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Chromonic Liquid Crystalline Phases of Pinacyanol Acetate: Characterization and Use as Templates for the Preparation of Mesoporous Silica Nanofibers Carlos Rodríguez-Abreu,*,†,‡,^ Carolina Aubery Torres,† and Gordon J. T. Tiddy§ †
Instituto de Química Avanzada de Catalu~ na, Consejo Superior de Investigaciones Científicas (IQAC-CSIC), Jordi Girona 18-26, 08034 Barcelona, Spain ‡ International Iberian Nanotechnology Laboratory (INL), Av. Mestre Jose Veiga, 4715-310 Braga, Portugal § School of Chemical Engineering & Analytical Science, University of Manchester, PO Box 88, Manchester M60 1QD, U.K.
bS Supporting Information ABSTRACT: We report on the self-aggregation of the cationic dye pinacyanol acetate and its use for the preparation of nanostructured silica via templated sol-gel reaction. The dye forms nematic and hexagonal chromonic liquid crystals at low concentrations in water (i.e., from 0.75 wt %); the type of counterion appears to play an important role in liquid crystal formation. From analysis of small X-ray scattering (SAXS) curves, it is inferred that dye aggregates have the morphology of hollow long tubes with one-molecule-thick walls; the diameter of the tubes does not to change much with concentration. The dye aggregates can be aligned by shear or by a magnetic field. The high-resolution 1H NMR spectra show that aggregation takes place over a range of concentrations rather than having a sharp “critical” aggregation. Within the aggregates the conjugated moiety, including the three-carbon link, is in close proximity to the aromatic groups of stack neighbors. On the other hand, dye aggregates direct the formation of silica nanofibers synthesized via solgel reaction, mimicking the elongated structures found in aqueous media. The nanofibers show a hierarchical organization; i.e., they contain hexagonal arrays of 3 nm cylindrical mesopores left after calcination of the templating molecules, and the pore walls are 2.7 nm thick. As the nanofibers form entangled networks, the obtained materials also show interparticle porosity. The present findings open new possibilities for the use of commercial cationic dyes in the synthesis of nanostructured materials.
1. INTRODUCTION When certain compounds with disklike or planar molecular shapes are dispersed in water, they show a strong tendency to aggregate into stacks, which may become ordered at higher concentrations, forming the so-called chromonic liquid crystals.1-3 The driving force for aggregation in this case is thought to be mainly enthalpic (which is different from conventional surfactants), resulting from strong attractive dispersion forces between aromatic groups and also, in some cases, from unlike charge attractions between ionic groups on the periphery. The relationship between the structure of the chromonic molecules and the bulk phase behavior is still obscure. Seemingly small differences in structure can have large effects on the phase diagram. Chromonic liquid crystals occur widely in aqueous dispersions of many formulated products such as pharmaceuticals4 and the dyes used for fabrics or inkjet printing,5 but they are usually not recognized as such. Several of the chromonic mesophases have structures not found for surfactants (e.g., hollow pipes).6 Moreover, these liquid crystals occur with nucleic acids and hence are r 2011 American Chemical Society
of fundamental importance for biological systems, and the staining and interaction of dyes and drugs with nucleic acids have been linked to their common aqueous phase behavior. Chromonic liquid crystals have also been used in material science for fabricating highly ordered thin films7 and anisotropic carbon nanomaterials.8,9 One advantage of chromonic liquid crystals is that they may form at relatively low concentration and can be aligned by external stimuli such as light,10 magnetic fields,6 and shear,11 which is important not only for structural control but also for the possible manipulation of some properties such as viscosity. Control of the orientational order is important for applications such as polarizers, alignment layers, optical compensators, retarders, filters, etc.3 On the other hand, self-assembly of templating molecules is one of the most widely used methods for the synthesis of Received: December 2, 2010 Revised: January 12, 2011 Published: February 11, 2011 3067
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Langmuir nanomaterials, since it is spontaneous and the associated energy cost is minimum.12,13 Among nanomaterials, mesoporous solids (with pore size between 2 and 50 nm) have attracted much interest due to their applications in catalysis, adsorption, encapsulation, photonics, separation, etc. Mesoporous materials with a high degree of ordering within their structure can also be used in turn as rigid and stable templates for the fabrication of other nanomaterials such as mesoporous carbons, nanowires, and nanotubes. There is an extensive literature on the synthesis of mesoporous solids using cooperative self-assembly between amphiphilic compounds and inorganic precursors. However, there is always a search for processes that combine both low consumption of templating molecules (for reduction of costs and environmental impact) and high quality of the resulting materials in terms of structure and stability. Accordingly, chromonic molecules are promising candidates due to their capacity to form aggregates at very low concentrations and with a high degree of ordering. To our knowledge, there is no report in the literature on the preparation of mesoporous solids using chromonic molecules as templates; therefore, this subject is novel and relevant. In this context, we present a report, to our knowledge the first of this kind, on the use of cationic chromonic liquid crystals as templates for the preparation of nanostructured silica fibers.
2. EXPERIMENTAL SECTION 2.1. Materials. Pinacyanol chloride (97%, Acros organics, Belgium), a cationic dye, was used in the experiments. The corresponding acetic salt (pinacyanol acetate, see Figure 1) was prepared by adding silver acetate to a solution of pinacyanol chloride in ethanol. After filtration of precipitated silver chloride, the solvent was evaporated in a Petri dish to obtain the solid pinacyanol acetate. The cation exchange was complete, as confirmed by elemental analysis. Tetraethyl orthosilicate (TEOS) was obtained from Sigma-Aldrich. Ultrapure water (resistivity =18.2 MΩ/cm) was used in all the experiments. 2.2. Methods. 2.2.1. Preparation of Pinacyanol Acetate Samples in Water. Mixing of viscous samples at high pinacyanol acetate concentrations was accomplished by repeated centrifugation through the narrow constrictions made in flame-sealed test tubes. Samples with very low concentration were prepared by dilution from more concentrated samples. 2.2.2. Synthesis of Silica. The procedure is similar to one reported in the literature.14 First, a solution of pinacyanol acetate in aqueous ammonia (25%) was prepared. TEOS, the silica precursor, was added to the solution, and the mixture was stirred for 90 min at 25 C and then for 90 min at 70 C. The precipitate obtained from the sol-gel reaction was recovered by filtration and dried. The resulting powder was calcined in air at 600 C for 6 h (heating rate =1 C/min). 2.2.3. Characterization 2.2.3.1. Microscopy. Polarized optical microscopy (POM) was performed with a Leica Reichert Polyvar 2. Scanning electron microscopy (SEM) images were taken with Hitachi TM-1000 and H-4100FE instruments at 15 kV. Transmission electron microscopy (TEM) images were collected with a JEOL JEM 1010 microscope at 80 kV. 2.2.3.2. Small-Angle X-ray Scattering (SAXS). The in-house instrument used consists of a SAXSess camera (Anton Paar, Austria) attached to a PW3830 sealed-tube anode X-ray generator (PANalytical). The monochromatic X-ray beam (λ = 0.1542 nm) was focused with a G€ obel mirror and a block (line) collimator. Samples were placed in a thermostated holder (TCS 120, Anton Paar). SAXS measurements were also carried out in the high-flux SAXS beamline at Elettra Laboratory (Trieste, Italy). The beamline optics
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Figure 1. Molecular structure of pinacyanol acetate. consists of a flat, asymmetric-cut double-crystal monochromator and a double-focusing toroidal mirror for point collimation. Bragg spacings were calculated from d ¼ 2π=q ð1Þ
where q is the scattering vector. For an array of cylinders in a hexagonal lattice, the cylinder diameter can be calculated from 2φ 1=2 ð2Þ D ¼ 2d pffiffiffi π 3 where φ is the volume fraction of cylinders (i.e., aggregates). 2.2.3.3. Nitrogen Sorption. Nitrogen sorption isotherms for porosimetry were determined at 77 K with a Micromeritics Tristar-3000 instrument. Samples were degassed and weighed prior to the measurement. The specific surface area was determined by applying the multipoint BET (Brunauer-Emmett-Teller) model.15 The pore size distribution was determined by the BJH (Barret-Joyner-Halenda) method.16 2.2.3.4. Nuclear Magnetic Resonance (NMR). For 2H NMR spectroscopy, samples were placed in NMR tubes with a diameter of 5 mm. Measurements were recorded at 25 C using a Bruker DMX-500 NMR spectrometer (Serveis Cientifico Tecnics, Universitat de Barcelona). 1H NMR spectra in D2O (4.8 ppm as a reference) were collected at 25 C on the DPX 400 MHz Bruker instrument of the Manchester Interdisciplinary Biocenter equipped with an autosampler. The samples were placed in NMR tubes with a diameter of 5 mm. The scan number (NS) of the acquisitions was varied from 100 until 2000 depending on sample concentration.
3. RESULTS AND DISCUSSION 3.1. Self-Aggregation of Pinacyanol Acetate. Pinacyanol chloride is a lower cost homologue to pseudoisocyanine chloride (PIC); the only difference between the two compounds is the length of the alkene bridge between the two quinaldine moieties, which is larger for pinacyanol chloride. PIC is known to form nematic and hexagonal aqueous chromonic liquid crystals.17,18 However, pinacyanol chloride is not sufficiently soluble in water to form liquid crystals; beyond a certain concentration, solid crystals precipitate. In order to increase the solubility in water, we exchanged the chloride counterion with acetate. It was found that at low concentrations in water (i.e., from 0.75 wt %) pinacyanol acetate forms a birefringent nematic N phase with characteristic schlieren optical textures, as seen in Figure 2. The dye nematic phases showed viscoelastic behavior, with shear thinning and banding (see Supporting Information, Figure S1), due to strong alignment of elongated aggregates. At concentrations of 4 wt % or higher, the samples were significantly more viscous and showed a grainy optical texture usually assigned to the M (hexagonal) phase. We also observed the formation of liquid crystals on contact of solid pinacyanol acetate with water. As the phases developed, there was a continuity of optical textures between N and M phases, suggesting some structural similarity. It is to be noted here that threadlike structures form when pinacyanol chloride is put in contact with aqueous acetic acid, as 3068
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Figure 2. POM textures (25 C) of pinacyanol acetate samples at different concentrations in water: (a) 1, (b) 2, (c) 6, and (d) 10 wt %.
observed under the optical microscope (see Supporting Information, Figure S2). SAXS spectra of selected samples measured with a line collimated instrument are shown in Figure 3. Several deep intensity minima are observed, suggesting very long aggregates with low polydispersity in shape and size.19 SAXS data from a point collimated instrument showed the same features (see Supporting Information, Figure S3). The positions of the minima, which do not change with concentration, are related to the cross-section dimension of scattering objects. Scattering data with well-defined minima have been also found in other planar molecules such as porphyrins.20 Several models were tested to fit the SAXS data;18,21-23 the one showing the best fit was that of monodisperse, infinitely long tubes with three layers in their walls, the innermost and outermost having the same electron density difference with respect to the solvent (water) and corresponding to the counterion (acetate) layers. Figure 4 shows the fitting of SAXS data (2 wt % pinacyanol sample) to this multiwall model; deviations could be attributed to the contribution of the structure factor to the scattering intensity as well as some polydispersity. Application of the generalized Fourier transform (GIFT) method24 to the data also gave evidence of a core-shell structure. We estimated from the curve fitting a tube diameter of 4.6 nm; we fixed a middle wall thickness of 0.3 nm, corresponding to the size of a benzene ring in the quinaldine moiety of the pinacyanol molecule. The multiple wall model is equivalent to the hollowchimney structure formed by molecular stacks proposed for anionic cyanine dyes by one of the authors6 (see Figure 4, inset), if we assume inner and outer charged (acetate) layers surrounding the stacked molecules with a distinguishable electron density difference; according to the our calculations, the thickness of those layers would be rather small. Taking into account that the pinacyanol molecular length is about 1.7 nm, each stacked rings would consist of around 8 molecules. A difference between the
Figure 3. SAXS profiles (25 C) at different concentrations from a line collimated instrument. The arrows indicate the relative positions of reflections (with respect to the first peak) for a hexagonal lattice.
Figure 4. Fitting of point-collimated SAXS data (2 wt % pinacyanol sample, 25 C) to a model of monodisperse multiwall tubes (see the text for details). The inset shows a sketch of the proposed hollow structure, similar to that reported in refs 2 and 6. 3069
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Langmuir inner and outer charged layers is highly likely because otherwise the charge density of the inner region would be very high due to the large fraction of space occupied by counterions. Hence, the concentration of charged groups on the inner layer will be much smaller than that of the outer layer. There is electron microscopy evidence for the formation of tubules in cyanine dyes,25 but having thicker walls with a bilayer molecular array. The Bragg spacing from the first, most intense X-ray diffraction peak (d0) is plotted as a function of dye concentration in
Figure 5. Bragg spacings from the first X-ray diffraction peak (d0) at 25 C (circles) and diameter of aggregates (D, squares) as a function of dye concentration within the hexagonal M phase region. The dashed line indicates the value of aggregate diameter calculated from the multiwall tube model.
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Figure 5. An increase of d0 with dilution is observed, as the M phase swells with water and the aggregates separate in direction perpendicular to the long axis. It is to be noted here that above 15 wt % hexagonal liquid crystals coexist with a solid phase. The diameter of aggregates calculated from the interlayer spacing within the M phase (see eq 2) practically does not change with concentration, and it is close to the diameter of the tubes calculated from the multiwall model discussed above. Namely, the M phase appears to be formed by the packing of the same aggregates forming the N phase. The variation of d0 as a function of the inverse of the volume fraction (1/φ) followed a linear tendency in a log-log plot, the slope being almost the same as the one reported previously by one of the authors6 for the hexagonal phase of an anionic cyanine dye (see Supporting Information, Figure S4), suggesting a close structural similarity between the two systems. 1 H NMR spectra of pinacyanol acetate solutions in D2O at different concentrations were recorded, and the results are presented in Table 1. The shifts associated with the methyl group of the acetate counterions show almost no change over the range of concentrations studied due to the low interaction between these groups and the rest of the aggregated molecules. By contrast, the chemical shifts of the other protons decrease with increasing dye concentration (see Figure 6), which points to the formation of molecular stacks with strong interaction among the aromatic rings. For the aromatic hydrogens even at the lowest concentration in Table 1, these chemical shifts are ca. 0.5 ppm smaller than the values reported for pinacyanol chloride in dimethyl sulfoxide (DMSO).26 As is well-known for chromonic systems, aggregation leads to smaller chemical shifts for the aromatic hydrogens because of shielding due to the presence of neighboring aromatic rings (“ring current effect”).27,28 Typically
Table 1. Chemical Shifts as a Function of Dye Concentration
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Figure 6. Relative change in chemical shift at 25 C of the proton signals as a function of pinacyanol acetate concentration.
there is a difference between the chemical shifts of monomers and aggregates of ca.1-1.8 ppm26,27 Only a single averaged spectrum is observed (rather than separate monomer and aggregate peaks) because of fast molecular exchange between the two states (>103 s-1). Clearly, the changes in chemical shifts show that there is a large increase in the fraction of aggregates present as the concentration is raised. But, from the change in chemical shifts in Table 1, it seems likely that a sizable fraction of both monomers and aggregates is present over the whole concentration range since there is no sign of a leveling out of the curves (Figure 6) at very high or low concentrations. Note that the largest changes are for the hydrogens in the 12 conjugated bonds at the bottom of molecule (Figure 6). Also, the shift change of the CH2 groups24,26 is reasonably large. Thus, we hypothesize that within the aggregates the conjugated bottom moiety, including the three-carbon link, is in close proximity to the aromatic groups of stack neighbors. Most of the solution spectra give narrow NMR peaks, with the line widths at half-height being slightly broadened only at the very highest measured concentrations (ca. 5 Hz at 1%). Thus, the aggregates formed should be relatively small—by comparison with previous results,23,24 say eca. 3 nm. However, more studies are needed to confirm this estimation. UV-vis spectroscopy (see Supporting Information, Figure S5) gives additional evidence of the formation of dye aggregates at very low concentration. Below ca. 0.000 675 wt %, spectra of pinacyanol acetate in water show mainly monomer vibronic bands at 550 and 600 nm, the same as those of pinacyanol chloride in water þ ethanol mixtures.29 However, above 0.000 675 wt %, the 600 nm band decreases, the 550 nm band blue-shifts, and a new band grows at ca. 525 nm; all these spectral changes are associated with the aggregation of pinacyanol acetate molecules. The splitting of the water deuterium signal (see Supporting Information, Figure S6) indicates the presence of a liquid crystalline phase aligned by the magnetic field; no splitting was found below this concentration; i.e., more diluted mixtures are isotropic. The signal becomes broader and the quadrupole splitting increases with dye concentration (Figure 7 and Supporting Information, Figure S6), indicating an increase in the order parameter. A detailed discussion of the NMR spectra of water in liquid crystals has been given previously.6,30 Briefly, the splitting arises from the fraction of water bound to the aggregates, in fast
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Figure 7. Deuterium quadrupole splitting values (Δ2H) for pinacyanol acetate samples as a function of dye/D2O mole ratio at 25 C. The dashed line indicates roughly the nematic-to-hexagonal phase transition.
Figure 8. SEM images of calcined silica synthesized from pinacyanol acetate solutions. The initial pinacyanol concentration in the reaction mixture was 3.5 wt %, and the aqueous ammonia/TEOS mass ratio was 4.6. Images a-c were taken with a Hitachi TM-1000 microscope, whereas image d was taken with a Hitachi H-4100FE instrument.
exchange with bulk water. The magnitude of the splitting should be proportional to the dye/water mole ratio at low dye concentrations, with the extrapolated curve passing through the origin. Figure 7 shows a rough agreement to this, but there are deviations at the very lowest and highest concentrations. These may be due to the occurrence of two-phase regions; further experiments are required to clarify this matter. 3.2. Characterization of Silica Samples. Figure 8 shows some SEM images of calcined silica samples prepared from pinacyanol acetate solutions. More images (including samples before calcination) can be found in the Supporting Information (Figure S7). It is to be noted here that pinacyanol acetate forms 3071
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Figure 9. TEM images of the sample shown in Figure 8. The region limited by the square in the picture on the left has been magnified 2.5 times in the inset. The scale bars correspond to 100 nm.
Figure 10. SAXS curve of the sample shown in Figure 9. Arrows indicate the relative peak positions (with respect to the first peak) for a hexagonal lattice. The initial pinacyanol concentration in the reaction mixture was 3.5 wt %. The inset shows schematically the structure before and after calcination.
chromonic liquid crystals even in the presence of ammonia, which is necessary for the sol-gel reaction. Long microscopic fibers can be observed before and after calcination; some of them are found isolated whereas others are entwined forming larger particles. It seems that the silica materials somehow mimic the long aggregates in the nematic liquid crystal phase (as inferred from SAXS results) and the threadlike structures observed in the aqueous solutions of the dye (see Supporting Information, Figure S2). As described later, such structures may derive from a hierarchical organization of the aggregates at different size scales, as it also occurs in certain surfactant systems.31 The formation mechanism in the present case appears to be different from those based on two-phase systems in which fiber growth is kinetically controlled.32 There appears to be a hierarchically organized structure that goes down to the nanometer range, as shown in Figure 9, where well-aligned tiny strips or channels (width ∼ 3 nm) are observed. A hexagonal array of pores is also noticeable in a section of the image (Figure 9, inset on left side). The above-mentioned pore array is confirmed by the SAXS spectrum of calcined silica (Figure 10), with peak position ratios √ 1: 3:2 corresponding to a hexagonal lattice. The Bragg spacing corresponding to the first peak is 5.4 nm, which is shorter than that of the same sample before calcination (6.3 nm, see Supporting Information, Figure S8); namely, there is a contraction of the structure. Note that such ordering is induced by the cooperative assembly with silica species during the synthesis, as the dye
Figure 11. Pore size distribution derived from the adsorption branch of the nitrogen isotherm of the sample shown in Figures 8 and 9.
solution (nematic phase) at the start of the sol-gel reaction shows no such indexed scattering peaks; moreover, the Bragg spacing of the noncalcined silica sample is very much shorter than the one corresponding to the initial reaction solution at 3.5 wt % of dye (nematic phase), strongly suggesting that charged oligomeric silicate species cause the separation of the silicate-chromonic aggregates from excess water by binding to the aggregates’ surface and partially neutralizing the long-range repulsions between them, as for surfactant templated systems. Consequently, there is a “condensation” into a so-called silicatropic liquid crystal during the polymerization and precipitation of the inorganic oxide. We should also indicate here that attempts to prepare mesoporous silica from anionic chromonics such as Sunset Yellow and disodium chromoglycate were unsuccessful, which stresses the importance of the cooperative electrostatic interactions between the charged groups of the templating molecules and the silicate species formed during the sol-gel process. The shape of the sorption isotherm of calcined silica (see Supporting Information, Figure S9) suggests the presence of both framework and interparticle porosity, the last reflected in the steep increase at high relative pressures. Note that fibers may be entangled forming a mesh (see Figure 8c). The specific surface area determined by applying the multipoint BET model15 was 230 m2/g. The analysis of the adsorption branch of the isotherm by the BJH method16 resulted in a relatively narrow pore size distribution (see Figure 11) with an average pore diameter of 3.1 nm, in good agreement with the channels observed in Figure 9. By subtracting the pore diameter from the lattice parameter derived from the SAXS peaks in Figure 10, we get a pore wall thickness of 3072
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Langmuir 2.7 nm. The pore size, which is much larger than the length of one pinacyanol acetate molecule, supports the model of multimolecular stacks derived from SAXS analysis. We could not determine the porosity of noncalcined samples. However, we cannot rule out the existence of mesoporosity even before calcination, although we do not have experimental evidence to confirm it yet. From our nitrogen sorption experiments, it was not possible to estimate accurately the microporosity, if any.
4. CONCLUSIONS Pinacyanol acetate forms nematic and hexagonal chromonic liquid crystals at low concentrations in water; the acetate counterion appears to play an important role in liquid crystal formation. Dye molecules stack forming hollow long tubes with one-molecule-thick walls; the aggregates can be aligned by shear or by a magnetic field, and their diameters do not to change much with concentration. The high-resolution 1H NMR spectra show that aggregation takes place over a range of concentrations rather than having a sharp “critical” aggregation concentration. Within the aggregates the conjugated moiety, including the three-carbon link, is in close proximity to the aromatic groups of stack neighbors Mesoporous silica microfibers can be prepared by templating the chromonic liquid crystals formed in aqueous mixtures of a cationic dye. The calcined materials show a hierarchical structure in which cylindrical mesopores (diameter ∼3 nm, pore wall ∼2.7 nm) forms a hexagonal array. As the nanofibers form entangled networks, the obtained materials also show interparticle porosity. ’ ASSOCIATED CONTENT
bS
Supporting Information. Optical and scanning electron microscopy images, SAXS and rheometry data, UV-vis and NMR spectra, sorption isotherms. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Present Addresses ^
International Iberian Nanotechnology Laboratory, Braga, Portugal.
’ ACKNOWLEDGMENT C.R.-A. is grateful to the Ministerio de Ciencia e Innovacion, Spain (Project CTQ2008-01979/BQU), and the 2007GB-004 bilateral cooperation program between Spain (CSIC) and RSC (UK) for financial support. Authors thank Dr. Lok Kumar Shrestha and Prof. Kenji Aramaki (Yokohama National University, Japan), Dr. Michael Rappolt (Elettra Laboratory, Italy), Antri Theodorou, Marina Sintyureva, and John Jones (University of Manchester, UK) and Serveis Cientificotecnics (Universitat de Barcelona, Spain) for their help in the experiments. ’ REFERENCES (1) (a) Lydon, J. E. In Handbook of Liquid Crystals; Demus, D., Goodby, J., Gray, G. W., Spiess, H.-W., Vill, V., Eds.; Wiley-VCH: Weinheim, 1998; Vol. 2B. (b) Lydon, J. Curr. Opin. Colloid Interface 1998, 3, 458. (c) Lydon, J. Curr. Opin. Colloid Interface 2004, 8, 480. (2) Lydon, J. J. Mater. Chem. 2010, 20, 10071.
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