A study of self-diffusion coefficient and prediction model of nano-confined supercritical water

The diffusion of nano-confined fluids plays a crucial role in nano-energy research. We developed three molecular models to calculate the diffusion behavior of both supercritical water (SCW) at 673-1173 K, 250 atm, and room water (300 K, 1 atm), confined in carbon nanotubes (CNTs) ranging from 9.49 to 50.17 Å. We analyzed the diffusion mechanism of water confined in various CNTs using the time coefficient. We calculated the self-diffusion coefficient of water in Fickian-like diffusion mode and examined the factors influencing it. The results indicate that in small-diameter CNT (7,7), SCW primarily follows a Fickian-like diffusion mode, while room temperature (300 K, 1 atm) water exhibits a superdiffusion mode. For CNT diameters larger than 20 Å, both room temperature water and SCW predominantly exhibit Fickian-like diffusion. Additionally, the self-diffusion coefficient of SCW increases linearly with temperature, displaying clear Arrhenius behavior. The self-diffusion activation energy of SCW in different types of CNTs shows a strong correlation with the hydrogen bond structure. Finally, we combined the saturated relationship between CNT diameter and self-diffusion coefficient to propose a predictive model for the self-diffusion coefficient of confined SCW. The model is simple, requiring only three parameters, with a mean absolute relative error of less than 6.5%.