SURFACE TENSION by the
RING METHOD
APPLICABILITY OF THE. DU NOUY APPARATUS RUDOLPH MACY Edgewood Arsenal, Maryland
I t i s possible by the ring method to obtain surface-tension
employ a cnlibration curve i n which determinations d e by the ring method are plotted againit correct values as tional Critical Tables. Two methods available are: (1) giuen in International Critical Tables. The calibration to apply correction factors according to the procedure procedure gives almost as good results as the exact method recornended by Harkins and his associates, or (2) to of Harkins.
data uniform with those @en as "correct" in Intema-
S
URFACE-tension data obtained by t8e capillary-height method are generally accepted as correct. For example, the surface tensions given in Internutional Critical Tables ( I ) by Harkins and his associates have been, "for the greater part," corrected to agree with certain "standard" values for water and benzene as determined by the capillaryheight method in air a t 760 mm. On the other hand, one can hardly doubt that the most rapid and simplest method for determining the surface tension of liquids is one of the ring methods made popular about fifteen years ago by the introduction of the du Nouy surface-tension apparatus (2, 3a, 3b, 4). It has been the experience of the writer that the remarkable convenience of this instrument for routine work and for such purposes as student instruction is largely nullified by the lack of proper information as to its use and the consequent loss of time required for one to become acquainted with it.
It is the purpose of this article to show that the du Nouy apparatus gives results uniform with the data in International Critical Tables, if the readings made with the instrument are corrected in accordance with the information given in the papers by Harkins and coworkers (5, 6), or if the instrument iscalibrated against a few liquids, the data for which are given in Intermtional Critical Tables. It is hoped that the paper will be of assistance to those who have put implicit faith in the claims made by the bulletins describing the du Nouy apparatus (3a, 3b), and to those whose work does not allow an opportunity for making a thorough examination of the characteristics of the instrument.* The apparatus discussed in this paper is the simple model described in reference (3a). The liquid is slowly * The Central Scientific Company has recently issued Bulletin No. 101, "The Ring Method for Surface and Interfacial Tensions" (no date), which contains an excellent bibliography and instructions for use of the Harkins correction factors (5.6).
raised by means of an adjusting screw until it touches the platinum ring. By turning a knob and increasing the torsion on the taut piano wire which supports the ring, the ring is then slowly lifted until it suddenly tears away from the liquid surface. The torsion on the piano wire, expressed in dynes, is a measure of the surface tension of the liquid. The relative surface tensions for different liquids are read on a dial. On the instrument used in this work the dial readings can be made to about 0.3 of a scale division, but on a later model (36) readings can be made much more accurately. DIRECT DETERMINATION OF SURFACE TENSION
According to instructions in the bulletin (3a) issued by the manufacturer, the instrument may he used for direct surface-tension measurements. This is done by forcing the ring hack to its zero position by adding weights to it, after the tearing of the liquid surface has taken place. The following experiment with ethyl acetate at 26'C. illustrates this procedure, and also shows how greatly the surface tension obtained differs from the accepted value. weightrequired to forcering back to zero = 0.223 g. = 27.3 dynes/cm. Surface tension * 0.223 2 x 4 In the calculation, the farce in dynes is divided by 8 because the ring has a mean perimeter of 4 cm., and there are two surfaces, one on each side of the wire, composing the ring. The accepted value for the surface tension of ethyl acetate at this temperature is 23.5
-,..-I,
r l . r n 0 ~/"rn
--L
.
It has been shown by Klopsteg (7, 8 ) that the proper technic in handling the instrument is to keep the ring in a constant level position, by lowering the liquid while
increasing the torsion on the wire. The surface tension of ethyl acetate, using this second procedure, was found to be 25.7 dynes/cm. This procedure gives lower results, in better agreement, but not in good agreement with accepted data. It is a tedious method, compared with the original procedure of simply lifting the ring, although i t is obviously more correct (4, 7 ) . The surface tensions determined by both methods for a number of liquids are shown in Table 1. In every case, except for water, the values are much too high, but the method of lowering the liquid gives the better data. The results obtained by the two methods are not quite parallel. For liquids with the usual surface tension of-about 30 to 40 dynes/cm. the "pull-ring" method gives data which are about 1.5 dynes/cm. higher than the results obtained with the "lower-liquid" method. For glycerol and water, however, the deviation rises to 2.3 and 2.7 dynes, respectively. METHOD OP STANDARDIZATION
In addition to the direct method of measurement of surface tension described in the preceding section, the instructions also recommend a standardization, so that the dial reading on the surface-tension apparatus can be converted directly to dynes/cm. This can be made by putting the ring in any definite position, taking the dial reading, and then placing weights on the ring and noting how much weight is required to return the ring to the zero position. For the instrument used in this work, a weight of 0.3 g. was equivalent to a reading of 54 dial units,
- .,
O.3 8 = 36.8 ddynes/em . 1 d i d unit' = 0.682 dyne/em.
Then 54 dial units
This value was used in converting the dial readings in Table 1 to the surface-tension figures given in columns 4 and 5 of the table. Surfarc lcnsion in dmrr/cm.
Did rooding pull Lower
PC.
(0) Pull
V)
(7)
Lower
I.C.T.
METHOD OP CRLIBRATION
Emor (7)
Prom
Col.
~.
17,,
Ring Liprid R h g Lipuid
rolucr
curve -Cot.
(6)
Ether kc-Bufyi alcohol Acetone Ethylacetate
24.0
29.0 27.0
24.5 25.0 28.0
38.3 36.0 28.1 24.6 22.5 * 0 1 22.5 39.8 37.0 27.2 25.2 23.1 -0.2 23.1 4 0 0 37.7 27.3 25.7 23.5 1;0.4 23.5
25.6
41.0 38.7 2 8 0 20.4 24.2 i0.4 24.2 47.0 44.5 32.1 30.4 28.2 * O O S 27.7 -0.5
-,.---.,.-.-"~25.0 n . . . . . , ho~
16.7 10.2 0 0 0
"1""-
Benzene ~ t ehloride Chloroben.ene A ~ I YirothioI cyanate
methane Bromobenzene Bennaldehyde ~niiine Nitrobenzene Ethylene Glycerol Wafer
19.8 18.4 16.5 -0.2
-
h
~
~
~
~
~
22.0 24.5
54.2 51.8 37.0 35.3 31.9 -0.3 55.5 53.0 37.9 36.1 32.8
32.0 +0.1 33.0 +0.4
25.0 26.0 24.0 25.0 22.0 27.0
56.5 60.0 60.7 64.3 70.0 70.0
33.9 36.3 36.5 39.0 42.4 42.7
54.0 58.0 58.5 61.7 67.2 67.7
38.6 4K9 41.5 43.8 47.8 47.8
36.8 s9.5 39.9 42.1 45.8 46.2
33.9 35.8 36.0 39.5 42.7 43.1
-0.3 -0.1 -0.5 -0.2 -0.4 -0.5
0 +0.5 +0.5 -0.5 -0.3 -0.4
22.0 77.2 74.2 52.6 50.6 47.6-10.5 47.2 -0.4 22.0 100 96.7 68.2 65.9 63.3 *3.0 63.5 +0.; 24.0 111 107 76.7 73.0 7 2 . 1 5 ~ 0 0 572.1
* The rearon for the use of the symbol (9)is explained at a later point io the paper.
A method of calibration which has been suggested (3a, 3b) is to obtain the dial reading a t which the ring pulls away from a water surface, and to use that value as a standard for other liquids. From Table 1 it will be seen that a reading 04107 dial units was obtained for water a t 24'C. The accepted value for the surface tension of water a t this tem~eratureis 72.15 dvnedcm. From the ratio 107/72.15 we should be ahie t o calculate the surface tensions of other liquids by simple proportion, after the dial reading has heen made a t the point when the ring is tom from the liquid surface. From Table 1, however, it is obvious that W b r is the worst &pato choose for cal&'ation purposes. Water is the only liquid whose surface tension is measured directly with moderate accuracy by the du Nouy apparatus; the results for all other liquids are high. is shown graphically in ~i~~~~ 1, in which the data point for water is indicated. If a straight line is drawn between the origin of the graph
and the point for water, this line will lie above the curve which is shown and will give high surface-tension data. A better way and perhaps the simplest way to cali-
-Y
At any rate, a calibration curve similar to that in Figure 1 is necessary to convert readings on the surfacetension apparatus to surface-tension data in dynes/cm. USE O F THE HARKlNS CORRECTION PACTOR
80
8
2 60
h 5
.- 40
8 $ z0 0
(0
20
40 60 Dial Reading
80
100
FIGURE1.-RELATION BETWEEN DIAL READING ON THE SwPACE-TENSIONAPPARATUS, AND S-ACE-TENSION DATAFKOM I N T E W I ~ NCRITICAL AL TABLES
The fact that the ring method is not a reliable method for the direct determination of surface tension has already been commented on extensively in the literature. For example, MacDougall (9) states, "I do not think that the simple theory of the experiment even with the procedure advocated by Klopsteg can lead to accurate values of the surface tension." Johlin (10) mentions that the value for benzene is high compared with the value for water, nsing the same ring. Other papers of interest are those cited in references (11, 12, 13, 14). It remained for Harkins and his co-workers (5, 6), however, to give the conclusive demonstration that the measurements made as so far described in this article are simply measurements of "the pull necessary to detach a ring from the surface of a liquid," and are not measurements of surface tension. The expression
brate the du Nouy apparatus is to obtain a calibration curve similar to that in Figure 1. This requires simply the dial readings for a series of,common organic liquids as well as for water, and the plotting of these readings against the accepted values for the surface tension at the corresponding temperature. Practically any of the common liquids of reasonable purity can be used for the calibration. No attempt was made to purify the liquids recorded in Table 1, although C.P. or u.s.p. products were employed in most cases. The smoothed data for the construction of the curve in Figure 1, as obtained for the instrument used by the writer, are shown in Table 2. In column 7 of Table 1 are given the surface-tension data read from this curve. It will be seen that they do not vary from the so-called "correct" values by more than 0.5 dyne/cm. This is good agreement when we consider that surface-tension measurements by different methods Often vary to the extent of as much as 3%. e The calibration curve in Figure 1 was obtained by the "lower-liquid" method. If the dial readings obtained by the "pull-ring" method are used instead, i t will be found that the curve does not go through the origin. It is possible, however, to construct a calibration curve using these measurements also. TABLE 2 SUOOTaBD D A T A BOP CALCBEATrON CUPYE OF FlOUPB
D i d Rcoding (Lmering ihr Licuidl
Svrfocr Tcnsia dmcr/cm.
1
where M i s the weight of liquid raised above the free surface of the liquid, and R is the radius of the ring measured from the center of the ring to the center of the wire, should not he equatedto y, the surface tension, but is better labeled by some other symbol, p.. The proper expression for the surface tension ( 7 ) is then where F is a factor which varies 6ith diierent rings and for different liquids. The factor for the size of ring is given by the ratio R/r, where r is the radius of the wire. The factor for the liquid is dependent on the ratio Ra/V, where V is the volume of liquid raised above the free surface of the liquid [or V = M/(D
