POLYMOLECULAR FILMS: MIXED FILMS WITH TWO OR MORE COMPONENTS. I FATTY ACIDSAND NON-POLAR SUBSTANCES~ ROBERT J. MYERS
AND
WILLIAM D. HARKINS
Department of Chemistry, University of Chicago, Chicago, Illinois Received June 11, 1956 I. INTRODUCTION
Monomolecular films of insoluble substances containing a polar group have been extensively examined by Harkins, Langmuir, Adam, Rideal, and other workers. Very little attention, however, has been paid to the more general case of polymolecular films composed of two or more substances. In 1925 Harkins and Morgan (6) examined the force-area characteristics of mixed films of stearic acid and phenanthrene, and showed that the stearic acid exists as a monomolecular film while the molecules of phenanthrene are mostly piled up over it, and the two together give a polymolecular film. de Haas (3) has recently examined films of mixtures of palmitic acid and paraffin, but the films exhibited quite anomalous characteristics In view of the great value of a knowledge of the behavior of polymolecular films as a fundamental approach to a generalized theory of surface films, as well as the application of mch knowledge to the practical problems of lubrication and cell-interface phenomena, cases in which polymolecular films exist beyond a doubt, the authors have undertaken an extensive investigation of these polymolecular films. Since the films studied both by Harkins and Morgan and by de Haas were solid films, it is quite likely that some of the phenomena exhibited by the films may have been due to this solidity, rather than to orientation effects alone. For this reason, in this investigation, liquid hydrocarbons were added to the polar substances to insure freedom for reorientation, if any, in the film. This paper is to be considered as a rapid preliminary survey of the general characteristics of films of fatty acids to which were added, before spreading, various quantities of tetradecane or Nujol, non-polar hydrocarbon liquids of relatively low vapor pressure. 11. APPARATUS AND PREPARATION O F FILMS
The film balance, of the type designed by Adam (1) and modified by Harkins and Freud (2) and Harkins and Fischer (4), has recently been Presented at the Thirteenth Colloid Symposium, held a t St. Louis, Missouri, June 11-13, 1936. 959
960
ROBERT J. MYERS AND WILLIAM D. HARKINS
further modified and is described in detail elbewhere ( 7 ) . The torsion head is shown in figure 1, and the film balance in figure 2. The recent improvpments may be briefly summarized as folloms: (1) A beam arrest, operating much as the halance beam arrest on an analytical balance, is mountcd on the torsion head as shown and serves to prel ent large movementh of the torsion wire-float assembly during a calibration or a determination of film pressure. This has rewlted in precise calibrations and exact check? of “zcro-point” reading,-. ( 2 ) The trough, 7 5 cm long and 25 4 em. ]de, i i supported by adjustable screws u hich in turn are niounted on amber posts for insulation purposes. The plate-glasq aides of the box are cowred nith fine (myper screens which are groundcd, togcthcr with the c~hiomiriiii-~)latccl frailion oik ( 3 ) Thc box (~oritniiiingthc filni ixdancc
FIG 1 The tuibion hcad ii encloscd in a larger thermostatically controlled chamber, which may also be closed and grounded, enabling the experinieiits to be conducted under any desired ga- and in total darkness, if necessary. (The M ork reported here was conducted at room teriiperature, prior to the construction of the thermoregulating devices ) (4) The surface is swept and the film compressed by means of barriers operated by rods extending t o the outfidc of the box. The threaded lead-screw driving the compression barrier-holder is fitted on the exterior with a mechanical revolution counter, and areas are read in terms of revolutions of the lead-screw Higher precision in area determinations is thus obtained ( 5 ) The electrodes for contact-potential measurement arc carried along the axis of the trough by a moving table Tvhich also serves as part of the top of the
POLYMOLECULAR FILMS
96 1
inner box. This moving top slides in grooved bars across the top of the box and is also moved by a lead-screw. (Potential measurements reported in this paper were made on the apparatus of Harkins and Fischer, as the potential circuit was under construction a t the time of the experiments.) This arrangement of the movable electrodes minimizes the number of moving parts over the trough, and in turn avoids to a great extent the danger of dirt falling upon the surface. (6) The divided drum attached to the torsion wire was mounted on the outside of the box, enabling all readings to be made on the exterior of the apparatus.
FIQ.2. The film balance
The fatty acids and the hydrocarbons were spread from ligroin as solvent. The ligroin was treated with activated silica gel followed by a distillation in an all-glass apparatus. The hydrocarbons, tetradecane and Nujol, were treated with fuller’s earth and then centrifuged. Blank tests made on these purified materials failed to show any traces of spreading substances. The saturated fatty acids were generously loaned for this work by Professor E. Emmet Reid of the Johns Hopkins University. They were prepared by Miss Jane Dick Meyer and are of the very highest purity. The oleic acid was prepared by Dr. Robert J. Moon in this laboratory, by evaporation in a high vacuum. It was exccptionally pure and free from saturated acids.
962
ROBERT J. MYERS AND WILLIAM D. HARKINS
The solutions for spreading were made by dissolving from 30 to 50 mg. of fatty acid in about 32 g. of ligroin. A special weight pipet designed b y Harkins and Freud was used in dropping the material on the surface. The capillary tip was ground flat according t o the directions of Harkins and Humphery (5) for the preparation of drop-weight tips. The drops formed slowly and broke off neatly, leaving a negligible quantity on the tip face. In this manner the evaporation error was entirely eliminated. A balance weighing t o five decimals was used, since an error of 0.2 mg. in preparing the solutions or in weighing the pipet may result in as much as 1 per cent error. Assuming that all the errors are additive, we feel that the results reported herein are reproducible t o less than 1 per cent. In some cases the shift in the F-A curve produced by the added hydrocarbon is quite small. T o eliminate the slight shift between curves caused by a weighing error between two solutions, the hydrocarbon was added directly to a weighed portion of the fatty acid solution, instead of attempting t o prepare a new solution of the fatty acid. 111. FORCE-AREA
RELATIONSHIPS O F FILMS OF FATTY BCIDS MIXED WITH TETRADECANE
The force-area curves of films of myristic, pentadecylic, stearic, and oleic acids mixed with various quantities of tetradecane or Nujol are shown in figures 3 to 9. All films were examined on 0.01 molar hydrochloric acid. The lack of any appreciable effect of tetradecane on the force-area curves is quite striking. The effect of longer time intervals between points is shown in experiments 88 and 89 (figure 3). If readings are taken 2 minutes after a film is adjusted to a certain increment of pressure, during which time the areas are decreased sufficiently to maintain the film pressure constant, the curve of experiment 88 (figure 3) is obtained. If the film is Leld at each presbure level for 4 minutes, the curve of experiment 89 (figure 3) is obtained. These and other experiments point definitely to a need for the development of a standard rate of compression. This problem will be studied during the course of these investigations. For this preliminary survey an arbitrary interval of 4 minutes vias adopted, except in those instances in which the “4-minute curve” was unchanged from the “2-minute curT7e.” The curves of figures 4 and 5 further illustrate the lack of effect of tetradecane on the force-area curves of myristic and pentadecylic acids. A t this point in the work it was felt that an evaporation of the tetradecane wab taking place. To prevent some of this evaporation, small quantities of dotriacontane2 were added to the fatty acid-tetradecane mixtures 2 The dotiiacontane was generously supp1it.d by Dr B J. Mair of the Xational Bureau of Standards, U. S. Department of Commerce, Wttshington, D. C.
963
POLYMOLECULAR FILMS
without appreciable effect. I n the case of myristic acid (experiment 100, figure 5) the addition of dotriacontane displaced the F-A curve to larger areas, but the curve was parallel to that without dotriacontane. Examination of the film showed that it was solid, hence it was feared that some
I
LO
1.
AREA
I1
ZI
PER HUECULE
11
AU'
FIQ.3. Stearic acid-tetradecane mixtures EXPlRIMENT NO.
0
0
e 0
91 85 88 89
MOLE RATIO
TEMPBlRATURE
IN
'c.
AT IN ~N~~
Stearic acid
Tetradwane
1 1 1 1
0
22.0
10 50.5 50.5
20.6 22 0
2 2 2
22.0
4
964
ROBERT J. MYERS AND WILLIAM D. HARKINS
of increasing the areas greatly a t low pressures, but as the film is compressed, the curve approaches that of myristic acid and collapses a t nearly the same pressure. It is worthy of note that upon slow compressions of this character (time intervals 4 minutes) the myristic acid films collapsed a t pressures but slightly above the kink point in the curve. This is quite different from the second curved portion above the kink in the F-A curve
EXPERIMENT NO.
0 0
116 97
e
117
8
111
COMPOSITION
Pentadecylic acid 1 mole pentadecylic acid and 16 moles tetradecane 0.77 g. pentadecylic acid and 1.0 g. Nujol 0.68 g. pentadecylic acid and 1.00 g. Nujol
TEYPERATURE IN
AT IN YINUQEE
22.1 22.8
4
22.1
4
24 4
4
'c.
4
obtained by Adam (1). The pressures at the kink point are approximately the same as those given by Adam, but the areas throughout are somewhat smaller. We have attributed this to the high purity of our materials, inasmuch as a small amount of short-chain impurity would influence the areas more than the collapse pressures. I n view of these results with tetradecane and dotriacontane it was felt
965
POLYMOLECULAR FILMS
that a more non-volatile liquid hydrocarbon would be preferable, as the error due to evaporation would be thereby lessened. The commercial hydrocarbon Nujol (Standard Oil Company of New Jersey) was selected for its purity, non-volatility, and low cost. I n working with Nujol, it was possible to place large pans of the hydrocarbon below the f h balance, in the interior of the box, through which a stream of nitrogen gas bubbled slowly, to saturate the air with Nujol vapor.
AREA
pu)
YOLECW Llf
FIQ.5. Mixed films containing myristic acid
0
98
1
0
99 100 101
1 1 1
0
0
0 12.46 12.46 76.21
0 0 1.10 1.10
22.3 22.4 22.6 22.6
4 4 4 4
IV. FORCE-AREA RELATIONSHIPS OF FILMS OF FATTY ACIDS MIXED WITH NUJOL
Quite contrary to the case of tetradecane, the addition of Nujol to fatty acids produced great changes in the force-area curves of expanded films. I n the case of stearic acid, a liquid condensed film, the addition of Nujol changed slightly the slope of the compression curve of stearic acid and facilitated collapse a t a lower pressure (experiment 110, figure 6), but the areas at zero compression were the same as stearic acid alone, within
966
ROBERT J. MYERS AND WILLIAM D. HARKINS
experimental error. It is quite possible that larger amounts of Nujol would effect a change in the F-A curve, and this will be investigated. In the case of myristic, pentadecylic, and oleic acids, the addition of Nujol produced marked effects on the F-A curve. Upon the addition of 1.67 g. of Nujol to 1.0 g. of myristic acid (experiment 106, figure 7), the character of the F-A curve is changed, giving larger areas a t lower pres-
EXPERIMENT NO.
0 0
91 93
e
110
COYPOSlTION
Stearic acid 1 mole stearic acid, 0.95 mole dotriacontane, and0.95mole tetradecane 1 g. stearic acid and 1.61 g. Nujol
TEM;yg,uRE A?'
I N MINCTEB
22.0 21.7
2 2
23.5
4
sures, the curve approaching that of myristic acid alone a t high pressures. No kink appears in the curve, and the area a t zero compression is nearly 10 A. U.2larger. The collapse point is the same as for myristic acid alone. Upon the addition of Nujol to give 9.08 g. of Nujol per gram of myristic acid, the curve cuts across the myristic acid curve a t 13 dynes, exhibits no
POLYMOLECULAR FILMS
967
’
kink, and continues without appreciable change in slope down to very small areas. The area a t zero compression is again some 10 A.U.2larger. With a concentration of 49.8 g. of Nujol per gram of myristic acid, and in addition 31.5 g. of tetradecane, the film exhibits even less adhesion to the water surface. The area a t zero compression is some 8 A.U.2 more than that of myristic acid alone. No kink point in the curve was observed, and upon compression to very small areas small lenses were visible to the eye.
FIQ.7. Mixed 6lms containing myristic acid and Nujol COMPOBITION IN ORAYI)
EXPBRIMBNT NO.
Myristio acid Tetradecsne
0
0
e
0
102 104 105 106
1 1
1 1
0 31.5 0 0
Nujol
0
49.8 9.08 1 67
TBMPBRATUBB IN
AT I N MINUTE8
22.8 21.8 22.4 22.9
4 4 4 4
‘c.
This was the only film on which lenses visible to the eye were noticed. It is quite possible that microscopic lenses may be present; the films are being reexamined by optical methods to determine if the upper surface is heterogeneous. It is to be understood that these films exhibit slight contractions all along the curve, and that the “4-minute” points do not necessarily represent equilibrium areas at that particular pressure. Other time intervals may very well give rise to other curvea, indicating again the
968
ROBERT J. MYERS AND WILLIAM D. HARKINS
great need for extended work on the character of a force-area curve as influenced by the rate of compression. Experiments 111 and 117 (figure 4) illustrate a most interesting effect of the addition of Nujol to pentadecylic acid. The curves are very similar in shape to that given by pentadecylic acid alone; in fact, are quite parallel throughout the expanded range. The area a t zero compression is some 9 A.U.2larger when 1.0 g. of Nujol per 0.77 g. of pentadecylic acid is pres-
1 0 0
EXPERIYBNT NO.
I
1 1
COMPOBlTION
Pentadecylic acid 0.77 g. pentadecylic acid and 1.00 g. Nujol
I
TEMPERATURE
'e.
I
1 E:! 1 IN
AT IN MINUTE8
4 4
ent. The kink occurs a t nearly the same area, but some 6 dynes higher, a t the same temperature. Beyond the kink a most interesting very flat portion appears which extends to a region around 21 A.U.2, where the curve meets that of pentadecylic acid and again rises, but with less slope. The presence of the Nujol appears to facilitate collapse a t a lower pressure. The flat portion of the curve suggested some sort of phase transition, and an examination of the contact potentials (4) of pentadecylic acid films with and without Nujol was undertaken to determine if any change
969
POLYMOLECULAR FILMS
occurred in the AV-A curves at this point. The results were most interesting, although they are to be considered as a very preliminary investigation of the contact potentials of the films (figure 8). The AV-A curve of the mixed film consists mainly of two straight lines which intersect at exactly the same area as the kink in the F-A curve. A very cursory examination of a pentadecylic acid film a t the same temperature showed a similar break in the AV-A curve at the same point, although the kink
AREA
0
e
PER
MOLECULE A U '
EXPBIRIYBINB NO.
COMPOSITION
TBIMPBIRATURE IN
AT I N MINUTE8
107 119
Oleic acid 0.54 g. oleic acid and 1.00 g.
25.4 24.1
4 4
"c.
Nujol
in the F-A curve came at an area some 2 A.U.2 smaller. In view of the. lack of sufficient data at this time, it appears hardly worth while to speculate on the nature of this interesting relationship. In the case of oleic acid (figure 9) the addition of Nujol caused a marked change in the film. The F-A curve crossed that of oleic acid at about 14 dynes, and extrapolated to an area at zero compression of about 70 A.U.2 as compared to 54 A.U.2 for the acid alone. Collapse set in much sooner when Nujol was present.
9 70
ROBERT J. MYERS AND WILLIAM D. HARKINS
V. CENEKAL DISCUSSION The ~ o r reported k here a a s done as a preliminary survey of the field For this reason it is not possible to formulate any exact explanation of the effects observed. A few points, however, seem quite definite a t this timr. The marked difference betvrren the effect of the addition of Nujol and that of tetradccane seems to indicate that almost all of the tetradecaiie evaporates from the surface, thus making it unsuited for an investigation of this character. The addition of solid hydrocarbons, such as dotriacontane or paraffin, on the other hand, may lead t o anomalous results, hence it is advisable to select liquid hydrocarbons of a high degree of iion-volatility The Nujol appears to affect most markedly the expanded portion of the P - A curve. I t is reasonable to suppose that in this region some of the hydrocarbon gets down to the water surface, increasing the area per molecule of fatty acid As pressure is applied, the hydrocarbon is probably squeezed up to the top of the film, where it may or may not exert a further influwce upon the film, depending upon the quantity of hydrocarbon present. If sufficient hydrocarbon is present, a second phase may be formed, which the fatty acid molecules may enter with ease, and the collapse point of the film is correspondingly lower. If but a small amount of hydrocarbon is present, it is quite reasonable to suppose that it will have but a slight influence on the collapse pressure. This analysis raises the question: When is the mixed film no longer to be considered as a film but as a second phase with polar material a t the oil-water interface? Only further work can give a suitable answer to this question. The lack of any effect of Nujol upon the lower region of the stearic acid curve is quite striking. It may be tentatively ascribed to the rather low concentration of Nujol present. The absence of lenses visible to the eye on all but one of the films seems to indicate at least a temporary stability of a thick film of a hydrocarbon oil with polar material a t the oil-water interface. Thick films of polar material alone have been shown to be very unstable, if sufficient area is present for a monomolecular film to be formed. There is, however, the distinct possibility that thick films such as those studied here may be quite stable. In any case, it seems evident that a further study of these polymolecular films promises results of the greatest interest, both theoretically and from the practical viewpoint. The presence of the hydrocarbon may facilitate phase changes in the film, merely by its action as a diluent. The results obtained with pentadecylic acid seem to indicate such an effect. It is more than likely that many obscure points regarding orientation in a monomolecular film may be clarified by aiding reorientation \\-it11the addition of a diluent.
POLYMOLECULAR FILMS
9il
VI. BUMMARY
A preliminary survey of films composed of fatty acids and tetradecane or Nujol has been made. The films containing Nujol exhibit many interesting characteristics, justifying further examination of films with two or more components. REFERENCES (1) ADAM,N . K.:The Physics and Chemistry of Surfaces. The Clarendon Press, Oxford (1930). (2) FREUD,B. B.: Dissertation, University of Chicago, 1927. (3) DE HAAS,J. J.: Chem. Weekblad SO, 347 (1933). (4) HAREINS,W.D., AND FISCHER, E. K . : J. Chem. Physics 1, 852 (1933). (5) HARKINS, W. D., AND HUMPHERY, E. C.: J. Am. Chem. SOC.38,228 (1916). W.D., AND MORGAN, J. W.: Proc. Natl. Acad. Sci. 11, 631 (1925). (6) HARKINS, (7) HARKINS, W.D., AND MYERS,ROBERT J . : J. Chem. Physics, November, 1936.
