Phase Transitions and Physical Properties of a Binary Mixture of Bicyclohexane Compounds, II: X-ray Diffraction Measurements

Abstract

X-ray diffraction data from a binary mixture of two isomeric nonpolar bicyclohexane compounds containing alkenyl side chains have been analyzed to determine the bond orientational order (BOO), orientational order parameters (OOP), layer thickness (in Smectic B phase), apparent molecular length (in nematic phase), and lateral molecular distance, D, as a function of temperature. The phase diagram shows the presence of a two-phase smectic B–nematic co-existing region. The orientational order parameter (OOP) values drop sharply with an increase in temperature within the co-existing phase, which is because the observed OOP value is the weighted mean OOP of the two phases. Interestingly, the BOO values are found to increase with an increase in temperature in the co-existing smectic, B–nematic phase. The transverse correlation lengths in the smectic, co-existing, and nematic phases have also been estimated. These values are found to diverge as the co-existing smectic B–nematic phase region is approached, indicating a second-order phase transition. This result has also been supported by entropy, density, refractive index, and OOP measurements.

Keywords:

• bicyclohexane
• binary mixture
• bond orientational order
• co-existing phase
• correlation length
• orientational order parameter

ACKNOWLEDGMENTS

The authors are thankful to S. Diele, Institute for Physical Chemistry, Martin Luther University, Halle, Germany, for fruitful discussions. We extend our gratitude to him for kindly allowing us to record the x-ray diffraction patterns for mixtures A and B on the 2-D area detector during one of the authors' (M. K. D) stay in Halle. We are also grateful to M/S Hoffmann-La-Roche and Co., Basel, Switzerland, for kindly donating the liquid-crystalline samples used in this work. One of the authors (B. D.) is thankful to the Council of Scientific and Industrial Research, New Delhi, India, for the award of a research associateship.