MEASt'REXfENT OF BOUNDllRY TENSION
671
T H E MEASVRETVIEST OF BOUSDARY T E N S I O S BY THE P E S D A S T - D R O P hIETHOD. I 'rHE ALIPHATIC ALCOHOLS'
GRAST \V SMITH
AKD
LEOTARD V. SORG
Department of Chemzstry, The Unzuerszty of Kansas City, Kansas Czty, .4fzssourz Recezr~edJ u l y 89, 1940
The importance of capillary phenomena in the technical and scientific world is well known. For nearly a century and a half, noteworthy studies have been produced which have established the theoretical and experimental foundations of this field of investigation. Yet the field is by no means exhausted. In particular, the interpretation of the behavior and structure of surfaces and interfacial bounda ries in terms of molecular structure and the various forces exirting in such regions has been the subject of many researches in recent years. Hauqer, Andreas, and Tucker ( 7 ) h a r e reviewed the importance of boundary-energy measurements in industry. These men (1) developed the "pendant-drop method" for the experimental measurement of boundary energy Jyhich was used in the present study. The objective of the present research a o r k has been threefold: ( a ) to construct and operate photographic and auxiliary equipment for the production of pendant-drop profiles from which measurements could be taken for the calculation of boundary energies; ( b ) to determine the accuracy of the above method by the use of highly purified calibrating liquids upon 1% hich highly dependable data have been obtained by other methods which are theoretically valid; and (c) to determine the surfaceenergy characteristic. of' a group of purified aliphatic alcohols embracing the normal compoundq from C1 to C12 and the readily obtainable isomers below CC It is recognized that a drop of fluid hanging pendant from a tube is a system in which an equilibrium of forces has been attained. The resultant of these forces has been such as to give the drop certain shape and surface characteristics peculiar to the particular system under consideration. The equation derived by Andreas, Hauser, and Tucker (1) for experimental use in determining boundary tensions is2
1 The material presented here formed part of a thesis submitted by Leonard V. Sorg to the Faculty of the University of Kansas City in partial fulfillment of the requirements for the degree of Master of Arts, June, 1940. 2 For the complete derivation of this equation, see reference 10.
672
GRANT W. SMITH AND LEONARD V. SORG
where y = the boundary tension, g = the acceleration of gravity, u = the difference in densities of the two fluids forming the interface, and d, = the maximum diameter of the drop, Le., at the equator. H is a function of the drop shape, defined as follows:
H
=
8($)2
where 8 = - and Y
RO = the radius of curvature a t the vertex of the drop, Le., the point of intersection of the axis of rotation with the surface of the drop. Another function of the drop shape is the “shape factor,’’ S.
S = d./d,
(3)
where d, is the diameter of a selected plane cutting the drop a t right angles to the axis of rotation a t a distance equal to d, from the vertex of the drop. The values of H have been determined experimentally as functions of S, the shape factor, by Andreas, Heuser, and Tucker by using measurements from photographs of drops of conductivity water a t 25°C. The accuracy claimed for the values in the published table is f 0.2 per cent. No attempt has been made in the present project to calculate a table of H-functions, but the liberty has been taken to use the table as presented by the above workers. DESCRIPTION O F APPARATUS
The apparatus used to obtain pictures of pendant drops is shown in a schematic diagram in figure 1 and also in figure 2. The equipment consisted of a.light source, a lens system for producing parallel rays, a photographic system, and auxiliary equipment. The light source was a Type H-4 General Electric high-pressure mercury-vapor lamp. The lamp was encased in a ventilated housing containing an iris diaphragm allowing the light to be directed on the lens system. A Wratten Xo. 62 filter was used on the cell compartment to make the light monochromatic. The primary lens was a 400-mm. Ldtz compound projection lens. It was placed in the light beam from the lamp so that the rays leaving it were parallel. The photographic system consisted of a Model I1 Korelle-Reflex camera. The barrel of a Leitz microscope using a 42-mm. objective was substituted for the lens system of the camera, and the camera-microscope
673
1IE;ZFTJREMENT O F BOUNDARY TENSIOK
was so mounted that a microfocusing adjustment was possible, using thc microscope gear system. -4I-mm. telecentric stop was placed a t the focal VIEWING TELECENTRIC
/ REFLEX CAMERA
MICROSCOPE OBJECTIVE
7 CELL HOUSING
MERCURY ARC LAMP
FIG.1. Schemntic di:igrnm of apparatus
FIG.2. Pendant-drop npps.rittua
point of the microscope objective to eliminate non-parallel rays from the light entering. The pendant drops were suspended from small 10-ml. Pyrex glass syringes with specially prepared tips of external diameter:: about 1.25
674
GFUKT W. SMITH AND LEONARD V. SORG
mm. to 1.75 mm. A thermostatic water bath was mounted on the optical bench between the primary lens and the camera. This bath was designed to enclose the Pyrex cuvette, 4 cm. wide x 7 cm. high x 2.5 cm. thick, with plane parallel sides and a cover with an opening to permit insertion of the syringe. Water a t constant temperature was rapidly pumped from a large auxiliary water bath to and from the cell housing by means of a centrifugal pump. The temperature in the cell housing was maintained constant a t 25.O"C. f 0.02". I n use, a small amount of sample was drawn into the syringe, which was then mounted so that it extended well into the cuvette. Several drops of liquid were expelled into the cuvette to establish vapor pressure equilibrium and eliminate evaporation from the drop under examination. The pendant drop was then brought into sharp focus on the viewing screen of t h r camera. With all adjustments thus made, a fresh drop was suspended from the tip, thus giving a surface free from contamination, and the expo-ure was then made. The degree of magnification of the drops by the lens system was determined by photographing the rulings on a Levy-Hausser counting chamber on each roll of film, Le., for each twelve pictures. By measuring the photograph, the magnification was calculated. Since each roll of film was processed separately, any shrinkage or expansion of the film due to processing was compensated for by measurement of the magnification for each roll. The average magnification was 8.273 times. Dimensions of the drop profiles were determined from the negatives by means of a measuring microscope equipped with a mechanical stage of special design. This was constructed so that it supported the picture between two glass plates for measurement, and also permitted easy manipulation of the picture for proper alignment with the measuring de7';"The two selected diameters of the pendant drop profile, d e and d,, were read on the microscope by means of a micrometer which could be read to 0.005 mm. The vertical distance from the tip of the drop could be read with an accuracy of 0.1 mm. It is to be noted in this connection that, owing to the shape of the pendant drop, the degree of precision required for determining this vertical distance is not so great as for the horizontal measurements of the diameters. THE MATERIALS FOR STUDY
I n the evaluation of the method, four substances mere used: benzene, carbon tetrachloride, toluene, and water. All of these compounds are readily purified, and data on the values of their surface energies are to be found in the literature. The benzene mas washed repeatedly with concentrated sulfuric acid, followed by sodium hydroxide solution and thorough washing with water. Two fractional crystallizations were made,
MEASUREMENT OF BOUNDARY TENSION
675
after which fractional distillation over metallic sodium was carried out. I n the first distillation, the middle one-third cut was taken and redistilled a second time. The fractional distillation was done in an all-glass fractionating column of high separating efficiency. A reflux ratio of 10 to 1 was maintained. The middle cut was selected for use. The toluene was taken from a C.P. supply and washed with concentrated sulfuric acid, followed by neutralization and washing with water. Two fractional distillations were made in the same manner as with benzene. The close-boiling middle cut was selected for use. A large sample of C.P. carbon tetrachloride was washed repeatedly with sodium hydroxide solution and then with water until neutral. Fractional distillation was carried out over anhydrous calcium sulfate. The middle cut was selected for use. Twice-distilled water was redistilled from alkaline permanganate through a block-tin condenser. This water was then treated with barium hydroxide and distilled from it. I n all distillations, only the middle third of the sample was retained for further treatment. In the last distillation from barium hydroxide, only a partial condensation was effected into a carbon dioxide-free atmosphere. This sample was used for the calibrating work. The alcohols were of the best quality obtainable from the Eastman Kodak Company, and the boiling point and the specific gravity a t 25.OoC. were determined on each one. If the properties thus determined were in good agreement with the data in the International Critical Tables, the alcohols were used without further purification. When evidence of impurities was obtained, a fractional distillation under a reflux ratio of 40 to 1 was made in a semi-micro high-efficiency column. All distillations made on the alcohols were over anhydrous calcium sulfate. RESULTS AND DISCUSSION
‘The results obtained with the calibrating substances are presented in table 1. The number of independent observations from which each mean value of the surface tension was obtained is shown, and also the mean deviation of the individual values from the mean. Before making comparisons of the data in table 1, a point of considerable importance should be recognized. With the exception of carbon tetrachloride, these reported data were taken by the capillary-rise method and the surfaces were exposed for a long time before equilibrium was considered to be established. I n this work, the data were obtained on drops with the surfaces only 10 sec. old, and therefore they may be considered to be very free from contamination, sinceevery effort was directed to insure this condition. Dorsey (3) has insisted that much of the data in the literature is the result of “old surfaces” which are likely to be
676
GRANT W. SMITH AND LEONARD V. SORG
contaminated. For this reason he suggests that, “other things being equal, more confidence may be placed in the higher values of surface tension.” Benzene and toluene have yielded values of the surface tension by this method of pendant drops which are somewhat higher than those reported for the capillary-rise method. Since unusual precautions were taken to prevent contamination and the measurements were made on fresh surfaces, the values of 28.6 for benzene and 28.2 for toluene a t 25.OoC. seem reasonable, in the light of Dorsey’s comment quoted above. The literature value of 26.1 for the surface tension of carbon tetrachloride was obtained by interpolation from the values given by Harkins for temperatures of loo, 20°, 30°, and 4OoC. The value of 26.0 at 25.0”C. was reported by Andreas, Hauser, and Tucker for the pendant-drop method. TABLE 1 Results for calibratzng lt9uids at 16.0”C. ~
MEAN COMPOUND
DEVIATION
1 Benzene Toluene Carbon tetrachloride Water
1
~
~ _ _ _ _ “C.
79.8 110.4 76.4
0.8776 0.8595
1 ,
dynea per centimefar
6 5
28 6 28 2
73 0
28 4 (8) 27 7 (8) 26 1 (5) (6)
I 72 1
0.21 0.14 0.10
0.23
The average value obtained in this work for the surface tension of water is 0.9 dyne per centimeter higher than that reported in the literature for 25.OoC. Ferguson (4) has surveyed the literature for values of the surface tension of water a t 15.0”C. and has shown that they vary from 72.78 to 74.30 dynes per centimeter for different workers whose work was comidered to be of high quality. He attributed the differences to the difficulty of obtaining really pure water and keeping it so during the determination of the surface tension.
Surface tensions of the alcohols The results of the measurements on the purified alcohols used in this study are shown in table 2. The densities are the average values of at least three separate determinations. The boiling points were obtained by using an inverted sealed capillary and noting the temperature a t which the vapor pressure within the capillary tube became less than atmospheric pressure. The thermometers used were carefully calibrated, and corrections on the boiling points were made for the temperature difference between the emergent stem and the boiling liquid. The surface-tension
3IEASlJREMEIiT OF BOUNDARY TENSION
677
data represent the average values of at least fire obserrations for each alcohol. The number of separate observations in each case is shown. Each observation represents a new lO-sec.-old drop a t equilibrium with air saturated with the alcohol vapor. It should be mentioned that each series of observations represents data taken a t different times, so that they are not all consecutive measurements. For comparison purposes, available data from the literature are included. TABLE 2
Results jor alzphatic alcohols at d5.0"C. MEAN DEVIATION
n-Butyl
tert-Amyl Methyl-n-propylcarbinol Isopropylmethylcarbinol Diethylcarbinol see-Butylcarbinol n-Hexyl n-Hept? 1 n-Octyl n-Sonyl n-Decyl
117 8 0 805
101 119 114 111 125 155 175 193 216 231
7 7 1 6 6 11 5 7
0 0 0 0 0 0 0
0 0 0
806 830 815 818 834' 816 819 813 824 827
6
21 2
24 2 (11)
25.2 (8)
7 6 6 6 6 6 6 8 5 6
25.3 23.3 22.4 24,l 23.0 21 1 25 1 25 8 27 1 26 7 28 1 28 6 28 9 29 6
25 8 (9); 24 1 (8) 24 2 (8) 25 6 ( 8 )
I
0.06 0.07 0.19 0.12 0.08 0.11 0.14 0.07 0.09 0.14 0.30 0.15 0.20 0.16 0.04 0.16 0.08 0.13 0 12
Most of the values from the literature were obtained by the capillaryrise method. The value of 25.8 dynes per centimeter given for 1-hexanol is from the work of Rovorka, Lankelma, and Stanford (9) in 1938 and was obtained on a very pure sample of the alcohol. This agrees with the value obtained in this work for 1-hexanol. I n contrast, the value of 24.1 dynes per centimeter, reported by Hennault-Roland (S), is designated simply as for "hexyl alcohol" without the knowledge of its purity. Since Hovorka gives the values of pure 2-hexanol and 3-hexanol as 24.3 and
678
GRANT W. SMITH A N D LEONARD V. SORQ
24.0 dynes per centimeter, it might be assumed logically that the value given by Hennault-Roland is for an impure sample containing isomers. Hennault-Roland has also reported on “octyl” and “heptyl” alcohols, aa shown in table 2. Since these are considerably different from the values resulting from this work, it may be that the value of the reported results suffers from impurities, as appears to be the case with the hexyl alcohol. Graphical representation of the surface tensions of the normal primary alcohols as a function of the number of carbon atoms is to be found in figure 3. An anomaly is observed in the values for the surface tensions
FIG.3. Surface tensions of :he normal primary alcohols
of methyl and ethyl alcohols. This is exemplary of the common observation that the first member of a homologous series is not typical in the properties that are possessed by the other members. It is very likely due to association of the molecules of methyl alcohol, which gives rise to a behavior similar to that of alcohols of higher molecular weight. I n regard to this behavior, it is recognized that the primary alcoholic group of methyl alcohol, while attached to a carbon atom, is not attached to the terminal carbon atom of a chain, since no chain exists. Beginning with ethyl alcohol, a chain of carbon atoms has come into existence and the alcoholic group is attached to the terminal carbon. Thus ethyl alcohol should fall in line with the higher alcohols, as is shown to be the case.
679
MEASUREMENTOFBOUNDARYTENSION
Regularity in the change of surface tension occurs until heptyl alcohol The observed anomaly in the values for heptyl and octyl
is reached.
TABLE 3 Effect o j conjigmation on the surface tension UUBFACl TENUION IX DYNES PEB CENTIYmIR AT
mm OF ALCOROL
1
Propyl
Primary. . . . . . . . . . . . . . . . . . . . . . . . . . . Secondary.. . . . . . . . . . . . . . . . . . . . . . . . Tertiary. . . . . . . . . . . . . . . . . . . . . . . . . . .
23.4 21.2
180.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21.2
Butyl
24.2 23.3 20.8 22.3
1
25.0'C.
-
Amyl
25.3 24.1 22.4 23.3
TABLE 4
The amyl alcohols uoaoL
CONFIQURATION
SURFACE TENSION AT 25.0"C.
dun? wr
centimeter
n-Amyl ....................... Methyl-n-propylcarbinol . . . . . .
..... .....
.....
Diethylcarbinol
C-C-G-C-C-OH
c-c-c-c-c
c-c-c-c-c
25.3 24.1
I
OH 24.1
I
OH C Isoamyl
...................
.....
\
c-c-c-oI~
/
23.3
C C Isopropylmethylcarbinol
......
.....
\
/
C tert-Amyl
.....................
sec-Butylcarbinol
.....
............. .....
c-c-c I
23.0
OH C
I I
C-C-C4H
22.4
C
c-c-c-c I
25.1
G-OH
alcohols is not due to accidental interchange of data, for this possibility was carefully checked. KO explanation worthy of strong support has been found for this behavior.
680
GRANT W. SMITH AND LEONARD V. SORG
The effect of configuration among isomers on the surface tension is brought out in tables 3 and 4. Table 3 shows the relative effects of attaching the alcoholic group t o a primary, secondary, or tertiary carbon atom. The presence of the alcoholic group in the primary position coincides with a relatively high surface energy. Movement of this group to the secondary carbon lowers the surface energy by approximately 1 dyne per centimeter. Similarly, changing from a secondary carbon atom to a tertiary carbon atom decreases the surface energy by approximately 2 dynes per centimeter, which is nearly twice the change in moving from a primary to a secondary position. More comprehensive data are shown for the amyl alcohols in table 4. I n addition to the effect noted above in going from the primary t o the secondary or 2-position, it is interesting that substantially no change in surface energy was observed in changing the alcoholic group from the 2-position to the 3-position. Similar results were obtained by Hovorka with the isomeric hexanols : 2-hexanol and 3-hexanol were found t o have surface tensions of 24.23 and 24.05 dynes per centimeter, respectively. The effect of branching of the carbon chain itself is brought out by considering isoamyl alcohol, isopropylmethylcarbinol, and tertiary amyl alcohol. As the branching becomes more pronounced, so as to make the length of the molecule less and the general structure more compact, it appears that a reduction of the surface tension occurs, with the tertiary grouping exhibiting the lowest value. It is difficult t o reconcile the high value of the surfsce energy of sec-butyl carbinol with that of the other primary alcohols studied, i.e., n-amyl and isoamyl alcohols. It appears that the presence of the methyl group on the carbon atom adjacent to the one containing the hydroxyl group gives rise to a higher surface energy value than occurs when the methyl group is located farther from the alcoholic group. SUMMARY
Suitable equipment has been constructed and assembled in order t o obtain photographic profiles of pendant drops from which measurements may be obtained for the calculation of surface tension. The method of pendant drops has been calibrated for accuracy by making measurements of surface tension on highly purified benzene, toluene, carbon tetrachloride, and water. Measurements of surface tension by the method of pendant drops have been obtained on the normal aliphatic alcohols from methyl alcohol to lauryl alcohol, and on all of the readily obtainable isomers below hexyl alcohol. The data have been presented and discussed from the viewpoint of the influence of the number of carbon atoms and of configuration.
LIGHT SCATTERING BY LIQUID FILMS
681
I n general, as the carbon content of the alcohols increases, their surface energies increase. Among the normal alcohols, methyl, octyl, and, to a lesser extent, heptyl alcohols show a someu-hat abnormal behavior. Configuration has been shown to have an important effect on the surface energy. Movement of the alcoholic group from the 1-position to the 2-position causes a lowering of the surface tension. Practically no further change is produced upon shifting the hydroxyl group to the 3-position. The tertiary alcohol grouping produces the lowest value of surface tension. Branching in the carbon chain has been shown to lower the surface tension. REFERESCES (1) ANDREAS,J. &I., HAUSER, E. A , , A N D TUCKER, W.B.: J. Phys. Chem. 42,1001-19
(1938). BIRCUMSHA L.~ L.: , J. Chem. soc. 121, 887-91 (1922). E. S . : T a t ] . Bur. Standards (U.S.), Sei. Papers, S o . 21, 563-95 (1926). DORSEY, FERGUSON, A , : Trans Faraday Soc. 17, 370-83 (1922). W.D . : Colloid Symposium Monograph 6, 17-40 (1928). HARKINS. W.D., B R O W NF. , E., A N D Daws, E. C. H . : J. Am. Chem. SOC.39, HARKIXS, 354-64 (1912). (7) HAUSER, E. A., ANDREAS,J. XI., A X D T U C K E RIT. , B.: Ind. Eng. Chem. 31, 32-5 (1939). (8) HESSAULT-ROLAKD, L . : Bull. S O C . chim. Belg. 40, 177-94 (1931). (9)HOVORKA, F., LABI(ELWA, H. P., A N D STANFORD, s. C . : J . Am. Chem. SOC.80, 820-7 (1930). (10) SORG,L. V.:M.A. thesis, The Cniversity of Kansas City, 1940. (11) TEKISEmfs, J., . ~ K DHESSAULT-ROL.ASD, L.: J. chim. phys. 27, 401-42 (1930). (2) (3) (4) (5) (6)
STUDIES OF SURFACE PROPERTIES BY T H E LIGHT SCATTERIXG OF DEPOSITED LIQUID FILMS VIKCEKT J. SCHAEFER Research Laboratory, General Electrzc C o m p a n y , Schenectady, S e w YoTk Recezved August 20, 1940
X h e n multilayers of barium stearate are built by lowering and raising a metal or glass plate through a water surface covered with a compressed monomoIecular film, either one or two monolayers are deposited with each complete dipping cycle, depending on the pH of the substrate solution. Blodgett has shown (3) that, below a critical alkalinity of the bath, two monolayers (Y-layers) are deposited for each round trip, while a t a pH above about 9.0 a layer is deposited on the down trip only (X-layer).
