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Langmuir 2000, 16, 8712-8718
Lyotropic Phase Behavior of Triethylammoniodecyloxycyanobiphenyl Bromide and the Influence of Added Thermotropic Mesogens† G. S. Attard,‡ S. Fuller,§,| O. Howell,‡ and G. J. T. Tiddy*,§,| Department of Chemistry, University of Southampton, Highfield, Southampton SO17 1BJ, U.K., Department of Pure and Applied Chemistry, University of Salford, Salford, Greater Manchester M5 4WT, U.K., and Department of Chemical Engineering, UMIST, P.O. Box 88, Manchester M60 1QD, U.K. Received March 1, 2000. In Final Form: May 29, 2000 The lyotropic mesophases formed by the “amphitropic” surfactant triethylammoniodecyloxycyanobiphenyl bromide in water have been studied using optical microscopy, X-ray diffraction, and NMR. Only a lamellar mesophase occurs, but this has a wide temperature and composition range. There is a marked decrease in the estimated bilayer thickness (decrease in alkyl chain “ordering”, increase of micelle surface curvature) with an increase of temperature, particularly at the lower surfactant concentrations. This is likely to arise from the occurrence of a defect-full lamellar phase with water-filled holes rather than a conventional structure with continuous bilayers. The influence on the liquid crystals of added thermotropic mesogens (pentylcyanobiphenyl and heptyloxycyanobiphenyl) has been compared with that of a conventional alkane, hexane. The mesogenic additives stabilize the (defective) lamellar phase, while hexane destabilizes it. The phase behavior and the NMR and the X-ray data all suggest that in the mesophase there are significant anisotropic attractive interactions between the oxycyanobiphenyl moieties, which are increased when the mesogenic additives are present. Some pointers for future directions in the search for true amphitropic mesophases are given.
Introduction Recently, interest has grown in an unusual class of materials that can exhibit both lyotropic and thermotropic mesomorphism,1-4 the so-called amphitropic mesogens. The dual mesomorphism arises because of their molecular structure. Typically, amphitropic mesogens are comprised of a polar headgroup (e.g., quaternary ammonium, or oligoethylene glycol group) which is attached to a rodlike dipolar unit (e.g., cyanobiphenyl) via a methylene chain. A consequence of this hybrid molecular structure is that the micellar aggregation behavior is modulated by the anisotropic dispersion and packing interactions of the rodlike dipolar units. These systems are of considerable interest because they offer the possibility of combining surfactant mesomorphism with the mesomorphism of conventional thermotropic systems. The relative contributions of the various interactions that are responsible for aggregation and self-organization can be probed by a judicious choice of chemical structure and geometry. To determine the influence of the various interactions involved, we have undertaken a systematic study, both of mixtures of standard thermotropic and lyotropic mesogens, and of novel surfactants where the two parts are chemically joined together. We examined first3 a conventional surfactant (octaethylene glycol hexadecyl ether, C16EO8) † Part of the Special Issue “Colloid Science Matured, Four Colloid Scientists Turn 60 at the Millennium”. ‡ University of Southampton. § University of Salford. | UMIST.
(1) Kunitake, T. Angew. Chem., Int. Ed. Engl. 1992, 31, 709. (2) Navarro-Rodriguez, D.; Frere, Y.; Gramain, P.; Guillon, D.; Skoulios, A. Liq. Cryst. 1991, 9, 321. (3) Corcoran, J.; Fuller, S.; Rahman, R.; Shinde, N.; Tiddy, G. J. T.; Attard, G. S. J. Mater. Chem. 1992, 2, 695. (4) Fuller, S.; Hopwood, J.; Rahman, R.; Shinde, N.; Tiddy, G. J. T.; Attard, G. S.; Howell, O.; Sproston, S. Liq. Cryst. 1992, 12, 521.
with an added thermotropic mesogen pentylcyanobiphenyl (5-CB). Here there was some solubilization of 5-CB in the surfactant lamellar phase, but no unusual combined mesophases were formed. Very little of the surfactant dissolved in the thermotropic nematic phase. Then we examined compounds where the thermotropic mesogenic group, a cyanobiphenyl moiety, was chemically attached to the surfactant hydrophobic chain.4 There have been previous reports on these materials, with much work on a variety of compounds by Tschierske et al.,5,6 on diolcontaining mesogens,7-9 and on double-chain quaternary ammonium compounds.10 Everaars et al.10 report on a series of compounds containing a single quaternary ammonium ion connected to two alkyl chains with terminal cyanobiphenyl groups. However, surprisingly, compounds containing two quaternary ammonium groups (“gemini surfactants”) are more common.4,11-17 In a previous paper11 we compared the lyotropic behavior of a series of gemini surfactants having chain-terminal oxycyanobiphenyl (OCB) (5) Neumann, B.; Sauer, C.; Diele, S.; Tschierske, C. J. Mater. Chem. 1996, 6, 1087. (6) Lindner, N.; Ko¨lbel, M.; Sauer, C.; Diele, S.; Jokiranta, J.; Tschierske, C. J. Phys. Chem. B 1998, 102, 5261. (7) Lattermann, G.; Staufer, G. Liq. Cryst. 1989, 4, 347. (8) Brezesinski, G.; Ma¨dicke, A.; Tschierske, C.; Zaschke, H.; Kuschel, F. Mol. Cryst. Liq. Cryst. Lett. 1988, 5, 155. (9) Diele, S.; Ma¨dicke, A.; Geissler, E.; Meinel, K.; Demus, D.; Sackmann, H. Mol. Cryst. Liq. Cryst. 1989, 166, 131. (10) Everaars, M. D.; Marcelis, A. T. M.; Sudho¨lter, E. J. R. Langmuir 1993, 9, 1986. (11) Fuller, S.; Shinde, N.; Tiddy, G. J. T.; Attard, G. S.; Howell, O. Langmuir 1996, 12, 1117. (12) Zana, R.; Benrraou, M.; Rueff, R. Langmuir 1991, 7, 1072. (13) Zana, R.; Talmon, Y. Nature 1993, 362, 228. (14) Alami, E.; Levy, H.; Zana, R.; Skoulios, A. Langmuir 1993, 9, 940. (15) Alami, E.; Beinert, G.; Marie, P.; Zana, R. Langmuir 1993, 9, 1465. (16) Alami, E.; Levy, H.; Zana, R.; Weber, P.; Skoulios, A. Liq. Cryst. 1993, 13, 201. (17) Dreja, M.; Gramberg, S.; Tieke, B. Chem. Commun. 1998, 1371.
10.1021/la000296m CCC: $19.00 © 2000 American Chemical Society Published on Web 08/10/2000
Lyotropic Mesophases
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Figure 1. Structure of 10-(4′-cyano-4-biphenyloxy)decyltriethylammonium bromide (OCB-C10NEt3).
moieties with that of the normal gemini materials. The major influence of the attached OCB was to favor a lamellar phase over hexagonal (H1) and bicontinuous cubic (V1) phases. We also observed two coexisting lamellar phases (in the binary surfactant-water systems) which might have been an indication of unusual mesophase structures. This we now believe to be associated with specific counterion binding of bromide ions in gemini (and other) surfactants18 rather than any influence of the OCB. The two lamellar phases were not present with a singlechain surfactant having an attached OCB unit.4 Here we present further studies on this single-chain “amphitropic” surfactant, 10-(4′-cyano-4-biphenyloxy)decyltriethylammonium bromide (OCB-C10NEt3, Figure 1) which avoids the complications of the two lamellar phases. Note that the effect on the mesophase structures of using a triethylammonium headgroup, rather than the usual trimethylammonium headgroup, has been established to be small.4,19,20 We are interested to investigate why the lamellar mesophase is so unstable with temperaturesthe lowest surfactant concentration at which it occurs changes from ca. 30 wt % at 40 °C to 60 wt % at 100 °C. (For the gemini surfactant the mesophase is even less stable.4) Is this a consequence of additional “amphitropic” order within the lamellar phase that is destroyed by temperature? Also, what is the effect of added thermotropic mesogens? We have studied the influence of added 5-CB and heptyloxycyanobiphenyl (7OCB), wellknown thermotropic mesogens, with mesophases at ambient temperatures. Both are expected to be solubilized in the bilayer interior where they could modify the structure. This behavior is compared to the changes induced by added hexane, which is also expected to be solubilized in the micelle interior. However, hexane is expected to promote the formation of a conventional micellar solution rather than mesophases. First, we report a fuller description of the surfactant/water phase behavior in the binary system than that given previously.4 Then we describe nuclear magnetic resonance (NMR) and X-ray diffraction measurements that help to elucidate the detailed structure of the lamellar phase. Finally the influence of the solutes on the aggregate structures is outlined. The results allow some suggestions to be made as to the structural requirements that might be necessary to obtain true amphitropic mesophasessi.e., mesophases which exhibit lyotropic and thermotropic order simultaneously! Experimental Section Materials. 10-(4′-Cyano-4-biphenyloxy)decyltriethylammonium bromide (OCB-C10NEt3) was synthesized as reported previously.4 4-Hydroxy-4′-cyanobiphenyl was coupled with R,ωdibromodecane to give R-bromodecyl-ω-4,4′-cyanobiphenyl, which was reacted with excess triethylamine to give OCB-C10NEt3. The purity was established by a single spot on thin-layer chromatography (TLC) and 1H NMR at 300 MHz. TLC was conducted on normal phase silica TLC plates that were soaked (18) Fuller S.; Tiddy, G. J. T.; Zana R. Unpublished results. (19) Fuller, S.; Attard G. S.; Tiddy, G. J. T.; Howell O. Manuscript in preparation. (20) Blackmore, E. S.; Tiddy, G. J. T.; J. Chem. Soc., Faraday Trans. 2 1988 84, 1115.
Figure 2. Phase diagram of OCB-C10NEt3 (wt %) with water (2H2O): isotropic solutions, L1, L2; lamellar phase, LR. The boundaries of the L2 “concentrated surfactant liquid” phase are dotted to indicate that this region was seen only in penetration scans. for 5 min in a 6 wt % solution of sodium bromide in methanol and then allowed to dry in air. The eluting mixture that gave the best resolution between the product and impurities consisted of methanol/water in the ratio (by volume) 1:9. The plates were viewed under a UV lamp and then developed using iodine vapor in order to visualize any non-UV absorbing species. Final purity to single spot on TLC was achieved by repeated recrystallization from dry ethanol with the addition of toluene. Water was deionized and double distilled. Heavy water (>99.7%) was obtained from Aldrich. Hexane (specially pure), 5CB, and 7OCB were obtained from BDH (now Merck) and used as received. Methods. Bulk samples were prepared by weighing the constituents into glass tubes that were sealed, and the contents were mixed by heating and centrifugation. Microscopy was carried out using a Carl Zeiss Jenaval polarizing microscope fitted with a Linkam THM600 hot stage and TMS 90 temperature control unit (accuracy (0.5 °C). X-ray diffraction measurements were made at the CLRC (then EPSRC) Daresbury laboratories on stations 7.2 for wide-angle scattering (60 Å). Calibration was carried out using standard samples provided (silica, graphite, sodium dodecyl sulfate for 7.2, rat-tail collagen for 8.2). NMR measurements (2H) were made using a Bruker AC 300 MHz spectrometer and variable-temperature probe operating at 46.07 MHz. The pulse sequence employed was that normally employed for highresolution spectra. Samples (nonspinning) were sealed in 5-7 mm tubes and stored above the Krafft boundary for days-weeks prior to measurement. Differential scanning calorimetry (DSC) studies were made using a Mettler TA 3000 thermal analysis system.
The Binary OCB-C10NEt3/Water System: Results Phase Diagram. The phase diagram of OCB-C10NEt3/ (Figure 2) was constructed by heating and cooling macroscopic samples in a water bath while observing them through crossed polaroid plates and by using optical microscopy with the Linkam hot stage where the samples were sealed to prevent water loss. To a lesser extent DSC was also used, particularly to verify the temperature at which the solid surfactant fully dissolved to form either micellar or lamellar phase (the Krafft boundary). In the absence of a solvent OCB-C10NEt3 forms no thermotropic mesophases and melts to an isotropic liquid at 110 °C. A metastable isotropic glass is formed on cooling to 48 °C. This (metastable) glass formation has been observed for a number of monoalkyl cationic surfactants 2H O 2
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with an OCB terminal group and also for similar “gemini” surfactants.19 In the latter case the glassy state can also be a thermotropic mesophase.19 The glass rapidly reverts to a crystalline solid plus solution or lyotropic mesophase when contacted with water; hence this phenomenon is not discussed further here. The only lyotropic mesophase observed is a lamellar phase, clearly identified from its optical texture and low viscosity. This phase exists from 22 wt % OCB-C10NEt3 to over 90 wt %. The penetration temperature of the lamellar phase (i.e., the lowest temperature at which it is seen) is 33.3 °C, and the Krafft boundary increases only slowly with concentration from 33.3 °C at 22 wt % OCB-C10NEt3 to 45 °C at 83 wt % before increasing rapidly. When viewed through crossed polaroid plates, the solid appears dark because light is scattered or reflected rather than being transmitted. The transition to a single lamellar phase is accompanied by the appearance of birefringence that is clearly observed through crossed polarizers. There is a rapid increase of the LR f L1 transition temperature with surfactant concentration: From 33 °C at 22 wt % until at ca. 60 wt % OCB-C10NEt3 the lamellar phase is still present at 100 °C. Between 40 and 47 wt % there appears to be a slight “knee” in the LR f L1 boundary where the phase transition becomes less dependent on composition. Repeated samples on different batches of surfactant gave identical results, showing that this is a real feature. Penetration scans,20 where water is placed in contact with the solid surfactant to set up a concentration gradient, were also employed to study the phase behavior. These clearly show that an isotropic L2 phase exists at high surfactant concentration. When the temperature reaches 77 °C, a narrow band of an isotropic, low-viscosity L2 phase is formed between the lamellar and solid phases. As the temperature is increased further, this band moves inward (i.e., to a higher surfactant concentration) and more of the solid “melts” until eventually at 110 °C only the L2 phase remains. [Note that the label L2 is used only to distinguish a second isotropic liquid phase. It does not imply any indication of the aggregate structure (e.g., reversed micelles) in this liquid phase.] No bulk samples were prepared in this region because we wished to conserve the limited quantities of the surfactant, and the mesophase was the major interest. Hence the boundaries of this region are very approximate and are shown as dotted lines. Also, the boundary between solid surfactant and the lamellar phase was determined with slow heating scans. It is possible that the temperature shown could be a degree or two too high, particularly at the higher concentrations, because of the slow dissolution of the solid. X-ray Diffraction Studies. A series of samples prepared with (normal) water were examined, mainly at two temperatures, 40 and 60 °C (well above the Krafft boundary), to investigate the effect of surfactant concentration on the lamellar phase dimensions (Tables 1 and 2 and Figure 3). Particularly at low concentrations and higher temperatures, only 1 order of reflection was seen (only do), although the samples were prepared and stored at temperatures above the Krafft boundary using conditions where multiple reflections are commonly observed with other systems. This might be a consequence of the unusual electron density of the nonpolar region, since the intensities of diffraction lines are related to the variation of the electron density through the unit cell, or because neighboring layers are only loosely ordered. The measurements give the average bilayer repeat distance (do), from which the thickness of the hydrocarbon region (dhc), the thickness of the aqueous region (daq), and the area per
Attard et al.
Figure 3. Values of do and dhc as a function of surfactant/ water mole ratio of OCB-C10NEt3/H2O in the lamellar phase at 313K (40 °C). Table 1. Dimensions of the Lamellar Phase for the OCB-C10NEt3/H2O System at 40 °C from X-ray Measurementsa wt % OCB-C10NEt3
mol ratio surfactant/water
do/Å
dhc/Å
a/Å2
28 42 47 50 56 58 63
0.0151 0.0281 0.0344 0.0387 0.0493 0.0535 0.0660
95.8 83.1 68.7 66.6 61.0 58.4 54.2
18.8 24.2 22.3 21.6 22.1 23.2 23.4
59.1 45.8 49.7 51.4 50.1 47.7 47.5
a Accuracy of d estimated to be (2 Å for values of ca. 100 Å, o falling to
