In Situ High-Pressure and High-Temperature Experiments on n-Heptane

Abstract

The Raman spectroscopy of n-heptane was investigated in a moissanite anvil cell at ambient temperatures and a diamond anvil cell under pressures of up to ~2000 MPa and at temperature range from 298 to 588 K. The results show that at room temperature the vibration modes, assigned to the symmetric and antisymmetric stretching of CH(3) and CH(2) stretching, shifted to higher frequency according to quasi-linearity with increasing pressure, and a liquid-solid phase transition occurred at near 1150 MPa. The high-temperature solidus line of n-heptane follows a quadratic function of P = 0.00737T(2) + 5.27977T - 1195.76556. Upon phase change, fitting the experimental data obtained in the temperature range of 183∼412 K to the Clausius-Clapeyron equation allows one to define the thermodynamic parameters of n-heptane of dP/dT = 0.01474T + 5.27977.

Phase Transition of Heptane under Pressure

“…It appears that the nonhydrostatic effects start small and gradually increase with increasing pressure. The hydrostatic limit of 3.5 GPa occurs where heptane is in its solid phase, as heptane freezes at 1.2 GPa. , Evidently, the solid phase remains soft enough at a lower pressure to serve as a hydrostatic medium below 3.5 GPa.…”

“…11 Heptane has a liquid-to-solid phase transition at 1.2 GPa. 12,13 Ma et al 14 reported observations of a new peak in the heptane X-ray diffraction (XRD) pattern appearing at a dspacing and pressure of 2.30 Å and 3 GPa, respectively, attributed to a rotator−rotator phase transition, along with a second phase transition at ∼7.5 GPa. The 3 GPa phase transition was apparently confirmed by Yamaguchi et al 15 based on IR transmission spectroscopy experiments.…”