Phase Transformations in Commercial Paraffin Waxes A REFRACTOMETRIC STUDY J L L I h i F. JOHVSOX California Research Corp., Richmond, Calif.
T
HE solid-st.ate polymorphism of long-chain n-paraffins has
been investigated by means of x-ray diffraction ( 4 ) , refractive index ( 2 , 1 6 ) , calorimetry (41, and dilatometry (12). I n general, the results show t'hat normal hydrocarbons in the molecular weight range of about C?&p to CBSH7B have tv-o principal cryst,al forms, with a solid-solid t,ransit'ion between room temperature and the melting point. I n some cases, x-ray diffraction apparently has detected more than two crystalline st.ates. Less information is available on commercial paraffin waxes, but other investigat'ors ( 1 4 ) have concluded that waxes exhibit the same type of crystal behavior as long-chain n-paraffins. This study, making use of the refractive index method, proves conclusively that solid-solid transitions take place in commercial WaXeE as well as in pure n-paraifins. The method also allows accurate measurement of the transition temperat,ure range. Because few properties of waxes in the solid stat'e can be measured conveniently and accurately, it is expected that use of the information obtained by this method will lead to a bett,er understanding of the behavior of waxes in the form in which t.hey are used. EXPERI>IENTAL WORK
I n order to measure t8herefractive indiceB of solid waxes in an Abbe refractomet'er, the wax is placed on the measuring prism which has been heated to a temperature above t'he melting point of the wax, and t.he temperatmureis allowed to decrease gradually while readings are taken a t frequent intervals. If the film makes good contact with the prism, refractive indices are measured after the wax has solidified. This method has been used by Seyer and Fordyce (IO)and Kest (16, 1 6 ) for long-chain paraffins and by Page ( 6 ) for &-axes. I n t.he present investigat,ion, temperatures were controlled by a Cenco water bath and measured by a fine wire thermocouple inserted directly betn-een the refractometer prisms. The solid wax is an oriented crystalline solid; therefore, two refractive indices are present for each phase, These lines are polarized a t 90" to one anot'her; hence, a polarizer cap was used on the refractomctmerto sharpcn the boundaries. An ordinary whit,e light source was used. The refractive indices reported are not absolute values, because it was not possible to calculate the corrections for elevated temperature measurements for the refractometer used, and there were insufficient known st'andards available a t these temperatures to calibrat'e the refractomet.er.
The repeatability of the measured refractive indices was about 10.0002 in the liquid state and =k0.0004 in the solid state. With one exception, the traneitions were reversible and the nieasurements were in good agreement between increasing and decreasing temperatures. Temperatures were measured to 1 0 . 1 " F. The n-octacosane and n-dotriacontane were purified by Fontana for use in other work ( 1 ) . The source and properties of the waxefi used are given in Table I. EXPERIMENTAL RESULTS
Refractive index-temperature curves were determined for pure compounds and commercial paraffin waxes. Tjpical graphs are shown in Figures 1 to 4. Individual points are not shown, because the scale is such that they fall within the n-idth of the line. The extraordinary and ordinary lines arising from the orientation of the wax are labeled 7~sand no, respective13 ; lor convenience the high temperature solid phase is labeled phase B, the low temperature phase, phase A . Table I1 compares the melting and transition temperatures determined by refractive index and cooling curves. The refractive indices of mixtures of octacosane and dotriacontane are given in Table 111and a phase diagram is shown as Figure V. I n Table IV the densities of dotriacontane calculated from the refractive index are compared with measured values from the literature. 1.58
I
I
1.56
1.40 i0
NE
,
ab
i
I
100
120.
I
t
I
140,
168
180
i
TEMPERATURE, F.
Figure 1.
Refractive Index z's. Temperature of n-Octacosane
DISCUSSION OF EXPERIMENTAL RESULTS
TABLE I. SOURCE AND PROPERTIES O F WAXES M.P.,
nn, 176O F. Source 1.4299 125/130 A l I P b refined wax sample 1 126.8 1.4263 125/130,AkfP refined wax, ;ample 2 118,4 1.4330 Distillation fraction of 125/130 A M P refined wax 143.8 1.4333 143/150 A M P refined wax, sample 1 E 143.1 1.4325 143/150 AMP refined wax, sample 2 F 160.1 1.4360 160/165 A M P refined wax G 182.2 1.4372C Distillation fraction of 1601165 A M P refined wax a From cooling curves. 5 American melting point. Extrapolated from measurement a t 185O F.
Wax A B C D
F.a 126.6
T h e refractive index-temperature curve for w-octacosane is shown in Figure 1. A sharp discontinuity in the refractive index occurs a t the melting point. Below the melting point, the solid was doubly refracting. At the transition temperature the refractive index increased by about 220 X 10-4 for both lines. The measured melting and transition temperatures agreed well Kith those determined from cooling curves. The behavior of dotriacontane was similar t o that of octacosane. There were sharp breaks in the refractive index a t the melting and transition temperatures. The data checked well
1046
May 1954
INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLE 11. COM'PARISON OF MELTING AND TRANSITION TEMPERATURES DETERMIXED BY REFRACTIVE INDEX AND COOLING CURVES
Melting Point, O F. By cooling By R.I. curve 156.8 156.7 142 3 142.4 162.7 152.7 126.6 126.9 126.8 126.8 118.7 118.4 143.8 143.9 144.4 143.1 159,Q 160.1 182.2 179.4
Materials n-Dotriacontane n-Ootacosane Stearic acid Wax A Wax B Wax C Wax D Wax E Wax F Wax G
Transition Point, ' F. By cooling By R.I. curye 146,7 146.6 127.9 127.2
.....
98-86 107-91 87-76 125-115 123 154-133
.....
Abo;; 98 About 97 About 81-70 120 123
...
...
due to impurities, or it might be a second-order transition. Failure to find first-order transitions does not agree with other investigators. LaTour (S), using x-ray techniques, found in transition a t 127" F. Singleton et al. ( I S ) reported two transitions a t 127' and 95' F., respectively. Schoon ( 9 ) found two different solid structures for stearic acid using electron diffraction, but determined no transition temperatures. There is no obvious explanation for the failure of the refractive index method to detect such transitions.
1.58
I
!
1.56
INDICES OF OCTACOSANETABLE 111. REFRACTIVE
1047
iI
NE
1.54
DOTRIACONTANE MIXTURES
176 170 160
coaane 1.4284 1.4295 1.4315
80 60% Octacosane 1.4289 1.4302 1.4322
49.85% Octacosane 1.4301 1.4313 1.4332
12 10% Octacosane 1,4310 1.4323 14345
155
1.4326
1.4333
1.4343
150 145
1.4330
1.4343 1.5230 1.4725 1.5250 1.4743 1,5270 1.4753 1.5200 1.4779 1 5312 1.4795 1.5558 1.5064 1.5578 1.5083
1.4354 1,5249 1.4740 1.5274 1.4756 1.5284 1.4773 1.5302 1,4790 1.5486 1.4975 1.5571 1.5071 1.5589 1,5090
1.4355 1,5264 1.4752 1,5283 1,4770 1.5529 1,4998 1.5537 1.6008 1.5546 1.5019 1.5555 1.5030 1.5645 1.5131 1.5659 1.5150
Tfmp.,
F.
?%e
1.4348 1.5246 1.4740 1.5265 1.4762 1.5283 1.4783 1.5521 1.5021 1.5622 1.5076 1.5641 1.5095
140 135 130 125 77 68
TABLEIV. Temp.,
F.
185.0 176.0 167.0 157.1 156.2 149.0 145.4 143.6 140.2 131.2 122.4 113.0 104.2 86.0 68 0 Q
b
100%
Dotriacontane 1.4318 1.4330 1.4350 1,5260 1.4755 1.5280 1.4770 1.5550 1.5035 1.5554 1.5045 1.5565 1.5057 1.5577 1,5070 1.5590 1.5080 1.5730 1.5195 1.5754 1.5216
1.40
I
I
I
80
60
I
100
I20
.
I
140
I
.
,
100
160
.
200
TCMPERATURE~~F.
Figure 2.
Refractive Index ws. Temperature of Stearic Acid
Figure 3 is a plot of the refractive index temperature curve for wax C, a distillation fraction of 125,430 AMP refined wax. The behavior was similar to that of normal hydrocarbons, except that the transition took place over a 10' temperature range from 87' to 76" F. Probably the wax has several components that have transitions in this temperature range, thus giving a smearedout type of transition. The refractive index was a convenient was of determining the temperature range of the transition, DENSITYOF LIQUIDAND SOLIDDOTRIACONTANE whereas it was very difficult to measure this by cooling curveB. Density The lack of sensitivity of the cooling curve method explains Refractive Calcd., the discrepancies in Table 11. Index, Experimental Lorentz-Lorenz
State Liquid Liquid Liquid Liquid Solid B Solid A Solid A Solid A Solid A Solid A Solid A Solid A Solid A Soljd A Solid A
nD
(11)
formula"
0,7744 0.7776 0.7807 0.7843 0.8613 0.8773, 0.8796b 0.9151 0.9175 0.9200, 0.9162b 0.9243 0.9290 0.9326 0.9343 0.9386 0.9439
I
*n.
"-I 1.56
Calculated from dag = 0.7840, nn = 1.4353 a t 158' F. Values from (8).
I
I42
1.40
with those of West (167, and the transition and melting temperatures agreed with those measured by cooling curves. The curve for stearic acid (Figure 2) showed no first-order transitions because there were no discontinuities in the solid refractive indices. The liquid line was visible for several degrees below the melting point, indicating impurities in the sample. The elope of the extraordinary ray was constant from the melting point to 141.4' F., a t which temperature the slope of the line changed appreciably, from -0.00050 to -0.00017 per ' F. The dope then remained constant down to room temperature. There was no change for the ordinary line, the slope remaining constant a t -0.00017 per O F. from the melting point to room temperature. This change in slope for the extraordinary line might be
.
,
.
,
I . I ,
.
,
,
I
-
.
I
.
Two whole waxes, A and B, which had been refined from different stocks, in the 125,430 AMP melting point range were measured. Wax A had a transition temperature range from 98" to 86" F., considerably higher than the distilled fraction, but otherwise the curves were similar. Wax B has a broad transition range, 107' to 91" F., but in addition a t about 97" F. (see Figure 4) a third line appeared. The cooling curve for this wax showed a break a t 98.6' F.; it is probable that this third line represents
INDUSTRIAL AND ENGINEERING CHEMISTRY
1048
the appearance of a second solid pliase and that the wax was a mixture of two crystalline solids. Waxes D and E were refined differently, but were both 143/150 AMP refined waxes. The curve for wax E was practically identical with that of a normal hydrocarbon, sharp breaks being observed a t both the transition and melting temperatures, 144.4" and 123" F., respectively. The only difference was that liquid was observed for several degrees below the melting point. Wax
1
I 1.56-
I
NE
I
I54 1.54-
x
.
2 1.52. z 1.50 y 1.50z
5 1.48.
I PHASEe2
No SOLID A
No
U
a
e 1.46-
I
1.44-
l
a
I Ilnuln L QUID
I
I
1.42-
.
1.404
60
, 80
.
, 100
.
I 7
'
120
,
I
.
140
TEMPERATURE,%
Figure 4.
,
t
le0
a solution having refractive indices b e h e e n those of the purc compounds. Below the melting point, the' solid is a solid solution of the B phases of octacosane and dotriacont,ane. This conclusion is based on the fact that only two refractive index linrs are observed. This corresponds t.0 the existence of only one phase, exactly as found for the pure compounds. Absence of a second phase cannot be definitely proved, because it is possil)le that such a phase might exist without giving a sufficient intense line to be visible in the refractomet,er. Below the transitmion point the refractive indices are lower than t,hose of either of the pure components a t corresponding temperatures. This is characterist,ic of the formation of solid solutions that differ appreciai)ly from ideality. X-ray diffraction work by Piper et al. (6) shoivcti that octacosane and dotriacont.ane form solid solutions at rooin temperature. This method of determining phase diagrams T ~ very convenient because only small amounts of sample were rcquired and the measured refract.ive indices indicated the nature of the solid phases. The Lorentx-Lorenx formula was used to calculate solid densities from the refractive indices and a measured density in t#hc liquid state. The refractive index for the sodium I1 linc. V:LS cmnlcu1,zted from the formula. of Pope ( 7 ) .
Y
4
,
I
180
Vol. 46, No. 5
200
Refractive Index TS. Temperature of Wax B
D had a melting point of 143 9" F., but the transition took place over the temperature range of 125' to 115" F. There was a 111stercsis loop in the beginning of the transition, with the values for the refractive index depending on whether the temperature was increasing or decreasing. T h r effect was fairly reproducible; on which such an effect hovw-er, this was the only w a studied ~ was observed.
Tahle IV contains the calculated densities, and for comparison, values measured by other investigators. The agreement ira; within about 1% for the solid densities. The measured temperature coefficients of the refract,ive indices of the waxes in t,he solid states r e r e much higher than normal, being of the same order of magnitude as for the liquids. Thus the calculated coeficirrrtq of espansion would be large compared to other solid materials. SUMMAR1
A
0 MELTING POINT
A TRANSITION P O I N T 155
/ I
Refractive index--temperature plots give a very sensitive met,hod for determining transition temperatures in commercial paraffin waxes. Typical waxes have solid-state transitions sirnilar to t'hose of normal st'raight-chain hydrocarbons in the sarnc molecular weight range. These transitions may occur over :L temperature range of 10' to 15' F. The transition temperatuw and width of the transition region show appreciable differences, depending on the crude source and method of refining the wax. The method is useful for making phase diagrams because only a small amount of sample is required, and the refractive index aids in identifying the solid phaees. Densities for solid waxes may be calculated TI-ith an accuracy within about 1%.
13Du 125
SOLI0 A
120
LITERATURE CITED
20
0
40
80
60
100
PER CENT DOTRIACONTANE
Figure
5.
Phase Diagram of Dotriacontaiie
Octacosane-
TYax F, a refined wax of 160jl65 dMP, had a wide transition temperature from about 133" t o 154O F. It was not possible to decide exactly where the transition began, because the slope of the extraordinary ray began t o change about 10" F. sooner than that of the ordinary ray. A very narrow distillation fraction of wax F totaling about 0.9% of wax F, labeled wax G, was prepared. This wax had no first-order transition, but the refractive index-temperature coefficient changed sharply a t 147-8" F , possibly indicating that a second-older transition occurs. Mixtures of n-octacosane and n-dotriacontane were measured and a phase diagram (Figure 5) was plotted from the measured melting and transition temperatures. Representative data are given in Table 111. From the refractive indices in this table, it ran be seen that above the melting point the two components foim
( I ) Fontana, B. J.,J . P h y s . Chem., 57, 222 (1963). (2) Kolvoort, E. C. H., J . Inst. Petroleum Technol., 24, 338 (19 (3) LaTour, F. D., Snn. phgs., 18, 199 (1932). (4) Mazee, W.hl., Rec. trau. chirn., 67, 197 (1948). ( 6 ) Page, J. RI., Jr., IND.EXG.CHEX, 28, 866 (1936). (6) Piper, S.H., et al., J . Biochern., 25, 2072 (1931). (7) Pope, J . Cltem. Soc. ( L o n d o n ) , 69, 1531 (1896). (8) Roeental, D., Bull. soc. c h i m . B ~ Q 45, . , 585 (1936). (9) Schoon, Th., 2. p h y s i k . C h e n ~ .B39, , 385 (1938). (IO) Seyer. W. F., and Fordyce, R., J . Am. Chem. Soc., 58, 2029 (1936). (11) (12)
Seyer, W. F., and Morris, W. d., IEid., 61, 1114 (1939). Seyer, ITr, F., Patterson, R. F., and Keays, J. L., Ibid., 66, 179
(13)
Singleton, W. S., Ward, T. L., and Dollear, F. G., J . Am. Oil
(1944).
Chemists' Soc., 27, 143 (1950).
Vorlander, D., and Selke, W., Z . physil;. Ckena., 129, 435 (1927). (16) West, C. D., IND. ENG.CHEK, SAL. ED.,10, 627 (1938). (1G) West, C . D., J . Am. Chem. Soc., 59, 743 (1937). (14)
RECEIVED for review September 29, 1953. ACCEPTED Jenuery 11, 1924. Presented before the Division of Petroleum Chemistry at the 124th lleeting of t h e AIIERICAN CHFIMICAL S o c m r u . C'hicago, Ill.
