PETERW. JEFFERS AND JEROME DAEN
2368
Studies on Spreading, Collapse, and Temperature and Compression Rate Effects on Monolayers of a,w-Dicarboxylic Acids’”
by Peter M. JeffersIband Jerome Daen Department of Chemistry, Lehigh Uniaersity, Bethlehem, Pennsylvania
(Received January 28, 1966)
Monolayers of a,w-dicarboxylic acids with 12, 14, 16, 20, and 24 methylene groups were studied under conditions of varying compression rate and temperature on both acidified distilled water and concentrated ammonium sulfate solution substrates. The surface pressure isotherms indicated that both carboxyls are bound to the substrate and that there is some buckling of the hydrocarbon chain. Measurements of surface potentials and the temperature dependence of the equilibrium spreading pressure support this conclusion. Acidified ammonium sulfate substrates stabilized the films, reduced solution, and caused an increase in the surface potential and collapse pressures; collapse areas, however, were essentially unaltered. Equilibrium spreading pressure measurements indicated that most portions of the spread monolayer isotherms refer to nonequilibrium states; this fact explains the n-A dependence on compression rate. A collapse mechanism involving a “rollingover” process is proposed.
Introduction The saturated, long-chain a,w-dicarboxylic acids, an interesting series of monolayer-forming materials, have not been extensively studied in the past. Adam and JessopZa reported that the dicarboxylic acids formed rather unstable gaseous films which contracted under low pressure to condensed films collapsing at 8 to 12 L&.2/molecule. Dervichian2b presented pressurearea isotherms for the dicarboxylic acids with 16 and 20 methylene groups (for convenience, diacids 16 and 20), which indicated that the films collapsed at 60 to 70 k2/molecule under a pressure of 7 to 10 dynes/cm. Some data on diacid monolayers have been given by Hotta and Isemura.3 The present paper will extend the results previously reported from this l a b ~ r a t o r y . ~Surface pressures and potentials for diacids 12, 14, 16, 20, and 24 will be presented for substrates of both acidified distilled water and concentrated ammonium sulfate solutions. Equilibrium spreading pressures will also be reported for several of the homologs. Compression rate effects and film collapse processes observed with the diacids will be interpreted in terms of the equilibrium pressures and a discussion of the monolayer collapse will be presented. The Journal of Physical Chemistry
Experimental Surface pressures were measured with an automatically recording null-type Wilhelmy surface balance and a surface tray machined from solid Teflon. Surface potentials were determined with an Ra226ionizing air electrode,j a fiber-type calomel electrode, and a Keithley Model 620 electrometer. Though surface pressures were measured a t 1 0 0 ~ orelative humidity, the existence of extensive parallel electrical leakage paths under saturated conditions required that surface potentials be investigated a t ambient room humidity. The pressures were measured to h0.05 dyne/“ ; surface potentials were read to a precision of f3 mv., and surface areas were estimated to within 1%. Isotherms of
(1) (a) Partially supported by a grant from the U. S.Army Research Office (Durham) ; (b) National Science Foundation Cooperative Fellow, 1961-1964.
(2) (a) Tu’. K. Adam and G. Jessop, Proc. Roy. SOC.(London), A112, 376 (1926); (b) D. G. Dervichian and M . Joly, J . Phys. Radium, 10, 375 (1939). (3) H . H o t t a and T. Isemura, Bull. Chem. SOC. Japan, 2 5 , 101 (1952). (4) P. M . Jeffers and J. Daen, Proceedings of the Fourth International Congress on Surface Activity, Brussels, 1964, in press. (5) J. H. Schulman, L. Nanis, and 1, 1960.
M.Baruch, NONR226(64), June
SURFACE PROPERTIES OF DICARBOXYLIC ACIDS
2369
9
8
7 6
gi s c 0
- 4 5
2
I 90
II 0 IS0 DIACID 20----- 90
150 110 WClD 24---
190 IS0 90
"O
DIACID I6
I70
II 0
130
170
Area. k.2,"olecule.
Figure 1. Force-area isotherms for various diacids on acidified distilled water a t a compression rate of 21.8 A.2/molecule min.
stearic acid obtained with this apparatus reproduced those reported by Gaines.6 Equilibrium spreading pressures were found by sprinkling finely powdered, crystalline diacid on a fixed area of 100 and measuring the resulting surface pressures with the Wilhelmy balance. The dicarboxylic acids were synthesized by Kolbe electrolysis and were spread from chloroform solutions of ca. 0.4 mg./ml. concentration. Distilled water was acidified to pH 2.0 with sulfuric acid. Reagent grade ammonium sulfate was used without purification to prepare substrate solutions having a salt concentration of 30-40% by weight ; these also were acidified with sulfuric acid. Details of the experimental techniques and material preparation have been given el~ewhere.~"
Results Pressure-area isotherms for films of diacid 16, 20, and 24 on acidified water substrates are presented in Figure 1. That the diacids formed liquid- or vaporexpanded films was indicated by the isotherms and the film compressibilities. With rising temperature, the films expanded smoothly and collapse pressures increased over a 15 to 20" temperature span, from 1-2 dynes/cm. to about 10 dynes/cm.; collapse areas were sensibly independent of temperature. No monolayer appeared to form below a temperature characteristic of each diacid. Diacid 32 did not seem to spread even a t 90". For diacid 24 a mono-
layer could be spread observably only above 53", while the corresponding temperature for diacid 20 was 41". At 3.4", diacid 16 appeared to be just above its apparent characteristic spreading temperature and, in passing, it may be remarked that a t this temperature diacid 14 formed a well-expanded film. Figure 2 illustrates the isotherms obtained for diacid 16 on acidified distilled water a t various compression rates. Generally, it was observed that all diacids exhibit a comparatively large decrease in collapse pressure and a slight increase in collapse area with decreasing compression rate. Both the surface potential AI' and the apparent dipole moment p measured for diacid 16 on the acidic substrate are displayed in Figure 3. As with the pressure-area isotherm, the surface potential rose smoothly from 200 i.2/molecule to collapse and was free from sharp breaks except a t collapse. Collapse points on both the pressure and the surface potential isotherms were coincident. Within experimental error the surface moment-area curve was linear from 200 i.Z/molecule to collapse. It may be noted that the pressure increased by a factor of about 10, the surface potential rose by a factor of 2.5, and the surface moment changed by 30% between 200 and 90 W.2/molecule. Hotta and Isemura's values3 for p of 410, 350, and (6) G. L. Gaines, J . Colloid Sci., 15, 321 (1960). (7) P. M~ Jeffers, Ph.D. Thesis, Lehigh University, Bethlehem, Pa.. 1964.
Volume 69, .\'umber
7
J u l y 1966
PETER M. JEFFERS A N D JEROME DAEN
2370
IS 14
8 7
e
12
6
IO 4 a
$5
m
54 c i 3
8 ; 71
2
S i 4
I
2
I40
I20
100
166
180
Area, .k.2/molecule.
Figure 2. Compression rate effects on *-A isotherms. Acidified distilled water substrate, diacid 16 a t 22" (in .&.2/molecule min.): a, 21.8; b, 10.9; c, 7.27; d, 2.91; e, 0.727; and f , 0.242. Acidified ammonium sulfate substrate, diacid 12 a t 26" (in A.2/molecule min.): A, 10.9; B, 2.9.
476
180.
140
-
426
i 5 100
-
d
s
-376 i
I
. I
1
1
100
I
120 .ires,
140
160
IO0
I
K.*/molecule.
Figure 3. Surface potential and moment of diacid 16 a t 22' and a compression rate of 10.9 A.2/molecule min.
320 mD. a t 100, 200, and 300 A.2/molecule for diacid 16 are within about 10% of the present measurements. These authors reported diacid 16 as forming a gaseous film, in agreement with the present work, but did not present any isotherms in their paper, thus preventing further comparisons. With diacids 12 and 14 at room temperature and with diacid 16 a t temperatures above 60°, there was extensive film loss from the surface. Films were spread on concentrated ammonium sulfate solutions (as, e.g., Brooks and AlexandeP) to clarify whether solution or vaporization was the major responsible factor. Loss from diacid 12 and 14 films was essentially eliminated, confirming the hypothesis of solution of these smaller molecules into the wat,er substrate. Moreover, on the salt substrate all of the diacid films were stable to much higher pressures with T attaining values of 20 dynes/ cm. in some cases. Solubility was reduced so dramatically that diacid 10 which was completely soluble when spread on water a t 25" formed a film on ammonium The Jourml of Physical Chemistry
sulfate which, when compressed a t a rate of 10.9 A.2/ molecule min., collapsed a t 16 dynes/cm., and a t an apparent area of 40 A.2/molecule. Partial but far from complete solution of the film is inferred from this small collapse area. The behavior of diacid 12 on ammonium sulfate solutions a t room temperature and a t two different compression rates is shown in Figure 2. Again, the trends noted on distilled water are evident, though on the salt substrate compression rate changes do not appear to displace the curves from each other. Isotherms for other diacids a t various temperatures are presented in Figure 4. In these instances, the effect of temperature was the same as that for water substrates. As before, diacids 20 and 24 apparently would not spread below certain temperatures, but the initial spreading temperature was appreciably depressed on ammonium sulfate. At 25", diacid 20 spread and was stable to 5 dynes/cm., while diacid 24 collapsed only a t 6 dynes/ cm. a t 51". Surface potential-area isotherms for diacids 12, 16, and 20 on ammonium sulfate are displayed in Figure 5 . The curves for diacids 12 and 16 were very similar and almost coincide within experimental limits; the shapes of the isotherms for diacid 16 on pure water and on salt are identical. For the diacid 16 film on ammonium sulfate, the curve, however, is displaced by 20 to 30 mv. Measured values of equilibrium spreading pressure for diacids 10, 12, 14, and 16 may be found in Table I.
---
--_-OUClD 20
60 !20 I60 DIACID24 100
140
Area, A. 2/molecule.
Figure 4. Force-area isotherms for various diacids on acidified ammonium sulfate a t a compression rate of 10.9 A.2/molecule min. ~~
(8) J. H. Brooks and A. E. Alexander, Proceedings of the Third International Congress on Surface Activity, Cologne, 1960, Vol I1 p. 196.
SURFACE PROPERTIES OF DICARBOXYLIC ACIDS
237 1
It can be seen that the equilibrium spreading pressures were considerably higher on the salt solution than on water. Spreading rates, as determined visually, for the diacids decreased with increasing chain length in agreement with Brooks and Alexander's findings for the fatty alcohols.8
sj
Discussion
i
I 1
80 J
I
'
I
1
180
I40
IO0
Area, A.z/moleoule.
Figure 5. Surface potentials of various diacids on ammonium sulfate at 25' and 10.9 %1.2/moleculemin.
Although the three smaller diacids were lost rapidly from spread films by solution into the substrate, the presence of excess surface crystal ensured the determination of acceptable equilibrium pressures. The recorded pressures were reached within 1 to 10 min. after the diacid crystals were applied to the surface and remained essentially constant thereafter.
Table I : Equilibrium Spreading Pressures (dynes/cm.) Diacid
10 12 14 16 24
,-----lo 20°
0.3 , . .
...
... ...
1.7 0.5 ,
.
,
.. .
...
Temperature, O C . 25' 31'
2.3 0.6,5.5" 0.07 0.04 ...
7
50'
2.7 1.0 0.25 0.12
7.0 4.4 2.0 0.4
...
...
60'
... 11.0"
... 6" Small"
' Film on (N&)zSO( substrate; all others on acidified distilled water.
To test equilibrium attainment with a diacid 10 film a t 25", the surface area was increased, then decreased by an amount sufficient to change the surface pressure momentarily by 0.5 dyne/cm. After the area increase, the pressure rose immediately to its original value, as expected, since spreading of diacid 10 was quite rapid and the reservoir of diacid crystal on the surface was sufficient to saturate the additional uncovered area. A slight decrease in pressure with time occurred shortly after the film was compressed; the original value of equilibrium pressure was not quite reached during the 15- 30-min. observation period, this experiment meaning that the recorded equilibrium pressure values may be low by no more than 0.3 dyne/cm.
That no monolayer can be compressed indefinitely is well known; there are a number of different usages, however, of the term "collapse" appearing in the literature. 9- l 3 It seems appropriate to term as collapse any process involving the transfer of molecules from the monolayer to any bulk phase. This definition would include solution, evaporation, and the formation of bulk crystals, but would not include reorientation within the film if unimolecular character were maintained. Under this definition, such processes as the LLlow pressure reversible collapse'' discussed by MacRitchie would not be called a collapse since the protein molecules as a whole appear to remain anchored to the water surface, but collapse would include both the evaporation and solution found by Brooks and Alexander'O in the fatty alcohol series. The distinction concerning collapse and reorientation evidently will be most significant in monolayers of macromolecular materials. I n this work, the diacid monolayers underwent collapse into a solid phase in addition to any accompanying dissolution or evaporation from the surface. Both solution and evaporation would depend on surface pressure, but only crystal formation would lead to the sharp break and rapid decrease in pressure observed with the diacids. Since the diacids melt only above 120°, the collapse was to a form of solid rather than to a liquid lens, although hydration of the carboxyls may have caused a different crystal structure from that usually found. I t is known that the crystalline structure of fatty alcohols is influenced by the presence of water. lo The large collapse areas observed with all of the diacids under the various experimental conditions imply a horizontal configuration with both carboxyl groups (9) N. K. Adam, Proc. Roy. Soe. (London), A101, 452 (1922). (10) J. H. Brooks and A . E. Alexander, "Evaporation Retardation by Monolayers," V. K. La Mer, Ed., Academic Press, Inc., New York, N. Y.,1962, p. 245. (11) F. MacRitchie, J . Colloid Sei., 18, 555 (1963). (12) R. J. Goldacre, "Surface Phenomena in Chemistry and Biology," J. F. Danielli, K. G . A. Pankhurst, and A. C. Riddiford, Ed.. Pergamon Press, New York, N. Y., 1958, p. 278. (13) A. W. Adamson, "Physical Chemistry of Surfaces," Interscience Publishers, Inc., New York, N. Y.,1960.
Volume 69, h'umber 7
July 1966
2372
PETERM. JEFFERSAND JEROME DAEN
anchored to the surface and with little vertical bowing of the hydrocarbon chain before collapse. Moreover, the maximum value of 1.1 for diacid 16 is 475 mD. Now, the surface potential for stearic acid a t 22 molecule is 400 mv. and this corresponds to a surface moment of 230 mD. Thus, the apparent diacid dipole moment is just slightly greater than twice the value for stearic acid. Recognizing that the major contribution to the surface moment is that of the carboxyl groups, the diacid surface potentials are rather direct proof that each carboxyl in the diacid is anchored to the surface. Additional perspective on these conclusions may be drawn from the energetics of the bulk-to-monolayer spreading process. In Table I1 are summarized thermodynamic quantities computed from the spreading pressure data already presented. As a result of the very low equilibrium spreading pressures and their small temperature coefficients, it is difficult to compute highly accurate values. It proved necessary to estimate areas for the lower molecular weight diacid equilibrium monolayers by extrapolation from isotherms for the larger diacids whose spread monolayers were essentially insoluble. These factors limit the reliability of the estimated energies and entropies to about f3071,. The heat of spreading was calculated using the Clausius-Clapeyron equation in the two-dimensional form proposed by Cary and Rideal14 and discussed by Alexander and Goodrichlj; free energies and enthalpies of equilibrium spreading were calculated following Harkins, Young, and Boyd.16 For comparison, corresponding quantities for several monofunctional materials measured by Boyd” are included. When, for example, entropies or latent heats of equilibrium spreading for the diacids are compared with corresponding properties of the monoacids, the in-
Table I1 : Est,imated Energies and Entropies of Spreading for Several Di:wids kcal./ mole
G,. kcal./ mole
H.. kcsl./ mole
8.. k e d . /
T. “C.
30 30 35 0 30 5 30 15 30 15 30
13 12 8.9 5.20 7 52 6.39 8.49 4.69 5 13 5.01 5.61
-0.47 -0.36 -0.18 -0.38 -1.25 -0.16 -0.89 -0.50 -0.77 -0.23 -0.52
12 12 8.7 4.82 6.28 6.23 7.60 4.19 4.36 4.78 5.09
42 40 29 19.0 24.8 23.0 28.0 16.3 16.9 17.4 18.5
Qe,
Material
Diacid 10 Diacid 12 Diacid 14 Tridecylic acid” Myristic
acidI7 Pentadecylic acid” Palmitic acid”
The Journal of Phg/s?hl Chemistry
mole
teresting and significant fact emerges that the former are approximately twice as large as the latter. On the basis of moles of carboxyl group spread, one may say that the entropy change for the diacids is roughly that of the monofunctional acid. I n other words, these thermodynamic results support the conclusion that both polar groups are bound to the substrate. It is somewhat surprising that the collapse areas do not show greater variation with chain length, since with a horizontal orientation the chain length might be expected to be an important determinant of the limiting area. Relatively slight flexibility of the extended hydrocarbon chain in the film is, of course, sufficient to account for this observation. The most obvious effect in passing from one member of the diacid series of the next is the relation of chain length to the temperature range of monolayer expansion. Thus the chain length effects are quite similar to those usually found in monofunctional homologous series. When it is recalled that stearic acid is stable to 40-60 dynes/cm. depending on compression rate, the diacid collapse pressure of less than 10 dynes/cm. may appear unusual. However, the difference in orientation must not be forgotten. With the upright hydrocarbon chain in stearic acid films, 40 dynes/cm. is equivalent to about 140 atm. in three-dimension$ pressure, while, assuming a film thickness of 6 A. for the horizontal diacids, 10 dynes/cm. corresponds to 165 atm. The strong dependence of the isotherms on compression rate indicates that the diacid monolayers are not equilibrium systems but are in fact metastable. Equilibrium spreading pressures for the diacids are indeed seen to be very low, so that most pressures attained on compression of the spread monolayers are above equilibrium values. Consistent with Cary and Rideal’s statement l 4 that the film-to-crystal transformation is frequently very slow, the diacid isotherms were very reproducible a t a given compression rate. As a result, the films can perhaps be best described as quasiequilibrium systems. The measurements of equilibrium spreading pressures suggest that the initial spreading temperatures found for the diacid monolayer may, under the present experimental conditions, coincide with the temperature where the equilibrium spreading coefficient of the solid (14) A. Cary and E. K. Rideal, Proc. Roy. Soc. (London), A109, 318 (1925). (15) A. E. Alexander and F. C. Goodrich, J . Colloid Sci., 19, 468 (1964). (16) W. C. Harkins, F. F. Young, and G. E. Boyd, J . Chem. Phya.. 8, 954 (1940). (17) G. E. Boyd. J . Phys. Chem., 62, 536 (1958).
SCRFACE PROPERTIES OF DICARBOXYLIC ACIDS
crystal becomes positive. Such a n observation is in agreement with Cary and Rideal's statement14 that films of behenic acid, which contract spontaneously at room temperature, become stable a t temperatures above that where the equilibrium spreading pressure becomes positive. In this connection, the work of Fernandez, Aenlla, arid Trillo,I8 who recently studied monolayers of diacid 20, a natmally occurrFg cork acid, at areas between 10,000 and 30,000 A.2/molecule, is pertinent. The fact that these workers could spread diacid 20 films at 15", while the present investigation indicated condensed films of diacid 20 to form only above 41", may be explained by the very dilute nature of the former system, the pressures measured being on the order of 0.002 dyne/crn. 'C'nder such conditions one would expect the decay toward the equilibrium state to be extremely slow. Thus, it appears that the characteristic temperatures observed in the experiments described here may very well be functions of the experimental conditions as are other monolayer phenomena. In other words, in such situations one must recognize the metastability when interpreting the observations. With stearic. acid, aggregates in the form of long wrinkles become visible upon collapse, l9 and similar visible aggregates are formed upon collapse of protein films.14 With the diacids there was no visible evidence of this sort. I t is likely that many tiny collapse sites were formed scattered over the entire surface; collapse proceeded into microcrystals rather than into a long, "folded" structure illustrated by stearic acid. For the diacids on water there was no tendency for collapsed films to respread. Several films were compressed just past the collapse point, then were expanded immediately and subsequently left undisturbed. After 4 hr., recompression showed the films to have collapsed further rather than to have respread. On concentrated ammonium sulfate, a film of diacid 20 was compressed at 5" until collapse began; at this instant the film was expanded. Immediate recompression showed that the film had respread almost completely during 1he expansion. The equilibrium spreading pressures were considerably higher on ammonium sulfate than on water, and with expansion of the diacid 20 film, the surface pressure fell below the equilibrium spreading pressure so that re-expansion should have occurred as observed. The stabilization of diacid films on ammonium sulfate, in addition to suppressing the solution process, is in accord with the results of Pankratov,20who noted both an expansion of the film and an increase in the surface potential with hexadecanol and ethyl palmitate monolayers on salt solutions.
237 3
The increased equilibrium spreading pressures on ammonium sulfate are consistent with greater stability of the spread monolayers, since the films spread on ammonium sulfate are less removed from equilibrium than are films on distilled water. The pressure-area isotherms and the compressibilities indicate the diacid films to be liquid-like around the region of collapse. In this vicinity, one would expect the monolayer to be composed of many small randomly oriented and distributed but somewhat internally ordered regions. The collapse sequence may then involve the reorganization of the small ordered regions to "microcrystals" with the initial stage involving only the rolling of one chain of molecules up onto the back of an adjacent chain, much as a layer of pencils would buckle under lateral compression perpendicular to their axes. Enough energy must be supplied by mechanical compression to tear the carboxyls from the mater surface to initiate collapse. Once begun, the energy of crystallization is probably sufficient to carry the process to the point of equilibrium between monolayer and crystal. This scheme is similar to that discussed by Kies and KimballZ1for stearic acid. However, since the diacid monolayer is much thinner than a stearic acid film, a shorter translation of the molecules pushed out of the surface is involved with the diacids. Although the equilibrium spreading pressure (and thus the theoretical limit of compression) decreases with increasing chain length,8 Ries and Kimball found the collapse pressure, under dynamic conditions, for n-hexatricontanoic acid to be 58 dynes/cm., 16 dynes/cm. greater than the collapse pressure for stearic acid. The low collapse pressures found with the diacids are consistent with this idea. This and the high stability of the diacid bulk crystal, as evidenced by very low equilibrium spreading pressures and low temperature coefficient of the spreading pressure, are probably responsible for the relative instability of the diacid films. No points of first-order phase change were observed. hloreover, since no measureable pressure was observed above about 170 A.2/molecule even at high temperatures, the surface vapor pressure of the diacid 20 and 24 liquid-expanded films must be less than 0.05 dyne/ cm. (18) S. G. Fernandez, E. 0. Aenlla, and J. 151 (1964).
JI. Trillo. Kolloid-Z.,
197,
(19) W. Rabinovitch, R. F. Robertson, and S. G. Mason, Can. J . Chem., 38, 1881 (1960). (20) A . Pankratov, Acta Physicochim. URSS., 10, 45 (1939). (21) H. E. Ries and W. A. Kimball, Proceedings of the Second International Congress on Surface Activity, Vol. I, Academic Press, New York, N. Y., 1957, p . 75.
Volume 69, Number 7 J u l y 1966
