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T H E JOURNAL OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY
Vol. 14, No. 7
T h e Surface Tension of Petroleum' 3 y C. K. Francis and H.T. Bennett COSDBN & Co.,TULSA, OKLAHOXA
The surface tension of petroleum f r o m oarious sections of the United States has been determined. I t is found that the value increases with the specific grauity. The small quantity of f a t t y acids and amorphous and crystalline wax commonly found i n petroleum products does not appear to influence the surface tension. The presence of high boiling fractions and products of high oiscosity tends to raise the surface tension of the lighter petroleum products, such as gasoline and naphtha. The surface tension decreases as the temperature increases, the surface tension of the oils tested decreasing approximately 0.05 dynes per cm. for each OF. increase in temperature. Surface tension in conjunction with other tests m a y aid in determining the lubricating and other values of a n oil.
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ANY properties of oils are to be considered when studying their behavior in machinery, and these properties are important in connection with measurements of volume and transportation through pipe lines. No single characteristics may with reason and safety be chosen by which to judge an oil for R specific purpose, but a combination of properties is usually selected and an interpretation for value made from it. The addition of surface tension to the other commonly recognized properties of oils will, it is thought, aid in such an interpretation. The facts presented in this article may, in themselves, appear to have a limited range of applicability, but i t is thought that logical conclusions as to practical application may be derived from our experiments. The value of certain oils is concerned with the interface oil-metal, and better data would be contained in measurements showing the relation between liquid and solid. The surface tension of any liquid is due to the cohesive action of the particles composing the liquid. These cohesive forces act in all directions below the surface and a particle in the interior of a liquid is equally attracted on all sides, but a particle in the surface layer is attracted inwards by the particles of liquid within its sphere of influence, the corresponding attraction by the few particles in the vapor space being negligible in comparison. Hence, a t the surface of every liquid there is a force, the so-called surface tension, which acts inward and causes the liquid to act as if it were covered by an elastic skin. The surface tension of petroleum from different sections of the United States, and of the products derived from the various crudes, was determined in order to ascertain whether a relation existed between the surface tension and any other physical properties of the oils. The measurements were made with the duNouy apparatus. The apparatus is simply a torsion balance, but instead of measuring the tension by means of weights, the torsion of wire is used to counteract the tension of the liquid film and to break it. A single reading on a dial supplies a figure from which, if the apparatus has been previously standardized with water, the surface tension may be obtained by a simple proportion.2 1 Received January 16, 1922. Presented before t h e Section of Petroleum Chemistry a t the 63rd Meeting of the American Chemical Society, Birmingham, Ala., April 3 t o 7, 1922. 2 J. Cen. Physiol., 1 (1919), 621.
METHOD OF OPERATING The watch glass or beaker to be used for containing the liquid is thoroughly cleaned in a mixture of potassium bichromate and sulfuric acid, and then rinsed in distilled water. The liquid to be tested is then poured into the watch glass which is placed upon the table G in the accompanying figure. The platinum wire H is held in a flame until all organic material is burned. Then, without touching it with the fingers, it is hooked to the lever D. The needle indicator A is brought by means of the knob B just in front of tlie point 0 on the dial. Then, by using the adjusting screw F, the torsion of the wire is modified until the lever D is just above the resting platform E, the distance between them not exceeding the thickness of a piece of very thin paper. The screw C being fixed in position, one will observe that an imperceptible movement of the knob B will bring the lever in contact with platform E practically without changing the reading on the dial. The apparatus is now ready for use.
The table G is raised slowly by means of the adjusting screw until the liquid touches the platinum ring H. One must be sure that a perfect contact exists. Then, by turning the knob B the torsion of the wire is controlled, and one keeps on turning until the ring is suddenly separated from the liquid by tearing off of the film. The reading is then made. The average of the two or more readings is noted and the instrument is standardized as follows: A small piece of clean paper is cut in such shape that, it can easily be slipped upon the platinum ring H, in the stirrup, after i t has been weighed. Weights are added on top of it, until the lever is forced down to its horizontal position, not quite in contact with the platform. It is now obvious that the sum of the weights, plus the weight of the paper, represents exactly in grams the strain of the wire. This being determined, we know the strain of the liquid film which was counteracted by the torsion of the wire. As the surface tension is expressed in dynes per cm., the number of grams must be multiplied by 981, and this product divided by the length
T H E JOURNAL OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY
July, 1922
of the circumference of the ring in order to have the number of dynes for every centimeter; but, as two films act on the ring, one outside and one inside, this result must be divided by two. Surface tension in dynes per cm. = where M = Weight in g. g = Acceleration in cm. per sec. L = Length of platinum wire in cm.
2L
Example Water a t 60' F. Weight necessary t o bring lever down, grams Weight of paper, grams Total weight Then 0.6390 X 981 8 = 78.357 Reading on dial 81
0.6260 0.0130 0.6390
Then any reading on the dial, multiplied by the ratio 78.3.57/81 or 0.967, will give the surface tension in dynes of any liquid tested. The results may be checked to 0.2 dyne per cm. by this method.
EXPERIMENTAL The surface tension of distilled water a t different temperatures was determined, in order that comparisons could be made between the surface tension of oils and water a t the same temperature, and that the duNouy figures could be compared with those obtained by any method. TABLE I-SURFACE TENSION O F WATER TEMPERATURE SURFACE TENSION O F. Dynes/Cm. 60 65 70 75 80 85 90
78.2 78.1 77.6 77.1 76.6 75.9 75.4
The surface tension and the gravity of petroleum were next determined. TABLE11-SURFACE TEXSION OF PETROLEUM FROM DIFFERENTFIELDS IN THE UNITEDSTATES BASE O F SPECIFIC SURFACE TENSION SOURCs OIL COLOR GRAVITY B6. Dynes/Cm. a t 8.5' F. Cabin Paraffin Yellow 0.7865 48.0 28 8 Creek, W,Va. Winnette, Very dark Mont. green 0.7937 46.4 29.0 Cushing, SemiVery dark Okla. paraffin green 0.8279 39.1 29.7 Osage, SemiOkla. paraffin Dark green 0.8403 36.6 30.7 SemiVery dark Bixby, Okla. paraffin green 0.8444 35.8 30.8 Homer, SemiVery dark paraffin green 0.8464 35.4 31.2 La.
....
The results indicate that a relationship exists between the surface tension and the specific gravity of petroleum and that the former increases as the latter increases. The physical constants of products from certain Oklahoma crudes were determined to ascertain the effect, if any, of gravity, volatility, and viscosity upon the surface tension. The tests indicate that the surface tension increases as the Baume gravity decreases and that the presence of highboiling products increases the surface tension. The surface tension appears to increase as the Saybolt viscosity increases, but the latter increases somewhat more rapidly than the former. The viscosity as determined by the Saybolt method is simply s measurement of the rate a t which the oil will rise in a small capillary tube when under a slight pressure. As the surface tension is frequently determined by noting the height to which a column of liquid will rise in a small capillary tube, it would be expected that the Saybolt thermo viscometer readings would be affected by surface tension as well as by viscosity. The results show also that the surface tension decreases as the temperature increases. It appears that the surface tension varies approximately 0.05 dyne per em. for each O F. change in temperature of the products tested.
TABLE 111-TESTS
ON
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LIGHTERFRACTIONS FROM PETROLEUM
SURFACE TENSION, DYNESPER CM. a t O F . Gravity Initial Mar. 60°F. 90'F. 60 70 80 85 90 62.4 108 ... . . . 24.4 60.6 106 125 2s:i 2 6 : i 2 5 : s ... 59.3 111 24.9 59.1 96 24.8 58.9 120 25.3 58.6 114 25.3 58.4 100 412 25.3 58.4 434 110 ... 25.6 436 57.4 131 25.7 57.3 100 27:s 2713 2 s : 9 ... 57.1 116 24.8 57.1 110 ... 25.7 56.8 118 143 27:s 2 7 : i 26:s ... 56.9 120 . . . . . . . . . . . . 25.8 ... 56.3 106 ... 25.8 ... 55.7 140 2 7 : s 2 7 : 5 2+:0 154 55.2 126 2s:3 ... 54.3 146 27.4 ... 48.32 132 28.2 46.7 224 29.2 ... 45.9 146 249 . . . 3 0 : o 2 9 ' 5 23:0 . . . 43.92 146 29.1 ... 42.2 320 326 iii 3 i : 9 3 i : 9 3 0 : 9 30,8. 41.5 344 575 419 312 3 2 . 7 3 1 . 9 3 1 . 2 ' 30.9 39.5 380 545 612 396 3 3 . 1 3 2 . 4 3 1 . 9 31.4 1 Viscosity determined with Saybolt Thermo Viscometer. 2 Cracked product.
Be. BOILINGPOINT,
F. VISCOSITY'
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As difficulty is sometimes experienced in pressing wax from wax distillate, on account of its poor crystalline structure, tests were made to ascertain if a relation existed between the surface tension and the crystalline structure of the wax in the distillates. The results are shown in Table IV. TABLEIV-TESTS O N WAX DISTILLATE Crystalline Viscosity a t Surface TenBe. Cold Structure of looo F. Say- sion Dynes / Gravity Flash Fire Test Wax Present' bolt Universal Cm. a t 8 5 O F . 31.8 255 315 62 Fair 51 34.4 Fair 31.8 280 350 56 35.0 61 Fair 29.9 275 340 53 34.6 85 Fair 27.5 295 380 68 35.5 113 315 49 Po& 31.6 260 34.4 52 Poor 31.4 284 322 62 34.7 70 Poor 370 440 77 36.2 28.1 153 320 56 57 30.6 250 Good 35.0 1 Determined by microscopic examination of the wax extracted from the war distillate by shaking with chloroform, cooling, and centrifuging t o remove the separated wax.
The wax was removed from two distillates, and t>hesurface tension of the oil before and after the removal of the wax was determined. No difference was noted. The addition of 10 per cent of petrolatum, and 9 per cent of 122" meltingpoint wax also failed to change appreciably the surface tension. The above tests indicate that the surface tension test cannot be used as a guide for determination of the quantity and the crystalline structure of the wax in distillates. The true relation between the surface tension and the lubricating value of an oil has not been determined, but it is evident that surface tension plays an important role in lubrication because it may aid in the maintenance of the oil film between the moving surfaces. In order to study the relation between the surface tension and the properties of lubricants, tests were made on oils of different viscosities, on vegetable oil, and on a mixture of fatty acid and cylinder stock. TABLE L7-TEsTs
PRODUCT Oklahoma lubricating oil Oklahoma lubricating oil Oklahoma lubricating oil Oklahoma lubricating oil Oklahoma lubricating oil Oklahoma cylinder stock California lubricating oil Castor oil
OF
LVBRICAETS
Saybolt Surface Universal Tension B6. Cold Viscosity Color Dynes / Gravity Test a t 100' K . P . A . Cm. 8 5 O F 27.1 23 155 236.0 27.2 25 160 336.2 26.2 26 185 436.0 25.8 25 240 636.4 25.5 19 260 37.5 23.8 24 1160 %en 37.3 1 9 . 7 -0 428 37,3 15.9 10 looat ;.540,4 . 210°F.
The above tests indicate that the great variations observed in the viscosity, the usual standard for determining the lubricating value of an oil, cannot be explained by the surface tension test of the oils as determined against air, because
