Nov., 1954
SPREADING OF HYDROCARBONS ON WATER
945
SPREADING OF HYDROCARBONS AND RELATED COMPOUNDS ON WATER BYJAMESE. SHEWMAKER,’ CARLE. VOGLER AND E. ROGER WASHBURN Department of Chemistry and Chemical Engineering, University of Nebraska, Lincoln, Nebraska Received November If, i B b l
The tendencies of several alkyl benzenes, isooctane and tetralin to spread on water have been investigated by two methods. Surface and interfacial tensions have been measured and the corresponding initial spreading coefficients have been calculated. With the exception of the tetralin, all of the materials have positive spreading coefficients. Spreading ressures for the spreading liquids have been measured with the hydrophile balance. The spreading pressures and initiar spreading coefficients are nearly the same for these liquids. Evidence is presented which indicates that the spreading ability acquired by aged tetralin is due chiefly to the formation of tetralin hydroperoxide.
The relative abilities of different liquids to spread on water as duplex films may be predicted from initial spreading coefficients which are related t o surface and interfacial tensions by an equation formulated by Harkinsa2 Harkins considered that duplex films must be a t least two molecules in thickness and probably are three or more molecules in thickness. He stated that the lower hydrocarbons may spread on water as duplex films which later change to non-duplex films or monolayers. Langmuir3 considered that monolayers containing long hydrocarbon chains behaved essentially as duplex films. It appears that unless a monolayer acts as a duplex film the concept of spreading coefficient, depending upon interfacial tension, is without significance for lens and monolayer type of spreading. Miller4 concluded that the spreading pressures for liquids which spread from lenses as monolayers should differ from initial spreading coefficients by the forces necessary to orientate and pack molecules in the films. The possibilities suggested by these viewpoints have made it seem worthwhile to carry out an experimental study of the spreading behavior of hydrocarbons including alkyl benzenes having different length and complexity of side chains. One of the hydrocarbons, tetralin (1,2,3,4-tetrahydronaphthalene) , was observed to be non-spreading in the pure form. A considerable spreading ability was acquired by tetralin which was allowed to age for some time after purification. Robertson and Waters6 have studied the autoxidation of tetralin and the decomposition of tetralin hydroperoxide. They have identified many of the products of these reactions and have determined the relative amounts in which some of them are formed. The chief products are tetralin hydroperoxide, a-tetralone and atetralol. It was decided to determine the surface activity of each of these materials in the pure form and in solution in tetralin and to determine which is the chief contributor to the spreading ability of aged tetralin. The kinetics of the oxidation has been further studied by Woodward and Mesrobiann6 Materials.-The hydrocarbons, ethylbenzene, normal and isopropylbenzenes, n-, sec- and t-butylbenzenea, and (1) E. I. du Pont de Nemours and Company Fellow 1949-1950; Standard Oil Company (Indiana) Fellow 1950-1951; present address: Standard Oil Development Co., Linden, N. J. (2) W. D. Harkins. J . Chem. Phya., 9, 552 (1941). (3) I . Langmuir, ibid.,1, 756 (1933). (4) N . F. Miller, THIS JOURNAL, 46, 102.5 (1941). (5) A. Robertson and W. A. Waters. J . Chem. SOC.,1574, 1578 ( 1948). (6) A. E. Woodward and R. B. Mesrobian, J . A m . Chem Soc., 7 6 , 6189 (1953).
isooctane (2,2,4-trimethylpentane) mere of the best grade obtainable from Eastman Kodak Co. When necessary they were purified by fractional distillations until satisfactory agreement was obtained with recently published values for density, refractive index and surface tension, or until constant values unchanged by further purification were obtained. An analytical grade of benzene was recrystallized until nine-tenths of the liquid solidified within the range 5.54 to 5.52’. A technical grade of tetralin was purified by treatment with sodium followed by repeated fractional distillations until satisfactory constants were obtained. Interfacial tensions with water probably provide the best indication of purity for such materials at least as far as surface active impurities are concerned. These values have not been previously determined for more than half of the hydrocarbons investigated and agreement with those that have been recorded was not as close as with the other constants. It was our practice to extract surface active jmpurities from the hydrocarbons with water until interfacial tensions were reproducible and constant. .Measured constants are recorded in Table I. Carefully redistilled water with a surface tension of 72.10 0.04 dynes/cm. at 25.0’ was used in all critical parts of the work. Tetralin hydroperoxide was synthesized by the method of Hock and Suaemih17 from purified tetralin. The final product, after several recrystallizations from petroleum ether (30-60’), melted a t 56.0”. The freshly purified hydroperoxide showed no surface activity when placed on pure water. The white, dry and nearly odorless crystals were stored in an atmosphere of nitrogen in a laboratory desk. After several months, during which time they were exposed only intermittently to diffuse light, they became yellow, sticky and very surface active. Because of this change it was necessary to recrystallize the material shortly before using. a-Tetralone was synthesized from r-phenylbutyric acid as described in “Organic Syntheses.”* The material was a pale yellow oil having an odor resembling that of peppermint, and boiling at 104’ under 2 mm. of mercury pressure. Its refractive index was 1.5669 and its density 1.0919 g./ em.* at 25.0’. Ita spreading pressure, measured with a hydrophile balance, was 14.0 dynes per cm. This value did not change during a month of aging, in a laboratory desk, even though the oil darkened slightly. The a-tetralol was prepared from a-tetralone by the lfeerwein-Ponndorf-Verley method of reduction. The atetralol produced was a light yellow oil which diatilled a t 107’ under two mm. of mercury pressure. Ita spreadin pressure was 19.4 dynesjcm. while its refractive index and density at 25.0’ were 1.5613 and 1.0750 g./cm.8, respectively. I t darkened slightly upon aging for several weeks, in a laboratory desk, but showed no measurable change in surface activity during this time. Methods.-The surface and interfacial tensions necessary for the calculation of spreading coefficients were measured by capillary rise methods. The instruments and methods of operation have been de~cribed.~Spreading pressures were measured with a hydrophile balance mounted in a coustant temperature cabinet. The barriers were moved along the tray of this balance by means of a motor driven mechanism mounted outside the cabinet. A small iron rod
+
(7) H. Hock and W. Susemihl, Ber., 66, 61 (1933). (8) E . L. Martin and L. F. Fieser, “Organic Syntheses,” Coll. Vol. 11, John Wiley and Sons, Inc., New York, N. Y . , p. 569. (9) L. F. Transue, E. R. Washburn and F. H. Kahler, J . A m . Chem. Soc., 64, 274 (1942).
JAMES E. SHEWMAKER, CARLE. VOGLERAND E. ROGER WASHBURN
946
Vol. 58
TABLE I PROPERTIES MEASURED AT 25.0' Material
Density, dcm.8
Benzene Tolueneg Ethylbenzene n-Propylbenzene Isopropylbenzene n-Butylbenzene sec-Butylbenzene t-Butylbenzene Isooctane Tetralin
0.8734 .8623 .8625 .8579 ,8577 .8560 8577 .8627 .6880 .9659
Refractive index, nD
1.4979
...
1.4931 1.4895 1.4889 1.4873 1.4877 1.4902 1.3890 1* 5393
with a pointed hydrophobic tip was permanently mounted to the tray to serve as a reference point in filling the tray so that depth of liquid might be reproduced. The experimental techniques have been described.10 Dean and Lill recognized the possibility that sorption of vapors from the spreading liquid by the surrounding stearic acid monolayer might interfere with the direct measurement of spreading, pressure of volatile liquids with the hydrophile balance. They concluded, however, that the area of monolayer exposed- to saturated vapor is too small to produce a detectable change in spreading ressure. The criterion would seem to be whether or not alrthe spreading liquid becomes compressed into a lens by the expanded monolayer. In this investigation care was taken to avoid such excessive compression.
Results.-Many measurements, twenty to thirty or more, of surface tension and interfacial tension were made on each of from three to eight different portions of the most carefully purified sample of each material. Between measurements the relative positions of the surfaces or interfaces were altered. This was done in several ways, by tilting the instrument, by altering the pressure on the liquid in the wide tube, or by altering the amount of liquid in the instrument. For the interfacial tensions the water and organic liquids were mutually saturated. The mutual solubility of the hydrocarbons and water was so slight that the densities of the pure liquids and of the liquids saturated with water did not differ by more than 0.0003 g . / ~ m . ~The individual determinations of surface tension did not differ from the recorded average values by more than 0.05 dyne/cm. The deviations were somewhat larger with interfacial tensions but the great majority of the individual measurements agreed with the recorded average within 0.1 dyne/cm. Average values are recorded in Table I. Different samples, even of the same liquid, did not always display the same pattern of spreading. Sometimes the added liquid seemed to spread as a layer of uniform thickness until stopped by the film pressure built up in the surrounding stearic acid monolayer. These broad lenses or layers would sometimes break up into many small lenses separated from each other by an invisible film. Some samples of each liquid studied, except tetralin, formed a lens, or a cluster of small lenses, surrounded by a very thin film which made no visible angle of contact along its outer boundary with the surrounding stearic acid. There was no measurable difference in the spreading pressures which (IO) E. R. Washburn and C. P. Keim, J . Am. Chem. Soc., 62, 1747 (1940). (11) R.B. Dean and F. S. Li, ibid., 5'2, 3979 (1950).
Surface tension, dynes/cm.
Interfacial tension. dynes/cm.
28.23 27.94 28.43 28.49 27.71 28.73 28 * 20 27.63 18.44 36.02
34.0 35.7 37.4 38.5 38.7 39.6 39.2 39.3 49.3 38.6
Spreading coefficient, dynes/cm.
9.9 8.5 6.2 5.1 5.7 3.8 4.7 5.2 4.4 -2.5
Spreading pressure, dynes/cm.
9.8 8.8 6.0 5.1 6.0 3.6 4.7 5.3 4.4 0.0
could be identified with the pattern of spreading shown by samples of the same material of equal purity. The deviations among a great many individual determinations of spreading pressures from the recorded averages were not greater than 0.2 dyne/cm. These values were obtained by adding dry hydrocarbons to the stearic acid film covered water surface. The values obtained with hydrocarbons saturated with water were not measurably different. The spreading pressures as measured with the hydrophile balance are direct measures of the tendencies of the hydrocarbons to spread on water in contact with air. Exactly, they are the minimum film pressures necessary to prevent the hydrocarbons from spreading from a lens as a thin layer. This is shown by the following development. The initial spreading coefficient for a hydrocarbon, b upon water, a, according to Harkins12is represented by S b / a = */a
- (Yb
+
Ys'b')
(1)
I n a similar manner the tendency for a hydrocarbon to spread upon water initially covered with a monolayer of stearic acid a* is given by the relation Sb/a*
= Ya*
- ( Y b + Ya'b')
(2)
and spreading will occur if this difference is greater than zero. The surface tension of the film covered water ya*may be represented by Ya* =
ya
-
(3)
7r
where T is the film pressure. A combination of these equations results in Sb/a* = Sb/a
-
ir
(4)
which indicates that the spreading of the hydrocarbon as a duplex film will stop when the film pressure of the stearic acid becomes equal to the initial spreading coefficient of the hydrocarbon. These spreading pressures should not be confused with the equilibrium film pressures of Harkins2 although in some cases they are not much different in magnitude. Harkins' equilibrium film pressures were measured in an atmosphere saturated with vapors of the spreading liquid, a condition not easily attained when the organic liquid is added to an open surface of water. The spreading pressures are definite quantities easily reproducible to within a few tenths of a dyne per centimeter. The fact that the measured spreading pressures for the hydrocarbons studied in this investigation agre? closely with initial spreading coefficients may indicate that these liquids spread on an open water
Y
Nov., 1954
SPREADING OF HYDROCARBONS ON WATER
surface as films which are either polymolecular in thickness or if monomolecular they are at least duplex in character. That is, they possess independent liquid-air and liquid-water boundary tensions. In any case the findings of this research neither confirm nor do they contradict the conclusion of Miller4 that the spreading pressure is greater than the initial spreading coefficient. Several of the liquids studied by Miller gave differences which are no larger than the experimental error. The difference between the spreading pressures of benzene and toluene is 1.0 while for toluene and ethylbenzene the difference is 2.8, almost three times as large. For ethylbenzene and n-propylbenzene the difference is 0.9 while for n-propylbenzene to n-butylbenzene a larger difference, 1.5, is again noted. This alternation of relative change in spreading pressure is so much larger than experimental error that it seems significant. The values for ethylbenzene and isopropylbenzene are identical and more nearly like that of the compact t-butylbenzene than any of the other alkyl benzenes. It is possible that these relationships are determined by ease and efficiency of packing the molecules in the surface layer or in the film, A fairly straight line is obtained when spreading pressures of the straight chain alkyl benzenes are plotted vs. number of carbon atoms in the chain. This line, when extrapolated, indicates that alkyl benzenes with straight chains containing more than seven carbon atoms should not spread upon water. A phenomenon which may be related to packing or molecular arrangement was encountered. When small amounts of benzene, toluene or isooctane were dropped on a stearic acid monolayer under an initial pressure somewhat less than the spreading pressure of the added liquid the pressure would increase instantly to the spreading pressure and remain there until the added liquid had nearly disappeared by evaporation. With the alkyl benzenes from ethylbenzene through the butylbenzenes the behavior was different. Shortly after the additions of these liquids, and while a large proportion of the added liquid still remained as a lens surrounded by a large film, the pressure began to decrease slowly but steadily. Simultaneously with this decrease the lenses of the alkyl benzenes began to break up into smaller lenses, slowly at first, then more rapidly as the pressure decreased. Subdivision and surface activity were very pronounced but could be greatly reduced or stopped altogether by stirring of the lens with a fine, freshly cleaned, glass rod or platinum wire or by adding new drops of spreading liquid. The stirring was associated with an increase of surface pressure to substantially the same value as that observed immediately after the initial addition of the drop. This rise was very rapid, often being complete after two or three gentle strokes of the rod through the lens. Sometimes more vigorous agitation was necessary to cause a return to the initial high value. If allowed to remain undisturbed the pressure would again decrease and stirring or the addition of fresh liquid would again bring it back to the original value. With the less volatile liquids several such pressure rises could be
947
produced during the existence of a single addition of spreading liquid. When drops of freshly purified tetralin were placed on a water surface covered with a continuous film of stearic acid under as low a film pressure as it was possible to measure with a hydrophile balance there was no detectable increase in film pressure due to the added tetralin. Upon aging in contact with a limited amount of air and exposed to laboratory light for a few days the tetralin developed a slight spreading pressure, too small to be measured with the film balance but indicated by the appearance of a drop of the liquid on the film covered water surface. The lens was much flatter than with freshly purified tetralin. Upon further aging measurable spreading pressures developed which, in a period of 120 days, appeared to reach a limiting value of about 21 dynes per cm. The rate of increase was greatly increased by placing the clear glass container of the tetralin in direct sunlight and greatly decreased by keeping the tetralin in a dark place. A sample of freshly purified tetralin, from which air was displaced with nitrogen, was sealed in a glass tube in a nitrogen atmosphere. No spreading ability was acquired in a month's time although the tube and contents were exposed to direct sunlight when possible during the month. Preliminary measurements were made with aging a t 40 and 55". Although spreading ability increased somewhat more rapidly a t the elevated temperatures than a t room temperature, the increases were small. This may indicate that a photochemical reaction, nearly independent of temperature, is important. The spreading behavior during the life of a drop deserves some mention. When a sample of aged tetralin was added to a water surface covered with a stearic acid film under low initial pressure, the drop would spread rapidly to a maximum pressure, depending on the age of the sample, then the pressure would decrease until it became essentially the initial film pressure. The maximum pressure was usually reached within 10 or 15 seconds after the addition of the drop while 15 minutes might be necessary for the return to the initial low pressure. Agitation of the tetralin layer during the slow decrease had little or no influence on the spreading pressure. The added tetralin drop contracted into a deep lens with almost no spreading ability. It seemed that water had extracted or destroyed the material which caused the aged tetralin to spread. Solutions of different concentrations of each of the oxidation products of tetralin were prepared in tetralin. The spreading pressures were measured immediately after the preparation and again after the solutions had aged for different periods of time, The results of typical experiments are recorded in Table 11. The pure tetralin, the values for which are recorded in the first horizontal line of Table 11, was stored in a 25-ml. glass-stoppered flask with greater exposure to more air than the solutions which were in small weighing bottles. This probably accounts for the relatively rapid increase in spreading pressure shown by the initially pure tetralin. The tetralin hydroperoxide causes the greatest increase in spreading ability while the a-tetralone
