Molecular dynamics simulation on the local density distribution and solvation structure of supercritical CO2 around naphthalene

- A solute-induced effect on the density enhancement of near-critical fluids is weak.
- Weak local density change indicates no formation of distinct cluster structure.
- High/low density region near the solute is from the fluctuation of neat solvent.
- Solute selects its surroundings depending on the extent of the attractive forces.
- Pre-existent density fluctuation amplifies the effect of short-range interactions.

Molecular Dynamics (MD) simulations were performed to understand the solvation structure and the mechanistic nature of the divergence of partial molar volume (v˜2∞) near the critical point. Isothermal-isobaric (NpT) simulations were carried out at Tr = 1.03 from pr = 0.87 to pr = 3.72 with the truncated and shifted Lennard-Jones and the Weeks-Chandler-Andersen (WCA) potentials. A statistical analysis on the structural characters was carried out to investigate the origin of the divergent v˜2∞ near the critical point. The result suggested that no distinct cluster structure exists. Rather, a solute enters the pre-existent high-density and/or low-density domain and its preference depends on the magnitude of the solute/solvent attractive interaction. The influence of the attractive forces is propagated via the density fluctuation throughout the entire system. Thus, the interplay between the solute/solvent attraction and the pre-existent density fluctuation is the mechanistic origin of the divergence of v˜2∞ near the critical point.

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