T H E STRUCTURE OF THE SURFACES OF SOLUTIONS AS SHOWN BY THEIR RESISTANCE TO THE SPREADING OF INSOLUBLE FILMS’ T. FOSTER FORD2 AND DONALD A. WILSOKa Department of Chemistry, Stanford University, Calijornia Received July 1 , 1038
Our knowledge of the surfaces of liquids has been derived mostly from the work of Langmuir and others on the phenomena presented by insoluble films on water. It has been commonly taken for granted that somewhat similar layers must exist upon the surfaces of solutions of all surface-active substances, even soluble substances. This assumption is based upon analogy, and upon the Gibbs prediction of positive adsorption with lowered surface tension. Elsewhere the authors have presented direct and indirect evidence for the existence of such soluble films (11, 10, 3, 4). This paper has to do with their direct quantitative study. In these experiments the surfaces of solutions of hydrocinnamic acid, caprylic acid, and phenol are compressed by means of oleic acid “piston films,” an adaptation of the recent method of Langmuir and Blodgett (1) for studying insoluble monolayers. Here the only films possible are those formed spontaneously by adsorption from the solutions themselves. The piston films are visibly retarded, and from curves showing compression against time, recorded by moving pirtrires, the properties of thc soluble films compressed are deduced. Thew seen1 to kw (Lapable of rbxiating in all the states of aggregation common to insoluble films. The actual amounts of solute in these soluble surface layers is found to agree quite closely with values for the absolute adsorption found by other experimental methods (4, 5, 6, 9), and with the adsorption predicted by the Gibbs equation. THE EXPERIMENTAL METHOD
The apparatus used is shown in figure 1. It consists of a circular dish 25 cm. in diameter for containing the solution, in an air-tight glass box,
with a capillary tube reaching to the center of the dish through which the Presented a t the Fifteenth Colloid Symposium, held a t Cambridge, Massachusetts, June 9-11, 1938. * Present address: Shell Development Company, Emeryville, California. Present address: Department of Physiological Chemistry, Srhool of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania. 1051
1052
T.
FOSTEn
FORD AND DONALD A. WIl.90N
oleie arid may be introduced beneath the suriarr of the solution when desired. The dish is a brass hoop, paraffin waxed, fitted over a glass disk beneath whieh is a film of paraffin wax eolored with lamp black to aid visibility. The interior of the apparatus is thoroughly paraffined to prevent contamination. The motion picture camera, not shown in figure 1, was mounted dirrrtly above t h r sprrading film. The temperature was 11C. in all c a m . I n each experiment dish and rapillary were first, thoroughly cleaned and rinsed with eonductivity watrr. Thr solution was then put into the dish, the surface awrpt with paprr strips, and thr enrlosore eovprrd for t h r
desired lrngth of time. Thr spreading. oil was introdurrd by trapping a droplet from an eye-dropper i n R roliimn of solution i n thr vrrtiral part of the capillary tubr outsidr thr cnrlnsurr so that solution and spreading oil ran into the dish by gravity. Brforr introdiwing tlir spreading oil, the cover was lifted just long. cnougli to dust. tlir surfarr of the solution with talc. All the neeessary data were takrn from the motion pirturr record by projerting sueeessivr frames onto a srrrrn. THE SPREADING O F OLEIC ACID ON WATER
The hrhavior of t h r olric arid piston wbrn rompressing thr surface of pure a a t r r is shown hy thr first romprrssion rurves in figiirrn 3 , 4 , 5 , and 6.
STRUCTURE O F SURFACES O F SOLUTIONS
1053
After a very short time the rate of spreading of the film in square centimeters per exposure is constant. That is, the area-time curve becomes a straight line, and, on water, it remains a straight line during the whole time of spreading. Evidently no sensible change in the retarding force due to water alone occurs as its free surface is reduced. Changes observed on solutions, then, must be due to the presence of solute films.
The rate os dzsentanglement On water the area-time curve is linear over most of its length, and on solutions large linear segments are observed. This constant rate of increaw of area of the piston film is probably determined by the rate of “disentanglement” of oleic acid molecules from a central globule of quite constant size, For solid nuclei of spreading Cary and Rideal (2) found the rate of spreading, or of disentanglement, t o be proportional t o the perimeter of the solid a t the plane of contact with the water surface. For liquid globules also they consider it probable that solution occurs only a t the circumference of contact. Oleic acid droplets released beneath water surfaces seem ‘ 3 form first single lenses which then suddenly break up into many smaller ones. This behavior was iioted by Cary and Rideal in studying the spreading of oleic acid over yery large surfaces of water, and it explains the gradual increase in the spreading rate shown by their data, since there was a n increase in the total perimeter of the retervoir. On some of our concentrated solutions, on which spreading was comparatively slow, a n upward swing of the area-time curves similar t o Cary and Rideal’s is noted. I n most cases, however, on the small areas studied by us the central globule evidently did not have time t o break up. I t s exact behavior fortunately has little bearing on the interpretation of our results. It may only be concluded that in the first part of the spreading on solutions the rate is dependent more on the nature of the reservoir than on the properties of the surface being compressed. The terminal linear velocity The data given here for both water and solutions show at the beginning of the spreading a constant linear velocity of the advancing edge. This must represent a terminal linear velocity. At this speed the resistance offered by the friction in the film and underneath the film, and by impact with chance obstructions ahead of the film, must equal the spreading pressure. This terminal velocity is observed only a t first, because only then is the reservoir in effect infinite. As the film increases in area the rate of spreading quickly becomes limited by the rate of disentanglement. THE RESULTS O F THE COMPRESSION O F SOLUBLE FILMS
Typical curves for the compression of soluble films by the oleic acid piston are shown in figure 2. The actual data obtained with various solu-
1054
T. FOSTER FORD AND DONALD A. WILSON
tions of hydrocinnamic acid, caprylic acid, and phenol are recorded in figures 3 , 4 , 5 , and 6. Many of the actual curves exhibit all of the characteristics formalized by the different segments in figure 2.
Terminal linear velocity At the beginning of the compression the linear velocity of the advancing edge of the piston film is constant. This is the terminal velocity discussed above. On a few of the more concentrated solutions the data suggest a slight acceleration of the linear velocity near the origin. Such an effect could
DISENTANGLEMENT CURVE
TERMINAL VELOCITY C U R E
TIME
FIG.2. Typical compression curves for soluble films
result from the fountain-like manner in which the oleic acid is introduced into the surface, since a certain time should be required for the formation of a lens having a stable perimeter. The necessity of accelerating the talc particles used for reference may also be a factor. In most cases these effects are masked in our experiments by attainment of the terminal velocity in less than the 0.076 see. elapsed between successive camera exposures. Disentanglement On almost all of the compression curves for soluble films a point is reached beyond which the area-time curve rather than the diameter-time
STRUCTURE O F SURFACEB OF SOLUTIONS
1055
Mob
FIQ.3. Compression curves for aged surfaces of hydrocinnamic acid solutions
FIQ.4. Compression curves for aged surfaces of caprylic acid solutions
1056
T. FOSTER F O R D AXD DOSA1.D A . W'ILSO?;
25
:m
5 c
g 15 +
E 10 ii
cc
?
Y4 5 0
0 5 10 15 20 25 30 35
40 45 59 55 60 65 70 55 80 EXPOSLIRE
FIG.5. Compression curves for aged surfaces of phenol solutions
FIG. 6. Compression curves for freshly swept surfaces
STRUCTURE OF SURFACES OF SOLUTIONS
1057
curve becomes a straight line. It has been assumed that a t this point the spreading rate becomes limited no longer by the terminal velocity but by the rate of disentanglement of molecules from the central globule. If the rate of disentanglement is assumed to be proportional t o a difference in spreading pressure between the piston film and the film compressed, then the soluble film must oppose the piston film with a constant pressure in the region where the area-time curve is linear. This portion of the curves, then, may be assumed to represent a transition from one surface phase to another. Presumably this change is from the two-dimensional gaseous state to a two-dimensional liquid state, a presumption confirmed by the nature of the subsequent changes. If the central globule always had the same perimeter on the different solutions, then the slopes of the disentanglement curves should be expected to be the same on solutions of the same solute regardless of concentration if two-dimensional condensation were occurring, and on solutions of different solutes the slopes should be proportional to the two-dimensional vapor pressures of the solutes. It Seems probable, however, that the perimeter of the central globule should itself be a function of the initial opposing pressure, as well as of the vapor pressure, and the slopes of the disentanglement curves should tend to decrease with increasing concentrations of solutions, as observed.
Rebound compression of the piston film Immediately after or during the period of disentanglement, most of the curves show a momentary break in the progress of the piston film. I n slow motion the films beem actually to stand still for an instant. A possible explanation is that at this point the piston film, which has been in the so-called liquid expanded state, is opposed by a force sufficient to compress it to a more compact form. At the hame time the soluble film may or may not be undergoing compression. When the piston film has been compacted, tile normal compression of the soluble film proceeds. From the existence of the “rebound” it is reasonable t o suppose that until compression to this point the soluble film was in some state more expanded than that of the piston film. That the point of rebound coincides roughly with the departure of the area-time curve from linearity would confirm the presumption that this point marks the completion of a transition from the gaseous to the liquid state. Another alternative explanation, depending upon the momentum of the body of the liquid set in motion toward the walls of the containing vessel by the advancing film during the period of its fastest motion, has been mentioned elsewhere ( 7 ) . The change from liquid to compact films The rounded portions of the curves, where neither the area nor the diameter is a linear function, apparently represent compression of the
1058
T. FOSTER FORD AND DONALD A. WILSON
“liquid” films to compact films. The compression of the compact films, in which the rate of reduction in area is comparatively low and may be assumed to depend only on the rate of escape of molecules from the films, is represented by the ftnal linear or approximately linear portions of the curves. By extrapolating these final linear portions of the curves backward we presumably obtain the total areas that all of the molecules in the soluble films a t any time would occupy if compressed into films as closely compacted as these curves represent. I n table 1 these minimum compact film areas are compared to the actual areas of the films for the particular point
TABLE 1 Ratios of horizontal C T O S S sections, A h , to vertical crosa sections, A,, of molecules as deduced from film experiments and as calculated from data from other sowces SOLUTION
CONCENTRATION
AQBID
rams per k i l o gram
Hydrocinnamic acid . . . . . . . . .
0.375 0.5 0.75 1.5 4.0
1.27 1.38 1.60 1.43 1.56
1.6 1.6 1.6 1.6 1.6
Caprylic acid. . . . . . . . . . . . . . . .
0.0089 0.0205 0.0420 0.0702 0.1656
1.88 1.89 1.89 1.95 1.86
2.4 2.4 2.4
Phenol. .....................
5.0 20.0 40.0 80.0
1.7 1.2 2.0 1.5
2.4 2.4 1.4
1.4 1.4
1.4
where the disentanglement curves cease to be linear. The ratios are seen to agree very well with the ratios between the vertical and horizontal cross sections of the molecules, as calculated from x-ray data or as deduced by analogy with data for insoluble films. From the apparently real discrepancy in the case of caprylic acid it may be concluded that many of these molecules already are oriented in the uncompressed surfaces. The conclusion from these results is that in the transition from the liquid t o compact films the essential change is in the “up-ending” of molecules in the surface. These results confirm the original presumption that, in the compression of soluble films as well of insoluble films, the transitions are from the
1059
STRUCTURE OF SURFACES OF SOLUTIONS
two-dimensional gaseous state, to the liquid state, to a solid or compact film state.
Adsorption accounted for by the surface films If extrapolating the compact film curves back t o zero time gives the total areas which could have been occupied by all the molecules in the original soluble films, if compacted, then in each case the number of grams of solute originally present a s a film per square centimeter of surface can TABLE 2 Values of adsorption for hydrocinnamic acid, caprylic acid, and phenol solutions as obtained from the areas of the compact solute Jilms at 11°C. and as calculated from surfacs tensions at ordinary temperatures CONCENTRA-
SOLUl'ION
Hydrocinnamic acid.
. . . . ..
Caprylic acid, , , . , . , . , , , . , .
Phenol . . . . , . . . . . . . . , . . . . , ,
TION
AQED
irame per kilogram
hours
0.375 0.5 0.75 1.5 4.0
122 16 43
0.0089 0.0205 0.0420 0.0702 0.1656
12 121 lo? 1 la ll+
5 20 40 80
Extrapolated iompact filma
6.4
1.7* 2.0* 3.2* 5.2* 7.6*
2.8 3.0 4.0
l.lt 2.2t 3.7t 5.6t 7.8t
4a 4+
34 9* 6% 11
2alculated frpm surface tenslon data
3.7 4.3
1.9 1.3 1.3 1.3
2.1: 4.2$ 4.4$ 4.4$
2.0t 4.2t 4.3t 2.3t
* Bhcon and Swain.,unout . hed results. t D.A. Wilson, unpublished results. $ From data of Harkins and Grafton (J. Am. Chem. SOC.47,1329 (1925)).
be calculated by assuming that the area per molecule in the filmis the same as the vertical cross-sectional area of the mol&ules as calculated from x-ray data. The results represent the amount of the Gibbs adsorption which is accounted for as existing in the surface as a film,in these cases. I n table 2 these values are compared with the theoretical values of adsorption as calculated from surface tensions. The failure of the calculated adsorption t o be wholly accounted for by the film measurements in every case might arise from a greater rate of escape of molecules from gas-
1060
T. FOSTER FORD AND DONALD A. WILSON
eous and liquid films than from compact filnib, which would cause the areas of compact films found by back extrapolation to be lower than the true areas. It can be concluded from these results that the adsorption predicted from the lowering of surface tension with increasing concentration is a t least largely accounted for by the existence of a soluble film in the surface. By the microtome method and by interferometric methods it appears t o have already been shown, a t least for the case of hydrocinnamic acid (6, 9, 5 ) , that the total amount of adsorption in a surface at equilibrium is in practical agreement with that predicted from the lowering of the surface tension. Apparently, then, the adsorption exists in the surface substantially as a soluble film, and there is here no evidence for a concentration gradient into the body of the solution a t equilibrium. THE COMPRESSIOPI’ O F FRESHLY SWEPT SURFACES
Compression curves for freshly swept iurfaces are shown in figure 6. Except on the most concentrated solutions, the behavior of the piston film is seen t o he practically the same as on pure water. Each surface was carefully swept several times with paper strips, by the same procedure as used in cleaning all the surfaces before beginning an experiment. By (Ifreshly swept” is meant that t h P time allowed for aging after sweeping was between 30 sec. and 2 min. From the results the conclusion might be drawn that the sweeping effectively removes any soluble as well as any insoluble film which may be on the surface, for the freshly swept surface appears to be almost identical with a pure water surface on dilute solutions and t o rarry only an attenuated gaseous film on concentrated solutions. This result also is a t least superficially in agreement with experiments by the microtome method (4,6). There it was found that the absolute adsorption of hydrocinnamic acid reached the true equilibrium value only after about 12 hr., and that when oiily a few minutes were allowed with this apparatus the observed adsorption was much reduced. However, a film could appear in the surface almost instantaneously by denudation of the underlying layers of solution and the microtome would iiot detect it. because this apparatus removes an average sample, including both film and dennded layers (8). The preqent experiments show either that there is no film on a freshly swept solution or that it is so readily displaced into the layers immediately beneath the surface as to offer no appreciable resistance t o the motion of the oleic acid piston. I t is difficult t o believe that there is no film upon or in the surface, because the surface tensions of freshly formed droplets are found to,be already greatly reduced and it is indeed from drop-weight surface tensions that we calculate the theoretical values of the absolute adsorption which the microtome and other methods confirm. It seems
STRGCTURE OF SURFACES OF SOLVTIONS
1061
possible, however, that in our experiments, where the surfaces were repeatedly swept with absorbent paper strips, the solute might have been almost completely removed from the surface layers by the sweeping, SO that the spreading actually occurred, in effect, on greatly diluted surfaces. For formation of a film corresponding t o the Gibbs adsorption on a 1.5 g. per kilogram hydrocinnamic acid solution total demdation t o a depth of 3500 A. would be necessary. This great depth of denudation might be used t o support the alternative assumption that the film exists but is readily displaced, because in the layers immediately under the film there are no solute molecules tending to replace those escaping from it. These points should be settled by measurements of surface tensions on freshly swept solutions, which are now in progress in this laboratory. SUMMARY
1. A new technique for the investigation of soluble films of positively adsorbed solutes upon the surfaces of their solutions has been developed. 2 . Films of solute molecules have been shown t o exist upon aged surfaces of solutions of caprylic acid, hydrocinnamic acid, and phenol, whereas the surfaces of freshly swept solutions seem more nearly to resemble water . 3. The analogy between these films of adsorbed solute molecules and two-dimensional insoluble films has been demonstrated. 4. The amount of solute in these soluble films is found to be of the same order of magnitude as predicted by the Gibbs theorem.
The authors wish t o express their thanks to Professor J. W. McBain for the interest he has taken in this work. REFERENCES (1) BLODGETT: J. Am. Chem. SOC.67, 1007 (1935). (2) CARP.AND RIDEAL:Proc. Roy. SOC.(London) A109, 301 (1925). (3) FORD:J. Phys. Chem. 40, 835 (1936). (4) FORD:Dissertation, Stanford University, 1936. (5) FORDAND RICBAIN:J. Am. Chem. SOC.68, 378 (1936). (6) MCBAIN,FORD,AND MILLS: To be communicated. (7) MCBAIN,FORD, AND WILSON:Kolloid-Z. 78, 1 (1937). (8) MCBAINAND HUMPHREYS: J. Phys. Chem. 36, 300 (1932). (9) MCBAINAND SWAIN:Proc. Roy. SOC.(London) A164, 608 (1936). (IO) MCBAINAND WILSON:J. Am. Chem. SOC.68, 379 (1936). (11) WILSONAND FORD:Nature 137, 235 (1936).
