Pressure and Temperature Tuning of Gas-Expanded Liquid Structure and Dynamics

Carbon dioxide-expanded liquids (CXLs) represent an important class of reaction media that provide tunability of mass transport, solvation, and solubility. Their properties have been demonstrated to provide advantages over traditional organic solvents. However, the molecular-level effects of the CO₂ expansion on the structure and dynamics of the liquid that lead to this result have not been fully explored. To address this question, we have used molecular simulations to examine the behavior of two CXLs relevant to the hydroformylation of 1-octene, which has been demonstrated to benefit from the use of gas-expanded reaction media. Specifically, the phase equilibrium properties of CO₂-expanded 1-octene and nonanal are calculated as functions of temperature and pressure using Gibbs ensemble Monte Carlo simulations to determine the pressure-composition phase diagrams and volume expansion. In addition, molecular dynamics (MD) simulations were conducted to compute the liquid structure, diffusion coefficients, and shear viscosities. The simulated phase diagrams are in excellent agreement with previous experimental data when available, validating the models used. The MD simulations reveal a direct, linear relationship between the liquid viscosity and the volume expansion, which has not been previously reported. In contrast, deviations from such a relationship are observed for the diffusion coefficient at large volume expansion, indicating that a single Stokes-Einstein relation cannot describe the behavior at all pressures.