DIELECTRIC PROPERTIES OF CHOLESTEROL DERIVATIVES

Transcription

1 PHYSICAL CHEMISTRY DIELECTRIC PROPERTIES OF CHOLESTEROL DERIVATIVES ATHAR JAVED 1, MUHAMMAD AKRAM 2, MUHAMMAD IMTIAZ SHAFIQ 3 1 Department of Physics, University of the Punjab, Quaid-i-Azam Campus, Lahore PAKISTAN, athar_pu@hotmail.com 2 Centre for Solid State Physics, University of the Punjab, Quaid-i-Azam Campus, Lahore PAKISTAN 3 Institute of Chemistry, University of the Punjab, Quaid-i-Azam Campus, Lahore PAKISTAN Received March 31, 2006 Frequency dependence of dielectric constant (ε ), dielectric loss factor (ε ), dielectric loss tangent (Tanδ) and conductance (G x ) are studied for the dilute solutions of four cholesterol derivatives namely, cholesteryl laurate, cholesteryl oleate, cholesteryl nonanoate and cholesteryl benzoate in the frequency range 330 Hz to 3 MHz. All measurements are made at room temperature. The dielectric data show that molecular orientation polarization has major contribution to the observed dielectric constant ε of cholesterol derivatives in solution. A peak observed in dielectric loss factor ε curves of the solutions has been attributed to the rotational relaxation of cholesterol derivative molecules. The frequency and the strength of the observed dispersion peak have been found to be related to the structure and size of the cholesterol derivative molecules. Key words: cholesterol derivatives, liquid crystals, dielectric constant, orientation polarization, rotational relaxation. 1. INTRODUCTION Liquid crystals are states of condensed matter whose symmetries lie between those of 3-dimensionally periodic crystals and isotropic liquids [1 3]. Thermotropic liquid crystalline phases are exhibited by a large number of organic compounds whose molecules have anisotropy of shape. A typical intermolecular energy responsible for the stability of the relevant order in the medium is comparable to the thermal energy, and thus liquid crystals are soft materials. Relatively weak interactions like those between molecular dipoles or chiral centers of appropriate molecules can give rise to new types of liquid crystals. The soft nature of the medium, coupled with anisotropic optical and 1 Correspondence author. Rom. Journ. Phys., Vol. 51, Nos. 7 8, P , Bucharest, 2006

2 820 Athar Javed et al. 2 dielectric properties gives rise to many electro-optic effects at relatively low voltages. These are exploited in liquid crystal displays (LCDs), which are the lowest power (~ 1 μw/cm 2 ) consuming flat panel devices and used in all calculators, laptop and palmtop computers, cell phones, full-size television screens, etc. Liquid crystal can be classified into three different groups, nematic, smectic and cholesteric depending on the level of order in their molecular structure. Liquid crystals in the nematic group are most commonly used in LCD production because of their physical properties and wide temperature range. In the nematic phase, liquid crystal molecules are oriented on average along a particular direction. By applying an electric or magnetic field, the orientation of the molecules can be manipulated in a predictable manner. This mechanism provides the basis for LCDs. Liquid crystals exhibit interesting viscoelastic properties [4, 5]. The study of viscoelastic modes in them is important not only in understanding the structural dynamics but also in selecting suitable materials for display devices. In liquid crystals, thermal fluctuations in the average direction of orientation of the molecules (the director) result in strong fluctuations in the dielectric tensor causing intense scattering of light. Liquid crystallinity was first observed for cholesteryl benzoate [6] and the most generally used classification is based on the fact that liquid crystalline phases are known based on pure compounds dependent on the temperature (thermotropic liquid crystals) and based on solvent-solute type systems where aggregates of molecules result in liquid crystallinity (lyotropic liquid crystals) [7]. Cholesteric liquid crystals have attracted much interest over the years due to their exciting optical properties [4]. Cholesterics have also found use in very diverse products ranging from thermometers to fabric dyes. There has been an effort for some time to produce and study well-aligned cholesteric liquid crystal polymers and their cross-linked networks-permanently stabilized cholesteric liquid crystals elastomers (CLCE). Much success has been achieved in developing cholesteric polymers [8 10], aligning them by electric and magnetic fields and by surface interactions with substrates. Theoretical work, describing CLCE and their response to mechanical deformation, predicted a number of new effects, including a very rich and complex evolution of photonic band structure [11, 12]. Y. Mao et al. [11], P. A. Bermel et al. [12] and K. A. Suresh [13] described the detail theory of a few experimental results of cholesteric liquid crystals. In this paper we study experimentally the dielectric properties of some cholesterol derivatives. Cholesterol derivatives are very useful cholesteric liquid crystals and are used in many advance fields. The chemical composition of cholesterol derivatives, used in the present study, with their temperature range of liquid crystalline phase is given in the Table 1 below.

3 3 Dielectric properties of cholesterol derivatives 821 Table 1 Chemical composition of four cholesterol derivatives with their temperature range of liquid crystalline phase Cholesterol Derivatives Molecular Formula Side Chain Temperature range of Liquid Crystalline Phase Cholesteryl Laurate C 39 H 68 O 2 C 12 H 23 O 2 77 C 89 C Cholesteryl Oleate C 45 H 78 O 2 C 18 H 33 O 2 48 C 50 C Cholesteryl Nonanoate C 36 H 62 O 2 C 9 H 17 O 2 76 C 79 C Cholesteryl Benzoate C 34 H 50 O 2 C 7 H 5 O C 150 C 2. EXPERIMENTAL DETAIL Cholesteric materials (cholesteryl laurate, cholesteryl oleate, cholesteryl nonanoate and cholesteryl benzoate) were purchased from Sigma-Aldrich Company, in powder form with 99% purity. The cholesteric liquid samples were prepared by weighing the appropriate amount of the solute and carbon tetra chloride (used as solvent). Four samples namely, sample L (cholesteryl laurate), sample O (cholesteryl oleate), sample N (cholesteryl nonanoate) and sample B (cholesteryl bebzoate) were prepared by dissolving appropriate amount of each material in carbon tetra chloride at room temperature and slowly poured into the cell of diameter 3 cm, for dielectric measurements. Dielectric constant (ε ), dielectric loss factor (ε ), dielectric loss tangent (Tanδ), and conductance (G x ) were measured in the frequency range, 330 Hz to 3 MHz at room temperature for all the four samples. The dielectric measurements were made using a bridge (TR-10C) in combination with a null detector (BDA-9) and an Oscillator (WGB-9) [14]. The complete set was TRS-10T of Ando electric company Japan. Measured quantities were the capacitance (C x ) and the conductance (G x ) of the specimen and dielectric constant (ε ) and dielectric loss factor (ε ) were calculated as [15]. Cx ε =, C a ε = Gt 2 x x ωd and Tanδ = ε /ε A ; A is the t area of the electrode in cm 2 and t is the thickness of the specimen in cm. C x is the electrostatic capacity of the specimen in pf; D is the effective electrode diameter, G x is the conductance of the specimen and ω= 2π f ( f is the measuring frequency). where C a is the capacitance of the cell with air and C a = ( )

4 822 Athar Javed et al RESULTS AND DISCUSSION Figs. 1 and 5(a) show the frequency dependence of dielectric constant ε for all the four samples and pure CCl 4 at room temperature. The comparison of dielectric constant values of all the four solutions with that of pure solvent shows that dielectric constant values of the solutions are slightly higher than those of pure solvent. From Fig. 5(a) it is clear that the dielectric constant values of pure CCl 4 remain constant with increase in frequency but there is slight monotonous decrease of ε values of the cholesterol derivative solutions with the increase in frequency. This is due to the fact that dielectric medium have orientation polarization, atomic polarization and electronic polarization, where as in case of pure CCl 4 Fig. 1 Frequency dependence of dielectric constant at room temperature for sample L, sample O, sample N, and sample B. Fig. 2 Frequency dependence of Log(ε ) at room temperature for sample L, sample O, sample N, and sample B.

5 5 Dielectric properties of cholesterol derivatives 823 (which is non-polar solvent) there is no orientation polarization and ε remains constant due to the contribution of atomic and electronic polarization of CCl 4. Figs. 2 and 3 show the frequency dependence of dielectric loss factor (ε ) and dielectric loss tangent (Tanδ) at room temperature for all the four samples. It is clear from the comparison of Figs. 2 and 5(b) that ε values of the cholesterol derivative solutions are slightly higher than those in pure CCl 4 as expected from the very low concentration of the solute in the solvent. A peak has been observed in dielectric loss factor curves of all the four samples which shift to lower frequencies with the side chain lengths of cholesteryl alkanoates. The peaks have been attributed to the rotational relaxation of the cholesterol derivative molecules in the solution. Cholesteryl oleate being heaviest reorients at the lowest frequency and cholesteryl nonanoate being lightest reorients at the high frequency. Fig. 3 Frequency dependence of Log(Tanδ) at room temperature for sample L, sample O, sample N, and sample B. Fig. 4 Frequency dependence of Log(G x ) at room temperature for sample L, sample O, sample N, and sample B.

6 824 Athar Javed et al. 6 The strength of the dispersion mechanism as indicated by the height of the loss peak also depends on the size of the side chain being maximum for cholesteryl laurate and minimum for cholesteryl nonanoate. Cholesteryl benzoate though least in molecular weight and side chain length has dielectric dispersion at lower frequency as compared to cholesteryl nonanoate which could be attributed to the presence of a bulkier benzene group in the side chain which is likely to experience more hindrance to its alignment in the applied alternating electric field. The strength of dispersion as indicated by the peak height is also much higher than that of cholesteryl nonanoate which reflects the large dipole moment of cholesteryl benzoate molecules. Similar results can be explained by comparing Figs. 3 and 5(c) because there is a little variation in ε values of the solution with frequency. Fig. 4 shows the frequency dependence of conductance at room temperature for all samples. It can be seen that G x values of all the four samples first decrease

7 7 Dielectric properties of cholesterol derivatives 825 Fig. 5 Frequency dependence of (a) ε, (b) ε, (c) Log(Tanδ), (d) Log(G x ) for carbon tetra chloride at room temperature. with increase in frequency and then increase very rapidly to many orders of magnitude. On the other hand, conductance curve of pure CCl 4 as seen from Fig. 5(d) increases monotonically with the increase in frequency. The conductivity values of the solutions are about eight orders of magnitude higher as compared to those in pure solvent at all frequencies which show that solute particles are the main source of electrical conductance in the cholesterol derivative solutions in CCl CONCLUSIONS Cholesteric liquid crystals have many applications as electro-optic materials in thin film and display devices because vertical and horizontal alignment of

8 826 Athar Javed et al. 8 liquid crystal molecules has different optical and dielectric properties. Frequency dependence of dielectric constant (ε ), dielectric loss factor (ε ), dielectric loss tangent (Tanδ) and conductance (G x ) are studied for the dilute solutions of four cholesterol derivatives (cholesteryl laurate, cholesteryl oleate, cholesteryl nonanoate and cholesteryl benzoate) in the frequency range 330 Hz to 3 MHz at room temperature. The dielectric data show that molecular orientation polarization has major contribution to the observed dielectric constant ε of cholesterol derivatives in solution. A peak observed in dielectric loss factor ε curves of the solutions has been attributed to the rotational relaxation of cholesterol derivative molecules. The frequency and the strength of the observed dispersion peak have been found to be related to the structure and size of the cholesterol derivative molecules. REFERENCES 1. D. Demus, J. Goodby, G. W. Gray, H. W. Spiess, V. Vill, Handbook of Liquid Crystals, Wiley-VCH, Vol. 1 (1998). 2. N. V. Madhusudana, Current Science, 80, (2001), T. Z. Rizvi, Journal of Molecular Liquids, 106, (2003), P. G. De Gennes and J. Prost, The physics of liquid crystals, 2nd edn., (Oxford University Press, Oxford, 1993). 5. S. Chandrasekhar, Liquid crystals, 2nd edn., (Cambridge University Press, Cambridge, 1992). 6. F. Reinitzer, Monatsh. Chem. 9, (1988), 421; for an English translation, see: Liq. Cryst. 5, (1989), C. Tschierske, Progress in Polymer Science, 21, (1996), H. Finkelmann and G. Rehage, Journal of the American Chemical Society, 186, (1983), N. A. Plate, Y. S. Freidzon, and V. P. Shibaev, Journal of Pure and Applied Chemistry, 57, (1985), R. Zentel, G. Reckert, S. Buslek, and H. Kapitza, Macromol. Chem. Phys., 190, (1989), Y. Mao, E. M. Terentjev, and M. Warner, Physical Review, E 64, (2001). 12. P. A. Bermel and M. Warner, Physical Review, E 65, (2002). 13. K. A. Suresh, Pramana- Journal of Physics, 61, (2003), Instruction Manual for type TR-10C, Dielectric Loss Measuring set, Ando Electric Co. Ltd., Japan, (1982). 15. Instruction Manual for type BDA-9, Null Detector, Ando Electric Co. Ltd., Japan, (1982).