Molecular Simulation Study on the Density Behavior of n‑Alkane/CO₂ Systems

The density and volumetric behavior of three typical n-alkanes (hexane, octane, and decane) influenced by different mole fractions of CO₂ injected in them at temperatures from 303 to 363 K and pressures from 3.8 to 8.67 MPa were investigated by performing molecular dynamics simulations. It is shown that the mass density first increases and then decreases with increasing CO₂ mole fraction. Correspondingly, the system volume only slightly swells at low CO₂ contents while suddenly expanding when the CO₂ mole fraction exceeds a value of ∼60%. The calculations of structural properties and interaction energies indicate that at low CO₂ mole fractions, there are a few CO₂ molecules existing in the gap of alkane molecules, resulting in poor compressibility, while at higher CO₂ concentrations, the CO₂ molecules begin to separate from the CO₂-saturated alkane phase and form a gas phase, leading to higher compressibility. Therefore, at high CO₂ mole fractions, the system density and volume can more easily be changed by temperature and pressure than that at low CO₂ mole fractions. In addition, since it is harder for alkanes with longer chains to separate from each other, the volume swelling decreases and the density increases with increasing carbon number of n-alkane chains. Finally, we found that the increase in CO₂ mole fraction, temperature, and the decrease in alkane chain length would promote the diffusion of both CO₂ and alkane molecules. However, the influence of pressure on molecular diffusion is very limited except when P = 8.67 MPa and T = 333 K, where CO₂ is in the supercritical state. This work is helpful for understanding the density and volumetric behavior of n-alkane/CO₂ mixtures at a molecular level and provides useful information for guiding carbon sequestration and CO₂-enhanced oil recovery.