Coefficient of Volume Expansion for Petroleum axes and Pure n- araffins P. R. TEMPLIN iMellon I n s t i t u t e , Pittsburgh, Pa.
I
T HAS become increasingly important in recent years to obtain fundamental knowledge concerning hydrocarbons of high molecular weight, which occur naturally as the waxy constituents of petroleum. Waxes have numerous commercial uses which depend on temperature-sensitive properties of a functional nature, such as tensile strength, sealing -- c strength, flexibility, expansion, 'L and dielectric strength. These properties show marked differA - Sample Bulb ences for the various crystalline B . Glass Seal C . Volume Calibrated m o d i f i c a t i o n s of the hydrocarCopillory Stem bons, and in the transition zone3 the properties change very rapidly. For many waxes these regions of transition occur near room temperature. One very specialized use of narrow wax fractions and pure hydrocarbon comA pounds takes advantage of the M large volume change a t the transition point to operate electrical 8 switches for controlling temperature. Information from x-ray studies by Mueller (14, 16), Piper and Malkin (IT),and others has conFigure 1. Dilatomtributed a great deal toward a eter better u n d e r s t a n d i n g of t h e changes in crystalline structure which occur over narrow temperature ranges for these n-paraffins. Density changes, solid transitions, and Crystalline forms Of waxes have been studied by many investigators making use of data obtained from density measurements, cooling cuwes, and microSCOPiC observations (3, 10, 11, 13, 20)by Car??euter (2) on the expansion of certain waxes and mixtures of narrow %Taxfractions produced very interesting data which emphasized the need for further investigation of the large volume changes that occur from the solid at room temperature t o the liquid phase. A more 13000 recent study by Koch and Concetta ( 8 )stresses the effect of composition on the expansion of binary mixtures TT hich include animal, vegeI2500 E' table, and mineral waxes. The volume meas9;I 2 0 0 0 urements obtained for these waxes were made at 10' C. intervals in the temperature range of t 25' t o 80' C. Seyer, Patterson, and Keays ( 3 2 ) 2 I1500 > measured the densities and located the transition points of the even-numbered h)drocarbons 000 from n-CleHsrto n-CarHioand the odd-numbered 20 compound n-C29HF0,and gave density curves 68 for these compounds for the temperature range of 0" to 90' c.
The present paper reports thermal expansion data for a number of petroleum waxes (paraffin and microcrystalline) and for one semipure and six pure n-paraffin hydrocarbons. The data include the total expansion associated with solid-solid and solid-liquid phase changes, the coefficient of volume expansion for each phase, freezing and transition temperatures, and the specific volume at these temperatures. Several correlations of volume data M-ith carbon number and limited data shoaing the effect of pressure on the freezing point of n-Cz, are also reported. Specific volumetemperature curves which are continuous from the liquid phase to below the solid transition are shown for each sample investigated. Similar data on five of the n-paraffins have been reported by Seyer, Patterson, and Keays ( $ 8 ) ; however, many of the expansion data are not given in the earlier report. One of the chief advantages derived from the use of these particular samples (API Research Project 42) for this work is that a considerable amount of data on physical properties of these same compounds has already been obtained and reported, which increases the value of additional data by virtue of the correlation powibilities. APP 4UATU 9
h dilatometer of the type shown in Figure 1 was used to measure the volume expansion of the sample. As mercury was the indicating fluid and the sample was in the liquid state part of the time, the inverted position of the bulb was necessary. The stem was made from 2-mm. precision-bore tubing which was volume-calibrated before it was sealed t o the sample bulb. The calibration vias made by determining the volume at each calibration point from the weights of small quantities of mercury withdrawn from the tube and density data for mercury. The molten sample was added to the bulb, before the stem was attached, by injection from a hypodermic needle and glass syringe. -4fter the sample was degassed thoroughly and nhile it was still at the degassing pressure ( < 1 micron), mercury was added to the bulb frcm a side-arm tube attached to the sample bulb and vacuum line. The dilatometer '(vas placed in a thermostated bath xvhich !vas temperatule, -+o,oo20,I-, for capable of maintaining a a considerable length of time. Temperature changes In the bath were measured with Beckmann differential thermometers, and an accurate temperature value at the start of the measurements was obtained with a platinum resistance thermometer. The measheight of the mercury column in the calibrated stem ured to 0.1 mm. by means of a vertical-type cathetometer 157ith a vernier adjustment.
I
0.
I
25
77
30 86
35
P5
43 104
45Tempermure, ' C 55 113Temperatvre 'F 131
60
65
70
75
140
149
158
167
Figure 2. Volume expansion of Gulf waxes
154
80 176
INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1956
155 I
PROCEDURE
Starting at approximately 10' C. above the melting point of the sample, equilibrium volume-temperature values were obtained at numerous points on the temperature scale down to about 10" C. below the temperature of transition from one crystalline form to another. The process was then reversed and equilibrium readings were continued as the temperature was raised, by similar increments, until the sample was again liquefied. At each temperature setting, hourly readings were taken until the level of the mercury in the dilatometer became constant. The total volume change was then found from the product of the mercury level change and the volume of the tube per unit length. The latter was an average value which made it necessary to compensate for the nonuniformity of stem bore by the use of a correction curve prepared by calibration in the manner described by Bekkedahl ( 1 ) . Corrections to compensate for the expansion of glass and mercury were then applied to give a final value representing the volume change of the sample alone The reliability of the method was checked by measuring the volume expansion of distilled water which had a solids content of < 1 p.p.m. and a conductivity value of 4 X 10-8 reciprocal ohm. A comparison of the observed data with specific volume data for water, taken from the Smithsonian tables, indicated that the method was accurate t o about 1 part in 10,000.
Table I. Sample Type grade
Inspection Data on Wax
60' E'. Viscositv a t 210" F.
0.7531 3 19 36 9
501&a% load 13 150-gram load 33 ASTM melting point, F. (D-87) 124 0
(D-721)
Molecularweight
I
1
and observed values indicated that sufficient time had been allowed for the sample to reach equilibrium. RESULTS
Petroleum Waxes. Starting with a reliable value for the liquid density of each wax sample, specific volume-temperature curves (Figure 2) were plotted from data obtained by the method described above. The paraffin wax curves are similar. Each one shows a sharp break a t the freezing point and a significant volume change, representing a change in crystal form, about 10" C. below the freezing point. The curve representing the volume expansion of microcrystalline wax shows no solid transition, and the phase change from solid to liquid covers a much greater temperature range than in the case of the paraffin wax. Although no solid-state transitions are apparent, there may be a continuous series of these transitions which fall within and are
Table 11. Physical Properties of Waxes 56.0 0.7547
56.1 0 7543
55.8 0.7555
46.6
3 22 37 0
3 51 37 9
3 59 38.1
12 59 68 6
9 28
6 19
5 18
18 36
131 8
134 3
143.8
0 34 376
0 22 377
0 36 387
8 52 587
127 6 0 48
358
(From expansion measurements)
0 7945
m-5)
Oil content, wt. % '
of these hydrocarbons
Microcrystalline Petrowax 125Amp 128Amp 133Amp 136Amp A
(DrV445)
Centistokes Sayboltseconds Penetration a t 77' F.
. . . here are basic data for a variety
Refined Paraffin
Test results Gravity (D-287) " A P I a t 2 1 0 ° F . 56.4 Specific gravity,
210' F./
Commercial uses of petroleum waxes develop from their temperature-sensitive properties
The point at which crystallization first occurred could be determined very precisely, as it was possible to maintain the sample in a delicate state of balance between freezing and melting for long periods of time. At this point, a decrease in temperature of only several thousandths of a degree resulted in crystal formation, whereas a similar rise in temperature caused the crystals to disappear. Above the freezing point and below the solid transition, the volume change with respect to temperature was small, making it possible to change the temperature by increments of about 1' C. without sacrificing the accuracy of the measurement. A temperature change of this magnitude, in the regions mentioned, required only about 2 to 4 hours for the sample to reach thermal equilibrium. When the sample wm in a transition state-i.e., solid-solid or liquid-solid transition-2 days or more were required to reach equilibrium for a temperature change of 0.01' to 0.02' C. When the sample was in such a transition state, the time to reach equilibrium was longer for a temperature decrease than for a comparable increase in temperature, As a mathematical check to determine if the values obtained actually were equilibrium values, the Guggenheim (6) method was used to calculate a number of equilibrium values from observed volume-time data. The very good agreement between calculated
Sample Refined Paraffin Type grade 125 Amp 128 Amp 133 Amp Crystalpoint, 'C. 51.6 53.2 55.9 Transition point, C. 34,O 35,5 38.4 AT required for complete phase change, C. At fusion 8.6 8.5 8.8 At transition 10.5 10.3 11.4 Expansion Coefficient,cc.1 g.(deg. Liquid 0.0011 0.0011 0.0010 Solid form A 0.0016 0.0014 0.0013 Solid form B 0,0010 0,0009 0.0008 Nonisothermal exoansion b At fusion cc./g. 0.1228 0.1237 0.1264
%
At transition cc./g.
%
Solid form B at transition
At fusion cc./g. T? At [ransition cc./g.
%
Solid form B to liauid a t crystal pdint cc./g.
%
Specific volume, cc./g. At crystal point At transition point a
10.6
10.7
11.0
136 Amp 56.8 40.5
7.7 12.0 0.0010 0.0014 0.0008
iMicrocrystalline Petrowax A
74.OU
...
35.0
...
0.0010 0 .'do09
0.1262 0 1372 11.0 12.5
0.0356 3.2
0,0369 3.3
0.0387 3.5
0.0390 3.5
...
0.1059 9.0
0.1119 9.6
0.1149 9.9
0,1150 9.9
0.1069 9.5
0.0252 2.3
0.0276 2.5
0.0300 2.7
0.0298 2 7
.. .
0.1436 0.1483 0.1536 0,1548 12.7 13.1 13.7 13.8
,..
0.1069 9.5
1.2796 1.2789 1 , 2 7 8 2 1 .2355O 1.1430 1.1413 1,1402 ... Values taken a t point of deviation on curve resulting from crystallization 1.2780 1.1420
of liquid sample. Exact point a t which crystals first appear is several degrees higher but extremely difficult to observe because of crystal size and hi h color of this microcrystalline wax. Expansion as it occurred over a temperature range (nonisothermal expansion) compared with expansion had it taken place entirely a t one temperature (isothermal expansion).
5
156
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 48, No. 1
solid transition, this difference may be due almost entirely t o the lack of a solid transition in the microcrystalline wax. I n order t o present the data in a more useful form, the values for specific volume were changed to gravity values, ".4PI. As the paraffin wax curves were similar in shape and very close t,ogether, a set of average gravity values was used to plot a curve representing all the paraffin waxes tested. From these data and from calculated conversion data, several curves were plotted (Figure 3), from which a variety of information can be obtained starting with a single observed liquid gravity Ft! per Lb x lo4 81 1 l 1 I : l I I I I I I I I l l / value. For example, from a given 45 50 55 60 liquid gravity value, the gravity of Lb, per Ft'. AI I I I 1 1 1 I l , l I l I ! I l I 1 1 the solid and the storage space 0 50 100 153 200 required for the material can Temperature, "F readily be found from this chart. Figure 3. Conversion curves for refined paraffin waxes Most of the standard test methods, designed t o measure functional propertiee of paraffin wax, specify a testing temperature of about i o " F. This temperature is slightly below the region of transition for paraffin waxes. A higher testing temperature would he most unsatisfactory, as in the transition region most of the functional properties change very rapidly. n-Paraffins. Expansion measurements were made on a semipure sample of n-G8 and on six n-paraffins of extremely high purity obt,ained from API Research Project 42. These samples were all from the even-carbon n u m b e r serics. I n d i v i d u a l volume-temperature c u r v e s f o r these compounds are given in Figures 4 to 10; the curves for the six pure materials are shown together in Figure ll. I summary of the data used for plotting these curves appears in Table 111. A comparison of the curves in Figures 4 and 8 indicates that masked by the long solidification range. The volume of paraffin hoth the impure and pure samples of n-CpBhave sharp freezing nax decreased very sharply a t the point where crystals first points and transition points. The temperature difference appear; however, the break in the microcrystalline wax curve betncen these points for the impure compound is more than comes several degrees below the crystallization point and the twice that for the pure material, although it is not so great as break is not very sharp. The differences between the two wax the difference shown by paraffin TTax (Figure 2). This effect tJpes can be accounted for in part by the high oil content of the of impuritv on the transition point is in accord with the findings microcrystalline wax, the greater molecular weight range of this of other investigators ( I d , 18). A comparison of the phypical way, nnd the fact that more than half of this material is composed properties obtained bv the volume expansion measurements is of compounds other than n-paraffins. given for the pure and impure n-octacosane samples in Table IV. Inspection data for these wax samples are given in Table I, and a summary of the physical properties determined from the The data show that the presence of impurity has the ability to decrease the expansion PT hich takes place a t both the melting expansion measurements is given in Table 11. It is seen from point and transition poini. A trend of this nature can be obthe latter table that whereas the coefficients of expansion are served from the expansion values fouiid in Tables I1 and IV as approximately the Fame for the two types of u-ax, the total expansion of the microcrystalline wax is less than that of the the impurity concentration increases in going from pure t o impure n-CLsand finally t o paraffin wax which contains only a minor paraffin wax. Furthermore, a3 the expansion difference is approyiniately equal to the volume change of the paraffin wax a t the amount of n-Czs.
157
INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1956
Table 111. Volume Expansion of n-Paraffins n-Cza
n-Czr
(PSC KO.540) Temp,, Sp. vol., cc./g. c. 1.2987 48.11 1.2980 47.49 1.2974 46.98 1.2971 46.72 1.2958 45.55 1.2946 44.51 1.2934 43.55 1.2952 45.04 1.2977 47.21 1.2982 47.64 1,2958 45.48 1.2939 43.7s 1.2968 46.43 1.2977 47.27 1.2985 47.86 1.2985 47.97 1,2997 48.94 1.3007 49.89 1.3012 50.29 1.3017 50.77 1 ,3032 51.97 1.2937 43.57 1.2915 41.74 1.2900 40.28 1.2882 38.88 1.2870 37.63 1.2860 36.77 1.2854 36.28 1.2850 36.00 1.1068 35.95 1.1035 35.87 1.0892 35.39 1.0872 35.24 1.0832 34.81 1.0799 34.09 1.0788 33.75 1.0776 32.11 1.0767 30.51 1.0770 31.35 1.0766 33.15 1,0747 30.16 1.0754 31.62 1.0758 32.68 1.0776 33.55 1.0811 34.52 1 0947 35.68 1,1099 36.00 1,1225 36.12 1.1471 36.24 1.2078 36.35 1,2855 36.41 1.2864 37,20 1.2881 38,68
Table IV.
(PSC No. 541) Temp., Sp. vol., C. cc./g. 1,2936 59,61 1 2925 58.65 1,2907 57.12 1.2885 55.93 1.2890 55.48 1,2886 54.86 1.2876 53.97 1,2866 53.19 1 .2863 52.92 1.2857 52.26 1.2849 51.54 1.2846 51.19 1.2837 50.34 1.2186 50.25 1,2092 50,22 1.1974 50.16 1.1843 50.05 1.1675 49.63 1,1583 48.76 1.1553 48.11 1.1531 47.49 1,1516 46.98 1,0908 46.72 1.0899 45.55 1.0891 44.51 1.0886 43.55 1.0894 45.04 1.0906 47.21 1.0895 47.64 1.0875 45.48 1.0866 43.75 1.0881 46.43 1.0884 47.27 1.0887 47.86 1.1299 47.97 1.1550 48.07 1.1593 48.94 1.1746 49.89 1.2380 50.29 1.2843 50.77 1 2856 51.97
Temp., 0
c.
61.13 61.11 61.07 61.04 59.93 59.16 58.11 57.61 57.44 56.86 57.68 57.88 58.04 58.60 59.52 60.21 60.60 61.79 63 08 59.94 58.33 56.50 55.97 55.94 55.92 55.89 55.87 55.84 55.79 55.69 55.48 54.90 53.81 52.11 53.27 52.70 51.89 51.15 50.30 48.37 46.64 47.68 49.39 50.57 51.31 50.19 45.66 48.75 50.75 47,69 45.73 46.77 48.24 50.18 51.55 51.76 I
n-Cns (PSC No. 106) Sp.vol., Temp., 0 c. cc./g. 51.99 1.2858 52,28 1.2857 52.54 1.2857 52.74 1.2857 52.84 1.2846 52.95 1.2836 53.07 1.2820 53.20 1.2813 53.37 1.2810 53.63 1.2802 53.83 1,2816 54.12 1,2819 54,61 1.2821 55.09 1.2831 55.21 1.2842 55.31 1.2850 65.42 1.2855 55.45 1.2866 59.61 1,2881 58.65 1,2845 57.12 1.2825 55.93 1.2799 55.48 1,2790 54.86 1.2570 53.97 1.2421 63,19 1.2326 52.92 1,2244 52.26 1,2154 51.54 1,2056 51,19 1.1900 50.34 1.1747 50.25 1.1593 50.22 1.1519 50.16 1.1466 50.05 1.1493 49.63 1.1477 48.76 1.0766 46.98 1.0757 45.55 1.0746 44.51 1.0733 43.55 1.0722 45.04 1.0729 47.21 1.0738 47.64 1.0736 45.48 1.0724 43.75 1.0710 46.43 1.0686 47.27 1,0703 47.86 1.0700 47.97 1.0682 48.07 1.0671 48.94 1.0677 49.89 1.0684 50.29 1.0695 50.77 1.0699 51.97 1.0699
Expansion of Pure and Impure n-Octacosane Semipure
Crystal point, ' C. Transition point, C.
AT " C .
E x p a h i o n coefficient, cc./deg./g. Liquid Crystalline form A Crystalline form 13 Volume change, cc./g A t fusion At transition
Pure n-Cze 61.1 58.1 3.0
n-Cza
61.1 54.1 7.0
0 0011 0.0020 0.0007
0 0010 0 0025 0 0011
0.1329 0.0774
0 1245 0 0534
The expansion curves for the n-paraffins are very similar in character; however, several differences should be pointed out. n-Cz4, n-CZ6, n G 3 ,and n-C32 (Figure 11) indicate that a solid first-order transition takes place a few degrees below the freezing point. n-Cp0 gives no transition point of this nature, and n-C36 shows two of these solid-state transitions. In spite of these differences, however, the volume change from the liquid state to the low-temperature solid form was very nearly the same regardless of whether there was one, two, or no solid transition at all. This information suggests that the A form of the solid does not exist for the n-C%oand that it freezes directly into the B or low-temperature solid form. It is possible that the A solid exists momentarily during the early stages of crystallization and then almost immediately changes to the B solid; how-
n-Czs
(PSC No. 176) Sp. vol., cc./g.
Temp.,
1.0699 1.0700 1.0707 1.0815 1.0933 1.1083 1,1250 1.1487 1.1491 1.1500 1.1506 1,1518 1.1549 1.1608 1.1633 1.1665 1,1705 1.1738 1.2842 1.2831 1,2817 1,2804 1,1757 1.1580 1,1512 1.1486 1,1479 1,1463 1.1447 1.0776 1.0770 1 0761 1.0756 1.0753 1.0750 1.0747 1.0743 1.0732 ' 1.0725 1.0721 1.0714 1.0722 1.0733 1,0734 1.0725 1.0714 1.0727 1.0732 1.0735 1.0734 1.0731 1.0736 1.0742 1.0742 1.0743 1.0748
70.64 69.49 69.20 69.07 68.77 68.22 67.24 66.18 65.27 64.73 64.67 63.59 62.35 61.44 64.46 64.70 65.05 65.39 66.74 69.32 69.13 65.83 64.86 65.03 64.41 61.13 61.11 61.07 61,04 60.93 61.01 60.98 60.95 60.92 60.89 60.86 60.83 60.79 60.46 59.93 59.16 58.11 57.95 57.79 57.67 57,61 57,44 56.86 55.52 53.04 54.39 56.20 57.68 57.88 57.99 58.04 58.11 58.60 59.52 60.21 60.60 61.01 61.06 61.79 63.08
c.
Sp. vol., cc./s. 2936 2924 2921 2919 2917 2911 2899 2889 2881 2874 2871 2861 2848 2839 2873 2877 2879 2882 2899 2925 2922 2888 2875 2877 2870 2838 2837 2100 1923 1668 1799 1738 1685 1648 1629 1607 1595 1581 1527 1496 1475 1454 1449 1445 1448 0722 0645 0671 0660 0645 0651 0661 0682 0715 0861 1129 1451 1460 1482 1507 1541 1937 2483 2846 2860
n-Ca? (PSC No. 157) , Temp., Sp. vol., c. cc./g. 2988 88,59 2978 84.63 2970 83.79 2965 83.03 2958 82.36 2944 81,12 2923 79.04 2911 78.18 2901 77.13 ,2896 76.80 2890 76.11 2890 75.84 2885 75,47 2877 74,90 ,2869 74.20 2864 73.65 2859 73,05 2823 69.55 ,2889 75,84 ,2887 75.46 ,2851 72.18 ,2845 71.92 ,2834 70.64 ,2820 69.49 ,2819 69.30 ,2581 69.26 ,2219 69.20 ,1843 69.14 ,1655 69.07 ,1507 68.77 .1466 68.22 ,1438 67.24 ,1415 66.18 ,1395 65.27 ,1384 64.73 ,0729 64.67 ,0700 63.59 ,0689 62.35 ,0677 60.51 ,0682 61.44 ,0701 64.46 ,0703 64.70 ,0704 64.80 ,0713 64.96 ,0720 65.05 ,0752 65.18 ,0790 65.25 ,0375 65.33 1072 65.39 ,1200 65.44 ,1360 65,52 1422 66,74 2820 69.32 ,2819 69.23 ,2829 70.13 ,2819 69.23 ,1786 69.13 ,1651 69.07 ,1406 65,83 ,1400 65.53 ,1397 65.48 ,1395 65.35 1384 64.86 ,1378 64.56 ,1375 64,41 ,1382 64.96 ,1276 65.09 .lo94 65.03 ,0870 64.41 ,0693 61.13 ,0692 61.11 .0691 61.07 ,0669 57.61 ,0652 54.39 ,0661 56.20 ,0671 58.04 ,0697 63.08
n-Css (PSC No. 190) Temp., SD. vol. C. &/g. 85.59 ,2880 ,2872 84,63 ,2863 83.79 ,2853 83.03 ,2845 82.36 ,283 1 81 12 ,2812 79.04 ,2802 78.18 ,2792 77.13 ,2789 76.80 ,2785 76.59 ,2785 76.39 ,2781 76.11 ,2780 75.93 ,1965 75.84 ,1825 75.81 ,1665 75.74 ,1554 75.66 ,1450 75.47 ,1391 74.90 ,1370 74.20 ,1358 73.65 ,0889 73.05 ,1376 74.52 ,1353 73.54 ,0902 73.46 ,0905 73.48 ,0904 73.50 ,0905 73.53 ,0873 72.41 ,0667 70.43 ,0662 69.55 .0663 70.14 .0668 70.90 ,0668 71.72 ,0856 71.96 ,0831 71.86 ,0819 71.82 ,0892 72.69 ,1363 73.96 ,2781 75.84 ,2787 76.40 ,2796 77.12 ,2802 77,70 ,0868 72.18 ,0863 71.92 ,279 1 76.26
.
I
75,57 73.54 72.34 70.95 69.83 71.62 71.87 72.16 72.24 72.44 73.02 73.24 73,65 73.76 73.80 73,83 74.48 75,94 75,85 75.87 75,88
1.1492 1.0958 1.0869 1.0687 1.0678 1.0685 1,0687 1.0703 1.0724 1 ,0887 1,0902 1 ,0505 1.0925 1.1153 1,1292 1.1352 1.1375 1,2785 1.2316 1.2657 1.2782
ever, if this is the caJe, the two processes are so closely integrated that they are indistinguishable. Each curve shows a smooth, rounded section at the lower end of the freezing zone, which is reproducible with either increasing or decreasing temperatures. This is contrary to what was expected from samples of high purity, which, it would seem, should show a sharp break a t the beginning and end of the freezing zone. This curvature is apparently an indication of a premelting effect which may be due to traces of impurity. Van Hook and Silver ( 2 4 ) state that premelting due to impurity of a similar nature is not significant; however, the expansion curves representing a pure compound, a semipure compound, and a very impure mixture (Figures 8, 4,and 2, respectively) show that the temperature range covered by this gradual volume change
158
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 48, No. 1
I I0500 30
32
34
36 Temperature
38
42
40
"C
TempsrolLra,
Figure 6.
Figure 5. Volume expansion of n-eicosane
I
, I
I 56
58
Tamperotwe ' C
Figure 7.
'C
Volume expansion of n-tetracosane
Volume expansion of n-hexacosane
decreases appreciably viith increasing sample purity. The effect described does not appear to be caused merely by the liquefaction of a portion of the sample, as visual observations failed to detect any sign of liquid in this region in spite of the fact that it involves approximately one third of the total solid-liquid expansion. Observations made in a previous investigation (18) indicate a change in the opacity of the sample in this region. According to x-ray data obtained by Mueller (16))the transition from the less symmetrical form into the hexagonal, close-packing arrangement is a continuous function of the temperature and takes place when the substance is solid. Ubbelohde (23) states that the premelting effect results from a change taking place within the solid phase near the melting point. Seyer and Morris ( 2 1 ) report that in the case of dotriacontane (dicetyl) the A phase first formed on freezing exists for a temperature range of only about 0.1" C. Piper and his associates (18) report two solid transitions for n-Cs6, n-Cz,, n-Csz, and n-C36; and the higher transition in each case is within about 1" C. of the freezing point, which corresponds closely to the gradual change in the solid found by the expansion measurements.
10500
54
I" 36
o--I%Lg--
I
I
56
60
Ternperotv e
62
64
"C
Figure 8. Volume expansion of n-octacosane
Long-chain n-paraffins can exist in any one of five crystalline forms. The molten sample freezes in the hexagonal form and then apparently changes t o the orthorhombic configuration. Below the solid-solid transition, the crystal can be of any one of several types. A monoclinic form may exist in either a stable or metastable variation, which can then change to the triclinic form; in some cases several forms have been found to exist at the same time. All of these crystal variations are reported in the literature; and while there is some disagreement as to the order of the changes, the consensus favors the order stated (5,9). At the transition, represented by the sharp break in the curve about 3" C. below the freezing point, a volume change of about 7% talres place (average values for the the pure n-paraffins showing this type of transition). The change in volume is due to the additive effect of the tilt in the chain axis reducing the distance between the 001 planes of the crystallite, the lateral compacting of the chains, and the closer approach of the end groups on neighboring chains. The second factor is perhape the greatest single contributor to the volume change, and it is made posqible by the restricted rotation about the chain axis ( 7 ) .
January 1956
159
INDUSTRIAL AND ENGINEERING CHEMISTRY 1.3000
1.2soo
l 72
70 Tempsroturc,
Figure 9.
~~~
~~~
-& 2
1
n-C24 1
36.4 50.3 36.2 32.'0 50.7
...
,. ,
. . . . . .
. . . . . .
Freezing and Transition Points of n-Paraffins Freezing (1) and Transition (2) Temper_atures, ' C. n-Czs n-Cza n-Ctz 2 1 2 1 . 2 2 1
48.1 47.9
61.1 01,2
56.0 53.2 56.8 48.8
n-Cas
1
2
73.8 65.5 75.9 58.1 69.3 62.5 54.0 69.5 73.9l74.1 65.2-65.4 7 5 . 6 57-57.4 6 9 . 2
50.6 56.2 6l:iO 56196 50.68 48:ll 56.2 52:54 51.2 40-41 58.0 45.5-46 . . 61'.2-3 54.2 50.7-8 47.0
... ...
...
...
...
.
...
. . . . ...
I
.
75.8
... ... ...
73.5
(From expansion measurements)
PBC No.
Crystal point First_trp$tidn n1,
-
0 0011
0 0011
0.OOll
190 36 75.9 73.8 2.1 72.3 0.0011 0.0011
0 0033
0 0022
0,0020
0.0021
0 0005
0 0005
0 0005
d .'d007
0 .'do05
0 2077
0 121.5 0 0662
0 1254 0 0780
0.1320 0.0774
0.1346 0,0692
1,2833 1.1547 ...
1 2800 I 1488
1.2837 1.1450
1.2818 1,1398
540 a
C. point,
20
O
l 80
l
l 82
the volume change noted represents a change in the crystal form. A second possibility is that the volume change is caused by the partial closing of the numerous fissures which appear throughout the sample below the transition point. Van Hook and Silver ($4)commented on the fact that such openings in the sample are incompletely filled by the indicating fluid. Expanaion measurements at high pressure may show whether void spaces arc a factor in producing this volume change. The volume below the transition point would be less than that observed if the fissures could be eliminated; however, there is little reason to believe that a decrease in the total volume of these voids should take place once they are formed. It was also observed that although all the samples had this fractured appearance below the transition
Table VI. Physical Properties of n-Paraffins CaFbon No.
!
Figure 10. Volume expansion of n-hexatriacontane
. . . . . . . . . . . . 56.1 51.5-52 61.1
36.6 36.6
l 76
~~~
Table V. Data Source
l
1emperoture. "C
"C
Volume expansion of n-dotriacontane
An anomaly which appears in the low-temperature solid form of 7t-Czo,n-Cza,n-Czs, and semipure n-Czs(Figures 5, 6, 7, and 4) is indicated by the parallel lines below the transition point on the expansion curves. In each case the upper line was established before the volume change occurred, and then repeated heating and cooling in this temperature range gave the data represented by the lower line. It is believed that this change is an irreversible transition, as at no time did i t take place from the lower level to the upper level. Mazee ( I d ) refers to a change of this nature as a slow, irreversible conversion of the monoclinic form obtained at room temperature to the stable triclinic crystalline form. The fact that several crystalline modifications have been found to exist at room temperature for the same sample also indicates that ~
] 76
74
C.
b.
Second transition point O C. Liquid expansion. o ~ . / g : / ~C. Solid expansion, cc./g./O C. Form A Form A2 Form R Expansion a t phase oliange, cc./g. Fusion First transition Second transition Specific volume, cc./g. Crystal point First transition point Second transition point Expansion, % Fusion First transition Second transition Low temp. solid a t transition point to liquid a t crystal Doint
36.4
... ...
0 0011
1,2863 ... ...
541 24 50.3 48.1 2.2
106 26
56 0 53 2 28
176 28 61.1 58.1 3.0
...
...
197 32 69.3 65.5 3.8
...
....
0.0020 0.002fi 0 0004 0.1377 0 0454 0.0200
1.2778 1.1361 1.0873
10.5 8.1 ...
10 9 73
11.5 7.2
11.7 6.5
, . .
...
12.1 4.2 1.9
19.3
17.9
19 5
20.2
19.7
19.7
19 3 ,
.
...
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
160
Vol. 48, No. 1
Figure 11. Volume expansion of n-paraffins
point, only the lower melting point compounds produced the volume change described above; and the magnitude of the change increased considerably with increasing carbon number from n-Cso to n-C2s. Only in the case of n-Cno did the curves indicate any subcooling tendency at the freezing point, although at every transition point subcooling occurred to a considerable degree. For this reason, only the transition points obtained on heating are reported, and these points refer to the intersection of the vertical line obtained at the transition, with the line representing the solid form above this phase change. This point is distinct and reproducible. The transition due to subcooling takes place several degrees below the point shown on the expan,'cion curves, and it does so very suddenly and completely once it starts. For this reason it was not possible to get equilibrium points on the curve between the two crystal forms on cooling, Thereas on heating, equilibrium points were obtained in the transition region indicated by the vertical line.
1.2953
1550
,1500 m
1.2900
3
(16), and Seyer, Patterson, and Keajs ($Z), although no satisfactory explanation has been given for this nonconformity to the general pattern. ,411 the samples tested except microcrystalline wax registered a sharp decrease in volume when the first crystals appeared in the melt. This volume change, which is caused by the compacting of the molecules during crystal formation, orcurred when only a very few trystals had formed; and the magnitude of the change is such that it cannot possibly be accounted for by the few visible crystals alone. I t seems almost certain that a considerable portion of the liquid sample becomes oriented in the crystal configuration at this point, and perhaps only a slight additional molecular packing is necesmry to make the crystals visible. Fujika (4),working with wax film5 on copper foils and plates, observed that the hexagonal crystalline form of the hgdrocarbon mixture remained at temperatures above the melting point of the sample. There is good agreement between the freezing point value8 obtained in this study and those reported in the literature; however, the transition temperatures do not show as good general agreement (Table V ) This is probably due to differences in the purity of the samples used for these investigations, as it has been shown that the transition point is much more sensitive to impurity than is the freezing point (18). This was also clearlr indicated in the present work by the freezing and transition point values for the pure and impure samples of n-octacosane (Table IV). Furthermore, as impurity lowers the transition point, it
2
-2
.g
,1650
,.1.2850
e i
-B
j
P 1.2800
,1400
>
1 1350
1.2750
20
25
30 Carbon Number
Figure 12.
Effect of carbon nuniber on specific volume
A comparison of the six curves in Figure 11 reveals that n-Ga is somewhat different from the other compounds with respect to the total volume change from liquid t o the B solid form. This change is considerably less for n-Czd than for any of the other pure n-paraffins tested. A difference in behavior between this and thp other n-paraffins ha8 been observed liy Molvoort, (9),Mueller
Figure 13. Effect of phase change on volume
January 1956
INDUSTRIAL AND ENGINEERING CHEMISTRY
might be said that the higher the transition point of a n-paraffin sample, the more pure i t is likely to be, especially if the melting point agrees with the accepted value for the compound. A summary of the physical properties obtained from the expansion measurements is given in Table VI for the pure compounds. The freezing and transition points of these samples are fartherest apart at n-Caz, and they have a tendency to converge above and below n-Csz as the carbon number increases or decreases. From this study it appears that the two points coincide at n-Czo and other reports indicate that the two curves should meet again in the n-Cto to n-Cae range (12, 18). A solid trmsition has been reported for n-C~o(22); however, most reports indicate that no major solid transition exists for this hydrocarbon.
161
0.023' C. for a pressure increase of one atmosphere. This information and the value given for the volume change a t the melting point were used in the Clapeyron equation to determine the heat of fusion for n-Czs. The calculation gave a value of about 45 cal. per gram with an estimated error of perhaps &lo%. This value is slightly higher than that obtained by calorimetric measurements. ACKNOWLEDGMENT
The writer is indebted to API Research Project 42, Pennsylvania State University, and to the director, R. W. Schiessler, for the loan of the pure compounds used in this work and for information concerning these samples. He wishes to express his appreciation to H. S. Frank, University of Pittsburgh, and to W. A. Gruse, W. E. Hanson, and W. P. Ridenour of Mellon Institute for their valuable suggestions and constructive criticism of the work as it progressed. A special feeling of gratitude is reserved for R. G. Capell of this institute for his continued interest and support of the project and his kind personal encouragement. This opportunity is taken to thank W. S. McClenahan and R. L. McLaughlin for the loan of the semipure sample of n-octacosane, S. M. Ohlberg for x-ray data pertinent to these hydrocarbons, and J. J. Kaufman for continuing the expansion measurements on several occasions when it was necessary for the writer to be absent from the laboratory. LITERATURE CITED
(1) Bekkedahl, Norman, J . Research Natl. Bur. Standards 43, 14556 (1949). (2) Carpenter, J. A . , J . Inst. Petroleum 12, 288-315 (1926). (3) Ferris, 8. W., and Cowles, H. C., IND.ENG.CHEM.37, 1054-62 (1945). (4) Fujika, Y . , Mem. Coll. Sci., Liniu. Kyota, Series A, 25, 119-25 (1949). (5) Gray, C. G., J . Inst. Petroleum 29,226-34 (1943). (6) Guggenheim, E. A,, Phil. Mag. 2,538 (1926). (7) Hoffman, J. D., and Decker, B. F., J . Phys. Chem. 57, 520-9 (1953).
Figure 14. Effect of pressure on freezing point of n-octaoosane
(8) Koch, J. R., and Concetta, Sister (9)
(IO)
The specific volume a t the transition point gradually decreases with increasing carbon number, as shown by Figure 12. The specific volume a t the freezing point, however, is discontinuous a t n-Ct8, which at present cannot be accounted for. This low value for specific volume is reflected by the break in the curve representing volume change a t the freezing point shown in Figure 13. The n-Czo value was not used for the curve, as the volume change for this sample apparently represents a combination of freezing and transition; and, as indicated earlier, this volume change is equivalent to the sum of the changes at the freezing and transition points for each of the other compounds. The curve representing the volume change a t transition shows a maximum at n-Cze. Extrapolation of the curve at both ends would intercept the abscissa scale at carbon numbers of about 20 and 40, which are also the approximate intersection points of the transition and melting point curves. The freezing point of n-paraffins is affected appreciably by moderate changes in pressure. The effect was determined by changing the pressure in the dilatometer and then making the necessary adjustments of the bath temperature to give the new freezing point of the sample. Figure 14 is a curve plotted from freezing point-pressure data for n-Cz8. The slope of the straight line indicates that the freezing point of this compound is raised
(11) (12) (13)
M.,Trans. Kentucky Acad. Sci. 13,104-10 (1950). Kolvoort, E. C. H., J . Inst. Petroleum 24, 338--47 (1938). Lord, H. D., Ibid., 25,263-76 (1939). Manee, W. M., Ibid., 35, 97-102 (1949). Mazee, W. M., Rec. trau. chim. 67,197 (1948). Morris, F. J., and Adkins, L. R., IND. ENG.CHEM.19, 301-2 (1927).
(14) (15) (16) (17) (18)
Mueller, A., J . Chem. Soc. 127, 599 (1925). Mueller, A., Proc. Roy. SOC.127, 417 (1930). Ibid., 138,514-30 (1932).
Piper, S. H., and Malkin, T., Nature 126,278 (1930). Piper, S. H., Chibnall, A. C., Hopkins, S. J., Pollard, A., Smith, J. A. B., and Williams, E. F., Biochem. J . 25,2072-94 (1931). (19) Schiessler, R. W., Kerr, C. H., Rytina, A. W., Weisel, C. A., Fischl, F., McLaughlin,R. L., and Kuehner, H. H., Proc. A m . Petroleum Inst. 26 (III), 254-302 (1946). (20) Scott-Harley, C. R., J . Inst. Petroleum 25,238-51 (1939). (21) Seyer, W. F., and Morris, W. J., J . Am. Chem. SOC.61,1114-17 (1939). (22) Seyer, W. F., Patterson, E. F., and Keays, J. L., Ibid., 66, 17982 (1944). (23) Ubbelohde, A. R., Trans. Faraday SOC.34, 282-99 (1938). (24) Van Hook, A., and Silver, L., J . Chem. Phys. 10, 68G90 (1942). ACCEPTED April 19, 1955. REOBIVED for review November 30, 1954. Division of Petroleum Chemistry, 125th Meeting ACS, Kansas City, Mo., Maroh-April 1954. From a thesis submitted to the Graduate Sohocl University of Pittsburgh, in partial fulfillment of the requirements for t h e degree of master of science.
