Anomalous Reversal of CO2-Induced Viscosity/Diffusivity Trends in Water: A Molecular Dynamics Study

Abstract Experimental viscosity data for dilute, water-rich aqueous CO2 are available only up to moderate temperatures, with reported measurements up to 449 K, leaving the temperature regime relevant to geothermal and carbon capture, utilization, and storage (CCUS) applications largely unexplored. Here, equilibrium molecular dynamics (MD) simulations were used to compute viscosity, self-diffusion coefficients, and density of CO2–H2O mixtures over 278–900 K and 10–30 MPa using multiple combinations of force fields for water and CO2. At 278–443 K, increasing the CO2 fraction increased viscosity and decreased molecular mobility, consistent with available low-temperature measurements. At 573 K, however, the trend reversed: at xCO2 = 5 mol %, the viscosity decreased by about 10% relative to pure water at 25 MPa, whereas the diffusivity increased by about 8% at 20 MPa. Above 700 K, composition effects became weaker and often comparable to the associated uncertainties. Analyses of number density, CO2 hydration, hydrogen bonding, tetrahedral order, and rotational relaxation show that the reversal behavior originates from the temperature-dependent volumetric and structural response of the water network to CO2 dissolution. The results identify a transport anomaly near 573 K that should be considered in modeling water-rich CO2-bearing fluids.