A Raman Spectroscopy and High-Speed Video Experimental Study: The Effect of Pressure on the Solid-Liquid Transformation Kinetics of N-octane

Abstract:

Phase transitions of minerals and rocks in the interior of the Earth, especially at elevated pressures and temperatures, can make the crystal structures and state parameters obviously changed, so it is very important for the physical and chemical properties of these materials. It is known that the transformation between solid and liquid is relatively common in nature, such as the melting of ice and the crystallization of mineral or water. The kinetics relevant to these transformations might provide valuable information on the reaction rate and the reaction mechanism involving nucleation and growth. An in-situ transformation kinetic study of n-octane, which served as an example for this type of phase transition, has been carried out using a hydrothermal diamond anvil cell (HDAC) and high-speed video technique, and that the overall purpose of this study is to develop a comprehensive understanding of the reaction mechanism and the influence of pressure on the different transformation rates. At ambient temperature, the liquid-solid transformation of n-octane first took place with increasing pressure, and then the solid phase gradually transformed into the liquid phase when the sample was heated to a certain temperature. Upon the cooling of the system, the liquid-solid transformation occurred again. According to the established quantitative assessments of the transformation rates, pressure and temperature, it showed that there was a negative pressure dependence of the solid-liquid transformation rate. However, the elevation of pressure can accelerate the liquid-solid transformation rate. Based on the calculated activation energy values, an interfacial reaction and diffusion dominated the solid-liquid transformation, but the liquid-solid transformation was mainly controlled by diffusion. This experimental technique is a powerful and effective tool for the transformation kinetics study of n-octane, and the obtained results are of great significance to the kinetics study of minerals or rocks related to solid-liquid phase transitions in geological environments.