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Langmuir 1996, 12, 6346-6350
Branched Fluorinated Nonionic Y-Shaped Surfactants. Effect of Molecular Geometry on Liquid Crystalline Phase Behavior Krystyna Kratzat,* Fre´de´ric Guittard,† Elisabeth Taffin de Givenchy,† and Aime´ Cambon† Institut fu¨ r Makromolekulare Chemie, Sonnenstr. 5, Universita¨ t Freiburg, D-79104 Freiburg, Germany, and Laboratoire de Chimie Organique du Fluor, Universite´ de Nice-Sophia Antopolis, Faculte´ des Sciences, B.P. no. 71, 06108 Nice Cedex 2, France Received August 8, 1996. In Final Form: September 25, 1996X The synthesis of branched partially fluorinated nonionic Y-shaped surfactants, with the general formula CFnC2N(EmM)2, where CFn denotes fluorinated alkyl chain F(CF2)n with n ) 4-10, C2 is C2H2, and EmM is oligooxyethylene monomethyl ether with m ) 2, 3, and their hydrocarbon analogs Cn+2N(EmM)2 (Cn equals hydrocarbon chain) is described. These surfactants are synthesized to analyze the influence of the fluorinated alkyl chain on the packing effects to amphiphilic layers in a binary surfactant/water system. The lyotropic phase behavior of these surfactants is investigated by polarizing microscopy. The liquid crystalline (LC) phase polymorphism of the fluorinated CFnC2N(E3M)2 surfactants strongly changes with n from hexagonal H1 phase (n ) 6) to lamellar LR phase (n ) 10). In contrast to other Y-shaped hydrocarbon oligooxyethylene surfactants CnG(EmM)2 (where G denotes a glycerol unit) which form only cubic I1 and H1 phases for n ) 12-16 and m ) 3-5, the phase behavior of the CFnC2N(EmM)2 surfactant/water systems shows a remarkable change in the LC phase polymorphism (from H1 to LR phase) with n and m. Compared to hydrocarbon surfactants these results distinctly confirm the influence of more rigid fluorinated alkyl chain on the phase behavior.
Introduction Due to the unique properties of fluorinated surfactants, they are indispensable in certain practical applications and of great theoretical interest for the study of surfactants and micellar systems. Understanding the relationship between molecular structure and the organization of micellar aggregate of surfactant/water systems as a function of temperature and concentration is very important to understanding macroscopic properties such as phase behavior, liquid crystalline phase morphology, and rheological behavior. The introduction of the packing parameter by Israelachvili et al.1 has provided a criterion to predict the shape of micellar aggregates of a given hydrocarbon surfactant well above the critical micelle concentration (cmc). The packing parameter is
F ) V1/Alc
(1)
where V1 is the volume, lc the critical chain length of the surfactant hydrophobic moiety, and A the average molecular area at the aggregate interface. The preferred micellar aggregate geometry for 0 < F e 1/3 are spheres, for 1/3 < F e 1/2 cylinders, for 1/2 < F e 1 bilayers. This approach is very helpful in interpreting the micellar polymorphism. In the past years the aqueous solutions of nonionic oligooxyethylene hydrocarbon surfactants have been studied intensively with respect to the micellar shape and liquid crystalline phase morphology.2-10 * Address correpondence to: Dr. Krystyna Kratzat, Institut fu¨r Makromolekulare Chemie, Universita¨t Freiburg, Sonnenstr. 5, D-79104 Freiburg, Germany. Tel: 0761/203-6282. Fax: 0761/28 69 04. † Universite ´ de Nice-Sophia Antopolis. X Abstract published in Advance ACS Abstracts, December 1, 1996. (1) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. N. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (2) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P. J. Chem. Soc., Faraday Trans. 1 1983, 79, 975. (3) Conroy, J. P.; Hall, C.; Leng, C. A.; Rendall, K.; Tiddy, G. J. T.; Walsh, J.; Lindblom, G. Progr. Colloid Polym. Sci. 1990, 82, 253.
S0743-7463(96)00781-0 CCC: $12.00
In contrast to the extensive information on hydrocarbon type surfactants, the knowledge of fluorinated surfactant micelles and lyotropic liquid crystals is limited. The substitution of the large and highly electronegative fluorine atom for the smaller hydrogen atom increases the amphiphilic nature of the surfactant and lowers surface tension and cmc. The fluorocarbon chain, with a lipophilic chain volume higher than that of the hydrocarbon chain, is stiffer and the interaction within the chains is weaker. Therefore a different micellar aggregate organizations will occur in the corresponding fluorocarbon and hydrocarbon surfactant/water systems. Like hydrocarbon chain surfactants, the fluorinated surfactants can form lyotropic liquid crystals in water. Tiddy et al.11-14 observed the formation of a hexagonal H1 phase, an intermediate phase, and a lamellar LR phase for ammonium and lithium perfluorooctanoate. A nematic ND phase and lamellar LR phase were found for cesium perfluorooctanoate15 and ammonium perfluorononanoate.16 Guo et al.17 observed the existence of a cubic phase for perfluoroheptanoic acid, its salts, and ethoxylated amides. (4) Kratzat, K.; Finkelmann, H. Liq. Cryst. 1993, 13, 691. (5) Kratzat, K.; Finkelmann, H. J. Colloid Interface Sci. 1996, 181, 542. (6) Kratzat, K.; Finkelmann, H. Colloid Polym. Sci. 1994, 272, 400. (7) Kratzat, K.; Schmidt, C.; Finkelmann, H. J. Colloid Interface Sci. 1994, 163, 190. (8) Kratzat, K.; Stubenrauch, C.; Finkelmann, H. Colloid Polym. Sci. 1995, 327, 257. (9) Kratzat, K.; Finkelmann, H. Langmuir 1996, 12, 1765. (10) Linemann, R.; La¨uger, J.; Schmidt, G.; Kratzat, K.; Richtering, W. Rheol. Acta 1995, 34, 440. (11) Tiddy, G. J. T. Symp. Faraday Soc. 1971, 5, 150. (12) Tiddy, G. J. T. Trans. Faraday Soc. 1972, 68, 653. (13) Tiddy, G. J. T.; Wheeler, B. A. J. Colloid Interface Sci. 1974, 47, 58. (14) Everiss, E.; Tiddy, G. J. T.; Wheeler, B. A. J. Chem. Soc., Faraday Trans. 1976, 72, 1747. (15) Boden, N.; Jackson, P. H.; McMullen, K.; Holmes, M. C. Chem. Phys. Lett. 1979, 65, 476. (16) Herbst, L.; Hoffmann, H.; Kalus, J.; Reizlein, K.; Schmelzer, U.; Ibel, K. Ber. Bunsenges. Phys. Chem. 1985, 89, 1050. (17) Guo, W.; Brown, T. A.; Fugo, B. M. J. Phys. Chem. 1991, 95, 1829.
© 1996 American Chemical Society
Branched Fluorinated Nonionic Y-Shaped Surfactants
Figure 1. Molecular structure of surfactants: (a) CF8C2N(E3M)2, (b) C10N(E3M)2, and (c) C12G(E3M)2.
For the present studies we prepared partially fluorinated Y-shaped nonionic oligooxyethylene surfactants CFnC2N(EmM)2. The hydrophilic part of these surfactants consists of two short oligooxyethylene chains linked via amine to the hydrophobic part that contains a partially fluorinated alkyl chain (Figure 1a). The synthesis and the phase behavior in water of the new CFnC2N(EmM)2 surfactants will be described with respect to the polymorphism of the lyotropic LC phases. An explanation of the phase behavior of the homologous series with systematic variation of the oxyethylene chains Em and alkyl chain CFn will be discussed. These results are compared to the phase behavior of the corresponding hydrocarbon Y-shaped surfactants: Cn+2N(EmM)2 and CnG(EmM)24 (where G denotes a glycerol unit) (Figure 1b,c) to answer the following questions: (i) how does the exchange of fluorine for hydrogen modifies the phase behavior in water and (ii) is the sequence of LC phase polymorphism determined by the molecular geometry of the fluorinated Y-shaped surfactants similar to the hydrocarbon Ysurfactants CnG(EmM)2 by varying the length n and m. 2. Experimental Section 2.1. Materials. The analytical data and the synthesis of compounds with a fluorinated tail, CFnC2N(EmM)2 with n ) 4, 6, 8 and m ) 2, 3, are described elsewhere.18 Tosylation of oxyethylene monomethyl ether was made according to standard procedure. The fluorinated compounds (with n ) 10; m ) 2, 3) and hydrocarbon homologues were synthesized according to the following procedure: synthesis of N,N-bis(bis(oxyethylene)oxymethyl)-2-(F-decyl)ethylamine, N,N-bis(tris(oxyethylene)oxymethyl)-2-(F-decyl)ethylamine and N,N-bis(tris(oxyethylene)oxymethyl)-2-alkylamine. To a solution of 10 mmol of 2-(F-decyl)ethylamine or alkylamine and 22 mmol of sodium carbonate in 25 mL of acetonitrile was added a solution of 22 mmol of tosylbis(oxyethylene)monomethyl ether or tosyltris(oxyethylene) monomethyl ether in 15 mL of acetonitrile. After the complete addition, the resulting mixture was left to reflux and followed with gas phase chromatography until removal of monoaddition product (52-100 h). The residue was filtered then the organic phase was evaporated. The resulting amine of the diaddition was isolated by acidobasic washing and was extracted with diethyl ether, dichloromethane, or chloroform according the lengthening and the nature of the tail. The evaporation after drying of the organic phase and distillation allows us to obtain the desired pure product. Analytical Data for CF10C2N(EmM)2. CF10C2N(E2M)2: MS (70 eV) 722 (1.1), 678 (100), 604 (18.6), 576 (3.2), 234 (4.7). Anal. Calcd for C22H26F21NO4: C, 34.43; H, 3.41; F, 51.99; N, 1.82. Found: C, 34.61; H, 3.32; F, 52.47; N, 1.76. (18) Guittard, F.; Taffin de Givenchy, E.; Cambon, A. J. Colloid Interface Sci. 1996, 177, 101.
Langmuir, Vol. 12, No. 26, 1996 6347 CF10C2N(E3M)2: MS (70 eV) 766 (0.5), 722 (100), 604 (15.6), 576 (3.8), 322 (4.7). Anal. Calcd for C26H34F21NO6: C, 36.50; H, 4.01; F, 46.63; N, 1.64. Found: C, 37.04; H. 3.91; F, 47.08; N, 1.51. H-NMR (CDCl /TMS) δ (ppm): 2.3 (m, 2H, CH R ), 2.75 (t, 3 2 F 4H, N(CH2CH2O)2), 2.9 (m, 2H, CH2CH2RF), 3.3 (s, 6H, 2*(OCH3)), 3.45-3.69 (m, 12H(m ) 2) or 20H(m ) 3), 2*[CH2O(CH2CH2O)m-1]). 19F-NMR (CDCl /CFCl ) δ (ppm): -81.2 (CF ), -114.5 (CF ), 3 3 3 2R -122.2 (5*CF2), -123.2 to -124.0 (2*CF2), -126.6 (CF2ω). Analytical Data for Cn+2N(E3M)2. C8N(E3M)2: MS (70 eV) 421 (M+,
