n-Heptane under pressure: structure and dynamics from molecular simulations.

Area of Science:

• Computational chemistry
• Materials science
• Chemical physics

Background:

• High-pressure spectroscopic experiments have prompted further investigation into n-heptane's phase behavior.
• Understanding phase transitions in alkanes is crucial for various chemical and industrial applications.

Purpose of the Study:

• To explore the liquid-solid and solid-solid phase transitions of n-heptane under varying pressures using molecular dynamics simulations.
• To investigate the molecular mechanisms underlying these transitions, particularly at high pressures.

Main Methods:

• Atomistic molecular dynamics simulations were conducted in the isothermal-isobaric ensemble.
• Simulations started from a stabilized solid phase at 300 K and 10 kbar.
• Pressure was systematically varied to observe transitions, complemented by normal-mode analysis for vibrational density of states.

Main Results:

• The liquid-solid transition was identified through the formation of gauche defects in the n-heptane molecular backbone.
• No evidence of a first-order solid-solid transition was observed at high pressures within the simulation model.
• Significant changes in the local environment and dynamics of methyl groups were detected.

Conclusions:

• Molecular dynamics simulations provide insights into n-heptane's phase behavior, particularly the liquid-solid transition.
• The absence of a clear solid-solid transition suggests complex behavior or limitations of the current model at extreme pressures.
• Observed changes in methyl group dynamics and vibrational properties warrant further investigation.