Dilatometric Study and Pretransitional Effect at Nematic to Isotropic Phase Transitions in Calamitic Lyotropics Madalina-R. PUICA a, Rodica MOLDOVAN b, Traian BEICA b, Stefan FRUNZA b, Wilfried SCHRANZ a a - University of Vienna, Institute of Experimental Physics, Strudlhofgasse no.4, 1090 Vienna, Austria b -National Institute of Materials Physics, Bucharest, R-76900, Romania Abstract A dilatometric study on the nematic to isotropic phase transition of lyotropic liquid crystal system: SLS/water/decanol, at a nematic calamitic concentration, is presented. The experimental results for the temperature dependence of the density at normal pressure, show within the limits of experimental precision (± 1×10 -5 g/cm 3 ), that the density has no jump at the nematic to isotropic phase transition. The obtained result is brought in connection with a pretransitional effect, observed for the first time at this lyotropic system, namely the appearance of isotropic elliptic domains in the nematic matrix below the transition temperature. The composition of the sample was chosen according to the phase diagram (Fig. 1) from literature 1, in order to obtain a calamitic lyonematic liquid crystal (Nc). Fig. 1: Partial phase diagram (wt %) for the system SLS/ water/ decanol 1 at 25°C. The dot gives the composition of the investigated nematic samples. Sample composition: N c : 25.05% (SLS) %(water) %(decanol) Experimental techniques The expansion of the liquid crystal as a function of temperature was investigated using a dilatometer with capillary tube (of volume  1.45cm 3 ). The length of liquid columns was measured using an Abbé comparing microscope (Zeiss) with accuracy of  1  m. The errors in the density variation determinations:  1  g  cm -3 (Fig. 2). The pretransitional effect for Nc mixtures, was checked using polarized light microscopy. The temperature was controlled with accuracy of 0.01°C using an HS1 Microscope hot stage (INSTEC). Photographs of the elliptic domains were taken with the aid of a digital image capture system provided with Kodak MegaPlus (Edmund Scientific) camera (Fig. 3). Experimental results The dilatometric measurements were made at a constant variation of the temperature with a rate of 1.5°C/hour. The temperature stability inside the micro chamber was  0.01ºC. By observing the dilatometer reservoir between cross-polarizers, the temperature transition (T 0 ) between nematic and isotropic phase was determined at: T 0 = 29.6°C. Fig. 4: a) The temperature dependence of the density  for the Nc solution. The straight line is the linear regression for the data,  L =  T. b) The deviation of the density  from the above linear variation  L. Usually when the temperature approaches the transition temperature, the sample, observed between crossed polarizers, is gradually getting dark until, at a given temperature, it becomes completely dark. For some samples, at a temperature of ( )°C below the transition temperature, we observed small elliptic isotropic domains in the nematic matrix (a pretransitional effect), as show in Fig. 5. Fig. 5: Images of the isotropic domains appeared in the nematic matrix of a lyo-nematic calamitic SLS/water/decanol mixture, within an 0.1°C interval under the nematic-isotropic transition temperature. The arrow points the liquid crystal orientation direction. Fig. 6: Size values of the major and minor axes of an elliptical isotropic domain, successively photographed. Continuous curves represent the fit with a second order equation. Conclusions -  -Since our experimental results put in evidence no density jump at the nematic to isotropic phase transition temperature (T N-I ), we may conclude that, in case that this jump exists, it must be smaller than the measurement resolution allowed by our method. Similar results were obtained by Boden and Joley 2 for another nematic system (CsPFO/water), whose behavior also presents no jump of density at the T N-I temperature. -  -For the first time it was evidenced a pretransitional effect for the lyotropic system SLS/water/decanol, at Nc composition. This could be related to the absence of density jump. Materials The lyotropic liquid crystal was obtained from sodium lauryl sulphate in water, with addition of decanol. Components used:  Sodium lauryl (dodecyl) sulphate (SLS) (Sigma 99% purity, molar mass M=288 g/mol)  - the chemical formula:  - the structural formula:  1-decanol (Merck, 99% purity, molar mass M=158 g/mol)  - the chemical formula:  tridistilled water (home-made). Fig. 2 Experimental set-up for density measurements. Fig. 3 Integrated system for polarized light microscopy 1 -Per-Ola Quist, B.Halle, I.Furo, J.Chem.Phys. 96, 3875 (1992) 2 -N. Boden, K.W. Jolley, Phys. Rev. A, 41, (1992) Reference Size evolution for an elliptic domain: