Molecular Dynamics Study of Structural and Transport Properties of Supercritical CO₂

Erik Zampieri, Matheus Polato Benatte Fernandes and Luiz Fernando da Costa Zonetti

Departamento de Física e Química, Universidade Estadual Paulista (UNESP), Faculdade de Engenharia, Caixa Postal 31, 15385-000, Ilha Solteira, SP, Brazil. And Instituto Federal de Educação, Ciência e Tecnologia de São Paulo (IFSP), Campus Birigui, Rua Pedro Cavalo, 709, Residencial Portal da Pérola II, 16201-407, Birigui, SP, Brazil.

Abstract
Carbon dioxide reaches its supercritical point at 304.13 K and 73.7 bar [1]. Where it exhibits intermediate properties between liquids and gases, such as high density and enhanced diffusivity [2]. Under these conditions, supercritical CO₂ (scCO₂) becomes a fluid of great interest for nanotechnology-related applications, including nanomaterials synthesis, surface modification, and transport in nanostructured systems [3]. In this work, the structural and dynamic properties of CO₂ in the supercritical regime are investigated using molecular dynamics simulations. A total of 2197 molecules were simulated in a cubic box of 70 × 70 × 70 ų using the NAMD software with the CHARMM36 force field, under the canonical ensemble, for temperatures ranging from 300 K to 410 K. The fluid structure was characterized through the radial distribution function (RDF) between carbon atoms (C–C), allowing the analysis of local organization at the nanoscale. It is observed that, with increasing temperature, there is a progressive decrease in the height of the first RDF peak, indicating a loss of structural order and a transition from a liquid-like regime to a more gas-like behavior. Molecular mobility was calculated from the mean square displacement (MSD), whose time evolution allowed the determination of the self-diffusion coefficient using Einstein’s relation. A significant increase in diffusion with increasing temperature was observed, as evidenced by the steeper slope of the MSD over time. This behavior is directly associated with the reduction of intermolecular correlations, resulting in greater molecular mobility in the supercritical regime. Overall, the results demonstrate a strong correlation between structure and transport at the nanoscale, providing fundamental insights for applications of supercritical CO₂ in nanostructured systems and advanced materials processing.

Keywords: Supercritical CO₂, Molecular Dynamics, Diffusion.

Acknowledgements: This study was also financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001.

[1] National Institute of Standards and Technology (NIST). Carbon Dioxide Thermophysical Properties. 2024.

[2] McHugh, M. A.; Krukonis, V. J. Supercritical Fluid Extraction: Principles and Practice. 2nd ed. Butterworth-Heinemann, 1994.

[3] Díaz-Reinoso, B.; Moure, A.; Domínguez, H.; Parajó, J. C. Supercritical CO₂ extraction and purification of compounds with antioxidant activity. Journal of Agricultural and Food Chemistry, 54 (7), 2441–2469, 2006.