compounds, particularly the more polar ones, differed considerably from the published values. This was no doubt partly due to the addition of the short collecting column. The catalog nevertheless served a useful purpose in enabling unknowns to be bracketed within certain limits. A number of experiments were conducted to obtain reproducible retention times in eluting known compounds. This required careful timing, particularly during the heating cycle. In cooling, equilibrium is approached after approximately 15 minutes so that essentially a steady state is reached if the column is cooled for 10 minutes or more prior to the 5-minute collection period. The heating curve continues to rise appreciably over a 20-minute period. However, experiments showed that a &minute heating period was sufficient to produce sharp peaks with reproducible retention times and this was the period adopted. Dry ice, rather than liquid nitrogen, was used as a coolant to avoid trapping oxygen, and a mixture of dry ice and organic solvent was avoided because of possible contamination of the sample with solvent vapor. The efficiency of the trapping system was good for most compounds though losses were observed for the Ca and lower hydrocarbons. For quantitative
work this aspect would require further investigation. Altshuller has shown that the sensitivity of the flame ionization detector is sufficient for the direct chromatographic determination of a number of atmospheric pollutants in urban localities (1, 2). However, samples as large as 5 ml. taken in the vicinity of this laboratory failed to show any peaks other than the initial combined methane, ethane, ethylene peak. Thus, in this case, the concentration step proved essential. It would also be useful in collecting samples of reactive compounds away from the laboratory where collection in plastic bags could lead to possible losses. LITERATURE CITED
(1) Altshuller, A. P., Bellar, T. A,, J. A i r Pollution Control Assoc. 13, 81 (1963). (2) Altshuller, A. P., Clemens, C. A., ANAL. CHEM.34, 466 (1962). (3) Amy, J. W., Dimick, K. P., 14th
Pittsburgh Conference on Analytical Chemistry and A plied Spectroscopy, Pittsburgh, Pa., d r c h 1963. (4) Bellar, T. A., Brown, M. F., Sigby, J. E. Jr., ANAL.CHEM.35, 1924 (1963). (5) Bei!ar, T.A., Sigby, J. E., Jr., 144th Meeting, ACS, Los Angeles, Calif., March 1963. (6) Boheman, J., Tanger, S. H., Perrett, R. H., Purnell, J. H., J. Chem. SOC. 1960, p. 2444. (7) Brenner, N., Cieplinski, E., Ettre,
L. S., Coates, V. J., J . Chromatog. 3,230 (1SflO). ~ _ _ _ .
(8) Brenner, N., Ettre, L. S., ANAL.CHEM. 31,1815 (1959). (9) Brown, I., Nature 188, 1021 (1960). (10) Farrington, P. S., Pecsok, R. L., Meeker, R. L., Olson, T. J., ANAL. CHEM.31, 1512 (1959). (11) Hoff, J. E.,Feit, E. D., Zbid., 35, 1298 (1963). (12) Hoff, J. E.,Feit, E. D., Zbid., 36, 1002 (1964). (13) Innes, W. B., Bambrick, W. E., Andreatch, A. J., Zbid., 35,1198 (1963). (14) Klaaus, P.J., Zbid., 33, 1851 (1961). (15) Lovelock, J. E., Lipsky, S. R., J . Am. Chem. SOC.82,431 (1960). (16) Martin, R. L.,ANAL.CHEM.34, 896 (1962). (17) Merritt. C.. Jr.. Walsh. J. T.. Ibid.. ' 34, 903 (1962): ' (18) Raupp, G.,2. Anal. Chem. 164, 135 (1958). (19) Rowan, R., ANAL. CHEM.33, 658 (1961). (20) Schenck, P. A., Eisma, E., Nature 199,170 (1963). (21)Scott, C. G., Phillips, C. S. G., Zbid., p. 66. (22) Suffis, R.,Dean, D. E., ANAL.CHEM. 34, 480 (1962). (23) West, P. W., Sen, B., Sant, B. R.,
Mallik, K. L., Sen Gapta, J. G., J. Chromutog. 6, 220 (1961). (24)Williams, I. H., 142nd Meeting, ACS, Atlantic City, N. J., September 1962.
RECEIVEDfor review June 17, 1965. Accepted September 20, 1965. Investi ation supported by research grant 609-7-54from the Department of National Health and Welfare.
80.
Analysis of Microcrystalline Waxes by Gas-Liquid Chromatography F. JOHN LUDWIG Research laboratory, Pefrolite Corp., St. louis, Mo. The hydrocarbon components in the range CZS to CSSof four microcrystalline waxes have been resolved on dual 2-foot columns packed with SE52 on Chromosorb G, using a programmed temperature dual flame ionization instrument. Distributions of the urea-adductible alkanes were calculated from the peak areas. Average molecular weights which were calculated from these distributions were in good agreement with the measured values. Molecular weight distributions for seven paraffin waxes and one synthetic wax were also obtained.
S
EVERAL QROUPS of
investigators have employed gas-liquid or gas-solid chromatography to analyze paraffi waxes. Scott and Rowel1 (11) separated the C16 to Cla n-alkanes in a refined paraffin wax by gas-solid chromatography a t temperatures up to 390' C. 1732
ANALYTICAL CHEMISTRY
on an alumina column containing 40% by weight of NaOH. Levy, Doyle, Brown, and Melpolder (6),using 8-foot columns packed with 10% microwax distillation residue on Chromosorb W. and column temperatures of 300' C., identified the C20 to C ~ homologs Z of n-alkanes, 2- to &methyl substituted alkanes, and 1-cyclohexyl or 1-cylopentyl substituted alkanes. O'Connor, Burrow, and Norris (9) isolated the n-alkane fractions in two paraffin waxes by adsorption on molecular sieves. Assuming equal thermal conductivities of the n-alkanes between C1s and (282, they calculated the molecular weight distributions of these components from chromatograms which were obtained on a 6-foot column of 20y0 silicone gum rubber on firebrick a t 300' C. The peaks were identified by superimposing those of pure (228, CS,and C32 n-alkanes on the wax chromatogram. Hista, et al. ( 4 ) discovered that high
boiling compounds including n-alkanes , polynuclear aromatic hydrocarbons, alcohols, esters, etc., could be eluted a t temperatures as much as 250' C. below their boiling points on columns containing 0.05% to 0.20/, liquid phase on glass microbeads. Using 2-meter columns of 0.5% SE30 or 0.5% Carbowax 20M on 20-micron glass beads, and a temperature programed from 55' to 315' C. a t a rate of 9O C./minute, Nikelly (8) was able to separate a 30component mixture of the even numbered normal alcohols, alkanes, and alkenes of carbon content 6 through 26 in 30 minutes. By means of relative response factors, he could analyze a Ca to Cle alchohol-hydrocarbon synthetic mixture with a 2 to 4y0 relative error. Dietz, Starnes, and Brown (1) analyzed the Cs to C17n-alkanes in more than 1000 paraffin wax samples with a precision of f1.O. They used a 20-foot column containing 0.5% polyphenyl ether substrate
on 80-mesh glass beads which was programed from 100" to 350' C. at 4" C./ minute. For waxes of high n-alkane content, they found that the weight per cent of each component could be calculated with satisfactory accuracy by assuming equal weight response factors. On their 20-foot glass bead column containing 0.5% SE30, a Cdl hydrocarbon could be eluted a t 300' C. in 1 hour. Thermal conductivity detection was employed in all of the above studies. The utility of the flame ionization detector for quantitative analysis of mixtures was reported by Ettre (d). He concluded that, for mixtures of substance with carbon numbers greater than 6 or 7 , the area per cent can be taken in good approximation as the concentration by weight regardless of the structure of the individual sample components. Levy and Paul (6) obtained the carbon number distributions for n-alkanes, isoalkanes, and cycloalkanes containing 19 to 36 carbon atoms in a 120-124" F. and a 13& 135" F. melting point wax by means of a dual column, dual flame ionization, programed temperature chromatograph. The columns were 12-foot '/s-inch copper tubing packed with 0.82y0 microcrystalline wax distillation residue on 80-100-mesh Diatoport S. The authors suggested that c%-c66hydrocarbons could be eluted if the liquid phase loading were reduced. Perkins, Laramy, and Lively (IO) were able to resolve a homologous series of the even-number alkanes from tetradecane through dopentacontane on 6-foot aluminum columns which were packed with 0.1% Apiezon L on 60-80-mesh glass microbeads and were programed from 40' to 350' C. a t 5.6" C./minute. I n the range Ce to Cz0, the maximum deviation between the peak area per cent and the theoretical weight per cent in two blends of homologous series of alkanes was 1% absolute or less a t each carbon number. All of the analyses which were summarized above were made on paraffin waxes. To date, no studies of the composition of microcrystalline waxes by means of gas-liquid chromatography have been reported. The characterization of a petroleum-derived wax as paraffin or as microcrystalline has been discussed in detail by Ferris (3). Although these terms are not precisely defined, in general micro-crystalline waxes have higher melt'ing points, larger viscosities, and contain higher contents of branched and cycloalkanesubstituted hydrocarbons than paraffin waxes. The initial attempts to separate the microcrystalline wax homologous series by gas-liquid chromatography in these laboratories were made about 2l/2 years ago using a 2-foot x 1/4 inch stainless
Table 1.
Physical Properties of Waxes Studied
No. av. mol. wt.
Ureaadductible,
M.P., a C. adductible hydrocarbons"
Name M.P., a C.a yo Petrolite C-700b 85.8 67 5 75 86.6 Be Sauare 170/175 Whitec 73.3 665 37 81.6 78.8 73.0 610 33 VictGy Amberc 650 25 72.5 60.5 Ultraflex WhiteC Sunoco 5512 67.2 444 5 These melting points were determined by observation of the intensity of the 13.7micron band in the infrared spectrum a t increasing temperatures, as described in reference (7).
* Derived from tank bottoms. Derived from petrolatums.
steel column packed with 2y0 SE30 on 60-80-mesh Chromosorb W, and a single flame ionization system. Because of column bleeding above about 310' C, quantitative analysis of hydrocarbons containing more than about 50 carbon atoms was not possible. However, peaks corresponding to C68 to CW hydrocarbons could still be detected above the background. An API hydrocarbon of 99.99+Y0 purity, n-tetratetracontane, eluted from this column at 265" C. as a single sharp peak with no evidence of cracking and with a zero base line over the entire range of temperature starting a t 50" C. With a dual column, dual flame ionization instrument, stable base lines up to 380" C. could be obtained using these short packed columns which had been conditioned a t 380-390" C. Then, quantitative analyses of the well resolved peaks of the urea-adductible hydrocarbons from microcrystalline wax urea-adductible fractions became possible. The results of these studies are reported in this paper.
EXPERIMENTAL
Apparatus. The F and M Co. Model 700, dual column, dual flame ionization attachment t o the Model 720 gas chromatograph was used. Peak areas were measured by means of a Model 201B Disc Integrator attached to the recorder. The Mechrolab 301A Vapor Pressure Osmometer was used to measure number average molecular weights in toluene a t 65" C. Materials. The microcrystalline waxes are commercial products which are manufactured by the Petrolite Corp., Bareco Division. The Sunoco paraffin waxes, which were provided by the Sun Oil Co., are commercially available waxes. Physical properties which partially characterize these waxes are summarized in Table I. The 99.99+% pure hydrocarbon standards were obtained from J. A. Dixon of the Pennsylvania State University. The sample of 4methyloctadecane and 2-methyl octadecane were purchased from K & K Laboratories, and
are of much lower purity than the API samples. The isoctane solvent was purified by percolation through a 4-foot X 3-inch column of activated silica gel. The urea adducts were prepared as follows: 20 grams of wax was dissolved in a mixture of 200 ml. of ccl4 and 10 ml. of methanol. Urea (130 grams) was added as a crystalline powder. The mixture was refluxed for 4 hours with stirring, filtered through Watman KO.4 filter paper, and the precipitate was washed with boiling CCl,. The filter cake was dispersed in 800 ml. of methanol by vigorous stirring, 800 ml. of water was added, and the suspension was stirred vigorously for 1 hour. The wax was filtered, dried, refiltered a t 100' C. and weighed. Columns. The 2-foot x 1/4-inch stainless steel columns were packed with 60- to 80-mesh acid-washed, silanized Chromosorb G, initially containing 3.Oy0 by weight of SE52. The columns were conditioned in helium by heating for about 10 hours at 300°, 4 hours a t 350', and finally, for 1 hour a t 375' C. Some loss of liquid substrate during this conditioning was apparent, but relatively little additional loss was observed during a wax analysis. The columns had a lifetime of about five months when used daily. Loss of column efficiency was evidenced by tailing of standard hydrocarbon peaks. One end of each column was inserted into the sample injection tube in the instrument in order to inject the solution directly onto the column packing by means of a Hamilton microliter syringe. GLC Conditions. Flame detectors' temperature 370" C. Injection ports (and initial 3 inches of columns) 340' C. Range lo2. Attenuation 1; manually reset to 2, 5 or 10 during the run. Gas flow rates, ml./minute: helium (60lbs. pressure) 70; hydrogen (20-lbs. pressure) 65; air (23-lbs. pressure) 600. Initial oven temperature 130' C. Temperature program rate 3' C./ minute. Sample size: Approximately 2 gl. of hot (65-70" C.) isoctane wax solution, about 2.5 to 3.0 grams of wax per gram of solvent. Molecular Weight Measurements. Volumetric flasks containing weighed amounts of wax and of toluene were placed in a 65' C. constant temperature bath. After solution was complete, samples were withdrawn into VOL. 37, NO. 13, DECEMBER 1965
1733
'C. INCREASING TCUPCRATURE-
Figure 1.
Urea-adductible fraction of
the syringes which are supplied with the Mechrolab 301A Vapor Pressure Osmometer. When the chamber was equilibrated, osmometer AR readings were taken at 65" C. The molecular weights were obtained from calibration curves of AR us. molal concentration using pure n-octadecane and benzil as standards. No concentration dependence was observed. Calculations. The per cent by weight of a given component in the homologous series was taken as the ratio of its peak area t o the summation of the areas of all peaks. The assumption is made here t h a t the results of Perkins et al. (10) are valid for higher molecular weight hydrocarbons. For the urea-adductible fractions or the parafin waxes, the base line between peaks was generally less than five chart units but not exactly zero. Hence, the area of a peak was measured between its points of inflection with the adjacent peaks. In the case of the urea-adductible fractions, this slight background is thought to be due to incomplete resolution of the members of a homologous series rather than to the presence of hydrocarbons of a different structure. To assign a molecular weight to a given peak in the chromatogram of a wax urea-adductible fraction, the wax sample and a mixture of (326, C32,. and CM n-alkanes were injected simultaneously onto the column. In the resulting chromatogram, the three nalkane peaks were superimposed exactly on three of the hydrocarbon peaks, and the number of intervening peaks corresponded to those expected for the homologous series. Then a calculated weight average molecular weight was obtained by summation of the product of the weight per cent and molecular weight of each component. 1734
ANALYTICAL CHEMISTRY
C-700microcrystalline wax,
gas-liquid chromatogram
L
II
-
10
-
9-
8-
7I
-
-
B $ 6
$ 5 -
4-
3-
2-
I-
,
22
I
/ , , 26
,
30
,
,
34
,
,
36
,
, 42
, \ , 46
, 50
, 54
CARBON NUMBER
Figure 2.
Distribution of n-alkanes in Sunoco Paraffin W a x 551 2
1
RESULTS AND DISCUSSION
The separation of the n-alkane homologs in the urea-adductible fraction of the microcrystalline wax C-700 on the 2-foot packed columns of SE52 on Chromosorb G is shown in Figure 1. The peaks which are enhanced when n-dotriacontane and n-tetratetracontane are chromatographed simultaneously with the wax sample are noted as n-Caz and n-Ctc in the chromatogram. For additional reference, the elution temperatures of 12 pure alkanes containing 6 to 44 carbon atoms are given in Table 11. An uncertainty of &lo C. in these temperatures arises from the rather small scale on the thermocouple readout meter. Similar chromatograms were obtained for the urea-adductible components of three other microcrystalline waxes, for seven paraffin waxes, and for one FisherTropsch wax. Good base line stability and the resolution of hydrocarbons containing up t o about 65 carbon atoms was achieved. Calculations of the molecular weight distribution from such chromatograms were made by the method described in the Experimental Section. These results are discussed in the following sections. ParafEn Waxes. The distribution of the straight-chain hydrocarbons in Sunoco wax 5512 is shown in Figure 2. Similar regular distributions were observed for six other Sunoco paraffin waxes of melting points 53" to 67" C. and molecular weights in the range 350 to 450. From 15 to 35 n-alkane components were present in these waxes in amounts greater than about 0.1%. The maxima in these distribution curves occurred in to CS4region. Small amounts of branched chain or other nonlinear hydrocarbons, which
c&
were not completely resolved from the n-alkanes on such short columns, were detected. Because the n-alkane peak areas could not be satisfactorily corrected for the contributions from these components, the calculated molecular weights were from 10 to 30 units larger than the measured values. These data were obtained to compare the molecular weight distributions in paraffin waxes with those in microcrystalline waxes, More accurate analyses of paraffin waxes have been obtained by the use of a longer column since higher elution temperatures could be used (1). The chief objective of this study, however, was the analysis of microcrystalline waxes for which short columns are necessary to elute hydrocarbons which contain 65-70 carbon atoms.
Table II.
GLC Elution Temperatures of Pure Alkanes
Microcrystalline Waxes. UNFRACWAXES. The chromatograms of the waxes melting in the C. range were distinctly 80-90' different than those of the waxes melting between 60-75" C. T h e lower melting waxes appear t o contain two, or possibly more, homologous series of hydrocarbons. One set of homologs eluted between about 190" and 300°, while the other eluted in the range about 230" to 380" C. Considerable overlap of peaks was observed between about 250' and 270" C. Because of a high background, probably due to unresolved peaks, molecular weight distributions could not be calculated. The higher melting waxes appear to consist predominantly of one homologous series. The peaks were well resolved, but above about 290" C. the base line was 10 to 15 chart units above zero, probably because of contributions from the nonlinear hydrocarbons. Even with this background, the average molecular weight of 695 which was calculated
Compound Temp., C.b +Hexane 35 %-Octane 46 n-Decane 56 n-Dodecane 76 93 n-Tetradecane n-Hexadecane 111 ZMethylheptadecane" 127 Octadecane 134 2-Methyltric~sane~ 175 n-Hexacosane" 199 n-Dotriacontane" 242 n-Tetratetracontane" 304 These hydrocarbons of 99.9+ % purity were obtained from J. A. Duton and were prepared under API Research Project 42. The other samples were commercial products. Initial oven temperature 30" C. program rate 3" C./minute.
TIONATED
from the distribution in (2-700 was only slightly larger than the measured value of 675. Similar results for the calculated molecular weights were obtained with two other higher melting microcrystalline waxes. More definite information about the distribution of hydrocarbons in microcrystalline waxes was obtained by gas-liquid chromatography of fractions which were separated by urea adduction. The urea-adductible contents and melting points are summarized in Table I. NONADDUCTIBLE FRACTIONS.Two types of chromatograms were obtained for the fractions of microcrystalline waxes which did not form urea complexes. Those from the lower-melting waxes contained two sets of overlapping peaks emanating from a large background which increased as the temperature was increased. I n contrast, the nonadductible hydrocarbons of the higher-melting waxes gave a single
-1 7
3
Figure 3. Distribution of urea-adductible hydrocarbons in C-700 microcrystalline wax
Figure 4. Distribution of urea-addudible hydrocarbons in Ultraflex White microcrystalline wax VOL. 37, NO. 13, DECEMBER 1965
1735
Table 111. Comparison of the Urea-Addudible Hydrocarbon Molecular Weights Calculated from GLC Distributions with Measured Values
Wax urea-adductible No. of Average molecular weight fraction detns. Calcd. Measured 4 663 f 1" 660 C-700 Be Square 170/175 4 605.5 f 0.5 603 Ultraflex White 3 572 f 4 570 Victory Amber 2 561 1 576 a The value 663 f 1 for (2-700, for example, indicates that in all four determinations the calculated molecular weights were in the range 662 to 664.
ity of these calculated weight average values and the agreement with the measured number average molecular weights are very good. These results indicate that the distributions must be approximately correct. The difference in magnitude between a weight average molecular weight M , and a number average molecular weight M , can be calculated for mixtures using the equations :
11.1 -~
z Wi
- Z(WiMi)
Table IV. GLC Elution Temperatures of Normal and Methyl Branched Alkanes
Elutifn Compound temp., n-Heptadecane 120 2-Methylheptadecane 123 n-Octadecane 127 2-Methyloctadecane 132 4Methyloctadecane 134-136* n-Nonadecane 134-136' a Initial oven temperature 90' C. Program rate 2' C./minute. * Broad unresolved peak. series of peaks on a slightly rising background. Accurate calculation of the GLC weight distributions was not possible in either case. Identification of the structures of these nonadductible components is difficult because of the lack of pure, high-molecular-weight, branched-chain alkanes. UREA-ADDUCTIBLE FRACTIONS. Coincidence of three of the peaks in the chromatogram of the urea-adductible fraction with those of the c26, Caz, and Cd4n-alkanes was observed for each of the four microcrystalline waxes. About 40 to 45 components representing the
homologs between about Cna to Cm were distinguished. The per cent by weight of the urea-adductible hydrocarbons in C-700 and in Ultraflex White as a function of carbon number is shown in Figures 3 and 4, respectively. To show the extent of the variations observed in the weight per cent, data for three different runs are plotted in Figure 3. Maximum differences of about 10% relative in a given value of the weight per cent were found, depending upon the resolution of the peaks. The irregularities in these distribution curves which appear in the vicinity of C-40 are thought to be real. These waxes undergo several processing steps including high-temperature distillation. In contrast, the distribution in a FisherTropsch synthetic wax was completely regular throughout the range C27 to c 6 7 with a maximum of about 4.8% a t C-40. In addition, as shown in Figure 2, no such anomaly was observed in the paraffin wax GLC distributions. The average molecular weights of the four urea-adductible fractions which were calculated from the GLC distributions are compared with the measured values in Table 111. The reproducibil-
M,
2 WiMi
= -
z Wi
where Wi is the weight of the ith component of molecular weight Mi. For the range of molecular weights in which the wax hydrocarbons fall, namely 400 to 900, the number average and weight average molecular weights which were calculated for arbitrary 5-, 9- or 10-component hypothetical mixtures differed by only above 3%. It would appear, a t first thought, that the GLC data could be used to draw some conclusions about the structural types4.e. normal or branched hydrocarbons-in the ureaadductible fractions. There are several results, however, which leave the conclusion that the urea-adductible alkanes are straight-chain compounds open to considerable doubt. Terres and Kath Sur ( l a ) observed that urea adduction can occur in hydrocarbons which have a methyl substituent in the 2, 3, 4, or 5 position if they contain linear chains of 11, 11, 15 or 16 carbon atoms, respectively. They also found that cyclic hydrocarbons which have linear side chains of more than 18 carbon atoms form urea adducts. Hence, the urea-
'C. INCREASING TEYPERATURE-
Figure 5.
1736
Urea-adductible fraction of Ultraflex White and mixture of pure alkanes, gas-liquid chromatogram
ANALYTICAL CHEMISTRY
adductible fractions of the microcrystalline waxes could include methylbranched or cyclic hydrocarbons as well as n-alkanes. To compare the elution temperatures of 2- or 4-methyl substituted hydrocarbons with those of n-alkanes on the 2-foot columns, a known mixture of straight chain and branched alkanes was chromatographed. The results are presented in Table 1V. The 4-methyloctadecane and its straight chain isomer, n-nonadecane, were not resolved. The elution temperatures of the %methyl substituted alkanes did not coincide with those of either the isomeric nalkane or the normal hydrocarbon containing one less carbon atom. To obtain further information, a mixture of 2-methyltricosane, n-hexacosane, ntetratetracontane and the urea-adductible hydrocarbons of Ultraflex White was separated on the 2-foot columns. The elutions of the n-Cs2 and n-Cu alkanes coincided with those of two of the wax components. The 2-methyltricosane, however, eluted a t a temperature between those of two wax homologs, as is evident from Figure 5 which, like Figure 1, is a photographically reduced reproduction of the original chromatogram. All of the above results were limited to hydrocarbons of molecular weights much lower than those of the major components in microcrystalline waxes.
Extrapolation of those results to higher molecular weight ranges is of questionable validity. Therefore, because of the lack of Ca-Cw branched chain hydrocarbon standards, the urea-adductible hydrocarbons of microcrystalline waxes can not be identified definitely from GLC data as n-alkanes, 2-, 3-, 4-, or &methyl substituted alkanes or longchain substituted cycloalkanes. Other techniques then, such as I R and NMR spectrometry, x-ray diffraction, etc., must be employed to elucidate the structures of the urea-adductible hydrocarbons. Measurements of the methyl and methylene absorbance ratios of these fractions and of pure n-alkanes in the molten state have been made (7). These results suggest, although do not conclusively prove, that the ureaadductible hydrocarbons in the highermelting microcrystalline waxes such as (3-700 are normal alkanes, and the adductible fractions in the lower melting waxes such as Ultraflex White contain hydrocarbons with a single methyl branch near the end of a long chain of methylene groups. Definite proof, however, of the structure of these hydrocarbons may not be achieved until pure, high molecular weight-e.g., C3,,-Cso-branched-chain and cycloalkyl substituted alkanes become available for chromatographic and spectrometric studies.
ACKNOWLEDGMENT
The author thanks 0. W. GrifEn and C. W. Holyoke for technical assistance, J. A. Dixon, Director of API Research Project 42, Pennsylvania State University, for samples of pure hydrocarbons, and F. E. Mange and W. J. Heintzelman for helpful suggestions in preparing this report. LITERATURE CITED
(1) Dietz, W. A., Starnes, P. K., Brown,
R. A,, Tech. Assoc. Paper Pulp Ind., Spec. Tech. Assoc. P u b l . 2, 33 (1963). ( 2 ) Ettre, L. S., J. Chromatog. 8, 525 (1962). (3) Ferris, S. W., Tech. ASSOC., Paper P u b Ind.. Svec. Tech. Assoc. Publ. 2, i ( i 9 6 3 j . * (4) . , Hista. C.. hlesserlv. J. P.. Reschke.
R. F., Fredericks, D."H.lCooke, W. D.; ANAL.CHEM.32,880 (1960). (5) Levy, E. J., Doyle, R. R., Brown, R. A,. hfebolder, F. W., Zbid.. 33. 698 (1961 j. (6) Levy, E. J., Paul, D. G., Ninth National Instrument Society of America, Analysis Instrumentation Symosium, April 29-May 1, 1963. (7P Ludwig, F. J., AXAL.CHEM.37, 1737 I
.
(1965). (8) Njkelly, J. G., Zbid., 34, 472 (1962). (9) 0 Connor, J. G., Burrow, F. H., Norris, M. S., Zbid., 34, 82 (1962). (10) Perkins, G., Jr., Laramy, R. E., Lively, L. D., Zbid., 35, 360 (1963). (11) Scott, C. G., Rowell, D. A., Nature 187, 143 (1960). (12) Terres, E., Nath Sur, S., BrennstoflChem. 38, 330 (1957). RECEIVEDfor review July 6, 1965. Accepted October 13, 1965.
Analysis of Microcrystalline and Paraffin Waxes by Means of Infrared Spectra in the Molten State F. JOHN LUDWIG Research laboratory, Pefrolife Corp., St. louis, Mo.
b The orthorhombic-hexagonal and hexagonal-liquid phase transition temperatures in paraffin waxes can be determined from the changes in intensity of the methylene chain rocking absorptions at 13.7 and 13.9 microns. Microcrystalline waxes show only a single solid-liquid transition. Microcrystalline waxes can be differentiated from paraffin waxes both from changes in the infrared spectra at increasing temperatures and from the magnitudes of the absorbance ratios of the methyl and methylene groups in the hexagonal crystalline state and/or in the molten state. Conclusions about the branching of the alkane chains in the ureaadductible fractions of microcrystalline waxes may be derived from a comparison of their adsorbance ratios with those of pure normal alkanes.
A
of different physical methods have been suggested for classifying petroleum wax as a paraffin wax or a microcrystalline wax (5). These include melting point, congealing point, boiling range, viscosity, molecular weight, index of refraction, crystal size, cooling curve, and urea reactive content. Infrared spectra of solid wax films, hob-ever, have not been included among these methods. This may be attributed to the fact that the CH2 deformation and rocking modes a t 6.8 and 13.9 microns, respectively, exist as doublets which depend upon the crystalline structure of the hydrocarbons (9, 1 6 ) . In solution or in the liquid phase, single bands are observed a t these wavelengths (1). The term molten state is used in this paper to refer to the liquid hydroNUMBER
carbon phase in order to avoid confusion with solution in a solvent. Broadhurst (2) has found that the solid phases of all n-alkanes with more than nine carbon atoms in the chain can be described in terms of four crystal structures : monoclinic, triclinic, orthorhombic, and hexagonal. In the evennumbered n-alkanes from CzZ to Car and in the odd-numbered n-alkanes from Cg to (243, the hexagonal state is stable a t temperatures just below the melting point. The orthorhombic state is the stable low temperature phase of odd n-alkanes above C9and, probably because of impurities, is the phase generally observed for all n-alkanes above about Cd0. The triclinic and monoclinic structures have been observed as stable low-temperature phases in the pure VOL. 37, NO. 13, DECEMBER 1965
1737
