, high melting, filtrate a t 55" F., crossed Nicols; D, high melting, filtrate a t 35 F.; E, low melting, first crystals from 25y0 solution; F, low melting, filtrate a t 45" F.; G, low melting, filtrate a t 35" F.; H, low melting, filtrate a t 35" F., crossed Nicols; I , low melting filtrate, a t 25' F.
1.
VOL. 49, NO. 4
APRIL 1957
755
A.
B.
C.
Figure 10. From melt, hollow slide, polarized light, 1OX. 6, hexacosane; C, n-octacosane; D, n-octacosane ( 1 00 X )
e
D.
perimental work with pure hydrocarbons appeared necessary. Crystallization of n-tetracosane (PSU541), n-hexacosane (PSU106), and n-octacosane (PSU176) and their blends from the melt was observed in the manner described earlier. Results. Crystals of these hydrocarbons formed from the melt, were large and frequently extended beyond the limits of the microscope field even at 10x (Figure 10). The bulk of the solid appeared as crystals which lay flat on the slide, although some were edgewise or tilted (Figure 10, A , B , and C). An entire crystal could seldom be brought into the field. These crystals grew in layers. Figure 1OD shows the layered structure of a flat crystal of n-octacosane. In Figure 10A the end of a large crystal of ntetracosane, seen edgewise, is splintering into layers. These crystals are soft and flexible and twist and bend readily to form unusual patterns some of which appear in Figure 10, B and C. However, the crystals showed no tendency to curl like those of paraffin wax, to form needles. Figure 11 shows that a mixture of equal parts of n-tetracosane and n-octacosane exhibits a different type of crystallinity. The crystals are much smaller and curl readily to form exactly the same type of crystal formed from paraffin wax melts. This type of tightly curled crystals must now be considered typical of sdid solutions of normal paraffins. Discussion. Dawson and others (7, 6, 7) have shown that growth of orthorhombic normal paraffin crystals proceeds by addition of layers. A layer one molecule thick is formed by addi-
756
tion of the long molecules side by side to form a first 001 layer. This appears to be a relatively rapid process. Addition of new layers is much slower and can proceed with facility, only from dislocations in the first layer laid down. This method of growth gives the characteristic thin rhombic crystals of these hydrocarbons from solution at low temperature. I t is probable that growth in the hexagonal lattice, above the transition point, proceeds by a similar mechanism, but no definite evidence on this point is available. Growth in this lattice is by layers, (Figure 1OD) but these layers are certainly more than one molecule thick. Lateral growth is random, giving crystals of very irregular outline. Molecules of the members of the normal paraffin series differ only in molecular length and can form solid solutions over a considerable composition range (27, 22). In development of a wax crystal by layers, it is probable that the first layer laid down consists of molecules of the highest melting components of the wax. Following layers on top of this contain increasing proportions of lower melting components. If such a succession of layers is formed above the transition point, on cooling, the contraction associated with transition will start in the first layer. This contraction involves only the b axis of the unit cell with the other axes remaining essentially unchanged. The strain produced by this contraction in a crystal layer is in a single direction parallel to the flat surface of the crystal. The obvious response of a plastic sheet such as these crystals are, to a unidirectional contraction in one surface is curling such as that observed. Curling does not stop further growth. The curled layers grow further laterally preserving the curled form to give the appearance of elongation of the needles. TVhile at any one point in the crystal surface the contraction is unidirectional, the symmetry of the hexagonal lattice is such that at other points the contraction may be a t an angle of any multiple of 60 degrees from that at the first point.
INDUSTRIAL AND ENGINEERING CHEMISTRY
A, n-tetracosane;
This can lead to the development of several needles from a single plate. This interpretation explains why the curled crystals are found in solid solutions of normal paraffins and not in the pure components crystallized from the melt. Hydrocarbons of other types in the proportion that they occur in paraffin wax appear to have little effect as evidenced by the small differences apparent in Figure 8. Fontana (70) has reported needle crystals formed from dilute solutions of pure normal paraffins in polar solvents just above the transition point. His photomicrographs show crystals of the same appearance and orthoscopic optical properties as those observed in this work on paraffin wax and blends of normal paraffins from the melt. Melts of pure normal paraffins gave no such crystals in this work. It is probable that differences in results are caused by differences in the purity of the compounds used. It is difficult to produce individual normal paraffins completely free from contamination with homologs, since recrystallization yields solid solutions rather than pure cormpounds as the solid phase. There is no means available for comparing the purity of the samples used by Fontana with these used in this work. His crystals were formed just above the transition point where, according to the theory of crystal growth outlined here, the effect of homologous impurities might be expected to be
Figure 11. Equal parts of n-tetracosane and n-octacosane from melt, 100 X, polarized light
P A R A F F I N W A X CRYSTALS greater than in crystallization from the melt. The method for forming needle crystals, outlined here, requires that transition start before solid solutions of normal paraffins are completely solidified. The binary phase diagrams reported by McCrone (20) do not suggest that this occurs extensively. His binary systems are, of course, much simpler than the complex series of solid solutions formed in paraffin wax. There are other unique difficulties in the behavior of solid solutions of normal paraffins. I t appears that these crystals grow by addition of monomolecular layers. The first layer laid down in an individual crystal will be richer in high melting components than later layers. The two-dimensional solid solution of the first layer will undergo transition before this occurs in the bulk of the crystal. Microscopic techniques will give only the properties of the bulk of the crystal. The detailed course of the curves in the phase diagram of the two-dimensional solid solution must remain a matter of conjecture, but phase changes must occur at temperatures higher than those observed for the three-dimensional solid solution of the entire crystal. Development of thin, misshapen crystals from solution of crude waxes below the transition point appears to result from inhibition of crystal growth. Addition of new layers seems to be the process most severely inhibited. Lateral growth of individual layers is limited little, if a t all, but the development of regular form in the layers may be seriously impaired. There is some evidence that two sets of inhibitors are involved-one preventing addition of new layers and the other interferring with regular lateral growth-but this point requires further investigation. The inhibitors are probably nonnormal hydrocarbons of higher molecular weight than the normal paraffins making up the bulk of the crystals. I t seems probable that progress in understanding paraffin wax crystallinity will depend on its further interpretation in terms of mechanism and kinetics of crystal growth. This approach appears more fruitful than that which attempts to attribute particular crystal types to the effect of individual components or groups of components.
Crystal Habit and Refining Properties of Paraffin Wax. Results of this work indicate that refining properties of paraffin wax are not properly related to any inherent tendency of the wax to crystallize in plate, needle, or malcrystalline form. Any paraffin wax can crystallize in one of two plate forms, depending on the temperature of crystallization, with other apparent forms being produced by distortion or trunca-
tion of these. The majority of these crystals represent normal paraffins. Refining qualities of crude waxes are probably related to size and stiffness of the crystals rather than essential differences in crystal habit. Small, thin crystals which deform readily under pressure tend to pack into an almost impermeable mass on a filter, reducing filter rates toward zero. Seen under the microscope, below the transition point, these crystals may appear as needles when viewed edgewise. Viewed flatwise the thin crystals would not offer sufficient contrast to be visible. Crossed Nicols would emphasize the appearance as needles. Curling of the soft, high temperature plates probably gives a structure of greater mechanical strength than that of the undeformed plates. T o this extent, the resulting needles can be considered essential to satisfactory sweating. Plates will appear in a slack wax if its oil content is high enough to depress the crystallization point into the transition region, since the crystals are now too stiff to curl readily (Figure 4). It is difficult to say whether this transition type of crystallinity is unsuitable for sweating because the high oil content, will hamper the operation. I n any case, such a situation reflects the quality of the cold-pressing operation which determines oil content of the slack wax. This step may have been impaired by formation at low temperatures of small malcrystals which are in reality tiny, thin plates. The size of wax crystals is determined by cooling rate and by the presence of crystal modifiers. Cooling rate has always been considered critical by refiner and its importance is obvious. Quantity of crystal modifiers can be controlled to some extent by close attention to the earlier steps of the refining procedure, distillation, solvent refining, and dewaxing. Acknowledgment
The author gratefully acknowledges the conscientious work of A. B. Lake in developing and printing the several hundred photomicrographs taken in connection with this program and the collaboration of T. W. Poesner in the motion picture work. F. P. Hochgesang and his associates in the research and development laboratories of the Socony Mobil Oil Co. were responsible for the mass spectrometer data. R. W. Schiessler, then director of API Research Project 42, kindly supplied samples of n-tetracosane, n-hexacosane, and n-octacosane. literature Cited (1) Anderson, N. G., Dawson, I. M., Proc. Roy. SOC.(London) 218A, 255 (1953).
(2) Buchler, C. C., Graves, G. D., IND. ENG.CHEM.19, 718 (1927). (3) Carventer, J. A.,’ J. Znst. Petroleum 12. 268 (1926).
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(4) Chamot. E. M., Mason. C. W., “Handbook of Chemical Micros: copy,” 2nd ed., p. 204, Wiley, New York, 1938. \
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(5) Clarke, E. W., IND.ENG.CHEM.43, 2526 (1951). ( 6 ) Dawson, I. M., Proc. Roy. SOC.(London) 214A 72 (1952). ( 7 ) Dawson, I. M., Vand, V., Ibid., 206A, 555 (1951). (8) Ferris, S. W., Cowles, H. C., IND. ENG.CHEM.37, 1054 (1945). (9) Ferris, S. W., Cowles, H. C., Henderson, L. M., Ibid., 23, 681 (1931). (10) Fontana, B. J., J . Phys. Chem. 57, 22 (1953). C. C., Petroleum (London) 7 , ( ) 1944). (12) Hoffman, J. D., Decker, B. F., J . Phys. Chem. 57, 520 (1935.) (13) Hubbard, B., Am. Mineralogist 30, 645 (1945). (14) Hughes, E. C., Hardman, H. F.,
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“Progress in Petroleum Technology,” p. 274, ACS, Washington, D. C., 1951. (15) Ivanovszky, L., Wredden, J. H., Petroleum (London) 9, No. 4, p. 8 2 ; No. 5, p. 110 (1946). (16) Katz, E.,. J . Znst. Petroleum 16, 870
(1930). (17) Ibid., 18, 37 (1932). (18) Kolvoort, E. C. H., Moser, F. R., Verver, C. G., Zbid., 23, 734 (1937). (19) Lund, H. A., Petroleum Processing 7, 326 (1952). (20) McCrone, W. C., WADC Tech. Rept. 54-349, Wright Air Development
Center, Wright-Patterson Air Force Base, February 1954. (21) Mazee, W. M., J . Znst. Petroleum 35,
97 (1949). (22) Mazee, W. M., Rec. trau. chim. 67, 197 (1948). Muller; A., ‘Proc.Roy. Soc. (London) IZOA, 437 (1928). Ibid., 127A, 417 (1930). Zbid., 138A, 514 (1932). Muller, A., Lonsdale, K., Acta Cryst. 1, 1928 (1948). (27) Nelson, W. L.. “Petroleum Refinery Engineering,”’ 3rd ed., pp. 320-5 McGraw-Hill. New York. 1949. Padgett, F. W.,’ Hefley, D.’ G., Henriksen, A., IND. ENG. CHEM.18, 832 (1926).
Piper, S. H., Chibnal, A. C., Hopkins, S. J., Pollard, A. F., Smith J. A. B., Williams, E. F., Biochem. J .
25, 2072 (1931 ). (30) Rhodes. F. H.. Mason. C. W.. Sutton, W. R., IND. ENG.’CHEM.19, 935 (1927). (31) Sachanen, A. N., Zherdeva, L. G., Vassilieff, N. A,, Natl. Petroleum News p. 29 (April 22, 1931); p. 1 3 (May 6, 1931). (32) Schoon, T., Ber. 72B, 1821 (1939). (33) Schoon, T., 2.physik. Chem. 39B, 385 (1938). (34) Smith, A. E., J . Chem. Phys. 21, 2229 (1953). (35) Tanaka, Y., Kobyaski, R., J. Fuc. Eng. Tokyo Imfierial liniu., 17, 289 (1938 ). (36) Tinaka,’Y., Kobayashi, R., Sadayuki, Ibid., 17, 275, 283 (1928). (37) Verma, A. R., “Crystal Growth and \
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Dislocation,” Academic Press, New York, 1953.
RECEIVED for review March 30, 1955 ACCEPTED October 18, 1956 VOL. 49,
NO. 4
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APRIL 1957
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