Analysis on the birefringence property of lyotropic liquid crystals below Krafft temperature

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1 Analysis on the birefringence property of lyotropic liquid crystals below Krafft temperature Radhakrishnan Ranjini, Murukeshan Vadakke Matham *, Nam-Trung Nguyen Department of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore , Singapore *Corresponding author address: (M.V. Matham). Abstract The effect of lyotropic liquid crystalline material, below Krafft temperature, in changing the polarization of the optical beam has been demonstrated. It is found that the lamella structures formed in the material below Krafft temperature are birefringent and this birefringence can be controlled by flowing it in rectangular channel under a suitable flow rate. Keywords: CTAB; Surfactant; Lyotropic liquid crystal; Optical properties; Birefringence; Polarization 1. Introduction Liquid crystalline materials have attracted much interest in the past two decades due to their potential applications in displays, temperature and pressure sensors, etc. Rheological properties of surfactants, a lyotropic liquid crystalline material, are studied extensively [1 5]. The growth of these aggregates into various shapes depends mainly on concentration and temperature. It is also noted that the properties of aqueous solutions of surfactants above and below a particular temperature known as Krafft temperature is completely different. Above this temperature, surfactant is highly soluble and micelles start to form. Below Krafft temperature, aqueous surfactant solution is in two phase form, hydrated crystals and water. It is identified that the hydrated crystals are formed due to the crystallization of hydrophobic alkyl chains and the hydrophilic head group of the surfactant molecule [6, 7]. The crystallization is favoured when the Van der Waals attraction energy exceeds thermal kinetic energy [6]. The properties of aqueous surfactant solution such as rheology, flow birefringence, etc. above the Krafft temperature was studied extensively [2 5]. However, the properties below Krafft temperature were inadequately investigated. Therefore, in this context we studied the birefringence property of a cationic surfactant, cetyltrimethylammonium bromide (CTAB) which is a lyotropic liquid crystalline material below Krafft temperature. 2. Experiment The schematic set up is shown in Fig. 1. Diode laser with a wavelength of 641 nm was used for the experiment. Here the flow cell was kept between crossed polarizer to analyze the

2 birefringence of CTAB solution. A linearly polarized laser beam was focused to the flow cell and the transmitted beam is again passed through a second polarizer (analyzer) with polarization axis perpendicular to the first polarizer. The output laser power was monitored using a power meter, connected to the computer for data analysis. The surfactant cetyltrimethylammonium bromide was purchased from Sigma Aldrich Chem. Co. and used without further purification. All solutions were prepared in double distilled water. A clear aqueous solution of CTAB was prepared initially and kept above Krafft temperature (25 C) for few hours. Later, this solution was cooled to below 25 C at which the hydrated crystals were formed by precipitation. This solution was kept for 2 days to attain equilibrium. Just before the experiment the turbid solution was stirred. The solution became glossy after stirring. A rectangular channel with 1-mm width and 2-mm height was fabricated using glass slides with a refractive index of Flow cell was connected to a syringe pump using silicone tubing with an outer diameter of 0.6 cm and an inner diameter of 0.3 cm. The channel was cleaned before introducing the surfactant. A magnetic stirrer was used to stir the solution in the syringe. Experiments were performed with and without stirring the solution in the syringe. The UV VIS absorption spectrum and optical microscope image of the CTAB solution were also taken. Absorption spectrum was measured on a Shimadzu UV-1601 PC spectrophotometer and the baseline was corrected with water. Optical microscope images were taken on a microscope (ZEISS, Axiotech ). Sample for the optical microscopic image was prepared by drop-casting the aqueous solution on a glass slide. 3. Results and Discussion Fig. 2 shows the UV VIS absorption spectrum of CTAB. The absorption spectra is characterised with a peak at 216 nm and a broad band centred around 275 nm. The spectrum clearly indicates that there is no absorption in the visible range. Fig. 3 shows the optical microscopic images of the hydrated crystals of CTAB. The metastable state (needle shaped) and stable state (plate like) of CTAB (Fig. 3A and B) are similar to those of cetylpiridinium chloride [6]. It was observed that the metastable stable transition takes place in few hours. In the experiment, the surfactant solution was introduced into the flow cell and the output laser power was monitored using a power meter. Fig. 4 shows the variation of the transmitted power with a constant flow rate of 80 ml/h. It was observed that the power increases to a maximum value and after few seconds it decreases exponentially. The initial increase in the power is attributed to the birefringence property of the hydrated crystals in the surfactant solution. Thus CTAB flow cell can be regarded as a birefringent medium with birefringence Δn = n e - n 0, where n e and n 0 are the refractive indices of extraordinary and ordinary rays that propagate with orthogonal polarizations. The change in the refractive indices will create a phase difference Φ between the two rays when it is emerged from the birefringent medium of a length L. When a particular flow rate was given, birefringence is varied which affects the light intensity. The transmitted light intensity can be written as

3 where I 0 is the input intensity, θ is the angle between the optical axis of the birefringent medium and the polarizer The exponential decrease in power after the initial increase is due to the absence of birefringent particles in the flow cell. Since these particles (hydrated crystals) are in the micron size range, within few seconds, they get settled in the syringe which is used to inject the solution into the flow cell. In order to avoid the settling of the particles in the syringe, CTAB solution was mixed continuously using a magnetic stirrer (Fig. 1). With this modified experimental set up the effect of flow rate (80 ml/h) on the output laser power was monitored for about 200 s as shown in Fig. 5. It was found that the output power remains constant after the initial increase, till the flow was switched off. The effect of different flow rates on the output power was also observed as shown in Fig. 6. At very low rates, there was not any significant change in the power and at higher flow rates, power fluctuations was large. These experimental results clearly indicate that CTAB surfactant solution below Krafft temperature shows birefringence. In this work the source and detector were placed in the same path and since the presence of CTAB results in power increase, the phenomenon of scattering will have only least effect. However, we have studied the effect of scattering using integrated sphere technique. The percentage reflectance from one face of the flow cell as a function of wavelength is shown in the Fig. 7. We targeted an interrogation area of about 1 cm 2 and we have observed that, diffuse reflection (caused by scattering) from the hydrated crystals in the flow cell is significantly low and negligible. 4. Conclusion In this paper the birefringence property of the lamella structures formed in the aqueous cationic surfactant below Krafft temperature was demonstrated. Using hydrated CTAB crystals it is possible to change the laser power by applying a suitable flow rate. This phenomenon can be used to develop a polarization rotator and variable optical attenuator that can find potential applications in optofluidics or lab-on-chip. Acknowledgements The authors acknowledge the financial support received through MOE-ARC 3/08 and NTU, Singapore for the research student support.

4 References [1] J.N. Israelachvili, Dj. Mitchell, B.W. Ninham, J. Chem. Soc. Faraday Trans. 72 (1975) [2] L. Coppola, R. Gianferri, I. Nicotera, C. Oliviero, G.A. Ranieri, Phys. Chem. Chem. Phys. 6 (2004) [3] F. Reiss-Husson, V. Luzzati, J. Phys. Chem. 68 (1964) [4] R. Cerf, H.A. Scheraga, Chem. Rev. 51 (1952) [5] J.P. Decruppe, R. Cressely, R. Makhloufi, E. Cappelaere, Colloid Polym. Sci. 273 (1995) [6] S. Sasaki, J. Phys. Chem. B 113 (2009) [7] D. Lavabre, V. Pradines, J.-C. Micheau, V. Pimienta, J. Phys. Chem. B 109 (2005)

5 List of Figures Fig. 1 Fig. 2 Fig. 3 Fig. 4 Fig. 5 Fig. 6 Fig. 7 Schematic diagram of the experimental set up. M-Magnetic stirrer, SP-Syringe pump and S-syringe UV VIS absorption spectrum of aqueous solution of cetyltrimethylammonium bromide Optical microscope image of the hydrated crystals formed in CTAB solution (A) metastable solid and (B) stable hydrated solid Variation of output laser power with time for 0.5 M aqueous CTAB solution at a constant flow rate of 80 ml/h The variation of laser power with time with constant stirring and at a flow rate of 80 ml/h Variation of power with time for different flow rates Variation of percentage reflectance with wavelength

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