Special issue research article Received: 14 April 2014
Revised: 15 May 2014
Accepted: 18 May 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/mrc.4091
NMR molecular dynamics study of chromonic liquid crystals Edicol Sunset Yellow doped with salts João P. de Almeida Martins,a Fabián Vaca Chávezb and Pedro J. Sebastiãoa,b* We investigate the effect of monoatomic salts on the molecular dynamics in the nematic and isotropic phases formed by the chromonic liquid crystal Edicol Sunset Yellow. The study was carried out using proton nuclear magnetic resonance relaxometry. To analyse the effect of incorporation of additional sodium chloride or lithium chloride on the solutions’ molecular dynamics, the spin-lattice relaxation time was measured for Larmor frequencies between 10 kHz and 100 MHz. In the nematic phase, the presence of additional sodium or lithium ions seems to contribute to an increase of the rotations/reorientations correlation times in comparison with the mixture without extra ions. The collective motions detected by proton NMR relaxometry are associated with collective fluctuations of molecules within the stacks in the nematic phase and with order parameter fluctuations in the isotropic phase. Copyright © 2014 John Wiley & Sons, Ltd. Keywords: NMR; dynamics; liquid crystals; choromonics; relaxation; salt
Introduction Unlike common lyotropics, chromonic mesophases are formed through the aggregation of non-amphiphilic molecules, which have a plank-like or disk-like polyaromatic central core with polar groups at the periphery [1,2] . Instead of forming micelles, those molecules stack on top of each other “face-to-face” to minimize the areas of unfavourable contact with water, leaving the ionic solubilizing groups at the aggregate/water interface. The mesogenic unit of chromonic mesophases are then columns of stacked molecules that lie in a continuum electrolyte fluid. For certain values of temperature and concentration, the molecular stacks still exist in the Isotropic (I) liquid phase. The most common mesophases in chromonic liquid crystals are the uniaxial nematic (N) phase and the hexagonal columnar (M) phase. Between two phases, in a transition from one phase to another, a two-phase state where the initial and final phases coexist is observed. For example, in the process of obtaining the N phase by cooling of an Isotropic (I) phase, we can observe a N C I state where domains of nematic substance coexist with an isotropic environment. In recent years, increasing attention has been given to chromonics in view of their potential technological applications such as the production of biosensors in the detection of antigens [3] and films with semi-conducting properties [4] . Ould-Moussa et al. reported the use of nematic chromonic phase to orient and disperse single-walled carbon nanotubes [5] and Yi and Clark showed the possibility of aligning the molecular stacks in chromonic liquid crystal phases when confined in cells patterned with line channels [6] . Therefore, the study of their physico-chemical properties is a matter of concern and is currently an active topic of research. Most studies reported in the literature have been focused on disodium cromoglycate (DSCG), which is a drug used on the treatment of asthma, but in the last years chromonic phases formed by Edicol Sunset Yellow (ESY), synthetic yellow azo dye used in
Magn. Reson. Chem. (2014)
the food industry, have been a subject of both theoretical and experimental studies [7,8] . Chami and Wilson reported results from molecular dynamics simulations that describe the dynamics and structure of aggregates of ESY molecules. Experimental works made use of techniques such as X-ray, optical microscopy, and NMR (1 H, 2 H and 23 Na) spectroscopy [9–11] . Zhou et al. measured the temperature and concentration of the Frank elastic constants splay (K1 ), twist (K2 ), and bend (K3 ) [12] using a magnetic Frederiks transition technique in the nematic phase [8] . Recently, proton NMR relaxometry and pulse gradient NMR techniques were used to study the molecular dynamics in the chromonic nematic and isotropic phases of the binary mixture composed by ESY and heavy water [13] . Here, we extend that work and we investigate the effect of sodium chloride and lithium chloride on the molecular dynamics of the same chromonic compound. The works reported in the literature are not just focused on the characterisation of chromonic liquid-crystalline systems formed by aqueous solutions but also on the effect of the addition of salts. Investigation using X-Ray, optical, rheologycal, NMR, and dielectric techniques were used to characterise the phase behavior and structure of the mixtures. Kostko et al. investigated the addition of sodium and potassium salts to the nematic phase of DSCG [14] . Park et al., Jones et al.,
* Correspondence to: Pedro J. Sebastião, Departamento de Física, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal. E-mail: [email protected] a Departamento de Física, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001, Lisboa, Portugal b Condensed Matter Physics Centre, Universidade de Lisboa, Av. Prof. Gama Pinto 2, 1649-003, Lisboa, Portugal
Copyright © 2014 John Wiley & Sons, Ltd.
J. P. de Almeida Martins, F. V. Chávez and P. J. Sebastião and Prasad et al. reported the addition of ionic compounds to the aqueous solution of ESY [10,11,15] .
Experimental Section Samples preparation Edicol Sunset Yellow, sodium chloride, and D2 O (99.98 atom % 2 H) were purchased from Sigma-Aldrich (Munich, Germany). Lithium chloride was obtained from Merck KGaA (Darmstadt, Germany). The received ESY was further purified according to the procedure reported in the literature [9] . The product was dissolved in Milli-Q water with a resistivity higher than 18.2 Mcm, followed by addition of ethanol. The dye precipitate was recovered and the process was repeated four more times. Finally, the ESY sample was dried at 60 °C, in an oven, to remove residual solvents. Heavy water, instead of normal water, was used to prepare the samples in order to make possible to distinguish solvent from solute molecules in the 1 H NMR measurements. These ESY/heavy water mixtures were prepared by weight of the chromonic molecules: 27.8 wt% and 26 wt%, respectively. The concentration of NaCl and LiCl was 0.5 mol/kg. In the following, the samples without salts will be designated as SY28 and SY26, the mixtures with salts will be designated as SY28-X and SY26-X, where X=Li or Na. The samples were placed in standard NMR tubes and flame sealed to avoid water evaporation. In Fig. 1 a schematic of chromonic nematic and isotropic phases (upper panels) and the corresponding textures observed on a microscope under cross polarisers (lower panels) are presented. The molecular structure of the NH hydrazone tautomer of the ESY is also shown. NMR measurements The proton spin-lattice relaxation rate, R1 . 1=T1 /, was measured over more four Larmor frequency, , decades, from 10 kHz to 300 MHz. For the frequency range 10–91 MHz, a variable-field iron-core magnet equipped with a Bruker Avance II console was used. At 300 MHz, R1 was measured with a Bruker Avance II spectrometer. The R1 measurements in the conventional spectrometers were made using the standard inversion recovery ./x . 2 /x,x Acq sequence. For frequencies below 10 MHz, the R1 data were obtained with a home developed fast field-cycling
Figure 1. Schematic of chromonic nematic and isotropic phases (upper panels) and the corresponding textures observed on a microscope under cross polarisers (lower panels). The molecular structure of the NH hydrazone tautomer of the Edicol Sunset Yellow is also shown.
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spectrometer [16] . A usual field cycle BP ! BE BE ! BD , where the polarisation field, BP , and detection field, BD , respectively, have the same value corresponding to the Larmor frequency of 8.9 MHz was used. The switching times were less than 3 ms. Time is varied between zero and about five times the expected value of the spin-lattice relaxation time for the low magnetic field value. A conventional =2 r.f. pulse was used to obtain the FID signal after each cycle. In all cases, the temperature was regulated and stabilised to within ˙0.2 °C using air flow.
Relaxation Mechanisms and Models Based on the assumption that the molecular motions present in a liquid crystal are statically independent or otherwise are characterised by distinct correlation times, the experimental relaxation rate R1 is usually assumed to be described by a linear combination of relaxation contributions, each one associated with a specific type of motion. Cross-terms can be neglected as the characteristic correlation times associated with those motions are usually considerably different. Then, the total relaxation rate can be expressed as R1 D R1,C C R1,IM (1) where C and IM refer to collective motions and individual molecular motions, respectively. Here, we summarise the relaxations models used to describe the results. More details can be found in ref. [17,18] . a) Collective motions. i) In the nematic phases of thermotropic and lyotropic liquid crystals, these motions are described as three-fold fluctuations of the director in laboratory axis frame and are known as order director fluctuations (ODF), where three-fold means that ODF propagate in the three dimensions. But in the case of the chromonic nematic phase, taking into account that the units are molecular stacks, it was shown that collective motions are described by elastic column deformations (ECD) [13,19] . The corresponding spectral density is Z Z q? dq? q4jj dqjj kT q?,h qjj,h JECD ./ D 2 , 2 q?,l qjj,l .2/2 2 q4jj C.K3 q4jjCBq2? /2 (2) where B and K3 are the elastic constants for bending and compression of the columns, respectively. q?,h and qjj,h are the components of the largest wave vectors of the deformation parallel and perpendicular to the columns, respectively. The shortest wave vectors are q?,l and qjj,l . High and low cut-off frequencies can be defined in terms of the limit wave vectors and the viscoelastic constants. ii) As it was shown previously, in the isotropic phase of ESY, order parameter fluctuations (OPF) can be present and the spectral density is [20,21] p Z xmax x dx JOPF ./ D AOPF 2 , (3) 2 xmin .2/ C x C o1 with o related to the coherence length of these fluctuations. The limits of integration are related to the
Copyright © 2014 John Wiley & Sons, Ltd.
Magn. Reson. Chem. (2014)
Effect of salts on the molecular dynamics of chromonic sunset yellow
Figure 2. (a) Spin-lattice relaxation rate profiles of the samples SY28, SY28-Na, and SY28-Li at 15 o C in the nematic phase. The inset shows a comparison between the two nematic phases at different Edicol Sunset Yellow concentration and temperatures. The solid lines are guide for the eyes. (b) Proton spectra corresponding to the samples shown in (a).
jumps, d is the molecular width, n is the density ˝ ˛ of 1 H spins. The mean-square jump distance r2 is related with˝ the ˛ self-diffusion constant D and D by the relation r2 D 6D D.
minimum and the maximum wave vectors and AOPF is a constant related with the viscoelastic properties of the liquid crystal in the isotropic phase. b) Individual molecular motions. i) Local rotations/reorientations. In the case of anisometric molecules, a model to describe the relaxation induced by reorientations of rigid elongated molecules was proposed by Woessner [20] . The spectral densities are given by 2 2 .˛ij /j2 4 X jdk0 jmj Jk ./ D ck hjD2km . /ji , 2 3 mD2 1C4 2 2 jmj a6ij (4) where aij are the inter-proton distance (2.5 Å in the case of ESY, from ref. [9] ) and ˛ij denotes the angle between the inter-proton vector and the long molecular axis. D2km is the second rank Wigner rotation matrix and the averages hjD2km . /ji can be expressed in terms of the second and fourth rank Legendre polynomials of the angle between the long molecular axis and the nematic director [22] . 2 dkm .˛ij / is the second rank reduced Wigner rotation matrix. c0 D 6, c1 D 1, and c2 D 4. The corresponding correlation times, jmj , can be expressed as 0 D s , 1 D .s1 C f1 /1 , and 2 D .s1 C 4f1 /1 , where f and s are two correlation times corresponding to molecular rotations/reorientations about the axis parallel to the long molecular axis and reorientations about an axis perpendicular to the molecular symmetry axis, respectively. In the present work, they correspond to reorientations parallel and perpendicular to the packing axis. ii) translational self-diffusion. We used the model proposed by Torrey [20] . The corresponding relaxation rate can be written as nD 1 D Cd 3 ŒJ .// C 4J .2/ , (5) T1 Diff d
where the dimensionless spectral density J ./ can be calculated analytically [20] . Cd D .1=2/ .3 0 2 „=.8//2 is the strength of the dipolar interaction, D is the average time between diffusion
Magn. Reson. Chem. (2014)
Results and Discussions Nematic phase Figure 2a illustrates the proton R1 NMRD profiles for the samples SY28, SY28-Li, and SY28-Na at 15 o C. The temperature corresponds to the nematic phase for all samples. The nuclear magnetic relaxation dispersion (NMRD) profiles present a rather small dependence on the presence of the salts. In fact, at high frequencies the data sets overlap for the three samples. The minor differences are observed at intermediate frequencies (1–10 MHz) and at low frequencies (below 50 kHz). Besides these small differences in the NMRD profiles, as a result of the presence of salts, the nematic order reflected on the separation of the outer lines in the 1 H spectrum shown in Fig. 2b clearly shows a difference. The dipolar splitting from the outer peaks is 7.13 kHz for the SY28 and 6.73 kHz for SY28-Li and SY28-Na. Following the procedure described by Edwards et al. [9] , who correlated the proton spectrum in the nematic phase of ESY with the protons present in the molecule, these splittings correspond to an order parameter of S = 0.57 for SY28 and S = 0.54 for the SY28-Li and SY-Na [23] . The inset in Figure 2a is a comparison between two nematic phases at different ESY concentration, 30% and 28%, and Table 1. Details of the fitting parameters according to the proposed models to describe the frequency and temperature dependence of R1 for the sample SY28 in the nematic phase
f [109 s] s [109 s] AECD [103 s2 ] D [1013 m2 /s] S
SY28
SY28-Na
SY28-Li
1.2* (1.2 ˙ 0.2) 4.1 ˙ 0.5 13.5 ˙ 0.6 5.5* (5.5 ˙ 0.7) 0.57
1.9 ˙ 0.4 6.2 ˙ 0.6 96 ˙ 5 4.7 ˙ 0.6 0.54
1.6 ˙ 0.3 4.7 ˙ 0.5 338 ˙ 17 4.3 ˙ 0.6 0.54
* Fixed value from ref. [13] . The value between parenthesis was obtained when this parameter was included in the fit (see text for details).
Copyright © 2014 John Wiley & Sons, Ltd.
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J. P. de Almeida Martins, F. V. Chávez and P. J. Sebastião
Figure 3. Fits of the R1 dispersions in the nematic phase corresponding to (a) SY28, (b) SY28-Na, and SY28-Li at 15 o C. The solid curve corresponds to the total fitting curve given by Eqn. (1), and the separate contributions for the proposed dynamic mechanisms are shown. See text for details.
temperatures, 23 o C and 15 o C, respectively. Qualitatively, the two profiles present a similar behaviour but at intermediate and low frequencies (. 10 MHz) they show some differences, whereas at high frequencies they look identical. In order to interpret the R1 data, a fit was performed using Eqn. (1) with a home written minimization software package [24] . For a detailed analysis, the two individual motions contributions in R1,IM are shown separately. Based on the fact, as pointed before, that the profiles of the samples SY28 and SY30 coincide at high frequencies, the fit of the former was carried out fixing the value of f to the corresponding one of SY30 at 23 o C from ref. [13] . Also, the self-diffusion coefficient was extracted from the same work as an extrapolation to 15 o C. The results of the fitting parameters are presented in Table 1 and the fitted experimental data are presented in Fig. 3 together with the total R1 curve and the individual relaxation contributions corresponding to the proposed dynamic mechanisms. On Table 1, between parenthesis, we also present the corresponding values of f and D for the sample SY28, which were obtained by fitting the data fixing the other parameters. As it is observed, the results support our assumptions of fixing them based on our previous findings. The estimation of the parameters’ errors was performed, taking into account the sensitivity of the global fit’s 2 (all data sets simutaneously) at the minimum, to a change of each fitting parameter individually, within a confidence level of 68% [25] . The slight decrease of the self-diffusion coefficient of the samples with salt agrees with the finding that the viscosity of ESY solutions increases with the addition of salts reported by Prasad et al. [15] . Also, the increase in the correlation times for the rotations/reorientations might be associated with the reported increase of viscosity. The strength of the ECD, AECD on Table 1, increases upon the addition of salts. This finding can be related not only to the increase on the viscosity but also related to the variation of the elastic constants.
Figure 4. Comparison of the spin-lattice relaxation rate profiles of pure Sunset Yellow/water solution at 24% and 26% in the isotropic phase. The solid lines are guide for the eyes.
Isotropic phase Figure 4 displays the NMRD profiles of the samples of pure ESY/D2 O solutions at 24% and 26% in the isotropic phase. As it can be observed, the spin-lattice relaxation profile obtained for the larger concentration solution SY26 is very close to the one obtained for the lower concentration SY24 at a lower temperature. Also, the profile at low frequencies obtained for the SY24
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Figure 5. Spin-lattice relaxation rate profiles of pure Sunset yellow/water solution and with the addition of 0.5 mol/kg of LiCl and NaCl salts in the isotropic phase at 50 °C. The solid lines are guide for the eyes.
Copyright © 2014 John Wiley & Sons, Ltd.
Magn. Reson. Chem. (2014)
Effect of salts on the molecular dynamics of chromonic sunset yellow
Figure 6. Fits of the R1 dispersions in the isotropic phase corresponding to (a) SY26, (b) SY26-Na and SY26-Li at 50 o C. The solid curve corresponds to the total fitting curve given by Eqn. (1) and the separate contributions for the proposed dynamic mechanisms are shown. See text for details.
Table 2. Details of the fitting parameters according to the proposed models to describe the frequency and temperature dependence of R1 for the sample SY26 in the isotropic phase at 50 o C SY26 9
f [10 s] 2.5* (2.5 ˙ 0.2) s [108 s] 4.4 ˙ 0.4 AOPF [103 s2 ] 435 ˙ 24
SY26-Na 2.5* (2.5 ˙ 0.2) 2.4 ˙ 0.4 769 ˙ 33
SY26-Li 2.5* (2.4 ˙ 0.2) 2.6 ˙ 0.4 533 ˙ 27
*Fixed value from ref. [13] . The values between parenthesis were obtained when these parameters were included in the fit (see text for details).
sample at a higher temperature is different than that of the other two profiles. In Fig. 5, the results of the R1 dispersion profiles of the samples SY26, SY26-Li, and SY26-Na at 50 o C in the isotropic phase are shown. The differences barely detected in the NMRD profiles in the nematic phase of the SY28, SY28-Li, and SY28-Na samples as the result of the presence of the salts are clearly more expressive in the shape of the profiles obtained in the isotropic phase of the samples SY26, SY26-Li, and SY26-Na. The differences are observed below 10 MHz, whereas above this frequency, the profiles are identical within the experimental uncertainty of ˙5%. In Fig. 6 are presented the fits performed using model Eqn. (1), considering the presence of order parameter fluctuations as previously referred and the results of the fitting parameters are presented on Table 2. In the same way as we proceeded in the previous section, Table 2 presents between parenthesis the values of f obtained by fitting the data fixing the other parameters. Here, the results also support our assumptions of fixing them based on our previous findings. As it was observed in a previous NMR relaxometry study of ESY [13] , in the isotropic phase, the self-diffusion relaxation mechanism presents a small contribution to the relaxation. The value of the self-diffusion constant considered in the fits was that measured for SY24 at 43 o C[13] . As the R1 profiles in the high frequency regime are similar, it is reasonable to consider that the fast molecular motions in the three samples are characterised by similar correlation times. In fact, very good fits were obtained using the same correlation time for the molecular rotations around the molecular axis perpendicular to the ESY molecular plane. The effect of the presence of the salts in the molecular dynamics are observed in the regions where order parameter fluctuations
Magn. Reson. Chem. (2014)
and reorientation of the molecules with respect to the stacks’ axis affect the relaxation dispersion, that is, at frequencies below 10 MHz. The values of the correlation times for the reorientations of the molecules about an axis perpendicular to the stacks’ axis decrease with the presence of the salts contrary to the effect of salts in the nematic phase. The fact that the correlation time S does not change by the same amount as a result of the presence of NaCl or the LiCl might be related with the differences between the two ions in the equilibrium and stabilisation of the ESY stacks. In the same way as in the nematic phase, the strength of the collective motions, AOPF on Table 2, increases upon the addition of salts.
Conclusions It is presented a proton NMR relaxation study of the molecular dynamics in two series of ESY systems prepared with two concentrations: 26 wt% and 27.8 wt%. Samples were prepared by adding 0.5 mol/kg of NaCl and LiCl salts in order to understand the effect of the presence of the additional NaC and LiC ions in the system. The samples prepared with 27.8 wt% ESY present nematic phases at 15 °C. The study of this samples in the isotropic phase would require the heating of the sample to temperatures so high that possible evaporation effects might not be negligible. Therefore, additional samples were prepared with 26 wt% ESY that present the isotropic phase at 50 °C. The influence of the salts in the molecular dynamics of the ESY systems was barely detected in the nematic phase of the ESY systems, but could be perceived more clearly in the isotropic phase. The presence of the salts contributes to modify the reorientations of the molecules about an axis perpendicular and parallel to the stacks’ axis. The collective motions in both nematic and isotropic phases are also affected. Acknowledgements Funding of this work was provided by Fundação para a Ciência e a Tecnologia (FCT), Project PEst-C/CTM/LA0025/2022 (Strategic Project - LA 25-2001/2012).
References [1] J. Lydon, Liq. Cryst. 2011, 38, 1663–1681. [2] P. J. Collings, A. J. Dickinson, E. C. Smith, Liq. Cryst. 2010, 37, 701–710. [3] S. V. Shiyanovskii, T. Schneider, I. I. Smalyukh, T. Ishikawa, G. D. Niehaus, K. J. Doane, C. J. Woolverton, O. D. Lavrentovich, Phys. Rev. E 2005, 71, 020702.
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J. P. de Almeida Martins, F. V. Chávez and P. J. Sebastião [4] V. G. Nazarenko, O. P. Boiko, M. I. Anisimov, A. K. Kadashchuk, Y. A. Nastishin, A. B. Golovin, O. D. Lavrentovich, Appl. Phys. Lett. 2010, 97, 263305. [5] N. Ould-Moussa, C. Blanc, C. Zamora-Ledezma, O. D. Lavrentovich, I. I. Smalyukh, M. F. Islam, A. G. Yodh, M. Maugey, P. Poulin, E. Anglaret, M. Nobili, Liq. Cryst. 2013, 40, 1628–1635. [6] Y. Yi, N. A. Clark, Liq. Cryst. 2013, 40, 1736–1747. [7] F. Chami, M. R. Wilson, J. Am. Chem. Soc. 2010, 132, 7794–7802. [8] S. Zhou, Y. A. Nastishin, M. M. Omelchenko, L. Tortora, V. G. Nazarenko, O. P. Boiko, T. Ostapenko, T. Hu, C. C. Almasan, S. N. Sprunt, J. T. Gleeson, O. D. Lavrentovich, Phys. Rev. Lett. 2012, 109, 037801. [9] D. J. Edwards, J. W. Jones, O. Lozman, A. P. Ormerod, M. Sintyureva, G. J. T. Tiddy, J. Phys. Chem. B 2008, 112, 14628–14636. [10] H. S. Park, S. W. Kang, L. Tortora, Y. Nastishin, D. Finotello, S. Kumar, O. D. Lavrentovich, J. Phys. Chem. B 2008, 112, 16307–16319. [11] J. W. Jones, L. Lue, A. P. Ormerod, G. J. T. Tiddy, Liq. Cryst. 2010, 37, 711–722. [12] P. G. de Gennes, J. Prost, The Physics of Liquid Crystals (2nd ), Clarendon Press, Oxford, 1993. [13] H. Cachitas, P. J. Sebastião, G. Feio, F. Vaca Chávez, Liq. Cryst. 2014, 41, 1080–1089.
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[14] A. F. Kostko, B. H. Cipriano, O. A. Pinchuk, L. Ziserman, M. A. Anisimov, D. Danino, S. R. Raghavan, J. Phys. Chem. B. 2005, 109, 19126–19133. [15] S. K. Prasad, G. G. Nair, G. Hegde, V. Jayalakshmi, J. Phys. Chem. B 2007, 111, 9741–9746. [16] D. Sousa, G. Domingos Marques, J. M. Cascais, P. J. Sebastião, Solid State NMR 2010, 38, 36–43. [17] R. Y. Dong, Nuclear Magnetic Resonance Spectroscopy of Liquid Crystals, World Scientific, Singapore, 2010. [18] P. J. Sebastião, C. Cruz, A. C. Ribeiro, Nuclear Magnetic Resonance Spectroscopy of Liquid Crystals, 2010, pp. 129–167. [19] S. Žumer, M. Vilfan, Mol. Cryst. Liq. Cryst. 1981, 70. [20] R. Dong, Nuclear Magnetic Resonance of Liquid Crystals, vol. 2, Springer-Verlag, New York, 1997. [21] F. Vaca Chávez, P. J. Sebastião, Y. Miyake, H. Monobe, Y. Shimizu, J. Phys. Chem. B 2012, 116, 2339–2346. [22] J. H. Freed, J. Chem. Phys. 1977, 66, 4183–4199. [23] J. W. Emsley, Nuclear Magnetic Resonance of Liquid Crystals, D. Reidel Publishing Company, Dordrecht, 1985. [24] P. J. Sebastião, Eur. J. Phys. 2014, 35, 015017. [25] F. James, M. Roos, CERN Program Library Long Writeup, 1992, D506.
Copyright © 2014 John Wiley & Sons, Ltd.
Magn. Reson. Chem. (2014)
Revised: 15 May 2014
Accepted: 18 May 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/mrc.4091
NMR molecular dynamics study of chromonic liquid crystals Edicol Sunset Yellow doped with salts João P. de Almeida Martins,a Fabián Vaca Chávezb and Pedro J. Sebastiãoa,b* We investigate the effect of monoatomic salts on the molecular dynamics in the nematic and isotropic phases formed by the chromonic liquid crystal Edicol Sunset Yellow. The study was carried out using proton nuclear magnetic resonance relaxometry. To analyse the effect of incorporation of additional sodium chloride or lithium chloride on the solutions’ molecular dynamics, the spin-lattice relaxation time was measured for Larmor frequencies between 10 kHz and 100 MHz. In the nematic phase, the presence of additional sodium or lithium ions seems to contribute to an increase of the rotations/reorientations correlation times in comparison with the mixture without extra ions. The collective motions detected by proton NMR relaxometry are associated with collective fluctuations of molecules within the stacks in the nematic phase and with order parameter fluctuations in the isotropic phase. Copyright © 2014 John Wiley & Sons, Ltd. Keywords: NMR; dynamics; liquid crystals; choromonics; relaxation; salt
Introduction Unlike common lyotropics, chromonic mesophases are formed through the aggregation of non-amphiphilic molecules, which have a plank-like or disk-like polyaromatic central core with polar groups at the periphery [1,2] . Instead of forming micelles, those molecules stack on top of each other “face-to-face” to minimize the areas of unfavourable contact with water, leaving the ionic solubilizing groups at the aggregate/water interface. The mesogenic unit of chromonic mesophases are then columns of stacked molecules that lie in a continuum electrolyte fluid. For certain values of temperature and concentration, the molecular stacks still exist in the Isotropic (I) liquid phase. The most common mesophases in chromonic liquid crystals are the uniaxial nematic (N) phase and the hexagonal columnar (M) phase. Between two phases, in a transition from one phase to another, a two-phase state where the initial and final phases coexist is observed. For example, in the process of obtaining the N phase by cooling of an Isotropic (I) phase, we can observe a N C I state where domains of nematic substance coexist with an isotropic environment. In recent years, increasing attention has been given to chromonics in view of their potential technological applications such as the production of biosensors in the detection of antigens [3] and films with semi-conducting properties [4] . Ould-Moussa et al. reported the use of nematic chromonic phase to orient and disperse single-walled carbon nanotubes [5] and Yi and Clark showed the possibility of aligning the molecular stacks in chromonic liquid crystal phases when confined in cells patterned with line channels [6] . Therefore, the study of their physico-chemical properties is a matter of concern and is currently an active topic of research. Most studies reported in the literature have been focused on disodium cromoglycate (DSCG), which is a drug used on the treatment of asthma, but in the last years chromonic phases formed by Edicol Sunset Yellow (ESY), synthetic yellow azo dye used in
Magn. Reson. Chem. (2014)
the food industry, have been a subject of both theoretical and experimental studies [7,8] . Chami and Wilson reported results from molecular dynamics simulations that describe the dynamics and structure of aggregates of ESY molecules. Experimental works made use of techniques such as X-ray, optical microscopy, and NMR (1 H, 2 H and 23 Na) spectroscopy [9–11] . Zhou et al. measured the temperature and concentration of the Frank elastic constants splay (K1 ), twist (K2 ), and bend (K3 ) [12] using a magnetic Frederiks transition technique in the nematic phase [8] . Recently, proton NMR relaxometry and pulse gradient NMR techniques were used to study the molecular dynamics in the chromonic nematic and isotropic phases of the binary mixture composed by ESY and heavy water [13] . Here, we extend that work and we investigate the effect of sodium chloride and lithium chloride on the molecular dynamics of the same chromonic compound. The works reported in the literature are not just focused on the characterisation of chromonic liquid-crystalline systems formed by aqueous solutions but also on the effect of the addition of salts. Investigation using X-Ray, optical, rheologycal, NMR, and dielectric techniques were used to characterise the phase behavior and structure of the mixtures. Kostko et al. investigated the addition of sodium and potassium salts to the nematic phase of DSCG [14] . Park et al., Jones et al.,
* Correspondence to: Pedro J. Sebastião, Departamento de Física, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal. E-mail: [email protected] a Departamento de Física, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001, Lisboa, Portugal b Condensed Matter Physics Centre, Universidade de Lisboa, Av. Prof. Gama Pinto 2, 1649-003, Lisboa, Portugal
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J. P. de Almeida Martins, F. V. Chávez and P. J. Sebastião and Prasad et al. reported the addition of ionic compounds to the aqueous solution of ESY [10,11,15] .
Experimental Section Samples preparation Edicol Sunset Yellow, sodium chloride, and D2 O (99.98 atom % 2 H) were purchased from Sigma-Aldrich (Munich, Germany). Lithium chloride was obtained from Merck KGaA (Darmstadt, Germany). The received ESY was further purified according to the procedure reported in the literature [9] . The product was dissolved in Milli-Q water with a resistivity higher than 18.2 Mcm, followed by addition of ethanol. The dye precipitate was recovered and the process was repeated four more times. Finally, the ESY sample was dried at 60 °C, in an oven, to remove residual solvents. Heavy water, instead of normal water, was used to prepare the samples in order to make possible to distinguish solvent from solute molecules in the 1 H NMR measurements. These ESY/heavy water mixtures were prepared by weight of the chromonic molecules: 27.8 wt% and 26 wt%, respectively. The concentration of NaCl and LiCl was 0.5 mol/kg. In the following, the samples without salts will be designated as SY28 and SY26, the mixtures with salts will be designated as SY28-X and SY26-X, where X=Li or Na. The samples were placed in standard NMR tubes and flame sealed to avoid water evaporation. In Fig. 1 a schematic of chromonic nematic and isotropic phases (upper panels) and the corresponding textures observed on a microscope under cross polarisers (lower panels) are presented. The molecular structure of the NH hydrazone tautomer of the ESY is also shown. NMR measurements The proton spin-lattice relaxation rate, R1 . 1=T1 /, was measured over more four Larmor frequency, , decades, from 10 kHz to 300 MHz. For the frequency range 10–91 MHz, a variable-field iron-core magnet equipped with a Bruker Avance II console was used. At 300 MHz, R1 was measured with a Bruker Avance II spectrometer. The R1 measurements in the conventional spectrometers were made using the standard inversion recovery ./x . 2 /x,x Acq sequence. For frequencies below 10 MHz, the R1 data were obtained with a home developed fast field-cycling
Figure 1. Schematic of chromonic nematic and isotropic phases (upper panels) and the corresponding textures observed on a microscope under cross polarisers (lower panels). The molecular structure of the NH hydrazone tautomer of the Edicol Sunset Yellow is also shown.
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spectrometer [16] . A usual field cycle BP ! BE BE ! BD , where the polarisation field, BP , and detection field, BD , respectively, have the same value corresponding to the Larmor frequency of 8.9 MHz was used. The switching times were less than 3 ms. Time is varied between zero and about five times the expected value of the spin-lattice relaxation time for the low magnetic field value. A conventional =2 r.f. pulse was used to obtain the FID signal after each cycle. In all cases, the temperature was regulated and stabilised to within ˙0.2 °C using air flow.
Relaxation Mechanisms and Models Based on the assumption that the molecular motions present in a liquid crystal are statically independent or otherwise are characterised by distinct correlation times, the experimental relaxation rate R1 is usually assumed to be described by a linear combination of relaxation contributions, each one associated with a specific type of motion. Cross-terms can be neglected as the characteristic correlation times associated with those motions are usually considerably different. Then, the total relaxation rate can be expressed as R1 D R1,C C R1,IM (1) where C and IM refer to collective motions and individual molecular motions, respectively. Here, we summarise the relaxations models used to describe the results. More details can be found in ref. [17,18] . a) Collective motions. i) In the nematic phases of thermotropic and lyotropic liquid crystals, these motions are described as three-fold fluctuations of the director in laboratory axis frame and are known as order director fluctuations (ODF), where three-fold means that ODF propagate in the three dimensions. But in the case of the chromonic nematic phase, taking into account that the units are molecular stacks, it was shown that collective motions are described by elastic column deformations (ECD) [13,19] . The corresponding spectral density is Z Z q? dq? q4jj dqjj kT q?,h qjj,h JECD ./ D 2 , 2 q?,l qjj,l .2/2 2 q4jj C.K3 q4jjCBq2? /2 (2) where B and K3 are the elastic constants for bending and compression of the columns, respectively. q?,h and qjj,h are the components of the largest wave vectors of the deformation parallel and perpendicular to the columns, respectively. The shortest wave vectors are q?,l and qjj,l . High and low cut-off frequencies can be defined in terms of the limit wave vectors and the viscoelastic constants. ii) As it was shown previously, in the isotropic phase of ESY, order parameter fluctuations (OPF) can be present and the spectral density is [20,21] p Z xmax x dx JOPF ./ D AOPF 2 , (3) 2 xmin .2/ C x C o1 with o related to the coherence length of these fluctuations. The limits of integration are related to the
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Magn. Reson. Chem. (2014)
Effect of salts on the molecular dynamics of chromonic sunset yellow
Figure 2. (a) Spin-lattice relaxation rate profiles of the samples SY28, SY28-Na, and SY28-Li at 15 o C in the nematic phase. The inset shows a comparison between the two nematic phases at different Edicol Sunset Yellow concentration and temperatures. The solid lines are guide for the eyes. (b) Proton spectra corresponding to the samples shown in (a).
jumps, d is the molecular width, n is the density ˝ ˛ of 1 H spins. The mean-square jump distance r2 is related with˝ the ˛ self-diffusion constant D and D by the relation r2 D 6D D.
minimum and the maximum wave vectors and AOPF is a constant related with the viscoelastic properties of the liquid crystal in the isotropic phase. b) Individual molecular motions. i) Local rotations/reorientations. In the case of anisometric molecules, a model to describe the relaxation induced by reorientations of rigid elongated molecules was proposed by Woessner [20] . The spectral densities are given by 2 2 .˛ij /j2 4 X jdk0 jmj Jk ./ D ck hjD2km . /ji , 2 3 mD2 1C4 2 2 jmj a6ij (4) where aij are the inter-proton distance (2.5 Å in the case of ESY, from ref. [9] ) and ˛ij denotes the angle between the inter-proton vector and the long molecular axis. D2km is the second rank Wigner rotation matrix and the averages hjD2km . /ji can be expressed in terms of the second and fourth rank Legendre polynomials of the angle between the long molecular axis and the nematic director [22] . 2 dkm .˛ij / is the second rank reduced Wigner rotation matrix. c0 D 6, c1 D 1, and c2 D 4. The corresponding correlation times, jmj , can be expressed as 0 D s , 1 D .s1 C f1 /1 , and 2 D .s1 C 4f1 /1 , where f and s are two correlation times corresponding to molecular rotations/reorientations about the axis parallel to the long molecular axis and reorientations about an axis perpendicular to the molecular symmetry axis, respectively. In the present work, they correspond to reorientations parallel and perpendicular to the packing axis. ii) translational self-diffusion. We used the model proposed by Torrey [20] . The corresponding relaxation rate can be written as nD 1 D Cd 3 ŒJ .// C 4J .2/ , (5) T1 Diff d
where the dimensionless spectral density J ./ can be calculated analytically [20] . Cd D .1=2/ .3 0 2 „=.8//2 is the strength of the dipolar interaction, D is the average time between diffusion
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Results and Discussions Nematic phase Figure 2a illustrates the proton R1 NMRD profiles for the samples SY28, SY28-Li, and SY28-Na at 15 o C. The temperature corresponds to the nematic phase for all samples. The nuclear magnetic relaxation dispersion (NMRD) profiles present a rather small dependence on the presence of the salts. In fact, at high frequencies the data sets overlap for the three samples. The minor differences are observed at intermediate frequencies (1–10 MHz) and at low frequencies (below 50 kHz). Besides these small differences in the NMRD profiles, as a result of the presence of salts, the nematic order reflected on the separation of the outer lines in the 1 H spectrum shown in Fig. 2b clearly shows a difference. The dipolar splitting from the outer peaks is 7.13 kHz for the SY28 and 6.73 kHz for SY28-Li and SY28-Na. Following the procedure described by Edwards et al. [9] , who correlated the proton spectrum in the nematic phase of ESY with the protons present in the molecule, these splittings correspond to an order parameter of S = 0.57 for SY28 and S = 0.54 for the SY28-Li and SY-Na [23] . The inset in Figure 2a is a comparison between two nematic phases at different ESY concentration, 30% and 28%, and Table 1. Details of the fitting parameters according to the proposed models to describe the frequency and temperature dependence of R1 for the sample SY28 in the nematic phase
f [109 s] s [109 s] AECD [103 s2 ] D [1013 m2 /s] S
SY28
SY28-Na
SY28-Li
1.2* (1.2 ˙ 0.2) 4.1 ˙ 0.5 13.5 ˙ 0.6 5.5* (5.5 ˙ 0.7) 0.57
1.9 ˙ 0.4 6.2 ˙ 0.6 96 ˙ 5 4.7 ˙ 0.6 0.54
1.6 ˙ 0.3 4.7 ˙ 0.5 338 ˙ 17 4.3 ˙ 0.6 0.54
* Fixed value from ref. [13] . The value between parenthesis was obtained when this parameter was included in the fit (see text for details).
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J. P. de Almeida Martins, F. V. Chávez and P. J. Sebastião
Figure 3. Fits of the R1 dispersions in the nematic phase corresponding to (a) SY28, (b) SY28-Na, and SY28-Li at 15 o C. The solid curve corresponds to the total fitting curve given by Eqn. (1), and the separate contributions for the proposed dynamic mechanisms are shown. See text for details.
temperatures, 23 o C and 15 o C, respectively. Qualitatively, the two profiles present a similar behaviour but at intermediate and low frequencies (. 10 MHz) they show some differences, whereas at high frequencies they look identical. In order to interpret the R1 data, a fit was performed using Eqn. (1) with a home written minimization software package [24] . For a detailed analysis, the two individual motions contributions in R1,IM are shown separately. Based on the fact, as pointed before, that the profiles of the samples SY28 and SY30 coincide at high frequencies, the fit of the former was carried out fixing the value of f to the corresponding one of SY30 at 23 o C from ref. [13] . Also, the self-diffusion coefficient was extracted from the same work as an extrapolation to 15 o C. The results of the fitting parameters are presented in Table 1 and the fitted experimental data are presented in Fig. 3 together with the total R1 curve and the individual relaxation contributions corresponding to the proposed dynamic mechanisms. On Table 1, between parenthesis, we also present the corresponding values of f and D for the sample SY28, which were obtained by fitting the data fixing the other parameters. As it is observed, the results support our assumptions of fixing them based on our previous findings. The estimation of the parameters’ errors was performed, taking into account the sensitivity of the global fit’s 2 (all data sets simutaneously) at the minimum, to a change of each fitting parameter individually, within a confidence level of 68% [25] . The slight decrease of the self-diffusion coefficient of the samples with salt agrees with the finding that the viscosity of ESY solutions increases with the addition of salts reported by Prasad et al. [15] . Also, the increase in the correlation times for the rotations/reorientations might be associated with the reported increase of viscosity. The strength of the ECD, AECD on Table 1, increases upon the addition of salts. This finding can be related not only to the increase on the viscosity but also related to the variation of the elastic constants.
Figure 4. Comparison of the spin-lattice relaxation rate profiles of pure Sunset Yellow/water solution at 24% and 26% in the isotropic phase. The solid lines are guide for the eyes.
Isotropic phase Figure 4 displays the NMRD profiles of the samples of pure ESY/D2 O solutions at 24% and 26% in the isotropic phase. As it can be observed, the spin-lattice relaxation profile obtained for the larger concentration solution SY26 is very close to the one obtained for the lower concentration SY24 at a lower temperature. Also, the profile at low frequencies obtained for the SY24
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Figure 5. Spin-lattice relaxation rate profiles of pure Sunset yellow/water solution and with the addition of 0.5 mol/kg of LiCl and NaCl salts in the isotropic phase at 50 °C. The solid lines are guide for the eyes.
Copyright © 2014 John Wiley & Sons, Ltd.
Magn. Reson. Chem. (2014)
Effect of salts on the molecular dynamics of chromonic sunset yellow
Figure 6. Fits of the R1 dispersions in the isotropic phase corresponding to (a) SY26, (b) SY26-Na and SY26-Li at 50 o C. The solid curve corresponds to the total fitting curve given by Eqn. (1) and the separate contributions for the proposed dynamic mechanisms are shown. See text for details.
Table 2. Details of the fitting parameters according to the proposed models to describe the frequency and temperature dependence of R1 for the sample SY26 in the isotropic phase at 50 o C SY26 9
f [10 s] 2.5* (2.5 ˙ 0.2) s [108 s] 4.4 ˙ 0.4 AOPF [103 s2 ] 435 ˙ 24
SY26-Na 2.5* (2.5 ˙ 0.2) 2.4 ˙ 0.4 769 ˙ 33
SY26-Li 2.5* (2.4 ˙ 0.2) 2.6 ˙ 0.4 533 ˙ 27
*Fixed value from ref. [13] . The values between parenthesis were obtained when these parameters were included in the fit (see text for details).
sample at a higher temperature is different than that of the other two profiles. In Fig. 5, the results of the R1 dispersion profiles of the samples SY26, SY26-Li, and SY26-Na at 50 o C in the isotropic phase are shown. The differences barely detected in the NMRD profiles in the nematic phase of the SY28, SY28-Li, and SY28-Na samples as the result of the presence of the salts are clearly more expressive in the shape of the profiles obtained in the isotropic phase of the samples SY26, SY26-Li, and SY26-Na. The differences are observed below 10 MHz, whereas above this frequency, the profiles are identical within the experimental uncertainty of ˙5%. In Fig. 6 are presented the fits performed using model Eqn. (1), considering the presence of order parameter fluctuations as previously referred and the results of the fitting parameters are presented on Table 2. In the same way as we proceeded in the previous section, Table 2 presents between parenthesis the values of f obtained by fitting the data fixing the other parameters. Here, the results also support our assumptions of fixing them based on our previous findings. As it was observed in a previous NMR relaxometry study of ESY [13] , in the isotropic phase, the self-diffusion relaxation mechanism presents a small contribution to the relaxation. The value of the self-diffusion constant considered in the fits was that measured for SY24 at 43 o C[13] . As the R1 profiles in the high frequency regime are similar, it is reasonable to consider that the fast molecular motions in the three samples are characterised by similar correlation times. In fact, very good fits were obtained using the same correlation time for the molecular rotations around the molecular axis perpendicular to the ESY molecular plane. The effect of the presence of the salts in the molecular dynamics are observed in the regions where order parameter fluctuations
Magn. Reson. Chem. (2014)
and reorientation of the molecules with respect to the stacks’ axis affect the relaxation dispersion, that is, at frequencies below 10 MHz. The values of the correlation times for the reorientations of the molecules about an axis perpendicular to the stacks’ axis decrease with the presence of the salts contrary to the effect of salts in the nematic phase. The fact that the correlation time S does not change by the same amount as a result of the presence of NaCl or the LiCl might be related with the differences between the two ions in the equilibrium and stabilisation of the ESY stacks. In the same way as in the nematic phase, the strength of the collective motions, AOPF on Table 2, increases upon the addition of salts.
Conclusions It is presented a proton NMR relaxation study of the molecular dynamics in two series of ESY systems prepared with two concentrations: 26 wt% and 27.8 wt%. Samples were prepared by adding 0.5 mol/kg of NaCl and LiCl salts in order to understand the effect of the presence of the additional NaC and LiC ions in the system. The samples prepared with 27.8 wt% ESY present nematic phases at 15 °C. The study of this samples in the isotropic phase would require the heating of the sample to temperatures so high that possible evaporation effects might not be negligible. Therefore, additional samples were prepared with 26 wt% ESY that present the isotropic phase at 50 °C. The influence of the salts in the molecular dynamics of the ESY systems was barely detected in the nematic phase of the ESY systems, but could be perceived more clearly in the isotropic phase. The presence of the salts contributes to modify the reorientations of the molecules about an axis perpendicular and parallel to the stacks’ axis. The collective motions in both nematic and isotropic phases are also affected. Acknowledgements Funding of this work was provided by Fundação para a Ciência e a Tecnologia (FCT), Project PEst-C/CTM/LA0025/2022 (Strategic Project - LA 25-2001/2012).
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