Article pubs.acs.org/JPCB
Orientational Order of a Lyotropic Chromonic Liquid Crystal Measured by Polarized Raman Spectroscopy Xuxia Yao,† Karthik Nayani,†,§ Jung Ok Park,†,§ and Mohan Srinivasarao*,†,‡,§ †
School of Materials Science and Engineering, ‡School of Chemistry and Biochemistry, and §Center for Advanced Research on Optical Microscopy, Georgia Institute of Technology, Atlanta, Georgia 30332, United States S Supporting Information *
ABSTRACT: Lyotropic chromonic liquid crystals are distinct from thermotropic nematics from a fundamental standpoint as the structure of the aggregating columns is a function of both the temperature and concentration. We report on the thermal evolution of orientational order parameters, both the second (=scalar) (⟨P200⟩ (=S)) and fourth (⟨P400⟩) order, of sunset yellow FCF aqueous solutions, measured using polarized Raman spectroscopy for different concentrations. The order parameter increases with the concentration, and their values are high in comparison with those of thermotropic liquid crystals. On the basis of Raman spectroscopy, we provide the strongest evidence yet that the hydrozone tautomer of SSY is the predominant form in aqueous solutions in the isotropic, nematic, and columnar phases, as well as what we believe to be the first measurements of (⟨P400⟩) for this system.
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INTRODUCTION Lyotropic chromonic liquid crystals (LCLCs) are a relatively new class of liquid crystals that have attracted considerable attention in recent years.1−3 Applications of these materials have been explored as polarizers,4−7 optical compensators,7,8 biosensors,9 precursors of aligned graphene,10 and templates for mesoporous nanofibers.11 Unlike conventional lyotropic amphiphiles which consist of a “hydrophilic” head group and a “hydrophobic” tail group, the main structural feature of LCLC molecules is the disklike or planklike rigid aromatic core with hydrophilic ionic or hydrogen-bonding solubilizing groups around it.1−14 It is believed that the driving force for aggregation is the π−π interactions of the rigid aromatic core. The aggregation process is believed to be isodesmic; that is, addition or removal of one molecule to a column is always associated with the same change of free energy.12−14 There are two well-characterized chromonic mesophases, the nematic N phase and the hexagonal M phase.1 It has been shown that, at the phase transition from the isotropic to the nematic phase, the aspect ratio L/D (length/diameter) of the aggregates is quite low,1−14 not following the predictions of the hard rod theory developed by Onsager for a system of rodlike molecules in solution to form an ordered phase.15−19 Sunset yellow FCF (SSY) is one of the most extensively studied LCLCs.13,14,20−24 Results from previous research on SSY show the aggregation energy to be around 7.25 kBT. Furthermore, the scalar order parameters,13,21,23 salt and pH effects on aggregation,20,22,24 rheology,25 and interactions with polymers have been explored.25,26 Determination of order parameters is useful in predicting several characteristic physical properties of a nematic fluid. For instance, it is possible to predict the flow behavior of the director, if the second (⟨P200⟩) and fourth (⟨P400⟩) moments of the orientational distribution © XXXX American Chemical Society
function are known. Polarized Raman scattering is one of the few experimental techniques which enables simultaneous measurement of both ⟨P200⟩ and ⟨P400⟩. In the present work, orientational order parameters, ⟨P200⟩ and ⟨P400⟩, in the nematic phase of SSY were quantified as a function of the concentration and temperature with the use of polarized Raman scattering. Furthermore, Raman spectroscopy was used to elucidate the predominant tautomeric form in water, since vibrational spectroscopy enables identification of a chemical structure as well as quantification of the order in the system, and we report on what we believe to be the first measurements of (⟨P400⟩) for this system.
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EXPERIMENT SSY was purchased from Sigma-Aldrich and purified by recrystallization using ethanol. Figure S1 in the Supporting Information shows the phase diagram of SSY in water. The flat capillaries (cross-sectional inner dimension 50 × 500 μm, length 5 cm) were purchased from Vitrocom and used after being cleaned with distilled water without any further special surface treatment. SSY solutions were filled into the flat capillary by capillary action in the isotropic phase (i.e., at high temperature) and subsequently cooled into the nematic phase, resulting in a monodomain sample. Silicone oil was placed on top of the SSY solutions to keep the SSY concentration constant by preventing water evaporation at high temperature. Both ends of the flat capillary were sealed with Permaoxy fastcure epoxy after the solution was filled (detailed steps are shown in the Supporting Information). A Kaiser 5000 Raman Received: February 27, 2016 Revised: April 12, 2016
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DOI: 10.1021/acs.jpcb.6b02054 J. Phys. Chem. B XXXX, XXX, XXX−XXX
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SSY molecules in the aggregates. This is further evidence of isodesmic aggregation of SSY molecules in water. As shown in Figure 1, strong bands are observed between 1300 and 1580 cm−1. Although one may assign one of the bands between 1380 and 1463 cm−1 to the NN stretching band, we rule out the possibility of these bands being attributed to NN stretching for the following reasons: It should be noted that the symmetric NN stretching mode in the transazobenzene usually induces a greater change in the polarizability than phenyl ring stretching does.27,29 Accordingly, the Raman band from the symmetric NN stretching is stronger than that from phenyl C−C stretching (the band at 1596 cm−1 in Figure 1). However, from the spectra we note that all bands between 1380 and 1463 cm−1 are weaker than the band at 1596 cm−1. Moreover, the band at 1390 cm−1 is most possibly from naphthalene stretching. The orange II dye has a structure and tautomerism very similar to those of SSY, while only one sulfonyl group is attached to the naphthalene ring. It exists in the hydrazone (≥95%) form in aqueous solution.30 The Raman spectrum of orange dye II is almost identical to that of SSY (see the Supporting Information, Table S6), which also confirms that the NH hydrazone structure of SSY is dominant in all phases. Lastly, the UV−vis absorption spectrum of an SSY dilute solution, showing strong absorption around 480 nm, provides evidence that the NH hydrazone tautomer is dominant. Abbott et al.30 reported that the OH group in the azo structure has a strong absorption between 400 and 440 nm, while the NH hydrazone form has a strong absorption between 475 and 510 nm. This agrees well with our deduction from the Raman spectra. Hence, we conclude that the SSY molecules form the hydrazone tautomer in the isotropic, nematic, and columnar phases. The degree of orientational order of the aggregated columns of SSY in the nematic N phase was obtained from polarized Raman scattering measurements. To minimize defects which can affect the accuracy of the measurement, the preparation of a monodomain sample is essential. While alignment is very critical, LCLCs are usually difficult to align. We achieve homogeneous planar alignment of an SSY aqueous solution in the nematic phase in a flat rectangular capillary. Macroscopic uniaxial alignment was observed by polarized optical microscopy (POM). Under crossed polarizers, the optical image of the sample has a 4-fold rotational symmetry in birefringence, as well as a high extinction ratio under crossed polarizers (I∥/I⊥ > 400), which indicates that SSY is uniaxially aligned. It should also be noted that the quality of the monodomain is sufficiently high that one can obtain conoscopic interference figures characteristic of a planar aligned, uniaxial single crystal with its optic axis in the plane of the sample (unpublished results, as seen in the Supporting Information). Two components of polarized Raman intensities, I∥ and I⊥, were measured, which represent the Raman scattering intensities when the polarization direction of the analyzer is parallel and perpendicular to that of the incident beam, respectively. Details about the experimental geometry and the expressions which relate polarized Raman intensities to the degree of orientation are described in the Supporting Information and elsewhere.31,32 The orientational order parameters were obtained by measuring I∥ and I⊥ with respect to the angle θ between the incident polarization direction and the director. Figure 2a depicts the angular dependence of the polarized Raman intensities (I∥(θ) and I⊥(θ)) measured using macroscopically aligned SSY in the nematic phase (1.0 M at 24.5 °C).
microscope, equipped with a 785 nm diode laser operating at 192 mW and with a 10× microscope objective attached to the fiber optic probe head, was used to measure Raman scattering. The sample temperature was controlled by a Linkam THM 600 hot stage with an accuracy of 0.1 °C. One advantage of using Raman microspectroscopy is that scattering only from SSY will be collected, as water is not Raman active.
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RESULTS AND DISCUSSION Sunset yellow FCF is an azo dye with two possible tautomeric forms: the NN azo form and the N−H hydrazone form (see Figure 1 for their tautomer structures). It has been shown by NMR spectroscopy that the hydrazone structure prevails in both dilute solutions and chromonic mesophases,21 which was later supported by the atomistic computer modeling.14 The structure of the SSY tautomer was characterized by identifying whether the band assigned to the NN stretching mode is present in its Raman spectrum.27 The presence or absence of the NN vibrational band would confirm the azo or the hydrozone tautomer, respectively. This provides a simple but definite method to elucidate the chemical structure because the vibrational energy is unique to the type of chemical bond in a molecule.28 The symmetric NN vibration is usually forbidden or weak in the infrared, but has an intense Raman peak. The effect of conjugation results in a decrease in the frequency of the NN mode. Usually, the Raman spectra of azo compounds which contain a NN bond show a strong band assigned to its stretching mode at around 1300−1580 cm−1. trans-Azobenzene has an intense NN stretching band at 1380−1463 cm−1,27 but the assignment of the NN stretch mode is more difficult for azonaphthols and azonaphthylamine than for azobenzene, as the NN band is overlaid by not only aromatic ring vibration bands but also the CN stretching bands arising from the tautomerism effect.27 A strong band seen at 1370− 1390 cm−1 is characteristic for most disubstituted naphthalenes, including azonaphthols.27 Figure 1 depicts the measured Raman spectrum for SSY aqueous solution in the isotropic phase. No significant changes
Figure 1. Raman spectrum of sunset yellow FCF in water (0.7 M concentration) at room temperature under parallel polarization.
in peak positions are observed in the nematic and columnar phases in comparison with the isotropic spectra (see the spectra of the isotropic and chromomic N and M phases, as well as the crystalline phase in the Supporting Information, Figure S4). This is probably a result of the similar microenvironment of B
DOI: 10.1021/acs.jpcb.6b02054 J. Phys. Chem. B XXXX, XXX, XXX−XXX
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Figure 2. (a) Intensity profiles of phenyl C−C stretching at 1596 cm−1 in a 1.0 M SSY aqueous solution at 24.5 °C, taken at 10° intervals over the entire 360° sample rotation. (b) Depolarization ratio profile, R(θ), for this sample, calculated from (a). The solid line is a fitted curve, as described in the text.
We analyzed the intensity of the band at 1596 cm−1, which is assigned to phenyl C−C bond stretching. The vibration of this mode is in the molecular plane. Therefore, the director is perpendicular to the direction of the vibrational symmetry of the phenyl ring. This is verified by the angular dependence of I∥(θ) and I⊥(θ). As we can see from Figure 2a, I∥(θ) has 2-fold rotational symmetry,with its highest value at θ = 90° and 270° and its minimum at θ = 0° and 180°. The angle θ = 0° is when the director is parallel to the polarization direction of incident light (see the Supporting Information). As a result, when θ = 90° and 270°, the phenyl C−C stretching is within the plane of the molecule and parallel to the polarization direction of the incident light; thus, the Raman band at 1596 cm−1 is most intense. Therefore, the SSY LCLC in a flat capillary has a planar alignment with the director perpendicular to the long axis of the capillary. By measuring the polarized Raman scattering of the SSY monodomain, both ⟨P200⟩ and ⟨P400⟩ of these stacking columns were determined. The depolarization ratio data in Figure 2b were obtained using the relation R(θ) = I⊥(θ)/I∥(θ). The order parameters ⟨P200⟩ and ⟨P400⟩, as well as the ratio of the differential polarizability, r = αxx/αzz′, can be extracted by fitting the experimentally obtained spectrum to the theoretical expressions of R (θ, ⟨P200⟩, ⟨P400⟩, r) (the derivation is shown in the Supporting Information) .33−35 The values of ⟨P200⟩, ⟨P400⟩, and r, calculated from the curve fitting of R(θ) (see the solid line in Figure 2b), are −0.41, 0.22, and −0.12, respectively. It should be noted that ⟨P200⟩ and ⟨P400⟩ values obtained this way represent the orientation distribution of the phenyl C−C stretching. Assuming that there is no correlation between the orientational fluctuations of the column and the orientational fluctuations of the molecules within the column relative to the aggregate axis, a simple relation between the order parameters of individual bonds and the stacking column, Plmn,column = Plmn/[Plmn(cos Ω)],36 can be used to calculate the order parameters of the stacking columns. Here, Ω is the angle between the principal axis of a Raman tensor and molecular chain (stacking column) axis. Plmn is the Legendre polynomial with P200(x) = (3x2 − 1)/2 and P400(x) = (35x4 − 30x2 + 3)/ 8.36 Therefore, Ω is the angle between the principal axis of the Raman tensor of the phenyl C−C stretching mode and the long axis of the stacking columns. The phenyl C−C stretching mode is strongly polarized within the molecular plane and perpendicular to the director. Accordingly, Ω is 90°, P200(cos Ω) is −1/2, and P400(cos Ω) is 3/8. Therefore, for the stacking columns in the 1.0 M SSY chromonics at 24.5 °C, ⟨P200⟩ is 0.82
and ⟨P400⟩ is 0.43. Both order parameters ⟨P200⟩ and ⟨P400⟩ increase as the concentration increases and decrease as the temperature increases. Table 1 shows the order parameters of the stacking columns of the SSY chromonics with various concentrations at 28.8 and 36.8 °C. Table 1. Concentration and Temperature Dependence of the Order Parameters of the Supramolecular Stacking Columns in SSY Chromonics temp (°C) ⟨P200⟩ ⟨P200⟩ ⟨P400⟩ ⟨P400⟩
28.8 36.8 28.8 36.8
1.0 M 0.79 0.78 0.50 0.49
± ± ± ±
0.02 0.02 0.08 0.09
1.05 M 0.82 0.80 0.58 0.52
± ± ± ±
0.02 0.02 0.11 0.13
1.1 M 0.85 0.84 0.62 0.59
± ± ± ±
0.03 0.03 0.02 0.02
Figure 3 depicts the temperature dependence of the order parameters for a 1.1 M SSY chromonic solution before the
Figure 3. Temperature dependence of the order parameters of 1.1 M SSY chromonics. Red points with error bars indicate the values of ⟨P200⟩ (=S), and the black line is the fitting with the equation S = S0(1 − T/TNB)β.42,43 Here TNB is the nematic−biphasic transition temperature (K). Blue squares with error bars indicate the values of ⟨P400⟩.
nematic-to-biphasic transition temperature, TNB, is reached. The value of ⟨P200⟩ varies between 0.79 and 0.85 in the nematic phase for the concentrations studied. These are higher than the values of 0.6−0.75 determined by an absorption measurement13 and 0.54−0.65 determined by an NMR measurement.21 However, an X-ray measurement23 reports values around 0.72− 0.9, which are close to our results. Furthermore, theoretical predictions by Taylor and Herzfeld for reversibly selfC
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The Journal of Physical Chemistry B assembling rodlike polydisperse systems for a volume fraction between 0.24 and 0.28 (between 1.0 and 1.2 M) at the phase transition also predict an order parameter of around 0.7− 0.8.37,38 It should be noted that the fairly high values for ⟨P200⟩ measured for lyotropic chromonic liquid crystals are quite reminiscent of the measurements for rodlike polymers in solution.39−41 For example, ⟨P200⟩ was measured to be 0.94 and constant over a wide range of concentrations (1 < ϕ/ϕ* < 2.5) for solutions of poly(p-phenylenebenzobisthiazole) (PBT) in methanesulfonic acid (MSA).40,41 Here ϕ and ϕ* are the volume fraction and the volume fraction of the polymer in solution that forms a nematic phase. The measured ⟨P200⟩ (=S) values were fitted by an empirical formula based on the mean field theory where the temperature dependence of the order parameter is described over the entire nematic N temperature range by the simple relation S = S0(1 − T/TNB)β.42,43 For 1.1 M SSY chromonics shown in Figure 3, S0 is calculated to be about 0.94 and β is about 0.045. Compared with many thermotropic nematic liquid crystals with β between 0.1 and 0.2,39,40 β for SSY chromonics is smaller, and hence, the order parameters are less sensitive to the temperature. It should be pointed out that chromonics experience a wider nematic− isotropic coexistence region before reaching the isotropic phase than thermotropic LCs, so the order parameters may be expected to continue to decrease with temperature in this coexistence region.
ACKNOWLEDGMENTS
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CONCLUSIONS In conclusion, we have confirmed that SSY in water predominantly exists in the hydrazone form over the entire concentration range that encompasses the isotropic, nematic, and columnar phases. We have also demonstrated that polarized Raman spectroscopy provides a means to determine the second and fourth rank order parameters for these supramolecular liquid crystalline phases, as a function of both the temperature and concentration in the nematic N phase. It should be noted that these are the first measurements of ⟨P400⟩ for any chromonic liquid crystal, and coupled with ⟨P200⟩ should enable the computation of the Leslie coefficients for SSY-based nematic fluids. The ⟨P200⟩ values are higher than those of conventional thermotropic liquid crystals, and those determined by absorption and NMR measurements, but closer to those determined by X-ray measurement and the prediction from the model by Taylor and Herzfeld for a reversibly selfassembling rodlike polydiperse system.37,38 ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.6b02054. Phase diagram of sunset yellow FCF in water, sample preparation steps, Raman experimental setup, Raman spectrum of SSY and orange II, temperature dependence of the band width of the 1596 cm−1 band, derivation and equation of the depolarization ratio, and conoscopic image showing the quality of the monodomain (PDF)
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This work is supported by a grant from the U.S. Office of Basic Energy Sciences, Department of Energy (DE-SC0001412). We thank Dr. Jinxin Fu for providing the conoscopic interference figures.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: 404-894-9348. Notes
The authors declare no competing financial interest. D
DOI: 10.1021/acs.jpcb.6b02054 J. Phys. Chem. B XXXX, XXX, XXX−XXX
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