Langmuir 1994,10, 2131-2138
2131
Monodisperse Perfluoroalkyl Oxyethylene Nonionic Surfactants with Methoxy Capping: Synthesis and Phase Behavior of Water/Surfactant Binary Systems Samuel Achilefu,? Claude Selve,* Marie-JosB StBbB, Jean-Claude Ravey, and Jean-Jacques Delpuech Universite de Nancy I - Campus Victor Grignard, Laboratoire &Etude des Solutions Organiques et Colloidales, LESOC - U A CNRS 406, B.P. 239, F-54506 Vandoeuvre-lbs Nancy Cedex, France Received October 14, 1993. In Final Form: April 12, 1994@ New monodisperse surfactantmolecules with a poly(oxyethy1ene)methoxy-capped chain for the hydrophilic head and a perfluoroalkyl moiety for the hydrophobic tail are synthesized. The principle of solid-liquid phase transfer reactions is used. Surface tension measurements of aqueous solutions ( y 20 mN m-l) show slow organization of the surfactant film at the waterlair interface for the fluorocarbon tail. Values of the criticalconcentrationand comparisons with the hydroxy-capped analogous surfactants are consistent with a small influence of the hydrophobic effect of the methoxy terminal group on the polar head. The results of phase behavior in aqueous binary systems are discussed. As expected, the replacement of OH group by an OMe group effectively reduces the hydrophilicity of the surfactant but does not change dramatically its phase behavior in water.
-
Introduction The synthesis and the physicochemical properties of perfluorinated nonionic surfactants are intensively investigated for biologicalu ~ e s . l -In ~ this area, compounds of the type F(CFZ),CHZO(EO),Hare currently studied in our l a b o r a t ~ r y . ~These -~ studies have shown that the binary systems (water/surfactant)and the ternary systems (waterlsurfactanvfluorinated oil) obtained with fluorinated surfactants are comparable to those of their hydrogenated homologues.6 In this paper, we have replaced the terminal hydroxyl group in the above compounds by a methoxy group in order to endow the hydrophilic moiety of these molecules with a chemical inertness comparable to that of its fluorinated end: with a view to practical applications it can be reasonably expected that the replacement of the terminal hydroxyl group by a methoxy group would effectivelyenhance their chemical stability. Indeed such methoxy-capped surfactants could then be used in the preparation of organized systems that would serve as reaction media. Consequently, a series of compounds with the structure F(CFZ),CHZO(EO)~CH~ were prepared' which present large chemical inertness, especially with respect to oxidizing agents.a On the other hand, it has been reported t Present address: University of Oxford, Inorganic Chemistry Laboratory, South Parks Road, Oxford OX1 3QR, England. * To whom correspondence should be addressed. Abstract published in Advance ACS Abstracts, June 15, 1994. (1) Riess, J. G.; Le Blanc, M. Pure Appl. Chem. 1982,54112, 2383. @
Angew Chem. Int. Ed. 1978,17,621. J . Chim. Phys. 1987,8419,1119.
Vth International Symposium on Blood Substitutes, San Diego, CA, March 1993. (2) Cambon, A.; Szonyi,F.; Matos, L.; Delpuech, J.-J.; Serratrice,G. Bull. SOC.Chim. Fr. 1986,6,965. Cambon, A.; Szonyi, F. J . Fluorine Chem. 1987, 36, 195. Ibid. 1990, 47, 235 and refs cited therein. (3) Afzal, J.; Fung, B. M.; O'Rear, E. A. J . Fluorine Chem. 1987,34, 385. (4) (a) Mathis, G.; Leempoel, P.; Ravey, J.-C.; Selve, C.; Delpuech, J.-J. J . Am. Chem. SOC.1984, 106, 6162. (b) Selve, C.; Delestre, C.; Achilefu, S.; Maugras, M.; Attioui, F. J . Chem. SOC.Chem. Commun. 1991, 13, 863 and refs cited therein. ( 5 ) Ravey J.-C.; Gherbi, A.; StBbB, M.-J. Progr. Colloid Polym. Sci. 1988, 76, 234. (6) Ravey, J.-C.; StBb6, M.-J. Progr. Colloid Polym. Sci. 1987, 73, 127. (7) Selve, C.; Achilefu, S. J . Chem. SOC.Chem. Commun. 1990, 13, 911. (8) Selve, C.; Achilefu, S. Info. Chimie. 1991, 326, 156.
Scheme 1. Synthesis of Methoxy-Capped Surfactants (see experimental section) HO(EO),H
Ba(EO)"H
BzO(EO),Me A "' HO(EO),Me YC&,CH20H
+ TsO(EO), Me
BzlO(EO),Me
A TsO(EO),Me YC#2,CH20(EO)nMe
with: Bzl = CgHg-CH2 : Y = F or H ;Ts = CH3-(C$i4)S(o)2 :EO = C2Hq-0 a (i)Bzl-C1, aqueous NaOH; (ii)Me2S04, aqueous NaOH; (iii) HDd; (iv) TsC1, pyridine; (v) powdered KOWp-dioxane.
that for the nonionic surfactants C,EO,, the presence of a terminal hydroxyl group is not necessary for obtaining good surface proper tie^.^ For example Tiddy et al.1° have recently discussed the effect of replacing a hydroxyl group by a methoxy group in hydrogenated systems: they indicated that at constant hydrophobic chain length, two to three supplementary EO units are required to obtain surface properties close to those of corresponding surfactants with a terminal hydroxyl group. This paper presents the preparation of methoxy-capped monodisperse fluorinated surfactants, the study of their surface properties in aqueous solutions, and the different types of organized binary systems they produce.
Synthesis A simplified scheme for the synthesis of the compounds, prepared by a "modular strategy method",'l has been shown in a communication' (Scheme 1). The procedure used for the preparation of the hydrophilic moiety, HO(C2H40lnMe,depends on the value ofn. Compounds with n = 2 and 3 are commercially available while those with n = 4-6 were prepared according to the pathways presented in Scheme 2. Different moieties of hydrophilic (9) Miesiac, I.; Szymanowski, J. Progr. Colloid Polym. Sci. 1988,76, 96. (10)Conroy, J.-P.; Hall, C.; Leng, C. A.; Fkndall, K; Tiddy, G. J. T.; Walsh, J.; Lindblom, G. Progr. Colloid Polym. Sci. 1990, 76, 96. (11) Rico, I.; Hajjaji-Srhiri,N.; Escoula, B.; Decastro Dantas, T. N.; Lattes, A.New.J . Chem. 1986,10/1,25andRico, I.; Duplaa, H.; HajjajiSrhiri,N.; Decastro Dantas, T. N.; Cecutti, C.; Lattes, A. Jorn. Comm. ESP.Deterg. 1986, 17, 535.
0743-746319412410-2131$04.50/00 1994 American Chemical Society
Achilefu et al.
2132 Langmuir, Vol. 10, No. 7, 1994 Scheme 2. Reaction Pathways for the Preparation of Monomethyl Ethers of Poly(oxyethy1ene)Glycols H(OC&)4OH
1
; : :5
1
c
NaOH
McOH
H(OC2H&,0Me
+ H(wH&OMe
MeOH
H(OC&&OMe
Scheme 3. Proposed Mechanism of the Condensation by Phase Transfer Reaction dioxsnc
['OH]
solide
Table 1. Methoxy-Capped Perfluoroalkyl Poly(oxyethy1ene)Surfactante YC,,.FdCHs),O(C2%O),Me codea Y m n yield(%)b I]" F 6 3 85 1.359 613Me 614Me F 6 4 83 1.373 615Me F 6 5 84 1.386 80 1.388 616Me F 6 6 F 7 2 82 1.343 712Me 83 1.357 713Me F 7 3 80 1.377 714Me F 7 4 F 7 5 78 1.379 715Me H 6 3 84 1.372 H613Me 78 1.384 H 6 4 H614Me H 8 4 77 1.362 H814Me 80 1.394 H815Me H 8 5 H 10 3 83 1.362 H1013Me
Rf 0.68 0.53 0.46 0.42 0.86 0.70 0.57 0.51 0.78 0.67 0.71 0.57 0.71
a Code: YmpnR with Y = fluorine or hydrogen, m = number of perfluorinated carbons in lipophilic moiety, p = number of hydrogenated carbons, n = number of oxyethylene units in polar head, and R = methyl or hydrogen capping the terminal oxygen of head. bYields calculated as a function of the equivalents of fluoroalkyl alcohol used.
+ [R(OCJQ~--OT~I soblion
properties. As it is well known, fluorinated surfactants reduce the surface tension of water from 72" l m to about 20 mN/m, a fairly low value. Measurement of the surface tension ( y ) of the aqueous solutions of these new compounds was, therefore, carried out. Moreover, since they could also be used to promote water-perfluorocarbon cosolubilization microe emulsion^)^ or to stabilize waterin-oil or oil-in-water emulsions, estimation of their hydrophobic-lipophilic balance is an important parameter in determining the possible applications of the new molecules. The phase behavior of the binary systems of these fluorinated surfactants in water was studied, and the binary phase diagrams (temperature versus concentration) of the surfactant in aqueous solutions have been compared with those of analogous perhydrogenated compounds. Hydrophilic-Lipophilic (fluoro) Balance
monomethyl oxyethylene ether with a well-defined number of n were prepared in this way. They were activated in the form of tosylates. The hydrophobic part, RFCH2OH, with RF = C6F13, C7F16, HCeF12, and HC8F16, are commercial products. Bridging of the hydrophilic and hydrophobic parts was effected by solid-liquid phase transfer reaction.12 The former transfer reaction requires a phase transfer agent but the reaction condensation is carried out in the absence of a exterior catalyst the tosyloxyethylene monomethyl ether plays the role of the phase transfer agent considerably favoring the substitution. The mechanism of the final reaction steps is shown in Scheme 3: The proposed intermediate complex is analogous to the one Ouchi et aZ.13 proposed in the synthesis of crown ether (template effect). The results of the condensation reaction are presented in Table 1. In this synthesis, the experimental setup is very simple. Better yields (up to 80%)are obtained compared with the relatively low yields (about 40%) that result from previous method^.^ Surfactant Properties
All these new compounds (for the codes, see Table 1) were prepared in view of their potential use as surfactants, hence the importance of examining some of their surface (12)Achilefu. S.:Selve. C. Svnth. Commun. 1990.20/6.799. (13)Ouchi,M.;Ynnoue,'Y.; Gnzaki, T.; Hakushi, T. J . Org. Chem. 1984,49,1408.
(HLB).
HLB values may be estimated by empirical calculations14 or determined by a variety of techniques, which depend more or less on some fundamental property of the molecule. Of all these methods, we adopted that of Griffin14which is defined for nonionic surfactants as: HLB = [H/(H L ) ] x 20 where H = molar mass of the hydrophilic head group; L = molar mass of the lipophilic chain. In this study, we used final products having essentially the structure shown below. Although we may anticipate that the terminal methyl group is less hydrophilic than the OH group, we shall consider it as a full part of the hydrophilic moiety, as far as the HLB number has to be evaluated. Thus for the compound 614Me [C6F13CH20(C2H40)4CH31rthe HLB can be calculated as follows: hydrophilic moiety ( H ) = O(C2&0)4CH3 = 207.25;lipophilic moiety (L)= C6F13CH2 = 333.07;molecular mass (H L ) = 540.32;hence HLB = (207.3h40.3)x 20 = 7.7. As calculated according to the Griffin equation,14the HLB is a number which can be used only in comparing various compounds within given series. For example, for hydrogenated compounds and ~ )20 "C, the for a PIT (phase inversion t e m p e r a t ~ r e lof HLB required to get stable emulsions with octane is about 10.6.15 For fluorinated nonionics, experiments show that the HLB required to stabilize emulsions with F-octane at ambient temperature is about 6-7,16 a number which is
+
+
~~
~
(14)Griffin, W.C.J . Soc. Cosmet. Chemist 1964,5,249and J.SOC. Cosmet. Chemist 1949.1. 311. (15)Buzier, M.; Ravey, J.4. J . Colloid Interface Sei. 1983,91,20. Ibid. 1986,103,594. (16)Ravey, J.-C.; Stkb6, M.-J. Colloids Su$. A 1994, 84, 11.
Langmuir, Vol. 10, No. 7, 1994 2133
Perfluoroalkyl Poly(oxyethy1ene)Nonionic Surfactants
13 4.5
-45
-5
4
-33
I
log c I F613Me
+
F614Me X F6lSMe 0 F616Me
Figure 1. Surface tension ( y ) versus log concentration (log C) curves for aqueous solutions of 613Me, 614Me, 615Me, and 616Me at 25 "C.
I
+
F714Me
*
F715Me
I
Figure 2. Surface tension ( y ) versus log concentration (log C) curves for aqueous solutions of 714Me and 715Me at 25 "C.
noticeably lower that the preceding one. Hence, to each surfactant ofthe present series (fluorinated tail, terminal methyl group), a HLB number can be ascribed in the same way as it is calculated for hydrogenated ethoxylated alcohol; but, for the moment, they cannot be mutually compared on a quantitative basis. Critical Concentrations. A critical concentration (CC) could be measured for each aqueous solution as the location of a well-marked break in the y us log C curves (Figures 1 and 2);for most of the hydrophilic surfactants at ambient temperature (25"C), these critical concentrations are true critical micellar concentrations (CMC),since for C > CC we effectively get the classical L1 isotropic
phase, which is the aqueous micellar phase.17 However, the aqueous solutions of the new surfactants presently investigated exhibit a liquid crystal or LZor LBbehavior (see binary phase diagrams): For this reason the term CC is preferred a t CMC since it represents the limits of solubility in water of the surfactant as a monomer. The surface tension decreases very rapidly until it reaches the critical concentration (CC) and then it remains constant. The data obtained from the plots y us log C are summarized in Table 2. (17) Matos, L.; Ravey, J.4.; Serratrice, G. J. Colloid Interface Sci. 1988,128, 341.
2134 Langmuir, Vol. 10,No. 7, 1994
Achilefi et al.
Table 2. Critical Concentrationof Some Surfactants Obtained by the Surface Tension Method at 26 "C code" 613Me 614Me 615Me 616Me 713Me 714Me 715Me H613Me H1013Me 613H 614H 615H 616H 714H CizE04Hd
molecular mass 496.27 540.32 584.37 628.43 546.28 590.33 634.38 478.28 678.31 482.24 526.29 570.35 614.40 576.30 362.55
104 cc
HLBG~ (mol dm-3) 6.58 1.03 7.67 1.18 8.60 1.53 9.40 2.35 5.97 7.02 0.25 0.34 7.92 6.82 3.40 4.81 6.19 0.93' 7.34 1.72c 8.32 1.97c 9.16 2.41' 6.71 O.4lc 10.66 0.64d
Ycc
(mN m-1)
18.2 18.5 19.5 21.5 17.0 18.3 18.0 36.0 29.0 17.0 17.5 18.5 19.5 16.5 28.6d
a See Table 1. Calculated with the Griffin equatiod4(see text). See refs 5 and 16. EO = CzH40; see ref 18.
Data for the corresponding surfactants with a terminal hydroxy group are also given in Table 2, taken from previous p a p e r ~ . ~AJcomparison ~ of the two sets of data show that amphiphiles with a terminal methoxy group are generally only slightly more hydrophobic, at least as far as critical concentrations are concerned, suggesting that the effectiveenergy oftransfer of OMe and OH groups from micelle to water are practically the same. On the other hand, the addition of one CF2 group to hydrophilic chain makes CC divided by a factor of about 5 (as it is well known, a factor of about 3 would correspond to the CH2 group). This examplifies the larger hydrophobicity of CF2 compared to CHz. However this factor is not as large as the one proposed in our preceding w o r k ~ . ' ~Probably, J~ this has to be ascribed to the limited precision and the small number of data (Le. 2) available in the present work. Surface Activity. Whereas most hydrogenated amphiphiles lower the surface tension of water to about 3040 mN/m,20the decrease is generally lower than 20 mN/m for fluorinated surfactant^.'^^^^ The change of y with the logarithm of the bulk concentration (C,mol dm-3) was measured a t fixed temperatures. This is examplified by the curves in Figure 1 for compounds 613Me, 614Me, 615Me, and 616Me and in Figure 2 for 714Me and 715Me. The lowering of the surface tension of pure water (ca. 72 mN/m at 25 "C) by addition of small amounts of the amphiphiles shows that they are surface-active agents. In most cases, it was measured till the critical concentration was reached. The CC indicates the concentration at which the surface tension has its lowest value and does not vary anymore. Characteristic values obtained for our compounds are between 17 and 20.5 d / m , which is within the range obtained with other perfluorinated nonionic surfactant^.'^^^^ Of particular note are the minimum surface tensions of the terminal hydrogen-substituted fluorocarbon compounds (H613Me and H1013Me) that are approximately the same as those of hydrocarbon surfactants. This emphasizes very strongly the exceptionally low surface energy of the CF3 terminal group and is consistent with the Zisman critical surface tension (18)Rosen, M. J.;Cohen, A. W.; Dahanayak, M.; Hua, X. J.Phrs. Chem. 1982,86,541. (19)Selve, C.; Ravey, J.-C.; Wbe, M.-J.; El Moujahid, C.; Moumni, E. M.; Delpuech, J.-J. Tetrahedron 1991,47/3,411-428. (20)Craine, L.;Greenblatt, J.; Woodson, S.; Hortelano, E.; Raban, M. J.Am. Chem. SOC.1983,105,7252. (21)(a) Shinoda, K.;Hato, M.; Hayashi, T. J.Phys. Chem. 1972,76, 909. (b) Meguro, K.;Ueno, M.; Esumi, K. In Non Ionic Surfactants: Physical Chemistrv: Schick.M. J.. Ed.: Surfactants Sciences Series. M. Dekker Inc.: New York; 1987;Vol. 23,p 114.
Table 3. Surface Properties of Some Surfactant Solutions Prepared (t = 25 OC) compound A,, (10-2 nm2) (or Az) nCc(mN m-l) 613Me 37 54 613H 35 55 614Me 54 42 614H 40 55 46 615Me 53 615H 54 43
values of terminal CF3, CFzH, CF2, CH3, and CHz, which ~~~~~ are 6, 15, 18, 22, and 31 mN/m, r e s p e c t i ~ e l y .This implies that for a liquid to wet a surface consisting of close-packed hydrocarbon groups with a terminal CH3, the liquid must have a surface tension of less than 22 mN/m. On the other hand, to wet a surface composed of close-packed fluorocarbon groups with terminal CF3 groups, the liquid must have critical surface tension values of less than 6 mN/m. If the compound is such that CFzH terminal group is on the surface, the critical value of 15 mN/m, which is 2.5 times as large as that for the CF3 surface, should be overcome in order to wet the surface. The unique character of the CF3 surface is, therefore, clearly shown, and 6 mN/m is the most nonwettable surface ever reported. We do not think that the decrease in the lowering of the surface tension of water by these substituted fluorocarbon surfactants is due to a decrease in the size of the terminal trifluoromethyl group (i.e. from FCFz to HCF2) because a similar result is observed when one of the trifluoromethyl fluorine atoms is replaced by chlorine,24which is a larger atom than fluorine. Semilogarithmic plots were used to obtain other adsorption parameters. The slope of the linear portion of each curve below CC was determined by the method of least mean squares (Figures 1and 2). From these slopes, and for concentrations below the CC, the maximum surface excess concentrations, rm (mol m-2), and the minimum can be calculated from the areas per molecule, Amin Gibbs equation:25
(x2),
r = -l-.L] 2.303R 6 log C TP
(1)
where (67 / 6 log C h p is the slope of the y versus log C plot (just before the CC) at constant (absolute) temperature, T (K), and pressure; R = 8.314 J mol-l K-l, and NA is the Avogadro's number. The values of A ~ and n surface pressure nminare listed in Table 3. Since the values of these parameters generally correspond to those around the CC, we shall denote them as A,, and n,,, respectively. The surface pressure is calculated from eq53: n,,= yo-ycc; where yo is the surface tension of the water (solvent), and y,, the surface tension of the aqueous surfactant solution at CC. Table 3 shows an increase in the minimum surface area per molecule (&) at the aqueous solution-air interface with increase in the number of oxyethylene units in the molecule. This is in agreement with the relationship observed for other polyoxyethylenated nonionic surfac(22)Fox, H.W.; Hare, E. F.; Zisman, W. A. J. Colloid Sci. 1963,8, 194. (23)Bryce, H.G. InFZuorine Chemistry;Simons,J. H., Ed.; Academic Press Inc.: New York, 1964;Vol. 5,p 308. (24) Bryce,H. G. InFluorine Chemistry;Simons,J. H., Ed.; Academic Press Inc.: New York, 1964;Vol. 5,p 295. (25)Moroi, Y.;Pramauro, E.; Gratzel, M.; Pelizetti, E.; Tundo, P. J. Colloid Interface Sci. 1979,69, 341 and refs cited therein.
Langmuir, Vol. 10, No. 7, 1994 2135
Perfluoroalkyl Poly(oxyethy1ene) Nonionic Surfactants Table 4. Melting Points (F)of L and Temperature Ranges ( T h- T-) for the ]Ls Phase compound F ["CI Tmin - T , ["CI 45 - 75 614H 75 615H 95 60 - 95 20 - 32 615Me 32 714H 85 50 - 85 714Me 25 15 - 25 42 715Me 35 - 42 68 51 - 68 CizE04H 27 24 - 27 ClzE04Me
tants18,26that A,,n-" is a constant. Values ranging from 21.7-24.8 A2 at 25 "C have been reported for the hydrogenated compounds CmH2,0(E0,)H with m = 12 and n = 2-8. This relation can be expressed as: A, = A,,nm;where A, is a constant. A better empirical equation has been proposed by Ravey et aL4 for perfluorinated polyoxyethylene nonionic surfactants such as C6F13CH20(E0,)H ( n = 2-7). A form of this relation is shown in eq 4: A,, = A,(n 1Y2;and they found A, = 15.5 k at 30 "C. Using eq 4, for a methoxy-capped surfactant, we obtainedd, = 18.8 k2.The& values for these fluorinated surfactants are lower than corresponding hydrogenated compounds, for whichA, = 23 k. It should be noted that the A,, of the methoxy-capped surfactants are, in general, slightly larger than those of their hydroxy analogues. This indicates the role of the methoxy group in the organization of the hydrated poly(oxyethy1ene)chain at the aidwater interface. The film pressure above CC (iz,) is another important adsorption parameter. Generally, fluorinated amphipbiles lower the surface tension of water to a larger extent than do their hydrogenated analogues. The effectiveness of this reduction decreases with increase in the number of oxyethylene units (Table 4). Longer chain perfluoroalkylated compounds at 25 "C in aqueous solutions can produce surface pressures as high as 55 mN m-l. For example, at 25 "C, the iz,, is 53.50 mN m-l for 614Me and 54.50 mN m-l for 614H. Rosen et aZ.16reported a value of 43.3 mN m-l for C12H250(E0)Jl at the same temperature. The film pressure for methoxy-capped or hydroxy terminal 1 mN m-l. But compounds is very similar: An, systematically n,,values for OMe surfactants are the lowest ones, so this has to be related to reduced hydrophilicity of this terminal group. So far, we have demonstrated that the replacement of the terminal hydroxyl group on the hydrophilic moiety of surfactants by a methoxyl group does not really perturb the dynamic structure of the former. Similar values of CC, A,,, nee, etc. were obtained for both series. However, the substitution of fluorine by hydrogen on the terminal carbon of perfluoroalkyl chain has a drastic effect on almost all the parameters determined. For example, they have higher CC and lower n,,values. The ycc obtained are similar to those of hydrogenated compounds, which suggests that their structure in aqueous solutions differs considerably from the completely perfluorinated hydrophobic homologues.
+
Phase Diagrams of Oxyethylene Surfactants The binary phase diagrams of 615Me, 714Me, and 715Me are shown in Figure 3, parts A, C, and E, respectively, for temperature below 55 "C. Most interesting are their comparisonswith the diagrams of the parent alcohols; two of them are reported in Figure 3 for 615H6 respectively. (B) and 714H6 (D), (26) Hsiao, L.;Dunning,H. N.; 60, 657.
Lorenz, P. B. J. Phys. Chem. 1958,
I
W
40
I
80
E
Figure 3. Binary phase diagram of wated615Me (A). Binary phase diagram of water/615H6 (B). Binary phase diagram of wated714Me (C).Binary phase diagram of water/714H6 (D). Binary phase diagram of water/715Me (E).
The phase diagrams of all the new compounds are very similar. They are characteristic of hydrophobic surfactants which are described by low HLB values (Table 2). At higher surfactant concentrations, an isotropic phase, noted L2 (the so called "reverse" micellar phase), is observed. On diluting at lower temperatures, a crystalline phase always appears. The examination of this phase in an optical microscope with crossed polarizers showed that, in all cases, it has a texture of lamellar liquid crystals. This phase is normally noted as 4. However, the melting point of La varies from one system to the other (Table 4). It is 32 "C for 615Me, 25 "C for 714Me, and 42 "C for 715Me. These results show that the addition of a fluoromethylene group (CF2) to the hydrophobic chain increases the melting temperature of the liquid crystals by about 10 "C. Similarly, increasing the number of oxyethylene units results in the augmentation of this temperature by 15 "C for each unit. This behavior is quite classical for moderately hydrophobic nonionics, whether they are fluorinated or not,1° methoxy-capped or not: this La phase is stabilized as well by thickening ofthe hydrophobic part as by the lengthening of the hydrophilic chain, although the physical origin may be different. The domain of existence of Lais very large in all cases, stretching from about 20-90% composition of 714Me and 715Me in water. As the hydrophobic chain length increases, the extent of the liquid crystal zone increases. When the liquid crystal phase disappears, the "reverse" micellar phase, La, extends into the zone composed of avery high percentage of water. This isotropic region is called L3. It is very sensitive to temperature. The temperature of emergence of L3 in the dilute region varies with the nature ofthe system: it is 20 "C for 615Me,
2136 Langmuir, Vol. 10, No. 7, 1994 15 "C for 714Me, and 35 "C for 715Me. In general, the variation of this temperature from one system to another follows the same trend as that observed for the maxima melting points of the lamellar liquid crystals. Comparison of the methoxy-capped surfactants with their hydroxyl analogues, RFCH20(EO),H, for example 615H (Figure 3B) and 714H (Figure 3D) shows that their phase behaviors are not very different6 a t low temperature: they have similar kinds and number ofphases. However, the melting temperatures of the hydroxylated compounds with an identical hydrophobic chain, as well as the L3 emergence temperatures, are noticeably higher than those for the corresponding methoxylated compounds: a difference of about 40 "C is observed. Ravey et ~ 1 . have ~ 3 ~ shown that a perfluoromethylene (CF2)group is equivalent to 1.7 methylene (CH3 groups, as far as surface properties are concerned. This rule may not be applied with high precision in the study of phase behavior of these compounds. Nevertheless, a closer look at the data shows that the fluorinated hydrophobic chain C6F13CH2is comparable to the hydrogenated chain C12H2.5, whether the terminal group of the ethylene glycol chain is OH or OMe. For the latter ones, investigated presently, this can be seen by comparing figures to data of Tiddy et aZ.10227Here, a general tendency is observed for all the compounds under comparison: (1)Increase in the number of EO in the hydrophilic chain shifts the melting point of the lamellar phase La and the region of existence of L3 by about +20 "C per EO unit. (2)Addition of the hydrophobic group, CF2, leads to an increase of these temperatures by about 10 "C.
Conclusion The effect of CH3 capping of the terminal OH in nonionic fluorinated surfactants appears quite small on the phase diagrams when the temperature is low. It is also negligible as far as the monomer solubility (CC) and the surface activity properties are concerned. This means that hydrogen bonding of water molecules to the hydrophilic end group is probably not essential. However when the temperature is raised, none of the isotropic or anisotropic phases can exist any more (except a t very low water content). In other words, the 0-methylated surfactants cannot be dispersed in water a t higher temperature, probably due to the lack of sufficiently strong hydrogen bonds. However, the sequence of the phases does not change, while only their temperature range is reduced. More specifically, it means that no bilayer structures can be formed in a stable way above about 30-40 "C. Indeed, let us recall that in the L3 phases, the local structures are surfactant bilayers, but, unlike in La phases, they are highly dispersed and disordered. Experimental Section Synthesis. Monobenzylationof Tetrakis(oxyethy1ene) Glycol.12 To a three-necked round-bottomed flask fitted with a condenser were added 97 g (0.5 mol; 4 equiv) of tetrakis(oxyethylene) glycol, 16 g (0.13 mol; 1 equiv) of benzyl chloride, and 500 mL of aqueous sodium hydroxide (50% w/w). This mixture was refluxed for 24 h with vigorous stirring. The resulting mixture was cooled and diluted with water (400 mL). Extraction was carried out with ether (4 x 200 mL) and the organic phase was dried over anhydrous sodium sulfate. The solvent was evaporated and the crude product distilled under vacuum. The ethylene glycol monobenzyl ether obtained was used in its crude form since trace amounts of the dibenzylated product that are also present do not affect subsequent reactions. (27) Mitchel, D. J.;Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P.J. Chem. SOC.Faraday Trans. 1 1983,79, 975.
Achilefi et al. However, for analytical purposes, the final product was purified by flash chromatography (Si021and eluted with AcOEt. Another simple method of purification used was to extract the final reaction mixture with petroleum ether (40-60 "C) in order to remove the dibenzylated products. The aqueous phase was extracted several times with diethyl ether, which was then washed with distilled water to remove traces of unreacted oxyethylene glycols. C&CH2(0C2H4)40H: MM = 284.35, obtained 33.3 g(90%); 7;' = 1.502;Rf(Ac0Et)= 0.48; bp 180 "C; IR (film) YOH = 3600-3300 cm-l; YCOC = 1150-1100 cm-l; 'H NMR (CDClJl'MS) 6 = 7.35 (5H, m, Ph), 4.56 (ZH, s, CHz), 3.60 (16H, m, O C Z H ~ )2.80 ~ ) , ( l H , s, OH). Methylation of Ethylene Glycol Monobenzyl Ether. l-Benzyl-16-methyl-l,4,7,10,13,16-hexaoxahexadecane [CsHsCH2(0C2H4)sOCH3]: 57 g (0.2 mol; 1 equiv) of tetrakis(oxyethylene) glycol monobenzylether in 80 mL of tetrahydrofuran was added to 28.4 g(0.3mol; 1.5 equiv) of 2-chloroethylmethyl ether. While vigorously stirring the mixture, 45 g (0.8mol; 4 equiv) of finely ground potassium hydroxide was added. The reaction mixture was then refluxed for 48 h. After reaction, the crude mixture was poured into water (300 mL) and extracted with ether (3 x 150 mL). The ether phase was dried over magnesium sulfate. Rotary evaporation of the solvent gave the desired product which was used without further purification. C&&H2(OC2H4).@CH3: MM = 342.43; obtained 63.1 g (92%); 7;' = 1.490; Rf (AcOEt) = 0.45. 1-Benzyl-13-methyl-1,4,7,10,13-pentaoxatridecane [C&&CH2(OC2H&OCH31: This product was obtainedby the same procedure after replacing 2-chloroethylmethyl ether by dimethyl sulfate. C6H&H2(OC&)40CH3: MM = 298.43; obtained 57.3 g (96%); 7: = 1.491; Rf (AcOEt) = 0.65. l-Benzyl-19-methyl-l,4,7,10,13,16,19-heptaox~onadecane [ C ~ H ~ C H Z ( ~ C ~ H ~This ) ~ ~compound C H ~ ] : was obtained by phase transfer reaction between the tosylate of tetraethylene glycol benzyl ether and diethylene glycol monomethyl ether in 50% aqueous sodium hydroxide. Tosylationof Tetrakis(oxyethy1ene)Glycol Monobenzyl Ether.12 To a three-necked flask fitted with a condenser, a thermometer, a n addition funnel, and containing 60 mL of pyridine was added 48 g (0.17 mol; 1 equiv) of tetrakis(oxyethylene)glycol monobenzyl ether. After cooling to 0 "C, 50 g (0.25 mol; 1.5 equiv) of tosyl chloride in 60 mL was added dropwise with constant stirring. The reaction mixture was left to stand a t room temperature for 4 h. The resulting solution was diluted with cold water and extracted with ether. In order to remove the pyridine present in the medium, the ether phase was acidifiedwith aqueous HCl(2 N). The organicphase was collected and neutralized with saturated aqueous sodium carbonate solution (2 x 30 mL) and washed with distilled water. After drying with anhydrous sodium sulfate, the ether was removed on a rotary evaporator. C6H5CHz(OC2H4)40S02C6H4-cH3: MM = 438.54; obtained 66.7 g (90%);7;' = 1.524; Rf (AcOEt) = 0.77; IR (KBr) vsoZ = 1200-1170 cm-l; YCOC = 1100 cm-l; lH NMR (CDClflMS) 6 = 7.34-7.75 (9H,m, 2Ph),4.55 (2H, s, CHz),3.60(16H,m, (oc&)4), 2.4 (3H, 8, CH3). Condensationof Diethylene Glycol Monomethyl Ether [H(OCZ&)ZOCH~] on l-Tosyl-13-benzyl-l,4,7,10,13-pentaoxatridecane [ C ~ H S C H ~ ( O C Z H ~ ) ~ O S O ~To C ~aC H ~ ] . three-necked round-bottomed flask fitted with a condenser were added 14.5 g (0.12 mol; 1 equiv) of tetrakis(oxyethy1ene) glycol tosyl monobenzylether, 61.4 g (0.14 mol; 1.2 equiv)of diethylene glycol monomethyl ether, and 5 equiv of aqueous sodium hydroxide solution (50% w/w) were added. This mixture was refluxed for 24 h with vigorous stirring. The resulting mixture was cooled and diluted with water (200 mL). Extraction was carried out with ether (4 x 100 mL) and the organic phase was dried over anhydrous sodium sulfate. The solventwas evaporated and the crude product distilled under vacuum. C&I&H2( O C Z H ~ ) ~ O CMM H ~ := 386.49; obtained 44 g (95%);7;' = 1.488; Rf(Ac0Et) = 0.41; IR (film) YCOC = 1150-1100 cm-l; YCH(ammat1c) = 3100-3000 cm-l; 'H NMR (CDClflMS) 6 = 7.35 (5H, m, Ph), 4.58 (2H, s, CHz), 3.70 (20H, m, (OC2&)5), 3.40 (3H, s, CH3). Anal. Calcd(Found): C% 62.15 (62.08);H% 8.87 (8.95);0%28.98 (28.97). Catalytic Hydrogenation of Poly(oxyethy1ene)Glycol 1-Benzyl n-Methyl Ether. A solution of 10 g (0.029 mol) of
Perfluoroalkyl Poly(oxyethy1ene)Nonionic Surfactants
Langmuir, Vol. 10, No. 7, 1994 2137
F613Me. 1,1,1,2,2,3,3,4,4,5,5,6,6-Tridecafluoro-8,11,14,17tetraoxaodadecane [ C ~ F ~ ~ C H ~ ( O C Z H ~ MM )~OM = ~496.27; I: obtained 10.2 g (85%); 7;' = 1.359; Rf (AcOEt) = 0.68. Anal. Calcd (Found): C% 33.88 (34.00); H% 3.45 (3.47); F% 49.78 (49.70). F614Me. 1,1,1,2,2,3,3,4,4,5,5,6,6-Tridecafluoro-8,11,14,17,20-pentaoxahenicosane[ C $ ' I ~ C H ~ ( O C ~ H ~ ) ~ MM OM~ =I540.32; : obtained 10 g (83%); 7;' = 1.373; Rf (AcOEt) = 0.53. F61SMe. 1,1,1,2,2,3,3,4,4,5,5,6,6-Tridecafluoro-8,11,14,17,20,23-hexaoxatetracosane [ C ~ F ~ ~ C H Z ( O C Z H ~ ) SMM O M ~=I : 584.37; obtained lOg(83%);r&'= 1.386;Rf(Ac0Et)= 0.46. Anal. Calcd (Found): C% 37.00 (37.90); H% 4.31 (4.38); F% 42.26 (41.58). F616Me. 1,1,1,2,2,3,3,4,4,5,5,6,6-Tridecafluoro-8,11,14,17,20,23,26-heptaoxaheptacosane[C$'13CH2(OCz&)60Me]: MM = 628.43; obtained: 9.6 g (80%);7;' = 1.388; Rf (AcOEt) = 0.42. F712Me. 1,1,1,2,2,3,3,4,4,5,5,6,6,7,7-Pentadecafluoro-9,12,15-trioxahexadecane [C7F1&H2(OCzH&0Me]: MM = 502.22; obtained 9.8 g (82%);7;' = 1.343;Rf (AcOEt) = 0.86. F713Me. 1,1,1,2,2,3,3,4,4,5,5,6,6,7,7-Pentadecafluoro-9,12,15,18-tetraoxanonadecane [ C ~ F ~ S C H ~ ( O C Z & ) ~ OMM M ~ ]= : 546.28; obtained 9.8 g (82%); 7;' = 1.357; Rf (AcOEt) = 0.70. Anal. Calcd(Found): C%32.98(33.22);H%3.14(3.05);F%52.17 (52.06). F714Me. 1,1,1,2,2,3,3,4,4,5,5,6,6,7,7-Pentadecafluoro-9,12,15,18,21-pentaoxadocosane)[ C ~ F ~ ~ C H ~ ( O C Z H ~ MM ) ~ O= M~I: 590.33; obtained 9.6 g (80%);:7 = 1.377; Rf (AcOEt) = 0.57. Anal. Calcd(Found): C% 34.59(35.25);H% 3.59 (3.60);F%48.27 (47.78). F71SMe. 1,1,1,2,2,3,3,4,4,5,5,6,6,7,7-Pentadecafluoro-9,12,15,18,21,24-hexaoxatetracosane [ C ~ F I ~ C H ~ ( O C ~ & ) ~MM OM~I: = 634.38; obtained 9.4 g (78%);7;' = 1.379; Rf(Ac0Et) = 0.51. Anal. Calcd (Found): C%35.97 (36.48);H% 3.97 (4.02);F% 44.92 (43.59). Compoundsof the Type HmpnMe. Spectral characteristics for all compounds type HmpnMe are identical: IR (film) YCH* = l-Tosyl-1,4,7-trioxaoctane[CH~(OC~H~)ZOSOZC~H~CH~I: 3000-2800 cm-l; YCOC = 1150-1100 cm-l; YCF = 1250-1150 MM = 274.36; obtained 3.6 g (85%);7;' = 1.505;Rf (AcOEt) = cm-l; lH NMR (CDClDMS) 6 = 6.10 (H, dt, HCFz), 4.10 (2H, 0.81. t, CHz), 3.70 (4nH, multiplet, (OCz&),), 3.35 (3H, s, CH3). l-Tosyl-1,4,7,10-trioxaundecane [CH~(0C2&)30S02Ce&H613Me. 1,1,2,2,3,3,4,4,5,5,6,6-Dodecafluoro-8,11,14,17-tetCH3l: MM = 318.30; obtained 4.1 g (86%); 7;' = 1.504; Rf raoxaoctadecane [ H C ~ F ~ ~ C H ~ ( O C Z H ~ ) MM ~-OM = ~478.28; ]: (AcOEt) = 0.68. obtained 6.1 g(84%);&'= 1.372;Rf(AcOEt)= 0.68. Anal. Calcd 1-Tosyl-1,4,7,10,13-pentaoxatetradecane [CHs(OC2H& (Found): C% 35.16 (35.33); H% 3.79 (3.73); F% 47.67 (46.49). OS02C6H4CH31: MM = 362.44; obtained 4.7 g (88%); 7;' = H614Me. 1,1,2,2,3,3,4,4,5,5,6,6-Dodecafluoro-8,11,14,17,201.502; Rf (AcOEt) = 0.50; IR (film) YCOC = 1150-1100 cm-l; pentaoxahenicosane [HCsFlzCH2(OC2&)40Me]: MM = 522.33; Y C H ( ~=~3100-3000 ~ ~ ~ ~ )cm-l; 'H NMR (CDClOMS) 6 = 3.38 obtained 6.12 g (78%);7;' = 1.384;Rf (AcOEt) = 0.67. (3H, s, CH301, 3.7 (20H, m, (C2H40)~),7.7-7.4 (4H, m, CS&), H814Me. 1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8-Hexadecafluoro-lO,2.42 (3H, s, CH3). Anal. Calcd (Found): C% 53.02 (53.32);H% 13,16,19,22-pentaoxatricosane [ H C S F ~ ~ C H ~ ( O C ~ H ~MM )~OM~]: 7.23 (7.33); S% 8.85 (9.17). = 622.34; obtained 7.19 g (77%);7;' = 1.362; Rf(Ac0Et) = 0.71. 1-Tosy1-1,4,7,10,13,16-hexaoxaheptadecane [CH3(OC&)5H816Me. 1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8-Hexadecafluoro-l0,OSOzC&CH3]; MM = 406.49; obtained 4.7 g(80%);7;' = 1.498; 13,16,19,22,25-hexaoxahexacosane [HCsF16CH2(OCz&)50Mel: Rf (AcOEt) = 0.41. MM = 666.40;obtained 8 g(80%);7;' = 1.394;Rf(AcOEt)= 0.57. 1-Tosy1-1,4,7,10,13,16,19-heptaoxaicosane [CH3(0Cz&)6HlOlSMe. 1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10-Eicos~uoroOSOzC6H4CH3]: MM = 450.55; obtained 5.0 g (75%); 7;' = 12,15,18,21-tetraoxadocosane [HCIOFZOCHZ(OCZ&)~OM~I: MM 1.49l;Rf(AcOEt)= 0.58. Anal. Calcd (Found): C%53.32 (52.62); = 678.31; obtained 8.4 g (83%);7;' = 1.362;Rf(Ac0Et) = 0.71. H% 7.61 (7.56); S% 7.11 (6.52). Anal. Calcd(Found): C%31.87 (31.88);H% 2.68(2.62);F%56.02 Example of Procedure for Condensation of lH,lH(56.45). Pertluoroalkyl Alcohols on (*ethylene) Glycol Tosyl Determination of Surface Tension. The surface tension Methyl Ethers. An amount of 3 g (0.052 mol; 4 equiv) of measurements were carried out with a Dognon-Abribat tensipowdered potassium hydroxide was added to a solution of 6.8 g ometer by the Wilhelmy plate technique.28Sets of measurements (0.018 mol; 1.2 equiv) of C H ~ ( O C ~ H ~ ) ~ O S O Z Cand ~ H ~5 CgH ~ were taken at 5 minintervals until no s i d c a n t change occurred. in (0.015 mol; 1equiv) of lH,lH-perfluorooctanol (C~F~SCHZOH) Solutions that contain pure surfactant molecules reached equi50 mL of dioxane. After refluxing the mixture for 48 h, it was librium and gave constant values in less than 20 min. When poured into cold water (300 mL) and extracted with ether (3 x impure surfactants were used, a continuous decrease in the 150 mL). Water was removed with magnesium sulfate. Rotary surface tension was observed even after 60 min. evaporationof the solventunder vacuum gave the desired product Determination of Phase Behavior of Binary Systems which may be used without firther purification. However, for (Water/Surfactant). The phase equilibria were examined analytical purposes, the final product was purified by flash between 3 and 60 "C for the compounds RFCHz(0C2H4),0Me in chromatography (SiOz) and eluted with ethyl acetate (AcOEt). almost the whole range of water/surfactant compositions by The condensations for all products were carried out with the isoplethal method (varying the temperature at constant comidentical procedure. position). Different aqueous surfactant solutions with known composition and contained in hermetically sealed tubes were Compoundsof the Type FmpnMe. Spectral characteristics placed in a thermostated water bath. This allowed temperature for all compounds type FmpnMe are identical: IR (film) Y C H ~= homogeneity of about f O . l "C to be obtained. The temperature 3000-2800 cm-'; YCOC = 1150-1100 cm-'; YCF = 1250-1200 cm-l; lH NMR (CDClOMS) 6 = 4.10 (2H, t, CHZ),3.70 [4nH, (28) Wilhelmy, L.Ann. Phys. 1863, 177, 199. multiplet, (OCZH~,],3.38 (3H, s, CH3). C ~ H & H Z ( ~ C ~ & ) S O Cin&60 mL of methanol was placed in a thermostated hydrogenation reactor fitted with a mechanical stirrer. To this solution was added 1 g (10%) of palladium activated on carbon under nitrogen. The reactor was then purged twice with nitrogen and once with hydrogen before leaving it under hydrogen pressure (70bars) at 60 "C for 3h. ARer reaction, the mixture was cooled to room temperature, and was purged twice with nitrogen before filtering on Celite. The solvent was evaporated and the product was obtained in a pure state. H(OCZH&OCH~:MM = 252.31; obtained 7.0 g (96%); 7;' = 1.451; Rf(Ac0Et) = 0.42; IR (film) YCOC = 1150-1100 cm-l; lH N M R (CDClOMS) 6 = 2.7 (lH, s), 3.70 (20H, m), 3.38 (3H, 5 ) . Anal. Calcd (Found): C% 52.36 (51.25); H% 9.59 (9.56); 0% 38.05 (38.15). The followingcompounds were obtainedby the same procedure. Tetrakis(oxyethy1ene) Glycol Monomethyl Ether [H(OCzH&OCH31 MM = 208.26; obtained 6.4 g (92%); 7;' = 1.446;Rf(AcOEt)=0.56. Anal. Calcd(Found): C%51.91(51.84); H% 9.68 (9.65); 0%38.41 (38.51). Hexakis(oxyethy1ene) Glycol Monomethyl Ether [H(OC2H4)60CH3]: MM = 296.36; obtained 6.1 g (80%);7;' = 1.455;Rf(Ac0Et)= 0.39. Anal. Calcd (Found): C% 52.69 (51.25); H% 9.52 (9.37); 0%37.79 (39.38). Tosylation of Pentakis(oxyethy1ene)Glycol Monomethyl Ether. An amount of 4.2 g (0.022 mol; 1.5 equiv) of tosyl chloride in 10 mL of pyridine was added dropwise to a solution of 3.7 g (0.0147 mol; 1 equiv) of pentakis(oxyethy1ene)glycol monomethyl ether in 5 mL of pyridine at 0 "C. f i r addition, the mixture was stirred at this temperature for 30 min and was then allowed to warm up t o room temperature and stirred for 1 hour. The mixture was then poured into 30 mL ofwater and the product was extracted with dichloromethane (3 x 20 mL). Workup was carried out as described above for the tosylation of tetrakis(oxyethy1ene) glycol monobenzyl ether. All the compounds are oils.
~
~~
2138 Langmuir, Vol. 10, No. 7, 1994 was progressively increased and allowed to stabilize between 1 and 2 h after each increment. The cloud points were obtained by noting the onset of turbidity for each sample at a given temperature. In order to determine the nature of each phase (isotropic, anisotropic), the samples were examined between crossed polarizers after each temperature increment. An optical microscope was used to evaluate the structure of the liquid crystals (mesophase). In constructing the phase diagrams, we used the arbitrary convention in which the temperature is placed on the ordinate and composition on the abscissa, with water to the left and surfactant to the right. The left and right borders represent the individual components at different temperatures. each point r ) , and corresponds within the diagram has the coordinates (XS, experimentally to adjusting the temperature of a given composi-
Achilefu et al. tion, Xs,to a specified value T. Experimentally defined lines are drawn within the diagram into regions. Each region corresponds either to isotropic or anisotropic monophase. Region boundaries drawn in dashed lines are known to exist but are difficult to define experimentally. Horizontal dashed lines are tie lines: they connect coexisting phases within any miscibility gap.
Acknowledgment. S. A. thanks CNOUS and CROUS for a grant. This work has been generously supported by a “research GBM contract”sponsored by “RBgionLorraineSGAR 594”. We thank Dr. A. Lantz and Dr. P. Durual (ATOCHEM Society) for furnishing us the fluorinated starting material.
