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Phase transition and thermal characterization of induced smectic phases in a ternary mixture
Journal of Molecular Liquids 202 (2015) 75–78
Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Phase transition and thermal characterization of induced smectic phases in a ternary mixture T.N. Govindaiah ⁎, H.R. Sreepad Post-Graduate Department of Physics, Government College (Autonomous), Mandya 571401, India
a r t i c l e
i n f o
Article history: Received 28 August 2014 Received in revised form 1 November 2014 Accepted 9 December 2014 Available online 11 December 2014 Keywords: Ternary mixture Temperature dependence Optical texture Optical anisotropy
a b s t r a c t Two or more multi-component system of didodecyl dimethyl ammonium bromide (DDAB), cholesteryl nonanoate (CN) and ethylene glycol (EG) exhibits an interesting different liquid crystalline cholesteric and induced smectic phases, such as SmA, SmC and SmB, sequentially when the specimen is cooled from its isotropic liquid phase. These phases have been characterized by using optical texture studies. The temperature variations of optical anisotropy, helical pitch measurements and electrical conductivity have been also discussed. XRD results have also been reported. © 2014 Published by Elsevier B.V.
1. Introduction Liquid crystals are a special class of soft materials characterized by so called mesophase, where they flow like an isotropic liquid yet possess a long-range orientational order and a complete or partial absence of positional order of building units that can either be individual molecules or their aggregates [1]. The two main types of liquid crystals are thermotropic liquid crystals and lyotropic liquid crystals. Thermotropic liquid crystals show mesophases depending on temperature and pressure. Their basic building units are usually individual molecules that have a feature of pronounced shape anisotropy, such as rods and disk. Thermotropic liquid crystals have been successfully used in display devices. Lyotropic liquid crystals are formed on the dissolution of lyotropic liquid crystal molecules in a solvent (usually water). A feature of lyotropic liquid crystals distinguishing them from thermotropic liquid crystals is the self-assembly of molecules into supramolecular structures that represent a basic unit of these mesophases [2,3]. The most common lyotropic liquid crystalline system is those formed by water and surfactants, such as soaps, synthetic detergents, and lipids. Surfactant molecules are formed by a hydrophilic part chemically bound to a hydrophobic part. Mixtures of these surfactant molecules with a solvent under certain conditions of temperature and relative concentrations produced the different types of liquid crystalline mesophases such as cholesteric, nematic, lamellar, discotic, twisted grain boundary (TGB) phase, and blue phase [4].
⁎ Corresponding author at: P.G. Department of Physics, Government College (Autonomous), Mandya 571401, India. E-mail address: [email protected] (T.N. Govindaiah).
http://dx.doi.org/10.1016/j.molliq.2014.12.012 0167-7322/© 2014 Published by Elsevier B.V.
In the present investigation, our aim is to study the ternary mixture of different compounds, namely, cholesteryl nonanoate (CN), Didodecyl dimethyl ammonium bromide (DDAB) and ethylene glycol (EG), which exhibits an interesting liquid crystalline cholesteric phase and induced smectic phases, such as SmA, SmC and SmB phases sequentially when they are cooled from isotropic phase. Optical, DSC, thermal and X-ray studies have been carried out to understand the intermolecular interactions in the mixture. 2. Experimental The compound didodecyl dimethyl ammonium bromide (DDAB) used in this investigation was obtained from the Basic Pharma Life Science Pvt., Ltd., India, and it was further purified twice by a recrystallization method using benzene as a solvent. Ethylene glycol (EG) was supplied from Kodak, Ltd., Kodak house, Mumbai, India. The cholesteryl nonanoate (CN) was obtained from M/s East Mann Organic Chemicals, USA. Mixtures of different concentrations of DDAB in CN + EG were prepared and were mixed thoroughly. These mixtures of various concentrations of DDAB in CN + EG were kept in desiccators for a long time. The samples were subjected to several cycles of heating, stirring, and centrifuging to ensure homogeneity. The phase transition temperatures of these concentrations were measured with the help of Leitzpolarizing microscope in conjunction with a hot stage. The samples were sandwiched between the slide and cover slip and were sealed for microscopic observations. The differential scanning calorimetry (DSC) thermograms were taken for the mixtures of all concentrations using Perkin-Elmer DSC II Instrument facility available at Raman Research Institute, Bangalore, India. The X-ray broadening peaks were obtained at different temperatures using JEOL diffractometer. The density
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and refractive indices in the optical region are determined at different temperatures by employing the techniques described by the earlier investigators [5,6]. Electrical-conductivity measurements of the mixture at different temperatures were carried out using digital LCR meter and a proportional temperature control unit. 3. Results and discussion 3.1. Phase diagram The ternary mixture of DDAB in CN + EG exhibits an interesting different liquid crystalline phases and the phase transition temperatures are measured by using Leitz-polarizing microscopic. The partial phase diagram shown in Fig. 1, which is obtained by plotting the concentrations against the phase transition temperatures of the mixture, which clearly illustrates that, the mixture of all concentrations of DDAB in CN + EG exhibit a SmA and SmB phases respectively at different temperatures, when the specimen is cooled from its isotropic liquid phase. The concentrations of the mixture from 5% to 22% and 32% to 60% of DDAB shows a cholesteric phase in addition of SmA, SmC and SmB phases, but in the concentration range from 8% to 50% of DDAB shows a schlieren texture of SmC phase.
a). Fingerprint pattern texture of cholesteric phase (250X).
3.2. Optical texture studies For the purpose of optical texture studies, the sample was sandwiched between a slide and cover glass, and then the optical textures were observed using a Leitz polarizing microscope in conjunction with a specially constructed hot stage. The concentrations ranges from 5% to 22% and 32% to 60% of the mixture are slowly cooled from its isotropic melt, the genesis of nucleation starts in the form of small bubbles and slowly grow radially, which form a fingerprint pattern texture of cholesteric phase with large values of pitch is shown in Fig. 2(a) [7,8]. On further cooling the specimen, the cholesteric phase slowly changes over to focal conic fan shaped texture, which is the characteristics of SmA phase as shown in Fig. 2(a). The concentrations from 8% to 50% of the mixture, the SmA phase changes over to schlieren texture of SmC. On further cooling the specimen, SmC phase changes over to SmB phase, which remains up to room temperature and then it becomes a crystalline phase [9]. 3.3. Optical anisotropy Results of this investigation are further supported by the optical studies. The refractive indices for extraordinary ray (ne) and ordinary ray (no) of the mixture were measured at different temperatures for the different concentrations using Abbe refractometer and precision goniometer spectrometer. The temperature variations of refractive indices
b). Focal conic fan shaped texture of SmA phase (250X). Fig. 2. Microphotographs obtained in between the crossed polars. a). Fingerprint pattern texture of cholesteric phase (250×). b). Focal conic fan shaped texture of SmA phase (250×).
for 20% of DDAB in CN + EG are shown in Fig. 3. The values of electrical susceptibility for 20% of DDAB in CN + EG have been calculated using Neugebauer relation [10] at different temperatures. The temperature variations of electrical susceptibility for the mixture are as shown in Fig. 4. From this figure, it can be observed that wherever there is a phase transition, the value of electrical susceptibility changes appreciably, which indicates that each change corresponds to induced mesomorphic phases. Further with increasing the temperature of given concentration of DDAB, the value of electrical susceptibility decreases, because the effective optical anisotropy associated with the molecules of DDAB also decreases. But here in this graph, we have observed an auxiliary peak in addition to the main peak, which illustrates that, the peak cannot be thought only due to change in the orientation of molecules. They can be attributed to changes in the dimension of molecules along with changes in orientation. 3.4. Helical pitch (smectic and cholesteryl layers) measurements
Fig. 1. Partial phase diagram for the mixture of DDAB in CN + EG.
The helical pitch measurements were performed on the cholesteric phase following the well-known Grandjean–Cano wedge method [11,
T.N. Govindaiah, H.R. Sreepad / Journal of Molecular Liquids 202 (2015) 75–78
77
Fig. 3. Temperature variation of refractive indices for the mixture of 20% DDAB in CN + EG. Fig. 5. The temperature variations of pitch for the mixture of 20% DDAB in CN + EG.
12]. The ternary mixture was taken in a wedge-shaped cell treated for homogeneous alignment. The two glass plates formed a small angle at the wedge. The mixture was cooled slowly (0.2 °C min−1) from isotropic cholesteric phase to smectic phase, which induces an array of equidistant Grandjean–Cano lines. The pitch of the cholesteric phase was determined by measuring the distance between the Grandjean–Cano lines as a function of temperature. As the temperature was lowered from the cholesteric phase to smectic phase, spacing between the lines are increased, indicating that the pitch in the cholesteric phase is also increasing. The temperature variation of pitch for the mixture of 20% DDAB in CN + EG is shown in Fig. 5. From this figure, it is evident that, the variation of pitch from cholesteric to smectic phase is smooth and continuous. But gradually, the value of pitch increases from 0.17 to 0.19 mm was observed upon cooling the sample from cholesteric phase to smectic phase. The value of the pitch increased steeply and reached a maximum of 0.5 mm at the cholesteric to smectic phase transition. But in the studies, we have noticed that, the sequence is Iso → Cho → SmA → SmC → SmB phases on cooling. Most of the data about the helical pitch have been given previously in Ref. [13]. The pitch is continuous at the cho → smectic transition in spite of a rather energetic transition. It increases on cooling to smectic phase and diverges on approaching the SmA and SmC phases. This divergence related to the second-order nature of the transition. It exhibits a steep decrease, close to cholesteric phase which is usually the characteristics of second-order SmA, SmC phase transitions.
Fig. 4. Temperature variation of electric susceptibility for the mixture of 20% DDAB in CN + EG.
3.5. Conductivity measurements Electrical-conductivity measurements help in getting better idea on the phase behavior with temperature. An abrupt increase or decrease of electrical-conductivity with temperature relates to the phase behavior of the lyotropic and thermotropic systems [14]. The temperature variations of electrical-conductivity are shown in Fig. 6. The changes were observed in electrical-conductivity, the values correspond to liquid crystalline phase transition of thermotropic and lyotropic systems respectively at different temperatures and they were also identified by DSC and optical texture studies. It was observed that, a change in electrical-conductivity at temperatures 70 °C, 85 °C, 110 °C and 127 °C respectively, which corresponds to phase transition from SmB → SmC, SmC → SmA and SmA → Cho phases. This type of behavior is generally observed in hexagonal, cubic and lamellar phase of lyotropic and thermotropic systems [15–17]. 3.6. Characterization of nano-aggregated grains The X-ray diffractometer traces obtained for the mixture of 20% DDAB in CN + EG at temperature 60 °C is shown in Fig. 7. The diffraction
Fig. 6. Temperature variation of electrical-conductivity σ (Ω−1 m−1) for the sample 20% of DDAB in CN + EG.
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4. Conclusions In light of the above results, we have drawn the following conclusions. The multi-component system exhibits an unusual sequence of phases, showing the formation of induced chiral smectic phases in the concentration range of 5% to 60% of DDAB in (CN + EG). The phase behavior is discussed with the help of phase diagram. The changes in value of electrical conductivity with the variation of temperature unambiguously correspond to smectic and cholesteric phases. Drastic changes in electrical conductivity are expected to be due to changes in the dimension of disks along with changes in the orientation order of the arrangement. The nano-aggregated grain size of liquid crystalline material for smectic-B phase comes out to be 45.3 nm. References Fig. 7. XRD traces obtained for the mixture of 20% DDAB in CN + EG.
peaks at this temperature correspond to smectic-B phase (by using JEOL diffractometer with the settings: TC4, CPS400, channel width 100 for λ = 1.934 Å). X-ray diffraction study is an important method to determine the nano-aggregated grain size of the molecules for different liquid crystalline phases [18,19]. The deviation from perfect liquid crystallinity leads to broadening of the diffraction peaks. In order to estimate nano-aggregated grain size of the molecules for different liquid crystalline phases corresponding to broadening of X-ray diffraction peaks we have used the Scherrer's formula L ¼ Kλ=β cosθ; where L is the nano-aggregated grain size, λ is the wave length of X-ray radiation (Fe: 1.934 Å), K is usually taken as 0.89, β is the line width at half maximum and θ is the diffraction angle. Usually with decrease of temperature [20,21], the nano-aggregated grain size of the molecules increases. The structure of molecular orientation for broken banded focal conic fan texture of smectic-B phase is stable and the molecular ordering of this phase shows three intense peaks. The nano-aggregated grain size of liquid crystalline material for smectic-B phase comes out to be 45.3 nm.
[1] M. Kleman, O.D. Lavrentovich, Soft Matter Phys, Springer- Verlag, New York, 2003. [2] A.M. Figueiredo Neto, S.R.A. Salinas, Phys. Lyotropic Liq. Cryst. Oxford University Press, 2005. [3] A.G. Petrov, Lyotropic State Matter, Gordon and Breach Publs., NJ, 1999. [4] T.N. Govindaiah, H.R. Sreepad, Nagappa, Mol. Cryst. Liq. Cryst. 574 (2013) 9–18. [5] Nagappa, S.K. Nataraju, D. Krishnamurti, Mol. Cryst. Liq. Cryst. 133 (1986) 31–54. [6] J. Thiem, V. Vill, F. Fischer, Mol. Cryst. Liq. Cryst. 170 (1989) 43–51. [7] D. Demus, C. Richter, Texture Liq. Cryst. Verlag Chemie, Weinheim, 1978. [8] Nagappa, D. Revanasiddaiah, D. Krishna Krishnamurti, Mol. Cryst. Liq. Cryst. 101 (1983) 103–127. [9] T.N. Govindaiah, H.R. Sreepad, Phase Transit. 87 (2014) 729–736. [10] H.E.J. Neugebauer, Can. J. Phys. 32 (1954) 1–8. [11] H.T. Nguyen, R.J. Tweig, M.F. Nabor, H. Isaert, C. Destrade, Ferroelectrics 121 (1991) 187–204. [12] A. Bouchta, H.T. Nguyen, M.F. Achard, F. Hardouin, C. Destrade, R.J. Tweig, N. Isaert, Liq. Cryst. 12 (1992) 575–591. [13] H.T. Nguyen, A. Bouchta, L. Navailles, P. Barois, N. Isaert, R.J. Twieg, A. Maaroufi, C. Destrade, J. Phys. II France 2 (1992) 1889–1906. [14] M. Marthandappa, Nagappa, K.M. Lokhanatha Rai, J. Phys. Chem. 95 (1991) 6369–6372. [15] J. Franeois, Kolloid Z.Z. Polym. 246 (1971) 606. [16] T.N. Govindaiah, H.R. Sreepad, P.M. Sathyanarayana, J. Mahadeva, Nagappa, Mol. Cyst. Liq. Cryst. 552 (2012) 24–32. [17] T.N. Govindaiah, H.R. Sreepad, Nagappa, Mol. Cyst. Liq. Cryst. 592 (2014) 91–98. [18] T.N. Govindaiah, H.R. Sreepad, Nagappa, Phase Transit. (2014), http://dx.doi.org/10. 1080/01411594.2014.966308 (ID: 966308). [19] V. Kumar, B. Joseph, P.W. Ramteke, A. Mani, F. Jahan, J. Chem. Pharm. Res. 3 (2) (2011) 226–233. [20] Mario Crosa, Valter Boero, Marinella Franchini-Angela, Clay Clay Miner. 47 (6) (1999) 742–747. [21] J.I. Langford, A.J.C. Wilson, J. Appl. Crystallogr. 11 (1978) 102–113.
Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Phase transition and thermal characterization of induced smectic phases in a ternary mixture T.N. Govindaiah ⁎, H.R. Sreepad Post-Graduate Department of Physics, Government College (Autonomous), Mandya 571401, India
a r t i c l e
i n f o
Article history: Received 28 August 2014 Received in revised form 1 November 2014 Accepted 9 December 2014 Available online 11 December 2014 Keywords: Ternary mixture Temperature dependence Optical texture Optical anisotropy
a b s t r a c t Two or more multi-component system of didodecyl dimethyl ammonium bromide (DDAB), cholesteryl nonanoate (CN) and ethylene glycol (EG) exhibits an interesting different liquid crystalline cholesteric and induced smectic phases, such as SmA, SmC and SmB, sequentially when the specimen is cooled from its isotropic liquid phase. These phases have been characterized by using optical texture studies. The temperature variations of optical anisotropy, helical pitch measurements and electrical conductivity have been also discussed. XRD results have also been reported. © 2014 Published by Elsevier B.V.
1. Introduction Liquid crystals are a special class of soft materials characterized by so called mesophase, where they flow like an isotropic liquid yet possess a long-range orientational order and a complete or partial absence of positional order of building units that can either be individual molecules or their aggregates [1]. The two main types of liquid crystals are thermotropic liquid crystals and lyotropic liquid crystals. Thermotropic liquid crystals show mesophases depending on temperature and pressure. Their basic building units are usually individual molecules that have a feature of pronounced shape anisotropy, such as rods and disk. Thermotropic liquid crystals have been successfully used in display devices. Lyotropic liquid crystals are formed on the dissolution of lyotropic liquid crystal molecules in a solvent (usually water). A feature of lyotropic liquid crystals distinguishing them from thermotropic liquid crystals is the self-assembly of molecules into supramolecular structures that represent a basic unit of these mesophases [2,3]. The most common lyotropic liquid crystalline system is those formed by water and surfactants, such as soaps, synthetic detergents, and lipids. Surfactant molecules are formed by a hydrophilic part chemically bound to a hydrophobic part. Mixtures of these surfactant molecules with a solvent under certain conditions of temperature and relative concentrations produced the different types of liquid crystalline mesophases such as cholesteric, nematic, lamellar, discotic, twisted grain boundary (TGB) phase, and blue phase [4].
⁎ Corresponding author at: P.G. Department of Physics, Government College (Autonomous), Mandya 571401, India. E-mail address: [email protected] (T.N. Govindaiah).
http://dx.doi.org/10.1016/j.molliq.2014.12.012 0167-7322/© 2014 Published by Elsevier B.V.
In the present investigation, our aim is to study the ternary mixture of different compounds, namely, cholesteryl nonanoate (CN), Didodecyl dimethyl ammonium bromide (DDAB) and ethylene glycol (EG), which exhibits an interesting liquid crystalline cholesteric phase and induced smectic phases, such as SmA, SmC and SmB phases sequentially when they are cooled from isotropic phase. Optical, DSC, thermal and X-ray studies have been carried out to understand the intermolecular interactions in the mixture. 2. Experimental The compound didodecyl dimethyl ammonium bromide (DDAB) used in this investigation was obtained from the Basic Pharma Life Science Pvt., Ltd., India, and it was further purified twice by a recrystallization method using benzene as a solvent. Ethylene glycol (EG) was supplied from Kodak, Ltd., Kodak house, Mumbai, India. The cholesteryl nonanoate (CN) was obtained from M/s East Mann Organic Chemicals, USA. Mixtures of different concentrations of DDAB in CN + EG were prepared and were mixed thoroughly. These mixtures of various concentrations of DDAB in CN + EG were kept in desiccators for a long time. The samples were subjected to several cycles of heating, stirring, and centrifuging to ensure homogeneity. The phase transition temperatures of these concentrations were measured with the help of Leitzpolarizing microscope in conjunction with a hot stage. The samples were sandwiched between the slide and cover slip and were sealed for microscopic observations. The differential scanning calorimetry (DSC) thermograms were taken for the mixtures of all concentrations using Perkin-Elmer DSC II Instrument facility available at Raman Research Institute, Bangalore, India. The X-ray broadening peaks were obtained at different temperatures using JEOL diffractometer. The density
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T.N. Govindaiah, H.R. Sreepad / Journal of Molecular Liquids 202 (2015) 75–78
and refractive indices in the optical region are determined at different temperatures by employing the techniques described by the earlier investigators [5,6]. Electrical-conductivity measurements of the mixture at different temperatures were carried out using digital LCR meter and a proportional temperature control unit. 3. Results and discussion 3.1. Phase diagram The ternary mixture of DDAB in CN + EG exhibits an interesting different liquid crystalline phases and the phase transition temperatures are measured by using Leitz-polarizing microscopic. The partial phase diagram shown in Fig. 1, which is obtained by plotting the concentrations against the phase transition temperatures of the mixture, which clearly illustrates that, the mixture of all concentrations of DDAB in CN + EG exhibit a SmA and SmB phases respectively at different temperatures, when the specimen is cooled from its isotropic liquid phase. The concentrations of the mixture from 5% to 22% and 32% to 60% of DDAB shows a cholesteric phase in addition of SmA, SmC and SmB phases, but in the concentration range from 8% to 50% of DDAB shows a schlieren texture of SmC phase.
a). Fingerprint pattern texture of cholesteric phase (250X).
3.2. Optical texture studies For the purpose of optical texture studies, the sample was sandwiched between a slide and cover glass, and then the optical textures were observed using a Leitz polarizing microscope in conjunction with a specially constructed hot stage. The concentrations ranges from 5% to 22% and 32% to 60% of the mixture are slowly cooled from its isotropic melt, the genesis of nucleation starts in the form of small bubbles and slowly grow radially, which form a fingerprint pattern texture of cholesteric phase with large values of pitch is shown in Fig. 2(a) [7,8]. On further cooling the specimen, the cholesteric phase slowly changes over to focal conic fan shaped texture, which is the characteristics of SmA phase as shown in Fig. 2(a). The concentrations from 8% to 50% of the mixture, the SmA phase changes over to schlieren texture of SmC. On further cooling the specimen, SmC phase changes over to SmB phase, which remains up to room temperature and then it becomes a crystalline phase [9]. 3.3. Optical anisotropy Results of this investigation are further supported by the optical studies. The refractive indices for extraordinary ray (ne) and ordinary ray (no) of the mixture were measured at different temperatures for the different concentrations using Abbe refractometer and precision goniometer spectrometer. The temperature variations of refractive indices
b). Focal conic fan shaped texture of SmA phase (250X). Fig. 2. Microphotographs obtained in between the crossed polars. a). Fingerprint pattern texture of cholesteric phase (250×). b). Focal conic fan shaped texture of SmA phase (250×).
for 20% of DDAB in CN + EG are shown in Fig. 3. The values of electrical susceptibility for 20% of DDAB in CN + EG have been calculated using Neugebauer relation [10] at different temperatures. The temperature variations of electrical susceptibility for the mixture are as shown in Fig. 4. From this figure, it can be observed that wherever there is a phase transition, the value of electrical susceptibility changes appreciably, which indicates that each change corresponds to induced mesomorphic phases. Further with increasing the temperature of given concentration of DDAB, the value of electrical susceptibility decreases, because the effective optical anisotropy associated with the molecules of DDAB also decreases. But here in this graph, we have observed an auxiliary peak in addition to the main peak, which illustrates that, the peak cannot be thought only due to change in the orientation of molecules. They can be attributed to changes in the dimension of molecules along with changes in orientation. 3.4. Helical pitch (smectic and cholesteryl layers) measurements
Fig. 1. Partial phase diagram for the mixture of DDAB in CN + EG.
The helical pitch measurements were performed on the cholesteric phase following the well-known Grandjean–Cano wedge method [11,
T.N. Govindaiah, H.R. Sreepad / Journal of Molecular Liquids 202 (2015) 75–78
77
Fig. 3. Temperature variation of refractive indices for the mixture of 20% DDAB in CN + EG. Fig. 5. The temperature variations of pitch for the mixture of 20% DDAB in CN + EG.
12]. The ternary mixture was taken in a wedge-shaped cell treated for homogeneous alignment. The two glass plates formed a small angle at the wedge. The mixture was cooled slowly (0.2 °C min−1) from isotropic cholesteric phase to smectic phase, which induces an array of equidistant Grandjean–Cano lines. The pitch of the cholesteric phase was determined by measuring the distance between the Grandjean–Cano lines as a function of temperature. As the temperature was lowered from the cholesteric phase to smectic phase, spacing between the lines are increased, indicating that the pitch in the cholesteric phase is also increasing. The temperature variation of pitch for the mixture of 20% DDAB in CN + EG is shown in Fig. 5. From this figure, it is evident that, the variation of pitch from cholesteric to smectic phase is smooth and continuous. But gradually, the value of pitch increases from 0.17 to 0.19 mm was observed upon cooling the sample from cholesteric phase to smectic phase. The value of the pitch increased steeply and reached a maximum of 0.5 mm at the cholesteric to smectic phase transition. But in the studies, we have noticed that, the sequence is Iso → Cho → SmA → SmC → SmB phases on cooling. Most of the data about the helical pitch have been given previously in Ref. [13]. The pitch is continuous at the cho → smectic transition in spite of a rather energetic transition. It increases on cooling to smectic phase and diverges on approaching the SmA and SmC phases. This divergence related to the second-order nature of the transition. It exhibits a steep decrease, close to cholesteric phase which is usually the characteristics of second-order SmA, SmC phase transitions.
Fig. 4. Temperature variation of electric susceptibility for the mixture of 20% DDAB in CN + EG.
3.5. Conductivity measurements Electrical-conductivity measurements help in getting better idea on the phase behavior with temperature. An abrupt increase or decrease of electrical-conductivity with temperature relates to the phase behavior of the lyotropic and thermotropic systems [14]. The temperature variations of electrical-conductivity are shown in Fig. 6. The changes were observed in electrical-conductivity, the values correspond to liquid crystalline phase transition of thermotropic and lyotropic systems respectively at different temperatures and they were also identified by DSC and optical texture studies. It was observed that, a change in electrical-conductivity at temperatures 70 °C, 85 °C, 110 °C and 127 °C respectively, which corresponds to phase transition from SmB → SmC, SmC → SmA and SmA → Cho phases. This type of behavior is generally observed in hexagonal, cubic and lamellar phase of lyotropic and thermotropic systems [15–17]. 3.6. Characterization of nano-aggregated grains The X-ray diffractometer traces obtained for the mixture of 20% DDAB in CN + EG at temperature 60 °C is shown in Fig. 7. The diffraction
Fig. 6. Temperature variation of electrical-conductivity σ (Ω−1 m−1) for the sample 20% of DDAB in CN + EG.
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4. Conclusions In light of the above results, we have drawn the following conclusions. The multi-component system exhibits an unusual sequence of phases, showing the formation of induced chiral smectic phases in the concentration range of 5% to 60% of DDAB in (CN + EG). The phase behavior is discussed with the help of phase diagram. The changes in value of electrical conductivity with the variation of temperature unambiguously correspond to smectic and cholesteric phases. Drastic changes in electrical conductivity are expected to be due to changes in the dimension of disks along with changes in the orientation order of the arrangement. The nano-aggregated grain size of liquid crystalline material for smectic-B phase comes out to be 45.3 nm. References Fig. 7. XRD traces obtained for the mixture of 20% DDAB in CN + EG.
peaks at this temperature correspond to smectic-B phase (by using JEOL diffractometer with the settings: TC4, CPS400, channel width 100 for λ = 1.934 Å). X-ray diffraction study is an important method to determine the nano-aggregated grain size of the molecules for different liquid crystalline phases [18,19]. The deviation from perfect liquid crystallinity leads to broadening of the diffraction peaks. In order to estimate nano-aggregated grain size of the molecules for different liquid crystalline phases corresponding to broadening of X-ray diffraction peaks we have used the Scherrer's formula L ¼ Kλ=β cosθ; where L is the nano-aggregated grain size, λ is the wave length of X-ray radiation (Fe: 1.934 Å), K is usually taken as 0.89, β is the line width at half maximum and θ is the diffraction angle. Usually with decrease of temperature [20,21], the nano-aggregated grain size of the molecules increases. The structure of molecular orientation for broken banded focal conic fan texture of smectic-B phase is stable and the molecular ordering of this phase shows three intense peaks. The nano-aggregated grain size of liquid crystalline material for smectic-B phase comes out to be 45.3 nm.
[1] M. Kleman, O.D. Lavrentovich, Soft Matter Phys, Springer- Verlag, New York, 2003. [2] A.M. Figueiredo Neto, S.R.A. Salinas, Phys. Lyotropic Liq. Cryst. Oxford University Press, 2005. [3] A.G. Petrov, Lyotropic State Matter, Gordon and Breach Publs., NJ, 1999. [4] T.N. Govindaiah, H.R. Sreepad, Nagappa, Mol. Cryst. Liq. Cryst. 574 (2013) 9–18. [5] Nagappa, S.K. Nataraju, D. Krishnamurti, Mol. Cryst. Liq. Cryst. 133 (1986) 31–54. [6] J. Thiem, V. Vill, F. Fischer, Mol. Cryst. Liq. Cryst. 170 (1989) 43–51. [7] D. Demus, C. Richter, Texture Liq. Cryst. Verlag Chemie, Weinheim, 1978. [8] Nagappa, D. Revanasiddaiah, D. Krishna Krishnamurti, Mol. Cryst. Liq. Cryst. 101 (1983) 103–127. [9] T.N. Govindaiah, H.R. Sreepad, Phase Transit. 87 (2014) 729–736. [10] H.E.J. Neugebauer, Can. J. Phys. 32 (1954) 1–8. [11] H.T. Nguyen, R.J. Tweig, M.F. Nabor, H. Isaert, C. Destrade, Ferroelectrics 121 (1991) 187–204. [12] A. Bouchta, H.T. Nguyen, M.F. Achard, F. Hardouin, C. Destrade, R.J. Tweig, N. Isaert, Liq. Cryst. 12 (1992) 575–591. [13] H.T. Nguyen, A. Bouchta, L. Navailles, P. Barois, N. Isaert, R.J. Twieg, A. Maaroufi, C. Destrade, J. Phys. II France 2 (1992) 1889–1906. [14] M. Marthandappa, Nagappa, K.M. Lokhanatha Rai, J. Phys. Chem. 95 (1991) 6369–6372. [15] J. Franeois, Kolloid Z.Z. Polym. 246 (1971) 606. [16] T.N. Govindaiah, H.R. Sreepad, P.M. Sathyanarayana, J. Mahadeva, Nagappa, Mol. Cyst. Liq. Cryst. 552 (2012) 24–32. [17] T.N. Govindaiah, H.R. Sreepad, Nagappa, Mol. Cyst. Liq. Cryst. 592 (2014) 91–98. [18] T.N. Govindaiah, H.R. Sreepad, Nagappa, Phase Transit. (2014), http://dx.doi.org/10. 1080/01411594.2014.966308 (ID: 966308). [19] V. Kumar, B. Joseph, P.W. Ramteke, A. Mani, F. Jahan, J. Chem. Pharm. Res. 3 (2) (2011) 226–233. [20] Mario Crosa, Valter Boero, Marinella Franchini-Angela, Clay Clay Miner. 47 (6) (1999) 742–747. [21] J.I. Langford, A.J.C. Wilson, J. Appl. Crystallogr. 11 (1978) 102–113.