Molecular dynamics simulation of the interaction between palmitic acid and high pressure CO2.

In this study, molecular dynamics simulation was used to explore the interaction characteristics of palmitic acid and CO2, and the effects of temperature and pressure on the solubility of palmitic acid in CO2 were investigated. In the range of 293-353 K and 5-30 MPa, the snapshot of palmitic acid distribution in CO2 shows that the molecular chain of palmitic acid in high-density CO2 system is more straight and more dispersed than that in low-density CO2 system. The radial distribution function further clearly shows that the solubility of palmitic acid in CO2 decreases with the increase of temperature and increases with the increase of pressure, which is consistent with the fatty acid solubility data reported in the literature and the setting rules of supercritical CO2 extraction process conditions. As the temperature decreases and the pressure increases, the interaction energy between palmitic acid and CO2 increases, which is conducive to overcoming the intermolecular force of palmitic acid and promoting dissolution. The solubility parameters of palmitic acid and CO2 can better reflect the trend of palmitic acid solubility changing with temperature and pressure, which can play a guiding role in the determination of process conditions and even the development of new processes.

Molecular-level investigation of plasticization of polyethylene terephthalate (PET) in supercritical carbon dioxide via molecular dynamics simulation.

The current study aims to use the molecular dynamics (MD) simulation method to discuss the glass transition behaviour and fractional free volume (FFV) of the pure polyethylene terephthalate (PET) and the plasticized PET induced by supercritical carbon dioxide (SC-CO2) sorption. The adsorption concentration reproduced through sorption relaxation cycles (SRC) was firstly estimated and in an order of magnitude with the known experimental results available in the reported literature. The FFV induced by SC-CO2 in PET polymer changes regularly, which is proportional to the capacity of SC-CO2 adsorption with the changes in temperature and pressure. The glass transition temperature (T g) was further estimated to be almost identical to the known experimental values and shows a gradually decreasing tendency with the increase of pressure. Meanwhile, the plasticization of PET polymer studied by radial distribution functions showed that CO2 molecules occupying the sorption sites on the PET backbone promoted plasticization by increasing the fluidity of the PET backbone chain.