Reorientational Motion and Phase Transitions of Cyclohexane in Restricted Geometries
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T. W. ZERDA AND YONG SHAO Texas Christian University, Physics Department, P. 0. Box 32915, Fort Worth, TX 76129 ABSTRACT Rotational motion of cyclohexane in the liquid and the solid plastic phase is studied using Raman light scattering. The results are compared with molecular dynamics simulations run for model pores of diameters similar to those used in the experiment. The presence of the surface layer and its effect on the relaxation times is discussed. The temperature of the solid-solid phase transition is determined from the analysis of the v21 band shape. It is shown that the depression of the cubic to monoclinic phase transition depends on the pore diameter and is different for modified and unmodified surfaces. It is suggested that molecules near the pore walls form the amorphous structure and only molecules near the center of the pore form crystallographic structure. INTRODUCTION Recent developments in the sol-gel processing enable the use of light scattering techniques to probe reorientational motion of molecules in the pores. Optically transparent glass of controlled porosity (pore diameters varying from 20 to 200 A) can be easily manufactured and impregnated with various liquids. It has been observed that when a liquid is placed in small pores, the freezing temperature shifts toward lower values, and the depression of the freezing temperature is inversely proportional to the pore radius. This effect has been observed for cyclohexane in different materials such as porous Vycor glass, silica powder, and KBr [1-3]. The presence of supercooled cyclohexane in model pores of diameters 30 A and 50 A has been confirmed by computer simulations [4]. In addition, molecular dynamic simulations also provided information on reorientational relaxation times for the tumbling and the spinning motions of cyclohexane in the pores. In this study we report our measurements of reorientational relaxation times for cyclohexane inside small pores of various sizes as a function of temperature, in both the liquid and the plastic phase, and compare the results with molecular dynamics simulations. Under atmospheric pressure cyclohexane freezes at 279.6 K and forms a plastic phase. This phase known as plastic crystal is characterized by rapid reorientations typically observed in the liquid state [5]. At 186 K, solid cyclohexane undergoes a transition from a plastic phase to a monoclinic ordered phase. The high temperature phase is a cubic crystal (a = 8.61 A, Z = 4; Fm3m) and it is formed immediately below the melting point at 279.6 K. The low temperature phase II is monoclinic crystal (a = 11.23 A, b = 6.44 A, c = 8.20 A, 3 = 108.830; Z = 4; C2/c). The cubic to monoclinic phase transition in the bulk phase has been extensively studied as a function of temperature and pressure [6-8] using different experimental techniques. In this study we concentrate on the analysis of Raman scattering of cyclohexane in small geometries. There is a limited number of experimental studies on solid-solid phase transitions inside small cavities. The a
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