November, 1929
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1093
Variation of Surface Tensions of Lubricating Oils with Temperature’ George Winchester and R. K. Reber DEPARTMENT OF PHYSICS, RUTGERS UKIVERSITY, h-EW
OME time ago i t became necessary in the course of an
S
investigation (24) to know the approximate surface tension of lubricating oil a t high temperature. A systematic search revealed very little m-ork on the s u r f x e tension of crude or refined oils and no data could be found bearing on the surface tension of oils a t high temperatures-e. g., 250’ to 300’ C. It is well known that the effectiveness of oils as lubricants diminishes rapidly a t high temperatures. What part surface tension plays in this is impossible to e determine so long as there is no knowledge of its ~ a l u under these conditions, Such knowledge seems very desirable as it has a direct industrial bearing. Two important applications that may be mentioned are the lubricating properties of oils and the capillary phenomena of oils under ground. Just what relation exists between the physical properties of oils and their value as lubricants has never been satisfactorily determined ( 7 ) . Although i t is quite certain that surface tension is not the most important factor in determining the relative lubricating value of oils, it is equally certain that some niinimum value of surface tension is necessary (14) and that some attention must be paid to this property when selecting a n oil for a particular use. For example, there is a yery intimate relation between the surface tension of a n oil and its ability t o form a n emulsion with water; a n oil of low surface tension will form smaller drops than one of high surface tension, and this is desirable in steam-cylinder lubrication. Furthermore, the lubricating power of an oil in siphon lubrication is determined in large measure by its capillary properties, Again a “low-viscosity” oil may be recommended to remedy certain lubricating difficulties. Now generally the low-viscosity oil has a lower surface tension and the improvement which follows may be due more t o the change in surface tension than to the change in viscosity, which usually gets the credit. The surface tension of a n oil doubtless has some influence on the formation and strength of thin oil films formed between nietsllic surfaces; and therefore differences in surface tension of lubricating oils may be expected to cause differences in tendencies to maintain these thin films. However, that is about all one may say a t present, for the nature and importsnce of surface tension in lubrication remrlin practically unknown even after years of widespread use by practically the entire world. A knowledge of surface tension a t high temperatures has also a bearing on the problems relating to the capillary concentration of oils and gas in porous rocks snd shales. The oil pressure under ground, the collection of oil into larger pockets, and the passage of oil from point to point are all intimately connected with surface tension (28). Oils are sometimes estimated to have come from as great a depth as 20,000 feet below the surface of the earth; the temperature a t such a depth must be very high. The temperature a t the bottom of the Lake Well in West Virginia 1s given as only about 70’ C. (depth, 7579 feet), but the tc,mperature gradient a t this depth is about twice whst it is at the surface. The temperatures reached in the experiments to be described were in the neighborhood of 300’ C., and they are too low to have much bearing on the problem of film lubri1
Received July 9, 1929.
BRUNSNICK,
N. J.
cation where the temperature of a certzin few molecules which are being squeezed out from between the metal surfaces msy be very high and may even approich the critical temperature (in the neighborhood of 550’ C . for some of these oils), when the tendency to maintain the film would drop to zero. However, it was felt, in the abyence of published data, that the variation of the surface tension over a wide range of temperature would be of value. Theory of Method
.
For the determinition of the surface tension the method of maximum bubble pressure (also czlled Jaeger’s method) was used. This method mas originsted by Cantor (4) and extensively used by Jaeger for a large number of substsnces. It consists of finding the lezst pressure t h s t will force bubbles of air (or gm) out of a narrow orifice of a tube dipping into the liquid to be measured. The elementary theory was given first by Cantor (4) and is simihr to that of the well-known capillary rise method. The pressure in any spherical cavity exceeds the outside pressure by 2 T / A , where T is the surface tension and A is the radius of the sphere. Thus, to a first approximation, the pressure required to force the bubble of air from the tube
Figure 1-Apparatus for Studyin? Variation of Surface Tensions of Lubricating Oils with Temperature
exceeds the hydro3tatic pressure a t the orifice by an amount 2 T / A . If this pressure is measured by a msnometer containing a liquid of density d , and if the difference in height of the liquid columns of the minometer arms is h, the inside pressure exceeds that of the atmosphere by dh. If d’ is the density of the liquid whose surface tension is to be found and h’ is the depth of the orifice below the surface, t h e hydrostatic pressure is given b y d’h‘. The surface tension is then given by the formula: (dh
2T - d’h’) = A
This formula is only approximate, as the bubble is never
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1094
exactly spherical. The reason for this is t h a t the bubble has an appreciable depth, and thus the hydrostatic pressure is different for different parts of the bubble. It was shown by Ferguson (8) that the true equation is of the form
2T - = where R
=
-*
A T
dh-d'
(h ' + -iA ) +- 1 2 R
A 3 6
d'h'
This correction is appreciable only when the diameter of the capillary tube is relatively large or when a dense liquid is being investigated. For the purpose of this experiment, with the tube used (diameter = 0.0% mm.) the correction was quite negligible. For example, the greatest value of the surface tension obtained with the approsimnte formula was 27.61 (for oil No. 1). Using Ferguson's formula the value obtained is 27.62 dynes. The difference is seen to be only about one-twentieth of the estimated experimental error. The difference between any two values given by the two formulas is uniformly less for all other temperatures for which the oils were investigated. Consequrntly, the simpler form of the equation was used in this experiment. Apparatus
The apparatus is shown in Figures 1 and 2. It consisted of a glass tube drawn down to capillary dimensions a t 0 and the other end connected to a water manometer, M , and an air reservoir, C, held at constant pressure. The source of the constant-pressure supply was a tank with a weighted, floating dome. The inside diameter of the orifice, 0, was nearly the same for all angles about the tube, showing that it was almost circular. The G greatest deviation from the mean diameter, found by averaging measurements taken a t every 20 d e g r e e s about the axis of the tube, was about one per cent. The manometer, M , consisted of tubes of about 2.5 cm. diameter, thus making it impossible for surface defects or impurities to affect t h e h e i g h t of t h e c e n t r a l part of the meniscus. Manometers of smaller bore were tried at first while working out the values of the surface tension of water, and it was found that, no matter by what process or Figure 2-Beaker and Asbestos h o w carefully the manometer Jacket was cleaned, the angle of contact between the water and the glass was a. variable depending upon the location of the surface of the water in the manometer, and also that i t varied from place to place around the tube a t the same height. The wide-bore manometer was considered decidedly superior to the narrower type. A valve in the pipe leading from the air reservoir, C, was used t o regulate the pressure in producing the bubble a t 0. The oil to be investigated.was contained in a Pyrex beaker (Figure 2) inside a n asbestos jacket, which consisted of 4 cm. of asbestos wool packed between the beaker and an outer asbestos shell. The asbestos jacket was placed upon a n electric heater, which in turn was supported on a stand with rack and pinion motion for delicate vertical adjustment. In operation the stand was carefully raised until the index rod, I , just made contact with the oil surface after the temperature had been raised t o the desired point. This
VOl. 21,
No. 11
was accomplished by cutting two small openings in the asbestos jacket near the top on opposite sides of I ; then when a source of light was placed in front of one of the openings, its rays were reflected from the surface of the oil through the other opening, and when the index and its image just met the rod was in contact with the surface. The distance between the end of the index rod, I , and the orifice, 0, was determined accurately by means of a cathetometer to be 4.422 cm. An asbestos board, DD', covered the entire apparatus. The stream of air was adjusted so that one bubble was formed about every 50 seconds. The temperature could be regulated by a variable resistance placed in series with the heater, and by proper adjustment it could be held constant, a t any point, for a considerable length of time. A cathetometer was used to read the manometer levels and could be read to 0.02 mm. Accuracy
I n order to test the accuracy obtainqble, a number of determinations of the surface tension of water were mnde. The results (mean = 7'2.70 dynes a t 20" C.) were in good agrement with the best values obtained by others (0,26,20). Duplications could be made to within 0.4 dyne, so that the greatest variation from the mean mas not more than 0.2 dyne. This accuracy compares favorably with that obtained by others using this method. Thus, Sauerwald and Drath (It?), Bircumshaw ( 2 ) , and Brown (S), working with liquid metals, all obtained variations of over 10 dynes. Of course, the error or variation in percentage is as great in this experiment as was found in the experiments cited, because the surface tension of oils (about 25 dynes) is much less than the surface tension of liquid metals (300 to 500 dynes). Almost all the error, howcver, was made in reading the manometer levels and in setting the surface of the oil in coincidence with the point of the glass rod. For this reason the error in dynes should be almost independent of the magnitude of the surface tension, and thus a higher percentage of accuracy should be possible with greater values of the surface tension. Oils Examined
Eight oils, differing considerably in general physical properties, were investigated. Table I gives the characteristic tests made by the oil companies who supplied the samples. All the oils were refined from a paraffin-base crude, except sample 5, which was refined from a naphthene-base crude. Table I-Physical
Data of Oils Examined
SAMPLE POURPOINT FLAsn POINT O F .
1 2 3 4 5 6-3FV 7 8
+-
c.
C.
35 30 25 25 75
1.1 -$-2 333. ..999
35
+'i:7
40
1.7
+ 4.4
FIREPOINT
282 282 321 316 274 285 274 216
c.
316 316 360 360
...
338
... ...
VISCOSITY AT 210°
C
SCC.
1 s2 140
230 210 1000 1 SO 179 68
Determinations
The surface tensions of these oils were determined a t intervals of about 20 degrees between 80" and 300' C. Accurate determinations could not be made at temperatures much lower than this, because the greater viscosity tended t o make the experimental value of the surface tension too large. The presence of a viscous yield could always be detected by increasing the speed of bublding. Above 80' C., however, no effects of this nature could be observed for most of the oils.
INDUSTRIAL A N D EN(XNEERING CHEMISTRY
November, 1929
I n order to calculate the hydrostatic pressure which the oils exerted on the escaping bubbles, it was necessary to know the densities a t each temperature. To determine these the density of each oil was found for a number of temperatures, and the results were plotted as shown in Figure 3. Results
The results are shown in Tables I1 and I11 and graphs. Not all the data obtained are included, as it was thought best to omit data on oils having essentially the same densitytemperature and surface tension-temperature curves as those already shown.
1095
When the surface tension was plotted as a function of the temperature very interesting results were obtained (Figure 4). It will be observed that the density-temperature curve of sample 2 (Figure 3) coincides with t h a t of sample 1, and the same is true of samples 3 and 4. Furthermore, all the density curves are practically parallel lines. As far as the investigation goes, it shows that if the density-temperature curves of two oils coincide their surface tension-temperature curves will coincide also. Also, if the density curves do not coincide then the surface tension curves do not have the same slope, indicating that some property other than density is effective in determining surface tension. It can be seen that in each the curve is a straight line.
Table 11-Density of Oils a t Various Temperatures DEKSITY TEMPERATURE DENSITY
0 875
TEYPERATUBB
c.
O
c.
Sample 1 81.4 107.2 147 0 185 7 225 4 254 0 285.5 311.0
Sample 4
0 864 0 847 0.824 0 800 0 775 0.758 0.739 0.722
80.8 122.9 157.2 197 0 217 2 284.0 318 7
0.873 0.842 0.817 0.794 0.760 0.741 0.725
45.5 73.3 108.3 132.2 163 3 191.1 222.8 226.7
Sample 2 60.3 115.6 154.5 194.5 240.5 280.5 305.5
0.870 0.815 0.824 0 so0 0 776 0 748 0.726 Sample 6
a 625
$ *
E 077s
9
0.701 0.770 0.767
Tension of Oils at Various Temperatures SURFACE SERFACE TEMPERATWRB TENSIOX TEMPERATURE TENSION O c. Dynes c. Dynes Sample 1 Sample 4 26.17 27.61 105 87.9 25.05 127.9 26 31 113.7 23.62 148.3 24 68 134.5 22.17 167.3 23 50 155.8 185.9 21.58 22.13 174.4 203.3 20 50 191.9 20 9 3 220.8 19 50 212.8 19.63 234.1 18.81 231,7 18 50 260.3 17 48 287.2 16.61 275.0 17.59 312.1 13.79 285 0 16 77 15.26 302.2 14.32 317.1 Sample 2 Sample 5 80 3 27.89 136.7 25.87 152 8 24 88 27 49 95 3 174.4 26.36 112 0 23 66 186.3 129 0 25 21 22.96 24 00 203.3 22 21 147 3 227.5 20 5 4 22 70 168 3 248 9 19 15 22 13 171 7 272.8 193.0 21 19 17 80 297 0 213 9 19 41 16 59 313 0 232.0 18.20 15.28 252 2 322.1 16.80 14.86 276 0 15 27 14.00 295.6 Sample 3 Sample 6 115 5 25 89 124.4 25.60 131 1 136 1 23 59 25.16 145 6 23 46 150 0 23 91 161.1 23 14 1B1 1 22 32 178.9 22.10 180 0 21 49 195 5 20 71 188 9 21.55 205 5 204 4 20 06 20 35 223 3 18 71 218 9 19.60 231.7 18 68 227 9 18 71 248 9 17.70 247 8 15 13 257.2 17.25 263 3 16 94 265.5 16.75 275 9 16 10 288 9 15 97 273.3 16.25 15 77 288 3 298.9 15 38 Table 111-Surface
It can be seen that for each oil the surface tension varies from about 25 or 26 dynes a t 100" C. to about 13 or 14 dynes a t 300" C. Thus in each case the surface tension a t 300" C. is just about half what it is a t 100" C. Furthermore, although the oils differed considerably in other properties, their surface tensions are about the same a t equal temperatures over this range.
0 72s
*C .L
Temperoture
Figure 3
Various formulas have been suggested for representing the variation of surface tension with temperature, but no general formula seems to hold in all cases. Many years ago Mendelyeev said that a perfect liquid should be characterized by two conditions. Expressed as formulas these are: v = V' (1 - kl) T = T' (1 - a t )
$1
where V and I" are the specific volumes a t temperatures t and 0, respectively, T and T' are the surface tensions a t the corresponding temperatures, and k and a are constants. Later Ramsay and Shields (15) from considerations of surface energy and Selby (19) from thermodynamics argued that liquids should obey the second of these equations. While it is certain that many liquids do not obey this simple relation, it is seen that over the range for which these properties were investigated, within the limits of error, the oils investigated strictly obeyed both these formulas. From the graphs of surface tension it can be observed that, although for each oil a number of points will not fall on any smooth curve, in each case these points are more symmetrical about a mean straight line than about any other smooth curve which could be drawn. I n almost every case the distance of the points above or below the line is less than the estimated masimum error. The fact that both the density and the surface tension are straight-line functions of the temperature may be suggestive of the nature of the dependence of the surface tension on density. There is much difference of opinion concerning this point. It is certain that surface tension is in some way intimately related to density, for a change in density corresponds to a change in the mean distance between the molecules. Thus the relation between density and surface tension must be largely dependent on the law of attraction between molecules for distances of the magnitude found in liquids.
INDUSTRIAL AA'D ENGINEERING CHEMISTRY
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Various estimates of this law of attraction between molecules have been made. Sutherland (21) gave reasons for believing that the force of attraction between molecules varies as the inverse fourth power of the distance between them. Chatly (5) suggested for solids an inverse sixthpower law. Kleeman (11) thinks an inverse fifth law the
Vol. 21, No. 11
exactly fit the data; for the mean distance between the molecules varies inversely as the cube root of the density. Kleeman (12), however, gives evidence to show that such a simple relation is generally not true, but that the surface tension is a function of both the density and the temperature, suggesting that the molecules shrink a t higher temperatures in order to remain stable with their greater rotational energies. Acknowledgment The authors avail themselves of this opportunity to express their thanks to the managements of the Standard Oil Company of New Jersey and the Vacuum Oil Company of New York for furnishing the samples of oils used in these experiments and for supplying the data on their physical characteristics.
E
a" s C
E
d
20
5
Literature Cited I5
200'F 93.C
300'f 149.C
400'F
500'F
600
204'C
260'C
316'C
Temperature Figure 4
F
most probable; and this was also the view which Maxwell ( I S ) reached from viscosity considerations. Kam (IO) favors a n inverse square law. Antonoff (1) from surface tension phenomena and Tomlinson (22) from work on the elasticity of solids suggest an inverse fourth-power law. Edser (6) considers a n inverse eighth law as being the most probable. Finally, Rosenhain (17) gives a n inverse cube law for forces between metallic atoms. I n these experiments both the density and the surface tension varied linearly with the temperature, so that the surface tension plotted against the density would also give B straight-line curve. Xow if the cause of the variation of the surface tension were attributed solely to change in density, it is seen that in this case an inverse cube law would
(1) -4ntonoff. Phil. -'dag,, [6] 36, 377 (1918) (2) Bircumshaw, Ibid., [71 6, 510 (1928). (3) Brown, Ibid., [7] 6, 1044 (1928). (4) Cantor, Wied. Ann., 47, 390 (1892). (5) Chatly, Proc. Phys. SOL.London, 27. ( 6 ) Edser, Brit. Assocn. Advzncemenl Sci. R e m . , 1922. (7) Engineering, 119, 109 (January 23, 1925); 122, 239 (August 20, 1826). (8) Ferguson, Phil. M a g . , [ 6 ] 23, 128 (1914). (9) Harkins and Brown, J . A m . Chem. Soc., 41, 499 (19191. (lo) Kam, Phil. Mag..[ 6 ] 37, 65 (1919). (11) Kleeman, Ibid., [ 6 ] 19, 783 (1910). (12) Kleeman, Ibid., [6] 21, 783 (1910). (13) Maxwell's Collected Papers, Vol. 2, p. 35. (14) Oshorne, Encyclopedia Britannica. 10th ed., Lubricants. (15) Ramsay and Shields, Phil. Trans. Roy. SOC.(London), 184, 647 (1893). (16) Richards and Carver, J . A m . Chem. Soc., 37, 1656 (1915). (17) Rosenhain, Cantor Lectures, 1025. (18) Sauerwald and Drath, Z . anorp. nllgem. Chem., 154, 79 (1926). (19) Selby, Phil. M a g . , [5]31, 430 (1891). (23) Sugden, J . Am. Chem. Soc.. 121, 865 (1922). (21) Sutherland, Phil. Mag., [ 5 ] 24, 113, 168 (1887). (22) Tomlinson, Ibid.,[71 6, 695 (1028). (23) Washburne, Bull. Am. I n s f . Mining En,.., 2368 (1914) (24) Winchester, Phys. Rev., 29, 911 (1927).
Refining of Shale Gasoline I-Relation
of Oxidation t o Colors and Gums Produced in Gasoline from Colorado Oil Shales' Robert A. Baxter 423 S I X T E E X T H
ST.,
I
N VIEW of the present great production of petroleum and comparatively low price of gasoline, it may be well to begin by saying that the purpose of this study is primarily the collection of information which may be useful in the refining of gasoline in general and not the immediate production of motor fuel in commercial quantities from oil shale. It is well known that the oils produced by the thermal decomposition of oil shales are similar to petroleum, but it is not quite so generally realized that those produced from shale oil are generally much more active. For this reason shale gasoline is a very desirable raw material to use in the study of color and gum formation, since it is frequently possible to get as great a change in a crude shale gasoline in a day as is possible in a month with most well gasolines. Several years ago we noticed that when shale gasoline was 1 Presented before t h e Division of Petroleum Chemistry a t t h e American Chemical Society, St. Louis, Mo., April 16 t o 19, 1928. Revised paper received June 6, 1929.
GOLDEX,COLU.
kept in tightly closed bottles away from air it did not change color or form gums to any appreciable extent, but that when the same material was exposed to air it darkened rapidly and soon deposited gums. Like many other discoveries, we soon found that this was well known (6). However, two different explanations had been offered for the gum formation-one group holding that the change was one of oxidation and the other group maintaining that it was one of polymerization. As a contribution to this discussion, there seem to be several additional pieces of information which should be presented, some of which may suggest methods of improving motor fuel quality and simultaneously reducing the cost of refining. The gasolines on which this work was done were derived from the richer beds of western Colorado shales, which were retorted either in the Ginet or Lamb retort or in a laboratory pot retort, and all of the oils were fractionated and treated in the laboratories of the Colorado School of Mines. It
