SURFACE TENSION by MAXIMUM BUBBLE PRESSURE HELMUT M. HAENDLER*
AND
WALDEMAR S. McGUIRE
Northeastern University, Boston, Massachusetts
HE determination of surface tension by the method liquid being measured proves satisfactory under the of maximum bubble pressure is an excellent experi- proper conditions. A more complete discussion of the ment for the undergraduate physical chemistry theory is given by Adam (2). The apparatus, which is shown in Figure 1, consists laboratory. The apparatus, with the exception of the glass bubbler, which is not expensive, can be assembled essentially of a glass bubbler suspended in a constant from materials found in the laboratory and shop. An temperature bath, a manometer, and a mercuryLasC: inexpensive substitute obviates the need of a cathe- pirator. The bubbler, Figure 2, can be purchased directly from tometer, as in the capillary height method. Surface tension measurements of different liquids can be made Scientific Supplies, Ltd., of London, or it can be made rapidly and accurately, and, if desired, the student can by a competent glass-blower. The size varies with the carry out the experiments at a series of temperatures dimensions of the other apparatus available; 15-25 with greater facility than with the capillary height cm. overall length is convenient. The two sections of the instrnment are joined by a ground-glass connection (a);the side-arm (b) is connected with the manometer and aspirator system; and (c) and (d) are the wide and narrow tubes, respectively. The narrow tube?@), which is of capillary dimensions, is made separately and tested before being joined to the tube (e). The di-
T
method. From this series of values the temperature equation can be derived by least squares, the critical temperature determined, and Eotviis' constant calculated. Cantor (2) and Jaeger (5, 6) originally devised and developed this method so that satisfactory determinations were possible. An air bubble was blown a t the bottom of a tube dipping into a liquid and the pressure required to burst the bubble measured. Sugden (8, 9) improved and extended the method, avoiding many of the experimental diffLculties by using two tubes of different diameter dipping to the same depth in the liquid. An empirical equation,
in which A is a calibration constant, PI - Pzis the measured pressure difference in dynes / sq. cm., r is the radius of the larger tube, and D is the density of the
ameter of this capillary should be such that i t gives a pressure diierence of 4-10 cm. of ethyl alcohol on the * Present address: University of Washington, Seattle, Wash- manometer when air is sucked through the tip, which is dipped into benzene to a depth of about 1 cm. The ington. 5911
appropriate capillary is joined to (e) so that it and the wide tube (c) end in the same horizontal plane. The open manometer consists of a U-shaped tube of mm. internal diameter Pyrex tubing, which is connected to the aspirator and bubbler. As a cathetometer is often not available in the laboratory a simple and efficient substitute can be constructed by mounting a vernier caliper on small metal blocks in such fashion that the knife edges used for measuring internal diameter come directly over the manometer tube. The height of the meniscus can then be measured directly to 0.01 cm.,with further estimation possible. The mercury aspirator recommended by Sugden is constructed by drawing out the tube of a 250-ml. separatary funnel to a fine capillary, the diameter of which should be such as to cause bubbles to form a t the rate of one per second on the bubbler tubes. The manometer, caliper, and aspirator are mounted on a wooden support. with the glass tubing supported by wires passing through holes in the support. The bubbler is placed in the laboratory constant temperature bath, or, if measurements a t more than one temperature are desired, a simple temperature bath with control to a t least O.l°C. can be constructed from a double-walled container, such as an old calorimeter tank, with a motor stirrer mounted on a separate support to reduce vibration. An immersion heater and a light bulb are operated manually and serve to heat and control the temperature. A 100°C. X '/lo thermometer is used. An excellent extension rod for mounting the bubbler or thermometer can be made by screwing a tripod leg into the socket left when the adjustable portion and jaws of a single buret clamp are removed. The instrument constant, A, is determined by measurement of pure, fractionated benzene, the surface tension of which is given by the equation
and substitution of the suitable values in equation ( 1 ) .
The density oi benzene a t varidus temperatures can be found in the Critical Tables (4). The density of the manometer alcohol, which is tinted with fuchsine, is determined a t a few temperatures between 20' and 30°C. and a graph or table constructed for intermediate temperatures. The pressure difference in cm.of alcohol is converted to dynes/sq. cm. As bubbles are formed on the large tube, the stopcock of which is open, there is an increase in pressure and then a sudden decrease as the bubble bursts. The reading is taken a t the lowest pressure, after which the stopcock is closed and the pressure of the bubbles formed on the small tube taken. The densitytemperature equation for various liquids to he tested can usually be found in the literature or can be determined for a range of temperature as a subsidiary experiment. The parachor of the compound can be determined after measurement and compared with known values, and the critical temperature and the constant can be calculated from data a t two diiTerent temperatures by means of the Ramsay and Shields' modification (7) of the Eotvos equation (3). Benzonitrile, methyl ethyl ketone, benzyl alcohol, bntyl chloroacetate, ethyleneglycol monoethyl ether, and m-iodotoluene were measured with the apparatus as described. The following liquids are recommended for original work on the part of the student: anisaldehyde, chloroacetone, o-dichlorobenzene, n-naphthyl isocyanate, tert.-butyl bromide, ethylaniline, eugenol, heptaldehyde, isopropyl bromide, n-propyl iodide, npropyl n-proprionate, o-chlorophenol, m-chloroaniline, p-cymene, o-iodotoluene, o-iodoanisole, methyl ptoluene sulfonate, and terpineol. As an example of the determinations possible, the measurement of benzyl alcohol may be considered. From 25-35°C. the values of the surface tension give the approximate equation, ST = 44.593 - 0.1268t; a parachor of 263.3 as compared with the known value of 266.1 ; a critical temperature of 455 "C. ; and a constant, k, of 2.15.
LITERATURE CITED
(1) ADAM,"The physics and chemistry of surfaces," pp. 12-8, 292424, Ckendon Press, Oxford, 1930. CANTOR, Wied. Ann., 47, 399 (1892). EliTvBs. ibid., 27, 448 (1886). Infernationel Critiml Tables. 3, 28.
(5) Jmcan, Akad. W&s. Wien, 100, IIA, 245, 493 (1891). (6) JAEGER, Z . anorg. C h . , 101, 1 (1917). (7) RAMSAY AND S ~ E L DZ. S ,physik. Chem.. 12,431 (1893). (8)
SUGDEN. J..Chem.
Sac., 121, 858 (1922).
(9) SUDDEN, ~ b s d . 125, , 27 (1925).
