(151i) Synchrotron X-Ray Reflectivity and Computational Simulations of Structural Organization of the Binary Mixture 5CB/8CB Liquid Crystals at Liquid Crystal-Air Interface

Authors: 
Sadati, M., The University of Chicago
Ramezani-Dakhel, H., University of Chicago
Bu, W., NSF's ChemMatCARS, University of Chicago
Sevgen, E., University of Chicago
Taheri Qazvini, N., University of Chicago
Liang, Z., University of Illinois at Chicago
Rahimi, M., University of Chicago
Lin, B., NSF's ChemMatCARS, University of Chicago
Roux, B., University of Chicago
Abbott, N. L., University of Wisconsin-Madison
de Pablo, J. J., Argonne National Laboratory
Eutectic mixtures of liquid crystals are commercially important because of their extended operating temperature range in the nematic phase. However, little is known about the precise molecular structure of these materials at their free surfaces. In this work, we have performed synchrotron X-ray reflectivity measurements accompanied by an advanced theoretical and computational analysis to study the structural organization of liquid crystals at the air-liquid crystal interface. The X-ray reflectivity was measured from two nematic (5CB) and smectic (8CB) liquid crystals and their binary mixtures at several temperatures, in the nematic phase and above the nematic-isotropic transition. Our computational simulations and X-ray reflectivity results indicate that in the case of 8CB nematic phase, incipient bulk smectic fluctuations are induced at the interface to form temperature-dependent multilayers at the interface. Such layers can extend far from the interface. However, the interface of 5CB in the nematic phase exhibits a relatively small number of layers. For the binary mixture 5CB/8CB liquid crystals, our results show that the molecular organization depends strongly on the 5CB content. Above 20% 5CB, the smectic properties of 8CB are suppressed and the surface induced smectic layering and the correlation length of the liquid crystal order decrease dramatically.