Feb., 1953
COLLOIDAL PROPERTIES OF HIGHLY FLUORINATED ALRANOIC ACIDS
247
COLLOIDAL PROPERTIES OF HIGHLY FLUORINATED ALKANOIC ACIDS BY C. H. ARRINGTON, JR., AND G. D. PATTERSON Contribution No. 316 f r m the Chemical Department, Experimental Station, E. I . du Pont de Nemours and Compang Wilmington,Delaware Received J u l y l.GO1066
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The hysical behavior of solutions of highly fluorinated alkanoic acids of the general formula H(CE)?,,COOH,where n varies $om 2 to 6, has been investigated. Small angle X-Ray scattering has revealed a micellar structure in concentrated solutions of the ammonium salts of these acids. X-Ray spacings corresponding to the I-spacings found for hydrocarbon soap solutions have been observed. The X-ray spacing which has been attributed to the over-all dimension of the micelle has not been found, The critical micelle concentrations of this series of highly fluorinated alkanoates are lower than those of hydrocarbon alkanoates of the same chain length. This behavior is attributed to the highly non-polar nature of the fluorocarbon chain. Characterization of monolayers of these alkanoic acids spread on a water substrate has indicated that comparatively weak intermolecular forces exist between the adjacent fluorocarbon chains. These monolayers behave as twodimensional gases at relatively high film pressures. The effect of the fluorocarbon chain on other physical properties such as viscosity, surface tension and dissociation constant of these acids is discussed.
The physical behavior of fluorocarbon compounds offers a special opportunity to study changes produced in an organic molecule by the substitution of the large, highly electronegative fluorine atom for the small, less electronegative hydrogen atom. Berry, et al.,l have reported the preparation of a series of aliphatic fluorine-containing alkanoic acids of the general formula H(CF&,COOH where n varies from 2 to 6. The compounds are completely fluorinated except for the terminal hydrogen atom. The nomenclature adopted in this paper is to describe the compound by the number of carbon atoms present and the nature of the functional group. Thus, hexadecafluorononanoic acid, with the formula H(CF2)&OOH, is designated as the C-9 fluorocarboxylic acid. The alkali and ammonium salts of the fluorocarboxylic acids have surface-active properties for chain lengths of n = 4 and greater. The hydrocarbon surface-active agents have been investigated extensively in recent years by many techniques including X-ray scattering, surface balance measurements and electron microscopy. The concept of a micellar structure has been firmly established from these investigations. The salts of long-chain fatty acids exist in dilute solution as individual molecules and ions, As the concentration increases, these ions associate themselves into a micellar structure. The initially formed micelle contains on the order of fifty molecules for a 16-carbon chain and is probably of spherical character with the hydrocarbon tails oriented into the interior of the micelle and the micelle surface composed of the charged portions of the ions. In solutions of higher concentration, it is believed that these micelles undergo further development into lamellar micelles in which the degree of order is very high and the system approaches the liquid crystalline state. The existence and some of the dimensions of these micelles can be found by low-angle X-ray scattering methods. The spherical micelles are formed in a fairly sharp concentration region. The location of this region can be determined by dye titrations, refractive index increment measurements and conductivity measurements. In this paper the results of an investigation carried out using standard methods to determine the micellar, monolayer and otlher properties of the (1) K. L. Berry, el ol., in presa.
highly fluorinated surface-active agents are given. The unusual properties of the fluorinated chain become evident in such an examination.
Experimental Details The preparation of the compounds used in this investigation is described by Berry.1 The compounds were recrystallized several times, but there may have been small amounts of the shorter- and longer-chain compounds in the samples used. The X-ray measurements were carried out on a smallangle vacuum camera of the same design used by Bolduan and Bear.2 The solutions were contained in thin-walled glass capillaries. The film-to-sample distance wm 150.05 mm. and the spacings were calculated from the Bragg expression which becomes d = 462.7/L where d is spacing and L is distance of separation of maxima. The distances of separation were determined by the use of a microdensitometer. The critical micelle concentrations were determined by the dye titration technique described by Corrin, et al.,a using Rhodamine GGDN, acridine orange and pinacyanol chloride as dyes. Most of the runs were made with visual detection of the end-point. It was found that the endpoint could be determined more accurately for Rhodamine 6GDN titrations by use of a photofluorimeter to follow changes in fluorescence. Critical micelle determinations were made also by the refractive index method described by Klevens4 using a differential refractometer. The surface balance measurements were made on a hydrophil balance with the film spread from a benzene solution. Surface tensions were determined on a Roller Smith surface tension balance No. 2349. It was found that for approximate measurements good results could be obtained quickly using the differential capillary method of Natelson.6 Dissociation constants were determined by potentiometric titrations using a Beckman pH meter, Model G, with a saturated calomel and a glass electrode.
Results and Discussion Characterization of Micelles by X-Ray Scattering.-Harkins and c o - w ~ r k e r shave ~ ~ ~examined the small angle X-ray scattering from a number of hydrocarbon soaps. They have found two main spacings which are designated the “I” and “M” spacings. The interpretation of these spacings is (2) 0. E. A. Bolduan and R. 6. Bear, J . AppZied Phys., 80, 983 (1949). (3) M.L. Corrin and W. D. Harkins, J . Am. Chcm. Soc., 69, 679, 683 (1947). (4) H.B. Klevens, THISJOURNAL,62, 130 (1948). (5) S. Natelson and A. H. Pearl, J . Am. Chem. SOC.,ST, 1520 (1935). (6) W.D.Harkins, R. W. Mattoon and M. L. Corrin, i b i d . , 68, 220 (1946). (7) R. W. Mattoon, R. S. Stearn and W. D. Harkins, J . Chem. Phya., 16, 644 (1948).
C. H. ARRINGTON, JR.,AND G. D. PATTERSON
248
.Val. 57
still under discussion, but Winsor* recently has TABLE I summarized the situation. The M spacing is SMALL-ANGLE X-RAY SCATTERINQ OF FLUOROCARBON AND believed due to the diameter of a small micelle of HYDROCARBON SOAPS spheroidal shape. Vaules of the M spacing are of Bragg I Zi% t i Z f n F spacing, the order of twice the length of the hydrocarbon Material soap A. A. chain and do not change with concentration. The C-9 Ammonium fluorocar2 . 5 ' 115.7' I spacing varies with concentration and is interboxylate [H(CF&COONH4] 5 . 0 79.9 preted by RilcBain'' and Winsor as being caused by b 10.3 74.6 the presence of lamellar micelles. The lamellar b 20.6 68.0 ' micelle is a sandwich consisting of regular layers of 6 38.5 ... soap molecules with water filling the space between Potassium myristate 12 36.8 67.4 the functional groups. Thus, the lamellar micelle 2.27 40.8 is an approach to a liquid crystalline phase. The Potassium myristnte' 4.47 41.5 I spacing gives the distance between adjacent water 7.16 40.7 layers in the sandwich. 9.82 40.2 71.0 X-Ray scattering data on solutions of the C-9 12.00 39.5 67.5 ammonium fluorocarboxylate are summarized in 13.74 39.6 66.0 Table I. The I spacings obtained for the fluoro16.47 40.2 62.5 carboxylate are of the same order as those observed a From ref. 7. No spacings could be observed. Doubtfor the hydrocarbon soaps. Some results of Mattoon' on potassium myristate are given in ful due to weak intensity. Table I for comparison. ate solutions. Exposures were made up to 40 A plot of the I spacings versus the logarithm of hours without detecting any evidence' of such a weight per cent. conoentration gives a straight line band. M band spacings were observed for potasfor the three highest concentrations. This is the sium myristate solutions using the same camera. same behavior observed by Harkinsa for hydro- This lack of an M spacing can be explained by concarbon soaps. sidering the electron densities in the soap micelle No M band was observed for the fluorocarboxyl- in the manner used by M ~ B a i n . McBain ~ calculated the electron density of the various segments of the molecule from scale models and constructed a diagram which is reproduced as Fig. la. This shows that the electron density increases markedly in passing from the water phase into the -M-------I M - ,carboxyl group region and then decreases sharply when the hydrocarbon *chain is reached. Figure COOH COOH 0'50 COOH COOH l b shows a similar plot for a fluorocarbon micelle. This figure indicates a gradual increase of electron density from the water phase through the carboxyl WATER group into the fluorocarbon chain. The M spacing g, 0.25 0 is assumed to arise from scattering by the carboxyl group peaks shown in Fig. la. Since these peaks are masked in a fluorocarboxylate by the high electron density of the fluorocarbon chain, diffraction 8 a from the carboxyl groups should no longer be obm served. The I spacing, which arises from scattering 0 between adjacent 'micellar layers, should still be present. This interpretation appears a reasonable one and is additional evidence for attributing the M spacing to the over-all micellar dimension. Determination of Critical Micelle Concentrations.-The critical micelle concentrations for several fluorocarbon compounds are summarized in Table 11. The dependence of the critical micelle
1
I
e
TABLE I1 CRITICAL MIeELLE CONCENTRATION O F HIQHLY FLUORINATED SURFACE-ACTIVE AGENTS
0.25
~
50 75 100 125 Distance, 1. Fig. 1.-Electron density plots of: a, hydrocarbon carboxylate micelle; b, fluorocarbon carboxylate micelle. The upper figure is a, the lower one b. 0
25
(8) P.A. Winsor, THISJOURNAL, 66, 391 (1952). (9) J. W. McBain and 0. A. Hoffman, ibid., SS, 39 (1949).
Compound
C M C p moles/l.
C-7 Ammonium fluorocarboxylate C-9 Ammonium fluorocarboxylate C-11 Ammonium fluorocarboxylate C-7 Fluorocarboxylic acid C-9 Fluorocarboxylic acid Potassium laurate Potassium myristate a CMC = critical micelle concentration.
0.25 .038 ,009 .15 .03 ,0255 ,0066
COLLOIDAL PROPERTIES OF HIGHLY FLUORINATED ALKANOIC ACIDS
Feh., 1053
concentration upon chain length is analogous to that of hydrocarbon compounds. The value of the critical micelle concentration for the C-9 ammonium fluorocarboxylate is roughly the same as that determined for potassium laurate (0.026 M ) . The fluorocarbon chain has a lower affinity for water thaii a hydrocarbon chain and as will be shown later the carboxyl group has the character of a strong acid. The net result is a molecule with a greater contrast between the polar and non-polar sections, producing a more efficient surface-active agent. Unfortunately, the solubility of the ammonium fluorocarboxylates decreases rapidly with increase in chain length. The C-11 compound has a solubility of less than 1% and the C-13 carboxylate is practically insoluble. A brief examination wassmade of a system of mixed micelles containing the C-9 ammonium fluorocarboxylates and potassium myristate. Figure 2 shows the variation of critical micelle concentration in this system as a function of the mole fraction of C-9 ammonium fluorocarboxylate. From the shape of the curve, it is evident that the micellar systems formed are of a complex nature.
240
centration curve observed for the ammonium C-9 fluorocarboxyla te. Behavior of Monolayers.-Attempts to spread monolayers of the shorter-chain fluorocarbon acids and alcohols on water were unsuccessful, probably because of their solubility. The C-11 fluoroalcohol [H(CF2)&H20H] could be spread on water and gave the force-area curve shown in Fig. 4. When the upper part of the curve is extrapolated to zero pressure, a close-packed area of 29 A.z is obtained.
Area per molecule, A.z Fig. 4.--Monolayer bchavior of C-11 fluoroalcohol (H(CFz)ioCHsOH).
This value is in good agreement with the expected cross-sectional area of a fluorocarbon chain. The C-13 fluorocarboxylic acid was spread on water and on 0.01 N hydrochloric acid. The results are given in Fig. 5 . On the acid substrate the fluorocarboxylic acid behaved normally and the results agree with the fluoroalcohol behavior. When the film was recompressed the curve was reproduced.
0.4 0.25
0.50
0,75
1.00
Mole fraction of ammonium C-9 fluorocarboxylate. Fig. 2.-Critical micelle concentrations of mixed micelles of potassium myristate and ammoniuni C-9 fluorocrtrhosylate.
Dependence of Surface Tension on Concentration.-The surface tension of solutions of hydrocarbon surface-active agents is characterized by a rise i n the region of the critical micelle concentrabion with decreasing concentration. Such a rise also has been observed for the fluorocarbon compounds. Figure 3 shows the surface tension conI
01
I
I
2
3
4
I
5
Concentration, wt %.
Fig. 3,--8urface tenaion of ammoniuni C-0 fluorocarboxylate solutions.
30
I
I
I
a 10
20
30
40
b 20
30
40
Area per molecule, b.2 Fig. 5.-Monolaver hehavior of C-13 fluorocarboxvlic acid on: a, antcr substrate; b, 0.01 N hvdrochloric acid substrate.
On a water substrate, the first compression of the fluorocarboxylic acid film gave a lower area per molecule than was observed on the acid substrate. Upon a,second compression, a much lower area per molecule was found with an extrapolated area of 19 per molecule. This behavior indicates that the fluorocarbon films have a greater stability on acid than on mater substrates. The hydrocarbon carboxylic acids are stable on both types of substrate, but generally have lower collapse pressures at lower pH's. There is some evidence that a rearrangement of molecular packing occurs on pB
250
C. H. ARRINGTON, ,JR.,
AND
change.1° The instability of the C-13 fluorocarboxylic acid film may be due to solubility in the water substrate. However, this compound appears to be almost entirely insoluble in water. The fact that the carboxyl group has a larger cross-sectional area than the hydrocarbon tail while the fluorocarbon chain has a greater cross-sectional area than the carboxyl group may be a factor. The surface balance data may be plotted as the product of force and area versus the force as shown in Fig. 6 . When this is done, marked differences are observed between fluorocarbon and hydrocarbon behavior. The myristic acid curve continuously decreases with decreasing pressure. Adam1’ has shown that if the myristic acid curve is extended to very low pressures it passes through a minimum and begins to rise toward a limiting pressure area value of 400. Similar behavior is also shown by Adam for the shorter-chain hydrocarbon carboxylic acids, revealing a general tendency to approach the value of 400 as a limit. This value of 400 is that to be expected for the product of pressure and area if the monomolecular film were behaving as a perfect gas. In Fig. 6 the fluorocarboxylic acid data indicate that this material approaches perfect gas behavior at relatively high pressures. The minimum and maximum exhibited by the fluorocarboxylic acids probably represent a change from two-dimensional liquid phase to twodimensional gas phase.
G. D. PATTERSON
VOl. 57
carbon compounds which also indicate a low order of intermolecular forces. Other Physical Properties.-A brief survey was made of other properties which might be markedly influenced by the fluorocarbon chain. The results are summarized in Table 111. The dissociation constants of the fluorocarboxylic acids are all of the indicating that these are strong order of 2.5 X acids of the same class as chloroacetic acid. The viscosities of the C-5 and C-7 fluorocarboxylic acids are substantially higher than those of the corresponding hydrocarbon compounds. However, the kinematic viscosity divided by the molecular weight is about the same for fluorocarbon as for hydrocarbon compounds. The surface tension of the pure fluoroacids is about 13 dynes/cm. This low value is another reflection of the low cohesion between neighboring fluorocarbon molecules. An electron micrograph was obtained by drying a solution of the ammonium C-9 fluorocarboxylate on a sample screen. The resulting picture showed a gel-like network with very dark regions. These dense regions were probably caused by the high electron density of the fluorine atoms. The structure was definitely not of the fibrous type found by Marton12for sodium laurate. TABLE I11 PHYSICAL PROPERTIES OF FLUOROCARBON COMPOUNDS Viscosity my@ poise
Density
cstistokes
v/
Mol. wt.
C-5 Fluorocarboxylic acid 84.3 1.664 5.07 0.021 ,036 C-7 Fluorocarboxylic acid 221.9 1.776 12.50 %-Valericacid” 22.4 0 939 2.38 .023 .036 43.6 0.918 4.76 Heptanoic acida Dissociation constants PK
C-5 Fluorocarboxylic acid C-7 Fluorocarboxylic acid C-9 Fluorocarboxylic acid Pressure, dynes/cm. Fig. 6.-Comparison of monolayer behavior of (3-13 fluorocarboxylic acid and myristic acid.
This indication that the fluorocarbon films have perfecf gas behavior at these pressures means that the intermolecular forces between fluorocarbon chains are very weak. This conclusion agrees with the evidence from boiling point data on fluoro(10) W. D. Harkins and R. J. Myers, J . Chem. Phys., 4,723 (1936). (11) N. K. Adam, “The Physics and Chemistry of Surfaces,” 3rd Ed., Oxford University Press, 1941, p. 117.
2.65 2.68 2.60
K X 10-8 2.23 2.08 2.51
Surface tension Surface tension, dynedam.
C-5 Fluorocarboxylic acid 13.0 C-7 Fluorocarboxylic acid 12.5 ” D a t a from “Lange Handbook of Chemistry,” 6th Ed., Handbook Publ. Inc., Sandusky, Ohio, 1946.
Acknowledgments.-Dr. E. P. H. Meibohm performed the X-ray scattering measurements and assisted in the interpretation of the results. (12) L. Marton, J. W. McBain and R. D. Vold, J . A m . Chem. S o c . , 63, 1990 (1941).
