New Catanionic Amphiphiles Derived from the Associative Systems (α

New phosphorous catanionic amphiphiles were easily prepared by an acid−base reaction between (α-hydroxyalkyl)-phosphinic or -phosphonic acid and ...
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Langmuir 2003, 19, 6678-6684

New Catanionic Amphiphiles Derived from the Associative Systems (r-Hydroxyalkyl)-phosphinic or (r-Hydroxyalkyl)-phosphonic Acid/ Cetyltrimethylammonium Hydroxide. Preparation, Characterization, and Self-Organization Properties Fatima Al-Ali, Alice Brun, Fernanda Rodrigues, Guita Etemad-Moghadam,† and Isabelle Rico-Lattes* Laboratoire des IMRCP (UMR CNRS No. 5623), Universite´ Paul Sabatier, 118, route de Narbonne, Baˆ t. 2R1, 31062 Toulouse Cedex 04, France Received March 12, 2003. In Final Form: June 6, 2003 New phosphorous catanionic amphiphiles were easily prepared by an acid-base reaction between (Rhydroxyalkyl)-phosphinic or -phosphonic acid and cethyltrimethylammonium hydroxide. In this way, we obtained, in nearly quantitative yields, bicatenary phosphinates and bicatenary and tricatenary phosphonates. The aggregation properties of these new catanionic amphiphiles were investigated at low concentrations in water (ranging from 1 to 5 mM), focusing in particular on spontaneous formation of different aggregate morphologies (vesicles, ribbons, tubules). Light and electron microscopy was used to identify the aggregate morphology of the catanionic amphiphiles. Moreover, these catanionic amphiphiles were able to organize into ordered lyotropic mesophases in concentrated water solutions, visualized by optical polarizing microscopy. Finally, the phosphorous catanionic amphiphiles, without rigid cores, were unexpectedly found to show thermotropic liquid-crystalline behavior. The various thermotropic mesophases were characterized by the formation of optical textures. Rigidification by ionic interactions could promote the formation of these thermotropic mesophases.

Introduction During the past few years, catanionic surfactant mixtures have received increasing attention because of the diversity of their aggregate microstructures (micelles, spontaneous vesicles, and lamellar phases)1 and because they can be obtained from a variety of surfactants available either from commercial sources or from original synthesis.2 When mixing surfactants, not only are their properties combined, but also in many cases, new properties are found. These are of both fundamental and commercial interest. Concerning phosphorous catanionic salts, very few examples have been reported in the literature.3-8 However, these compounds can be considered as analogues of phospholipids, the main components of biological membranes. Recently, we synthesized in good yields a series of (Rhydroxyalkyl)-phosphinic and -phosphonic acids9 and we reported their aggregation properties.10 Following this, * Corresponding author. Tel: +33 (0) 5 61 55 62 70. Fax: +33 (0) 5 61 55 81 55. E-mail: [email protected]. † This paper is dedicated to Dr. Guita Etemad-Moghadam who directed this work and regrettably died on March 7, 2002. (1) (a) Brasher, L. L.; Kaler, E. W. Langmuir 1996, 12, 6270. (b) Salkar, R. A.; Mukesh, D.; Samant, S. D.; Manohar, C. Langmuir 1998, 14, 3778. (c) Blanzat, M.; Perez, E.; Rico-Lattes, I.; Lattes, A.; Gulik, A. Chem. Commun. 2003, 2, 244. (2) Tondre, C.; Caillet, C. Adv. Colloid Interface Sci. 2001, 93, 115. (3) Minardi, R. M.; Schulz, P. C.; Vuano, B. Colloids Surf., A 2002, 197, 167. (4) Minardi, R. M.; Schulz, P. C.; Vuano, B. Colloid Polym. Sci. 1998, 276, 589. (5) Minardi, R. M.; Schulz, P. C.; Vuano, B. Colloid Polym. Sci. 1996, 274, 669. (6) Yamauchi, K.; Yoshida, Y.; Moriya, T.; Togawa, K.; Kinoshita, M. Biochim. Biophys. Acta 1994, 1193, 41. (7) Lerebours, B.; Watzke, H. J.; Fendler, J. H. J. Phys. Chem. B 1990, 94 (4), 1632. (8) Mehreteab, A.; Loprest, F. J. J. Colloid Interface Sci. 1988, 125 (2), 602.

we prepared a new series of phosphorous catanionic salts. The bicatenary phosphinates and bicatenary or tricatenary phosphonates were prepared by simple acidbase reaction between phosphinic or phosphonic acid and 1 or 2 equiv of cetyltrimethylammonium hydroxide. These new catanionic systems show similar behavior to phospholipids. Moreover, R-hydroxyl derivatives (phosphinic and phosphonic acids, esters, and salts) are an interesting class of compounds with potential biological activity (inhibition of enzymes and metalloenzymes and antiviral, antibacterial, and fungicidal properties).11-15 Here, we report the preparation, the characterization, and the selfassociation properties of several new series of phosphorous catanionic amphiphiles. Experimental Section Materials. The starting products were commercially available from large chemical suppliers. Phosphonic and phosphinic acids were synthesized in our laboratory. (9) Albouy, D.; Brun, A.; Munoz, A.; Etemad-Moghadam, G. J. Org. Chem. 1998, 63, 7223. (10) Brun, A.; Albouy, D.; Perez, E.; Rico-Lattes, I.; EtemadMoghadam, G. Langmuir 2001, 17, 5208. (11) (a) Patel, D. V.; Rielly-Gauvin, K.; Ryono, D. E. Tetrahedron Lett. 1990, 31, 5587. (b) Patel, D. V.; Rielly-Gauvin, K.; Ryono, D. E. Tetrahedron Lett. 1990, 31, 5591. (c) Einhau¨sser, T. J.; Galansky, M.; Vogel, E.; Keppler, B. K. Inorg. Chim. Acta 1997, 257, 265. (12) Sikorski, J. A.; Miller, M. J.; Braccolino, D. S.; Cleary, G.; Corey, S. D.; Font, J. L.; Gruys, K. J.; Han, C. Y.; Lin, K. C.; Pansegrau, P. D.; Ream, J. E.; Schnur, D.; Shah, A.; Walker, M. C. Phosphorus, Sulfur Silicon 1993, 76, 115. (13) Stowasser, B.; Budt, K. H.; Jian-Qi, L.; Peyman, A.; Ruppert, D. Tetrahedron Lett. 1992, 33 (44), 1625. (14) (a) Ruel, R.; Bouvier, J.-P.; Young, R. N. J. Org. Chem. 1995, 60, 5209. (b) Kalir, A.; Kalir, H. H. The Chemistry of Organophosphorus Compounds; Hartley, F. R., Ed.; Wiley & Sons: New York, 1996; Vol. 4, Chapter 9, p 767 and references therein. (15) Baylis, E. K. Eur. Pat. Appl. EP 614,900, 1994; Chem. Abstr. 122, 106126y.

10.1021/la0344231 CCC: $25.00 © 2003 American Chemical Society Published on Web 07/12/2003

Catanionic Amphiphiles from Associative Systems General Methods. Spectra were recorded using the following instruments. IR spectra: Perkin-Elmer IR-FT 1600 (samples were studied in the condensed phase (KBr)). 1H, 13C, and 31P NMR spectra: Brucker AC200 or 400WM. Melting points were recorded on an Olympus 8 × 50 microscope with a Mettler Toledo FP82HT hot stage ((0.1 °C). Synthesis of Surfactants. The synthesis and the characterization of phosphorous amphiphiles, (R-hydroxyalkyl)-phosphinic and -phosphonic acids, were already described in a previous work.9 The general methods are indicated below. (R-Hydroxyalkyl)-phosphinic Acid 1 and (R-Hydroxyalkyl)phosphonic Acid 2. The heating for 4 h at 85 °C or sonication of heterogeneous mixtures of 50% aqueous hypophosphorous acid and long-chain aldehydes in the presence of catalytical amounts of hydrochloric acid leads to the corresponding (R-hydroxyalkyl)phosphinic acid 1 with good yields (60-85%). The oxidation of phosphonic acid 1 in the presence of a stoichiometric amount of dimethyl sulfoxide (DMSO) and catalytic amounts of iodine at 60 °C for 4-5 h leads to the corresponding (R-hydroxyalkyl)phosphonic acid 2 with good yields (65-85%). Cetyltrimethylammonium Hydroxide (CTAOH). Cetyltrimethylammonium bromide (CTAB) was recrystallized from an acetone/methanol (1/1) mixture. CTAOH was prepared from CTAB using an ion-exchange resin (Amberlite IRA-900) (converted to its OH- form by stirring it with 0.1 M NaOH solution). The resulting aqueous solution was freeze-dried to give a white powder that was stored under an argon atmosphere until used. Yield 95%; 1H NMR (400.137 MHz, D2O at 70 °C) δ 3.79 (m, 2H, CH2N+), 3.6 (s, 9H, (CH3)3N+), 2.29 (m, 2H, CH2-CH2N+), 1.79 (m, 26H, CH2), 1.38 (t, 3JHH ) 6.5 Hz, 3H, CH3). Catanionic Salts. (1) Bicatenary Cetyltrimethylammonium (RHydroxyalkyl)-phosphinates 3. A solution of cetyltrimethylammonium hydroxide (1 equiv) in distilled water was added dropwise to a solution of (R-hydroxyalkyl)-phosphinic acids 1 (0.24 mmol, 1 equiv) in n-propanol/water (75/25 v/v). After 12 h of stirring at 40 °C, the solvents were evaporated to dryness under a vacuum. The catanionic products 3 were obtained quantitatively as white powders. (1.a) Bicatenary Cetyltrimethylammonium (R-Hydroxyoctyl)phosphinates 3a. mp 192 °C; 31P NMR (200 MHz, D2O) δ 30 (d, 1J 1 PH ) 500 Hz); H NMR (400.137 MHz, D2O at 70 °C) δ 7.25 (d, 1J PH ) 493 Hz, 1H, P-H), 3.98 (m, 1H, CH-P), 3.79 (m, 2H, CH2N+), 3.62 (s, 9H, (CH3)3N), 2.27 (m, 2H, CH2-CH2N+), 2.04 (m, 2H, CH2-CHP), 1.79 (m, 36H, CH2), 1.4 (t, 3JHH ) 6.3 Hz, 3H, CH3), 1.37 (t, 3JHH ) 6.8 Hz, 3H, CH3); IR (KBr, cm-1) 2366.7, 2340.7 (ν PH), 1166.6 (νa PO2-), 1051.6 (νs PO2-). (1.b) Bicatenary Cetyltrimethylammonium (R-Hydroxydecyl)phosphinates 3b. mp 189 °C; 31P NMR (200 MHz, D2O) δ 30 (d, 1J 1 PH ) 500 Hz); H NMR (400.137 MHz, D2O at 70 °C) δ 7.22 (d, 1J PH ) 493 Hz, 1H, P-H), 3.9 (m, 1H, CH-P), 3.79 (m, 2H, CH2N+), 3.62 (s, 9H, (CH3)3N), 2.23 (m, 2H, CH2-CH2N+), 2.01 (m, 2H, CH2-CHP), 1.79 (m, 40H, CH2), 1.37 (t, 3JHH ) 6.3 Hz, 3H, CH3), 1.34 (t, 3JHH ) 6.8 Hz, 3H, CH3); IR (KBr, cm-1) 2325 (ν PH), 1172 (νa PO2-), 1046 (νs PO2-). (1.c) Bicatenary Cetyltrimethylammonium (R-Hydroxydodecyl)phosphinates 3c. mp 187 °C; 31P NMR (200 MHz, D2O) δ 30 (d, 1J 1 1 PH ) 500 Hz); H NMR (400.137 MHz, D2O) δ 7.24 (d, JPH ) 493 Hz, 1H, P-H), 3.95 (m, 1H, CH-P), 3.85 (m, 2H, CH2N+), 3.65 (s, 9H, (CH3)3N), 2.25 (m, 2H, CH2-CH2N+), 2.15 (m, 2H, CH2-CHP), 1.81 (m, 44H, CH2), 1.4 (m, 6H, 2 CH3); IR (KBr, cm-1) 2364.7, 2340.1 (ν PH), 1167.1 (νa PO2-), 1045.3 (νs PO2-). (1.d) Bicatenary Cetyltrimethylammonium (R-Hydroxytetradecyl)-phosphinates 3d. mp 196 °C; 31P NMR (200 MHz, D2O) δ 30 (d, 1JPH ) 500 Hz); 1H NMR (400.137 MHz, D2O) δ 3.95 (m, 1H, CH-P), 3.85 (m, 2H, CH2N+), 3.7 (s, 9H, (CH3)3N), 2.3 (m, 2H, CH2-CH2N+), 2.1 (m, 2H, CH2-CHP), 1.86 (m, 48H, CH2), 1.45 (m, 6H, 2 CH3); IR (KBr, cm-1) 2358.9, 2341.6 (ν PH), 1167.2 (νa PO2-), 1042.8 (νs PO2-). (1.e) Bicatenary Cetyltrimethylammonium (R-Hydroxyhexadecyl)-phosphinates 3e. mp 182 °C; 31P NMR (200 MHz, D2O) δ 30 (d, 1JPH ) 500 Hz); 1H NMR (400.137 MHz, D2O) δ 7.26 (d, 1J PH ) 496 Hz, 1H, P-H), 3.95 (m, 1H, CH-P), 3.66 (s, 9H, (CH3)3N), 1.82 (m, 52H, CH2), 1.39 (m, 6H, 2 CH3); IR (KBr, cm-1) 2370.9 (ν PH), 1163.4 (νa PO2-), 1054.5 (νs PO2-). (1.f) Bicatenary Cetyltrimethylammonium (R-Hydroxyoctadecyl)-phosphinates 3f. mp 200 °C; 31P NMR (200 MHz, D2O) δ 30

Langmuir, Vol. 19, No. 17, 2003 6679 (d, 1JPH ) 500 Hz); IR (KBr, cm-1) 2320 (ν PH), 1166.7 (νa PO2-), 1082.8 (νs PO2-). (2) Bicatenary Cetyltrimethylammonium (R-Hydroxyalkyl)phosphonates 4 and Tricatenary Cetyltrimethylammonium (RHydroxyalkyl)-phosphonates 5. We utilized the same procedure as that described previously. Using the acid-base reaction in n-propanol/water (75/25 v/v) between (R-hydroxyalkyl)-phosphonic acids 2 (1 equiv, 0.24 mmol) and CTAOH (1 or 2 equiv), we prepared bicatenary (R-hydroxyoalkyl)-phosphonates 4 and tricatenary (R-hydroxyoalkyl)-phosphonates 5, respectively. (2.a) Bicatenary Cetyltrimethylammonium (R-Hydroxydodecyl)-phosphonates 4c. mp 210 °C; 31P NMR (200 MHz,D2O) δ 20 (s); 1H NMR (400.137 MHz, CD3OD) δ 3.59 (m, 1H, CH-P), 3.13 (s, 9H, (CH3)3N), 1.81 (m, 2H, CH2-CH2N+), 1.64 (m, 2H, CH2CHP), 1.3 (m, 44H, CH2), 0.92 (t, 3JHH ) 6.8 Hz, 6H, 2CH3); IR (KBr, cm-1) 1151 (νa PO2-), 1038, 1024 (νs PO2-). (2.b) Bicatenary Cetyltrimethylammonium (R-Hydroxytetradecyl)-phosphonates 4d. mp 214 °C; 31P NMR (200 MHz, D2O) δ 20 (s); 1H NMR (400.137 MHz, CD3OD) δ 3.53 (m, 1H, CH-P), 3.13 (s, 9H, (CH3)3N), 1.8 (m, 2H, CH2-CH2N+), 1.66 (m, 2H, CH2CHP), 1.3 (m, 48H, CH2), 0.92 (t, 3JHH ) 6.8 Hz, 6H, 2CH3); IR (KBr, cm-1) 1148 (νa PO2-), 1079, 1034 (νs PO2-). (2.c) Bicatenary Cetyltrimethylammonium (R-Hydroxyhexadecyl)-phosphonates 4e. mp 210 °C; 31P NMR (200 MHz, D2O) δ 20 (s); 1H NMR (400.137 MHz, CD3OD) δ 3.62 (m, 1H, CH-P), 3.13 (s, 9H, (CH3)3N), 1.81(m, 2H, CH2-CH2N+), 1.64 (m, 2H, CH2-CHP), 1.3 (m, 52H, CH2), 0.92 (t, 3JHH ) 6.8 Hz, 6H, 2CH3); IR (KBr, cm-1) 1149 (νa PO2-), 1038 (νs PO2-). (2.d) Bicatenary Cetyltrimethylammonium (R-Hydroxyoctadecyl)-phosphonates 4f. mp 210 °C; 31P NMR (200 MHz, D2O) δ 20 (s); 1H NMR (400.137 MHz, CD3OD) δ 3.57 (m, 1H, CH-P), 3.13 (s, 9H, (CH3)3N), 1.8 (m, 2H, CH2-CH2N+), 1.62 (m, 2H, CH2-CHP), 1.3 (m, 56H, CH2), 0.92 (t, 3JHH ) 6.8 Hz, 6H, 2CH3); IR (KBr, cm-1) 1147 (νa PO2-), 1071, 1039 (νs PO2-). (2.e) Tricatenary Cetyltrimethylammonium (R-Hydroxydodecyl)-phosphonates 5c. mp 196 °C; 31P NMR (200 MHz, D2O) δ 20 (s); 1H NMR (400.137 MHz, CD3OD) δ 3.62 (m, 1H, CH-P), 3.13 (s, 18H, 2(CH3)3N), 1.81 (m, 4H, 2CH2-CH2N+), 1.64 (m, 2H, CH2-CHP), 1.3 (m, 72H, CH2), 0.92 (t, 3JHH ) 6.8 Hz, 9H, 3CH3); IR (KBr, cm-1) 1152 (νa PO32-), 1067, 1042 (νs PO32-). (2.f) Tricatenary Cetyltrimethylammonium (R-Hydroxytetradecyl)-phosphonates 5d. mp 204 °C; 31P NMR (200 MHz, D2O) δ 20 (s); 1H NMR (400.137 MHz, CD3OD) δ 3.59 (m, 1H, CH-P), 3.13 (s, 18H, 2(CH3)3N), 1.8 (m, 4H, 2CH2-CH2N+), 1.63 (m, 2H, CH2-CHP), 1.3 (m, 76H, CH2), 0.92 (t, 3JHH ) 6.8 Hz, 9H, 3CH3); IR (KBr, cm-1) 1146 (νa PO32-), 1090, 1022 (νs PO32-). (2.g) Tricatenary Cetyltrimethylammonium (R-Hydroxyhexadecyl)-phosphonates 5e. mp 200 °C; 31P NMR (200 MHz, D2O) δ 20 (s); 1H NMR (400.137 MHz, CD3OD) δ 3.52 (m, 1H, CH-P), 3.13 (s, 18H, 2(CH3)3N), 1.8 (m, 4H, 2CH2-CH2N+), 1.63 (m, 2H, CH2-CHP), 1.3 (m, 80H, CH2), 0.92 (t, 3JHH ) 6.8 Hz, 9H, 3CH3); IR (KBr, cm-1) 1139 (νa PO32-), 1028, 1010 (νs PO32-). (2.h) Tricatenary Cetyltrimethylammonium (R-Hydroxyoctadecyl)-phosphonates 5f. mp 202 °C; 31P NMR (200 MHz, D2O) δ 20 (s); IR (KBr, cm-1) 1162 (νa PO32-), 1037 (νs PO32-). Preparation of Aggregates in Dilute Solution. Aggregates of compounds 3-5 were prepared by simple dissolution in water for low concentrations (ranging between 1 and 5 mM). Bicatenary phosphonates 4c and 4d are less soluble in water. We prepared their aggregates by the evaporation method. They were dissolved in methanol (30 mg/100 mL). The solvent was removed in a rotary evaporator. Aggregates were prepared by adding the appropriate volume of water to the layer of surfactants on the wall of the flask with vigorous vortexing for a few minutes to yield final surfactant concentrations of 1 or 2.5 mM. Surface Tension Measurements. Critical aggregation concentrations (cac’s) were measured in water at 25 °C ((0.1 °C) with a Lauda (TD1) tensiometer using the stirrup detachment method. Light Scattering Experiments. The aggregate diameters were evaluated by dynamic light scattering using a Malvern Instruments Zetasizer 3000, suitable for samples containing particles from 1 nm to 10 µm. Electron Microscopy. The formation of supramolecular aggregates was observed by transmission electron microscopy using a JEOL JEM 200 CX electron microscope, operating at 200

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Scheme 1. Synthesis of (r-Hydroxyalkyl)-phosphinic 1a-f and (r-Hydroxyalkyl)-phosphonic 2a-f Acids

Scheme 2. Synthesis of Bicatenary Cetyltrimethylammonium (r-Hydroxyalkyl)-phosphinates 3a-f

kV. Aliquots of solution were applied to carbon-coated Formvar grids, negatively stained with a 2% (w/v) solution of ammonium molybdate (pH ) 5). Light Microscopy. The aggregates that were sufficiently large were observed by light microscopy. Aggregate suspensions in water were imaged at room temperature by placing a drop of the suspension between a glass slide and coverslip. The light microscope used was an Olympus CH-2 microscope. Polarized Light Microscopy. The lyotropic and thermotropic mesophase textures were studied using an Olympus CH-2 polarizing microscope equipped with a hot stage ((0.1 °C), linked to a Physitemp TS-4 ER temperature controller and a Sony CCD-IRIS/RGB color video camera Hyper HAD, connected to a Sony camera adapter CMA-D2. Samples were prepared by steadily dissolving the crystals in water, to reach a concentration range of 40-80% w/w.

Results and Discussion The preparation of (R-hydroxyalkyl)-phosphinic 1 or -phosphonic 2 acid was described proviously,9 and the general method is shown in Scheme 1. We used phosphorous acids 1 and 2 for the preparation of catanionic amphiphiles 3-5. Mixing phosphinic acid 1 or phosphonic acid 2 with cetyltrimethylammonium in n-propanol/water (75/25 v/v) for 12 h under stirring at 40 °C led, in nearly quantitative yields, to new series of phosphorous catanionic amphiphiles 3-5. The bicatenary phosphinates (Scheme 2) or phosphonates (Scheme 3, step 1) resulted from 1:1 acid-base mixtures. Tricatenary phosphonates (Scheme 3, step 2) were obtained by neutralizing the two acid functions of phosphonic acid 2 by 2 equiv of cetyltrimethylammonium. Characterization of Surfactants. We characterized the new phosphorous catanionic amphiphiles by NMR

Table 1. Micellization Parameters for Compounds 3a-f, 4c, and 5c compound

Cn

cac (mM)

γcac (mN/m)

3a 3b 3c 3d 3e 3f 4c 5c

8 10 12 14 16 18 12 12

0.3 0.1 0.04 0.03 0.03 0.02 0.04 0.02

29 27 23 23 32 45 23 25

studies (31P and 1H) and IR spectroscopy. 1H NMR spectra of catanionic phosphinate 3 were recorded in D2O at 70 °C. In these conditions, the aggregates dissociate and the resolution is better. Catanionic phosphonates 4-5 are opalescent and viscous in water; we plotted their spectra in CD3OD. In the IR spectra for catanionic amphiphiles 3 derived from phosphinic acid, we identified the frequency characteristic of P-H bands and the PO2- anion (see experimental part). Compared to phosphinic acids, we observed a reduction in the P-H vibration frequency attributed to the formation of intermolecular hydrogen bonds, between the PdO and P-H groups in the catanionic derivatives. For example, the characteristic band of P-H occurs at 2399 cm-1 for phosphinic acid 1b. This band moves to 2325 cm-1 for catanionic derivative 3b. For catanionic phosphonates 4, we identified the characteristic bands of the PO2- anion. Absorption bands of the PO32anion characterize catanionic phosphonates 5 (see experimental part). Surface Tension Measurements. Table 1 lists the cac’s for catanionic amphiphiles 3a-f, 4c, and 5c. Figure 1 gives the graphic data for bicatenary compounds 3. Compounds 4 and 5 have viscoelastic behavior at low concentrations (0.1 mM). It was only possible to measure the cac for compounds 4c and 5c, their solutions being sufficiently fluid. Concerning derivatives 3, all compounds exhibited a cac. Lengthening the side chain of phosphinate decreased the cac between compound 3a (n ) 8) and compound 3d (n ) 14) and increased their surfactant powers, reducing the surface tension of water from 75 to 23 mN/m. Above 14 carbon atoms, the cac remained fairly constant. It thus seems that for this length, the chain folds and does not contribute any quitter stabilization by hydrophobic interaction. However, for compounds 3e and 3f we observed an increase in surface tension from 23 to 45 mN/m. This paradoxical situation is also often observed with nonionic surfactants where the γ only increases slightly until the alkyl chain has reached a certain

Catanionic Amphiphiles from Associative Systems

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Scheme 3. Synthesis of Bicatenary Cetyltrimethylammonium (r-Hydroxyalkyl)-phosphonates 4 (Step 1) and Tricatenary Cetyltrimethylammonium (r-Hydroxyalkyl)-phosphonates 5 (Step 2)

Figure 1. Variation of the surface tension vs the concentration of aqueous solutions of bicatenary catanionic phosphinates 3a-f at 25 °C.

length.16,17 These results are in agreement with previous work done in our laboratory on catanionic glycolipid analogues of galactosylceramide.18 We have realized the surface tension measurements for these compounds 3e and 3f at 50 °C, and they presented the same behavior observed at 25 °C. Therefore, we can conclude that increase in surface tension cannot be related to solubility problems. In fact, the compounds 3e and 3f are very soluble in water at 25 °C. However, they do not present a classical behavior and form several morphologies of aggregates in addition to micelles obtained in classical cases. These aggregates are also present at 50 °C. This can explain the observed situation. In fact, these compounds having a more significant hydrophobic part tend to form nonclassical aggregates as shown later in Figure 2. Compounds 4a and 5a self-organize at very low concentrations (cac ) (16) Laakel, N.; Rubini, P.; Rodehu¨ser, L. New J. Chem. 1991, 15, 345. (17) Rosen, M. J. Surfactants and interfacial phenomena; J. Wiley & Sons: New York, 1989; see also references therein. (18) Blanzat, M.; Perez, E.; Rico-Lattes, I.; Prome, D.; Prome, J. C.; Lattes, A. Langmuir 1999, 15, 6163.

Figure 2. Transmission electron micrographs of aggregates formed from catanionic salts 3 in water: (a) spherical closed vesicles of 3e at 5 mM, (b) giant vesicle of 3e at 1.25 mM, (c) helices of 3d at 5 mM, (d) at a greater magnification of helices of 3d at 5 mM, (e) ribbons of 3f at 5 mM, and (f) at a greater magnification of ribbons of 3f at 5 mM.

0.03 mM) and have a high surfactant activity. These values are comparable with those for the bicatenary phosphinate having the same length of alkyl chain. The ammonium salts of compounds 1a-c and 2a-c are sufficiently water soluble to allow measurement of the critical micellar concentrations (cmc’s). We observed the absence of micellization for compounds 1a and 2a. The cmc’s of compounds 1b, 1c, and 2b were 40, 9.8, and 45 mM, respectively, in aqueous NH4OH solution at pH 10.10 We observed that catanionic phosphorous salts 3-5 lowered the cmc of phosphinic 1 or phosphonic 2 acids by a factor higher than 100. Moreover, the cac of catanionic salts 3-5 is considerably lower than those observed for similar phosphorus catanionics. For example, disodium dodecane-

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Table 2. Diameters of the Supramolecular Aggregates Determined by Dynamic Light Scattering and Formed by Catanionic Compounds 3-5 in Pure Water compounds

concentration (mM)

aggregate size range (nm)

3b 3c 3d 3e 3f 4c 4d 4ea 4fa 5d 5e 5f

5 5 5 5 5 2.5 2.5 2.5 2.5 2.5 2.5 2.5

30-430 10-20, 130-540 220-1800 337-1341 207-2600 280-780, 1700-3000 66-3600 47-4600 55-2400, 5000 60-100, 400-1000, 4000-6000 124-150, 1000-2000 221-330, 1300-1600

a

Aggregates prepared by the evaporation method.

phosphonate/dodecyltrimethylammonium hydroxide micellized at 17.7 mM. Study of Aggregates in Dilute Solutions. We prepared the aggregates of compounds 3-5 by simple dissolution in water for low concentrations (between 1 and 5 mM). Bicatenary phosphonates 4c and 4d were less soluble in water, and their aggregates were prepared by the evaporation method19 (see experimental part). Dynamic light scattering studies were carried out on surfactants 3-5 and are reported in Table 2. The major compounds exhibited spontaneous formation of aggregates. A broad distribution of aggregate-sized particles (20-2600 nm) was observed for the phosphinate catanionics 3 and (60-6000 nm) for the phosphonate catanionics 4-5. The aggregate morphologies of the surfactants were determined by transmission electron microscopy (TEM) for compounds 3 and by light microscopy for compounds 4-5 where the dominant population has a sufficient size. We observed a large variety of morphologies (vesicles, tubules, helices, and ribbons) for catanionic salts 3-5 and in particular for catanionic derivatives 4 and 5 that all formed the various types of aggregates. Moreover, the aggregates were giant. This broad range of structures was also found to occur in new sugar catanionic surfactants containing a fluorescent coumarinic moiety. 20 We show in Figures 2 and 3 the large variety of assembly morphologies. A population of vesicles with diameters generally ranging from 60 to 100 nm was observed for bicatenary phosphinate 3e (Figure 2a), with, in some cases, diameters of over 1000 nm (Figure 2b). Giant vesicles were observed for all compounds 4 and 5. We found giant vesicles with diameters ranging from 7 to 8 µm formed by tricatenary phosphonate 5d (Figure 3a) and from 3 to 12 µm for bicatenary phosphonate 4e (Figure 3b); helices with a diameter of 330 nm and a length of 7 µm (Figure 2c) were also observed for bicatenary phosphinate 3d, and at a greater magnification, we observed series of 130 nm in diameter (Figure 2d). We also observed ribbons (Figure 2e,f) with a width of 800 nm and a length of 8 µm for compounds 3f; giant tubules with a diameter of 4 µm were observed by optical microscopy for compounds 4d (Figure 3c). (19) (a) Bangham, A. D. J. Mol. Biol. 1965, 13, 238. (b) Bonaccio, S.; Walde, P.; Luisi, P. L. J. Phys. Chem. 1994, 98, 6661. (c) Walde, P.; Wick, R.; Fresta, M.; Mangone, A.; Luisi, P. L. J. Am. Chem. Soc. 1994, 116, 11649. (d) Sujatha, J.; Mishra, A. K. Langmuir 1998, 14, 2256. (e) Monnard, P. A.; Berclaz, N.; Conde-Frieboes, K.; Oberholzen, T. Langmuir 1999, 15, 7504. (20) Blanzat, M.; Massip, S.; Spe´ziale, V.; Perez, E.; Rico-Lattes, I. Langmuir 2001, 17, 3512.

Figure 3. Optical micrographs of aggregates formed from catanionic salts 4 and 5 in water at 2.5 mM: (a,b) spherical closed giant vesicles of 5d and 4e and (c) tubules of 4d.

The theory relating the structure of amphiphiles to the shapes of aggregates and presented by Israelachvili21 could not be applied directly in the cases of catanionic systems for which repartition of charges is not homogeneous at the interface of aggregates. Therefore, it involves modification in curvature of aggregates leading to the modification in the morphology and the properties of these systems.22,23 These various aggregate morphologies (especially ribbons, tubules, and helices) obtained for catanionic amphiphiles 3-5 are usually observed for chiral amphiphilic molecules. In contrast, in our case, the phosphinate and phosphonate anions are racemic. However, the presence of a stereogenic center in these anions could involve a certain chiral recognition in the interactions between the counterions and create chiral microdomains causing the formation of these aggregates (ribbons and tubules). Moreover, the formation of ribbons was recently highlighted for catanionic mixtures of sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide.24 Lyotropic Mesomorphic Behavior: Polarized Light Microscopy. Self-organization of compounds 3-5 in water was studied at higher concentrations by means of the phase penetration technique of Lawrence,25 in which pure surfactant (as catanionic salt) was brought into contact with water: the sample was prepared directly between a microscope slide and a coverslip, by placing a drop of water on a layer of catanionic salts 4-5, and was slowly heated. Thus, as the water penetrated the amphiphilic sample, mesophases developed around the anhydrous bulk of the sample with characteristic optical textures which can be identified using a polarizing microscope.25b,c This method allows the observation of all the successive phases in equilibrium with excess surfactant by increasing the temperature and enables observa(21) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (22) Dubois, M.; Deme´, B.; Gulik-Krzywicki, T.; Dedieu, J. C.; Vautrin, C.; De´sert, S.; Perez, E.; Zemb, T. Nature 2001, 14, 672. (23) Kaler, E. W.; Murthy, A. K.; Rodriguez, B. E.; Zasadzinski, J. A. Science 1989, 245, 1371. (24) Bergstro¨m, M.; Pedersen, J. S. Langmuir 1999, 15 (7), 2250. (25) (a) Lawrence, A. S. C. Liquid Crystals 2, Part 1; Brown, G. H., Ed.; Gordon and Breach: London, 1969; p 1. (b) Auvray, X.; Perche, T.; Petipas, C.; Anthore, R.; Marti, M. J.; Rico, I.; Lattes, A. Langmuir 1996, 8, 2671. (c) Van Doren, H. A.; Wingert, L. M. Recl. Trav. Chim. Pays-Bas 1994, 113, 260.

Catanionic Amphiphiles from Associative Systems

Figure 4. Patterns obtained from polarizing microscopy for some binary systems H2O/catanionic salts 3-5 (bar ) 100 µm): (a) lamellar mesophase, H2O/3e at 70 °C; (b) lamellar mesophase, H2O/5f at 50 °C; (c) lamellar mesophase, H2O/4c at 60 °C; (d) coexistence of lamellar and hexagonal mesophase, H2O/5d at 60 °C; (e) hexagonal mesophase, H2O/5d at 60 °C; (f) lamellar mesophase, H2O/5d at 40 °C.

tion of successive intermediate phases in a single experiment. As the slide and the coverslip were not previously treated, the samples are polydomains. Therefore, the junctions between the microdomains led to optical defects in polarizing microscopy. They are used to identify the type of mesophase. All catanionic amphiphiles 3-5 self-organized into mesophases in concentrated aqueous solutions at room temperature. Phase transitions were observed upon heating. We observed for all catanionic salts 3-5 lamellar lyotropic mesophases that were characterized by the presence of typical optical textures (Maltese crosses and oily streaks). These phase transitions were observed upon heating at 40, 50, or 60 °C up to 90 or 95 °C. Cooling from 95 °C, these phases persisted down to 30 °C. Figure 4 shows some examples of the textures observed for the bicatenary and tricatenary phosphorous catanionic salts 3-5. We observed oily streaks (Figure 4a,b) and Maltese crosses and oily streaks (Figure 4c). However, for tricatenary phosphonate 5b we observed the coexistence of lamellar and hexagonal phases. In fact, the same textures characteristic of a lamellar phase, Maltese crosses and oily streaks, were observed at 50 °C. Upon heating samples of 5b to 60 °C, we observed the coexistence of the two phases: a lamellar phase identified by Maltese crosses and oily streaks and a hexagonal phase identified by fanshaped patterns (Figure 4d). Cooling from 95 °C, the two phases coexist from 70 to 30 °C. Images e and f of Figure 4 show the two phases (hexagonal and lamellar) observed after cooling at 60 and 40 °C, respectively.

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Thermotropic Mesomorphic Behavior: Polarized Light Microscopy. Mesophases formed by a pure compound, in the absence of solvent and under the influence of temperature alone, are known as thermotropic mesophases. They are considered as intermediate states between the crystalline solid and the isotropic liquid. In fact, both principles, order and mobility, are combined in liquid-crystalline phases. Compounds able to form thermotropic mesophases are known as mesogenic molecules. The driving forces for the formation of mesophases are the anisotropic dispersion interactions caused by the anisotropy of the mesogens. Moreover, the simultaneous presence, in these molecules, of a rigid part (often a polyaromatic core) and of a flexible part (alkyl chains for example) is noted. The rigid part makes it possible to ensure the cohesion of the system, while the flexible part provides a certain fluidity to the mesophase.26 The samples were placed between a slide and a coverslip in the absence of solvent and were observed by optical microscopy with polarized light. The sample was heated to above the isotropic solid-liquid transition TSI of the sample (uniform homeotropic phase completely black) and then gradually cooled from the isotropic phase. The passage to the isotropic phase occurs at high temperature (182 °C < T < 196 °C for phosphinates 3, 210 °C < T < 240 °C for phosphonates 4 and 5). The various thermotropic mesophases were thus characterized by their optical textures. All catanionic salts 3-5 formed a liquid-crystalline thermotropic phase. This is a rare property for compounds without a rigid core. The optical textures observed on cooling from the isotropic liquid are typical for lamellar mesophases (Maltese crosses, oily streaks) and could correspond to smectic phases. These observations by optical microscopy in polarized light will require confirmation by X-ray diffraction for a more exact characterization of the nature of these mesophases. Figure 5 shows some of the textures observed for phosphorous catanionic amphiphiles 3-5. The isotropic-mesophase transition of the bicatenary phosphinate 3b starts at 152 °C with the formation of oily streaks (Figure 5a). Concerning bicatenary phosphinate 3c, bicatenary phosphonate 4f, and tricatenary phosphonate 5d, we observed conical defects after cooling: Maltese crosses and oily streaks accompanied by homeotropic zones (Figure 5b-d). These textures are characteristic of lamellar mesophases. Focal-conic fan textures, a common feature of lamellar phases and that could be smectic mesophases, appeared when samples of compounds 4e were cooled from the isotropic phase at 185 °C and remained stable down to room temperature (Figure 5e). These thermotropic mesophases formed by all the phosphorous catanionic amphiphiles have a broad domain of existence whose temperature range extends over several tens of degrees reaching down to room temperature. The thermotropic liquid-crystalline behavior of phosphorous catanionic amphiphiles 3-5 without rigid cores was rather unexpected. In the literature, we only found a few articles that describe the formation of thermotropic mesophases by phosphorous amphiphiles, of the phosphonium ion type.27 The smectic mesophases formed involve a specific packing arrangement of the ionic layers (26) (a) Goodby, J. W. Handbook of Liquid Crystals; Wiley-VCH: New York, 1998; Vol. 2A, Chapter I, pp 3-21 and references therein. (b) Goodby, J. W.; Mehl, G. H.; Saez, I. M.; Tuffin, R. P.; Mackenzie, G.; Auze´ly-Velty, R.; Benvegnu, T.; Plusquellec, D. Chem. Commun. 1998, 2057 and references therein. (27) (a) Kanazawa, A.; Tsutsumi, O.; Ikeda, T.; Nagase, Y. J. Am. Chem. Soc. 1997, 119, 7670 and references therein. (b) Abdallah, D. J.; Robertson, A.; Hsu, H. F.; Weiss, R. G. J. Am. Chem. Soc. 2000, 122, 3053. (c) Abdallah, D. J.; Weiss, R. G. Chem. Mater. 2000, 12, 406.

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aptitude to form strong ionic interactions between the polar heads: the phosphinates or phosphonates and the ammoniums. Moreover, the presence of the R-hydroxyl group favors intramolecular and intermolecular hydrogen bonding and contributes to the formation and stabilization of the thermotropic mesophases. Conclusion

Figure 5. Optical micrographs for some catanionic salts 3-5 observed after cooling from the isotropic liquid (bar ) 100 µm): (a) oily streaks, 3b at 151 °C; (b) Maltese crosses, 3c at 113 °C; Maltese crosses and oily streaks, 4f at 180 °C; (d) Maltese crosses, 5d at 99 °C; (e) focal-conic fan textures, 4e at 104 °C.

and hydrophobic domains. The greater ionic interaction between polar heads and counteranions rigidifies the layers and contributes to the formation of a smectic layer structure. The original behavior of phosphorous catanionic amphiphiles 3-5 leading to the formation of a thermotropic liquid-crystalline phase could be attributed to their

In this work, we present new phosphorous catanionic amphiphiles. These new bicatenary phosphinates, bicatenary phosphonates, and tricatenary phosphonates were easily prepared by acid-base reactions between phosphinic or phosphonic amphiphiles and cetyltrimethylammonium hydroxide. In aqueous media, they have weak critical aggregate concentrations (cac < 0.3 mM). Electron and optical microscopy observations of these surfactants in water showed a large variety in the morphologies of the assemblies occurring in a single sample: vesicles, helices, ribbons, and tubules. The presence of a stereogenic center in the phosphorous anions could involve a certain chiral recognition and seems to be at the origin of the bending force leading to the helices, ribbons, and tubules usually observed for chiral surfactants. At higher concentrations, the catanionic amphiphiles formed lamellar lyotropic phases. Moreover, thermotropic mesophases were observed for all the catanionic phosphorous amphiphiles. Rigidification by ionic interaction between polar heads could explain the unexpected behavior of these compounds without a rigid core. The thermotropic mesophases observed were lamellar and stable over a broad range of temperatures right down to room temperature. The selforganization of these catanionic salts enabled the preparation of a range of colloidal materials that have a high potential for various biological and technological applications. Acknowledgment. We thank L. Datas for assistance in transmission electron microscopy measurements and M. Mauzac for his useful comments on optical microscopy data of lyotropic and thermotropic mesophases. LA0344231