7184
Langmuir 2008, 24, 7184-7192
Dichroism in Dye-Doped Colloidal Liquid Crystals Nuttawisit Yasarawan and Jeroen S. van Duijneveldt* School of Chemistry, UniVersity of Bristol, Cantock’s Close, Bristol BS8 1TS, U.K. ReceiVed March 18, 2008. ReVised Manuscript ReceiVed May 1, 2008 Nematic liquid crystals were obtained in sterically stabilized suspensions of rodlike particles of sepiolite clay, with an average length up to 900 nm and aspect ratio up to 40. In agreement with computer simulations for hard spherocylinders, the isotropic-nematic transition shifted to lower volume fractions with increasing aspect ratio. However, the coexistence gap was broadened noticeably due to particle polydispersity. The sepiolite crystal structure includes channels filled with zeolitic water, which can be replaced by indigo dye molecules. The indigo molecules are constrained inside the zeolitic channels to be aligned along the long axes of the rods. As a result, the colloidal nematic phase showed a marked dichroism, with an order parameter up to 0.5 for magnetically aligned samples, similar to typical values for dye-doped thermotropic liquid crystals.
Introduction As was already predicted by Onsager,1 rodlike colloidal particles can form a nematic phase on increase of concentration. Many experimental systems are now known that exhibit this Onsager transition, including mineral, organic, and virus particles.2–16 Work on mineral liquid crystals was reviewed by Gabriel and Davidson.17 Recently, the first example of this transition using a natural clay, sepiolite, was reported.18 Sepiolite and palygorskite are clay minerals with rodlike particle shape, with zeolitic channels running along the particles. The present paper shows how dichroic colloidal liquid crystals can be obtained by dye doping of sepiolite rods. Previous studies have been carried out on dye molecules doped into various kinds of molecular sieves, in particular zeolites such as AlPO4-5 and MCM-41, with the aim of obtaining materials with interesting optical properties or pigments with a high resistance to thermal and chemical degradation. Various methods of dye loading were used, including absorption either from solution or by solid-state reaction, ion exchange processes, and * Corresponding author. Telephone: +44 (0)117 928 7665. Fax: +44 (0)117 925 0612. E-mail:
[email protected]. (1) Onsager, L. Ann. N.Y. Acad. Sci. 1949, 51, 627–59. (2) Bawden, F. C.; Pirie, N. W.; Bernal, J. D.; Fankuchen, I. Nature 1936, 138, 1051–52. (3) Buining, P. A.; Lekkerkerker, H. N. W. J. Phys. Chem. 1993, 97, 11510– 16. (4) Dong, X. M.; Kimura, T.; Revol, J. F.; Gray, D. G. Langmuir 1996, 12, 2076–82. (5) Dogic, Z.; Fraden, S. Phys. ReV. Lett. 1997, 78, 2417–20. (6) Purdy, K. R.; Fraden, S. Phys. ReV. E 2004, 70, 0617031–38. (7) Purdy, K. R.; Varga, S.; Galindo, A.; Jackson, G.; Fraden, S. Phys. ReV. Lett. 2005, 94, 0578011–14. (8) Li, L. S.; Marjanska, M.; Park, G. H. J.; Pines, A.; Alivisatos, A. P. J. Chem. Phys. 2004, 120, 1149–52. (9) Li, L. S.; Walda, J.; Manna, L.; Alivisatos, A. P. Nano Lett. 2002, 2, 557–60. (10) Lima, M. M. D.; Borsali, R. Macromol. Rapid Commun. 2004, 25, 771– 87. (11) Folda, T.; Hoffmann, H.; Chanzy, H.; Smith, P. Nature 1988, 333, 55–56. (12) Fraden, S.; Hurd, A. J.; Meyer, R. B.; Cahoon, M.; Caspar, D. L. D. J. Phys. (Paris) 1985, 46, 85–113. (13) Kajiwara, K.; Donkai, N.; Hiragi, Y.; Inagaki, H. Makromol. Chem. 1986, 187, 2883–93. (14) Davidson, P.; Garreau, A.; Livage, J. Liq. Cryst. 1994, 16, 905–10. (15) Pelletier, O.; Davidson, P.; Bourgaux, C.; Livage, J. Europhys. Lett. 1999, 48, 53–59. (16) Lemaire, B. J.; Davidson, P.; Petermann, D.; Panine, P.; Dozov, I.; Stoenescu, D.; Jolivet, J. P. Eur. Phys. J. E 2004, 13, 309–19. (17) Gabriel, J. C. P.; Davidson, P. AdV. Mater. 2000, 12, 9–20. (18) Zhang, Z. X.; van Duijneveldt, J. S. J. Chem. Phys. 2006, 124, 154910– 16.
Figure 1. Structure of indigo.
in situ dye synthesis inside nanopores.19–23 In certain cases, the dye molecules are orientationally ordered inside the pores (see for instance ref 21). In this paper it is shown how, starting from such a material, macroscopic orientational control over dye molecules can be obtained by alignment of the host particles into a colloidal liquid crystal. In ancient Maya civilization (8th-16th century A.D.), a blue pigment was used in artifacts that was remarkably resistant to chemical or physical degradation. It is known as Maya Blue, and artifacts retain a vivid color even today.24–28 The nature of this pigment has been the subject of speculation but it is now accepted that it consists of rodlike nanoparticles of natural palygorskite clay doped with indigo (Figure 1). This dye was originally extracted from the leaves of Indigofera suffruticosa.29 The pigment is resistant to treatment with strong acids, bases, and organic solvents. The origin of the high color stability of Maya Blue remains somewhat controversial, but it is widely claimed that the indigo enters the zeolitic channels, replacing zeolitic water, following heat treatment of indigo-clay mixtures to 120-190 °C for several hours24,30 (see however also the recent work by Chiari et al.27). (19) Shim, T. K.; Kim, D.; Lee, M. H.; Rhee, B. K.; Cheong, H. M.; Kim, H. S.; Yoon, K. B. J. Phys. Chem. B 2006, 110, 16874–78. (20) Kim, H. S.; Lee, S. M.; Ha, K.; Jung, C.; Lee, Y. J.; Chun, Y. S.; Kim, D.; Rhee, B. K.; Yoon, K. B. J. Am. Chem. Soc. 2004, 126, 673–82. (21) Bruhwiler, D.; Calzaferri, G. Micropor. Mesopor. Mat. 2004, 72, 1–23. (22) Calzaferri, G.; Huber, S.; Maas, H.; Minkowski, C. Angew. Chem. Int. Ed. 2003, 42, 3732–58. (23) Schulz-Ekloff, G.; Wohrle, D.; van Duffel, B.; Schoonheydt, R. A. Microporous Mesoporous Mater. 2002, 51, 91–138. (24) Van Olphen, H. Science 1966, 154, 645–46. (25) Jose´-Yacama´n, M.; Rendo´n, L.; Arenas, J.; Puche, M. C. S. Science 1996, 273, 223–25. (26) Del Rio, M. S.; Martinetto, P.; Reyes-Valerio, C.; Dooryhee, E.; Suarez, M. Archaeometry 2006, 48, 115–30. (27) Chiari, G.; Giustetto, R.; Druzik, J.; Doehne, E.; Ricchiardi, G. Appl. Phys. A-Mater. Sci. Process. 2008, 90, 3–7. (28) Littmann, E. R. Am. Antiq. 1980, 45, 87–100. (29) Chiari, G.; Giustetto, R.; Ricchiardi, G. Eur. J. Mineral. 2003, 15, 21–33. (30) Giustetto, R.; Xamena, F.; Ricchiardi, G.; Bordiga, S.; Damin, A.; Gobetto, R.; Chierotti, M. R. J. Phys. Chem. B 2005, 109, 19360–68.
10.1021/la800849y CCC: $40.75 2008 American Chemical Society Published on Web 06/14/2008
Dichroism in Dye-Doped Colloidal Liquid Crystals
Figure 2. Crystallographic structure of sepiolite and its channels with zeolitic water molecules inside (redrawn after Pecharroman et al.64).
The channels in palygorskite have dimensions 7.3 × 6.3 Å2, which is just sufficient to accommodate indigo, with a theoretical width of 5.0 Å.29,31 Sepiolite [Si12Mg8O30(OH)4(OH2)4 · 8H2O,32 see Figure 2] has a structure similar to palygorskite, with zeolitic channels that are somewhat wider at 3.6 × 10.6 Å2.33 It has been reported that sepiolite can be used to prepare a Maya Blue analog.24,34 Previously it was shown how sterically stabilized suspensions of sepiolite can be prepared, showing a phase behavior similar to that anticipated for hard rods.18 In the present work, it is shown how the same result can be achieved using sepiolite rods that have been doped with indigo dye. The narrow pores in sepiolite restrict the orientations of the dye molecules inside the pores. Furthermore, formation of the colloidal nematic phase offers a route toward a self-assembled, aligned state of these rods. This novel material is shown below to have a significant dichroism, with order parameter S ≈ 0.5 being observed for well-aligned samples.
Experimental Section (1) Preparation of Indigo-Doped Sepiolite Rods. Ten grams of sepiolite clay powder (Pangel S9, purified grade supplied by Tolsa) was added to 500 mL of toluene with stirring. After that the sepiolite suspension was ultrasonicated for 1 min (Ultrawave U500D, Ultrawave) and homogenized for 1 min using a high-shear mixer (T18 basic Ultra-Turrax, IKA) to give a slurry. Then 5 g of indigo granules (C16H10N2O2, Acros Organics) were ground and dissolved in 500 mL of toluene. The indigo solution was mixed with the clay slurry and slowly stirred overnight. Several previous studies used a dry mixing route in the preparation;24,30,31,34 however, this was found to be less effective at achieving a homogeneous color than the wet mixing method. The mixture was centrifuged at 10 000 rpm (∼17 000g) for 1 h using a centrifuge (Sorvall RC-5B, Sorvall) operating at 25 °C. The supernatant was noticeably lighter in color than the original indigo solution, suggesting that the majority of indigo was adsorbed onto the clay particles. The supernatant was removed and the sediment was heated in vacuum at 200 °C overnight to allow zeolitic water33,35,36 to be replaced by indigo. The dry indigo-clay mixture was crushed with a pestle and mortar. In order to remove the excess indigo adsorbed onto the outside of the clay particles, the indigo-clay powder was loaded into a cellulose thimble (31) del Rio, M. S.; Picquart, M.; Haro-Poniatowski, E.; Van Elslande, E.; Uc, V. H. J. Raman Spectrosc. 2006, 37, 1046–53. (32) Brauner, K.; Preisinger, A. Tschermaks Mineral. Petrog. Mitt. 1956, 6, 120–40. (33) Torro´-Palau, A.; Ferna´ndez-Garcı´a, J. C.; Orgile´s-Barcelo´, A. C.; PastorBlas, M. M.; Martı´n-Martı´nez, J. M. Int. J. Adhes. Adhes. 1997, 17, 111–19. (34) Ovarlez, S.; Chaze, A. M.; Giulieri, F.; Delamare, F. C. R. Chim. 2006, 9, 1243–48. (35) Kuang, W. X.; Facey, G. A.; Detellier, C.; Casal, B.; Serratosa, J. M.; Ruiz-Hitzky, E. Chem. Mater. 2003, 15, 4956–67. (36) Nagata, H.; Shimoda, S.; Sudo, T. Clay Clay Min. 1974, 22, 285–93.
Langmuir, Vol. 24, No. 14, 2008 7185 for Soxhlet extraction with hot chloroform for 72 h. The resulting blue clay powder was left in air until dry and crushed again. (2) Preparation of Suspensions of Indigo-Doped Sepiolite Rods. Ten grams of the indigo-clay powder was dispersed in 400 mL of toluene, ultrasonicated for 1 min, and then homogenized for 1 min using a high-shear mixer. Five grams of SAP-230TP, a synthetic poly(isobutylene)-based stabilizer (Infineum, UK; hereafter referred to as SAP), was dissolved in 100 mL of toluene. SAP has a polyalkylamine anchor group that binds to mineral surfaces. It has previously been used as a steric stabilizer to prepare nonaqueous suspensions of silica spheres,37,38 boehmite rods (γ-AlOOH),39 and gibbsite platelets (Al(OH)3).40 The SAP solution in toluene was mixed with the indigo-clay suspension and was left stirring overnight. The suspension was centrifuged at 3000 rpm (∼1500g) for 45 min. The sediment was redispersed in toluene and will be referred to as the “3K fraction”. The supernatant was centrifuged again at 10 000 rpm for 1 h; the sediment was redispersed in toluene and will be referred to as the “10K fraction”. In order to remove excess SAP, both 3K and 10K fractions were separately centrifuged at 10 000 rpm for 1 h. The supernatant toluene containing SAP was removed and the sediment redispersed in toluene. The particle concentrations were determined by drying known amounts of the suspensions. (3) Preparation of Suspensions of Undyed Sepiolite Rods. In order to prepare undyed sepiolite suspensions, 10 g of Pangel S9 clay powder was added to 1 L of 0.02 M cetyltrimethylammonium bromide (CTAB, C19H42BrN, Acros Organics) solution in deionized water (Purelab Ultra, Elga). The mixture was left stirring overnight and then centrifuged at 10 000 rpm for 1 h. The supernatant with nonadsorbed CTAB was removed and replaced with fresh water to redisperse the sediment. The suspension obtained was centrifuged at 10 000 rpm for 1 h and the supernatant removed once more. The sediment of CTAB-treated clay was dried at 100 °C in vacuum overnight, carefully ground, and added to 300 mL of toluene. The suspension of CTAB-treated clay in toluene was stirred for 4 h, ultrasonicated for 1 min, and homogenized using a high-shear mixer for 1 min. After that the same procedure of SAP treatment as used for the indigo-doped clay was applied. The SAP-treated clay suspension was centrifuged at 3000 rpm for 45 min. Only the supernatant was kept and centrifuged again at 10 000 rpm for 1 h and the sediment was redispersed in toluene (referred to as 10K fraction). Finally, excess SAP was removed using the same centrifugation process as used for indigo-doped clay. Previously, the combination of quaternary ammonium surfactant and polymer treatment was used in the preparation of Laponite and montmorillonite clay suspensions by Leach et al.41 In the present work, electron microscopy showed that the pretreatment of sepiolite rods with CTAB effectively dissociated the sepiolite clusters. Without this pretreatment, rod clusters were present after the SAP treatment. However, unlike the undyed rods, the surfactant treatment of the blue rods with CTAB was not necessary, as most of the blue rods in the specimen appeared well-separated and the amount of rod clusters was negligible. (4) Elemental Analysis. Both untreated sepiolite and indigodoped sepiolite powders were analyzed for their carbon content using an elemental analyzer (Carlo Erba EA1108, Carlo Erba Instruments) with an estimated accuracy of (0.3% of the measured mass, and an average over at least three measurements was taken. (5) Surface Area Measurement and Thermal Gravimetric Analysis. Nitrogen gas adsorption experiments were carried out at 77 K using a Quantachrome Autosorb-1 instrument on untreated sepiolite dried at 100 °C in vacuum overnight and on indigo-doped sepiolite. The measurement was made for several different pressures (37) Pathmamanoharan, C. Colloids Surf. 1988, 34, 81–88. (38) Smits, C.; Briels, W. J.; Dhont, J. K. G.; Lekkerkerker, H. N. W. Prog. Colloid Polym. Sci. 1989, 79, 287–292. (39) Buining, P. A.; Veldhuizen, Y. S. J.; Pathmamanoharan, C.; Lekkerkerker, H. N. W. Colloids Surf. 1992, 64, 47–55. (40) van der Kooij, F. M.; Lekkerkerker, H. N. W. J. Phys. Chem. B 1998, 102, 7829–32. (41) Leach, E. S. H.; Hopkinson, A.; Franklin, K.; van Duijneveldt, J. S. Langmuir 2005, 21, 3821–30.
7186 Langmuir, Vol. 24, No. 14, 2008 and the BET isotherm was employed to describe the adsorption. Thermogravimetric analysis of untreated sepiolite powder was performed on a thermogravimetric analyzer (Q500, TA Instruments) at a heating rate of 5 °C min-1, from 28 to 800 °C, under an atmosphere of prepurified nitrogen. (6) Transmission Electron Microscopy (TEM). TEM of undyed and dyed sepiolite rods was achieved using a transmission electron microscope (JEOL JEM 1200-EX, JEOL) operating at a 120 kV accelerating voltage. A drop of dilute suspension in toluene (less than 0.001 wt %) was pipetted onto carbon-coated copper grids and the solvent was allowed to evaporate. At least 200 particles from the TEM micrographs were counted to determine the particle size distributions. In order to account for the effect of the stabilizing SAP polymer layer, 8 nm was added to the length and diameter.38,40 (7) Light and Small-Angle X-ray Scattering. Light scattering measurements on dilute (0.01 wt %) suspensions of the indigodoped clay particles were made using a Malvern 4800 Autosizer (Malvern Instruments) equipped with an avalanche photodiode detector and a 532 nm laser source. Cylindrical glass cuvettes (540.110-QS, Hellma) filled with the suspensions were thermostated at 25 °C before starting the measurements. Toluene used as a solvent was filtered through a Whatman PTFE syringe filter with a pore size of 0.2 µm to remove dust. The time-averaged scattered intensity was measured as a function of scattering angle ranging from 30° to 130° with respect to the incident light path. The scattered intensities detected were plotted as a function of scattering wave vector (Q), defined by the relation 4πn/λ sin θ/2, where n, λ, and θ are the refractive index of the solvent, the laser wavelength in vacuum, and the scattering angle, respectively. Small-angle X-ray scattering (SAXS) measurements were made using a home-built setup at the H.H. Wills Physics Laboratory, University of Bristol. Nickel-filtered Cu KR radiation (λ ) 1.54 Å) was used and the scattering pattern was recorded using a multiwire area detector42 for Q ranging from 0.02 to 0.1 Å-1. The sample to detector distance was calibrated using a silver behenate standard.43 Data were taken on a 1 wt % indigo-doped sepiolite suspension (10K fraction), filled into a glass capillary with a 1 mm path length. The intensity scattered from pure toluene (solvent) was subtracted. Scattering from more dilute samples (0.1 wt %) was too weak to allow accurate background subtraction. The experimental light scattering and X-ray scattering intensity profiles were compared to the theoretical form factor for rodlike particles.44 (8) UV-Visible Spectrophotometry. UV-visible spectra of both pure indigo solutions and indigo-doped sepiolite rods in toluene were measured in 1 cm path length quartz cuvettes (21/Q/10, Starna), from 500 to 800 nm using a UV-visible spectrometer (HewlettPackard Agilent 8453E, Agilent Technologies). A toluene background was subtracted. (9) Macroscopic Observation of Isotropic-Nematic Phase Transition. Suspensions were filled by capillary action into rectangular capillaries with internal dimensions 0.02 cm × 0.4 cm × 10 cm (W3520-100, Vitrocom). Epoxy glue was employed to seal both ends of the capillaries after filling. The capillaries were left standing for 10 days to investigate their phase behavior. After that the samples were observed between crossed polarizers. In addition, each suspension was also filled into rectangular cells with internal dimensions 1 mm × 10 mm × 45 mm (1/G/1, Starna). The resulting phase behavior was compared to that observed in the capillaries. (10) Microscopic Observation of Phase Transition. The phase transition process in concentrated suspensions of indigo-doped sepiolite rods was followed using a polarizing microscope (Optiphot2, Nikon) equipped with polarizing filters and color video camera (JVC) linked to a personal computer. A 20 wt % indigo-doped sepiolite suspension was homogenized, filled into a capillary, and (42) Bateman, J. E.; Connolly, J. F.; Stephenson, R.; Flesher, A. C.; Bryant, C. J.; Lincoln, A. D.; Tucker, P. A.; Swanton, S. W. Nucl. Instrum. Methods Phys. Res., Sect. A 1987, 259, 506–20. (43) Huang, T. C.; Toraya, H.; Blanton, T. N.; Wu, Y. J. Appl. Crystallogr. 1993, 26, 180–84. (44) Richardson, R. M. Programme LineFit, Department of Physics, University of Bristol.
Yasarawan and Van DuijneVeldt observed in the polarizing microscope during phase separation. Polarizing micrographs of the suspension were taken immediately and then every 24 h until the nematic phase was completely formed. (11) Particle Alignment Experiments. Indigo-doped sepiolite suspensions of 5 and 8.5 wt % were filled into rectangular capillaries and placed in a 9 T magnetic field oriented along the capillaries for 1 week. After the samples were removed from the magnet, they were observed immediately between crossed polarizers as well as through a single polarizing filter with the plane of polarization either parallel or perpendicular to the capillaries, denoted as 0° and 90°, respectively. The observations were repeated 1 week later. The same samples were also imaged using a polarizing microscope. (12) Transmittance Measurement of Polarized Light. Light from an unpolarized HeNe laser (λ ) 632 nm) was passed through an angle-adjustable polarizing filter and then through a capillary containing the sample. The transmitted intensity was recorded using an amplified photodiode as a function of the orientation of the polarization plane. The transmittance (T) was determined following the relation: T ) Vs/Vb, where Vs and Vb are the voltages measured from the sample and a capillary filled with toluene, respectively. The absorbance (A) then followed as A ) -log T.
Results and Discussion (1) Elemental Analysis. Elemental analysis showed that the carbon content of the untreated sepiolite (bare rods) and indigodoped sepiolite (blue rods) after heat treatment was 0.4 and 33.0 wt %, respectively. The tiny amount of carbon detected in the bare rods is possibly due to a trace amount of carbonate minerals that is not completely removed.45 Following Ovarlez et al.,34 the blue rods were washed with tetrahydrofuran at room temperature. The indigo-treated sepiolite was dispersed in tetrahydrofuran and then the dispersion was centrifuged at 10 000 rpm for 1 h. The blue supernatant solvent was removed and replaced with fresh solvent. The sediment was redispersed and then the same centrifugation procedure was repeated until the supernatant was clear. This washing process only reduced the carbon content to 22.0 wt %. A more vigorous procedure was then employed, consisting of Soxhlet extraction of the blue rods with hot chloroform28 for 24, 48, and 72 h. This caused the carbon content in the blue rods to drop much further to 11.2, 3.7, and 3.3 wt %, respectively. Allowing for the innate carbon in the bare rods, the additional carbon content due to the presence of indigo is 2.9 wt % (equivalent to 4.0 wt % indigo). It was also found that further continuation of the extraction did not result in a further decrease of the carbon content. The indigo-extracted rods remained turquoise blue. Some of the indigo molecules that are extractable may be loosely adsorbed onto the external rod surfaces, whereas indigo molecules adsorbed into the rod channels may not be removed by the extraction. The heat treatment is essential in order for indigo to fill the sepiolite channels; if prepared without heat treatment, indigo can be extracted from resulting mixture more easily: after 72 h of extraction only less than 0.5 wt % of carbon remained in the rods. The TGA results discussed below also show that the heat treatment is necessary to displace zeolitic water. For comparison, the carbon content due to the indigo in the rod channels can be estimated on the basis of the assumption of zeolitic water replacement. According to the sepiolite crystal structure determined by Brauner and Preisinger,32 16 molecules of zeolitic water per unit cell can be removed when heating the clay at 200 °C, resulting in a unit cell mass reduction of sepiolite from 2584 to 2296 g mol-1. Assuming a three-cell superstructure ((a, b, 3c) in the Z-direction) as for palygorskite,29,30 the mass of the sepiolite superstructure will become 6888 g mol-1 after the departure of zeolitic water, and two molecules of indigo (45) Aznar, A. J.; Gutierrez, E.; Diaz, P.; Alvarez, A.; Poncelet, G. Microporous Mater. 1996, 6, 105–14.
Dichroism in Dye-Doped Colloidal Liquid Crystals
Langmuir, Vol. 24, No. 14, 2008 7187
Table 1. BET Surface Area Analysis from N2 Adsorption Isotherms sample undyed sepiolite dried undyed sepiolite dried indigo-doped sepiolite extraction indigo-doped sepiolite extraction for 72 h
BET surface area (m2g-1)
at 100 °C overnight at 200 °C overnight before Soxhlet
187 270 114
after Soxhlet
135
could fit into this structure, which corresponds to 384 g mol-1 of carbon. Therefore, the maximum carbon content due to indigo adsorbed into the sepiolite channels is expected to be 5.2 wt % (equivalent to 7.1 wt % of indigo). The experimental carbon content is somewhat lower at 3.3 wt %, suggesting that either the preparation does not completely fill all channels with indigo or that some of the indigo is extracted out of the channels. If indigo only adsorbed into grooves on the outside of the sepiolite particles, as was suggested recently for Maya Blue,27 a lower coverage up to 1.7 wt % indigo would be expected, in disagreement with the results obtained here. After treatment with SAP, the particle carbon content in the 10K fraction had increased to 14.2 wt %. Allowing for the presence of indigo, the increase of carbon content corresponds to a coverage of 0.16 g SAP per g of indigo treated rods. On the basis of the BET area reported below, this corresponds to a coverage of 1.2 mg/m.2 For comparison, the same stabilizer adsorbed onto boehmite rods gave a coverage of 1.0 mg/m2.39 Initially a preparation route was followed without a Soxhlet extraction step. This resulted in SAP- and indigo-treated rods that formed a sediment, showing no birefringence. This was interpreted as implying that the particles were not stabilized effectively, as well-stabilized rods tend to form a birefringent (nematic) sediment.3,18 Presumably the presence of excess indigo on the outside of the particles prevented an effective SAP coating from taking place. (2) BET Surface Area Analysis. As shown in Table 1, the BET surface areas of the undyed sepiolite dried at 100 and 200 °C overnight are 187 and 270 m2 g-1, respectively. The latter value is close to values reported elsewhere.35,46 The increase in the surface area after heating at 200 °C is ascribed to loss of zeolitic water, consistent with the TGA results discussed below. Heating sepiolite at 100 °C only partially removes the zeolitic water, and the BET area is correspondingly lower. The indigodoped sepiolite before Soxhlet extraction has a BET surface area of 114 m2 g-1. Similar to the effect of zeolitic water occupation, the decrease in the surface area compared to other samples is ascribed to the zeolitic channels now being filled with indigo. Equating the BET area in this case with the outer surface area of the clay rods, the rod diameter can be estimated as D ≈ 4/AspF, where Asp is the specific surface area of sepiolite (in m2 g-1) and F the sepiolite density (2.1 × 106 g m-3),47 giving D ≈ 16.7 nm, in reasonable agreement with the value obtained from TEM analysis shown below. After Soxhlet extraction of the indigodoped clay with hot chloroform for 72 h, the surface area was found to have increased again to 135 m2 g-1, which may reflect that some indigo was indeed extracted from the channels. (3) Thermogravimetric Analysis of Sepiolite. The weight loss as a function of temperature (TGA curve) and the corresponding differential weight loss with temperature (DTG curve) for a 5 mg bare sepiolite sample are given in Figure 3. (46) Knapp, C.; Gil-Llambias, F. J.; Gulppi-Cabra, M.; Avila, P.; Blanco, J. J. Mater. Chem. 1997, 7, 1641–45. (47) Balci, S. Clay Min. 1999, 34, 647–55.
Figure 3. TGA and DTG curves for bare sepiolite (heating rate ) 5 °C/min).
Figure 4. Transmission electron micrographs taken from three different types of sterically stabilized rod-like particles: (a) 10K fraction of indigodoped sepiolite, (b) 3K fraction of indigo-doped sepiolite, and (c) undyed sepiolite.
Four distinct stages of weight loss were observed and are attributed as follows. Increasing the temperature from room temperature to 140 °C resulted in a weight loss of 9.2%. This large weight loss possibly corresponds to the departure of water adsorbed onto the outer surfaces together with the zeolitic water from the channels. For comparison, the zeolitic water content is predicted to be 11.1% on the basis of the sepiolite composition derived by Brauner et al.32 The second and third stages, each with a 2.8% weight loss, were observed when sepiolite was heated further up to 290 and 620 °C, respectively (with loss peaks at 238 and 467 °C). Because the observed weight loss for these stages is almost the same, it may be inferred that the first and second molecules of the structural water, which are coordinated to the magnesium ions at the edge of the octahedral layers, are successively removed. The total weight loss due to the removal of the coordinated water also agrees well with the predicted value of 5.6%. The final stage observed when heating sepiolite beyond 620 °C corresponds to the loss of the octahedral hydroxyl groups that are attached to the inner magnesium ions. These results agree well with other thermal gravimetric studies of sepiolite.34,35 (4) Transmission Electron Microscopy and Particle Size Distributions. In Figure 4 are shown TEM micrographs taken from three different types of colloidal rod suspensions: (a) 10K and (b) 3K fractions of the indigo-doped sepiolite rods and (c) undyed sepiolite rods. The clay particles have a square cuboidal shape18 and it is therefore convenient to define the rods by their length (L) and diameter (D). The average values evaluated from the micrographs are reported in Table 2. In order to account for the thickness of the stabilizing SAP polymer layer, 8 nm was
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7188 Langmuir, Vol. 24, No. 14, 2008
Table 2. Particle Dimensions of Three Sepiolite Rod Batches Obtained from Electron Micrographs, Together with Corresponding Isotropic and Nematic Phase Boundaries
a
suspension
〈L*〉a (nm)
〈D*〉a (nm)
〈L*〉/〈D*〉
φI
φN
indigo-doped sepiolite (10K fraction) undyed sepiolite (SAP-treated) indigo-doped sepiolite (3K fraction)
340 ((37%) 660 ((40%) 860 ((40%)
21 ((17%) 22 ((23%) 21 ((19%)
16 30 41
0.030 0.021 0.012
0.191 0.123 0.104
Values were obtained by adding 8 nm (SAP polymer layer thickness) to the TEM result.
Figure 7. UV-visible absorption spectra of indigo solutions in toluene of concentration (a) 2.7 × 10-4 wt %, (b) 5.4 × 10-5 wt %, and (c) 1.1 × 10-5 wt %.
Figure 5. Length (a) and diameter (b) distributions determined from TEM micrograph analysis for three batches of sepiolite rods.
Figure 8. UV-visible absorption spectra in toluene of indigo-doped sepiolite suspension (solid line) and undyed sepiolite suspension (dashed line).
Figure 6. Static light scattering (circles) and small-angle X-ray scattering (squares) measured from 0.01 wt % indigo-doped sepiolite rods (340nm blue rods) in toluene. A theoretical form factor for rods with length 300 nm, diameter 20 nm, and length polydispersity 40% is shown (solid line).
added in order to obtain the effective length and diameter (denoted L* and D*).38,40 The particle diameters are all around 21 nm, whereas the rod length varies for the three batches. The resulting aspect ratios are 16 and 41 for the 10K and 3K fractions, respectively, whereas the undyed sepiolite has an intermediate aspect ratio of 30. The length and diameter distributions of the rods in the suspensions were also determined and are shown in Figure 5a,b. Given the similar diameter distributions, the rod suspensions are referred to using their average lengths below. (5) Light and Small-Angle X-ray Scattering. The 340 nm blue rods in toluene were characterized using light scattering and SAXS (Figure 6). The light-scattering data approach the I ∝ Q-1 behavior expected for rodlike objects, whereas the SAXS data satisfy the Porod law I ∝ Q-4, expected for objects with
smooth surfaces.48 For comparison, the theoretical form factor for rigid rods is shown49 with a length, diameter, and length polydispersity of 300 nm, 20 nm, and 40%, respectively. Given the particular dimensions of the particles studied here, these experiments do not allow refining the determination of the particle length and diameter. (6) UV-Visible Spectrophotometry. UV-vis absorption spectra of both indigo in solution and indigo-doped sepiolite suspension in toluene are shown in Figures 7 and 8, respectively. At the concentrations studied, the indigo-doped sepiolite spectra obeyed the Lambert-Beer’s law, and accordingly, the results have been presented in the form of an absorptivity (absorbance per concentration per path length). Dilute indigo solutions with concentrations up to 2.7 × 10-4 wt % (8.9 × 10-6 M) were used, and the suspensions had concentrations up to 0.16 wt % of the treated particles. The main feature in the indigo spectrum is the peak at 600 nm, which is ascribed to indigo monomers in solution. Already at the concentrations used here, an additional peak around 710 nm becomes visible, due to formation of indigo dimers. The same effect was studied in detail by Miliani et al. for indigo solutions in CHCl3.50 The resulting spectra appear very similar; indeed, at 298 K they found the monomer peak at 600 nm, and (48) Roe, R. J. Methods of X-ray and Neutron Scattering in Polymer Science; Oxford University Press: New York, 2000. (49) Berne, J. B.; Pecora, R. Dynamic Light Scattering ; John Wiley & Sons: New York, 1976. (50) Miliani, C.; Romani, A.; Favaro, G. Spectrochim. Acta Part 1998, 54, 581–88.
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Figure 9. Nematic phase formation for various concentrations of the indigo-doped sepiolite suspensions (860 nm blue rods) in rectangular cells: (a) 8 wt %, (b) 10 wt %, (c) 15 wt %, (d) 18 wt %, and (e) 25 wt %. Samples were observed between crossed polarizers (orientations indicated by arrows). The dark area near the bottom of samples c-e is due to a reflection.
a dimer peak appeared at 700 nm at a concentration of 7 × 10-6 M. Comparing with Figure 8, the indigo-doped sepiolite shows a shallow peak at 615 nm, which is ascribed to indigo monomers. The indigo confined in the zeolitic channels of sepiolite cannot associate in the same way as in solution; however, indigo molecules can approach closely in a head-to-tail fashion. This results in a red-shifted absorption,51 which is observed at 650 nm. Previously, an absorption band due to the presence of dehydroindigo around 440 nm was proposed by Domenech et al.52 for indigo-doped palygorskite; however, this feature is not observed in the present work. For comparison, data obtained using an undyed sepiolite suspension is included in Figure 8. No absorption band is observed; however, the (apparent) absorption decreases with increasing wavelength, as a result of light scattering by the particles. The peak absorptivity of indigo monomers in solution at 600 nm is about 254 (wt %)-1 cm-1, whereas the main absorption peak of the indigo-doped sepiolite at 650 nm is 2.30 (wt %)-1 cm-1. Given that the indigo content in these rods is about 4.5 wt % (see above), this amounts to a peak absorptivity per mass unit of indigo of 51 (wt %)-1 cm-1, about 20% of the peak absorptivity of indigo in solution. Recall, however, that the indigo molecules in the sepiolite are interacting and few free monomers appear to be present. A significantly weaker peak absorptivity for dimers was also noted in indigo solutions.50 (7) Macroscopic Observation of Isotropic-Nematic Phase Transition. When the indigo-doped sepiolite suspensions were freshly prepared, they were homogeneous liquids, showing a wide range of blue colors depending upon the concentration of the dyed rods. The dilute samples flowed easily when vibrating the sample cell, while the concentrated ones (more than 10 wt %) were more viscous. Flow-induced birefringence of the indigodoped sepiolite suspensions was not obvious between crossed polarizers because of their deep blue color, whereas dilute undyed sepiolite suspensions showed vivid birefringence when the sample (51) Kasha, M. Radiat. Res. 1963, 20, 55–70. (52) Domenech, A.; Domenech-Carbo, M. T.; Pascual, M. J. Phys. Chem. B 2006, 110, 6027–39.
Figure 10. Phase diagram showing the relationship between the relative volume of the nematic phase and the volume fraction of the three types of rods: 340-nm indigo-doped rods (triangles), 660-nm undyed rods (circles), and 860-nm indigo-doped rods (squares).
tube was shaken. After having been left for 10 days, concentrated blue rod suspensions (both 340 and 860 nm rods) in rectangular cells showed a phase separation. As seen in unpolarized light, each showed a relatively darker blue phase at the bottom and a lighter blue one on the top. Figure 9 shows suspensions of the 860 nm blue rods observed between crossed polarizers. A birefringent lower phase (nematic) is observed and the isotropic upper phase is black. The proportion of the nematic phase as a fraction of the total volume is shown in Figure 10 for the three sample batches studied. Results are plotted as a function of the volume fraction of rods (φ), calculated as
φ)
(mp/Fp) (mp/Fp) + (ms/Fs)
where mp, ms, Fp, and Fs are the mass of particles, mass of solvent, density of particles, and density of solvent, respectively. The trends for the three batches of rods are very similar and differ only in the positions of the isotropic-nematic (I-N) coexistence regions. As summarized in Table 2, the coexistence regions shift to lower volume fraction with increasing particle aspect ratio. Computer simulations of the isotropic-nematic phase boundary as a function of aspect ratio were carried out by Bolhuis and
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Figure 11. Phase coexistence data, expressed as density variable c (see the text) as a function of reciprocal aspect ratio, D/L. Previous data on colloidal rods3,18 are also included. Simulation data by Bolhuis and Frenkel53 are included for comparison as well as the Onsager limiting values,54 indicated by crosses. The solid line is to guide the eye. Circles and squares refer to the isotropic and nematic boundaries, respectively.
Frenkel for spherocylinders with a length L and diameter D.53 In order to compare the experimental phase boundaries with these predictions, the particle number density is evaluated using F ) φ/V0, where the volume of one particle V0 ) π[(LD2/4) + (D3/6)]. By plotting the reduced density variable c ) 1/4π(L*)2D*F against D*/L* (Figure 11), the data can be shown together with the Onsager limit, L/Df∞, where the isotropic and nematic phase boundaries take the values c ) 3.29 and 4.19, respectively.54 Note that for all experimental data the effective particle dimensions (allowing for the thickness of the stabilizing layer) are used. Computer simulations predict that the I-N transition should shift to lower rod volume fractions as the aspect ratio L/D increases; this translates into c values that are only weakly dependent on the aspect ratio.53 This trend is also observed in the experimental data. Compared to computer simulation data, the experimental phase boundaries are noticeably further apart (cI is lower and cN is higher). The same was observed in the previous studies,3 and the fractionation experiments in ref 18 suggest that the length polydispersity of the rods is likely to account for these observations. As in previous work, all three batches of rods had a relative length polydispersity around 40% (see Table 2). There is no evidence that the I-N transition is any different for the indigo-doped rods compared to the undyed ones. (8) Polarizing Microscopy Observation of the I-N Phase Transition Process. For samples contained in capillaries, the nematic layer took around 2 weeks to settle out, noticeably longer than for the 1 mm thick rectangular cells. However, the relative nematic volume in the capillaries was comparable to that in the rectangular cells. This may be related to the size of nematic domains becoming a significant fraction of the capillary thickness of 200 µm. The evolution of the nematic phase in the concentrated indigo-doped sepiolite rods in the capillary can be observed in the series of polarizing micrographs shown in Figure 12a-e. Initially the sample was isotropic. Within a day, tiny nematic droplets nucleated. These droplets grew, forming large nematic domains after 2 days. In the third day, these domains coalesced together. The final structure (shown after 10 days) shows the Schlieren texture characteristic of the nematic phase. (9) Magnetic Alignment of Nematic Phase. Suspensions of indigo-doped sepiolite suspension (860 nm rods) were filled into capillaries and initially showed no birefringence. A sample at 8.5 wt % was kept in a 9 T magnetic field for 1 week. The appearance of the sample immediately after removing it from (53) Bolhuis, P.; Frenkel, D. J. Chem. Phys. 1997, 106, 666–87. (54) Lekkerkerker, H. N. W.; Coulon, P.; Vanderhaegen, R.; Deblieck, R. J. Chem. Phys. 1984, 80, 3427–33.
Figure 12. Polarizing micrographs taken from 20 wt % 860-nm indigodoped sepiolite suspension loaded in a capillary (a) immediately after preparation and after (b) 1 day, (c) 2 days, (d) 3 days, and (e) 10 days.
the magnet is shown in Figure 13. Between crossed polarizers (panel a) the sample showed a vividly birefringent (nematic) lower phase. This nematic phase appeared free of defects, unlike the samples discussed above, where no magnetic field was applied. Sepiolite is known to have its slow axis (highest refractive index) along the particle length.55 Using a first-order (530 nm) retardation plate in the polarizing microscope, the colloidal rods were found to be aligned along the capillary length. This shows that the nematic director was aligned along the magnetic field (i.e., vertically in the images). The fact that the particles can be aligned in a magnetic field is in all likelihood due to anisotropy of the magnetic susceptibility.56 The isotropic-nematic phase separation was not quite complete in this sample, with some birefringent specks still being visible in the upper layer. Observations using polarized light show the aligned nematic phase was dichroic (panels b and c). It appeared blue in vertically polarized light and transparent in horizontally polarized light. This observation is discussed in detail below. Similar alignment observations were obtained for a 5 wt % sample. The same series of observations was repeated 1 week after the sample was removed from the magnetic field (Figure 13d-f). The nematic phase observed between crossed polarizers (panel d) now showed horizontal striations with a length of 1.7-2 mm and spacing of 0.25-0.3 mm, whereas these features were not obvious in the polarized light observations (panels e and f). Assuming the lower phase remained nematic, these observations imply that domains of nematic formed perpendicular to the capillary. Some nematic domains were still visible in the isotropic phase as before; however, comparison of panels e and f now showed that the (55) Kauffman, A. J. Am. Mineral. 1943, 28, 512–20. (56) Connolly, J.; van Duijneveldt, J. S.; Klein, S.; Pizzey, C.; Richardson, R. M. J. Phys.-Condens. Matter 2007, 19, 156103–19.
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Figure 14. Transmittance as a function of polarization angle with respect to the capillary axis for the nematic phase occurring in 5 and 8.5 wt % indigo-doped sepiolite suspensions aligned in a 9 T magnetic field for 1 week. The measurement was made both immediately after removing the magnetic field (solid squares ) 5 wt %; solid circles ) 8 wt %) and 1 week later (open squares ) 5 wt %; open circles ) 8 wt %).
16° with respect to the long molecular axis.57 Hence, the absorption will be close to a maximum when the polarization direction is parallel to the long axis of the indigo molecules. In the bulk indigo solutions and in isotropic blue rod suspensions, indigo molecules point in all directions and hence no dichroism is observed. However, in the (magnetically aligned) nematic phase, thanks to the orientational order of the rods, combined with the constrained orientations of indigo molecules inside their zeolitic channels, dichroism can be observed. In Figure 14, at both concentrations the transmittance showed its minimum and maximum at polarization angles of 0° and 90°, respectively, indicating that the average orientation of the transition dipole of the indigo molecules, and by implication the nematic director, were oriented along the capillary length (i.e., the magnetic field direction). Previously, the infrared absorption dichroism of aligned pyridine-treated sepiolite films had been proposed by RuizHitzky58 as evidence of molecular access of pyridine into the sepiolite channels. The transmission data allow evaluation of the order parameter (S) of the transition dipoles, using the relation59
S) Figure 13. Photographs of 8.5 wt % indigo-doped sepiolite suspension taken immediately after removing the magnetic field, (a) between crossed polarizers and through light polarized (b) parallel and (c) perpendicular to the capillary axis. The orientation of the polarizers is indicated by arrows. Panels d-f show the same sample 1 week after removing the magnetic field.
orientation of these domains had become randomized, whereas they were initially aligned in the magnetic field. Six months later, the appearance of this sample had changed little: the striations were still present, with the same spacing, and had grown slightly longer to 2.5-2.8 mm. (10) Dichroism of Magnetically Aligned Nematic Phase. In order to quantify the dichroism of the nematic phase, transmittance measurements were made using a HeNe laser (the wavelength of 632 nm being close to the peak absorption (see Figure 8). Figure 14 shows the transmittance as a function of the polarization angle of the incident light (with respect to the capillary axis) for the 5 and 8.5 wt % blue rod suspensions after alignment in the magnetic field for 1 week. Observation of dichroism in dye molecules such as indigo requires the transition dipole for the absorption process to be aligned. For a single (gas phase) molecule of indigo, the transition dipole is predicted to be around
A|| - A⊥ A|| + 2A⊥
(1)
where A| is the absorption for light polarized along the capillary axis and A⊥ is measured perpendicular to this axis. As shown in Table 3, immediately after removal from the magnetic field, the 5 and 8.5 wt % samples had S values of 0.51 and 0.42, respectively. One week after removing the magnetic field, the indigo alignment had deteriorated somewhat, with S values now being 0.46 and 0.41. Given the visual appearance of the 8.5 wt % sample in Figure 13d-f, this result is not surprising. For comparison, the same experiment was carried out on samples of the same concentrations that were not placed in a magnetic field. This produced a nematic phase that was not dichroic; the transmission was independent of the polarization state of the incident light. This shows that the alignment discussed above was indeed the result of the magnetic field, rather than other influences such as sedimentation. The angle-averaged absorbance (〈A〉) of 5 and 8.5 wt % blue rods without applied field was 1.0 and 1.3, respectively. These values compare well with the orientational averages Aiso ) (A| + 2A⊥)/3 for the fieldapplied samples, of 0.92 and 1.24, respectively. A number of effects are likely to contribute to the orientational disorder of the transition dipoles observed, with the experimental (57) van Faassen, M.; de Boeij, P. L. J. Chem. Phys. 2004, 120, 8353–63. (58) Ruiz-Hitzky, E. J. Mater. Chem. 2001, 11, 86–91. (59) Raj, D. Mater. Chem. Phys. 1996, 43, 204–11.
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Table 3. Order Parameter (S), Absorption Maximum (A|) and Minimum (A⊥), Isotropic Absorbance (Aiso), and Angle-Averaged Absorbance (〈A〉) Calculated from the Transmission Data of the Blue Rod Suspensions 5 wt %
8.5 wt %
conditions
A|
A⊥
Aiso
S
A|
A⊥
Aiso
S
without applied field immediately after removing field 1 week after removing field
1.85 1.88
0.45 0.53
〈A〉 ) 1.0 0.92 0.98
0.51 0.46
2.29 2.78
0.72 0.88
〈A〉 ) 1.3 1.24 1.51
0.42 0.41
values for S after 1 week outside the magnetic field lying in the range 0.4-0.5 (immediately after removing the sample from the magnet, slightly higher values of S were observed, which may in part be due to an enhanced nematic order with the field applied; a quantitative analysis of this effect, such as in ref 56 will not be attempted here). First of all, the nematic phase of the colloidal clay rods can be characterized by a nematic order parameter SN. For hard spherocylinders with a comparable aspect ratio L/D ) 40, the nematic order parameter for the nematic phase in coexistence with the isotropic phase was predicted to be SN ) 0.83. This value varied little with L/D in the range 10-50.53 A second contribution is that the transition dipole of indigo lies at an angle of about 16° with the long axis of the indigo molecule. If indigo molecules were perfectly aligned, but able to rotate freely around their axes, as is probably the case here if the rods can rotate about their long axes, this would give rise to an order parameter for the dipoles:60
Sdip ) (1/2)(3 cos2 θ - 1)
(2)
where θ is the angle between the transition dipole and the long axis of dye molecule, resulting in Sdip ) 0.89. Previously, the sepiolite particles were found to have a square cross section18 and local biaxial ordering of the rods, restricting particle rotations, is therefore not likely. Finally, the sepiolite channels have width of 10.6 Å,33 whereas the molecular dimensions of indigo are about 5 and 12 Å.29,31 The channel width would allow orientational disorder of the indigo molecules in the channels, with a maximum angle of 32°. If all indigo molecules assumed this maximum angle, this further source of disorder would give rise to an order parameter Schan ) 0.58. Assuming that the different contributions to disorder are all uncorrelated, the overall S can be estimated as their product, giving S ) 0.43, in agreement with the observed values.56,61,62 Perfect alignment of the indigo molecules along the channels (Schan ) 1) would result in S ) 0.74, much higher than the values observed, so the experiments suggest that this is not the preferred orientation of indigo inside the channels. It would be of interest to attempt similar experiments using palygorskite, as in the original Maya Blue pigment. Palygorskite has narrower zeolitic channels than sepiolite of 6.4 Å,29 and as a result, the maximum misalignment of indigo inside the channels would only be 7°, corresponding to Schan ) 0.98. So far no colloidal nematic phase has been demonstrated using palygorskite particles, however.
Conclusions Suspensions of well-dispersed sepiolite clay particles in toluene were obtained using a two-stage surface treatment with surfactant and polymeric stabilizer. As in previous work,18 these particles (60) Kelly, S. M. Flat Panel Displays: AdVanced Organic Materials; RSC Materials Monographs; The Royal Society of Chemistry: Cambridge, UK, 2000. (61) Weisstein, E. W. MathWorldsA Wolfram Web Resource. http:// mathworld.wolfram.com/SphericalHarmonicAdditionTheorem.html. (62) Warner, M. Mol. Phys. 1984, 52, 677–90.
showed an isotropic-nematic transition on increasing concentration. The phase behavior as a function of aspect ratio (up to L/D ) 40) followed predictions for hard spherocylinders, although the coexistence region was broadened noticeably due to the polydispersity of the rod lengths. By heating mixtures of sepiolite and indigo, zeolitic water in the mineral can be replaced by the dye molecules. Initially some of this indigo was on the outside of the particles, but this could be removed using Soxhlet extraction. The remaining amount of indigo was in reasonable agreement with estimates based on the crystal structure of sepiolite. The indigo doping did not affect the liquid crystal phase behavior of the colloidal rods. Due to their close confinement inside the zeolitic channels, the orientations of the indigo molecules were slaved to those of the clay rods. As a result, the colloidal nematic phase formed by dye-doped particles was found to be dichroic, with values for the order parameter around 0.4-0.5 observed for magnetically aligned samples. High order parameter dichroic dyes are necessary to achieve a good contrast ratio in guest-host LCD technology, where dyes are dissolved in a thermotropic liquid crystal. Typical values for the order parameter in such systems, determined at the peak absorption wavelength, are in the range 0.2 and 0.8.60 The novel route for orienting dye molecules presented here yields similar values. The present work only addresses indigo-doped samples, although many other species can be absorbed into sepiolite, and orientational ordering was also obtained using methylene blue.58 The novel type of material presented here may therefore be of interest for potential display device applications. The principle of controlling dye molecule orientation using colloidal particle manipulation may also offer opportunities for spectroscopic studies. Finally, if analogous particles with oriented fluorescent dyes could be developed,21 this would enable high resolution characterization of particle orientations using confocal imaging methods in solution; such an approach has already been employed to visualize the orientation of zeolite particles deposited onto patterned surfaces.63 Acknowledgment. N.Y. was supported through a Thai Government scholarship. Samples were kindly provided by Dr. Manuel Perez (Tolsa S.A.) and Dr. Peter Dowding (Infineum, UK). We gratefully acknowledge Prof. Rob Richardson for access to the SAXS instrument and for a critical reading of the manuscript. In addition we thank Mr. Les Corbin (BET surface area analysis), Mr. Desmond Davis (elemental analysis), Mr. Jean-Charles Eloi (TGA measurement), Prof. Julian Eastoe (access to the polarizing microscope), and Drs Karen Edler and David Fermin (zeolite references). LA800849Y (63) Bossart, O.; Calzaferri, G. Microporous Mesoporous Mater. 2008, 109, 392–97. (64) Pecharroman, C.; Esteban-Cubillo, A.; Montero, I.; Moya, J. S.; Aguilar, E.; Santaren, J.; Alvarez, A. J. Am. Ceram. Soc. 2006, 89, 3043–49.
