Exploring critical size and critical temperature: Insights into adsorption and diffusion in nano confinement slits

Abstract

Nanoscale confinement affects unconventional oil and gas development, materials science, chemistry, and energy storage. Size and temperature significantly affect adsorption and diffusion, but the critical size and temperature for nano-confinement effects are not yet clear. In this study, molecular dynamics simulations were employed to investigate the n-heptane adsorption and diffusion characteristics of hydroxylated quartz in nano-confinement, the analysis elucidated that both size and temperature effects have important influences on adsorption and diffusion, the critical size is 6 nm, and the critical temperature is 303 K for the nano-confinement effect. Beyond 6 nm, a free adsorption layer appears, with constant absolute adsorption capacity and density peak value, and increased diffusion with increasing size and temperature, the interaction energy remains relatively constant with varying slit sizes. Below 6 nm, nano-confinement forms a fully adsorbed layer, absolute adsorption capacity decreases with increasing size, the interaction energy increases monotonically with decreasing slit sizes, the diffusion coefficient in the Z-direction is low to zero; The density peak value of the first adsorption layer increases and the diffusion coefficient decreases with decreasing size below 303 K; Below 6 nm and above 303K, the two values show little correlation with size and exhibit somewhat random fluctuations, suggesting that temperatures exceeding 303 K weaken the nanoconfinement effect. This paper provides certain theoretical guidance for the development of fluids in nanopores such as shale oil.

Cited as：Zhang, X., Ye, Q., Shao, S., Zhou, Y., Zhu, W., A, H., Hu, W. Exploring critical size and critical temperature: Insights into adsorption and diffusion in nano confinement slits. Computational Energy Science, 2024, 1(2): 117-126. https://doi.org/10.46690/compes.2024.02.05

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