J. Phys. Chem. 1983,87,3289-3297
3289
Fluorocarbon Surfactants. Phase Equilibria and Phase Structures in Aqueous Systems of a Totally Fluorinated Fatty Acid and Some of Its Salts Krlster Fontell+and Bjorn Llndman"z Division of Food Technology, and Division of Physlcal Chemistry 1, Chemical Center, University of Lund, S-220 07 Lund, Sweden (Received: February 1, 1982; First Revision Received: July 2, 1982; In Final Form: February 9, 1983)
The phase equilibria of two-component systems of water and heptadecafluorononanoic acid and its salts were investigated between 20 and 100 O C , the main aim of the study being to create a basis for further investigations. The counterions studied were lithium, sodium, cesium, ammonium,tetramethylammonium, dimethylammonium, and diethylammonium. The rough phase diagrams which were established using polarizing microscopy and low-angle X-ray diffraction showed the existence of regions with liquid crystalline structures besides regions with micellar solutions. There are marked differences in phase equilibria in comparison with the corresponding hydrocarbon systems. These differences are discussed on the basis of current theoretical models of amphiphile systems and the properties of fluorocarbon chains, hydrophobicity, van der Waals radius, C-F bond distance, and trans-gauche equilibria. The acid and the dimethyl- and diethylammonium salts have a low aqueous solubility while they have very extensive regions with ("ideally" swelling) lamellar liquid crystalline structure. It has been suggested previously that "giant" micelles form in the aqueous solutions of these compounds but this seems doubtful in view of the present results. With reference to theoretical models it is proposed that these compounds are essentiallynonionized and the much reduced electrostatic repulsions, either due to hydrogen bonding between counterion and surfactant ion or due to proton transfer resulting from changed acidity constants, should favor a lamellar structure with respect to one composed of spherical micelles. The other salts are ionized and have a considerable aqueous solubility above a critical temperature. The liquid crystalline phases that occur in these latter systems include other structures than the normal lamellar one.
Introduction There is a considerable interest in the physicochemical properties of surface-active fluorocarbon derivatives, but the studies have so far mainly been confined to dilute solutions. These substances are extremely surface active, and the cmc values of the totally fluorinated surfactants are considerably lower than those of the corresponding hydrocarbon compounds (for instance, the cmc's of the potassium salts of octanoate are 4.0 X lo-' and 2.9 X M, respectively, and those of decanoate 1 X lo-' and 9 X lo4 M, respectively).' The foremost contributions to the knowledge of the association behavior of fluorocarbon surfactants are from the schools of Mukerjee2 and Shinoda.3 Muller et al.4 performed a series of 19FNMR chemical shift studies of totally and partially fluorinated surfactants and Ulmius and Lindman5 studied analogous systems by 19FNMR relaxation. In a recent paper, Hoffmann et al.6bclaim that they have found evidence for the existence of "giant micelles" in aqueous solutions of totally fluorinated nonanoate when the counterion is a partially alkylated ammonium ion or magnesium. Such micelles are difficult to reconcile with current theories of micellization7and, if real, the existence and nature of such giant micelles in dilute systems would be of great theoretical interest. However, a prerequisite to understand these phenomena is to have basic information on the phase equilibria involved and thus on the phase diagrams. It is to be noted that rather few studies of phase diagrams of systems containing a totally fluorinated surface-active compound and water are known. The published diagrams that the present authors are aware of are the pentadecafluorooctanoate systems C7F15COONH4 and C7F15COOLipresented by Tiddy et al.,8C7Fl,COOCs by Boden et al.,9 and C7F15COON(CH4)4 by Hedge et al.IO 'Division of Food Technology. *Division of Physical Chemistry 1.
In order to broaden this base and to elucidate the influence of the counterion, a study was undertaken of the phase equilibria of aqueous systems of heptadecafluorononanoic acid and some of its salts. The counterions studied were lithium, sodium, cesium, ammonium, tetramethylammonium, dimethylammonium, and diethylammonium. In this paper we will present rough phase diagrams showing the behavior of the aqueous systems between 20 and 100 "C and report some information obtained about the internal structures of the liquid crystalline phases encountered. An attempt is made to rationalize the observed phase diagrams in view of the theory of (1) Mukerjee, P.; Myaels, K. J. Natl. Stand. Ref. Data Ser. (U.S., Natl. Bur. Stand.) 1971,No. 36. (2)Mukerjee, P.; Mysels, K. J. ACS Symp. Ser. 1976, No. 9,231. Mukerjee, P.; Yang, A. Y. S. J . Phys. Chem. 1976,80,1380. (3)Nakayma, H.; Shinoda, K. Bull. Chem. SOC.Jpn. 1967,40,1797. Hato, M.; Shinoda, K. Nippon Kagaku Zasshi 1970,91,27.Shinoda, K.; Hato, M.; Hayaehi, T. J . Phys. Chem. 1972,76,909. Kuneida, H.;Shinoda, K. Ibid. 1976,80,2468. (4)Muller. N.: Birkhahn. R. H. J. Phvs. Chem. 1968. 72.583:1967.71. 957. 'Muller,'N.;'Platko,F.'E. Ibid. 197"1,75,547. Muller; N.;'Simsohn; M. Ibid. 1971,75, 942. (5)Ulmius, J.; Lindman, B. J . Phys. Chem. 1981,85,4131. (6)(a) Hoffmann, H.; Tagesson, B. Z . Phys. Chem. (Frankfurt am Main) 1978.110.113. (b) Hoffmann. H.: Ulbricht. W.: Taesson. B. Ibid. 1978,'113,17. (c) Bauernschmitt, D.;Hoffmann; H.'Mabomol. Chem. 1980,181,2384. (d) Hoffmann, H.; Platz, G.; Rehage, K.; Ulbricht, W. Ibid. 1981,182,541. (e) Hoffmann, H.; Platz, G.; Ulbricht, W. J . Phys. Chem. 1981,85,1418.(0 Schorr, W.; Hoffmann, H. Ibid. 1981,85,3160. (7)WennerstrBm, H.; Lindman, B. Phys. Rep. 1979,52,1. Lindman, B.; WennerstrBm, H. Top. Curr. Chem. 1980,87,1. (8)(a) Tiddy, G. J. T. J . Chem. Soc., Faraday Trans. 1 1972,68,608. (b) Tiddy, G.J. T. Symp. Faraday SOC.1971,5 150. (c) Tiddy, G. J. T. J . Chem. Soc., Faraday Trans. 1 1972,68,653. (d) Ibid. 1972,68, 670. (e) Tiddy, G.J. T.; Wheeler, B. A. J . Colloid Interface Sci. 1974,47,59. (0Everiss, E.; Tiddy, G . J. T.; Wheeler, B. A. J . Chem. SOC.,Faraday Trans. 1 1976, 72, 1747. (9) Tiddy, G.J. T. Ibid. 1977, 73, 1731. (h) Morris, P. G.; Mansfield, P.; Tiddy, G . J. T. Faraday Symp. Chem. SOC. 1979,13,38. (9)(a) Boden, N.; Jackson, P. H.; McMullen, K.; Holmes, M. C. Chem. Phys. Lett. 1979,65, 476. (b) Tiddy, G.J. T. Phys. Rep.. 1980,57,1. (IO) Hedge, P. M.; Thomas, R. K.; Mortimer, M.; White, J. W. J . Chem. SOC.,Faraday Trans. I 1980,76,236.
0022-3854/83/2087-320Q~Q~ .5Q/Q 0 1983 American
Chemlcal Society
3290
The Journal of Physical Chemistty, Vol. 87,No. 17, 1983
Fontell and Lindman
TABLE I : X-ray Diffraction a n d Other Characteristics of t h e Crystalline Fluorocarbon C o m p o u n d s X-ray spacings, A . low-angle region ( h k l ) compd C,F,,COOH C,F,,COOLi C,F,,COONa C,F,,COOCs C,F,,COONH, C,FI,COON(CH,), C8F17C00NH2(CH, C8F
17C00NH,(C2HS
12
12
mol wt
mp, "C
001
463.1 469.1 485.1 596.0 480.0 536.1 508.2 536.1
77 244-245 282 225 182 195 44-45 53-54
26.6 26.2 25.6 27.0
Jonsson and Wennerstrom." Our intention is to perform in the future more detailed studies of the conditions in the various systems by different experimental techniques.
Materials and Methods Heptadecafluorononanoic acid (perfluorinated nonanoic acid) was obtained from Riedel-de-Haen, West-Germany. The compound was mostly used as received, since recrystallization from water or ethanol affected the results very little. There was no indication in 19FNMR spectra of the presence of appreciable amounts of branched isomers. The lithium, sodium, and cesium salts were prepared by dissolving the acid in ethanol and neutralizing with the corresponding base in an aqueous solution. The alcohol was removed from the aqueous solution by gentle heating and the solution was thereafter freeze-dried. The ammonium salt was prepared by bubbling dry ammonia through an ether solution of the acid and the precipitate was dried under vacuum. The tetramethyl-, dimethyl-, and diethylammonium salts were obtained by dissolving the acid in ether and neutralization with the corresponding base, in either aqueous or ether solution. After evaporation of the ether, the salts were dried under vacuum or freezedried. All salts were crystalline. Their molecular weights were checked by titration with perchloric acid in nonaqueous environment using Oracetblue B as indicator. The melting points and some other characteristics are given in Table I. The X-ray diffraction powder patterns gave for the crystals of the acid and the alkali-metal salts long-range repeats of 26-27 A and a set of wide-angle spacings between 2 and 5 A. This suggests that the crystal structures are similar to those of hydrocarbon fatty acids and their salts which form crystals where the length of one axis is close to twice the length of the molecule.12 The structures are presumably similar for the ammonium, tetramethyl-, dimethyl-, and diethylammonium salts, but one has to assume that the chains in these crystals are tilted or that they are interdigitated as the long-range repeats had values of 11-17 A (Table I). Samples were prepared by weighing appropriate amounts of the compounds and water in ampules which were flame-sealed. After equilibration under shaking and occasional use of gentle heat the gross appearance was observed by the naked eye at different temperatures up to about 120 OC without and between crossed polaroids. The results were checked on a polarizing microscope equipped with a Koffler hot stage. Rough temperatureconcentration diagrams were constructed based upon these (11)(a) Jonsson, B.; WennerstrBm, H. J. Colloid Interface Sci. 1981, 80,482. (b) Jbnsaon, B.; WennerstrBm, H. Chem. Scr. 1980, 15, 40. (12)Fontell, K. in 'Thermodynamic and Transport Properties of Organic Salts"; Franzosini, P., Sanesi, M., E&.; Pergamon Press: Oxford, 1980; JUPAC Chem. Data Ser., No., 28,p 343.
002
003
13.1
13.1 12.7
8.9 8.4
11.3 18.5 16.2 17.0
7.9 8.0
wide-angle region 4.8. 4.5. 4.0 4.9; 4.5; 4.3 5.4, 5.1, 4.9 4.2, 3.6, 3.1, 2.7, 2.4, 2.1 5.25, 5.05, 4.6, 4.2, 3.6 4.6, 4.5, 4.05 6.0, 5.4, 4.6 6.5, 4.9, 4.4, 4.0
TABLE 11: Partial Specific Volumes of t h e Fluorocarbon Surfactants in Lamellar Liquid Crystalline Phases
compd
partial specific vol, cm3/g
teomp, C
C,Fl7COOH C,F,,COONH, C,F,,COONH2( CH, 1 2 C8F17C00NH2(C2H,5)2
0.516 0.504 0.580 0.605
20 25 20 20
observations. Note that the results refer to conditions in closed ampules. The accuracy in the determination of the boundaries varied from system to system and also on the part of the phase diagram studied. Most care was devoted to the systems of the nonanoic acid and its dimethyl- and diethylammonium salts, especially at high water contents. The temperature boundaries were accurate to *0.5 "C at room temperature and slightly above but the precision decreased with the increase in temperature. In order to confirm the preliminary structure assignments based upon the texture observations in the polarizing microscope samples from the different phase regions were studied between 20 and 70 "C by low-angle and wide-angle X-ray diffraction t e ~ h n i q u e s . ' ~ J ~ ~ Density determinations required for the evaluation of the structural parameters were made pycnometrically. The size of the pycnometer vials was 1.5-2 cm3. The measurements showed that the partial specific volume for water is close to unity while it varies for the acid and the ammonium and dimethyl- and diethylammonium compounds between 0.5 and 0.6 cm3/g (Table II).14
Results Description of the Phase Diagrams. The purpose of the present study was to locate the regions of stable isotropic solution phases and liquid crystalline phases and to report structural data for the liquid crystalline phases. Our aim has not been to determine the precise position of the phase boundaries or to study in detail the intermediate two- and three-phase regions. Neither have we been interested in the phases and structures occurring below the chain melting temperature. The phase diagrams are thus sketchy and incomplete. Between the isotropic aqueous solution regions and the molten or solid crystalline state of nonaqueous fluorocarbon surfactant, various birefringent liquid crystalline phase regions are encountered in the temperature-con(13)Fontell, K.in "Liquid Crystals and Plastic Crystals"; Gray, G. W., Winsor, P. A., Eds.; Horwood: Chichester, 1974;Vol. 2,p 80. (14)(a) Fontell, K.Proceedings of the 7th Scandinavian Symposium on Surface Chemistry, Sept 1981,Holte, Denmark"; Birdi, K. S., Ed.; Technical University: Lyngby, Denmark, 1982;p 205. (b) Fontell, K.In 'Surfactants in Solution"; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, in press.
Fluorocarbon Surfactants
W
u L W
+ a
\
I !
a E
2
Flgure 1. Approximate phase diagram for the system of the diethylamine salt of heptadecafluorononanoic add and water. Notations: L (L,, L2), isotropic solution; LC, liquid crystalline phase, subscript indicates the internal structure; LC L, etc., two-phase regions; T,, concentrationdependent melting point curve of the (fiuoro)carbon chains.
+
I
Figure 4. Approximate phase diagram for the system of the ammonium salt of heptadecafiuorononanoic acid and water. Notations as in Figure 1.
2c LC Lam
,'
,' '
I
' I
/
'
I
'
,
u
50 Ce F,,C 00 NH2 (C H,
'0
W-
a 12
, '10w
2
+
Flgure 2. Approximate phase diagram for the system of the dimethylamine salt of heptadecafluorononanoicacid and water. Notations as in Figure l .
10
4
L
F
,
, I ' '
,
, , I .
,
,
, ,
'
'
LC
I
I
, I
0
E
#
1
Figure 5. Approximate phase diagram for the system of the tetramethylammonium salt of heptadecafiuorononanoic acid and water. Notations as in Figure 1. The niveau of the T , curve is uncertain; see discussion in text.
m
L W
n.
-
I
50 C 8 F, 7 C 00N iC H, I L, o/' w
+ 3
2
I
/
I
I
LC Lam
W L
/
I
I
b
U
'
e
-
-
~
I
1
50 C o F,,COOH ,'IO w Flgure 3. Approximate phase diagram for the system of heptadecafiuorononanoic acid and water. Notations as in Figure 1.
centration diagrams (Figures 1-8). Samples from liquid crystalline regions gave several sharp "crystalline" reflections in the low-angle region of the X-ray diffractograms. In addition, there was a broad diffuse reflection corresponding to a Bragg spacing of 4.8-5 A in the wide-angle region. The sharp reflections in the low-angle region and the broadness and diffuseness of the reflection in the wide-angle region are indication of the liquid crystalline nature of the ~amp1e.l~
The general appearance of the diagrams changes with changes in the melting temperature of the particular fluorocarbon surfactant and the concentration dependence of the T,curve in the system.16J7 (15) Liquid crystallinesamples of an amphiphiliccompound and water give besides sharp low-angle reflections a broad diffuse wide-angle reflection corresponding to a Bragg value of 4.5 A. This latter reflection is similar in nature and position to that obtained for liquid paraffins and is considered to represent the average distance between randomly oriented liquid hydrocarbon chains. The Occurrence of such a reflection in aqueous liquid crystalline systems is taken as an indication that the hydrocarbon regions of the sample are in liquid state,13however, with the difference that the polar head groups are anchored at the interfaces existing between the olar and nonpolar regions. The increase of the Bragg value to 4.8-5 l i s a natural consequence of the bulkiness of the fluorocarbon chains.
3202
Fonteil and Lindman
The Journal of Physical Chemistry, Vol. 87, No. 17, 1983
u U
a.
L
L W 2
c 3
+
L m
m
W
a
W
5
a E
b-
b-W
L
C, F,, C 00C s , O/o w
Flgure 8. Approximate phase diagram for the system of the cesium salt of heptadecafluorononanolc acid and water. Notations: MOP, magnetically orientabie phase; others as in Figure 1.
In a presentation of the individual phase diagrams it is perhaps most illustrative to start with those showing the existence of an “ideal” liquid crystalline lamellar structure.18J9 The diethylammonium salt has the phase diagram given in Figure l. The solubility in water as isotropic solution is low at room temperature,
